ups breakers

APC by Schneider Electric 01/2012 edition p. 1

Key factors in UPS installations

Contents Introduction ..................................................................... 2 Using this guide .............................................................. 3 Overview of protection solutions .................................. 4

Protection solutions ..............................................................................4 Accompanying software and services ..................................................5

UPSs in electrical installations ...................................... 6 Function of each component in the installation.....................................6 Essential installation parameters ..........................................................7 Sources of information in setting up installation specifications .............8

Basic notions on installations with UPSs ..................... 9 Need for high-quality and high-availability power .................................9 Supply systems with UPSs ...................................................................10 UPS power quality ................................................................................11 UPS power availability ..........................................................................13 Selection of the configuration ...............................................................16

Power calculations .......................................................... 17 Elements required for power calculations .............................................17 Ratings of single-UPS configurations ...................................................19 Ratings of parallel-UPS configurations .................................................22

Control of upstream harmonics ..................................... 24 UPSs and upstream harmonic currents for different input rectifiers .....24 Filtering of upstream harmonics for Graetz bridge rectifiers .................25 Selection of a filter ................................................................................27

System earthing arrangements ...................................... 30 Background information on system earthing arrangements .................30 Applications in UPS installations ..........................................................32

Protection ........................................................................ 35 Protection using circuit breakers ..........................................................35 Selection of circuit breakers .................................................................38

Cables ............................................................................. 43 Selection of cable sizes ........................................................................43 Example of an installation ....................................................................44

Energy storage ............................................................... 45 Storage technologies ............................................................................45 Selection of a battery ............................................................................46 Battery monitoring ................................................................................47

Human-machine interface and communication ........... 49 Human-machine interface (HMI) ..........................................................49 Communication ....................................................................................49

Preliminary work ............................................................ 51 Installation considerations ....................................................................51 Battery room .........................................................................................52


Introduction

Growing needs for high-quality and high-availability power Problems related to the quality and availability of electrical power have become vitally important due to the key role of computers and electronics in the development of many critical applications. Disturbances in distribution systems (micro-outages, outages, voltage sags, etc.) can result in major losses or safety hazards in a number of activities such as: • Sensitive process industries, where a malfunction in the control/monitoring systems can result in production losses. • Airports and hospitals where faulty operation of equipment can represent a serious danger to human life. • Information and communication technologies, where the necessary level of reliability and dependability is even higher. Data centers require high-quality, "no-break" power 24/365, year after year and without halts for maintenance. UPS protection systems are now an integral part of the value chain of many companies. Their level of availability and power quality have a direct effect on the service continuity of operations. Productivity, the quality of products and services, the competitiveness of the company and site security depend on the smooth operation of the UPS. Failure is not an option. APC by Schneider Electric - a complete solution covering all needs APC by Schneider Electric offers a complete range of power-protection solutions to meet the needs of all sensitive applications. These solutions implement communicating software and products incorporating state-of-the-art technology offering the highest levels of reliability. They are backed by complete services based on unique expertise, worldwide presence and use of the most advanced techniques and technologies. APC Global ServicesTM, with 40 years of experience on customer sites, accompanies your installation throughout its life cycle, from design and start-up to operation and upgrades, wherever they may be. Uninterruptible power supplies (UPSs) are of course a central part of these solutions. They supply high-quality, high-availability continuous power with built-in, advanced communication interfaces that are compatible with both electrical and computer environments They are often used in conjunction with other communicating products such as active harmonic conditioners, transfer switches, distribution switchboards, battery-monitoring systems and supervision software. Taken as a whole, this offering provides a complete and effective answer to the protection problems that arise in sensitive installations. For data centers, on-demand solutions integrate the physical infrastructure including server racks, UPSs, electrical distribution, cooling and security along with the associated software. A guide to assist professionals dealing with electrical installations for critical applications APC by Schneider Electric has made a large part of its know-how available in this design guide. Its purpose is to assist in designing and installing complete, optimised power-protection solutions, from the utility line through to the final load, corresponding to the quality and availability requirements of your critical applications. It is intended for all professionals dealing with this type of installation, including: • Independent design offices and engineering firms, • End-user design departments, • Installers, • Project managers, • Facility managers, • Computer system managers, • Financial or purchasing managers.


Using this guide

Structure of this document

Finding information Information may be located in a number of ways: • The general contents at the start of the guide, • The overview on pages 4 and 5 of chapter” Key factors in UPS installation”, which presents the products, communication systems, software and services that are all part of protection solutions.

Chapters • Chapter” Key factors in UPS installation” presents on pages 6 and 7 the role of UPSs in electrical installations and indicates the main parameters that must be taken into account. The remainder of the chapter guides you through the selection process for a solution by determining the main elements of an installation with a UPS. • Chapter “Selection of the UPS configuration” presents a number of practical examples in view of selecting a configuration, from a simple, single-UPS unit through to installations offering exceptionally high levels of availability. • Chapter “eliminate harmonic currents” presents solutions to eliminate harmonic currents in installations. • Chapter” Technical review” provides background technical information for devices and notions mentioned in other parts of the guide. Finally, to facilitate the preparation of projects:

Cross references The various chapters contain cross references (indicated by the symbol ) to other parts of the design guide presenting more in-depth information on specific topics. Refererences to technical articles (White Papers - WP) are indicated by the following symbol together with the number of the White Paper in question.

Chap. 1: Key factors in UPS installation Chap. 2 : Selection of the UPS configuration Chap. 3 : Eliminate harmonic currents Chap. 5 : Technical review

See WP no.


Overview of protection solutions

Power protection solutions

Fig. 1.1. APC by Schneider Electric products.


Overview of protection solutions

Accompanying software and services

Fig. 1.2. APC by Schneider Electric software and services.


UPSs in electrical installations

Function of each component in the installation

Fig. 1.3. Functions of the components in installations with UPSs.


UPSs in electrical installations (cont.)

Essential installation parameters

Fig. 1.4. Main parameters for the components in installations with UPSs.

APC by Schneider Electric GUIPR537UK - 01/2012 edition p. 8

UPSs in electrical installations (cont.)

Sources of information in setting up installation specifications The diagrams on the previous pages provide a general overview of the components and various parameters in installations with UPSs. It is now time to go into more detail. The table below indicates: the order in which the subjects are presented in this chapter the choices that must be made the purpose of each decision with the indication of the pages concerning the relevant elements in this chapter where additional information on each subject may be found in the other chapters of this design guide.

Choices Purpose See Additional information SeeMono or multi-source architecture and configuration of UPS sources

Determine the installation architecture and UPS configuration best suited to your requirements in terms of energy availability, upgrades, operation and budget.

Selection of the UPS

configuration

Examples and comparison of 13 typical installations, from single-UPS units to high-availability architectures.

Selection of the UPS

configurationp. 5

Supplying sensitive loads. Technical review

p. 2 UPS configurations. Technical

review p. 23

Engine generator sets. Technical review p. 35

UPS power rating Determine the rating of the UPS unit or parallel units (for redundancy or capacity) required, taking into account the distribution-system and load characteristics.

Key factors in UPS

installations p. 17

UPS make-up and operation. Technical review p. 14

Control of upstream harmonics

Reduce voltage distortion on the upstream busbars to acceptable levels, depending on the power sources likely to supply the UPS system.

Key factors in UPS

installations p. 24

Elimination of harmonics in installations.

Eliminate harmonic currents

Harmonics Technical review p. 38

System earthing arrangements

Ensure installation compliance with applicable standards for the protection of life and property and correct operation of devices. Which system earthing arrangements are required for which applications?

Key factors in UPS

installations p. 30

Upstream and downstream protection using circuit breakers

Determine the breaking capacity and the ratings of the circuit breakers upstream and downstream of the UPS, solve any discrimination problems.

Key factors in UPS

installations p. 35

Connections Limit voltage drops and temperature rise in the cables, as well as harmonic distortion at the load inputs.

Key factors in UPS

installations p. 43

Battery Operation on battery power (backup time) must last long enough to meet user requirements.

Key factors in UPS

installations p. 45

Energy-storage solutions and batteries.

Technical review p. 31

Communication Define UPS communication with the electrical and computer environment.

Key factors in UPS

installations p. 49

Preliminary work (if any)

Construction work and ventilation must be planned, notably if there is a special battery room.


Standards Be aware of the main applicable UPS standards.


Electromagnetic compatibility Technical review p. 26


Basic notions on installations with UPSs

Disturbances in distribution-system power Public and private utilities supply electricity whose quality may be reduced by a number of disturbances. These disturbances are inevitable due to the distances involved and the wide variety of connected loads. The origin of disturbances includes: • the distribution system itself (atmospheric conditions, accidents, switching of protection or control devices, etc.), • user equipment (motors, disturbing devices such as arc furnaces, welding machines, systems incorporating power electronics, etc.). These disturbances include micro-outages, voltage sags, overvoltages, frequency variations, harmonics, HF noise, flicker, etc. through to extended outages.

Disturbances in distribution-system power, see Ch. 5 p. 3.

Requirements of sensitive loads Digital equipment (computers, telecom systems, instruments, etc.) use microprocessors that operate at frequencies of several mega or even giga Hertz, i.e. they carry out millions or even billions of operations per second. A disturbance in the electrical supply lasting just a few milliseconds can affect thousands or millions of basic operations. The result may be malfunctions and loss of data with dangerous (e.g. airports, hospitals) or costly consequences (e.g. loss of production). That is why many loads, called sensitive or critical loads, require a supply that is protected against distribution-system disturbances. Examples. • industrial processes and their control/monitoring systems - risk of production losses. • airports and hospitals - risks for the safety of people. • information and communication technologies - risk of halts in processing at a very high hourly cost. Many manufacturers of sensitive equipment specify very strict tolerances (much stricter than those for the distribution system) for the supply of their equipment, one example being CBEMA (Computer Business Equipment Manufacturer’s Association) for computer equipment.

Sensitive loads, see Technical review p. 2 "Supply of sensitive loads". Costs incurred by the quality of electrical power Over 50% of failures for critical loads are due to the electrical supply and the hourly cost of downtime for the corresponding applications is generally very high (fig. 1.5). It is therefore vital for the modern economy, which is increasingly dependent on digital technologies, to solve the problems affecting the quality and the availability of the power supplied by the distribution system when it is intended for sensitive loads.

Examples of hourly costs of failures mobile telephones - 40 kEuros. airline reservation systems - 90 kEuros. credit-card transactions - 2.5 MEuros. automotive assembly line - 6 MEuros stock-market transactions - 6.5 MEuros.

Fig. 1.5. Origin and cost of system failures due to the electrical supply.

Humanerror

Supplyproblems

Equipmentfailure

Nuisance tripping(circuit breaker, etc.)

15 %

20 %

20 %

45 %

Need for high-quality and high-availability power


Basic notions on installations with UPSs (cont .)

Purpose of UPSs UPSs (uninterruptible power supply) are designed to meet the needs presented above. First launched in the 1970s, their importance has grown in step with the development of digital technologies. UPSs are electrical devices that are positioned between the distribution system and sensitive loads. They supply power that is much more reliable than the distribution system and corresponds to the needs of sensitive loads in terms of quality and availability.

UPSs, see Technical review p. 4 "The UPS solution".

Types of UPSs The term UPS covers products with apparent power ratings from a few hundred VA up to several MVA, implementing different technologies. That is why standard IEC 62040-3 and its European equivalent ENV 62040-3 define three standard types (topologies) of UPS. UPS technologies include: • Passive standby, • Interaction with the distribution system, • Double conversion. For the low power ratings (< 2 kVA), the three technologies coexist. For higher ratings, virtually all static UPSs (i.e. implementing semiconductor components, e.g. IGBTs) implement the double-conversion technology. Rotary UPSs (with rotating mechanical parts, e.g. flywheels) are not included in the standards and remain marginal on the market.

Types of UPSs, see Technical review p. 9 "Types of static UPSs". Double-conversion static UPSs This is virtually the only type of UPS used in high-power installations due to their unique advantages over the other types: • complete regeneration of the power supplied at the output, • total isolation of the load from the distribution system and its disturbances, • no-break transfer (where applicable) to a bypass line. • The operating principle (fig. 1.6) is presented below. • during normal operation, a rectifier/charger turns the AC-input power into DC power to supply an inverter and float charge a battery. • the inverter completely regenerates a sinusoidal signal, turning the DC power back into AC power that is free of all disturbances and within strict amplitude and frequency tolerances. • if the AC-input power fails, the battery supplies the power required by the inverter for a specified backup time. • a static bypass can transfer the load without a break in the supply of power to a bypass line to continue supplying the load if need be (internal fault, short-circuit downstream, maintenance). This "fault-tolerant" design makes it possible to continue supplying power to the load in "downgraded mode" (the power does not transit the inverter) during the time required to re-establish normal conditions.

Double-conversion UPSs, see Technical review p. 14 "Components and operation".

Fig. 1.6. Double-conversion static UPS

Supply systems with UPSs



Power quality of double-conversion UPSs By design, double-conversion solid-state UPSs supply to the connected loads a sinusoidal signal that is: • high quality because it is continuously regenerated and regulated (amplitude ± 1%, frequency ± 0.5%), • free of all disturbances from the distribution system (due to the double conversion) and in particular from micro-outages and outages (due to the battery). This level of quality must be ensured, whatever the type of load. Voltage quality for linear loads What is a linear load? A linear load supplied with a sinusoidal voltage draws a sinusoidal current having the same frequency as the voltage. The current may be displaced (angle ϕ) with respect to the voltage (fig. 1.7). Examples of linear loads Many loads are linear, including standard light bulbs, heating units, resistive loads, motors, transformers, etc. They do not contain any active electronic components, only resistors (R), inductors (L) and capacitors (C). UPSs and linear loads For this type of load, the UPS output signal is very high quality, i.e. the voltage and current are perfectly sinusoidal, 50 or 60 Hz.

Purely resistive load Load with inductor and/or capacitor

Fig. 1.7. Voltage and current for linear loads. Voltage quality for non-linear loads What is a non-linear load? A non-linear (or distorting) load supplied with a sinusoidal voltage draws a periodic current that has the same frequency as the voltage, but is not sinusoidal. The current drawn by the load is in fact the combination (fig. 1.8) of: • a sinusoidal current called the fundamental, at the 50 or 60 Hz frequency, • harmonics, which are sinusoidal currents with an amplitude less than that of the fundamental, but a frequency that is a multiple of the fundamental and which defines the harmonic order (e.g. the third order harmonic has a frequency 3 x 50 Hz (or 60 Hz) and the fifth order harmonic has a frequency 5 x 50 Hz (or 60 Hz)). The harmonic currents are caused by the presence of power-electronic components (e.g. diodes, SCRs, IGBTs) which switch the input current. Examples of non-linear loads Non-linear loads include all those that have a switch-mode power supply at their input to supply the electronics (e.g. computers, variable-speed drives, etc.).

Effect of harmonics (H3 and H5 in this

example).

Voltage and current drawn by a single-phase switch-mode power supply (computers).

Fig. 1.8. The current drawn by non-linear loads is distorted by the harmonics.

Power quality of UPSs



Harmonic spectrum of the current drawn by a non-linear load The harmonic analysis of a non-linear current consists in determining (fig. 1.9): • the harmonic orders present in the current, • the relative importance of each order, measured as the percentage of the order.

Hk% = distortion of harmonic k = rms valueof harmonick

rms valueof the fundamental

Voltage and current harmonic distortion Non-linear loads cause both current and voltage harmonics. This is because for each current harmonic, there is a voltage harmonic with the same frequency. The 50 Hz (or 60 Hz) sinusoidal voltage of the UPS is therefore distorted by the harmonics. The distortion of a sine wave is presented as a percentage:

THD* % = total distortion = rms value of all theharmonic krms value of the fundamental

* Total Harmonic Distortion. The following values are defined: • TDHU % for the voltage, based on the voltage harmonics, • TDHI % for the current, based on the current harmonics (fig. 1.9). The higher the harmonic content, the greater the distortion. Practically speaking, the distortion in the current drawn by the load is much higher (THDI approximately 30%) than that of the voltage at the input (THDU approximately 5%).

Input current of a three-phase rectifier.

Harmonic distortion levels

H5 = 33% H7 = 2.7%

H11 = 7.3% H13 = 1.6% H17 = 2.6% H19 = 1.1% H23 = 1.5% H25 = 1.3%

THDI = 35% (see calculation ch. 5, p. 41)

Harmonic spectrum and corresponding THDI.

Fig. 1.9. Example of the harmonic spectrum of the current drawn by a non-linear load.

Non-linear loads, see "Elimination of harmonics in installations" and Technical review p. 38 "Harmonics". UPSs and non-linear loads Harmonics affect the sinusoidal voltage at the UPS output. Excessive distortion can disturb the linear loads connected in parallel on the output, notably by increasing the current they draw (temperature rise). To maintain the quality of the UPS output voltage, it is necessary to limit its distortion (THDU), i.e. limit the current harmonics that produce voltage distortion. In particular, it is necessary that the impedance (at the UPS output and in the cables supplying the load) remain low. Limiting the distortion of the output voltage Due to the free-frequency chopping technique employed, the impedance at the output of UPSs from APC by Schneider Electric is very low, whatever the frequency (i.e. whatever the harmonic order). This technique virtually eliminates all distortion in the output voltage when supplying non-linear loads. The quality of the output voltage is thus constant, even for non-linear loads. Practically speaking, installation designers must: • check UPS output values for non-linear loads and, in particular, make sure that the announced level of distortion, measured for standardised non-linear loads as per standard IEC 62040-3, is very low (THDU < 2 to 3%), • limit the length (impedance) of the output cables supplying the loads.

UPS performance for non-linear loads, see Technical review p. 43.



What is meant by availability? Availability of an electrical installation Availability is the probability that the installation will be capable of supplying energy with the level of quality required by the supplied loads. It is expressed as a percentage.

Availability (%) = ( )1 100− ×MTTRMTBF

The MTTR is the mean time to repair the supply system following a failure (including the time to detect the cause of the failure, repair it and start the system up again). The MTBF is the mean time between failures, i.e. the time the supply system is capable of ensuring correct operation of the loads. • Example. An availability of 99.9% (called thee nines) corresponds to a 99.9% chance that the system will effectively carry out the required functions at any given time. The difference between this probability and 1 (i.e. 1 - 0.999 = 0.001) indicates the level of non-availability (i.e. one chance out of 1000 that the system will not carry out the required functions at any given time).

Fig. 1.10. MTTR and MTBF. What is the practical signification of availability? Down-time costs for critical applications are very high (see fig. 1.5). These applications must obviously remain in operation as long as possible. The same is true for their electrical supply. The availability of the energy supplied by an electrical installation corresponds to a statistical measurement (in the form of a percentage) of its operating time. The MTBF and MTTR values are calculated or measured (on the basis of sufficiently long observations) for the components. They can then be used to determine the availability of the installation over the period. What are the factors contributing to availability? Availability depends on the MTBF and the MTTR. • Availability would be equal to 100% if the MTTR is equal to zero (instantaneous repair) or if the MTBF is infinite (operation with no breakdowns). This is statistically impossible. • Practically speaking, the lower the MTTR and the higher the MTBF, the greater the availability. From "3 nines" to "6 nines" The critical nature of many applications has created the need for much higher levels of availability for electrical power. • The "traditional" economy uses power from the public utility. An average-quality distribution system with HV backup offers 99.9% availability (3 nines), which corresponds to eight hours of non-availability per year. • Sensitive loads require an electrical supply capable of providing 99.99% availability (4 nines), which corresponds to 50 minutes of non-availability per year. • The computer and communication equipment in data centres requires 99.9999% availability (6 nines), which corresponds to 30 seconds of non-availability per year. This level is the means to ensure, without risk of major financial loss, operation of infrastructures 24/365, without shutdown for maintenance. It is a step toward a continuous supply.

UPS power availability



The "traditional" economy uses

public-utility power offering 99.9% availability, i.e. 3 nines.

Sensitive loads require a 99.99% level of availability, i.e. 4 nines.

Data centres require 99.9999%, i.e. 6 nines.

Fig. 1.11. Evolution in the level of availability required by applications. How can availability be improved? To improve availability, it is necessary to reduce the MTTR and increase the MTBF. Reduce the MTTR Real-time fault detection, analysis by experts to ensure a precise diagnosis and rapid repair all contribute to reducing the MTTR. These efforts depend on the key factors listed below. Quality of service • International presence of the manufacturer. • International availability of services. • The number, the qualification and the experience of service teams. • The installed product base and the experience gained. • Easy to maintain, modular UPSs • The resources and the proximity of the technical support. • Local availability of original spare parts. • High-performance manufacturer methods and tools. • Remote diagnostics. • Training in courses adapted to customer needs. • Quality and availability of documentation in the local language.

APC Global ServicesTM offers a complete range of consulting services, training and audits to provide users with the knowledge required for system operation, diagnostics and level-one maintenance.

APC Global ServicesTM

Reduce the MTTR Increase availability

Fig. 1.12. The quality of service is an essential factor in high availability. UPS communication capabilities • User-friendly interface providing easy operating diagnostics. • Communication with the electrical and computer environment.

Communication and supervision of UPSs from APC by Schneider Electric, see . UPS communication.



Increase the MTBF This goal depends primarily on the factors listed below. Selection of components with proven reliability • Products with certified design, development and manufacturing processes. • Performance levels certified by recognised, independent organisations. • Compliance with international standards on electrical safety, EMC (electromagnetic compatibility) and performance measurement.

With 40 years experience and protecting 350 GVA of critical power, solutions from APC by Schneider Electric have proven their value to the major industrial companies. All products comply with the main international standards and their level of performance is certified by recognised organisations.

Certified quality and reliability Increase the MTBF Increase availability

Fig. 1.13. The proven reliability of products increase the MTBF and availability. Built-in fault tolerance Fault tolerance makes possible operation in a downgraded mode following faults that may occur at different levels of the installation (see fig. 1.14). During the time required to repair, the load continues to be supplied and generates revenues.

Fig. 1.14. Fault tolerance increases availability. Installation maintainability This is the capacity to isolate (de-energise) parts of the installation for maintenance under safe conditions, while continuing to supply the load. It should be possible: • in the UPS, due to the static bypass and maintenance bypass, • in other parts of the installation, depending on the architecture.

Direct supply of the load during maintenance. Automatic, no-break transfer of the load to the bypass line following a downstream internal fault or overload.

Fig. 1.15. Static bypass and manual maintenance bypass.

Immediate tripping: - detection and alarms - identification of causes - corrective action



APC by Schneider Electric solutions ensure fault tolerance and maintainability by implementing: • double-conversion UPSs capable of transferring the load to the Bypass AC input via the automatic bypass and equipped with a maintenance bypass, • redundant, multi-source UPS configurations with STS units. Key factors to the availability of installations with UPSs A few years ago, most installations were made up of single-UPS units and the number of parallel systems was small. The applications requiring this type of installation still exist. However, the shift toward high availability requires use of configurations offering redundancy at a number of levels in the installation (see fig. 1.16).

Source redundancy: availability even during long utility outages. UPS redundancy: reliability, easier and safer maintenance. Redundant distribution with STS units: maximum availability.

Fig. 1.16. The required levels of availability have resulted in the use of redundancy on a number of levels in the installation. This trend has led designers, depending on the criticality of the loads and the operating requirements, to take into account some or all of the key factors listed below. Reliability and availability Propose a configuration corresponding to the level of availability required by the load, comprising components with proven levels of reliability and backed up by a suitable level of service quality. Maintainability Ensure easy maintenance of the equipment under safe conditions for personnel and without interrupting operation. Upgradeability It must be possible to upgrade the installation over time, taking into account both the need to expand the installation gradually and operating requirements. Discrimination and non propagation of faults It must be possible to limit faults to as small a part of the installation as possible, while enabling servicing without stopping operations. Installation operation and management Make operations easier by providing the means to anticipate events via installation supervision and management systems. Prerequisite step in establishing installation specifications The selection of a configuration determines the level of availability that will be created for the load. It also determines the possible solutions for most of the factors listed above. The configuration may be single or multi-source, with single or parallel UPS units and with or without redundancy. Selection of the configuration is the initial step in establishing installation specifications. To assist in making the right decision, chapter 2 is entirely devoted to this subject. It compares the various configurations in terms of availability, protection of the loads, maintainability, upgradeability and cost.

Configuration selection based on typical installations corresponding to different levels of availability, see Selection of the UPS configuration

Selection of the configuration


Power calculations

Installation considerations Type of load supplied Linear loads (cos ϕ) or non-linear loads (power factor). These characteristics determine the power factor at the UPS output. Maximum power drawn by the load under steady-state conditions For a load, this is the power rating. If a number of loads are connected in parallel on the UPS output, it is necessary to calculate the total load when all the loads operate at the same time. Otherwise, it is necessary to use diversity to calculate the most unfavourable operation in terms of the power drawn. In-rush currents under transient conditions or for a short-circuit downstream The overload capacity of a UPS system depends on the time the overload lasts. If this time limit is exceeded, the UPS transfers the load to the Bypass AC input, if its voltage characteristics are within tolerances. In this case, the load is no longer protected against disturbances on the distribution system. Depending on the quality of the Bypass AC power, it is possible to: • use the Bypass AC input to handle current spikes due to switching of devices or downstream short-circuits. This avoids oversizing the system, • disable automatic transfer (except for internal faults), while maintaining the possibility of manual transfers (e.g. for maintenance). UPSs from APC by Schneider Electric operate in current-limiting mode. By spacing switching of devices over time, it is generally possible to handle in-rush currents without having to transfer to the Bypass AC power. If the in-rush current exceeds the limiting threshold (e.g. 2.33 In for MGE Galaxy 9000 UPSs) for a few periods (but less than one second), the UPS current limits for the necessary time. This downgraded operating mode may be acceptable, for example, for a cold start (on battery power, utility power absent). Power of a UPS Rated power of a UPS This rating, indicated in the catalogues, is in the output power. It is indicated as an apparent power Sn in kVA, with the corresponding active power Pn in kW, for a: • linear load, • load with a cos ϕ = 0.8. However, last-generation UPSs from APC by Schneider Electric can supply loads with a cos ϕ = 0.9 leading. Calculation of the rated power Pn (kW) = 0.8 Sn (kVA). rated active power This calculation depends on the output voltage of the UPS and the current drawn by the load, where:

Sn (kVA) = UnIn 3 in three-phase systems Sn (kVA) = VnIn in single-phase systems For a three-phase UPS, U and I are rms line values, for a single-phase UPS, V is a phase-to-neutral voltage, where: Un = phase-to-phase voltage Vn = phase-to-neutral voltage Un = Vn 3 For example, if Un = 400 volts, Vn = 230 Volts. Power and type of load The two tables below present the equations linking the power, voltage and current, depending on the type of load (linear or non-linear). The following symbols are used: • instantaneous voltage u(t) and current i(t) values, • the corresponding rms values U and I, • ω = angular frequency = 2 π f where f is the frequency (50 or 60 Hz), • ϕ = displacement between the voltage and the current under sinusoidal conditions.

Elements required for power calculations


Power calculations (Cont.)

Linear loads Three-phase Single-phase Sinusoidal voltage u(t) = U 2 sin ωt between phases v(t) = V 2 sin ωt phase to neutral

U = V 3 Displaced sinusoidal current i(t) = I 2 sin (ωt - ϕ) phase current

Current crest factor 2 Apparent power S (kVA) = UI 3 cos ϕ S (kVA) = VI

Active power P (kW) = UI 3 cos ϕ = S (kVA) cos ϕ P (kW) = VI cos ϕ = S (kVA) cos ϕ

Reactive power Q (kvar) = UI 3 sin ϕ = S (kVA) sin ϕ Q (kvar) = VI sin ϕ = S (kVA) sin ϕ

S = P Q2 2+

Non-linear loads

Sinusoidal voltage The regulated UPS voltage remains sinusoidal (low THDU), whatever the type of load.

u(t) = U 2 sin ωt between phases v(t) = V 2 sin ωt phase to neutral

U = V 3 Current with harmonics

i(t) = i1(t) + Σihk(t) total phase current i1(t) = I1 2 sin (ωt - ϕ1) fundamental current

ik(t) = Ihk 2 sin (kωt - ϕk) k-order harmonic

I = I I I I12

22

32

42+ + + + .... rms value of the total current

C = peak current value / rms value Current crest factor

THDI = I I I I

I12

22

32

42

1

+ + + + .... Current total harmonic distortion

Apparent power S (kVA) = UI 3 S (kVA) = VI

Active power P (kW) = λ UI 3 = λ S (kVA) P (kW) = λ VI = λ S (kVA)

Power factor λ =

P kWS kVA

( )( )

UPS percent load This is the percentage of the rated power that is effectively drawn by the load.

Load (%) = S kVAS kVAload

n

( )( )

Recommendation: take into account growth in loads

It is advised to leave a margin (excess power) when setting the rated power, particularly if a site expansion is planned. In this case, make sure the percent load on the UPS is still acceptable after the expansion. UPS efficiency This factor determines the power drawn by the UPS on the upstream distribution system, i.e. the consumption. It may be calculated as:

η (%) = P kWP kWUPSoutput

UPSinput

( )( )

For a given power rating, a high level of efficiency: • reduces power bills, • reduces heat losses and, consequently, ventilation requirements.



It is possible to calculate the efficiency at full rated load, i.e. with a 100% load.

ηn (%) = P kW

P kWn

UPSinput

( )( )

The rated active power of the UPS is obtained by multiplying the rated apparent power Sn (kVA) by 0.8 (if λ > 0.8) or by λ (if λ< 0.8). The efficiency can vary significantly depending on the percent load and the type of load. The installation designer must therefore pay attention to two aspects of efficiency.

Recommendation 1: check the efficiency for non-linear loads The presence of non-linear loads tends to reduce the power factor to values below 0.8. It is therefore necessary to check the efficiency value for standardised non-linear loads. This check is recommended by standards IEC 62040-3 / EN 62040-3.

Recommendation 2: check the efficiency at the planned percent load Manufacturers generally indicate the efficiency at full rated load. However, its value may drop if the percent load is lower (1). Attention must therefore be paid to UPSs operating in an active-redundancy configuration, where the units share the total load and often operate at 50% of their full rated load, or less. (1) A UPS is optimised to operate at full rated load. Even though losses are at their maximum at full rated load, the efficiency is also at its maximum. In a standard UPS, losses are not proportional to the percent load and the efficiency drops sharply when the percent load drops. This is because a part of the losses is constant and the relative percentage of this part increases when the load decreases. To obtain high efficiency at low load levels, the constant losses must be very low.

Due to their design, UPSs from APC by Schneider Electric have very low constant losses and as a result, the efficiency is virtually stable for loads from 30 to 100%.

UPS efficiency, see Technical review p. 20.

Single-UPS configurations These configurations comprise a single, double-conversion UPS unit (see fig. 1.17). The overload capacity at the UPS output is indicated by a diagram (the example below is for the MGE Galaxy 9000 range). In the event of an internal fault or an overload exceeding UPS capacity, the system automatically transfers to the Bypass AC input. If transfer is not possible, UPSs from APC by Schneider Electric current limit for overloads greater than the maximum value (e.g. 2.33 In peak for one second for Galaxy 9000, which corresponds to a maximum sine wave with an rms value of 2.33 / 2 = 1.65 In). Beyond one second, the UPS shuts down. A set of disconnection switches is available to isolate the UPS for maintenance in complete safety.

Fig. 1.17. Single double-conversion static UPS unit and example of an overload curve.

Ratings of single-UPS configurations



Power levels under steady-state conditions A UPS is sized using the apparent rated output power Sn (kVA) and an output power factor of 0.8. These conditions correspond to an active rated power of Pn (kW) = 0.8 Sn (kVA). In real-life situations, a UPS supplies a number of loads with an overall power factor λ that is often not 0.8 due to the presence of non-linear loads and means to improve the power factor; • If λ ≥ 0.8, the UPS is still limited to Pn (kW), • If λ < 0.8, the UPS is limited to λ Sn (kW) < Pn (kW). Consequently, selection of the power rating in kVA must take into account the active power supplied to the loads. The active power is determined by following the four steps below. 1 - Apparent and active power drawn by the loads The first step is to evaluate the power requirements of the load. The table below must be drawn up for the k loads to be supplied.

Load Apparent rated power (kVA)

Input power factor λ (or cos ϕ)

Active rated power (kW)

Load 1 S1 λ1 P1 = λ1 S1 Load 2 S2 λ2 P2 = λ2 S2 … Load i Si λi Pi = λi S i … Load k Sk λk Pk = λk S k Total S λ P = λ S (1) S is not the sum of

Si. (2) λ must be measured or calculated.

(3) P = λ S = Σ λi S i

(1) S is not the sum of Si because: - it would be necessary to calculate the vectoral sum if all the loads were linear, using the angles of the different cos ϕ, - some of the loads are not linear. (2) λ must be measured on site or evaluated on the basis of past experience. (3) P = λ S = Σ λi S i because the active power is added (no displacement). 2 - Rated apparent power of the UPS (Sn) The second step is to select a UPS with an apparent-power rating sufficient to cover the load requirements (in kVA). Under the given conditions, the suitable rated apparent power for the UPS is: Sn(kVA) > S. where S = P / λ. In the UPS range, select the UPS with a rated power Sn (kVA) just above S. If reserve power is required and the selected rating is too close to S, select the next highest rating. 3 - Check on the active power The third step is a check to ensure that the selected power rating can cover the load requirements in kW under the stipulated operating conditions. For the selected rating, the UPS will supply the rated active power Pn (kW) = 0.8 Sn (kVA) • If λ ≥ 0.8, make sure that Pn (kW) > P, i.e. that the UPS can supply the additional power required, otherwise select the next highest rating. • If λ < 0.8, the power supplied by the UPS is sufficient because Pn (kW) > λ Sn (kVA), i.e. the selection is correct. 4. - Percent load The fourth step is a check to ensure that the percent load is acceptable now and in the future, given the desired operating conditions. The percent load is: Load = S / Sn(kVA) . It must be sufficient to cover any increases in the load or if there are plans to expand the system to become redundant.



Power levels under transient conditions Load in-rush currents It is necessary to know the in-rush current of each load and the duration of the transient conditions. If a number of loads risk being turned on at the same time, it is necessary to sum the in-rush currents. Necessary checks It is then necessary to check that the planned UPS power rating can handle the in-rush currents. Note that the UPS can operate for a few periods in current-limiting mode (e.g. 2.33 In for one second for an MGE Galaxy 9000). If the UPS cannot handle the in-rush currents, it is necessary to decide whether it is acceptable to transfer to the Bypass AC input when the transient conditions occur. If transfer is not acceptable, it is necessary to increase the power rating.

Review of in-rush currents, see Technical review p. 37. Example The example below is simply to illustrate the point and does not correspond to a real situation. The purpose is to indicate the required steps. The installation is made up of three 400 V three-phase loads connected in parallel: • Computer system - S1 = 4 x 10 kVA (4 identical 10 kVA loads), λ = 0.6 for all the loads, in-rush current 8 In over four periods 50 Hz (80 ms) for each load, • Variable-speed drive - S2 = 20 kVA, λ = 0.7, in-rush current 4 In over five periods (100 ms), • Isolation transformer - S3 = 20 kVA, λ = cos ϕ = 0.8, in-rush current 10 In over six periods (120 ms).

Total power consumed by

the loads P (kW) = 54 kW

4 x 10 kVA 20 kVA 20 kVA λ1 = 0.6 λ2 = 0.7 cos ϕ = 0.8

Rated apparent output power Sn(kVA) Active power

Pn(kW) = 0.8 Sn(kVA)

Power factor λ at UPS output for all loads

Maximum active output power (that the UPS can

supply to the loads) λ Sn (kVA)

Fig. 1.18. Example of an installation. Power levels under steady-state conditions 1 - Apparent and active power drawn by the loads Below is the table that should be drawn up.

Load Rated apparent power (kVA)

Input power factor Rated active power (kW)

Computer system 40 0.8* 32* Variable-speed drive 20 0.7 14 LV/LV transformer 20 0.8 16 Total S λ = 0.68

measured or estimated P = 54 kW

* average of new top of the range systems with power factor 0.9 and older equipment with power factor between 0.7 and 0.8. 2 - Rated apparent power of the UPS S = 54 / 0.68 = 79.4 kVA A Galaxy PW UPS with a sufficient rating should be selected. The 80 kVA rating would not be sufficient, i.e. the 100 kVA rating should be selected or higher if a site extension is planned.



3 - Check on the active power • The UPS can supply the loads 100 x 0.68 = 68 kW > 54 kW. 4 - Checks on the percent load and rated current • The percent load is therefore 79.4 / 100 = 79.4%. • Rated current of the UPS - Sn (kVA) = UI 3 , i.e. I = 100 / (400 x 1.732) = 144 A. In-rush currents under transient conditions The loads should be started up one after the other to avoid combining the in-rush currents. It is necessary to check that the UPS can handle the in-rush currents. The rated currents are calculated as S (kVA) = UI 3 , i.e.: • Computer system - In = 10 / (400 x 1.732) = 14.4 A, i.e. 8 In ≈ 115 A for 80 ms • Variable-speed drive - In = 20/(400 x 1.732) = 28.8 A, i.e. 4 In ≈ 115 A for 100 ms • Transformer - In = 20 / (400 x 1.732) = 28.8 A, i.e. 10 In = 288 A for 120 ms • A 100 kVA MGE Galaxy PW UPS has an overload capacity of 120%, i.e. 151 A x 1.2 = 173 A for 1 minute and 150%, i.e. 151 A x 1.5 = 216 A for 1 minute • Operation in current-limiting mode at 2.33 In, i.e. 335 A for one second. If the four computer loads (10 kVA each) are started one after the other, the 20% overload capacity of the UPS is sufficient (173 A -1mn > 115 A - 80 ms). If the four loads are started simultaneously, the in-rush current would be 4 x 115 = 460 A > 335 A. The system would current limit for 80 ms. For the variable-speed drive, the overload capacity is sufficient. For the isolation transformer (288 A for 120 ms), the overload capacity is again sufficient. Parallel-UPS configurations Purpose of parallel connection Parallel connection of a number of identical units is the means to: • increase the power rating, • establish redundancy that increases the MTBF and availability. Types of parallel connection Two types of UPS units can be connected in parallel. • Integrated parallel UPS units - each UPS unit includes an automatic bypass and a manual maintenance bypass. The manual bypass may be common to the entire system (in an external cubicle). • Parallel UPS units with an SSC - the static-switch cubicle comprises an automatic bypass and a maintenance bypass that are common for a number of parallel units without bypasses (see fig. 1.19). True modular parallel systems are also available, made up of dedicated and redundant modules-power, intelligence, battery and bypass, all engineered into a design that is easily and efficiently serviceable. Power modules can be easily added as demand grows or as higher levels of availability are required. There are two types of parallel configurations: • Without redundancy - all the UPS units are required to supply the load. Failure of one unit means the entire system shuts down (not recommended), • With redundancy N+1, N+2, etc. - the number of UPS units required for the load is equal to N. All the UPS units (N+1, N+2, etc.) share the load. If one UPS unit shuts down, the remaining units (at least equal in number to N) continue to share the load.

Typical configurations and characteristics, see Ch. 2.

Fig. 1.19. UPS system with parallel-connected units and a static-switch cubicle (SSC).

Ratings of parallel-UPS configurations



Power levels in redundant parallel configurations In a redundant parallel configuration made up of identical units, the units share the load. The power rating of each unit does not depend on the level of redundancy, but must be calculated to continue supplying the load even if redundancy is completely lost. Active redundancy: • improves availability, • increases the overload capacity, • reduces the percent load on each UPS unit. The power level is determined by following the same four steps as for a single-UPS configuration. 1 - Apparent and active power drawn by the loads The same type of table is used as that for a single UPS (see Ch1 p. 20). The result is the apparent power S that must be supplied to the load. 2 - Rated apparent power of the UPS units (Sn) in the configuration Consider a level of redundancy N + K (e.g. 2 + 1), which means: - N units (e.g. 2) are required to supply the load, - K units (e.g. 1 extra unit) ensure redundancy. Each UPS unit must be sized to enable the system as a whole to operate without redundancy, i.e. with N operational units and K units shut down. In this case, the N units must each have an apparent power rating Sn (kVA) such that: Sn(kVA) > S / N. Select in the UPS range the power rating Sn (kVA) just above S/N. If reserve power is required or the selected rating is too close to S, select the next highest rating. 3 - Check on the active power For the selected rating, the UPS will supply the active rated power Pn (kW) = 0.8 Sn (kVA) • if λ ≥ 0.8, make sure that Pn (kW) > P, i.e. that the UPS can supply the additional power required, otherwise select the next highest rating. • if λ < 0.8, the power supplied by the UPS is sufficient because Pn (kW) > λ Sn (kVA), i.e. the selection is correct. 4 - Percent load With redundancy, the UPS units share the load according to the equation S / (N+K). The percent load for each unit when there is redundancy is therefore: TL = S / (N + k) Sn(kVA) . In a non-redundant system, it is calculated as: TL = S / N Sn(kVA). It must be sufficient to cover any increases in the load. Example This example will use the results from the last example and we will suppose that the loads are critical, i.e. redundancy is required. • The total load is 54 kW with an overall power factor for all the loads of 0.68, i.e. S = 54 / 0.68 = 79.4 kVA. • If 2+1 redundancy is used, two units must be capable of supplying the load. Each must will have to supply S / 2 = 79.4 / 2 = 39.7 kVA. • An MGE Galaxy PW UPS with a sufficient rating should be selected. The 40 kVA rating would not be sufficient, i.e. the 50 kVA rating should be selected or higher if a site extension is planned. • If redundancy is not available, the two UPS units must be capable of supplying the load. • This is the case because 2 x 50 x 0.68 = 68 kW > 54 kW. • During operation, the percent load will be: - with redundancy, i.e. with 3 UPS units sharing the load: 79.4 / 3 x 50 = 52.9%, - without redundancy, i.e. with only 2 UPS units sharing the load: 79.4 / 2 x 50 = 79.4%.


Control of upstream harmonics

Role of the input rectifier UPS units draw power from the AC distribution system via a rectifier/charger. With respect to the upstream system, the rectifier is a non-linear load that causes harmonics. In terms of harmonics, there are two types of rectifiers. Standard rectifiers These are three-phase rectifiers incorporating SCRs and using a six-phase bridge (Graetz bridge) with standard chopping of the current. This type of bridge draws harmonic currents with orders of n = 6 k ± 1 (where k is a whole number), mainly H5 and H7, and to a lesser degree H11 and H13. Harmonics are controlled by using a filter (see fig. 1.20). PFC-type transitor-based controlled active rectifiers These transistor-based active rectifiers have a regulation system that adjusts the input voltage and current to a reference sine wave. This technique ensures an input voltage and current that are: • perfectly sinusoidal, i.e. free of harmonics, • in phase, i.e. with a power factor close to 1. With this type of rectifier, no filters are required.

Clean transitor-based rectifiers, see Ch. 4.

All high-power UPS ranges from APC by Schneider Electric (except MGE Galaxy PW and and MGE Galaxy 9000) use PFC type controlled active rectifier technologies and therefore do not generate harmonics.

Fig. 1.20. Input rectifier and harmonics.

UPSs and upstream harmonics


Control of upstream harmonics (Cont.)

Goals of harmonic filtering This section concerns only the MGE Galaxy PW and MGE Galaxy 9000 ranges and UPSs with conventional Graetz bridge rectifiers. A "clean" upstream system The goal is to ensure a level of voltage distortion (THDU) on the busbars supplying the UPS that is compatible with the other connected loads. The UTE recommends limiting the THDU to: • 5% when the source is a generator, • 3% when the source is a transformer to take into account 1 to 2% of THDU which may already be present on the HV distribution system. This recommendation may differ for each country. Practically speaking, solutions for voltage distortion (THDU) must be implemented in a manner specific to the country where the installation is located. Easy combination with an engine generator set The goal is to make possible a UPS/engine generator set combination with no risk of increasing the level of harmonics when the load is transferred to the generator. This risk exists because the generator has a source impedance lower than that of a transformer, which increases the effects of harmonics. High power factor at the rectifier input The goal is to increase the input power factor (generally to a level higher than 0.94). This reduces the consumption of kVA and avoids oversizing the sources. Installation complying with standards The goal is to comply with standards concerning harmonic disturbances and with the recommendations issued by power utilities. • Standards on harmonic disturbances (see table 1.2) - IEC 61000-3-2 / EN 61000-3-2 for devices with an input current ≤ 16 A/ph. - IEC 61000-3-4 / EN 61000-3-4 for devices with an input current > 16 A/ph. • Standards and recommendations on the quality of distribution systems, notably: - IEC 61000-3-5 / EN 61000-3-5, - EN 50160 (Europe), - IEEE 519-2 (United States), - ASE 3600 (Switzerland), - G5/3 (U.K.), etc.

Standards on harmonics, see "UPS standards" in Technical review p. 29. Table 1.2. Example of harmonic-current limitations as per guide IEC 61000-3-4 / EN 61000-3-4 for devices with an input current > 16 A/ph (stage 1, simplified connection).

Harmonic % of H1 (fundamental) H3 21.6% H5 10.7% H7 7.2% H9 3.8% H11 3.1% H13 2.0% H15 0.7% H17 1.2% H19 1.1% H21 ≤ 0.6% H23 0.9% H25 0.8% H27 ≤ 0.6% H29 0.7% H31 0.7% ≥ H33 ≤ 0.6% Even orders ≤ 0.6% or ≤ 8/n (n even order)

Types of harmonics filters Harmonics filters eliminate certain orders or all orders, depending on their technology. The following types are available.

Filtering of upstream harmonics for UPSs with Graetz bridge rectifiers



Passive LC filters • non-compensated • compensated • non-compensated with contactor Double-bridge rectifier Phase-shift filter THM active filter (Active 12-pulse technology).

Filtering and parallel connection When a number of UPS units are connected in parallel and depending on the type of filter used, it is possible to install: • an individual filter on each UPS unit, • a common filter for the entire parallel configuration. The goal is to achieve a balance between cost and effectiveness, taking into account the acceptable levels of harmonic distortion. The comparison tables for the various solutions (Ch. 1, p. 28) are helpful in making a selection. Combination of LC filters and generator The generator can supply only relatively low capacitive currents (10 to 30% of In). When an LC filter is installed, the main difficulty lies in the gradual start-up of the rectifier on generator power, when active power is equal to zero and the generator supplies only the capacitive current for the filter. Consequently, the use of LC filters must be correctly analysed to ensure that operation complies with manufacturer specifications. Below is a method for selection of LC filters, using as an example a generator derating curve, similar to those provided by manufacturers.

Fig. 1.21. Derating curve for a generator, as a function of the installation power factor. The curve in the figure above, provided as one example among many, shows the power derating as a function of the operating point, for a given generator. For a purely capacitive load (λ = 0), the power available is equal to only 30% of the rated power (point A). If we assume an apparent power rating such that Pn generator = Pn rectifier, the meaning of points A, B, C, D, E and F is the following: A: reactive power corresponding to the capacitive current of a non-compensated filter, B: reactive power corresponding to the capacitive current of a compensated filter, C: operating point at start-up with a non-compensated filter with contactor, D: operating point at the rated load with a non-compensated filter, E: operating point at the rated load with a compensated filter, F: operating point at the rated load, without a filter or with a phase-shift filter.



Example Consider a non-compensated filter with a 300 kVA generator and a 200 kVA MGE Galaxy PW UPS. The power rating of the rectifier, taking 87% as the efficiency value (1 / 0.87 = 1.15), is 1.15 times that of the inverter, i.e. 200 x 1.15 = 230 kVA. The capacitive current of the non-compensated filter is 230 x 30% (1) = 69 kVA. The reactive power that the generator can handle (point A) is 300 x 0.3 = 90 kVA. The filter is therefore compatible with the generator. (1) The value of 30% has been determined experimentally.

Selection parameters for a filter Overall effectiveness - reduction in distortion (THDI and THDU) The effectiveness depends on the harmonic orders filtered and the degree to which they are attenuated or eliminated. It is measured by the THDI at the rectifier input. The impact on the THDI determines the level of the THDU. It is necessary to check the performance at the planned percent load, given that many UPS systems operate at percent loads between 50 and 75%. Improvement in the power factor λ The filter improves the power factor (generally to a level higher than 0.92). Compatibility with an engine generator set It is also necessary to check the performance with the planned source(s), either a transformer or an engine generator set. This is because the generator has an output impedance lower than that of a transformer, which increases the effects of harmonics. Suitable for parallel-UPS configurations Depending on the type of filter, it is possible to install one on each UPS unit or set up a single filter for overall elimination of harmonics. Efficiency Consumption of the filters can slightly modify the efficiency of the installation as a whole. Flexibility for set-up and upgrades Filters are generally specific to a UPS and may be factory-mounted or installed after installation. The SineWave conditioner provides overall elimination of harmonics and great flexibility in the configuration. Dimensions It is necessary to check whether the filter can be installed in the UPS cabinet or in a second cabinet. Cost It impacts on the effectiveness of the filter and must be weighed against the advantages obtained. Compliance with standards It is necessary to determine compliance with standards, in particular IEC 61000-3-4, in terms of the individual harmonic levels indicated in the texts. Comparison table of solutions The following tables list the elements for comparison, with a general comment on use of each type of solution. Table 1.3 presents individual solutions for single-UPS configurations. These solutions may also be used in parallel configurations. Table 1.4 presents overall solutions for entire configurations.

Selection of a filter



Table 1.3. Comparison of individual harmonic-filtering solutions.

Type of filter Criterion

LC non-compensated

LC compensated LC with contactor Double bridge Built-in THM

Diagram

Fig. 1.22a Fig. 1.22b Fig. 1.22c Fig. 1.22d Fig. 1.22e

Reduction in distortion THDI at 100% load THDI at 50% load

7 to 8% 10%

7 to 8% 10%

7 to 8% 10%

10% 15%

4% 5%

Harmonics eliminated H5, H7 H5, H7 H5, H7 H5, H7, H17, H19 H2 to H25 Power factor λ at 100% load λ at 50% load

0.95 1

0.95 1

0.95 1

0.85 0.8

0.94 0.94

Compatibility with generator

* ** ** ** ***

Efficiency of filter *** *** *** * ** Flexibility, upgradeability * * * * *** Cost *** *** *** * ** Dimensions *** *** *** * *** Connection in parallel with UPS

* * * * **

Fig. 1.22f Fig. 1.22g Fig. 1.22h Fig. 1.22i Fig. 1.22j Compliance with guide IEC 61000-3-4

no no no no yes

General comment Solution suitable for installations without an engine generator set.

Solution suitable for installations with an engine generator set. The added inductor load reduces the capacitive power that must be supplied by the engine-generator set.

Solution suitable for installations comprising an engine generator set with a power rating lower than that of the UPS. The LC line is switched in by the contactor at a preset value corresponding to an inverter percent load that is acceptable for the engine generator set.

Solution suitable for installations with gensets

Solution suited to sensitive installations or with changing load levels. The most effective and the most flexible solution. Does not depend on the percent load or the type of upstream source.

*** Excellent ** Good * Sufficient



Table 1.4 Comparison of overall solutions.

Type of filter Criterion

SineWave Phase-shift filter

Diagram

Fig. 1.23a Fig. 1.23b Fig. 1.23c Fig. 1.23dReduction in distortion THDI at 100% load THDI at 50% load

4% 5%

< 10% 35% with 1 UPS shut

down


down


downHarmonics eliminated H2 to H25 Power factor λ at 100% load λ at 50% load

0.95 1

0.8 0.8

Compatibility with generator

*** **

Efficiency of filter *** ** Flexibility, upgradeability *** * Cost *** *** Dimensions *** * Compliance with guide IEC 61000-3-4

yes yes

General comment Solution suited to sensitive installations or with changing load levels. The most effective and the most flexible solution. Does not depend on the percent load or the type of upstream source.

Solution cannot be modified. Suited to installations with more than two parallel-connected UPS units.

*** Excellent ** Good * Sufficient

UPSUPS UPS

AC input

Load

SW


System earthing arrangements

Protection of persons against electrical contact International standards require that electrical installations implement two types of protection of persons against the dangers of electrical currents. Protection against direct contacts The purpose of this form of protection is to avoid "direct" contact between persons and intentionally live parts (see fig. 1.24). It includes the points listed below. • isolation of live parts using barriers or enclosures offering a degree of protection at least equal to IP2X or IPXXB. • opening of the enclosure (doors, racks, etc.) must be possible only using a key or a tool, or following de-energising of the live parts or automatic installation of a screen. • connection of the metal enclosure to a protective conductor. Protection against indirect contacts and system earthing arrangements The purpose of this form of protection is to avoid "indirect" contact between persons and exposed conductive parts (ECP) that have become live accidentally due to an insulation fault. The fault current creates in the exposed conductive parts (ECP) a potential that may be sufficient to cause a dangerous current to flow through the body of the person in contact with the exposed conductive parts (see fig. 1.24). This protection includes the points listed below. • mandatory earthing of all exposed conductive parts (ECP) that may be accessed by the user. The protective conductor is used for connection to the earth. It must never be interrupted (no breaking devices on the protective conductor). The interconnection and earthing techniques for the exposed conductive parts (ECP) determine the system earthing arrangement (SEA) for the installation. • disconnection of the supply when the potential of the ECPs risks reaching dangerous levels. Interruption is carried out by a protection device that depends on the selected system earthing arrangement (SEA). It often requires residual-current devices (RCD) because the insulation-fault currents are generally too low to be detected by standard overcurrent protection devices.

Fig. 1.24. Direct and indirect contacts. Types of system earthing arrangements (SEA) There are three types of system earthing arrangements (SEA). • Isolated neutral (IT). • Earthed neutral (TT). • Exposed conductive parts connected to the neutral (TN with TN-C and TN-S). The first two letters indicate how the neutral and the ECPs of the loads are connected.

First letter Second letter Third letter (for TN) Connection of the neutral Connection of the ECPs Type of protective

conductor T = earthed neutral T = exposed conductive parts

earthedC = Common neutral and

protective conductor (PEN) S = Separate neutral (N) and

protective conductor (PE) I = isolated neutral N = exposed conductive parts

connected to the neutral IT, TT or TN systems TN-C or TN-S

Background information on system earthing arrangements


System earthing arrangements (Cont.)

System earthing arrangements (SEA) Isolated neutral (IT)

The source neutral is: - either isolated from the earth (isolated neutral), - or connected to the earth via a high impedance res (impedant neutral). The exposed conductive parts (ECP), all protected by the same breaking device, are earthed (earth electrode resistance RA).

E.g. Phase-to-ECP fault in a load. Uo is the phase-to-neutral voltage in the distribution system (230 V). Current of the first fault RA= 10 Ω and Zres= 3500 Ω (approximately), Id = Uo / (RA + Zres) = 66 mA. Voltage of the first fault Ud = Uo x RA / (RA + Zres) = 0.66 V. This potential is not dangerous. The fault must be detected by an IMD (insulation monitoring device), located by a fault-locating device and repaired. Current of the second fault A second fault occuring before the first fault has been repaired results in the flow of a phase-to-phase or phase-to-neutral short circuit. It must be cleared by the overcurrent protection devices within the time limits set by the standards.

Fig. 1.25. IT system. Earthed neutral (TT)

The source neutral is earthed. The exposed conductive parts (ECP), all protected by the same breaking device, are earthed (earth electrode resistance RA).

E.g. Phase-to-ECP fault in a load. Uo is the phase-to-neutral voltage in the distribution system (230 V). Fault current E.g. RA = 10 Ω and RB = 5 Ω Id = Uo / (RA + RB) = 15.3 A Fault voltage Ud = Uo x RA / (RA + RB) = 153 V This potential is dangerous (> 50 V). The fault must be cleared by the protection devices within the times set by the standards. The fault current is low and must therefore be detected by a residual-current protection device (RCD) that actuates the protective device immediately upstream. The operating current of the RCD and the time required to clear the fault are set by the standards.

Fig. 1.26. TT system. Exposed conductive parts connected to the neutral (TN)

The source neutral is directly earthed. The installation ECPs are connected to the neutral and consequently to the earth via the protective conductor (PEN). This arrangement transforms all insulation faults into phase-to-neutral short-circuits. The potential of the protective conductor is maintained close to that of the earth by numerous connection points.

Impedance of the fault loop Zb = ZABCDEF (part of circuit ABCDEF) Zb ≈ ZBCDE ≈ 2 ZDE because ZBC = ZDE (BC and DE are identical, the fault impedance is negligible) E.g. A load supplied by a 50 mm² copper cable that is 50 metres long (phase and PE). Zb = 2 ρ L / S where ρ = 22.5 Ω. mm2/m Zb = 2 x 22.5 10-3 x 50 / 50 = 45 mΩ. Fault voltage A voltage drop of 20% is permissible for the phase-to-neutral voltage Uo, i.e. UBE = 0.8 Uo. In that ZBC = ZDE, the potential of the ECPs rises to Ud = UBE / 2 = 0.8.Uo / 2 = 92 V Fault current Id = 0.8 Uo / Zb = 0.8 x 230 / 45 10-3 = 4089 A Breaking is carried out by the overcurrent protection devices within the times set by the standards. The fault current depends on the impedance of the fault loop. Care must be taken to ensure that at all points in the system, the fault current is greater than the operating threshold of the protection devices.

Fig. 1.27. TN-S system (the basic principle is identical for the TN-C system).

L1L2L3N

PE

Ud

Zres

RAId

L1L2L3N

PE

UdRB RAId

L1L2L3

PEN

Ud

FE

C

D

BA Id



Comparison of system earthing arrangements (SEA)

Type of SEA IT (isolated neutral) TT (earthed neutral) TN-S (ECP to neutral) TN-C (ECP to neutral) Operation Signalling of first insulation

fault. Location and elimination of the first fault. Disconnection for the second fault.

Disconnection for the first insulation fault.

Disconnection for the first insulation fault occurs Separate neutral (N) and protective conductor (PE).

Disconnection for the first insulation fault. Common neutral and protective conductor (PEN).

Protection of persons Interconnection and earthing of ECPs. First fault: - very low current, - monitoring/indication by an IMD. Second fault: - potentially dangerous current, - interruption by overcurrent protection devices (e.g. circuit breaker).

Earthing of ECPs combined with use of residual-current devices (RCD). First fault: - leakage current is dangerous, but too low to be detected by the overcurrent protection devices, - detection by the RCDs combined with breaking devices.

Interconnection and earthing of ECPs and neutral imperative. first fault: - fault current, - interruption by overcurrent protection devices (e.g. circuit breaker).

Interconnection and earthing of ECPs and neutral imperative. First fault: - fault current, - interruption by overcurrent protection devices (e.g. circuit breaker).

Specific equipment Insulation-monitoring device (IMD) and fault-locating device.

Residual-current devices (RCD).

For long distances, RCDs must be used.

Advantages and disadvantages EMC

Solution offering the best continuity of service (the first fault is signalled). Requires competent surveillance personnel (location of the first fault). High EMC performance, very low currents in the earth cable.

Easiest solution to design and install. Mandatory use of RCDs. Different earth electrodes (distant sources). Highly sensitive to lightning strikes.

High installation costs for high power ratings. Difficult to design (calculation of the loop impedances). Flow of high fault currents. High EMC performance, low current in the PE during normal operation.

Reduced installation costs (one less conductor). Difficult to design (calculation of the loop impedances). Flow of high fault currents. Low EMC performance, high currents in the PEN (connections between ECPs).

Use Installations requiring continuity of service, e.g. hospitals, airports, industrial processes, ships. Installations and premises where there is a risk of fire or explosion, i.e. mines, etc.

Commercial and residential premises, public lighting, schools, etc.

Large commercial premises, tall buildings, etc. Industries without continuous processes (IT system). Supply of computer systems.

Large commercial premises, tall buildings, etc. Industries without continuous processes (IT system). Supply of computer systems.

ECP = Exposed conductive parts.

Specific aspects in systems with UPSs Implementation of the above protection systems in installations comprising a UPS requires a number of precautions for a number of reasons: • The UPS plays two roles: - a load for the upstream system, - a power source for the downstream system, • When the battery is not installed in a cabinet, an insulation fault on the DC system can lead to the flow of a residual DC component. This component can disturb operation of certain protection devices, notably RCDs used for the protection of persons. Protection against direct contact All APC by Schneider Electric UPS installations satisfy the applicable requirements because the equipment is installed in cabinets providing a degree of protection IP 20. This is true even for the battery when it is housed in a cabinet. When the battery is not installed in a cabinet (generally in a special room), the measures presented at the end of this chapter should be implemented.

Applications in UPS installations



Protection against indirect contact Selection of a system earthing arrangement A basic protection measure required by the standards is the creation of a standardised system earthing arrangement both upstream and downstream of the UPS. The two systems can be the same or different if certain precautions are taken. In an existing installation to which the UPS is added, the upstream system is already determined. Selection of the downstream system, either the same or a different one, depends on its compatibility with sensitive loads. The table on the previous page provides the necessary elements to compare the various standardised system earthing arrangements.

Caution, local regulations may prohibit certain types of system earthing arrangements. Selection of the breaking devices Above and beyond the interconnection and earthing of the exposed conductive parts in compliance with a standardised system earthing arrangement, the protection of persons must be ensured by breaking devices selected according to the system earthing arrangement. These devices must cause tripping of the overcurrent protection devices in the event of an insulation fault. Tripping may: • be directly provoked by suitable settings on the overprotection devices (circuit breakers, fuses), • or require (mandatory for the IT system) use of residual-current devices (RCD) that may or may not be built into the circuit breaker. The RCDs are required to detect the insulation-fault currents that are often too low to trip standard overcurrent protection devices.

Check local requirements concerning the safety of electrical installations. Types of systems for UPSs The possible systems depend on: • the existing or selected system upstream of the UPS, • the system downstream of the UPS for which selection may be determined by: - reuse of the same system as upstream, - the presence of isolation transformers upstream or downstream which make it possible to change the system earthing arrangement, - the loads (e.g. computer systems require a TN-C or TN-s system), - the organisation of the downstream distribution system, with static transfer switches (STS), • certain requirements imposed by standards, e.g. the protective conductor PE or PEN must never be interrupted to ensure flow of the fault current. A TN-C system (non-interrupted PEN) can be installed upstream of a TN-S system (separate N and PE conductors), but not the contrary. UPSs are increasingly designed without transformers, offering advantages in terms of weight, size and efficiency. Transformerless technology also makes it possible to modulate the voltage for improved adapatation to all types of loads, in particular non-linear loads with harmonics. Transformerless technology has an impact on the use of system earthing arrangements. For more information see White Paper - WP 98: "The Elimination of Isolation Transformers in Data Center Power Systems"). Many different cases may be encountered depending on the upstream and downstream earthing arrangements and the type of UPS. Your APC by Schneider Electric representative has a complete set of diagrams for all system earthing arrangements and UPS ranges concerned. The MGE Galaxy PW and MGE Galaxy 9000 ranges are designed with isolation transformers. All the other ranges use transformless technology with the neutral recreated electronically. The following pages show some examples for MGE Galaxy PW and MGE Galaxy 5000, 7000 and 9000 UPSs. For other cases, contact your APC by Schneider Electric representative to obtain the applicable diagram.

See WP 98



Output transformer (MGE Galaxy PW and 9000)

No output transformer (MGE Galaxy 5000 and 7000))

Separate Normal and Bypass AC inputs. Common Normal and BP

inputs. Fig. 1.28. Standard diagrams. Identical systems upstream and downstream

Same system upstream and downstream IT or TT or TN-S.

Distributed neutral on the two lines.


Distributed neutral on the bypass line only.

Same system upstream and downstream TN-C


Distributed neutral. MGE Galaxy PW and 9000 MGE Galaxy 5000 and 7000

Fig. 1.29. A few examples with the same system upstream and downstream. Different systems upstream and downstream

Change in earthing systems to IT or TT or TN-S downstream.


Change in earthing systems to IT or TT or TN-S downstream.


Change in earthing systems

to TN-C downstream.Change in earthing systems

to TN-C downstream. MGE Galaxy PW and 9000 MGE Galaxy 5000 and 7000

Fig. 1.30. A few examples with different systems upstream and downstream.


Protection

The protection system for installations with UPS units presented here will implement circuit breakers. Below is a presentation of the main characteristics of circuit breakers and their trip units. The part number mentioned as examples pertain to Schneider Electric circuit breakers. Other characteristics, such as limiting thermal stress and current, are among the strong points of the Compact NSX range of circuit breakers, but will not be discussed here.

For further information, see the Schneider Electric low-voltage and medium-voltage distribution catalogue and the "Electrical installation guide". Trip units Technology There are two types of trip units: • thermal-magnetic, • electronic. Construction • built-in (thermal-magnetic only). • interchangeable. Comparison Thermal-magnetic trip units are simple and inexpensive. Electronic trip units offer more precise and comprehensive settings for better adaptation to installations and their requirements. The table below sums up the characteristics of both types of trip units for circuit-breakers from 1 to 630 A and should enable you to solve most of the problems commonly encountered (from 1 to 400 kVA). Figure 1.31 presents the characteristic curves for the trip units.

Protection Symb. Definition Availability Overload protection

(thermal or long delay) (1)

Ir Overload current setting. All trip units.

Long delay (2) tr Applies a long tripping delay (e.g. for motor starting).

Electronic trip units (e.g. Micrologic 2, 5, 6).

Short-circuit protection (magnetic or short delay) (3)

Im or Isd

Short-circuit current setting. On electronic trip units, Isd is a function of Ir (generally 2 to 10 Ir).

All trip units.

Short delay (4) tm or tsd

Applies a short tripping delay (e.g. for time discrimination with downstream circuit breaker).

Electronic trip units (e.g. Micrologic 5, 6).

Short-circuit protection, instantaneous trip (5)

Ii Instantaneous short-circuit setting. Depends exclusively on trip-unit rating (e.g. protection of static switches).

Electronic trip units (e.g. Micrologic 5, 6 ).

(1) Ir is the thermal protection threshold (sometimes written Ith) of thermal-magnetic trip units or the long-delay protection threshold of electronic trip units. These thresholds are defined by an inverse time curve that depends on the selected setting. (2) tr is the time delay of the long-delay thermal protection for a given value of Ir. (3) Im is the magnetic threshold of thermal-magnetic trip units and Isd the short-delay threshold of electronic trip units. (4) tm is the time delay (adjustable or fixed) of the magnetic protection of thermal-magnetic trip units and tsd the time delay (generally adjustable) of the short-delay protection of electronic trip units. (5) Ii is the instantaneous tripping threshold.

Protection using circuit breakers


Protection (Cont.)

Fig. 1.31. Circuit-breaker time/current curves (Icu is the ultimate breaking capacity).


Protection (Cont.)

Discrimination, cascading, current limiting Discrimination Discrimination results from correct circuit-breaker selection and setting such that, if a fault occurs, it trips only the first upstream circuit breaker. Discrimination thus limits the part of the installation affected by the fault to a strict minimum. There are a number of types of discrimination summed up in the table below and illustrated on the previous page. Current limiting When a high fault current hits the circuit breaker, the breaker contacts separate under the electrodynamic forces, an arc is created and its resistance limits the short-circuit energy. Cascading When a short-circuit occurs downstream of the installation (see fig. 1.32), the fault current also flows through the upstream circuit breaker which current limits, thus attenuating the current applied to the downstream circuit breaker. The breaking capacity of the latter is thus reinforced.

Discrimination Concerns Principle Current

discrimination

All types of trip units

The fault current is lower than the upstream threshold setting. Ir upstream > Ir downstream and Im or Isd upstream > Im or Isd downstream

Time discrimination

Electronic trip units only (e.g. Micrologic)

Delays upstream tripping by the long-time (Ir) and short-time (Im or Isd) delay.

Energy discrimination

Compact NSX and NS

Arc pressure upstream is not sufficient to trip the upstream circuit breaker, but it is sufficient to trip the downstream circuit breaker.

Zone-selective interlocking

Compact NSX 100 to Masterpact with Micrologic trip units

Delays upstream tripping if the short-circuit is also detected downstream. A pilot wire connects the upstream and downstream trip units.

Fig. 1.32. Upstream/downstream discrimination and cascading.


Protection (Cont.)

Rating The selected rating (rated current) for the circuit breaker must be the one just above the rated current of the protected downstream cable. Breaking capacity The breaking capacity must be selected just above the short-circuit current that can occur at the point of installation. Ir and Im thresholds The table below indicates how to determine the Ir and Im thresholds to ensure discrimination, depending on the upstream and downstream trip units. Remark. Time discrimination must be implemented by qualified personnel because time delays before tripping increase the thermal stress (I2t) downstream (cables, semi-conductors, etc.). Caution is required if tripping of CB2 is delayed using the Im threshold time delay. Energy discrimination does not depend on the trip unit, only on the circuit breaker. Ir and Im thresholds depending on the upstream and downstream trip units

Type of downstream circuit

Ir upstream / Ir downstream ratio

Im upstream / Im downstream ratio

Im upstream / Im downstream ratio

downstream trip unit all types magnetic electronic distribution > 1.6 > 2 > 1.5 asynchronous motor > 3 > 2 > 1.5

Special case of generator short-circuits Figure 1.33 shows the reaction of a generator to a short-circuit. To avoid any uncertainty concerning the type of excitation, we will trip at the first peak (3 to 5 In as per X"d) using the Im protection setting without a time delay.

Fig. 1.33. Generator during a short-circuit.

Selection of circuit breakers


Protection (Cont.)

Example Consider the example used to determine the UPS power rating (Ch. 1 p. 21) with a number of parallel-connected 400 V three-phase loads, namely: • Computer system - S1 = 4 x 10 kVA, λ = 0.6, in-rush current 8 In over four periods (80 ms), • Variable-speed drive - S2 = 20 kVA, λ = 0.7, in-rush current 4 In over five periods (100 ms), • Isolation transformer - S3 = 20 kVA, λ = 0.8, in-rush current 10 In over six periods (120 ms). The three loads represent 54 kW with a power factor of 0.68. In chapter 1, p. 21, an MGE Galaxy PW was selected, with a power rating of 100 kVA, I = 100 / (400 x 3 ) = 144 A.

630 kVA transformer

Determine CB1 and CB2

Determine the most powerful CB3 for

discrimination Total power consumed by

the loads P (kW) = 54 kW

40 kVA 20 kVA 20 kVA λ = 0.6 λ = 0.7 cos ϕ = 0.8

400 kVA generator

Rated apparent output power

100 kVA In = 144 A

Power factor at UPS output for all loads

λ = 0.68

Maximum active output power (that the UPS can

supply to the loads) λ Sn (kVA) = 68 kW

Fig 1.34. Example of an installation. The goal is to select circuit breakers CB1 and CB2, and the most powerful circuit breaker CB3 compatible with discrimination requirements, given that the upstream installation includes the following: • 20 kV / 400 V transformer with a power rating of 630 kVA, • 400 V engine generator set with a power rating of 400 kVA, • Transformer to MLVS link, five meters of aluminium cable 4 x 240 mm2 per phase, • Busbars to circuit breaker link, four meters using three copper bars 400 mm² per phase. Calculation of CB1 and CB2 ratings and breaking capacities The breaking capacity depends on the short-circuit currents downstream of CB1 and CB2 at the level of the main low-voltage switchboard (MLVS). Most often, this upstream short-circuit value is supplied by the utility. It can also be calculated. It is necessary to determine the sum R of the resistances upstream and the sum X of the reactances upstream of the considered point. The three-phase short-circuit current is calculated as:

Isc 3-ph = U

R X3 2 2+

U is the phase-to-phase no-load voltage (load voltage + 3 to 5%). R = Σ Rupstream and X = Σ Xupstream

In this example, we simply indicate the general method with a number of simplifications to shorten the calculations.

For more detailed information, see the Cahier Technique document no. 158 "Calculation of short-circuit currents" from Schneider Electric.


Protection (Cont.)

Upstream system Ra, Xa

Sources Rtr Xtr

Source output to MLVS cable link

Rc, Xc General circuit breaker

Rd, Xd MLVS busbars

Rb, Xb

Fig. 1.35. Calculation of short-circuit current for CB1 and CB2. It is necessary to calculate the resistances and reactances upstream of CB1 and CB2 in figure 1.34. Distribution system upstream of the transformer • Psc = upstream short-circuit power = 500 MVA = 500 x 106 VA • U20 = phase-to-phase no-load voltage on the transformer secondary winding = 400 V, + 3%, i.e. 410 V • Rup = resistance upstream ≈ 15% Xup, negligible given Xup • Xup = reactance upstream with respect to transformer secondary winding

Xup = UPsc

202

= 410

500 10

2

6x= 0.288 mΩ

Rup ≈ 0 and Xup = 0.33 mΩ. Transformer • Sn = rated apparent power 630 kVA • In = rated current = 630 / U 3 = 630 103 / (400 x 3 ) = 909 A • Usc = transformer short-circuit voltage = 4% • Pcu = transformer copper losses in VA

Rtr = transformer resistance = Pcu

In3 2 ≈ 20% Xtr, negligible given Ztr

Xtr ≈ Ztr = transformer impedance = USn

x Usc202

= 4102 x 0.04 / 630 103 = 10.7 mΩ

Rtr ≈ 0 and Xtr = 10.7 mΩ. Cables linking the transformer to the MLVS • Length 5 meters • Cross-section 240 mm² • ρ = resistivity at the normal temperature of the conductors copper: ρ = 22.5 mΩ.mm2/m, aluminium: ρ = 36 mΩ.mm2/m • Xc = conductor reactance (typically 0.08 mΩ/m) = 0.08 x 5 = 0.4 mΩ

Rc = cable resistance (copper) = ρ LS

= 22.5 x 5 / (4 x 240) = 0.12 mΩ

Rc = 0.12 mΩ and Xc = 0.4 mΩ. General circuit breaker Typical values Rd ≈ 0 et Xd = 0.15 mΩ.


Protection (Cont.)

Busbars • Xb = busbar reactance (typically 0.15 mΩ/m) = 0.15 x 4 = 0.6 mΩ

• Rb = busbar resistance = ρ L / S= 22.5 x 4 / (3 x 400) = 0.075 mΩ (negligible)

Rb ≈ 0 and Xb = 0.6 mΩ. Transformer Isc at the level of CB1 and CB2 • R = Total upstream resistance = 0.12 mΩ • X = Total upstream reactance = 0.33 + 10.7 + 0.4 + 0.15 + 0.6 =12.18 mΩ R can be neglected, given X.

Isc 3-ph = U

R X3 2 2+≈

UX3

= 4103 1218 10 3x x. − = 19.4 kA

Note. A rough estimate is provided by the short-circuit current on the transformer terminals, assuming that the upstream short-circuit power is infinite. ISCT = on transformer terminals = In / Usc = 20 In = 20 x 909 = 18.2 kA Generator Isc at the level of CB1 and CB2 • rated apparent power of the generator = 400 kVA • rated current of the generator = 400 / U 3 = 400 103 / (400 x 3 ) = 577 A • X"d = short-circuit voltage of the generator = 10% It is decided to trip at 5 In (see fig. 1.33). ISCG = on the generator terminals = 5 In = 5 x 577 = 2.9 kA Continuous current of CB1 This is the current at the UPS input. It is necessary to multiply the UPS rating by 1.2 to take into account the efficiency, i.e. 120 kVA. Iinput = 120 / U 3 = 120 103 / (400 x 3 ) = 173 A Continuous current of CB2 This is the continuous current of the loads supplied via the bypass, i.e. 54 kW with a power factor of 0.68 for an apparent power S = 54 / 0.68 = 67.5 kVA. Iload = 67.5 / U 3 = 120 103 / (400 x 3 ) = 97 A Energising current of the largest load The loads must be energised at different times. The highest inrush current is that of the 20 kVA transformer, i.e. In = 28.8 A and 10 In = 288 A - 120 ms. Calculation of the maximum static-switch current This is the short-circuit current at the level of CB3, which is practically that of CB2. Selection parameters The table below sums up the various values calculated. Parameter Value transformer short-circuit current 19.4 kA generator short-circuit current 2.9 kA rectifier current (UPS input) 173 A continuous load current downstream of the UPS 97 A energising current of the largest load 288 A - 120 ms maximum static-switch current 19.4 kA

Characteristics of CB1 and CB2 Characteristic D1 D2 Breaking capacity > 19.4 kA, i.e. 25 kA > 19.4 kA, i.e. 25 kA Continuous current > 173 A, i.e. 200 A > 97 A, i.e. 125 A Ir threshold > 173 A +20% > 97 A + 20% Im threshold > 173 A + 20% and

< 2.9 kA - 20% > 288 A +20% and < 2.9 kA - 20%

20% represents here the typical tolerance range of circuit-breaker settings.


Protection (Cont.)

Characteristics of the most power circuit breaker CB3 possible

Sources

Incomer circuit breakers (input)

Static bypass Negligible impedance

Outgoing circuit breakers (output) Isc at CB3 ≈ Isc at CB2

Fig. 1.36. Calculation of the short-circuit current at CB3. Operation with bypass power • Breaking capacity The highest short-circuit current downstream of CB3 is virtually that of CB2 because it is assumed that the outgoing circuits are near the UPS. Consequently, the breaking capacity of CB3 is also 25 kA. • The rating is determined by the largest load, i.e. the 4 x 10 kVA of the computer system with a continuous current of: Iload = 40 / U 3 = 40 103 / (400 x 3 ) = 57 A A 60 A device should be selected. • Settings A majority of the loads is of the distribution type, i.e. the Ir threshold of CB3 must be less than 97 A / 1.6, i.e. < 61 A. The Im threshold must be less than 1847 / 2, i.e. < 900 A. Operation without bypass power In this case, the short-circuited UPS limits its current to 2.33 In for one second. For APC by Schneider Electric UPSs of the MGE Galaxy range, experimental results have determined that the highest rating of CB3 must be less than 0.5 In to ensure discrimination. This is the case for the circuit breaker for the computer loads. 60 A < 0.5 x 144= 72 A


Cables

Cable temperature rise and voltage drops The cross section of cables depends on: • permissible temperature rise, • permissible voltage drop. For a given load, each of these parameters results in a minimum permissible cross section. The larger of the two must be used. When routing cables, care must be taken to maintain the required distances between control circuits and power circuits, to avoid any disturbances caused by HF currents. Temperature rise Permissible temperature rise in cables is limited by the withstand capacity of cable insulation. Temperature rise in cables depends on: • the type of core (Cu or Al), • the installation method, • the number of touching cables. Standards stipulate, for each type of cable, the maximum permissible current. Voltage drops Maximum values The maximum permissible voltage drops are: • 3% for AC circuits (50 or 60 Hz), • 1% for DC circuits. Selection tables The tables below indicate the voltage drop in percent for a circuit made up of 100 meters of copper cable. To calculate the voltage drop in a circuit with a length L, multiply the value in the table by L/100. If the voltage drop exceeds 3% on a three-phase circuit or 1% on a DC circuit, increase the cross section of the conductors until the value is within tolerances. Voltage drop for 100-meter cables • Sph - the cross section of the conductors • In - rated current of the protection devices on the circuit Three-phase circuit (copper conductors) 50-60 Hz - 400 V three phase, cos ϕ = 0.8, balanced 3-ph + N system

Sph (mm2) 10 16 25 35 50 70 95 120 150 185 240 300 In (A) 10 0.9 16 1.2 20 1.6 1.1 25 2.0 1.3 0.9 32 2.6 1.7 1.1 40 3.3 2.1 1.4 1.0 50 4.1 2.6 1.7 1.3 1.0 63 5.1 3.3 2.2 1.6 1.2 0.9 70 5.7 3.7 2.4 1.7 1.3 1.0 0.8 80 6.5 4.2 2.7 2.1 1.5 1.2 0.9 0.7 100 8.2 5.3 3.4 2.6 2.0 2.0 1.1 0.9 0.8 125 6.6 4.3 3.2 2.4 2.4 1.4 1.1 1.0 0.8 160 5.5 4.3 3.2 3.2 1.8 1.5 1.2 1.1 0.9 200 5.3 3.9 3.9 2.2 1.8 1.6 1.3 1.2 0.9 250 4.9 4.9 2.8 2.3 1.9 1.7 1.4 1.2 320 3.5 2.9 2.5 2.1 1.9 1.5 400 4.4 3.6 3.1 2.7 2.3 1.9 500 4.5 3.9 3.4 2.9 2.4 600 4.9 4.2 3.6 3.0 800 5.3 4.4 3.8 1000 6.5 4.7

For a three-phase 230 V circuit, multiply the result by 3 . For a single-phase 208/230 V circuit, multiply the result by 2.

Selection of cable sizes


Cables (Cont.)

DC circuit (copper conductors) Sph (mm2) 25 35 50 70 95 120 150 185 240 300 In (A) 100 5.1 3.6 2.6 1.9 1.3 1.0 0.8 0.7 0.5 0.4 125 4.5 3.2 2.3 1.6 1.3 1.0 0.8 0.6 0.5 160 4.0 2.9 2.2 1.6 1.2 1.1 0.6 0.7 200 3.6 2.7 2.2 1.6 1.3 1.0 0.8 250 3.3 2.7 2.2 1.7 1.3 1.0 320 3.4 2.7 2.1 1.6 1.3 400 3.4 2.8 2.1 1.6 500 3.4 2.6 2.1 600 4.3 3.3 2.7 800 4.2 3.4 1000 5.3 4.2 1250 5.3

Special case for neutral conductors

In three-phase systems, the third-order harmonics (and their multiples) of single-phase loads add up in the neutral conductor (sum of the currents on the three phases). For this reason, the following rule is applied - neutral cross section = 1.5 x phase cross section. Calculation example

Consider a 70-meter 400 V three-phase circuit, with copper conductors and a rated current of 600 A. Standard IEC 60364 indicates, depending on the installation method and the load, a minimum cross section. We shall assume that the minimum cross section is 95 mm2. It is first necessary to check that the voltage drop does not exceed 3%. The table for three-phase circuits on the following page indicates, for a 600 A current flowing in a 300 mm2 cable, a voltage drop of 3% for 100 meters of cable, i.e. for 70 meters: 3 x 70/100 = 2.1%, less than the 3% limit. A identical calculation can be run for a DC current of 1000 A in a 10-meter cable with a cross section of 240 mm². The voltage drop for 100 meters is 5.3%, i.e. for ten meters: 5.3 x 10/100 = 0.53%, less than the 1% limit.

Example of an installation

Fig. 1.37. Connection of cables.


Energy storage

Energy storage in UPSs UPSs require an energy-storage system to supply the inverter with power if utility power fails or is no longer within tolerances. The stored energy must have the following characteristics: • electricity that is immediately available to ride through micro-breaks, short voltage drops and utility outages, • sufficient power level to supply the entire load, i.e. a rating equivalent to that of the UPS system itself, • backup time, generally about ten minutes, suited to the needs of the loads and to any other sources available (e.g. an engine generator set for long backup times).

Fig. 1.38. Simplified diagram of a UPS with backup energy storage. Available technologies The various technologies currently available are the following: • batteries: - sealed lead-acid, - vented lead-acid, - nickel cadmium, • ultracapacitors, • flywheels: - traditional units turning at low speeds (1500 rmp) and combined with engine generator sets, - medium-speed (7000 rpm) or high-speed (30 to 100 000 rpm) units. Comparison of technologies Batteries are by far the most commonly employed solution today. They are the dominant solution due to low cost, proven effectiveness and storage capacity, but nonetheless have a number of disadvantages in terms of size, maintenance and the environment. Ultracapacitors do not yet offer the necessary performance levels. Flywheels operating at high speeds constitute a possible technology in terms of their power ratings (40 to 500 kW), for short backup times (12 seconds to 1 minute). Figure 1.39 shows the fields of application for the different technologies.

For more information, see White Paper WP 65: "Comparing Data Center Batteries, Flywheels, and Ultracapacitors".

Fig. 1.39. Characteristics in terms of power ratings and backup times.

Storage technologies

See WP 65


Energy storage (Cont.)

The table below compares the different solutions in terms of their capacity to meet the energy-storage requirements of static UPSs.

Criteria for comparison Technology Sealed lead-acid

batteries Vented lead-acid batteries

Ni/Cad batteries Ultracapacitors Flywheels

Power ****

**** **** * ***

Backup time *** 5 minutes up to several hours

**** 5 minutes up to several hours

* 5 minutes up to several dozen minutes

* a few seconds

** a few dozen seconds

Purchase price **** low

*** low to medium

** high

* cost multiplied by 2 or 3 compared to batteries, for 10 seconds of backup time

* cost multiplied by 8 compared to batteries, for 10 seconds of backup time

Implementation / installation / start-up Requires a special room

*** no

** yes

* yes

**** no

** yes

Temperature * * ** **** *** Service life ** ** *** **** *** Footprint ** ** ** **** *** Maintenance Frequency / time required

*** low

** medium

* high

**** none

* long servicing times

Maturity of the technology for UPSs **** **** **** ** *** **** excellent *** good ** fair * poor Flywheels APC by Schneider Electric offers flywheel energy storage systems on request. This solution is suitable to complement batteries in that it may be used to ride through short disturbances without calling on battery power, thus preserving the battery. Use without a battery is possible, but the backup time is only a dozen seconds. For certain applications, such a short backup time is insufficient to start an engine generator set. Types of batteries The batteries most frequently used in UPSs are: • sealed lead-acid, also called gas-recombination batteries, • vented lead-acid, • nickel cadmium. Lithium-polymer batteries are currently being studied for use in UPSs. Solutions using this technology should be available in two to three years.

Types of batteries, see Ch. 5 p. 32 "Energy storage - Types of batteries". For use in conjunction with its UPS ranges, APC by Schneider Electric recommends sealed lead-acid batteries. Selection of a battery depends on the following factors: • operating conditions and requirements (special room, battery cabinet, racks, etc.), • required backup time, • cost considerations. Backup time APC by Schneider Electric offers: • standard backup times of 5, 10, 15 or 30 minutes, • custom backup times that can reach a number of hours. Selection depends on: • the average duration of power-system failures, • any available sources offering long backup times (engine generator set, etc.), • the type of application.

Selection of a battery



The following general rules apply. • Computer systems Battery backup time must be sufficient to cover file-saving and system-shutdown procedures required to ensure a controlled shutdown of the computer system. Generally speaking, the computer department determines the necessary backup time, depending on its specific requirements. • Industrial processes The backup-time calculation should take into account the economic cost incurred by an interruption in the process and the time required to restart. • Applications requiring long backup times An engine generator set can back up a battery if long outages occur, thus avoiding the need for very large batteries. Generally speaking, use of an engine generator set becomes feasible for backup times greater than 30 minutes to one hour. The combination must be carefully studied to optimise the generator rating and ensure correct operation.

Combination with an engine generator set, see Ch. 5 p. 35 "Engine generator set". Service life APC by Schneider Electric offers batteries with service lives of 5 or 10 years, or longer.

Battery service life, see Ch. 5 p. 33. Comparison between types of batteries Sealed lead-acid batteries (gas-recombination) These are the most commonly used batteries for the following reasons: • no maintenance, • easy implementation, • installation in all types of rooms (computer rooms, technical rooms not specifically intended for batteries, etc.). Vented batteries This type of battery (lead-acid or Ni/Cad) offers certain advantages: • long service life, • long backup times, • high power ratings. Vented batteries must be installed in special rooms complying with precise regulations (see Ch. 1 p. 51 "Preliminary work") and require appropriate maintenance. UPSs from APC by Schneider Electric include advanced battery-monitoring systems. Battery monitoring on MGETM GalaxyTM UPSs DigiBatTM The DigiBatTM battery-monitoring system is an assembly of hardware and software, installed as standard on UPSs of the MGE Galaxy range from APC by Schneider Electric and offering the following functions: • automatic entry of battery parameters, • optimised battery service life, • protection against excessive discharges, • regulation of the battery floating voltage depending on the temperature, • limitation of the battery current, • continuous evaluation of available power taking into account the battery age, the temperature and the percent load, • forecast of battery service life, • periodic, automatic tests on the battery, including a check on the battery circuit, an open-circuit test, a partial-discharge test, etc.

DigiBat, see Ch. 5 p. 34 "Battery Management".

Battery monitoring



Environment sensor unit Battery operating parameters and particularly the temperature affect battery life. The Environment Sensor, easy to install and combined with a Network Management card (SNMP/Web), makes possible monitoring of temperature/humidity and the status of two contacts via SNMP or the web. It also initiates equipment shutdown if necessary.

Detection and prevention of battery failure for MGETM GalaxyTM UPSs In spite of the advantages of sealed lead-acid batteries, over time, all batteries will fail due to ageing. Without rigorous monitoring, the true integrity and capacity of a battery remains unknown. Battery-monitoring techniques have a major impact on reliability and can be used to define the best strategy for replacement, resulting in a better level of protection. APC by Schneider Electric also offers continuous, cell by cell, battery-monitoring systems with software and communication capabilities. These systems can be implemented by the user or integrated in the Teleservice offer. B2000 battery-monitoring system The B2000 system offers continuous, overall monitoring of the main battery parameters. That includes the voltage, current, temperature and any drift detected during charge and discharge cycles. It issues an alarm when tolerance levels are overrun. Automatic recording of discharges, whether planned or unplanned, is also available for data analysis. The monitoring system can help detect possible problems before the battery fails and thus enhance availability of UPS energy. Cellwatch battery-monitoring system General battery maintenance may not be sufficient to ensure correct operation, notably for mission-critical applications where there is no room for error. Between periodic tests (generally once every three months), a cell may suddenly fail. A valve-regulated sealed lead-acid cell can fail in just a few days after a periodic test. The cause is the chemical reactions that take place in the cell following charge and discharge cycles. These cycles occur even if the protection system is not in operation. What is more, corrosion can affect the entire connection system of the battery string, inside or outside of the cell. It was therefore necessary to do more than simply check the voltage. The research carried out showed that the internal resistance or the impedance of the cell is a good indicator of its status, in that it reveals both deterioration and any physical problems. The Cellwatch monitoring system uses this system based on cell impedance to monitor each cell. It provides reliable monitoring of the service life of each cell. APC battery management system for SymmetraTM UPSs The APC battery management system, available for UPSs of the Symmetra range from APC by Schneider Electric, ensures your batteries are optimally charged and ready for use. This browser-accessible, 1U rackmountable system combines battery monitoring and testing with individual boost charging for peak battery performance. Integration into your preferred building management system or use of a Web browser provides visibility of the health and status of your batteries. This system makes it possible to solve battery problems before they affect availability.

APC by Schneider Electric 01/2012 Edition p. 49

Human-machine interface and

communication

General characteristics The human-machine interface on the UPS must be user-friendly, easy to use and multi-lingual (adjustable to the user's language). It is generally made up of a mimic panel, a status and control panel, and an alphanumeric display. A password-protected personalisation menu may be available for entry of installation parameters and access to detailed information. Example The HMI typically offers the functions listed below. On and Off buttons • delayed to avoid erroneous operations. • with an option for a remote EPO (emergency power off). • independent with respect to the rest of the display Status LEDs that clearly identify: • normal operation (load protected), • downgraded operating mode (malfunction), • dangerous situations for the load (load not protected), • operation on battery power. Alarms • alarm buzzer and buzzer reset button. • battery shutdown warning. • general alarm. • battery fault. A screen providing: • access to measurements - input power (voltage, current, frequency). - battery (voltage, charge and discharge currents, remaining backup time, temperature). - inverter output (phase-to-neutral voltage, current, frequency, active and apparent power, crest factor). • access to history logs - log containing time-stamped events. - curves and bargraphs of the measured values. High availability for critical applications requires communicating protection equipment The UPS system, essential for mission-critical equipment, must include communication features that keep operators continously informed, wherever they may be, of any risk of compromising the operating security of the system so that they can take immediate action. To ensure power availability, the UPS communication features provide the following four essential functions:

Supervision / monitoring of all installed UPSs via software. Notification via the network and the Internet. Controlled shutdown (local or remote, automatic or manual) of protected

applications. Teleservice via a modem and telephone line to a support centre.

Human-machine interface (HMI)

Communication


Human-machine interface and

communication (Cont.)

APC by Schneider Electric solutions Communication cards • Network Management Card (Ethernet) - Web monitoring - Email notification - SNMP MIB & Traps - Server protection with Network Shutdown Module - Supervision with Enterprise Power Manager or ISX Central - Environment monitoring with Environment Sensor (T°, H%, Inputs) • Modbus – Jbus card (RS232 & RS485) - Monitoring • Teleservice card (Modem) - Alerts - Monitoring - Diagnostics - Reporting • Relay card (contacts) - Indications Management software • Enterprise Power Manager & ISX Central (software & server) Software solutions to manage all installed UPSs via IP networks, web compatible and accessible from any web browser. • NMS Integration kits (Network Management System) Integration in NMSs such as HP OpenView, IBM Tivoli, CA Unicenter, etc. • Network Shutdown Module - Software module for safe system shutdown.

Fig. 1.40. The communication cards combined with supervision software offer a wide range of functions.


Preliminary work

The main elements that must be taken into account for the UPS installation are the following: • plans for site modifications, any preliminary work (notably for a battery room), taking into account: - the dimensions of equipment, - operating and maintenance conditions (accessibility, clearances, etc.), - temperature conditions that must be respected, - safety considerations, - applicable standards and regulations, • ventilation or air-conditioning of rooms, • creation of a battery room. Dimensions Layout of UPS cabinets and enclosures should be based on precise plans. The physical characteristics of UPSs from APC by Schneider Electric that may be used to prepare the plans are presented in chapter 4. They indicate, for each range: • the dimensions and weights of: - UPS and centralised-bypass cabinets; - battery cabinets, - any auxiliary cabinets (autotransformers, transformers, filters, etc.), • minimum clearances required for cabinets and enclosures to ensure optimal ventilation and sufficient access. Ventilation, air-conditioning Ventilation requirements UPSs are designed to operate within a given temperature range (0 to 40°C for UPSs from APC by Schneider Electric ) that is sufficient for most operating conditions without modifications. However, UPSs and their auxiliary equipment produce heat losses that can, if no steps are taken, increase the temperature of a poorly ventilated room. What is more, the service life of a battery is heavily dependent on the ambient temperature. The service life is optimal for temperatures between 15° C and 25° C. This factor must be taken into account if the battery is installed in the same room as the UPS. A further consideration is the fact that UPSs may be installed in the same room as computer equipment which often has more severe requirements concerning operating-temperature ranges. Selecting a type of ventilation For all the above reasons, a minimum amount of ventilation is required, and where applicable air-conditioning, to avoid any risk of excessive temperature rise in the room due to the heat losses. Ventilation can be by: • natural convection, • forced exchange by a ventilation system, • installation of an air-conditioning unit. Selection depends on: • the heat losses that must be evacuated, • the size of the room. The thermal characteristics of UPSs from APC by Schneider Electric are indicated in chapter 4 and may be used to calculate ventilation needs. They mention for each range: • the heat losses of cabinets and any filters installed, • the volume of air output by a ventilation system.

Installation considerations


Preliminary work (Cont.)

IP degree of protection and noise level Degree of protection (IP) UPSs must operate in an environment that is compatible with their degree of protection (IP 20 for UPSs from APC by Schneider Electric), defined by standard IEC 60529/EN 60529. The presence of dust, water and corrosive substances must be avoided. Noise level UPSs must produce a low level of noise, suited to the room where they are installed. Measurement conditions for the level of noise indicated by the manufacturer must comply with standard ISO 3746 (measurement of noise).

Where possible and if desired, the battery should be installed in a cabinet. Battery-cabinet dimensions are indicated for each UPS range, depending on the rated power. However, for very high-power UPSs, batteries are generally installed in special rooms (electrical room). Batteries must be installed in compliance with international standards, local regulations and standard IEC 60364. Battery installation method The criteria determining the battery-installation method are the following: • available floor space, • the weight that the floor can handle (kg/m2), • ease of access and maintenance. The following three methods are used. Battery installed directly on floor This is the most simple arrangement. However, a large battery room is required, given: • the large amount of floor space occupied by the battery, • the insulated flooring (duck board), which is mandatory if the voltage exceeds 150 volts. Battery on racks The battery cells are installed on a number of different levels, off the floor. When determining the height between each rack, it is necessary to take into account the space required to check battery levels and fill the battery cells easily. A minimum height of 450 mm is recommended. Battery on tiers This installation method is similar to the preceding. It is the most convenient method for checking battery levels. Battery-room features Whatever the installation method selected, the battery installation must comply with the following requirements (the numbers indicate the elements shown in figure 1.40). Floor and walls (1) • The floor must slope to an evacuation trough which leads to a holding tank. • Protection coating against acid on the floor and walls, up to a height of at least 0.5 meters. For example, asphalt for lead-acid batteries, PVC or chlorine-based paint for alkaline batteries.

Battery room


Preliminary work (Cont.)

Ventilation (2) • calculation of throughput The volume of air to be evacuated depends on the maximum load current and the type of battery. In installations comprising a number of batteries, the quantities of air that must be evacuated are cumulative. - vented batteries d = 0.05 x N x Im, where d - throughput in cubic meters per hour, N - number of battery cells, Im - maximum load current in amperes. - sealed battery The ventilation conditions in a general-purpose room are sufficient. • safety An automatic device must stop battery charging if the ventilation system fails. • location Air must be drawn out from the top of the battery room. Layout of cells (3) Layout must inhibit simultaneous contact with two bare parts presenting a voltage greater than or equal to 150 V. If the above condition cannot be met, terminal shields must be installed and connections must be made using insulated cables. Service flooring (4) If the voltage exceeds 150 V, special flooring is required. It must offer sure footing, be insulated from the floor and offer at least one meter of walkway around the battery. Battery connection (5) Connections must be as short a possible. Battery-protection circuit breaker (6) The circuit breaker is generally installed in a wall-mounted enclosure. Fire-fighting equipment (7) Authorized fire extinguishers include power, CO2 or sand. Safety equipment (8) The safety equipment must include protective glasses, gloves and a source of water. Inspection equipment (9) • Hydrometer. • Filling device. • Thermometer. Sensors (10) • Hydrogen detector. • Temperature sensor.

Fig. 1.41. Layout of battery room

ups breakers

Documents

ups power availability

ups configurations

ups power quality

power calculations

selection of circuit

highavailability power

electrical installations

installation specifications