In this post we are going to see, how the High Impedance Restricted Earth fault protection scheme works by using Metrosil and Stabilizing Resistors.
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. The Differential Protection for transformer can’t cover 100 percent of its winding. 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. Like this Differential Protection provides Protection for 80 percent of the winding and rest of the protection will be provided by REF.
For non critical transformers, only Differential protection may be used.
Why REF protection is called Restricted Protection?
It provides protection in the zone between Transformer Star side winding and it’s Neutral Terminal which is earthed. It senses the fault current only in this particular zone so it is called restricted protection.
How to implement High Impedance Restricted Earth fault protection?
REF protection is implemented using a set of Phase Current Transformers and Neutral Current transformer along with Metrosil and Stabilising Resistors as shown below.
REF relay is nothing but an over current relay which is connected in parallel with the three phase CTs and one neutral CT.
The CT secondaries ( phase and neutral CTs) are wired to cancel each other’s currents during external faults and to drive all secondary current to the relay when the fault is inside the protected zone. (Same as Differential Protection)
High impedance Restricted Earth Fault protection is the voltage based operating protection.
The connections of Metrosil, Stabilizing Resistor and neural CT are shown in above figure.
Purpose of Metrosil:
It is Voltage Limiting Non-Linear Resistor. Non-Linear resistors are connected in parallel with the relay circuit to limit the peak voltage developed across the high impedance components during internal faults to a safe level below the insulation level of the current transformers, relay and interconnecting leads.
Purpose of Stabilizing Resistor: RS
The stabilizing resistor is connected in series with the over current relay and this combination is connected in parallel with the CT branches. It makes sure the relay will not trip during an external fault.
Purpose of Neutral CT:
This is the Class-PS current transformer connected between the Neutral of transformer and earth. Any unbalance current flows through the neutral of the transformer is sensed by Neutral CT.