ABB RELION REX640 Technical Manual page 935

1MRS759142 F
underfrequency situation, the load-shedding trips out the unimportant loads to
stabilize the network. Thus, loads are normally prioritized so that the less important
loads are shed before the important loads.
During the operation of some of the protective schemes or other system
emergencies, the power system is divided into small islands. There is always a load
- generation imbalance in such islands that leads to a deviation in the operating
frequency from the nominal frequency. This off-nominal frequency operation is
harmful to power system components like turbines and motors. Therefore, such
situation must be prevented from continuing. The frequency-based load-shedding
scheme should be applied to restore the operation of the system to normal
frequency. This is achieved by quickly creating the load - generation balance by
disconnecting the load.
As the formation of the system islands is not always predefined, several load-
shedding relays are required to be deployed at various places near the load centers.
A quick shedding of a large amount of load from one place can cause a significant
disturbance in the system. The load-shedding scheme can be made most effective if
the shedding of load feeders is distributed and discrete, that is, the loads are shed
at various locations and in distinct steps until the system frequency reaches the
acceptable limits.
Due to the action of load-shedding schemes, the system recovers from the
disturbance and the operating frequency value recovers towards the nominal
frequency. The load that was shed during the disturbance can be restored. The
load-restoring operation should be done stepwise in such a way that it does not
lead the system back to the emergency condition. This is done through an operator
intervention or in case of remote location through an automatic load restoration
function. The load restoration function also detects the system frequency and
restores the load if the system frequency remains above the value of the set
restoration frequency for a predefined duration.
REX640
Technical Manual
Protection functions
935