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Research Group&

Power Electronics and HVDC for 2030

Prof Tim Green

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Research Group&

Examples of Power Electronics

owerontrol

Research Group&Sylwin Alpha VSC Converter Station

Siemens MMC Technology at 864 MW at ±320 kV

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Research Group&Power Electronics for

Distribution Networks

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Research Group&Traditional Distribution

Network Control

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Research Group&Is Power Electronics

Business-as-Usual?

Generation

Transmission Distribution

End-Use

Not “is it universal?” but “is it part of the standard set of solutions?”

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Research Group&How far has power electronics got

in energy networks?

•  End Use •  Energy efficiency and controllability call for power electronics

•  Generation •  Power electronics established as the means to optimise wind turbine

operation for maximum energy yield •  Solution has relatively high capital cost but the cost/benefit case is clear

•  Transmission •  HVDC established as the means to operate long / high-capacity cables •  Solution has relatively high capital cost but the cost/benefit case is clear •  HVDC ratings expansion and system integration still challenging •  FACTS (reactive power compensators, series compensators) can increase

utilisation of transmission assets

•  Distribution •  Several “electronic substation” research themes being pursued •  Demonstration projects by network operators

Distribution is unconquered territory for power electronics but the advance now seems to be underway

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Research Group&

Source: Prof. Xiaoxin Zhou （周孝信） Birmingham, October 2015

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Research Group&Transmission System Expansion

and Interconnectors in the UK

Sizewell

Pembroke

Osbaldwick

RowdownBeddington

ChessingtonWest

Landulph

Abham

Exeter

Axminster

Chickerell

Mannington

Taunton

Alverdiscott

Hinkley Point

Bridgwater

Aberthaw

Cowbridge

PyleMargam

SwanseaNorth

CardiffEast

Tremorfa

Alpha SteelUskmouthUpper Boat

Cilfynydd

ImperialPark

Rassau

Whitson

Seabank

Iron Acton

Walham

Melksham

Minety DidcotCulham

Cowley

Bramley

Fleet

Nursling

Fawley Botley Wood

Lovedean

Bolney

NinfieldDungeness

Sellindge

Canterbury

E de F

Kemsley

Grain

Kingsnorth

Rayleigh Main

Littlebrook

Tilbury

Warley

Barking

W.HamCity Rd

BrimsdownWaltham

Ealing

Mill HillWillesden

Watford

St Johns

Wimbledon

New Hurst

Elstree

Rye House

N.Hyde

Sundon

Laleham

Iver

Amersham Main

WymondleyPelham

Braintree

BurwellMain

Bramford

EatonSocon

Grendon

EastClaydon

Enderby

Walpole

NorwichMain

Coventry

Berkswell

Rugeley

Cellarhead

IronbridgeBushbury

Penn

Willenhall

OckerHill

KitwellOldbury

Bustleholm

Nechells HamsHall

BishopsWood

Feckenham

Legacy

Trawsfynydd

Ffestiniog

Dinorwig

Pentir

Wylfa

Deeside

Capenhurst Frodsham

Fiddlers

Rainhill

KirkbyListerDrive

Birkenhead

WashwayFarm

Penwortham

Carrington

SouthManchester

Daines

Macclesfield

Bredbury

Stalybridge

Rochdale

WhitegateKearsley

Elland

Stocksbridge

WestMelton

AldwarkeThurcroft

BrinsworthJordanthorpe

Chesterfield

Sheffield CityNeepsend

Pitsmoor

Templeborough ThorpeMarsh

Keadby

WestBurton

Cottam

HighMarnham

Staythorpe

Stanah

Heysham

Padiham

Hutton

BradfordWest Kirkstall Skelton

Poppleton

Thornton

Quernmore

Monk

EggboroughFerrybridge

Killingholme

SouthHumberBank Grimsby

West

Drax

Lackenby

GreystonesGrangetown

Saltholme

Norton

Spennymoor

Tod PointHartlepool

Hart Moor

Hawthorne Pit

Offerton

West BoldonSouth ShieldsTynemouth

StellaWest

Harker

Eccles

Blyth

IndianQueens

Coryton

RatcliffeWillington

Drakelow

Shrewsbury

Cross

Weybridge

Cross

Wood

North

FrystonGrange

Ferry

Winco Bank

Norton Lees

Creyke BeckSaltend NorthSaltend South

Hackney

BaglanBay

LeightonBuzzard

PatfordBridge

Northfleet EastSinglewell

Fourstones

Humber Refinery

SpaldingNorth

West Thurrock

ISSUE B 12-02-09 41/177619 C Collins Bartholomew Ltd 1999

Dingwall

Dounreay

Newarthill

Cumbernauld

Kincardine

WishawStrathaven

KilmarnockSouth

Ayr

Coylton

Inveraray

HelensburghDunoon

Inverkip

DevolMoor

Hunterston

Sloy

Fort William

Bonnybridge

Neilston

Ceannacroc

Conon

Fort AugustusFoyers

Inverness

Stornoway

Elvanfoot

Kaimes

Glenrothes

Westfield

Grangemouth

Longannet

Linmill

Bathgate

Errochty Power Station

TornessCockenzie

Keith

Thurso

FasnakyleBeauly

Deanie

Lairg

Shin

Nairn

Kintore

Blackhillock

Elgin

Keith

Peterhead

Persley

Fraserburgh

InvergarryQuoich

CulligranAigas Kilmorack

GrudieBridge

Mossford

OrrinLuichart

Alness

Brora

CassleyDunbeath

Mybster

St. Fergus

Strichen

Macduff

Boat ofGarten

Redmoss

Willowdale

Clayhills

Dyce

Craigiebuckler

Woodhill

Tarland

DalmallyKillin

Errochty

Tealing

GlenagnesDudhope

Milton of CraigieDudhope

Lyndhurst

CharlestonBurghmuir

Arbroath

Fiddes

Bridge of Dun

Lunanhead

St. Fillans

Finlarig

LochayCashlie

Rannoch

TummelBridge

Clunie

Taynuilt

Nant

Cruachan

PortAnn

Carradale

Auchencrosh

Lambhill

ClydesMill

Glenluce

NewtonStewart

Maybole

Dumfries Ecclefechan

Berwick

Hawick

Galashiels

Dunbar

Meadowhead

Saltcoats

HunterstonFarm

SP TRANSMISSION LTD.

Kilwinning

Currie

Cupar

Leven

Redhouse

Glenniston

SCOTTISH HYDRO-ELECTRICTRANSMISSION

Telford Rd.Gorgie

KilmarnockTown

Busby

Erskine

Strathleven

MossmorranDunfermline

Broxburn

Livingston

Whitehouse

ShrubhillPortobello

Devonside

StirlingWhistlefield

SpangoValley

Ardmore

Broadford

Dunvegan

NGC

Easterhouse

EastKilbrideSouth

Gretna

Chapelcross

THE SHETLAND ISLANDS

Tongland

GlenMorrison

Clachan

400kV Substations275kV Substations400kV CIRCUITS275kV CIRCUITS

Major Generating Sites Including Pumped Storage

Connected at 400kVConnected at 275kVHydro Generation

TRANSMISSION SYSTEM REINFORCEMENTS

Langage

BlacklawWhitelee

Iverkeithing

Marchwood

BickerFenn

Coalburn

REINFORCED NETWORK

Under Construction or ready to startConstruction subject to consents

Very strong need case

Series Capacitors

RedbridgeTottenham

Strong need case

Future requirement, but no strongneed case to commenceat present

“The Western Bootstrap” 420 km HVDC Subsea Route Hunterston to Connah’s Quay Contract awarded to Siemens ±600 kV, 2,200 MW, CSC

“BritNed” HVDC Subsea Route Kent to The Netherlands Built by Siemens ±500 kV, 1,000 MW

Deeside to Ireland, 500 MW, at ±200 kV

“IFA“ Sellindge to Bonningue-lès-Calais, 2,000 MW at ±270 kV CSC

Ayrshire to NI, 500 MW

“The Eastern Bootstrap” HVDC Subsea Route Peterhead to Tyneside Status unclear.

“Nemo”, 1,000 MW VSC

“NSN”, 1,400 MW, Blythe to Norway 730km, 2€Bn

“IFA2“Chilling to Tourbe, 1,000 MW

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Research Group&

Offshore Wind Power Projects

London Array – 640 MW - Operational

Thanet – 300 MW - Operational

Dogger Bank – 7,200 MW - Approved

Hornsea 1– 1,200 MW – Approved Hornsea 2– 1,800 MW – Submitted

Great Gabbard – 504 MW - Operational

Firth of Forth– 3,600 MW - Approved

UK Peak demand: 60,000 MW UK Offshore installed wind capacity: 4,050 MW

UK Offshore wind under construction or approved: 11,000 MW UK Onshore installed wind capacity: 8,080 MW

Plus other renewables by 2030: 25,000 MW

180 km

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Research Group&

How Might a North Sea Grid Look

Manyideasonwhatagridmightlooklike:

Onecommonfeature:HVDClinksareconnectedatvariousnodestoformnetworkswithrou:ngop:ons

EWEA

Friends of the SuperGrid

European Commission

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Research Group&

Power Electronics in Grids

•  Traditional electrical machines present a voltage source behind an impedance to a fault

•  Thermal time-constant of machine is such that large fault currents can be tolerated for several seconds.

•  Inertia is such that power imbalance can be tolerated for circa 200 ms

•  Decades of experience on using sequence-set decomposition for asymmetric faults and on identifying sub-transient reactance etc.

•  Power electronics present a voltage or current source as dictated by action of limiters in control loops

•  Almost no tolerance of over-current without over-rating •  Little ability to absorb power mismatch •  Fault-ride through needs to be a designed-in feature

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Research Group&

Inertia Deficit in AC Systems

•  At times of high renewable energy production, conventional generators are stood down and the real physical inertia of the AC system is reduced.

•  Rates of change of frequency become higher and frequency deviations larger

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Research Group&

AC Grid Strength •  South East England is where several HVDC interconnectors land and is a

region that has little synchronous plant. •  The short circuit ratio is low and reactive current during a fault is sought. •  Control interactions possible in weak grid •  Cascading failure through failure to ride-through network faults is a risk.

To Netherlands (BritNed)

To Belgium (Nemo Link)

To France (ElecLink)

To France (IFA)Source: System Operability Framework 2014, National Grid

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Research Group&

Challenges for HVDC

•  Increase ratings •  Decrease losses, footprint and cost •  Incorporate services on top of energy transfer •  Provide short-term ratings to accommodate

services and emergency action

•  Realisation of DC networks

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Research Group&

Modular Multi-Level Converters

•  Each cell is switched to add voltage into circuit or bypass

•  Always have half the cells providing voltage but change proportion between upper and lower arms

•  Upper and lower arms act as potential divider

•  AC current splits equally between upper and lower arms

•  DC current circulates through both arms

•  AC terms sum to zero at DC bus and DC current is smooth

•  Controlling the various current flows is the main task

Graphics from: http://en.wikipedia.org/wiki/HVDC_converter

Iarm =½IAC+⅓IDC

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Research Group&

MMC Rating Example

Rating 600 MW DC bus voltage ±300 kV DC current 1 kA AC voltage (line) 330 kV AC frequency 50 Hz SM voltage 1.5 kV ±20% SMs per arm 400 Total number IGBTs 4800 SM capacitor 4.37 mF Total number Capacitors 2400

NCapacitors = NSM = 6VDCVSM

NIGBT = 6×NIGBT per SM ×NSM perarm

=12VDCVSM

NSM perarm =V̂ArmVSM

=VDCVSM

Number of Sub-Modules per arm

Number of SM capacitors

Number of IGBTs

In practice, more than 400 cells per arm would be installed to provide redundancy and allow continued operation after a reasonable number of cell faults

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Research Group&P/Q Capability Chart:

Synchronous Machine

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Research Group&

MMC Converter Ratings

Siemens MMC Technology at 864 MW at ±320 kV

Alstom Sub-Module at circa 1,000 A and 1,700 V

Voltage ripple on sub-module capacitor

Commutation current limit of IGBTs

Temperature of IGBTs

Modulation limit Available arm

voltage

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Research Group&

P/Q Capability of MMC

In general, reactive power capability is limited Common design is for Q=±0.3 pu for P=±1.0 pu Limits on P/Q capability of MMC depend on AC line voltage.

Paul Judge

At 1.05 PU Line Voltage At 0.95 PU Line Voltage

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Research Group&Converter Ratings and

Design for Overload CanwetemporarilyexpandtheP/Qenvelopetoachieveoverload?

Oneexample:runcontrolledcircula?ngcurrenttoreducesuppresspeaks

Paul Judge

At 0.95 PU Line Voltage

Designpenaltyissmallifreac?vepowerrequirementduringoverloadisdecreased

Increasedlossesmeanthisisnotusedduringnormalopera?on

P=1.30pucanbeachievedwithmodestincreaseinsub-modulenumbers

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Research Group&Using Overload Rating for

Loss of In-feed Mitigation Assume pair of HVDC interconnectors of ±500 kV and 2.5 kA (2.5 GW) One link suffers an outage. Overload capability (set at 30%) of the other link is used to reduce the loss of in-feed and reduce the frequency error This action gives time for further action to be planned

120.104.88.072.056.040.0 [s]

6000.

4700.

3400.

2100.

800.0

-500.0

3.18 GW With Overload

2.5 GW No Overload

DC Power Flows in HVDC LinksLink A Outage (-2.5 GW)[MW]

DIgSILENT

120.104.88.072.056.040.0 [s]

51.0

50.6

50.2

49.8

49.4

49.0

49.36 Hz

49.53 Hz

Grid FrequencyLink A Outage (-2.5 GW)[Hz]

DIgSILENT

Claudia Spallarossa et al.

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Research Group&

Devicejunc?ontemperaturesmaybecomeanissueduringoverload.DynamicRa?ngshouldbeusedtoprovidelargeamountofextrapowerduringstartofsystemeventsthenreducetoasteady-stateoverloadra?ng

Thermal Implications of Converter Overload Paul Judge

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Research Group&

Avoiding Large Loss of In-feed from DC

•  Weshouldavoidsingle-pointfailuretoavoidlargeloss-of-infeedrisk;

•  Metallicreturncableallowsbi-poleconfigura?ontobeswitchedtomono-poleathalf-power

•  Ormono-poleAACandFB-MMCcanprovidesamehalf-poweropera?onbyopera?ngat½VDC

•  Thereareotherriskstoguardagainsttoo(transformerfaultsetc.)

Phil Clemow

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Research Group&Operation of Fault-Blocking Converter

with a Pole-to-Ground Fault

1)Faultcollapsesvoltageononepole

2)BriefcurrentspikecausedbyDCbuscapacitorsdischargingintothefault.Currentsthenwell-controlled

3)Cellvoltagesandarmcurrentsarewell-controlled

4)Converteroperatesindefinitelyat50%powerandcon?nuesreac?vepower

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Research Group&

Hybrid IGBT-Thyristor Structures

•  IdeasarebeinggeneratedbyGEGrid(Alstom)andothers

•  Isthisthewaytobreaksomeofthedensity-losses-costcompromises?

•  DoesthishelpusinterfaceCSCandVSC?

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Research Group&

UHV Challenges

•  Converter Density •  Valve halls are essentially air-insulted •  Size of semiconductors, heatsinks and capacitors are substantial •  But the shields and clearance distance substantial too •  The size of a platform “top-side” determines cost as much as the weight.

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Research Group&

UHV Challenges /2

•  Cable Voltage Transients •  Excursions beyond the nominal cable voltage might happen due to ripple,

fast changes in power-flow, fault conditions or deliberate over-loading •  We could pour effort (and cost) into curtailing these in the power electronics •  But what is the optimal system solution? •  Is there scope for using short-term over-voltage operation, as well as over-

current, to provide emergency support?

•  Mono-Pole Operation •  What grounding arrangements are feasible?

•  There are some important opportunities for co-design of HVDC sub-systems to get a better overall system

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Research Group&

Power Electronics in Distribution Networks

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Research Group&What Drives Interest in PE

in Distribution Networks?

Growth of distributed generation, notably PV, and electric vehicle charging cause:

• Excessive current flow in lines and transformers

• Excessive voltage drop or rise • Changes to load cycles,

peak:average ratio etc. • Possible power quality problems

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Research Group&

Network Capacity Studies

XxY=353mx292m

QP30257

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Detailed studies of LV networks show that for suburban networks heat pumps and EV chargers will depress the voltage Operation experience in several countries already showing voltage rise difficulties with PV

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Research Group&

Traditional Network Reinforcement

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Research Group&What are the options for avoiding or

postponing traditional reinforcement?

Uprating cables and transformers is disruptive, expensive and hard to achieve in confined spaces. What are we seeking to achieve? •  Prevent voltage limits impinging before thermal limits

•  By increasing the control options in the network •  Even up loading between phases

•  Dynamically rebalancing voltage and flows to avoid limit on individual phase •  Even up loading between feeders

•  Route power between feeders •  Even up loading between transformers

•  Create new routes power between substations or parts of substations •  Alleviate power quality problems

Can power electronics fulfil these functions well? Can a business case be made?

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Research Group&

Existing Voltage Control

•  Transmission v Distribution •  X:R ratio much lower – R×P

matters •  Few sources of Q anyway •  Radial structure favours tap-

changers

•  Problems •  No tap-changers at 11kV / 0.4 kV

substations •  All control at 33 kV / 11 kV

substations •  All feeders subject to same

change

V

d

Feeder withgeneration eased down

Feeder with heavy load eased up

11kV33kV

ΔV ≈RP + XQ

V

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Research Group&

Some LV Feeder Options

Heavily loaded feeder

Soft open-point Relieves feeder loading

and regulates

Electronic Transformer Regulates each feeder

Feeder Regulator Regulates one feeder

Mid-Feeder Regulator Regulates section of feeder

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Research Group&

DC Use of AC Cables

•  Similar case to HVDC capacity improvement •  Utilisation of maximum voltage continuously, rather than

just at peaks. •  Given converter at point-of-load, could raise line

voltage •  Cable insulation is actually 1,000V in many cases.

But •  How will a cable used on AC for 50 years age if now

used for DC? •  How do you use the cores fully in DC if cable is 3-

core plus neutral/earth? •  How will a DC feeder be protected?

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Research Group&

Soft Open Points at 400 V

FUN-LV is a Low Carbon Networks Fund Project to demonstrate SOPs. Led by UK Power Networks with Riccardo, Imperial College, CGI and TurboPower Systems

A 2-port 400 kVA SOP installed in street cabinet in Brighton

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Research Group&

SOP Construction and Testing

A 3-port SOP installed in a substation

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Research Group&HV Power Electronics:

Construction Techniques •  Projects Equilibrium and ANGLE-DC will

demonstrate power electronics at HV and higher power

•  Constructing power electronics for 10s kV and 10s MVA falls between two existing formats

•  MVDC Converter Stations

•  Machine Drives

•  The volume, losses, cooling arrangements, costs and system integration issues are formidable 18MW, ±30kV MMC

7MW, 6.9kV Drive

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Research Group&

Remarks on PE in Distribution

•  Use-cases for power electronics in distribution networks are becoming clearer

•  Focus is on easing voltage and line-flow limitations to growth of network use

•  Avoiding processing all of the power, all of the time looks important

•  Voltage control not primarily a reactive power issue •  Network compatibility in terms of control format, fault

management, treatment of unbalance etc. is important •  A whole-life, whole-system view of costs is needed. •  Physical volume and inefficiency are impediments.

•  Insulation systems that allow more compact designs and still allow good cooling would really help the overall system design

•  Again, it’s a system-of-system optimisation problem