Page 170 - Practical Power System and Protective Relays Commissioning

170 Practical Power System and Protective Relays Commissioning

Negative-sequence impedance:

Z 2 5 Z 1
Zero-sequence impedance:
Z 0 can be calculated or determined by testing.
Synchronous generator sequence impedances are now described.
The positive-sequence impedance of a generator is approximately equal
to: Xs (steady state reactance).
The negative-sequence impedance of a generator is approximately equal
to: Xsv (sub transient reactance).
The zero-sequence impedance of a generator is approximately equal to:
XL (leakage reactance).

16.4 SYMMETRICAL THREE-PHASE FAULT ANALYSES
This type of fault is a symmetrical fault as it is on the three phases at the
same time as shown below (Fig. 16.11):
V R 5 V Y 5 V B ; I R 1 I Y 1 I B 5 0
E
I 1 5
Z s 1 Z 1

16.4.1 Unsymmetrical Single-Phase Fault Analyses

This fault is unsymmetrical as it is on one phase and earth, and can be ana-
lyzed as follows (Fig. 16.12):
V R 5 0; I Y 1 I B 5 0; I 1 5 I 2 5 I 0 ; I R 5 3I 1

E
I 1 5
3Z s 1 2Z 1 1 Z 0
I F 5 3 I 0

FIGURE 16.11 Three-phase fault.