Application Notes P54x/EN AP/La4
MiCOM P543, P544, P545 & P546
(AP) 6-13
AP
2.1.4.3 High set differential setting
When inrush restrain is enabled, a high set differential protection becomes active. This is
provided to ensure rapid clearance for heavy internal faults with saturated CTs. Because
high set is not restrained by magnetizing inrush, hence the setting must be set such that it
will not operate for the largest inrush currents expected. It is difficult to accurately predict the
maximum anticipated level of inrush current. Typical waveforms peak values are of the
order of 8-10x rated current. A worst case estimation of inrush could me made by dividing
the transformer full load current by the per unit leakage reactance quoted by the transformer
manufacturer.
2.1.5 Mesh corner and 1½ breaker switched substations
Whe
re a line is fed from a mesh corner or 1½ breaker switched substation, as shown in
Figure 1, then two options are available for CT co
nnections to the relay. The first is by
paralleling the two sets of line CTs into a common input, ‘A’. The second is by using two
separate inputs for each set of line CTs, ‘B’. The P544 and P546 relays are designed with
an additional set of input CTs specifically for this purpose.
In the case of a through fault as shown, the relay connected to circuit ‘A’ should see no
current and as such, will remain stable. Under this condition, it should be noted that no bias
is produced in the relay. To ensure relay stability, the two sets of line CTs should be as near
as identical in all characteristics, and equally loaded, such that the relaying connection is at
the equipotential point of the secondary leads.
In the case of circuit ‘B’ no differential current should result. A large bias current will
however exist, providing a high degree of stability in the event of a through fault. This bias
will also ensure stability where CTs are not closely matched. Therefore, circuit ‘B’ is the
preferred connection for such applications and so the P544 and P546 relay models would
normally be specified.
A B
F
I
F
I
F
87 87
Stub
bus
inputs
Bus 2
Bus 1
P1009ENa
P1009ENa
Figure 1 Breaker and a half switched substation
2.1.6 Small tapped loads (tee feeds)
Whe
re transformer loads are tapped off the protected line it is not always necessary to install
CTs at this location. Provided that the tee-off load is light, differential protection can be
configured for the main line alone. The settings ‘Phase Char’, ‘Phase Time Delay’ and ‘TMS’
or ‘Time Dial’ in table 3 allow the differential element to time grade with IDMT overcurrent
relays or fuses protecting the tap. This keeps stability of the differential protection for
external faults on the tee circuit.
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