two days before i was Doing demonstration of Equipment (Winding Ohm-meter) which inject Dc current up to 50A to measure Resistance of transformer or any inductive object which are used in Electrical power substation of 765KV. we have taken shut-down & we had connect all the necessary cables to kit as well as the transformer.
my Equipment have LCD Display to see the results as well as Discharge Facility with LED indication.
after completing all the connection i noticed that Discharge LED is glowing & display also flickers on my Equipment remember i have not turn on the kit kit yet. & now if i turn on the Equipment then then complete it Equipment will be Completely burn out.
this problem is occurring due to induction voltage present on Transformer buss bar.
the induction voltage may be arround 60-70V AC.
if anybody have any technique or any Circuit which can suppress this AC voltage & Pass DC current.
please help me to resolve this problem. please help me

 

Wouldn't it be required to short opposite transformer leads before working on it?

Also, this sounds like something that should be covered in operation manual. A capacitor would be the typical device for bypassing AC.

 

This sounds like it might be a bit out of the general level here. I might suggest checking on a professional engineering forum. Try eng-tips.com

 

can you load the circuit under test to swamp the induced current?

 

Wouldn't it be required to short opposite transformer leads before working on it?

Also, this sounds like something that should be covered in operation manual. A capacitor would be the typical device for bypassing AC.

Thank you my Friend for your valuable reply.
I am sending you the exact connection diagram of the demonstration.
please note that my Equipment inject 50A DC Current with open Circuit Voltage of 60 VDC. i was suppose to check winding resistance between R-Y phase but after making all the connection Discharge LED on Equipment Start Glowing some noise start to come from Equipment.

induction voltage may be arround 150VAC to 200VAC. in diagram i have connected C1, C2, C3 as you said. suppose if i connect all three capacitor X2 type may be 100KPF/250VAC on all three bushing then is it possible to bypass induction voltage to ground & after that can i start the test?

please find attach Connection Diagram File.
if you want additional information then don't hesitate to ask.
please reply me.
Thank you

 

Is the transformer secondary (LV) totally isolated from any load?

 

Is the transformer secondary (LV) totally isolated from any load?

no, transformer bushing was not open but yes when we test on LV side we didn't face any type of problem. test conducted properly & results is also coming properly.
Actually the induction is present on HV side only & due this high induction we are facing these problems.
please check the connection diagram with this message? in this diagram i have connected few capacitors (C1,C2,C3) to nullify induction AC voltages.
can you please check & tell me is this technique is correct or not? or else if have any idea then please give me it will help me to overcome by this problem as well as it will increase my knowledge.
please reply me.
Thank you.

 

You show low voltage side open.

It would seem to me that it should be shorted during dc test of high voltage side. Or at least during discharge time.

Are direction included with test device for discharging stored energy after test?

This seems like a dangerous test and specific instruction for test should accompany test meter.

 

no my friend i have just shown in diagram. actually it was connected tofurther stage. bushing of transformer were not open neither HV side nor LV side. at that time we had taken shutdown of transformer only.

we never short circuit any of the winding during test as well as discharge process.

Test device discharge the test object only in one direction.

the only think we are doing during the test is are as follows.
1) During the test on HV side Winding, we connect Winding on which want to test to ground through Grounding rod. then we connect our test leads to Test Equipment & then Test Object. then we remove Grounding rod to start the test.but when we remove the Grounding rod we face induction problem which i already discussed.
2) During the test on LV side winding we didn't connect any grounding rod because no induction we present on LV side winding & we performed our Test Easily.

please not that i am facing these problems in 765KV substation only i have perform many demonstration below 765KV & all were successful.

the circuit i have uploaded with Three Capacitors is this technique work or not?

if you need additional information regarding this then don't hesitate to ask.
Thank you.
Best Regards.

 

If you don't isolate the LV side then surely any voltage induced there will be multiplied by the transformer ratio and appear on the HV side? Although the induced voltage may not have been a problem on the LV side, when it gets multiplied it does become a problem on the HV side.

Edit: What is the transformer ratio?

 

Ground the non-polarity leg of the HV winding phase under test.

Better yet, test two windings at once. That is if your test set has two potential channels.
This lets you utilize the turns ratio of the transformer boost your test current.
Attention needs to be paid to the phasor diagram to ensure you're putting the same phases in series.

The test set is detecting a voltage in the test circuit which is why you're getting the warning light.

 

If you don't isolate the LV side then surely any voltage induced there will be multiplied by the transformer ratio and appear on the HV side? Although the induced voltage may not have been a problem on the LV side, when it gets multiplied it does become a problem on the HV side.

Edit: What is the transformer ratio?

The transformer ratio is 1.91 i.e 765KV input(HV side) & 400KV ouput (LV side)

 

Ground the non-polarity leg of the HV winding phase under test.

Better yet, test two windings at once. That is if your test set has two potential channels.
This lets you utilize the turns ratio of the transformer boost your test current.
Attention needs to be paid to the phasor diagram to ensure you're putting the same phases in series.

The test set is detecting a voltage in the test circuit which is why you're getting the warning light.

can you please explain the meaning of "Ground the non-polarity leg of the HV winding phase under test"
we have tried this also but before stating the kit induction come into the kit. & discharge LED start giving indication without turning on the kit.
suppose if i connect capacitors between test winding (both side) & ground so is it possible to neutralize induction voltage to ground? so that i can start the kit & carry out the measurement.
Awaiting your valuable feedback.

 

Can't you try using caps as suggested above but use a sacrificial cheap DMM, instead of your expensive kit, just to see if that method eliminates the induced voltage?

 

can you please explain the meaning of "Ground the non-polarity leg of the HV winding phase under test"
.

If you're measuring the resitance between H1 & H2 then ground H2 as it's the non-polarity leg (where the black test lead goes). If your next test is H2 to H3 then ground H3 first then remove the ground on H2.

That warning light is illuminated when there is a possibility that voltage exists across the test leads. It's as a safety warning to the operator so no one grabs a test lead while the circuit is still charged as well as it shows when the test circuit is discharged

I've been in hot yards before and had the same warning on my Vanguard WRM-40. After talking with Hai at Vanguard, we continued with testing like normal. I did jumper both the positive and negative current leads to ground at the test set and the light went off.
I still gave it a few extra minutes before moving the no load tapchanger. Even causing some small arching on the contacts is the last thing you want to do.

Remember: When around energized lines, that ground is the first thing to go on and the last thing to come off because a floating winding can build up a charge that can kill you.

I snagged this image from an old Multi-Amp manual showing how one leg or phase is grounded.

I hope that helped,

Mike