Application - ABB RELION 620 Series Technical Manual

Section 4
Protection functions
4.3.2.5
334
blocking is activated. In the "Block all" mode, the whole function is blocked and the timers
are reset. In the "Block TRIP output" mode, the function operates normally but the TRIP,
OPR_FRQ and OPR_FRG outputs are not activated.

Application

An AC power system operates at a defined rated frequency. The nominal frequency in
most systems in the world is 50 Hz or 60 Hz. The system operation is such that the
operating frequency remains approximately at the nominal frequency value by 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
1MAC504801-IB E
620 series ANSI
Technical Manual