use of statcom for offshore wind stability & grid compliance

Use of STATCOM for Offshore wind stability & grid compliance

C Smith 15/10/09

This document contains confidential and proprietary information of Converteam and must not be used for any purpose other than that for which it is supplied by Converteam. Its contents must not be disclosed to any other

person nor copied in whole or in part without the prior written consent of Converteam

CUSTOMISED TECHNOLOGY FOR CUSTOMER SUCCESS© 2009 Converteam UK Ltd. All rights reserved COMMERCIAL IN CONFIDENCE 15th September 2009 2

Introduction

Grid code requirements Options for VAr compensation for offshore windfarms STATCOM Case Study Additional benefits of STATCOM


UK Grid Code MVAr Wind Farm Requirements

Reactive Power “ Q “

MVArs versus power current “ P “

0 %+ 30 %- 50 %

Lag

50 %

100 %

+ 50 % Lead

0 %- 30 %

Requirement is for a rated +/- 0.95 PF.

This is +/- 33 % MVArs at 100 % AC volts.

Below 20 % power MVArs can be zero.P

Continuous control required

Requirement is for MVArs defined at a +/- 5 % AC supply voltage level.

Below 95 % AC supply voltage the MVArs are not defined but are to be at the plant’s ability.

For optimal system design the MVArs current should be constant for 0 to 110 % AC supply voltage.

MVArs versus network voltage “ V “

UK grid rating point

V

Q min Q max

100%

Voltageat PCC


Solutions to meet grid codes

Mechanically switched reactors/capacitors (MSR/C)

Cost effective Cannot provide continuous control

Static VAr Compensation (SVC)

Cost Effective for large MVAr rating Not constant current below rated voltage

STATCOM (Static Synchronous compensator)

Constant current down to 20% voltage Most expensive per MVAr of technologies

Combination Switched devices/STATCOM Cost effective Keeps all the advantages of STATCOM


Converteam’s MV ProVAr STATCOM

DC capacitors

DC grounding switchIGBT phase power stack

Terminals to transformer

Controller PECe

Cooling unit with standby pump

Terminals to motor


Design decisions - example

Nominal offshore wind farm2 off 132kV step out cables of 90km each to meet UK regulator (OFGEM) recommendations72 turbines in 8 strings Connection agreement of 260MWCapacitive element of step out cables 99 MVArTwo design options reviewed

Option 1 – MSR/C and Wind turbines

Option 2 - MSR/C, Wind turbines and STATCOM

132kV

Y

Step outcables

33kV

132kV

Y

33kV

Y

36 Turbines 36 Turbines

Compensationequipment

PCC

On shore

Off shore


Example – reactive power requirements

Load flows give following span 99 MVAr reactors to compensate

for step out cables 20MVAr capacitive point A 72MVAr inductive point C

Option 1 Six 15MVAr & one 9 MVAr

reactors 72MVAr from Turbine (step out

cables limited)Option 2

five 15MVAr and one 9MVAr inductor

One 15MVAr STATCOM 72MVAr from Turbine (step out

cables limited)


Options review-based on example

Option 1 – MSR/C and Wind turbines Scheme Cost of £x

Option 2 - MSR/C, Wind turbines and STATCOM Scheme cost of 1.5 x £x (50% cost addition)

Easy decision based on cost? Cost difference compared to farm complete CAPEX is approximately

0.3% of total CAPEX % Differential will reduce as wind farm becomes larger Additional benefits for offshore


Offshore distribution system stability

Example from a study of a UK offshore wind farm Long cables introduce resonances Number of turbines connected alters circuit impedance. Resonance point moves

10 Drives15 Drives20 Drives25 Drives

7th harmonic 12th harmonic


Impact on offshore distribution system

Significant amplification of existing grid harmonicsWind Turbine converters have very high control bandwidthPossibility of random/nuisance trips

At At PCCPCC

At wind At wind turbine turbine

converterconverter

Vol

tage

Vol

tage

Harmonic PCC (%) WTG (%)

9 1 4

11 1 20

13 1 3


Solutions

Reduce wind turbine converter controller bandwidth Reduces control performance Could impact on Grid Fault Ridethrough capability

Passive Filter Difficult to design for all combinations Costly

Active Damping Can be added to STATCOM with additional software

Vol

tage

Vol

tage

Active filter offActive filter off Active filter onActive filter on


Frequency response

Grid generation/loads changing Wind farms to play a more active role in Ancillary service HydroQuebec grid code now calling for synthetic inertia from wind

farms At present requires derating of wind turbines output

Loss of revenue Energy storage can be added to STATCOM Possible technologies

Supercapictors Flow cells Flywheels

Technology only just becoming commercially viable Our STATCOM example would give 5% at full generation or 15%

capability based on utilisation factor (40%)


Converteam ProVAr With Energy Store

Each cap store: 250 units, 125 in series. Net 2.6 F. 11 MJ available.

Provides fast MVAr and MW support for the AC supply down to zero retained AC supply voltage.

Provides fast acting current support to assist in fault recovery and AC supply frequency control.

ACsupply

Transformer

SupercapDC

energystore


Conclusions

Temptation to choose least cost option of mechanically switched devices

STATCOM provide additional advantages: Active damping to improve wind farm stability Frequency response can be retrofitted when

commercially viable

This document contains confidential and proprietary information of Converteam and must not be used for any purpose other than that for which it is supplied by Converteam. Its contents must not be disclosed to any other

person nor copied in whole or in part without the prior written consent of Converteam

www.converteam.com

Thank you for your attention

http://www.converteam.com/

use of statcom for offshore wind stability & grid compliance

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