UNDER/OVER FREQUENCY PROTECTION:

UNDER/OVER FREQUENCY PROTECTION:

Over frequency results from the excess generation and it can easily be corrected by reduction in the power outputs with the help of the governor or manual control.

 Under frequency operation: Under frequency occurs due to the excess. During an overload, generation capability of the generator increases and reduction in frequency occurs. The power system survives only if we drop the load so that the generator output becomes equal or greater than the connected load. If the load increases the generation, then frequency will drop and load need to shed down to create the balance between the generator and the connected load. The rate at which frequency drops depend on the time,
amount of overload and also on the load and generator variations as the frequency changes. Frequency decay occurs within the seconds so we can not correct it manually. Therefore automatic load shedding facility needs to be applied.

These schemes drops load in steps as the frequency decays. Generally load shedding drops 20 to 50% of load in four to six frequency steps. Load shedding scheme works by tripping the substation feeders to decrease the system load. Generally automatic load shedding schemes are designed to maintain the balance between the load connected and the generator. The present practice is to use the under frequency relays at various load points so as to drop the load in steps until the declined frequency return to normal. Non
essential load is removed first when decline in frequency occurs. The setting of the under frequency relays based on the most probable condition occurs and also depend upon the worst case possibilities.
During the overload conditions, load shedding must occur before the operation of the under frequency relays. In other words load must be shed before the generators are tripped.