Aging assesment of transformer pressboard insulation by

Aging Assesment of Transformer Pressboard Insulation by Micro-strip Ring Resonator at GHz Frequencies

W.M.S.C Samarasinghe, J.R.S.S Kumara, M.A.R.M Fernando

High Voltage Research Group

and Aruna U.A.W. Gunawardena Microwave Research Group

Department of Electrical and Electronic Engineering,University of Peradeniya, Peradeniya 20400,

Sri Lanka

ABSTRACT Condition assessment of insulation is widely used to identify the state of aging of power transformers. Traditionally, condition assessment is done using dielectric response measurements conducted at low frequencies. However, due to long testing time required for these methods, attention has now been paid towards measurements at microwave (above 1 GHz) frequencies. In this paper, we establish a correlation between dielectric properties and the state of aging of transformer pressboard samples. This is useful as it paves way to develop techniques to assess the state of aging by measuring dielectric properties at high frequencies. Dielectric measurements are made by using a microstrip ring resonator. The state of aging is determined by measuring the tensile index. Tests were carried out on dry and wet oil-impregnated pressboard samples thermally aged at laboratory conditions for durations up to 45 days. The results were compared with field aged samples collected from a 33 kV sealed type distribution transformer of 18 years in service. It was found that the permittivity values had a good correlation with respective tensile index values whereas loss tangent values showed limited correlation for both dry and wet samples. The results of field aged samples matched well with the established correlation curves. It can be concluded that the permittivity of pressboard samples estimated from the ring resonator can be effectively used to evaluate the state of aging of transformer insulation.

Index Terms - Aging, condition assessment, transformer pressboard, ring resonator, permittivity, loss tangent, high frequencies, microwave.

1 INTRODUCTION

POWER transformer is one of the most crucial and expensive equipment in electric power systems. The life time of a power transformers mostly depends on the condition of insulation [1, 2]. In transformers, there are two types of insulation: liquid (mineral oil) and solid (pressboard and paper). The transformers with solid insulation are dried during the production process to reduce the moisture level to a value less than 0.5%. However, during the operation of the transformers, the solid insulation is subjected to various stresses like thermal, electrical, mechanical, and chemical [3, 4]. As a result, their insulation ages resulting decomposition of the cellulose material so that the excess moisture and other by-products are usually formed. In addition, the moisture content inside the transformer can increase due to leakages in the tank.

Long-term aging may even result in transformer failure, severely affecting the power system reliability. As a result, aging assessment of power transformers has become an important research topic [1-8].

The aging condition of transformer insulation can be assessed by traditional methods such as dielectric response measurements, loss tangent (tanδ) measurement at power frequency, and insulation resistance [9] measurement. Dielectric response measurement is the most common among the above methods. In dielectric response measurement polarization /depolarization currents in the time domain (TD) and/or the complex permittivity values in the frequency domain (FD) are used to evaluate the state of aging of insulation [7-9]. However, the long testing time required to observe the complete dielectric spectrum is the main drawback of these methods. For example, for depolarization measurements in TD, insulation should be initially charged at least for 10,000 s. On the other hand, in FD, it usually takes

Manuscript received on 11 September 2016, in final form 14 February 2017, accepted 14 February 2017. Corresponding author: J.R.S.S. Kumara.

DOI: 10.1109/TDEI.2017.006322

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 24, No. 3; June 2017 1923

Authorized licensed use limited to: Ramesh Fernando. Downloaded on June 15,2020 at 05:36:48 UTC from IEEE Xplore. Restrictions apply.

around 12 hrs to measure the complex permittivity from 0.1 MHz to 1 kHz [7]. However, long testing time results in long interruptions and variations in insulation temperature within the measurement. The temperature varies because the insulation will no longer be in an equilibrium state once the transformer is de-energised. The attention is now paid towards the response measurements conducted at high frequencies to overcome these issues. Such investigations conducted at THz frequencies are reported in [10].

Our previous work [11], presented a method to estimate the moisture content in transformer pressboard insulation using a ring resonator at GHz microwave frequencies. In this paper, the same ring resonator method is used to estimate the state of aging of transformer pressboard samples. Dry and wet oil- impregnated pressboard samples were thermally aged in the laboratory and dielectric properties (loss tangent and permittivity) were estimated using the ring resonator method. The state of aging was estimated by measuring the tensile strength of the samples. In addition to the laboratory aged samples, field aged pressboard samples collected from an 18 years old sealed type distribution transformer (33/0.4 kV) were also tested. Finally, the results were analysed to establish a correlation between the state of aging and the dielectric properties estimated at high frequencies.

2 METHODOLOGY

2.1 ESTIMATION OF DIELECTRIC PROPERTIES BY RING RESONATOR METHOD

A detailed description of the ring resonator circuit and the method of estimation of dielectric properties can be found in our previous publication [11]. However, key features are summarized here for easy reading. The test sample (one layer of pressboard sample) is mounted on a ring resonator circuit fabricated on NX9320 printed circuit board (PCB) substrate manufactured by Neltec. This is shown in Figures 1a and 1b. The scattering parameter S21 (insertion loss) is then observed using a vector network analyser (VNA). When the pressboard sample is mounted on the resonant circuit, the microwave signal propagates through both the PCB substrate and the test sample. Therefore, the frequency response of S21 depends on the effective relative permittivity value of the pressboard - PCB substrate composite. The relation shown in equation (1) was used to estimate effective relative permittivity, εreff using observed resonant frequency fres.

reff

resr

nCf

2 (1)

where n is the harmonic number which is an integer (n =1, 2, 3...), C is the velocity of the light in vacuum, and r is the mean radius of the ring [11, 12]. By using equation (2), the relative permittivity of the pressboard sample (εrPB) can be found since the permittivity of the substrate (εrSUB) is known. In equation (2), q1 and q2 are filling factors which depend on the physical dimensions of the ring resonator circuit [11, 12].

221

21

1 )1(

)1(

qqq

qq

rPBrPBrSUBreff

(2)

Equation (3) was used to estimate the loss tangent of the pressboard sample, tan δPB. Parameters PSUB,1, PSUB,2 and PPB,2 are constants which can be found directly as explained in [11,12]. Loss tangent of the PCB substrate, tan δSUB is a known value. Parameters QU,1 and QU,2 are the unloaded Q factors with and without the test sample (pressboard) respectively.

SUBPB

SUBSUB

uuPBPB P

PP

QQP tan

111tan

2,

2,1,

122,

(3)

The ring resonator used for this study has a fundamental frequency of 1 GHz. The inner and outer radii of the ring resonator were 25 mm and 26.85 mm respectively. The PCB substrate used to fabricate the resonator has a relative permittivity of 3.2 and loss tangent of 0.0024 at 10 GHz. The ring resonator design was validated by finite element simulations. More detail about the ring resonator can be found in our previous publication [11].

As presented in [11], the proposed method was validated by comparing the permittivity values of two known PCB substrate material samples: NX9320 (εr= 3.2±0.4 at 10GHz) and FR4 (εr= 4.3±0.2 at 10 GHz). The permittivity values

(a)

(b)

Figure 1. Ring resonator test set-up: (a) Schematic diagram, (b) Photograph.

1924 W. M. S. C. Samarasinghe et al.: Aging Assesment of Transformer Pressboard Insulation by Micro-strip Ring Resonator


estimated at 8.28 GHz are well within the range specified by the manufacturers data sheets at 10 GHz.

2.2 ESTIMATION OF AGING STATE BY TENSILE STRENGTH MEASUREMENTS

During the aging process cellulose polymer gets decomposed or de-polymerized. Accordingly, the average length of the polymer chains may reduce during aging. Degree of polymerization (DP) and/or tensile strengths are widely used to estimate aging state of the transformer pressboard samples [13]. When the pressboard sample is extensively aged, those properties may reduce by a factor of 50% [13, 14]. In our work, tensile strength was used to evaluate the aging state of the pressboard samples. Rectangular pieces of 8 cm x 1 cm (length x width) pressboard samples were tested according to BS-EN-ISO1924 [13]. Maximum tensile strength was then measured using the Houns Field Tensometer. In order to have a better accuracy this measurement was repeated three times and the average value was considered. In order to compare samples with different thicknesses, TI was also estimated with an uncertainty of ± 3.4 % (see equation (4)).

)/( gNmgW

F

g

TSTI (4)

Where g is the grammage g/m2, F is the mean maximum tensile force in Newtons and W is initial width of the sample in meters. The measurement rate of elongation was selected as 10 mm/min and the test was carried out at laboratory conditions of 28°C temperature and 81% relative humidity.

3 LABORATORY AGED SAMPLES This section presents the application of the proposed

method for the pressboard samples aged in laboratory.

3.1 SAMPLE AGEING

Five number of 11 cm x 11 cm, Class A type, 2 mm thick pressboard samples were used for laboratory aging. Two different sets of samples (dry and wet) were prepared. For dry samples, pressboards were kept inside a conventional air circulating oven (sized 74 cm x 67 cm x 90 cm, volume of 115 litres) at 105 °C for 24 hours. Simultaneously, virgin mineral oil (manufactured by IOC, density 0.8405 g/ml and loss tangent 0.00022) was also treated using the same method. Dried pressboard sample sets were then kept in a vacuum oven at 1-1.5 kPa and 30 °C for 24 hours for further moisture evaporation. Afterwards, the samples were impregnated in treated mineral oil. The wet samples were impregnated directly (without drying in the oven). Both pressboard samples were aged inside the air circulating oven at 130 °C up to 45 days. In each day, the samples were heated only for 8 hours and allowed to rest during remaining 16 hours. Heating and resting procedure was adopted to simulate heating power transformers during night peak and resting during day times (day time power demand is about 50 % lower than night peak) [15].

3.2 MEASUREMENTS

The dielectric properties of the samples were obtained by the ring resonator method at room temperature of 28 ºC and a RH of 81 %. The measurements were conducted at the beginning of the aging process (at 0 days) and at three different aging stages. Namely, after 15, 30, and 45 days of aging. In order to obtain the state of aging, the tensile strength measurements were also conducted for each of these cases.

Figures 2 and 3 show the insertion loss response of the ring resonator at 0, 15, 30 and 45 days of aging for dry and wet samples respectively. The resonance peaks of the insertion loss response appear at the fundamental as well as at harmonic frequencies. Accordingly, the figures show the loss responses of second to fifth harmonics i.e. 1.5 to 6 GHz. Here, the fundamental component was neglected because its response was quite abnormal compared to others. The harmonics are separated by 1 GHz, which is the designed fundamental frequency of the ring resonator. As described in [11], the relative permittivity of the test sample can be extracted from the resonant frequency of S21 response. The loss tangent can be obtained from the 3 dB bandwidth and peak amplitude of the same response. Figures illustrate that the resonant frequencies shifted to left side when the samples are aged. The same effect can be obtained in all second to fifth harmonic frequencies for both dry and wet samples. Therefore, a clear increase of the effective permittivity could be noted with the aging time (see equation 1). Thus the increases of the effective permittivity values were directly reflected by the relative permittivity of the pressboard samples since the permittivity of the PCB substrate material was constant. On the other hand, the -3 dB values do not show a significant variation with

Figure 2. Variation of insertion loss responses of dry pressboard samples with respect to aging time.

Figure 3. Variation of insertion loss responses of wet pressboard samples with respect to aging time.



respect to aging time. Thus the loss tangent could not be expected to vary significantly with aging. A detailed analysis is presented in Section 3.3.

3.3 RESULTS

The frequencies, -3 dB bandwidths, and insertion losses at resonance were obtained from the responses in Figures 2 and 3. The relative permittivity and the loss tangent values were calculated based on the theory described in section (2.1). Figures 4 and 5 illustrate the variation of relative permittivity with the harmonic frequency for dry and wet samples respectively. In general, the permittivity values reduce with the harmonic frequencies. Thus, rather than a single permittivity value, we consider four different values, namely the permittivities at the second and the fifth harmonic frequencies, the ratio of permittivites at second to fifth harmonic, and the average. Tables 1 and 2 show the summary of the dielectric properties with respect to aging times for dry and wet samples respectively.

It can be observed that for dry pressboard samples, the resonant frequency had reduced with the aging time. Accordingly, the calculated permittivity values showed an increasing trend. On the other hand, the -3 dB bandwidth of the insertion loss of the second harmonic frequency had increased with the aging time. However, in case of loss tangent there is no clear variation with aging time. Similar trend could be observed for wet samples.

Figure 4 and 5 show the variation of estimated relative permittivity with the frequency for dry and wet samples, respectively, aged over different durations. It can be observed that initial permittivity values (at 0 days) are 1.2 and 2.1 for dry and wet samples respectively. High permittivity value in the wet sample is due to ingress moisture ingress in the sample. After 45 days of aging, these values increased to 8.5 and 8.7 for dry and wet samples respectively. When samples were aged at 130ºC, moisture and other by-products may form and results higher permittivity values. In contrast to the permittivity variations, the loss tangent values did not show significant changes. The reason for such observations was unclear and a problem topic for further investigations. Therefore, in the proposed method, attention was drawn to permittivity variations.

According to Figures 4 and 5, the permittivity values of

second harmonic frequency rapidly increased with the aging time. In comparison, the increase of permittivity at fifth harmonic frequency was limited. It is also interesting to note that, when samples were non-aged or lightly aged (i.e. 0 and 15 aging days) the permittivity values did not significantly change with the harmonic frequency. However, when samples were heavily aged at 30 and 45 days, permittivity significantly varied with the frequency. This non-linear nature can be seen by the ratio of ϵr at F2 to ϵr at F5 given in Tables 1 and 2. For example, when samples were non-aged or lightly aged, the ratio was approximately 1 whereas the values were in the range from 2.6 to 2.8 when samples were aged at 30 and 45 days. Moreover, the average values of the estimated permittivity had increased with the aging time (see Tables 1 and 2). The above mentioned characteristics were similar for both dry and wet pressboard samples. Considering all these observations, it is concluded that the average permittivity and the ratio of εr at F2 to εr at F5 could be effectively used for analysing the aging condition.

Table 1. Variation of the dielectric properties of the dry pressboard samples with respect to aging time.

Aging time / Days 0 15 30 45

F2 (2nd harmonic

resonant freq./ GHz

2.26 2.14 1.78 1.71

3dB bandwidth of the response at F2 / GHz

0.018 0.051 0.031 0.053

Insertion loss at F2 / dB

-28.3 -32 -30.7 -33.1

tanδ at F2 0.27 1.17 0.37 0.7

r at F2 / GHz 1.2 2.3 7.2 8.5

r at F5/ GHz 1.26 2.2 2.59 3.33

r average/ GHz 1.23 2.25 4.28 5.33

r at F2/ r at F5 1 1.1 2.8 2.6

Figure 5. Variation of estimated relative permittivity of wet pressboardsamples for second to fifth harmonics at different ageing times.

Figure 4. Variation of estimated relative permittivity of dry pressboardsamples for second to fifth harmonics at different aging times.



Figure 6. Variation of tensile index (TI) with aging time of dry and wetpressboard samples.

Figure 7. Damaged 100 kVA, 33/0.4 kV field aged sealed distribution.

Table 2. Variation of the dielectric properties of the wet pressboard samples with respect to aging time.

Aging time / Days 0 15 30 45

F2 (2nd harmonic

resonant frequency)/ GHz

2.16

2.13 1.76 1.70

3dB bandwidth of the response at F2 / GHz

0.049

0.076 0.04 0.059

Insertion loss at F2 / dB

-33.5

-35.1 -31.9 -33.8

tanδ at F2 1.92 1.86 0.51 0.79

r at F2 / GHz 2.1

2.5 7.6 8.7

r at F5/ GHz 2.1 2.5 2.8 3.4

r average/ GHz 2.1

2.4 4.6 5.5

r at F2/ r at

F5 1

1 2.7 2.6

Figure 6 shows the variation of TI with respect to the aging time. Both dry and wet samples were included in the figure for comparison. In both dry and wet samples, a clear decreasing trend could be observed with the aging time. Initial TI values of the dry and wet samples were 114 Nm/g and 93 Nm/g respectively. These values are comparable with the values reported in [16] for oil impregnated paper samples. Note that, as expected, lower TI value could be noted in wet samples compared to dry ones. After 45 days, the respective TI values of dry and wet samples were 45.7 and 41 respectively. The values were in the same range confirming that the samples were aged to a similar level irrespectively of the initial moisture contents. When tensile strength reduces to 50 % from its initial values, the sample is considered as significantly aged [14]. Therefore in this study aging duration was limited to 45 days.

4 FIELD AGED SAMPLES This section presents the application of the proposed

method for the pressboard samples aged in field.

4.1 SAMPLE COLLECTION

In order to verify the proposed method, field aged samples were collected from a 100 kVA, 33/0.4 kV, 50 Hz, sealed type three phase distribution transformer. This transformer had been in service for 18 years and it was damaged on 16th March 2016 due to a lightning strikes came through the 33 kV side. One phase of the transformer and the corresponding 33 kV surge arrestor in the primary side were also damaged. Bottom part of the transformer tank had been blasted and as a result the mineral oil had drained from the transformer. Figure 7 shows a photograph of the damaged transformer.

The transformer was brought to the high voltage laboratory of the University of Peradeniya on 17th March 2016. Part of transformer tank casing was removed and the damaged transformer was carefully inspected. It was found that in phase c, the windings were damaged whereas there were no visual damages to the other phases. The main insulation between the low voltage (LV) and high voltage (HV) windings were corrugated pressboard material so that they were not used for the investigation. Different sets of flat pressboard samples were found vertically installed between phases. Nine number of 11 cm x 11 cm size samples were collected from locations between phases a and b.

4.2 MEASUREMENTS

The samples of the field aged transformer were tested using a micro-strip ring resonator. The permittivity and loss tangent values of the samples were estimated. The tensile tests were also conducted on these nine samples (1 cm x 8 cm pieces) to assess the state of aging.

Figure 8 illustrates the insertion loss responses of the nine samples for the second to fifth harmonic frequencies. When compared with laboratory aged samples (Figures 2 and 3), the insertion loss responses were similar in shape. It can be observed that the resonant peak values and -3 dB bandwidth values (especially the second harmonic frequency) were at



Figure 9. Variation of estimated average relative permittivity of dry, wetlaboratory samples and field aged samples with respect to TI.

1

2

3

4

5

6

406080100120

εraverage

TI / Nmg‐1

εr_avg dry (Lab aged)

εr_avg wet (Lab aged)

εr_avg (Field Aged)

similar levels. We assumed that the pressboard samples were aged in a similar manner irrespectively of their location inside the transformer.

4.3 RESULTS

Table 3 shows the estimated permittivity values of the nine samples. The results included (a) relative permittivity at second harmonic frequency (F2), (b) relative permittivity at the fifth harmonic frequency, (c) ratio of relative permittivity at second harmonic to fifth harmonic frequency and (d) the average permittivity values.

Table 3. Permittivity values of the pressboard samples collected from the field-aged transformer.

Sample Number r at F2 r F5 r at F2/

r at F5 r

average

S1 2.48 2.40 1.03 2.40 S2 1.92 1.86 1.03 1.88 S3 2.25 2.08 1.08 2.13 S4 2.36 2.22 1.07 2.25 S5 2.24 2.08 1.08 2.13 S6 1.60 1.57 1.02 1.56 S7 2.02 1.95 1.04 1.98 S8 1.92 1.91 1.01 1.91 S9 2.36 2.17 1.09 2.22

Average 2.13 2.03 1.05 2.05

According to the results, the relative permittivity values of

each sample at the second harmonic frequency and the fifth harmonic frequency were close. As a result, the ratio of εr at F2 / εr at F5 was around 1 and the average values were also closer to values at F2 and F5. Calculations showed that the nine samples have an average TI of 70.9 Nm/g with a standard deviation of 8.95. The initial TI values of the field aged samples were not known. However, according to typical TI values in Figure 6, the samples were equivalent to 15 days of laboratory aging.

5 CORRELATION BETWEEN DIELECTRIC PROPERTIES AND AGING STATE

This section aims at establishing a correlation between

dielectric properties estimated from ring resonator method and the aging state estimated from TI. Figures 9-11 show the permittivity (average, F2 and ratio F2/F5) variation with respect to TI values for Laboratory aged dry samples and wet samples. Field aged samples are also plotted on same figures.

As shown in Figures 9 and 10, the average permittivity increases when the sample is aged. The increase is significant at lower TI values. The results of field aged samples matched well with those of laboratory aged samples. The same results were reflected in permittivity ratio between F2 and F5. When samples were non-aged or lightly aged, the ratio was around 1 whereas the values increased to around 3 when the samples were aged. Again the field aged samples matched closely with the laboratory aged ones. The proposed method clearly demonstrates a correlation between dielectric relative permittivity and TI. All three parameters: average permittivity value, permittivity at F2 and the ratio between ϵr at F2 to ϵr at F5 frequencies provide useful information for estimation of state of aging.

Properties of the field aged samples are well agreeing with the relative permittivity to TI correlations obtained for lab aged samples (see Figures 9, 10 and 11). According to the results it is reasonable to conclude that the tested transformer had not aged severely, i.e. up to 50%. According to above

Figure 10. Variation of estimated relative permittivity at second harmonic ofdry, wet laboratory samples and field aged samples with respect to TI.

1

3

5

7

9

406080100120

εr atF2

TI / Nmg‐1

εr at F2 dry (Lab aged)

εr at F2 wet (Lab aged)

εr at F2 (Field aged)

Figure 8. Variation of 2nd to 5th resonant peaks of nine field aged pressboardsamples.



Figure 11. Variation of ratio of estimated relative permittivity at secondharmonic to fifth harmonic frequency of dry, wet laboratory samples and fieldaged samples with TI %.

0.5

1

1.5

2

2.5

3

406080100120

εratF2/ε

r atF5

TI / Nmg‐1

εr at F2 / εr at F5 dry (Lab aged)

εr at F2 / εr at F5 wet (Lab aged)

εr at F2 / εr at F5 (Field aged)

observations, it is clear that proposed micro-strip ring resonator method can be used to evaluate the state of aging of transformer solid insulations.

6 CONCLUSIONS The insertion loss response of a multilayer micro-strip ring

resonator was used to estimate relative permittivity and loss tangent values of a pressboard sample, which is used as the upper layer of the ring structure. The ring resonator was designed to resonate at a frequency of 1 GHz and the dielectric properties were evaluated at 2nd to 5th harmonics.

Virgin wet and dry pressboard samples were thermally aged at the laboratory condition for a duration of 15, 30 and 45 days. The differences in the state of aging due to different aging times were verified by measuring the tensile index of samples. With the particular heating and resting procedure followed in this study, 45 days of aging resulted in a 50% reduction of tensile index from its initial value.

The estimated relative permittivity values increased with the aging time for both dry and wet samples. After 45 days of aging, the estimated relative permittivity values have increased at least by a factor of 4 compared to its initial values. Furthermore, it was observed that the average relative permittivity value and the ratio of εr at 2nd harmonic to the εr at 5th harmonic could be effectively used for estimating the state of aging. However, the variations of loss tangent values were not well correlated with the aging duration. Both dry and wet samples showed similar characteristics.

Correlations established between the relative permittivity and the tensile index showed that relative permittivity increases while tensile index is decreasing. When the sample was considerably aged (i.e. tensile index is halved) a rapid increase of permittivity could be observed. This confirms the possibility of estimating state of aging using permittivity values. The results of field aged samples matched well with the established correlations between relative permittivity and tensile index.

After considering all the aspects, it can be concluded that the ring resonator method can be used to assess the state of aging of a power transformer insulation by estimating relative

permittivity values at GHz frequencies. In future this method can be used for condition monitoring of transformers in service, by installing several ring resonators inside the transformer at the manufacturing stage and providing provision for external measurements.

ACKNOWLEDGMENT The authors would like express their deep gratitude to

Lanka Transformers PLC and Ceylon Electricity Board for providing transformer oil, pressboard samples and field aged pressboard samples.

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W.M.S.C. Samarasinghe (S’15) was born in Peradeniya, Sri Lanka in 1990. He received the B.Sc. Eng. degree from the University of Peradeniya, Sri Lanka in 2014. At present, he is an instructor at the Department of Electrical and Electronic Engineering, University of Peradeniya. He is an associate member of the Institution of Engineers, Sri Lanka. His research interests include condition monitoring, Insulation materials, Dielectric aging and microwave engineering.

J.R.S.S. Kumara (M’13-SM’16) was born in Ratnapura, Sri Lanka in 1979. He received the B.Sc. Eng. (Hons) degree in electrical and electronic engineering in 2004 and M.Phil. degree in 2007, both from the University of Peradeniya (UOP) Sri Lanka. He obtained his Tec. Lic and Ph.D. degree in 2009 and 2012, respectively from Chalmers University of Technology, Gothenburg, Sweden. Currently he is a senior lecturer in the Department of Electrical and Electronic Engineering, University of Peradeniya.

Currently he is the Treasurer of IEEE Sri Lanka central region subsection and IEEE PES Sri Lanka Chapter. His research interest includes polymeric insulations, gas discharge simulations, high voltage testing and power system modelling.

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, an International Professional Engineer and fellow of Institution of Engineers Sri

Lanka. 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.

A.U.A.W. Gunawardena (M’89-SM’12) is a Senior Lecturer attached to the Department of Electrical and Electronic Engineering, University of Peradeniya, Sri Lanka. He obtained his B.Sc.Eng. degree from the University of Peradeniya in 1985 and his Masters (M.Eng.Sc.) and Ph.D. degrees from the University of New South Wales, Australia and the University of Queensland Australia, in 1992 and 1997 respectively. His main research interests are in the field of microwave engineering. His other areas of interest

include signal processing, antenna design and radar systems. He is the founder of the IEEE MTT-S Sri Lanka chapter and was the chair for two consecutive years from 2013 to 2104. He is a member of IEEE Microwave and Theory, Signal Processing, Antennas and Propagation, and Geo-science and Remote Sensing societies.



Documents

Aging assesment of transformer pressboard insulation by