WHAT IS UNDERFREQUENCY LOADSHEDDING?
The purpose of loadshedding is to balance load (customer demand)and generation (power plant capacity) during a sudden drop of a generating unit. A sudden loss of a generating unit produces a system overload which is a condition where there is more load than generating capacity. This overload causes a drop in frequency (below 60 Hertz) and is known as underfrequency.
GPA has devised a system protection scheme that would automatically drop loads when an underfrequency condition occurs. This is to avoid a total collapse of the system, otherwise known as an island-wide blackout. When the protection scheme is initiated, load is shed a block at a time isolating power delivery to predetermined areas until the power system stabilizes. There are three blocks of load, big enough to save the system upon loss of a Cabras unit, MEC unit, or a Tanguisson unit. The frequencies at which these load blocks are shed are 59.1 hertz (Hz), 58.8 hertz, and 58.5 hertz with a time delay of 0.1 seconds (six cycles).
Selected feeders are assigned to these frequencies on a 10 outage rotational basis. For example, a feeder assigned now to 59.1 Hz will be on 58.5 Hz the following outage rotation.
As the frequency recovers the load is restored also in small blocks to avoid an underfrequency condition and maintain a stable power system. The restoration is done by supervisory control at the Power System Control Center.
A listing of the stages and feeders on the Underfrequency Loadshedding schemes can be found on the following link.
Underfrequency Loadshedding Scheme
Download 8 kB pdf |
