Megger

It is trying to find a circuit between the phase and ground. 200meg is a high resistance value and indicates the insulation is sound at 1000V.

Is that substation operational now?

Yes, the megger applies 1000V to the circuit under test and measures the resistance. Hence for 200Mohm insulation resistance, the current flow is: i = V/R = 1000/2E8 = 5E-6 or 5pA (0.005mA)

Does this answer your question?

... except that 5E-6 A is 5µA or 0.005mA. That's 5 micro-Amperes. 5pA (5 pico-Amperes) is 5e-12 A.

Woops! Thanks for pointing out my mistake dk!

The Transformer insulation values are very less. It is required more than 1000 M ohms in all conditions.

If we apply some dc voltage 1000V definatly the current flows when a closed circuit.

Our requrement insulation resistance should be high. It very less means some closing ckt happned (leakage current) due to moisture in Windings.

If HV-E or LV-E is very low (5Mohms) then somewhere it is grounding.

Better to connduct Transformer ratio test, winding restance test & magnetic balance test. If all these tests found normal then open the winding assembly, keep drying in oven,Re-assemble and do oil filtration.

Re-test the Insulation resistance and charge the TX

I hope this helps to you.

Dear Sir,

Before finding an answer to your problem some data are required.

(1) Did you filter the oil when you did the maintenance work.This may improve the

insulation value.

(2) You can apply very low voltage on the primary with the secondary shorted as

this may drive out the moisture ( for this the winding should be lifted from the

tank)

(3) While doing the test (2) check for localised hot spots for inter winding shorts,

for any smoke / for any sparks.

(4) Check the Silica Gel colour. Dry out & use it in the breather.

(5) Check the colour of the oil ( Pale yellow & clear). If the colour is slightly black or

has burnt smell - Oil change is recommended.

These may solve your problems. If not go in for ratio test etc.

Manroop.Chennai.

Megger is actually a mega-ohmmeter and it generates voltage ( in this case 1000V) which is applied on the insulation and the leakage resistance of the insulation is measured .This is obtained by the ratio of applied voltage by the leakage current passing thru the insulator and this computation is done by the meter itself . The applied voltage is generally double the voltage which the insulator will be subjected to in working conditions . In ideal case the insulation resistance will be very high , but if the insulation resistance shows low value then it indicates more leakage current thru it , and will require repair /replacement .Megger testing should always be done after isolating the equipment from power supply .

Since there is no perfect insulators, and due to the inherent reactances associated with any AC circuits, a finite amount of Leakage Current will always flow. In high voltage applications, the need for a megger to verify the integrity or test how good these insulators are, is a must.

Actually, the OP did not give enough information to draw an informed judgment of the transformer and ACB condition from the limited test results with 1000V megger. What are the primary and secondary voltage ratings of the transformer and the rating of the circuit breaker? What type of insulation in each?

Also, what measurement do the test results indicate? On the transformer, primary winding to ground, primary to secondary, or secondary to ground? Or both windings tied together, to ground?

200Meg may be fine for a LV transformer winding. I would expect higher resistance for 5kV or higher. 5Meg for the circuit breaker is fair for LV (600V or less), but might not be unusual considering a breaker taken from service. The condition of the breaker at time of test (just removed from service, after cleaning, etc.) or the environmental conditions (humidity & contamination) are not mentioned. If this is a medium voltage breaker, 5Meg is too low, and often indicative of being dirty or contaminated, or that the arc chutes have absorbed a lot of moisture.

Since the transformer capacity was given as 15MVA, this is likely a substation transformer, and a better test would be a Doble Insulation Power Factor or dissipation factor (tan-delta) test, which is an AC test at somewhat higher voltage (up to 12kV) that can give much more information about the condition of the winding insulation and the oil. A DC megger test gives far too little information to make a good judgment about the condition of the transformer and what is the cause of the readings.

Thank you all the substation is 110/11KV exclusivily for a steel plant.I am a trainee here. I never know that electricity can pass through anything (insulator). What about vacuum oh is it the reason for the introduction of vacuum circuit breaker.