Condition assessment of stator insulation during drying ...

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 20, No. 6; December 2013 2081

1070-9878/13/$25.00 © 2013 IEEE

Condition Assessment of Stator Insulation during Drying, Wetting and Electrical Ageing

M. A. R. M. Fernando, W. M. L. B. Naranpanawa, R. M. H. M. Rathnayake

Department of Electrical and Electronic Engineering University of Peradeniya

Peradeniya 20400, Sri Lanka

and G. A. Jayantha Generation Projects

Ceylon Electricity Board Colombo Sri Lanka

ABSTRACT

Stator insulation failure due to moisture ingress is one of the common reasons for generator failures. Non-destructive tests such as DC ramp, insulation resistance (IR) and frequency domain spectroscopy (FDS) are usually conducted to assess the conditions of such insulations. This paper presents a laboratory investigation of generator spare windings under drying, wetting and cyclic electrical ageing conditions. First Dc ramp, IR and FDS tests were conducted on asphalt-mica and epoxy-mica insulated spare windings to obtain initial finger print values. Then the drying effects was investigated by DC ramp and FDS measurements conducted on asphalt-mica insulated winding samples by drying them up to three days at 80 C. The wetting effect was studied by wetting the polyester-mica insulated winding samples in a water bath up to one week at 25 C. Further a correlation has been established between the moisture content and FDS results by drying and wetting of two asphalt-mica insulated samples at 70 C and the room temperature respectively. The ageing effects was obtained by electrically stressing two asphalt-mica and two epoxy-mica insulated winding samples at 300% of the rated voltage up to 9 weeks in a cyclic manner. For all cases, conductivity () and parameters representing the response function were estimated by the Davidson-Cole and inverse power dependence functions. The estimated parameters under DC ramp, IR and FDS tests were in similar range for new samples. During ageing and drying, a clear variation was noted only on the values and FDS results. It was found that FDS results and estimated parameters provide useful information about the condition of generator insulation.

Index Terms — Condition assessment, stator insulation, electrical ageing, moisture, frequency domain spectroscopy.

1 INTRODUCTION

GENERATORS are the most important and the essential part of a power system and their failures severely affect the power system reliability. It has been reported that about 56% of the generator failures are attributed to stator winding insulation problems [1, 2]. The stator winding insulation of generators are usually exposed to thermal, electrical, mechanical and environmental stresses during their operation and failures of such winding insulations depends on either single or combination of these stresses. The main root

cause for insulation failures can be categorized as 31% due to ageing, 25% due to contamination and 22% due to internal partial discharges [1]. The insulation damages by contamination is mainly dominated by the moisture absorption within or outside from the generator. Due to hygroscopic nature of many types of the insulation systems, moisture can be drawn into the body of the insulation from the atmospheric air (in open type ventilation) or from leakage of water from the coolers (in close ventilation system) of the generator stator. Moisture in combination with partly conductive contamination such as oil, carbon dust and brake dust (which are already deposited on the winding insulation and ventilation paths) can accelerate thermal Manuscript received on 17 September 2012, in final form 22 March 2013.

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2082 M. A. R. M. Fernando et al.: Condition Assessment of Stator Insulation during Drying, Wetting and Electrical Ageing

deterioration or thermal ageing of the generator stator winding insulation [3, 4].

Due to the operating conditions, generators can be subjected to higher than normal stresses due to temperature, voltage and mechanical force. However, the electric stress due to voltage plays an important role in insulation deterioration compared to other stresses such as thermal, mechanical, thermo-mechanical etc. [4 - 6]. It is important to investigate how the insulation behaves under those electric stresses in combination with the presence of moisture. When the insulations are exposed to high humidity condition, natural wetting may take place. Sometimes generators face accidental flooding situation where generator windings are submerged in water [7]. Then the winding insulations are exposed to the forced wetting condition. In such situations, the windings are dried to remove the absorbed moisture [8]. This paper addresses the cases of drying, wetting and cyclic ageing aspects.

Different non-destructive test methods are presently used to assess the condition of stator winding insulations [5, 9-17]. Some of the important test methods are insulation resistance (IR) measurement [9], DC ramp testing [10, 11], polarization/depolarization current measurements [12, 13], frequency domain spectroscopy (FDS) [5,17], loss tangent measurements [14], and partial discharge measurements [15, 16] etc. Out of them, IR and DC ramp tests can easily be conducted in the field to provide rough estimation about the condition of the insulation. However, by these tests the parameters such as conductivity (), permittivity () and dielectric responses functions (f(t)) characterizing the condition of the insulation cannot be estimated accurately [11]. On the other hand, the dielectric response in time and frequency domains as measurements of PDC and FDS respectively provide useful information [5, 12] despite the testing time takes longer time i.e. 3-4 hours compared to DC ramp and IR tests. In addition, PDC and FDS correlate to each other so that with one set of measurements, the other set can be obtained by Fourier transform, Hamon approximation etc., [5, 18, 19]. If the correlations can be established among the different test methods, parameters such as f(t), and can be effectively utilized to assess the condition of the insulation with the aid of one or two test methods.

This paper also presents a laboratory investigation of generator spare windings under drying, wetting and cyclic electrical ageing conditions. DC ramp and FDS measurements are widely used to assess the insulation condition.

2 TEST SAMPLES The Sri Lankan power utility, Ceylon Electricity Board

(CEB) has been engaged in power generation, transmission and distribution since 1950s. There are 16 hydro power stations consisting of 39 generators installed in different eras covering different insulation types i.e. asphalt-mica, polyester-mica and epoxy-mica. In this work, seven different sets of spare windings were selected covering

different ages of installation. The details of the windings are given in Table 1. Those spare windings had been stored and exposed to indoor atmospheric conditions from the year of installation of their counterparts.

3 NON-DESTRUCTIVE TEST METHODS

3.1 TIME DOMAIN MEASUREMENTS In time domain measurements, the insulation is subjected to a constant or ramped dc voltage. According to the

Maxwell’s equations (t

DJxH

) and with

corresponding substitutions for current density J and electric flux density D, the time variation of current density can be written as

t

tP

t

tEtEtxHtJ

0 (1)

The E and the P are electric stress and polarization respectively. The first, second and third terms represent the contribution from conduction, capacitive and polarization currents respectively. The polarization part is contributed by its rapid and slow phenomena as

t

dtEftEtP0

00 1 (2)

Where f(t) is the response function of the dielectric insulating material. By combining equations (1) and (2), the current density can be written as

t

dtEftEt

tEtJ0

0 (3)

When a voltage U(t) is applied, the current becomes

t

dtUftUt

CtUC

tI0

00

0 (4)

The C0 is geometric capacitance and the f() represents the response function. By measuring the current one could separate capacitive, conduction and polarization components.

If the voltage is a ramped one with as the gradient the voltage becomes U(t) = t. Since most of the generator

Table 1. Details of the spare windings.

Generator/ Power station

Power [MVA]

Voltage [kV]

Year of installation

Insulation

G1/ Wimalasurendra

31.25 11 1957 Asphalt M

G2/Samanala 46.9 12.5 1969 Asphalt M

G3/Ukuwela 20 12.5 1973 Polyester M

G4/Bowathenna 47 11 1980 Polyester M

G5/Randenigala 85 12.5 1985 Epoxy M

G6/Kotmale 90 13.8 1986 Epoxy M

G7/ New Laxapana

72 12.5 2012 Epoxy M

M – Mica insulation



Figure 1. Current – voltage characteristics for asphalt mica and epoxy micaspare windings under DC ramp test.

0

20

40

60

80

100

120

140

160

0 3 6 9 12 15

Voltage [kV]

Cu

rre

nt

[uA

]

0

2

4

6

8

10

12

14

16

Asphalt mica Epoxy mica

insulation obeys Curie-Von Schweidler (CvS) model i.e. f(t) = At-n [11], the response function can be approximated according to [11, 18, 19]. Then current components for DC ramp test and IR tests can be written as in equation (5) and (6) respectively.

ntn

AtCtI 1

00 1

(5)

nAttCtUtI )()(

00

(6)

3.2 FREQUENCY DOMAIN MEASUREMENTS

In frequency domain measurements, the insulation is subjected to an AC voltage with different frequencies and the response is recorded with respect to the frequency. The current voltage relationship can be written as

UjCCjUjCjI

UjCjI

"'"'

"'0

0

(7)

Where ^"' Fj , F^() is the

Fourier transform of f(t). In frequency domain, it is easy to interpret the results as loss tangent as

'

"

'

"tan 0

C

C (8)

By considering both time and frequency domain results, the key parameters representing the status of the insulation are the conductivity (), the permittivity (), the response function (f(t)) and the loss tangent. The Fourier transform of the CvS model f(t)=At-n can be used to fit frequency domain results.

In comparison to CvS model, we use Davidson-Cole (1+jτ)-β and the inverse power dependence A-n functions [18] to model the real and imaginary components of permittivity. Then the corresponding equations can be written as in equation (9) and (10). The imaginary permittivity (”) component for inverse power dependence was calculated as the Kramers-Kronig (K-K) transformation of that of real part [20].

The current components for DC ramp test and IR tests can be written as the corresponding inverse Fourier transform as shown in equations (11) and (12) respectively.

nAj 1Re)(' (9)

2)1(cot

1Im)(0

''

nA

j

n

(10)

n

tnnA

tt

CtIn

21cos1

)( 00

0

(11)

1

1

00

21cos1

)(

)()(n

t

tnnA

et

t

CtUtI

(12)

3.3 COMPARISON OF TIME AND FREQUENCY DOMAIN MEASUREMENTS

To illustrate the comparison of different test methods, two generator windings i.e. asphalt-mica (G1), and epoxy-mica (G6) in Table 1 were selected. The selected tests were DC ramp, and IR measurements in time domain and FDS in the frequency domain. The DC ramp test was conducted from 0 kV to 15 kV (125% of the rated voltage) at a rate of 1 kV/minute by using DCR50 Adwel DC ramp tester. The IR test was conducted at 5 kV and the measurements were taken in every minute for 15 minutes megger ohm-meter. The voltage level and the duration were selected to represent the measurement of IR and polarization index. According to the practical possibility two electrode configurations was used for both tests. The FDS test was



Figure 3. Variation of loss tangent with respect to frequency for asphaltmica and epoxy mica spare windings under FDS test.

0.01

0.1

1

10

100

0.0001 0.001 0.01 0.1 1 10 100 1000

Frequency [Hz]

loss

Tan

ge

nt


4a. dc ramp test

4b. IR test

4c. FDS permittivity ’

4d. FDS permittivity ”

Figure 4. Comparison of measured and modeled curves for new epoxy mica insulated samples during dc ramp, IR and FDS tests

0

0 .1

0 .2

0 .3

0 .4

0 .5

0 .6

0 5 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0T im e (s )

Cur

rent

(u

A)

D C R a m p M o d e l

1

1 0

0 .0 0 1 0 .0 1 0 .1 1 1 0 1 0 0 1 0 0 0F re q u e n c y (H z )

Per

mitt

ivity

(e'

)

e 'M o d e l - T h re e R e la x a tio n sM o d e l - S in g le R e la xa tio n

0 .0 1

0 .1

1

1 0

0 .0 0 1 0 .0 1 0 .1 1 1 0 1 0 0 1 0 0 0F re q u e n c y (H z )

Per

mitt

ivity

(e"

)

E ' '

M o d e l - S in g le R e la x a t io n

M o d e l - T h re e R e la x a t io n s

Figure 2. Variation of permittivity with respect to frequency for asphaltmica and epoxy mica spare windings under FDS test.

1

10

100

0.0001 0.001 0.01 0.1 1 10 100 1000

Frequency [Hz]

Pe

rmit

ivit

y


conducted with three electrode system at 200 V by using Insulation Diagnostic Analyzer (IDA200). The frequency was varied from 1 kHz to 1 mHz.

Figures 1-3 show the comparison of DC ramp and FDS results (permittivity and loss tangent) for the tested windings respectively. All the tests were conducted at room temperature of about 25 C. The DC ramp test results indicated the condition of insulation at a comparatively higher voltage level i.e. up to about 125% of the nominal voltage. In comparison, FDS results show responses of the insulating material at a low voltage level in a broader range in frequency domain. The asphalt-mica samples are usually less resistive to moisture absorption compared to epoxy mica samples. Hence asphalt-mica samples showed higher level of current in DC ramp tests. Further it showed a higher losses and permittivity values especially in the low frequency range. The DC ramp and FDS results showed a good agreement in this regards.

For further investigation, epoxy-mica (G7) in Table 1 was selected to compare correlation among the estimated parameters of the tested methods. DC ramp voltage test, IR test and FDS test were conducted on the samples. The measured curves were fitted by model curves and the parameters were estimated. The estimated

parameters were permittivity at infinity (at 1 kHz in our case), dc conductivity () and parameters describing the response function. Two different models were used for the response function: (1) Curie-Von Schweidler (CvS) model (according to equations (5) and (6)) [11]. (2) Davidson-Cole and inverse power dependence functions (according to equations (9)-(12)) [20].

Table 2. Comparison of the estimated conductivity () and dielectric response function parameters by Davidson-Cole and inverse power dependence functions under different test methods

Test Method

(pS/m)

A (x10-3)

n ∆ε β

Dc ramp 0.27 70 0.43 0.56 2.58 0.13

IR 0.28 60 0.43 0.53 2.58 0.10

FDS 0.29 60 0.41 0.50 2.63 0.15



Figure 5. Current- voltage characteristics of DC ramp test for asphaltmica winding samples before and after oven drying.

0.1

1

10

100

1000

0 3 6 9 12 15

Voltage [kV]

Cu

rren

t [u

A]

Before drying After drying

Figure 6. Real and imaginary permittivity of FDS test for asphalt micawinding samples before and after oven drying.

0.01

0.1

1

10

100

1000

10000

0.0001 0.001 0.01 0.1 1 10 100 1000

Frequency(Hz)

Per

mit

ivit

y

e' Before Drying e" Before Drying e' After Drying

e" After Drying Model

Here for FDS measurement parameters were estimated by considering single and three relaxations. The estimated parameters were Δ, τ and β for Davidson-Cole function, A and n for inverse power dependence function. It was found that the second option (Davidson-Cole and inverse power dependence) gave the better fitting. The measured and fitted curves are shown in Figure 4. The summery of the estimated results are shown in Table 2. It is clear that estimated parameters for different methods were in similar range. Out of the second option, three relaxations gave the better fitting compared to single relaxation. However, for the simplicity, single relaxation was used for further modeling work.

Since dc ramp test is conducted with two electrode arrangement, the effect of surface current is usually added to measured signal. Therefore the parameter estimation by DC ramp result gives good accuracy if the leakage component is very small i.e. for a new sample like G7. Thus the FDS measurements were mostly used in parameter estimation under different tested conditions such as drying wetting and ageing etc.

4 EFFECTS OF DRYING AND NATURAL WETTING

In Sri Lanka, all the hydro power stations are situated and operated at high humid ambient conditions where ambient temperature can vary from 20 to 40 °C and humidity can vary from 0% to 95%. The authors, through their field experience, have identified that the main cause for the insulation deterioration is due to moisture absorption by long shutdowns without generator heaters or minor water leaks from the generator coolers. In addition, accidental water penetration into the insulation due to submerging a power station has also been reported [7, 8].

The effect of moisture on the insulation is twofold due to physical and chemical mechanisms. Chemical degradation due to moisture is a slow process where ions are produced by the reaction of water with composite

material (i.e. epoxy, mica) while physical degradation is caused by the formation of water layers around the fillers. As a result of water absorption, interfacial polarization at the interfaces between mica and epoxy resin as well as the interface between glass fiber and epoxy resin occurred [5, 21]. It is very important to confirm that the stator winding is in dry condition before returning to the service after accidental ingress of water or after detection of low insulation resistance due to absorption of moisture. Hence, the laboratory experiments were conducted on selected winding samples (see Table 1) to investigate the effects moisture under drying and wetting conditions. DC ramp and FDS tests were used to evaluate the condition.

4.1 DRYING

In 1994, 47 MVA, 12.5 kV, 50 Hz generator G4 (after 14

years in service starting from 1980) was submerged in water for more than 24 hours as a result of an accident that occurred during the opening of the main inlet valve [8]. It was required to dry the stator core and winding (polyester-mica) for a period of 688 hours at 80 ˚C. Afterwards, the power station was restored and commissioned for commercial operation. Similar drying procedure was adopted in the laboratory testing.

An asphalt-mica insulated winding sample (G1 in Table 1) was dried inside an oven (2.6 m x 1.7 m x 0.8 m) for nearly three days to see the drying effect. Initially, the oven temperature was increased from 29˚C to 80 ˚C at a rate of 5 ˚C per hour and was kept at 80 ˚C for 48 hours. Finally the sample was allowed to cool naturally under closed condition of the oven for further duration of 18 hours. DC ramp, IR and FDS tests were conducted initially and at the end of the tests for comparison.

Figures 5 and 6 show the DC ramp (leakage current) test and FDS (complex permittivity) test results before and after drying. In both figures, removal of moisture from the



Figure 8. Real and imaginary capacitance of FDS test for asphalt micawinding samples before and after wetting.

0.01

0.1

1

10

0.0001 0.001 0.01 0.1 1 10 100 1000

Frequency [Hz]

Cap

acit

ance

[n

F]

C' - Initial C" - Initial

C' - After 168 hrs C" - After 168 hrs

Figure 7. Current- voltage characteristics of DC ramp test for asphalt micawinding samples before and after wetting.

0.0

0.5

1.0

1.5

0 3 6 9 12 15

Voltage [kV]

Cu

rren

t [u

A]

Before Wetting After Wetting

winding was clearly indicated by reduction of leakage current in DC ramp test and reduction of imaginary part of the capacitance in FDS test respectively. In the DC ramp test, the initial leakage current level at 15 kV was 135 A. After drying, it dropped to 3.5 A at the end of the test. In the IR one minute test conducted at 5 kV, the initial measured IR value at one minute was 174 MΩ at 5 kV. At the end of drying, the IR value increased to 8.8 GΩ. The corresponding PI values before and after drying were 1 and 4.1 respectively. The FDS also showed clear reduction of ” (losses) i.e. order of 2 times at high frequencies and order of 10 times at low frequencies (see Figure 6). However, the reduction of the permittivity i.e. at 1 kHz, was less significant as 9%.

Table 3 shows the variation of estimated parameters by Davidson-Cole and inverse power dependence functions. The fitted curves are included in Figure 6. The conductivity and parameters describing the dielectric response function indicated clearly the drying process by reducing conductivity as well as the response function parameters.

4.2 NATURAL WETTING

In 1990, another incidence was reported in Ukuwela power station (after 17 years of service from 1973) [8], where the lower part of the end windings up to the bottom core level (level of the semi conductive winding) of 20 MVA, 12.5 kV, 50 Hz generator had been exposed 2-3 times to heavy leakages from the generator cooling water lines in the generator chamber. The polyester-mica insulated spare winding (see G3 in Table 1) was tested in the laboratory for similar

moisture injection process as happened in the field. The winding was submerged only up to the lower limit of the semi conductive layer). The DC ramp and the FDS tests were conducted before and after wetting. The total period of wetting was 168 hours.

Figures 7 and 8 show the comparison of DC ramp and FDS test results for the tested polyester-mica insulated winding before and after moisture ingress. According to DC ramp test results, the LC (Leakage Current) levels under ramped DC voltage increased slightly about 10%. In comparison the FDS results gives very small variation of C’ i.e. about 5% increase at 1 kHz, but the values increased with the reduction of the frequency and at 1 mHz the increased level was about 40%. The dielectric losses showed a shifting of loss peak rather than increasing the loss magnitude. The loss peak at wetted sample was at around 4 Hz.

5 MOISTURE ESTIMATION Since moisture is one of the main causes for degradation of insulation, it is very important if the moisture content (MC) level inside the generator insulation could be estimated quantitatively. In this work, a method is proposed to estimate the MC level of the generator insulation using its correlation to FDS results i.e. loss tangent. Thus by setting up boundary for MC level, FDS measurements can be used as condition assessing tool. In this study, ten numbers of 5cm long asphalt-mica insulated winding samples were prepared from the spare winding of G1 of table 1 for MC level estimation. All samples were initially at relatively “wet” condition. Five samples were used for moisture measurements from sample weights whereas the other five samples were used for the FDS measurements. Since the weight of the copper conductor of the winding was dominant part, the weight measurements were conducted without the copper conductor. To facilitate that, the ground wall insulation was separated from the copper conductors with a cut. Conductors and the ground wall insulation were tied

Table 3. The estimated conductivity () and dielectric response function parameters by Davidson-Cole and inverse power dependence functions during drying

Treatment (pS/m)

A

n ∆ε β

Before drying 87.0 2.34 0.52 19.5 5.3 0.35

After drying 3.6 1,5 0.64 10.4 6.0 0.33



Figure 9. Permittivity (’) as a function of the frequency at moisturecontents at 70 C on asphalt mica samples.

1

10

100

0.001 0.01 0.1 1 10 100 1000Frequency (Hz)

Per

mit

tivi

ty

1.70% 1% 0.75% 0.30% 0.10% Model

Figure 10. Loss tangent as a function of the frequency at moisture contentsat 70 C on asphalt mica samples.

0.001

0.01

0.1

1

10

100

1000

0.001 0.01 0.1 1 10 100 1000

Frequency (Hz)

Los

s T

ange

nt

1.70% 1% 0.75% 0.3 0.10%

together so that it was possible to separate the insulation for weight measurements easily. The sample weight was about 30-40 g. The weight was measured by using an electronic scale of accuracy of 10 mg so that the accuracy of the MC level was about 0.25%. The samples were dried inside an oven (sized 0.6 m x 0.6 m x 0.6 m) with the copper conductors and during weight measurements the conductor was removed.

The samples were dried at a temperature of 70C, i.e. typical temperature of the particular generator winding at rated loading condition. By drying the first set of samples moisture was removed until the measured

change of the weight was negligible (dry weight) and MC levels were calculated using the dry weight. Three-electrode configuration was used for the FDS measurements on the second set of samples. The copper conductor of the sample was connected to the HV terminal whereas 1.5 cm wide aluminum foil strip was tied in the middle of the ground wall for the LV terminals. A guarding ring was formed on both ends of the ground wall in order to remove surface leakage current from measurements. The FDS measurements were conducted on them from 1 kHz to 1 mHz at 200 V.

After the samples were dried, they were kept for naturally wetting for 15 days at room temperature of 28 C and RH of 75%. The MC levels were calculated similar to drying process. Figures 9 and 10 show the frequency variation of permittivity and loss tangent during drying periods (at 70 C) respectively. The estimated moisture content values were also included in the plots. Table 4 shows the variation of estimated conductivity and parameters describing the dielectric response function under different moisture condition. The loss tangent plot shows a clear increase of losses with the increase of the MC i.e. from the MC=0.3% to MC=1.7% shows increase of about 10 times. However, the changes in the capacitance were very small and were in the same order at high frequencies i.e. at 1 kHz in our case. According to the estimated parameters, higher level conductivity and response parameters were observed at the highest MC level i.e. 1.7% whereas no significant variation could be seen at other MC level. In general, the estimated parameters do not show a good correlation with the MC level.

6 CYCLIC ELECTRIC AGEING

When the generators are in service, insulation deterioration is mainly caused by electrical stresses whereas other stresses contribute to create defects in the insulation [5]. Thus in this study, winding samples were electrically stressed to see the effects on deterioration of the insulation. In Sri Lanka, around 50% of the power generation is contributed by hydro-power stations located in the hilly part of the country. The hydro-catchments areas get rainfall mainly from south-west monsoons during January to April periods, the rainfall intensity is not very significant due to north-east monsoons so that the majority of the generators are operated with plant factors of 30-60%. Hence, the operation pattern is more or less cyclic (for the day peak and night peak) unless there is heavy rainfall. In order to maintain this realistic condition, the winding samples were aged in a cyclic behavior. Each day, the samples were stressed about 12 hours and afterwards, the samples were rested under room temperature of 25 °C and RH more than 70% (worst case scenario without the operation of generator heaters).

The conditions were so created to simulate the moisture absorption into the winding during the resting period.

Table 4. The estimated conductivity () and dielectric response function parameters by Davidson-Cole and inverse power dependence functions for different moisture contents of asphalt mica samples

Moisture content [%]

(pS/m)

A (x10-3)

n ∆ε β

1.70 150 90 1.01 3.8 1.2 0.37

1.00 4.8 32 0.85 2.2 1.2 0.30

0.75 12.9 50 1.59 2.0 2.0 0.44

0.30 22.9 20 1.54 0.7 0.9 0.40

0.10 13.7 50 185 0.4 1.4 0.24



Figure 11. Schematic diagram of the ageing set-up.

Figure 12. Variation of permittivity with respect to frequency for asphaltmica insulated winding samples under FDS test during ageing.

1

10

100

1000

0.0001 0.001 0.01 0.1 1 10 100 1000

Frequency (Hz)

Per

mit

tivi

ty (

E')

Initial 168 hrs 672 hrs 1008 hrs

1176 hrs 1344 hrs 1512 hrs Model

Figure 13. Variation of loss tangent with respect to frequency for asphaltmica insulated winding samples under FDS test during ageing.

0.01

0.1

1

10

100

0.0001 0.001 0.01 0.1 1 10 100 1000Frequency (Hz)

Lo

ss

Ta

ng

ent

Initial 168 hrs 672 hrs 1008 hrs

1176 hrs 1344 hrs 1512 hrs

Two samples of 40 cm long asphalt-mica and two samples of 40 cm epoxy-mica insulated winding were prepared from the spare windings G1 and G6 taken from Wimalasurendra and Kotmale power stations (see Table 1). The samples were prepared in order to get a three electrode configuration for FDS measurements similar to the samples used for moisture estimation by cutting from the winding sample. The exposed insulation sides were applied an anti-tracking class S varnish to avoid absorption of moisture. Figure 11 shows the schematic diagram of the ageing set up.

The asphalt-mica insulated samples were first stressed up to 1007 hours and then rested for 168 hours and afterwards aged further 332 hours. The epoxy-mica insulated samples

were first stressed up to 672 hours and rested for 336 hours and afterwards stressed further 836 hours. The atmospheric conditions were the room temperature of 25 C and RH of about 70%. The FDS measurements were conducted in every 168 hours at the room temperature. For both set of samples the applied voltages were 300% of the nominal voltage of the samples (i.e. 11 kV for asphalt-mica and 13.8 for epoxy-mica).

Figures 12 and 13 indicate the variation of permittivity and loss tangent against the frequency for asphalt-mica insulated samples. Figures 14 and 15 indicate the variation of permittivity and loss tangent against the frequency for epoxy-mica insulated samples. Tables 5 and 6 show the variation of estimated conductivity and parameters describing the dielectric response function for asphalt-mica and epoxy mica insulated winding samples under cyclic electric stress.

In general, the insulation properties deteriorated with ageing time for both asphalt-mica and epoxy-mica insulated winding samples. The ageing effects were clearly indicated by increasing losses (see Figures 13 and 15) and the permittivity (see Figures 12 and 14). Similarly, when comparing the modeled parameters, the conductivity levels also increased whereas polarization component characterized by the dielectric response function did not show any significant change (see Tables 5 and 6). When comparing the asphalt-mica and epoxy-mica insulated windings, the asphalt-mica had weaker insulation properties confirmed by having higher levels of loss tangent, permittivity, conductivity and the dielectric response function parameters compared to epoxy-mica insulated windings. For asphalt-mica insulated windings the loss tangent values varied between 2 to 5 times than those of epoxy-mica for high frequencies to low frequencies. The permittivity values also showed an order of 5 times for asphalt-mica windings. When comparing the modeled parameters, the estimated conductivity showed significant level for asphalt-mica by having a level order of 10 times than the epoxy-mica windings (see Tables 5 and 6). The other parameters did not show a clear trend. The results

Table 5. The estimated conductivity () and dielectric response function parameters by Davidson-Cole and inverse power dependence functions for asphalt-mica samples during ageing.

Ageing time

[hour]

(pS/m)

A

(x10-3) n ∆ε β

168 2.99 310 0.69 36 42.6 0.21

332 2.27 240 0.84 33 36.6 0.22

504 0.06 540 0.64 31 13.0 0.34

840 13.0 601 0.70 30 10.3 0.22

1007 14.9 333 0.76 28 8.7 0.25

1176* 53.0 400 0.90 31 13.6 0.22

1344 14.0 223 0.76 48 11.7 0.31

1512 50.0 78 1.10 3 4.9 0.29

* includes 168 hours of resting



Figure 14. Variation of permittivity with respect to frequency for epoxy mica insulated winding samples under FDS test during ageing.

1

10

100

0.001 0.01 0.1 1 10 100 1000

Frequency (Hz)

Per

mit

tivi

tyInitial 168 hrs 672 hrs 1008 hrs

1176 hrs 1344 hrs 1848 hrs Model

Figure 15. Variation of loss tangent with respect to frequency for epoxy micainsulated winding samples under FDS test during ageing.

0.001

0.01

0.1

1

10

100

0.001 0.01 0.1 1 10 100 1000

Frequency (Hz)

Lo

ss T

ang

en

t

Initial 168 hrs 672 hrs

1008 hrs 1176 hrs 1344 hrs

1848 hrs

were in the similar order as previous similar investigations on epoxy mica [5] and epoxy/asphalt mica [11] samples.

According to asphalt-mica insulated samples, the loss tangent values first reduced and then increased with ageing time showing the initial drying and then ageing process (see Figure 13.). The permittivity values were also initially reduced and then increased (see Figure 12) confirming the similar results as the loss tangent. The estimated response function parameters also showed similar tendency. However, the estimated conductivity values did not show a significant variation during the initial ageing periods (see Table 5) and a significant change with resting after 1000 hours. After resting, losses and permittivity values increased showing moisture absorption to the winding under high humid condition. The estimated conductivity and response function also clearly showed similar pattern to loss tangent and permittivity. Extended ageing after resting showed a significant variation in the parameters.

Compared to asphalt-mica, epoxy-mica windings were relatively dry. The losses increased with ageing time (see Figure 15). Similarly permittivity values were also increased with ageing (see Figure 14). The estimated parameters also fallowed similar behavior (see Table 6). The resting time also had a similar effect as asphalt-mica i.e. moisture absorption during resting. However, further

ageing was not clearly highlighted for epoxy-mica winding samples compared to asphalt-mica samples. According [5], the dielectric response parameters do not show a clear variation when epoxy samples were stressed electrically. Our study also confirmed those results under cyclic ageing.

7 CONCLUSIONS

Three diagnostic tests DC ramp, insulation resistance (IR) and frequency domain spectroscopy (FDS) measurements are proposed in this paper to assess the condition of generator insulation. Results of the study are concluded as follows:

1. When the insulation is new, estimated conductivity and response function parameters are in similar order. Estimated parameters of wet or aged samples show differences in DC ramp/IR tests which are due to the difficulty in separating the leakage current component.

2. Absorbed moisture in generator insulation can be removed by oven-drying. The drying effect is clearly indicated in both DC ramp and FDS results by reduction of current and complex permittivity. Estimated conductivity values also show a clear reduction.

3. Apart from being in contact with humid environment, moisture can also be absorbed to generator insulation due to partial or full submergence in water. However, the wetting effect due to such condition of polyester-mica insulation is clearly indicated by FDS results with a loss peak and increase permittivity values. Neither DC ramp test nor estimated parameters show a significant change.

4. Moisture content (MC) of generator stator insulation can be roughly estimated as a percentage of weight using the fact that increase of MC levels correlates with the increase of loss tangent in FDS measurements.

5. Cyclic electrical ageing shows combined effects of moisture and electric stress on insulation. During the period of electrical ageing, drying effect of insulation is followed by the ageing effect. During resting period of the cycle, absorption of moisture to the insulation can be

Table 6. The estimated conductivity () and dielectric response function parameters by Davidson-Cole and inverse power dependence for epoxy mica samples during ageing.

Ageing time [hour]

(pS/m)

A (x10-3)

n ∆ε β

168 0.40 0.01 2.19 1.2 9.0 0.16

332 0.30 2.3 1.2 2.0 0.8 0.23

504 0.30 28.3 0.43 2.5 24.8 0.22

672 8.00 7.3 1.18 2.6 18.2 0.22

1008* 10.50 1.0 1.49 5.4 1.4 0.31

1176 9.04 220 1.26 2.9 2.7 0.27

1340 3.29 2.0 1.75 1.6 3.9 0.21

1508 2.85 2.7 1.47 3.4 1.5 0.21

1844 6.16 5.3 1.3 1.5 0.9 0.26

* includes 336 hours of resting



observed. Accordingly, the loss tangent values reduce during drying and increase during ageing and moisture absorption. However, variation of the estimated conductivity and response function parameters are not very significant.

6. The FDS results can be effectively fitted by Davidson- Cole and inverse power dependence functions compared to Curie-Von Schweidler model. The modeling by Davidson-Cole function with more relaxations give better fitting compared to single relaxation.

As far as drying and ageing of generator stator winding insulation are concerned, it can be concluded that FDS measurements and estimated parameters provide more conclusive information than DC ramp or IR test results.

ACKNOWLEDGMENT The authors would like to express their deep gratitude to

the Department of Electrical and Electronic Engineering, University of Peradeniya and Ceylon Electricity Board for providing assistance to conduct the tests.

REFERENCES [1] R. Brutsch, M. Tari, K. Frohlich, T. Weiers, and R. Vogelsang,

“Insulation failure mechanisms of power generators”, IEEE Electr. Insul. Magazine, Vol. 24, No. 4, pp. 17-25, 2008.

[2] CIGRE Study Committee SC11, EG11.02, “Hydro-generator Failures– Results of the Survey,” 2003.

[3] T.P. Hong, O. Lesaint, and P. Gonon, “Water Absorption in a Glass-M ica- Epoxy Composite I: Influence on Electrical properties”, IEEE Trans. Dielectr. Electr. Insul, Vol. 16, pp. 1-10, 2009.

[4] G.G. Stone, E.A. Boulter, I. Culbert, and H. Dhirani, Electrical Insulation for Rotating Machines-Design, Evaluation, Ageing, Testing and Repair, IEEE Press, ISBN 0-471-44506-1, 2004

[5] M. Farahani, H. Borsi, and E. Gockenbach, “Dielectric Response Studies on Insulating Systems of Rotating Machines”, IEEE Trans. Dielectr. Electr. Insul, Vol. 13, pp. 383-393, 2006.

[6] M. Farahani, H. Borsi, E. Gockenbach, K. Schafer, and M. Kaufhold, “Behavior of Machine Insulation Systems Subjected to Accelerated Thermal Ageing Test”, IEEE Trans. Dielectr. Electr. Insul, Vol. 17, pp. 1364-1372, 2010.

[7] G.A. Jayantha, M.A.R.M. Fernando, and C.M.B. Ekanayake, “Monitoring of Moisture on Stator Winding Insulation During Drying”, 4th IEEE Int’l. Conf. Industrial and Information Systems (ICIIS), Kandy Sri Lanka, Paper No. PSEHV3-4, 2009.

[8] G.A. Jayantha and M.A.R.M. Fernando, “Field Experience of Generator Stator Insulation Monitoring and Failures in Sri Lanka”, 7th IEEE Int. Conf. Industrial and Information Systems (ICIIS2012), Madras India, Paper No. 171, Aug. 2012.

[9] IEEE Std 95-2002, “IEEE recommended practice for testing of ac machinery (2300 V and above) with high direct voltage”, 2002.

[10] W. Mcdermid, and J.C. Bromley, “The Ramp Test - Its Origins Development and Application”, IEEE Trans. Dielectr. Electr. Insul, Vol. 17, pp. 1470-1478, 2010.

[11] E. David, T. Godin, J. Bellemare, and L. Lamarre, “Modeling of the Dielectric Response of a Stator Winding Insulation from a DC Ramp Test”, IEEE Trans. Dielectr. Electr. Insul, Vol. 14, pp. 1548-1558, 2007.

[12] E. David, R. Soltani, and L. Lamarre, “PDC Measurements to Assess Machine Insulation” IEEE Trans. Dielectr. Electr. Insul, Vol. 17, pp. 1364-1372, 2010.

[13] S.A. Brumiwatt, “On-site Non-destructive Dielectric Response Diagnosis of Rotating Machines”, IEEE Trans. Dielectr. Electr. Insul, Vol. 17, pp. 1453-1460, 2010.

[14] K. Younsi, P. Neti, M. Shah, J.Y. Zhou, J. Krahn, K. Weeber, and C.D. Whitefield, “On-line Capacitance and Dissipation Factor Monitoring of Stator Insulation”, IEEE Trans. Dielectr. Electr. Insul, Vol. 17, pp. 1441- 1452, 2010.

[15] J.K. Nelson and J. Stein, “A Field Assessment of PD and EMI Methodology Applied to Large Utility Generators”, IEEE Trans. Dielectr. Electr. Insul, Vol. 17, pp. 1411-1427, 2010.

[16] M. Levesque, E David, C. Hudon, and M Belec, “Effect of Surface Degradation on Slot Partial Discharge Activity”, IEEE Trans. Dielectr. Electr. Insul, Vol. 17, pp. 1428-1440, 2010.

[17] N. Taylor and H. Edin, “Stator End-winding Currents in Frequency-domain Dielectric Response Measurements”, IEEE Trans. Dielectr. Electr. Insul, Vol. 17, pp. 1489- 1498, 2010.

[18] A.K. Jonscher, Dielectric Relaxation in Solids, Chelsea Dielectrics Press, London UK, ISBN 0-9508711-0-9, 1983

[19] A. Helgason and U. Gafvert, “Dielectric Response Measurements in Time and Frequency Domain on High Voltage Insulation with Different Response”, Int’l. Sympos. Electr. Insul. Materials, Toyohashi, Japan, pp. 393-398, 1998.

[20] C.M.B. Ekanayake, Diagnosis of Moisture in Transformer Insulation – Application of Frequency Domain Spectroscopy, Ph.D. Thesis, Chalmers University of Technology, Sweden 2006.

[21] R. Soltani, E. David, L. Lamarre, “Impact of Humidity on Dielectric Response of Rotating Machine Insulation System”, IEEE Trans. Dielectr. Electr. Insul, Vol. 17, pp. 1479-1488, 2010.

M. A. R. M. Fernando (M’07-SM’10) was born in Colombo, Sri Lanka in 1966. He received the B.Sc. Eng. degree from the University of Peradeniya, Sri Lanka in 1993, the Tech. Lic., degree from the Royal Institute of Technology, Stockholm Sweden in 1997 and the Ph.D. degree from the Chalmers University, Gothenburg, Sweden in 1999. At

present, he is a Professor in the University of Peradeniya. He is a chartered Engineer and an International Professional Engineer. He was the founder chair of IEEE Sri Lanka Power and Energy Society Chapter in 2010, the general chair of IEEE fourth International Conference on Industrial and Information Systems in 2009 and the chair of IEEE Sri Lanka central region subsection in 2009/2010. His research interests include condition monitoring, alternative insulation, problems related to outdoor insulation.

W. M. L. B. Naranpanawa and R. M. H. M. Rathnayake are instructors of Department of Electrical and Electronic Engineering, University of Peradeniya, Sri Lanka

G. A. Jayantha was born in Matara, Sri Lanka in 1959. He received the B.Sc. Eng. degree from the University of Peradeniya, Sri Lanka in 1983. He started his career as the engineer in-charge of Udawalawe power station Sri Lanka and later worked as the senior electrical engineer of Samanalawewa Hydro Power project (120 MW) and

Kukuleganga Hydro power project (70 MW). During 1993-2004 he worked as the senior Electrical engineer of Mahaweli Hydro Power Complex in Sri Lanka. During 2004 -2011 he worked as the Chief engineer in-charge of condition monitoring of generators, transformers and HV circuit breakers of all the hydro power stations in Sri Lanka. From 1993 he was involved in fault diagnosing, major repairs and overhaul work of hydro power stations in Sri Lanka. He has served in many technical committees of hydro power projects in Sri Lanka. From 2011 he has been working as the Deputy General Manager of Generation projects where he is in-charge of refurbishment and modernization of hydro power stations in Sri Lanka. He has more than 28 years of service as a hydro power engineer. He is a qualified charted engineer. Condition monitoring and fault diagnosing of generators and transformers are his main research interest.


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