Page 288 - Practical Power System and Protective Relays Commissioning

282 Practical Power System and Protective Relays Commissioning

Set to 1.5 for residential loads;
F 5 reset ratio 5 0.85 for electromechanical relays and set to 0.9 0.95
for static and digital relays;
I L Max 5 maximum expected load;
I set 5 2 2.5 I L Max .
The performance of overcurrent relays is checked by calculating the
sensitivity factor, S, which is the ratio of the minimum short circuit current
following at the relay in the end protected line in a radial system to the relay
setting as follows:
S 5 I sc minimum=I set should be . 1:5

19.1.2 TRANSMISSION LINE OVERCURRENT PROTECTION

Fig. 19.1.1 illustrates the overcurrent protection of a high-voltage transmis-
sion line
The circuit data include:
Overcurrent relay type MCGG22.
Application:
Backup protection 500 kV, fault level at end B 5 12,000 MVA, current
setting of relay at A 5 800 A.
I s 5 800 A current transformer (CT) ratio 5 1600
1
1
I s 5 800 5 0:5A
1600
Plug setting multiplier (PSM)

I f MVA F
PSM 5 -I F 5
I s O3 3 V
12; 000
I F 5 5 13:856 KA
O3 3 500

FIGURE 19.1.1 Overcurrent protection of a high-voltage transmission line.