Island operators face new challenges20 July 2001
Guernsey’s diesel engines have an excellent performance record. But the island’s new interconnector will require a very different operating philosophy.
Like several other island communities around the world, marine-derivative low speed two stroke diesels have been the mainstay of Guernsey’s growing electricity supply for the past 20 years or so. Over that time Guernsey Electricity (formerly known as the States of Guernsey Electricity Board) has built up an in-house expertise and capability for the maintenance and operation of such machines that is world class, with a strong emphasis on self-sufficiency and high reliability. This independence of spirit is as you would expect on an island which is a British dependency but has its own legislature (the States of Guernsey), currency and tax regime.
With a population of 65 000, Guernsey is located in the English Channel about 130 km from the south coast of England and some 50 km from France. In recent years the island’s importance as an international financial centre has been growing, with the consequent need for an ever more reliable electricity supply to run the computers. The peak load, which occurs in winter, now stands at around 65 MWe.
There are five low-speed nine-cylinder Sulzer two stroke cross head engines on Guernsey. At Vale C station there are three 150 rpm Sulzer 9RNF68 machines, commissioned in the period 1979-82 and rated at 12.2 MWe each. There is also at Vale C a 125 rpm Sulzer 9RTA58, rated at 14.2 MWe. This was commissioned in 1987 and, motivated by high oil prices at the time, was fitted with a small (430 kW) gas turbine generator powered by the exhaust gas. This Sulzer “Efficiency Booster” is designed to squeeze another 3 per cent extra electricity output per unit of fuel, resulting in an efficiency of some 45.39 per cent for this unit. Across the road, in Vale D, there is another Sulzer 9RTA58. This 136 rpm machine was commissioned in 1993. It is rated at 14.5 MWe and also has the Efficiency Booster.
In the old Vale B station there are in addition three elderly Mirrlees 12KVM medium speed (500 rpm) diesels, rated at about 3.5 MWe apiece. These are of 1965-70 vintage and due to be retired next year.
All the Vale machines run on residual heavy fuel oil and the station is achieving an overall efficiency of over 42 per cent.
The success of Guernsey Electricity’s operation and maintenance strategy for its low speed diesels is reflected in a recent benchmarking exercise. This was conducted by Electricité de France among operators of comparable plants (very often on exotic islands), and the results were reported at the April 2001 annual meeting of the Mobil-sponsored Power User Group for Reciprocating Engines, held, appropriately enough, on Guernsey. This benchmarking study shows Guernsey Electric’s machines consistently in the top places in terms of such indicators as availabilities, outage rates, specific fuel consumption, manpower per installed MW, cost per kWh, O&M costs etc.
Scrupulous attention to detail coupled with the very strong in-house technical capabilities appear to be among the main contributors to this outstanding performance record. “We do lots of small things, but we do them right”, says Alan Richards, deputy power station manager and primary instigator of the plant’s maintenance philosophy. He is also prone to what he describes as “pickiness”, healthy scepticism (for example about what fuel suppliers and viscometers tell you), and a propensity for analysis and research. The strategy seems to have paid off.
Alan says that the station’s fuel laboratory is perhaps the key to its success, for example identifying fuel that is not within the contract specifications and carrying out regular plots of temperature v viscosity. The contract viscosity for fuel is 40 cSt at 100 °C. But on several occasions fuel batches have been found to depart from this, which means that fuel injection is not optimal, resulting in reduced efficiency unless adjustments are made to final injection temperature.
Guernsey Electricity has carried out original research on the relationship between fuel quality and engine performance. This work has shown, among other things, that there is a very narrow band of fuel temperature which achieves the right viscosity for a particular design of fuel injection nozzle and thus achieves optimum combustion performance.
Additionally, at Vale, combustion peak pressure is checked for each cargo of fuel and injection timing adjustments made in order to achieve correct levels. “In some cases we have advanced injection by up to 2 degrees to keep peak combustion pressure within specifications,” says Alan Richards. “Such testing and adjustment has kept specific fuel consumption rates at ‘test-bed’ levels throughout the life of the engines” (about 182 g/kWh for the 9RTA58 machines, according to the Guernsey Electricity 2000 annual report).
On other occasions, “we found fuel with very high CCAI [Shell Calculated Carbon Aromaticity Index] – nearly explosive – and had to retard injection.”
In fact, notes Alan Richards, there have been adjustments to injection timing with just about every new cargo of fuel and on some occasions batches have been returned, which concentrates the mind of the supplier.
The station has also taken a pioneering role in the field of lubrication. Recent years have seen a steady decline in the sulphur content of fuel oil, which affects acidic wear rates in engines. The sulphur content of the fuel oil used at Vale, for example, used to be around 3.5 per cent but is now down to about 1 per cent. This has required a corresponding change in lubricating oil formulations.
In collaboration with its long-time lubricant supplier, Mobil, Guernsey Electricity has been actively involved in field trials of new grades of lubricant at Vale, following their initial testing on a six-cylinder Sulzer RTA 38 that Mobil has installed at its Gravenchon research facility in France. For example, in one of Vale C station’s nine cylinder engines, two cylinders were used to trial one formulation, two other cylinders operated with another new lubricant, while the remaining five cylinders operated with the standard product.
As a result, in consultation with engine supplier Sulzer, it has proved possible to increase the period between major overhauls from 6000 hours to about 13000 hours, with consequent cost savings.
Another example of self-sufficiency is that the Vale power plant manages all its own fuel injection nozzle servicing in-house.
Guernsey Electricity has also designed its own control system, allowing all the generating plant and the island’s distribution system to be monitored and controlled from a single control room at the Vale site. Major investment has been put into ensuring that the I&C has been continually upgraded over the years.
End of isolation
However, Guernsey’s diesel generators now face a new challenge, indeed an altogether different and uncertain era, following completion of a submarine cable link.
In July 1998 the Jersey Electricity Company and Guernsey Electricity formed the Channel Islands Electricity Grid Company (CIEG) – a 50/50 joint venture to construct and operate the link for the import of electricity from France into Jersey and Guernsey. Among the reasons for building the link were to increase reliability of electricity supply on the island, to lessen exposure to fluctuating international oil prices, and to reduce emissions from local generation (by getting French nuclear stations to do the generation whenever possible, as opposed to diesel engines in Guernsey). Low speed two stroke diesels provide high efficiency on low grade fuels over a wide load range because of the long dwell time and high combustion temperatures, but the penalty is relatively high NOx emissions. Selective catalytic reduction can be fitted but this is very expensive, perhaps costing as much as £1 million to install and a roughly similar amount per year to operate.
The laying of the new cable was also an opportunity to boost data transmission capacity, by installing fibre optics in the power cables and by laying additional fibre optic cables alongside the power cable.
In November 2000 a cable, capable of carrying 60 MWe, was completed, for the first time ending Guernsey’s electrical isolation and linking it to its old rival Jersey and via Jersey to France, giving the islands access to France’s relatively cheap nuclear generated electricity as well as to the wider European grid.
The presence of the new interconnector has markedly changed the role of the low speed diesels, from baseload, which is what Guernsey Electricity and the original designers intended, to essentially standby, with stop-start mode occasionally. This is an unusual operating regime for these large machines, which weigh about 400 t each and need a fair amount of time for temperatures to stabilise when the load changes.
A whole new set of issues must now be addressed and the learning curve is steep. “We are being asked to use the engines in a way that they were not originally designed for,” says Alan Richards, “we are trying to make baseload engines do standby work”, he told the Power User Group meeting. Startup/shutdown cycles are much more arduous than baseload operation, tending to shorten component life, and, as Alan Richards says, “400 tons is an awful lot of metal to heat up and cool down.”
One thing that he has decided is that rather than being laid up “the engines must be kept in warm standby, with the jackets at 50 °C, or they won’t last.”
Corrosion in an idle engine is a concern, with the large amounts of liquids present and the possibility that bacteria will build up at the raised temperatures.
The Guernsey experience of this new regime is likely to figure prominently at next year’s Power Users Group meeting. This is due to be held on the Isle of Man, which also has a new interconnector to the mainland.
New gas turbine
Another factor that may influence the fate of the low speed diesels at Vale is that Guernsey Electric is currently installing a new gas turbine generating set there. The new machine is an Alstom Cyclone, being manufactured in Lincoln, UK, with black start diesel generator. It is to be used in simple cycle mode to provide emergency standby and peak lopping.
The Cyclone, which will have a dry low emissions combustion system to keep NOx levels down, will join three other existing gas turbine generating sets at Vale, two Thomassen GE Frame 5 machines with a capacity of 19.5 MWe each that were commissioned in 1996 and 1997 respectively, and an old Stal Laval 13.1 MWe machine, of 1972 vintage, shortly to be retired. Once the new 11 MWe Cyclone is installed, due this summer, with handover scheduled for September, total gas turbine capacity at Vale will be about 60 MWe (all operating on gas oil). This is roughly equivalent to the capacity of the new link.
The turbine generating sets, which can be started up much more quickly than the big diesels but are much more expensive to run, can be seen as a sort of insurance policy. The prudence of this approach was demonstrated in March when the new link failed unexpectedly (due to a switch termination fault), disrupting supplies from France and resulting in a power cut of about 40 minutes duration, not something the people of Guernsey are used to. The Frame 5’s had to be brought into operation and subsequently the low speed diesels.
In addition to the interconnector the Vale plant must now contend with yet another change in its operating environment: commercialisation. Under a policy approved by the States of Guernsey in March 2000, government control over Guernsey Electricity will be relaxed to allow the adoption of a more commercial approach – although it will not be privatised. Commercialisation is due to come into effect on 1 October. It will be interesting to see how this form of liberalisation works out in practice and how the low speed diesels will fare in this new competitive world - perhaps another topic for the Mobil Power User Group’s next meeting.