- When the primary winding is delta connected or has unground star point, earth faults on secondary side are not reflected on primary side.
- Under such circumstances, an earth fault relay connected in residual circuit of three CT's on primary side operates on internal earth faults in primary winding only.
- Because earth fault on secondary side do not produce zero sequence current on primary side.
- Therefore for faults on star connected earthed secondary winding of power transformer restricted earth fault protection is used.
- Such a protection provides high speed tripping under fault condition.
- The fig shows the star connected side is protected by restricted earth fault protection.
- When an earth fault occurs beyond the transformer (point f1) , the current I2 and current I1 flows in CT secondary's.
- Therefore the resultant current in earth fault relay is negligible.
- But When Earth Fault occurs within the transformer star connected winding (point f2), only current I2 flows and I1 is negligible. hence only current I2 flows through the earth fault relay.
- Thus Restricted earth fault relay does not operate for earth fault beyond the protected Zone of the transformer.
- When fault occurs very near to the neutral point of the transformer, the voltage available for driving earth fault current is small. hence fault current would be low.
- If the Relay is set too sensitive to sense such faults, then it may operate for External faults and switching surges also.
- Therefore relay setting adjusted in such that it operates for earth fault current of the order 15 % of rated winding current.
- Such setting protects restricted portion of the winding. hence the name restricted earth fault protection.
Overcurrent Protection of transformer
- For Transformer below 5MVA , differential protection is uneconomical . For such transformers, over current protection is employed as main protection against phase faults.
- For small distribution transformer below 500KVA, overcurrent protection may be provided by means of fuse on HV side; as such transformer are installed in unattended sub-stations, circuit breakers and relay are not provided.
- For transformers above about 5MVA, if differential protection is used as a main protection, over current protection is used in addition. as a backup for sustained through faults and earth fault protection is provided in addition to phase fault protection.
MCQ on Restricted earth fault protection and overcurrent protection
1. For transformers below ......., Differential protection is uneconomical.
(A) 10 MVA
(B) 5 MVA
(C) 30 MVA
(D) 20 MVA
Answer: B) 5 MVA
2. Which type of protection provided for Transformer below 5 MVA ?
(A) Differential Protection
(B) Overcurrent protection
(C) Both (A) & (B)
(D) None of above
Answer: (B) Overcurrent protection
3. Which type of protection used for Distribution transformer below 500 KVA ?
(A) Overcurrent protection
(B) Fuse on HV side
(C) Differential protection
(D) None of above
Answer: (B) Fuse on HV side
4. Transformer above 5 MVA which protection used with differential protection as backup?
(A) Overcurrent and earth fault protection
(B) Overcurrent protection only
(C) Earth fault protection Only
(D) None of above
Answer: (A) Overcurrent and earth fault protection