Under Frequency Operation of Power System

An established grid operating at a certain frequency (say 60 Hz) would have a certain inertia associated with this particular frequency. Given that it's possible to instantaneously drop the system frequency without losing sync, then, if we operate at a lower frequency, can we get some extra power injected to the system?

The grid standard for power quality dictates the frequency deviations allowable. Keep in mind that some electrical equipment are frequency sensitive. On the other hand inertia is a mechanical property of rotating masses which resist change in rotational speed. Inertia is the first response of the electrical grid against system load changes or shutdown of big generating units upon trips. As more inertia a system possess the less steep in the initial rate of frequency change. Speed governors will stop the frequency deviations in a stable operation.

Conventional generators are electrically protected using under frequency and volts/hertz (functions 81 and 24) respectively to avoid over fluxing in the generators and will prevent prime movers operation close to critical speeds. However synthetic inertia can be obtained from wind turbines type 3 and type 4 through proper control algorithms and from proper inverters arrays as part of flexible alternating current systems, FACTS.

Power System inertia plays its role during dynamic power system disturbance. The load inertia also plays part in recovery of frequency post disturbance as new load-generation balance takes place. The balancing exercise of conventional generation vs renewable generation might need matching inertia for various kinds generating resources including storage devices in order to keep frequency in reasonable band.

If the frequency is instantaneously dropped it means that all of a sudden the system got overloaded. The rotating kinetic energy of the generators will initially supply the extra load (due to sudden drop in frequency). Governors will start working to give a boost to the generation of the units - as generation of units go up frequency tends to become 60 Hz and the governor action stops as soon as the system frequency reaches to 60 Hz. So extra power will be injected to the system from the generators.

There are some other factors to consider with a change in operating frequency.

There are often regulatory requirements on the system operator to maintain system frequency and time error within certain bands. In New Zealand, the normal frequency band is 50 Hz +/- 0.2 Hz and time error should be kept within +/-5 seconds with time error being zero at least once per day.

The New Zealand System Operator is currently carrying out studies to assess the operational implications of reducing system inertia. I understand similar studies are being or have been carried in the UK and probably elsewhere. These studies once complete could help your own research.

Storage (battery or other) and particularly, EV charging can offer opportunities to manage system frequency. EV charging modulation in response to changes in system frequency can provide a fast response to stabilize frequency.

Leave your comment