ABB RED615 Technical Manual page 515

1YHT530004D05 D
615 series
Technical Manual
a small margin. The safe margin of operation is usually less than ±0.5 Hz. The
system frequency stability is one of the main concerns in the transmission and
distribution network operation and control. To protect the frequency-sensitive
electrical equipment in the network, departure from the allowed band for safe
operation should be inhibited.
Any increase in the connected load requires an increase in the real power
generation to maintain the system frequency. Frequency variations form whenever
there are system conditions that result in an unbalance between the generation and
load. The rate of change of the frequency represents the magnitude of the
difference between the load and generation. A reduction in frequency and a
negative rate of change of the frequency are observed when the load is greater than
the generation, and an increase in the frequency along with a positive rate of
change of the frequency are observed if the generation is greater than the load. The
rate of change of the frequency is used for a faster decision of load shedding. In an
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.
Section 4
Protection functions
509