Restricted Earth Fault (REF) protection is basically a Differential Protection. The only difference in between the Differential Protection and REF Protection is that, latter protection is more sensitive as compared to the former protection scheme. In earlier posts we have already discussed Differential Protection of Transformer and various characteristics of Differential Protection. In this post we will focus on Restricted earth Fault protection.
For the sake of understanding REF Protection, we take a Transformer of configuration DYn i.e. HV side of Transformer is Delta connected while the LV side is Start connected and neutral is grounded solidly.
As shown in figure above, there are a total of four Current Transformers (CTs), three CTs connected in each phase i.e. R, Y and B and one CT connected in neutral. The secondary of these four CTs are connected in parallel. The parallel connected CT secondary are then connected to REF Relay Coil. Basically REF protection Relay element is an over current element.
Under balanced condition i.e. under normal operation the sum of currents through the secondary of CTs will be zero and current in neutral CT will also be zero. But as soon as a fault takes place in the secondary winding of Transformer, the current in R, Y and B phase will no longer be balanced. Also under earth fault a current will flow through the neutral CT. Because of this unbalance, the summation of current will not be zero but it will have some finite value and hence the relay will pick up. It shall be noted that for a fault outside the Transformer i.e. for through fault Restricted Earth Fault Protection will not operate as in this case of through fault, the vector sum of currents in CT secondary will be zero. This is the reason; such kind of protection scheme is for restricted zone and hence called Restricted Earth Fault Protection.
Now, it is normal to ask that Differential Protection is also a zone protection and it shall operate for any internal fault in Transformer, then why do we need extra Restricted Earth Fault Protection?
This is really a very smart question. See, what happens is, the setting of differential protection is normally kept at 20%. So, differential relay shall pick if the differential current exceeds 0.2 A. Now let us consider a case where earth fault occurs just near the neutral point as shown in figure below.
Since the location of fault is very near to the neutral point, the voltage driving the fault current will be very less and hence the reflection of such a low current in primary side of transformer will also will be low. Thus in such case, Transformer differential protection may not operate as its setting is quite high at 20%. Therefore for protection of Transformer from such a fault we need more sensitive protection scheme which is implemented by using Restricted Earth Fault Protection. The sensitivity of REF protection is superior as compared to Differential Protection. Normally the setting of REF protection is kept as low as 5%. Basically the sensitivity of REF protection increases as we are using CT in neutral of transformer and whenever an earth fault take place it is damn sure that current will complete its path through the neutral and hence increasing the sensitivity of REF protection.