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5.0 Functions and Descriptions
5.1 Circuit Breaker Failure Protection
Circuit breaker failure protection (CBFP) is used to generate a tripping signal via selected output
relay after a preset time delay if the fault has not been cleared after the activation of tripping signal
through trip contact relay R1. Thermal overload is excluded from fault condition for CBFP.
CBFP output is usually used to trip the upstream circuit breaker or to trip a redundant tripping
circuit of the same circuit breaker.
CBFP function can be assigned to relay R2 to R4.
There is an option to enable or disable Aux 1 or 2 as a fault condition (if Aux 1 or 2 is assigned to a
digital input as a tripping source). Prolonged activation or latched Aux input could cause the
undesirable activation of CBFP relay, in this case set ‘Include Aux’ to ‘No’ to disable the condition.
Timing diagram for CBFP. (R4 function set to CBFP, delay 0.5s.)
5.2 Inrush Blocking
The inrush blocking function can be used to block the overcurrent (selectable: I>, I>>, I>>>), earth
fault (Io>, Io>>, Io>>>) and negative sequence overcurrent (I2>, I2>>) starting due to inrush
current with second harmonic component. During transformer energization, the second harmonic
component of the inrush current is high. The relay activates inrush blocking for any phase with
second harmonic current ratio above the Inr H2 Ratio. In general, a setting of 15 to 20% can be
applied. A too high setting may result in the overcurrent tripping during transformer energization. A
too low setting may prevent tripping during some internal transformer faults with significant second
harmonic current. 0.2In minimum fundamental current is required for operation of inrush blocking
function.
The reset delay (tReset) of the Inrush Blocking once the 2nd harmonic level falls below the set
threshold can be set depends on the transient inrush duration. Typically from 0.1seconds (for
100kVA transformer) to 1.0 second (for a large transformer)
Trip relay
R1
Fault
condition
CBFP relay
R4
CBFP delay
0.5s
Fault
Fault
<0.5s
