P54x/EN AP/La4 Application Notes
(AP) 6-12
MiCOM P543, P544, P545 & P546
AP
2.1.4.1 CT ratio correction
In many cases the HV and LV current transformer primary ratings will not exactly match the
transformer winding rated currents. The CT correction factor must be set to ensure that the
signals to the differential algorithm are correct to guarantee current balance of the differential
element under load and through fault conditions. To minimize unbalance due to tap changer
operation, current inputs to the differential element should be matched for the mid-tap
position. If there is not mismatch between the CTs, the CT correction factor should be set to
1:1.
The compensated current values should be arranged to be as close as possible to relay
rated current to provide optimum relay sensitivity
When a Star/Delta software interposing CT is chosen, no additional account has to be taken
for the
3 factor which would be introduced by the delta winding. This is accounted for by
the relay.
2.1.4.2 Phase correction and zero sequence current filtering
Selection
of the phase correction settings will be dependant on the phase shift required
across the transformer and on zero sequence filtering elements with CT correction factors,
the phase correction is applied to each relay. Providing replica interposing CTs in software
has the advantage of being able to cater for line CTs connected in either star as well as
being able to cater for zero sequence current filtering
To aid selection of the correct setting on the relay menu, some examples of selection of
phase compensation factors are shown in the following table:
Vectorial compensation
(relay setting)
Transformer
connection
Transformer phase
shift
HV LV
Dy1 - 30o Yy0 (0 deg) Yd11 (+30 deg)
Yd1 - 30o Yd1 (-30 deg) Yy0 (0 deg)
Dy5 - 150o Yy0 (0 deg) Yd7 (+150 deg)
Yd5 - 150o Yd5 (-150 deg) Yy0 (0 deg)
Dy7 + 150o Yy0 (0 deg) Yd5 (-150 deg)
Yd7 + 150o Yd7 (+150 deg) Yy0 (0 deg)
Dy11 + 30o Yy0 (0 deg) Yd1 (-30 deg)
Yd11 + 30o Yd11 (+30 deg) Yy0 (0 deg)
YNyn 0o Ydy0 (0 deg) Ydy0 (0 deg)
As shown in the table, a delta winding is introduced with the Y side software interposing CT.
This provides the required zero sequence trap, as would have been the case if the vector
correction factor had been provided using an external interposing current transformer.
Whenever an in zone earthing connection is provided, a zero sequence trap should always
be provided. For instance if a YNyn power transformer is in the protected zone, there will be
some difference between HV and LV zero sequence magnetizing current of the transformer.
This is normally small, but to avoid any problems with any application the above rule for zero
sequence traps should be applied with earthed windings.
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