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Transformer tripping on Standby Earth Fault (SEF) protection if neutral of transformer is grounded.

kssr (Electrical) (OP)
10 Sep 12 0:35
A 66/6.9kV YNyn0 transformer with 66kV side solidly grounded when connected to a 66kV solidly grounded system is tripping on Standby Earth Fault protection. My doubts are:
1. Can grounded equipment like a transformer be connected to a grounded system.
2. Can there be one than one earth point in the power system.
3. Can the source as well as loads be grounded. If yes, why?
4. If the source and loads can be grounded, then what should be the optimum SEF protection setting to prevent tripping in normal service.
5. Similarly, if there are more than one generator in the electrical system, can all the generators be grounded at neutral point. If yes, what should be Earth Fault/ Standby Earth Fault settings of the Generator. If no, why and what is the recommended arrangement for the electrical system to continue to work as a grounded system in all possilbe eventualities.
Request to provide guidance in this aspect please.
inpran (Electrical)
12 Sep 12 12:33
kssr,
Please share more info as to what is the rating of the transformer? What is the application? When the Transformer trips on SEF, is there a fault observed on any other feeders/bays in your sub-station? Do you have the settings for the relay available and can you upload the Disturbance Recorder here?
Regarding your other quiries,
1. Yes you can have your transformer neutral earthed in a grounded system.
2. There can be more than one earth in the sense that, generator's neutral can be grounded, transformer's neutral can be grounded, cable's armour can be earthed at one end etc.
3. It sure can be.
4. Data you have presented here insufficient to make such an important decision. What are the protections you have for the transformer?Rating of the transformer?
5. Can you please clarify what you mean by " all the generators be grounded at neutral point"?
totoboodj (Electrical)
23 Sep 12 7:59
kssr,

The system is 66/6.9KV. Are there generators? and if there are, i supposed they are connected to 6.9KV bus? If similar generators are connected to 6.9KV bus then probably when you energize them you must connect them to a single NGR. This is what you probably mean in your question #5. Please provide more info.

toto

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