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Section 6
Differential protection
1.
Manual reset command is given to the IED
2.
DC power auxiliary supply to the IED is interrupted (that is, switched-off)
However, if it is required to have mechanically latched tripping and lock-out in the circuit
breaker closing circuit, then it is recommended to use one dedicated lockout IED for each bay.
Such mechanical lock-out trip IEDs are available in the COMBIFLEX range (for example RXMVB2
or RXMVB4 bistable IEDs).
From the application point of view lock-out trip IEDs might have the following drawbacks:
•
The trip contacts will remain closed. If the breaker would fail to open the tripping coil will
be burnt and the DC supply short-circuited.
•
The trip circuit supervision (TCS) IEDs will reset and give alarm for a failure in the trip
circuit if the alarm is not opened by the lock-out IED or a double trip circuit supervision is
recommended where the trip circuit is supervised with two alternatively, TCS IEDs.
6.1.3.11
Contact reinforcement with heavy duty relays
There is sometimes a request for heavy duty trip relays. Normally the circuit breaker trip coils,
with a power consumption of 200 to 300 W, are provided with an auxiliary contact opening the
trip circuit immediately at breaker tripping. Therefore, no heavy duty breaking capacity is
required for the tripping relays. Nevertheless heavy duty trip relays are still often specified to
ensure trip circuit opening also if the circuit breaker fails due to a mechanical failure or a lack
of energy for operation. This can particularly occur during site testing. In this case it is
recommended to use COMBIFLEX RXMH 2 or RXMVB 2 heavy duty relays.
6.1.3.12
Trip circuit supervision
Trip circuit supervision is mostly required to supervise the trip circuit from the individual bay
IED panel to the circuit breaker. It can be arranged also for the tripping circuits from the
busbar protection.
However, it can be stated that the circuit from a busbar protection trip relay located in the
busbar protection panel is not so essential to supervise as busbar faults are very rare
compared to faults in bays, especially on overhead power lines. Also it is normally a small risk
for faults in the tripping circuit and if there is a fault which affects only one bay and all other
bays are thus correctly tripped meaning that the fault current disappears or is limited to a low
value.
6.1.4
Two-zone busbar arrangements
6.1.4.1
General
Busbar differential protection application principles for typical busbar arrangements, up to
two differential zones, are shown and described in the following sections.
6.1.4.2
Single busbar arrangements
The simplest form of busbar protection is a one-zone protection for single busbar
configuration, as shown in Figure 53. When different CT ratios exist in the bays compensation
is done by setting the CT ratio individually for each bay.
The only requirement for busbar protection is that the protection scheme must have one
differential zone. For any internal fault all circuit breakers must be tripped, which will cause
loss of supply to all loads connected to the station.
112
1MRK 505 370-UEN D
M12120-3 v1
M12120-4 v3
M12121-3 v3
M12121-4 v3
M6640-3 v2
M6641-3 v5
Busbar protection REB670
Application manual
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