Experimental study using design of experiment of pollution layer effect ...

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H. Terrab and A. Bayadi: Experimental Study Using Design of Experiment of Pollution Layer Effect on Insulator Performance

Experimental Study Using Design of Experiment of Pollution Layer Effect on Insulator Performance Taking into Account the Presence of Dry Bands Hocine Terrab and Abdelhafid Bayadi Electrical Engineering Department Faculty of Technology Ferhat ABBAS University Setif - 1 -, Algeria

ABSTRACT Performance of outdoor insulator under polluted conditions depends specially in the contamination layer configuration and conductivity, which makes important to take them into account during the conception and the design of new insulator. This paper presents an experimental study of the flashover voltage of polluted insulator as a function of pollution layer parameters such as; conductivity, layer length, position, number and width of dry bands. Many configurations of pollution distribution are studied using design of experiment methodology. Parameters effects and their interactions have been investigated and evaluated using ANOVA variance analysis statistical technique. The relationship between pollution parameters and the flashover voltage are modeled and analyzed using response surface methodology. Results show how much the flashover voltage of non-uniformly polluted surface is mainly influenced by length of contamination layer and conductivity. Moreover, the obtained statistical models of flashover voltage are adequate with experimentation results. Such information can be exploited to optimize the design of glass insulator used in polluted areas, by making suitable design to create much and wider dry bands in the middle of the insulator surface. Index Terms - Flashover, insulator contamination, non-uniform pollution, dry band, ANOVA analysis, design experiment, statistics.

1 INTRODUCTION THE performance of outdoor insulators is a key factor in the determination of power systems reliability. The insulators not only must withstand normal operating voltage, but also must avoid flashovers from occurring. The reduction of its performance is mainly affected by the pollution contamination deposits from air-borne, which can form a conducting or partially conducting surface layer when fog or wet, which enhances the chances of flashover. It is clear that the severity and the configuration of the pollution layer on the insulator depend mainly in the interaction between the environmental operating condition and the insulator shape. This makes the quantification of its effect much difficult because of the unpredictable behavior of environmental parameters. Therefore studying and testing insulator performance under different configuration of pollution uniform, non uniform and discontinuous is useful for the conception and the design of new insulators [1, 2]. Most of researches made to resolve this subject are carried out using uniform distribution of contamination, which cannot reflect the real behavior of pollution in the field which is far to be uniform [3, 4]. Manuscript received on 31 December 2013, in final form 22 March 2014, accepted 25 April 2014.

Therefore, studying insulator performance under uniform pollution distribution is not sufficient [5, 6]. The effect of pollution distribution on insulator performance is not yet practically treated because of many contradictions between researchers, concerning the difference between the flashover voltage under uniform and non-uniform pollution distribution. [7, 8] found that the insulator flashover strength under non uniform pollution distribution is higher than that obtained under uniform case, while [9] proved the opposite. This contradiction requires further investigation. The mechanism of flashover of polluted insulator is investigated by many researchers, static and dynamic models have been developed [10-14]. It has been shown that the flashover voltage is affected by various parameters acting simultaneously such us; the severity of pollution, the configuration of pollution and the voltage polarity [5, 9]. Generally, dry bands formation on the contaminated and wetted insulators is unavoidable. It can contribute largely in the discontinuity of the pollution layer. It leads to the distortion of the potential distribution on the surface of insulators which makes the applied electric field on these dry bands higher. However, the withstand of polluted insulators is directly related to the dry bands formation [15, 16]. [17] shows that the creation of a dry band is closely associated with corona discharge on the top surface, and the

DOI 10.1109/TDEI.2014.004542

IEEE Transactions on Dielectrics and Electrical Insulation

Vol. 21, No. 6; December 2014

partial discharges on the surface prevents the formation of a thin film of water in areas where the glass is thinnest. Statistical modeling also is already used to make a relation between pollution severity, leakage current and flashover voltage [3, 18, 19]. Statistical analysis is a powerful tool for predicting the degree of pollution severity, dimensioning insulator, selection of the insulation levels and making decisions of washing program in polluted areas [3, 20]. Using the design of experiments methodology is the better way to obtain the maximum of information with minimum of experiments to be carried out without influencing the quality of the results [21-23]. Using mathematical processing of design of experiment data enables a rather accurate evaluation of multi -factor effects and interactions. In this framework, the paper presents an experimental study of the effect of pollution layer distribution and its conductivity on the performance of insulator surface, taking into account the layer length and its position, the dry bands number and its width. This investigation is based on the use of the design of experiment methodology and the analysis of variance statistical technique to evaluate and to determine the effect of each factor. The response surface methodology is used to model the relation between the studied factor and the flashover voltage of insulation. This study presents a lot of advantages because it reduces the gain of time, money and labor involved in insulator laboratory tests for conception and designing.

2 EXPERIMENTAL SETUP AND PROCEDURE Experimental work is carried out in the LGEG Laboratory at the University of Guelma. The used experimental setup consists of an AC generator, a test cell, The AC test voltage is provided by a HAEFELY transformer type (5 kVA, 100 kV, 50 Hz), a control panel OT 276 and the flashover voltage is recorded through a digital measuring instrument DMI 551 (Figure 1). The insulator model is a rectangular plane made of glass having 6 mm thickness, 50 mm width and a total length of 150 mm. The ground electrode consists of a band of steel in contact with the experimental model. The HV electrode consists of a point made of steel. The total distance between electrodes is equal to 100 mm (Figure 2). The pollution is prepared using a simple electrolyte of water and sodium chloride. In first time, the insulating surface is cleaned with water and dried with paper tissue. Then, it is perfectly cleaned with alcohol to ensure the perfect cleanliness of the studied model. Spraying solution is used to obtain the desired configuration indicated in tables 1 and 2. The pollution layer control is achieved by masking the part of insulator that we want to save clean, then we apply 5 pulverizations from 50cm of distance using adequate sprayer, this technique can allow a good reproduction of pollution layer thickness (Figure 3). The applied voltage was increased using constant speed of the slope until the flashover occurs. This process was repeated for all configurations indicated in tables 1 and 2. In order to realize a reliable study, values of flashover voltage reported in this work are the average of twenty five repetition measurements for each case. The obtained flashover voltages are converted to the standard conditions of pressure, temperature and humidity according to IEC 60060-1[24].

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Table 1. Experimental results of non uniform pollution layer flashover voltage using Taguchi design.

Conductivity (mS/cm) 10 10 10 10 10 10 10 10 10 50 50 50 50 50 50 50 50 50 90 90 90 90 90 90 90 90 90

Length layer (mm) 20 20 20 50 50 50 80 80 80 20 20 20 50 50 50 80 80 80 20 20 20 50 50 50 80 80 80

position Ground Middle HV Ground Middle HV Ground Middle HV Ground Middle HV Ground Middle HV Ground Middle HV Ground Middle HV Ground Middle HV Ground Middle HV

Flashover voltage (kV) 41.38 42.71 41.03 30.66 32.2 30.46 19.67 20.23 20.1 38.84 40.41 37.04 26.66 28.01 26.678 17.33 18.975 18 36.45 37.915 35.792 26.22 27.1 26 13.8 15.3 15.58

Figure 1. Experimental setup, SG: voltage control unit, HV transformer, R: current limiting resistor (2.5 MΩ), V: voltage measurement DMI 551, C1 and C2: capacitive voltage divider (100 pF), In: Insulator model, Ca: Camera, PC: personal computer.

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H. Terrab and A. Bayadi: Experimental Study Using Design of Experiment of Pollution Layer Effect on Insulator Performance

(A) Figure 2. Flat insulator model.

The experimental work was divided into three series: the first experiment is aimed to investigate the conductivity effect on flashover voltage for uniform pollution distribution. The second experiment was to investigate the effect of non uniform pollution on insulator flashover voltage. Three parameters of pollution layer are studied: conductivity, length and position. In this second series, the adopted experimental design is L27 Taguchi standard orthogonal array. Parameters to be studied and the attribution of the respective levels are indicated in Table1. Three levels for each parameter are chosen. Three conductivities (10, 50 and 90 mS/cm), three widths of pollution layer (20, 50 and 80 mm) and three positions are adopted to obtain non uniform pollution (pollution band in HV electrode side, Band in ground electrode side and the last in the middle of insulator model). Positions are coded by values 1, -1 and 0 respectively to HV, ground and middle position (Figure 4a). Table 2. Experimental results of discontinuous non uniform pollution layer flashover voltage using factorial mixed design.

Conductivity Number of dry (mS/cm) bands Width (mm) 10 1 4 10 1 8 10 1 12 10 2 4 10 2 8 10 2 12 10 3 4 10 3 8 10 3 2 90 1 4 90 1 8 90 1 2 90 2 4 90 2 8 90 2 2 90 3 4 90 3 8 90 3 12

Flashover voltage (kV) 37.6 38.05 38.83 38.8 39.71 41 41.22 42.08 42.5 35.39 37.22 38.24 36.34 38.37 39 37.64 38.98 40.97

(B) Figure 3. Spraying procedure: (A) non uniform layer of 20 mm in the middle of insulator model, (B) discontinuous non uniform layer of 50 mm with one dry band in the middle position.

The third experiment series was to investigate the effects of discontinuous non uniform pollution layer on the dielectric strength of insulating surface. Two parameters from second series are fixed, the length of layer and the position. Three parameters of discontinuous pollution layer are studied: dry bands number, width and conductivity. In this series, an L18 mixed factorial design is adopted as an experimental design. Parameters to be studied and the attribution of the respective levels are indicated in Table 2. Two conductivities are used (10 and 90 mS/cm), three widths of dry bands (4, 8 and 12 mm) and the number of dry bands is varied from 1 to 3 bands. The pollution is located in the middle of insulator with fixed length of 50 mm (Figures 4b and 4c).

 

Figure 4. Pollution configuration: (A) non uniform layer of 50 mm in the middle of insulator model, (B) discontinuous non uniform layer of 50 mm with one dry band in the middle position, (C) discontinuous non uniform layer of 50 mm with two dry bands in the middle position.

IEEE Transactions on Dielectrics and Electrical Insulation

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Vol. 21, No. 6; December 2014

RESULTS AND DISCUSSIONS

3.1 UNIFORM POLLUTION DISTRIBUTION This part of work is aimed to study the effect of pollution conductivity on flashover voltage for the uniform distribution case. The results in (Figure 5) shows that flashover voltage characteristic is non linear and decreases with increasing of pollution conductivity. This result is experimentally obtained for three levels of pollution severity: light, medium and heavy, represented by 10, 50 and 90 mS/cm respectively. This can be  explained  by  the  fact  of  increasing  conductivity  of  the  electrolyte  leads  to  conduct  more  leakage  current  on  insulator  surface.    Joule  heating  effect  causes  more  water  evaporation  which  leads  to  create  dry  bands  where  the  current  density  is  highest.  The  electric  field  in  the  electrolyte increase and it will be supported by these dry  bands. The electric field in the air immediately above this  point  has  approximately  the  same  value.  Once  the  ionization  level  in  this  air  is  reached,      discharge  occurs  and its elongation makes more probable of total flashover  for lower applied voltage.

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where, Vfov is the flashover voltage C is the pollution conductivity in (mS/cm) P-value is the probability of testing the significance of null hypothesis S is the standard deviation F is the standard “F” statistic R² is the residual sum of squares R² (adj) is the adjusted residual sum of squares DF is the degrees of freedom SS is the sum of squares MS is the mean sum of squares A high value of R2 (adj) indicates that the model can explain the variability over a wide range and a high “F” ratio and low “P” value means that the model is highly significant. The developed models were validated by analyzing model coefficients, predicted values and using prior experience, and physical theory.

Figure 5. Flashover voltage variation for uniform pollution distribution.

A quadratic model by regression is developed to estimate flashover voltage of uniform pollution in function of contamination layer conductivity. The generic form of quadratic regression models is given by the equation as follow:

Figure 6. Normal probability plots of flashover voltage for uniform pollution distribution.

Table 3. Statistical results of flashover voltage of uniform pollution layer based model.

The test of Anderson-Darling and the normal probability plots of the residuals versus the predicted response for the flashover voltage of insulator under uniform pollution are plotted in (Figure 6). The data closely follows the straight line. The null hypothesis is that the data distribution law is normal and the alternative hypothesis is that the data distribution is non-normal. Using the P-value which is higher than alpha of 0.05 (level of significance which is 0.82 in this case). So, we cannot reject the null hypothesis. That means the data follow a normal distribution which implies the adequacy of the proposed model.

The regression equation is Vfov = 20.7806 – 0.111186 C + 0.0004554 C2  Source DF SS MS F P-value Reg 2 17.7042 8.85208 406.378 0.002 Error 2 0.0436 0.02178 Total 4 17.7477 S = 0.14759 R2 = 99.8 % R2 (adj) = 99.5 %

3.2 NON-UNIFORM POLLUTION DISTRIBUTION In this part, focus is made to investigate the effect of non uniform pollution layer parameters, such as the conductivity, the layer length and the position on the insulator surface dielectric strength. Their influence is identified using ANOVA analysis in order to study the non-uniform pollution layer influence on the flashover strength of the insulating

aX

a X

Table 3 shows the statistical results of the flashover voltage based model in case of uniform pollution distribution. The proposed model is adequate with determination factor 99.5 %.

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H. Terrab and A. Bayadi: Experimental Study Using Design of Experiment of Pollution Layer Effect on Insulator Performance

surface. Using L27 Taguchi standard orthogonal array, the experimental results are given in Table 1. This plan was developed for establishing the quadratic model of flashover voltage under non uniform pollution. The model is given as follows: aX

a X

a XX

where Y is the desired response: flashover voltage. a0 is constant. ai , aii and aij represent the coefficients of linear, quadratic and cross-product terms respectively. Xi reveals the coded variables that correspond to the studied parameters. The test for significance of model coefficients is performed to verify the goodness of fit for the obtained model. The variance analysis is used to summarize these tests. Then, the main effects, interactions and normal probability plots corresponding to ANOVA analysis were performed. These plots are used to determine the effect of pollution layer characteristics on the flashover voltage of non-uniformly polluted insulator and they are illustrated in (Figure 7). The statistical significance of the fitted quadratic models is evaluated using the P -values of ANOVA. Values are given in Table 4. When P -values are less than the threshold of 0.05 (or 95% confidence), the obtained model is considered to be statistically significant. It proves that the chosen terms in the model have significant influence on the responses. The R² coefficients of determination is an important coefficient because it represents the ratio of the explained variation to the total variation and is a measure of the degree of fit, When R² approaches to unity, the obtained model is better. Table 4. Analysis of variance for flashover voltage of non uniformly polluted insulator. Term

DF

seq SS

adj SS

adj MS

F

P-value

PC %

C

1

108.94

108.94

108.94

221.28

0.000

4.9674

L

1

2060.43

2060.43

2060.43

4184.99

0.000

93.9506

P

1

0.01

2.28

2.28

4.62

0.043

0.0004

P*P

1

10.68

10.68

10.68

21.68

0.000

0.4870

5.48

0.029

L*P

1

2.70

2.70

2.70

Error

21

10.34

10.34

0.49

Total

26

2193.1

0.1231 0.4715 100

where, C is the pollution conductivity; L is the pollution layer length; P is the position of layer; DF is the degrees of freedom; F is the standard “F” statistic; P-value is the probability of testing the significance of null hypothesis; SS is the sum of squares; SS adj is the adjusted sum of squares; MS is the mean sum of squares; PC is the percentage of contribution.

Figure 7. Main effects and interactions plots for non uniform pollution layer characteristics.

From Table 4, it can be seen that the L, C, P, P*P and the product L*P are significant terms on flashover voltage of non uniform pollution distribution. Also, the most significant factor on the flashover strength is the length of pollution layer with contribution of 93.95 % in the total variation. The next largest contribution on flashover voltage comes from the conductivity with a contribution of 4.97 %. Finally, the influence value of the position and its interactions with the layer length is less than 0.61 %, it does not represent a significant contribution on flashover voltage. In Figure 7, the main interaction effects are plotted. It is clearly observed that the length of pollution layer strongly affects the flashover voltage. The dielectric strength of insulator decreases with increasing of the conductive layer length. Also, the conductivity of pollution layer has a significant decreasing effect. Dielectric strength of insulator surface is improved by decreasing pollution conductivity. Finally, there is small difference observed on flashover voltage due to the pollution position, we notice that flashover strength is higher when pollution layer is located in the middle position of model than HV and ground position. The insulating model has the highest dielectric strength when the conductive layer is smaller and located in the middle position. The explanation of the conductivity effect has been already illustrated in the section 3.1. The decreasing effect of pollution layer length on the dielectric strength can be explained by a significant reduction of the clean band width. The electric field applied on these bands increases and reaches the breakdown strength of these bands for lower applied voltage, which reduce strongly the flashover

IEEE Transactions on Dielectrics and Electrical Insulation

Vol. 21, No. 6; December 2014

strength. That can be electrically explained by the reduction of the total equivalent impedance of the insulator model proportionally to the increase of the conductive layer length. This impedance has a dominant capacitive behavior when the polluted layer is small, it starts to be more resistive and lower capacitive with increasing of the conductive layer length. This reduction of the total equivalent impedance leads to reduce the flashover strength of insulator [4]. The presence of the conductive layer in the middle position increases slightly the flashover strength comparatively to both configuration HV and ground, this can be explained by the presence of two dry bands near the electrodes which leads to decrease the maximum electric field applied on each one and increase the surface dielectric strength. That can be electrically explained by the fact of the total equivalent impedance of the polluted insulator is the highest one if the conductive layer length is smaller and located in middle of insulator. In the interaction between L and P, it is shown that the slight effect of position start to be neglected when the length of layer increase until 80mm when the flashover voltage for three configurations practically the same. That is related to the significant reduction of clean band width, and the electric field reaches the breakdown for small applied voltage for the three configurations. The test of Anderson-Darling and normal probability plots of the residuals versus the predicted response of the flashover voltage of insulator under non uniform pollution is plotted in (Figure 8). The data closely follows a straight line. The null  hypothesis is that the data distribution law is normal and  the  alternative  hypothesis  is  that  the  data  distribution  is  non‐normal. Using the P-value which is higher than alpha of 0.05 (level of significance which is 0.134 in this case). So we cannot reject the null hypothesis. That means the data has a normal distribution which implies the adequacy of the proposed model. The initial analysis of the obtained response from the RSM (response surface methodology) takes into account all parameters effects and their interactions. The model is reduced by eliminating terms with no significant effect on the response through the backward elimination process. The final

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quadratic model is represented as follow: Vfov 50.112 0.062 C 0.357 L 0.808 P 1.334 P P 0.016 L P With,

R² = 99.5 %

R² (adj) =99.4 %

Figure 9 represents the experimental and the calculated values obtained from the quadratic model for 27 experimental vectors of the test set. The calculated flashover voltages based on the developed statistical model are in a good accordance with the experimental values. We show that the calculated results vary in an accepted error margin.

Figure 9. Representation of the actual and the estimated values from quadratic model.

3.3 DISCONTINUOUS NON-UNIFORM POLLUTION DISTRIBUTION In this part, focus is made to investigate the effect of discontinuous non uniform pollution layer parameters, such as the conductivity, the number of dry bands and its width on the insulator surface dielectric strength. Their influence is evaluated using ANOVA analysis in order to study the behavior of the insulating surface dielectric strength. The L18 factorial mixed design is adopted for this case, the experimental results are given in Table 2. Table 5. Analysis of variance for flashover voltage for discontinuous non uniformly polluted insulator.

Figure 8. Normal probability plots of flashover voltage for non uniform pollution distribution.

Term

DF

seq SS

adj SS

adj MS

F

C

1

17.287

1.134

1.134

11.89

P-value PC % 0.005

26.976

N

1

27.180

18.398

18.398

192.91

0.000

42.410

W

1

15.300

2.288

2.288

23.99

0.000

23.875

C*N

1

1.748

1.748

1.748

18.33

0.001

2.722

14.90

0.002

C*W

1

1.421

1.421

1.421

Error

12

1.144

1.144

0.095

Total

17

64.082

where, C is the pollution conductivity; N is the number of dry bands; W is the Width of dry bands.

2.217 1.785 100

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H. Terrab and A. Bayadi: Experimental Study Using Design of Experiment of Pollution Layer Effect on Insulator Performance

From Table 5, it is shown that the C, N, W, C*N, and the product C*W are significant terms on flashover voltage of discontinuous non-uniformly pollution distribution. The most significant factor on the flashover voltage is the number of dry bands with contribution of 42.41 % in the total variation, the next largest contribution comes from the conductivity with a 26.976 %. The last contributions come from the width of dry bands with 23.875 %. The interactions of conductivity with the width and the number of dry bands have a contribution of 4.94 %. In Figure 10 the main and interaction effects are represented. It is shown that the increase of the number of dry bands has an increasing effect on the flashover voltage. The conductivity of pollution layer also has an important and decreasing effect as like as obtained in 2nd case. Dielectric strength of insulator surface  is  improved  by  increasing  of  the  dry  bands  width  in  the  middle  of  conductive  layer. Finally, there is small variation observed on flashover voltage due to the interactions between factors. The insulating surface reaches its highest dielectric strength when the conductive layer contains a lot of wider dry bands located in the middle position of the sample. The explanation of the conductivity effect has been already presented in the previous case of uniform distribution. The increasing effect of dry bands width in middle section of insulator on the dielectric strength is related to the reduction of the maximum field strength applied on these clean bands. The increasing effect of the number of dry bands on the dielectric strength is related to the presence of many dry bands

Figure 11. Normal probability plots of flashover voltage for discontinuous uniform pollution distribution.

created at the same time in the middle section of insulator that have ability to weaken the field strength of one another. Therefore, for many dry bands created at the same time, the field strength on each one is usually less than that of each one when it is individually exists. Consequently, the dry bands created at the intermediate section may be considered as a potential barrier which may effectively weaken the strength near the electrode and may obstruct the formation of large voltage drop on their outer dry bands. The test of Anderson-Darling and normal probability plots of the residuals versus the predicted response for the flashover voltage of insulator under discontinuous non uniform pollution is plotted in (Figure 11). The  data  closely  follows  the  straight  line.  The  null  hypothesis  is  that  the  data  distribution  law  is  normal  and  the  alternative  hypothesis  is  that  the  data  distribution  is  non‐normal. Using the P-value which is higher than alpha of 0.05 (level of significance which is 0.533 in this case), so, we cannot reject the null hypothesis. That means the data have a normal distribution which implies the adequacy of the proposed model. The initial analysis of the obtained response from the response surface methodology takes into account all factors effects and their interactions. The model is reduced by eliminating terms with no significant effect on the response through the backward elimination process. The final quadratic model is represented as follow: With,

Figure 10. Main effects and interactions plots for discontinuous non uniform pollution characteristics.

R² = 98.2 %

R² (adj) = 97.5 %

Figure 12 represents the experimental and the calculated values obtained from the quadratic model for 18 experimental vectors of the test set. The calculated flashover voltages based on the developed statistical model found a good accordance with the experimental values. We show that the calculated results vary in an accepted error margin.

IEEE Transactions on Dielectrics and Electrical Insulation

Vol. 21, No. 6; December 2014

Figure 12. Representation of the actual and the estimated values from quadratic model.

4 COMPARISON OF DIFFERENT POLLUTION CONFIGURATIONS EFFECTS In this part, comparison is made between the results obtained in section 3.1, 3.2 and 3.3 for different pollution configurations shown in table 1 and 2. Other tests for pollution conductivity (30, 50 and 70 mS/cm) are done to accomplish the comparison which are not used for the statistical analysis in previous sections. These tests are done following the same procedure presented in section 2. (Figure 13) shows the difference between the flashover voltage of uniform, non uniform and discontinuous non uniform pollution distribution. Results show how that the distribution of the pollution layer influences the insulator dielectric strength, the flashover voltage values of insulator under non uniform distribution are higher than those obtained under uniform pollution. Moreover, for the same length of pollution layer on the surface, it is shown that the flashover voltage is higher in the discontinuous distribution case. The dielectric strength of insulator surface increase with increasing of dry bands number, the value of flashover voltage for pollution layer of 50 mm located in the middle position with 3 dry bands can reach the flashover value of pollution layer of 20 mm on insulator surface. From this result, it is found that the dielectric strength of insulator can be improved if the distribution of pollution layer on its surface is not uniform. In addition, if the conductive layer contains much and wider dry bands the dielectric strength will be higher. 5 PROPOSED DESIGN OF INSULATOR Recently, ceramic insulators with RTV silicone rubber coating was adopted for several substations and transmission lines in different country, because it have proved their effectiveness in preventing contamination outages in coastal and industrial areas. RTV coating can remarkably suppress the surface activity in condensation case. In other hand, coating can present some problems in service such as; the poor adhesion which is one of the main problems due to difficult application conditions in the field in which the surfaces were

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Figure 13. Comparison between Vfov under uniform and non uniform distribution.

either moist or inadequately cleaned which require qualified skills, then, until now, there is no true and fast way to establish the end-of-life of a RTV coating [25, 26]. Moreover, in the case of light rain, the recorded level of activity has been similar for both coated and non-coated insulators and ageing under this condition need further investigation. [27]. For the above reason, it can be mentioned that using glass insulator still available for areas characterized by light rain and low pollution activity or when coating is not a profitable solution technically and economically. This makes improving glass insulator better choice when coating is not required. In this part, an attempt is made to optimize the design of glass insulator. The proposal is based on the combination between the obtained results and the results showed in (Figure 14) (dry band formation) [16]. It is clear that the insulator has the best performance when the pollution layer is located in the middle of the insulator, which can obtained by covering the insulator surface near cap and pin electrodes by a thin layer of hydrophobic silicone rubber to prevent the creation of wet polluted film in these areas. Moreover, from (Figure 14) we observe that the dry bands formation starts on the top surface where the glass is the thinnest and between the pin and the first rib by corona discharges effect. That prevents the formation of a thin film of water in these areas.

Figure 14. Dry bands formation on top surface insulator by corona discharges process.

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H. Terrab and A. Bayadi: Experimental Study Using Design of Experiment of Pollution Layer Effect on Insulator Performance

From the mentioned results and literature of the mechanisms of dry bands generation on the surface of polluted insulator, it seems possible to improve the design of outdoor insulator through the increase of the ribs number on the bottom surface. Also, by increasing the distance between them where the glass is thinnest. This conception leads to create a lot of wider dry bands on the top surface which increase the dielectric strength of insulator. The new design based on changing the ribs form, number and its dimensions. With saving the real dimensions of insulator such as the diameter, the height and the leakage distance. Nevertheless, this  proposal  needs  further  investigations  which  present  our  future  perspective. 

REFERENCES [1] [2]

[3]

[4]

[5]

6 CONCLUSION From this work, we show that the insulator flashover strength depends on the distribution and the conductivity of pollution layer on its surface; it decreases strongly with the increase of the pollution layer length and the conductivity. The insulating surface has the best dielectric strength when the conductive layer is smaller and located in the middle of insulating surface. Furthermore, the dielectric strength of discontinuous non-uniformly polluted surface increase with increasing the number of dry bands and its width successively. The dry bands can be considered as a potential barrier in the conductive layer which makes the flashover voltage of discontinuous non uniform pollution is higher than the non uniform and uniform distribution whatever the configuration of pollution. The obtained results may be useful to optimize the design of insulator for polluted areas. For example, improve the shape of insulators by adding thin layer of silicone near electrode to make difficult to create a thin polluted film on ground and HV position. Moreover, making an appropriate design to increase the number and the width of dry bands created of in the middle section of insulator by changing ribs form, number and dimensions. The use of design of experiments seems a very interesting tool for studying and predicting the surface characteristics of insulator. Eventually, the proposed experimental results and the statistical approaches were performed for laboratory model, which bring a reliable methodology to optimize the design of outdoor insulator. Moreover, it can be extended efficiently for studying other high voltage insulator.

ACKNOWLEDGMENT This work was achieved in the electrical engineering laboratory at the University of 8 May 1945 of Guelma, Algeria. The authors thank all members of the LGEG laboratory for their help. Thanks are extended to Professor Andrea Cavallini from LIMAT laboratory at Bologna University, Italy and Dr. Khaider Bouacha from Souk Ahras University for providing encouragements and important suggestions.

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IEEE Transactions on Dielectrics and Electrical Insulation

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2495 Hocine Terrab was born in 1988. He received the B.Sc. and M.Sc. degrees in electrical engineering from the University of 8 May 1945, Guelma, Algeria, in 2009 and 2011, respectively. He joined Setif University as a Ph.D. degree student in 2011. He is currently working on the pollution effect of outdoor insulation.

Abdelhafid Bayadi obtained the degree of Ingénieur d’Etat in electrical engineering in 1991. The degree of magister in electrical engineering in 1996 from the Electrotechnics Institute of Setif. He joined the same institute in 1998 as an Assistant Professor. He obtained the Ph.D. degree in high voltage engineering in 2005. In the same year, he was appointed a Senior Lecturer and in 2010, a Professor in electrical engineering at Setif University. His research interests are in power system modeling and simulation, power system transients, high voltage engineering, insulation systems, and power system overvoltage protection. He has co-authored over 30 publications. He is a member of the Steering Committee of the National Conference on High voltage (CNHT).