HRC fuse breaking capacity:- 80 KA
SF6 breaking capacity:- 30 KA
ACB breaking capacity :- 50 KA
hence fuses are used for S.C. protection and Relays for O.L. protection.
T1 and T2 reduce the fault current ( say from 400 A to 10 A ) which is the utility of these transformers.
Protections used in 132 KV/ 6.9 KV switchyard:-
- Lightening arrestor 120 KV, 10 KA
- IDMT O/C Relay + IDMT E/F Relay ( 51 + 51 N)
- IDMT O/C Relay + Instantaneous Relay + IDMT E/F Relay ( 51 + 50 + 51 N)
- Transformer Differential Relay(87)
- Restricted E/F Relay + Standby E/F Relay (64 R + 51 G)