Alstom MiCOM P546 - 1.1.3 CT ratio correction (all models); 1.1.4 Protection of transformer feeders (P543 and P545); 1.1.4.1 Transformer magnetizing inrush and high set differential setting (P543;P545)

P54x/EN OP/La4 Operation
(OP) 5-20 MiCOM P543, P544, P545 & P546
OP
When applying a three end scheme with ends local (L), remote 1 (R1) and remote 2 (R2),
the differential current is calculated as follows:
d = L + R1 + R2 - (jVL Bs/3) - (jVR1 Bs/3) - (jV R2 Bs/3)
d = {L - (jVL Bs/3) } + { R1 - (jV R1 Bs/3) }+ { R2 - (jV R2 Bs/3) }
d = Local relay current + remote 1 relay current + remote 2 relay current
Where Bs is the total teed line positive sequence susceptance
i.e. Bs = Bs from L-Tee + Bs from R1 – Tee + Bs from R2 - Tee
The display of currents in the 'Measurements 3' column will be affected by this feature when
selected.
1.1.3 CT ratio correction (all models)
To en
sure correct operation of the differential element, it is important that under load and
through fault conditions, the currents into the differential element of the relay balance. There
are many cases where CT ratios at each end of the differential protection are different. Ratio
correction factors are therefore provided. The CT ratio correction factors are applied to
ensure that the signals to the differential algorithm are correct.
1.1.4 Protection of transformer feeders (P543 and P545)
MiCOM P5
43/P545 relays can be applied when transformers are located in the differential
zone. In order to obtain the correct performance of the relay for this application, MiCOM
P543/P545 is provided with:
 Phase compensation to take unto account any phase shift across the transformer,
possible unbalance of signals from current transformers either side of windings, and the
effects of the variety of earthing and winding arrangements. In P543 and P545,
software interposing CTs (ICTs) are provided to give the required compensation.
 Inrush restrain to cater for high levels of magnetizing current during inrush conditions.
 CT ratio correction factor as mentioned in section 1.1.3 to match the transformer winding
rated currents if needed.
Note: The P544 and P546 relays do not include any of the above features,
except CT ratio mismatch compensation, and as such would not be
suitable for the protection of in-zone transformer feeders.
On P543 or P545 relays where capacitive charging current compensation is available, there
is a setting to select if capacitive charging current compensation is used or if interposing CTs
are used.
1.1.4.1 Transformer magnetizing inrush and high set differential setting (P543/P545)
The m
agnetizing inrush current to a transformer appears as a large operating signal to the
differential protection. Special measures are taken with the relay design to ensure that no
maloperation occurs during inrush.
Figure 7 shows a transformer magnetizing ch
aracteristic. To minimize material costs, weight
and size, transformers are generally operated near to the ‘knee point’ of the magnetizing
characteristic. Consequently, only a small increase in core flux above normal operating
levels will result in a high magnetizing current.