Insulation Resistance

[ASHISHRAMAN]

I checked the insulation resistance value between transformer winding and earth putting 2.5 KV DC, and i got 16 mega ohm as IR. if increase the voltage to 5 KV, what would be the new IR value ? Higher than 16 mega ohm or lower ?

 

[ASHISHRAMAN]

Sorry , small mistake IR is 16 giga ohm not mega ohm

 

[itsmoked]

I'm not the guy to answer but I was going to say... run away! Glad you're talking giga-ohms.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[electricpete]

The test that you describe (looking for change in resistance with change in voltage) is similar to what is done on rotating equipment.... where you measure current vs time. It's called dc step voltage test and described in IEEE 95. Timing of the measurement during the test is important because there are multiple components of current present during the test: absorptive, capacitive, resistive. The first two vary with time after an increase in voltage, the third is typically the one you're most interested in.

For rotating machines, the current vs voltage tends to be constant with voltage increases (when measurements are timed properly ... typically one minute after each step) for a normal healthy motor in the normal voltage ranges (up to and beyond operating voltage). That corresponds to constant resistance with voltage. If the winding is unhealthy, then the current starts to increase greater than linearly. That corresonds to decreasing resistance with voltage.

Since dry transformer insulation is somewhat similar to rotating machinery insulation, I would think a dry transformer insulation test would act similar to a rotating machine test as described above. For an oil filled transformer, all bets are off.

Of course if you are aiming to test your transformers insulation condition, I would stick to more standard tests. That leads to the question: why do you ask?

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(2B)+(2B)' ?

 

[prc]

1) Why you increase the voltage unless you have some doubt ? Normally it will be done at same voltage as earlier cases to get correct bench mark.
2) If insulation is dry, there will be no difference in IR with increased voltage. But wet insulation will result lower IR at higher voltage (more than 25 % reduction is a cause for concern)
3) Voltage application shall be for the same time, generally one minute.