Abnormal System Voltage Variations

From your measurement table, it seems you should check your Transformer grid resistance as well as individual earrth pit resistance.

at time B you found 415 V between N-E & i guess Neutral & earth should be at the same potential.

Dear Abele -

yes you are right, the neutral and earth are at the same potential, transformer neutral

are solidly earth, and the earth pit resistance are well within the range, showing less

then 1 ohm.

What is the resistance for all grid connected earth Pits?

Something is wrong here:

But, I'm confident that you will prevail.

I would be looking for a dodgy neutral link, and to throw away that high input impedance digital voltmeter in favour of a lower impedance moving coil one.

Agree with Cabtree, use a lamp tester or something similar.

I agree look very closely at the Earth - Neutral links. If they are common then they should always be at the same potential.

From your chart I see this is the case at Time A but with Time B the N remains constant as do the Phase - Phase voltages and the Earth volts rise to a phase to phase voltage.

To me this means that some how your earth & possibly the Neutral are getting fed from somewhere. It has to be triggered by something as I cannot believe it is random. Try to understand what is being switched in your network when it rises - you may in fact be getting a feed from another source. If it happens when something is switched then that may be the problem.

You may also have any 1 of the transformers with a faulty Neutral so I would completely disconnect each one (in turn) and test it. While you are doing that, monitor the voltage as before - if it happens with the unit off line then move to the next unit. If they all pass then it is likely you have something else in your network that is supplying phase volts to the N & E.

You have no choice but to work through this little by little & elliminate all possibilities. As indicated by others - make sure ALL your earth connections are in good condition before anything else, then do the N to E connections.

Good luck.

if the fault is common to the whole system than it looks like a neutral earthing issue not related to the transformer or cables.

I had similar problem sometime ago but the voltage variations was not so high.check and confirm your neutral earth resistance readings if practical. or make new earthing pits of required readings and connect up to existing ones.I did the second option. pls do highlight whatever you did to solve your problem for our knowledge.

Thanks to all my fellow members for the guidance and suggestion uploaded in the forum.

This was a case of a giant Textile industry in Mumbai, As there are numbers of equipment associated with the power supply directly, checking individual points were very difficult.

But we have done isolations of department and verified the same but unable to come to any conclusion.

Now this company is not functioning due to relocation to new area. Further checking won't be possible.

I am thanking all my members, who responded to this forum, as these replies has given me a confidence that we were working on the same track as you people have suggested.

Thanks once again

Subhash

hello subh.

pls chek the winding connection first follow the wyedelta star connection if correct! then retighning all teminals and pls make sure all terminals are dry and clean , the problem you have same we incounter before just try to do what i say it, and try do the test again. but if same result contact the manufacturer,