Page 218 - Practical Power System and Protective Relays Commissioning

218 Practical Power System and Protective Relays Commissioning

18.6.4.1 Effect of System Impedance Ratio (SIR) on Distance
Protection Measurements

The system impedance ratio (SIR) term refers to the ratio of source imped-
ance of the power system behind the relay over the impedance of the trans-
mission line, as follows:
Z s
SIR 5
Z L
As shown in Fig. 18.6.1, the transmission lines can be classified based on
the value of SIR as follows:

1. Transmission line with high SIR ratio (SIR ...);
2. Transmission line with low SIR (SIR ,,, ).
Refer to Fig. 18.6.12 for explanation of the effect of SIR on distance
relay measurements at end A.

FIGURE 18.6.12 Effect or system impedance ratio (SIR) on distance protection measurements.
(A) Impedance relay measurement; (B) short electrically line SIR more than 4; and (C) Long
electrically line SIR less than 0.5.