BECKWITH ELECTRIC M-3425A, Instruction Book

Page: 206 / 482

4–75
System Setup and Setpoints – 4
Implementation of the generator breaker failure
function is illustrated in Figure 4-54. The breaker
failure timer will be started whenever any one of
the designated output contacts or the external
programmed breaker failure initiate status input are
operated. The timer continues to time if any one of
the phase currents are above the 50BF-Ph pickup
setting or if the 52b contact indicates the breaker is
still closed; otherwise, the timer is reset.
Since current in the generator high side CT which
energizes the 50BF protection (I
A
, I
B
, I
C
) might not
extinguish concurrently with the breaker opening for
faults between the CT location and the generator
breaker, a possible area of mis-operation exists.
Usually the risk of faults in this limited area is small
enough to be ignored but should be considered.
50BF-Neutral Element
: This instantaneous
overcurrent relay is energized from the generator
neutral CT (See Figure 4-18, One-Line Functional
Diagram). This function is internally in series with
a breaker “b” contact (IN1) to provide logic for the
breaker flashover protection (see Figure 4-54).
HV Breaker Failure (limited) The breaker failure
function may be used for a unit breaker rather
than a generator breaker. It is limited in that it has
no fault detector associated with the unit breaker.
Output contact operation would occur if any of the
initiate contacts close and the 52b contact indicated
a closed breaker after the set time delay.
This operation is chosen by disabling the neutral
element, disabling the phase element, and
designating initiating inputs and outputs and a time
delay setting.
50BF Generator Breaker Failure/HV Breaker
Flashover
The Generator Breaker Failure/HV Breaker
Flashover function (50BF) is applicable when a
generator breaker is present and line side generator
CTs are being used. The 50BF-Ph phase detector
element (if enabled) is used for breaker failure and
the 50BF -N (if enabled) provides breaker flashover
protection by providing an additional breaker failure
initiate which is only active when the breaker is
open. For high impedance grounded applications,
the 50BF-N function is inapplicable and must be
disabled. Ranges and increments are presented
in Figure 4-55.
50BF-Ph Generator Breaker Failure
: When the
M-3425A Generator Protection Relay detects an
internal fault or an abnormal operating condition, it
closes an output contact to trip the generator breaker
or the unit HV breaker. When a generator breaker is
used, protection is available for the instance where
it fails to clear the fault or abnormal condition. Such
generator breaker failure protection output contacts
must be connected to trip the additional necessary
breakers to isolate the generator from the system.
The breaker-failure condition is usually detected
by the continued presence of current in any one or
more of the phases after a trip has been sent to the
breaker. However, the current detector (50BF-Ph)
may not always give the correct status of the
breaker, especially for generator breakers. This is
because faults and abnormal operating conditions
such as ground faults, overexcitation, over/under
frequency, and reverse power may not produce
enough current to operate the current detectors. For
this reason, the breaker status input 52b contact
must be used, in addition to the 50BF-Ph, to provide
adequate breaker status indication.
Phase Initiate Enable
IN1 (52b)
Logic high when breaker closed
Output Initiate
Input Initiate
Neutral Initiate Enable



Logic high
when breaker open
Figure 4-54 Breaker Failure Logic Diagram