flashover process and frequency analysis of the leakage current on insulator model under non-uniform...

M. A. Douar et al.: Flashover Process and Frequency Analysis of the Leakage Current on Insulator Model

1070-9878/10/$25.00 2010 IEEE

1284

Flashover Process and Frequency Analysis of the Leakage Current on Insulator Model under non-Uniform Pollution Conditions

M. A. Douar, A. Mekhaldi and M. C. Bouzidi

Laboratoire de Recherche en Electrotechnique, Laboratoire de Haute Tension Ecole Nationale Polytechnique of Algiers

10 Avenue Hassen Badi, B.P 182, El-Harrach, 16200 Algiers, Algeria.

ABSTRACT

In this paper, we present results dealing with the non-uniform pollution carried out under 50 Hz applied voltage on a plane model simulating the 1512 L outdoor insulator largely used by the Algerian Company of Gas and Electric Power (SONELGAZ). Many configurations in non-uniform pollution are studied in the ENPs (Ecole Nationale Polytechnique dAlger) High Voltage Laboratory in order to analyze the impact of polluted layer distribution on the insulator dielectric performances. The polluted solution has a conductivity of 1.2 mS/cm obtained with distilled water and NaCl. Our investigations are particularly focused on the on line monitoring of both position and width of the contaminated layer. The flashover voltage and the leakage current magnitude have been investigated in order to study the flashover process on this insulating surface. A video apparatus is used to reflect the parallel discharges behavior, appearing when the polluted layer reaches a critical width. Phase angle values between applied voltage and leakage current (LC) signals at the fundamental frequency (50 Hz) are calculated using the Fast Fourier Transform (FFT) spectral analysis. Phase angle measurements indicate that the equivalent impedance of the insulator behaves like RC circuit with a high capacitive effect engendered by the pre-established clean band. This effect decreases when electric discharges occur at a particular voltage level. The Discrete Wavelet Transform (DWT) is adopted for the leakage current decomposition in several time-frequency bands. The STD-MRA (Standard deviation-Multi Resolution Analysis) of these frequency bands is calculated and is employed to choose the most interesting details that detect both position and width increasing of the conducting layer. Reported results show that the pollution surface state and the severity of this conducting layer deposited on insulator surface could be identified from the STD-MRA representation of leakage current frequency bands. It was shown that the high frequency band of the leakage current increases before the final flashover when the polluted layer is located in the middle of the plane model. It was established that a good correlation has been found between the insulator state surface and details of the leakage current obtained through the DWT decomposition. In fact, these details provide relevant information on both position and width of the polluted layer non-uniformly distributed on the insulator surface.

Index Terms Flashover process, parallel discharges, pollution severity, wavelet transform, plane model, leakage current, phase angle, monitoring.

1 INTRODUCTION

IN the power industry, outdoor insulators are widely employed to maintain electrical insulation ranging from distribution to transmission lines and to support the mechanical load between a conductor and the ground in power apparatus systems. Most of the time, insulators are subjected to sustained moisture and soluble and non-soluble contamination (dust and sand, chemical products and salt)

caused by natural or anthropogenic parameters [1]. These factors lead to the formation of a conducting layer on the insulator surface that significantly reduces the dielectric performance of the insulator. In fact, the pollution layer repartition on insulating surfaces is generally non-uniform [2-4] because of the insulators shape, their position with regard to the high voltage conductor, their height related to the ground, their arrangement, the weather conditions and the electrostatic attraction of particles deposited on the insulating surface in presence of the electric field. These non-uniform polluted layers can be formed by group, by sector or periodical [5]. As Manuscript received on 4 November 2010, in final form 8 April 2010.

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 17, No. 4; August 2010 1285

an example, the polluted layer can be deposited at back sections of insulators shed when dusts transported by the wind are deposited aerodynamically on the outdoor insulator surface.

The insulator pollution phenomenon is the major problem which is responsible of a significant flashover voltage decrease. This one depends on many parameters such as the nature of the deposit density under wetted conditions. However, the understanding of flashover phenomenon is still complex despite of many researches and experiments carried out [6-9] in order to understand the electric discharges development on the polluted surface leading to the flashover under wetted and contaminated conditions. The electric discharges appearance can be considered as a precursor sign just before the final insulator flashover [10-13]. Thus, it becomes necessary to develop diagnostic methods of the pollution severity on the insulator surface in the viewpoint of minimizing risks of damage to operating systems and reducing costs of maintenance. The aim of diagnostics is, in general, to get relevant information about the state of technical systems [14]. In insulator applications this means that diagnostics should be an aid in making decisions about if and when maintenance or replacement should be done. One practical method of flashover prevention and pollution characterization [15] consists of the soluble deposit density estimation evaluated by equivalent salt deposit density (ESDD) and non-soluble deposit density (NSDD); they are generally measured by using pilot insulator. The analysis of frequency characteristics of the leakage current, visual inspections in situ, thermal, acoustic and optical measurements, measurements of electric field distribution, LIF (Laser Induced Fluorescence) measurement for the biological life detection on the insulator surface and the microstructure based evaluation of field aged and new porcelain suspension insulators are the most interesting diagnostic methods [16, 17]. Examinations made up on leakage current waveforms showed that it is an efficient tool to collect information about the surface state of polluted insulators by analyzing current peaks when electric discharges occur on the insulating surface and performing an FFT (Fast Fourier Transform) analysis to extract its frequency characteristics [18]. Aulia et al [19] experiments results showed that the leakage current THD (Total Harmonic Distortion) increases when electric discharges occur under both salt fog and cement dust contamination if pulverization time is relatively long, then a combination of the two pollution types will increase the probability of the flashover occurrence. Meghnefi et al [20] analyzed the leakage current signal of a porcelain standard station insulator during a glaze ice accretion. The results showed that icing rate measurement can be achieved by using the time evolution of the third and fifth harmonic, as well as the phase angle between leakage current and applied voltage. Suda [21] established a method for monitoring contamination deposited on cap and pin insulators by studying leakage current waveforms and their frequency characteristics, and classified the transition of leakage current waveforms into six stages in order to predict flashover. The application of Discrete Wavelet Transform (DWT) on the leakage current is considered as another diagnostic tool for pollution severity

characterization in order to understand the behavior of time-frequency components of the leakage current especially when electric discharges occur in contaminated conditions. Chandrasekar et al [22] applied the DWT decomposition and calculated the STD-MRA (Standard Deviation- Multi Resolution Analysis) distortion ratio pattern of the leakage current. Reported results on both silicon rubber and porcelain insulators showed that the pollution severity of outdoor insulators could be identified through this ratio. Although, the DWT decomposition was greatly employed to remove the noise associated to the PD measurement [23-26] and no researchers were interested to estimate both position and width increasing of the non uniform polluted layer by using the DWT decomposition of current in order to characterize the severity of the polluted layer width and to diagnose the surface state of the polluted insulator before the final flashover. In this work, we study the impact of both position and width increasing of the non uniform polluted layer on frequency characteristics obtained with the DWT decomposition of the leakage current which passes through this polluted layer. We have particularly examined three non uniform pollution configurations on the insulating surface of the plane model which simulates the 1512 L outdoor insulator largely used by the Algerian company of Gas and Electric Power (SONELGAZ). The first configuration is obtained by contaminating a region of the insulator near the high voltage (HV) electrode. The second one is obtained by contaminating a region of the insulator near the ground electrode. The third one consists of contaminating the middle zone of the insulating surface. The FFT (Fast Fourier Transform) spectral analysis is performed on both leakage current and voltage waveforms at the fundamental frequency (50 Hz) for phase angle analyzing when the polluted layer width increases. The STD-MRA representation is employed to analyze the behavior of frequency components of the leakage current and at the same time, to detect components that give estimation about both position and width of the non-uniform polluted layer for each tested configuration. These investigations make it possible to establish the location of polluted layer which is distributed non-uniformly on the insulator surface. This is done by diagnosing the influence of both position and width of the contaminating layer on the dielectric performances of the plane model by means of the wavelet transform.

2 EXPERIMENTAL ARRANGEMENT Experiments are carried out in the ENPs (Ecole Nationale

Polytechnique of Algiers) High Voltage Laboratory test station (Figure 1), on a plane model reproducing the 1512 L outdoor insulator (Figure 2). The experimental setup is composed of a high voltage test transformer (50 kVA, 50 Hz), a TEKTRONIX digital oscilloscope of 500 MHz bandwidth for leakage current and applied voltage waveforms recording, a PC (personal computer) records data related to different widths of the polluted layer and a video apparatus SONY DCR-SR 45 is used to reflect the parallel discharges appearance when the polluted layer reaches a critical length that significantly decreases the dielectric performances of the insulator. Dimensions of the 1512 L outdoor insulator are

M. A. Douar et al.: Flashover Process and Frequency Analysis of the Leakage Current on Insulator Model 1286

Figure 3. Spraying method for both polluted bands: near the HV electrode and near the ground one.

Figure 4. Spraying method with a pollution band in the middle of the planemodel.

given in Table 1. In fact, the studied model is made of glass (500 mm x 500 mm x 3 mm). Two aluminum electrodes (50 cm x 3 cm) are cut up and connected to both high voltage and ground bounds. The distance between the two electrodes represents the leakage path of the 1512 L outdoor insulator (29.2 cm). First, the insulating surface is cleaned with tap water and dried with papers. Then, it is perfectly cleaned with some 70 alcohol to ensure a perfect neatness of the studied model.

Figure 1. Measurement of both voltage (U) and leakage current (I).

Leakage current waveforms are recorded through a shunt of

2 k inserted between the test sample and earth. A coaxial cable is used to recover leakage current data sent to the

oscilloscope. The sample frequency is 250 kHz in order to have 2 signal periods and to visualize clearly transitions of the leakage current during arcing phenomenon. The contamination of the insulating surface is non-uniform by spraying a solution composed of distilled water and NaCl having a conductivity of 1.2 mS/cm, 5 times on each side of the plane model and at 50 cm distance from this insulator (Figures 3 and 4). This salt solution is sprayed over the insulating surface in order to obtain three non uniform pollution configurations: the first one is a polluted band near the HV electrode, the second one is located near the ground electrode and the third one is situated in the middle of the plane model. Several widths of the polluted layer (X = 5, 10, 15, 20 and 25 cm) are adopted for these non-uniform configurations (Figures 3 and 4). The applied voltage level equals 27 kVrms in the case of both HV and ground polluted bands and approximately 40 kVrms for the middle polluted layer. These two levels correspond to the beginning of preliminary discharge phenomena that can be considered as the first stage of flashover process.

Table 1. Technical data of the 1512 L outdoor insulator.

Name D(mm) N(mm) P(mm) Leakage distance(mm) 1512 L 255 16 127 292

(a)

(b) Figure 2. (a) 1512 L insulator profile, (b) laboratory plane model profile.


Figure 5. Flashover voltage characteristics for the three configurationsaccording to the ratio between the polluted layer width and the total leakagedistance.

3 RESULTS AND DISCUSSIONS In this section, we are interested in the flashover voltage measurement, the leakage current magnitude variation, the phase angle analysis between leakage current and voltage waveforms at the fundamental frequency and parallel discharges appearance in the air gap formed between the polluted layer and electrodes during the flashover process. These discharges occur for a particular applied voltage level and when the polluted layer width reaches a critical value. These investigations make it possible to diagnose the surface state of the polluted insulator in order to prevent the insulator flashover and to characterize the pollution severity.

3.1 THE FLASHOVER VOLTAGE This part is aimed to study the influence of both position and increasing width of the polluted layer on the insulator flashover voltage in order to establish the polluted layer discontinuity impact on the dielectric performances of the insulating surface. Figure 5 shows flashover variations (which represents an average of four values) when the polluted layer is located near the HV electrode, near the ground one and when this conducting layer is situated in the middle of the plane model. Flashover voltage values are plotted versus the ratio between the polluted layer width and the total leakage distance, to establish the increasing width effect of the polluted layer on the dielectric performances of the plane model by taken into account its total leakage path for the three non uniform configurations.

From Figure 5, we notice that the flashover voltage

characteristic is practically linear when the polluted layer is located on both HV electrode and ground electrodes. It decreases with increasing of the polluted layer ratio. For the ground polluted band, the flashover voltage values are higher than those obtained for the HV polluted band especially for both 0.51 and 0.85 ratios. These results infer that the polluted insulator surface is more strengthen in the case of ground

configuration for both previous ratios. The difference in the flashover voltage values for the HV band is about 5.2% in the case of the ratio 0.61 and 7% in the case of the ratio 0.85, comparing to those of the ground polluted band.

When the polluted layer is located in the middle of the plane model, the flashover voltage characteristic is also linear and decreases with increasing of the polluted layer ratio. However, this decrease is less emphasized than the two previous configurations and flashover values are the highest one except for the polluted ratio 0.85. This result obviously shows that the insulating model has the highest dielectric strength when the polluted layer is distributed in the middle of its insulating surface.

For each non-uniform configuration, the flashover voltage value is the highest for the lowest ratio 0.17 when we compare to other ratios (0.34, 0.51, 0.68 and 0.85). The flashover value reaches a minimum value when the polluted layer width equals 25 cm which corresponds approximately to 85% of the total leakage distance. This ratio would correspond to the most favorable case to the rapid formation of electrical arcs. These observations were made up by other researchers [27]. The appearance of these electric discharges particularly in the pre-established clean band can be explained by a significant reduction of the clean band width up to a critical value (representing 25 cm) by which the intensity of the electric field in this dry band reaches the breakdown strength and thus, discharges occur in this clean gap.

3.2 THE LEAKAGE CURRENT MAGNITUDE ANALYSIS

In order to show the influence of polluted layer position and

its width on the leakage current magnitude characteristics, many tests have been carried out with different applied voltage levels (3, 9, 15, 21 and 27 kVrms ) when the pollution band is near the HV electrode and when it is near the ground. For the middle polluted band, applied voltage levels are 3, 9, 15, 21 and 40 kVrms. Figure 6 shows leakage current magnitude variations with increasing of both the applied voltage level and the polluted layer width for the three configurations. Video observations show that the flashover arc occurs in the air with the formation of electric discharges just before the final flashover when the polluted layer width is less than 20 cm. When the polluted layer width reaches 25 cm, the flashover process is characterized by the formation of parallel discharges for the three configurations. The number of these discharges increases in the clean air gap with increasing of the voltage level, then some electric sparks appears in the polluted region of the plan model after the formation of some dry bands in which the electric field intensity reaches the breakdown strength causing the appearance of other electric discharges. Finally, parallel discharges progress on the polluted layer until the total flashover with increasing of the applied voltage level. We have particularly noticed that the flashover arc occurs in the air because of the presence of the pre-established clean air gap located between the polluted band and electrodes (HV or ground).

When the polluted band is near the HV electrode (Figure 6a), the leakage current magnitude increases with increasing of both the polluted layer width and the applied voltage level.


(a)

(b)

(c) Figure 6. Leakage current magnitude characteristics with increasing of bothpolluted layer width and applied voltage level, (a) HV band, (b) ground band,(c) middle band.

The current magnitude reaches a maximum value when the applied voltage equals 27 kVrms and when the polluted band width represents 85% of the total leakage distance. These results represent the most favorable case to the formation of parallel discharges on the polluted surface especially in the air gap located between the polluted layer and the ground electrode. The increase of the leakage current magnitude can be explained by a significant reduction of the total equivalent impedance of the insulator proportionally to the increase of the polluted layer width. Figures 7 and 8 show the flashover process in the case of both 20 and 25 cm contaminated layer widths when the polluted band is near the HV electrode.

When the polluted layer is near the ground electrode, experimental results show the same as HV polluted configuration (Figure 6b) that the current magnitude increases with increasing of both polluted layer width and applied voltage level. Although, laboratory observations show that the current magnitude is higher than that obtained for the HV polluted band especially when the polluted layer ratio equals 0.68 for both 21 and 27 kVrms voltage levels. The difference between both HV and ground current values approximately equal 12.8% for 21 kVrms and equals 11% for 27 kVrms with regard to HV polluted configuration. For the ground polluted band, the current magnitude is the highest when the polluted layer ratio reaches 0.85 and the applied voltage equals 27 kVrms. Figure 9 shows that the flashover process is achieved with the electric discharges formation for the ratio 0.85 and the voltage 27 kVrms) in the air gap located between the HV electrode and the ground polluted layer. This systematically induces a large elevation of the leakage current magnitude

(a) (b)

(c) (d) Figure 7. Flashover stages in the case of HV polluted band at 20 cm layer width, (a) arc ignition at 44 kVrms ,(b) arc progression at 50 kVrms ,(c) arc formation in the air gap just before the flashover at 52 kVrms, (d) flashoveroccurs at 54 kVrms.

(a) (b)

(c) (d) Figure 8. Flashover stages in the case of HV polluted band at 25 cm layer width, (a) discharges appear at 25 kVrms, (b) Increase of the discharges number at 28 kVrms,(c) pre-flashover arc ignition at 33 kVrms, (d) flashover occurs at 40 kVrms.


(a) (b)

(c) (d) Figure 10. Flashover stages when the polluted band is in the middle of theplane model at 25 cm layer width, (a) discharges appear at 33 kVrms, (b) Development of parallel discharge activities at 36 kVrms, (c) electric sparks appear at 40 kVrms, (d) Flashover occurs at 45 kVrms.

which can be considered as a precursor sign of the imminent flashover at a particular applied voltage level. Although, the flashover arc for the ratio 0.68 is not preceded with any electric sparks and formation of discharges.

Experimental observations carried out when the polluted band is in the middle of the plane model (Figure 6c) show also that the leakage current magnitude increases with increasing of the polluted layer width and the applied voltage level. However, these current values are the lowest comparatively to previous configurations, indicating that the total equivalent impedance of the polluted insulator is the highest one if the polluted width reaches 15, 20 and 25 cm and when the applied voltage equals 9, 15 and 21 kVrms.

This significant reduction in the leakage current magnitude can be explained by the decrease of the insulator conduction effect which is similar to the point-point electrodes system

when two insulated barriers are introduced between these electrodes because of the presence of two pre-established dry zones. The rapid increase of current magnitude for the voltage 40 kVrms and the polluted ratio 0.85 is engendered by the occurrence of both parallel discharges and electric sparks (Figure 10) that short-circuit both air gaps formed by the middle band and electrodes (HV and ground). Thus, discharge phenomena lead to significantly decrease the equivalent impedance of the plane model. Consequently, we conclude that the parallel discharges formation in air gaps located between electrodes and the polluted layer is achieved when 80 % to 85% of the total insulator leakage distance is polluted. For the HV band configuration, discharges appear when the applied voltage equals 21 kVrms. For the ground band configuration, this is done when the voltage reaches 27 kVrms .For the middle polluted band, discharges appear when the applied voltage is greater than 33 kVrms. These electric discharges increase significantly the current magnitude inducing a probable flashover arc on the polluted surface for other high applied voltage levels and at the same time, creating dry bands in the polluted region which are the result of water evaporation leading to the formation of some electric sparks.

3.3 PHASE ANGLE CHARACTERISTICS

In this section, we study phase angle variations between leakage current and voltage waveforms at the fundamental frequency in order to understand the impact of the polluted layer repartition on the insulator equivalent circuit changes. The phase angle analysis can be useful to establish the behavior of the equivalent circuit of the plane model when it is more resistive or more capacitive and to distinguish different polluted layer dispositions on the insulating surface. The phase angle calculation between the leakage current and the applied voltage is obtained through the FFT spectral analysis. Figure 11a shows phase angle variations for the HV polluted layer case at different applied voltage levels. Experimental observations show that the phase angle value decreases with increasing of the conducting layer width. This clearly shows that the equivalent circuit behavior of the plane model is greatly resistive and poorly capacitive with increasing of this polluted layer. For layers 5, 10 and 15 cm width, the applied voltage has no influence on the phase angle current-voltage which is characterized by no significant variation and its numerical value is high as 76 to 80 degrees. This result is in good agreement with that found by other researchers [28, 29].When the conducting layer width is higher than 15 cm, the phase angle current-voltage tends to clearly decrease according to the increase of both the polluted layer width and the applied voltage level. The equivalent circuit of the plane model becomes more resistive than capacitive. In the case of 20 and 25 cm width, the phase angle value is the smallest when the applied voltage level respectively equals 21 and 27 kVrms indicating that the equivalent circuit is highly resistive when parallel discharges occur in the air gap and short-circuiting this pre-established gap, leading to decrease the capacitive effect of the plane model.

(a) (b)

(c) Figure 9. Flashover stages in the case of ground polluted band at 25 cm layer width, (a) discharges appear at 27 kVrms, (b) Increase of the discharges number at 38 kVrms,(c) flashover occurs at 42 kVrms


From Figure 11b, we distinguish that obtained results for the ground polluted band are practically similar to those obtained in the case of the HV polluted band. The phase angle decreases with increasing of both conducting layer width deposited on the insulating surface and applied voltage level. The decrease of phase angle appears clearly when the polluted layer ratio is greater than 0.51. However, the phase angle value for the ground polluted band is lower than that obtained for the HV band when the polluted ratio is less than 0.51; it equals 72 degrees for the ratio 0.34. These results indicate that the polluted plane model is more resistive when the polluted layer is located near the ground electrode. Electric discharges begin to appear when the polluted layer ratio reaches 0.85 and the applied voltages equals 27 kVrms. The occurrence of electric discharges causes more significant decrease in the phase angle value. Thus, the plane model impedance behaves like a resistance when electric discharges initiate on that insulating area non-uniformly polluted.

For the last non-uniform configuration (middle band), Figure 11c shows that the phase angle value decreases with increasing of the polluted middle band width inferring that resistive effect increases when the insulating area tends to be fully polluted. When the applied voltage level is lower or equal to 21 kVrms, the increase of the applied voltage level has no specific effect on the phase angle value current-voltage. In the other hand, if the applied voltage reaches 40 kVrms, the phase angle value becomes to significantly decrease because of thermal excitement of ionized particles on the polluted plane surface, and preliminary superficial discharges that increase the resistive behavior of the polluted plane model. However, the phase angle decrease is less accentuated when the middle region of the insulator is contaminated inferring a high capacitive behavior of the total equivalent impedance and indicating a better dielectric strength and performance comparatively to previous non-uniform configurations. The ground polluted band configuration presents a critical situation because the plane model has the lowest dielectric performance when we consider low phase angle values for low applied voltage levels (Figure 11b).

4 FREQUENCY CHARACTERISTICS OF THE LEAKAGE CURRENT

In this section are examined frequency bands of the leakage current obtained after the Discrete Wavelet Transform (DWT) decomposition to establish the behavior of these frequency bands with the different geometrical constraints when the non uniform layer position changes and its width increases. This study is aimed to correlate the appropriate frequency band with the insulator state surface by on line monitoring both position and width of the polluted layer. This analysis is an aid to make rapid decisions about engaging a cleaning process of the outdoor insulator in service.

4.1 DISCRETE WAVELET TRANSFORM Basically, the discrete wavelet transform consists of the signal decomposition in frequency bands with minimizing calculation time and is easy to implement by means of the multi-resolution analysis [30]. It is expressed by equation (1) as follows: represents the mother wavelet scaled by the parameter a0m and shifted by the coefficient nb0(a0) m. a0 and b0 are fixed values with a0 > 1 and b0 > 0, m and n are positive integers. When a signal is decomposed with the DWT multi resolution analysis, it passes through the DWTs filter pair LF (Low Frequency) and HF (High Frequency) called QMF (Quadrature Mirror Filter). The signal is subjected to the down sampling algorithm (Figure 12) inducing the separation of the signal in low frequency bands known as approximation and in high frequency bands called details. The approximation is decomposed for the second time when it passes through the DWTs filter pair. The signal reconstruction procedure is achieved through the up-sampling algorithm representing the

(a)

(b)

(c)

Figure 11. Phase angle current-voltage characteristics with increasing of both polluted layer width and applied voltage level, (a) HV polluted band, (b) ground polluted band, (c) middle polluted band.

(1)


Inverse Discrete Wavelet Transform (IDWT) which is considered as the inverse process of the DWT decomposition.

As illustrated in Figure 12, the DWT decomposition operated on the input signal S gives rise to a series of coefficients representing several details and one approximation, which can be represented by CN and DN, DN-1,DN-2,,D1,D2 where C, D and N respectively represent approximation, detail and the final decomposition level. In fact, while a signal is decomposed, it is halved every time it passes through the filter pair (LF and HF). Thus, the input signal S is left with a length of (1/2)L, (1/4)L, (1/8)L,..,(1/2N)L of the original length at level 1,2,3.., N. Then, the corresponding frequencies of levels 1, 2, 3.., N are (1/2)fs, (1/4)fs, (1/8)fs..,(1/2N)fs, where fs is the sampling frequency of the input signal S, and L is the original length of the signal S. Finally, when a signal is subjected to the DWT decomposition, it is decomposed in an approximation component A1 and a detail component D1, then approximation A1 is decomposed into A2 and D2 at the next step and so on. Consequently, a hierarchical decomposition is obtained and can be mathematically represented by equations (2) and (3) as follows: h and g are the quadrature mirror filters. n is a positive integer described in equation (1).

4.2 THE SELECTION OF THE MOTHER WAVELET The selection of the analyzing wavelet is the most important

task for a better signal examination to understand transition phenomena like discharge peaks observed on the leakage current which passes crossing the outdoor insulator in wetted and contaminated pollution conditions. It is well known that Daubechies 4 and Daubechies 6 are the most appropriate wavelets to detect high frequency in short time perturbation. On the other hand, Daubechies 8 and Daubechies 10 are appropriate wavelets to analyze long time perturbation [31]. In this work, we have chosen the Daubechies 4 wavelet (Figure 13) for the leakage current signal decomposition in order to

identify energy transitions when the polluted layer width increases. The complex mother Daubechies wavelet is expressed by equation (4).

5 RESULTS OF THE WAVELET

DECOMPOSITION For the three non-uniform pollution configurations, the leakage current signal is subjected to the multi-resolution decomposition with the db 4 wavelet as an analyzing wavelet until the level 10 which is calculated in agreement with [23] (Table 2). Table 2. Frequency bands of the DWT detailed components of leakage current.

DWT detailed components Frequency band (kHz)

D1 62,5 - 125

D2 31,25 - 62,25

D3 15,625 - 31,25

D4 7 ,8125 - 15,625

D5 3,90625 - 7 ,8125

D6 1,953125 - 3,90625

D7 0,765625 - 1,953125

D8 0,488281 - 0,765625

D9 0,244140 - 0,488281

D10 0,12207 - 0,244140

Frequency bands calculated for each detail (D1 to D10) of the DWT decomposition are given in table 2. The standard

0 2 4 6

-0.50

0.51

1.5

Time (s)

Mag

nitu

de

(a)

1 2 3 4 5 6 7 8-0.5

0

0.5

1

(b)

1 2 3 4 5 6 7 8-1

-0.5

0

0.5

1

(c)

Figure 13. The Daubechies 4 wavelet, (a) time representation of the Daubechies 4 wavelet (db4), (b) Low pass decomposition filter, (c) High pass decomposition filter.

(2)

(3)

(4)

Figure 12. Tree structure of the DWT including filtering and downsampling.


deviation values are calculated for all details of the leakage current in order to identify energy transitions observed on the leakage current waveform when the polluted layer width increases leading to the formation of electric discharges between the polluted layer and electrodes. The standard deviation can be considered as a measure of the energy present in the signal with zero mean [32, 33]. The standard deviation is given by the equation (5) as follows:

dn represents a sample of the signal d, is the standard deviation of the signal d, mn is the mean of the signal d and N represents the length of d.

5.1 HIGH VOLTAGE POLLUTED BAND Figure 14 shows different leakage current waveforms obtained for the five studied width of the polluted layer when the applied voltage level is maintained equal to 27 kVrms. We notice that leakage current distortions are too much important and greatly caused by the significant capacitive effect of the clean band that amplifies magnitudes of the leakage current odd harmonics. In the other hand, peaks with high magnitude are remarkable on leakage current waveforms and tend to increase with increasing in the layer width. These peaks announce the beginning of preliminary discharges on the polluted insulator surface.

Figure 14e shows a typical leakage current waveform when electric discharges start on the polluted surface. Electric discharges deform strongly the leakage current waveform and increase its peaks number and magnitude. This can be explained by a significant reduction in the capacitive effect proportionally to the increase of the polluted layer width activating preliminary discharges that tending to the formation of electric discharges in the air gap formed by the polluted layer and the ground electrode.

The DWT decomposition operated on leakage current waveforms makes it possible to identify frequency bands that can be used to correctly diagnose the state surface of the polluted insulator. Figure 15 shows the STD-MRA representation for the corresponding details, calculated with

(a) (b)

(c) (d)

(e)

Figure 14. Leakage current waveforms obtained with a pollution band nearthe HV electrode at 27 kVeff. (a) Layer 5 cm, (b) Layer 10 cm, (c) Layer15 cm, (d) Layer 20 cm,(e) Layer 25 cm with electric discharge activities.

(a) (b)

(c) (d)

(e)

Figure 16. D10 magnitude characteristics with increasing of the polluted layer width near the HV electrode at 27 kVrms, (a) ) Layer 5 cm, (b) Layer 10 cm, (c) Layer 15 cm, (d) Layer 20 cm, (e) Layer 25 cm with electric dischargeactivities.

Figure 15. Standard deviation representation for corresponding leakage current details when the polluted band is near the HV electrode at 27 kVrms

(5)


MATLAB software. Otherwise, this representation constitutes a good tool for the frequency band increasing detection to estimate both position and width increase of the non-uniform polluted layer. The STD-MRA representation shows that the detail D10 increases with the increase of the polluted layer located near the HV electrode (Figure 16). In fact, the detail D10 contains the third harmonic of the leakage current characterized by a significant magnitude increase when electric discharges occur on the polluted surface of the plane model for the 25 cm polluted layer width. This result is in good agreement with observations of other researchers [22] in the case of uniform pollution band deposited on the insulating surface of both porcelain and polymeric insulators. This means that the detail D10 is correlated with both increase of the HV polluted layer width and beginning of electric discharges. Figure 16.e shows the trend followed by the detail D10 when electric discharges occur on the polluted layer near the HV electrode.

5.2 GROUND POLLUTED BAND For this non uniform pollution configuration, leakage current waveforms for each polluted layer width are practically similar to those obtained when the polluted band is located near the HV electrode (Figure 17). However, the leakage current magnitude for different polluted layer widths is lower than that obtained for the HV polluted band with a little reduction in the number and magnitude of the high frequency peaks. This suggests that the

total equivalent impedance of the plane model in the case of the ground polluted band is higher than that formed by the HV polluted band. Figure 17.e shows a typical leakage current waveform when electric discharges occur on the polluted insulator surface especially when the polluted layer width reaches 25 cm at the 27 kVrms applied voltage level.

The STD-MRA representation (Figure18) shows for this configuration that details D10 and D8 increase with increasing of the contaminating layer width and are liable to the ground polluted band especially D8 (Figure 20). In the case of the 25 cm width, the D10 magnitude is the highest when parallel discharges occur on the polluted surface as the same as the HV polluted band configuration. Variations of D10 and D8 leakage current details are respectively shown in figures 19 and 20.

(a) (b)

(c) (d)

(e)

Figure 17. Leakage current waveforms with a pollution band near the groundelectrode at 27 kVrms.,(a) Layer 5 cm, (b) Layer 10 cm, (c) Layer 15 cm, (d) Layer 20 cm, (e) Layer 25 cm with the electric discharges appearance.

Figure 18. Standard deviation representation for corresponding details when the polluted band is near the ground electrode at 27 kVrms.

(a) (b)

(c) (d)

(e)

Figure 19. D10 magnitude characteristics with increasing of the polluted layer width near the ground electrode at 27 kVrms, (a) ) Layer 5 cm, (b) Layer 10 cm, (c) Layer 15 cm, (d) Layer 20 cm, (e) Layer 25 cm with electric discharge activities.


5.3 MIDDLE POLLUTED BAND

For this latest non uniform configuration, current magnitude is relatively lower than previous configurations; this means that the total equivalent impedance is the high est. We notice that the magnitude of leakage current peaks

increases with increasing of the polluted layer width with high frequency occurrence. When electric discharges start on the polluted layer, the applied voltage level is near to that of the total flashover (Figure 21e), the leakage current magnitude increases with significant distortions observed on its waveform. The leakage current magnitude passes through zero for brief moments when electric discharges start on the polluted insulator (Figure 21e), inferring that the applied voltage level is not sufficient to activate these discharges in the air gap formed by the polluted layer and electrodes.

The STD-MRA representation for the five studied widths of the non uniform layer (5, 10, 15, 20 and 25 cm) shows that details D8 and D1 have a magnitude that increases with increasing of the polluted layer width (Figure 22). In the case of the 25 cm polluted layer, we observe that the D10 magnitude is significant with regard to other details when electric discharges occur on the polluted layer but its variation is random according the increase of middle polluted band. The trend followed by details D8 and D1 are respectively represented in Figures 23 and 24. From observations made up on leakage current frequency bands, the appearance of an extreme activity of preliminary discharges is noticed if we consider the high frequency detail D1 that increases rapidly when the polluted layer located in the middle of the plane model increases. This means that preliminary discharge activities are more developed for this non uniform configuration than the others just before the total flashover and that the detail D1 is good correlated with the middle polluted band. Concerning the detail D10, we conclude that variations of this frequency band of leakage current which contains the third harmonic can be used as a prediction tool of electric

(a) (b)

(c) (d)

(e)

Figure 20. D8 magnitude characteristics with increasing of the polluted layerwidth near the ground electrode at 27 kVrms, (a) ) Layer 5 cm, (b) Layer10 cm, (c) Layer 15 cm, (d) Layer 20 cm, (e) Layer 25 cm with electricdischarge activities.

(a) (b)

(c) (d)

(e)

Figure 21. Leakage current waveforms with a pollution band in the middleof the plane model at 40 kVrms, (a) Layer 5 cm, (b) Layer 10 cm, (c) Layer15 cm, (d) Layer 20 cm, (e) Layer 25 cm with electric discharge activities.

(a)

(b)

Figure 22. Standard deviation of the leakage current frequency bands when the polluted band is in the middle of the plane model, (a) layers 5, 10,15 and 20 cm at 40 kVrms, (b) layer 25 cm at 40 kVrms before the flashover.


(a) (b)

(c) (d)

(e)

Figure 24. Magnitude variations of the detail D1 with a polluted band in the middle of plane model at 40 kVrms. (a) Layer 5 cm, (b) Layer 10 cm, (c) Layer15 cm, (d) Layer 20 cm,(e) Layer 25 cm with electric discharge activities.

discharges activation to prevent against the total insulator flashover.

6 CONCLUSION The present work was devoted to study the transversal

repartition impact and the pollution severity of the non-uniform polluted layer on the plane model behavior. This plane model simulates the 1512 L outdoor insulator greatly used by the company SONELGAZ in Algeria. According to experiments carried out in the High Voltage Laboratory, the following conclusions can be drawn:

1) Investigations carried out on the flashover voltage showed

that its value decreases linearly with increasing of the

polluted layer width. The insulating surface has the best dielectric strength and performance when the conducting layer is placed in the middle of the insulating surface and has the lowest one in the case of the ground polluted band. When the polluted layer is localized near both HV and ground electrodes the flashover occurs for practically same values. The flashover voltage reaches a minimum value for the three polluted bands configurations when 85% of the total insulator leakage distance is polluted.

2) The leakage current magnitude study reveals that its

magnitude increases with increasing of both conducting layer width and applied voltage level. Parallel discharges occur in the pre-established clean air gaps formed by the polluted layer and electrodes. This happens when the contaminated layer reaches a critical ratio of 0.85 for both 27 kVrms(HV and ground polluted bands) and 40 kVrms(for the middle polluted band) voltage levels. This phenomenon increases significantly the leakage current magnitude indicating how closer the insulator flashover is.

3) The phase angle analysis between leakage current and

voltage waveforms at the fundamental frequency allows understanding the equivalent circuit behavior of the insulator. We establish that the capacitive effect is more dominant than the resistive one in the case of a small polluted layer width for the three non-uniform configurations but decreases with increasing of this width. The electric discharges occurrence in clean air gaps increases greatly the resistive effect of the insulator equivalent impedance by short-circuiting these gaps. For all polluted layer configurations, the applied voltage influence appears when the polluted layer ratio is greater than 0.51. In the case of ground band, the phase angle is the lowest and decreases with the voltage increase. For the middle polluted band, the increase of voltage is negligible before the occurrence of preliminary discharge phenomena. Referring to these results, the phase angle analysis makes it possible to distinguish the three non-uniform pollution configurations.

4) The DWT decomposition of leakage current has identified

the most interesting frequency bands in order to study the behavior of these bands for each non uniform configuration. The STD-MRA representation identifies frequency components that increase with increasing of the contaminated layer width. This latter representation is aimed to optimize the study of leakage current in order to correlate both position and width of the polluted layer with the appropriate leakage current detail.

5) The on line monitoring of the insulator state surface is also

possible by using the DWT decomposition of leakage current. When the polluted band is near the HV electrode, the STD-MRA representation shows that detail D10 is correlated with the increase of the HV polluted layer band. In fact, this signal is very interesting for electric discharges detection on the polluted insulator surface because it contains the third harmonic of leakage current which

(a) (b)

(c) (d)

(e)

Figure 23. Magnitude characteristics of the detail D8 with a polluted band inthe middle of plane model at 40 kVrms, (a) Layer 5 cm, (b) Layer 10 cm, (c) Layer 15 cm, (d) Layer 20 cm, (e) Layer 25 cm at the starting of electricdischarges.


significantly increases with the electric discharges occurrence. When the polluted band is near the ground electrode, details D10 and D8 increase with increasing of the polluted layer width. This means that the detail D8 is particularly in good relationship with this non-uniform configuration. When the polluted band is located in the middle of the plane model, frequency bands D8 and D1 detect the width increase of the polluted layer. This means that detail D1 is correlated with the middle polluted band. The monitoring of insulator state surface is also possible by using the STD-MRA representation in the case of an unknown polluted configuration. The pollution phenomena of outdoor insulators have been

well known and well studied for a long time by different scientists (elaboration of mathematical models, analysis of leakage current harmonics etc). However, the leakage current signal decomposition using the DWT theory allows obtaining quickly more precise and probative information, for localizing the polluted area and assessing its severity on the insulator surface: this is the newness of the present study. Consequently, this analysis being more efficient presents lot of advantages: gain of both money and time and minimizing risks of accidents during human maintenance operations.

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Mohammed Adnane Douar was born in Stif, Algeria on 1st June 1986. He received the degree on Engineer in Electrical Engineering in July 2009 from Ecole Nationale Polytechnique (ENP) of Algiers. He is currently pursuing a Magister degree in Electrical Machines at Ecole Nationale Polytechnique of Algiers.

Abdelouahab Mekhaldi was born in Algiers, Algeria in May 1958. He received the degree of Engineer in 1984 in electrical engineering and the Magister in high voltage engineering in 1990 from Ecole Nationale Polytechnique (ENP) of Algiers. He also obtained the Doctorate in 1999 in electrical engineering (high voltage engineering) from Ecole Nationale Polytechnique (ENP) of Algiers. He is

currently a Professor at ENP of Algiers, where he has been giving lectures as Assistant lecturer since 1984 and moreover supervising research in the field of high voltage engineering since 2000. His principal research is discharges phenomena, insulators pollution, polymeric cables insulation, lightning, artificial intelligence application in HV insulation diagnosis and electrical field calculation. He has published about seventy reports. He is a member of the Algerian HV power systems Association ARELEC (National Algerian Comity of CIGRE), the ENP Elders Association ADEP, and IEEE member.

Mohamed Chrif Bouzidi was born in Biskra, Algeria on 16 August 1986. He received the degree of Engineer in 2009 in electrical engineering from Ecole Nationale Polytechnique (ENP) of Algiers. He is currently pursuing the 2nd year of Reaserch Master in electrical systems at Institut Nationale Polytechnique of Toulouse (INPT) in France.

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