2-16
Specifications
Date Code 20010515
SEL-2BFR, -2/BFR Instruction Manual
If all three trip inputs assert, the 62FC pickup timer starts. The three-phase trip input is latched
and may be deasserted after a single quarter-cycle assertion. Following trip input assertion and
62FC pickup time, the 62FC timer output asserts. The timer output remains asserted for one
cycle. Relay word bit FBF asserts if 50FT picks up and asserts during the one-cycle window.
The 62TT pickup timer also times for three-phase faults. Thus, the 62FC pickup timer provides
an operational advantage only if it is set such that:
62FC pickup time delay setting < 62TT pickup time delay setting
Otherwise, the 62TT pickup timer will time out first for a three-phase fault.
The 50FT element is disabled while the 62TT or 62FC pickup timers are timing. Thus, the 50FT
reset time does not have to be taken into account as part of the safety margin incorporated into the
62TT and 62FC pickup time-delay settings. See
Section 5: Applications
for more details.
Note:
In a double-breaker arrangement (i.e., ring-bus or breaker-and-a-half) there are two
breaker sources to the line, Figure 2.7. In this example, fault current distributes
unequally between Breakers 1 and 2. Fault current through Breaker 1 is below breaker
failure overcurrent element pickup levels. The majority of the fault current flows through
Breaker 2. If Breaker 2 clears before Breaker 1 then the fault current increases
dramatically through Breaker 1.
If Breakers 1 and 2 both fail to interrupt fault current and Breaker 1 has Scheme 6 active, breaker
failure protection problems can result. The breaker failure protection scheme at Breaker 2 tries to
trip the immediate breakers surrounding it, including Breaker 1. Breaker 1 is also failed, so the
breaker failure trip from Breaker 2 to Breaker 1 is ineffectual.
Breaker failure timer 62TT (or 62FC) times out in Scheme 6 for Breaker 1 and the one-cycle
window becomes active to let the 50FT overcurrent element pickup if sufficient fault current is
present. After the one-cycle window, the 50FT element is turned off again. Because of the
unequal fault current distribution between the Breakers 1 and 2, and Breaker 2 is failed, no fault
current above the 50FT overcurrent element pickup level is detected through Breaker 1. The one-
cycle window passes and no breaker failure tripping is initiated for Breaker 1.
The immediate breakers to the right of Breaker 2 eventually clear due to operation of the Breaker
2 breaker failure protection scheme and then fault current levels increase dramatically through
Breaker 1. The one-cycle window for Breaker 1 has already passed so Scheme 6 does not initiate
breaker failure tripping for Breaker 1 in this scenario.
The failure to trip load or line-charging current scheme (see Figure 2.8 and accompanying
discussion) should be enabled to provide breaker failure protection for this scenario if it is a
concern for Scheme 6 applied to such breaker arrangements.
Figure 2.1 and accompanying discussion (Scheme 1) address operating concerns similar to this
scenario. Scheme 1 may be better suited to operate in such a scenario.
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