Control of The National Grid - Dynamics of The UK National Grid

Dynamics of The UK National Grid

The system frequency target is normally set at 50.00 Hz. Frequency changes as the balance between demand and generation alters. The Grid controller attempts to balance the two by instructing the generators, but there is a range of frequencies between which no instructions will be issued, say 49.95 to 50.05 Hz.

All generators and synchronous machines that are connected to the Grid are locked in to the system frequency. If the demand increases, the frequency will start to fall. There is only a relatively small amount of kinetic energy in the rotating parts of the generators so the governor system of the turbines driving the generators respond by very rapidly increasing the energy input to restore the balance. The characteristic of the governor can be varied to allow a small deviation from the target frequency rather than attempting to return to exactly 50.00 Hz. This results in a more stable system.

The fuel input of a gas turbine can be adjusted very rapidly. With a steam turbine, the energy within the boiler provides a relatively large reserve, and the fuel input to the boiler (or nuclear reactor heat output) can be increased after a minute or two to restore the equilibrium. The combination of some rotational kinetic energy and some potential energy stored in the boiler pressure parts permits a beneficial flywheel effect overall. National Grid Co. needs to quantify the amount of kinetic energy in the system in order to make stability calculations reflecting different scenarios.

The power required to drive a machine is proportional to its speed (depending on the machine characteristics). When the system frequency drops the power of most rotational machinery being driven will drop, which also helps to stabilise the situation. The opposite effects take place when demand falls.

In an AC system it is not desirable to allow voltages to vary more than a small amount with demand. Voltage variations are not a normal power control mechanism (they are a reactive power control mechanism) The generator and Grid system voltages are maintained within quite close limits, otherwise the system can become unstable and parts of it fall out of synchronism, with adverse consequences.

Voltages can be reduced deliberately at the final distribution stages of the system as the first stages of deliberate load reduction, and the system can also be deliberately run at a low frequency (down to 49.00 say) to reduce demand before having to cut customers off. Such tactics have not been deployed very much in recent decades, but it was a regular feature before the 1970s and may well become common again in a decade or two.

The financial consequences for energy users will depend entirely on their individual circumstances. They are only paying for energy used but they may suffer from reduced product outputs or quality control problems.

The fundamentals of AC grid systems everywhere are the same so all will have very similar characteristics provided that there is not a shortage of generating capacity to meet demand.