Interpratation of the online Partial Discharge Measurement Result 2

[powering2002]

Dear All,
we have installed online partial discharge measurement in our generator (2x 100 MW). From the last 2 month measurement result, it was found that partial discharge measurement value in one of the phase ( V phase) exceed 18 nanocoulomb. But this value occurs rarely (sometimes), not continuosly. Is it safe for us to operate our generator in that condition? How much is the maximum value of the allowed partial disharge?
Thanks for your attention.

 

[GuntherA]

not sure about generators but for transformers its in the order of some pico coulombs, I'd be alarmed if it was nano coulombs.

 

[electricpete]

I work with the Iris equipment which reads out in millivolts (80pF capacitor).


=====================================
(2B)+(2B)' ?

 

[electricpete]

Sorry, somehow I hit "submit: while I was still typing.

I was going to proceed to wonder how to convert your charge number to a voltage number.
I wonder if the capacitance provides a fair conversion between your picocoulombs and my millivolts.

V = Q / C = 18E-9 Coulombs / 80E-12 Farads = 225 volts = 225,000 millivolts.
Hmmm.

I don't know if that conversion makes any sense but would be ridiculously high if it's in the ballpark.

Were you able to determine the pulse phase distribution (where in relation to the fundamental phase to ground volts). Could be the signal you're looking at is something other than traditional pd within the generator (you may be picking up other phenomena associated with the power system).

If it really is valid pd coming from the generator, typically the only type of pd that goes to ridiculously high magnitudes is discharge between the slot and the semi-con associated with very loose coils. In that case you would see positive predominance of the partial discharge (by the Iris / US terminology, I understand it's called negative predominance in Europe).


=====================================
(2B)+(2B)' ?

 

[electricpete]

Are your sensors bus couplers or slot couplers?
Would it be an air-cooled machine?

=====================================
(2B)+(2B)' ?

 

[iwright]

Can you post some more information about your generator? Voltage, air or hydrogen cooled and type of sensor? High PD on a hydrogen cooled machine is worse than high PD on an air cooled machine, because hydrogen under pressure supresses PD. High PD on a low voltage machine is also worse than high PD on a high voltage machine because the thinner insulation will fail sooner.

There is no maximum value for online PD, the PD isn't your problem it is a symptom of your problem. If these high readings are intermittent, what is happening when you get the spikes? Are you load cycling, is the system voltage higher at these times, if it is a hydrogen cooled machine is it operating at rated pressure? The company I work for has a couple machines with very high continuous PD, we are monitoring them and we are planning to rebuild them but we would never shut a unit down just because of high PD.

Talk to the supplier of your test equipment, they have experts that can help interpreting your results.

 

[powering2002]

Dear iwright, we use hydrogen cooled generator.

 

[electricpete]

sensors - coupling capacitors or slot couplers?

=====================================
(2B)+(2B)' ?

 

[Addy71]

I think the more important questions are:
1. What is the coupler rating - 80 pF, 500 pF, 1 nF, 2 nF or 8 nF? - The higher the coupler rating; the larger the measured signal will be.

2. What is the frequency band in which the measurement is taken? - A lower "low cut-off" frequency will give you a higher pC/nC value.

3. What does the phase-resolved pattern look like? At times; you get a high PD value but looking at the signal you can identify that it is electrical noise coupling in; not actual PD. This can also help identify the location of the PD; which is vital for determining the criticality.

4. What is the max. PD level noticed on the other two phases?

5. Have you tried correlating the times at which the high PD occurred with other activities/conditions in the plant?

I have seen machines with 18 nC; but not common. If possible; please upload your PRPDA graphs & trends here.

Regards,

Aditya

 

[powering2002]

Dear electricpete and Addy71, I will try to find the desired data. Thanks...