Solutions for Power Quality and Productivity

As in many other sectors of society, thepower business environment is undergoing dramatic change.

All round the world, previously passiveElectricity Consumers are becomingactive Power Customers, a trend whichis particularly evident in newly deregulated power markets.

Ever increasing competition, in bothpower utility and industrial markets, isputting the focus onto the importance ofPower Quality in the power supply.

Power Quality putsyou in high demand

Increasingly, industrialists are realising that electricity should be viewed as one among manycommodities vital for their business. As such itshould be subject to the same demands concer-ning quality control, reliability of supply, etc., asany other commodity.

For industry, Power Quality is all about assuredproductivity and cost control.

For utilities, Power Quality is all about PowerCustomer preference when choosing supplier.

With this increased awareness, previously hiddencosts of poor quality supply are surfacing. Forinstance:

The interruption of an industrial process due to apower outage or voltage dip can result in verysubstantial additional costs to the operation.These include lost productivity, labour costs forcleaning and restart, damaged or poor qualityproducts, delays in delivery, reduced customersatisfaction, and possibly even damage to pro-duction equipment. The industrial power consumer and grid owner can expect to enterinto negotiations concerning responsibilities andliabilities in the near future.

Power quality –a question of value

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Power Quality – a concern for everybody

Where power quality is lacking, disturbancessuch as voltage flicker and harmonics may causeproblems for both domestic and industrial con-sumers, far from the source of the problem.Ultimately, the offending equipment will becomea concern for many consumers and not just forthe owner of the equipment or grid.

Problems and impactThe quality of a power supply is largely synony-mous with the voltage quality. The voltage provided at a given connection point should beas close to the nominal value as possible, andthe wave form a pure sine form, free fromharmonics and other disturbances.

On a network, variations normally occur in voltage level, due to varying load conditions.This variation is kept within certain limits, andequipment connected to the network can handlethis without problems.

Lightning strikes or grounding faults, caused bytrees falling onto power lines for example, canhowever, lead to a drop in voltage outside the

tolerance band for short or extended periods.Even though the cause is local, the resultantvoltage drops and sags can affect consumershundreds of kilometers away. And complex orsensitive industrial processes like paper, plastics,or semiconductor manufacturing can be disrup-ted, even by quite limited voltage disturbances.

Electric arc furnaces (EAFs) are used by steelmakers all over the world. As heavy users ofpower, steel works are important customers forgrid owners and power producers.

An uncompensated EAF in operation drawslarge amounts of reactive power from the grid.

For the steel maker this leads to unnecessaryloss of efficiency and productivity.

For the grid owner, an uncompensated EAF isa major source of disturbance, such as voltagefluctuations, harmonics, and phase unbalance,which may cause problems for other customerson the grid. What’s more, the reactive powerdrawn by the uncompensated furnace leads to

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power losses and limits the flow of useful, revenue bringing, active power in the network.

On weaker networks, large motors can causesuch large voltage depressions when switched on,that the motor itself may not be able to operate.

Harmonics, caused by non-linear loads like rectifiers in power electronics or arc furnaces,lead to unnecessary losses and call for expensivemodifications to equipment. In serious cases,high levels of harmonics can even disrupt control systems and electronic equipment.

Fortunately, the problem of poor or insufficientpower quality, as highlighted above, can bedealt with.

ABB Power Systems offers a portfolio of so-lutions to power quality problems, which arepresented in this brochure. In addition to solvingpower quality problems, our solutions often helpimprove the industrial process and increase yourproductivity and, with it, your competitiveness.

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MinicapIn systems where relatively large loads areconnected close to the remote end of a radialfeeder, severe voltage drops and voltage fluc-

tuations can occur at varying load levels. Insome instances, large rotating machines on weakfeeders cause such large voltage depressionsduring start-up, that it becomes impossible forthe machine itself to reach operating speed.This can be the problem in remote mines,crushers, or mills, for example.

Applying series capacitor technology can be aneconomically attractive solution to these kindsof problem. The ABB Power Systems product rangeincludes a pole-mounted series capacitor specially adapted to distribution lines:

- Minicap, designed to support power supply up to 36 kV

Minicap helps to minimise undervoltage con-ditions resulting from heavy loads, mitigate over-voltage conditions due to sudden load rejection,and reduce reactive power consumption. Bycontinuous and instantaneous voltage control,Minicap fulfils the same stringent demands forprecision and operational reliability as any otherform of modern distribution equipment.

SVC-Q and MinicompLoads where the demand for reactive powerrapidly varies, such as rolling mills, mine hoists,cranes, welding machines, and arc furnaces,cause rapid voltage fluctuations. If the effectsare sufficiently large, production or operationaldisturbances, such as relays being tripped byvoltage depressions, can occur. The best wayof counteracting this type of voltage variationsis controlled reactive power compensation.

ABB Power Systems offers two solutions tothis type of problem:

- Static Var Compensators, tailored to suit theneeds of large loads (typically > 10 MVA up to several 100 MVA)

- Minicomp, aimed at medium power loads, which have reactive power demand from 1 to 20 Mvar

Both types of equipment use a combination ofreactors, capacitors, and thyristor valves, to provide controlled reactive power output. Sincemost of the undesired voltage variations arenormally caused by reactive power, the SVCand Minicomp are extremely effective in com-bating voltage variations caused by the normal operation of equipment.

Voltage fluctuations

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Flicker

What is flicker?Voltage variations give rise to fluctuations inintensity of lighting sources in homes, offices,and other buildings. At certain frequencies,even tiny light variations can be experienced ashighly irritating by many people. Such distur-bances are referred to as flicker. Flicker as aPower Quality issue has attracted a great deal ofmedia attention. Around the world, lower toler-ance limits on flicker are gradually beingimplemented.

Electric arc furnaces found in the steel industry,are a significant source of flicker. Other sources include welding machines andwind power generators.The violent forces released in the arc furnaceprocess impose a wide band of disturbances ofa truly stochastic nature on the power grid.

The only way to reduce such disturbances successfully is to continuously measure and correctly counteract rapid changes by extremelyfast compensation.

SVC Light – A Real Flicker TerminatorThe SVC Light is ABB Power Systems’ innovativesolution to the problem of flicker. The SVC Light

uses a voltagesource converter to provide the necessary reactivepower. The con-verter is equippedwith IGBTs whichenable it to have aswitching frequencyin the kHz range,and a response timeof less than one millisecond. This speed makes the SVC Lightmuch more efficient - typically by a factor of 2-3, compared with conventional solutions - in combating flicker. The SVC Light is similar in principle to a STATCOM.

The SVC Light stabilises the voltage on the furnace bus. From a production perspective,this means more power can be used in the furnace, which in turn can be used to increaseproductivity.

Additional benefits include reduced electrodeconsumption, reduced energy consumptionper melt and less erosion of the furnace refractory lining.

EAF in operation

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Red: Flicker, uncompensatedBlue: Flicker, SVC Light in operation

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great lenghts to prevent faults on their systems,they cannot be eliminated altogether. Sincefaults (and, therefore, voltage sags) are inevitable,it is important for customers to make sure thatcritical equipment sensitive to voltage sags isadequately protected. This is where DVR(Dynamic Voltage Restorer) comes in.

Despite the local nature of the cause, voltagedips and sags can be felt hundreds of kilometersaway. Even a relatively modest reduction involtage of, say, down to 70 per cent of nominalin one phase, may trip entire paper machines.Other types of industries affected include semi-conductor manufacturing, textile, pulp, andplants having critical hydraulic installations.

Compensating voltage variations over timeperiods ranging from minutes up to hours by changing taps on transformers has beenstandard practice in power systems for manyyears. For compensating voltage dips and sagson a millisecond time scale, however, con-ventional tap changers are far too slow.

ABB offers DVR as solution to dips and sagsproblems.

Voltage Dips and Sags

DVR

A voltage sag is a rapid decrease in RMS voltagemagnitude, typically lasting between 10 msand 1 second, and usually caused by a remotefault somewhere on the power system. Voltagesags are a very important power quality problemfacing many process industries. Equipmentused in modern industrial plants such asprocess controllers and adjustable speed drivesis becoming increasingly sensitive to voltagesags as the complexity of the process andequipment increases.

These events, random in character, are mainlycaused by weather conditions affecting thetransmission grid such as lightning strikes aswell as the impact from wind and ice, which inturn interfere with industrial distribution systems.Contamination of insulators and accidentsinvolving construction or transportation activitiesalso cause faults.

Faults resulting in voltage sags can occur onthe utility system or within the plant. The largemajority of faults on a utility system are singleline-to-ground faults. Three phase faults aremore severe, but less common. The voltagesag condition lasts until the fault is cleared bya protective device. Although utilities go to

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support complete outages. For the load to ridethrough a complete outage, a shunt connectedvariation of the DVR can be installed, – a DUPS,Dynamic Uninterruptable Power Supply.

System AnalysisBefore design of a DVR is undertaken, a systemanalysis should be performed. Important issuesto define are:

- Type of faults against which the criticalload (single phase to ground, phase tophase or three phase faults) has to be protected and where faults occur .

- Size and duration of the voltage dips.- Size and power factor of the critical load.- Possibilities to separate feeders with

important loads.- Voltage correction requirements at the

critical load (percent of the voltage andresponse time).

- Transformer connections (Y/y, Y/d, type of grounding, etc.).

With the information above, design of the most suitable and cost effective DVR can be carried out.

DVR – Dynamic Voltage RestorerThe function of the DVR is illustrated above.In the event of a voltage dip, the power elec-tronic converter injects the appropriate voltagerequired into the supply bus to compensate forthe sag. It effectively acts as a buffer to theload and prevents unacceptable disturbances.The rapid control cycles and millisecondswitching speed of the converter afford fastand accurate control of the voltage experi-enced by the load. This can be critical inprocesses like semiconductor manufacturing,where a single voltage sag may cause the lossof production, and with it, very high costs.

A DVR would typically have sufficient energystorage capacity to compensate a 50 per centthree-phase voltage dip for up to 10 cycles, theperiod normally required for fault clearance.Capacitors serve as the energy storage device.For coping with longer dips, an energy storageof larger capacity can be added to the DVR.ABB’s DVR covers a power range from 3 MVAup to 50 MVA.

Although a DVR may be rated to compensateup to a 90 per cent voltage dip, it does not

Container housed DVR for mitigation of voltage sags in sensitive industrialprocess. Load rating: 4 MVA.

Harmonic filtersIn the ideal case, the voltage and current waveforms follow perfect sine curves. In real net-works, non-linear loads such as drive systems,electric arc furnaces, rectifiers, and increasingly,home and office appliances, cause harmonicsand non-sinusoidal wave forms. The harmonicsgive rise to losses in, for instance, transformercores and motors. They can also cause other,more serious problems, however, when inter-fering with control systems and electronic equipment.

Harmonics can be reduced by filtering. For large,easily identifiable sources of harmonics, conven-tional filters designed to meet the demands ofthe actual application, are the most cost efficientmeans of eliminating harmonics and remainingwithin stipulated limits. These filters consist ofcapacitor banks with suitable tuning reactorsand damping resistors.For small and medium size loads, active filters,based on power electronic converters withhigh switching frequency, may be a moreattractive solution.

UnbalanceFor several reasons, it is preferable for the loadto be shared equally by all three phases of theAC system. However, large single phase loadsand unequal loading due to other reasons, cause unbalance in the system. Too much un-balance may have a disturbing or even damagingeffect on generators, rotating machines and other equipment.

Depending on whether the unbalance is fixedor dynamic, appropriate compensation shouldbe applied.

SVC is widely used for dynamic load balancingin conjunction with railway feeding.

If the load unbalance is fixed and known, fixed compensation can be designed using acombination of capacitors and reactors.

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Harmonics and Unbalance

StudiesStandard products can be used in applicationswith limited power demands. For higher power,it is often more cost effective to optimise thesolution to the specific system. It is also importantto investigate the effects on system resonancescaused by the introduction of new equipment.Modern computer simulation is an indispensableaid for this. An additional benefit of computersimulation is the facility to investigate the what-ifeffects of different faults and contingencies onthe system, without putting the live system at risk.ABB Power Systems has extensive experienceof applying computer simulation to solve PowerQuality problems, using both standard commercialsoftware packages and special programs develop-ed in-house. Simulation results are regularly validated against measurements in full scale applications and cases.

CapabilitiesABB Power Systems has delivered Power Quality solutions and systems for over thirtyyears. This experience is borne by a dedicatedorganisation, employing some 100 people, withthe project management, engineering, procure-ment, shipping, and all other know-how resourcesnecessary for effective project performance. ABBPower Systems implements projects all roundthe world, often to demanding deadlines. Projectinstallations include turn-key deliveries, as wellas other divisions of responsibility, according tothe specific wishes of each customer. A special Customer Support department takescare of customer needs for service contracts, service visits, and spare parts.Naturally, ABB Power Systems has quality certification to ISO 9001 standard.

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ABB Power Systems – your quality partner

ABB Power Systems ABS-721 64 Västerås, SwedenTelephone: +46 (0)21 32 40 00Telefax: +46 (0)21 18 31 43e-mail: info.sepow@se.abb.comhttp://www.abb.com/powersystems

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