Dvr and Upqc

8/13/2019 Dvr and Upqc

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DayDay--44

Custom PowerCustom Power

Compensating DevicesCompensating DevicesArindam 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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Load Compensation usingLoad Compensation using

DSTATCOMDSTATCOMThe primary goals of a DSTATCOM areThe primary goals of a DSTATCOM are to cancel the effect of poor load power factorto cancel the effect of poor load power factor

such that the current drawn from the sourcesuch that the current drawn from the sourcehas a near unity power factor.has a near unity power factor.

to cancel the effect of harmonic contents into cancel the effect of harmonic contents inloads such that current drawn from theloads such that current drawn from thesource is nearly sinusoidal.source is nearly sinusoidal.

to offset the effect of unbalanced loads suchto offset the effect of unbalanced loads suchthat the current drawn from the source isthat the current drawn from the source isbalanced.balanced.

In addition, it can also eliminate dc offset inIn addition, it can also eliminate dc offset inloads.loads.


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DSTATCOMDSTATCOM -- FunctionFunction

Inject a set of currents such that theInject a set of currents such that thesource currents aresource currents are balanced andbalanced and

sinusoidal.sinusoidal. If the load is unbalanced then the zeroIf the load is unbalanced then the zero--sequence current circulates in the pathsequence current circulates in the path

joining the neutral of the load (joining the neutral of the load (nn) and the) and thecompensator (compensator (nn) such that the current) such that the currentflowing through the neutral (flowing through the neutral (NN) is zero.) is zero.

Additionally power factor correction can alsoAdditionally power factor correction can alsobe performed such that the DSTATCOMbe performed such that the DSTATCOMsupplies part or whole of the reactive powersupplies part or whole of the reactive power

required by the load.required by the load.


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DSTATCOMDSTATCOM

Reference GenerationReference Generation

Generate a set of reference currentsGenerate a set of reference currents iifafa**,,iifbfb

** andand iifcfc**..

These currents when injected into theThese currents when injected into thesystem will simultaneously perform thesystem will simultaneously perform thefollowing three tasks.following three tasks.

a)a) Force the zeroForce the zero--sequence source current tosequence source current tobe zero, i.e.,be zero, i.e.,

0=++ scsbsa iii

This guarantees that the zeroThis guarantees that the zero--sequencesequencecurrent flowing through the source neutralcurrent flowing through the source neutral

is zero in a 3is zero in a 3--phase, 4phase, 4--wire systemwire system


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DSTATCOMDSTATCOM


b)b) Force the angle between the source voltageForce the angle between the source voltageand current to be a specified value, i.e.,and current to be a specified value, i.e.,

( ) ( )( ) ( )

( ) ( )

+=

+=

+=

scsc

sbsb

sasa

iv

iv

iv

In the above,In the above, is preis pre--specified.specified. IfIf = 0, the source voltage and load= 0, the source voltage and load

current are in phase.current are in phase. This will be calledThis will be called

the unity power factor (the unity power factor (upfupf) operation.) operation.


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DSTATCOMDSTATCOM ReferenceReference

GenerationGeneration

c)c) The DSTATCOM must not absorb/generateThe DSTATCOM must not absorb/generateany real power in the steady state, i.e., it onlyany real power in the steady state, i.e., it onlysupplies zerosupplies zero--mean oscillating power.mean oscillating power.


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The power condition can be written asThe power condition can be written as

lavscscsbsbsasa piviviv =++

In the aboveIn the abovepplavlav is the average power drawnis the average power drawnby the load.by the load.

To calculated it on line, 3 HallTo calculated it on line, 3 Hall--effect voltageeffect voltageand 3 current transformers are required.and 3 current transformers are required.

A lowA low--passpass ButterworthButterworth (or(or ChebyshevChebyshev) filter) filtercan be used. It will have longer settling time.can be used. It will have longer settling time.

Alternatively a moving average filter settlesAlternatively a moving average filter settles

in just half (or full) cycle.in just half (or full) cycle.


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SinceSincecbakiii sklkfk ,,, ==

The reference currents areThe reference currents are( )

( )

( )

3tan,

222

222

222

++

+=

++

+=

++

+=

lav

scsbsa

sbsasclcfc

lav

scsbsa

sascsblbfb

lav

scsbsa

scsbsalafa

pvvv

vvvii

p

vvv

vvvii

p

vvv

vvvii


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DSTATCOMDSTATCOM IdealIdeal

CompensationCompensation

In a similar way the algorithm can be applied toIn a similar way the algorithm can be applied to

3p, 3w systems with both star and delta3p, 3w systems with both star and deltaconnected loads.connected loads.

== -- 303000

= 30= 3000


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DSTATCOM StructureDSTATCOM Structure

A DSTATCOM is realized by a VSC. The VSCA DSTATCOM is realized by a VSC. The VSCis supplied by a dc capacitor. Theis supplied by a dc capacitor. TheDSTATCOM must meet the followingDSTATCOM must meet the following

requirements:requirements: It should be able to inject distorted andIt should be able to inject distorted andnegativenegative--sequence currents.sequence currents.

It must be able to provide a circulating pathIt must be able to provide a circulating pathfor the zerofor the zero--sequence load current suchsequence load current suchthat it does not travel towards the source.that it does not travel towards the source.

It must be able to regulate the voltage dcIt must be able to regulate the voltage dccapacitor such that no external batterycapacitor such that no external batterysource is required.source is required.


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A common dc capacitor makes the distributionA common dc capacitor makes the distributionof power easier.of power easier.

The transformers provides isolation in additionThe transformers provides isolation in additionto voltage step down.to voltage step down.

My favoriteMy favoritestructurestructure


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Suitable for dc compensation.Suitable for dc compensation. Two capacitors (Two capacitors (CCdcdc11 andand CCdcdc22) required.) required.

An additional chopper circuit containingAn additional chopper circuit containing SSchch11,,SSchch22,, RRpp andand LLpp required.required.


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CurrentCurrent IIPP builds up inbuilds up in LLpp in the directionin the direction

shown.shown. Also the voltageAlso the voltageVVdcdc22 falls.falls. OnceOnce II

PPbuilds up to a desired level,builds up to a desired level, SS

chch22isis

opened.opened. The currentThe currentIIPP discharges throughdischarges through DDchch11

charging the capacitorcharging the capacitor CCdcdc11.. The voltageThe voltage VVdcdc11 rises.rises.

Normally the switchesNormally the switchesSSchch11 areare SSchch22 open.open. SupposeSuppose VVdcdc11 drops anddrops and

VVdcdc22 rises.rises. SSchch22 is closed.is closed.

Ip


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Zero sequence current is routed to pathZero sequence current is routed to path nnnn.. This must be supplied by the 4This must be supplied by the 4thth leg.leg.

Complexity and switching losses increase.Complexity and switching losses increase.


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DSTATCOM ControlDSTATCOM Control

The reference current equations also getThe reference current equations also getmodified in whichmodified in whichpplavlav is replaced by the sumis replaced by the sum

pplavlav ++pplossloss..

losslavscscsbsbsasa ppiviviv +=++

The dc voltage supplying the DSTATCOMThe dc voltage supplying the DSTATCOMmust be held constant irrespective of themust be held constant irrespective of thelosses in the circuit.losses in the circuit.

This can be accomplished by drawingThis can be accomplished by drawingpower from the ac system to compensatepower from the ac system to compensatefor the losses.for the losses.

The power equation then must be modifiedThe power equation then must be modifiedto incorporate the losses asto incorporate the losses as


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DSTATCOM ControlDSTATCOM Control

VVrefref is the reference dc voltage andis the reference dc voltage and VVdcdc,,avav isisthe average dc voltage.the average dc voltage.

Any deviation of the capacitor voltage fromAny deviation of the capacitor voltage fromthe reference is due to losses.the reference is due to losses. The PI controller loop draws the loss fromThe PI controller loop draws the loss from

the ac system to hold the voltage constant.the ac system to hold the voltage constant.

avdcref

IPloss

VVe

dteKeKp

,=

+=

HereHerepplossloss is generated by dc capacitoris generated by dc capacitorcontrol loop.control loop. A PI controller is used for generating thisA PI controller is used for generating this

DSTATCOMDSTATCOM


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DSTATCOMDSTATCOM

HysteresisHysteresis ControlControl

The reference currents are tracked in aThe reference currents are tracked in a

hysteresishysteresis band.band.

iDSTATCOM O ti


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Simulation results of the DSTATCOM operation.Simulation results of the DSTATCOM operation.

DSTATCOM OperationDSTATCOM Operation

DSTATCOM O tiDSTATCOM O ti


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DSTATCOM OperationDSTATCOM Operation

SimulationSimulation

ExperimentalExperimental

DSTATCOM Connected toDSTATCOM Connected to


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weak ac supply pointweak ac supply point

The a load is supplied by a feeder.The a load is supplied by a feeder. The DSTATCOM is connected at PCCThe DSTATCOM is connected at PCC

((terminalterminal) at the end of the feeder.) at the end of the feeder.



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The reference generation algorithm assumesThe reference generation algorithm assumesbalanced supply.balanced supply.

The left figure shows when the measuredThe left figure shows when the measuredterminal voltages are used and the right figureterminal voltages are used and the right figureshows when fundamental voltages are used.shows when fundamental voltages are used.



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To bypass the switch frequency harmonicTo bypass the switch frequency harmonic

generated, capacitorgenerated, capacitorCCff is used (left).is used (left). TheThe hysteresishysteresis tracking oftracking of iiff results in anresults in an

unstable feedback system (right).unstable feedback system (right).

Alternate control strategy required.Alternate control strategy required.

DSTATCOMDSTATCOM S it hi C t lS it hi C t l


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DSTATCOMDSTATCOM Switching ControlSwitching Control

One approach is state feedback control usingOne approach is state feedback control using

linear quadratic regulator (LQR).linear quadratic regulator (LQR). This will require onThis will require on--line generation of all theline generation of all the

reference states.reference states.

The LQR design is robust to parameterThe LQR design is robust to parameteruncertainties.uncertainties.

The output feedback approach is simpler butThe output feedback approach is simpler butcannot guarantee stability.cannot guarantee stability.

BuAxx +=Kref

+

DSTATCOMDSTATCOM Performance forPerformance for


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DSTATCOMS CO Performance fore o a ce o

NonNon--Stiff SystemsStiff Systems

With LQR control the source currents becomeWith LQR control the source currents becomesinusoidal.sinusoidal.

To achieve this, the PCC (terminal) voltagesTo achieve this, the PCC (terminal) voltagesare forced to be sinusoidal.are forced to be sinusoidal.

DSTATCOMDSTATCOM Performance forPerformance for


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NonNon--Stiff SystemsStiff Systems

The unbalance and harmonics in the sourceThe unbalance and harmonics in the source

voltages get reflected in the source currents.voltages get reflected in the source currents.

DSTATCOM in VoltageDSTATCOM in Voltage


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DSTATCOM in VoltageDSTATCOM in Voltage

Control ModeControl Mode

TheThe TheveninThevenin equivalent of the system looking atequivalent of the system looking at

BusBus--3 is the same as those shown in Slides 2453 is the same as those shown in Slides 245or 247.or 247.

We can then use a state feedback or outputWe can then use a state feedback or output

feedback controller to hold the voltage of Busfeedback controller to hold the voltage of Bus--3.3.

Let us assume thatLet us assume thatwe would like towe would like to

correct the voltagecorrect the voltageof Busof Bus--3.3.

Voltage Control ModeVoltage Control Mode


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( ) ( ) += dtvvKvvKp

dcavdrefIdcavdrefpshref

The magnitude of the bus voltage can beThe magnitude of the bus voltage can bechosen arbitrarily.chosen arbitrarily.

The phase angle (The phase angle () should be controlled to) should be controlled toensure the power flow.ensure the power flow. Note that the dc capacitor must be able toNote that the dc capacitor must be able to

supply the DSTATCOM while maintaining itssupply the DSTATCOM while maintaining itsvoltage by drawing power from the ac system.voltage by drawing power from the ac system. Two PI controllers are designed in an innerTwo PI controllers are designed in an inner

power and outer voltage loop configuration.power and outer voltage loop configuration. The outer loop regulates the dc voltage asThe outer loop regulates the dc voltage as



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The inner loop set the angle by adjustingThe inner loop set the angle by adjustingthe power difference asthe power difference as

( ) ( ) += dtppCppCshavshrefIshavshrefP



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o tage Co t o odeg

ResultsResults

The source isThe source isbalanced but thebalanced but theload is unbalancedload is unbalanced

& distorted.& distorted. The uncompensatedThe uncompensatedbus voltage (bus voltage (vvtata) is) is

distorted (topdistorted (toptrace).trace). The compensatedThe compensated

bus voltage (bus voltage (vvtata) is) isdistortion freedistortion free(bottom trace).(bottom trace).


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gg

ResultsResults

The sourceThe sourcevoltages arevoltages are

unbalanced (topunbalanced (toptrace) and thetrace) and theload is bothload is both

unbalanced andunbalanced anddistorted.distorted.

The DSTATCOMThe DSTATCOM

makes the busmakes the busvoltages balancedvoltages balanced(bottom trace).(bottom trace).

Dynamic Voltage RestorerDynamic Voltage Restorer


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Dynamic Voltage RestorerDynamic Voltage Restorer

(DVR)(DVR)

In addition, it can also tightlyIn addition, it can also tightlyregulate the voltage at theregulate the voltage at the

load bus.load bus.

A DVR is aA DVR is a

seriesseriesconnectedconnecteddevice.device.

It protectsIt protectssensitive loadssensitive loadsfrom sag,from sag,

swell orswell orsource voltagesource voltagedistortion.distortion.

DVRDVR Voltage SupportVoltage Support


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DVR Voltage Supporto age uppo

A fault in either locations shown will results inA fault in either locations shown will results ina voltage sag in the feeder supplying the load.a voltage sag in the feeder supplying the load. The DVR adds a voltage in series to offset theThe DVR adds a voltage in series to offset the

sag.sag.

SENSITIVE

LOAD

DVR

DVR InstallationsDVR Installations


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DVR InstallationsDVR Installations

In August 1996, Westinghouse Electric CorporationIn August 1996, Westinghouse Electric Corporationinstalled world's first DVR in Duke Power Company'sinstalled world's first DVR in Duke Power Company's12.47 kV substation in Anderson, South Carolina to12.47 kV substation in Anderson, South Carolina toprotect to an automated rug manufacturing plant.protect to an automated rug manufacturing plant.

The next commissioning of a DVR was done byThe next commissioning of a DVR was done byWestinghouse in February 1997 inWestinghouse in February 1997 in Powercor'sPowercor's 22 kV22 kVdistribution system at Stanhope, Victoria, Australia.distribution system at Stanhope, Victoria, Australia.

This was done to protect a diary milk processingThis was done to protect a diary milk processingplant.plant. The saving that may result from the installation ofThe saving that may result from the installation of

this DVR is estimated at over $100,000 per year.this DVR is estimated at over $100,000 per year.

The first ever platform mounted DVR was installedThe first ever platform mounted DVR was installedto protect Northern Lights Community College andto protect Northern Lights Community College andseveral other smaller loads in Dawson Creek, Britishseveral other smaller loads in Dawson Creek, British

Columbia, Canada.Columbia, Canada.

The Study SystemThe Study System


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The Study Systeme Study Syste

The DVR is represented by voltage sourcesThe DVR is represented by voltage sources vvfafa,,vvfbfb andand vvfcfc..

Zla

Zlb

Zlc

n

vsa

vsb

vsc

v a

v b

v c

vt vl

isa

isb

isc

+

+

+

+

+

+

+ +

PCC

DVR StructuresDVR Structures


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The dc storage is one important issue in theThe dc storage is one important issue in theDVR design.DVR design.

Schematic diagrams of (a) rectifier supportedSchematic diagrams of (a) rectifier supportedand (b) capacitor supported are shown.and (b) capacitor supported are shown.

Other possibility is battery back up/support.Other possibility is battery back up/support.


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CapacitorCapacitor

Supported DVRSupported DVR


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Supported DVRSupported DVR

The compensator power is zero in the steadyThe compensator power is zero in the steady

state.state.

Capacitor SupportedCapacitor Supported

DVR ImplementationDVR Implementation


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DVR ImplementationDVR Implementation

The DVR on the left has a filter capacitor inThe DVR on the left has a filter capacitor inparallel with the transformer secondary.parallel with the transformer secondary.

The DVR on the right has an LC filter connectedThe DVR on the right has an LC filter connected

in the primary side of the transformer.in the primary side of the transformer.

Capacitor SupportedCapacitor Supported

DVR LimitationsDVR Limitations


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DVR LimitationsDVR Limitations

Without active power exchange with the acWithout active power exchange with the acsystem, the capacity of the DVR to compensatesystem, the capacity of the DVR to compensate

a deep sag gets limited.a deep sag gets limited.

Rectifier SupportedRectifier Supported

DVRDVR


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DVRDVR

Power flow is unidirectional, i.e., from thePower flow is unidirectional, i.e., from therectifier to DVR and not the other wayrectifier to DVR and not the other way

round.round.

Difficult for the DVR to absorb powerDifficult for the DVR to absorb powertransiently.transiently.

During deep sag, if the rectifier outputDuring deep sag, if the rectifier outputcapacitor voltage exceeds the peak of thecapacitor voltage exceeds the peak of theinput ac voltage, the rectifier will cut off.input ac voltage, the rectifier will cut off.

The rectifier generates harmonics at itsThe rectifier generates harmonics at itsinput terminals.input terminals. Solutions: Use UPQC.Solutions: Use UPQC.

Unified Power QualityUnified Power Quality


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Unified Power QualityUnified Power Quality

Conditioner (UPQC)Conditioner (UPQC)


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Custom Power ParkCustom Power Park


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Custom Power ParkCustom Power Park

Mini Custom Power ParkMini Custom Power Park


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L-1 L-2 L-3DSTAT

1sv 2svPF AF

STS

CPP BusB1 B2 B3 B4

DG

Cf

STS is GTO based.STS is GTO based. B1, B2 areB1, B2 are SCBsSCBs, while B3, B4 are mechanical., while B3, B4 are mechanical.

Mini Custom Power ParkMini Custom Power Park


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Loads LLoads L--2 and L2 and L--3 and the CPP bus voltage are3 and the CPP bus voltage arebalanced sinusoidal irrespective of Lbalanced sinusoidal irrespective of L--1 being1 being

unbalanced and distorted.unbalanced and distorted.


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Thank youThank you

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Dvr and Upqc