Schweitzer Engineering Laboratories SEL-2BFR, Instruction Manual

Page: 37 / 240

Date Code 20010515 Specifications 2-17
SEL-2BFR, -2/BFR Instruction Manual
2 1
DWG: IM2007
DATE: 06-07-00
Figure 2.7: Double-Breaker Arrangement Provides Two Fault Current Sources
Protection While Tripping Load or Line-Charging Current
Figure 2.8 illustrates the logic that protects against breaker failure while tripping load current.
This logic contains two sections. The first section detects breaker operation by dropout of
sensitive phase overcurrent elements. The second section detects breaker operation by dropout of
the 52A input. Both sections of logic operate the Relay Word LPF bit for pending failure
operations and the Relay Word LBF bit for failure operations.
To use the overcurrent section of the logic, set the 50LD overcurrent element to pickup for load
or line-charging current. With this sensitive setting, the 50LD element is picked up whenever the
protected breaker is closed. When the trip input is asserted for two consecutive quarter-cycles,
the 62LP and 62LD timers start. If the 50LD overcurrent element does not drop out before 62LP
expires, the LPF (Load Current Pending Failure) bit asserts in the Relay Word. If the current
remains above the 50LD threshold for the duration of 62LD, the LBF bit in the Relay Word
asserts. When the phase current drops below the 50LD threshold, both timers reset.
Because this logic is very sensitive, the trip input must be asserted for two consecutive quarter-
cycles before this logic acknowledges the input. This feature provides security against incidental
trip input assertions.
In some applications, current through the closed breaker may be below the minimum setting of
the 50LD element. In these instances, enable the relay to provide breaker failure protection based
upon the 52A input state. When you set the E62A setting equal to Y or 1, the relay starts the
62AF and 62AP timers each time any TRIP input is asserted for two consecutive quarter-cycles
while the protected breaker is closed (52A input asserted). The timers run until one of the
following occurs:
The 52A input deasserts, indicating that the breaker opened, or
Some breaker failure detection mode operates the relay 86BF TRIP outputs, or
The 62AF timer expires.
If the 52A input deasserts before either timer expires, both timers stop then are reset the next time
the breaker closes. If the breaker fails in some manner detected by other logic in the relay and the
86BF TRIP output asserts, the 62A timers are disabled. When the 62AF timer expires, the Relay