Description of Operation
GARD 8000 Distance Relay
RFL Electronics Inc.
Figure 9-61. Rate of change of frequency
9.18.4
UNDERVOLTAGE BLOCK UNIT
The undervoltage unit supervises the frequency units and blocks them for undervoltage conditions.
This unit asserts when the voltage drops below 100% of set value, resetting at 105% of the setting if
this condition persist during a time period of 10 cycles.
Note that the relay can not determine the frequency for voltage below 10 V so for these conditions all
frequency elements are blocked.
9.18.5
APPLICATION OF FREQUENCY UNITS
When load and supply for a given portion of power systems are unequal, or not distributed equally, any
type of a sudden generation or transmission deficiency may cause cascading outages. When the load
in a balanced power system significantly exceeds generation, the system can survive only if enough
load is separated from the system with a shortage in generation to cause generator output to be equal to
or slightly above the connected load.
Variations in frequency are caused by unbalance between generation and load. The reason for the
unbalance is often one of the following:
•
Break up of the power system in sections
•
Unbalance between generation and load due to lack of running reserves
•
Loss of generation, trip of substations or lines for important interconnections
August 1, 2009
9-69
973.334.3100
