Page 209 - Practical Power System and Protective Relays Commissioning

Protection Relays Chapter | 18 209

FIGURE 18.5.11 Grading method for a ring main circuit.

18.5.6 IEC CURVES AND ANCI/IEEE CURVES OVERCURRENT
RELAY CHARACTERISTIC
a. IEC 60255 curves overcurrent relay characteristic (Table 18.5.4)
b. ANCI/IEEE curves overcurrent relay characteristic (Table 18.5.5)

TABLE 18.5.4 IEC 60255 Curves Overcurrent Characteristic
Relay Characteristic Equation (IEC 60255)
0:14
Standard inverse (SI) t 5 TMS 3
I 0:0221
13:5
Very inverse (VI) t 5 TMS 3
I 2 1
80
Extremely inverse (EI) t 5 TMS 3
I 221
120
Long time standard earth fault t 5 TMS 3
I 2 1

TABLE 18.5.5 ANCI/IEEE Curves Overcurrent Characteristic
Relay Characteristic Equation (IEC 60255)

TD 0:0515
IEEE moderately inverse t 5 1 0:114
7 I r 0:0221
TD 19:61
IEEE very inverse t 5 1 0:491
7 I r 221

(Continued)