Light duties for high powered link

19 October 2001

Several recent HVDC link projects are using the Siemens light triggered thyristor DC control valve.

An efficient and reliable HVDC control valve developed by Siemens is starting to find significant applications around the world. Having been reliability tested in the USA (see panel, page 26) and installed in the Moyle Ireland/Scotland link (the first in the world to be completely equipped with the this high voltage semiconductor technology) it has now earned a contract worth 350 million euros to Siemens for the construction of converter stations in a new 940 km 3000 MW/ ±500kV HVDC link from Anshun to Zhaoqing, China, coming into operation in 2004.

The system of direct light triggering developed by Siemens uses a 10 µsec 40 mW light pulse to activate the thyristors. The device also incorporates overvoltage protection so that it is self-protecting if the forward voltage exceeds the maximum permitted limit. The light pulse is carried by fibre optics from the valve control directly to the thyristor gate itself.

This is simpler than the hybrid optical-electronic type that has become the standard, in which light signals are carried to the thyristors, but each is triggered electrically, requiring a light-to-electrical conversion circuit. With the separate external circuits to provide a forward overvoltage protection function these amount to 80 per cent of the components. Further, in the light triggered thyristor (LTT) all firing and monitoring electronics are at earth potential and therefore accessible during system operation. But although directly triggered valves are simpler and more efficient, they are also more expensive. The real benefits are reliability and reduced losses.

The direct-light-triggered thyristor

Conventional hybrid thyristor valve technology uses electrically triggered thyristors which need a pulse with a peak power of several watts. This pulse has to be generated by a separate circuit for each thyristor. In turn, this circuit, which needs an auxiliary power supply, is activated at earth potential by optical signals from the valve control. Substituting direct light-firing significantly reduces the number of electrical and electronic components in the thyristor valve, increasing reliability, lengthening maintenance intervals and eliminating some problems of electromagnetic compatibility. It ought also to reduce problems associated with the long term availability of replacement electronic components.

The triggering system

It is essential that all the thyristors, although at different potentials, are triggered simultaneously. The earliest circuits used powerful gate pulse amplifiers and magnetic pulse transformers in each thyristor valve to achieve this. Later, the hybrid opto-electrical designs improved matters but still required an individual signal for each thyristor.

By using laser diodes as direct light sources the light pulse transmission system can be greatly simplified compared to electrically triggered thyristors or earlier LTT technologies using infrared diodes (IRED). It is now possible with stable, moderately priced 3W laser diodes to turn on up to 14 LTTs simultaneously. Laser diodes can have a life of more than 40 years compared to ten years with LEDs and require 85 per cent fewer triggering cables, even with 100 per cent redundancy.

  As the cables can be made from low attenuation monofibres, they may have a length of 100 m or more. This makes it possible to move the cabinet containing the light sources away from the thyristor valves and integrate it into the control equipment room, simplifying layout and access.

In the module, the incoming pulse is spread out to the thyristors via a multimode star coupler (MSC) and fibre optic cables sited in a groove in the thyristor housing and arranged to point directly at the gate of the thyristor without further connections.

The Moyle link

The 500 MW HVDC Moyle transmission link is about to be commissioned and will connect Northern Ireland to the European system by the end of this year. It officially opens for business in April 2002.

The connection brings to life a scheme that has been under consideration for more than thirty years, and in its present form for ten years. The process has been long and somewhat painful, partly because of the number of parties involved, partly owing to the necessity to satisfy stringent environmental standards. It started in 1991, with the aim of completing the project in 1994-5. Although consents were first applied for in 1993, consent for the overhead lines was not received until 1997, and full consent for the converter stations not until March 2000. Part of the process was an exhaustive environmental study commissioned to assess the effects of all elements of the Interconnector - studies of ecology, archaeology, forestry, tourism and recreation, together with noise surveys and an assessment of electric and magnetic field strengths were undertaken by specialists. The undersea aspects, including marine biology, war graves, shipwrecks and safety to mariners, were also studied.

The converter stations contract was awarded to Siemens in 1999, the cables contract to Alcatel (Nexans Norge) in the same year and the overhead line (Scotland) contract to Balfour Kilpatrick in 2000.

The perceived advantages of the interconnector from the point of view of owner Viridian, which until the 300 MW North-South connector was restored in 1995 was serving a population of around 1.6 million in a self-contained and isolated system, are that it will gain access to a competitive market, with the potential to reduce generation costs; and enhanced security and supply quality. In fact trading has already started – 320 MW of capacity has been contracted for the January to March quarter 2002, and Moyle has a 125 MW, 70 month contract with Scottish Power already in place. Current work includes the commissioning of pole 2 and the completion of rock dumping where the undersea cable runs over shallow sediment.

The cable and substations

The interconnector consists of two monopolar submarine HVDC cable links operating in parallel. The converters have a power rating of 2x250 MW in either direction, referenced at the rectifier station. The dc operating voltage of each monopole is 250 kV, the ac sides of the converters are connected to the 275 kV networks on each side. The nominal direct current is 1000 A per monopole. Both dc cable systems, which connect the two converter stations, consist of 55 km of undersea cable and 8.5 km of underground cable.

The cable is of the well proven mass-impregnated paper insulated type but, in a development of the conventional design, includes an integrated return conductor (IRC). This design, by Nexans, has a metallic coaxial layer integrated in the HVDC cable to form the return path for the current and also to work as a part of the torsion balanced armouring. The solid core endows greater rigidity, although in any case an oil filled cable was environmentally unacceptable. Maximum design temperature is 50 °C, with design losses at 50 W/m at 1000 A. The fibre optic cable for control and communications between the converter stations is integrated into the polyethylene sheath of the IRC.

Adjacent to each valve hall is a dc hall, in which the dc switchyard including the measuring equipment, the smoothing reactor and the cable sealing end are located. The ac switchyard in Ballycronan More converter station is designed as a double busbar arrangement fed by the existing 275 kV transmission system. Both outdoor switchyards are designed for a short circuit current of 31.5 kA.

The six transformers at each substation are of the single-phase three-winding type rated 96/48/48 MVA each and with windings for star and delta configurations. A tap changer with 8x1.25 per cent steps is used to keep the valve side voltage at the ideal value. The transformers are located directly adjacent to the valve hall with their dry type valve side bushings penetrating the valve hall walls. It is not necessary to install separate dc wall bushings with this arrangement.

Thyristor valves

Conventional thyristor valves for HVDC converter stations are assembled from high voltage semiconductor devices with a peak blocking voltage of 8 kV and a rated current of up to 4000 A dc for transmission voltages up to 500 kV dc. In this case thyristor valves are arranged in three branches (one for each phase) with four valves in each branch connected in series and fed from a single-phase three-winding transformer. The quadruple valves are suspended from the ceiling of the valve hall with the high voltage connections at the bottom. Each valve consists of 39 thyristors in series connection. The valves are water cooled, each thyristor running at 1 kW. Triggering is by light pulses generated by laser diodes at ground potential and applied directly to the thyristor gate.

Power quality

The necessity is that power quality must not fall as a result of the interconnector. To this end three triple tuned ac filters rated at 59 MVAr each and tuned to the predominant 3/12/24 harmonics in combination with one 59 MVAr shunt capacitor cover the similar demands on reactive power and harmonic currents filtering of both stations. To compensate for the reactive power demand of the 64 km transmission line in Scotland, two 59 MVAr filters tuned to the 12 harmonic will be installed in Auchencrosh. Each filter branch can be switched individually by means of circuit breakers with two interrupter units. This design satisfies both the maximum limit of switching overvoltage and the requirements for harmonic current suppression.

Control and performance

Since the converter stations will be unmanned, the link is designed for fully automatic remote operation from NIE’s dispatch centre including automatic load scheduling operation. The control system combines the functions for control, supervision and protection of the link in its normal operating mode of absolute power control. Control functions usual for modern HVDC transmissions are also available, including delta power, emergency power, direct current, frequency limit, stability, and automatic reactive power control functions.

The main performance objective of the Moyle link is to establish low losses and a very high availability and reliability combined with a low maintenance requirement throughout the expected lifetime of more than 30 years. In fact losses are guaranteed at less than 1.35 percent, and energy availability at more than 99.6 per cent. The control system is fully redundant in order to meet these high demands.

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