CERC Staff Paper

8/6/2019 CERC Staff Paper

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GridSecurityNeedForTighteningofFrequencyBand&OtherMeasure

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CentralElectricityRegulatoryCommission

NewDelhi

GRIDSECURITYNEEDFORTIGHTENINGOFFREQUENCYBAND

&OTHERMEASURES

CERCSTAFFPAPER

March2011


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GRIDSECURITYNEEDFORTIGHTENINGOFFREQUENCYBAND

&OTHERMEASURES

CERCSTAFFPAPER

1. IntroductionPower frequency reflects the loadgenerationbalance in thegridataparticular instant.Frequency is

oneofthemostimportantparametersforassessmentofthesecurityofpowersystemandthequality

ofpowersupplyinanygrid.Itistobemaintainedwithinthespecifiedrangeinwhichalltheelectrical

equipmentsaredesignedtoperformsafelyandefficiently.Handling imbalances isan integralpartof

market design. In India, the balancing market is frequency dependent and market design must

complementreliability.Thisdiscussionpaperfocusesongridfrequencyandotherissuesrelatedtogrid

securityinIndia.

2. PowerSupplyScenarioinIndiaIndiaisoneofthelargestsynchronousinterconnectionsintheworld.Theinstalledgenerationcapacity

ason31stJan2011 is170.23GW.Therevisedtargetforgenerationcapacityaddition in11thfiveyear

plan is 62 GW. The projected peak shortage and energy shortage in March 2012 is 6.5% and 2%

respectively.


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Table1:AnticipatedPowerSupplyScenarioin2012

Period Peak

Demand

(MW)

Peak

Availability

(MW)

Deficit()/

Surplus(+)

(MW)

Deficit()/

Surplus(+)

(%)

201112 152746 142765 9981 6.5

Energy

Requirement

(MU)

Energy

Availability

(MU)

Deficit()/

Surplus(+)

(MU)

Deficit()/

Surplus(+)

(%)

201112 968659 948836 19823 2%

Demandasper17th

ElectricPowerSurvey

Source:PowerScenarioataGlance,January2011, CEA

3. StandardsforPowerFrequencyinIndiaAspertheIndianElectricityRules1956(amendedupto25

thNov2000),thepermissiblerangeforgrid

frequency was +/ 3 % of nominal i.e. 48.5 Hz to 51.5 Hz. The permissible frequency ranges (by

manufacturers) for operation of various makes of steam turbine are shown in Figure 1. The nominal

frequencyofoperationinIndiangridis50.0HzandthepermissiblefrequencybandspecifiedbyIndian

ElectricityGridCode(IEGC)is49.5Hzto50.2Hzw.e.f3rd

May2010.


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Thenormaloperatingfrequencyrangeallowedbythe IndianElectricityGridCodetill31stMarch2009

was49.0to50.5Hz.ThefrequencybandwastightenedbyCERC insubsequentamendmentsto IEGC.

Thefrequencybandw.e.f3rd

May2010is49.5Hzto50.2Hz.

Table3:OperatingrangeforfrequencyspecifiedinIEGC

SNo. Period OperatingRange(inHz)

1 Till31stMarch2009 49.0to50.5

2 1stApril2009to2

ndMay2010 49.2to50.3

3 w.e.f3rd

May2010 49.5to50.2

SimilarlytheUImechanismhasbeenrevisedseveraltimesinthepasttoimprovethefrequencyprofile

inthegrid.ThevariousstagesofevolutionoftheUImechanismhasbeendisplayedasAnnexure.The

continuousimprovementinthegridfrequencyprofileisevidentfromthefrequencyprofilerecordedin

theNorthEastWestgridinthepastfewyearsasdisplayedinfigure2and3below.

ER / NEW GRID MAXIMUM AND MINIMUM FREQUENCY JANUARY'98 ONWARDS

47.0

47.5

48.0

48.5

49.0

49.5

50.0

50.5

51.0

51.5

52.0

52.5

53.0

53.554.0

Jan-

98

Jan-

99

Jan-

00

Jan-

01

Jan-

02

Jan-

03

Jan-

04

Jan-

05

Jan-

06

Jan-

07

Jan-

08

Jan-

09

Jan-

10

Jan-

11

Date -->

Hz-->

MAXIMUM MINIMUM

Figure1:MaximumandminimumfrequencyrecordedinNEWgrid

MaximumFrequency

MinimumFre uenc


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SR GRID MAXIMUM, MINIMUM FREQUENCY PLOT FROM APRIL-2001 ONWARDS

47.5

48.0

48.5

49.0

49.5

50.0

50.5

51.0

51.5

Apr-01

Sep-01

Mar-02

Sep-02

Mar-03

Sep-03

Mar-04

Sep-04

Mar-05

Sep-05

Mar-06

Sep-06

Mar-07

Aug-07

Feb-08

Aug-08

Feb-09

Aug-09

Feb-10

Aug-10

Feb-11

DAY ---->

FREQUENCY

IN

HZ--->

MAXIMUM MINIMUM

Figure2:MaximumandMinimumfrequencyrecordedinSRgrid

5. SignificantdevelopmentsandconcernsinIndiangridinnearfutureThepowersysteminIndiaisexpandingatafastpacetomeettherequirementsofIndianeconomy.Few

significantdevelopmentsinthepowersystemthatnecessitatediscussiononthefrequencystandardsin

theIndiangridhavebeenhighlightedbelow.

a) GrowingsizeofInterconnectionIndian grid is presently demarcated into five regional grids. Four out of the five grids (except

Southern Grid) are operating in synchronism since August 2006. There are plans to integrate the

Southern Grid with the rest of the grid through synchronous ties in near future (presently it is

through asynchronous HVDC link). Indian grid is also striving to expand by establishing

interconnectionwithneighbouringcountries.Bhutanisalreadysynchronouslyinterconnectedwhile

NepalhasseveralasynchronoustieswiththeIndiangrid(ACradiallinks).Aprojectforestablishing

asynchronous ties with Bangladesh through HVDC backtoback link is already under progress. A

MaximumFre uenc

MinimumFrequency


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tighter operating band for frequency is essential for secure operation of a large synchronous

interconnection.

b) SizeofGeneratingUnitsandgenerationcomplexPresently the size of the largest generating unit in India is 660 MW and very soon 800 MW unit

(Mundra UMPP) and 1000 MW unit (Nuclear unit at Kudunkulam) are also expected to be

synchronized.

Table4:Populationofdifferentsizeofgenerators

SNo. UnitSize Numberofunits

1 800MW and>800MW Expectedsoon

2 660MW 2

3 600MW 4

4 540/500/490MW 45

5 300/330/380MW 13

6 250/210/200MW 244

7 140/150/170/185MW 74

8 100/110/120MW 205

9 Lessthan100MW 983

10 Total 1570

Table5:Numberoflargegenerationcomplex

SNo. Sizeofgenerationcomplex Number

1 4000MWandabove UMPPsexpectedsoon

2 2000MWto4000MW 7

3 1000MWto2000MW 36

4 Below1000MW 325

5

Total

368


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Table6:Incidentsofcompletepowerstationtrip(May2010toJan2011)

S

No.

Generation

loss Number

of

instances

1 >2000MW TrippingofstationssuchasKorbaand

VindhyachalbeforeMay2010

2 1000MWto2000MW 4

3 500MWto1000MW 3

The system is generally planned for outage of single largest unit outage. However number of

incidentswherethecompletepowerstationhastrippedisalsosignificantlyhigh.Infactalmostall

largepowerstationsinthecountryhavegoneunderforcedoutageatleastonce.Thegridfrequency

needs to be maintained at level so that the system is able to bear the impact of such large but

credible contingencies, even if it is with the help of suitable protection schemes such as load

sheddingschemes initiatedbyUnder frequency,RateofChangeof frequencyandUnderVoltage.

Thesettingofautomatic loadsheddingthroughunderfrequencyandrateofchangeoffrequency

relaysisshowninTablesbelow.

Table7:UnderFrequencyRelaysettingsadoptedinIndia

Region StageI StageII StageIII

NorthernRegion 48.8 Hz 48.6Hz 48.2Hz

WesternRegion 48.8Hz 48.6Hz 48.2Hz

EasternRegion 48.5Hz 48.2Hz 48.0Hz

Northeasternregion 48.8Hz 48.5Hz 48.2Hz

SouthernRegion 48.8Hz 48.5Hz 48.2Hz


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Table8:RateofChangeofFrequencyRelaysettingadoptedinIndiangrid

Region StageI StageII StageIII

NR 0.1Hzpersecand49.9Hz 0.2Hzpersecand49.9Hz 0.2Hzpersecand49.9Hz

WR 0.1Hzpersecand49.9Hz 0.2Hzpersecand49.9Hz

SR 0.3Hzpersecand49.5Hz

(Alarm)

0.3Hz persecand49.3Hz

(Trip)

c) GrowingexpectationsofconsumersAlargeinterconnectioncaterstoadiversecategoryofconsumers.Anarrowoperatingrangeforgrid

frequency significantly reduces wear and tear in electrical machines and thus increases their life.

Moreover, with the increase in the proportion of sophisticated consumer loads such as process

industries,tractionlocomotivesandsiliconloadstheexpectationofabetterpowerqualityfromthe

gridisalsorising.

d) ImpactoffrequencyonvoltageIt has been observed from studies that increase in frequency results in increase in voltage and

decreaseinfrequencyresultsindecreaseinvoltage.InNortherngridithasbeenobservedthatone

Hzfrequency increase/decrease isequivalentto8kV increaseordecrease involtage. Thiswould

haveasignificantimpactontransmissionlossesandefficiencyinthegrid.


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Figure3:Impactofgridfrequencyongridvoltage

a) FrequencyfluctuationsandprimaryresponsefromgeneratorsIthasbeenobservedthat thepowernumber inNEWgridandSRgrid is1800MWperHertzand

1020MW per Hertz respectively. Tighteningof frequencyband by0.1 Hz may imply reduction in

demand met by approximately 180 MW in NEW Grid and approximately 102 MW in SR Grid.

However, grid operation within a narrow range would encourage utilities to provide primary

responsefromtheirgenerators.Thiswouldhelp inarrestingthewidevariations infrequencyand

network loadings during sudden change in injection/withdrawal from the grid. Thus improved

securityandefficiencygainsobtainedthroughtighteningoffrequencybandaremuchlarger.

b) VolumeofunscheduledInterchangeAfrequencydependentunscheduled interchangemechanism is inplace inthe Indiangrid.Awide

operating range of frequency creates room for large volume of unscheduled interchanges. The

volumeofUIandtheScheduledInterchangesisshowninfigurebelow.


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Figure4:VolumeofScheduledinterchangeandUnscheduledInterchange

Several market players participate in the Indian market with a highly unbalanced portfolio. The

unpredictability inthebehaviourofsuchmarketplayershasserious implications forgridsecurity.

ThereforeCERChastakenseveralinitiativestoencouragemarketplayerstoshiftfromUnscheduled

InterchangetoScheduledInterchange.


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Figure5:TrendofweightedAveragePrices

A comparison of UI rate with the weighted average energy price in the bilateral and collective

transactions intheNEWgridrevealsthattheUIratehasbeen lowerthanthenegotiatedprice in

bilateral transactions and the discovered price in the Power Exchange. This is a positive

developmentandimpliesthatbuyersarewillingtopayapremiumforscheduledinterchangethat

provides higher certainty. Therefore it is desirable to encourage the reliance on scheduled

interchange through the available price signals and by enforcement of the UI volumes cap as

mandated in the CERC regulations on Unscheduled Interchanges. The SERCs could also consider

similarmeasures.

c) IntegrationofrenewableenergyIntegrationofrenewableenergyinthegridisoneofthebiggestthrustareas.Thecontributionfrom

windenergyishighestintherenewableportfolio.Theinstalledwindgenerationcapacityason30th

Sep 2010 is 6070 MW. This is expected to be 13065 MW by Dec 2012. Considering the high


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variabilityandunpredictabilityofgenerationfromrenewable,theinjectionfromwindenergycanbe

safelyabsorbedinthegridonlyifthefrequencyinthegridismaintainedinacomfortablerange.

d) SimulationandmodellingThe focus in power system and electricity market operation planning is gradually shifting from

empiricalstudiestoquantitativeanalysisusingsimulationsstudies.Thesoftwareusedinmodelling

generally assumes a stable frequency. A stable frequency regime would facilitate comparison of

simulatedandactualsystembehaviour.

2. ComparisonofpermissiblefrequencyinothercountriesandviewsinIndiaIt is evident from the previous sections that the wide range of permissible frequency by design has

economicaswellassecurityconcerns ina largegrid. Inthiscontext itthepermissibledeviationfrom

thenominalfrequencyprevailinginothercountrieshasbeenexaminedandisshownisTablebelow.

Table9:PermissibleFrequencybandinothercountries

Country/Interconnection Nominal

frequency

(Hz)

PermissibleFrequency

Band(Hz)

Permissible

Deviation(%)

EasternInterconnection(US) 60 59.95 60.05 +/0.083%

Nordiccountries 50 49.9 50.1 +/ 0.2%

WesternInterconnection(US) 60 59.856to60.144 +/0.24%

Europe 50 49.8 50.2 +/ 0.4%

India 50 49.550.2 1%/+0.4%

OtherSAARCcountries 50 49.5 50.5 +/ 1%

The intent of CERC on the desirable frequency band as mentioned in CERC order on ABT dated 4th

January2000.


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Quote

5.9.8Anotherpointforconsiderationiswhetherchargesforoverdrawalshouldbethesameat49.0

Hzandevenbelow49.0Hz. It shouldbenoted that thedeclaredfrequency in India is50Hz.An

integratedpowersystemshouldoperatewithagridfrequencyhoveringaround50Hz. Inpractice

however, thefrequency range in India has been 48.5 Hz to 50.00 Hz. This is not desirablefor

achieving interconnected/integrated operation of the grid. With the additions to generation

capacities, it ishopedthattheremaynotbeadropbelow49Hz.. Infacttheattempt

should be to further narrow down the range with more generating capacities coming up and

redundancycreated.

Unquote

The issue also finds a mention in the Minutes of Meeting of 4th Coordination Forum held on 17th

August2009

Quote

It was highlighted in the presentation that low frequency situations are also resulting in sub

standardgridvoltages.Afterthediscussion,it was generallyfeltthattherewasaneedtofurther

narrowdownthepermissiblefrequencyrangefrom49.5Hzto50.3Hzw.e.f.January2010andfrom

49.8Hzto50.2Hzw.e.f.January2011.

Unquote

3. IssuesfordiscussionwithrespecttotighteningoffrequencybandTheissuesthatneedtobediscussedareasbelow:

a) Tighteningofoperatingfrequencybandisdesirable Whatshouldbenextoperatingrangeforfrequency?

Presentlytheoperatingrangeis49.5to50.2Hz


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SuggestedsolutionbyCERCstaff

Inyear2011 : 49.7to50.2HzwithaStepSizeof0.01Hz

Inyear2012 : 49.8to50.1HzwithaStepSizeof0.01Hz

DeviationsfromscheduleareinevitableandthedeviationsfromschedulewouldbesettledattheUIrate.WhatshouldbethepermissiblevolumeofUnscheduledInterchangeforacontrol

areaasapercentageofitsschedule?

Nochangeproposed.

4. OtherissuesrelatedtogridsecurityPowerSystemvisualizationandsituationalawarenessarevitalfordecisionmakingatthecontrolcentre.

This requires reliable and fast communication between the generating stations/substations and the

control centre. With the rapid expansion in the power system infrastructure in India a robust

communicationinfrastructureisalsorequiredforpowersystemoperationandcontrol.Themajorissue

inthisregardistheurgentneedforanexclusiveRegulationoncommunicationsystemforPowerSector

x


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References

a. CEA,17thElectricPowerSurveyb. CEA,PowerScenarioataGlance,January2011c. IndianElectricityRules1956,asamendedupto25thNovember2000d. CERC,IndianElectricityGridCodeRegulation2010,April2010e. CERC,UnscheduledInterchangechargesandRelatedMattersregulations,2010f. NREB,ReportoftheTaskForceonFrequencyControl,December1992g. DwarkaNath,UnderfrequencyTrendrelayasPowerSystemSaviour,InternationalSeminaronGridStabilityandLoad

Management,Nagpur,January1996

h. NRPC,ReportoftheSubgroupconstitutedbyNorthernRegionOperationCoordinationSubcommitteetoreviewUnderFrequencySchemeinNorthernRegion,January2005

i. CERC,AnnualReportoftheMarketMonitoringCellofCERC,2009j. S.K. Sooneeetal., FrequencyResponseCharacteristicsofan InterconnectedPower SystemACase StudyofRegional

GridsinIndia,6thInternationalR&DConferenceonSustainableDevelopmentofWaterandEnergyResources,February

2007

k. S.K.Sooneeetal.,ReactivePowerandSystemfrequencyRelationshipaCaseStudy,7thInternationalR&DConference,February2009

l. CERCorderonABT,January2000m.Minutesof4thCoordinationCommitteeMeeting,August2009


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