SVC Light: a powerful flicker mitigator

Rolf Grünbaum, Tomas GustafssonABB Power Systems AB

SE-721 64 Vasteras, SwedenPhone:+46 21 32 40 00; Fax: +46 21 18 31 43

Thomas JohanssonABB Power T&D Company Inc.

Raleigh NCPhone: (919) 856 2379; Fax: (919) 856 2412

Key Words: EAF, Power quality, SVC Light, VSC,IGBT, Flicker mitigation, Harmonic filtering, loadbalancing, productivity improvement.

INTRODUCTION

With the advent of continuously controllablesemiconductors for high power, voltage sourceconverters far into the tens of MVA range havebecome feasible. With ABBs SVC Light concept,VSC (Voltage Source Converters) and IGBT(Insulated Gate Bipolar Transistors) have beenbrought together to offer possibilities hithertounseen for power quality improvement in the highpower range.

This opens up new options for power qualitycontrol in areas such as voltage flicker from electricarc furnaces ranging from a couple of tens of MVAup to ratings exceeding 100 MVA.

The paper describes the SVC Light, itstechnology as well as application, and comments onits abilities for power quality enhancement bydecreasing or eliminating phenomena such asvoltage flicker, harmonic distortion and phaseunbalance.

As a current example of the usefulness of SVCLight, an installation for flicker mitigation has beeninstalled at Hagfors steel mill in Sweden.

POWER QUALITY: A MATTER INFOCUS

Modern society relies heavily upon electricity.With deregulation, electricity has become acommodity as well as a means for competition.Power quality, as a consequence, is coming intofocus to an extent hitherto unseen. Industry as wellas commercial and domestic groups of users simplydemand power quality. Flickering lamps and TVsets are no longer accepted, nor are deratings orinterruptions of industrial processes due tounsufficient power quality.

In fact, the interruption of an industrial processfor instance due to a voltage sag can result in verysubstantial costs to the operation. These costsinclude lost productivity, labour costs for cleaningand restart, damaged product, reduced productquality, delays in delivery, and reduced customersatisfaction. Thus, costs previously hidden in poorpower quality are brought forward to their facevalue.

Another example: for a steel maker, an electricarc furnace (EAF) is a piece of equipment needed tomake his product. For the grid owner and for thesupplier of electricity, the EAF user is a subscriberto power, i.e. a customer, but in the worst case alsoa polluter of the grid. Out of the EAF may wellcome an abundance of distortion such as voltagefluctuations, harmonics and phase unsymmetry.Also, the grid may be subject to carrying largeamounts of reactive power, which is unintended andgives rise to transmission and distribution losses aswell as impedes the flow of useful, active power inthe grid.

Disturbances emanating from any particular loadwill travel far, and, unless properly remedied,spread over the grid to neighbouring facilities.

Thus, for instance, voltage flicker and harmonicsmay turn up far away from their source and disturbother consumers, urban as well as industrial, andbecome a nuisance to many. At the end of the day,the disturbing equipment will therefore become anissue to many and not just to the owner of theparticular equipment. We are then talking aboutlack of power quality.

Fig. 1. Disturbances travel far.

Fortunately, there are means to deal with theproblem of poor or unsufficient power quality ingrids. One obvious way is to reinforce the powergrid by building of new lines, installing new andbigger transformers, or moving the point ofcommon coupling to a higher voltage level.

Such measures, however, are expensive andtime-consuming, if they are at all permitted. As amatter of fact, there is a tendency in the oppositedirection at present in some places, with P.C.C.s insome cases being moved to lower voltage levels inthe grid.

A simple, straightforward and cost-effective wayof power quality improvement in such cases as wellas similar is to install equipment especiallydeveloped for the purpose in immediate vicinity ofthe source(s) of disturbance.

As an additional, very useful benefit, improvedprocess economy will often be attained as well, andas a matter of fact enable the said investment to turnout a profitable measure.

Voltage flicker

An electric arc furnace is a heavy consumer notonly of active power, but also of reactive power.Also, the physical process inside the furnace(electric melting) is erratic in its nature, with one orseveral electrodes striking electric arcs betweenfurnace and scrap. As a consequence, theconsumption especially of reactive power becomesstrongly fluctuating in a stochastic manner (Fig.2).

The voltage drop caused by reactive powerflowing through circuit reactances in the electrodes,electrode arms and furnace transformer thereforebecomes fluctuating in an erratic way, as well. Thisis called voltage flicker and is visualized mostclearly in the flickering light of incandescent lampsfed from the polluted grid.

Fig. 2. Reactive power consumption of EAF.

Spectral analysis confirms that lamp flickercaused by EAF action is severe around frequenciesfor which the human eye is particularly sensitive,i.e. below 20 cycles. And for certain, flicker is avery annoying sensation and becomes easily asource of complaint.

The International Union of Electroheat (UIE) incooperation with IEC has defined a quantity forexpressing flicker severity, Pst. According to thisterminology, Pst = 1 means that in a group ofpeople, half can observe light flicker that the groupis being exposed to. According to the IECrecommendations the Pst planning level for highand extra high voltages should be 0.8 or less.

SVC LIGHT

SVC Light is a flicker mitigating device. It achievesthis by attacking the root of the problem, the erraticflow of reactive power through the supply griddown into the furnaces. The reactive powerconsumption is measured, and correspondingamounts are generated in the SVC Light andinjected into the system, thereby decreasing the netreactive power flow to an absolute minimum. As animmediate consequence, voltage flicker is decreasedto a minimum, as well.

Important added benefits are a high and constantpower factor, regardless of load fluctuations overfurnace cycles, as well as a high and stable busRMS voltage. These benefits can be capitalized asimproved furnace productivity as well as decreasedoperational costs of the process in terms of lowerspecific electrode and energy consumption andreduced wear on furnace refractories.

To parry the rapidly fluctuating consumption ofreactive power of the furnaces, an equally rapidcompensating device is required. This is brought

about with state of the art power electronics basedon IGBT (Insulated gate bipolar transistor)technology. With the advent of such continuouslycontrollable semiconductor devices capable of highpower handling, VSC (Voltage Source Converters)with highly dynamic properties have becomefeasible far into the tens of MVA range.

The function of the VSC in this context is a fullycontrollable voltage source matching the busvoltage in phase and frequency, and with anamplitude which can be continuously and rapidlycontrolled, so as to be used as the tool for reactivepower control (Fig.3).

Fig. 3. VSC: a controllable voltage source.

The output of the VSC is connected to the ACsystem by means of a small reactor. By control ofthe VSC voltage (U2) in relation to the bus voltage(U1), the VSC will appear as a generator or absorberof reactive power, depending on the relationshipbetween the voltages. To this controlled reactivepower branch, an offsetting capacitor bank is added

in parallel, enabling the overall control range of theSVC Light to be capacitive.

The reactive power supplied to the network canbe controlled very fast. This is done only bychanging the switching pattern in the converterslightly. The response time is limited mainly by theswitching frequency and the size of the reactor.

The controllability of IGBTs also facilitatesseries connection of devices with safeguardedvoltage sharing across each IGBT. This enablesSVC Light to be directly connected to voltages inthe tens of kilovolts range. Thanks to this, itbecomes unnecessary to parallel converters in orderto achieve the power ratings needed for arc furnacecompensation (typically tens of MVA).

UDDEHOLM TOOLING SVC LIGHTINSTALLATION

Uddeholm Tooling at Hagfors in mid Sweden isa steel producer with its metallurgical process basedon scrap melting in an electric arc furnace (EAF)and subsequent refining by means of a ladle furnace(LF). The EAF is rated 31,5 MVA with a 20%temporary overload capability, whereas the LF israted 6 MVA plus a 30% overload capability. Bothfurnaces are fed from a 132 kV grid via anintermediate voltage of 10,5 kV (Fig.4).

The feeding grid is relatively weak, with a faultlevel at the P.C.C. of about 1000 MVA. It isobvious that this is quite unsufficient to enableoperation of the two furnaces while upkeepingreasonable power quality in the grid.

Fig. 4. Single-line diagram of EAF feeding networkand compensation.

The SVC Light (Fig.4) is rated at 0 - 44 Mvar ofreactive power generation, continuously variable.This dynamic range is attained by means of a VSCrated at 22 MVA in parallel with two harmonicfilters, one rated at 14 Mvar existing in the plantinitially and one installed as part of the SVC Lightundertaking, rated at 8 Mvar. Via its phase reactors,the VSC is connected directly to the furnace busvoltage of 10,5 kV. During the energisation of theVSC, the DC capacitors are charged via thecharging resistors. While the DC capacitors arecharged, the by-pass switch is closed.

M

Vm

Vm

Vm

INC.

VSVC-Q CONTAINER

LOAD(FURNACES)

MAINCB

FC's

EAF BUS

CAPACITORS

DC BUS PRE-INS. RESISTORSBY-PASSSWITCH

VALVES

REACTORS

PT's

CT's

INSTRUMENTTRANSFORMERS

USED IN THEVSVC-Q CONTROL

Fig. 5. Main circuit diagram of Hagfors SVC Light.

VOLTAGE SOURCE CONVERTERS

The input of the Voltage Source Converter isconnected to a capacitor, which is acting as a DCvoltage source. At the output, the converter iscreating a variable AC voltage. This is done byconnecting the positive pole, neutral or the negativepole of the capacitor directly to any of the converteroutputs. In converters that utilise Pulse WidthModulation (PWM), the input DC voltage isnormally kept constant. Output voltages such as asinusoidal AC voltage can be created. Theamplitude, the frequency and the phase of the ACvoltage can be controlled by changing the switchingpattern.

In SVC Light, the VSC uses a switchingfrequency greater than 1 kHz. The AC voltageacross the reactor at full reactive power is only asmall fraction of the AC voltage, typically 15 %.This makes SVC Light close to an ideal tool for fastreactive power compensation.

IGBT

For SVC Light the IGBT has been chosen as themost appropriate power device. IGBT allowsconnecting in series, thanks to low delay times forturn-on and turn-off. It has low switching losses andcan thus be used at high switching frequencies.Nowadays, devices are available with both highpower handling capability and high reliability,making them suitable for high power converters.

As only a very small power is needed to controlthe IGBT, the power needed for gate control can betaken from the main circuit. This is highlyadvantageous in high voltage converters, whereseries connecting of many devices is used.

Series connecting of IGBTs

At series connection of IGBTs, a proper voltagedivision is important. Simultaneous turn-on andturn-off of the series connected devices areessential. In SVC Light, the turn-off and turn-onsignals are distributed to the individual IGBTsthrough a high bandwidth fiber optic system. AllIGBT positions are also equipped with a specialtype of gate unit, which turns the IGBTs on and offwith a short delay time and at a controlled dV/dt.After turn-off of a IGBT valve or a diode valve, asmall voltage difference will be seen betweendifferent IGBT positions. Control of this differencein voltage is done by proper design of the resistiveand capacitive voltage dividers.

powersupply

gateunit

voltagedivider

fiber optics

powersupply

gateunit

voltagedivider

fiber optics

powersupply

gateunit

voltagedivider

fiber optics

IGBT valvecontrol unit

Fig. 4 The IGBT valve, with a number of series-connected IGBTs switched simultaneously

maincontroller

Fig. 6 The IGBT valve, with a number of series-connected IGBTs switched simultaneously

In addition to this, every IGBT position isequipped with an over-voltage monitoring system.This system makes it possible to detect if any IGBTposition behaves in an unnormal way already duringthe delivery test. If this would happen, such deviceswill be exchanged.

The converter valve

The converter topology for SVC Light is a threelevel configuration. In a three-level converter theoutput of each phase can be connected to either thepositive pole, the midpoint or the negative pole ofthe capacitor. The DC side of the converter isfloating, or in other words, insulated relative toground. Using PWM, the converter will create avery smooth phase current, with low harmoniccontent. The three-level topology also gives lowswitching losses. This means high converterefficiency and high current capability.

U

U

Fig. 5a: The principle of a 3-level converter (one phase) Fig 5b: Neutral Point Clamped converter (one phase)

U

U

Fig. 7a The principle of a 3-level converter Fig. 7b Neutral Point Clamped converter (one phase)

The three-level converter used is a so calledNeutral Point Clamped (NPC) configuration. Thisconfiguration includes four IGBT valves and twoDiode valves in every phase leg. The DC capacitoris divided into two series connected branches. Thedifferent valves are built by stacking the devices ontop of each other (between coolers) and by applyingan external pressure to the stack.

The IGBT valves are water cooled using ABBPower Systems well proven system with deionizedwater. Water cooling gives a compact converterdesign and high current capability. Besidescustomer benefits, a compact design also hasanother advantage. This means that the loopinductance between the IGBT valves and the DCcapacitors can be kept low, which is beneficial froma loss point of view. In the Hagfors installation thevalves are designed to handle approximately 1300Arms phase current continuously and at transientconditions 1700 Arms.

ABB Power Systems are now ready to deliverSVC Light for direct connection to voltages up to36 kV AC.

Fig. 8. Photograph of IGBT stacks.

MACH2, the control and protection system

To fulfill the requirements regarding very fastand extensive data processing, ABB Power Systemshas developed the MACH2 control and protectionsystem. It is based on industrial standard PChardware (operating system Windows NT) tofacilitate an open system easily integrated intoexisting systems at the steel works and externallyaccessed directly if desired.

The strategy of building the control andprotection system based on open interfaces assuresthat future improvements in the fast developingfield of electronics can be used. The system consists

basically of three units, the Main Computer, the I/Orack, and the VCU (Valve Control Unit). See Fig.9below. The communication between the units isperformed via industrial standard type busses.

I/O rack

PCI

CPUPent ProC&P(slow)

PS801C&P(fast)

LAN

TDM

CP/FP

PS820SUP

CAN

Main Computer

CENTRAL UNIT OPTO UNIT

I/O SIGNAL

CONNECTOR

I/O SIGNAL

CONNECTOR

FIBER OPTIC

CONNECTOR

FIBER OPTIC

CONNECTOR

8+8 IR DIODES

8+8 DETECTORS

QHLD 505 QHLD 506CENTRAL UNIT

QHLD 505

IR DRIVERS

DETECTORS

Valve Control

Fig. 9. MACH2 for SVC Light.

The system is operated from an OWS (OperatorWork Station), which can be a standard PC. TheMMI (Man Machine Interface) is the world´s mostused graphic control package for Windows NT -InTouch. InTouch is a flexible software, in whichspecial customer adopted MMI can easily bedesigned.

MITIGATION OF HARMONIC,TRANSIENT, AND NEGATIVE PHASE

SEQUENCE DISTORTION

The possibility for the SVC Light to “follow” thestochastically varying furnace current gives anenormous opportunity to reduce voltage flicker.However, with its quick response, also otherunpleasant disturbances can easily be reduced. Therecording below shows the initial operation of theSVC Light when paralleled to the EAF in Hagfors

steel works. Signals show the SVC Light ability totrack the rapid changing of the furnace current.

Fig. 10. Tracking of rapid changes of the EAFcurrent.

Harmonic filtering

Very often harmonic generations from ElectricArc Furnaces are of a great magnitude. Largeamounts of harmonics could create unwantedheating of machines as well as resonance problems.

The SVC Light concept offers an active filterwhere depending on the application harmonics up tothe 11:th order would be damped. With an activefilter, the risk of dangerous parallel resonancesbetween network components will be minimized.

Fast transients damping

With a switching frequency higher than 1 kHz,even fast transients can be damped. In someinstallations switching transients from a furnacetransformer could for example lead to unexpectedproblems. With the SVC Light technology, thefurnace operation with all switching transients will

be compensated. The recording below shows theinrush current of the furnace transformer in Hagforsand the SVC Light output current response.

Fig. 11.Inrush current of EAF transformer and SVCLight output current response.

Load balancing

For several reasons it is desirable that the load isshared equally by the three phases of the ACsystem. Too much unbalance may have disturbingor even damaging effects for example on generatorsor other rotating machines. With the traditionalSVC or new SVCLight concept, the system willfrom the network be balanced in all three phases.

POWER FACTOR CORRECTION:ECONOMIC BENEFITS FOR THE

OPERATOR

Since SVC Light is a reactive powercompensator, it can also be used to benefit forpower factor correction in the steel plant. Itsdynamic response ensures that the power factor can

Reactive current component

0.35 0.37 0.39 0.41 0.43 0.45 0.47 0.49(Time)

(A)

Response SVCLightFurnace current

Reactive current component

-

-

-

-

0.2 0.25 0.3 0.35 0.4 0.45 0.5

(Time)

(A)

Response SVCLightInrush current

be kept high and constant under strongly varyingconditions of reactive power demand of the plant.This yields valuable benefits for the owner of theplant, not only in the way of minimized billing forreactive power, but also in a decrease of activesystem power losses.

Also, the minimizing of reactive power flowthrough the plant brings about a very importantstabilization of the EAF bus voltage at a high level,which means that more active power can be takenout of the furnace than otherwise. This can becapitalized on by means of increased furnaceproductivity as well as lower specific productioncosts (electrode consumption, etc.).

A concrete increase of active power output of theEAF has in fact been demonstrated in the Hagforscase, please see under “Field measurements” below.

PRELIMINARY FLICKER FIELDMEASUREMENTS

Field measurements have been performed inorder to evaluate the performance of the SVC Lightat Hagfors site. So far, the measurements havemostly been focusing on flicker.

Investigations at site show that the installedauxiliary current transformer in the EAF circuit hastoo small a power rating. The low rating leads tosaturation during the bore down period of the melt.This problem will be corrected, however at thisstage no flicker measurements with a correct CT arepresent.

The recordings and evaluation presented belowshow flicker mitigation of 3.4 times. The flickermitigation is expected to increase to a factor around4.5 to 5.0 times when new auxiliary transformershave been installed.

The flicker measurements have been performedaccording to the UIE/IEC method. The method iswell known in Europe, Canada, South America andSouth Africa and is also getting more common inthe United States.

The bar chart (Fig. 12) shows flicker generationfrom the EAF with and without SVC Light. Thechart also shows when the furnace is in operation.

Fig. 12. Flicker generation with/without SVC Light.

Statistical evaluation

In order to evaluate flicker generation with andwithout SVC Light a statistical evaluation has beenperformed. The figures below show histograms forthe following three cases:

Figure 13a: Furnace in operation, without SVCLight. Pst(95%) = 3.90 p.u.

13b: Furnace in operation, with SVC Light.Pst(95%) = 1.21 p.u.

13c: Background flicker generation.Pst(95%) = 0.41 p.u.

Hagfors, 25-27/5-99

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

5.5

6

11:4

925

-maj

13:4

925

-maj

15:4

925

-maj

17:4

925

-maj

19:4

925

-maj

21:4

925

-maj

23:4

925

-maj

1:49

26-m

aj3:

4926

-maj

5:49

26-m

aj7:

4926

-maj

9:49

26-m

aj11

:49

26-m

aj13

:49

26-m

aj15

:49

26-m

aj17

:49

26-m

aj19

:49

26-m

aj21

:49

26-m

aj23

:49

26-m

aj1:

4927

-maj

3:49

27-m

aj5:

4927

-maj

7:49

27-m

aj9:

4927

-maj

11:4

927

-maj

13:4

927

-maj

Time/ Date

Pst (

10 m

in)

EAF in operation

*) *) *)

*) SVCLight in operation

Fig. 13a. Flicker generation without SVC Light.

Fig. 13b. Flicker generation with SVC Light inoperation.

Fig. 13c. Background flicker generation.

According to the UIE (International Union ForElectroheat), the recommend praxis to evaluateflicker severity from multiple sources is per thefollowing formula:

( )m

i

mSTiST PP

/1

��

���

�= �

where m is the summation coefficient.

The summation coefficient considers the risk ofcoincident furnace operation.The factor is recommended to vary from 1 up to 4.m=4 is used, when furnaces specifically run in orderto avoid coincident melts. m=1 is used when there isa very high occurrence of coincident voltagechanges.

The background flicker is generated by manysources, and will therefore appear more or lessconstant over the day. To deduct the backgroundflicker, an “m” factor of 1 should be used. Howeverto perform the evaluation in a very conservativeway, a factor of two is used.

In the case of flicker generated by the EAF onlyand without background flicker, we get with noSVC Light in operation:

( )[ ] ( )[ ] 88.341.090.3 5.0222/123

21 =−=−= STSTSTA PPP

p.u.

The residual flicker value with the SVC Lightand EAF in operation and without backgroundflicker is:

( )[ ] ( )[ ] 14.141.021.1 5.0222/123

22 =−=−= STSTSTB PPP

p.u.

Histogram/ without SVCLight

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 4 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 5

Pst(10 min)

.00%5.00%10.00%15.00%20.00%25.00%30.00%35.00%40.00%45.00%50.00%55.00%60.00%65.00%70.00%75.00%80.00%85.00%90.00%95.00%100.00%

Pst (95%) = 3.90 p.u.

Mean value = 1.61 p.u.

Standard deviation = 1.22 p.u.

Cumulative percentage

Histogram/ with SVCLight

00.

05 0.1

0.15 0.

20.

25 0.3

0.35 0.

40.

45 0.5

0.55 0.

60.

65 0.7

0.75 0.

80.

85 0.9

0.95 1

1.05 1.

11.

15 1.2

1.25 1.

31.

35 1.4

1.45 1.

51.

55 1.6

1.65 1.

71.

75 1.8

1.85 1.

91.

95 2

Pst(10 min)

.00%5.00%10.00%15.00%20.00%25.00%30.00%35.00%40.00%45.00%50.00%55.00%60.00%65.00%70.00%75.00%80.00%85.00%90.00%95.00%100.00%

Pst (95%) = 1.21 p.u.

Mean value = 0.46 p.u.

Standard deviation = 0.36 p.u.Cumulative percentage

Histogram/ background generation

00.

020.

040.

060.

08 0.1

0.12

0.14

0.16

0.18 0.2

0.22

0.24

0.26

0.28 0.3

0.32

0.34

0.36

0.38 0.4

0.42

0.44

0.46

0.48 0.5

0.52

0.54

0.56

0.58 0.6

0.62

0.64

0.66

0.68 0.7

0.72

0.74

0.76

0.78 0.8

Pst (10 min)

.00%5.00%10.00%15.00%20.00%25.00%30.00%35.00%40.00%45.00%50.00%55.00%60.00%65.00%70.00%75.00%80.00%85.00%90.00%95.00%100.00%

Pst (95%) = 0.41 p.u.

Mean value = 0.21 p.u.

Standard deviation = 0.11 p.u.

The flicker mitigation by the SVC Light is:

4.314.188.3 ===

STB

STASVCLight P

PR p.u.

Increased furnace power

An arc furnace requires a stable voltage supplyfor optimum performance. An SVC Light caninstantaneously compensate the random reactivepower variations, and hence the voltage variations,of an EAF.

Reactive power compensation by an SVC Lightor an SVC helps to obtain the following productionbenefits:

A higher and stabilised voltage level at thefurnace busbar, giving:- Shorter melt down times- Reduced energy losses- Reduced electrode consumption.

Fig. 14 shows measurements of the active powerconsumption in Hagfors without and with dynamiccompensation.

Fig.14. Active power consumption at Hagforswithout and with dynamic compensation.

Through dynamic compensation, the voltage onthe furnace busbar is stabilised. The stabilisedvoltage increases the available furnace input power,Fig. 14.

SUMMARY

The appearing of continuously controllablesemiconductors capable of handling high currentsand voltages has opened up for a wholly new classof highly dynamic power electronic devices.Voltagesource converters far into the tens of MVA rangewith high dynamic response based on IGBTtechnology are now a reality. An important as wellas sophisticated application for this new technologyis mitigation of severe voltage flicker from electricarc furnaces.

An SVC Light, a full scale installation for flickermitigation based on IGBT equipped voltage sourceconverter equipment has been implemented in asteel mill in Sweden. As an additionally valuablebenefit, the available melting power of the electricarc furnace has been increased.

REFERENCES

1. B. Bijlenga, R. Grünbaum, and Th. Johansson,“SVC Light – a powerful tool for power qualityimprovement”, ABB Review, No. 6, 1998.

Hagfors, furnace power

0

4

8

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16

20

24

28

32

36

40

Act

ive

pow

er (M

W)

Without SVCLight With SVCLight