Page 190 - Practical Power System and Protective Relays Commissioning

192 Practical Power System and Protective Relays Commissioning

7. Dependability
This means that the relay should operate for faults in its zone in the
correct way and at the correct time, with selectivity, sensitivity, and
speed, to give a proper trip for the fault in its zone.
8. Sensitivity
Sensitivity is critical to being able to detect faults. There are three
independent components for the measured input quantities for the relay:
accuracy, precision, and resolution.

18.3.2 ZONES OF PROTECTION
The general philosophy of protection is to divide the power system into pro-
tective zones that can be protected adequately with a minimum number of
disconnections.
These zones are overlapped to avoid any possibility of unprotected
areas in the power system and can be divided into the following protec-
tive zones:
Generator zone, Zone B in Fig. 18.3.2;
Transformer zone, Zone A in Fig. 18.3.2;
Busbars zone, Zone C in Fig. 18.3.2;
Transmission and distribution circuits zone, Zones D and E in
Fig. 18.3.2;
Motors zone, Zone F in Fig. 18.3.2.
Fig. 18.3.2 shows the protection zones and their overlapping.

18.3.3 STEPS AND INFORMATION REQUIRED TO DESIGN A
GOOD PROTECTION SYSTEM

1. System configuration based on a single line diagram (SLD).
2. Existing system protection based on a protection SLD (PSLD).
3. Existing operation procedures, practices, and possible extensions.
4. Fault study, with the maximum and minimum fault levels during summer
and winter.
5. Maximum load, CT ratios, VT ratios.
6. System parameters, such as transmission line impedances, transformer
impedances.
7. CT and VT locations, connections, and ratios.