ABB RELION REX640 Technical Manual page 859

1MRS759142 C
REX640
Technical Manual
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
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