How to check insulation of a field of electrostatic precipitator?

[SA07]

Hi
We have an electrostatic precipitator with field 1,2,3. We have a problem with field 2. The panel is tripping with UV. We did an open circuit test. The panel + transfo seems good. We suspect a short inside field 2. How can we do an insulation test to confirm this? While the plant is operating, there are charges on field 2. If we connect a megger on the field it might get damaged. Any idea if there is a special device which can test the insulation of electrostatic precipitator field?

 

[waross]

The electrostatic precipitator is probably running at a much higher voltage than a megger can produce.
The first rest may be a visual inspection for arc marks or bridging at the support insulators.
To check the value of the insulation you may be able to use a high voltage test set, intended for testing high voltage cables.
But, if the test shows a bad insulator, you still have to locate the faulty insulator.
You may try switching the connections if that is easily possible.
That is swap the leads between field 1 and field 2.
That will rule out a faulty panel.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[SA07]

We cannot do an inspection now as the plant, boiler are operating. We wanted to do an insulation test to confirm the problem is inside the field. Then during next plant stoppage, we will do an inspection inside the field to locate the fault. The operating voltage of the ESP is around 55 kV.

 

[waross]

I assume (With the dangers that that implies.) that you are feeding a lower voltage to 55 kV transformers to supply each field.
Insulation testing generally involves applying a high test voltage and monitoring the current.
You have a high voltage transformer applying a high voltage to the field and apparently the field is failing the test.
I repeat, if the panel and the transformer are good, then you have done a high voltage insulation test and the field has failed the test.
A test is normally done with a voltage higher than the normal operating voltage, but if the field is failing at operating voltage, there is no need for a higher voltage test.
I hope that your testing was done by disconnecting the final connection to the field so that the rectifiers were included in the panel and transformer test.
Are you able to inspect ant test the rappers without an outage?
You may have failed or failing rappers that are failing to clear a dust buildup.
You may want to try an extended use of the rappers if the rappers normally run on a schedule rather than continuously.
Other than that, you may need an outage.
This may be a management decision;
The cost of an outage compared to the cost of fines for exceeding emission limits.
Disclaimer; My last close encounter with a large ESP was over 50 years ago. My advice is based on general principles as I remember them.




--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[crshears]

And my last exposure to ESPs was 40 years ago . . . but what what Bill is saying sounds spot-on.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[FacEngrPE]

I concur with Bill. My only experience with percipitators were much smaller units, used in a welding shop, the collected material included some flaky material. We found that it was very easy to create a short between the charging wires and the plates.

Bridging should be less of a problem in a large flyash collector. An extended run of the shakers has a reasonable chance of clearing your problem, without interrupting operations.

 

[LotsOTrouble]

We never try to test the field with the unit online.
You may want to make sure flyash hoppers are pulling properly (and vacuum out if not).
If they are, you probably have a downed wire or a plate that is loose/broken and contacting the field.

Good luck!

 

[waross]

Welcome to Eng-Tips LotsOTrouble.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Dopler]

Have you verify high voltage cables? As I remember, those where isolated by isolating oil impregnated paper, the connecting device to EP field has a reservoir that has to be oil filled to have good isolation in cable.

Hope this helps.

 

[SA07]

We just did our annual maintenance. We put all rappers in service for about 3 days and check everyday that everything was ok inside the penthouse and in the fields. Everything was ok. We do not have hv cable inside oil reservoir. The HV cable comes out of the transformer, enters a changeover switch ( to select between earth and the ESP field), then enters the ESP field. We checked this part, everything was ok. We will stop in about one week. We will check inside. I will inform you what we find inside/ the issue.

 

[Dopler]

When entering EP penthouse, there must be some porcelain bushings, inspect them, may be there could be somthing.

 

[SA07]

Hi

We found a welding rod between the discharge and collecting plate electrodes. It fell down while maintenance work was done.
We removed it, energised the field. It is ok now.

 

[waross]

Thanks for sharing the solution with us.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[sunnysky]

https://en.wikipedia.org/wiki/Electrostatic_precipitator#Collection_efficiency_(R)