The earth fault relay must be very sensitive for sensing the fault. Such settings protect a restricted portion of the winding. Hence the name restricted earth fault protection. The stabilising current is connected in series with the relay for avoiding the magnetising inrush current. Even if the fault occurs nearby neutral the relay will protect the winding unlike differential protection.
Your email address will not be published. Save my name, email, and website in this browser for the next time I comment. This relay can be implemented for only star connected transformer or star connected alternator.
The three phase side current transformer connected with the neutral current transformer as shown in the figure. The neutral transformer is called as NCT. Normally NCT comes inside of the transformer. Note: We should use separate CT or separate protection core for restricted earth fault relay. The CT output should not be shared with any other electrical equipment.
This condition is applicable for both phase CT and neutral CT. During Normal Operation the balance current in the sum of all phase CT is equal to the current flow in the neutral.
It would be interesting if other site users have opinion on this and can add to the discussion. I want to just ask that how we can restrict the earth fault detection of a Particular Critical System let us consider an alternator and what is the Logic Behind that? How the CTs are arranged in Differential Protection what is the logic behind the arrangement. Anybody can advise me on these? On a through fault if the CTs saturate the currents will be different and the relay will see a zone fault where one does not exist.
This could result in a nuisance trip. Some modern relays are able to compensate for differences in CT performance. This would need to be investigated on a case by case basis. Is the polarity of connection samefor both protection schemes?
I think the intent is that both mean the same thing. Which is the correct or better to use may be open to interpretation. The primary difference is in the type of relay used. High impedance relays have a high input impedance and draw very little current under normal conditions. Low impedance relays are the opposite. There is a lot of debate on which to use and some theory. My view is that low impedance is preferred.
It's simpler, there is more flexibility in the CTs and inputs and coupled with all the functions of a modern numerical relay it eliminates a lot of problems that exist in high impedance systems. It could be many things. Similarly, if your CTs are becoming saturated they can become unbalanced and give problems. You should also make sure that the CT ratios, phase shift, logic, etc. It is difficult to remotely diagnose this type of issue.
If you are also able to get a recording of the voltages and currents during an incident this will help you a lot. Hi Steven, "As the winding fault position moves towards the neutral, the magnitude of the current seen on the primary rapidly decreases and could potentially not be detected limiting the amount of winding which can be protected ".
Can you please explain why..? I have a few queries also it may sound stupid. Query 1. I am bit confused between the operation of REF and Standby earth fault relay. Can you help me in understanding with some diagrams or any links to refer.?
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. So, differential relay shall pick if the differential current exceeds 0. 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. Therefore for protection of Transformer from such a fault we need more sensitive protection scheme which is implemented by using Restricted Earth Fault Protection.
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.
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