Different protective relays

 Different protective relays are used for different power system equipment protection

Now let’s have a look on which different protective relays are used in different power system equipment protection schemes

Relays for Transmission & Distribution Lines protection

SL	Lines to be protected	Relays to be used
1	400 KV Transmission Line	Main-I: Non switched or Numerical Distance Scheme Main-II: Non switched or Numerical Distance Scheme
2	220 KV Transmission Line	Main-I: Non switched distance scheme (Fed from Bus PTs) Main-II: Switched distance scheme (Fed from line CVTs) With a changeover facility from bus PT to line CVT and vice-versa.
3	132 KV Transmission Line	Main Protection: Switched distance scheme (fed from bus PT) Backup Protection: 3 Nos. directional IDMT O/L Relays and 1 No. Directional IDMT E/L relay.
4	33 KV lines	Non-directional IDMT 3 O/L and 1 E/L relays.
5	11 KV lines	Non-directional IDMT 2 O/L and 1 E/L relays.

Relays for Transformer protection

SL	Voltage Ratio and Capacity of Transformer	Relays on HV Side	Relays on LV Side	Common Relays
1	11/132 KV Generator Transformer	3 nos Non-Directional O/L Relay 1 no Non-Directional E/L Relay and/or standby E/F + REF Relay	– -	Differential Relay or Overall differential Relay Overflux Relay Buchholz Relay OLTC Buchholz Relay PRV Relay OT Trip Relay WT Trip Relay
2	13.8/220 KV 15.75/220 KV 18/400 KV 21/400 KV Generator Transformer	3 nos Non-Directional O/L Relay 1 no Non-Directional E/L Relay and/or standby E/F + REF Relay	– -	Differential Relay or Overall differential Relay Overflux Relay Buchholz Relay OLTC Buchholz Relay PRV Relay OT Trip Relay WT Trip Relay
3	220 /6.6KV Station Transformer	3 nos Non-Directional O/L Relay 1 no Non-Directional E/L Relay and/or standby E/F + REF Relay	3 nos Non-Directional O/L Relay	Differential Relay Overflux Relay Buchholz Relay OLTC Buchholz Relay PRV Relay OT Trip Relay WT Trip Relay
4	Gen-volt/6.6KV UAT	3 nos Non-Directional O/L Relay	3 nos Non-Directional O/L Relay	Differential Relay Overflux Relay Buchholz Relay OLTC Buchholz Relay PRV Relay OT Trip Relay WT Trip Relay
5	132/33/11KV upto 8 MVA	3 nos O/L Relay 1 no E/L Relay	2 nos O/L Relays 1 no E/L Relay	Buchholz Relay OLTC Buchholz Relay PRV Relay OT Trip Relay WT Trip Relay
6	132/33/11KV above 8 MVA & below 31.5 MVA	3 nos O/L Relay 1 no Directional E/L Relay	3 nos O/L Relay 1 no E/L Relay	Differential Relay Buchholz Relay OLTC Buchholz Relay PRV Relay OT Trip Relay WT Trip Relay
7	132/33KV, 31.5 MVA & above	3 nos O/L Relay 1 no Directional E/L Relay	3 nos O/L Relay 1 no E/L Relay	Differential Relay Overflux Relay Buchholz Relay OLTC Buchholz Relay PRV Relay OT Trip Relay WT Trip Relay
8	220/33 KV, 31.5MVA & 50MVA 220/132KV, 100 MVA	3 nos O/L Relay 1 no Directional E/L Relay	3 nos O/L Relay 1 no Directional E/L Relay	Differential Relay Overflux Relay Buchholz Relay OLTC Buchholz Relay PRV Relay OT Trip Relay WT Trip Relay
9	400/220KV 315MVA	3 nos Directional O/L Relay (with dir.highset) 1 no Directional E/L relay. Restricted E/F relay 3 nos Directional O/L Relay for action	3 nos Directional O/L Relay (with dir.highset) 1 no Directional E/L relay. Restricted E/F relay	Differential Relay Overflux Relay Buchholz Relay OLTC Buchholz Relay PRV Relay OT Trip Relay WT Trip Relay Over Load (Alarm) Relay

Points to be remembered in respect of protection of transformers

i. No Buchholz relay for transformers below 500 KVA capacity

ii. Transformers up to 1500 KVA shall have only Horn gap protection

iii. Transformers above 1500 KVA and upto 8000 KVA of 33/11KV ratio shall have one group control breaker on HV side and individual LV breakers if there is more than one transformer.

iv. Transformers above 8000 KVA shall have individual HV and LV circuit breakers.

v. The relays indicate above shall be provided on HV and LV

vi. LAs to be provided on HV & LV for transformers of all capacities and voltage class.

vii. OLTC out of step protection is to be provided where Master follower scheme is in operation

viii. Fans failure and pumps failure alarms to be connected.

ix. Alarms for O.T., W.T., Buchholz (Main tank & OLTC) should be connected.