Electric “flashover” or insulation breakdown has occurred on “hot stick” safety tools used on live AC transmission lines at Manitoba Hydro in 1997 and 2002. Investigations showed pollution flashover as the cause, whereby leakage currents cascade into flashover. Prior to reinstating live-line work with mitigation procedures, DC voltage experiments suggested an atypical flashover uncharacteristic of pollution flashover without leakage currents, which may require a different mitigation strategy. In this thesis, statistical analysis shows that relative humidity has a greater correlation than voltage with the type of flashover. Labeled a “fast flashover”, it seems to be distinct from pollution flashover, although not statistically significant. A time-stepping computer model was developed to calculate a critical voltage for flashover as a function of relative humidity. However, lack of data prevents the model from making firm conclusions. A list of recommended research is proposed to remedy these deficiencies to allow future model refinement.

Electric “flashover” or insulation breakdown has occurred on “hot stick” safety tools used on live AC transmission lines at Manitoba Hydro in 1997 and 2002. Investigations showed pollution flashover as the cause, whereby leakage currents cascade into flashover. Prior to reinstating live-line work with mitigation procedures, DC voltage experiments suggested an atypical flashover uncharacteristic of pollution flashover without leakage currents, which may require a different mitigation strategy. In this thesis, statistical analysis shows that relative humidity has a greater correlation than voltage with the type of flashover. Labeled a “fast flashover”, it seems to be distinct from pollution flashover, although not statistically significant. A time-stepping computer model was developed to calculate a critical voltage for flashover as a function of relative humidity. However, lack of data prevents the model from making firm conclusions. A list of recommended research is proposed to remedy these deficiencies to allow future model refinement.

Power industries in transmission and distribution level are obligated to maintain and replace their electrical equipment. Maintaining the quality and continuity of supply is their priority to avoid customers' complaints and financial penalisation. Live-line working as one of the most important methods of maintenance has been used since the 1900s where the new methods in 1960s made the live-line workers enabled to work on the higher voltage levels up to 800kV. Various industries adopt different techniques to calculate the minimum approach distance (MAD) during the live-line work. A suitable method reduces the risk to live-line workers and provides adequate safety distances between the live parts and linesmen. Therefore, setting an appropriate safety distance between the linesmen and live parts ensures the safety of the workers and minimise the risk of flashover. In this thesis, different methods of calculation of the minimum approach distance are described, and results from overvoltage simulations are used as an input to the methodology outlined in IEC 61472. Also, this thesis highlights and investigates the impact of a range of factors within 400kV transmission line on the minimum approach distance (MAD). Factors examined include the time to crest of the overvoltage (wave shape), the fault type, the probability of occurrence of each type of fault, fault level and the type of overhead line and towers. Furthermore, the minimum approach distances and also associated risk due to each factor and scenario have been calculated. The calculated risk in this thesis presents the risk of failure of a gap against the switching overvoltages due to the simulation of sources of overvoltage. A new set of estimated equations is developed to consider the influence of wave shape in the calculation of the minimum approach distance (MAD). This thesis does not propose a method to replace the international standards, but it could be used in many situations including where utility companies wish to develop a complete understanding of the risk associated with live-line working. Calculation of the minimum approach distance (MAD) within the National Grid UK is based on the methodology described in the IEC 61472, whereas EDF Energy uses the IEEE method to calculate the minimum approach distance. The choice of a smaller / larger minimum approach distance (MAD) using different methods will have an impact on the risk associated with live-line working. Previous works intend to investigate the magnitude of switching overvoltages on one part of a network and calculate the appropriate minimum approach distance for the work in that section. This work is based on the examination of the switching overvoltages under the worst case scenarios. As a result, the simulated overvoltages in this work are higher than expected overvoltages in National Grid network. Also as in practice, the magnitude of switching overvoltages in National Grid network is controlled by different protections equipment therefore, the simulated results and the calculated minimum approach distances in this work are very conservative.

This thesis is based on the investigative studies performed on fiberglass crossarm, which will be used in the distribution line structures as an additional insulation component. Lightning impulse voltage tests were conducted on the fiberglass crossarm alone to determine its critical flashover (CFO) voltage, and also the critical flashover voltage of 15 kV polymer suspension insulator plus the fiberglass crossarm. The Added CFO voltage of the fiberglass crossarm to the insulator was determined with polymer suspension insulator as the primary insulation component and the fiberglass crossarm as the secondary insulation component. Added CFO voltage was evaluated for 1 ft through 5 ft length of the fiberglass crossarm for positive and negative polarities, under dry and wet condition. The thesis also presents a comparison of the obtained results made with the previous results of the wood crossarm and polymer crossarm with the polymer suspension insulator.

No description available.

Pollution is the single largest cause of transmission/distribution line outages, next to lightning, which result in expensive power outages. A major significance of the problem is that it can repeatedly occur even at normal working voltages. As a result, it has become the most detrimental factor affecting the safe operation of extra and ultra high voltage (EHV/UHV) transmission lines and substations. In reality, the phenomenon of pollution-induced flashover is a very complex, and vexatious problem that continues to challenge high voltage engineers even today. In spite of knowing this phenomenon for the past several decades, a solution has remained still elusive. Although there exist some remedial measures, there are associated limitations, which will become evident on long runs. The guaranteed solution seems to be the washing and cleaning of insulators for which utilities spend significant amount of money. Therefore, there is a need to develop a suitable mitigation technique which is cost effective and yield better performance in the field. Motivated by this, the present study is undertaken and it essentially aims to seek simple alternative solutions for the problem for strings with ceramic insulator discs. All the ten types (normal and anti fog) ceramic insulators, which are commonly employed in our country, are considered in the study. Amongst the several controllable and uncontrollable physical quantities leading to the pollution flashover phenomena, the maximum surface field identified as on of the major influencing factor. In fact, the field concentration near the pin can lead to early formation of dry band and scintillation/partial arcs. Considering this, it is intended to seek possible minimization of the maximum surface field occurring at the pin region. This is expected to yield enhanced pollution/contamination flashover strength and in addition, show an improvement in normal operation. The intended study requires a detailed knowledge on field distribution. However, the required data is found to be rather scarce. In view of this a detailed study on field distribution is taken up for all the ten types of disc insulators used in our country. For the problem under investigation, the governing equation has been identified for both clean and polluted conditions along with pertinent boundary conditions. Considering the open geometry nature of the problem along with presence of multiple dielectrics, Surface Charge Simulation (SCSM) methodology was found to be most suitable and hence adopted for the work. In particular, the Galerkin method with piecewise linear interpolation function is employed in the formulation. The method employed and the codes developed are verified with suitable examples. First, a detailed quantification of the field distribution under clean conditions is made for all the ten types of discs in single disc and string configuration. Subsequently, the task of reducing the maximum surface field gradient, which occurs at the pin, is attempted. Several consideration lead to an artificial extension of pin as one of best feasible choice. However, any attempt to extend the pin would lead to some reduction in total creepage length, possible enhancement of bulk stress in air and enhancement of stress in triple junction. After a careful study, involving several experimental trials, a novel field control element (FCE) is developed both for normal and anti-fog types of insulator discs. From the electric field simulation study, it is shown that the use of field control element for uniform pollution deposition prevailing under laboratory test conditions yields a significant reduction of maximum surface field for discs by about 47 to 54%. Similarly a reduction of about 37% to 55% in case of 3-disc string (for 33 kV class), 30% to 52% in case of 9-disc string (for 132 kV class), 27% to 52% in case of 14-disc string (220 kV class), 27% to 54% in case of 23/20 disc string (for 400 kV class) and 41% to 48% in case of 35/29 disc strings (for 765kV class) is achieved respectively for different strings. It is anticipated that this will lead to retardation in inception of scintillations/partial arcs, which in turn can reduce the risk of pollution induced flashover. Subsequently, it was aimed to experimentally evaluate the impact of field control element on the performance of disc/string under normal and that during polluted conditions. For this a national level unique artificial pollution test facility as per the international standards has been established for conducting pollution studies on disc insulator/strings up to 132kV system voltage (The rating of test source: 150kV/2A,100kV/3A,50kV/6A of 300kVA). Experimental investigations for the normal operation involving dry and wet power frequency flashover strengths, lightning impulse strength, radio interference level (RIV), visible discharge inception (Corona) level and voltage distribution along the string are carried out. The study showed that with the insertion of field control element, performance under normal condition is maintained and in fact noticeably improved in certain cases. Subsequently pollution flashover strength is evaluated using solid layer and cold fog methods. The pollution flashover strength exhibited an overall improvement 15 to 20% for all types of discs considered in the study. Similarly an improvement of 16 to 19% and 12-14% is observed for 3-disc string and 6-disc string respectively. The reasons for relatively lower gain in the pollution flashover strength as compared to reduction in maximum surface field are investigated. In particular, the reduction in improvement with number of discs in a string is dealt with. The non-uniform wet-ting against highly non-uniform drying of insulator surface and discs in a string, are identified as the cause for deviation. For an experimental verification of the same, the pollution layer resistance of individual discs in a 3-disc string is measured prior to and immediately after flashover. It is shown that the resistances of the different units become grossly different even though initial values were substantially the same. The values measured immediately after flashover show that the resistance of the top unit develops voltage enough to result in its flashover and subsequently, the flashover of the whole string. In summary, the main contribution of the present work is the development of novel field control element (FCE) for both normal and anti-fog type of ceramic disc insulators, which will enhance their pollution flashover strength. The actual cost of these elements is estimated to be about 1-2% of the cost of the disc, while the gain in strength is shown to be more than 12 - 20%.

No description available.

abstract: Transmission voltages worldwide are increasing to accommodate higher power transfer from power generators to load centers. Insulator dimensions cannot increase linearly with the voltage, as supporting structures become too tall and heavy. Therefore, it is necessary to optimize the insulator design considering all operating conditions including dry, wet and contaminated. In order to design insulators suitably, a better understanding of the insulator flashover is required, as it is a serious issue regarding the safe operation of power systems. However, it is not always feasible to conduct field and laboratory studies due to limited time and money. The desire to accurately predict the performance of insulator flashovers requires mathematical models. Dynamic models are more appropriate than static models in terms of the instantaneous variation of arc parameters. In this dissertation, a dynamic model including conditions for arc dynamics, arc re-ignition and arc motion with AC supply is first developed. For an AC power source, it is important to consider the equivalent shunt capacitance in addition to the short circuit current when evaluating pollution test results. By including the power source in dynamic models, the effects of source parameters on the leakage current waveform, the voltage drop and the flashover voltage were systematically investigated. It has been observed that for the same insulator under the same pollution level, there is a large difference among these flashover performances in high voltage laboratories and real power systems. Source strength is believed to be responsible for this discrepancy. Investigations of test source strength were conducted in this work in order to study its impact on different types of insulators with a variety of geometries. Traditional deterministic models which have been developed so far can only predict whether an insulator would flashover or withstand. In practice, insulator flashover is a statistical process, given that both pollution severity and flashover voltage are probabilistic variables. A probability approach to predict the insulator flashover likelihood is presented based on the newly developed dynamic model. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2016

abstract: Overhead high voltage transmission lines are widely used around the world to deliver power to customers because of their low losses and high transmission capability. Well-coordinated insulation systems are capable of withstanding lightning and switching surge voltages. However, flashover is a serious issue to insulation systems, especially if the insulator is covered by a pollution layer. Many experiments in the laboratory have been conducted to investigate this issue. Since most experiments are time-consuming and costly, good mathematical models could contribute to predicting the insulator flashover performance as well as guide the experiments. This dissertation proposes a new statistical model to calculate the flashover probability of insulators under different supply voltages and contamination levels. An insulator model with water particles in the air is simulated to analyze the effects of rain and mist on flashover performance in reality. Additionally, insulator radius and number of sheds affect insulator surface resistivity and leakage distance. These two factors are studied to improve the efficiency of insulator design. This dissertation also discusses the impact of insulator surface hydrophobicity on flashover voltage. Because arc propagation is a stochastic process, an arc could travel on different paths based on the electric field distribution. Some arc paths jump between insulator sheds instead of travelling along the insulator surfaces. The arc jumping could shorten the leakage distance and intensify the electric field. Therefore, the probabilities of arc jumping at different locations of sheds are also calculated in this dissertation. The new simulation model is based on numerical electric field calculation and random walk theory. The electric field is calculated by the variable-grid finite difference method. The random walk theory from the Monte Carlo Method is utilized to describe the random propagation process of arc growth. This model will permit insulator engineers to design the reasonable geometry of insulators, to reduce the flashover phenomena under a wide range of operating conditions. / Dissertation/Thesis / Doctoral Dissertation Engineering 2016

M.Sc. / The need for a fire-induced flashover (power line arcing to the ground) probability index for Eskom transmission (high voltage power) lines became evident soon after the installation the Advanced Fire Information System (AFIS) in 2004. AFIS is a satellite based fire detection system that utilizes polar and geostationary satellite sensors to detect fires as small as 50 m x 50 m in size. As soon as a fire is detected by either, the Terra, and Aqua Moderate Resolution Imaging Spectro-radiometer (MODIS) or Meteosat Second Generation (MSG) geostationary satellites close to any of the 28 000 km of Eskom transmission lines, a cell phone and email text warning is sent out to line managers responsible for the management of the particular section of line affected. Between 3000 - 6000 fires are recorded annually close to Eskom transmission lines with a fire-induced flashover rate of 100 - 150 transmission line trips per year. Fire-induced flashovers occur when the air around high voltage transmission lines are ionised due to a hot flame (> 500° C). As the air becomes conductive, electricity can move from the line to the ground in the form of a lightning flash. Studies have shown that one flashover can cause an average of three voltage depressions (dips) on the electrical transmission system, and each voltage depression can cause damage to a customer’s production ranging between R5000 and R150000 per dip. The aim of this study was to develop a prediction model with the ability to accurately predict fire-induced flashover occurrences on Eskom transmission lines in order to reduce the large amount of false alarms (SMS and email messages) produced annually by AFIS. The prediction model in the form of a probability index was derived from a combination of remote sensing satellite products as well as weather forecast variables. With the MODIS active fire product as base layer, weather forecast variables in the form of air temperature, relative humidity, wind speed and wind direction, as well as topographical elevation and a satellite derived vegetation condition product served as input to the predictor data set of the model, while flashover statistics for 2007 provided the target data set within a Classification and Regression Tree (CART) analysis. iii The prediction capabilities for each of the variables were evaluated based on their prediction accuracy and Receiver Operation Characteristic (ROC) value in terms of the validation data set. Wind speed, relative humidity, wind direction and air temperature were shown to have the highest predictor importance and were used to develop the probability index calculated from a logistic regression analysis. The Fire-induced Flashover Probability Index (FIFPI) was tested through simulations of predictor variables and was also compared to existing Fire Danger Indices (Willis et al. 2001). The FIFPI was able to outperform most of the standard Fire Danger Indices (FDI’s) with only the McArthur Grassland Index (MK 4) which demonstrated some prediction capability. The importance of wind direction as an environmental component in the prediction of flashovers became clear as it tended to decrease the misclassification rate from 4.45% when only wind speed, relative humidity and temperature were used to 3.87% when wind direction was added. The research has shown that wind speed, wind direction, relative humidity and temperature can be used as an indicator of possible fire-induced flashovers underneath Eskom transmission lines. However, additional research is needed to verify the results from 2007. Ideally at least 3 years of data should be used.