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Offshore grids and HVDC

Janaka Ekanayake and Tim Green

Content

· Main drivers for offshore grid

· Technologies

­ Current Source Converter based ­ Voltage Source Converter based

· Multi-terminal

Current Offshore Wind Farms in the UK

972MW installed capacity, further 3.5GW being implemented in 2010/11 All AC Radial connections to shore

Wind turbine rating of between 2 ­ 3.6 MW (except Beatrice ­ 5 MW)

Nine offshore wind farm zones licences which could generate upto 32 GW was recently announced. Some of them are situated as far as 195 km

Transmission Possibilities

Turbines Generate at ~1kV with a rating of between 2 to 5MW

Transformers inside Turbines step voltage up to 33kV

Maximum Rating of 33kV Cable is 40MW Large Offshore wind farm capacity > 100MW Multiple 33kV cables High Voltage DC

Wind farm connecting point Onshore converter Offshore converter

High Voltage AC

DC link cable

Main AC grid Offshore wind farm

Reasons for choosing HVDC transmission The break-even point between AC and offshore

Capital Costs

Wind generator and its power electronic interface

DC transmission depends on the distance to shore and the power rating.

Main costs:

Grid transformer

HVAC ­ cables HVDC ­ converter stations

Wind farm side converter

Network side converter

Increasing cable length and ratings HVDC more economical

Comparison of Distance vs. Cost for a 1 GW offshore wind farm

3-core 132kV AC cable x5 (with compensation)

Cost per MW (£k)

Break-even point for 132 kV AC and 2x500 MW DC Transmission

Break-even point for 220 kV AC and 2x500 MW DC Transmission

Ref: Econnect, East Coast Transmission Network Technical Feasibility Study. 2008, Crown Estate.

Distance from shore (km)

2x 1core 300kV bipolar HVDC

Planned offshore connections - UK

· Two HVDC offshore transmission links to strengthen the north to south interconnection are proposed. · Alternatives are on-land reinforcement of North South circuits as well as more effective management of existing assets · Offshore wind farm connections on a multi-terminal configuration is possible

European Super Grid

`Oceans of Opportunity', published Sept 2009, EWEA

Currently existing Currently planned Under study Under study-EWEA recommendation EWEA recommended grids by 2020 EWEA recommended grids by 2030

A widespread electricity network across Europe would have a variety of advantages: · Deregulation of energy markets · Regional balancing of energy generation · Increased security of supply · Reduced dependency on fuel imports · Smoothing of load spikes due to the continental spread of time zones

7

HVDC technologies

Current Sourced Converter (CSC) Thyristor based

Voltage Sourced Converters IGBT based

Bipole 1 Bipole 2 Control Building Filters

Filters

Cable Route

SVC 2

400kV GIS Substation

SVC 1

Inside a thyristor valve hall

Naptune HVDC station Long Island

Current Source versus Voltage Source

· · AC side

­ ­ Established voltage required Reactive power required by converter

AC side

­ ­ Inductor to couple two voltage sources Reactive power supply not required Capacitors holds the voltage constant Direct current can reverse giving power reversal DC capacitor (& converters) feeds the fault Self-commutated (IGBT, GTO)

·

DC side

­ ­ ­

·

DC side

­ ­ ­

­

Inductor for constant current Direct current is unidirectional Power reversal through voltage reversal Inherent fault (dc side) current limiting

Line commutated (Thyristor)

· · ·

Switches

­

·

·

Switches

­

Power ratings up to 7,000MW

Power power ratings up to 400 MW Several converters can operate on the same DC network

Why Use Voltage Source HVDC

· · · · · · Freedom to operate at any P and Q combination Ability to operate in a weak grid and even black-start Fast acting control No commutation-failure regions to be avoided Can use polarised cables Good sine wave-shapes; reduced filter components

But · Ratings presently very much lower than classic current source · Power losses are higher

Evolution of HVDC technologies

MW or kV

source: IEEE Power and Energy Magazine

VSC Operation

DC source Electronic switch

vdc 2

S1 Load

· Voltage source converter creates an AC voltage from a DC voltage (using pulsewidth modulation or similar)

vL

vdc 2

S2

Squire wave

vdc 2

Fundamental sinusoidal component

· The voltage difference across the interface reactor is the key to operation

vdc 2

S1-ON S2-OFF S1-ON S2-OFF

Pulse Width Modulation

Modulation signal (50 Hz)

to S1 to S2 Comparator

Carrier triangular waveform (2 kHz)

Inverter

vdc 2

Fundamental sinusoidal component

vdc 2

Power Flow Control with VSCs

Converter 1 DC link AC

C

Converter 2

Reactive power

Q

vdc

AC

Converter current rating

P

P

Active power

· With a controllable voltage source, in-phase and inquadrature current can be created at will · Proper choice of converter AC voltage leads to any combination of P and Q on AC side.

P-Q capability

· Full real power rating available in either direction · Independent reactive power at both converters · But reactive power limited by voltage considerations and overall current rating of converter

source: IEEE Power and Energy Magazine

Series Valves and Multi-Level

A Series Valve · Voltages required of HVDC are far above the rating of semiconductor devices (approx ±150kV against 5kV) · Some sort of series connection is needed

­ Direct series connection (ABB approach) ­ Multi-level converters (Siemens approach)

1 2

VDC

Multi-Level Power Converters

Multi-terminal HVDC

· Might be used with collection from multiple offshore wind farms in Round 3 · Might use multiple infeed sites onshore

ii

i

Multi-Terminal Configurations

· Provides some redundancy; but does the cost-benefit analysis support investment

· It is not certain that interconnecting DC links will be economic:

­ Cables and cable laying are very expensive so shortest routes to shore are favoured ­ The redundancy offered by multi-terminal systems may not bring significant savings in "spilled" energy in the event of an outage

· Three technical issues

­ Power flow control ­ Fault management

­ Grid-code compliance

VSC HVDC With DC Faults

· A fault on the DC side draws uncontrolled currents through the converter diodes

­ The fault could be "cleared" with AC circuit breakers but this would be needed on all terminals and require a shutdown of the whole system ­ Solution might be DC breakers or in different converter topologies that can block the fault current themselves

I Fault

Fault

Concluding Remarks

· HVDC connections are now considered to strengthen the existing network and for offshore connections. · VSC HVDC has several key operational advantages and allows greater network control · Unrestricted control of DC power at each node makes multi-terminal DC feasible · DC Fault-blocking required for multi-terminal operation · However, use of off-shore DC network to support onshore AC network opens a great many questions

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