not impact the speed or sensitivity of the protection elements,
operates during all load conditions, and correctly allows tripping
during an evolving external-to-internal fault condition.
The dual-breaker line terminal supervisory logic essentially
determines if the current flow through each breaker is either
forward or reverse. Both currents should be forward for an internal
fault, and one current should be forward and one reverse for an
external line fault. The supervisory logic uses, on a per-phase
basis, a high-set fault detector (FDH), typically set at 2-3 times the
nominal rating of the CT, and a directional element for each CT
input to declare a forward fault, for each breaker. The logic also
uses, on a per-phase basis, a low-set fault detector (FDL), typically
set at 1.5-2 times the nominal rating of the CT, and a directional
element to declare a reverse fault, for each breaker.
Tripping is permitted during all forward faults, even with weak
infeed at the dual-breaker terminal. Tripping is blocked for all
reverse faults when one breaker sees forward current and one
breaker sees reverse current. During an evolving external-tointernal fault, tripping is initially blocked, but when the second
fault appears in the forward direction, the block is lifted to permit
tripping.
Line Differential:Line differential protection is prone to tripping
due to poor CT performance on dual-breaker terminals, as the
error current from the CTs is directly translated into a differential
current. The only possible solution for traditional line differential
relays is to decrease the sensitivity of the differential element,
which limits the ability of the differential element to detect low
magnitude faults, such as highly resistive faults.
The L90 Line Differential Relay supports up to four three-phase
current inputs for breaker failure, overcurrent protection, and
metering for each circuit breaker. The relay then uses these
individual currents to form the differential and restraint currents
for the differential protection element.