21
interposing CT. Output windings S1–S2 and S3–S4 must be connected in series to
give 43 turns, thus:
I
s/
√
3
I
p
5 x 43
√
3 x 4.62
Table 1 indicates that primary taps nos. 3–6 will give 27 turns.
Primary turns (Tp) =
= 26.86 say 27 turns.
Tp =
x Ts
138kV winding:
Based on 400MVA the corresponding current would be:
400 x 10
6
√
3 x 138 x 10
3
Secondary current from 138kV current transformers:
1674 x 5
1200
The 1200/5 star connected line CTs are associated with the 138kV star winding and
thus the necessary transition to delta connection must be made by means of an
interposing CT.
The output windings S1–S2 and S3–S4 of the interposing transformer must be
connected in series to give a total of 43 turns (see Table 1).
The number of primary turns (Tp) should be
Tp =
x Ts =
= 17.73 say 18T
Table 1 indicates that primary tap nos. 2—9 should be used, with link 6—X removed
and terminal 5 connected to terminal X.
I
n =
= 1674A
I
s =
= 6.975A say 7A
I
s/
√
3
I
p
A
B
C
To
Relay
To 500kV
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
6
6
6
3
3
3
5 x 43
√
3 x 7
