1

The Benmore - Haywards HVDC Link

National Historic Heritage Workshop- Engineering Heritage

3 to 5 August 2004

Wellington

Marshall ClarkTranspower New Zealand

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Milestones1950 Bill Latta forecasts NZ supply and demand1950 ASEA awarded contract for world’s first undersea HVDC link

to Swedish island of Gotland1954 Gotland HVDC link commissioned1956 BICC reports on viability of Cook Strait crossing1958 Trial length of cable laid off Oteranga Bay1960 Construction of Benmore dam commences1961 HVDC link approved1964 Cable laying commences1965 First full power transmitted – 1 April 19651965 Official opening – 15 May 19651976 Link modified to allow power to be transmitted south1992-1993

Upgraded HVDC link commissioned with addition of 700 MWthyristor converters

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International Context

• Only 12 HVDC mercury arc valve transmission schemes were ever commissioned

• The New Zealand scheme was notable for its high voltage and long length of submarine cable

• By end 2005, there will only be one other HVDC scheme still operating with mercury arc valves

Transpower currently plans to maintain its mercury arc valves until at least 2010

5Scheme was controversial

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Cook Strait Cable Cross Section1. Hollow duct - N2 filled

2. Copper conductor

3. Conductor screen

4. Dielectric - paper tape

5. Dielectric screen -copper tape

6. Lead sheath

7. Reinforcement bedding

8. Reinforcement - steel tape

9. Armour bedding tape

10. Bend restricting armour

11. Anti-corrosion serving

12. Anti-teredo protection - brass

13. Armour bedding

14. Steel wire armour

15. Serving - jute string

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N2 filled hollow duct

Copper conductor

Dielectric - paper tape

Lead sheath

Reinforcement - steel tape

Bend restricting armour

Anti-teredo protection - brass

Steel wire armour

Serving - jute string

Cook Strait Cable

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9Cook Strait Cable

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Fighting Bay Cable Terminal Station

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HVDC line

12Line insulators replaced live

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Overview Single Line Diagram

14Haywards

15Benmore

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Mercury Arc Valve

17Mercury arc valve group

18Mercury arc valve hall

19Mercury arc valve workshop

20Clean workshop

21Haywards DC switchyard

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Smoothing reactor

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Valve Damping Resistor

24Base isolation scheme

25Haywards AC harmonic filters

26Haywards resistor-reactor units

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How the HVDC link is now used

• Mix of hydro/thermal generation• South Island hydro capacity exceeds demand in that island• HVDC transfer is determined by market dispatch tools based on

solving on priced half-hourly offers for generation• HVDC transfer can vary rapidly with market conditions• variability of transfer is unprecedented in 39 year history of

HVDC in New Zealand

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South Island Generation

DC Transfer (-)

Mid Waitaki

Manapouri

Clutha

Upper Waitaki

-1200.0 -1000.0 -800.0 -600.0 -400.0 -200.0

0.0200.0400.0600.0800.0

1000.01200.01400.01600.01800.02000.02200.02400.02600.02800.0

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Day

Average (Period) M

W

Upper Waitaki

Clutha

Manapouri

Other SI Hydro

Mid Waitaki

DC Transfer (-)

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Mercury Arc Valve Performance

• MAV’s require reasonable thermal stability• frequent start/stops, rapid rates of change of load and

inverter/rectifier mode changes increase rate of valve disturbances– commutation failures– arc backs– arc-throughs

• disturbances lead to increasing internal deterioration• increasing risk of valve faults and consequential failures• some constraints have been imposed on dispatch patterns to

preserve remaining life

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Scheduled Energy Unavailability

0

2

4

6

8

1993 1995 1997 1999 2001 2003

Year

SEU

(%)

MAV

Thyristor

Forced Energy Unavailability

00.20.40.60.8

11.21.4

1993 1995 1997 1999 2001 2003

Year

FEU

(%)

MAV

Thyristor

Operational Performance

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Forced Outage Count

010203040506070

1993 1995 1997 1999 2001 2003

Year

FOC MAV

Thyristor

Mercury arc valves inherently suffer more operational performance problems than the modern thyristor replacement

Operational Performance

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Risks of ownership

• Mercury presents an on-going safety and environmental hazard, although it is well controlled at present

• There is a rapidly diminishing pool of international expertise in the maintenance of mercury arc valves (support from the manufacturer ceased in 1970 !)

• Much of the equipment is unique and spares are very limited• Custom made solutions are required for failures of some

components - this carries its own risk• The equipment does not have the same seismic strength as

modern equipment• The operational performance is less than the modern equivalent

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Planning for replacement

• Transpower is preparing plans for replacement of the mercury arc valves and associated converter station equipment

• These plans will form part of a major Grid Upgrade Plan to be submitted to the Electricity Commission next year

• Following consultation, if approval is given, a major project will be launched to replace the mercury arc valve converter stations by around 2010

• The original converter stations will remain in service until thecompletion of the commissioning of the new equipment

• A lengthy period of decommissioning and decontamination is likely to be required

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Preservation for the Future

• The New Zealand HVDC scheme has been an extremely important part of our electricity system since 1965

• The scheme represented world-leading technology in its time, and involved very challenging engineering work

• The history is well documented in the books:– People, Politics & Power Stations– White Diamonds North

• The mercury arc valves themselves are the icons of this scheme• Efforts should be made to preserve at least one• An Electricity Transmission Heritage Society has been formed

with industry support• Plans must be made for some suitable memorial

351990 Engineering Heritage Plaque