Power Quality Ghosh

8/20/2019 Power Quality Ghosh

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DayDay--33

Power Quality & CustomPower Quality & Custom

PowerPowerArindam GhoshArindam Ghosh

Dept. of Electrical EngineeringDept. of Electrical Engineering

Indian Institute of TechnologyIndian Institute of Technology

KanpurKanpur, India, IndiaEE--mail:mail: aghoshaghosh@@iitkiitk.ac.in.ac.in


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Power Quality (PQ)Power Quality (PQ)

•• The termThe term electric power quality electric power quality broadlybroadlyrefers to maintaining a near sinusoidal powerrefers to maintaining a near sinusoidal power

distribution bus voltage at rated magnitudedistribution bus voltage at rated magnitudeand frequency.and frequency.

•• In addition, the energy supplied to aIn addition, the energy supplied to a

customer must be uninterrupted from thecustomer must be uninterrupted from thereliability point of view.reliability point of view.

•• It is to be noted that even though powerIt is to be noted that even though powerquality (PQ) is mainly a distribution systemquality (PQ) is mainly a distribution systemproblem, power transmission systems mayproblem, power transmission systems may

also have an impact on the quality of power.also have an impact on the quality of power.


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Causes of PQ DeteriorationCauses of PQ Deterioration

They can be divided into two categories.They can be divided into two categories.•• Natural Causes:Natural Causes: Faults or lighting strikes onFaults or lighting strikes on

transmission lines or distribution feeders,transmission lines or distribution feeders,

falling of trees or branches on distributionfalling of trees or branches on distributionfeeders during stormy conditions, equipmentfeeders during stormy conditions, equipmentfailure etc.failure etc.

•• Due to Load or Transmission Line/FeederDue to Load or Transmission Line/FeederOperation:Operation: TransformerTransformer energizationenergization,,capacitor or feeder switching, powercapacitor or feeder switching, power

electronic loads (UPS, ASD, converters etc.),electronic loads (UPS, ASD, converters etc.),arc furnaces and induction heating systems,arc furnaces and induction heating systems,switching on or off of large loads etc.switching on or off of large loads etc.


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PQ Problems and CausesPQ Problems and Causes


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PQ Problems and CausesPQ Problems and Causes


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PQ StandardsPQ Standards


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PQ MonitoringPQ Monitoring

•• Power quality variations are classified asPower quality variations are classified aseithereither disturbances disturbances oror steady statesteady statevariations variations ..

•• DisturbancesDisturbances pertain to abnormalities inpertain to abnormalities inthe system voltages or currents due tothe system voltages or currents due tofault or some abnormal operations.fault or some abnormal operations.

•• Steady state variationsSteady state variations refer torefer to rmsrmsdeviations from the nominal quantities ordeviations from the nominal quantities orharmonics.harmonics.

•• Power quality variations are monitored byPower quality variations are monitored bydisturbance analyzers, voltage recorders,disturbance analyzers, voltage recorders,harmonic analyzers etc.harmonic analyzers etc.


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•• The input data for any power qualityThe input data for any power qualitymonitoring device is obtained throughmonitoring device is obtained through

transducers like CT, PT, Halltransducers like CT, PT, Hall--effecteffecttransducers etc.transducers etc.

•• Disturbance analyzers and disturbanceDisturbance analyzers and disturbance

monitors are instruments that aremonitors are instruments that arespecifically designed for power qualityspecifically designed for power qualitymeasurements.measurements.

•• There are two categories of these devicesThere are two categories of these devices−− conventional analyzers and graphicsconventional analyzers and graphics--

based analyzers.based analyzers.


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•• Conventional analyzers provide information likeConventional analyzers provide information likemagnitude and duration of sag/swells,magnitude and duration of sag/swells,under/under/overvoltagesovervoltages etc.etc.

•• GraphicGraphic--based analyzers are equipped withbased analyzers are equipped withmemory such that the realmemory such that the real--time data can betime data can besaved.saved.

•• The advantage of this device is that the savedThe advantage of this device is that the saveddata can be analyzed later to determine thedata can be analyzed later to determine the

source and cause of the power quality problems.source and cause of the power quality problems.•• These analyzers can also graphically presentThese analyzers can also graphically present

the realthe real--time data.time data.


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•• Harmonic data are analyzed with the help ofHarmonic data are analyzed with the help ofDSPDSP--based harmonic or spectrum analyzers.based harmonic or spectrum analyzers.

They can perform fast Fourier transformThey can perform fast Fourier transform(FFT) by sampling real(FFT) by sampling real--time data.time data.

•• These analyzers can simultaneously measureThese analyzers can simultaneously measure

the voltage and currents such that harmonicthe voltage and currents such that harmonicpower can be computed.power can be computed.

•• They can also sample the signals at a veryThey can also sample the signals at a veryhigh rate such that harmonics up to abouthigh rate such that harmonics up to about5050thth order can be determined.order can be determined.


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•• Flicker monitoring is done through IECFlicker monitoring is done through IECflickermeterflickermeter..

•• These meters measure the instantaneousThese meters measure the instantaneousflickering voltage. This is called theflickering voltage. This is called theinstantaneous flicker level (IFL).instantaneous flicker level (IFL).

•• The recorded IFL is then stored andThe recorded IFL is then stored andstatistical operations on these data arestatistical operations on these data are

performed to determine short term (10performed to determine short term (10min) flicker severity index and long termmin) flicker severity index and long termflicker severity index.flicker severity index.


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TransientsTransients

•• Impulsive transientsImpulsive transients

like lightning arelike lightning areunipolarunipolar in nature.in nature.•• Oscillatory transientsOscillatory transients

are bipolar and areare bipolar and arecaused bycaused bytransformertransformer

energizationenergization,,capacitor orcapacitor orconverter switchingconverter switching

etc.etc.

•• Transients are of two typesTransients are of two types –– impulsive andimpulsive andoscillatory transients.oscillatory transients.


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Short DurationShort Duration

Voltage Variations Voltage Variations

•• Voltage sagVoltage sag is a fundamental frequencyis a fundamental frequencydecrease in the supply voltage for adecrease in the supply voltage for a

short duration (5 cycles to one minute).short duration (5 cycles to one minute).•• Voltage swellVoltage swell is defined as the increaseis defined as the increase

of fundamental frequency voltage for aof fundamental frequency voltage for ashort duration.short duration.

•• AnAn interruptioninterruption occurs when the supplyoccurs when the supply

voltage (or load current) decreases tovoltage (or load current) decreases toless than 0.1 per unit for a period of timeless than 0.1 per unit for a period of timenot exceeding 1 minute.not exceeding 1 minute.


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Voltage Sag, Swell Voltage Sag, Swell

& Interruption& Interruption


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Example of Sag and SwellExample of Sag and Swell

•• A 1LG fault is createdA 1LG fault is createdat 0.04 s in Feederat 0.04 s in Feeder--1.1.

•• While phaseWhile phase--a voltage sags, the other two phasesa voltage sags, the other two phases

swells.swells.


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Long DurationLong Duration


•• These are theThese are the rmsrms variations in the supplyvariations in the supplyvoltage at fundamental frequency for periodsvoltage at fundamental frequency for periods

exceeding 1 minute.exceeding 1 minute.•• Classifications:Classifications:

–– overvoltages overvoltages –– undervoltages undervoltages

–– sustained interruptions sustained interruptions

•• AnAn overvoltageovervoltage (or(or undervoltageundervoltage) is a 10% or) is a 10% ormore increase (or decrease) inmore increase (or decrease) in rmsrms voltagevoltage

for more than 1 minute.for more than 1 minute.


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•• In a weak system the switching off of aIn a weak system the switching off of a

large load or thelarge load or the energizationenergization of a largeof a largecapacitor bank may result in ancapacitor bank may result in an overvoltageovervoltage..

•• AnAn undervoltageundervoltage is the result of an event,is the result of an event,

which is a reverse of the event that causeswhich is a reverse of the event that causesovervoltageovervoltage..

•• The termThe term brownout brownout is often referred asis often referred assustained periods ofsustained periods of undervoltageundervoltage due todue toutility strategy to reduce power demand.utility strategy to reduce power demand.


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•• When the supply voltage is zero for aWhen the supply voltage is zero for a

period of time in excess of 1 minute, theperiod of time in excess of 1 minute, thelong duration voltage variation is calledlong duration voltage variation is calledsustained interruptionsustained interruption..

•• Typical causes of sustained interruptionsTypical causes of sustained interruptionsvary from place to place.vary from place to place.

•• Human intervention is required duringHuman intervention is required duringsustained interruptions for repair andsustained interruptions for repair andrestoration.restoration.


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Voltage Imbalance Voltage Imbalance

•• This is the condition in which the voltages ofThis is the condition in which the voltages ofthe three phases of the supply are not equalthe three phases of the supply are not equal

in magnitude or equally displaced in time.in magnitude or equally displaced in time.

•• The primary cause is the singleThe primary cause is the single--phase loadsphase loadsin threein three--phase circuits. These are howeverphase circuits. These are however

restricted to within 5%.restricted to within 5%.•• Severe imbalance (greater than 5%) canSevere imbalance (greater than 5%) can

result during single phasing conditions whenresult during single phasing conditions whenthe protection circuit opens up one phase ofthe protection circuit opens up one phase ofa threea three--phase supply.phase supply.


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Waveform DistortionWaveform Distortion

•• Classifications:Classifications:–– Dc offsetDc offset

–– harmonicsharmonics–– notchingnotching

•• The major causes ofThe major causes of dc offsets dc offsets are geomagneticare geomagnetic

disturbance and halfdisturbance and half--wave rectification.wave rectification.•• The offsets due to geomagnetic disturbancesThe offsets due to geomagnetic disturbances

are especially severe in higher latitudes.are especially severe in higher latitudes.•• Poor grounding can also result in dc offsets.Poor grounding can also result in dc offsets.

•• Effects: Transformer saturation and heating.Effects: Transformer saturation and heating.


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HarmonicsHarmonics

•• Classifications:Classifications:–– Integer harmonicsInteger harmonics

–– SubharmonicsSubharmonics

–– InterharmonicsInterharmonics

•• For a fundamental frequency ofFor a fundamental frequency of f f 00,, integerinteger

harmonicsharmonics have frequency components that arehave frequency components that areinteger multiples ofinteger multiples of f f 00, i.e.,, i.e., nf nf 00, where, where n n is ais apositive integer.positive integer.

•• Causes of integer harmonics are powerCauses of integer harmonics are powerelectronic equipment and loads, like ASD, UPSelectronic equipment and loads, like ASD, UPS

etc.etc.


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HarmonicsHarmonics

•• SubharmonicsSubharmonics are those components that areare those components that arebelow the fundamental component, i.e.,below the fundamental component, i.e., mf mf 00 forfor0 1.1.

•• CycloconvertersCycloconverters mainly causemainly cause interharmonicsinterharmonics..

•• InterharmonicsInterharmonics are rather difficult to detect.are rather difficult to detect.

•• Harmonics can cause damages to powerHarmonics can cause damages to power

apparatus and appliances.apparatus and appliances.


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Harmonics (THD)Harmonics (THD)

A measure of harmonic content in a signal isA measure of harmonic content in a signal isthethe total harmonic distortion total harmonic distortion ((THD THD ). The). Thepercentagepercentage THD THD in a voltage is given byin a voltage is given by

1

2

2

V

V

THD nn∑

∞

==

wherewhere V V n n

denotes the magnitude of thedenotes the magnitude of the n n thth

harmonic voltage andharmonic voltage and V V 11 is the magnitude ofis the magnitude ofthe fundamental voltagethe fundamental voltage


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Harmonics SpectrumHarmonics Spectrum


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NotchingNotching

•• Cause: operation ofCause: operation ofpower electronicpower electronicconverters.converters.

•• Occurs when currentOccurs when currentcommutates from onecommutates from onephase to otherphase to other

causing a momentarycausing a momentaryshort circuit betweenshort circuit betweenthe two phases.the two phases.

•• The maximum voltageThe maximum voltageduring notchesduring notchesdepends on thedepends on the

system impedance.system impedance.

•• The frequencyThe frequencycomponents that arecomponents that areassociated with notchesassociated with notches

are usually very high.are usually very high.


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Voltage Fluctuations Voltage Fluctuations

•• These are systematic random variations inThese are systematic random variations insupply voltages.supply voltages.

•• A very rapid change in the supply voltage isA very rapid change in the supply voltage iscalledcalled voltage flicker voltage flicker ..

•• This is caused by rapid variations in currentThis is caused by rapid variations in current

magnitude of loads such as arc furnaces inmagnitude of loads such as arc furnaces inwhich a large inrush current flows when thewhich a large inrush current flows when thearc strikes first causing a dip in the busarc strikes first causing a dip in the bus

voltage.voltage.•• Other customers that are connected to theOther customers that are connected to the

same bus face regular severe voltage drops.same bus face regular severe voltage drops.


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Frequency variationsFrequency variations

•• These variations are usually caused by rapidThese variations are usually caused by rapidchanges in the load connected to thechanges in the load connected to thesystem.system.

•• The maximum tolerable variation in supplyThe maximum tolerable variation in supplyfrequency is often limited withinfrequency is often limited within ±± 0.50.5 Hz.Hz.

From the nominal frequency of 50 or 60 Hz.From the nominal frequency of 50 or 60 Hz.•• The frequency is directly related to theThe frequency is directly related to the

rotational speed of the generators.rotational speed of the generators.

•• Thus a sustained operation outside theThus a sustained operation outside thetolerable frequency range may reduce thetolerable frequency range may reduce the

life span of turbine blades on the shaft.life span of turbine blades on the shaft.


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Power Acceptability CurvesPower Acceptability Curves

•• These curves quantify the acceptability ofThese curves quantify the acceptability ofsupply power as a function of duration versussupply power as a function of duration versus

magnitude of bus voltage disturbances.magnitude of bus voltage disturbances.•• Most popular curve was originally developed byMost popular curve was originally developed by

Computer Business Equipment ManufacturersComputer Business Equipment ManufacturersAssociation (CBEMA) to set limits to theAssociation (CBEMA) to set limits to thewithstanding capabilities of computers.withstanding capabilities of computers.

•• The CBEMA curve has however become a deThe CBEMA curve has however become a defacto standard for measuring the performancefacto standard for measuring the performanceof all types of equipment and power systems.of all types of equipment and power systems.


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CBEMA CurveCBEMA Curve


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CBEMA CurveCBEMA Curve

•• In the CBEMA curve there are two tracesIn the CBEMA curve there are two traces −−one forone for overvoltageovervoltage and the other forand the other forundervoltageundervoltage..

•• These show the percent bus voltage deviationThese show the percent bus voltage deviationfrom the rated voltage against time.from the rated voltage against time.

•• The region below the upper trace and aboveThe region below the upper trace and abovethe lower trace is the acceptable range. Thisthe lower trace is the acceptable range. Thisregion defines the tolerance level.region defines the tolerance level.

•• Example anExample an overvoltageovervoltage of very short durationof very short durationcan be tolerable if it is in the acceptablecan be tolerable if it is in the acceptable

region.region.

PQ P blPQ P bl


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PQ ProblemsPQ Problems

Some of the major concerns of bothSome of the major concerns of bothcustomers and utility arecustomers and utility are

•• Poor load power factorPoor load power factor•• Harmonic contents in loadsHarmonic contents in loads•• Notching in load voltagesNotching in load voltages

•• Dc offset in load voltagesDc offset in load voltages•• Unbalanced loadsUnbalanced loads

•• Supply voltage distortionSupply voltage distortion•• Voltage sag/swellVoltage sag/swell•• Voltage flickerVoltage flicker

Load Power FactorLoad Power Factor


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Load Power FactorLoad Power Factor

•• A significant drop in the feeder voltage isA significant drop in the feeder voltage iscaused when the magnitude of the loadcaused when the magnitude of the load

currentcurrent I I s s is large.is large.•• There will also be a large amount ofThere will also be a large amount of I I s s 22R R s s loss associated with high heat dissipation inloss associated with high heat dissipation in

the feeder.the feeder.

N li L dN li L d


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Nonlinear LoadsNonlinear Loads

In the distribution system shown, the nonlinearIn the distribution system shown, the nonlinearload (Loadload (Load--2) will cause distortion in voltages2) will cause distortion in voltages

of buses 2 and 3 and all the currents.of buses 2 and 3 and all the currents.



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ff f i


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Effects of HarmonicsEffects of Harmonics

•• The presence of harmonics can causeThe presence of harmonics can causeadditional losses in induction motors,additional losses in induction motors,especially when they are operating close toespecially when they are operating close to

their rated values, resulting in increasedtheir rated values, resulting in increasedheating.heating.•• the supply voltage is used for timingthe supply voltage is used for timing

purposes in many cases like digital clocks.purposes in many cases like digital clocks.Power electronic equipment like phasePower electronic equipment like phasecontrolled thyristor circuits use the zerocontrolled thyristor circuits use the zero

crossing of the supply voltage to generatecrossing of the supply voltage to generatetrigger pulses. A distorted voltage waveformtrigger pulses. A distorted voltage waveformcan create false triggering of the timingcan create false triggering of the timing

circuits.circuits.

S l V lS l V lt


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Supply VoltageSupply Voltage

DisturbanceDisturbance

The disturbances in the supply voltage can haveThe disturbances in the supply voltage can havean adverse impact on the customers.an adverse impact on the customers.

Examples:Examples:•• Even a small duration voltage interruption canEven a small duration voltage interruption can

cause relay tripping and stopping a processcause relay tripping and stopping a process

line resulting in many hours of production loss.line resulting in many hours of production loss.•• Even a short duration outage can causeEven a short duration outage can cause

defects in semiconductor processing.defects in semiconductor processing.

•• A sustainedA sustained overvoltageovervoltage can cause domesticcan cause domesticlights to burn out faster and can put stresslights to burn out faster and can put stresson capacitors.on capacitors.



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DisturbanceDisturbance

•• Voltage spikes or transientVoltage spikes or transient overvoltageovervoltage cancancause permanent damage on capacitorscause permanent damage on capacitors

thereby burning power supply or otherthereby burning power supply or othersemiconductor components of computers,semiconductor components of computers,TVs, VCRs and household appliances.TVs, VCRs and household appliances.

•• SustainedSustained undervoltageundervoltage or even a few cycleor even a few cyclevoltage sag can cause motors to stall.voltage sag can cause motors to stall.•• Voltage flicker can be very annoying to theVoltage flicker can be very annoying to the

human eyes as it causes incandescent lampshuman eyes as it causes incandescent lampsto flicker. This can cause headaches, nauseato flicker. This can cause headaches, nauseaor migraine.or migraine.

C t PC t P


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Custom PowerCustom Power

•• The termThe term Custom PowerCustom Power (CP)(CP) pertains to thepertains to theuse of power electronic controllers for poweruse of power electronic controllers for power

distribution systems.distribution systems.•• Just as the FACTS controllers improve theJust as the FACTS controllers improve the

reliability and quality of power transmissionreliability and quality of power transmission

systems, the custom power enhances thesystems, the custom power enhances thequality and reliability of power that isquality and reliability of power that isdelivered to customers.delivered to customers.

•• Since the custom power devices improve theSince the custom power devices improve thepower quality, they can also be called powerpower quality, they can also be called powerquality enhancing devices as well.quality enhancing devices as well.

UtilityUtility Customer InterfaceCustomer Interface


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UtilityUtility--Customer InterfaceCustomer Interface

•• The feeder current will be unbalanced andThe feeder current will be unbalanced anddistorted.distorted.

•• Voltages of BusVoltages of Bus--2 will also be unbalanced and2 will also be unbalanced anddistorted affecting the loads connected todistorted affecting the loads connected tothese buses. (Assumption Busthese buses. (Assumption Bus--1 is a stiff bus).1 is a stiff bus).

Let the load connectedLet the load connectedto Busto Bus--3 be unbalanced3 be unbalanced

and nonlinear. Thenand nonlinear. Then

UtilityUtility--CustomerCustomer


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Ut tyy Custo eInterfaceInterface -- SolutionsSolutions

•• Customer at BusCustomer at Bus--3 installs a shunt device to3 installs a shunt device tocompensate for the unbalance and distortion.compensate for the unbalance and distortion.

•• Instead the customer pays penalty for notInstead the customer pays penalty for notcomplying.complying.•• However unbalance and distortion will persist.However unbalance and distortion will persist.

•• Alternative: Utility connects a shunt device.Alternative: Utility connects a shunt device.•• BusBus--1 is a stiff bus, not affected by unbalance1 is a stiff bus, not affected by unbalance

or distortion.or distortion.

•• Therefore place a shunt controller at BusTherefore place a shunt controller at Bus--2 for2 forvoltage control.voltage control.

•• This will correct for upstream current as well.This will correct for upstream current as well.

Custom Power DevicesCustom Power Devices


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•• Custom power devices are of two typesCustom power devices are of two types ––those used for isolation & protection andthose used for isolation & protection andthose used for compensation.those used for compensation.

•• Network reconfiguring typeNetwork reconfiguring type–– Static Current Limiter (SCL)Static Current Limiter (SCL)

–– Static Circuit Breaker (SCB)Static Circuit Breaker (SCB)–– Static Transfer Switch (STS)Static Transfer Switch (STS)•• Compensating typeCompensating type

–– Distribution STATCOM (DSTATCOM)Distribution STATCOM (DSTATCOM)–– Dynamic Voltage Restorer (DVR)Dynamic Voltage Restorer (DVR)

––

Unified Power Quality Conditioner (UPQC)Unified Power Quality Conditioner (UPQC)



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Custom Power Devices

•• Static Current Limiter (SCL)Static Current Limiter (SCL) limits a faultlimits a faultcurrent by quickly inserting a seriescurrent by quickly inserting a seriesinductance in the fault path.inductance in the fault path.

•• Static Circuit Breaker (SCB)Static Circuit Breaker (SCB) breaks abreaks afaulted circuit much faster than afaulted circuit much faster than amechanical circuit breaker.mechanical circuit breaker.

•• Static Transfer Switch (STS)Static Transfer Switch (STS) is connectedis connectedin the bus tie position when a sensitive loadin the bus tie position when a sensitive load

is supplied by two feeders. Itis supplied by two feeders. It

protects theprotects the

load by quickly transferring it from theload by quickly transferring it from thefaulty feeder to the healthy feeder.faulty feeder to the healthy feeder.



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•• Distribution STATCOM (DSTATCOM)Distribution STATCOM (DSTATCOM) this isthis isshunt connected device thatshunt connected device that can operate incan operate intwo modes:two modes:

–– Current ControlCurrent Control: In this mode the: In this mode theDSTATCOM acts as an active filter, powerDSTATCOM acts as an active filter, power

factor corrector, load balancer etc. Thesefactor corrector, load balancer etc. Thesefunctions are called the load compensation.functions are called the load compensation.

–– Voltage ControlVoltage Control: In this mode the: In this mode the

DSTATCOM can regulate a bus voltageDSTATCOM can regulate a bus voltageagainst any distortion, sag/swell, unbalanceagainst any distortion, sag/swell, unbalanceand even short duration interruptions.and even short duration interruptions.



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•• Dynamic Voltage Restorer (DVR)Dynamic Voltage Restorer (DVR) is a seriesis a seriescompensating device. It is used forcompensating device. It is used forprotecting a sensitive load that isprotecting a sensitive load that is

connected downstream from sag/swell etc.connected downstream from sag/swell etc.It can also regulate the bus voltage at theIt can also regulate the bus voltage at theload terminal.load terminal.

•• Unified Power Quality Conditioner (UPQC)Unified Power Quality Conditioner (UPQC)this device, like the UPFC, consists of twothis device, like the UPFC, consists of twovoltage source inverters. The capabilitiesvoltage source inverters. The capabilities

of this device are still unexplored. Howeverof this device are still unexplored. Howeverit can simultaneously perform the tasks ofit can simultaneously perform the tasks ofDSTATCOM and DVR.DSTATCOM and DVR.

Static Current Limiter (SCL)Static Current Limiter (SCL)


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( )( )

An SCL is a parallel connection ofAn SCL is a parallel connection of

•• an antian anti--parallel gate turnparallel gate turn--offoff thyristorthyristor (GTO)(GTO)switch withswitch with snubberssnubbers•• a currenta current limtinglimting inductorinductor

•• a zinc oxide (a zinc oxide (ZnOZnO) arrester) arrester

SCLSCL -- OperationOperation


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SCLSCL OperationOperation

•• A GTO can be switched off at any time byA GTO can be switched off at any time byapplying a negative gate pulse.applying a negative gate pulse.•• Therefore it can interrupt a currentTherefore it can interrupt a current

instantaneously.instantaneously.•• AA thyristorthyristor switches off only when theswitches off only when the

current through it changes polarity.current through it changes polarity.

•• An antiAn anti--parallelparallel thyristorthyristor switch is in aswitch is in acurrent limiter will keep on conducting tillcurrent limiter will keep on conducting tillthe next zero crossing irrespective of thethe next zero crossing irrespective of theinstant of occurrence of the fault.instant of occurrence of the fault.

•• This will defeat the purpose for which aThis will defeat the purpose for which acurrent limiter is installed.current limiter is installed.



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pp

•• Under normal (Under normal (unfaultedunfaulted) operating conditions,) operating conditions,thethe GTOsGTOs are gated for full conduction.are gated for full conduction.

•• Once a fault occurs, theOnce a fault occurs, the GTOsGTOs are turned offare turned offas soon as the fault is detected.as soon as the fault is detected.

•• A GTO can respond within a few microseconds.A GTO can respond within a few microseconds.

•• Once theOnce the GTOsGTOs are turned off, the faultare turned off, the faultcurrent is diverted to thecurrent is diverted to the snubbersnubber capacitorcapacitorthat limits the rate of rise in voltage acrossthat limits the rate of rise in voltage acrossthethe GTOsGTOs..

•• The voltage across the antiThe voltage across the anti--parallel GTOparallel GTOswitch rises until it reaches the clamping levelswitch rises until it reaches the clamping level

established by theestablished by the ZnOZnO arrester.arrester.



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pp

•• The same voltage also appears across theThe same voltage also appears across thecurrent limiting reactor.current limiting reactor.•• Once the clamping level of the voltage isOnce the clamping level of the voltage is

reached, the current across the reactor willreached, the current across the reactor willrise linearly.rise linearly.

•• This linear rise will continue till it becomesThis linear rise will continue till it becomes

equal to the instantaneous level of currentequal to the instantaneous level of currentflowing in the line.flowing in the line.

•• Thus the current will be limited by totalThus the current will be limited by total

effective series impedance, i.e., by aeffective series impedance, i.e., by acombination of the impedance of the limitingcombination of the impedance of the limitingreactor and the faulted feeder impedance.reactor and the faulted feeder impedance.



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•• The peak of the fault current is limited toThe peak of the fault current is limited toabout 100 A.about 100 A.

•• The peak of fault current can go up to 1000 A.The peak of fault current can go up to 1000 A.

SCLSCL – – Function ofFunction of

SnubbersSnubbers


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•• Without theWithout the snubbersnubber circuit, the limitingcircuit, the limitinginductor comes in series with the feederinductor comes in series with the feederinductance once the GTO switch is turned off.inductance once the GTO switch is turned off.

•• The initial condition of the limiting inductorThe initial condition of the limiting inductorcurrent is zero, while the feeder currentcurrent is zero, while the feeder currentflows through the feeder reactance.flows through the feeder reactance.

•• However when these two inductance come inHowever when these two inductance come inseries, the current through these twoseries, the current through these twoinductances must be same.inductances must be same.

•• Therefore the limiting inductor must beTherefore the limiting inductor must beforced to instantaneously carry the feederforced to instantaneously carry the feedercurrent.current.

SCLSCL – – Function ofFunction of

SnubbersSnubbers


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•• In the presence of theIn the presence of the snubbersnubber circuit, thecircuit, thefault current is diverted to thefault current is diverted to the snubbersnubber

capacitor once the GTO switch is switched off.capacitor once the GTO switch is switched off.•• The current through the limiting inductor isThe current through the limiting inductor is

allowed build up slowly as discussed before.allowed build up slowly as discussed before.

•• To achieve this, aTo achieve this, alargelarge L L ((di di / / dt dt ) must) must

be applied acrossbe applied acrossthe switch therebythe switch therebycausing a damagingcausing a damaging

voltage spike.voltage spike.

Static Circuit Breaker (SCB)Static Circuit Breaker (SCB)


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•• TheThe GTOsGTOs are theare thenormal current carryingnormal current carryingelements.elements.

•• With the detection ofWith the detection ofa fault, they go througha fault, they go througha number of suba number of sub--cyclecycle

autoauto

reclosereclose

operations.operations.

•• For a persistent fault, theFor a persistent fault, the GTOsGTOs are turned offare turned offand theand the thyristorsthyristors are turned on.are turned on.

•• The fault current now starts flowing throughThe fault current now starts flowing throughthe current limiting inductor.the current limiting inductor.

•• The fault current is eventually cut off byThe fault current is eventually cut off byblocking theblocking the thyristorsthyristors..

SCBSCB – – Alternate Topology Alternate Topology

•• The current in theThe current in the


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•• The current in theThe current in the

normal (normal (unfaultedunfaulted) state) stateflows through the VCB.flows through the VCB.•• With the detection of aWith the detection of a

fault, simultaneously thefault, simultaneously theGTOsGTOs are turned on andare turned on andan open signal is given toan open signal is given to

the VCB.the VCB.•• For high speed contact parting the VCB usesFor high speed contact parting the VCB useselectromagnetic repulsion.electromagnetic repulsion.

•• The fault current starts flowing through theThe fault current starts flowing through theGTO switch and when the current is completelyGTO switch and when the current is completelycommutated, it is interrupted by turning thecommutated, it is interrupted by turning theGTO switch off.GTO switch off.

Coordination Issues with StaticCoordination Issues with Static

Limiting and TransferringLimiting and Transferring


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•• Let 1 is an SCB while 4Let 1 is an SCB while 4is a conventionalis a conventionalbreaker.breaker.

•• For a faultFor a faultdownstream from 4,downstream from 4,breaker 1 will operatebreaker 1 will operate

before breaker 4.before breaker 4.•• This will disconnect both faulty and healthyThis will disconnect both faulty and healthyfeeders supplied byfeeders supplied by T T 11..

•• A potential installation point of an SSB is theA potential installation point of an SSB is thebusbus--tie location 3.tie location 3.•• This will require no coordination with anyThis will require no coordination with any

other protection device.other protection device.




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•• Example, for a fault in the transformerExample, for a fault in the transformer T T 11side of the system, the SSB will open theside of the system, the SSB will open thebus tie thereby preventing transformerbus tie thereby preventing transformer T T 22

from feeding the fault.from feeding the fault.•• An SCB can be connected at location 7.An SCB can be connected at location 7.

••

A fault on the load side can be quicklyA fault on the load side can be quicklyisolated by the SCB without affecting theisolated by the SCB without affecting theother protective devices.other protective devices.

•• The best position for the placement of aThe best position for the placement of acurrent limiter is at the output of the maincurrent limiter is at the output of the mainincoming transformers, i.e., locations 1 and 2.incoming transformers, i.e., locations 1 and 2.




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•• The fault at any part of the network will beThe fault at any part of the network will belimited without causing any coordinationlimited without causing any coordinationproblem.problem.

•• The current tap settings of the downstreamThe current tap settings of the downstreamovercurrentovercurrent relays can then be set at lowerrelays can then be set at lowervalues.values.

•• A limiter at the bus tie location 3 can be mostA limiter at the bus tie location 3 can be mostbeneficial as it will have lower losses underbeneficial as it will have lower losses undernormal operating conditions.normal operating conditions.

•• Since the current flowing through thisSince the current flowing through thisposition for a fault at any part of the circuitposition for a fault at any part of the circuitis maximum, the rating of the device at thisis maximum, the rating of the device at this

location must be very high.location must be very high.

Requirements of SCLRequirements of SCL


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•• It must limit a short circuit current such thatIt must limit a short circuit current such thatthe current does not exceed the interruptingthe current does not exceed the interruptingrating of any downstream protecting device.rating of any downstream protecting device.

•• It must maintain a fault current within aIt must maintain a fault current within aspecified limit till a downstream device clearsspecified limit till a downstream device clearsthe fault.the fault.

•• It must allow sufficient fault current to flowIt must allow sufficient fault current to flowsuch that downstreamsuch that downstream overcurrentovercurrent protectionprotection

devices can isolate the fault.devices can isolate the fault.•• The limiter must reset automatically after aThe limiter must reset automatically after a

fault clearance.fault clearance.

Static Transfer Switch (STS)Static Transfer Switch (STS)


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An STS is used for protecting a sensitive loadAn STS is used for protecting a sensitive load

from sag/swell, fault in the preferred feeder.from sag/swell, fault in the preferred feeder.

STSSTS – – FunctioningFunctioning


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•• Usually the load is supplied by the preferredUsually the load is supplied by the preferredfeeder and the load current flows throughfeeder and the load current flows throughthe switchthe switch Sw Sw 11..

•• When a deep voltage sag or interruption isWhen a deep voltage sag or interruption isdetected in this feeder, the switchdetected in this feeder, the switch Sw Sw 22 isisturned on.turned on.

•• Once the load current starts flowingOnce the load current starts flowingthrough the switchthrough the switch Sw Sw 22 the switchthe switch Sw Sw 11 isisturned off.turned off.

•• This switching action is calledThis switching action is called makemake--beforebefore--break (MBB)break (MBB)..

STS – Make-Before-Break (MBB) Operation


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Load CurrentLoad Current

PreferredPreferredfeederfeedercurrentcurrent

AlternateAlternatefeederfeeder

currentcurrent

STS – MBB OperationDuring Fault


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•• The MBB operation of the STS will cause a faultThe MBB operation of the STS will cause a fault

current to be supplied by the alternate feedercurrent to be supplied by the alternate feederbefore the switchbefore the switch Sw Sw 11 is cut off.is cut off.•• The path for this current is indicated by theThe path for this current is indicated by the

dotted line.dotted line.

STS – Incorrect TransferDuring Fault


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•• The alternate feeder feeds the faultThe alternate feeder feeds the faultthrough switchesthrough switches Sw Sw 11 andand Sw Sw 22..

STS – Correct TransferDuring Fault


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•• The current through switchThe current through switch Sw Sw 11 goes togoes tozero at the next available zero crossing.zero at the next available zero crossing.

•• TheThethyristorsthyristorsof theof theswitchswitch Sw Sw 11

are blockedare blockedas soon asas soon asthe fault isthe fault is

detected.detected.

STS – Correct TransferDuring Fault


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•• The load current stabilizes within aboutThe load current stabilizes within aboutone cycle.one cycle.

•• TheThethyristorsthyristors ofofthe switchthe switchSw Sw 22 arearegated aftergated after

the faultthe faultcurrentcurrentbecomesbecomes

zero.zero.

Sag/Swell DetectionSag/Swell Detection


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•• ForFor subcyclesubcycle transfer, ideally it is desirabletransfer, ideally it is desirableto detect any voltage sag/swell almostto detect any voltage sag/swell almostinstantaneously.instantaneously.

•• This however can only be achieved in theThis however can only be achieved in thecase of balanced sags and cannot becase of balanced sags and cannot beachieved for sag in one or two phases.achieved for sag in one or two phases.

•• The next best option is to detect this withThe next best option is to detect this withas little delay as possible.as little delay as possible.

•• An algorithm is discussed next in which itAn algorithm is discussed next in which itonly takes only two consecutive samples foronly takes only two consecutive samples forsag/swell detection.sag/swell detection.



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•• Let three unbalanced voltage waveforms beLet three unbalanced voltage waveforms bedenoted bydenoted by v v a a ,, v v b b andand v v c c ..•• Their correspondingTheir corresponding phasorphasor values arevalues are

denoted bydenoted by V V a a ,, V V b b andand V V c c ..•• TheThe phasorphasor symmetrical components aresymmetrical components are

(subscripts 0, 1 and 2 respectively represent(subscripts 0, 1 and 2 respectively represent

zero, positive and negative sequences)zero, positive and negative sequences)

°=

=

120

2

2

2

1

0

,~~

~

1

1

111

31

~~

~

j

c

b

a

a

a

a

ea

V

V

V

aa

aa

V

V

V



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•• Let us define the following instantaneousLet us define the following instantaneoussymmetrical componentssymmetrical components

=

c

b

a

a

a

a

v

vv

aa

aa

v

vv

2

2

2

1

0

1

1111

3

1

•• All the three quantities on the left handAll the three quantities on the left hand

side are time varying.side are time varying.•• The vectorThe vector v v a a 22 is complex conjugate of theis complex conjugate of thevectorvector v v a a 11..


•• The instantaneous vectors can be decomposed asThe instantaneous vectors can be decomposed as


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he nstantaneous vectors can be decomposed asmp

( )[ ]

( )[ ] ~~32

~~

32

1

0

∗

∗

−−

=

−−

=

t jt j

a

t jt j

a

e Be F j

v

e H e H j

v

ω ω

ω ω

•• Then it can be shown thatThen it can be shown that

6

~~ and

6

~~,

6

~~

210 BV F V H V aa ===a



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•• The vectorsThe vectors H H ,, F F andand B B can be computedcan be computedfrom two consecutive values of thefrom two consecutive values of theinstantaneous vectorsinstantaneous vectors v v a a 00 andand v v a a 11..

•• These vectors are computed from theThese vectors are computed from theinstantaneous values of the measuredinstantaneous values of the measuredvoltagesvoltages v v a a ,, v v b b andand v v c c ..

•• From the vectorsFrom the vectors H H ,, F F andand B B , the, the phasorphasorzero, positive and negative sequencezero, positive and negative sequence

components can be calculated.components can be calculated.•• From theFrom the phasorphasor sequence components thesequence components thehealth of the system can be determined.health of the system can be determined.


•• The plot shows the transfer operation for anThe plot shows the transfer operation for an


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The plot shows the transfer operation for anThe plot shows the transfer operation for anangle unbalance of 3angle unbalance of 300 in preferred feederin preferred feedervoltage.voltage.


•• The plot shows the transfer operation for anThe plot shows the transfer operation for an


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p f p fp p

11% sag in the magnitude of phase11% sag in the magnitude of phase--bbpreferred feeder voltage.preferred feeder voltage.

STSSTS – – Total Transfer TimeTotal Transfer Time


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•• The total transfer time is the duration fromThe total transfer time is the duration fromthe inception of voltage sag/swell or fault inthe inception of voltage sag/swell or fault inthe preferred feeder to the load beingthe preferred feeder to the load being

completely transferred to the alternatecompletely transferred to the alternatefeeder.feeder.•• The load circuit parameters also influence theThe load circuit parameters also influence the

total transfer time.total transfer time.•• Also the phase difference between the twoAlso the phase difference between the twosupplying source and feeder impedance willsupplying source and feeder impedance will

affect the transfer time.affect the transfer time.•• The gating strategy of the switches in bothThe gating strategy of the switches in both

preferred and alternate feeders can play anpreferred and alternate feeders can play an

important role.important role.

Power Quality Ghosh

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