eue 404-module 1-lec 1

Chapter One: Introduction to Power Quality 1

Lecture Notes for Electrical Utility Engineering 624 Power Quality Phenomena, ECEN 5787 Electrical & Computer Engineering Department Electrical & Computer Engineering Department Curtin University of Technology University of Colorado at Boulder Perth, WA Boulder, Colorado, USA

Copyright 2005 by Ewald F. Fuchs & Mohammad A.S. Masoum

Chapter 1. Introduction to Power Quality

1.1. Definition of Power Quality 1.2. Causes of Disturbances in Power Systems 1.3. Classification of Power Quality Issues

1.3.1. Transients 1.3.2 Short-Duration Voltage Variations (Interruption, Sags (dips), Swells) 1.3.3. Long-Duration Voltage Variation (Sustained Interruption, Under-voltage, Over-voltage) 1.3.4. Voltage Unbalance [4-123] 1.3.5. Waveform Distortion (Dc Offset, Harmonics, Inter-harmonics, Notching, Noise) 1.3.6. Voltage Fluctuation and Flicker (Flicker) [4-103] 1.3.7. Power Frequency Variations

1.4. Definitions and Measures used for Power Quality 1.4.1. Harmonics 1.4.2. The Average Value of a Nonsinusoidal Waveform 1.4.3. The rms Value of a Nonsinusoidal Waveform 1.4.4. Form Factor (FF) 1.4.5. Ripple Factor (RF) 1.4.6. Harmonic Factor (HF) 1.4.7. Lowest Order Harmonic (LOH) 1.4.8. Total Harmonic Distortion (THD) 1.4.9. Total Inter-harmonic Distortion (TIHD) 1.4.10. Total Subharmonic Distortion (TSHD) 1.4.11. Total Demand Distortion (TDD) 1.4.12. Telephone Influence Factor (TIF) 1.4.13. C-Message Weights 1.4.14. V.T and I.T Products 1.4.15. Telephone Form Factor (TFF) 1.4.16. Distortion Index (DIN) 1.4.17. Distortion Power (D)

1.5. Effects of Poor Power Quality on Power System Devices 1.6. Standards Referring to Power Quality

1.6.1 IEC 61000 Series of Standards for Power Quality 1.6.2. IEEE-519 Standard for Power Quality

1.7. Harmonic Modeling Philosophies [5] 1.7. 1. Time Domain Simulation [5] 1.7. 2. Frequency (Harmonic) Domain Simulation [5] 1.7. 3. Iterative Simulation Techniques [5] 1.7. 4. Modeling Harmonic Sources

1.8. Power Quality Improvement Techniques 1.8.1. High Power Quality Equipment Design [8] 1.8.2. Harmonic Cancellation [8] 1.8.3. Concept of a Dedicated Isolation Line or Transformer 18.3.1. Application Example I (Disturbance Mitigation by Isolation Transformers) 1.8.4. Optimal Placement and Sizing of Capacitor Banks 1.8.5. Deterating Power System Devices 1.8.6. Harmonic filters (passive, active, hybrid) and active power conditioners

1.8.6.1. Application Example II (Harmonics produced by 12-Pulse Converters) 1.8.6.2. Application Example III (Filter Design for Example II to Meet IEEE-519) 1.8.6.3. Application Example IV (Several Users on a Single Distribution Feeder

2 Power Quality Phenomena in Power Systems and Electric Machines (b y E.F. Fuchs & M.A.S. Masoum)

Chapter 1

Introduction to Power Quality

The subject of power quality is very broad by nature. It covers all aspects of power system engineering from transmissions and distribution level analysis to user end type problems. Therefore, electric power quality has become the concern of utilities and end users, as well as manufacturers. They must work together for developing solution to power quality problems:

• Public service managers and designers must build and operate systems that take into account the interaction issues between customer facilities and the power system. Electric utility must understand the sensitivity of the end-use equipments to the quality of the voltage.

• The customers must learn to respect the rights of their neighbors and control the quality of their nonlinear loads. Studies show that the best and the most efficient solution to power quality problems is to control them at their initial source. Customers can perform this by careful selection and control of their nonlinear loads and by taking appropriate actions to control and mitigate pollutions and harmonics before entering the power system.

• Manufacturers and equipment engineers must work to make equipments that are compatible with the real-world power system. This might mean lower level of harmonic generation or less sensitivity to voltage distortions.

This chapter introduces the subject of electric power quality. After a brief definition of power quality and its causes, detailed classification of the subject is presented. A section is included on the terminology and definitions used for power quality. A section is also presented for the important subject of standards referring to power quality. The rest of the chapter is about issues that will be covers in the incoming chapters including impacts of poor power quality on power system and appliances, modeling and mitigation techniques for power quality phenomena. 1.1. Definition of Power Quality Electric power quality has become an important part of power systems and electric machines. In recent years, the subject has attracted the attention of many universities and industries and a number of books have been published in this relatively new and exciting field [1-10]. In despite of tremendous papers, articles, books… published in the area of electric power quality, there is no universal agreement about its definition! However, nearly everybody accept that it is a very important aspect of power systems and electric machinery with direct impacts on efficiency, security and reliability. Various sources use the term "power quality" with different meaning. It is used as synonymous with "supply reliability", "service quality", "voltage quality", "current quality", "quality of supply" and "quality of consumption". Judging by the different definitions, "power quality" is generally used to express the quality of voltage and/or the quality of current and can be defined as: "the measure, analysis and improvement of the bus voltage to maintain a sinusoidal waveform at rated voltage and frequency". The subject includes all momentary and steady state phenomena.


1.2. Causes of Disturbances in Power Systems Although, a flood of literature on power quality issues is now available but most engineers, facilities managers and consumers remain unclear as to what constitutes a power quality problem. Furthermore, due to the power system impedance, any current (or voltage) harmonic will result in the generation and propagation of voltage (or current) harmonics and affects the entire power systems. Figure 1.1 illustrates the impact of current harmonics generated by a nonlinear load on a typical power system with linear loads.

Figure 1.1 Propagation of harmonics (generated by a nonlinear load) in power systems

What are the origins of the power quality problem? Some references [7] divide the harmonic sources into three categories: "small and predictable (e.g., domestic and residential causing harmonics)", "large and random (e.g., arc furnaces producing voltage fluctuations and flicker)" and "large and predictable (e.g., static converters of smelters and HVdc transmission causing characteristic and uncharacteristic harmonics, as well as harmonic instability). However, the likely answers to this question are: "unpredicted events", "the utility", "the customer", and "the manufacturer". Unpredicted Events: Both public service and customer agree that more than 60% of power quality problems are generated by natural and unpredicted phenomena [4]. Some of these events include: faults, lightning surge propagation, resonance, ferroresonance and geomagnetically induced currents (GICs) due to solar flares. This issue is considered as a utility related problem. The Utility: There are three main sources of poor power quality related to the utility:

• The Point of Supply Generation- Although, synchronous machines generate near perfect sinusoidal voltage (normally at 11 kV), there are power quality problems originating at the generating plants which are mainly due to: maintenance activity, planning, capacity and expansion constraints, scheduling, events leading to forced outages, and load transferring from one substation to another.

• The Transmission System- Relatively few power quality problems originate on the transmission system. Typical power quality problems originating on the transmission system are: galloping (under high wind conditions resulting in supply interruptions and/or

Harmonic currents flowing through the system impedance results in Harmonic Voltage Distortion at the point of common coupling

Non-linear load does not draw a sinusoidal current from a perfectly sinusoidal voltage source, i.e. solid state drives, switched-mode power supplies, fluorescent lamps

Source voltage

Harmonic Voltage Distortion imposed on another customer

Cables, overhead lines and transformers of the distribution system make up the system impedance

Point of common coupling

Customer with non-linear load

Normal loads


random voltage variations), lightning (resulting in a spike or transient over-voltage), insulator flashover, voltage dips (due to faults), interruptions (due to planned outages by the utility), transient over-voltages (generated by capacitor (inductor) switching actions and/or lightning), transformer energizing (resulting in inrush currents that are rich in harmonic components), improper operation of voltage regulation devices (can lead to long duration voltage variations), slow voltage variations (due to a long-term variation of the load caused by the continuous switching of devices and load), FACTS devices and HVDC systems.

• The Distribution System- Typical power quality problems originating on the transmission system are: voltage dips, spikes and interruptions, transient over-voltages, transformer energizing, improper operation of voltage regulation devices, and slow voltage variations.

The Customer: Customer loads constitute a considerable portion of power quality problems in today’s power systems. Some customer related problems are: harmonics (generated by non-linear loads such as power electronic devices and equipments, renewable energy sources, FACTS devices, adjustable speed drives, UPS, fax machines, laser printers and computers, fluorescent lights, poor power factor (due to highly inductive loads such as induction motors, AC units, etc), flicker (generated by arc furnaces), transients (mostly generated inside a facility due to device switching, electrostatic discharge and arcing), improper grounding (causing most reported customer problems), frequency variations (when secondary and backup power sources, such as diesel engine and turbine generators are used) and misapplication of technology, wiring regulations and other relevant standards. Manufacturing Regulations: There are two main sources of poor power quality related to the manufacturing regulations:

• Standards- A lack of standards for the testing, certification, sale, purchase, installation and use of electronic equipment and appliances is a major contributor to power quality problems.

• Equipment Sensitivity- The proliferation of “sensitive” electronic equipment and appliances is one of the main reasons for the escalation of the power quality problem. The design characteristics of these devices, including computer-based equipment have increased the incompatibility of a wide variety of these devices with the electrical environment.

Power quality therefore must necessarily be tackled from three fronts, namely:

• The Utility: In their design, maintenance and operation of the power system. • The Customer: In their wiring, system grounding practices and the use of modern electronic

devices. • The Manufacturer: In the design of electronic devices that generate less electrical

environment polluting agents while being immune to anomalies on the power supply line.

1.3. Classification of Power Quality Issues For solving any power quality problem it is necessary to understand and classify this relatively complicated subject. This section is based mainly on the power quality classification and information of references [4] and [7]. There are different classifications for power quality issues; each using a specific factor to categorize the problem. Some of them devise the subject to "steady-state" and "non-steady state" phenomena. In some category (e.g., ANSI C84.1) the most important factor is the "duration of the event". Other categories (e.g., IEEE) use the wave shape (duration and magnitude) of each event to classify power quality problems. Some standards (e.g., IEC) use frequency range of the event for the classification.


For example IEC 61000-2-5 uses the frequency range and divides the problems into three main categories; "low frequency" (<9 kHz), "high frequency" (>9 kHz) and "electrostatic discharge" phenomena. In addition, each frequency range is divided into "radiated" and "conducted" disturbances. Table 1.1 shows the principal phenomena causing electromagnetic disturbances according to IEC classifications [7]. All these phenomena are considered as power quality issues, however, the two conducted categories are more commonly addressed by the industry.

Table 1.1. Main phenomena causing electromagnetic and power quality disturbances [4,7]

Conducted Low-Frequency Phenomena

• Harmonics, inter-harmonics • Signaling voltage • Voltage fluctuations • Voltage dips • Voltage unbalance • Power frequency variations • Induced low frequency voltages • d.c. in a.c. networks

Radiated Low-Frequency Phenomena

• Magnetic fields • Electric fields

Conducted High-Frequency Phenomena

• Induced continuous wave (CW) voltages or currents • Unidirectional transients • Oscillatory transients

Radiated High-Frequency Phenomena

• Magnetic fields • Electric fields • Electromagnetic field

- Continuous waves - Transients

Electrostatic discharge phenomena (ESD) Nuclear electromagnetic pulse (NEMP)

We can use the magnitude and duration of events and set a plane to classify the power quality events, as shown by Figure 1.2. In the magnitude-duration plot we have nine different parts [9]. Various standards give different names to events in these parts. The voltage magnitude is split into three regions:

• interruption: voltage magnitude is zero, • under-voltage: voltage magnitude is below its nominal value, • over-voltage: voltage magnitude is above its nominal value.

The duration of these events is split into four regions: • very short, • short, • long, • very long.

It is obvious that the borders in this plane are somewhat arbitrary and the user can set them according to the standard that is used.


IEEE standards use several additional terms (as compared with the IEC terminology) to classify power quality issues. Table 1.2 provides information about categories and characteristics of electromagnetic phenomena defined by IEEE-1159. These categories are briefly introduced in the remaining parts of this section.

Figure 1.2. Magnitude-duration plot for classification of power quality events [9]

1.3.1. Transients Power system transients are undesirable, fast and short-duration events that produce distortions. Their characteristics and waveforms depend upon the mechanism of generation and the network parameters (e.g., resistance, inductance and capacitance) at the point of interest. The word surge is often considered synonymous with transient. Transients can be classified with their many characteristic components such as "amplitude" , "duration", "rise time", "frequency of ringing polarity", "energy delivery capability", "amplitude spectral density", "position with respect to the mains waveform" and "frequency of occurrence". Transients are usually classified into two categories; impulsive and oscillatory (Table 2.1). An impulsive transient is a sudden non power frequency change in the steady state condition of voltage, current or both, that is unidirectional in polarity (Fig.1.3). The most common cause of impulsive transients is lightning. Impulsive transients can excite the neutral frequency of the system. An oscillatory transient is a sudden non power frequency change in the steady state condition of voltage, current or both, that includes both positive and negative polarity values. Oscillatory transients occur for different reason in power systems such as "appliance switching", "capacitor bank switching (Fig.1.4)", "fast-acting over-current protective devices" and "ferroresonance (Fig.1.5)".


Table 1.2. Categories and characteristics of electromagnetic phenomena in power systems as defined by IEEE-1159 [4,7]

Categories Typical spectral content

Typical duration

Typical voltage

magnitude 1.1. Impulsive

• Nanosecond • Microsecond • Millisecond

5 ns rise 1 µs rise 0.1 ms rise

< 50 ns 50 ns – 1 ms > 1 ms

1. Transient

1.2. Oscillatory • Low frequency • Medium frequency • High frequency

< 5 kHz 5 – 500 kHz 0.5 – 5 MHz

0.3 – 50 ms 20 µs 5 µs

0 – 4 pu 0 – 8 pu 0 – 4 pu

2.1. Instantaneous • Interruption • Sag • Swell

0.5– 30 cycles 0.5– 30 cycles 0.5– 30 cycles

< 0.1 pu 0.1 – 0.9 pu 1.1 – 1.8 pu

2.2. Momentary • Interruption • Sag • Swell

0.5 cycle – 3 s 30 cycles– 3 s 30 cycles– 3 s

< 0.1 pu 0.1 – 0.9 pu 1.1 – 1.4 pu

2. Short-duration variation

2.3. Temporary • Interruption • Sag • Swell

3 s – 1 min 3 s – 1 min 3 s – 1 min

< 0.1 pu 0.1 – 0.9 pu 1.1 – 1.2 pu

3.1. Sustained interruption

> 1 min 0.0 pu

3.2. Under-voltage > 1 min 0.8 – 0.9 pu

3. Long-duration variation

3.3. Over-voltage > 1 min 1.1 – 1.2 pu

4. Voltage imbalance

Steady state 0.5% – 2%

5.1. d.c. offset Steady state 0% – 0.1% 5.2. Harmonics 0 – 100th H Steady state 0% – 20% 5.3. Inter-harmonics 0 – 6 kHz Steady state 0% – 2% 5.4. Notching Steady state

5. Waveform distortion

5.5. Noise Broad- band Steady state 0% – 1% 6. Voltage fluctuation

< 25 Hz Intermittent 0.1% – 7%

7. Power frequency variations

< 10 s


Figure 1.3. Impulsive transient current caused by lighting stroke [4-Fig.2.1]

Figure 1.4. Low-frequency oscillatory transient caused by capacitor bank enegization [4-Fig.2.3]

Figure 1.5. Low-frequency oscillatory transient caused by ferroresonance of an unloaded transformer [[4-Fig.2.4]]


1.3.2 Short-Duration Voltage Variations This category encompasses the IEC category of "voltage dips" and "short interruptions". According to IEEE-1159 classification, there are three different kinds of short-duration events (Table 1.2): "instantaneous", "momentary" and "temporary". Each category is divided to "interruption", "sag" and "swell’. Principle cases of short-duration voltage variations are: fault conditions, large load energization and loose connections. Interruption: Interruption occurs when the supply voltage (or load current) decreases to less than 0.1 pu for less than one minute, as shown by figure 1.6. As the causes of interruption we can pointed to "equipment failures", "control malfunction", "blown fuse" or "breaker opening". The difference between long interruption (sustained interruption) and interruption is that, in the long interruption the supply is restored manually but during the interruption the supply is restored automatically. Interruption is usually measured by its duration. For example, according to the European standards (EN-50160): - short interruption: up to 3 minutes, - long interruption: longer than 3 minutes. However, based on the IEEE standard (IEEE-1250): - instantaneous interruption: between 0.5 cycles to 30 cycles, - momentary interruption: between 30 cycles to 2 seconds, - temporary interruption: between 2 seconds to 2 minutes, - sustained interruption: longer than 2 minutes.

Figure 1.6. Momentary interruptions due to a fault and subsequent recloser operation [4-Fig.2.5]

Sags (dips): Sags are short duration reductions in the rms voltage between 0.1 and 0.9 pu, as shown by figure 1.7. There is no clear definition for the duration of sag but it is usually determined 0.5 cycles to 1 minute. Voltage sags are usually caused by:

• energization of heavy loads (e.g., arc furnace), • starting of large motors, • single line to ground faults,


• load transferring from one power source to another. Each of these cases may cause sag with a special (magnitude and duration) characteristic. For example, if a device is sensitive to voltage sag of 25% it will be affected by the motor starting [9]. Sags are main reasons for malfunctions of electrical low voltage devices. Uninterruptible power supply (UPS) or power conditioners are mostly used to prevent voltage sags.

Figure 1.7. Voltage sag caused by a single line to ground (SLG) fault [4-Fig.2.6]

Swells: The increase of voltage magnitude between 1.1 to 1.8 pu is called swell, as shown by figure 1.8. There is no definite definition for the duration of swells but usually the most accepted duration is from 0.5 cycles to 1 minute [5]. Swells are not as common as sags and their main causes are:

• switching off a large load, • energizing a capacitor bank , • voltage increase of the unfaulted phases during a single line to ground (SLG) fault [8].

In some textbooks the word momentary over-voltage is used as a synonym for the term swell. As in the case of sags, UPS or power conditioners are typical solutions to limit the effect of swell [8].

Figure 1.8. Instantaneous voltage swell caused by a single line to ground (SLG) fault [4-Fig.2.8]


1.3.3. Long-Duration Voltage Variation According to standards (e.g., IEEE-1159, ANSI-C84.1), the deviation of the rms value of voltage from the normal value for longer than 1 min is called long-duration voltage variations. The main causes of long-duration voltage variations are load variations and system switching operations. IEEE-1159 divides these events in three categories (Table 1.2): "sustained interruption", "under-voltage" and "over-voltage". Sustained Interruption: Sustained (or long) interruption is the most severe and the oldest power quality event at which voltage drops to zero and does not come back automatically. According to IEC definition the duration of sustained interruption is more than three min but for IEEE definition the duration is more than one min. The number and duration of long interruption is a very important characteristic to measure the ability of power system to deliver a good service to customers. Most important causes of sustained interruptions are:

• fault occurrence in a part of power systems with no redundancy or with the redundant part that is out of operation,

• an incorrect intervention of a protection relay leading to a component outage, • scheduled (or planned) interruption in a low voltage network with no redundancy.

Under-voltage: Under-voltage condition occurs when the rms voltage decreases to 0.8-0.9 pu for more than one minute. Over-voltage: Over-voltage is defined as an increase in the rms voltage to 1.1-1.2 pu for more than one minute. There are three types of over-voltages [9 of Frahbakh]:

• power frequency over-voltages generated by an insulation fault, ferroresonance, faults on alternator regulator, tap changer transformer or over compensation,

• lightning over-voltages, • switching over-voltages produced by rapid modifications in the network structure such as

opening of protective devices or switching on capacitive circuits. 1.3.4. Voltage Unbalance When voltages of a three phase system are not identical in magnitude and/or the phase differences between them are not exactly 120 degree, voltage unbalanced (or imbalanced) has occurred [8]. There is two ways to calculate the degree of unbalance:

• divide the maximum deviation from average of three phase voltages with the average of three phase voltages,

• compute the ratio of the negative (or zero) sequence component to the positive sequence component [5].

Main causes of voltage unbalanced in power systems are: • unbalanced single phase loading in three phase system, • untransposed overhead transmission lines [6-Farh], • blown fuses in one phase of three phase capacitor bank, • sever voltage imbalance (e.g., > 5%) can result from single phasing conditions.

1.3.5. Waveform Distortion


A steady-state deviation from a sine wave of power frequency is called waveform distortion [5] There are five primary types of waveform distortion: "dc offset", "harmonics", "inter harmonics", "notching" and "noise’. Fourier series is usually used to analysis the nonsinusoidal waveform. Dc Offset: The presence of a dc current and/or voltage component in an ac system is called dc offset [5]. Main causes of dc offset in power systems are:

• geomagnetic disturbances [4,5] causing geomagnetic induced currents (GICs), • application of rectifiers and other electronic switching devises.

Main detrimental effects of dc offset in alternating networks are: • half cycle saturation of transformers core, • activating even and odd harmonics, • decreasing the life of transformer, machines and electromagnetic appliances, • additional heating in the appliances, • electrolytic erosion of grounding electrodes and other connectors.

Harmonics: Harmonics are sinusoidal voltages or currents with frequencies that are integer multiples of the power system frequency (usually, f = 50 or 60 Hz). Periodic nonsinusoidal wave forms posse Fourier series and can be decomposed into the sum of fundamental component and harmonics [9]. The frequency of harmonics are integer multiple of the fundamental frequency (e.g., the frequency of nth harmonic is n×f). Main sources of harmonics in power systems are [9]:

• industrial nonlinear loads (Fig.1.9) such as power electronic equipment (drives, rectifiers, inverters) or loads using electric arcs (arc furnaces, welding machines, lighting),

• domestic loads with switching mode power supplies (Televisions, computers, fluorescent and energy savings lamps, …).

Some detrimental effects of harmonics are [8]: • maloperation of control devices and protective relay, • extra losses in capacitors, transformers and rotating machines, • Additional noise from motors and other apparatus, • telephone interference, • activating shunt and series resonance (due to the power factor correction capacitor and cable

capacitance) frequencies, resulting in voltage amplification even at a remote point from the distorting load.

Recommended solutions to reduce and control harmonics are applications of "high pulse rectification", "passive, active and hybrid filters" and "active power conditioners". Inter-Harmonics: The frequency of the inter-harmonics are not integer multiple of the fundamental frequency. Inter-harmonics appear as discrete frequencies or as a band spectrum. The main source of inter harmonic waveform are: "static frequency converters", "cycloconverters", "induction motors" and "arcing devices". The effect of inter-harmonics is not well known. However, inter harmonics have been included in various harmonic standards such as the IEC 61000-4-7 and the IEEE 519.

Notching:


A periodic voltage disturbance caused on the line voltage waveform by the normal operation of power electronics devices when current is commutated from one phase to another. During this period, there is a momentary short circuit between two phases pulling the voltage as close to zero as permitted by system impedances.

Figure 1.9. Current wave form and harmonic spectrum of an adjustable- speed drive [4-Fig.2.10]

Notching is repetitive and can be characterized by its frequency spectra (Fig.1.10). The frequency of this spectrum is quite high. Usually it is not possible to measure it with equipment which is normally used for harmonic analysis. Notches cam impose extra stress on the insulation of transformers, generators and sensitive measuring equipments. Notching can be characterized by the following properties:

• notch depth- average depth of the line voltage notch from the sinusoidal waveform at the fundamental frequency,

• notch width- the duration of the commutation process, • notch area- the product of notch depth and width, • position on the sinusoidal waveform where the notches occur,

Some standards (e.g., IEEE) set limits for notch depth and duration (with respect to the system impedance and load current) in terms of the notch depth, the THD of supply voltage and the notch area for different supply systems.

Figure 1.10. Voltage notching caused by a three phase converter [4-Fig.2.11]


Noise: Noise is defined as unwanted electrical signals with broadband spectral content lower than 200 kHz superimposed upon the power system voltage or current in phase conductors, or found on neutral conductors or signal lines. Noise may result from faulty connections in transmission or distribution system, arc furnace, electrical furnace, power electronic devices, control circuits, arcing equipments, loads with solid state rectifiers, improper grounding, turning off capacitor banks and adjustable speed drives. The problem can be mitigating by using filters, line conditioners and isolation transformers. Noise disturbs electronic devices such as micro computer and programmable controllers. The problem can be mitigated by using filters, isolation transformers, and line conditioners. 1.3.6. Voltage Fluctuation and Flicker Voltage fluctuations are systemic variations of the voltage envelope or random voltage changes, the magnitude of which dose not normally exceed specified voltage ranges (e.g., 0.9 to 1.1 pu as defined by ANSI C84.1-1982). Voltage fluctuations are divided in two categories:

• step voltage changes, regular or irregular in time, • cyclic or random voltage changes produced by variations in the load impedances.

Voltage fluctuation degrade the performance of the equipments using capacitors, control system and instable the internal voltage and currents in electronic equipment. However, voltage fluctuations less than 10% can not affect the equipment. Main causes of voltage fluctuation are: "pulsed power output", "resistance welders", "start-up of drives", " arc furnaces", "drives with steeply-changing loading" and "rolling mills". Flicker: Loads exhibiting continuous and rapid variations in the load current magnitude can cause voltage variations that are often referred to as flicker (Fig.1.11). Flicker has been described as "continuous and rapid variations in the load current magnitude which causes voltage variations". The term flicker is deriver from the impact of the voltage fluctuation on lamps such that they are perceived to flicker by the human eye. This is caused by an arc furnace; one of the most common causes of the voltage fluctuations on utility transmission and distribution system. The difference between voltage fluctuation and flicker is that voltage fluctuation is an electro magnetic phenomenon while flicker is an undesirable result of the voltage fluctuation in some loads and the environmental conditions are very important factors to determine the flicker. However, the two terms are often linked together in some standards.


Figure 1.11. Voltage flicker caused by arc furnace operation [4-Fig.2.12]

1.3.7. Power Frequency Variations The deviation of the power system fundamental frequency from its specified nominal value (e.g., 50 or 60 Hz) is defined as power frequency variation. If the balance between generation and demand (load) is not maintained properly, the frequency of power system will deviate because of changes in the rotational speed of the generators. The amount of deviation of the frequency and the duration of it depends on the load characteristics and response of the generation control system to load changes. Faults of the bulk power transmission system can also cause frequency variations out side of the accepted range for normal steady-state operation of the power system. �

1.4. Definitions and Measures used for Power Quality This section briefly introduces some of the most commonly used definitions and measures of electric power quality as used in this book and as defined in standard documents. Main sources for power quality terminologies are IEEE Std 100, IEC Std 61000-1-1, CENELEC Std EN 50160 and the UIE "guide to quality of electrical supply for industrial installations". Appendix C of reference [9] also presents a fine survey of power quality definitions. 1.4.1. Harmonics Nonsinusoidal current and voltage waveforms occur in today’s power systems due to equipments with non-linear characteristics such as transformers, rotating electric machines, FACTS devices, power electronics components (e.g., rectifiers, triacs, thyristors with capacitor smoothing which are used extensively in PCs, audio and video equipments), switching mode power supplies, compact fluorescent lamps, induction furnaces, adjustable ac and dc drives, arc furnaces, welding tools, renewable energy sources and HVDC networks. Main effects of harmonics are maloperation of control devices and protective relay, telephone interferences, additional line losses (at fundamental and harmonic frequencies), decreasing the lifetime and increasing losses in public service equipments (e.g., transformers, rotating machines, capacitor banks) and customer devises. The periodic nonsinusoidal waveforms posse Fourier series. Each terms in the Fourier series is called the harmonic component of the distorted waveform. The frequency of harmonics are integer multiple of the fundamental frequency. Therefore, a nonsinusoidal voltage or current waveform can be defined as:

......+)t(v+)t(v+)t(v+)t(v+V=)+thcos(V+V=)t(v )4()3()2()1(dc

1=hho

)h(rmsdc (1-1A)�


......+)t(i+)t(i+)t(i+)t(i+I=)+thcos(I+I=)t(i )4()3()2()1(dc

1=hho

)h(rmsdc (1-1B)

where o is the fundamental frequency, h is the harmonic order. Also� h)h()h( ,I,V �and� h are

the amplitude and phase shift of voltage and current for the hth harmonic. Even and odd harmonics of a nonsiusoidal function correspond to even (e.g., 2, 4, 6, 8…) and odd (e.g., 3, 5, 7,9,…) components of its Fourier series. Harmonics of order 1 and 0 are assigned to the fundamental frequency and the dc component of the waveform, respectively. When both positive and negative half-cycles of the waveform have identical shapes, the Fourier series contain only odd harmonics. This is the usual case of voltages and currents of power systems. The presence of even harmonics is often a clue that there is something wrong, either with the load equipment or with the transducer used to make measurement. There are notable exceptions to this such as half-wave rectifiers, arc furnaces (with random arcs) and the presence of Geomagnetically Induced Currents (GICs) in power systems. Triplen Harmonics: Triplen harmonics are the odd multiples of the third harmonic (h=3, 9, 15, 21,…).These harmonic orders become an important issue for grounded-wye system with current flowing on the neutral. Two typical problems are overloading the neutral and telephone interference. For the system of perfectly balanced single phase loads, fundamental current components in the neutral are found to be zero but the third harmonic components are three times the third-harmonic-phase currents because they naturally coincide in phase and time. Transformer winding connection has a significant impact on flow of triplen harmonic currents from single-phase nonlinear loads. For the grounded wye-delta transformer, the triplen harmonic currents enter the wye side and since they are in phase, they add in the neutral. The delta winding provides ampere-turns balance so that they can flow but they remain trapped in the delta and do not show up in the line currents on the delta side. This type of transformer connection is the most common employed in utility distribution substations with the delta winding connected to the transmission feed. Using grounded-wye winding in the both sides of transformer allows balanced triplens to flow from the low voltage system to the high voltage system unimpeded. They will be present in equal proportion on both sides. Subharmonic: Subharmonics have frequencies below the fundamental frequency. There are rarely subharmonics in power systems. Resonance between the harmonic currents or voltages with the power system capacitance and inductance may cause subharmonics. They may be generated when a system is highly inductive (such as an arc furnace during start up) or when the power system contains large capacitor banks for power factor correction or filtering [7 of FRAHZAD]. Inter-harmonics: The frequency of the inter-harmonics are not integer multiple of the fundamental frequency. Inter-harmonics appear as discrete frequencies or as a band spectrum. Main sources of inter-harmonic waveform are static frequency converters, cycloconverters, induction motors and arcing devices. Inter-harmonics have recently been included in various power quality standards such as the IEC 61000-4-7 and the IEEE 519. However, many important related issues such as the range of frequencies to be considered are left out.


Characteristic and Uncharacteristic Harmonics: The harmonics of orders 12k+1 (positive sequence) and 12k-1 (negative sequence) are called characteristic harmonics. The amplitudes of these harmonics are inversely proportional to the harmonic order. Filters are usually used to reduce characteristic harmonic of the large power converter. When the ac system is weak and the operation is not perfectly symmetrical uncharacteristic harmonics appear. It is not economical to reduce the uncharacteristic harmonics with filters; therefore, even a small injection of the harmonic currents can, via parallel resonant conditions, produce very large voltage distortion levels. Positive, Negative and Zero Sequence Harmonics: Assuming a positive-phase (abc) sequence balanced three-phase power system, the expressions for the fundamental currents are:

)240-tcos(I=)t(i

)120-tcos(I=)t(i

)tcos(I=)t(i

oo

)1(cc

oo

)1(bb

o)1(

aa

(1-2)

The negative displacement angles indicate that the fundamental phasors rotate counterclockwise rotation in the space-time plane. For the third harmonic currents we have:

)t3cos(I=)720-t3cos(I=)240-t(3cosI=)t(i

)t3cos(I=)360-t3cos(I=)120-t(3cosI=)t(i

)t3cos(I=)t(i

o)3(

co

o)3(

co

o)3(

c)3(

c

o)3(

bo

o)3(

bo

o)3(

b)3(

b

o)3(

a)3(

a

(1-3)

This equation shows that the third harmonic phasors are in phase and have zero displacement angles between them. The third harmonic currents are known as zero sequence harmonics. The expressions for the fifth harmonic currents are:

)120-t5cos(I=)1200-t5cos(I=)240-t(5cosI=)t(i

)240-t5cos(I=)060-t5cos(I=)120-t(5cosI=)t(i

)t5cos(I=)t(i

oo

)5(c

oo

)5(c

oo

)5(c

)5(c

oo

)5(b

oo

)5(b

oo

)5(b

)5(b

o)5(

a)5(

a

(1-4)

Note that displacement angles are positive, therefore, the phase sequence of this harmonic is clockwise and opposite to that of the fundamental. The fifth harmonic currents are known as negative sequence harmonics. Similar relationships exist for other harmonic orders. Table 1.3 categorizes the harmonics ib terms of their respective sequence orders.

Table 1.3. Phase sequences of power systems harmonic.

Harmonic order Phase sequence 1, 4, 7, 10, 13, 16, 19, … Positive 2, 5, 8, 11, 14, 17, 20, … Negative 3, 6, 9, 12, 15, 18, 21, … Zero

Note that the harmonic phase-shift angle has the effect of altering the shape of the composite waveform (e.g., adding a third harmonic component with 0 degree phase shift to the fundamental results in a composite waveform with maximum peak-to-peak value while 180 degree phase shift will result in a composite waveform with minimum peak-to-peak value), the phase sequence order of the harmonics is not affected.


1.4.2. The Average Value of a Nonsinusoidal Waveform The average value of a pure sinusoidal waveform is defined as:

T

oave dt)t(i

T

1=I (1-5)

For the nonsinusoidal of Eq.1-1 we have, T

o

)4()3()2()1(T

oave dt.....]+)t(I+)t(I+)t(I+)t(I[

T

1=dt)t(i

T

1=I (1-6)

Since all harmonics are pure sinusoidal waveforms, the average value of a nonsinusoidal function is equal to its dc value:

dcave I=I (1-7)

1.4.3. The rms Value of a Nonsinusoidal Waveform The rms value of a pure sinusoidal waveform is defined as:

2

I=I

2

1=dt)t(cosI

2

1=dt)t(i

T

1=I max2

max

T

o

22max

T

o

2rms (1-8)

For the nonsinusoidal of Eq.1-1 we have,

2/121

)2(max

)1(max

322)3(

max222)2(

max122)1(

max

T

o

2/123

)3(max2

)2(max1

)1(max

T

o

T

o

2rms

}dt......]+)+t2cos()+tcos(II2+.....

...+)+t3(cos)I(+)+t2(cos)I(+)+t(cos)I[(T

1{=

}dt]...+)+t3cos(I+)+t2cos(I+)+tcos(I[T

1{=

dt)t(iT

1=I

(1-9)

This equation contains two parts: • First part is the sum of squares of harmonics:

)+tp(cos)I( p22)p(

max

H

1=p

(1-10)

• Second part is the sum of products of harmonics:

qp),+tqcos()+tpcos(II qp)q(

max)p(

max

H

1=p

H

1=q

(1-11)

After some simplifications, the average of the second part is zero and the first part becomes:

....+t2d)]+t2cos()I([+td)]+tcos()I([=

t2d])+tp(cos)I([=I

22)2(

max

2

o1

2)1(max

2

o

p22)p(

max

H

1=p

2

orms

(1-12)

Therefore, the rms value of a nonsinusoidal waveform is:

2/12)H(rms

2)3(rms

2)2(rms

2)1(rmsrms ])I(+.....+)I(+)I(+)I([=I (1-13)

If the nonsinusoidal waveform contains dc values, then:


2/12)H(rms

2)3(rms

2)2(rms

2)1(rmsdcrms ])I(+.....+)I(+)I(+)I(+I[=I (1-14)

1.4.4. Form Factor (FF) Form factor is a measure of the shape of waveform and is defined as:

ave

rms

I

I=FF (1-15)

Since the average value of a pure sinusoidal waveform is zero, its average over one half cycle is used in the above equation. As the harmonic contents of the waveform increases, its FF will also increase. 1.4.5. Ripple Factor (RF) Ripple factor is a measure of the ripple content of the waveform and is defined as:

dc

ac

I

I=RF (1-16)

where 2dc

2rmsac )I(-)I(=I . It is easy to show that:

1-FF=)I(

)I(-

)I(

)I(=

I

)I(-)I(=RF 2

2dc

2dc

2dc

2rms

dc

2dc

2rms

(1-17)

1.4.6. Harmonic Factor (HF) The harmonic order of the hth harmonic which is a measure of the individual harmonic contribution is defined as:

)1(rms

)h(rms

h I

I=HF (1-18)

Some references [8], call HF the Individual Harmonic Distortion (IHD). 1.4.7. Lowest Order Harmonic (LOH) It is that harmonic component whose frequency is closest to the fundamental one and its amplitude is greater than or equal to 3% of the fundamental component. 1.4.8. Total Harmonic Distortion (THD) The most common harmonic index used to indicate the harmonic content of a distorted waveform with a single number is THD. It is a measure of the effective value of the harmonic components of a distorted waveform, which is defined as the rms of the harmonics expressed in percentage of the fundamental component:

)1(

2=h

2)h(

I

)I(

=THD (1-19)

A commonly cited figure of 5% is often used as a dividing line between high and low distortion level. However, the exact limit is determined by the selected standard. The ANSI standard recommends truncation of THD series at 5kHz, but most practical commercially available instruments are limited to about 1.6kHz (due to the V.T and I.T transformer used and the word length of the digital hardware [3]). Main advantages of THD are:

• It is a common use for the quick measure of distortion. • It is being readily calculated.


Some disadvantages of THD are: • It does not indicate amplitude information. • The detail information of full spectrum is lost.

THD is related to the rms value of the waveform as follows [4]:

2)1(

2=h

2)h(rms THD+1I=)I(=I (1-20)

THD can be weighted to indicate the amplitude stress on various system devices. The weighted distortion factor adapted to inductance is an approximate measure for the additional thermal stress of inductances such as coil and induction motors [7, Table 2.4]:

THD Adapted to Inductance = )1(

50

2=h

2)h(

ind V

h

)V(

=THD (1-21)

where 2......1= . On the other hand, the weighted THD adapted to capacitors is an approximate measure for the additional thermal stress of capacitors directly connected to the system without series inductance [7, Table 2.4]:

THD Adapted to Capacitor =

( ))1(

50

2h

2)h(

cap V

)V(h

THD∑

=×

(1-22)

Because voltage varies only few percent, the voltage THD is nearly always a meaningful number. This is not the case for current; a small current may have a high THD but not be a significant threat to the system. 1.4.9. Total Inter-harmonic Distortion (TIHD) This factor is equivalent to the THD, but defined for inter-harmonics [7]:

)1(

n

1=i

2)i(

I

)I(

=TIHD (1-23)

where "i" is the total number of inter-harmonics and "n" is the total number of frequency bins present including subharminics (e.g., inter-harmonic frequencies that are less than the fundamental frequency). 1.4.10. Total Subharmonic Distortion (TSHD) This factor is equivalent to the THD, but defined for subharmonics [7]:

)1(

S

1=s

2)s(

I

)I(

=TSHD (1-24)

where "S" is the total number of frequency bins present below the fundamental frequency. 1.4.11. Total Demand Distortion (TDD) Due to the mentioned disadvantages of THD, some standards (e.g., IEEE 519:1992) have defined the total demand distortion factor. This term is similar to THD except that the distortion is expressed as a percentage of some rated or maximum value (e.g., load current magnitude), rather than as a percentage of the fundamental current:


( )rated

50

2h

2)h(

I

I

TDD∑

== (1-25)

1.4.12. Telephone Influence Factor (TIF) Telephone influence factor (which is jointly proposed by Bell Telephone Systems (BTS) and Edison Electric Institute (EEI) and is widely used in USA and Canada), determines the interface of power systems harmonic on telecommunication systems. It is a variation of THD in which the root of the sum of the squares is weighted using factors (weights) which reflect the response of human ear [3]:

( )

( )1=i

2)i(

1=i

2)i(i

V

Vw

=TIF (1-26)

where iw are the TIF weighting factors obtained by physiological and audiological tests, as shown by Table 1.4. They also incorporate the way current in a power circuit induces voltage in a adjacent communication system. 1.4.13. C-Message Weights The C-message weighted index is very similar to the TIF except that the weights ic are used in

place of iw [3]:

( )

( )

( )

rms

1=i

2)i(i

1=i

2)i(

1=i

2)i(i

I

Ic

=

I

Ic

=C (1-27)

where ic are the C-message weighting factors (Table 1.4) that are related to TIF weights by:

iio w=cfi5 . The C-message could also be applied to bus voltage.

1.4.14. V.T and I.T Products The THD index doesn’t give information about the amplitude of voltage (or current) so some systems like BTS - EEI system use I.T and V.T products. The I.T and V.T products are alternative indexes to THD, incorporating the voltage or current amplitude:

V.T= 1=i

2)i(i )Vw( (1-28)

I.T= 1=i

2)i(i )Iw( (1-29)

where iw are listed in Table 1.4.

1.4.15. Telephone Form Factor (TFF) Two weighting systems widely used by the industry for interference on telecommunication system are [7]:

• The sophomoric weighting system proposed by the international consultation commission on telephone & Telegraph System (CCITT) used in Europe.


• The C-message weighting system proposed jointly by Bell Telephone Systems (BTS) and Edison Electric Institute (EEI), used in USA and Canada.

These systems acknowledge that the harmonic effect is not uniform over the audiofrequency and use measured weighting factors to account for this non-uniformity. They take into account the telephone equipment and the sensitivity of the human ear to provide a reasonable indication of the interference from each harmonic. The BTS and EEI systems describe the level of harmonic interference in terms of telephone influence factor (Eq.1-26) or C-message (Eq.1-27) while the CCIT system uses Telephone Form Factor (TFF):

∑∞

=

=1h

2)h(hh)1(

)V(PKV

1TFF (1-30)

where 800/hK h = is a coupling factor and hP is the harmonic weight [7 (Fig.2.5)] divided by 1000.

Table 1.4. Telephone interface ( iw ) and C-massage ( ic ) weighting factors [3]

Harmonic order (h, f1=60Hz)

TIF weights ( iw )

C weights ( ic )

Harmonic order (h, f1=60Hz)

TIF weights ( iw )

C weights ( ic )

1 0.5 0.0017 29 7320 0.841 2 10.0 0.0167 30 7570 0.841 3 30.0 0.0333 31 7820 0.841 4 105 0.0875 32 8070 0.841 5 225 0.1500 33 8330 0.841 6 400 0.222 34 8580 0.841 7 650 0.310 35 8830 0.841 8 950 0.396 36 9080 0.841 9 1320 0.489 37 9330 0.841

10 1790 0.597 38 9590 0.841 11 2260 0.685 39 9840 0.841 12 2760 0.767 40 10090 0.841 13 3360 0.862 41 10340 0.841 14 3830 0.912 42 10480 0.832 15 4350 0.967 43 10600 0.822 16 4690 0.977 44 10610 0.804 17 5100 1.000 45 10480 0.776 18 5400 1.000 46 10350 0.750 19 5630 0.988 47 10210 0.724 20 5860 0.977 48 9960 0.692 21 6050 0.960 49 9820 0.668 22 6230 0.944 50 9670 0.645 23 6370 0.923 55 8090 0.490 24 6650 0.924 60 6460 0.359 25 6680 0.891 65 4400 0.226 26 6790 0.871 70 3000 0.143 27 6970 0.860 75 1830 0.0812 28 7060 0.840 83.3 840 0.0336


1.4.16. Distortion Index (DIN) The DIN index is commonly used in standards and specification outside North American. It is also used in Canada and is defined as [3]:

DIN= 1+THD

THD=

)V(

)V(

2

1=i

2)i(

2=i

2)i(

(1-31)

For low levels of harmonics, one can apply Taylor series expansions, then:

)THD2

1-1(THDDIN (1-32)

1.4.17. Distortion Power (D) Harmonic distortion complicates the computation of power and power factor because voltage and current equations (and their products) contain harmonic components. Under pure sinusoidal conditions, there are four standard quantities associated with power: • Fundamental apparent power ( 1S ) = the product of the rms fundamental voltage and current.

• Fundamental active power ( 1P ) = the average rate of delivery of energy.

• Fundamental reactive power ( 1Q ) = the portion of the apparent power that is out of phase with P.

• Power factor (or displacement factor) = 11 SP The relationship between these quantities is defined by the power triangle:

21

21

21 )Q(+)P(=)S( (1-33)

If voltage and current waveforms are nonsinusoidal (Eq.1-1), the above equation does not hold because S contains cross terms in the products of Fourier series which correspond to voltages and currents of different frequencies, while, P and Q correspond to voltages and currents of the same frequency. It has been suggested to account for these cross terms as follows [3]:

2222 D+Q+P=S (1-34)

where,

Apparent power = 2)h(rms

H

,...3,2,1,0=h

2)h(rms

H

,...3,2,1,0=hrmsrms )I()V(=IV=S (1-35)

Total real power = hhhh)h(

rms)h(

rms

H

,...3,2,1,0=h

-=where),cos(IV=P (1-36)

Total reactive power = )sin(IV=Q h)h(

rms)h(

rms

H

,...3,2,1,0=h

(1-37)

Distortion power = )]-cos(IV 2-)I()V+)I()V[(=D nm)n(

rms)m(

rms2)m(

rms2)n(

rms2)n(

rms2)m(

rms

H

1+m=n

1-H

0=m

(1-38)

Also, the fundamental power factor in the case of sinusoidal voltage and non sinusoidal currents is defined as [6]:

21

21)Q(+P

P=cos (1-39)

and the harmonic displacement factor is defined as [6]:


221

2 D+)Q(+P

P= (1-39)

The power and displacement factor quantities are shown in addition to the power quantities in Fig.1.12.

Figure 1.12 Vector diagram of different parameters of electric power under nonsinusoidal conditions [6, Fig.2-14]. 1.5. Effects of Poor Power Quality on Power System Devices Poor electric power quality has many harmful effects on power system devices and end-user appliances. What makes this phenomenon so insidious is that very often its effects are not known until failure occurs. Therefore, insight into how disturbances are generated and interacted within a power system and how they affect components is important for preventing failures. Even if failures don’t occur, poor power quality and harmonics increase losses and decrease the lift time of power system components. Some of the main detrimental effects of poor power quality include:

• Harmonics add to the rms and peak value of the waveform. This means equipments could receive a damagingly high peak voltage and may be susceptible to failure. High voltage may also force power system components to operate in the saturation regions of their characteristics, producing additional harmonics and disturbances. The waveform distortion and its effects are very dependant on the harmonic phase angles. The rms value can be the same but depending on the harmonic phase angles, the peak value can be different.

• There are adverse effects from heating, noise, and reduced life on capacitors, surge suppressors, rotating machines, cables and transformers, fuses and customers’ equipment (ranging from small clocks to large industrial loads).

• Distribution companies are particularly concerned that distribution transformers may need to be derated to avoid premature failure from saturation effects (caused by harmonics).

• Additional losses of transmission lines, cables, generators, AC motors and transformers (at fundamental and harmonic frequencies).

• Failure of power system components and customer loads due to unpredicted disturbances, voltage and/or current magnifications, harmonic resonances and ferroresonaces.

• Malfunction of controllers and protective devices such as fuses and relays. • Iinter-harmonics can perturb ripple control signals and at subharmonic levels can cause

flicker. • Harmonic instability [7] caused by large and unpredicted harmonic sources such as arc

furnaces. • • ………. (Any more effects)????


Impacts of poor power quality on power systems and its components will be discussed in detail in the incoming chapters. 1.6. Standards Referring to Power Quality Much documentation for power quality standards have been generated by different organizations and institutes. These documents come in three levels of applicability and validity: guides, recommendations and standards [3]:

• Power quality guides are illustration and exemplary procedures that contain typical parameters and representative solutions to commonly encountered power quality problems.

• Power quality recommended practices recognize that there are many solutions to power quality problems and an indicated solution over others. Any operating limits that are indicated by recommendations are not required, but should be targets for designs.

• Power quality standards are formal agreements between industry, user and the government as to the proper procedure to generate, test, measure, manufacture and consume electric power. In all jurisdictions, violation of standards could be used as evidence in court of law for the purpose of litigation.

Usually the fist passage of a power quality document is done in the form of the guides that are often based on an early document from an industry or government group. Guides are then prepared and edited by different working groups. A recommended practice is usually an upgrade of a guide, and a standard is usually an upgrade of a recommended practice. The main reasons for setting guides, recommendations and standards in harmonically puluted power systems are:

• To keep disturbances to user equipment within permissible limits. • To provide uniform terminology and test procedures for power quality problems. • To provide a common basis on which a wide range of engineering is referenced.

There are many standards and related documents which deal with the power quality issues. A frequently updated list of available documents on power quality issues will simplify the search for appropriate information. Table 1.5 includes some of the commonly used guides, recommendations and standards on electric power quality issues. The mostly adopted documents are:

• The North American Standards that are adopted by many countries of North and South America: a) Institute of Electrical and Electronic Engineering (IEEE) b) American National Standards Institute (ANSI) c) Military Specifications (MIL-Specs) published by US department of defense and

Canadian Electric Association (CEA) • British Standards (BS) • European (Standards) Norms (EN) • International Electrotechnical Commission (IEC) • Computer Business Equipment Manufacturers Association (CBEMA) curves • Information Technology Industry Council (ITIC) curves [4(Fig.2.13),9(Fig.5.9)] • UIE-DWG [7, page 20] • CENELEC [7, page 20] • CISPR [6, page 7] • VDE [6, page 1] = a German standard • NEMA {7, page 20] • SEMI F47 Curves


Table 1.5. Some guides, recommendations and standards on electric power quality

Source Coverage IEEE and ANSI Documents:

IEEE 4: 1995 Standard techniques for high-voltage testing IEEE 100: 1992 Standard dictionary of electrical and electronic terms IEEE 120: 1989 Master test guide for electrical measurements in power circuits IEEE 141: 1993 Recommended practice for electric power distribution for industrial plants.

Effect of voltage disturbances on equipments within an industrial area IEEE 142: 1993 (The Green Book)

Recommended practice for grounding of industrial and commercial power systems.

IEEE 213: 1993 Standard procedure for measuring conducted emissions in the rang of 300 kHz to 25 MHz from television and FM broadcast receivers to power lines.

IEEE 241: 1990 (The Gray Book)

Recommended practice for electric power systems in commercial buildings.

IEEE 281: 1994 Standard service conditions for power system communication equipment. IEEE 299: 1991 Standard methods of measuring the effectiveness of electromagnetic shieldings

enclosures. IEEE 367: 1996 Recommended practice for determining the electric power station ground

potential rise and induced voltage from a power fault. IEEE 376: 1993 Standard for the measurement of impulse strength and impulse bandwidth. IEEE 430: 1991 Standard procedures for the measurement of radio noise from overhead power

lines and substations. IEEE 446: 1987 (The Orange Book)

Recommended practice for emergency and standby systems for industrial and commercial applications (e.g., power acceptability curve [3, Fig.2-26], CBEMA curve)

IEEE 449: 1990 Standard for ferroresonance voltage regulators. IEEE 465 Test specifications for gas tube surge protective devices IEEE 472 Event recorders IEEE 473: 1991 Recommended practice for an electromagnetic site survey (10 kHz to 10 GHz). IEEE 493: 1997 (The Gold Book)

Recommended practice for the design of reliable industrial and commercial power systems.

IEEE 519: 1992 or 1993 Recommended practice for harmonic control and reactive compensation of static power converters.

IEEE 539: 1990 Standard definitions of terms relating to corona and field effects of overhead power lines.

IEEE 859: 1987 Standard terms for reporting and analyzing outage occurrences and outage states of electrical transmission facilities.

IEEE 944: 1986 Application and testing of uninterruptible power supplies for power generating stations.

IEEE 998: 1996 Guides for direct lightning stroke shielding of substations. IEEE 1048: 1990 Guides for protective grounding of power lines. IEEE 1057: 1994 Standards for digitizing waveform recorders. IEEE P1100: 1992 (The Emerald Book)

Recommended practice for powering and grounding sensitive electronic equipment in commercial and industrial power systems.

IEEE 1159: 1995 Recommended practice on monitoring electric power quality. Categories of power system electromagnetic phenomena.

IEEE 1250: 1995 Guides for service to equipment sensitive to momentary voltage disturbances. IEEE 1346: 1998 Recommended practice for evaluating electric power system compatibility with

electronics process equipment.


IEEE-P-1453 Flicker IEEE/ANSI 18: 1980 Standards for shunt power capacitors IEEE/ANSI C37 Guides for surge withstand capability (SWC) tests IEEE/ANSI C50 (1982) Harmonics and noise from synchronous machines IEEE/ANSI C57.110: 1986

Recommended practice for establishing transformer capability when supplying nonsinusoidal load currents

IEEE/ANSI C57.117: 1986

Guides for reporting failure data for power transformers and shunt reactors on electric utility power systems.

Table 1.5. (continue)

Source Coverage IEEE and ANSI Documents (continue):

IEEE/ANSI C62.45: 1992 (IEEE 587)

Recommended practice on surge voltage in low voltage AC power circuits. Including guides for lightning arresters applications.

IEEE/ANSI C62.48: 1995 Guides on interactions between power system disturbances and surge-protective devices.

ANSI C84.1: 1982 American national standard for electric power systems and equipment voltage ratings (60 Hz).

ANSI 70 The national electric code ANSI 368 Telephone influence factor ANSI 377 Spurious radio frequency emission from mobile communication equipment International Electrotechnical Commission (IEC) Documents: IEC 38: 1983 Standard voltages IEC 816: 1984 Guides on methods of measurement of short duration transients on low voltage

power and signal lines. Division of equipments susceptible to transients. IEC 868: 1986 Flicker meter- functional and design specifications. IEC 868-0 (1991) Flicker meter- evaluation of flicker severity. Evaluates the severity of voltage

fluctuation on the light flicker. IEC 1000-3-2:1994 Electromagnetic compatibility Part 3: Limits Section 2: Limits for harmonic

current emissions (equipment absorbed current ≤ 16A per phase) IEC 1000-3-6:1996 Electromagnetic compatibility Part 3: Limits Section 6: Emission limits

evaluation for perturbing loads connected to MV and HV networks IEC 1000-4:1991 Electromagnetic compatibility Part 4: Sampling and metering techniques EN 50160:1994 Voltage characteristics of electricity supplied by public distribution systems IEC/EN 60868-0 Flicker meter implementation IEC 61000 standards on EMC

On Electromagnetic Compatibility (EMC)

British Standards (BS) and European Norms Documents: BS5406 (based on IEC 555 part 2)

Control harmonic emissions from small domestic equipment

Other Documents: ER G5/3 The basis of standards in some other (mostly commonwealth) countries, but it

does not include notching and burst harmonics

G5/4 :2001 limiting harmonic voltage distortion levels on public networks at the time of connection of new non-linear loads to ensure compatibility of all connected equipment

UIE-DWG-2-92-D Produced by the Distribution Working Group (DWG) of Union Internationale Electrothermie (UIE). Including guides for measurements of voltage dips and short circuit interruptions occurring in industrial installations.

UIE-DWG-3-92-G UIE guides for quality of electrical supply for industrial installations. Including types of disturbances and relevant standards.

CBEMA Curves: 1983 Produced by the Computer Business Equipment Manufacturer Association for the design of the power supply for computers and electronic equipments.

ITI Curves (new CBEMA curves)

Information Technology Industry Council (the new name for CBEMA) application note, available at http://www.itic.org/iss_pol/techdocs/curve.pdf


1.6.1 IEC 61000 Series of Standards for Power Quality The IEC 61000 (or EN 61000) series is one of the most commonly used references for power quality in Europe that contains six parts, each with standards and technical reports [7]:

• Part 1 (General)- Two sections covering application and interpretation aspects of EMC • Part 2 (Environment)- Twelve sections on classification of the electromagnetic environment

and compatibility levels for different environments. Some aspects of this document include harmonic compatibility levels for; public LV systems (IEC 61000-2-2), industrial plant (IEC 61000-24) and public MV systems (IEC 61000-2-12).

• Part 3 (Limits)- Eleven sections covering emission limits for harmonics and other disturbances. Some aspects of this document include harmonic current emission limits for equipment connected at LV with low (less than 16 A per phase) current (IEC 61000-3-2), flicker (IEC 61000-3-3), harmonic current emission limits for equipment connected at LV with high (more than 16 A per phase) current (IEC 61000-3-4) and assessment of emission limits for distorting loads in MV and HV power systems (IEC 61000-3-6).

• Part 4 (Testing and Measurement Techniques)- Thirty one sections on describing standard methods for testing equipment for emission of and immunity to the different disturbances. Some aspects of this document include harmonic and inter-harmonic measurements and instrumentation (IEC 61000-4-7), dips and interruptions (EN 61000-4-11), inter-harmonics (EN 61000-4-13) and power quality measurement methods (IEC 61000-4-30).

• Part 5 (Installation and Mitigation Guidelines)- Seven sections covering earthing, cabling, mitigation and degrees of protection against EM disturbances.

• Part 6 (Generic Standards)- Five sections covering immunity and emission standards for residential, commercial, industrial and power station environments

: EN 61000-3-2 introduces power quality limits (Table 1.6) for four classes of equipments: • Class A: Balanced three-phase equipment and all other equipment, except those listed in

other classes. • Class B: Portable tools • Class C: Lighting equipment, including dimming devices • Class D: Equipment with a "special waveshape" and an active input power of 75 to 600W

Table 1.6. Harmonic limits defined by the EN 61000 standards for different classes of equipments

Harmonic Order (h)

Class A [A]

Class B [A]

Class C [% of fundamental]

Class D [% of fundamental]

2 1.08 1.62 2 3 2.30 3.45 30× 3.4 4 0.43 0.65 5 1.44 2.16 10 1.9 6 0.30 0.45 7 0.77 1.12 7 1 8 0.23 0.35 9 0.40 0.60 5 0.5 10 0.18 0.28


11 0.33 0.50 3 0.35 12 0.15 0.23 13 0.21 0.32 3 0.296

14-40 (even) 1.84/h 2.76/h

15-39 (odd) 2.25/h 3.338/h

3 3.85/h

1.6.2. IEEE-519 Standard for Power Quality The American standards (ANSI and IEEE) do not have such a comprehensive and complete set of power quality standards as the IEC. However, their standards are more practical and provide theoretical background on the phenomena. This has made them very useful reference documents, even outside of the United States. IEEE-Std 519 (1993) is the IEEE recommended practices and requirements for harmonic control in electric power systems. It is one of the well-known documents for power quality limits. IEEE 519 is more comprehensive than IEC 61000-3-2 but is not a product standard. First official version of this document was published in 1981. Product testing standards for the US are now considered within TC77A/WG1 (TF5b), but are also discussed in IEEE. EU had a short term demand to unify testing standards and is a few years ahead of the US. Current direction of TC-77 working group is towards a global IEC standard for both 50/60 Hz and 115/230 V. Comparison of IEC and IEEE power quality limits are show in Fig.1.13.

Figure 1.13 Comparison of IEC and IEEE limits for power quality [from internet]

IEEE 519 contains thirteen sections, each with standards and technical reports [11]:

• Section 1 (Introduction and scope)- including application of the standard. • Section 2 (Definition and Letter Symbols) • Section 3 (References)- including standard references. • Section 4 (Converter Theory and Harmonic Generation)- including documents for

converters, arc furnaces, static var compensators, inverters for dispersed generation, electronic control, transformers and generators.

• Section 5 (System Response Characteristics)- including resonance conditions, effect of system loading and typical characteristics of industrial, distribution and transmission systems.

• Section 6 (Effect of Harmonics)- detrimental effects of harmonics on motors, generators, transformers, capacitors, electronic equipments, meters, relaying, communication systems and converters.

0.00

0.50

1.00

1.50

2.00

2.50

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39

Europe IEC61000-3-2 Class-A type products

US 120 V per IEEE-Std-519 Table 10.1

European Harmonic Limits & IEEE recommended limits for US

Europe IEC61000-3-2 Class-D type products @ 300 Watt

Amp

US 120 V per IEEE-Std-519 Table 10.3


• Section 7 (Reactive Power Compensation and Harmonic Control)- including converter power factor, reactive power compensation and control of harmonics.

• Section 8 (Calculation Methods)- including calculations of harmonic currents, telephone interference, line notching, distortion factor and power factor.

• Section 9 (Measurements)- for line notching, harmonic voltage and current, telephone interface, flicker, power factor improvement, instrumentation and statistical characteristics of harmonics.

• Section 10 (Recommended Practices for Individual Consumers)- including standard impedance, customer voltage distortion limits, customer application of capacitors and filters, effect of multiple sources at a single customer and line notching calculations.

• Section 11 (Recommended Harmonic Limits on the System)- including voltage distortion limits on various voltage levels, TIF limits vs. voltage level and IT product.

• Section 12 (Recommended Methodology for Evaluation of New Harmonic sources) • Recommended 13 (Bibliography)- including books and general discussions.

IEEE-519 sets limits on the voltage and current harmonics distortion at the point of common coupling (PCC, usually the secondary of the supply transformer). The total harmonic distortion at the PCC is dependant on the percent of harmonics distortion from each non-linear device with respect to the total capacity of the transformer and the relative load of the system. There are two criteria that are used in IEEE 519 to evaluate harmonics distortion:

• Limitation of the harmonic current that a user can transmit/inject into utility system (THDi). • Limitation of the quality of the voltage that the utility must furnish the user (THDV).

The interrelationship of these two criteria shows that the harmonic problem is a system problem and not tied just to the individual load that requires the harmonic current. Tables 1.7 and 1.8 list the harmonic current and voltage limits based on the size of the user with respect to the size of the power system to which the user is connected [11]. The short-circuit current ratio (

SCR ) is defined as the ratio of the short circuit current (available at

the point of common coupling) to the nominal fundamental load current (Fig.1.14):

L

SC

SCI~I~

=R (1-40)

Thus as the size of the user load decreases with respect to the size of the system, the larger is the percentage of harmonic current the user is allowed to inject into the utility system. Table 1.8 lists the amount of voltage distortion [11] that is acceptable from a utility to a user as specified by the IEEE 519. To meet the power quality values of Table 1.8, cooperation among all users and the utility is needed to insure that no one user deteriorates the power quality beyond these limits. The values in Table 1.8 are low enough to insure that equipment will operate correctly.

Figure 1.14 Equivalent circuit of power system and nonlinear load. ZS is small (or ISC is large) for strong system for strong systems and ZS is large (or ISC is small) for weak systems


Table 1.7. IEEE 519 harmonic current limits [11] for nonlinear loads at the point of common coupling (PCC) with other loads at voltages of 2.4 to 69 kV

Maximum harmonic current distortion at PCC in % of fundamental Harmonic order (odd harmonics)

LSC II 11<h 17<h11 23<h17 35<h23 35h

THD

< 20* 4.0 2.0 1.5 0.6 0.3 5.0

20-50 7.0 3.5 2.5 1.0 0.5 8.0

50-100 10.0 4.5 4.0 1.5 0.7 12.0

100-1000 12.0 5.5 5.0 2.0 1.0 15.0

> 1000 15.0 7.0 6.0 2.5 1.4 20.0 *) All power generation equipment is limited to these values of current distortion, regardless of the actual

LSC II .

Even harmonics are limited to 25% of the odd harmonic limits above. Where: =ISC

maximum short circuit current at PCC.

=ILmaximum load current (fundamental frequency) at PCC.

Foe PCC’s from 69 to 138 kV, the limits are 50 percent of the limits above. A case-by-case evaluation is required for PCC’s of 138 kV and above.

Table 1.8. IEEE 519 harmonic voltage limits [11] for power producers (public utilities or co-generators)

Harmonic voltage distortion % at PCC

2.3 to 69 kV 69 to 138 kV > 138 kV

Maximum for individual harmonics 3.0 1.5 1.0

Total harmonic distortion (THD) 5.0 2.5 1.5

1.7. Harmonic Modeling Philosophies [5] For the simulation and modeling of conventional power system, its dynamic operation is normally subdivided into well-defined quasi steady-state regions. Differential equations representing system dynamic in each region are transferred into algebraic relations, and the circuit is solved at fundamental frequency (50 or 60 Hz) in terms of voltage and current phasors. Modern power systems have many nonlinear components and loads that produce voltage and current harmonics.By definition, harmonics result from periodic steady state conditions and therefore, their simulation should also be formulated in terms of harmonic phasors. Considering the complex nature of many nonlinear loads (sources) and their couplings with the harmonic power flow, sophisticated modeling techniques are required for accurate simulation. Three techniques are usually used for harmonic analysis of power system in the present of nonlinear loads and/or components: time domain simulation, frequency (harmonic) domain modelling and iterative procedures. The latest may use time domain, frequency domain or some combination of time and frequency domain techniques to achieve a more accurate solution (e.g., the main structure of many


harmonic power flow algorithms are based on a frequency domain technique while nonlinear loads are model in time domain). 1.7. 1. Time Domain Simulation [5] Dynamic characteristics of power system are represented in terms of nonlinear sets of differential equations that are normally solved by numerical integration. There are two commonly used time domain techniques:

• State variable – extensively used for the simulation of electronic circuits. • Nodal analysis – commonly used for electromagnetic transient simulation of power system.

Two main limitations attached to the time domain methods for harmonic studies are: • They usually require considerable computing time (even for small systems) for the

derivation of harmonic information. This involves solving for the steady state condition and then applying the Fast Fourier Transformation.

• There are some difficulties in time domain modeling of power system components with distributed or frequency-dependent parameters.

Electromagnetic transient program (EMTP) is one of the well-known time domain programs that are widely used for transient and harmonic analyses. 1.7. 2. Frequency (Harmonic) Domain Simulation [5] The most commonly used model in frequency domain assumes a balanced three-phase system (at fundamental and harmonic frequencies) and uses single-phase analysis, a single harmonic source and a direct solution. The injected harmonic currents by nonlinear power plant are modeled as constant current sources to make a direct solution possible. In the absence of any other comparable nonlinear loads, the effect of a given harmonic source is often assessed with the help of equivalent harmonic impedances. The single source concept is still used for harmonic filter design. Real-world power systems are usually asymmetric. This justifies the need for multiphase harmonic models and power flow that considerably complicates the simulation procedures. For the more realistic cases that more than one harmonic source is present in the power system, the single source concept can still be used, provided that the interaction between them can be ignored. In theses cases, the principle of superposition is used to compute the total harmonic distortion throughout the network. In many cases, it can be assumed that only the fundamental frequency and its harmonics are present. This type of analysis, the harmonic domain simulation, is actually a restriction of the frequency domain modeling to integer harmonic frequencies. 1.7. 3. Iterative Simulation Techniques [5] In many modern networks, due to the increased power ratings of nonlinear elements (e.g., HVDC systems, FACTS devices, renewable energy sources and industrial and domestic nonlinear loads) as compared to the system short circuit power, application of superposition (as applied by frequency domain techniques) is not justified and will result in inaccurate solutions. In addition, due to the propagation of harmonic voltages and currents, injected harmonics from each nonlinear load is a function of that from other sources. For such systems, accurate results can be obtained by iteratively solving nonlinear equations describing system steady state conditions. At each iteration, the harmonic domain simulation techniques can be applied, with all nonlinear interactions included. Two important aspects of the iterative harmonic domain simulation techniques are:

• Derivation of system nonlinear equations- The system is partitioned into linear regions and nonlinear devices (described by isolated equations). System solution is then predominantly a solution for the boundary conditions for each nonlinear devise. Many techniques have been proposed for device modeling including time domain simulation to steady state [reference #12 of [5]], analytical time domain expressions [reference #11&13 of [5]],


waveform sampling and FFT [reference #14 of [5]] and harmonic phasor analytical expressions [reference #15 of [5]].

• Solution of nonlinear equations- Early methods used the fixed point iteration procedure of Gauss-Seidel that frequently diverges. Some techniques replace the nonlinear devices at each iteration by a linear Norton equivalent (which might be updated at the next iteration). More recent methods make use of Newton-type solutions and completely decouple device modeling and system solution. They use variety of numerical analysis improvement techniques to accelerate the solution procedure.

1.7. 4. Modeling Harmonic Sources As mentioned above, an iterative harmonic power flow algorithm is usually used for the simulation of the distorted power system. At each iteration, harmonic sources need to be accurately included and their model must be updated at the next iteration. For most harmonic power flow studies it suitable to treat harmonic sources as (variable) harmonic currents. At each iteration of the power flow algorithm, the magnitudes and phase angles of these harmonic currents need to be accurately updated. This is performed based on the harmonic couplings of the nonlinear load. Different techniques have been proposed to compute and update the values of injected harmonic currents, including:

• Use measurements to compute them. • Use published data. • Assume they are inversely proportional to the harmonic order (e.g., Ih=1/h). • Use a more sophisticated Thevenin or Norton equivalent model. • Use an iterative nonlinear (time and/or frequency based) model for a detailed simulation of

the harmonic-producing load. 1.8. Power Quality Improvement Techniques Nonlinear loads produce harmonic currents that can travel to other locations in the power system and eventually back to the source. Therefore, harmonic current propagation produces harmonic voltages thought out the power systems. Many mitigation techniques have been proposed and implemented to keep the harmonic voltage and current contents within the recommended levels are:

• High power quality equipment design • Harmonic cancellation • Concept of dedicated isolated line or transformer • Optimal placement and sizing of capacitor banks • Deterating power system devices • Harmonic filters (passive, active, hybrid) and active power conditioners

The usual practice is that if at PCC harmonic currents are not within the permissible limits, the costumer with the nonlinear load is to take some countermeasure actions. However, if harmonic voltages are above recommended levels public service will have to take appropriate actions to improve the power quality. Detailed analyses of improvements techniques for power quality are presented in chapters 8 to 10. 1.8.1. High Power Quality Equipment Design [8] The use of nonlinear and electronic-based devices is steadily increasing and it is estimated that they will constitute more than 70% of power system loading by year 2010. Therefore, demand is increasing for the designers and product manufacturer to produce devices that generate lower distortion and for end-users to go the extra miles for selection of better quality devices. These actions have already been started in many countries, as reflected by improvements in fluorescent lamp ballasts, inclusion of filters with energy saving lamps, improved PWM adjustable speed


controls, high power quality battery chargers, switching mode power supplies and uninterruptible power sources. 1.8.2. Harmonic Cancellation [8] There are some relatively simple techniques that use transformer connections to employ phase-shifting for the purpose of harmonic cancellation, including:

• A delta-delta and a delta-wye transformer (or a multiple phase-shifting transformer) for supplying some harmonic producing loads in parallel (such as six pulse rectifiers) to eliminate 5th and 7th harmonic components.

• Transformers with delta connection to trap and prevent triplen harmonics from entering power systems.

• Transformers with zigzag connection to produce high phase order voltages (as applied in high phase order static converters).

• Other phase-shifting techniques to cancel higher harmonic orders, if required. • Any others points to be mentioned????????????

1.8.3. Concept of a Dedicated Isolation Line or Transformer Isolated line or transformers are used to attenuate the high frequency noise and transient as they attempt to pass from one side to the other. Therefore, disturbances are prevented from reaching the load and any load-generated noise and transients are kept from reaching the rest of the power system. However, some common mode and normal noise can still reach the load. Isolated transformers with (single or multiple) electrostatic shields, are more effective in eliminating common mode noise. Inter-harmonics (caused by induction motor drives) and voltage notching (due to power electronic switching) are two examples of problems that can limited to the load side by an isolated transformer. They can also attenuate capacitor switching and lightning transients coming from the utility system and prevent annoyance tripping of adjustable speed drives and other equipments. Isolated transformers do not totally cure voltage sags or swells. However due to the inherent large impedance, their presence between PCC and the source of disturbance (e.g., system fault) will lead to relatively shallow sags. An additional advantage of isolated transformers is that they allow the user to define a new ground reference that will limit neutral-to-ground voltages at sensitive equipments. 18.3.1. Application Example I (Disturbance Mitigation by Isolation Transformers) Figure 1.15 shows a typical modern distribution system with linear and nonlinear loads. The nonlinear load (labeled as "distorting nonlinear load") consists of two squirrel-cage induction motors used as chiller-compressors for a building’s air conditioning system. This load produces inter-harmonics currents that generate inert-harmonic voltage drops across the system’s impedances resulting in the inter-harmonic content of the line-to-line voltage of the induction motors as given by Table 1.9. Some of the loads (e.g., monitors of a closed-circuit television security system) are very sensitive to disturbances and must be protected against poor power quality conditions. These loads are labeled as "sensitive loads". Three case studies are considered:

• Case#1- distorting load and sensitive loads are fed from same pole transformer (Fig.1.16). • Case#2- a dedicated 1:1 isolation transformer is used between the distorting load and

sensitive loads (Fig.1.17). • Case#3- a dedicated isolation pole transformer is used between the distorting load and

sensitive loads (Fig.1.18).


Table 1.9. Inter-harmonics of phase current and line-to-line voltage generated by a three-phase induction motor [12]

Inter-harmonic fh [Hz]

Inter-harmonic amplitude of phase current [%]

Inter-harmonic amplitude of line-to-line voltage [%]

1128 7 0.40

1607 10 0.40

1730 10 0.55

Figure 1.15. Overall (per phase) one-line diagram of the distribution system used in application example I


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Figure 1.18. Case#3 of application example I: use of a dedicated isolation pole transformer between distorting (nonlinear) load and sensitive loads. Computations are shown on Figs.1.19 and 1.20 for the above three cases. As illustrated, for

2load1load V=V the 1128th inter-harmonic is %2.2=%)100(120

)12064.122(=V1128

, while for 3loadV this

inter-harmonic is only %006.0 . This demonstrates the effectiveness of a dedicated line or transformer, other than a 1:1 transformer.

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Figure 1.20. Equivalent circuit referring all lumped reactances to the secondary of the pole transformer 1.8.4. Optimal Placement and Sizing of Capacitor Banks It is well known that proper placement and sizing of shunt capacitor banks in distorted networks can result in reactive power compensation, voltage regulation, power factor correction and power/energy loss reduction. The capacitor placement problem consists of determining the optimal numbers, types, locations and sizes of capacitor banks such that minimum yearly cost due to peak power/energy losses and cost of capacitors is achieved, while the operational constraints are maintained within the required limits. Most of the reported techniques for capacitor placement assume sinusoidal operating conditions. These methods include: nonlinear programming, near global methods (genetic algorithms, simulated annealing, tabu search, artificial neural networks and fuzzy theory. All these approaches ignore the presence of voltage and current harmonics [13-14]. Optimal capacitor bank placement is a well-researched subject. However, very limited attention is given to this problem in the presence of voltage and current harmonics. Some of the recent publications have taken into account the presence of distorted voltages for solving the capacitor placement problem. These investigations include: exhaustive search, local variations, mixed integer-nonlinear programming, heuristic methods for simultaneous capacitor and filter placement, maximum sensitivities selection, fuzzy theory and genetic algorithms. According to newly developed investigations based on fuzzy and genetic algorithm [13-14], proper placement and sizing of capacitor banks in polluted power systems with nonlinear loads can results in lower system losses, greater yearly benefits, better voltage profile, prevention of harmonic resonances, as well as improved power quality. Simulation results for the standard 18-bus IEEE distorted distribution system show that proper placement and sizing of capacitor banks can limit voltage and current harmonics and decrease its THD to the recommended levels of IEEE 519, without application of any passive or active filters. For cases where the construction of new bank locations is not feasible, it is possible to perform the optimization process without defining any new capacitor bank locations.


Therefore, it is highly recommended to reexamine capacitor bank sizes and locations before taking any major steps for power quality mitigation. 1.8.5. Deterating Power System Devices Power system components must be derated when supplying harmonic loads. Commercially buildings have drawn the most attentions in recent years due to the increasing use of different nonlinear loads. According to the IEEE dictionary, derating is defined as “the intentional reduction of stress/strength ratio (e.g., real or apparent power) in the application of an item (e.g., cables, transformer, electrical machines), usually for the purpose of reducing the occurrence of stress-related failure (e.g., reduction of lifetime due to increased temperature beyond rated temperature)”. As has been discussed in Section 1.5 harmonic currents and voltages result in harmonic losses increasing the temperature rise. This rise beyond its rated value results in a reduction of lifetime as will be discussed in Chapter 6. There are several techniques for determining the derating factors (functions) of appliances for nonsinusoidal operating conditions (as discussed in Chapter 2), including:

• From tables in standards and published researches (e.g., ANSI/IEEE Std C57.110 for transformer derating).

• From measured losses. • By determining the K-Factor. • From the FHL-Factor .

1.8.6. Harmonic filters (passive, active, hybrid) and active power conditioners One means of ensuring that harmonic currents will not unduly interface with the rest of the power system is to place filters at location near or close to nonlinear loads. The main function of a filter is either to draw off harmonic currents, block them from entering the power system, or compensate them by locally supplying harmonic currents. Due to the lower impedance of the filter in comparison to the impedance of the source, harmonic currents will circulate between the load and the filter and do not affect the entire system. If other frequencies are to be controlled (e.g., for an arc furnaces), additional tuned filters are required. Harmonic filters are broadly classified into passive, active and hybrid structures. These filters can only compensate for harmonic currents and/or harmonic voltages at the installed bus and do not consider the power quality of other buses. New generations of active filters are active power conditioners that are capable of minimizing the power quality of the entire system. Passive filters are made of passive component (inductance, capacitance, and resistance) tuned to the harmonic frequencies that are to be attenuated. The values of inductors and capacitors are selected to provide low impedance paths at the selected frequencies. Passive filters are generally designed tp remove one or two specific harmonics (usually 5th and 7th). They are relatively inexpensive compared with other means for eliminating harmonic distortion, but also suffer from some inherent limitations, including:

• Interactions with the power system. • Forming resonance circuits with source impedance (at fundamental and/or harmonic

frequencies). This may result in a situation that is worse than the condition being corrected. It may also result system or equipment failure.

• Changing characteristics (e.g., their notch frequency) due to filter parameter variations. • Unsatisfactory performance under variations of nonlinear loads. • Compensating limited number of harmonics. • Not considering the power quality of the entire system.


• Creating the sharp parallel resonance at a frequency below the notch frequency. This resonance frequency must be safely away from any significant system harmonic. Passive filters are commonly tuned slightly lower than the attenuated harmonic to provide a margin of safety in case there are some changes in system parameters (due to temperature variations and/or failures). For this reason filters are added to the system starting with the lowest undesired harmonic. For example installing a seventh-harmonic filter usually requires that a fifth-harmonic filter also be installed.

Designing passive filters is a relatively simple but tedious matter. Passive For the proper tuning of passive filters, the following steps should be carried:

• Model the power system (including nonlinear loads) to indicate the location of harmonic sources and the orders of the injected harmonics. A harmonic power (load) flow algorithm (chapter 7) should be used, however for most applications with a single dominated harmonic source, a simplified equivalent model and hand calculations are adequate.

• Place the hypothetical harmonic filter(s) in the model and reexamine the system. Filter(s) should be properly tuned to the dominated harmonic frequencies.

• If unacceptable results are obtained, change filter location(s) and modify parameter values until results are satisfactory.

In addition to power quality improvement, harmonic filters can be configured to provide power factor correction. For such cases, the filter in designed to carry resonance harmonic currents, as well as, the normal fundamental current of the system. Active filters use active power conditioning to compensate for the unwanted harmonic currents. They actually replace the portion of the sine wave that is missing in the nonlinear load current by detecting the distorted current and using power electronic switching devices to draw harmonic currents (of such magnitudes, frequencies and phase shifts) from the source. Their main advantage over passive filters is their fine response to changing load and harmonic variations. Active filters can be used in very difficult circumstances where passive filters can not operate successfully because of where the parallel resonance lies. They can also address more than one harmonic at a time and combat other power quality problems such as flicker. They are particularly useful for large, distorting loads fed from relatively weak points on the power system. But they are relatively expensive and not applicable in small facilities. 1.8.6.1. Application Example II (Harmonics produced by 12-Pulse Converters) Fig.1.121 shows a large industrial plant such as an oil refinery or chemical plant [11] being serviced from a utility with transmission voltage of 115 kV. The demand on the utility system is 50 MVA and 50% of its load is twelve-pulse static power converter load. Table 1.10 lists the harmonic currents (Ih) given in p.u. of the fundamental current based on h

cX =

��DQG� � ��RI�VL[-pulse and twelve-pulse converters. In an ideal twelve-pulse converter, the magnitude of some current harmonics (typed bold in Table 1.10) is zero. However, for actual twelve-pulse converters, the magnitudes of these harmonics are normally taken as ten percent of the six-pulse values [11].


Figure 1.21 One-line diagram of a large industrial plant fed from transmission voltage [11] used in application example II Table 1.10. Harmonic current (Ih) generated 6-pulse and 12-pulse converters [11] based on

12.0=Xhc

and O30=

Harmonic order (h)

Ih for 6-pulse converter [p.u.]

Ih for 12-pulse converter [p.u.]

1 1.000 1.000 5 0.192 0.0192 7 0.132 0.0132

11 0.073 0.073 13 0.057 0.057 17 0.035 0.0035 19 0.027 0.0027 23 0.020 0.020 25 0.016 0.016 29 0.014 0.0014 31 0.012 0.0012 35 0.011 0.011 37 0.010 0.010 41 0.009 0.0009 43 0.008 0.0008 47 0.008 0.008 49 0.007 0.007


Calculating Current Harmonics at PCC#1: At PCC#1, we have:

• MVA67.16=3

MVA50=

3

I~

V~

=Sph

• kV4.66=3

kV115=Vph

• )kV4.66(3

MVA50=

V

S=I

ph

ph

ph

• IL = Iph = 251 A.

• At a short-circuit ratio of: RSC = 40=I

I

L

SC (at PCC#1)

• The system’s impedance is: Zsys* = 0.5% = 0.005 p.u. , *) 10 MVA base

• 1#PCC

SCI = RSC IL = 40Â��

• 1#PCCSCI = 10,000A.

According to Table 1.7, for PCCs from 69 to 138 kV, the current harmonic limits should be divided by two. Therefore, for ISC/IL = (20 – 50):

Ihlimit = %5.32

0.7 = for 5 ��K��

Ihlimit = %8.12

5.3 ≅ for 11 ��K��

Ihlimit = %3.12

5.2 ≅ for 17��K��

…………………………

The actually occurring harmonic currents are for Ispc = A5.125=2251 for 12-pulse converter (SPC

means static power converter):

actual5I = Ispc Â��

= 125.5 Â�� $ actual7I = 125.5 Â�� $ actual11I = 9.15A actual13I = 7.15 A

…………….. with IL = 251A

%100I

I=I

L

actual5%actuak

5

%actual5I = 0.96% Allowed limits are

%actual7I = 0.66% 3.5%


%actual11I = 3.65% Allowed limits are

%actual13I = 2.85% 1.8%

……………………….. As can be seen, actual

11I and actual13I are too large!

Calculating Voltage Harmonics at PCC#1: Calculation of short circuit (apparent) power SSC:

• ISC = IL Â�5SC = 251 Â�� $

• SCL

SC R=I

I

• SSC = SCph IV3

• = kAkV

04.103

1153 ⋅⋅

• SSC = 2,000 MVA Checking:

• SSC = MVASC = ]MVAbaseatunitperin[Z

]MVA[S

syst

base

• SSC = MVA000,2=.u.p005.0

MVA10

Calculation of voltage harmonics (at PCC#1) for the above-computed current harmonics:

• Vh = base

h

I

I(h) Â�=syst Â��

• Sbase = 10MVA (all three phases)

• Ibase = phase

basephase

V

S

• Ibase = AkV

MVA2.50

3/115

3/10 =

Harmonic (actually occurring) voltages:

%100005.052.50

41.2=V5

%120.0=V5

%100005.072.50

65.1=V7 Allowed limits are 1.5% for individual

%115.0=V7 harmonic voltages

%0.1=V11

%93.0=V13 Also:

• %5.2THD itlimV


• %64.1=%100V

V

=THDphase

,...7,5=h

2h

actualV

Calculating Current Harmonics at PCC#2: At PCC#2, we have:

• Sph = MVA67.16

• Vph = kV968.7=3

kV8.13

• Iph = kV968.7

MVA67.16

• Iph = 2,092A

• At a short-circuit ratio of: RSC = 7.8=I

I

L

SC (at PCC#2)

• The system’s impedance is: Zsyst* = 2.3% = 0.023 p.u., *) 10 MVA base

• 2#PCCSCI = RSC Â IL= 8.7 Â��$

• 2#PCCSCI = 18.2kA.

Calculation of short circuit (apparent) power SSC:

• Sphase = MVA67.16=3

MVA50

• Vphase = kV97.7=3

kV8.13

• IphaseL = kV8.13

3

3

MVA50

• IphaseL = 2.092kA • ISCphase = RSC Â�,phaseL • ISCphase = 8.7 Â�� N$

• ISPCphase = kA046.1=2

IphaseL

• RSC = 7.8=092.2

2.18=

I

I

phaseL

SCphase

• Zsyst = 2.3% = 0.023 p.u. Actually occurring harmonic currents at PCC #2 for 12-pulse converter:

A08.20=0192.01046=I actual5

A81.13=0132.01046=Iactual7

A35.76=073.01046=Iactual11

A62.59=057.01046=I actual13

……………………..


or in percent

%100I

]A[I=I

phaseL

actual5%actual

5

%96.0=%1002092

08.20=I %actual

5

%66.0=I %actual

7

%65.3=I %actual

11 Above limits (see analysis

%85.2=I %actual13 for PCC #1)

Calculating Voltage Harmonics at PCC#2:

Vh= %100Z)h(I

Isyst

base

h .

where 3/kV8.13

3/MVA10=Ibase

A4.418=Ibase

%100)023.0)(5(4.418

08.20=V5

%552.0=V5 Below limit of 3%

%100)023.0)(7(4.418

81.13=V7

%531.0=V7 %61.4=V11 Above limit of 3%

%26.4=V13 Note that on the 13.8 kV bus, the current and voltage distortions are greater than recommended by IEEE-519. A properly sized harmonic filter applied on the 13.8kV bus would reduce the current distortion and the voltage distortion to within the current limits and the voltage limits on the 13.8kV bus. 1.8.6.2. Application Example III (Filter Design for Example II to Meet IEEE-519 Requirements) The circuit of Fig.1.21 is now augmented with a passive filter, as shown by Fig.1.22.


Figure 1.22. One-line diagram of a large industrial plant fed from transmission voltage (Fig.1.21) with a passive filter placed at PCC#2 System Analysis: At harmonic frequencies, the circuit of Fig.1.22 can be approximated by the equivalent circuit shown by Fig.1.23. This circuit should be analyzed at each frequency of interest by calculating series and parallel resonances.

Figure 1.23. The equivalent circuit of Fig.1.22 at harmonic frequencies.

For series resonance fI~

is large, while for parallel resonance fI~

and sysI~

are large. The major

impedance elements in the above circuit respond differently as frequency changes. The impedance of the transmission line Zline is a complex relationship between the inductive and capacitive reactances. Using the fundamental frequency resistance R and inductance of the transmission line, however, gives acceptable results. For most industrial systems Zt and Zline can be approximated by the short-circuit impedance if low-frequency phenomena are considered. The impedance versus frequency characteristic of a transformer: It depends upon design, size, voltage etc. Its load loss, I2R, will constitute 75% to 85% of the total transformer loss and about 75% of this is not frequency dependent (skin effect). The remainder varies with the square of the frequency. The no-load loss (core loss) constitutes between 15% and 25% of the total loss and, depending upon flux density, the loss varies as f 3/2 to f 3. From this, with


reactance increasing directly with frequency (inductance L is assumed to be constant), it can be seen that the harmonic )RX( hh ratios will be less than the fundamental frequency )RX( 11 ratio:

)R

X(<)

R

X(

1

1

h

h .

If fundamental frequency )RX( 11 ratio is used, there will be less damping of the high-frequency current than in actuality. Adjacent capacitor banks: If there are large capacitor banks or filters connected to the utility system, it is necessary to consider their effect. Converter as a harmonic generator: The converter is usually considered to be a generator of harmonic currents ih, and is considered to be a constant current source. Thus Zconv is very large and can be ignored. If the converter is a constant voltage source Zconv should be included. Circuit analysis: Using Ohm’s and Kirchhoff’s laws (Fig.1.23):

linehthsysh Z+Z=Z

fsysh I~

+I~

=I~

syshsysfhf ZI~

=ZI~

,

or

)Z

Z(I

~=I

~

fh

sysh

sysf

fhsys I~

-I~

=I~

)Z

Z(I

~-I

~=I

~

fh

sysh

syshsys ,

or

hfh

sysh

sys I~

=)Z

Z+1(I

~

hfh

syshfh

sys I~

=)Z

Z+Z(I

~,

or

hsyshfh

fhsys I

~)

Z+Z

Z(=I

~.

Correspondingly,

hsyshfh

sysh

f I~

)Z+Z

Z(=I

~.

Define

syshfh

fhsysh Z+Z

Z= ,


syshfh

sysh

fh Z+Z

Z= .

Then

hsyshsys I~

=I~

,

hfhf I~

=I~

. Because of

fsysh I~

+I~

=I~

1=+ fhsysh

1RWH� WKDW� sysh� DQG� fh� DUH� FRPSOH[� TXDQWLWLHV�� ,W� LV� GHVLUDEOH� WKDW� sysh be small at the various harmonics. Typical values for a series tuned filter are (at the tuned frequency hf1)

0sysh 6.80-045.0= , (for series tuned filters)

0fh 6.2+994.0= , (for series tuned filters).

Parallel resonances occur between Zfh and Zsysh�LI� sysh�DQG� fh are large at the tuned frequency (hf1). Typical values are

°9.92-67.16=sysh ,

°6.83+75.16=fh .

The approximate 180° phase difference emphasizes why a parallel resonance cannot be tolerated at a frequency near a harmonic current generated by the converter: a current of the resonance frequency will excite the circuit and a 16.67 p.u. current will oscillate between the two energy storage units, the system impedance Zsysh and that of the filter capacitors Zfh. $�SORW�RI� sysh�YHUVXV�K�LV�D�XVHIXO�GLVSOD\�RI�ILOWHU�SHUIRUPDQFH��)UHTXHQWO\�D�SORW�RI�ORJ�� sysh) is

more convenient. The harmonic voltage hV~

is

ffhsyssyshh I~

Z=I~

Z=V~

fsyshfhsyshh I~

)Z+Z(=V~

hfhsyshfhsyshh I~

)Z+Z(=V~

hsyshfh

sysh

syshfhsyshh I~

)Z+Z(

Z)Z+Z(=V

~

hsyshsyshh I~

Z=V~

,

or

hsyshfh

sysh

syshfhsyshfh

fhh I

~)Z+Z(

Z)Z+Z(

)Z+Z(

Z=V

~

fhsyshfh

sysh=

)Z+Z(

Z

hfhfhh I~

Z=V~

1.8.6.3. Application Example IV (Several Users on a Single Distribution (Radial) Feeder) Figure 1.124 shows a utility distribution feeder that has four users along the radial feeder [11]. Each user sees a different value of short circuit or system size. Note that

]baseMVA10at.u.p[Z

MVA10=MVA=S

sysSCSC


There is one type of transfoUPHU�� -Y), therefore, 6-pulse static power converters are used only.

Figure 1.24. Overall one-line diagram the distribution system feeder containing four users with six-pulse converters [11] used in application example IV


Calculation of Harmonic Current for user #1 (Case A, no filter): For user #1 we have:

• MVA350=SSC

• MVA67.116=3

S=S SC

SCphase

• phaseSCphaseSCphase VI=S

• kV959.7

MVA67.116=

V

S=I

phase

SCphase

SCphase

• kA65.14=ISCphase

• MVA5.2=Sload

• MVA833.0=3

S=S load

loadphase

• phase

loadphaseloadphase V

SI =

• A7.104=kV959.7

MVA833.0=I loadphase .

For this user, there is a 25% static power converter (SPC) load:

• A2.26=4

A7.104=I loadSPC

• 140=A7.104

kA65.14=

I

I=R

loadphase

SCphase

SC .

Therefore, harmonic currents for the 6-pulse static power converter of user #1 are:

• A03.5=)192.0(2.26=I ]A[5 � %8.4=%100A7.104

A03.5=%100

I

I=I

loadphase

]A[5

[%]5

• A46.3=)132.0(2.26=I ]A[7 %3.3=7.104

46.3=%100

I

I=I

loadphase

]A[7

[%]7

• A91.1=I ]A[11 %82.1=I [%]11

• A49.1=I ]A[13 %42.1=I [%]13

………………… ………………… Calculation of Harmonic Current for user #2 (Case A, no filter): For user #2, we have

• MVA300=SSC

• MVA100=3

S=S SC

SCphase

• kV9585.7=Vphase


• kA56.12=kV9585.7

MVA100=

V

S=I

phase

SCphase

SCphase

• MVA5=Sload

• MVA667.1=S loadphase

• phase

loadphase

loadphase V

S=I

• A5.209=I loadphase .

The 50% static power converter (SPC) load of this user yields:

• A73.104=I loadSPC

• 95.59=A5.209

A12560=

I

I=R

loadphase

SCphase

SC

• 60R SC .

Therefore, harmonic currents for the 6-pulse static power converter of user #2 are:

• A11.20=)192.0(73.104=I ]A[5 �� %6.9=%100A5.209

A11.20=%100

I

I=I

loadphase

]A[5

[%]5 ,

• A82.13=)132.0(73.104=I ]A[7 �� %6.6=%100A5.209

A82.13=%100

I

I=I

loadphase

]A[7

[%]7 ,

• A64.7=I ]A[11 �� %65.3=I [%]11

• A96.5=I ]A[13 �� %9.2=I [%]13

………………… ………………… Calculation of Harmonic Voltages Vh (Case A, no filter): The harmonic equivalent circuit of Fig.1.24 in p.u and ohms is shown by Figs.1.25(a) and 1.25(b), respectively.


Figure 1.25. Harmonic equivalent circuit of Fig.1.24 with impedances expressed in; (a) p.u , (b) ohms Using the harmonic equivalent circuits of Fig.1.24, we have:

• MVA10=Sbase

• MVA3

10=Sbasephase

• kV9585.73

MVA10=

V

S=I

phase

basephase

base

• A83.418=Ibase

• base

base

base

phase

base I

V=

I

V=Z

Note that Vbase = Vphase , therefore:


• 19=83.418

9585.7=Zbase

Voltage harmonics are computed using the total current harmonics. For example, for the fifth harmonic we have: A03.5=I 1#5

A18.25=I+I 2#51#5

A11.20=I 2#5 The fifth voltage harmonic amplitudes for users 1 and 2 are:

• )18.25()5(5428.0+)03.5()5(18.4=V 1#PCCuser5

=V 1#PCCuser5 173.35V

=V 1#PCCuser5 2.17%

• )18.25()5(5428.0+)11.20()5(18.2=V 2#PCCuser5

=V 2#PCCuser5 287.39V

=V 2#PCCuser5 3.61%,

The total fifth harmonic voltage is: • =)18.25()5(5428.0=V PCCtot5 68.22V

%100kV96.7

22.68=V PCCtot5 = 0.86%.

Exercise 1.1: For the application example IV (Fig.1.24), calculation harmonic currents associated with users #3 and #4. Are they within the permissible power quality limits of IEEE 519? Exercise 1.2: For the application example IV (Fig.1.24), calculation harmonic currents associated with users #3 and #4. Are they within the permissible power quality limits of IEEE 519? References 1. W. Shephered, P. Zand, "Energy Flow and Power Factor in Nonsinusoidal Circuits", Cambridge University Press, 19??. 2. J. Arrilaga, D.A. Bradley, P.S. Bodeger, "Power System Harmonics", John Wiley & Sons, 1985. 3. G.T Heydt, “Electric Power Quality”, Star in a Circle Publications, 1991 (Purdue University). 4. R.C. Dugan, M.F. McGranaghan, H.W. Beaty, “Electrical Power Systems Quality”, McGraw-Hill, 1996 (Electrotek Concepts Inc. + Electric Light & Power). 5. J. Arrillaga, B.C. Smith, N.R. Watson, A.R. Wood, “Power Systems Harmonic Analysis”, John Wiley & Sons, 1997 (University of Canterbury, New Zealand).


6. J. Schlabbach, D. Blume, T. Stephanblome, “Voltage Quality in Electrical Power Systems”, Originally Published in German by VDE-Verlag in 1999, English Edition by The Institution of Electrical Engineers, UK, 2001 (University of Applied Science of Bielefeld). 7. J. Arrillaga, N.R. Watson, S. Chen, “Power System Quality Assessment”, John Wiley & Sons, 2000 (University of Canterbury, New Zealand + Nanyang Technology University, Singapore). 8. C. Sankaran, “Power Quality”, CRC Press, 2002 (??). 9. M.H.J. Bollen, "Understanding Power Quality Problemes", IEEE Press Series on Power Engineering, 2000 (Chalmers University of Technology, Gothenburg, Sweden). 10. ????????, "Power Quality Primer", ????? (will be completed later). 11. C. Dffey, R.P. Stratford, “Updates to Harmonic Standard IEEE -519: IEEE Recommended Practices and Requirements for Harmonic Control in Electric Power Systems”, IEEE Trans. on Industrial Applications, Vol.25, No.6, 1989. 12. E.F. Fuchs, D. Roesler, M.A.S. Masoum, “Are Harmonic Recommendations According to IEEE and IEC Too Restrictive?”, IEEE Trans. on Power Delivery, USA, Paper #TPWRD-00022-2003, Accepted for Publication. 13. M.A.S. Masoum, A. Jafarian, M. Ladjevardi, E.F. Fuchs, W.M. Grady, “Fuzzy Approach for Optimal Placement and Sizing of Capacitor Banks in the Presence of Harmonics”, IEEE Trans. on Power Delivery, USA, Paper # TPWRD-00316-2002.R1, Accepted for Publication. 14. M.A.S. Masoum, M. Ladjevardi, A. Jafarian, E.F. Fuchs, “Optimal Placement, Replacement and Sizing of Capacitor Banks in Distorted Distribution Networks by Genetic Algorithms”, IEEE Trans. on Power Delivery, USA, Paper # TPWRD-00028-2003.R1, Accepted for Publication.