Restricted Earth Fault Protection of Transformer


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.

Also Read,

Transformer Differential Protection

Percentage Differential Protection - Slope in Differential Protection

Harmonic Restraining in Differential 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.

Comments

Post a Comment

Popular posts from this blog

Concept of Subtransient, Transient & Steady State

Image
The concept of Subtransient, Transient and Steady State arises in case of fault in an Alternator. Let us assume a sudden short circuit in three phase of alternator. The fault current will flow in all the three phases of alternator and its waveform will be as shown in figure below. When the alternator is short-circuited, the currents in all the three-phases rise rapidly to a high value of about 10 to 18 times of full load current, during the first quarter cycle. The flux crossing the air gap is large during a first couple of cycles. The reactance during these first two or three cycle is least and the short circuit current is high. This reactance is called subtransient reactance and is denoted by X". The first few cycles come under sub-transient state. After a first few cycles, the decrement in the r.m.s. value of short circuit current is less rapid than the decrements during the first few cycles. This state is called the Transient  State and the reactance in th...
Read more

Open Circuit and Short Circuit Characteristics of Synchronous Machine

Image
Open Circuit Test and Short Circuit Test are performed on a Synchronous Machine to find out the parameters of Synchronous Machine and hence to have an idea of their performance.  Open Circuit Test of Synchronous Machine is also called No Load, Saturation or Magnetizing Characteristics for the reason which will be clear after going through the post. For getting the Open Circuit Characteristics of Synchronous Machine, the alternator is first driven at its rated speed and the open terminal voltage i.e. voltage across the armature terminal is noted by varying the field current. Thus Open Circuit Characteristic or OCC is basically the plot between the armature terminal voltage E f versus field current I f while keeping the speed of rotor at rated value. It shall be noted that for OCC, the final value of E f shall be 125% of the rated voltage. Figure below shows the connection diagram for performing the Open Circuit Test of Alternator. As clear from the figure a...
Read more

Transformer Cooling Classes

Image
Losses in the transformer are of the order of 1% of its full load kW rating. There are mainly two types of losses in a Transformer i.e. Core Loss and Ohmic Loss. Because of these losses the winding of transformer as well as Core gets heat up and we need to provide sufficient cooling to remove heat and maintain a constant winding and Core temperature. Various types of cooling methodologies are used for Transformer but before going to each methodology, we will have a look at the General Method of designating a particular cooling method i.e. class. In the nameplate of Transformer, four letter represents the cooling method of the Transformer. The above table is self explanatory, therefore if you see ONAN on the Name Plate of Transformer then it means that Oil with flash point less than 300° is used as cooling media and Natural convection is used for removing heat. In secondary side of cooling mechanism, Air with Natural convection is used by employing Radiator. ...
Read more