Date Code 20000421 Applications 5-19
SEL-251, -2, -3 Instruction Manual
Figure 5.12: Underfrequency or Undervoltage Trip Condition Time Duration Limit
RESIDUAL CURRENT UNBALANCE ALARM
The NDEM bit asserts when current exceeds ground/residual demand threshold setting NDEM.
Using SELOGIC Control Equations:
A2(1234)= NDEM
Output contact A2 asserts for residual current unbalance. It can also warn of impending ground/-
residual overcurrent relay pickup due to residual current unbalance.
All demand ammeters use the same time constant (DATC = settable from 5 to 60 minutes).
BLOCK VOLTAGE REGULATING LOAD-TAP-CHANGER OPERATION
Load-tap-changers should not operate when a reclosing cycle is in progress because fault current
should not pass through a tap-changer in motion. Also, if the voltage input to the load-tap-
changer is on the load side of the feeder circuit breaker, the load-tap-changer will see zero
voltage during the reclosing relay open intervals and will erroneously start to move toward the
highest tap possible in an attempt to raise the voltage.
The 79CY bit asserts when the reclosing relay is in the reclose cycle state.
Using SELOGIC Control Equations:
A3(1346)= 79CY (Output contact A3 should be configured as a "b" contact)
To Next Page
