Insulation Resistance of generator stator drops when the turbo-generator is charged with stator coil cooling water. To what value does it drop it? How come the gen is safe for production at that IR? To add detail, when the generator stator coil cooling water circuit is charged, if we measure the insulation resistance of the stator (phase to earth, say), the value is very less as compared to when it was before cooling water charging (due to the small levels of conductivity). I've been told, though this is not recommended to be measured, the value can be even as low as 1 Mega Ohm. I am curious as to why/how a generator can be declared ready for generation eventhough its insulation resistance in this condition is as low as noted above?

If that resistance can be considered to be reasonably linear with respect to voltage, it will only cause a parasitic current of a few mA, so as long as that does not cause any discontinuity in the flow of coolant, it should not cause concern. The alternator rated voltage for the larger generators are generally less than 30kV line to line, so the line to earth voltage will be less than around 20kV, corresponding to 20mA at 1MOhm. This will then be "shared" between the three phases, so only around 7mA per phase.

You generally will only employ water cooled stators for generators of several MW rating, so in terms of other parasitic currents caused by capacitive coupling to the surrounds, and the like, it will hardly be measurable during operation.

Bharathram,

IEEE Std 43-2000 (revised 2013), "Recommended Practice for Testing Insulation Resistance of Rotating Machinery" addresses the question this way:

"...To obtain insulation resistance measurements for a directly water-cooled winding, the water should be removed and the internal circuit thoroughly dried... In general, if the water is not removed, then the conductivity of the water should be greater than 0.25 μs/cm. More information should be available in the winding manufacturer’s manual..."

In your case only the manufacturer's guidance should be relied upon.

Alan

For your information, after removing the water for performing your insulation test, sometime vacuum is applied to dry your water circuit. BE CAREFUL to NOT DO the insulation resistance test while the vacuum is applied. In such case the insulation resistance drops to very very low values and you can flashover your stator just by testing it... which is quite not a good issue of course!
Never forget to break your water system vacuum before applying any test voltage to your stator then.

Thanks for your inputs!
The query was more theoretical than practical, as we know the harm testing with water system charged can cause. Thanks again!