I
nt
r
oduct
i
on
Wind and photovoltaic(PV)energy sourcesare usefulwaysto reduce CO2emissionsin the electricity sector.However,distribution grid systemsare facing challengesofhow bestto accommodate wind and PV energy on alarge scale because middle and low voltage distribution gridswere notoriginally designed for thispurpose.In thisstudy,grid operationsin Japan and Germany are investigated,with afocuson key rules thatenable wind and PV systemsto be integrated into the powernetwork on alarge scale by comparing energy policiesin the two countries.
First,bidirectionalgrid operation in the 50Hertztransmission zone in Germany isexamined.This research focuseson reverse powerflow (Rückspeisung)from low voltage (LV)and medium voltage (MV) gridsto high voltage (HV)grids.The relationship between reverse powerflow and feed-in from wind and
Wi
nd
and
PV
Ener
gy
I
nt
egr
at
i
on
i
n
Di
st
r
i
but
i
on
Gr
i
ds
and
Bi
di
r
ect
i
onal
Gr
i
d
Oper
at
i
ons
:
A
Compar
at
i
ve
Anal
ysi
s
i
n
J
apan
and
Germany
As
a
mi
TAKEHAMA
ⅰAbstract:Grid operationsand key rulesenabling integration ofwind and photovoltaicsystemsinto the powernetwork system in Japan and Germany were analyzed in thisstudy.The research focuseson reverse powerflow from low voltage (LV)and medium voltage (MV)gridsto extra-high voltage (EHV) grids.The resultsshowed thatGerman grid operatorsflexibly transmitand distribute electricity in a bidirectionalway,depending on the poweroutputfrom renewable electricity.German grid operators prioritize purchase ofrenewable electricity and transmititto the upstream grid asnecessary.Excess electricity from wind and PV systemsisphysically transmitted to upstream gridsand to neighbouring zonesasapriority.In Japan,grid operation isunidirectionaland usesadownward powerflow based on electricity from nuclearpowerand large-scale coalpowerplants.Nuclearelectricity hasthe firstpriority overallothertypesofelectricity to be transmitted downward from 500 kV gridsto the lowestvoltage gridsand to inter-zone tie-lines.Renewable electricity hasno legaldefinition with regard to upstream transmission.Because the Renewable Energy Actin Japan doesnotoblige utility companiesto expand theirgrid capacity,there are few opportunitiesforrenewable energy sourcesin the grid system. Therefore,rulesforupstream transmission and reinforcementofupstream gridsare required forutility companiesin Japan.
Keywords : wind energy, photovoltaic energy, grid integration, reverse power flow, upstream transmission,bi-directionalgrid operation,verticalgrid load,priority transmission,grid capacity development
PV energy willbe examined.Grid datain the 50Hertzzone isused in thispartofthe study.Second,the key rulesthatenable reverse powerflow from wind and PV energy in Germany willbe discussed,including rulesconcerning grid connection,transmission and upstream transmission ofrenewable electricity underthe Erneuerbare Energien Gesetz (EEG: Renewable Energy Sources Act)1,2. Finally, rules concerning connection,transmission,distribution and grid expansion underthe feed-in tarifflaw in Japan willbe analyzed.The reformsnecessary forthe successofthislaw willbe discussed through acomparison with grid integration policiesin Germany.
1
Gr
i
d
oper
at
i
on
wi
t
h
hi
gh
penet
r
at
i
on
of
wi
nd
and
PV
ener
gy
i
n
Ger
many
1.1 Negative verticalgrid load and reverse powerflow
A positive verticalgrid load (VGL)indicatesload flow from the 380 kV transmission grid to the 110 kV distribution grid with avoltage decrease through transformers,whereasanegative VGL indicatesreverse powerflow from the 110 kV grid to the 380 kV grid with avoltage increase through transformers.3The value ofVGL in thisstudy isdefined asthe totalnetsum ofelectricity through alltransformersand tie-lines thatare directly connected to the 380 kV grid in the controlzone ofeach transmission system operator (TSO). Reverse power flow can occur in some transformer substations with a large capacity of interconnected wind and PV systems,while some urban orindustrialareashave apositive VGL atthe same time.Even ifthe aggregated VGL in aTSO controlzone isnegative,there isapossibility thattransformer substationsin some areaswillstillhave apositive VGL.VGLsare given in unitsofMW/15 min.4
In thispaper,VGL,tie-line transmission,and internationaltransmission in the 50Hertzand TenneT zoneswillbe examined using grid datain unitsof15 min,with the relationship between feed-in from wind and PV energy asapriority.In the 50Hertzzone,wind energy capacity isaround 13,4000 MW and PV capacity is7,400 MW.Dataforcontrolareaload,VGL,wind feed-in,PV feed-in,internationalexchange,and feed-in managementare shown in unitsofMW/15 min,according to Sections13(1)-EnWG and 13(2)-EnWG (German Energy Industry Act).The grid load foreach transmission line in the 50Hertzzone isshown in MW/hour.Alldataare available atthe website of50HertzGmbH.5
1.2 Verticalgrid load and voltage levels ofwind and PV energy integration
The magnitude and timesofthe negative VGL have increased since 2011 in the 50Hertzzone (Fig.1). The amountofnegative VGL accounted for0.68% ofthe totalsum ofelectricity transmitted between the 380 kV grid and 110kV grid,and hoursofnegative VGL accounted for3% ofannualhoursin 2013.Brown coal, large hydro,and hard coalplantsare connected to the 380 kV transmission gridsin the 50Hertzzone. However,91% ofwind capacity and almostallPV capacity in the 50Hertzzone isinterconnected to the 110kV and lowervoltage grid (Fig.2).Around 76% ofPV capacity in the 50Hertzzone isconnected to the MV grid,which indicatesthatfeed-in from wind and PV energy satisfiesthe load demand in the LV and MV grids.
The EEG obligesGerman grid operatorsto feed-in and trasnmitelectricity from renewable energy sources(RES)asapriority.Therefore,when feed-in from RES exceedsthe load demand in the LV (0.23 or 0.4 kV)and MV (20 or35 kV)grids,the excesselectricity istransmitted to the HV grid (110 kV)with a voltage increase.When feed-in from wind and PV energy exceedsthe load in the 110 kV grid,excess electricity istransmitted to the 380 kV grid with avoltage increase.
changescaused by reverse powerflow.In addition,almostalltransformersat380kV/110kV,HV/MV and MV/LV are equipped with both-direction protection systemsto dealwith reverse powerflow.Both-direction protection systemshave been installed in the pastdecade.6Thisisbecause the EEG (2004 version)obliged German grid operatorsto transmitrenewable energy electricity asapriority.Reverse powerflow at 380kV/110kV transformersoccursonly when wind and PV feed-in isextremely high and the controlarea load islow.AtHV/MV and MV/LV transformers,however,reverse powerflow occursquite often in distribution areasin which alarge capacity ofwind and PV systemsare interconnected to the grid and the load in the distribution areaislow.7
1.3 Correlation ofwind feed-in,PV feed-in and reverse powerflow
Feed-in from wind and PV systemscan vary according to weatherconditions.However,the EEG requiresgrid operatorsto feed-in electricity from wind and PV systemsasapriority.Therefore,TSOsmust
Fig.2.Integrated voltage levels ofrenewables (50Hertz,Oct.2014)
Source:Calculated from EEG-Anlagenstammdaten8
Fig.1.Verticalload (50Hertzzone,Jan.2011―Oct.2014)
ensure system stability and reliability ofthe grid,and mustadaptto changesofwind and PV feed-in atthe same time.Residualload (RL)indicatesifwind and PV electricity isfed-in to the grid asapriority,and what amountofthe controlareaload mustbe satisfied with feed-in from conventionalpowerplantsand other renewable powerplants.
RL = L − (W + PV) (1)
The RL isdefined in Equation (1),where L isthe load in the controlzone [MW].W isthe wind feed-in, and PV isthe photovoltaicfeed-in [MW].High feed-in from wind and PV energy and alow load in acontrol areacausesalow ornegative residualload and alow ornegative VGL (Figs.3,4,5).VGL clearly decreases
Fig.3.Verticalgrid load and high feed-in from wind energy (20―28 Dec.2013,50Hertzzone)
Source:Calculated from 50Hertzgrid data
Fig.4.Verticalgrid load,exportand transmission to the TenneT zone (20―29 Dec.2013,50Hertzzone)
atatime oflow residualload.Thisisbecause asfeed-in from wind and PV energy in the distribution grid (LV,MV.110 kV)satisfiesthe load in each grid,feed-in from the 380 kV to 110 kV grid decreases. Electricity supplied to the 380 kV grid ismainly from brown coal,hard coal,and large hydro powerplantsin the 50Hertzzone.There isno nuclearpowerplantin thiszone.There isaclearcorrelation ofthe residual load and VGL in the 50Hertzzone (Fig.6).When wind feed-in ishigh,VGL dereases,and the amountof wind feed-in mainly determinesthe VGL in the 50Hertzzone (Fig.7).
1.4 Reverse powerflow and use ofdomestic tie-line transmission
Atatime ofvery high wind and PV feed-in,the VGL in the 50Hertzzone becomesnegative atalmost
Fig.5.Grid situation ofhigh wind and PV feed-in,low load,verticalload and curtailment according to Section 13(2)-EnWG (German Energy Indestry Act)(22–28 March,2013,50Hertzzone).
Source:Calculated from 50Hertzgrid data
Fig.6.Residualload and VGL (50Hertzzone,2013)
Source:Calculated from 50Hertzgrid data
Fig.7.Wind feed-in and VL (50Hertzzone,2013)
all380kV/110kV transformersubstations,exceptin large citiessuch asBerlin and Hamburg.Excess electricity in the 110kV grid istransmitted to the upstream 380 kV grid asreverse powerflow and to the neighbouring TenneT zone and internationalzones(Denmark,Poland,and Czech Republic).The capacity fordomestictie-line trasnmission from the 50Hertzzone to the TenneT zone totalsaround 5000 MW under stable conditions.9On the otherhand,the capacity allocation forcross-bordercongestion managementin the 50Hertzzone isaround 2000 MW (from 50Hertz)with fourcross-borderconnection hubs.
The amountofdomestictie-line transmission from the 50Hertzzone to the TenneT zone isestimated using Equation (2).
Tr-TenneT = (G − L)− Exp (2)
where Tr-TenneT isthe transmission from the 50Hertzzone to the TenneT zone [MW],G isthe generated amountin the controlzone,L isthe controlareaload [MW],and Exp isthe netexportfrom the control zone [MW].
Correlationsofwind feed-in with domestictie-line transmission from the 50Hertzzone to the TenneT zone (Fig.8)and with netexport(Fig.9)are similar.The amountofdomestictie-line trasnmission reached itsmaximum limitofcapacity (Fig.8)and the netexportreached itsmaximum trasnmission limit(Fig.9).
1.5 Tie-line transmission to the TenneT zone and wind feed-in
In ahigh wind feed-in situation,50Hertztransmission usesthe following three routesto transport excesselectricity from wind energy to the TenneT zone:(a)No.413,No.414 (Remptendorf-Redwiz),(b) No. 449 (Vieselbach-Mecklar), No. 450 (Eisenach-Mecklar), and (c) No. 491, No. 492 (Wolmirstedt -Helmstedt).Thisisbecause alarge capacity ofthe wind energy systemsislocated in northern areasin the 50Hertzzone.
The grid loadsoftransmission linesatNo.413,No.414,No.449,and No.450 between 50Hertzand TenneT are shown in Figs.10(a)and 10(b).Wind and PV feed-in and TSO intervention forsystem security
Fig.8.Wind feed-in and transmission to TenneT zone
(Oct.– Dec.2013,50 Hertzzone)
Source:Calculated from 50Hertzgrid data
Fig.9.Wind feed-in and netexportin the (50Hertzzone,2013)
underSection 13(1)-EnWG and 13(2)-EnWG (German Energy Industry Act)and Section 11-EEG are shown in Fig.10(c).The amountofpowerregulated underSection 13(1)-EnWG mainly correspondsto redispatch from conventionalpowerplants.The amountofpowerregulated underSection 13(2)-EnWG in conjunction with Section 11-EEG ismainly curtailment(feed-in management)ofRES electricity.
The solid and dotted linesin Fig.10(a-c)show 50% and 70% capacity use,which meansthe grid loads were 50% and 70% of the transmission capacity of each line, respectively. A proviso is that the real transmission capacity can vary according to real-time grid situations.The linesfor50% and 70% capacity use in thispaperonly indicate the minimum value [MW]foreach line in the year2012.According to an explanation given by 50HertzTransmission,grid use atlessthan 50% capacity hasno problemsin acase of line failure.However,acapacity ≥50% to ≤70% isclose to the limitifline failure occurs.Ifthe capacity use is> 70%,itisatthe limitin acase ofline failure.10
The 50Hertzzone integratesalarge capacity ofwind and PV energy,in addition to 10 GW forbrown coalplants.Therefore,the 50Hertzzone often hasexcesselectricity transmission to the TenneT zone and to cross-bordertransmission,exceptatatime ofvery low wind feed-in,although the grid load in Fig.10 does notindicate the direction ofelectricity flow from orto the 50Hertzzone.
Fig.10.Grid load ofdomestic tie-lines between 50Hertzand TenneT in Dec.2012 by line in hourly values.
(a)Line No.449,No.450,(b)Line No.413,No.414,(c)wind and PV feed-in and TSO interventionsforsystem security (redispatch and curtailmentsunders.13(1)and 13(2)EnWG (Energy Industry Act).
Source:Calculated from 50Hertzgrid data (b)
(c) (a)
The grid load atNo.413,No.414,No.449 and No.450 often reached 50% ofthe transmission capacity. The grid load ofNo.413 exceeded 50% capacity use in 1663 hoursand No.414 in 1620 hoursof7336 observed hours from 1st March to 31st Dec. 2012 (Fig. 11). Based on Figs. 8, 10 and 11, 50Hertz Transmission usesdomestictie-linesto the maximum limitto transmitexcesselectricity from wind and PV energy.
1.6 Bidirectionalgrid opertations to feed-in renewable electricity in Germany
A large negative VGL in the 50Hertzzone iscaused by combined effectsoffeed-in from wind and PV energy (Fig.12).High levelcapacity use in domestictie-linesbetween 50Hertzand TenneT signifiesthat 50Hertztransmission system operatorsuse domestictie-linesfully to transmitexcesselectricity from wind and PV energy asapriority (Fig.11).Reverse powerflow occursquite often from LV to MV gridsand from
Fig.11.Grid load ofinter-zone transmission lines,No.413 and No.414,sorted in orderofgrid loads (March to Dec.2012,50Hertzzone).
Source:Calculated from datafor50Hertzgrid load flow by line
Fig.12.Negative verticalgrid load,wind and PV feed-in in the 50Hertzzone (5th–12th May 2014)
Fig.13.Reverse power flow from middle volatage to high voltage grids (Energie NetzMitte zone,measured at HV/MV transformersubstations,2013)
Source:Calculated from datafrom Energie NetzMitte
Fig.14.Reverse powerflow from renewable energy plants and voltage levels
MV to HV grids(Fig.13).However,very few distribution grid operators(DSOs)disclose theirreverse powerflow (Rückspeisung)datain MV grids(20 kV)orLV grids(230 V and 400 V).
These datasuggestthatGerman grid operatorscarry outbidirectionaland flexible grid operationsto feed-in and transmitrenewable energy electricity to upstream gridsasapriority,ifnecessary.11A simplified structure ofbidirectionalgrid operation and integrated voltage levelsofrenewable energy plantsin Germany isshown in Fig.14.Thisfigure isbased on severalinterviewswith DSOsand TSOs.
2
Gr
i
d
i
nt
egr
at
i
on
r
ul
es
t
o
enabl
e
bi
di
r
ect
i
onal
gr
i
d
oper
at
i
ons
i
n
Ger
many
The key rulesforgrid connection,purchase and transmission ofrenewable electricity and bidirectional grid operationsunderthe EEG (2014 version)are summarized in thissection.The EEG requiresGerman grid operatorsto immediately and even physically,asapriority,purchase,transmitand distribute allofthe electricity produced from renewable energy sourcesorfrom mine gas(section 11(1)-EEG).The dutiesof priority purchase,transmission and distribution are imposed on grid operatorswho are directly connected to renewable powerplantsand on upstream TSOsconnected with renewable plantsindirectly (section 11(5) 1-EEG).These definitionsrequire upstream TSOsto transmitelectricity from renewable energy physically as apriority.Thisenablesreverse powerflow ofexcesselectricity from renewable energy from the lower voltage grid to the highervoltage grid.
The EEG requiresthatGerman grid operatorsimmediately optimize,reinforce and expand their networkswith the available technology to ensure purchase,transmission and distribution ofelectricity from renewable energy (section 12(1)-EEG).The duty ofgrid reinforcementiseven imposed on upstream grid operatorsup to avoltage levelof110 kV,even ifrenewable powerplantsare notdirectly connected with them (section 12(1)-EEG).Grid operatorsare obliged to provide grid connectionsto renewable power plants, even when purchase of renewable electricity is only possible by optimizing, reinforcing and expanding the grid (section 8(4)-EEG).
Itshould be noted that,according to the EEG,even upstream grid operatorsare obliged to carry out grid reinforcement.Hosting capacity increase and optimization ofvoltage controlin the distribution grid (LV,MV and 110 kV grids)are essentialto enable reverse powerflow from wind and PV feed-in in lower voltage grids.Thisisbecause reverse powerflow from excesselectricity from wind and PV systemsoften causesvoltage risesin distribution gridsand transformers,especially atMV/LV and HV/MV transformers in localdistribution grids.12Therefore,thismandatory capacity expansion imposed on grid operators, including upstream grids,isimportantto achieve reverse powerflow from wind and PV energy systemsin distribution grids.
The operatorcan bring the compensation costforfeed-in management(curtailment)into the calculation ofgrid charges.However,the grid operatorbearsthe compensation costsoffeed-in management,ifthe operatordid notexhaustalloptionsforoptimizing,reinforcing,and expanding the grid (section 15(2)-EEG). These strictlegaldefinitionsmake German grid operatorsreinforce the grid system to ensure purchase and transmission ofrenewable electricity.These definitionsare the legalbasisofenabling reverse powerflow of renewable electricity and bidirectionalgrid operationsin Germany.
3
Gr
i
d
i
nt
egr
at
i
on
pol
i
ci
es
f
or
wi
nd
and
PV
ener
gy
i
n
Japan
underthe Acton SpecialMeasuresConcerning ProcurementofElectricity from Renewable Energy Sources by Electricity Utilities(hereinafter,the Renewable Energy Act)are summarized in thissection.
First,itshould be noted thatin Japan the transmission grid systemshave yetto be separated from generation businesses.The nine majorgeneration companies(GeneralUtility Companies;hereinafter,the utility companies)own and operate the grid systemsin theircontrolzones.These nine companiesare ‘vertically integrated’ power companies that operate all aspects from generation, transmission and distribution to retailofelectricity (Fig.15).These utility companiesown 90% ofthe totalgeneration capacity in Japan,including nuclearpowerplants(Fig.16).
There are no legaldefinitionsconcerning priority connection and purchase ofrenewable electricity underthe Renewable Energy Act.There isaproviso thatwhen there isarisk to stable supply and grid reliability,the utility companiescan rejectaconnection requestfrom renewable operators(Article 5, paragraph 1,(2)-Renewable Energy Act).Six ofthe nine utility companiessetacapacity restriction on PV and wind energy integration in each controlzone (Figs.17 and 18).There are no legaldefinitionsofthe ‘priority connection’and ‘priority transmission’ofrenewable electricity underthe act.The six utility companiesin Fig.18 are allowed to setcurtailment(outputreduction)‘withoutcompensation’to PV operatorswhen integration ofPV capacity in each controlzone exceedsthe restriction amount(Article 6, paragraph 7.EnforcementRegulationsin the Renewable Energy Act).13
The Renewable Energy Actdoesnotobligate utility companiesto expand and reinforce the grid system to accommodate renewable energy plants.To increase integration ofwind and PV systems,suitable capacity expansion and reinforcementoflow voltage grids(0.2 kV),6.6 kV gridsand 66 kV gridsare urgently needed,forthe following reasons.The voltage levelsin the Japanese grid system are 500,275,154,66,22 and 6.6 kV and 100 and 200 V (with some regionalvariationsin controlzones).The inter-zone tie-linesare mainly at500 kV grids.Nuclearand large fossilfuelplantsare connected to 500 kV grids.Hard coal generatorsand large hydro powergeneratorsare connected to 500 kV,275 kV or154 kV grids.In contrast, many wind energy systemsare interconnected with 66 kV or154 kV grids.Many PV systemsare connected to low voltage,6.6 kV and 66 kV grids.Therefore,to accomodate wind and PV systems,itisnecessary to expand and reinforce low voltage (0.2 kV),6.6 kV,66 kV and 154 kV grids.
When wind and PV systemsare integrated into the grid on alarge scale and in aperiod ofhigh feed-in from wind and PV energies,excesselectricity in each trasnformersubstation willcause reverse powerflow. Thisflow willcause voltage increasesattransformersand distribution grids,especially in the low voltage and 6.6 kV grids.Therefore,both grid capacity and transformercapacity mustbe sufficiently expanded and reinforced.Both-direction protectionssystemsand voltage controlequipmentmustbe optimized to enable reverse powerflow and upstream trasnmission with controlofvoltage levelsin apermissible range.In addition,the regulatory framework in Japan requiresutility companiesto be obliged to expand and reinforce theirgridsattheirown expense,including upstream voltage gridssuch asthe 66 kV and 154 kV grids,ifit isindeed necessary to accomodate the renewable energy plants.
To enable reverse powerflow attransformersubstationsin the 6.6 kV and 66 kV grids,upstream transmission isessential.However,the Renewable Energy Actdoesnotoblige the utility companiesto prioritize transmission ofrenewable electricity to upstream grids.
Wind and PV system operatorsare strictly limited concerning use ofinter-zone tie-linesunderthe rules ofOCCTO (Organization forCross-regionalCoordination ofTransmission Operators,Japan).Renewable energy operatorsare required to use batteries,electricity storage measuresorcombination ofother electricity sourcesto stabilize feed-in from wind and PV systems14.
In atime ofcurtailment(suppression ofoutputfrom generators),renewable electricity hasapriority only overfossilfuelplants.However,nuclearelectricity alwayshasthe firstpriority overallotherelectricity to feed-in to trasnmission gridsin each zone (underthe Renewable Energy Act)15and to inter-zone tie-lines (underthe OCCTO Network Code).Thiscode statesthat“long-term costrecovery generation”hasthe first priority in transmission to inter-zone lines.The network code definesnuclearenergy as“long-term cost recovery generation”.16
Fig.15.Grid structure and inter-zone tie lines in Japan
Source:Sammarized from the Interim Reportofthe Committee on the MasterPlan forReinforcementofInter -Zone Tie-Lines(PowerSystem Reform Committee,Ministry ofEconomy,Trade and Industry),2012.Lood data based on the Federation ofElectricPowerCompaniesofJapan.
Fig.16.Generation capacity in Japan in 2004 and 2014
Source:Calculated from powergeneration statisticsfrom METI,feed-in tariff scheme datadisclosure from METI,and Trendsin PhotovoltaicApplicationsfrom IEA-PVPS
According to the Implementing Regulationsin the Renewable Energy Act(hereinafter“Implementing Regulations”),when feed-in from renewable energy generatorsexceedsdemand in each controlzone,utility companiescan impose curtailments(suppression)ofoutputon renewable electricity.Utility companiescan escape from paymentofcompensation forcurtailmentforup to 360 hoursayearforPV systemsand 720 hoursayearforwind energy systems.17In addition,the regulation exemptsthe following ‘designated utility companies’from the paymentofcompensation withoutlimitation ofhours:Hokkaido,Tohoku,Hokuriku, Shikoku, Chugoku and Kyushu utility companies for PV systems, and Hokkaido and Tohoku utility
Fig.17.Capacity restrictions on wind energy integration by majorutility companies
Source: Data from METI New Energy Subcommittee, Federation of Electric Power CompaniesJapan.
Fig.18.Capacity restrictions on PV energy integration,requests for connection,and installed capacity
Source: Data from METI New Energy Subcommittee, Federation of Electric Power CompaniesJapan.
companiesforwind energy systems.18
4
Concl
usi
ons:
Bi
di
r
ect
i
onal
gr
i
d
oper
at
i
on
i
n
Ger
many
,
uni
di
r
ect
i
onal
gr
i
d
oper
at
i
on
i
n
Japan
Grid datain Germany show thatGerman grid operatorsflexibly transmitand distribute electricity in a bidirectionalway,depending on the poweroutputfrom renewable electricity.Grid operatorsprioritize purchase ofrenewable electricity and transmititto the upstream grid asnecessary.Excesselectricity from wind and PV systemsiseven physically transmitted to the upstream grid and to neighboring zonesasa priority.German renewable energy law obligatesgrid operatorsto expand and develop grid capacity “withoutdelay”to accommodate renewable electricity.Thislegalframework hasenabled bidirectionalgrid operations.In contrast,grid operation in Japan isunidirectionaland hasadownward powerflow based on supply from nuclearpowerand large-scale coalpowerplants.Nuclearelectricity hasthe firstpriority overallother typesofelectricity to be transmitted downward from 500 kV gridsto lowestvoltage gridsand inter-zone tie lines.The utility companiesgive renewable energy systemsvery limited accessto the grid,and renewable electricity hasno legaldefinition in regard to upstream transmission.Because the Renewable Energy Actin Japan doesnotoblige utility companiesto expand theirgrid capacity,there are few opportunitiesfor renewable energy sourcesto penetrate into the grid system in Japan.
The following reformsare necessary in grid integration policiesforwind and PV energy on alarge scale in Japan.First,generation businessesmustbe legally separated from grid operation businesses(not only operation ofinter-zone tie lines,butalso transmission and distribution gridsin each zone).Second, utility companiesmustbe obliged to connectrenewable energy sourcesto the grid asapriority overall otherenergy sources.Aslong asnuclearpowerplantssupply base powerasthe firstpriority,wind and PV electricity cannotpenetrate into the electricity supply on alarge scale.Third,utility companiesmustbe obliged to expand and reinforce theirgrids,including upstream grid systems,in areasonable period oftime to ensure purchase ofrenewable electricity,even ifgrid expansion and reinforcementare necessary to build anew connection with arenewable powerplant.In particular,grid expansion and reinforcementfor6.6 kV, 66 kV and 154 kV gridsare essentialto ensure wind and PV energy integration.Fourth,utility companies mustbe obliged to transmitelectricity from renewablesto upstream gridsphysically asapriority ifthisis necessary.Rulesforupstream transmission and grid reinforcementofupstream gridsmustbe jointly imposed on utility companies.Capacity expansion ofgridsisimportantforutility companies,both to enable reverse powerflow from excesselectricity oflowervoltage grids,and to controlvoltage levelsin a permissible range according to grid technicalcodes.Capacity expansion isurgently needed forlower voltage gridswith which renewable energiesare interconnected,and forupstream grids.Fifth,the capacity restriction on grid integration ofPV and wind energy mustbe abolished.Sixth,compensation mustbe paid forrenewable energy operatorswhen curtailmentisseton renewable electricity in each zone.
Acknowledgment
Thiswork wassupported by JSPS KAKENHIGrantNumber26340125 and by the Society ofRitsumeikan SocialSciences.
Notes
1 Gesetzfürden Ausbau erneuerbarerEnergien (Erneuerbare-Energien-Gesetz-EEG,2014). 2 Gesetzfürden Vorrang ErneuerbarerEnergien (Erneuerbare-Energien-Gesetz-EEG,2012).
3 The voltage levelsin the German powersystem are superhigh voltage (SHV)fortransmission gridsat380 kV or220 kV,high voltage (HV)fordistribution gridsat110 kV,medium voltage (MV)forlocaldistribution grids at20 kV (and 35kV in some regions,since there are regionaldifferencesin MV levels),and low voltage (LV) forlocaldistribution gridsat400 V and 230 V,mainly forsmall-scale consumers.
4 50Hertz:Verticalgrid load.available from http://www.50hertz.com/de/Kennzahlen/Vertikale-Netzlast
5 Dataused in thisstudy:(a)50Hertzzone in 2013:Generation ouput,Verticalgrid load,Controlareaload, Wind feed-in,PV feed-in,Maßnahmen nach §13.1 EnWG,§13.2 EnWG,Load flowscross-border,in MW/15 min.(b)50Hertzzone:Grid load by line,hourly datain MW.
6 Based on interviewswith FraunfoherIWES (in Aug.2015),Hanse Werke (in May.2014),StromnetzHamburg (in Sep.2015),50Hertz,and Amprion (in Sep.2014).
7 Stetz,T.,Kraiczy,M.,et.al.Technicaland economicalassessmentofvoltage controlstrategiesin distribution grids.Prog.Photovolt.Res.Appl.(2013),DOI:10.1002/pip.2331
8 50HertzGmbH:available from http://www.50hertz.com/de/EEG/Veroeffentlichung-EEG-Daten. 9 Information from 50HertzGmbH.Actualtransmission capacity can change according to grid conditions. 10 50Hertz:Explanation on grid load and capacity usage:http://www.50hertz.com/en/Grid-Data/Grid-load 11 Stetz,T.,Kraiczy,M.Diwold,K.etal.Transition from unidirectionalto bidirectionaldistribution grids.I
EA-PVPS,task 14 (2014).
12 Concerning the voltage rise caused by high PV penetration in the localdistribution grid,the necessary grid expansion measuresare explained in the following papers:Stetz,T.,Kraiczy,M.Diwold,K.Country-specific analysis,Germany.IEA-PVPS Task 14,Subtask 2:High PenetrationsofPhotovoltaicSystemsin Distribution Grids(2014).Stetz,T.,Kraiczy M.,et.al.,Technicaland economicalassessmentofvoltage controlstrategiesin distribution grids.Prog.Photovolt.Res.Appl.(2013),DOI:10.1002/pip.2331.
13 Concerning the Designated Utility Companies shown in Fig.18 and the Okinawa Power Company. Implementing Regulationsforthe Acton SpecialMeasuresConcerning ProcurementofElectricity from Renewable Energy Sourcesby Electnicity Utilities.
14 OCCTO:Network Codesofthe Organization forCross-regionalCoordination ofTransmission Operators, Japan.Article 202,Paragraph 3-(i),(amended April2016).
15 Implementing Regulationsforthe Acton SpecialMeasuresConcerning ProcurementofElectricity from Renewable Energy Sourcesby Electricity Utilities.Article 6,Paragraph 1,Point3-(1).
16 OCCTO,Network Codesofthe Organization forCross-RegionalCoordination ofTransmission Operators, Japan.Article 202,Paragraph 1-(vi),(vii),Article 210,Paragraph 1 (amended April2016).
17 Implementing Regulationsforthe Acton SpecialMeasuresConcerning ProcurementofElectricity from Renewable Energy Sourcesby Electricity Utilities.Article 6,Paragraph 1,Point3-(1).
本稿は，風力・太陽光発電システムを配電網に大量連系するための規則と系統運用について，日本とドイ ツを比較した。特に，配電網から送電網への逆潮流に焦点を当てて分析した。 ドイツでは，系統運用者（送電会社および配電会社）は，風力・太陽光発電からの電力を，出力状況に応じ て柔軟に，低圧および中圧系統から高圧系統へ，さらには，特別高圧系統に逆潮流させる双方向潮流対応の系 統運用を行っている。ドイツの系統運用者は，再生可能エネルギー電力を最優先で給電し，かつ，上位電圧系 統に優先送電する義務を負う。系統運用者は，風力・太陽光発電からの給電が配電網内の需要を上回る場合， 上位電圧系統（送電網）に，物理的にも電力を上流送電し，他の送電区域に地域間送電する。 これに対して日本では，長期固定電源（原子力，大型水力，地熱発電）の電力を５００kV送電系統から低圧系 統に，下方一方向潮流で配電する系統運用である。日本では，原子力発電からの電力を他の全ての電源に優 先して，給電・配電する。地域間送電線の使用においても，日本では，原子力発電の電力を最優先で送電する 規則となっている。 日本では，再生可能エネルギーからの電力を，下位電圧系統から上位電圧系統へ優先的に上流送電（逆潮 流）させる規則は整備されてない。日本の再エネ特措法は，一般電気事業者に電力網の系統容量の拡張を義 務付けていないため，系統容量の拡張が不十分なままで，再生可能エネルギー電源の連系を制約している。 再生可能エネルギー電源を大量に電力網に連系させるには，一般電気事業者に対して，再生可能エネルギー 電力の上流送電と電力網の系統増強を義務付ける必要がある。 キーワード：風力発電，太陽光発電，系統連系，逆潮流，上流送電，双方向潮流対応，垂直負荷，優先送電，系統 増強
