Restricted Earth Fault Protection Philosophy

"WHAT IS RESTRICTED EARTH FAULT PROTECTION??". BUT MORE IMPORTANT THAN "WHAT" IS "WHY" RESTRICTED EARTH FAULT PROTECTION

The name suggest the principle. It is restricted. What does it mean and why we need such restriction. Lets go deeper.

It is restricted between a certain zone. That means the protection function will work only within a specified zone that is pre-determined.

Suppose you have selected a certain boundary, this restricted earth fault protection (REF) will detect earth fault only within that range. Earth fault out side the boundary will not be detected by the REF scheme.

Protection against earth fault is very much essential for the machine having a core such as generator, transformer & motors. The most occurred and severe earth fault happens when insulation of winding get damaged, and it touches the core of the machine. Since core is kept earthed, this situation causes an Earth fault.

It is also a high resistance earth fault. The core is a high resistance element. so fault current is generally low due to the high fault resistance.

So, if the fault current is low why we need to worry??

We need to be worried because of the two reasons.

1.  Small Earth fault current is difficult to detect.

2.  If the earth fault current is beyond tolerable range of the core then it can cause damage of the core. (for the very same reason it is necessary to specify earth fault withstand current  of any machine. 

Say, if a generator is specified as 
Earth fault withstand-100 A, which means the core  of the generator can withstand 100 A current.

An industrial power system is consists of huge nos. of generators and transformers. Since these two machine delivers power they act as source. So any earth fault returns through their grounded neutral. Although motor has core material but since motor is a load and not the source hence earth fault inside motor is also detected through the feeder earth fault protection, as described in our blog post-

https://electricaltechnologyrishi.blogspot.com/2018/08/ct-connection-to-sense-earth-fault.html

REF shall be applicable for Generator and Transformer. Lets have a look on Circuit diagram of a REF protection of transformer.

REF protection/https://electricaltechnologyrishi.blogspot.com

Here we can see that to implement REF scheme four (4) nos. of CTs are required. Three (3) line CTs(LCTs) are connected residually and the fourth one connected to the neutral, and these two set of CTs (LCTs & NCT) are connected deferentially. 

So the most important part of the REF protection is it is a DIFFERENTIAL EARTH FAULT protection.

Now let us have look what is the zone under protection, and beyond protection.

Protection zone of REF/
https://electricaltechnologyrishi.blogspot.com

The picture beside shows the zone under protection, and zone beyond protection. That means is an earth fault occurs within the range of the protected zone it shall be detected. But Earth fault outside the protected zone shall not be detected.

SPECIFIC APPLICATION:

If we place the Line CTs at the bushing of generator or transformer terminal this will only sense earth fault inside the generator or transformer. This scheme is specifically used to detect if the earth fault is within the transformer or generator's winding or it is outside of the winding. 

Now let us see how the REF protection works.

Earth Fault inside and outside of the Protected zone/https://electricaltechnologyrishi.blogspot.com

The above picture shows application of REF, in the left hand side Fault has occurred inside of the protected zone for which the relay will respond & in the right hand picture the Fault has occurred outside of the protected zone for which the relay will not respond. Let us see how......

As mentioned earlier since REF is nothing but a Differential earth fault protection so it actuate on the differential value between the NCT output & sum of LCT output.

I _relay=∑I_{LCT_SECONDARY} ˜ I_{NCT_SECONDARY}

If you see the left hand picture you will find that earth fault current is only passing through the NCT
So, ∑I_{LCT_SECONDARY} =0
Only NCT output current flows through the relay, causing activation of the relay.

Now for the right hand side picture, the Lct 2 & NCT shall have same primary current. If CT ratio of all Lct & NCT are the same their secondary current will also be same., hence differential current will be 0. For which the will not be activated.

In all the pictures we have seen a resistance connected in series with the REF relay. It is called Stabilizing resistor. Function and requirement of this element shall be discussed in the next article, but due to presence of this resistance this type of REF protection is called HIGH IMPEDANCE RESTRICTED EARTH FAULT PROTECTION.

And for High Impedance Earth Fault Relaying it is mandatory that Ratio of all Line CT & NCT are same.

Since the protection is differential protection hence PS class Ct is required. knee point voltage, magnetizing current, & Ct resistance shall also be identical for all Lct & NCT.

WHAT IF THE RATIOS ARE NOT SAME?

Suppose for the given example the transformer rating is 11/6.9 kV, 10 MVA, Neutral is earthed through a resistance (Neutral Grounding Resistor) of 10 Ohm.

Full load current of the trafo secondary is=10*1000/(1.732*6.9)=836.76 A. Say line CT ratio taken as 1000/1 A

Since there is NGR earth fault current is=6900/(1.732*10)=398.38 say 400 A, 
NCT ratio taken 400/1 A. 

If CT selection is done like that then for an earth fault occurring out side of the protected zone 

Lct secondary current shall be=(400*1/1000)=0.4 A (Earth fault current of 400 A)
& NCT secondary current shall be (400*1/400)=1 A

So, there will be a differential current value even if the fault is outside of the protected zone, causing mal-operation of the REF scheme.

So, for high impedance REF it is mandatory to select all the Line CT & NCT identical.

N.B-In case the CT ratios are not identical then we have to opt for Biased REF scheme, which is also known as Low Impedance REF, which is beyond scope of this article.

So, This is how REF scheme works. Generally employed scheme for REF is the High Impedance REF scheme.

REF is used for Generators & transformers. However very small rated distribution transformers and generators may not have REF scheme.

In the upcoming Articles we shall discuss how CT selection is done for REF, desirable parameters of NCT & LCTs, function & rating selection of Stabilizing resistor, setting method of High Impedance REF etc. 

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