During the normal operation of a grid, it is possible in which the load exceeds the generation. If this happens the frequency of the system goes down. The standard frequency is 50 Hz. But the frequency could go down to about 49.0 Hz. After this value, it is not advisable to allow it to decrease it any further since it could cause the system to lose synchronism and lead to ultimate collapse of the system. As output, we go for purposeful shedding of load, known as load shedding. The load shedding is a procedure of reducing load on the grid so as to save the grid as a whole.
Load shedding could be completed in two ways:
1. Automatic: For this reasons automatic under-frequency relays are installed. These relays carry out automatic shedding of load if the frequency falls below a certain level.
2. Manual: Special guidelines have been given through RLDCs/SLDCs for the load shedding at various frequency levels. These guidelines depend upon the grid parameters at the particular instance as well as some fixed guidelines for frequency falling below a particular limit or area-wise/consumer category-wise shedding.