Showing papers on "Partial discharge published in 2011"

Partial discharge behavior within a spherical cavity in a solid dielectric material as a function of frequency and amplitude of the applied voltage

Abstract: Modeling of the partial discharge (PD) process allows a better understanding of the phenomena. In this paper, a simulation model for spherical cavities within a homogeneous dielectric material has been developed. The model is implemented using Finite Element Analysis (FEA) software in parallel with a mathematical package. This method provides many advantages over previous PD models because discharge events can be simulated dynamically and the electric field in the cavity can be calculated numerically. The model has been used to study the effect of different amplitudes and frequencies of the applied voltage and simulation results have been compared with experimental measurement results. It is found that certain model parameters are dependent on the applied stress and parameters that clearly affect PD activity can be readily identified, these parameters include; the electron detrapping time constant, the cavity surface conductivity, the initial electron generation rate and the extinction voltage. The influence of surface charge decay through conduction along the cavity wall on PD activity has also been studied.

High Voltage and Electrical Insulation Engineering

Abstract: Chapter 1 Introduction Chapter 2 Electric Fields, Their Control and Estimation Chapter 3 Field Dependent Behavior of Air and Other Gaseous Dielectrics Chapter 4 Lightning and Ball Lightning, Development Mechanism, Deleterious Effects, Protection Chapter 5 Electrical Properties of Vacuum as High Voltage Insulation Chapter 6 Liquid Dielectrics, Their Classification, Properties, and Breakdown Strength Chapter 7 Solid Dielectrics, Their Sources, Properties, and Behavior in Electric Fields

Dielectric properties of epoxy based nanocomposites for high voltage insulation

Abstract: Epoxy polymer with micro, nano and micro + nano silica fillers have been evaluated for their electrical performance in high voltage insulation applications. The dielectric strength of these samples was measured in accordance with the ASTM D-149 standard. Dielectric spectroscopy was used to understand the role of space charge and interfaces in these materials. The results of dielectric spectroscopy suggest that significant improvement in the electrical performance can be expected by using samples containing nanofillers and micro + nanofillers when compared to materials containing only microfillers. However, the dielectric strength measurement showed no statistically significant improvement for the nanofilled samples. Techniques other than dielectric breakdown may be required to adequately characterize differences in the electrical performance of the dielectrics. For example, a partial discharge test using a highly non-uniform field may be more useful as it would correspond to simulate actual service conditions.

Simultaneous measurement of partial discharges using IEC60270 and radio-frequency techniques

Abstract: Partial discharge (PD) measurement is an established condition monitoring technique used to facilitate the detection of incipient faults in high voltage electrical insulation systems such as gas-insulated switchgear and power transformers. By simultaneously recording partial discharge using both the IEC60270 'apparent charge' measurement technique and the more recent Radio Frequency method, which measures the energy radiated from the discharge, it is thought that more information can be gained about the nature of the PD source. Combined measurement has been carried out on various common PD source topologies recreated under laboratory conditions using PD test cells in a range of insulating media including SF6, oil and epoxy resin. Test cell geometries include floating electrodes, free particles and protrusions. By plotting apparent charge against the energy of the RF signal for a large number of PD pulses, the relationship between the two techniques can be quantified for a given test configuration. It has been found that the correlation between the two techniques produces characteristic patterns specific to each defect type. It is envisioned that combined RF/IEC measurements will contribute to a more widespread acceptance of the RF technique in terms of its ability to quantify PD severity by relating the RF energy to the more widely accepted IEC60270 pC level.

Surface charge accumulation on cylindrical polymeric model insulators in air: simulation and measurement

Abstract: The time characteristics of the surface charge accumulation process on cylindrical polymeric model insulators under dc stress were investigated in a partial discharge free, dry air environment. In order to simulate the influence of different dielectric and electric material properties on surface charge accumulation, a tool was developed which considers also the nonlinear behavior of the electric conduction mechanism within the gas volume. The results of simulation have been verified by measurements of the surface potential distribution along the samples at different ambient temperatures.

Research on the Typical Partial Discharge Using the UHF Detection Method for GIS

Abstract: Gas-insulated switchgear (GIS) plays an important role in power systems, not only because it is maintenance free and has less of an influence on environmental and industrial conditions, but also due to its low space requirements and high reliability. In order to monitor the online condition of GIS and diagnose existing defects, the ultra-high frequency (UHF) methods are widely used for detecting partial discharge (PD) owing to its sensitivity and anti-interference capability. In this paper, five types of typical PD sources are designed and fabricated to imitate the defects in GIS online. In order to distinguish different types of PDs, some feature parameters from the UHF signals are extracted and discussed. Generally, the conventional phase-resolved PD (PRPD) and pulse-sequence analysis methods are adopted to discriminate PD sources. However, sometimes it is not convenient to employ these methods because of the absence of phase information. In this research, nine feature parameters realized by processing the UHF pulse magnitude sequence q and time interval sequence Δt are introduced. The parameters can represent exactly the physical characteristics of different PDs. Based on these parameters, a new approach is developed through which pattern classification of PD can be achieved effectively without employing the phase information of the applied voltage. The clustering results obtained by performing statistical product and service solutions indicate that there is evident difference in the characteristic coefficients.

Modeling of partial discharge activity in spherical cavities within a dielectric material

Abstract: In this paper, a model for a spherical cavity within a homogeneous dielectric material was developed using finite element analysis (FEA) software. The model was used to study the influence of different cavity conditions on local electric field distribution and consequent PD activity. The study involved variation of cavity diameter and cavity location within the dielectric material. In addition, experimental measurements were undertaken using epoxy samples containing single spherical cavities of different diameters.

Aging condition assessment of transformer oil-paper insulation model based on partial discharge analysis

Abstract: This paper presents aging condition assessment of oil-paper transformer insulation based on partial discharge analysis in order to realize statistical parameters reduction. The extracted feature factors of this proposed model were used to identify oil-paper samples with different aging degrees. An accelerated aging test was implemented using artificial oil-paper samples with an internal flat air gap. During the aging test, partial discharge signal acquisition was conducted periodically. In the new model, conventional statistical parameters of phase resolved partial discharge (PRPD) patterns were analyzed using principal component and factor analysis (PCFA), and a group of new features constituted by the extracted factors was obtained. These factors were not only independent of one another, they had their own specific properties. To a great extent, these factors represent information on PRPD patterns through a limited number of variables. Through the use of the new features extracted from PCFA method, the clustering and discriminating results of the samples with different aging stages provided significantly referenced information on the condition assessment of oil-paper insulation.

The Use of Partial Discharges as an Online Monitoring System for Underground Cable Joints

Abstract: With their vast amount of joints, underground cable systems are expensive to maintain; therefore, measurement equipment must be cost-effective. This study proposes the long-term placement of partial-discharge (PD) monitoring units at cable joints. The unit consists of a peak detect circuit and a digital signal processor for acquiring and converting the sensors' PD signals into discharge sequence data, whose data are then provided to a remote computer for analysis and display of the condition of the cable joint. Finally, a cable joint with artificial defects was tested by accelerated aging tests to verify the performance of the equipment. Two characteristics, namely, the average discharge quantity (Qave) and the discharge phase region, are proposed and plotted against each other to render an evolution locus, which reveals a shape of an upside-down letter N. The locus allows a judgment to be made on the state of the insulation in the cable joint. These results further justify work to further develop the method as an online cable monitoring system.

Diagnostic of HVDC systems using partial discharges

Abstract: This paper presents a methodology for the diagnosis of HVDC systems based on partial discharge analysis. In particular, the characteristics of partial discharge phenomena from different test objects are highlighted, with the aim of providing tools enabling automatic identification of the type of PD, as well as noise and disturbance rejection, for on-line and/or off-line PD testing of HVDC equipments. The results show that, using a fuzzy identification system based on higher-order moments of the PD magnitude and inter-time distributions, corona phenomena can be identified easily, whereas it is sometimes difficult to distinguish between internal and surface partial discharge sources. For this purpose, the characteristics of internal and surface partial discharge phenomena as a function of influence variables as humidity and temperature are reported. By exploiting these characteristics through on-line/off-line PD monitoring systems, accurate identification of the PD phenomena affecting an electrical apparatus can be attained.

Choice of optimal voltage for more electric aircraft wiring systems

Abstract: Increased electrical power demands are being experienced on the new generation of aircraft due to an increased reliance on electrical technology such as air conditioning, de-icing systems and electrical flight control actuation. Distribution of power at higher voltages is therefore now being seen in modern aircraft to avoid the penalties incurred due to high wire weights. Voltages have increased past the minimum of Paschen's law resulting in a risk that life limiting partial discharge damage can occur in the insulation systems. This study uses a theoretical analysis backed by experimental results to investigate the optimal operating voltage of a wiring system. In addition it proposes a methodology for optimising the operating voltage level based on an analysis of the power carrying capability of wiring within a fixed volume system and the derivation of the wire weight as a function of voltage. The work done has shown that the optimal operating point for an aircraft power system does not imply the use of the highest voltage possible. A trade-off between wire weight and power transfer is required and furthermore the use of direct current systems can result in higher power transfers than conventional three phase/400 Hz alternating current (AC) systems.

Effect of packing solid material on characteristics of helium dielectric barrier discharge at atmospheric pressure

Abstract: The influence of a solid material (e.g., Al2O3) on the physical characteristics of helium dielectric barrier discharge (DBD) has been investigated in a cylindrical plasma reactor. The helium DBD with the solid packing shows a multiple current-peak effect, which can be ascribed to the enhancement of charge density due to charge deposition on the Al2O3 pellet surface. The combination of plasma and the solid increases both the peak-to-peak voltage and breakdown voltage of the discharge at a fixed input power but decreases the discharge temperature. It is also found that the presence of the solid has a weak effect on the electron temperature and density in the discharge.

Calibration of on-line partial discharge measuring system using Rogowski coil in covered-conductor overhead distribution networks

Abstract: Partial discharge (PD) detection has been regarded as one of the most effective on-line predictive maintenance test and diagnostic tool for the condition monitoring of high voltage (HV) equipment. In this study, a methodology for detecting on-line PDs produced because of falling trees on the covered-conductor (CC) overhead distribution lines is introduced and calibration of PD measuring system is carried-out. The Rogowski coil is used as a PD sensor which is non-intrusive and superior to the conventional PD detectors. The experimental set-up was arranged in the HV laboratory for real-time analysis and a pulse calibrator was used to calibrate the PD measuring system. Few real-life PD measurements have been taken and it is revealed that PD magnitudes and signals bandwidth may vary under various circumstances. The calibrated on-line PD measuring system can be used to detect and measure the amount of PDs produced because of falling trees on CC lines, thus improving the reliability and safety of the distribution networks as well as reducing visual inspection work after storms.

Polymer nanocomposites in high voltage electrical insulation perspective: a review

Abstract: Polymer nanocomposites have attracted wide interests in various industries. This new class of material is capable of providing significant improvements in combined electrical, thermal and mechanical properties. Although the potential use of polymer nanocomposites in electrical insulating industry has only recently begun to be explored, a great number of researches have been conducted with regards to high voltage electrical insulation performance. However, it is found that the fundamental physics and chemistry concerning the property enhancement due to the incorporation of nanocomposites is still poorly understood, and there is still room for improvement in this research area. This paper serves to highlight some of the past developments of polymer nanocomposites and to inspire some potential fields that can be explored in high voltage electrical insulation perspective. Effect of the electrical discharges that causes the surface tracking and partial discharge phenomena on the polymer nanocomposites are the main subjects to be discussed in this paper.

Surface discharge initiated by immobilized metallic particles attached to gas insulated substation insulators: process and features

Abstract: In order to study the developing process of surface discharges initiated by immobilized metallic particles attached to Gas Insulated Substation (GIS) insulators under gradually-raised applying voltages, long-term laboratory tests were conducted on a well-established 220 kV test assembly to observe the entire evaluation process of surface discharges from its inception to gradual expansion and the eventual occurrence of surface flashover Real-time detection was conducted to record conventional impulse current (CIC) signals, UHF signals, ultrasonic signals and light emission signals from PD Features of surface discharges at different developing stages were captured from the discharge trend curves, scatter diagrams, two-dimensional histograms, and grey-scale maps of PD The test findings indicate that i) surface discharge initiated by immobilized metallic particles attached to GIS insulators undergoes three developing stages from corona discharge domination in the incipient stage, to co-existence of corona discharge and surface discharge in the expansion stage, and finally the surface discharge domination in the final stage; ii) the shape and phase distribution shown in scatter diagram, histogram and grey-scale map could be used as effective criteria to infer different evolutionary stages of surface discharges produced by immobilized metallic particles attached to GIS insulators

The Characteristics of UV Strength According to Corona Discharge From Polymer Insulators Using a UV Sensor and Optic Lens

Abstract: Inspections and diagnoses of corona discharges are important in order to prevent electrical faults of external insulation in power systems. This paper studies the measurement of ultraviolet rays (UV) strength of corona discharges on polymer insulators using a UV sensor with an optic lens. The data have been compared with the images of a UV camera. The UV signal by corona discharge was detected from more than 60 kV, which was equivalent to around 37.5% of the breakdown voltage. The UV strength increased with increasing high voltage and the supplying dc voltage. Also, UV image was observed. However, the longer the distance was, the greater the reduction in UV signals would be. The robust, inexpensive sensor can detect corona discharge activity on polymer insulators when the applied voltage exceeds 37.5% of the breakdown voltage.

Numerical analysis of plasma evolution on dielectric barrier discharge plasma actuator

Abstract: Time evolution of the discharge plasma in the dielectric barrier discharge (DBD) plasma actuator was simulated by the simple fluid model in which the electron and single positive ion species were considered. The characteristics of the discharge plasma evolution were investigated in detail, and the following results were obtained. When the positive-going voltage is applied, the streamer discharge is formed periodically. The periodically formed streamer expands from the exposed electrode, and its length becomes longer than the previous one. Periodic breakdown of the gas and step-by-step plasma expansion are also observed during the negative-going voltage; however, the streamer is not formed and the breakdown frequency is much higher. The simulation results with a triangular applied voltage waveform show the same characteristics as observed in the experiment; large discharge current spikes are observed during both the positive- and negative-going voltage phase, and the plasma in the negative-going voltage phase expands more smoothly than that in the positive phase because of its higher breakdown frequency. It was shown that even the simple numerical model could provide valuable insights into the physics of DBD plasma actuator; this indicates that the positive ions and electrons play a prominent role in determining the general characteristics of the plasma evolution.

Partial-Discharge Characteristics of Free Spherical Conducting Particles Under AC Condition in Transformer Oils

Abstract: One of the major problems in the liquid insulation is the particle contamination, which can seriously degrade the withstand voltage of the insulation. The movement of conducting particles under operation voltage can generate incipient discharges that can lead to the complete failure of the insulation system during operation. The partial-discharge characteristics of the free spherical conducting particle in the quasiuniform electric field were studied in this paper in order to better explain the partial-discharge (PD) measurement signals for particle movement and implications for insulation diagnostics in power transformers. The phase-resolved partial-discharge (PRPD) pattern was measured and the PD inception voltage, PD charges, and PD frequency were studied under different applied voltages. The influence of oil aging degree to particle PD characteristics was also studied in this paper. It is observed that the PD pattern is different in a different particle motion stage and the number of particles has a significant influence on PD frequency and a slight influence on PD pattern. The particle is inactive in severely aged transformer oil. The PD charges and PD frequency (pulses number per half ac cycle) reduce with the aging degree of the oil sample.

Measurement and analysis of surface partial discharges at semi-square voltage waveforms

Abstract: The fast switching in PWM drive applications exerts complex transient stresses on the cable and motors, creating conditions for different partial discharge form initiation. The mechanism of surface PD on dielectric materials (ethylene-propylene rubber and thermosetting insulation) under semi square voltage has been investigated. In the rod-plane electrode arrangement positive polarity PWM-like pulses with variable rise time (800 ns and longer), and frequency (0.1 Hz to 2 kHz) have been used. Experiments show correlation of PD inception voltage and its time range within a semi-square voltage period with slew rate of applied voltage and its switching frequency. Partial discharges appear not only at rising/falling slope of the voltage pulses but also during the voltage plateau, even when the rod electrode potential becomes zero. The main process taking place during surface PD action on insulating materials is the interaction of space charge near the rod electrode with the polarized dielectric. As a result it was revealed that in such conditions PD can occur within a whole period. This phenomenon is relevant to understand the destructive processes in insulating materials under PWM-like pulses action.

Runaway electrons and x-rays from a corona discharge in atmospheric pressure air

Abstract: The characteristics of a corona discharge in atmospheric pressure air are studied using pulsed power generators that produce voltage pulses of different durations, polarities and shapes. The characteristics are measured in the single pulse, batch, and repetitively pulsed modes. It is shown that no matter what the voltage pulse polarity is, a corona discharge starts developing as a conical diffuse discharge near the electrode tip with a voltage rate of increase of 10 15 Vs 1 across an electrode of small curvature radius. With lower voltage rate of increase ( 10 13 Vs 1 or lower), one or several diffuse jets develop from this electrode. The diameter of the jets at their front is less than 1mm and depends on many factors (voltage pulse amplitude and increase, inter- electrode gap width, pulse repetition rate, etc). It is found that at long voltage pulse durations, the radiation spectrum of the corona discharge changes, and the bands and lines of the material of the electrode appear in the UV region at 200-300nm. It is demonstrated that a runaway electron beam in a corona discharge is generated and detected at a distance several times greater than the brightly glowing plasma region of the corona discharge. It is shown that x-rays are generated from a corona discharge at high pulse repetition rates of up to 1kHz.

Evaluation of UHF method for partial discharge measurement by simultaneous observation of UHF signal and current pulse waveforms

Abstract: The partial discharge - current pulse waveform analyzing (PD-CPWA) system can measure partial discharge (PD) current pulses continuously in high speed, and is being used for the study of PD mechanisms in SF6 gas in a laboratory. On the other hand, the ultra high frequency (UHF) method is becoming popular for the on-site monitoring of gas-insulated switchgears (GIS). The former can detect PD with high accuracy and make it possible to analyze and understand discharge physics in detail. The latter is suitable to detect PD in actual power equipment, even in noisy site condition. Clarification of coincidence of PD characteristics measured with both methods is very important for further application of the UHF method. In this paper, PD current pulses generated from artificial defects in SF6 gas were synchronously measured with both methods. Measured data were analyzed and compared in many PD characteristics. As a result, many characteristics coincided well between them. However, the scatter of PD and the limited equipment performance of the UHF method may cause some detection failures. An appropriate and advanced use of the UHF method was pointed out.

Simulation of Partial Discharge in High Voltage Power Equipment

Abstract: In high voltage (HV) electrical power system, variety of solid, liquid and gaseous materials are used for insulation purpose to protect the incipient failure inside the HV power equipment. Among these the solid insulation is widely used for high voltage power equipment HV electrical power system. Most of insulating materials are not perfect in all respect and contains always some impurities. The presence of air bubble is one of such impurities in insulating materials and highly undesirable for such type of insulation which causes a local weak zone inside the insulator. Insulation of the HV power equipment gradually degrades inside the insulator due to cumulative effect of electrical, chemical and thermal stress. Due to the high voltage stress the weak zone inside the insulator causes the partial discharge (PD) which is known as local electrical breakdown. As a result the insulation properties of such materials are enormously degrades its quality due to the PD. In this work, the simulation of PD activity due to presence of a small cylindrical void inside the solid insulation material of high voltage power equipment is studied with the MATLAB Simulink platform.

Experience with UHF partial discharge detection and location in power transformers

Abstract: Ultra-high frequency (UHF) methods for detecting and locating partial discharges (PD) are well established for gas insulated equipment. This paper provides an overview of their application to power transformers, discussing practicalities, capabilities and challenges. In recent years, UHF PD detection has proved helpful to manufacturers as a tool for enhancing quality control. The technique can now form a valuable addition to the suite of on-line monitoring technologies available for power transformers. A key aspect discussed in this paper is the installation of sensors, which must have an electromagnetic ‘view’ into the Faraday cage of the tank. Locating PD sources by time-of-flight methods in transformers requires accurate spatial resolution in three dimensions in the presence of large conducting components inside the tank that often block the line of sight between PD source and sensor. For this reason, instead of an ‘empty box’ representation, modeling techniques are used to account for the internal structure of the transformer so that observed time differences among PD signals received at different sensors can be interpreted more realistically. The PD location procedure is illustrated by means of practical results, which indicate that PD sources can be located typically to within 0.3 m in a power transformer.

Discussion on possible additions to IEC 60897 and IEC 61294 for insulating liquid tests

Abstract: IEC 60897 (determination of lightning impulse breakdown voltage, LIBV) and IEC 61294 (determination of partial discharge inception voltage, PDIV) are designed with the purpose of evaluating insulating liquids' electrical property in a divergent field, and their current versions have existed for about two decades without revision. In view of the interest in the use of natural and synthetic esters in transformers, the suitability of IEC 60897 and IEC 61294 to evaluate new insulating liquids and the potential revisions are discussed here. Through comparative tests of mineral oil and esters, it is proposed that for IEC 60897, V–T (breakdown voltage–time to breakdown) curve should be obtained at overstressed voltages, i.e. voltages higher than LIBV, preferably under positive polarity, as a complement of LIBV. Similarly for IEC 61294, counting partial discharge (PD) numbers at overstressed voltages, i.e. voltages higher than PDIV, is proposed as a complement of PDIV. Overall, overstressing the liquids helps provide more information about electrical property of insulating liquid and thus can better assess a liquid for the purpose of insulation design. According to the above proposed procedure, both esters are found to be inferior to mineral oil in terms of lightning impulse property and PD behaviour.

Simulation of partial discharge sequences using fluid equations

Abstract: Simulation methods employing continuity equations to describe particle transportation are used to examine the process of single discharge. However, it is difficult to simulate partial discharge (PD) behaviour accurately by these methods, because it will take too much computation time to simulate consecutive PD sequences. In this report, in order to save computation time, a simple three-dimensional network is used to simulate PD sequences in a small void under ac voltage. The model considers charge generation, charge transport and charge disappearance, and electrical field redistribution in three dimensions. The continuity equations for electrons, positive ions and negative ions, including the effects of ionization, attachment, recombination, electron diffusion and the transport of charge under electrical field are solved simultaneously with Poisson's equation. With this model, the phase-resolved PD pattern and the current forms of each PD are obtained. Moreover, considering the propagation process of each PD and its influence on surface charge distribution, the fall part of PD current form and the effect of previous discharge on the subsequent one are also studied.

Device for testing insulating property of high-voltage oiled paper

Abstract: The invention relates to the technical field of high voltage and insulation, in particular to a device for researching and testing an insulating property of high-voltage oiled paper. Aiming at the positions of frequent faults during the operation of a converter transformer, the device simulates a ball-board, a needle-board, a board-board, faces and suspension discharge electrode models; and the typical high-voltage oil/oiled paper insulating experiments can be realized under direct current (DC) voltage, alternating current (AC) voltage and AC-DC voltage so as to acquire partial discharge characteristics and breakdown characteristics of the major insulation of the converter transformer.

Partial discharge parameters to evaluate the insulation condition of on-line located defects in medium voltage cable networks

Abstract: Partial discharge analysis has a long record as reliable diagnostic tool to assess the integrity and the quality of electrical insulation of power systems. On-line partial discharge monitoring systems for live medium voltage cable connections have recently been introduced in Dutch grids and also a few worldwide. To enhance the applicability of on-line cable diagnostics there is a need to develop a PD interpretation approach suitable for on-line monitoring. This paper discusses various PD parameters, including PD charge magnitude, PD occurrence rate and PD charge density and their related statistical values such as mean and maximum, used to interpret the PD activity. Trend watching of these parameters is employed to study the degradation state of cable connections and their components. The approach is applied to field data obtained over a year through monitoring several live circuits. In this paper, three examples are presented. Weak spots were detected and failures were prevented by early warning.

Partial Discharge Behaviours and Breakdown Mechanisms of Ester Transformer Liquids under ac Stress

Abstract: Mineral oil has been widely used in liquid insulation of power transformers. However, it is poorly biodegradable and could cause serious contamination to the environment if a spill occurs. With increasingly strict environmental rules and regulations, there is considerable interest from the Utilities to apply esters in power transformers as substitutions to mineral oil. In order to use esters in large power transformers, their dielectric properties should be thoroughly investigated. This PhD thesis covers the experimental studies on the dielectric properties of a type of synthetic ester (Midel 7131) and a type of natural ester (FR3) in both uniform and divergent electric fields, using a mineral oil (Gemini X) as the benchmark.The dielectric properties of transformer liquids in uniform fields were investigated using breakdown voltage tests. The breakdown voltages of esters should be at least similar to that of mineral oil to allow a replacement in transformers. To obtain a fair comparison, the AC breakdown voltages of well-processed transformer liquids were tested, and their distributions were statistically analyzed. Since the breakdowns of transformer liquids in uniform fields are caused by the weakest-links, conditions representative of in-service transformer liquids were also considered by testing the effects of extraneous factors, such as particles, water and electrode area.The divergent fields were produced by sharp needle electrodes with tip curvatures of a few micrometers. The dielectric properties of transformer liquids in such fields were studied using several methods. A traditional PD detector was used to study the partial discharge characteristics of insulating liquids, such as the inception voltages and the repetition rates. A high speed camera was utilized to identify the streamer generation, propagation and breakdown phenomena. An oscilloscope was used to investigate the current signals associated with these phenomena. The breakdown voltages of transformer liquids were also determined in the divergent field at various gaps. Furthermore, the fault gases in transformer liquids under partial discharge faults were determined and analyzed.The following findings and conclusions can be made from the research in this thesis:* The AC dielectric strengths of esters in uniform fields are similar to that of mineral oil when they are in a well-processed condition. When practical liquid conditions are considered, the AC dielectric strengths of esters are higher than that of mineral oil.* The partial discharge behaviours at overstressed voltages can be used to differentiate various transformer liquids. Esters are relatively inferior to mineral oil in terms of higher discharge amplitude, higher discharge repetition rate and more negative partial discharges.* Mineral oil possesses a higher ability to suppress the propagation of negative streamers. Thus, the AC dielectric strength of mineral oil in the divergent field is relatively higher than those of esters.* Esters generate the same types of fault gases due to electrical discharge as mineral oil, but in relatively larger amounts.