Alstom MiCOM P545 - 3.1.4 Three winding transformer in zone with different rated CTs example

Application Notes P54x/EN AP/La4
MiCOM P543, P544, P545 & P546
(AP) 6-53
AP
Susceptance settings:
For Ends A and B
With a VT ratio 275 kV/110 V and CT ratio 4000/5
RCT = 800
RVT = 2500
B = 310 S
Secondary susceptance = 310 S x RVT/ RCT = 968 S
For End C
With a VT ratio 275 kV/110 V and CT ratio 1200/5
B = 310 S
Secondary susceptance = 310 S x RVT/ RCT = 3.22 mS sec.
3.1.4 Three winding transformer in zone with different rated CTs example
P543 and P5
45 relays are suitable for the protection of three winding transformers in zone.
An example is shown in Figure 17.
100 MVA/100
MVA/30MVA Transformer, Ynyn0d1, 400 kV/110 kV/30 kV
HV, 400 kV CT ratio - 600/1
MV, 110 kV CT ratio - 1200/1
LV, 30 kV CT ratio - 2000/5
P543/5
Yd1 x 4.16
P543/5
Yy0 x 1
Ch 1
Ch 2
Ch 1
Ch 2
Ch 1
Ch 2
2000/5
30 kV
110 kV
400 kV
600/1
Digital communication link
Digital communication link
Digital communication link
1200/1
P1503ENc
P543/5
Yd1 x 3.81
Ynyn0d1
100MVA/100MVA/30MVA
Figure 17 Three winding transformer in zone application
These three relays must be rated differently, i.e. 1A for HV and MV side and 5 A for 30 kV
side. This does not present a problem for P54x relays as the digital signals representing the
currents are in pu.