Line Imbalance Fault

You say you are reading these voltages "across the phases". It looks like you are reading phase to neutral. Can you confirm this?

No sorry these are line to line voltage readings from VT's on the 400V system

VTs eh? One long shot....is the VT connected in open delta ? That is often done to save one winding. If so, a connection error of one winding can result in such errors in readings. However, you should have ended up with a zero reading in one position. So, this may not be the case for you. Nevertheless, worth reversing one winding beginning to end and trying....

Is the HV still reading correct? If it is, then disconnect load and test LV again, this will tell you if the problem is in the transformer or not...You may have a partially shorted winding....

Dear all,

We had a similar voltage unbalance on the LV side of 20kV/420V 3 phase transformer with a healthy MV supply.

We found the unbalance was caused by the no-load tap changer which was burned by mis-operation.

Measure the winding resistances by using a milli-Ohm meter, if the resistances are not balanced, call a transformer specialist.

Regards

Thanks for the comments guys. Sorry I was mistaken earlier in saying they were line to line voltages - they were of course line to neutral.

As far as I'm aware the HV supply is still ok. It is most likely that the connector on the HV side is the problem as this has sustained damage. It could be the transformer itself I guess but no reason to suspect this has been damaged.

You have a very dangerous condition. If the transformer termination blows, which it sounds to be highly likely. somebody could get killed. Get an electrical engineer now before someone dies!

Rainman thanks for that completely useless advice. This is why I usually don't bother with forums like this because of people like you making stupid comments. The whole system was immediately de-energised isolated and earthed when the fault was found. I am not able to get the system tested for a few days so hoped somebody could advise in advance based on the VT readings what the fault is likely to be i.e. would loss of one phase on HV side with the delta star wiring of trafo result in these voltage readings on LV side. So far nobody has been able to help with this.

Dear Markilton,

You may carryout two simple tests to detetrmine whether there is any fault(apart from insulation failure) in the Trafo winding and these are :

Magnetic Balance Test

and

Turn Ratio test.

Thanks,

Manindra

Since you have already done what I suggested and not told us I could say that you are wasting our time. How can you do further tests if it is dead or have you made a faulty? piece of equipment alive again without testing? I have survided 40years of front line electrical engineering how long do you think you would last?

Best wishes

You've been in the industry 40 years and you don't know how to carry out tests on a dead system? As I've clearly explained I believe the HV connector is faulty so an IR and continuity test will tell us whether this is the case. I'm expecting the tests to show we have an open phase in the connector but thought somebody could do the 3 phase calcs based on the observed imbalanced voltage readings to support this. Just forget about it this is wasting my time

Dear Markilton, I don't know that you are being fair here, both because you have claimed testing you did not tell the forum, - and been unfair to Rainman as a consequence, but also your implication of testing ie continuity, is not really enough in today's environment, - I am not an engineer, I design renewable energy systems where I have to do the work of all 3 major engineering branches, - civil, mechanical and electrical, and death be on my head if I fail, but I would think you are using a Mega, and not HiPot testing on your dead system, from your words, - so you will not find some faults, perhaps crucial faults, - justifying Rainman's concern. Cheers, Geoff Thomas.

It is hard to give a precise answer, because the measured voltages will be determined by the types of loads connected to the secondary of the transformer. A 3-phase motor with a very light load will continue to run after the loss of one input phase and will induce voltage back into the open third phase line. However, the current and voltage imbalances in the motor will most likely cause significant overheating and lead to its failure even if its line current remains less than the nameplate. I worked with a customer who had a 5% voltage imbalance burn-up a 500Hp motor--the utility crews spent almost 5 hours searching for the cause before they found an open phase in the primary a ways down the street (it had been damaged during a tornado a year earlier but had only failed that month).

Ball-park opinion--open phase close to your utilization equipment; probably on the primary side of the transformer, with the transformer's windings contributing the voltages you have measured on the open phase's secondary wiring.

--John M.