Lecture 3_Jan 16_2015_Power Electronis Application in Power System

8/9/2019 Lecture 3_Jan 16_2015_Power Electronis Application in Power System

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POWER ELECTRONIC APPLICATIONS

IN POWER SYSTEM

.Professor, Electrical Engineering Department

, -


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PQ PROBLEMS MITIGATION USING

POWER ELECTRONICS CONTROLLERSDC Power System – 1880, Edison, simple to visualize, dealswith real quantities, voltage, current and resistance, transmission

AC Power System – about same period, Nikola Tesla, involves both, .

faces challenging problems of black out, brown out, steady state andtransient stability, reactive power, harmonics power etc. Theseproblems become severe when ac systems were pooled on common

grid.

Due to growing demand of wide variety of load the transmission

lines are increasingly stressed to maintain stability margin andreliability factors. On the other hand, distribution network issubjected to problems of unbalance load, poor power factor,

, .devices for better, reliable, fast and flexible control of power intransmission network and reliable supply of power to the load.


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COMPENSATING DEVICES

Based on the this requirement, the compensating devices can bebroadly classified into two categories .

s ev ces – s contro ers are as ca y g power semiconductor technologies and have been instrumental in the providingfast, reliable and efficient operation of power system [1-4]. The FACTs

. .compensating devices and shunt compensating devices. The FACTsdevices in power system facilitates the following advantages.

Custom Power Devices – The concept of custom power is theemployment of power electronic or static controllers in medium voltagedistribution systems for the purpose of supplying a level of reliability

to power quality variations. Custom power devices, or controllers, includestatic switches, inverters, converters, injection transformers, master control modules, and/or energy storage modules that have the ability toper orm curren n errup on an vo age regu a on unc ons n adistribution system to improve reliability and/or power quality


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POWER CONTROL : CONVENTIONAL METHODS

)(sin 1221

X V V

P

Power transmitted over two ac interconnected link:

Thus power transmitted over ac link can be influenced by threeparameters: Voltage, impedance, and voltage angle difference.The Conventional methods used to control these parametersmechanically are:

A) shunt reactors to improve voltage (V) by injecting reactive power atdesired location

B) switched series capacitors to reduce reactance (X) of transmissionline

-Owing to mechanical nature of control, the above methods providepower flow solution only under steady state or slowly load changingconditions. The dynamic state or fast load changing conditions, the

problems are usually handled by over-design. This results in under utilization of transmission lines.


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FLEXIBLE AC TRANSMISSION SYSTEMs

Kee in the above roblems in mind the ower

system engineers realized that fast and reliablecontrol of ac power using the existing.

best achieved by advance power electronic

based controllers. These are referred as FlexibleAC Transmission System (FACTs).

An im roved utilization of existin transmissionlines is possible through application FACTscontrollers.


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FLEXIBLE AC TRANSMISSION

Schematic FACTs devices


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VARIOUS FACTs CONTROLLERS

Broadly, FACTs controllers based on the type of compensation,can be classified into the following categories.

SHUNT COMPENSATION (control voltage)

SERIES COMPENSATION (control reactance, voltage angle)

SHUNT-SERIES COMPENSATION (control reactance, voltage andangle)


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SHUNT COMPENSATION USING FACTS

A shunt compensation is mainly used for reactive power and voltagecontrol. The static var compensator (SVC) generates or absorbs shuntreactive power at its point of connection.

SVC comprised of 1. Thyristor Controlled Reactor (TCR)2. Thyristor Switched Capacitor (TSC)

3. Fixed Capacitor (FC) + TCR

4. Gate-Turn-Off (GTO)-based voltage sourceconverter (VSC) : STATCOM

L/2 C L/2

T2T1 T2T1

L

T2T1

VSC

L/2

Vdc

TCR TSC FC+TCR STATCOM


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ADVANTAGES of SHUNT

COMPENSATION (SVC)i) Dynamic voltage stabilization: increases power transfer capability,

reduced volta e variation

ii) Synchronous stability improvement: increases transient stability,improved power system damping

iv) Steady state voltage support

► SVCs are rated in such a way that they are capablesystem voltage by at least 5%. Thus they can dynamicallyoperate from 10% to 20% of the short circuit power at pointof common coupling.

► SVCs are generally placed at mid point of high voltage,

substations.


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SERIES COMPENSATION USING FACTS

The principle of the series compensation is to compensate thevoltage drop in the line by an inserting the capacitive voltage or in

line. The voltage series capacitor is proportional and in phasequadrature with the line current. The reactive power support isproportional to the square of the current.

THE ADVANTAGES OFFERED BY SERIES COMPENSATION ARE:1. Steady state voltage regulation and preventing voltage collapse

Capacitive voltage drop is proportional to line current Voltage regulation

By reducing line reactance, it prevents voltage collapse

2. Improving transient stability

Due to series capacitor, X reduces and P- curve shifts upward and this


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SCHEMES OF SERIES COMPENSATIONSeries compensation can be achieved through fixedseries capacitor of controllable series capacitor offeringmore versatility. Two important schemes are given below.

CThyristor Switched Series compensationTSSC comprised of reverse connected thyristor

in shunt with the capacitor. The operating

T2

T1principle is that the degree of seriescompensation is controlled in a step like mannerby increasing or decreasing number of seriescapacitors. The compensation may use number

TSSC

C

of such units.

T1

Thyristor Controlled Series CompensationTCSC is comprised of reactor in parallel with

sections of capacitor bank. The combination allows

TCSC

T2

reactive capacitance over a wide range.


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SERIES COMPENSATION USING FACTS

The modification of voltage magnitudes and/or phase shift by adding acontrol voltage is an important concept. It gives the basis of a new

.

It is comprised of high speed semiconductor switches such as GateTurn Off (GTO) thyristors, voltage source inverter (VSI) – synchronized

.

The application of VSI to inject a phase quadrature voltage in lines

yields a fast controllable phase shifter for active power control.

aV aV

bcV bcV

tat c ync ronous er esCompensator (SSSC)

bV

cV bV

cV

caV

abV aV aV

VSC

Conventional Phase shifter SSSC

Vdc


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SHUNT-SERIES COMPENSATION

USING FACTS: UPFCThe functions of an SSSC (series

(Control of Reactance, Voltage, Angle)

compensa or an a s un

compensator) can be combined toproduce a Unified Power Flow Controller (UPFC).

STATCOM and SSSC share a commondc energy source, which acts as an

energy buffer. In steady state, no energyis drawn from the dc link ca acitor.

The UPFC provides simultaneous, real-time control of all three basic power transfer parameters (voltage, impedance

VSC 2VdcVSC 1

SSSCSTATCOM

an p ase ang e n any com na on ooptimize the transmitted. To compensatecontroller losses, the STATCOM is sooperated that it draws the compensating

UPFC

act ve power rom t e connecte ac us.


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GENERAL ADVANTAGES OFFERED BY

Greater control of power – so that it flows on prescribedransm ss on ne wor

Secure loading of transmission lines – (but not overloading)nearer to their thermal limits

Greater ability to transfer the power between the controlledareas- so that generation reserve margins reduce

Prevention of cascading outages by limiting the effects offaults and equipment failures

Damping of power system oscillations – which could damageequipment and/or limit the usable transmission capacity

Transient Stability – FACTS devices improve transient stability


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HVDC & FACTs

s ron c ac o power ransm ss on

history that after more than half acentury of Edison’s pioneering work, dctransmission was re-invented withmo ern power e ec ron cs ec no ogy osolve the problem of long transmissionline, i.e. high voltage dc transmission(HVDC)

The HVDC technology began in 1954has now grown steadily to the current 600 kV line and about 4000 A capacity.HVDC technolo in turn has rovidedthe basis for the development of FACTs


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WHY HVDC SYSTEMS?

HVDC is used to transmit large amounts of power over lon distances or for interconnections between

e r v ng ac ors o use ec no ogy are:

asynchronous grids

HVDC possesses inherent ability to control transmittedower in efficient and d namic wa

An overhead DC transmission line with its towers can bedesigned to be less costly per unit of length than anequivalent AC line designed to transmit the same level of electric power

Due to higher controllability HVDC systems are alsoused for stabilization of AC network

Import electric energy into congested load areas. Inareas where new generation is impossible to bring intoservice to meet load growth or replace inefficient or decommissioned plant, underground DC cabletransmission is a viable means to import electricity.


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HVDC OPERATIONRdId

dr V diV

Converter

transformer

Converter

transformer

The basic concept in the control of an HVDC transmission is the possibility

to vary direct voltage across the converter by varying the phase position of

c ema c o monopo ar ransm ss on sys em

The firing angle can be changed over the range 0-180 deg. For 0-90 deg.Range, converter acts as rectifier and for 90-180 deg. Range, it acts asinverter.

At rectifier end, voltage is set to a higher direct voltage than that presentedby inverter to make the inverter valve to conduct and to flow the power

The direct current (Id) is forced alternatively into different phase windingsof the transformer connected to the converter operating as inverter and thusactive power is delivered to the ac network


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HVDC TOPOLOGIES-I

Back to Back HVDC

Multi-terminal HVDC


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HVDC TOPOLOGIES-II

Monopolar HVDC system Bipolar HVDC systems

In Monopolar HVDC, dc power is transmitted over single power conductor andground forms the return path. It consists of a single conductor connecting oneor more 12-pulse converter units in series or parallel. For relatively small ratings(50-100MW) back to back links are of monopolar design.

A bipolar HVDC system consists of two 12 pulse units in series with electrodelines (positive and negative). The two conductors change their polarity for bi-direction power flow. Bipolar HVDC systems are used for large interconnections


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HVDC SYSTEMS IN INDIA

HVDC technology as compared toFACTs, is suitably exploited in India. InIndia there are five separate regions, with

a disparity of resource and demand anda wide variation of operating frequencyand voltage on a day-to-day basis. Tosynchronize any two was difficult for tworeasons: the need to maintain stabilityand the organization necessary toschedule power exchange. The needs of resource a ocat on ave owever demanded an ability to transfer power between them. To the large extent, theseproblems were solved using HVDC

ec no ogyIt is found that the grid would be

substantially strengthened through

capacity HVDC and EHVAC lines.


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CUSTOM POWER DEVICES► Like FACTs devices, Custom Power Devices are also power electronic based controllers in medium voltage distribution

power quality that is needed by electric power customerssensitive to power quality variations.

us om power ev ces, or con ro ers, nc u e s a cswitches, inverters, converters, injection transformers, master control modules, and/or energy storage modules that have theabilit to erform current interru tion and volta e re ulationfunctions in a distribution system to improve reliability and/or power quality.

,active power filters/conditioners. A DVR called dynamic voltagerestorer is a series compensation device and used for regulationof bus connected to the sensitive loads


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MITIGATION USING CUSTOM POWER DEVICES

Once PQ problems are known, the next step is to mitigate them. Inthe following sections, the various strategies/techniques of PQproblems mitigation will be highlighted. The ultimate aim of any

problem. The PQ problems mitigation using custom power devicesinvolves the following.

Power factor correction

Harmonics elimination

Unbalance load mitigation

Volta e sa /swell and interru tion

mitigation

Voltage transient mitigation

Flicker mitigation


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CUSTOM POWER DEVICES CLASSIFICATIONn a roa sense cus om power ev ces can e c ass e n o ree

categories:

Shunt devices; generally called as DSTATCOM in currentcontrol mode or shunt active power filter. The main objective is to

inject the current at the point of common coupling in order toprovide:Load balancingPower factor correctionHarmonic eliminationVoltage regulation

genera y ca e as ser es ac ve powerfilter or DSTATCOM in voltage control mode. These are alsoknown as Dynamic Voltage Restorer (DVR). Their main objectiveis to inject the voltage in series with feeder in order to provide theo ow ng:

Voltage regulation (sag/swell)Elimination of voltage harmonics


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DSTATCOM STRUCTURE

DSTATCOM consists of

Schematic of DSTATCOM (a) Conventional (b) Power converter based

Power converter (voltage or current source inverter)DC storage capacitors to support converter operationInterfacing inductors through which DSTATCOM isconnected to the point of common coupling


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POWER CONVERTER TOPOLOGIES

s ng vo age source nver erss ng vo age source nver ersVSIs are lighter, cheaper

elements is more efficienteasily expandable to multi-level versions

Using current source invertersUsing current source inverters

C Is are more reliable and ault tolerant

Voltage source inverter

well suited for accurate control

DisadvantagesHigher losses, higher initial cost

VSIs are however referred over CSIs,because of the VSIs are more efficient,

lower in initial cost than the CSIsCurrent source inverter


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Classification of Active Power Filters

Supply systems influence the choice of filter topology wo w re supp y

Three phase three wire supplyDelta- connected loadStar-connected load

3-phase, 4-wire supply

Active power filter topology

Series active power filterSeries active and shunt passive power filter (Hybrid filters)Unified ower qualit conditioner (UPQC)

Implementation uses following types of convertersVoltage source inverters (VSIs)

Active power filter with hybridenergy source


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Shunt active power filter [Akagi 94] :The aim of the compensator is to inject the

TOPOLOGIES

armon c currents to cance out t e armon ccurrents of the load.mainly used at load distribution centres.

LOADlisis

v

f i

widely used configuration to eliminate theharmonics in currents due to nonlinear loads

Series active ower filter [DVR] :

cvThe compensator is connected before theload in series with the mains through amatching transformer.

used to eliminate the voltage distortions and tobalance and regulate the terminal voltage.The Dynamic Voltage Restorer is a

LOADlisi

s f V _ +

par cu ar mp emen a on or e m na ngvoltage swells and sags on sensitive equipment.

propagation caused by resonance with the line

impedance and passive shunt filters cv


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Hybrid Power Active FilterHybrid Power Active FilterTOPOLOGIES

The required rating of the series active filter isconsiderably smaller than that of conventional one

lisisv

f V _ +

ma n y use at oa str ut on centres.

widely used configuration to eliminate theharmonics in currents due to nonlinear loads

Shunt passivefilter

cvUnified Power Quality ConditionerUnified Power Quality Conditioner

LOADlisi

sv f V _ +

f i

Combination of both active shunt and activeseries filtershares sin le common ca acitor inductor

Hybrid

cv

However they are expensive and theircontrol is rather complex because of large

er es ac ve

filter

un ac ve

filter number of switching devices and thecoordination between the two modes

UPQC


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CONTROL ALGORITHMS FOR ACTIVE SHUNTCONTROL ALGORITHMS FOR ACTIVE SHUNT

The FFT techniquesThe FFT techniques

The capacitor error voltage and PI controllerThe capacitor error voltage and PI controller

Instantaneous reactive power theory(pInstantaneous reactive power theory(p--q theory)q theory)Generalized reactive power theoryGeneralized reactive power theory

Theor of instantaneous s mmetrical com onentsTheor of instantaneous s mmetrical com onents


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REALIZATION OF DSTATCOMREALIZATION OF DSTATCOM


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SOME IMPORTANTSOME IMPORTANTreeree - - ase opo og esase opo og es

3-phase, 4-wire compensated distributionsystem with generalized active power filter


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Three-phase, three-leg topologyw c s orage capac ors

T0 PCC

a cb

S1a

S3a

S4a S

1a

+

-

vc1

S1b

S3b

S4b

S1b

+

-v

c2

n'

S1c

S3c

S4c S

1c

+

-v

c3

(a) It uses three capacitors and it is very difficult to regulatethe capacitor voltages

(b) It uses 12 power switches.


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Three Independent Single-phase VSI suppliedfrom a common dc stora e ca acitor

Each VSI is connected to the network through a transformer

It contains three H-bridge VSIs that are connected to a commonc s orage capac or

The purpose of including the transformers is to provide isolation betweenthe inverter legs. This prevents the dc storage capacitor from being shorted

LOAD

sai laiPCC

through switches in different inverters

LOAD

LOAD

faiscv

e opo ogy owever s nosuitable for compensation ofloads containing dc components inaddition to ac com onents in the

dcC VSIsload current. The presence ofisolation transformers does notallow the dc component of the


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THREE-PHASE, THREE-LEG TOPOLOGY

LOAD

LOAD

sai laiPCCThe structure consists of

three-phase VSI connected to

faiscv scicommon c capac tor

The topology is not suitable

for loads containing zeroC

CSIs

sequence curren s as zerosequence currents through thepath N-n.


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NEUTRAL CLAMPED INVERTER TOPOLOGY

sai

sbi

sci

sav

sbv

scv

lai

lbiNLOAD

LOAD

lci

It uses two identicalcapacitors and the neutral of

3S 5S 1C

+

1 cv

fai fbi fci1i

1S R f

VSI

i

source an oa s c ampe at t ecommon point of the capacitor.

The topology does not use thea

c

-

2C

4S 6S 2S ch

L b

f

'n+

-2

cv

so a on rans ormer encecan be used so dc offset in theload current can be compensated,rovided the two ca acitors

2i

voltage are at their referencevalue.


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3-PHASE 4-LEG TOPOLGY

When the compensator is working,zero sequence current is routed to athn-n’ containing switching frequencyharmonics.

Using fourth leg of the inverter, thiscurrent is tracked to prevent anycurrent to flow in the supply current.

How the current in path n-n’ containsunbalance, harmonics and also theswitching frequencies of the inverter.So tracking this current is extremely

.high bandwidth. This not only increasesthen cost but also the switching lossesin the inverter.


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NEUTRAL CLAMPED INVERTERCHOPPER TOPOLOGY

This topology was proposed as apart of my research work and calledas neutral clamped inverter chopper

sai

sbi

sav

sbv

v

lai

lbiN n

LOAD

LOAD

topology.

It can compensate the unbalance,harmonics and dc offsets in load

S S +

fai fbi fci

scsc lc

S

1i

LOAD

S R

VSIChopper currents.

Due to dc offset current in load

currents, the dc currents also passoi

ac

-1 cv

8 D

7

4S 6S 2S ch

R ch L b

chi

L f 'n

+ cv

t roug t e c capac tors. evoltage across capacitors ismaintained constant by separate PIcontrol loo . But due to DC o set

8S 2C

2i

-

current the voltage of individualcapacitors drift.

Volta e drift roblem ofcapacitors is solved using choppercircuit.


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DYNAMIC VOLTAGE RESTORER

Hardware structure wise, DVRonly slightly differs fromDSTATCOM

DVR is connected intodistribution system via a seriesconnected transformer which alsoprovi es iso ation. Due to itsseries connected topology, DVRprovides complimentary set of

DSTATCOM.

DVR is capable of injectingcontinuousl var in series volta e

A DVR operation under a voltage dip condition

without taking any real power. Thisis achieved by injecting voltage inphase quadrature as indicated in

e agram

SOLID STATE BREAKERS (SSB)


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SSB device is based on the gate turn-off

SOLID STATE BREAKERS (SSB)

thyristor, which utilizes several anti-parallelpairs of the switches which are seriesconnected to achieve line rating.

SSB does not operate in the same way as aconventional circuit breaker. It interrupts faultcurrents by monitoring both steady currentand rate of change of current, and onlyinterrupts when the onset of a fault is

.

The SSB in its present form is not likely toreplace the conventional circuit breaker.

However it has a number of applications,provide uninterrupted power by providingrapid transfer to a secondary feeder or limitreactive in-rush currents by pulse widthmodulating the current.

Schematic of solid state state breaker

SSB consists of three elements: The GTO element is the normal current carrying elementand in the event of a fault will go through a number of sub-cycle auto-reclose operations.If this is not sufficient to clear the fault, the GTO element goes open circuit and fault

,operate. To protect power electronics devices a zinc oxide arrestor is used to shortcircuit the device in the event of lightening or switching transient.


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SOLID STATE TRANSFER SWITCH (STS)

SUMMARY


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SUMMARY

A wide range of compensating devices to mitigatePQ problems are discussed.

The modern compensators are power electronicbased controllers which are very fast and accuratein operation. These compensators are classifiedinto two categories i.e. FACTs devices and custompower devices.

HVDC technology, Flexible AC TransmissionTechnology is not sufficiently exploited in India andthere is much scope for the applications of these

. . , ,level for efficient and reliable control of power.

Documents

Lecture 3_Jan 16_2015_Power Electronis Application in Power System