Date Code 20010515
Specifications
2-11
SEL-2BFR, -2/BFR Instruction Manual
The front-panel targets indicate breaker failures based on the outputs from each of the basic
breaker protection schemes, regardless of whether you have enabled the scheme for protection.
This can become important when testing the relay, because targets may assert for conditions not
under test if they indicate that a failure has occurred.
S
IGNAL
P
ROCESSING
The relay filters the current and voltage channels and samples the signals four times per power
system cycle. The relay stores the analog low-pass filtered samples for event reporting. The
microprocessor digitally filters each signal using the CAL or FOD digital filters explained below.
The relay stores digital filter outputs for magnitude calculations.
The relay uses a simple, effective CAL digital filter with the properties of a double differentiator
smoother. Let the latest four samples of one analog signal be X1, X2, X3, and X4. Then the
filter is defined:
P = X1 – X2 – X3 + X4.
This filter eliminates dc offsets. When all samples are set to the same value, the filter output is
zero. The filter also eliminates ramps, which you may verify by setting the samples equal to 1, 2,
3, 4. Again, the output is zero.
Every quarter-cycle, the relay computes a new value of P for each input. The current value of P
combines with the previous value (renamed Q) to form a Cartesian coordinate pair. This pair
represents the input signal as a phasor (P, Q). The relay processes these phasor representations of
the input signals.
The digital and analog filter net frequency response is centered on system frequency and
completely rejects dc and double-frequency harmonics. The computer determines the magnitude
of each voltage and current from the digital filter output.
For a faster pickup and dropout response time, the 50FT element uses a First Order Difference
(FOD) filter (P = X1 – X2) rather than the CAL filter.
L
OGIC
D
ESCRIPTION
The SEL-2BFR Relay provides protection for several circuit breaker (CB) failure modes:
Failure to clear a fault (six available schemes)
Failure to trip under load
Failure of the breaker 52A contact to indicate operation
Failure to complete trip sequence due to trip resistor(s) remaining inserted
Failure to complete close sequence due to close resistor(s) remaining inserted
Failure to close
Failure while open: breaker pole flashover detected
The relay provides reset logic for applications with and without motor-operated disconnects. The
relay also provides logic for a verified 52B output. The following sections describe the logic for
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