Alstom MiCOM P546 - 3.1.2 Transformer feeder examples

P54x/EN AP/La4 Application Notes
(AP) 6-50 MiCOM P543, P544, P545 & P546
AP
If VT is connected, a facility exists to overcome the effects of the line charging current. It will
be necessary in this case to enter the line positive sequence susceptance value. This can
be calculated from the line charging current as follows (assuming a VT ratio of 33 kV / 110 V):
ch = 25 x 2.5 A = 62.5 A
Susceptance B =
C = ch/V
B = 62.5 A/(33/
3 ) kV primary
B = 3.28 x 10-3 S primary
Therefore set:
B = 3.28 mS primary (= 2.46 mS secondary)
s1 may now be set below the value of line charging current if required, however it is
suggested that Is1 is chosen only sufficiently below the charging current to offer the required
fault resistance coverage as described in section 2.1.2 Where charging current is low or
negligibl
e, the recommended factory default setting of 0.2 n should be applied.
3.1.2 Transformer feeder examples
Ratio corre
ction example
:
P543 and P545 relays are suitable for the protection of transformer feeders. An example is
shown in Figure 15.
20MVA Tran
sformer, Dyn1, 33/11 kV
HV CT ratio - 400/1
LV CT ratio - 1500/1
Digital communication channel
400/1
350A 1050A
0.875
0.7
1500/1
0° 30°
Dyn1
20MVA 33/11kV
P1011ENa
Yy0 x1.14
Yd11 x1.43
Figure 15 Typical transformer feeder circuit
It is necessary to calculate the required ratio correction factor to apply to the relays’ at each
line end.
33 kV full load current = 20 MVA/(33 kV.
3 ) = 350 A
Secondary current = 350 x 1/400 = 0.875A
11 kV full load current = 20 MVA/(11 kV.
3 ) = 1050 A
Secondary current = 1050 x 1/1500 = 0.7A
Each of these secondary currents should be corrected to relay rated current; in this case 1A.
HV ratio correction factor 1/0.875 = 1.14 [Setting applied to relay]
LV ratio correction factor 1/0.7 = 1.43 [Setting applied to relay]