Restricted Earth fault Relay & Differential Protection Relay ?

A Standby earth fault relay is nothing but an earth fault protection used to protect the generator, transformer, and motor from the external earth fault (earth fault outside the zone. for example grid side earth fault).
What is Symmetrical Faults and Unsymmetrical Faults ?
Symmetrical Fault:
Symmetrical faults means all three phase line shorted with ground and the magnitude of the load current is same in all three phases with 120 deg phase displacement each other. It is the most severe type of fault involving largest current, but it occurs rarely.
Symmetrical Fault types:
1.Three-phase short circuit fault (LLL)- Three line of the conductor short with each other.
2.Three-phase-to-ground fault (LLLG)- Triple Line-to-ground fault (LLLG)- A triple line-to-ground fault occurs when three conductors fall on the ground or come in contact with the neutral conductor.
Unsymmetrical faults
Unsymmetrical faults are normal fault which means the three phase lines become unbalanced (unequal currents with unequal phase shifts in a three phase system.) and they do not have the equal phase displacement each other’s.
Unsymmetrical faults types:
1.Single line-to-ground fault (LG)- In single line-to-ground fault, one conductor comes in contact with the ground or the neutral conductor. Single line to ground fault is the most frequently occurring fault (60 to 75% of occurrence)
2.Line-to-line fault (LL)- A line-to-line fault occurs when two conductors are short circuited. This type of fault occurrence ranges from 5 to 15%.
3.Double Line-to-ground fault (LLG)- A double line-to-ground fault occurs when two conductors fall on the ground or come in contact with the neutral conductor. This type of fault occurrence ranges from 15 to 25% of occurrence
In this, one of the relays is used to protect our system against L-G, L-L-G, L-L-L-G faults. The relay is called the Standby Earth fault relay.
Why it is called standby earth fault relay?
It is backup protection for restricted earth fault (REF) relay. It operates the CB when REF is failed to trip the circuit, Heavy earth fault outside of the REF protective Zone, and all other earth faults. Simply we can say, it is standby protection for all other earth faults.
Standby Earth fault Relay Construction:
In Neutral side, one current transformer will be installed at a neutral side of the transformer or alternator. The current reference will be taken from the neutral CT. On the panel side, and over a current element will be directly connected with the CT reference.
Standby Earth fault Relay Working Function:
During Normal condition, there is zero current flow in the neutral.Apply Kirchhoff’s current law,
I1 + I2 + I3 = IN = 0.With this condition, the relay does not operate the trip circuit associated with the relay. During the abnormal conditions, when there is a fault in phase R, hence the unbalance fault current goes to the Neutral. The CT senses the unbalance fault current and if the fault current exceeds the preset value, the relay operates.
Note: In practical, always some amount of current flows in the Neutral typically 1-15 Amps depending upon the rating of the equipment i.e for 25MW alternator IN= 10 -15 amps at full load. Because all the phases are exactly equal in magnitude and phase.
What happens if the standby earth fault relay operates?
The standby earth fault protection creates a class A trip. Hence, Relay 51N operates, Master trip relay 96 operates, and the circuit breaker 52 trips.
Restricted Earth fault Protection in Transformers & Generators
Transformers and Generators are voltage sources. They are traditionally protected by an Overcurrent + Earth fault relay, normally mounted in the breaker panel which is not adequate
Hence a separate scheme is required to detect internal earth faults. This scheme is called Restricted Earth Fault (REF) scheme. Fault currents in Zone A are limited by the impedance of the equipment in the zone - for transformers and generators it is very low - the fault currents can rise very fast and damage the equipment. Hence REF protection is of utmost importance for generators and transformers.
Why is this called Restricted E/F ?
The name Restricted is derived since the objective of the protection is to detect the earth fault is restricted to the specific zone, starting from the breaker to the machine terminals.
Why REF protection is required while using Differential protection scheme?
The Principle of REF protection is same as Differential Protection.
REF Protection is also called as Single Phase Differential Protection.
As Differential Protection initial setting is usually around 20 percent to avoid CT Errors and all. So if the fault current is below 20 percent it can’t provide protection even for the internal faults.
So we are using REF protection with lower setting usually around 5 to 10 percent.
For non critical transformers, only Differential protection may be used.

[@nishantadake9390]

Language not understand but concept clear from slides👌

[@allinone.4you]

Thanks brother.. Wil try to provide in English also in future

[@rajeshgummidi235]

Sir pls make video on how to read control diagram

[@allinone.4you]

Thank u brother..Will provide soon one by one based on response..Which control diagram u want?

[@rajeshgummidi235]

@@allinone.4you sir if we see vcb panel, relay panel, MCC panel or any other schematic ( control)diagram how read and understand that diagrams.

[@MadanSingh-un9ym]

Hindi

[@allinone.4you]

I don't know much hindi brother