Unusually high capacitance of HV motor winding

[rotman]

We recently carried out checking measurements on two HV cage motors (4000 HP) after their rewinding (VPI process). What surprised us, is that capacitance of stator winding insulation is very high – ca.100 times higher than in previously rewound motors of the same type. But all parameters taken as criteria for insulation quality (insulation resistance, polarization index, coeff.of dielectric losses tg delta and increment of tg delta) are well within acceptable limits.
We checked our measuring equipment (Schering bridge) and it’s OK. Repair workshop assets, that they changed nothing in materials and process.
We faced for the first time such case. Could anybody help us to interpret this (possible reasons of high capacitance and can this cause problems during motors’ operations) ?

 

[electricpete]

Interesting. To my knowledge capacitance isn't usually taken as any indicator of insulation quality unless it changes (on the same motor/insulation) or perhaps imbalanced among phases.

The physical parameters which might cause capacitance to go up:
Increasing number of turns
increasing area at inteface of copper/iron
decreasing ground insulation thickness
increasing dielectric capacitance of the resin materials.

It is hard to imagine which of these could change. Particularly if the previously rewound motors of the same type were sister units (made at same time by same manufacturer to same specs).

The fact that tan delta and tan-delta-tipup are not high would seem to rule out gross contamination-type problems.

One thing that occurs to me is that high capacitance equates to low impedance.... sort of like an ac analogy to low insulation resistance. As we know a low insulation resistance (or impedance) is the combination of many parallel paths, and a single path with a low resistance (or impedance) will dominate the measurement. So it seems possible that perhaps you have an isolated problem which is dominating the measurement (perhaps a very low ground insulation thickness at one location in the slot only). Measurement of all three phases isolated from each other (if not already done) could help to prove or disprove this hypothesis.... if one phase is dramatically different from the others than it is likely a localized problem... if all the same than some other explanation.

It would also seem adviseable to request some additional testing if not already done in order to identify any problem before you accept the motor. Consider: partial discharge testing, lights-out test, dc hi-pot by step-voltage test (consider your max voltage carefully), corona probe inspection, ozone monitoring, pdma test, etc. If it were my motor I think I would enlist the help of a consultant.

 

[jbartos]

Suggestions:
1. One possible explanation is a different insulation of the winding conductor that can be caused by the different physical dimensions, and different material property of the insulation than specified.
2. Some flaw in the measurement of capacitance

 

[cbarn24050]

Hi, are you measuring with the motor windings connected in star?