where
X
Zt¼ZtabþZtbcþZtca½¼ct000
000
0002
6
43
7
5½¼dt1
nt100
010
0012(^64)
3
(^75)
½¼At
1
3 nt
2 1 1
12 1
1 12
2
6
4
3
7
5
½Bt ¼
1
3
P
Zt
2 ZtabZtcaþZtbcZtca 2 ZtabZtbcþZtbcðÞZtabþZtca 0
2 ZtbcZtcaZtbcðÞZtabþZtbc 2 ZtbcðÞZtabþZtca ZtbcZtca 0
ZtabZtca 2 ZtcaðÞZtabþZtbc ZtabZtbc 2 ZtbcZtca 0
2
(^64)
3
(^75)
where
X
Zt¼ZtabþZtbcþZtca
21.1.5.5 Thevenin Equivalent Circuit
The study of short-circuit studies that occur on the load side of a transformer bank requires the three-
phase Thevenin equivalent circuit referenced to the load side terminals of the transformer. In order to
determine this equivalent circuit, the Thevenin equivalent circuit up to the primary terminals of the
‘‘feeder’’ transformer must be determined. It is assumed that the transformer matrices as defined above
are known for the transformer connection in question. A one-line diagram of the total system is shown
in Fig. 21.22.
The desired Thevenin equivalent circuit on the secondary side of the transformer is shown in
Fig. 21.23.
In Fig. 21.22 the system voltage source [ELNABC] will typically be a balanced set of per-unit voltages.
The Thevenin equivalent voltage on the secondary side of the transformer will be:
½¼Ethabc ½At ½ELNABC (21:155)
The Thevenin equivalent impedance in Fig. 21.23 from the source to the primary terminals of the feeder
transformer is given by
½Zthabc ¼½At½ZsysABC½dt þ½Bt (21:156)
Source [Z sysABC]
[ELNABC]
[IABC]
[Iabc]
[VLNabc]
FIGURE 21.22 Total system.