Surge Cap without Discharge Resitor for Motor Protection.

[eaaf]

Good Day.

I have a question.
I'm usign surge protection for the 4.16 Kv motors in our plant. Usign Surge Cap an LA's, near of the motor conections.
I want to use an insulation meter, installed in the motor control, so we need to change the surge cap for another ones, that don't have the discharge resistor; in order to the meter can read the insulation resistance for the cables and motor.
Is there any concern about the used of thes kind of surge cap?
I think about the moment when the motor start again, the caps are almost charged, what will happen in this moment? Could I have problems?

Thanks in advance.

Regards.

EAAF

 

[electricpete]

We always disconnect the surge cap for performing motor insulation resistance / polarization index test. Otherwise you might not be able to apply full test voltage initially, which messes up the timing of the test.

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(2B)+(2B)' ?

 

[electricpete]

IEEE 43-2000 Rev 2006 (IEEE Recommended Practice for Testing Insulation Resistance of Rotating Machinery) states:
The connection leads, brush rigging, cables, switches, capacitors, surge arresters, voltage transformers, and other external equipment may greatly influence the insulation resistance reading. It is recommended that measurements of the insulation resistance be made with all external equipment disconnected and grounded.


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(2B)+(2B)' ?

 

[electricpete]

Is there any concern about the used of thes kind of surge cap? [no discharge resistor]

As far as I know, the only issue would be personnel safety. As long as your staff knows to properly ground the cap prior to maintenance, there are no problems I know of. We use a hot-stick that has it's own attached discharge resistor... then after grounded attach safety grounds.

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(2B)+(2B)' ?

 

[electricpete]

I guess the name for the tool I mentioned is "grounding stick" rather than "hot stick". It includes insulated pole, insulated wire running out to the end with resistor in series... the other end of insulated wire gets attached to ground.

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(2B)+(2B)' ?

 

[eaaf]

Pete.
Thanks a lot for all the information that you post.

The meter that I want to use, is an equipment that every time that the motor stops take a measure for the insulating resistence of the set (cables and motor), and hold the voltage applied until the starts; looking for a abnormal reading, in order to do not permit the start of the motor, if it occurs. Is our concer because of the grade of contamination that the motor has. This one is a open type.

We are looking to detect a low reading before the motor get energize, to prevent a ground fault in the windings.

We has a complete protocol for the grounding of the equipments using the grounding stick that you mentioned before.

Sorry for not including this information before.

I was lookign for any issue if the discharge resistor is not present in the caps.
For the formal testing of the windings we use the appropriate equipment when it stops.

Regards.

EAAF.

 

[electricpete]

Beyond personnel protection, I guess there are at least 2 questions to ask:
1 - will trapped charge can cause a mis-operation of motor protection during start.
2 – will trapped charge interfere with your resistance measurement.


#1 I’m not sure about. My initial reaction was that it would not be a problem based on my ASSumption that our plant surge caps had no discharge resistors. However I went back and looked at a photo of the nameplate (attached) and found out that ours do indeed have internal discharge resistors.

The answer to #2 depends on how long you apply dc voltage. Certainly after some period of time, the cap will come into equilibrium with the applied voltage and initial stored charge no longer affects the measurement.


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(2B)+(2B)' ?

 

[magoo2]

I really don't think there's a lot of use for surge capacitors at 4.16 kV. At higher voltages, sure, but not at 4.16 kV. What is the switching device? Is it a vacuum switch?

 

[eaaf]

Pete.
Thanks a lot.

I have the same concerns about it. All my Caps are with discharge resistors.
And the DC voltage will be apply from 20 minutes to one hour; I think just the time that the protective relay stablish depending of the thermal capacity used in the motor before the stops.

Magoo2.

Yes the switching device has vaccum contactor, so we use surge caps and LA's near the motor. This is the protection that almost of our motors above 500 HP in 4.16 KV has.

Regards.

 

[eaaf]

Good day.

For complements to the question that pete wrote:

No.1 The manufactures indicated that don't have no complaints due to a mis-operation of the protections after the chage of the surge capacitors.

No. 2. Talking with the manufacturer of the instrument, he indicated to me that the instrument at the beginning of the measurement discharge the capacitor's AC charge, and then apply the DC test voltage.

I will test this surge caps and the instrument, then I will feedback you, how well its works.

Regards.

EAAF