Alstom MiCOM P546 - 1.1.4.2 Phase correction and zero sequence current filtering

P54x/EN OP/La4 Operation
(OP) 5-22 MiCOM P543, P544, P545 & P546
OP
P1007ENa
V
+ jm
Ie
- jm
+2jm
Ie
V
j
j
Steady
state
Switch on at
voltage zero
Figure 8 Magnetizing inrush waveforms
1.1.4.2 Phase correction and zero sequence current filtering
To com
pensate for any phase shift between two windings of a transformer, it is necessary to
provide phase correction. This was traditionally provided by the appropriate delta connection
of main line CTs. Phase correction is provided in the P54x relays via software interposing
CTs.
In addition to compensating for the phase shift of the protected transformer, it is also
necessary to mimic the distribution of primary zero sequence current in the protection
scheme.
The advantage of having replica interposing CTs is that it gives the P54x relays the flexibility
to cater for line CTs connected in either star or delta, as well as being able to compensate
for a variety of system earthing arrangements
Figure 9 shows the need for zero sequence current filtering
for differential protection across
a transformer. The power transformer delta winding acts as a ‘trap’ to zero sequence
current. This current is therefore only seen on the star connection side of the transformer
and hence as differential current.
The filtering of zero sequence current has traditionally been provided by appropriate delta
connection of main line CT secondary windings. In the P54x relays, zero sequence current
filtering is automatically implemented in software when a delta connection is set for a
software interposing CT. Where a transformer winding can pass zero sequence current to
an external earth fault, it is essential that some form of zero sequence current filtering is
employed. This would also be applicable where in zone earthing transformers are used.