Grid Frequency and Speed - Effects on Power Generation

Electric power generators connected to the electricity transmission and distribution grid function not individually but as part of a team of generators. The key factor that is common to the grid and the individual generator is the frequency.

Frequency is the change in direction of the current flow in an AC (alternating current system). In US and many other countries the grid frequency is 60 Hertz (cycles per second). In India, Europe and may other countries the frequency is 50 Hertz.

The frequency is directly linked to the speed of rotation of the generators. The generators on the 50 Hz systems rotate at a speed of 3000 rpm. This is because the rotor in the generator is a single magnet with two poles. 3000 rpm is 50 revolutions per second, or in every second the single magnetic field cuts the stator coils 50 times.

In US and other countries where the frequency is 60 Hz the generators rotate at 3600 rpm.

An exception is in the case of large nuclear power plants where they use a 4-pole magnet in the generator rotor so the speed is half at 1500 rpm for producing 50 Hz (1800 rpm in US). This is required to overcome mechanical limitations on the steam turbine side.

The grid frequency is not a fixed value; it keeps changing within a narrow range. Allowable variation of the grid frequency is in a small range of ± 0.5 Hz or less. This is ± 30 rpm. At any point of time all the generators connected to the grid run at the same speed or in a "synchronized" mode.

Governors or speed controls on turbines or diesel engines control the speed of individual generators.

As the electrical load on the grid increases the generators tend to operate at a lower speed. This is compensated for by the turbine governors which feed more steam to the turbines, thereby increasing the speed. If one of the turbine generators cannot increase the speed due to steam capacity limitations, another generator on the grid will compensate for this. When all the generators reach their input capacity limitation, the grid will start operating at a lower frequency than the 50 Hz. This is an indication that the grid is overloaded and demand changes are required.

Coal fired thermal power plants are slow to respond to load changes because of thermal inertia, whereas hydropower plants or gas turbines respond much faster. A good grid will have a proper mix of all types of generators so that it can quickly control the load.

Induction machines like motors are more susceptible to the effects of grid changes than pure resistive appliances like heaters and lamps.

Most of the electric motors are synchronous machines that run at constant speed governed by the frequency. So when the frequency changes the speed changes. For a motor connected to a pump or fan, the change in rotation will affect the performance of the connected equipment.