Generator diagnostic-Partial discharge

[behemot]

Could anyone tell me something about acceptable level of patrtial discharge in generators. Is there any documents that says about it? Do you think that PD measurment it is useful testing methods for generators?.

Thanks in advance for your help

 

[electricpete]

For Iris coupling capacitor equipment, an appropriate alarm level for a 13.2kv air-cooled machine is roughly 400 millivolts.

It will vary by:
voltage level
cooling type
sensor type (coupling cap or slot coupler)
equipment vendor (Iris, Adwell, Cutler Hammer)

Iris publishes rough limits based on statistical analysis (worst 10%) for each class of machines (voltage level, cooling type, sensor type).

Of course trending with historical results and comparison with similar machines are the most effective techniques. In my database I compensate for the effects of temperature and humidity by determining temperature coefficient and relative humdity coefficient of partial discharge through linear regression analysis of pd results for each machine. Then I can trend the corrected results with the variability introduced by those variables removed.

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[gordonl]

Do you also monitor and compare based on loading of the machines? I'm looking at putting a system in shortly and was thinking that machine loading would be important, but perhaps temp (stator I assume) would naturally compensate for this.

 

[electricpete]

Machine loading would be a good variable in most cases.

I work at a 2-unit nuke plant. We have 30 13.2kv motors monitored with PD. The vast majority of the time, loading is at fixed level corresponding to normal full-power plant lineup. So I don't track machine loading. But relatively small winding temp changes resulting from ambient temperature changes seem to have a big impact on pd. If anyone is interested, I can provide some of my temperature coefficients and correlation factors (R^2).

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[behemot]

Thanks for the information but I'm using Haefely PD detector that measure charge in coulombs not in volts.