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Heating of stator windings 1

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appunni

Electrical
Feb 11, 2003
52
Sir,
We have one 18MVA and 144 MVA units. In seperate occasions the megger values of stator windings were found low. The xd'',xd' and xd values of these units are about 15,25 and 90 percent respectively. These units are working in 11 kv system and the power is stepped up to higher voltage levels through delta /star generator transformers.
Can I heat the stator windings by applying 3phase,440v on the 3 phase windings of the units? Will the impedances of the windings be synchronous reactance of the unit even if the unit is standstill and the exciter is off?
What are the prcatical methods to pass current through stator windings in order to improve megger values?
I request for your valualble comments.
with regards,
appunni
 
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Don’t think 0.04 puV us going to do much heating, particularly with the stator having very high reactance at low voltage—very little heating {watt dissipation} may occur.

Watt/phase-angle and ammeters on the 440V circuit will likely demonstrate this.
 
The economical way to heat gen winding during stand still is to apply dc current.

During spinning, short circuit run is the best. If short ciruciting is not possible, even unexcied run for few hours will improve IR due to windage losses.

Recently, I improved the IR of 20 MW generator rotor from 1 meg to over 100 meg just by mechanical spin for about an hour.
 
Thanks for your replies.

To Busbar: What will be the impedance of the stator windings at that time? Will it be the synchronous impedance?
To edison123: Recently we tried to improve IR value of one 60MW unit stator winding from 5Megohms by mechanical spinning without excitation and after closing water cooling supply to stator windings. But failed to improve eventhough the stator temperature rose from 40 degree to 55 degree. I request your expert comments in this regard.
With regards,
appunni
 
appunni,

If your generator cooling is close circuit, then you need to vent the inside air after the mechanical spin / short circuit run to let out the moist air. This will improve your IR. This is normally done by opening the inspection doors/covers.
 
Maybe the winding insulation is bad and not wet. Also you said:

Recently we tried to improve IR value of one 60MW unit stator winding from 5Megohms by mechanical spinning without excitation and after closing water cooling supply to stator windings.

Without excitation? I have no experience with this but without excitation doesn't seem right to me. No excitation means no current flow and no heating????
 
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