ABB 615 Series ANSI, Technical Manual

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1MAC050144-MB C
Section 4
Protection functions
615 series ANSI 275
Technical Manual
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