pq tutorial

Power Quality Briefing Paper www.leonardo-energy.org

February 2007

Power Quality Tutorial Angelo Baggini, Engineering Consulting & Design

1 Introduction .................................................................................................... 2

2 Power Quality Phenomena.......................................................................... 2

2.1 Harmonics .............................................................................................................. 2

2.2 Short interruptions (< 1min) .............................................................................. 2

2.3 Long interruptions (> 1min) ............................................................................... 3

2.4 Voltage dips and swells ...................................................................................... 3

2.5 Transients, surges (switching, lightning) ...................................................... 3

2.6 Flicker ...................................................................................................................... 4

2.7 Unbalance ............................................................................................................... 4

2.8 Voltage value (long term undervoltages and overvoltages) ..................... 4

2.9 Earthing and EMC................................................................................................. 5

3 Power Quality problems .............................................................................. 6

3.1 Circuit breakers and RCD’s nuisance tripping ............................................. 6

3.2 Computers lock up ............................................................................................... 6

3.3 Computers or other electronics are damaged .............................................. 6

3.4 Data loss ................................................................................................................. 6

3.5 Lights flicker, blink, or dim ................................................................................ 6

3.6 Loss of synchronization of processing equipment..................................... 7

3.7 Motors or other process equipment malfunctions ...................................... 7

3.8 Motors or process equipment are damaged ................................................. 7

3.9 Noise interference to telecommunication lines............................................ 7

3.10 Relays and contactors nuisance tripping................................................... 7

3.11 Transformers and cables overheating ........................................................ 7

4 Power Quality solutions .............................................................................. 8

4.1 Back-up generator(s) ........................................................................................... 8

4.2 Dynamic voltage restorers ................................................................................. 8

4.3 Harmonic filter (passive)..................................................................................... 8

4.4 Isolation transformers ......................................................................................... 8

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4.5 Line conditioners or active filters .................................................................... 9

4.6 Multiple independent feeder .............................................................................. 9

4.7 Oversizing equipment (transformers, motors) and cables (line and especially neutral conductors) .................................................................................... 9

4.8 Shielding and grounding .................................................................................. 10

4.9 Static transfer switches .................................................................................... 10

4.10 Static Var Compensator ................................................................................ 12

4.11 Surge protectors on key pieces of equipment ........................................ 12

4.12 Uninterruptible power supply (UPS) devices .......................................... 13

4.13 Voltage stabilisers .......................................................................................... 13

5 PQ phenomena and design issues.......................................................... 14

6 References.................................................................................................... 14

1 I n t r o d u c t i o n A short tutorial to introduce the subject of power quality phenomena, the problems they cause and their solutions.Power Quality phenomena

2 P o w e r Q u a l i t y P h e n o m e n a

2.1 Harmonics Harmonic frequencies are integral multiples of the fundamental supply frequency, i.e. for a fundamental of 50 Hz, the third harmonic would be 150 Hz and the fifth harmonic would be 250 Hz. Harmonic distorted waveform is clearly not a sine wave and that means that normal measurement equipment, such as averaging reading rms-calibrated multi-meters, will give inaccurate readings. Note also that there could be also many zero crossing points per cycle instead of two, so any equipment that uses zero crossing as a reference will malfunction. The waveform contains non-fundamental frequencies and has to be treated accordingly. When talking about harmonics in power installations it is the current harmonics that are of most concern because the harmonics originate as currents and most of the ill effects are due to these currents. No useful conclusions can be drawn without knowledge of the spectrum of the current harmonics but it is still common to find only the total harmonic distortion (THD) figures quoted. When harmonics propagate around a distribution system, that is, to branch circuits not concerned with carrying the harmonic current, they do so as voltages. It is very important that both voltage and current values are measured and that quoted values are explicitly specified as voltage and current values. Conventionally, current distortion measurements are suffixed with ‘I’, e.g. 35% THDI, and voltage distortion figures with ‘V’, e.g. 4% THDV. Harmonic currents have been present in the electricity supply system for many years. Initially they were produced by the mercury arc rectifiers used to convert AC to DC current for railway electrification and for DC variable speed drives in industry. More recently the range of types and the number of units of equipment causing harmonics have risen sharply, and will continue to rise, so designers and specifiers must now consider harmonics and their side effects very carefully.

2.2 Short interruptions (< 1min) Disappearance of the supply voltage in all phases by some definitions also defined as in “..one or more phases”. Usually qualified by an additional term indicating the voltage drop or



retained voltage and duration of the interruption (e.g. Momentary, Temporary, or Sustained). Values depend on standard. Short interruptions:

IEEE = - �Un < 90% duration from 20 ms to 1 minute EN= - �Un < 99% duration from 20 ms to 3 minutes

By some definitions short interruption is interpreted as a special kind of voltage dip

2.3 Long interrupt ions (> 1min) Disappearance of the supply voltage in all phases by some definitions also defined as in “...one or more phases”. Usually qualified by an additional term indicating the voltage drop or retained voltage and duration of the interruption (e.g., Momentary, Temporary, or Sustained). Values depend on the source of the interruption. Long interruptions :

IEEE = - �Un < 90% duration > 1 minute EN= - �Un < 99% duration > 3 minutes

2.4 Voltage dips and swells A voltage dip is a short-term reduction in, or complete loss of, RMS voltage. It is specified in terms of duration and retained voltage, usually expressed as the percentage of nominal RMS voltage remaining at the lowest point during the dip. A voltage dip means that the required energy is not being delivered to the load and this can have serious consequences depending on the type of load involved. Voltage sags - longer-term reductions in voltage – are usually caused by a deliberate reduction of voltage by the supplier to reduce the load at times of maximum demand or by an unusually weak supply in relation to the load. Motor drives, including variable speed drives, are particularly susceptible because the load still requires energy that is no longer available except from the inertia of the drive. In processes where several drives are involved individual motor control units may sense the loss of voltage and shut down the drive at a different voltage level from its peers and at a different rate of deceleration resulting in complete loss of process control. Data processing and control equipment is also very sensitive to voltage dips and can suffer from data loss and extended downtime. There are two main causes of voltage dips; starting of large loads either on the affected site or by a consumer on the same circuit and faults on other branches of the network. Voltage swells - temporary increase in RMS value of AC voltage with the magnitude of the retained voltage between 110% and 180% of the rated voltage.

2.5 Transients, surges (switching, l ightning) Pertaining to or designating a phenomenon or a quantity, which varies between two consecutive steady states during a time interval that is short compared to the time scale of interest. A transient can be a unidirectional impulse of either polarity or a damped oscillatory wave with the first peak occurring in either polarity. Causes include switching or lightning strikes on the network and switching of reactive loads on the consumer’s site or on sites on the same circuit. Transients – surges can have magnitudes of several thousand volts and so can cause serious damage to both the installation and the equipment connected to it. Electricity suppliers and telecommunications companies go to some effort to ensure that their incoming connections do not allow damaging transients to propagate into the customers’ premises. Nevertheless, non-damaging transients can still cause severe disruption due to data corruption. The generation and influence of transients is greatly reduced and the efficiency of suppression techniques greatly enhanced where a good high



integrity earthing system has been provided. Such an earthing system will have multiple ground connections and multiple paths to earth from any point so ensuring high integrity and low impedance over a wide frequency band.

2.6 Fl icker Flicker are short duration voltage changes, resulting from switching, short-circuits and load changing. The permissible magnitude of light flicker is regulated by International Standards, based on perception criteria. Excessive flicker can cause migraine and is responsible for some instances of the so-called ‘sick building syndrome’.

2.7 Unbalance A three-phase power system is called balanced or symmetrical if the three-phase voltages and currents have the same amplitude and are phase shifted by 120° with respect to each other. If either or both of these conditions are not met, the system is called unbalanced or asymmetrical. It is implicitly assumed that the waveforms are sinusoidal and thus do not contain harmonics. In most practical cases, the asymmetry of the loads is the main cause of unbalance. At high and medium voltage level, the loads are usually three-phase and balanced, although large single- or dual-phase loads can be connected, such as AC rail traction (e.g. high-speed railways) or induction furnaces (large metal melting systems employing highly irregular powerful arcs to generate heat). Low voltage loads are usually single-phase, e.g. PCs or lighting systems, and the balance between phases is therefore difficult to guarantee. In the layout of an electrical wiring system feeding these loads, the load circuits are distributed amongst the three-phase systems, for instance one phase per floor of an apartment or office building or alternating connections in rows of houses. Still, the balance of the equivalent load at the central transformer fluctuates because of the statistical spread of the duty cycles of the different individual loads. Abnormal system conditions also cause phase unbalance. Phase-to-ground, phase-to-phase and open-conductor faults are typical examples. These faults cause voltage dips in one or more of the phases involved and may even indirectly cause overvoltages on the other phases. The system behaviour is then unbalanced by definition, but such phenomena are usually classified under voltage disturbances, which are discussed in the corresponding application guides, since the electricity grid’s protection system should cut off the fault.

2.8 Voltage value ( long term undervoltages and overvoltages) Long term voltage variations generally last for periods over several seconds. They are not the result of system faults. They may be caused by load variations, system switching operations and general system voltage regulation practices. Long term voltage variations can be either overvoltages or undervoltages, depending on the cause of the variation. They can influence voltage value sensitive equipment e.g. this under/overvoltage protected or voltage controlled equipment e.g. motors, overstress insulation.

2.8.1 UNDERVOLTAGES An undervoltage is a decrease in the rms ac voltage (to less than 90% of rated voltage) at the power frequency for a duration longer than several seconds (or even 1 minute).

2.8.2 OVERVOL TAGES An overvoltage is an increase in the rms AC voltage (to more than 110% of rated voltage) at the power frequency for a duration longer than several seconds (or even 1 minute).



2.9 Earthing and EMC Earthing of installations and equipment is an issue that crosses the boundaries of the various disciplines involved in the construction and equipping of a modern commercial or industrial building. In general any earthing system needs to satisfy three demands:

• Lightning and short circuit: the earthing system must protect the occupants, prevent direct damage such as fire, flashover or explosions due to a direct lightning strike and overheating due to a short-circuit current.

• Safety: the earthing system must conduct lightning and short-circuit currents without introducing intolerable step-voltage and touch-voltages.

• Equipment protection and functionality: the earthing system must protect electronics by providing a low impedance path to interconnect equipment. Proper cable routing, zoning and shielding are important aspects and serve the purpose of preventing sources of disturbance from interfering with the operation of electrical equipment.

Although requirements for these three aspects are often specified separately, the implementation of them requires an integrated systems approach. Every piece of electrical and electronic equipment produces some electromagnetic radiation. Similarly, every piece of equipment is also sensitive, to a greater or lesser extent, to electromagnetic radiation. If everything is going to work, the cumulative level of radiation in an environment must be rather less than the level that will disrupt the operation of the equipment working in that environment. To achieve this goal, equipment is designed, built and tested to standards to reduce the amount of radiation that is emitted and increase the amount that can be tolerated. EMC is defined in the IEC 61000 series as:

“The ability of an equipment or system to function satisfactorily in its electromagnetic environment without introducing intolerable electromagnetic disturbances to anything in that environment.”

Maintaining this compatibility in practice requires great care in the design and implementation of the installation and the earthing system. Detailed guidance will be given in later sections of this Guide; here a general overview only is presented. In traditional electrical engineering separate earthing systems were used, for example, signal earth, computer earth, power earth, lightning earth etc. etc. In today’s electrical engineering new insights have been gained on the aspect of earthing and grounding and its relation to instrument protection. The concept of separate earthing systems has been abandoned and the international standards now prescribe one overall earthing system. There is no such thing as ‘clean’ and ‘dirty’ earth. This single earthing concept means in practice that protective earth (PE) conductors, parallel earthing conductors, cabinets and the shields and screens of data or power cables are all interconnected. Also steel construction parts and water and gas pipes are part of this system. Ideally all cables entering a zone must enter at one point at which all screens and other earth conductors are connected. To reduce interference on equipment the earthing loops between cable-screens and other earthing structures must be kept small. Bonding cables against metal structures makes these structures act as parallel earthing conductors (PEC). Parallel earthing structures are used both for data and power cables. Examples are, in ascending order of effectiveness: earthing wires, cable ladders, flat metal surfaces, cable trays or ultimately metal pipes. The PEC reduces the impedance of the loop formed by the cable and the earthing network. The earthing resistance to mother earth is mostly not important for the protection of equipment. A



very effective form of a PEC is a densely woven or completely closed cable screen with a large metal cross-section, connected all around at both ends of the cable. To keep the impedance of bonding connections in the earthing network small for high frequencies, litz wire (stranded, individually insulated) or metal strips with a length to width ratio smaller than 5 must be used. For frequencies higher than 10 MHz round wires should not be used. A raised floor can serve as a good equipotential plane. The copper grid underneath it must have a maximum spacing of 1.2 metres and be connected to the common bonding network via many equipotential bonding conductors. The grid should be connected to a 50 mm2 copper ring placed around the raised floor area, within the boundaries of the floor, at 6 metre intervals. Power and signal cables should be at least 20 cm apart and where they cross, they should do so at right angles.

3 P o w e r Q u a l i t y p r o b l e m s

3.1 Circuit breakers and RCD’s nuisance t ripping Inrush currents may trip circuit breakers. Circuit breakers may not correctly sum the current contained in the fundamental and various harmonics and so trip erroneously, or not at all when they should. Leakage currents may reach thresholds that cause residual current devices to trip.

3.2 Computers lock up Earth current originating in the equipment results in a voltage drop between the equipment and true earth. Although small, this noise voltage may be significant compared with the signal voltages (of a few volts) on which IT equipment operates. PC hardware is designed to minimise sensitivity to this kind of disturbance but it cannot be eliminated entirely, especially as the noise frequency rises. Modern communications protocols have error detection and correction algorithms built in, requiring retransmission of erroneously received data - and consequently reducing the data throughput. As a result, PCs will often slow down or lock-up, a frequent phenomenon in today’s office environments.

3.3 Computers or other electronics are damaged Should lightning and switching strikes occur “close” to computers and other electronic equipment, they may completely damage the equipment if insufficient surge protection is available.

3.4 Data loss Earth leakage currents cause small voltage drops along the earthing conductor. In a TN-C network, the combined earth-neutral conductor will constantly carry significant current, dominated by triple-n harmonics. Due to the increasing use of low-voltage signals in IT equipment, bit error rate increases up to the point where the entire network locks up or freezes. How many large and small, privately owned networks experience this phenomenon almost on a weekly basis? Where this happens, , the network locks up, e-mail services fail and it is no longer possible to print for no apparent reason.

3.5 Lights f l icker , bl ink, or dim Short duration voltage changes, resulting from switching, short-circuits and load changing can result in light flicker. The permissible magnitude of light flicker is regulated by International Standards and is based on perception criteria. The light blinking and dimming may also be a result of shorter and longer voltage sags, which are the consequence of large load fluctuation and too low local electrical system short circuit power.



3.6 Loss of synchronizat ion of processing equipment Severe harmonic distortion can create additional zero-crossings within a cycle of the sine wave; this can affect sensitive measurement equipment. Synchronisation of process control equipment in continuous manufacturing may be disturbed and PLC devices may lock up.

3.7 Motors or other process equipment malfunct ions Voltage harmonics cause extra losses in direct line-connected induction motors. The 5th harmonic creates a counter-rotating field, whereas the 7th harmonic creates a rotating field beyond the motor’s synchronous speed. The resulting torque pulsing causes wear and tear on couplings and bearings. Since the speed is fixed, the energy contained in these harmonics is dissipated as extra heat, resulting in premature ageing. Harmonic currents are also induced into the rotor causing further excess heating. The additional heat reduces the rotor/stator air gap, reducing efficiency even further. Variable speed devices cause their own range of problems. They tend to be sensitive to dips, causing disruption of synchronised manufacturing lines. They are often installed some distance from the motor, and cause different electromagnetic interference and voltage spikes due to the sharp voltage rise times. Special care has to be taken at start-up of motors after a voltage dip when the motor is normally operating at close to full load. The extra heat from the inrush current at start-up may cause the motor to fail. Optimum sizing of motors should mitigate this phenomenon.

3.8 Motors or process equipment are damaged In extreme cases when the motor is operating in a high harmonic content environment it may be damaged because of overheating.

3.9 Noise interference to telecommunication l ines If the electrical noise cannot be reduced to a low enough level it will produce interference signals, which exceed telecommunication immunity level increasing transmission error rates.

3.10 Relays and contactors nuisance tripping Relays and contactors are sensitive to voltage dips and can often be the weakest link in the system. It has been established that a device may drop out during a dip even when the retained voltage is higher than the minimum steady state hold-in voltage. The resilience of a contactor to dips depends not only on the retained voltage and duration but also on the point on the waveform where the dip occurs, the effect being less at the peak.

3.11 Transformers and cables overheating Harmonics produce extra eddy current losses. These losses lead to transformers and cables overheating. Additionally triple n harmonics add up and flow in neutral conductors and delta windings of transformers creating additional extra heat.



4 P o w e r Q u a l i t y s o l u t i o n s

4.1 Back-up generator(s) Engine generating sets (EGS) usually consist of one or more internal diesel combustion engines as the source of mechanical energy, a generator to convert mechanical to electric energy, accelerators, control and regulation systems and switchgear. This type of equipment may be designed for relatively long-term operation, say up to several hours or days, or may be designed for continuous operation. EGSs are available in a wide range of power ratings, usually from a few tens of kW up to few MW. Instead of diesel engine there is sometimes installed a gas turbine to drive generator of a large power, in the range of few MW or more. Generator sets driven by gas turbines are often designed as sources of power for peak lopping or as co-generation plants. The EGSs are also used for special applications where no power network is available, such as marine applications, or where a short-term, high demand requirement exists, such as major televised sports events. These kinds of using of EGSs will not be discussed in this guide. EGSs can operate in two different manners, distinguished here as group I and group II.

4.2 Dynamic voltage restorers Where heavy loads or deep dips are concerned a Dynamic Voltage Restorer (DVR) is used. This device is series coupled to the load and generates the missing part of the supply; if the voltage dips to 70%, the DVR generates the missing 30%. DVRs are normally expected to support the load for a short period and may use heavy-duty batteries, super capacitors or other forms of energy storage such as high-speed flywheels. DVRs cannot be used to correct long term under and over voltage.

4.3 Harmonic f i lter (passive) Passive filters are used to provide a low impedance path for harmonic currents so that they flow in the filter and not the supply. The filter may be designed for a single harmonic or for a broad band depending on requirements. Sometimes it is necessary to design a more complex filter to increase the series impedance at harmonic frequencies and so reduce the proportion of current that flows back onto the supply. Simple series band stop filters are sometimes proposed, either in the phase or in the neutral. A series filter is intended to block harmonic currents rather than provide a controlled path for them so there is a large harmonic voltage drop across it. This harmonic voltage appears across the supply on the load side. Since the supply voltage is heavily distorted it is no longer within the standards for which equipment was designed and warranted. Some equipment is relatively insensitive to this distortion, but some is very sensitive. Series filters can be useful in certain circumstances, but should be carefully applied; they cannot be recommended as a general purpose solution.

4.4 Isolat ion t ransformers As mentioned previously, triple-N currents circulate in the delta windings of transformers. Although this is a problem for transformer manufacturers and specifiers - the extra load has to be taken into account – it is beneficial to systems designers because it isolates triple-N harmonics from the supply.

Figure 1 - Delta star isolation transformer. S - Supply, L - Load.



The same effect can be obtained by using a ‘zig-zag’ wound transformer. Zig-zag transformers are star configuration auto transformers with a particular phase relationship between the windings that are connected in shunt with the supply.

4.5 Line condit ioners or active f i lters The idea of the active harmonic conditioner is relatively old, however the lack of an effective technique at a competitive price slowed its development for a number of years. Today, the widespread availability of insulated gate bipolar transistors (IGBT) and digital signal processors (DSP) have made the AHC a practical solution. The concept of the AHC is simple; power electronics is used to generate the harmonic currents required by the non-linear loads so that the normal supply is required to provide only the fundamental current. The load current is measured by a current transformer, the output of which is analysed by a DSP to determine the harmonic profile. This information is used by the current generator to produce exactly the harmonic current required by the load on the next cycle of the fundamental waveform. In practice, the harmonic current required from the supply is reduced by about 90%. Because the AHC relies on the measurement from the current transformer, it adapts rapidly to changes in the load harmonics. Since the analysis and generation processes are controlled by software it is a simple matter to programme the device to remove only certain harmonics in order to provide maximum benefit within the rating of the device. A number of different topologies have been proposed and some of them are described below. For each topology, there are issues of required components ratings and method of rating the overall conditioner for the loads to be compensated.

4.6 Mult iple independent feeder Where the power requirement is high and the cost is justified, as in the case of continuously operating plant such as paper or steel making, two independent connections to the distribution grid may be provided. This approach is only effective if the two connections are electrically independent, i.e. a predictable single failure will not cause both network connections to fail at the same time. It depends on the network structure, and often, this requirement cannot be met without the use of very long (and expensive) lines. The use of two independent connections from the distribution network does not mean that other reserve supplies are unnecessary. This type of measure is unlikely to reduce the number or severity of voltage disturbances however, because the networked nature of the distribution system allows dips – the effect of faults - to propagate over very long distances.

4.7 Oversizing equipment (t ransformers, motors) and cables ( l ine and especially neutral conductors)

Transformers are affected in two ways by harmonics. Firstly, the eddy current losses, normally about 10% of the loss at full load, increase with the square of the harmonic number. In practice, for a fully loaded transformer supplying a load comprising IT equipment the total transformer losses would be twice as high as for an equivalent linear load. This results in a much higher operating temperature and a shorter life. In fact, under these circumstances the lifetime would reduce from around 40 years to more like 40 days! Fortunately, few transformers are fully loaded, but the effect must be taken into account when selecting plant. The second effect concerns the triple-N harmonics. When reflected back to a delta winding they are all in phase, so the triple-N harmonic currents circulate in the winding. The triple-N harmonics are effectively absorbed in the winding and do not propagate onto the supply, so delta wound transformers are useful as isolating transformers. Note that all other, non triple-N, harmonics pass through. The circulating current has to be taken into account when rating the transformer.


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Harmonic voltage distortion causes increased eddy current losses in motors in the same way as in transformers. However, additional losses arise due to the generation of harmonic fields in the stator, each of which is trying to rotate the motor at a different speed either forwards or backwards. High frequency currents induced in the rotor further increase losses. Where harmonic voltage distortion is present motors should be de-rated to take account of the additional losses. Alternating current tends to flow on the outer surface of a conductor. This is known as skin effect and is more pronounced at high frequencies. Skin effect is normally ignored because it has very little effect at power supply frequencies but above about 350 Hz, i.e. the seventh harmonic and above, skin effect will become significant, causing additional loss and heating. Where harmonic currents are present, designers should take skin effect into account and de-rate cables accordingly. Multiple cable cores or laminated busbars can be used to help overcome this problem. Note also that the mounting systems of busbars must be designed to avoid mechanical resonance at harmonic frequencies.

4.8 Shielding and grounding Earthing of installations and equipment is an issue that crosses the boundaries of the various disciplines involved in the construction and equipping of a modern commercial or industrial building. In general any earthing system needs to satisfy three demands:

• Lightning and short circuit: the earthing system must protect the occupants, prevent direct damage such as fire, flashover or explosions due to a direct lightning strike and overheating due to a short-circuit current.

• Safety: the earthing system must conduct lightning and short-circuit currents without introducing intolerable step-voltage and touch-voltages.

• Equipment protection and functionality: the earthing system must protect electronics by providing a low impedance path to interconnect equipment. Proper cable routing, zoning and shielding are important aspects and serve the purpose of preventing sources of disturbance from interfering with the operation of electrical equipment.

Although requirements for these three aspects are often specified separately, the implementation of them requires an integrated systems approach. Shielding is the use of a conducting and/or ferromagnetic barrier between a potentially disturbing noise source and sensitive circuitry. Shields are used to protect cables (data and power) and electronic circuits. They may be in the form of metal barriers, enclosures, or wrappings around source circuits and receiving circuits.

4.9 Static transfer switches AC power supply systems - fast switching in the AC circuits by means of a static-switch. Apart from the basic component of AC highly reliable power supply systems (UPS), as a next step fast static switches can be used to increase the reliability of power supply.



uP

AC(1)

N

AC(2)

N

AC(OUT)

N

PE Figure 2 - Block diagram of static-switch and the oscillogram of the output voltage AC (Out) Figure 2 shows a diagram of the static-switch and an example of the oscillogram of power supply reconnection from AC circuit (1) to AC circuit (2). The control system of static-switch should provide very quick switching of power networks; there are switches with switching time below 6ms and return to basic power supply of 0.2ms. This ensures the uninterrupted operation of the loads even those, which are very sensitive to short supply voltage decays. Where the standard contactors are used, disruptions in power supply can last for as long as tens or even hundreds of milliseconds. Another advantage of static switches is that they do not generate switching overvoltages like standard contactors. So their application in environments sensitive to overvoltages, like these with inductive loads is recommended. Static switches are also used for operation in three-phase networks for currents of a dozen or so to a few hundred amperes. An important feature of static switches is a very high instantaneous overload capacity (e.g. 2000%/20ms).

Figure 3 - Fast bypassing of a short circuit by means of the static-switch

If there are several circuits supplied by the UPS and a short circuit occurs in one of them, then the input terminals of the UPS are earthed and the rest of the circuits is not supplied. This lasts until the fuses in the shorted circuit are blown. Very quick switching of the bypass circuit ensures very quick reaction of the fuse and provides much shorter break in power supply of other circuits. Returning to power supply from the UPS is very quick, too. This guarantees uninterruptible operation of the correctly operating loads connected to the UPS. If two UPS systems are used (fig. 4a), the reliability of power supply of lines 1L and 2L can be significantly increased by installation of two additional static switches (fig.4b).



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a) b)

1AC

3AC

1L

2L

1UPS

2UPS

2AC

1AC

3AC

1L

2L

1UPS

2UPS

2AC

Figure 4 - Improvement of power supply reliability by means of the static-switches In this way we create a system in which each of the UPS units supplies ‘its own’ loads. In the event of a failure of one of the UPS, a static switch ensures very quick switching in such a way that the UPS which is operating at this moment correctly supplies both 1L and 2L circuits. Instead of 2AC line, a power generator can be used, which ensures supply of lines 1L and 2L even in case of a long voltage decay in the power network. The system shown also provides easy maintenance and service, which is important for a long lasting operation of the power supply system.

4.10 Static Var Compensator Finally, special fast-acting power electronic circuits, such as ‘Static Var Compensators’ can be configured to limit the unbalance. These behave as if they were rapidly changing complementary impedances, compensating for changes in impedance of the loads in each phase. Also, they are capable of compensating unwanted reactive power. However, these are expensive devices, and are only used for large loads (e.g. arc furnaces) when other solutions are insufficient. The impact of cyclic loads, such as spot welders, can be mitigated by the use of a static VAR compensator that corrects power factor ‘on the fly’ and reduce the impact on the system.

4.11 Surge protectors on key pieces of equipment These are nonlinear devices to limit the amplitude of voltage on a power line. The term implies that the device stops overvoltage problems (i.e. lightning). In actuality, voltage clamp levels, response times and installation determine how much voltage can be removed by the operation of an arrester. To protect power supplies against lightning strikes it is necessary to use lightning current arresters. For a low residual voltage the lightning current arresters have to be coordinated with surge voltage arresters. Lightning arrester spark gaps and varistor arresters that are connected in series, have been used for lightning and surge voltage protection in low voltage systems for the last two decades. With relatively low internal energy conversion the spark gaps are capable of dischargingseveral times non-destructive even in the event of very high lightning energiesy. After sparkover they enable a low residual voltage level. The sparkover voltage with generally 4 kV is quite high. Therefore before the spark gap sparkover or upon non-reaching the sparkover voltage, a varistor arrester has to take over the voltage limitation. On the one hand, residual voltages that exceed the isolation coordination allowed for the devices in question to be protected, may not be generated; on the other hand varistors may not be



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overloaded. The values for the isolation coordination can be taken from the IEC 60364-4-443. In order to avoid the power overload of the varistor arresters, both arresters – the spark gap as well as the varistor - are coordinated with each other. According to conventional procedure, an additional voltage is generated to counter the voltage dropping over the varistor by an inductivity that is switched between both arresters.

4.12 Uninterrupt ib le power supply (UPS) devices UPS systems are now commonly used as standby power supplies for critical loads, where the transfer time must be very short or zero. Static UPS systems are easily available in ratings from 200 VA to 50 kVA (single-phase) and from 10 kVA up to about 4000 kVA (three-phase). As well as providing a standby supply in the event of an outage, UPSs are also used to locally improve of power quality. The efficiency of UPS devices is very high, with energy losses ranging from 3% to 10%, depending on number of converters used and type of secondary battery. The basic classification of UPS systems is given in the standard IEC 62040-3 published in 1999 and adopted by CENELEC as standard EN-50091-3 [1]. The standard distinguishes three classes of UPS, indicating the dependence of the output voltage and output frequency from the input parameters:

• VFD (output Voltage and Frequency Dependent from mains supply), • VI (output Voltage Independent from mains supply), • VFI (output Voltage and Frequency Independent from mains supply).

However, in practice this classification closely corresponds to classification by internal structure:

• passive standby, • line interactive, • double conversion.

Classification according to the standard [1] VFD VI VFI UPS solution Passive

standby Line interactive Double

conversion Cost lowest medium highest

Voltage regulation none limited Yes Frequency regulation none none Yes

Transfer time short zero Zero Table 1 - Classification and characteristics of standard classes of UPS.CLASSIFICATION

4.13 Voltage stabi l isers Most voltage dips on the supply system have a significant retained voltage, so that energy is still available, but at too low a voltage to be useful to the load. This section discusses voltage dip mitigation equipment that deals with this type of dip. No energy storage mechanism is required; they rely on generating full voltage from the energy still available at reduced voltage (and increased current) during the dip. These devices are generally categorised as automatic voltage stabilisers. Other types of equipment are available to deal with dips where the retained voltage is zero and are described in another section of this Guide. This section gives a basic description of each type of automatic voltage stabiliser. The advantages and disadvantages of each type have been listed to enable the appropriate choice of voltage stabiliser to be made for the particular application.



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The main types of automatic voltage stabilisers are as follows: • Electro-mechanical • Ferro-resonant or constant voltage transformer (CVT) • Electronic step regulators • Saturable reactors (Transductor) • Electronic voltage stabiliser (EVS).

An important point to note in the selection of an automatic voltage stabiliser is that the chosen solution must solve the particular problem without creating additional problems. One example of this would be to connect a ferro-resonant stabiliser to the output of an inferior generator to reduce voltage variations. The net result would be adversely affected by the frequency fluctuations of the inferior generator that would produce an AC voltage change of 1.5 % for each 1 % change of frequency.

P Q p h e n o m e n a a n d d e s i g n i s s u e s PQ phenomena, problems and solutions approached by LPQI have been processed to define a PQ - design issue table which has been used for interview design and installation model design.

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DI-01 Power supply PS X X X X X X X X X DI-02 Scheme SY X X X X X DI-03 Transformers and reactors EQ X X X DI-04 Motors EQ X X X X X X X DI-05 PFC units EQ X X DI-06 Cables EQ X X X DI-07 Protection devices EQ X X DI-08 Ground systems EQ X X X DI-09 Lighting EQ X X X X DI-10 Plugs FC -- -- -- -- -- -- -- -- --

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