P54x/EN AP/La4 Application Notes
(AP) 6-54
MiCOM P543, P544, P545 & P546
AP
It is necessary to calculate the required ratio correction factor (CF) as well as the phase
correction factor for each end. To choose the appropriate vector compensation, it is
necessary to account for phase current and zero sequence current filtering as explained in
example 3.1.2.
To
calculate the correction factor range, it is necessary to use the same MVA base for the
three sides of the transformer although the third winding actually has a lower rated MVA.
This is to ensure secondary current balance for all conditions.
For HV side: 100 MVA/ (400 kV.3) = 144.34 A.
Secondary current = 144.34 x 1/600 = 0.24 A
For MV side: 100 MVA/ (110 kV.3) = 524.86 A.
Secondary current = 524.86 x 1/1200 = 0.44 A
For LV side: 100 MVA/ (30 kV.3) = 1924.5 A.
Secondary current = 1924.5x 5/2000 = 4.81 A
Each secondary current should be corrected to relay rated current, in this case 1A for HV
and MV side and 5 A for 30 kV side
HV ratio correction factor = 1/0.24 = 4.16
MV ratio correction factor = 1/0.44 = 2.29
LV ratio correction factor = 5/4.81 = 1.04
To choose the vector compensation connection, it should be noted that the Wye connected
HV line is in phase with the MV line current and leads the LV line current by 30
o
. Therefore
for LV side, the phase shift must be compensated.
To account for the zero sequence current filtering in the case of an external earth fault, it is
necessary to connect the Wye connected power transformer windings to an interposing
current transformer (internal relay ICT) to trap the zero sequence current (the secondary side
being connected delta).
To account for both vector compensation and zero sequence current filtering, the following
vectorial compensation setting is recommended:
For HV side = Yd1 (-30 deg)
For MV side = Yd1 (-30 deg)
For LV side = Yy0 (0 deg)
Note: It is not necessary to include the 3 factor in the calculation as this is
incorporated in the relay algorithm.
P543 and P545 relays are suitable for transformer applications, as such an inrush restrain is
provided on these relay models. By enabling inrush restrain, an additional current differential
high setting (Id High set) becomes enable.
When the inrush restrain feature is enabled, it is necessary that this function is enabled in
the relay at each line end (3 ends).
For the differential calculation the same recommended settings for the previous examples
are recommended:
Is1 = 0.2 In
Is2 = 2 In
K1 = 30%
K2 = 100%
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