Leakage Current Patterns on Artificially Polluted Porcelain and Polymer Insulator

Ceramic insulators, specifically porcelain and glass insulators, have widely been used all over the world since the 19th century [1,2]. However, when such insulators are energized and exposed to different pollution environments under different climatic and weather conditions, leakage current (LC) develops along their surfaces [3-7] and eventually cause flashovers. Various remedial measures have been adopted by power utilities to mitigate this problem. Among the common methods are: (a) increasing insulator creepage (leakage) distance, (b) reducing supply voltage, (c) insulator washing (cleaning), (d) oiling or greasing (silicone) the insulator surface, and (e) insulator replacement. Composite insulators, also referred to as non-ceramic or polymeric insulators, have also been used during the last six decades as an alternative to the ceramic insulators [8-10]. The housing materials most often used have been silicone rubber (SIR) [11] and ethylene-propylene-diene-monomer based rubber (EPDM) [12]. Water repellent (hydrophobic) properties of composite insulators, light weight, resistance to vandalism are among the main reasons for their usage. This has been proven by positive outcomes of numerous studies performed in laboratory and field conditions [8,9,13-16]. However, due to exposure to harsh service stresses such as corona discharges, UV radiation and chemical attacks etc., the housing materials may lose hydrophobic properties and develop surface degradation (tracking, erosion). In addition to the ever-increasing use of composite insulators, polymeric coatings (mainly RTV silicone rubber) [17] and semi-conducting glazes [18] have also been adopted for improving performance of ceramic and glass insulators [19].

Leakage current frequency components are frequently used as a tool for surface condition monitoring on polymeric insulation material and their pollution severity. Fast Fourier Transform (FFT) is one of the methods that are applied for the analysis, but it has some limitation in non-stationary signal. This paper presents analysis of leakage current on polymeric insulation material for high voltage application in frequency domain and time-frequency representation. Tracking and erosion test (Inclined Plane Test (IPT)) complying with BS EN 60587-2007 is conducted on polymeric insulation to select a set of different leakage current patterns from capacitive, resistive and discharge. Then, the leakage current patterns are analysed in frequency domain and time-frequency representation using Fast Fourier Transform and spectrogram technique, respectively. It is found that the surface condition of polymeric insulation material state can be classified accurately by using spectrogram compared to Fast Fourier Transform.

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In the present study the performance of porcelain and RTV SIR coated porcelain insulators has been investigated in field and laboratory conditions, by leakage current measurements. In field conditions, by the use of the appropriate equipment it was possible to continuously monitor a group of twelve 150kV porcelain insulators, installed in two high voltage substations, of the Transmission System in Crete. Ten of them were coated with RTV SIR. In addition simultaneous measurements of meteorological parameters were performed, allowing the correlation of the LC measurements to the environmental conditions. The leakage current measurements performed indicated two periods of intense surface activity. In the case of porcelain, the summer period and especially the months From August to October, represent the period of intense surface activity. During the rest of the year the recorded LC levels are remarkably lower. This monthly distribution comes in agreement with the observed pollution flashovers distribution, for the period 1969-2005. The opposite activity distribution is observed for the RTV SIR coated insulators. In this case the levels of surface activity in the summer period are remarkably low and the period of intense activity for the coatings is observed during the winter. It is worth mentioning however that even in this case the levels of activity are remarkably lower than the corresponding levels in the case of porcelain, for the same time period. The opposite behavior of porcelain and coated porcelain insulators can be correlated to the environmental conditions and especially the wetting mechanism present. During the summer, insulator wetting is possible as the result of two mechanisms, the hydroscopic behavior of the pollution layer and condensation. Both mechanisms are capable of wetting the total leakage creepage distance, without cleaning the insulators surface in the same time. As a result a critical amount of pollution can be formed on the insulator surface, considering also the low levels of precipitation. So in the case of porcelain the formation of surface conductivity is possible in contradiction to the RTV SIR coated insulator, where the formed surface hydrophobicity is maintained. On the other hand during the winter, light precipitation can support the development of surface activity, since it is possible to disturb the surface condition. In the case of porcelain this will result to the cleaning of the surface. However in the case of RTV SIR coatings a loss of hydrophobicity is observed which allows the development of surface activity, considering also that cleaning is more difficult in this case. It is worth noticing however that in all cases the observed activity on the RTV SIR coated insulators is remarkably lower than the corresponding activity in the case of uncoated porcelain insulators. The leakage current waveforms for finite time periods are also included in the information provided by the field measurements performed. In the case of porcelain insulators, the analysis of the corresponding waveforms indicated that the current required for the formation of dry bands depends on the wetting mechanism present. In the case of mechanisms such as condensation, a current in the range of 1 – 2mA is capable to support the formation of dry bands. However in the case of precipitation the necessary current is higher, reaching a level of 15mA. Further the formation of dry bands is reflected to the leakage current waveform by zero current periods which are observed between the current conduction periods. This behavior indicates that the observed activity can be considered as a sequence of independent current pulses. Additionally the FFT analysis correlates the non linear current behavior to an increased 150Hz component. These characteristics have been traced in all the waveforms recorded, in the range from 14mA to 150mA. In the case of RTV SIR coated insulators the recorded waveforms are in large extent similar to the waveforms on the porcelain insulators. However additional phenomena, correlated with the existence of areas which maintain the hydrophobic behavior. In addition the non linear behavior is enhanced in this case, something that results in higher levels of a current component at 150Hz. The performance of the employed RTV SIR coatings was also investigated in laboratory conditions. Two tests were performed in a salt fog chamber, were the stress conditions are similar to the conditions observed in the case of light rain. The material performance observed and the corresponding measurements performed in both tests, indicate that the formulations tested, the endurance of the ATH filled coatings is higher than the silica filled, both in the case of corona and dry band discharge stress. The difference observed can be correlated with the action of each filler type. However the influence of the filler action needs to be further investigated.

The current paper presents an alternative and innovative technique to predict the severity of pollution of high voltage insulator using a higher harmonics component with up to the 7^th component of leakage current. The leakage current was measured using a current transformer and a shunt resistor. Next, laboratory tests were conducted on glass and porcelain insulators with artificial pollution under salt-fog pollution state which is further represented by three levels, namely light, medium, and high contamination. In this case, the formulation of a new severity of harmonic index refers to a ratio of the sum of 5^th and 7^th to the 3^rd harmonic component. More importantly, the new index managed to provide more accurate results when used as a diagnostic tool for the levels of pollution, compared to the ratio of the total harmonic distortion (THD) to the number of odd harmonics components (n) as the boundaries. In this case, the insulators were found to be in a clean and normal condition when the K_(5+7)/3 value was greater than 3%. Contrastingly, the insulators were in an extreme condition when the K_(5+7)/3 was lower than 3%. Nevertheless, there is a high probability of a flashover in glass and porcelain insulators if the K_(5+7)/3 value is less than 2%. The present study shows the possibility of utilizing the value of strange harmonics up to the 7^th component of leakage current as the parameter for the monitoring of leakage current in overhead insulators in the presence of contamination. Overall, it can be concluded that the 3^rd, 5^th, and 7^th harmonics details extracted from the leakage current act as a good indicator for the level of contamination.

This work is devoted to study the influence of the increasing of the width of the polluted band on insulators used in desert regions, by measuring the leakage current through the insulators surface. The Recurrent Plot technique (RP) is used to analyze the leakage current (LC) signal for monitoring the insulators surface state. This technique is applied directly on the leakage current waveforms, and the high frequency components are extracted from the leakage current signal using the wavelet transform technique. We also used the Recurrent Quantification Analysis (RQA) technique to quantify the RP characteristics of the LC signal and high frequency components, by computing the Recurrence Rate (RR), the Determinism (DET) and the Laminarity (LAM).

This paper describes the effects and the aging of silicone compound coatings on ceramic insulators under artificial tropical climate. The improvement of the properties of the ceramics insulators was investigated. The effects of temperature, humidity and various artificial pollutants were also investigated. The experimental results show that application of silicone compound coatings suppressed leakage currents significantly under contaminated environment. The hydrophobicity of the insulators was also improved drastically. Up to 50/spl deg/C the coating performed well and the leakage current was remain constant. Under humidity of more than 90% higher leakage current and lower flash over voltage were observed. The coating on the wet-polluted insulators results in a very poor improvement of the insulator properties. This showed coating method affected the performance of the coating.

Due to the rising voltage level of the power lines, the occurrence of pollution-induced flashover of the line insulators is becoming a serious issue. In this experimental study, the contamination flashover performance of three kinds of high voltage insulators energized by HVAC is evaluated. The samples were polluted artificially with five particular types of contaminants. Clean steam fog is used in order to wet the polluted samples. A comparison between the insulator flashover characteristics by measuring their surface conductivity, flashover voltages, Equivalent salt deposit density, and Non-soluble deposit density is presented. Results show that the highest flashover voltage (FOV) is found to be in dry state by the composite insulator for sand is 96 kV, and for porcelain (both fresh and serviced) insulators are 85 kV and 78 kV respectively, and least obtained for salt mixture, 88 kV for composite, 75 kV for a fresh piece of porcelain insulator and 63 kV for serviced porcelain insulator. Whereas, under wet conditions, these values of flashover voltages for the three samples are reduced by 17.7%, 22% and 30.7% for sand, and for salt mixture, values of FOV are decreased by 43.0%, 48%, and 46% respectively. Comparison analysis suggests that the composite insulators perform 5% better than porcelain insulators, even in the heavily polluted environment. This study may provide a reference to select suitable insulators for a specific area.

This study presents a novel technique for estimation of contamination level as well as flashover voltage of outdoor insulators based on dielectric dissipation factor evaluated from the applied sinusoidal excitation and corresponding surface leakage current. Existing literature shows that surface leakage current of insulator is infested with odd harmonics though the applied voltage is sinusoidal in nature. The presence of harmonic contents in leakage current is due to non-linear characteristics of the insulator surface owing to formation of dry bands. The nonlinear characteristic of the insulator surface depends on its contamination level which in turn affects the corresponding flashover voltage. Therefore, the harmonic configuration in leakage current can be correlated with the contamination level on insulator surface and corresponding flashover voltage. Considering the fact, in this contribution, a non-linear equivalent circuit of insulator has been proposed based on harmonic components of the leakage current. For each harmonic component of the leakage current flowing through the insulator system, energy lost and energy stored in the system is calculated, which eventually enables to calculate dielectric dissipation factor. The proposed technique is applied for experimental investigation on 11 kV disc porcelain insulator artificially contaminated in the laboratory under controlled environmental conditions.

Contamination flashover as a result of pollution severity leading to costly outages plagues insulator performance. Apart from the problem of flashover, failure of insulators as a result of ageing has also lead to service disruption. Leakage current (LC) is a good measure of insulator surface condition and thus its magnitude has been used as a parameter for monitoring insulator flashover and pollution severity, but this parameter is not effective in monitoring ageing insulator. Furthermore, field monitoring and diagnosis of ageing insulators has not been exploited. This paper presents an overview of the ageing mechanism of both ceramic and polymeric insulators. Previous studies on the correlations of LC with pollution severity, environmental parameters and insulator performance are discussed. A review of recent studies on relationship between LC with ageing insulators and the possible future areas of research on monitoring and diagnosis of ageing insulators are presented.

The demand of electrical energy has been increasing worldwide since the start of the 21st century. The focus on energy demand, efficiency increase, and loss control is one of the major concerns. Following the approach to control the power loss specifically in underdeveloped countries in the power distribution network where polymer insulators are installed. This research aims to study the impact of industrial pollutants by adopting accelerated aging techniques on polymer insulators installed in coastal regions. An experimental chamber is designed to model the artificial effects of the environment on polymer insulators. The polymer insulator was kept in an experimental chamber for 45 days to apply the environmental effects on it. Different environmental effects are applied in the experimental chamber including temperature, humidity, pollutants, and ultraviolet light. These environmental effects are controlled through different designated sensors that are managed by a data acquisition setup. After the accelerated aging process of polymer insulators in the experimental chamber the leakage current has been measured between healthy and contaminated polymer insulators. The healthy insulator shows 52e−9 amp leakage current while the contaminated insulator shows 149e−9 amp of leakage current after the aging process. The experimental results also concluded that the environmental impact on polymer insulators in the coastal regions is significant. Therefore, to avoid power loss due to leakage current it is recommended to have periodic cleaning of the polymer insulators specifically in the coastal regions that are installed in the power distribution network. This will help to reduce the power loss due to contamination in environment.

Although polymer insulators show excellent insulation characteristics, especially under contaminated conditions due to hydrophobicity on their surface, it is reported that corona discharges generated at high electric field portions such as energized end of a 500-kV polymer insulator, damaged the surface of its sheath. Due to hydrophobicity, voltage distribution along a long rod type polymer insulator is very non-uniform even under wet conditions. So water droplets deposited on hydrophobic surface of energized end of a polymer insulator for higher voltage application are subjected to high electric field, and deformed under such electric field. Corona discharges may generate from the tips of deformed water droplets. We investigated fundamental mechanism of corona discharges from water droplets on the energized end of a hydrophobic polymer insulator under AC and DC voltages. Behavior of water droplets was carefully examined by a high speed video camera and leakage currents were measured for detailed analysis.

Resumo A resistência mecânica e a propriedade dielétrica de isoladores cerâmicos são características fundamentais para o suporte das redes de distribuição de energia elétrica. Porém, tanto a umidade quanto o teor de poluentes na atmosfera podem vir a ser prejudiciais e causar falhas na entrega de energia por elevação da sua corrente elétrica de fuga. O vidrado do isolador tem grande afinidade com a água resultante da interação da superfície do vidro com a atmosfera. No presente trabalho, foi realizada a deposição de filmes de nitreto de alumínio sobre o vidrado de amostras de isoladores elétricos cerâmicos, via plasma "magnetron sputtering" pulsado, de corrente contínua, e com uma mistura dos gases argônio e nitrogênio e de Ti sobre isoladores tipo pino para classes de tensão de 15 kV. A morfologia dos filmes e o desempenho dos isoladores foram medidos por MEV-FEG/EDS, AFM, máxima corrente de fuga RMS e grau de hidrofobicidade. O comportamento dos filmes depositados medido pelo grau de hidrofobicidade reduziu nas amostras de CM6 para CM4, com AlN, e de CM7 para CM3 quando foi depositado o Ti. A máxima corrente de fuga RMS medida nos isoladores sem e com filme nanométrico à base de Ti foi superior nos dispositivos sem a proteção, alcançando valores medidos de 5 mA e com desligamento do sistema computadorizado de medida (após 3 dias em Sauípe e 3 meses em Pituba) em consequência desta ter superado o intervalo de segurança de curto circuito estabelecido nos dispositivos eletrônicos de proteção (surtos superiores a 300 mA). Com a deposição dos filmes em 7 isoladores, foram verificadas alterações de comportamento em dois destes, com picos de máxima corrente de fuga atingindo 2,4 mA no campo. A maioria dos isoladores analisados teve como variações de correntes de fuga RMS, no período de medida, entre 0,2 e 2 mA, em condições adversas de clima (com chuva e diferentes velocidades de vento).

The flashover process during ice-melting period of polluted and ice-covered insulator string was studied by experiment in this paper. Arc development, evolution of leakage current and its frequency representation (FFT) were analyzed. The results indicated that the flashover process can be divided into five stages, named initial stage, intermittence local arc stage, arc propagation stage, critical flashover stage and flashover stage. Each stage has distinct discharge phenomenon and a fixed level of leakage current peak value. Both the nonlinearity and the phase angle difference between leakage current and the applied voltage decrease with the raising of leakage current. At the initial stage, the leakage current peak value is less than 10 mA and hardly has any harmonic content. When the arc is intermittent, the harmonic content of the leakage current is the largest. The leakage current waveform is smooth and has fewer harmonics at critical flashover stage.

This paper introduces the characteristics of outdoor conventional and polymeric insulators subjected to natural pollution conditions near industrial areas in Egypt. The wetting property, flashover characteristics, as well as the insulators' hydrophobicity are thoroughly investigated for porcelain and silicone rubber insulators exposed to fertilizer pollution for about two years. Test results show that silicone rubber insulators keep the pollution layer longer than porcelain insulators. Moreover. Noticeable reduction in the contact angle is noticed for silicone rubber insulators. The electrical performance of the polluted insulators was affected by exposure to fertilizer pollution. However, after washing, silicone rubber insulators still have superior performance in flashover voltage compared to porcelain insulators.