The Evaluation of Daily Comparative Leakage Currents on Porcelain and Silicone Rubber Insulators Under Natural Environmental Conditions

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Received January 23, 2021, accepted January 28, 2021, date of publication February 8, 2021, date of current version February 18, 2021.

Digital Object Identifier 10.1109/ACCESS.2021.3057626

The Evaluation of Daily Comparative Leakage Currents on Porcelain and Silicone Rubber Insulators Under Natural Environmental Conditions

WALUYO , (Member, IEEE), DINI FAUZIAH, AND ISMAIL MUHAMMAD KHAIDIR

Department of Electrical Engineering, Institut Teknologi Nasional Bandung (Itenas), Bandung 40124, Indonesia Corresponding author: Waluyo ([email protected])

ABSTRACT In this research, the leakage current (LC) and applied voltage waveforms on samples of installed outdoor porcelain and silicone rubber (SIR) insulators were measured by using a storage digital oscilloscope concurrent with humidity, temperature, and illuminance measurements. Essentially, the measurements were conducted daily, every three hours, for thirty days, and the data were analyzed using exponential regressions, box plots, and correlation coefficients. The porcelain and SIR insulator leakage currents varied from 23.7 µA to 28.8 µA and 8.4 µA to 10.35 µA for median values, respectively, while for outlier excluded values, they varied from 20.3 µA to 40.0 µA and 5.1 µA to 17.1 µA, respectively. Both leakage currents slightly increased as the exposure duration of the insulator rose, while the ratio decreased very slightly.

Additionally, the leakage currents decreased moderately and slightly as the temperature and illuminance increased, respectively. However, the parabolic leakage currents increased as the humidity rose, especially at high humidity. Therefore, the porcelain insulator was more susceptible than the daily SIR, while the SIR insulator was more highly influenced by aging of the exposure duration. The rainy condition contributed to the highest leakage currents on both insulators, whereas the drizzle condition contributed to the highest leakage current on the SIR insulator. The total harmonic distortion (THD) and phase angle of the porcelain insulator leakage currents experienced differences of 5.8% and 7.8 degrees in the minimum and maximum humidity, respectively, but both quantities were less for the SIR insulator.

INDEX TERMS Humidity, leakage current, porcelain, silicone rubber.

I. INTRODUCTION

High voltage outdoor insulators have an important role in electrical power transmission and distribution systems, including operational reliability, safety, and efficiency [1].

However, their functions are mostly influenced by environmental conditions [2], [3]. Common insulator materials include glass and porcelain, which are ceramic materials that have been used for hundreds of years. Moreover, composite and cast cycloaliphatic epoxy resin (epoxy resin, ER), as a polymeric category, has been used in the last few decades [3], [4]. These insulators have sheds for reducing the electric field stress and tolerating the accumulation of pollution [5].

Currently, research conducted on insulator leakage cur- rents based on daily time functions is limited. Vosloo et al. [6]

The associate editor coordinating the review of this manuscript and approving it for publication was Chong Leong Gan .

illustrated the average peak leakage currents on some insula- tors, and Roman et al. [7] monitored the insitu dc leakage current magnitudes, namely, rain, humidity, and temperature, on composite and glass insulators. According to Castillo- Sierra et al. [8], the ceramic insulator leakage currents were measured using a clamp meter, as were the temperature and humidity, followed by a regression analysis. Further research by Vosloo and Holtzhausen [9] showed that the peak leakage current of porcelain insulators, including precipitation, relative humidity (RH), ultraviolet (UV) radiation, temperature, and wind speed, was recorded for one week using linear and nonlinear multiple regression analyses.

In research conducted by Pieterse et al. [10], the average ac, dc(+), and dc(−) peak leakage currents of the insulator creepage distance were recorded at 1175 mm and 615 mm.

Chrzan [11] reported a daily maximum amplitude of the insulator leakage currents for porcelain and silicone rubber,

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while Wang et al. [12] investigated the daily LC magnitudes, humidity, temperature, and nonlinear (exponential) curve fitting of the leakage currents versus humidity on a glass insulator string. Vosloo and Holzhausen [13] investigated the leakage currents on EPDM, SIR, and glass insulators, and discovered that the glass insulator has the highest leakage at night, followed by EPDM and silicone rubber. In a further study conducted by Chrzan et al. [14], the ratio of variations in the leakage current between the silicone coating and the porcelain insulators was investigated. According to research conducted by Elombo et al. [15], dc(−) yielded the maximum absolute peak daily leakage current at night compared to dc(+) and ac. Vosloo et al. [16] reported a maximum peak of negative and positive leakage currents on SIR and EPDM, which were relatively high and low for approximately half at the beginning and at the end, respectively. Additionally, Vosloo and Holtzhausen [17] presented the porcelain LC and temperature, which were recorded daily. The results showed that during the daytime, the temperature rose while the leakage current decreased. Vosloo et al. [18] presented the average maximum absolute LC on glass, EPDM, and SIR insulators, which were initially high but later reduced.

Based on the literature reviews above, it is necessary to conduct further research on the daily leakage current behav- iors under some natural environmental parameters, such as humidity, temperature, and illuminance. Essentially, the measurements were not only the magnitudes or peak values, but also the waveforms. Therefore, an oscilloscope was used to obtain the phase angles. It is also necessary to determine the correlation between the weather conditions and the leakage currents. To obtain a distinctive comparison, the samples used were in the ceramic and nonceramic insulator categories.

Moreover, in performing the data analysis some statistical tools were used.

The objective of this research was to determine the waveforms of the leakage current and the daily comparative patterns of porcelain and silicone rubber. It was also used to find the trending insulator leakage currents over thirty days and their reaction to humidity, temperature, and illuminance.

Consequently, the typical insulator leakage currents under some weather conditions based on visual observations were obtained. The THD and phase angle comparisons of the leakage currents were presented under the minimum and maximum environmental conditions. Then, exponential regressions and box plot statistical tools were used to analyze the data trends and data scattering, respectively. Moreover, correlation coefficients were used to analyze the relationship between the leakage current magnitudes and environmental factors.

II. RESEARCH METHOD

In this research, a comparative undertest new pin-type porcelain and SIR insulators were used as representatives of the ceramic and nonceramic categories, respectively. For the porcelain insulator, the leakage distance, dry arcing distance and minimum pin height are 178 mm, 114 mm, and 127 mm,

FIGURE 1. Undertest pin-type of (a) porcelain and (b) SIR insulators.

respectively, at an applied voltage of 11.5 kV. However, for the SIR insulator, the structure height, insulating distance, minimum creepage distance and diameter of the shed are 215 mm, 125 mm, 280 mm, and 148/118 mm, respectively, at a rated voltage of 10 kV. The porcelain and SIR insulators for the research are shown in Fig. 1. Both insulator ratings were almost same.

The insulators were installed outside, next to The Depart- ment of Electrical Engineering building, Institut Teknologi Nasional Bandung, 23 PHH Mustafa Street Bandung, West Java, Indonesia, with coordinates of 6 ^◦53’51’’S, and 107^◦38’8’’E.

Digital light (lux), temperature, and humidity meters were used for the outdoor measurements during the conducted research. The setup of the measurement diagram is shown in Fig. 2, where a step-up transformer supplied the undertest insulators with 220 V/7.5 kV. In principle, the leakage currents of the insulators were measured by using a storage digital oscilloscope on channel 1 through a series resistor.

The leakage currents were the voltage drops divided by the resistance. The applied voltages were measured on channel 2 through a voltage divider and a differential probe. There- fore, both the leakage current and applied voltage waveforms were displayed and saved. Furthermore, the peak and effective (root-mean-square, rms) values, the sinusoidal purity, and the phase angle were observed. Finally, the resistive and capacitive dominant characteristics of the insulators were represented. The files were saved in csv (comma separated values) and JPEG (joint photographic experts group) formats, and the csv files were further used for analysis and waveform charts. These measurements were conducted daily every three hours at approximately 00:00, 03:00, 06:00, 09:00, 12:00, 15:00, 18:00 and 21:00 for thirty days between October 9^th and November 7^th, 2019, with the intent to obtain various parameters and weather conditions of the natural environment.

The first analysis was conducted to obtain the FFT (Fast Fourier Transformer) spectrum using OriginPro software.

After that, the THDs, or distortion factor (harmonic factor),

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Waluyo et al.: Evaluation of Daily Comparative Leakage Currents on Porcelain and Silicone Rubber Insulators

FIGURE 2. Measuring diagram set-up.

TABLE 1. Interpretation of correlation coefficient sizes.

of the insulator LCs were computed by using equation (1) [19]–[20]. When the THD value is close to zero, then the leakage current waveforms will be close to the pure sinusoidal waveform, and vice versa.

THD = q

I₃²+ I5²+ I7²+ . . . I1

(1) Moreover, the phase angles, which were the angles between the leakage current and the applied voltage wave (θ ), were computed as shown in equation (2), where 1t and T are the time difference on both the waves and wave period, respectively [20]. Because the location of the research measurements was not considerably polluted and because the insulators were still in a new condition, the THD values and the phase angles generally did not undergo much change.

Hence, both parameters were computed under some extreme conditions.

θ = 1t

T x360^◦ (2)

Moreover, the correlations of the leakage currents, their ratios, and insulator impedances to the environmental parameters were analyzed using a correlation coefficient analysis, as stated in equation (3). The correlation coefficient is the standardized version of the covariance and is dimension- less [21].

ρ (x, y) = cov (x, y)

√var (x) · var (y) (3) The rule of thumb for the interpretation of the correlation coefficient size is listed in Table 1 [22].

Generally, the data on one parameter were not inherently linear to the asymptotic patterns on the other parameters.

Therefore, exponential models were employed to determine

FIGURE 3. Sample waveforms of the JPEG picture.

the regressions [12], [23], [24], as shown in equation (4), where C and D are the constant and exponential constant of the regression, respectively.

y = Ce^Dx (4)

Graphically, the box plot, also known as the box-and- whisker plot, was used for the analysis and, combining some numerical summaries. The box height is the interquartile range (IQR), while the horizontal line in the inside box corre- sponds to the sample median. Additionally, the data beyond the upper and lower quartiles are outliers, which are 1.5 times the IQR distance [21]. Hence, the box plot represents the value distribution, which helps to understand the hypothesis test [25]. If there are extreme values (outliers), then the median is a better measure of the center [25].

Finally, the analysis was conducted as qualitative observations of the weather conditions with respect to the LC magnitudes and insulator corona discharge currents among the remaining leakage currents.

III. RESULTS AND DISCUSSION A. LEAKAGE CURRENT WAVEFORMS

Fig. 3 shows one sample of the waveforms of the JPEG files displayed on the oscilloscope screen. On the file, the relative humidity measured on the porcelain insulator was 28.6%.

The first and second waveforms are the leakage current and applied voltage, respectively, and it can clearly be seen that the leakage current waveform leads to the applied voltage.

Therefore, the insulator can be described to contain a capacitive property. The voltage and frequency values are displayed on the right side of the oscilloscope screen, whereas the V/div and time/div are shown at the bottom.

Excel software was used to convert the displayed wave- forms of the csv files into real values for further analy- sis. Hence, the yielded leakage current waveform samples of the porcelain and silicone rubber insulators under two extreme relative humidity conditions, which include 28.6%

and 91.0%, are shown in Fig. 5. The results showed that the

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FIGURE 4. Leakage current waveforms at two extreme humidities: (a) Porcelain, 28.6%RH, (b) Silicone rubber, 28.6%RH, (c) Porcelain, 91.0%RH, (d) Silicone rubber, 91.0%RH.

effective and peak leakage currents for the porcelain insulator at 28.6%RH, were 21.2 µA and 31.2 µA, and those of silicone rubber were 7.1 µA and 12.8 µA, as shown in Fig. 4(a) and 4(b), respectively. Additionally, at 91.0%RH, the effective and peak leakage currents for the porcelain insulator are 35.8 µA and 45.6 µA, while those of the silicone rubber insulator are 14.5 µA and 27.2 µA, as shown in Fig. 4(c) and 4(d), respectively.

Consequently, the rms and peak leakage currents increased, with differences of 14.6 µA and 14.4 µA, respectively. This was recorded because of the difference in the humidity on the porcelain insulator. Meanwhile, on the silicone rubber insulator, the differences were 7.4 µA and 14.4 µA for the effective and peak leakage currents, respectively. Therefore, the peak of the leakage current wave, compared to its effective value, is almost doubled in the silicone rubber insulator due to the tapering of the waveform. Nevertheless, the difference in the porcelain insulator’s effective leakage current was higher than that of the silicone rubber.

B. LC VARIATIONS AT THE TIME OF INSTALLATION Fig. 5(a) shows the daily temperature based on the box plot and median values. The typical lowest temperature was 22.2

◦C at approximately 06:00 am and increased to 31.5 ^◦C

at noon. After the peak values, the temperature gradually decreased until 06:00 in the morning.

Furthermore, Fig. 5(b) shows that the typical daily relative humidity was 80.8% and occurred at approximately 06:00 am. However, the humidity decreased to a typical value of 33.2% at noon and gradually increased again to its peak at 06:00. This analysis indicates that the temperature and humidity were opposite of one another.

Fig. 5(c) shows the typical daily illuminance with minimum and maximum values of 0.9 and 87,050 lux at 03:00 and 12:00, respectively. Between 06:00 and 18:00 the daily illuminance was high, with the highest value and wide range obtained at midday due to the influence of sun radiation.

According to Fig. 6(a), the daily average rms leakage currents of the porcelain were higher than those of the silicone rubber insulators. The chart shows that the lowest values were recorded at 09:00 until 12:00, with minimum average values of 23.4 µA and 8.1 µA for the porcelain and silicone rubber insulators, respectively. The highest average values recorded at 00:00 were 40.5 µA and 14.0 µA for porcelain and SIR insulators, respectively.

Furthermore, the time trends for the daily box plot median values of the porcelain and SIR insulator leakage currents are shown in Fig. 6(b). Based on the chart, the lowest value

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FIGURE 5. Daily (a) temperature, (b) humidity and (b) illuminance.

of the porcelain insulator of 23.7 µA was recorded between 09:00 and 12:00, at midday, while the highest median value of 28.2 µA was obtained at 0:00, at midnight. This result shows that the trending pattern of the SIR insulator was in line with that of porcelain, but its values were approximately one- third that of porcelain. In essence, the lowest median value of 8.4 µA was obtained between 9:00 and 12:00, while the highest value of 10.35 µA was recorded at 0:00 midnight. The variations in the leakage currents for outlier excluded values were 20.3 µA to 40.0 µA and 5.1 µA to 17.1 µA for the porcelain and SIR insulators, respectively. Both the average and median values were proportional.

FIGURE 6. Daily (a) average and (b) box plot median of the insulator leakage currents.

Moreover, this daily leakage current pattern (i.e., the reduction during the day and the increment at night) can be described to be in line with previous studies in [8]–[10], [12], [15], [17], [18], [26]. Nevertheless, the values were different.

This was caused by the presence of a high humidity at night and a low humidity during the day due to solar radiation.

Additionally, there was an increase in the leakage current at a local time of approximately 6:00 in the morning (compared to others) with median values of 27.95 µA and 9.95 µA for the porcelain and silicone rubber insulators, respectively. This phenomenon was caused by the accumulation of moisture on the insulator surfaces due to the high humidity, which began to decrease in the early morning due to sunshine.

In addition, this case was in line with [10], [12], [15], [17], [18], [26] because the rise in the local time occurred between 04:00 and 08:00. According to Pieterse et al. [10], it occurred on the relatively low creepage distance of 615 mm, rather than the high creepage distance of 1175 mm. The research conducted by Wang et al. [12] and Zhao et al. [26] described that the increase in the leakage current was probably caused by the increase in humidity. However, Vosloo et al. [17], [18] described that it was caused by wet conditions at critical wetting times (3:00 to 5:00), which eventually increased the surface conductivity.

The data analysis used in this research shows that the average and median box plot methods are comparable. This

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FIGURE 7. Daily (a) average and (b) box plot median porcelain-to-silicone rubber insulator leakage current ratio.

can be evidently seen in Fig. 7(a) and 7(b), which describe the daily average porcelain-to-silicone rubber insulator leakage current ratio. According to Fig. 7(a), the values were small variations, and they reached a maximum of 2.90 at 9:00, while the minimum value was 2.79 at 03:00, during dawn.

Furthermore, Fig. 7(b) also shows that the values tend to exhibit small variations, which are slightly between 2.8 and 2.9. This indicated that for the new porcelain and silicone rubber insulators the leakage current variations can be com- pared with one another. In accordance with Chrzan et al. [14], the Isir/Iporc ratio was almost constant and slightly changed when there was rain.

Fig. 8(a) shows the average and median regressions of the porcelain and silicone rubber insulator leakage currents for thirty days. During this time, both leakage currents tended to increase slightly. Essentially, this phenomenon was in line with [7], [8], [11], [12], [27]–[30]; however, both trends were different from one another.

Based on the average values, the leakage current of the porcelain insulator has a higher basis value of 24.529 µA, which is approximately three times the value of silicone rubber of 8.1294 µA. However, the exponential coefficient of the porcelain insulator leakage current of 0.0052 is lower than that of silicone rubber, which is 0.0109. When the chart is further extrapolated, there will be an intersection on day 194, with leakage currents of 67.2 µA.

FIGURE 8. Thirty-day duration of (a) average and (b) box plot median porcelain and silicone rubber insulator leakage currents.

Additionally, on the median basis, the leakage current of the porcelain insulator has a higher basis value of 23.637 µA, which is approximately three times the value of silicone rubber with 7.523 µA. Meanwhile, the exponential rate of the porcelain insulator leakage current of 0.0061 is lower than that of silicone rubber, which is 0.0128. When the chart is further extrapolated, an intersection will occur on day 171 with leakage currents of 67.1 µA. Based on both extrapolations, the leakage currents will almost be the same from 6 to 7 months.

Furthermore, the box plot median leakage currents for the porcelain and silicone rubber insulators are shown in Fig. 8(b). It is clearly shown that the increasing trend of the porcelain insulator is lower than that of the silicone rubber.

Additionally, the average rates of the increasing leakage currents are 0.1052 µA/day and 0.1500 µA/day for the porcelain and silicone rubber insulators, respectively. Based on this phenomenon, the silicone rubber insulator was influenced more by the exposure time duration of installation than the porcelain insulator was.

The median-based regression and box plot of the porcelain- to-silicone rubber insulator leakage current ratio are shown in Fig. 9(a) and 9(b), respectively. Based on the chart, it is clear that the ratio tended to decrease slightly, with an average decreasing rate of −0.046/day. Consequently, when extrap- olation of the regression was done, the ratio reached unity

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FIGURE 9. Median values (a) regression and (b) box plot of thirty-day porcelain-to-silicon rubber insulator leakage current ratios.

on day 194. After that, the leakage current of the silicone rubber insulator was higher than that of the porcelain. Thus, the increase in the leakage current on the porcelain insulator is not proportional to that of SIR. This is because the values of the leakage currents on the porcelain become lower than those on the silicone rubber insulator as the exposure time increases, assuming that there is no considerable influence of pollution or other factors.

C. LC VARIATIONS AGAINST THE NATURAL TEMPERATURE The porcelain and silicone rubber leakage currents versus the measured temperature are shown in Fig. 10(a). Accord- ing to the chart, the minimum and maximum temperatures measured were 20.6^◦C and 32.5^◦C, respectively. Based on the data, the average porcelain leakage current magnitude was 3.058 times higher than that of silicone rubber. This is reasonable because the regression constants are 62.3 and 26.1 for the porcelain and silicone rubber leakage currents, respectively. Therefore, when the insulator leakage currents are extended, there is no intersection.

Fig. 10(b) shows the box plot median values of the porcelain and silicone rubber insulator leakage currents versus the classified temperatures. Visually, the leakage currents decreased slightly as the temperature increased. According to the calculations, the temperatures chosen in the middle of the ranges showed that the average rates for the decreas-

FIGURE 10. Leakage currents: (a) regressions, (b) box plot medians and (b) median-based regressions versus temperature.

ing leakage currents were −0.413µA/^◦C and −0.058µA/^◦C for the porcelain and silicone rubber insulators, respectively.

This is evident because the decreasing rate of the porcelain leakage current was higher than that of the silicone rubber.

Thus, the porcelain insulator is more susceptible to temperature influence than silicone rubber is. Visually, the insulator leakage currents decreased as the environmental temperature increased, and this phenomenon is in line with [8], [12], [17].

Castillo-Sierra et al. [8] and Vosloo and Holtzhausen [17]

stated that at noon the temperature rose with a decrease in the leakage currents. Wang et al. [12] used exponential regressions with various constants to reduce the leakage currents as the temperature increased. In Fig. 10(c), the median- based regressions are shown, where the median values of

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FIGURE 11. Porcelain-to-silicone rubber leakage current ratio on (a) box plot medians and (b) median-based regression versus temperature.

the porcelain insulator were higher than those of the silicone rubber, with an average ratio of 2.773. In addition, the average decreasing leakage currents due to the temperature rise were −0.413µA/^◦C and −0.058µA/^◦C for the porcelain and silicone rubber insulators, respectively. Although the absolute average decreasing leakage current of the porcelain was higher than that of the silicone rubber, the regression would not experience an intersection. This is because, with an assumed maximum normal ambient temperature of 48^◦C, the exponential constant value of the porcelain insulator leakage current regression was −0.0183, which was slightly lower than that of the silicone rubber, which was −0.0185.

The box plot and median-based regression on the porcelain-to-silicone rubber leakage current ratio versus the temperature are shown in Fig. 11(a) and 11(b). Visually, the box plot median slightly decreased with an average ratio of −0.025/^◦C as the temperature increased. Based on the regression, the ratio slightly decreased with respect to the installation temperature by an exponential constant of

−0.003. Hence, the porcelain insulator was more vulnerable to temperature, as one of the environmental factors, than silicone rubber was.

D. LC VARIATIONS AGAINST NATURAL HUMIDITY

The scattered plot of the porcelain and silicone rubber leakage currents versus humidity, as well as the exponential regressions, are shown in Fig. 12(a). Visually, both leakage currents

increased considerably as the relative humidity rose, such that they tended to give a parabolic approach. The leakage currents of the porcelain insulators were always higher than those of silicone rubber, with bases of 17.75 µA and 5.85 µA, respectively. Even though the exponential constant of the porcelain was higher than that of silicone rubber, they did not coincide at 100% relative humidity.

The box plot median values of the leakage currents for the porcelain and silicone rubber insulators are shown in Fig. 12(b). Fig. 12(c) shows the median-based regressions, which indicate that the leakage currents increase drastically as the humidity increases, especially at 70% RH and above.

It is distinctively clear that at a high humidity, the chart of the leakage currents tended to be parabolic. This phenomenon was in line with [8], [12], [20], [24], [26]–[27], [31]–[33] and implied in [34]. According to Castillo-Sierra et al. [8] and Vosloo and Holtzhausen [17], the leakage currents were high and low at night and noon, respectively. Jiang et al. [20] and Samakosh and Mirzaie [27] stated that the amplitudes of the leakage current waveforms rose as the humidity increased.

Subsequently, Zhao et al. [26], Wang et al. [12], Cui and Ramesh [24], Fontana et al. [31], and Li et al. [33] stated that the leakage currents rose with an increasing humidity and tended to exhibit parabolic or quadratic patterns.

The average increases in the leakage currents to the relative humidity were 0.1785 µA/%RH and 0.0892 µA/%RH for the porcelain and silicone rubber insulators, respectively.

The results therefore showed that the vulnerability of porcelain to humidity was approximately twice as high as that of silicone rubber. The influenced part was the insulator surface was due to the influence of the outdoor humidity.

Furthermore, the porcelain insulator surface is glazed, which is hydrophilic, with a strong affinity for water. This means that it has the ability to mix, dissolve, or be attracted to water; therefore, an electrolytic film covers the insulator [6].

Meanwhile, the SIR insulator is silicone-based, which tends to maintain a hydrophobic surface, even when it is severely contaminated. The hydrophobic characteristic means is water repellent, resistant to wetting, or opposite of hydrophilic. The water forms separate droplet beads, thereby preventing continuous conductive layer formation [6]. Never- theless, the hydrophobic characteristics are not stable. This is because as the exposure time increases, these characteristics tend to decrease due to erosion and other degradations [6].

Fig. 13(a) shows the scatter plot of the porcelain-to- silicone rubber leakage current ratio versus the humidity. The chart shows that the regression tended to slightly decrease as the relative humidity increased, with an exponential constant of −0.107, whereas Fig. 13(b) shows the box plot median values of the ratio. The average rate was −0.003077/%RH, which indicated that the ratio decreased very slightly as the relative humidity rose. This therefore shows that the rate of increase in the porcelain leakage current due to the increase in humidity was higher than that of silicone rubber, which was especially reduced at approximately 70% and above.

Additionally, Fig. 13(c) shows the median-based regression

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FIGURE 12. Porcelain and silicone rubber leakage currents on (a) regressions, (b) box plot medians and (b) median-based regressions versus humidity.

of the ratio, which indicated that it was slightly reduced as the humidity increased.

E. LC VARIATIONS AGAINST NATURAL ILLUMINANCE Fig. 14(a) shows the porcelain and silicone rubber rms leakage current versus the illuminance. The leakage currents slightly decreased as the illuminance increased, with an exponential constant of −3.10⁻⁶ on both the porcelain and silicone rubber insulator leakage currents. Fig. 14(b) shows the box plot median values for the leakage current versus the illuminance. Essentially, the reduction in the leakage currents was caused by the effect of the rising temperature.

This further indicated that the implementation of the box plot medians on high illuminance has minimum values of 23.9 µA and 8.5 µA for porcelain and silicone rubber insulators, respectively. There is currently no direct relationship between

FIGURE 13. Porcelain-to-silicone rubber leakage current ratio on (a) regressions, (b) box plot median and (c) median-based regression versus humidity.

the leakage current and illuminance. However, there is a correlation between the leakage current and ultraviolet (UV) radiation, where the leakage current decreased as the UV radiation increased [17]. In addition, the increase in UV radiation causes the temperature to increase [9], [13], [17]

and the humidity to decrease [13], [17]. Hence, increasing the temperature will reduce the leakage currents, which is in line with [8], [12], [17].

Fig. 15(a) shows the porcelain-to-silicone rubber leakage current ratio versus the illuminance, with a very small exponential constant of 10⁻⁷. Therefore, the ratio can be described to be fairly constant for the illuminance variation with a value of 2.82. This is also indicated by the box plot medians, where the values experienced small variations that slightly increased and decreased again, as shown in Fig. 15(b).

The correlation coefficients among environmental parameters, leakage currents, and insulator impedances are shown

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FIGURE 14. Porcelain-to-silicone rubber leakage current ratio on (a) regressions and (b) box plot medians versus humidity.

in Table 2. Visually, the humidity highly positively influenced the porcelain leakage current (IPor), with a value of 0.885, followed by the silicone rubber leakage current (ISIR), with a value of 0.671. However, the humidity negatively influenced the porcelain and silicone rubber impedances (ZPorand ZSIR) with values of −0.907 and −0.514, respectively. In addition, the temperature negatively influenced the porcelain and silicone rubber leakage currents with values of −0.853 and

−0.614, respectively. However, it also positively influenced the porcelain and silicone rubber impedances with values of 0.884 and 0.528, respectively. Finally, the illuminance negatively influenced the porcelain and silicone rubber leakage currents with values of −0.727 and −0.495, respectively, and positively influenced the porcelain and silicone rubber impedances with correlation coefficients of 0.776 and 0.408, respectively.

Based on these results, the three environmental parameters were weak in influencing the porcelain-to-silicone rubber leakage current ratio (IPor/ISIR). The influence of the temperature on the correlations of the porcelain insulator leakage current and impedance were highly negative and positive. However, its influence on the silicone rubber insulator leakage current and impedance was moderately negative and positive. Furthermore, the influence of humidity on the correlations of the porcelain leakage current and impedance were highly positive and negative, respectively. However, its

FIGURE 15. Porcelain-to-silicone rubber leakage current ratio on (a) the regression and (b) box plot median versus illuminance.

TABLE 2.Correlations among environmental parameters, insulator leakage current and impedance.

influence on the silicone rubber insulator leakage current and impedance was moderately positive and negative, respectively. The influences of illuminance on the porcelain leakage current and impedance were moderately negative and positive, respectively, while its influence on the silicone rubber insulator and impedance were low-negative and low-positive, respectively. Finally, the temperature and illuminance influenced the porcelain-to-silicone rubber leakage current ratio as a slightly positive category, and the humidity influenced it as a slightly negative category.

F. LC VARIATIONS DUE TO NATURAL WEATHER

Visual inspections were conducted under natural weather conditions, such as bright, cloudy, overcast, rainy, drizzle, wet, and foggy conditions. Fig. 16(a) shows the typical temperatures in various weather conditions, which were classified into one of two categories, namely, above and below

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FIGURE 16. (a) Temperature, (b) humidity, and (c) illuminance of various weather conditions.

25^◦C. Those above were bright, cloudy, and overcast, while the remaining ones were rainy, drizzle, wet and foggy.

This was similar to the typical humidity, as shown in Fig. 16(b). The weather conditions lower than 70% RH were bright, cloudy, and overcast, while the remaining weath- ers were rainy, drizzle, wet, and foggy.

Furthermore, Fig. 16(c) shows the illuminance in various weather conditions. The bright weather had the largest range of data, while drizzle, wet and foggy weather was obtained at night.

The natural weather conditions were correlated with the leakage currents. Hence, the average porcelain insulator leakage currents of the obtained weather conditions are shown

in Fig. 17(a). The highest leakage currents for the rainy condition usually occurred at 15:00 and 21:00, with average values of 40.4 µA and 42.3 µA, respectively. Fig. 17(b) shows the average leakage currents of the silicone rubber insulator due to some weather conditions. The results showed that the highest average leakage currents were under rainy and drizzle conditions at 15:00 (rainy) and at 21:00 (rainy and drizzle). Fig. 17(c) shows the average insulator leakage currents under various weather conditions. The chart shows that the weather that contributed to the highest leakage current of the porcelain insulator was rainy, while the weather that contributed to the silicone rubber insulator included drizzle and rainy. According to Fig. 17(d), the box plot median value shows that the leakage current on the porcelain insulator in rainy weather was 41.3 µA, while that of silicone rubber in rainy and drizzle weather was 15.8 µA. Based on these results, it can be concluded that the highest leakage currents due to rain were in line with [12], [34]. The drizzling weather contributed to the highest leakage current on the silicone rubber insulator because the surface moisture was slightly trapped, since it was not as smooth as the glazed surface of the porcelain insulator.

Rainy, drizzle, wet and foggy weather contributed to the high leakage currents and humidity, in line with Fig. 16(b).

Fig. 18(a) shows the average porcelain-to-silicone insulator leakage current ratio at various times and weather conditions. According to the chart, the highest ratio was in the overcast weather at 15:00. In addition, Fig. 18(b) and Fig. 18(c) show the average and median based porcelain-to- silicone rubber insulator leakage current ratios for the overall data. According to the charts, the highest ratio was also overcast weather, which indicated that the increasing leakage current on the porcelain insulator is significantly higher than that of the silicone rubber.

Based on the box plot median values, the comparative porcelain and silicone rubber leakage currents were 25.3 µA and 9.1 µA, respectively. In addition, the first quartile, third quartile, and interquartile range for the porcelain and silicone rubber insulator leakage currents were 23.95 µA and 28.35 µA, 4.4 µA and 8.3 µA, and 10.45 µA and 2.15 µA, respectively. Visually, both the magnitude and range indicated that the variation in the porcelain insulator leakage current was higher than that of the silicone rubber. Therefore, the porcelain insulator was more susceptible than the silicone rubber.

Additionally, the comparative box plot median values for porcelain and silicone rubber impedances were 296 M and 819.5 M, respectively. The first quartile, third quartile, and interquartile range for both insulators are 163.25 M, 312.75 M and 49.5 M, and 711.25 M, 899.25 M and 188 M for porcelain and SIR, respectively.

G. THD AND PHASE ANGLE ON THE MAXIMUM AND MINIMUM CONDITIONS

Table 3 shows the comparative THD and the phase angle of the leakage currents under minimum and maximum

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FIGURE 17. Leakage current: (a) average porcelain, (b) average SIR, (c) average LCs and (d) box plot medians at various times and weather conditions.

FIGURE 18. Porcelain-to-silicone rubber leakage current ratios:(a) average for various weather conditions, (b) average for various weather conditions and (c) box plot median for various weather conditions.

conditions. At a minimum humidity of 28.6% RH, the THDs of the leakage currents were 24.5% and 29.2% for the porcelain and silicone rubber insulators, respectively. At a maximum humidity of 91% RH, the leakage current THDs were 18.7% and 27.2% for the porcelain and silicone rubber insulators, respectively. This indicated that the decrements in the leakage current of the porcelain and silicone rubber insulators were only 5.8% and 2%, respectively. Additionally, at the minimum illuminance of 0.39 lux, the THDs of the leakage current were 25.9% and 26.8%

for porcelain and silicone rubber, respectively, while at a maximum illuminance of 95,300 lux, the THDs were both

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29.2%. Thus, the increases were 3.3% and 2.4% for the porcelain and silicone rubber insulators, respectively. The THDs at a minimum temperature of 20.6^◦C were 23.4%

and 24.2% for the porcelain and silicone rubber insulators, respectively, while at a maximum temperature of 32.5^◦C, the THDs were both 29.2%. Hence, the increases were 5.8%

and 5.0% for the porcelain and silicone rubber insulators, respectively.

Essentially, the THD reduction with an increasing humidity would cause the conductivity of the insulator surface to increase, relatively. Nevertheless, the THD reduction on the porcelain insulator was higher than that on the silicone rubber. This implied that the increasing conductance on the porcelain insulator surface was higher than that on the silicone rubber. Additionally, the THD reduction of the leakage currents due to a high humidity was in line with [20].

Moreover, the increase in the illuminance and temperature raised the THD of the leakage currents on both the porcelain and silicone rubber insulators. Nevertheless, the increment on the porcelain insulator was slightly higher than that on the silicone rubber. This indicated that the porcelain insulator was more influenced by the illuminance and temperature than the silicone rubber was.

At a minimum humidity of 28.6% RH, the leakage current phase angles were 79.4 degrees and 81.0 degrees for the porcelain and silicone rubber insulators, respectively, while at a maximum humidity of 91.0% RH the phase angles were 71.6 degrees and 81.0 degrees for the porcelain and silicone rubber insulators, respectively. This indicated that the decre- ment for the porcelain was 7.8 degrees, but the SIR insulator did not decrease. The reduction in the phase angle was due to the increase in the relative humidity, which was in line with [20] and implied in [27]. Therefore, this indicated that the properties of the porcelain insulator were more susceptible to a reduction due to high humidity than those of the silicone rubber insulator.

At a minimum illuminance of 0.39 lux, the leakage current phase angles were 78.4 degrees and 79.2 degrees for the porcelain and silicone rubber insulators, respectively, while on a maximum illuminance of 95,300 lux, the leakage current phase angles were 80.8 degrees for both insulators. Thus, the decrements in the porcelain and silicone rubber insulator leakage currents were 2.4 degrees and 1.6 degrees, respectively. Additionally, at a minimum temperature of 20.6 degrees, the leakage current phase angles were 74.2 degrees and 81.0 degrees for the porcelain and silicone rubber insulators, respectively, while at the maximum temperature of 32.5 degrees, the leakage current phase angles were 80.8 degrees and 81.0 degrees for porcelain and silicone rubber insulators, respectively. Hence, the increments in the porcelain insulator leakage current were 6.6 degrees, but the silicone rubber did not change. Moreover, the high phase angle of the leakage current at a high humidity on the nonceramic insulator was still in accordance with [32]. This result showed that the increase in the illumination and temperature caused a slight increase in the leakage current phase angle.

TABLE 3.Comparative THDs and phase angles for extreme humidity, illuminance, and temperature.

Nevertheless, the influence of the illuminance was slightly lower than that of the temperature due to humidity.

Although the humidity and temperature were opposite, they significantly influenced the porcelain leakage current phase angle and the THD. The high phase angle meant that the insulators had a relatively low surface conductance; therefore, the capacitive property was more dominantly visible, followed by a high THD.

H. CORONA DISCHARGE CONDITIONS

Along these measurements there were three occurrences as presumed corona discharges, with the measured discharge currents including 149.0 µA, 148.5 µA and 143.3 µA, and 45.2 µA, 45.4 µA and 45.3 µA for the porcelain and silicone rubber insulators, respectively. These values were the highest among others, and hissing sounds were produced during the measurements. Essentially, all three measurements were recorded at approximately midnight, when the humidity was generally quite high, with intermittent surges in the leakage current waveforms. Nevertheless, the magnitude of the corona discharge currents that occurred in silicone rubber was lower than that of the porcelain insulator. Two examples of corona discharge current waves are shown in Fig. 19(a) and 19(b) for the porcelain and silicone rubber insulators, respectively, with waves that were accompanied by intermittent occurrences circled in the figures. These discharge currents were in line with [31], [35], [36]. This is because, according to Fontana et al. [31], the ionization current increased drasti- cally, and according to Palangar et al. [35], this increase was due to polluted and high humidity conditions. Chandrasekar et al. [36] stated that it had short-duration discharges.

Fig. 20(a) shows the outlier positions of the discharge currents on the porcelain insulator. Additionally, Fig. 20(b) shows the outlier positions of the discharge currents on the silicone rubber insulator. Moreover, the hissing sound produced on the SIR insulator when measuring the discharge currents was less than that of porcelain.

Fig. 21 shows the THD and phase angle samples on the normal leakage and discharge currents. The chart shows that both the THD and phase angle decreased as the normal leakage currents increased. This reduction in the phase angle is in line with [36]. Essentially, the discharge currents were always much higher than the normal leakage currents. Thus,

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FIGURE 19. Leakage currents on (a) porcelain, 149.0 µA and (b) SIR, 45.4 µA insulators accompanied by discharge currents.

the discharge currents of the porcelain insulator were also much higher than that of silicone rubber. This means that as the normal leakage currents increased, the THDs and phase angles tended to decrease, except for the THD of the silicone rubber insulator. Furthermore, the THDs and phase angles increased again on the discharge currents. Thus, the THDs and phase angle would be minimal for the maximum values of the normal leakage currents. However, both quantities tended to rise in the low leakage and high discharge currents, except for the THD of silicone rubber, which was more immune than porcelain. Additionally, the phase angle and THD tended to decrease as the leakage currents increased before rising again.

This is because the discharge current of the intermittent local arc is stronger, as implied in [35] and [20], respectively.

In corona discharges, a high THD means a high current magnitude, which is far from the pure sinusoidal waveform.

These were also dominated by high capacitive characteristics.

I. CORRELATION ON THE ENVIRONMENTAL PARAMETERS The figures below show the relationship among the environmental factors in order to strengthen their influence on the leakage currents. Fig. 22(a) shows the chart of the relative humidity function and temperature, which clearly shows that when the temperature increased, the humidity decreased significantly. Additionally, this phenomenon was explained by the box plot median in Fig. 22(b). When the typical temperatures of the box plot range were chosen, the average

FIGURE 20. Outlier positions as discharge currents on (a) porcelain and (b) SIR insulators.

FIGURE 21. THD and phase angle on the normal leakage and discharge currents.

rate of relative humidity reduction to the temperature was

−4.7%RH/^◦C. Consequently, the increase in the temperature caused a decrease in the humidity, as well as in the leakage current. This result was therefore in line with [7]–[9], [12].

Finally, Fig. 23 shows the temperature and relative humidity versus the illuminance. The chart shows that as the illuminance increased the temperature also increased, but the humidity decreased. Based on previous references, an increase in illuminance, which is proportional to UV radiation, will cause an increase in temperature [9], [13], [17] and a reduction in the humidity [13], [17], and insulator leakage currents [17].

The correlation coefficients between the illuminance and temperature, temperature and humidity, and illuminance

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FIGURE 22. Relative humidity as a function of temperature on (a) regression and (b) box plot medians.

FIGURE 23. Temperature and relative humidity versus the illuminance.

and humidity were 0.813, −0.943 and −0.817, respectively.

Thus, the correlation coefficient between the temperature and humidity had the highest negative category, while the correlation coefficient between the illuminance and temperature and the illuminance and humidity had high positive and high negative categories, respectively.

IV. CONCLUSION

In this research, the daily leakage current and applied voltage measurements on porcelain and silicone rubber insulators were evaluated using a storage digital oscilloscope. This was accompanied by environmental factors, namely, humidity, temperature, and illuminance. The results showed that the daily leakage current patterns were similar for both porcelain

and silicone rubber insulators, but their magnitudes were different. In addition, the highest leakage currents occurred at approximately 00:00 (midnight), while the lowest currents occurred between 09:00 and 12:00. The local increase in the leakage currents occurred in the morning at approximately 06:00 because of the high humidity; as a consequence, moisture accumulated on the insulator surfaces.

The leakage currents tended to slightly increase over thirty days. Additionally, the porcelain insulator tended to be influenced by the daily environmental factor, whereas the silicone rubber insulator tended to be influenced by the outdoor installation’s duration of exposure. Thus, the daily porcelain-to- silicone rubber leakage current ratio tended to be constant, but slightly decreased as the exposure duration increased.

Furthermore, the leakage currents were considerably reduced as the temperature increased; likewise, the ratio of the leakage current also decreased slightly as the temperature increased. The increasing humidity caused the leakage currents to significantly increase, especially above 70%RH, while the increase in the illuminance caused the leakage current to decrease slightly. Therefore, the results showed that the porcelain leakage current was most influenced by humidity with a high positive correlation. The correlation levels between the temperature, humidity, illuminance and leakage currents were −0.853, 0.885, and −0.727 for the porcelain insulator and −0.614, 0.671, and −0.495 for the silicone rubber insulator, respectively. Therefore, the order for the environmental parameter influences on the insulator leakage currents was the humidity, temperature, and illuminance, as positive, negative, and negative correlations, respectively.

The increase in humidity caused the leakage current THD and the phase angle of the porcelain insulator to decrease more significantly than those of silicone rubber. Consequently, as the leakage current magnitudes increased, both the THD and phase angle decreased. Therefore, the high humidity increased the porcelain insulator surface conductance, and vice versa for the temperature. However, it had less of an effect on the silicone rubber insulator.

As the leakage currents further increased to become corona discharge currents, both the THD and phase angle rose again. The corona discharges occurred at midnight due to the high humidity, low temperature, and low illuminance; hence, the currents were extremely high as statistical outliers. The corona discharge waveforms were far from pure sinusoidal waveforms.

The rainy condition led to the highest leakage currents of the porcelain and silicone rubber insulators, while the drizzle condition led to that of the silicone rubber insulator.

REFERENCES

[1] K. O. Papailiou and F. Schmuck, Silicone Composite Insulators: Materials, Design, Applications. Berlin, Germany: Springer-Verlag, 2013, p. 1.

[2] J. S. T. Looms, Insulators for High Voltages. London, U.K.: The Institution of Engineering and Technology, 2006, pp. 1–2.

[3] R. S. Gorur, E. A. Cherney, and J. T. Burnham, Outdoor Insulators.

Phoenix, AZ, USA: Ravi S. Gorur, Inc., 1999, pp. 16–21.

(16)

[4] G. Orawski, ‘‘Overhead lines,’’ in Electrical Engineer’s Reference Book, 16th ed., M. S. Laughton and D. F. Warne, Eds. Burlington, MA, USA:

Newnes, 2003, pp. 30-10–30-11.

[5] M. Farzaneh and W. A. Chisholm, Insulators for Icing and Polluted Envi- ronments. Hoboken, NJ, USA: Wiley, 2009, pp. 25–26.

[6] W. L. Vosloo, R. E. Macey, and C. de Tourreil, The Practical Guide to Outdoor High Voltage Insulators. Bedfordview, South Africa: Crown Publications, 2008, pp. 5–6.

[7] M. Roman, R. R. van Zyl, N. Parus, and N. Mahatho, ‘‘in-situ monitoring of leakage current on composite and glass insulators of the cahora bassa HVDC transmission line,’’ SAIEE Afr. Res. J., vol. 110, no. 1, pp. 4–10, Mar. 2019.

[8] R. Castillo-Sierra, O. Oviedo-Trespalacios, J. E. Candelo, and J. D. Soto-Ortíz, ‘‘Modeling leakage current of ceramic insulators subject to high pollution levels for improving maintenance activities,’’

Revista DYNA, vol. 85, no. 204, pp. 364–371, Jan. 2018.

[9] W. L. Vosloo and J. P. Holtzhausen, ‘‘The prediction of insulator leakage currents from environmental data,’’ in Proc. IEEE AFRICON. 6th Africon Conf. Afr., George, South Africa, Oct. 2002, pp. 2–4.

[10] P. J. Pieterse, A. I. Elombo, G. N. J. Mouton, H. J. Vermeulen, J. P. Holtzhausen, and W. L. Vosloo, ‘‘A coastal insulator pollution test station for the evaluation of the relative ageing performance of power line insulators under AC and DC voltage,’’ in Proc. 17th Int. Symp. High Voltage Eng., Hannover, Germany, Aug. 2011, Paper C-007/144.

[11] K. L. Chrzan, ‘‘Leakage currents on naturally contaminated porcelain and silicone insulators,’’ IEEE Trans. Power Del., vol. 25, no. 2, pp. 904–910, Apr. 2010.

[12] J. Wang, Y. Xi, C. Fang, L. Cai, J. Wang, and Y. Fan, ‘‘Leakage current response mechanism of insulator string with ambient humidity on days without rain,’’ IEEE Access, vol. 7, pp. 55229–55236, 2019.

[13] W. L. Vosloo and J. P. Holzhausen, ‘‘A comparison of glass cap and pin, silicone rubber and EPDM rubber insulators over a four day period at Koeberg insulator pollution test station,’’ in Proc. IEEE Africon. 5th Africon Conf. Afr., Cape Town, South Africa, Oct. 1999, pp. 743–748.

[14] K. L. Chrzan, W. L. Vosloo, and J. P. Holtzhausen, ‘‘Leakage current on porcelain and silicone insulators under sea or light industrial pollution,’’

IEEE Trans. Power Del., vol. 26, no. 3, pp. 2051–2052, Jul. 2011.

[15] A. I. Elombo, J. P. Holtzhausen, H. J. Vermeulen, P. J. Pieterse, and W. L. Vosloo, ‘‘Comparative evaluation of the leakage current and aging performance of HTV SR insulators of different creepage lengths when energized by AC, DC+ or DC–in a severe marine environment,’’ IEEE Trans. Dielectr. Electr. Insul., vol. 20, no. 2, pp. 421–428, Apr. 2013.

[16] W. L. Vosloo, J. P. Holtzhausen, and A. H. A. Roediger, ‘‘Leakage current performance of naturally aged non-ceramic insulators under a severe marine environment,’’ in Proc. IEEE AFRICON, Stellenbosch, South Africa, Sep. 1996, pp. 489–495.

[17] W. L. Vosloo and J. P. Holtzhausen, ‘‘The effect of thermal characteristics of power line insulators on pollution performance,’’ in Proc. IEEE 6th Africon Conf. Africa, George, South Africa, Oct. 2002, pp. 609–612.

[18] W. L. Vosloo, F. Engelbrecht, N. Heger, and J. P. Holtzhausen, ‘‘Implemen- tation of 220 kV in-service insulator performance test tower in Namibia,’’

in Proc. AFRICON, Windhoek, South Africa, Sep. 2007, pp. 1–5.

[19] IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems, IEEE Industry Applications Society/Power Engineering Society, IEEE Standard SM 519, 1992.

[20] X. Jiang, Y. Shi, C. Sun, and Z. Zhang, ‘‘Evaluating the safety condition of porcelain insulators by the time and frequency characteristics of LC based on artificial pollution tests,’’ IEEE Trans. Dielectrics Electr. Insul., vol. 17, no. 2, pp. 481–489, Apr. 2010.

[21] F. M. Dekking C. Kraaikamp, H. P. Lopuhaa, and L. E. Meester, A Modern Introduction to Probability and Statistics: Understanding Why and How.

London, U.K.: Springer-Verlag, 2005.

[22] D. E. Hinkle, W. Wiersma, and S. G. Jurs, Applied Statistics for the Behavioral Sciences, 5th ed. Boston, MA, USA: Houghton Mifflin, 2003.

[23] M. H. Kutner, C. J. Nachtsheim, J. Neter, and W. Li, ‘‘Introduction to nonlinear regression and neural networks,’’ in Applied Linear Statistical Models, 5th ed. New York, NY, USA: McGraw-Hill, 2005, ch. 13.

[24] L. Cui and M. Ramesh, ‘‘Prediction of flashover voltage using electric field measurement on clean and polluted insulators,’’ Int. J. Electr. Power Energy Syst., vol. 116, Mar. 2020, Art. no. 105574.

[25] A. Holmes, B. Illowsky, and S. Dean, Introductory Business Statistics.

Houston, TX, USA: OpenStax, Rice University, 2018.

[26] C. Zhao, H. Mei, M. Zhu, H. Dai, L. Wang, and Z. Zhou, ‘‘Devel- opment of contamination flashover pre-warning system and analysis of operating experience,’’ IEEE Trans. Dielectr. Electr. Insul., vol. 22, no. 4, pp. 2234–2241, Aug. 2015.

[27] J. Dadashizadeh Samakosh and M. Mirzaie, ‘‘Analysis of leakage current characteristics during aging process of SiR insulator under uniform and longitudinal non-uniform pollution conditions,’’ Measurement, vol. 147, Dec. 2019, Art. no. 106862.

[28] A. R. Verma, S. R. B., and R. Chakraborty, ‘‘Multistress aging studies on polymeric insulators,’’ IEEE Trans. Dielectr. Electr. Insul., vol. 25, no. 2, pp. 524–532, Apr. 2018.

[29] M. R. M. Fernando and S. Gubanski, ‘‘Ageing of silicone rubber insulators in coastal and inland tropical environment,’’ IEEE Trans. Dielectr. Electr.

Insul., vol. 17, no. 2, pp. 326–333, Apr. 2010.

[30] K. Wieczorek, ‘‘Influence of the shape of composite insulators housing on their aging resistance in presence of high DC voltage and clean fog,’’

Przegla¸d Elektrotechniczny (Electrotechnical Review), vol. R.94, no. 10, pp. 81–84, Oct. 2018.

[31] E. Fontana, S. C. Oliveira, F. J. M. M. Cavalcanti, R. B. Lima, J. F. Martins-Filho, and E. Meneses-Pacheco, ‘‘Novel sensor system for leakage current detection on insulator strings of overhead transmission lines,’’ IEEE Trans. Power Del., vol. 21, no. 4, pp. 2064–2070, Oct. 2006.

[32] Y. Tu, B. Gong, C. Wang, K. Xu, Z. Xu, S. Wang, F. Zhang, and R. Li,

‘‘Effect of moisture on temperature rise of composite insulators operating in power system,’’ IEEE Trans. Dielectr. Electr. Insul., vol. 22, no. 4, pp. 2207–2213, Aug. 2015.

[33] J. Y. Li, C. X. Sun, and S. A. Sebo, ‘‘Humidity and contamination severity impact on the leakage currents of porcelain insulators,’’ IET Gener., Trans- miss. Distrib., vol. 5, no. 1, pp. 19–28, Jan. 2011.

[34] C. S. Richards, C. L. Benner, K. L. Butler-Purry, and B. D. Russell,

‘‘Electrical behavior of contaminated distribution insulators exposed to natural wetting,’’ IEEE Trans. Power Del., vol. 18, no. 2, pp. 551–558, Apr. 2003.

[35] M. Faramarzi Palangar, M. Mirzaie, and A. Mahmoudi, ‘‘Improved flashover mathematical model of polluted insulators: A dynamic anal- ysis of the electric arc parameters,’’ Electr. Power Syst. Res., vol. 179, Feb. 2020, Art. no. 106083.

[36] S. Chandrasekar, C. Kalaivanan, A. Cavallini, and G. Montanari, ‘‘Investi- gations on leakage current and phase angle characteristics of porcelain and polymeric insulator under contaminated conditions,’’ IEEE Trans. Dielectr.

Electr. Insul., vol. 16, no. 2, pp. 574–583, Apr. 2009.

WALUYO (Member, IEEE) was born in Magelang, Indonesia, in 1969. He received the master’s and Ph.D. degrees in high voltage engineering and technology from the Institut Teknologi Bandung, Bandung, Indonesia, in 2002 and 2010, respectively. He is currently an Associate Profes- sor with the Department of Electrical Engineering, Institut Teknologi Nasional Bandung. His research interests include high voltage engineering and technology, power transmission, and automation systems.

DINI FAUZIAH was born in Kuningan, Indone- sia, in 1993. She received the master’s degree in high voltage engineering and technology from the Institut Teknologi Bandung, Bandung, Indonesia, in 2017. She is currently an Academic Staff with the Department of Electrical Engineering, Institut Teknologi Nasional Bandung. Her research interests include high voltage engineering and technology, smart grids, and numerical analyses.

ISMAIL MUHAMMAD KHAIDIR was born in Kuningan, Indonesia, in 1997. He received the bachelor’s degree in electrical engineering from the Institut Teknologi Nasional Bandung, Ban- dung, Indonesia, in 2020. He currently works for a private company. His research interests include high voltage engineering and electrical installation.