22
13.45kV winding:
Based on 400 MVA the corresponding current would be:
400 x 10
6
√
3 x 13.45 x 10
3
NB
In this case the transformer winding (13.45kV) is delta connected, and the
associated line current transformers are star connected. There is thus no
phase angle compensation required, the associated interposing CT may be
star connected and taps selected to reduce the 17.17A output to 5A.
Using S1–S2, and S3–S4 in series as output winding, the primary tap becomes:
Primary turns (Tp) =
x Ts
Tp =
= 12.52 say 13T
Table 1 indicates that primary taps nos. 2–8 will give 13 turns, with link X–5 instead
of X–6.
I
n =
= 17170.2A
I
s =
= 17.17A
17170.2 x 5
5000
I
s
I
p
S3
A
B
C
To 138kV
line
current
transformer
P1
P1
P1
A
B
C
N
S1
S1
S1
S2
S2
S2
S3
S3
S4
S4
S4
P2
P2
P2
9
5
x
2
9
5
x
2
9
x
2
5
A
B
C
To
Relay
To 13.45kV
line
current
transformer
P1
P1
P1
A
B
C
N
S1
S1
S1
S2
S2
S2
S3
S3
S3
S4
S4
S4
P2
P2
P2
N
8
5
x
2
8
5
x
2
8
5
x
2
43 x 5
17.17
