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On abc-model and on Pedersens model

Michael G. Danikas

Democritus University of Thrace

Dept. of Electrical and Computer EngineeringPower Systems Laboratory

Xanthi, GREECE

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More on the comparison between the abc model andPedersens model

Comparison extended to experimental results from scientificliterature

Results taken from the following: Shoba and R. S. Nema, Internal partial discharge and

breakdown characteristics of thin polypropylene films, Ann. Rep.CEIDP, Pocono Manor, USA, October 1993, pp. 408-411 [12]

J. H. Mason, The deterioration and breakdown of dielectricsresulting from internal discharges, Proc. IEE, vol. 98, pp. 44-59,

1951 [13] B. Kuebler, Investigations of partial discharges measuring

techniques using epoxy resin samples with several voids, Conf.Rec. IEEE Int. Symp. Electr. Insul., Montreal, Canada, June 1978,pp. 82-85 [14]

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[12] - Test specimens consisting of three sheets of polypropylene sandwiched between plane- plane electrodes [13] - cylindrical cavity - uniform electrode arrangement [14] - spherical cavities Evidently Pedersen model estimates are closer to

experimental data than those of the abc model General observation: with increasing cavity dimensions, the charge

values (both experimental and calculated) become larger This is to be expected since the charge in the abc-model increases

proportionally with surface area whereas in Pedersens model itincreases proportionally with the volume of cavity

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[12] R. Shoba and R. S. Nema, InternalPD and breakdown characteristics of thin

polypropylene films, Ann. Rep. CEIDP,Pocono Manor, USA, October 1993, pp.408-411.

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[13] J. H. mason, The deteriorartion andbreakdown of dielectrics resulting from

internal discharges, Proc. IEE, vol. 98,pp. 44-59, 1951.

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At least initially, presence of streamer mechanism in enclosed cavities Physical interpretation: Townsends second ionization coefficient takes

very small values in an enclosed cavity because the cavity does not comeinto contact with any metallic surface (Buchalla, 1996)

Initial conditions: streamer mechanism prevails (Devins (1984), Morshuis(1995))

Aged conditions: Townsend mechanism prevails (Devins (1984)) Practical consequences of the above: very fast PD are to be expected upon

switching the high voltage (provided that the inception field for PD hasbeen reached), so we need appropriate PD detectors to detect events ofrise time in the range of 1 nsec)

Increasing divergence between experimental and theoretical values ascavity dimensions increase Why? Because for both models is assumed that the whole of the cavity

discharges (not necessarily true!) Recombination effects also possible (?) (Kuffel and Zaengl)

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[14] B. Kuebler, Investigations of partialdischarges measuring techniques using

epoxy resin samples with several voids,Conf. Rec. IEEE Int. Symp. Electr. Insul.,Montreal, Canada, June 1978, pp. 82-85.

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With the exception of Kueblers data [14],results indicate that the calculated charges (bothfrom abc-model and Pedersens model) are

larger than the experimental charges There is an increasing divergence between the

calculated charge values and the experimentalones with increasing cavity dimensions

This is due to the fact that for both models itis assumed that the whole of the cavity isdischarged

In reality, this assumption is not necessarily true

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Possible directions for further work

Is the gas in an enclosed cavity air-like? (see Gjaerdeswork (1994)on pressure variation in cavities and

gaseous byproducts) How Pedersens model behave in a humidity

environment? Transition from cavity PD to electrical treeing? What sort of PD cause electrical treeing? Is there a

critical PD magnitude causing initiation of electrical treefrom a cavity and what is its relation to otherexperimental parameters, such as gas pressure andinner wall chemistry?

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Pedersens model relates the charge q induced on the measuring electrodeby a PD to some important parameters

q = k0(i - El) 0

Pedersens model is based on electromagnetic theory The abc(or capacitance) model is based on the calculation of the charge

of a PD by taking into account the respective capacitances of the enclosedcavity, the dielectric next to it and the rest of the dielectric

The abc(or capacitance) model approach is old and proved itsusefulleness on several occasions

We know by now that Pedersens model is related to streamer-like PD andthe abc(or capacitance) model more to Townsend-like PD

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A thorough search in technical literaturedoes not bring necessarily a wealth of

data on enclosed cavities as one wouldhope

Therefore, a comparison between the two

aforementioned models may be based onsome selected papers