Hot Hipot test on 100amp 3phase system

[Lovelec]

Hi everyone,
I am new at this, so please forgive any ignorance that comes through.
I am tasked to setup a test procedure to conduct dielectric withstand test on a 480v 3phase system. Because of all the internal contactors and solid state relays, the DUT has to be powered up. In the literatures, it is recommended to power up the DUT through an isolation transformer when conducting a Hot Hipot test in order to protect the return line of the tester against excessive current. Coming up with a 3 phase isolation transformer with high enough a current (50-100A) is a daunting and expensive choice, and my last option.
Q1- Are there any less expensive alternatives for conducting this Hot Hipot test safely? Can I for example use an isolation transformer for the tester instead, or some other way to protect the return circuitry of the tester.
Q2- Can I use three single phase isolation transformers instead of one giant 3phase, and if so, how should I wire it?

 

[roydm]

What do you need the isolation transformer for? Hi-pot testing is done with the mains power OFF.
Roy

 

[Lovelec]

The DUT is quite complicated and it has a lot of relays inside that cannot be mechanically activated, so we have to actually supply power to it to create a complete path.

 

[edison123]

I agree with Roy. No need of isolation transformer (that a 3 phase, why ?).

AC Hipot Procedure

1. Disconnect the supply to the 480 V system.

2. Ground the panels

3. Isolate electronic parts.

4. Use a single phase, ac hipot trafo with a stepless variable trafo feeding into the primary of the hipot trafo with all over-current protection (the KVA of these trafos depend on the expected charging current of the system - It is normally a few milliamps)

5. Apply the hot end of the hipot trafo to the parts to be tested and the low end to the ground.

What is test voltage level, 2 KV ?

 

[Lovelec]

I wonder if I am missing something. Unless I supply voltage to the DUT, the majority of my system will be isolated because of the contactor and the many relays. Are you suggesting each of these subsystems to be tested individually? I am reading the article bellow, last page.

http://www.asresearch.com/events-training/est/docs/pdfs/HotHipot.pdf

 

[waross]

Are you sure you want to do this?
A test intended for small appliances may not scale up to a 480 volt industrial system.
Hire a professional testing service.
Possibly we are using differing definitions of "system".
Attempting to "HiPot" an energised 480 V system may result in the death of you and some of those around you.
You need more help than you can get on this forum.
The technical issues are few compared to the safety issues that are not mentioned in your reference.
RIP

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[edison123]

I agree with waross. You should hire a testing professional to do this.

The manual you referred to has some points which I disagree with. You seem to have misread it on top of it.

Remember, not everything put out on the net is vetted. They are all worth exactly how much you paid for them...

 

[Lovelec]

The article is by the people whose test equipment we are using. could you clarify what I am misreading? I am just curious how a professional test agency would tackle this problem. I agree that the Hipot test is mostly used for small appliances, but CE cert agency we are dealing with is asking for 2KV dielectric withstand test. I really appreciate your concern about my safety, and believe me, I am not going to hook up a single wire until I understand everything that might happen around it. And you might also be quite right that this is totally out of our league.

 

[edison123]

I will give two examples - both involving safety. There are many others but I am too lazy to type'em out.

When comparing ac hipot over dc hipot, (the tabulated talking points)

AC Hipot Advantages

1. Slow ramping of test voltage isn't necessary... Really ? Do they advocate sudden dumping of full test voltage on the DUT. Sure way to kill the DUT.

2. It is unnecessary to discharge the DUT after ac testing....Really, really ? Sure way to kill yourself.

 

[davidbeach]

What are you going to hi-pot? Normally you apply your test voltage to the supply circuit to verify that it is suitably insulated from ground. If you conduct the "test" you are talking about you will elevate the supply voltage by the test voltage and you will be "testing" far more than the device you intend to test. Best to do it right and not look for short cuts.

You could apply jumper around all of the contactors so that both paths out of the contactor are energized during the hi-pot test.

 

[waross]

What are you testing? Is it bigger than a bread box? Can you put it in the test shop?
When you are connected to one phase and are testing with 480x2+1000 V, only the phase under test will see 1980 V to ground.
The voltages on the other two phases will be varying between 480+1000V and 480x3+1000 V, or 1480 V to 2440 V.
This may not be the test you had in mind. You may find some marginally rated components this way.
If you persist, consider davidbeach's suggestion. Jumpers and test with no supply power.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

I see it usually done one of three ways.

Jumper out the relays or contactors so that the test voltage gets applied to all the power wiring and components.

Apply control voltage to the package and energize the relays or contactors so that the test voltage gets applied to all the power wiring and components. Make sure the control and power are isolated from each other. You will be working with voltage present doing this test.

Apply the test voltage to each individual section of power wiring so that all parts eventually get the test voltage applied to them.

 

[Lovelec]

Those were good suggestions. Our device is quite large, has a huge step down transformer, a PLC along with sensors and actuators. I think I want to take the path of jumpering the relays and the contactor. This opens up another area: design for testability. I can picture a set of terminals dedicated for jumpering relays only to be used during testing. The contactor might have a mechanical actuation capability. I have called the tester manufacturer for their advice too, but the guru is on vacation till next week. So, your suggestions are invaluable in my education.
Now, on single phase DUT, I have seen diagrams that show the test voltage applied to both Line and Neutral leads at the same time. Can one do the same on 3 phase systems and tie the 3 lines together and apply the test voltage at the same time?

 

[waross]

Seek professional help.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Lovelec]

Some of my friends tell me the same thing "seek professional help", but I think they mean something totally different. The bottom line is that not many people are born as experts. We learn as we live and asking questions and being open to the advice of those with experience makes this learning fun. That is why I appreciate all the suggestions and challenges you guys are offering. Keep them coming, because I am learning a ton.

 

[waross]

Hello Lovelec.
Please do not take my gruff comments too hard. I respect your intelligence and knowledge. However it is your field experience that I am a little concerned about. Your questions seem to reflect a good theoretical basis but less field experience.
I think that we want you to be very careful. Teaching yourself on the job can be lethal. A lot of us older members have horror stories of testing and commissioning mistakes and accidents. A lineman I have known for 8 or 10 years who was dependable and conscientious made a tiny mistake last year. He is now in a wheelchair, having lost an arm and a leg. I have pictures but you don't want to see them.
We are not trying to belittle you. We are asking you to be very careful. You don't want a capacitor blowing your face off because you overlooked an over voltage and you don't want to discover subtle little back feeds that put lethal voltages where you don't expect them.

Bill
--------------------
"Why not the best?"
Jimmy Carter