Restricted Earth Fault Protection:
Normally Earth fault is nothing but an unintended contact between the conductor body to ground is called earth fault see here the main article. The earth fault may occur due to various reason. When fault occurs the fault current flow to the power system, and this may return through the earth. The fault current will be very high typically more than 50kA depending upon the voltage rating. This much fault current may damage the electrical equipment seriously. To protect our system against from external earth fault, we use standby earth fault protection. At the same time, to sense the internal fault such internal winding earth fault, we use restricted earth fault relay. When an external fault in the star side of the transformer or alternator leads to current flowing in the phase conductor and the phase Current Transformer reads the fault current of the faulty phase and the same time a balancing current flows in the neutral current transformer (NCT), hence as per sechem the vector sum of the phase current and Neutral current is Zero. therefore, the resultant current in the relay is zero. So this REF relay will not be operated for external earth fault. Restricted earth fault relay is called sensitive earth fault relay. It uses to protect the transformer/alternator against internal fault such as internal earth fault, earth leakage fault etc.
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Restricted Earth fault Relay Connection arrangement:
This relay can be implemented for only star connected transformer or star connected alternator. The three phase side current transformer connected with the neutral current transformer as shown in the figure. The neutral transformer is called as NCT. Normally NCT comes inside of the transformer. But for alternator we have to implement the Neutral CT separately on NGT or NGR panel.
Note: We should use separate CT or separate protection core for restricted earth fault relay. The CT output should not be shared with any other electrical equipment. This condition is applicable for both phase CT and neutral CT.
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Restricted Earth Fault Protection operation:
During Normal Operation the balance current in the sum of all phase CT is equal to the current flow in the neutral. Hence the total current flow in the relay operating coil is zero. Then relay remains inoperative. It circuit creates a balance bridge. But during internal earth fault the neutral current transformer only carries the unbalance fault current and the same current will be read by the neutral CT. Then the balance bride become unbalance, hence there is some unbalance current flow through the relay operating coil. if the unbalance fault current reaches the pre-set value of the relay then the Restricted Earth Fault trips the circuit breaker.
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Cause of restricted earth fault relay:
- 64 R- (ANSI Code for Restricted earth fault relay) relay operates.
- 86 H (Master trip heavy fault operates).
- Main equipment circuit breaker trips.
- Turbine trips
- Block out.
Why Restricted earth fault relay is called as Sensitive earth fault relay
The restricted earth fault relays are very sensitive for detecting internal faults. A small amount of unbalance current leads to tripping of electrical equipment.
Why REF is called Restricted Earth fault Relay?
Restricted earth fault protection is kind of Zone protection. It does not detect the fault current outside of the protective zone. Because if the fault occurs outside of the zone means, the same fault read by the phase CT and the corresponding unbalance current read by the Neutral CT, therefore as per the relay scheme the both unbalance current cancels each other, the relay will be inoperative. Since it is called restricted earth fault.
- If a through fault occurs means, due to that any one of the CT core saturated means the relay operates the tripping mechanism. The relay stability become reduced. We can eliminate by adding suitable stabilizing resistor in series with the operating coil.
- REF needs identical current transformer including ratios and burden. For higher size electrical equipment, the neutral CT and phase CT ratio will be changed, So that, we have to install addition Current transformer to match the ratio difference. That is called interposing current transformer ICT.
- Current transformer output should not be shared.
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