proceedings of the xivth international symposium on

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PDC Measurements at Generator Bars

C. Sumereder, M. Muhr, R. Woschitz

Graz University of Technology

Institute of High Voltage Engineering and System Management

Inffeldgasse 18, 8010 Graz, Austria

[email protected]

Abstract: The dielectric response function of an

electric insulation system can be determined by the

measurement of the polarization and depolarization

current (PDC). Beside partial discharge and dissipation

factor measurements the time domain dielectric

response function can be taken to gain information

about the aging process and the condition of the

insulation medium. The best results for the condition

evaluation can be achieved at the frequency domain

description in the shape of the complex capacitance and

dissipation factor. For this reason the dielectric responsefunction has to be converted with the Discrete Fourier

Transformation (DFT).

Until now the PDC method was successfully applied to

determine the water content of oil-paper systems in

transformers. This paper should give a view to PDC

measurements at insulation systems for rotating

machines and the implication for the condition

evaluation. First results of the diagnosis with the PDC

method are discussed and a comparison to classical

dielectric measurements is given.

Dielectric Measurements at Generator Bars

Dielectric measurements are a very important tool to

evaluate the condition of electric insulation systems,

different standards were state of the art, e.g. the testing

of the insulation resistance [1] or the dissipation factor.

The test methods can be divided in AC tests with (0.1)

50/60 Hz or higher and DC methods.

Figure 1: Equivalent circuit and DC currents for

insulation systems [1]

The most popular dielectric measurements were

dissipation factor, insulation resistance (polarization

index) and partial discharge test. Beside these there

were also the absorption (polarization) current,

conduction current, geometric capacitance current andsurface leakage current. Figure 1 shows the equivalent

circuit of an insulation system and the measured

currents during insulation resistance test according to

[1].

The total current of insulation resistance measurements

is the sum of leakage, conductance, capacitance and

absorption current, according to equation {1}. The

quantitative characteristic of each is illustrated in

figure 2.

IT = IC + IG + IL + IA {1}

Figure 2: Types of currents at DC resistance

measurement of insulation systems [1]

The absorption current decays at a decreasing rate. The

current vs. time relationship is a function according to

equation {2}; it may be plotted as a straight line on a

double logarithmic scale graph.

IA = K. t n {2}

IA absorption Current

K function (insulation system, test voltage)

t time of applied direct voltage

n characteristic function of insulation system

The absorption current consists of two components,

which are due to the polarization of the impregnating

materials and the gradual drift of electrons and ions

through most organic materials. Organic molecules,

such as epoxy, polyester, and asphalt, tend to change

orientation in the presence of a direct electric field. It

usually takes several minutes after application of the

electric field for the molecules to become reoriented.

Proceedings of the XIVth International Symposium on High Voltage Engineering,

Tsinghua University, Beijing, China, August 25-29, 2005

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The conduction current in well-bonded polyester and

epoxy-mica insulation systems normally is zero unless

the insulation has become saturated with moisture.

The surface leakage current is constant over time, where

a high level can be caused by moisture or some othertype of partly conductive contamination present in the

machine.

The quantity of each current is dependant of the used

insulation materials and factors as geometrical

arrangement. The characteristic of the total current

admits statements about the condition of the insulation

system. E.g. a low percentage of the leakage and/or

conduction current can be interpreted as cleanliness and

dryness, if the windings were wet or contaminated the

absorption current is relatively smaller. For clean and

dry rotating machine insulation, the insulation

resistance is between about 30 s and a few minutesprimarily determined by the absorption current.

Dielectric Response Function

The measurement of dielectric response function and its

interpretation for the condition evaluation is a very old

method. Beginning in the year 1889 [2] physicist started

to investigate the polarisation and depolarisation

behaviour of insulants. In 1927 a contribution about the

anomaly of dielectrics was mathematical described. The

so called Curie - von Schweidler Law [3] describes the

dependence of currents during polarization and

depolarization as linear function in double logarithmic

scale. The theory and results from measurements are

shown in figure 3.

Figure 3: dielectric response function in theory and real

The dielectric response of an electrical insulation

system at the PDC method is measured in time domain.

The polarization and depolarization current in

dependence of time is recorded, transformed and

evaluated. Several measurements have shown that the

so called Curie - von Schweidler law does not meet thisbehavior in real, because dielectrics were heterogeneous

systems. The characteristic of the polarization

behaviour depends on the geometry and insulation

conditions.

The time domain response function is converted to the

frequency domain description, the Discrete Fourier

Transformation (DFT) is applied [4]:

00

)()(

UC

titf d

=

= dtetff tj )()(

The advantage of this mathematical procedure is that

the frequency domain is a complex description of the

response function where the parameters conductivity,

permittivity and polarization can be described according

to the formula:

=

+

=+=

rrr

r

rPOLL

j

*

0

0tantantan

{3}

with: conductivity, tan dissipation factor, r* complex permittivity

The dissipation factor can be expressed according to

{3} where the conductivity and the real part of the

complex permittivity show a very low dependence to

frequency. The imaginary part has a strong dependence

to frequency caused of different polarization mechanism

in the insulating medium. The results of a simulation for

the variation of the parameters , c and c areillustrated in figure 4 and 5. The rise of the conductivitycauses a change in the shape of the tan function in thelower frequency area. The process of a rising

conductivity should be characteristic for degradation of

the insulating medium. The rise of c and c proceeds

in the opposite way. A rising of c, which can be

interpreted as a rise of the relative permittivity, causes a

parallel decrease of tan and vice versa for c.

Figure 4: Variation of dissipation factor in dependence

of conductance

Up to now there were many considerations done about

possible connection between the dielectric relaxation



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and condition or residual disruptive strength of the

insulation medium, but all this expectations could not

be fulfilled, no physical parameter is known which can

represents an absolute parameter. For this reason the

dielectric parameter and r were taken into accountfor the evaluation of the measurements.

Figure 5: Variation of dissipation factor in dependence

of parameters c and c

Taking a look to the equivalent circuit of the generator

bar insulation several serial and parallel resistances and

capacitances can be observed (pancake model). This

insulation system is built of two components: mica tape

and resin. The R0 represents the geometric resistance,

C0 the vacuum capacitance and Ri, Ci the polarization

mechanism. The size of these parameters is determined

by the proportion of the used components. The per cent

by volume of mica at resin rich bars is about 70% and at

VPI bars 85-95%. The dielectric strength is mainlydestined by the electric field strength and the resulting

voltage distribution. Figure 6 illustrates the electric field

strength in the material components resin and mica tape

(left) and the equivalent circuit with the parallel linked

geometric resistances and vacuum capacitances (right).

Figure 6: Dielectric model of a generator bar and

equivalent circuit of heterogeneous dielectrics

This equivalent circuit meets the theory of the Maxwell

two-layer model: The dissipation factor is dependant on

the temperature and the power frequency for a dielectric

with orientation polarization; a temperature rise isequivalent to a decrease of the power frequency. A

higher temperature disturbs the orientation process as

well as the dipoles can not follow the field at high

frequencies [5]. The parallel linked Ri and Ci describe

further polarization effects, Ri delays the polarisation

and depolarisation processes of the serial linked Ci.

Test Objects and Measurements

The test objects are generator bars of resin rich

technology. The generator bars were in operation in

different periods of time. The preparation of the test

objects and arrangement of the measuring equipment

can be seen in figure 7. The PDC measurements were

done periodically approximately all 100 h.

CXTest Voltage

Grounded Electrodes

Measuring Air Gap

Electrode

Figure 7: Preparation of generator bars and connection

of power supply test lead for dielectric measurements

The air gap was necessary to separate the geometric

resistance and vacuum capacitance of the total

resistance and capacitance values (with generator bar

ends) on the one hand and on the other to prevent

leakage currents.

Results

For the evaluation of the test results the polarization and

depolarization currents, the complex capacitance,

insulation resistance, dissipation factors were

investigated in detail. In diagram 1 the depolarization

currents of one generator bar in dependence of load

time was observed. The current shows an almost linear

decreasing characteristic. In dependence of load time

the Depolarization current were rising.

Diagram 1: Depolarization Currents

In diagram 2 the insulation resistances of the same

generator bar were shown in dependence of load andmeasuring time. The Resistances were calculated:



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IR = Uc/IPOL, IR = Uc/(IPOL-IDEPOL), in the diagram the

R values are illustrated because of the very small

differences between R and R. From this diagram the

Polarization Index (PI) could be calculated. For all

curves the PI was within the demanded values

according to the standard [1]. According to the Current

measurements the insulation resistances had the similar behaviour. In dependence of load time the resistances

get smaller and smaller.

Diagram 2: Insulation Resistances

In diagram 3 the real part of the complex capacitance is

shown. The c rises with load time in the lower

frequency areas. At higher frequency there was almost

no change in quantity.

Diagram 3: Complex Capacitances, Real Part

The processes of aging concerning the imaginary part of

the complex capacitance is shown in diagram 4. The

curves rise with load time over the whole frequency

area. The dissipation factor in dependence of load time

is illustrated in the diagram 5. The curves show a risingbehaviour with load time over the total frequency area.

Diagram 4: Complex Capacitances, Imaginary Part

Diagram 5: Dissipation Factors

Conclusions

The results of the PDC measurements and the

calculation of dielectric parameters can be summarized

as follows:

The theoretical considerations about the behaviour of

dissipation factor in dependence of resistivity and

complex capacitance were met very well. The resistance

of the generator bars falls, the complex capacitance

raised with load time. The mathematical formulation of

dissipation factor according to equation 3 is confirmed.

It was observed that the generator bars fail out without

sudden. This behaviour can be interpreted that the

mechanism which causes the damage and finally the fall

out happens very fast.

With the PDC method the aging behaviour of anelectrical insulation system can be observed. It is an

integrative method, for this reason no absolute

statements (as a withstand voltage test or PD

measurement enables) about the condition of the

observed system can be done. It is important to do

periodical measurements.

The PDC method can be applied at generator bars in

general. The application to measure a whole winding

respectively a whole generator has to be verified. Future

measurement should show

References[1] IEEE Std 43-2000, Recommended Practice for

Testing Insulation Resistance of Rotating Machinery

[2] J. Curie, Recherches sur la Conductibilite des Corps

Cristallises, Annales de Chimie et de Physique, 1889

[3] E. von Schweidler, Studien ber die Anomalien im

Verhalten der Dielektrika, Annalen der Physik 1907,

24, 711-770

[4] A. Helgeson, Analysis of Dielectric Response

Measurement Methods and Dielectric Properties of

Resin-Rich Insulation During Processing, Doctor

Thesis, Kungl Tekniska Hgskolan Stockholm Sweden,

2000, ISSN 1100-1593

[5] A. Kchler, Hochspannungstechnik, VDI Verlag1996, ISBN 3-18-401530-0



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