Load Effect on PD Test

[Lenz81]

Ladies and Gentlemen

As We know Partial discharges occur when the local electrical field intensity exceeds the dielectric strength of the dielectric involved, resulting in localized ionization and breakdown , i was wondering how current will effect PD ?

 

[electricpete]

Does the question refer to cables or rotating machines?

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(2B)+(2B)' ?

 

[Lenz81]

generally ,lets say both .

 

[electricpete]

Increasing load tends to increase mechanical compression of insulation in the slot section based on 1 - copper tends to get hotter than iron; 2 - even if temperature of copper and iron increased the same, copper has higher thermal expansion coefficient than iron. This effect can tend to shrink voids in the insulation or between insulation and iron or copper smaller which tends to decrease partial discharge. I see a pretty good correlation between temperature and pd on my machines and I attribute it mostly to this effect.

For endwinding pd caused by for example surface contamination on endwindings, according to the teachings of Iris, the effect of temperature is not as repeatable (this type of pd could increase with temperature or decrease with temperature).

Iris also mentions that certain types of severe discharge associated with wearing away the carbon on side of the coil can increase with load since vibration of coils increases with load.

For cables I have no idea.

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(2B)+(2B)' ?

 

[electricpete]

By the way the correlation I referred to above is negative... i.e. increasing temperature results in decreased partial discharge on machines.... that is what I see most often. We have 30 13.2kv machines that we trend every quarter for last 10+ years.

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(2B)+(2B)' ?

 

[Lenz81]

Thanks , current would effect thermal condition of cable also i was wondering how magnetic field caused by current would effect void.

 

[electricpete]

My guess would have been that it does not affect partial discharge.

However, google led me to this:

http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=5658222

Influence of leakage magnetic fields on partial discharge activity in power transformers

Marti?nez-Tarifa, J.M.; Rivas-Conde, J.; Robles, G.; Sanz-Feito, J.; Escuela Politec. Super., Univ. Carlos III de Madrid, Leganes, Spain

This paper appears in: Dielectrics and Electrical Insulation, IEEE Transactions on
Issue Date: Dec. 2010
Volume: 17 Issue:6
On page(s): 1724 - 1730
ISSN: 1070-9878
INSPEC Accession Number: 11684872
Digital Object Identifier: 10.1109/TDEI.2010.5658222
Date of Current Version: 06 December 2010
Sponsored by: IEEE Dielectrics and Electrical Insulation Society

Abstract

Partial discharges are a clear ageing agent on insulating materials used in high voltage electrical machines. For this reason, there is increasing interest in measuring this phenomenon in an effort to forecast unexpected failures in electrical equipment. Despite the fact that normalized tests have been carried out for years, the effect of leakage magnetic fluxes in transformers and rotating machines stator windings has not yet been considered. The authors show that common stray fluxes in typical high voltage machines modify partial discharge activity, so the interaction between ionization processes and magnetic fields is analysed. This might have a clear effect on the results extracted from on-line and off-line partial discharge measurements.

I am a member of DEIS, so I was able to download for free (but can't post it, since it's copyrighted). On my first quick scan through the article, the only thing I noticed is that they have done experiments and provided statistical analysis of the results. When I get a chance, I will read it closer to try to understand how significant is the effect and what is the physical mechanism for this effect. Are you an IEEE DEIS member?

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(2B)+(2B)' ?

 

[Lenz81]

Thanks for great response , no i am not , i would appreciate .