x/EN AP/Na7
-14 MiCOM P74
2.1.1.7 Busbar Protection Tripping Times
2.1.1.7.1 Tripping times
In the P742 and P743, there are options to use a “high speed, high break” contact board.
As the closing time of a standard relay is around 5ms and the operation of the high speed
contact is less than 1ms, this option can be used to speed-up the tripping time by around
4ms.
2.1.1.7.2 Busbar Protection Tripping Time delay in the CU
In the CU, the 87BB tripping time can be delayed by a settable time, the fault has to be
detected by the phase element only.
That allows, for example the clearance of the fault by a fuse on the fault feeder before
tripping the whole zone.
2.1.1.7.3 Busbar Protection Tripping Time delay in a PU
In all the PUs, the 87BB tripping time can be delayed by a settable time.
That allows, for example in Generation, a sequential tripping of all feeders connected to the
faulty zone.
2.1.1.8 Busbar Protection Tripping order PU logic
In all the PUs, there are options to block the 87BB protection trip order coming from the CU.
The logic is based either on the I>2 high set phase overcurrent function or on the IN>2 high
set neutral overcurrent function, each with the following options:
• Phase and earth fault element (87BBP&N blocking),
• Only the 87BB phase element (87BB/P blocking),
• Only the 87BB SEF element (87BB/N blocking),
• A combination of the different functions (I>2 & 87BBP&N, I>2 & 87BB/P, I>2 & 87BB/N).
The 87BB blocking function has a settable drop-off timer from 200ms to 600s by step of
100ms.
2.2 Additional protections
2.2.1 Dead Zone protection (DZ)
On a feeder, if the isolators or the breaker is open, a dead zone (or end zone) is said to exist
between the open element and the CT. The P74x peripheral units can protect this zone with
the Dead Zone protection. This is a simple time delayed overcurrent and earth fault element
which is only active when a dead zone is identified in the local topology.
2.2.1.1 Setting guidelines
The trip order must be mapped in the PSL to an output relay to send an intertrip to the other
end of the line if the breaker is on the busbar side.
It is advised to also map it to the breaker relay trip contacts.
The busbar diff will trip if the nominal bias current is less than:
IDZ fault x
i.e. K2% = 65%, IDZfault x 0.538.
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