unsymmetrical open circuit voltages

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panthripu

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Hi Everybody
I need a lot of information on the question i m going to ask.I asked this in other forums but no satisfactory answer.
My excitment started when one day i observed that in one of our transformer (whose secondary winding is delta connected) , the no load voltage is very unbalance with respect to ground.If i check the voltage on secondary side without connecting load to it. It shows 220,220,20. With load these voltages becomes balanced.
I know in transformers with delta secondary , there is no reference to check the voltages.The only thing which is reference is the capacitance with ground.
When i megger the secondary side with cable connected , it gives me reading in gega ohms.
I have two questions.
1) By checking the no load voltages of our transformer , can i say that some of the insulator is week.If yes , then what about the megger value which is very high.
2) If i am using no load voltage in our system for some calculations . Is it possible to make these unbalance voltages balanced in no load conditions.
I guess if i connect some resistors in each phase with ground , it may neglect the ground capacitance as these resistors will behave like small load on transformer.
 
1. No

2. First, I would ask why are you using phase to ground voltages? That number is pretty meaningless in an ungrounded system. Second, can you use L-L voltages divided by sqrt(3)? Third, I suspect there are no ground detectors on the system and there should be. These ground detectors will have some impedance to ground. Use 3 - 240V light bulbs for the ground detectors. See if that is enough impedance

cf
 
To elaborate on CF's good suggestion:

Connect a 240 Volt light bulb from each phase to ground. If the delta is truly floating, with no grounds on the system, each light bulb will glow equally at reduced brilliance. When one phase has a ground fault, that bulb will go out and the other two will burn bright. You can locate the fault by turning off loads one at a time until the lights go back to equal briliance.

Measuring V-G voltages with no reference is interesting but not useful.
 
unsym. voltages

unsym. voltages

thanks to all of you.
We monitor the phase to ground voltage and can see them on our HMI.
So we dont need the lamps to confirm the grounding.
We know that if one phase is grounded, the voltage of that phase becomes zero and other two phase equals the line voltage.
i remember in old time the voltage of third phase used to be around 100V in no load condition.Which now became 20 V only.
Now does it show that some insulator is weeken.To kill our doubt we meggered the secondary side and found the value 11 Gohms.
Just for clarifying the picture to all of you........Its a arc furnace transformer.The secondary side which inclueds, secondary bushing, water cooled cables,electrod arm, etc. are water cooled.So generally if we megger the system with water running in system gives zero megger value.
 
panthripu said:
...We monitor the phase to ground voltage and can see them on our HMI.
So we dont need the lamps to confirm the grounding.
....
Click to expand...

The idea of the lamps is to put a small load on the transformer output and stabilize the voltage to ground. Perhaps the HMI sensors already do that. Do you know the sensor impedance? If it is megohms, they likely are not contributing to stabilizing the transformer voltage.

panthripu said:
...To kill our doubt we meggered the secondary side and found the value 11 Gohms.
Just for clarifying the picture to all of you........Its a arc furnace transformer.The secondary side which inclueds, secondary bushing, water cooled cables,electrod arm, etc. are water cooled.So generally if we megger the system with water running in system gives zero megger value.
Click to expand...

panthripu said:
...If i check the voltage on secondary side without connecting load to it. It shows 220,220,20. ...
Click to expand...
Just curious: When you did the secondary voltage check, was the water running?

cf
 
unsym. voltages

unsym. voltages

yes , we monitor the no load/open circuit voltage with running water.
Is it possible to pin point that where this capacitance is getting unsymmtrical.
 
unsym. voltages

unsym. voltages

Hi
I agree with cold fusion , If i put three lamps ,one in each phase with ground.It will act as small load and then symm. the no load voltage.
But we have already three transformers connected one each phase to ground
600/100V to measure this voltage. Why these measuring transformers are not acting as load ? As it has been stated that the impedence of these measuring transformer may be very high.
 
unsymmetrical open circuit voltages

A few points I would throw out.
An arc furnace transformer in it's use takes a beating. Every time the electrodes are plunged into metal the transformer sees nearly short circuit current so it has a tremendous magnetic attraction and windings, with time, do move and can cause damage.
The Meger is placing a D.C. voltage on the windings and the insulation breakdown voltage is not an issue from what you report.
The A.C. voltage reading on an ungrounded secondary is indeed dependant on the capacitance from the wiring to ground. The capacitance, with the winding movement, can change and then the open circuit voltage will change.
I would think you have a sign of a mechanicaly damaged transformer.I would contact the manufacturer for some advice or thier suggestions on testing and inspecting the unit.
 
unsym. volt

unsym. volt

Thanks Martin
I would really appriciate someone who has worked in transformer manufacturing to put some more light on the issue being discussed.
Some people says based on there experience , If it is delta wired transformer on secondary , the no load voltage is not issue untill you see that the voltage of a particular phase becomes zero and other phase increases by root 3 times. Which as per me is correct.
 
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