Normalising Switchgear Insulation Resistance Measurement to 20degC

[JezNZ]

Hi,

Does someone have reference to the appropriate formula for normalising IR test values to 20degC?

For reference we have extended an 11kV Metal-Clad Switchgear bus, and while happy with the test results on the extension, the test results on the coupled board are low compared to the factory installed.

Thanks

 

[che12345]

Dear Mr. JezNZ (Electrical)(OP)23 Mar 22 08:24
"....Does someone have reference to the appropriate formula for normalising IR test values to 20degC? For reference we have extended an 11kV Metal-Clad Switchgear bus, and while happy with the test results on the extension, the test results on the coupled board are low compared to the factory installed...."
1. In your case it is regarding the insulation resistance between the bars to the frame(earth) and between bars. It is understood that the bars are insulated from the frame by insulation material. In usual cases, the insulator insulation properties are NOT significantly affected by the temperature, say between 10-90 degC. Therefore, the readings in Mohm/Gohm are taken at any convenient or room temperature; without having to state the temperature of the bar conductor.
2. However, the insulation resistance of motor winding is highly affected by the winding temperature. Therefore, the reading taken at any temperature is to be normalized to 20degC; in order to make a meaningful comparison with the subsequent readings, on the same temperature basis. There are chart suggesting the factor for conversion from any other temperature to 20degC.
3. There are rule-of-thumb suggesting the acceptable minimum resistance of (ohm per volt). Any value that is lower than the suggested should be investigated. A single value is NOT informative of the condition. Numerous values taken over a period of time would be of greater usefulness.
Che Kuan Yau (Singapore)

 

[JezNZ]

Thanks che12345, very informative answer.

Is there also any consideration/correction of humidity?

For reference, we have:
the existing switchgear reference at initial install (2006) - 500+ GOhm
the new switchgear (non-coupled) measured at approx 250 GOhm
the coupled switchgear measured at approx 100 GOhm

Unfortunately we did not measure the existing switchgear prior to coupling. The testing was down on a wet, high-humidity day so it was expected that values would be lower, however the technicians noted the much lower of the coupled board and are considering to propose to the client that they should begin to undertake 3-yearly bus chamber clean out and repeat insulation test. However due to the conditions, comparing the 2006 result and 2022 result don't seem of any use.

 

[che12345]

Dear Mr.JezNZ (Electrical)(OP)23 Mar 22 19:44
"...#1. Is there also any consideration/correction of humidity? For reference, we have: the existing switchgear reference at initial install .....Unfortunately we did not measure the existing switchgear prior to coupling. #2. The testing was down on a wet, high-humidity day so it was expected that values would be lower, however the technicians noted the much lower of the coupled board and are considering to propose to the client #3. that they should begin to to undertake 3-yearly bus chamber clean out and repeat insulation test. ...."
#1. Modern days insulators used in 11kV systems are NOT significantly affected by the humidity of the surrounding air during testing when conducted in [indoor say within 10-40degC]. Attention !, there shall be NO condensation on the insulator. Therefore, the relative humidity (RH) of the surrounding air during testing is NOT recorded. This means no correction needed.
#2. See above #1., the variation would be insignificant/irreverent.
#3. A valid and valuable proposal. If readings are taken say yearly, the trend would be more informative; instead of base on a single value.
Che Kuan Yau (Singapore)

 

[prc]

IR is only a basic check and better methods are used for condition monitoring, at least in the case of liquid immersed equipment, like a transformer. When you are coupling old switchgear with low IR with a new one of high IR, two resistances are getting paralleled and you will get the net resistance. This is the reason IR of a single-phase transformer will be three times more than a three-phase transformer of an identical insulation system.

Compared to liquid immersed switchgear, air-insulated switchgear will have high IR. Temperature correction will be more for liquid immersed units - transformers for every 10C rise in temperature IR will come down by half.

So you have to arrive at the temperature correction factor depending on the design of the equipment. You can check IR at different ambient temperatures and estimate the correction factor for your equipment. Still better check IR at the same ambient temperature. In the case of air-insulated equipment, ambient moisture levels also come in to picture affecting IR values.

 

[che12345]

@ prc (Electrical)24 Mar 22 01:30
"...... In the case of air-insulated equipment, ambient moisture levels also come in to picture affecting IR values...."
1. It is understood that ambient temperature, humility , air pressure and air pollution etc, etc do affect the IR value of air-insulated equipment. In reality, some have significant effect but others negligible; during testing conducted in indoor/testing station. For this reason, you don't see IR value qualify it with the ambient %RH, Air-pressure/altitude of the testing station, degree of air pollution etc., etc. IR test may be conducted indoor without any restriction. Attention ! without any condensation on the insulator.
2. We would be grateful if you would advise which Standard IEC/ANSI/NEMA/UL/NEC .... or any IR instrument manufacturer that states the IR value shall be qualified with the %Rh ... air pressure etc., etc.
Che Kuan Yau (Singapore)

 

[prc]

1) I never said air pressure will affect IR values. Only mentioned about moisture level or dew point of atmosphere.

2) You will get the details in IEEE std 43-2013 Recommended Practice for testing IR of electrical machinery. For transformer, IR values refer to C57.152 & C57.125

3) Manufacturer's booklet is the famous booklet " A stitch in time" originally published by AVO in the 1970s but the latest version is by Megger. You can get it from their website.