Load following and frequency control

As noted in the Introduction, we have one form of 'intermittency' that results from the day-to-night variation in the demand for electricity. In the UK, this demand is met in two ways. Nuclear plants, plus a small number of fossil fuel plants, provide the base load. Five years ago, combined-cycle gas turbine plants using natural gas provided the base load from the fossil fuel sector. Today, it is the most efficient coal-fired units that operate around the clock. On a typical day the combination of nuclear and coal, with some CCGT plants, might be supplying 20 gigawatts (GW) to 30GW of power.

Although there is a big difference between the day and night demand for power, which leads to some plants having to be shut down at night, the demand for power during the day is never constant. These particular variations in load can be met using coal-fired steam plants, the more efficient of which will operate on a 24-hour basis. Output from steam plants can be reduced to about 50 per cent of design without much difficulty. But when in load-following mode, plants are expected to maintain grid voltage and frequency at 230 volts and 50 hertz (Hz). As noted, this is not difficult with steam plant as the steam flow to the turbines can be quickly changed through actuation of the governors. In making these changes, temperature and pressure do not vary substantially. The main effect is that by operating at reduced output, efficiency goes down slightly. Pressurized water reactors should also be able to offer some help with frequency control; but if load following is needed from the nuclear sector, reactors of the boiling water type would seem to offer more capability. The UK has none of these.

The frequency issue is more problematic for CCGTs. A modern CCGT consists of a gas turbine and steam turbine, which are, in big modern units, connected by the same shaft, driving on to an alternator, which nominally runs at 3000 revolutions per minute. The alternator, and everything else in such a machine, is locked to the grid at the nominal frequency of 50Hz. If there is a sudden increase in demand on the grid, all the alternators throughout the grid network will slow down slightly until more power can be delivered. On a coal-fired steam plant, this is fairly easy; power can be increased by opening the throttle to the steam turbine. It is a bit more difficult for CCGTs. The slowing-down of the alternator will slow down the gas and steam turbines; unfortunately, the result is that the power output from the gas turbine drops. Less air is taken in by the gas turbine compressor and less fuel can be burned.

This is clearly a dangerous situation if one has a grid in which all the power is coming from CCGTs since the drop in grid frequency, caused by the demand for power, will lead to a drop, rather than an increase, in the power output. The situation is not quite as bad as it may seem since the voltage in the system will drop, and this will reduce the demand for power from many consumers. Many consumers will have experienced such 'brown-outs' when there has been a lack of capacity, perhaps due to the weather or industrial disputes.

Not all CCGTs are quite as susceptible as the big tandem shaft designs described above. Some 'merchant power' plants in the UK, designed for meeting varying loads, have independent alternators for the gas turbines and steam turbines. A typical set-up would be two gas turbines feeding into one HRSG, which supplies one steam turbine set. In such a case, the steam turbine and its alternator can be operated independently.

'Aero-derived' gas turbines, based on turbojets, intended for grid reinforcement and which are not of the CCGT type, do not suffer from the frequency problem, which is one reason why they are so useful for local reinforcement of the grid. They have completely separate power turbines to drive the alternator. Hence, if the grid runs slow or fast, there is no direct effect on the engine or its power output.

Whereas in a coal-fired steam plant it takes some time after the burners are lit for steam to start to be produced and put to the turbines, in a CCGT, some power will come from the gas turbine within about 15 minutes from startup. In industry parlance, the gas turbine is then 'synchronized'. It will take somewhat longer for the HRSG to get hot enough to produce steam; but both the gas and steam turbines of a CCGT can be up to full power within about an hour. Efforts to cut this time can damage the HRSG; but, on balance, CCGTs are probably better at meeting the bulk of the daytime load than steam plant.

Against this ability to start up quickly, the biggest shortcoming of CCGTs is the drop in efficiency at low loads, the cause being the drop in turbine temperature as power is reduced. Temperatures can be maintained down 80 per cent of the design rating by partially closing the inlet guide vanes of the compressor, thus reducing the flow of air along with the fuel supply. As a result, although less fuel is being burned, turbine inlet temperatures are maintained. Because of this characteristic, some operators have the view that although CCGTs are good for meeting the more stable part of the daytime peak, as well as being very good for base-load operation, they are not so efficient at compensating for highly variable loads. Such views are, of course, often coloured by individual experience with specific designs.

A new problem, related to the demand for power and its availability from CCGTs, is the increased air-conditioning load in the UK, which goes up on hot days. Unfortunately, the power output of CCGTs tends to fall at these times. High air temperatures reduce air density. Less fuel can be burned in the gas turbine, and the physical mass of air through the gas turbines drops away - so output declines. A drop-off in power can be overcome by burning extra fuel; but this can lead to turbine inlet temperature limits being exceeded and will result in some reduction in blade life. Palliatives include cooling of the inlet air; but this may not be economic if plants are only operating part-time.

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