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Associate

Join Date: Nov 2016
Posts: 28

Insulation Resistance

01/10/2021 11:17 AM

hello

When we performing the insulation test for power transformer like 132/6.6 kv

we test insulation between (high winding to earth ) , (low winding to earth) and

(high winding to low winding) my question is which of these three tests should its

resistance be higher compared to the other two tests and why.In other words, how

are the insulation resistance values of the three tests gradually

thanks

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Guru

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#1

Re: insulation resistance

01/10/2021 11:48 AM

You aren't comparing the insulation resistances with each other, you are comparing each insulation resistance value against a required minimum. The requirement for measurements involving the higher voltage winding will be higher than for the lower voltage winding.

" Do’s and don’ts when measuring insulation resistance of a transformer

Insulation Resistance Measurement

This test is performed at or above rated voltage to determine if there are low resistance paths to ground or between winding to winding as a result of winding insulation deterioration. The test measurement values are affected by variables such as temperature, humidity, test voltage, and size of transformer.

This test should be conducted before and after repair or when maintenance is performed. The test data should be recorded for future comparative purposes. The test values should be normalized to 20°C for comparison purposes.

The general rule of thumb that is used for acceptable values for safe energization is 1 MΩ per 1000 V of applied test voltage plus 1 MΩ. Sample resistance values of good insulation systems are shown in Table 1.

TABLE 1 – Typical Insulation Resistance Values for Power and Distribution Transformers

Transformer Winding Voltage (kV)Winding Ground (MΩ)
22°C30°C40°C50°C60°C
6.64002001005025
6.6 – 1980040020010050
22 – 45100050025012565
≥ 66120060030010075

https://electrical-engineering-portal.com/measuring-insulation-resistance-transformer

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Associate

Join Date: May 2007
Location: Serbia
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#2

Re: Insulation Resistance

01/11/2021 2:02 PM

Insulation resistance between high and low voltage winding usualy should be sum of high and low winding resistance to the earth but it is not important .

It should meet the minimum values ​​for voltage level and as a Rixter stated strongly depends on the temperature of the winding at which the measurements are made

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#3

Re: Insulation Resistance

01/14/2021 2:01 PM

A <...test...> without a predetermined criterion that it is intended to demonstrate is actually called a “measurement”.

The reasoning for carrying out a “test” without that criterion being established beforehand is abstruse.

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