Control of Power and frequency

In power systems it is essential to keep the frequency and the voltage close to their rated values. The frequency is controlled by controlling the balance between the power supplied to the system and the power taken from it. Figure 2.16 shows a transmission system with a prime mover driving a generator, and a motor driving a mechanical load. Table 2.5 gives examples of prime movers and loads.

The power P_in supplied to the system is determined by the prime mover(s). In a steam‐turbine generator, the steam valves arc the main means of control. Of course, if the valves arc opened wide, the boiler must be able to provide sufficient steam (at the correct pressure and temperature) to develop the required power. This means that the boiler control must be coordinated with the steam valves. Similarly, in a wind turbine the power transmitted to the generator is determined by the wind speed and the blade pitch, which can be varied to control the power to the required level.

The power P_out taken from the system is determined by the mechanical and electrical loads. For example, consider a direct‐connected induction motor driving a pump. The motor rotates at a speed determined by the intersection of its torque/ speed characteristic with the pump’s torque/speed characteristic. Since the motor torque/speed characteristic is very steep near synchronous speed, the motor tends to run near synchronous speed and the torque is then determined by the requirements of the pump (depending on the pressure head and the flow rate). So the power is jointly determined by the pump and the motor. With passive electrical loads (such as lighting and heating), the power supplied to the load depends on the voltage and the load impedance.

Fig. 2.16 Transmission system with prime mover, generator, motor and load. The voltages at both ends of the transmission system are assumed to be controlled, so the symbol E is used instead of V. At the sending end, the voltage is E_s; at the receiving end, E_r.

Table 2.5 Examples of prime movers and mechanical loads

Examples of prime movers	Examples of mechanical loads
Steam turbine‐generator (coil, oil, gas, nuclear, etc. - i.e. ‘thermal’) Hydro‐electric turbine generator Wind turbine generator Diesel engine	Pumps (water, sewage, process fluids, foods, etc.) Fans and blowers (air-moving) Compressors Machinery, hoists, conveyors, elevators

It is evident that and are determined quite independently. Yet in the steady state they must be essentially equal, otherwise energy would be accumulating some‐ where in the transmission system (Losses in the transmission system are assumed to be negligible for the purposes of this discussion). The power system operator can control but s/he has no control over , since customers can connect and disconnect loads at will. The power system operator does not even have any practical means of measuring for the entire system, and in any case, even if this parameter was available, there may be several generating stations in the system, so it appears to be somewhat arbitrary as to what contributions should be supplied by the individual generating stations at any instant.

In the short term (i.e. over a period of a fraction of a second), it is the frequency control that ensures that , and this control is effected by maintaining the speed of the generators extremely close to the nominal value. Suppose the power system is in a steady state and . Suppose that the load increases so that more power is taken from the system, tending to make . The prime mover and the generator will tend to slow down. Therefore the prime mover has a governor (i.e. a valve controller) that increases when the frequency is below the rated value, and decreases when the frequency is above the rated value.

In an isolated power system with only one generator, the governor has a relatively simple job to do, to maintain the speed of the generator at the correct synchronous speed to hold the frequency constant. But what happens in a power system with multiple generators? In this case usually there is a mixture of power stations. The large ones which produce the most economical power are usually best operated at constant power for long periods, without varying their contribution to . Apart from the economics, one reason for this is that if the power is varied, the temperature distribution in the turbine, boiler, and generator will be affected, and ‘thermal cycling’ is considered undesirable in these very large machines. So these generators have a relatively steep or insensitive governor characteristic, such that the frequency would have to change by quite a large amount to change the contribution to (Quite a large amount’ might mean only a fraction of 1 Hz). Elsewhere in the power system, or sometimes in the same power station, there are special generators assigned to the task of frequency control. These generators have very flat governor characteristics such that a tiny change in frequency will cause a large swing in power. They are usually gas turbine powered, up to 20 MW or so, but very large rapid‐response generators are sometimes built into hydro‐electric pumped‐storage schemes. For example, the Dinorwic power station in North Wales has a rating of 1800 MW and can change from zero to maximum power in a few tens of seconds.

The rapid‐response generators in a large interconnected power system (such as the United Kingdom system) are used for frequency control in the short term (over a few minutes or hours). They provide a time buffer to allow the larger power stations to vary their contribution. As the total system load changes during the day, the frequency is maintained almost constant, within 0.1 Hz. Averaged over 24 hours, the frequency is kept virtually dead accurate.

(In fossil‐fuel power stations two‐pole generators predominate, and the speed is in a 50‐Hz system or 3600 in a 60‐Hz system. In nuclear power stations, four‐pole generators are more common, running at 1500 (1800 at 60 Hz). In hydro plants, the generators have larger numbers of poles with speeds in the range .)

Some of the generators in a large system may be operated at light load in a state of readiness or ‘spinning reserve’, in case the system load increases suddenly by a large amount. This can happen, for example, at the end of television transmissions when the number of viewers is exceptionally high.

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