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`The development of grid code requirements for new
`and renewable forms of generation in Great Britain
`
`Antony Johnson and Nasser Tleis,
`Network Design, National Grid Company,
`NGT House,Warwick Technology Park,Warwick, England, CV34 6DA
`Tel:- +44 (0) 1926 655466, Fax:- +44 (0) 1926 656521
`http://www.nationalgrid.com
`Antony.Johnson@ngtuk.com
`Nasser.Tleis@ngtuk.com
`
`ABSTRACT
`This paper presents the development of Grid Code requirements for new and renewable
`forms of generation in Great Britain (GB). After briefly describing the background to the GB
`Transmission System and the volume of renewable generation which is anticipated to
`connect in the future, the paper discusses the Grid Code and Grid Code change process. In
`particular, the paper discusses the interfaces with key stakeholders, the technical issues
`considered in connecting other generation technologies to the Transmission network and
`the rationale for the final Grid Code requirements. The technical issues discussed include
`fault ride through, frequency range, frequency response, power/frequency characteristic,
`reactive range, voltage control and power quality. The paper concludes with the
`experiences gained in the connection of wind generation projects to the GB Transmission
`System both before and after the approval of new proposals to the GB Grid Code.
`
`Keywords: Grid Code, Technical Performance Requirements, Renewable Embedded
`Generation.
`
`I INTRODUCTION
`The National Grid Company (NGC) is a privately owned utility which owns and operates the
`high voltage Transmission Network in England and Wales. NGC is a wholly owned subsidiary
`of National Grid Transco (NGT) and is the holder of the GB (Great Britain) Transmission
`Licence. NGT also own and operate the high pressure gas transmission network in Great
`Britain. The company has a number of operations in both gas and electricity throughout the
`world in particular the North Eastern United States. Following the introduction of the British
`Electricity Trading and Transmission Arrangements (BETTA), introduced on 1 April 2005,
`NGC became responsible for operation of the entire Transmission System in Great Britain
`(England and Wales, and Scotland). The transmission networks in Scotland remain under the
`ownership of the Scottish Transmission companies (Scottish Power Transmission (SPT) and
`Scottish Hydro-Electric Transmission Limited (SHETL)).
`Figure 1 shows the layout of the GB Transmission System. Full details of the GB
`Transmission System including network structure, demand and generation backgrounds are
`detailed in [1]. Under its Transmission Licence, NGC has an obligation to ensure a safe, secure
`and economical transmission system and ensure non discrimination amongst System Users.
`
`

`

`(cid:127) (cid:127) ..
`
`©
`©
`©
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`THE DEVELOPMENT OF GRID CODE REQUIREMENTS FOR NEW AND
`RENEWABLE FORMS OF GENERATION IN GREAT BRITAIN
`
`To ensure NGC continues to meet these requirements, its activities are overseen by the
`Government Regulator “Ofgem”.
`
`BOUNDARY OF INFLUENCE
`AS AT OCTOBER 2004
`
`TRANSMISSION SYSTEM
`AS AT 1st DECEMBER 2003
`
`400kV Substations
`275kV Substations
`132kV Substations
`400kV CIRCUITS
`275kV CIRCUITS
`132kV CIRCUITS
`
`Major Generating Sites
`Including Pumped Storage
`
`Dounr eay
`
`Thur so
`
`Mybste r
`
`Stornoway
`
`Cassley
`
`Dunbea th
`
`Lairg
`
`Shin
`
`Brora
`
`Mossfo rd
`
`Grudgie
`Bridge
`
`Conon
`
`Luichart
`
`Orrin
`
`D ingwall
`
`Alness
`
`Deanie
`
`Cull igran
`
`Alga s K ilmorack
`
`Inv erne ss
`
`Nairn
`
`Elgin
`
`Keith
`Keith
`
`Macduff
`
`S trichen
`
`Beauly
`
`Blackhill ock
`
`Fraser burgh
`
`Fras erbur gh
`
`S t. Ferg us
`
`P eterhead
`P eterhead
`
`Fasn akyle
`
`Great
`Glen
`
`Quo ich
`
`Fort
`William
`
`Kilchrennan
`
`Dalm ally
`
`Foye rs
`
`Fort
`August us
`
`Inv erga rry
`
`B oat of
`Garten
`
`K intore
`
`Dyce
`
`Wood
`Hill
`
`Tarla nd
`
`Craig iebuckle r
`
`Persl ey
`
`Willowdale
`
`Clay hills
`
`Redmos s
`
`Fiddes
`
`Ranno ch
`
`Err ochty
`
`Pitio chry
`
`Tum mel
`B ridge
`
`Cash ley
`
`Lochay
`
`Killin
`
`Finlarig
`
`C lunie
`
`Bridge of Du n
`
`Tealin g
`
`Luna Head
`
`Dudhope
`
`Arbroa th
`
`Lyndhurst
`
`Milton of Crai gie
`
`C harlston
`B urghmuir
`
`Glenagne s
`
`Dudhop e
`
`Nant
`
`Inver aray
`
`Clachan
`
`Sloy
`
`St. Fillans
`
`B raco
`
`Glenrot hes
`
`Westfield
`
`Port
`Ann
`
`Devol
`Moor
`Inver kip
`
`Wind yhill
`
`B onnyb ridge
`
`Kincar din e
`
`Easte rhouse
`
`Lon ganne t
`
`Gra ngem outh
`
`S ighthill
`
`Torn ess
`
`Cocke nzie
`
`Dunoo n
`
`Neil son
`
`Newa rthill
`
`Cume
`
`Smea ton
`
`Carr ada le
`
`H unterst one
`
`A yr
`
`Wis haw
`
`Strathaven
`
`Linmill
`
`E lvanf oot
`
`K ilmarnock
`S outh
`
`Coylt on
`
`Ecc les
`
`A uchencr osh
`
`Harke r
`
`Fours tone s
`
`S tella
`W est
`
`Blyth
`
`Tynemouth
`South Shields
`
`West Boldon
`
`Off erton
`
`Spennymoo r
`
`Hawt horn e Pit
`
`Hart M oor
`
`Hartl epool
`
`Salth olme
`
`Tod Point
`Grangetown
`
`N orton
`
`Gre ystones
`
`Lacke nby
`
`Hutton
`
`H eysham
`
`Que rnmore
`
`Stanah
`
`Padih am
`
`Popple ton
`
`Os baldwic k
`
`Tho rnton
`
`Bradfo rd
`West
`
`Kirkstall
`
`Skelt on
`Gran ge
`
`Monk
`Frys ton
`
`Connected at 400kV
`Connected at 275kV
`Hydro Generation
`
`Creyke Beck
`
`Salt end Nor th
`
`Salt end Sou th
`
`Penworth am
`
`E lland
`
`Roch dale
`
`Ferryb ridge
`
`K earsley
`
`Whit egate
`
`Templeboro ugh
`
`Drax
`
`E ggb orough
`
`Killingholme
`H umber Ref ine ry
`
`Keadby
`
`Wyl fa
`
`P entir
`
`Dinorwig
`
`Ffes tiniog
`
`Trawsfyn ydd
`
`Was hway
`Farm
`
`Kirkby
`
`Liste r
`Drive
`
`Rain hill
`
`S outh
`Manchest er
`
`Birkenhead
`
`Capenh urst
`
`Deesid e
`
`Carring ton
`
`Fiddle rs
`Ferry
`Frodsh am
`
`Staly brid ge
`
`Bredbur y
`
`S tocksbri dge
`Win co Bank
`Neepse nd
`Sheff ield C ity
`
`Daine s
`
`Maccle sfield
`
`Jordant horpe
`
`Cheste rfield
`
`West
`Melton
`
`Pits moor
`
`Tho rpe
`Marsh
`
`Aldwarke
`
`Thurcr oft
`
`Brins worth
`
`Norto n Lees
`
`H igh
`Marnham
`
`Stayth orpe
`
`S outh
`Humber
`Bank
`
`Grimsb y
`West
`
`West
`Burto n
`
`Cottam
`
`Legacy
`
`Cell arhead
`
`Willington
`
`Ratcl iffe
`
`Rugel ey
`
`D rakelow
`
`Ironbrid ge
`
`Shrewsbur y
`
`P enn
`
`Bush bury
`Willenhall
`
`Bustl eholm
`
`Nechells
`
`Hams
`H all
`
`Enderby
`
`Ocker
`H ill
`
`Oldbury
`
`K itwell
`
`Berks well
`
`Covent ry
`
`Walp ole
`
`Spald ing
`North
`
`N orwich
`Main
`
`Bishop s
`Wood
`
`Feckenham
`
`Grendon
`
`Eaton
`Soco n
`
`Burwe ll
`Main
`
`Rass au
`
`Walh am
`
`Imperial
`Park
`
`P atford
`B ridge
`
`East
`Clay don
`
`Leighto n
`Buzz ard
`
`Cowley
`
`Sundon
`
`Pelha m
`
`Wymo ndley
`
`B raintree
`
`Rye House
`
`Brim sdown
`
`Wal tham
`Cross
`
`S izew ell
`
`Bramf ord
`
`Pembro ke
`
`Swa nsea
`North
`Bagla n
`
`Bay
`
`Margam
`
`Pyle
`
`Cilfyn ydd
`
`U skmo uth
`
`Whitson
`
`Upper Boat
`
`A lpha Stee l
`
`S eab ank
`
`Iron Act on
`
`Minety
`
`Cowb ridge
`
`Aberthaw
`
`Trem orfa
`
`C ardiff
`East
`
`Melksham
`
`Braml ey
`
`A mersham Main
`
`Watford
`
`Elst ree
`
`Hackn ey
`
`Culham
`D idcot
`
`Iver
`
`N.Hy de
`
`Mill Hill
`Willesden
`
`Ealin g
`
`Tottenham
`
`Redbrid ge
`
`Barki ng
`
`Laleham
`
`St Jo hns
`Wood
`
`City Rd
`
`W.Ham
`
`Hurst
`
`New
`Cross
`
`West
`Weyb ridge
`
`C hessingto n
`
`Rowd own
`Beddin gton
`
`Fleet
`
`Wimb led on
`
`Alverdi scott
`
`H inkley Poin t
`
`Bridgwater
`
`Taunton
`
`Axmi nst er
`
`Exeter
`
`C hickere ll
`
`Indian
`Que ens
`
`Land ulph
`
`Abham
`
`Nursl ing
`
`B oln ey
`
`Mannington
`
`Fawle y
`
`Botle y Wo od
`
`Lovedean
`
`N infield
`
`Peak Demand = 58.8 GW
`Minimum Demand = 22GW
`
`Figure 1.
`
`Overview of the GB Transmission System.
`
`Warle y
`West Thurr ock
`North fleet East
`
`Rayl eig h Ma in
`
`Coryt on
`
`Tilbury
`
`Grain
`
`S ingle well
`
`N orthfleet
`West
`
`Littlebrook
`
`K ingsnorth
`
`K emsley
`
`Canterbu ry
`North
`
`Sellindge
`
`E de F
`
`Dungen ess
`
`2. PRESENT AND FUTURE TRENDS FOR WIND GENERATION IN GREAT BRITAIN
`Following the Kyoto protocol, the UK Government set a target of 10% of electricity
`requirements in England and Wales to be sourced from renewable generation technologies by
`2010, which has subsequently increased to 15% by the year 2015. In Scotland, the Scottish
`Parliament have a commitment to achieving 18% renewable generation by the year 2010,
`increasing to 40% by the year 2020. Whilst this target includes all forms of renewable
`
`

`

`(cid:141)
`
`SHETL
`
`•
`• •
`•• •
`
`•
`
`11GW
`
`6GW
`
`•
`
`NGC
`
`8GW
`
`• ROUND 1 SITES OFFSHORE
`ROUND 2 AREAS OFFSHORE
`
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`generation, a large proportion of this is expected to come from wind, especially since, to
`encourage renewable generation, the UK Government introduced a number of incentives.
`These include additional payments to generators who supply “Green Energy” through the
`issue of “Renewable Obligation Certificates” (ROC’s), the requirement for Suppliers to source
`a proportion of their energy from renewable sources and the issue of grants for the
`development of renewable generation projects.
`In Scotland, the naturally good wind resource and its geographic characteristics have
`resulted in interest amongst a number of developers for onshore sites. By the end of December
`2004, the Scottish Transmission Licensees had received a total of nearly 17 GW of wind farm
`applications. It should however be noted that a number of these projects would be considered
`as speculative and may not proceed to the construction phase.
`In England and Wales, interest has also been shown in onshore wind farms although to
`date, these have tended to be less than 50 MW in size. The majority of growth is expected to be
`in offshore wind farms around the England and Wales coast. These have been promoted
`through the Round 1 and Round 2 Government offshore wind farm programmes. Under
`Round 1, developers have leased offshore seabed sites (less than 12 nautical miles from the
`coast) for the purposes of developing wind farms up to a maximum of 30 turbines. Under the
`Round 1 programme some 1.3 GW of offshore wind generation are expected to connect over
`the next few years. Round 2 is a much larger programme with tendering resulting in the
`connection of some 7.1 GW of additional offshore wind farm generation. A graphical
`representation of these figures is shown in Figure 21, which portrays both onshore and
`offshore wind farm projects.
`
`Figure 2.
`
`Potential future Wind Farm Connection Activity in Great Britain as of 31 December 2004
`
`1Figure 2 details the Round 1 and Round 2 development sites for offshore wind farms. Further details are
`available from:- http://www.thecrownestate.co.uk/34_r1_r2_windfarm_location_maps_04_08_13.pdf
`
`

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`THE DEVELOPMENT OF GRID CODE REQUIREMENTS FOR NEW AND
`RENEWABLE FORMS OF GENERATION IN GREAT BRITAIN
`
`In respect of these programmes and other projects, NGC has witnessed increasing activity
`from interested developers in connecting wind farms to the GB Transmission System.
`
`3. THE GRID CODE AND THE NEED FOR CHANGE
`Under the terms of its Transmission Licence, NGC is required to maintain a Grid Code. The Grid
`Code was originally developed in 1989 to facilitate privatisation of the UK Electricity Supply
`Industry in 1990. The Grid Code defines the technical and operational requirements on all
`Users of the GB Transmission System and was based on previous decades of network design
`and operational experience. Originally, the technical requirements for generating plant were
`specified in terms of large synchronous machines.
`Wind turbines rely on generator technologies [2] which exhibit very different operational
`characteristics to their synchronous counterparts. Since the operational characteristics of a
`transmission system are largely dictated by the generation and demand, it was important to
`ensure that NGC could continue to meet its statutory obligations (including the Security and
`Quality of Supply Standards, specified under its Licence) whilst facilitating the connection of
`the forecast large volumes of wind generation. Consequently it was recognised that the Grid
`Code was vague in its treatment of renewable generation technologies and changes would be
`required to provide a transparent set of technical requirements to all users. NGC had also
`observed the growth of wind generation in Germany and Denmark and the corresponding
`technical requirements that had been introduced in these and other countries [5], [6], [8].
`
`4. THE GRID CODE CHANGE PROCESS
`The Grid Code applies to all users of the GB Transmission System and is an industry wide
`document whose governance is managed by the Grid Code Review Panel (GCRP). This panel
`represents a cross section of the industry and includes representatives from generating
`companies, distribution companies, inter-connector operators, transmission companies and
`large customers. The Regulator attends meetings as an observer. Any member of the Grid
`Code Review Panel including NGC, can propose a change to the Grid Code although changes
`must be subject to a consultation process and approved by the Regulator before
`implementation.
`
`4.1. The grid code change process for renewable generation
`In September 2002, NGC submitted a paper to the Grid Code Review Panel proposing a change
`to the Grid Code in respect of renewable generation. To minimise future work, the terms of
`reference included all forms of renewable generation, in addition to wind farms. A working
`group (the Generic Provisions Working Group (GPWG)) was established which included
`representatives from generating companies, distribution companies, inter-connector
`operators, trade associations (such as the British Wind Energy Association) and NGC with the
`Regulator attending as an observer. The aim of the working group was to develop a set of Grid
`Code proposals for new and renewable forms of generation suitable for industry wide
`consultation.
`Key parts of the process involved in developing the Grid Code proposals both in England
`and Wales and Scotland are shown in Figure 3. A comprehensive account of this process is
`detailed in [3] [4].
`
`

`

`OctobeiJoilt-::·Ckfubli-jjOJ--1
`England & Wales~
`.Q.urnp followed by Industry
`Wide Consultation D/03 and
`r~_Q!:! .. ~.o_.Q_fg~_
`
`r------<11>-----, i consultations in Scotland and
`
`March - Doc ember 2002
`Two industry wide
`
`i
`
`report to Ofgem
`
`November 2003 - Marth 2004
`I Three Transmission Licensees in Great I
`! Britain align requirements and furtho- !
`! consult with wind twbine manufacturo-s !
`'
`on capabilities & costs
`'
`
`~ ,
`
`March - April 2004
`
`, L,
`l ll1/:J~1~:~~dJ;~~:~~IJ l
`
`June - August 2004
`Industry Wide Consultation
`H/04 forEnglandandWales
`Grid Code
`- Report to Of gem for approval
`
`June -~
`Industry Wide Consultation
`SA2004 for Scottish
`, -~------, Grid Code-Report to Ofgem
`for approval
`
`1
`
`N<mmb,,- 2004 - Fetrua,y 2005
`Ofgemconsult ona combined GB Grid
`Code basis with a Regulatory Irrq:iact
`Assessment
`
`Man:h- Jme 2005
`i Ofgemconsider Consultation Responses
`'
`and apirove Grid Code proposals
`(subject to minor changes) an I June
`2005
`
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`Figure 3.
`
`Diagram of the Grid Code Change Process for Renewable Generation.
`
`The key stages to note as part of the Grid Code development process included, alignment
`of the Scottish and the England and Wales proposals in preparation for a single GB Grid Code,
`detailed discussions with turbine manufacturers, fully open dialogue with all stakeholders at
`discussion forums arranged by the Regulator, full industry wide consultations for all
`interested parties and a Regulatory Impact Assessment.
`
`5. EXPERIENCES OF ADOPTING A GRID CODE TO INCLUDE REQUIREMENTS FOR
`RENEWABLE GENERATION
`The declared aim of all stakeholders involved in the Grid Code process was to enable wind
`generation to connect to the GB Transmission System without undue restriction whilst
`ensuring the continued safety, security and economy of supply. When the Generic Provisions
`Working Group was first established in October 2002, there was a significant difference of
`opinion between the stakeholders. These could be summarised into four key interest groups:-
`
`I.
`
`II.
`
`III.
`
`IV.
`
`Wind Farm developers were reluctant to accept any requirements on the basis of
`additional cost incurred and that technology was still at the proving stage.
`Existing System Users including nuclear power stations and those having no wind
`generation projects were keen to maintain safety, security and economy of supply
`and ensure a level playing field amongst Users.
`Manufacturers were generally supportive of a clear set of technical requirements as
`a basis for increasing market share.
`Utilities required the development of technical requirements for wind generation to
`meet Transmission Licence obligations for maintaining system security and
`stability.
`
`As the process developed, these views started to converge as more information was
`gained from manufacturers’ turbine generator capabilities, experience overseas, additional
`costs and a continuous review of the technical requirements based on transmission system
`
`

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`THE DEVELOPMENT OF GRID CODE REQUIREMENTS FOR NEW AND
`RENEWABLE FORMS OF GENERATION IN GREAT BRITAIN
`
`need. When the H/04 and SA2004 proposals were submitted to the Regulator in late August
`2004, there had been significant movement by all stakeholders. At this stage however, the
`impending introduction of a single electricity market (BETTA) scheduled for 1 April 2005,
`needed a single GB Grid Code, irrespective of the changes required for renewable generation.
`In view of this, the proposals from consultation H/04 and SA2004 were redrafted within the
`framework of the combined GB Grid Code. Ofgem then held a further industry wide
`consultation in January and February 2005, releasing a Regulatory Impact Assessment at the
`same time. After due consideration of the responses from Users to the consultation, Ofgem
`approved the Grid Code proposals (subject to minor changes) to become effective from 1 June
`2005. The full determination by the Regulator is available in [13].
`
`6. SPECIFIC TECHNICAL ASPECTS OF THE GRID CODE REQUIREMENTS
`The aim of the Grid Code modifications was to develop a set of technical requirements which
`would meet the minimum security and stability needs of the transmission system, ensure non
`discrimination between technology types and enable maximum flexibility for developers. For
`this reason, all the technical requirements were based at the point of connection to the Public
`Electricity System rather than at the turbine terminals. Before any Grid Code proposals could
`be drafted, the Transmission Licensees had to understand the behaviour, operation and
`characteristics of non-synchronous generator technologies such as wind turbines. This was
`achieved through information received at seminars, conferences, literature searches of Grid
`Code work, liaison with manufacturers and detailed system modelling studies.
`A number of key issues were identified including fault ride through, frequency range,
`frequency control, power frequency characteristic, voltage / reactive control, power quality
`and data requirements. The technical issues and rationale behind these requirements are
`briefly described below.
`
`A. Fault ride through
`In the event of a significant three-phase transmission system fault, the voltage at the point of
`fault will be zero on all three phases. Due to the small impedance of the transmission lines, the
`voltage depression will be experienced across large geographical areas of the Transmission
`system as shown in Figure 4.
`Where a generating plant is directly connected to a faulted circuit, it will be disconnected,
`i.e. ‘lost’. However, generation connected to adjacent healthy circuits separated by circuit
`breakers would be expected to remain connected and running. Synchronous generating plant
`is required to continue to operate through the transient voltage depression that accompanies
`a network fault provided the fault is cleared in sufficient time to prevent pole slipping.
`However, some early designs of non synchronous generator technologies which employed
`power electronic converters were susceptible to tripping even if the terminal voltage only fell
`below 80% of nominal for periods shorter than that expected for transmission fault clearance.
`Under the Security and Quality of Supply Standards, NGC currently cater for a maximum
`instantaneous generation loss of 1320 MW. From the results shown in Figure 4, and the
`potential risk of wind farm tripping under short duration low voltage conditions, it possible
`that 1320 MW of generation could trip with the additional loss of neighbouring wind farms
`connected to healthy circuits. The corresponding fall in system frequency could result in
`excursions to limits approaching the threshold of the automatic low frequency demand
`disconnection scheme, which if operated would result in wide spread customer demand
`disconnection. Alternatively, the connection of wind generation would have to be
`
`

`

`(cid:127)
`(cid:143)
`(cid:143)
`(cid:127)
`(cid:143)
`(cid:143)
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`(cid:127)
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`significantly constrained thereby resulting in a shortfall against government energy targets
`with additional constraint issues for future conventional generation which intended to
`connect adjacent to future wind-farm sites. Not only would it prevent future opportunities to
`developers, but also it would limit the market for manufacturers.
`
`Scotlan
`d
`
`Location of
`Fault
`
`Fault Location
`
`15 - 30% Volts
`
`40 - 50% Volts
`
`60 - 70% Volts
`
`80 - 90% Volts
`
`0 - 15% Volts
`
`30 - 40% Volts
`
`50 - 60% Volts
`
`70 - 80% Volts
`
`East Coast Round 2 Connection Area
`
`Figure 4.
`
`Network retained voltage during a transmission system fault.
`
`In consideration of these issues, NGC developed a set of technical requirements based on
`the minimum stability needs of the Transmission System. They where supported through
`system studies, manufacturer liaison meetings (Appendix 2 Section 7 [11]), publicly available
`literature and the requirements specified by overseas utilities [5], [6], [7] and [8]. Although
`alternatives, such as fast disconnection and reconnection of turbines, were initially
`investigated, these were rapidly rejected on the basis that they would not meet the stability
`requirement. The fault ride through requirements are detailed in the Grid Code [14]
`Connection Conditions clause, CC.6.3.15, and can be summarised as follows:-
`
`1)
`
`The plant should remain connected and transiently stable for any fault on the 400 or
`275 kV Transmission System for a total fault clearance time of up to 140 ms.
`
`

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`THE DEVELOPMENT OF GRID CODE REQUIREMENTS FOR NEW AND
`RENEWABLE FORMS OF GENERATION IN GREAT BRITAIN
`
`Following fault clearance, the plant should generate at least 90% of the pre-fault
`active power which should be available within 0.5 seconds of recovery of the
`voltage at the connection point. During the period of the fault, the plant is required
`to generate maximum reactive power without exceeding the rating of the machine.
`In addition, the plant shall be required to remain connected and transiently stable
`for other disturbances which could include remote faults cleared in backup
`operating times. To achieve this objective, the plant is required to remain connected
`and transiently stable for supergrid voltages above the heavy black line shown in
`Figure 5.
`
`Supergrid Vo ltage
`(% of Nominal)
`
`208
`
`2)
`
`3)
`
`90
`85
`80
`
`15
`
`
`
`0.14s
`
` 1.2s
`
`
` 2.5s
`
` 3 minutes
` Supergrid Voltage Duration
`
`Figure 5.
`
`Voltage Duration profile.
`
`4)
`
`5)
`
`For slow clearing faults, the plant is required to achieve the same requirements as
`defined in item 2) above, although the active power restoration is only required in
`proportion to the retained supergrid voltage. In addition, 90% of the active power
`must be achieved within 1 second of restoration of the supergrid voltage to nominal
`levels. An allowance is made if the wind speed has dropped after the disturbance
`has been cleared.
`Relaxation’s were added to these requirements in the situation where less than 5%
`of the turbines were running or under very high wind speed conditions when more
`than 50% of the turbines in a wind farm had been shut down.
`
`While specific to Great Britain, the above requirements are consistent with those of other
`international utilities. A large number of manufactures advised that solutions were available
`which would result in minor additional costs. In addition, independent studies [9]
`demonstrated that fault ride through capability was cost effective for customers if large
`penetration levels were to be accommodated within the system.
`
`

`

`......,._,. ---
`
`.,..~
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`
`209
`
`B. Frequency range
`The NGC Transmission System operates to a nominal frequency of 50 Hz. Currently all the
`plant of system users is required to operate over a continuous frequency range of 47.5 – 52.0
`Hz and 47.0 – 47.5 Hz for a period of 20 seconds.
`The ability of generating plant to maintain operation over this frequency range is required
`in order to (i) avoid a frequency collapse for additional generation losses, and (ii) minimise the
`risk of partial or total system shutdowns during larger system disturbances. Since the system
`frequency is dependant on the total generation and load, it is essential that all generating plant
`is capable of operating over the same frequency range. On this basis, the same frequency
`range was applied to renewable generating plant as was to synchronous generating plant.
`Manufacturers were able to confirm that their plant was capable of meeting such a
`requirement.
`
`C. Power frequency characteristic
`Whilst the generation connected to the System is required to operate over the full frequency
`range, it is essential that the power output of the generating plant over this range is controlled
`to minimise the effect of a system frequency collapse or over-speed situation. Existing
`synchronous generating plant is required to maintain 100% of its active power output between
`49.5Hz – 50.4 Hz. Between 49.5 to 47.0 Hz the power output should not fall by more than pro-
`rata with frequency (i.e. at 47.0 Hz, the power output should be no less than 95% of that at 49.5
`Hz). For frequencies in excess of 50.4 Hz, the power output should be reduced by at least 2% of
`output for each 0.1 Hz rise in system frequency above 50.4 Hz.
`The requirement for the power output not to fall by more than pro-rata with frequency
`below 49.5 Hz is to ensure that the additional generation loss is not greater than the natural
`drop in demand that accompanies falling system frequency. The requirements which exist for
`synchronous plant were therefore applied to renewable generating plant (including wind
`farms). Similar requirements had been adopted in Germany [5] and manufacturers confirmed
`such a requirement would not create technical or commercial issues.
`
`D. Frequency control
`One of the principle requirements of the Electricity Safety, Quality and Continuity
`Regulations 2002 is that there should not be a permanent change in system frequency outside
`the statutory limits of 50.0 Hz ± 0.5 Hz.
`
`5 0 .4
`
`5 0 .3
`
`5 0 .2
`
`5 0 .1
`
`5 0
`
`4 9 .9
`
`4 9 .8
`
`4 9 .7
`
`4 9 .6
`
`4 9 .5
`
`4 9 .4
`
`FREQUENCY (Hz)
`
`Frequenc y Trace, 2 6-M ay-200 3
`
`Pr(cid:0)im(cid:0)ar(cid:0)y R(cid:0)es(cid:0)po(cid:0)ns(cid:0)e (cid:0)0 (cid:0)- (cid:0)30(cid:0) s(cid:0)ecs(cid:0)
`
`Se(cid:0)co(cid:0)nd(cid:0)ar(cid:0)y R(cid:0)es(cid:0)po(cid:0)ns(cid:0)e 30(cid:0) s(cid:0)ec(cid:0)s (cid:0)- (cid:0)30(cid:0) m(cid:0)in(cid:0)s(cid:0)
`
`5 0 .4
`
`5 0 .3
`
`5 0 .2
`
`5 0 .1
`
`5 0
`
`4 9 .9
`
`4 9 .8
`
`4 9 .7
`
`4 9 .6
`
`4 9 .5
`
`4 9 .4
`
`FREQUENCY(Hz)
`
`00:37:00
`
`00:36:00
`
`00:35:00
`
`00:34:00
`
`00:33:00
`
`T IM E ( G M T)
`
`frequency
`
`target freq
`
`Figure 6.
`
`Secured credible instantaneous generation loss of 1175 MW.
`
`

`

`F""~,.,, I i
`
`:
`
`'
`
`Time(c)
`
`~ : \ ............ ~ -:--: - - -~
`
`r,
`
`T n -------r.:1~:::~:::,"""'=·"''"'""'""'~,m~--- T o
`
`I
`
`T,
`
`()um'lltd
`WinclPo""'c-r
`(MW)
`
`RHDQOSI! fi:om a Wind Farm for a Hieb SV.mm....uealllD&Y..
`i ...... / \ .. (············\······.·\r-=
`
`j
`
`_.,,.._.. ..... --..
`
`••• Polenlllal Peak Outpd
`
`-
`
`RHponslvti Output
`
`Frcqumcy
`Oni.tion
`(1h)
`• 0.5
`
`I
`
`: v
`
`Timc(t)
`
`\.,__
`
`1in1t(t)
`
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`
`210
`
`THE DEVELOPMENT OF GRID CODE REQUIREMENTS FOR NEW AND
`RENEWABLE FORMS OF GENERATION IN GREAT BRITAIN
`
`In order to achieve this, it is essential for generating plant to be equipped with a governor
`which will control MW output following a sudden rise or fall in system frequency. Figure 6
`illustrates an actual instantaneous generation infeed loss secured by frequency responsive
`generation plant and some demand management.
`From an international perspective, the GB power system (and hence its overall inertia) is
`comparatively small, certainly in relation to the wider European UCTE system. On this basis,
`frequency control capability is an essential requirement from all forms of generation
`(including wind generation), especially because of the range in system demand that can be
`experienced and the expected volume of wind generation.
`It is acknowledged that active power from a wind farm will be constrained by wind speed,
`however this will not restrict the ability of a wind farm to provide frequency response. In
`Figure 7a, the delivery of low frequency response is illustrated. The top red dotted line
`indicates the power that could be exported by a wind farm extracting the maximum available
`power from the wind, whilst the lower blue line indicates the effect of part loading through the
`action of blade pitching. As the system frequency changes, the power output will increase (or
`decrease) in proportion to the frequency. In contrast, Figure 7b shows the delivery of high
`frequency response only. For this restricted service, the spilled renewable energy is small, as
`the wind farm is free to operate at full output at a nominal system frequency of 50 Hz.
`
`Figure 7a.
`
`Illustration of high frequency response from a Wind Farm.
`
`Discussions with wind turbine manufacturers and publicly available literature [10]
`demonstrated that frequency control could be implemented within a wind farm. On this basis,
`and in view of the construction phase of the Round 2 projects, the Grid Code requires that wind
`farms in operation after 1 January 2006 shall be capable of contributing to frequency control.
`
`Figure 7b.
`
`Illustration of high frequency response from a Wind Farm.
`
`

`

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`WIND ENGINEERING VOLUME 29, NO. 3, 2005
`
`211
`
`There was concern from wind farm developers that they would be commercially
`penalised for de-loading before providing frequency response, as no benefit would be gained
`from a corresponding reduction in fuel cost and even more the loss of incentives (eg ROC’s).
`However, the current market arrangements allows the Generator to specify the price (the
`‘bid’) at which they are willing to be de-loaded. Once de-loaded, the Generator will also
`receive two additional payments for providing frequency response (holding and response
`energy payments). Therefore. the economics of de-loading for provision of frequency
`response could be reflected in the Generation Bid prices. After 1 October 2005, the holding
`price is also set by the plant owner. This situation is demonstrated in Figure 7a, where the
`lower blue line indicates the wind farm is operating to a defined contract position in the GB
`electricity market. Under some circumstances, e.g. minimum demand, provision of high
`frequency response without de-loading may be an attractive service.
`
`E. Reactive power and voltage control
`Reactive power and voltage control are required in order to protect plant from over-voltages,
`facilitate the transfer of active power, maintain adequate voltage quality and stability, and
`assist in securing the system under post fault conditions. The principle method of voltage
`regulation on the HV Transmission System is through the control of the excitation system on
`synchronous generating units. Whilst synchronous generating units have an inherent
`capability to provide reactive power, a wind farm comprised of pure induction generators and
`collection network could result in the uncontrolled absorption or generation of reactive
`power onto the transmission network. In order to control and provide voltage quality to
`customers, additional reactive support may be needed from adjacent synchronous generation
`or dedicated reactive compensation plant. In developing th