Analysis of partial discharges and their development into creeping discharges at solid/gas interface under AC voltage

A new model that can reproduce main stochastic features of partial discharge (PD) activity at AC and DC voltages was proposed. The type of PD activity because of microdischarges in small cavities present in dielectric materials was considered. Three different criteria were used to simulate an initiation of partial discharge inside voids. The simplest criterion of threshold type was used also to describe a decay of plasma in voids and subsequent decrease in conductivity to zero. After AC voltage was applied to solid dielectric, the narrow peaks of current in external circuit were observed in our simulations. Every peak corresponds to a moment of PD in a void. The behaviour of cavities in dielectric liquid under DC voltage was also simulated. In this case, PD activity is possible even under DC voltage because of both elongation of microbubbles present in a liquid and diffusion of charge carriers from the surface of a bubble into a liquid.

In this paper a novel approach to model partial discharges (PD) activity taking place inside a spherical void in epoxy resin system has been traced. The approach is based on a time varying conductance of the inner void, subjected to multistress conditions: voltage, temperature and pressure. A simple lumped circuit macro-model simulates the global effects of PD activity: the different parameters influencing the discharge phenomenon in the void are taking into account by using a physical approach resulting in a time varying conductance inside the circuit. The evaluated PD activity has been compared with experimental and simulated one for the accessible and inaccessible part of the system. A discussion on the observed changes in PD activity has been reported.

The principal cause of damage in underground power cable installations is partial discharge (PD) activity. PD is a localized non-linear phenomenon of electrical breakdown that occurs in the insulating medium sitting between two conducting materials, which are at different potentials. The damage to the insulating material is induced by the AC voltage to which the insulator is subjected during the discharge process, and it can be directly or indirectly measured by the charge displacement across the insulation and the cavity defect. Non-invasive detection techniques that help in identifying the onset of the discharge process are required as PD is a major issue in terms of maintenance and performance of underground power installations. The main locations of failure are the accessories at points of connection such as terminals or splices. In this article, a study of electrical detection of PD and image processing of thermal pictures is presented. The study was carried out by controllably inducing specific failures in the accessories of the installation. The temporal evolution of the PD signals was supported with thermal images taken during the test in order to compare the PD activity and thermal increase due to failure. The analysis of thermographic images allows location of the failure by means of intensity-based texture segmentation algorithms. This novel technique was found to be suitable for non-invasive detection of the PD activity in underground power cable accessories.

This paper deals with the study of the AC breakdown voltage (BDV) and partial discharges (PDs) activity of synthetic ester-based nanofluids (NF) with two kinds of carbonic nanoparticles (NPs), namely graphene (Gr) and fullerene (C₆₀); the synthetic ester (SE) being Midel 7131. The BDV measurement was achieved at various concentrations of NPs and different electrodes gap distances, while the partial discharges activity was studied only at the optimal concentration that gave the best improvements of BDV. First, a detailed improved procedure for preparing our NFs is presented. Then, the zeta potential measurements are performed on these NFs, and their stability is checked. After the preparation and characterization of samples, the BDVs and PDs parameters are measured according to IEC 60156 (modified gap distance) and IEC 60270 standards, respectively. Finally, conformity of the experimental data with Normal and Weibull distributions is examined, and the BDV at cumulative probabilities of 1% and 50% are then deduced. It is shown that BDV outcomes for all studied liquids obey both Normal and Weibull distributions, and the BDVs at cumulative probabilities of 1% and 50% are improved. Moreover, adding Gr and C₆₀ nanoparticles at different concentrations enhances the BDV values for investigated electrode gaps (0.1 to 2.0mm). The best improvement is obtained with a concentration of 0.4 g/L for 0.5mm and 0.7mm electrode gap with fullerene and Graphene nanoparticles, respectively. For a 2mm gap distance, the best improvements are of about 12.67% and 16.64% with 0.4g/L of C₆₀ and 0.3 g/L of Gr, respectively. It is also shown that the addition of C₆₀ significantly reduces the activity of partial discharges compared to pure SE, while the addition of Gr destroys the partial discharges resistance of pure SE.

It is well known that the occurrence and evolution of partial discharge phenomena within electrical insulation can lead to their destruction. Early detection of this phenomenon can avoid catastrophic failures of electrical equipments installed on transmission, distribution and generating stations. In this paper, two practical experiences using radio frequency scanning to detect partial discharge activity in electrical equipments are described. In the first case, partial discharge activity in two of three phases of a 13.8 kV air isolated bus duct was detected using radio frequency scanning after suspicions of PD activity in a 115 kV GSU power transformer and a 160 MVA generator was detected using acoustic emission and electromagnetic interference measurements tests with radio frequency current transformers. The second case describes the use of radio frequency scanning to detect partial discharge and corona activity inside a 34.5 kV metal clad switchgear located on a 500 kV substation. Although the electrical interference in the area was the main concern to get a reliable measurement, digital filtering and wide band scanning of the radio frequency scanning technique showed high efficacy to discriminate partial discharge activity inside the switchgear. Final results confirmed the abnormal condition of a circuit breaker in the switchgear.

When detecting the presence of partial discharge (PD) activity in the insulation system in high-voltage equipment, the excitation voltages at variable frequency have been widely used instead of power-frequency (50/60 Hz) sinusoidal voltage in order to reduce the charging power. This work reviews the relevant research on PD activity at very low frequency (VLF) method, including sinusoidal or cosine-rectangular voltage shape, and damped AC (DAC) method. Based on the research history and development status, some major PD characteristics, such as PD inception voltage (PDIV), PD amplitude, PD charge, PD phase-resolved pattern, and several hot issues, such as surface charge decay and statistical time lag, have been discussed. Moreover, the advantages, disadvantages, and applied conditions of two reviewed methods has been summarized. Finally, the prospects have been made on the main development trends of this research field in the future.

Due to environmental concerns regarding the use of mineral oil, biodegradable oil is increasingly being used as an alternative dielectric fluid. This paper presents results of experiments performed in the laboratory on the use of biodegradable oils for transformer application. The investigations cover two important diagnostic techniques for insulation assessment: (i) an investigation of partial discharge (PD) activity and characteristics in such oils and (ii) an investigation of the standard hydrocarbon dissolved gas products produced due to transformer faults. The fundamental aim of the investigation was to provide information as to whether the existing analysis techniques of PD fault pattern recognition and dissolved gas analysis methods, developed for mineral oil, are valid when used for insulation assessment with biodegradable transformer oil. The experiments were conducted on test samples to simulate three common types of transformer faults: low-energy PDs, high-energy arcing/sparking, and overheating. For the purpose of comparison, the same tests were performed using a mineral oil (Shell Diala MX) and a biodegradable oil (Envirotemp FR3). Also, each oil type was tested at three different moisture levels and with or without the presence of solid insulating materials (pressboard). PD activity was monitored using the standard IEC60270 phase-resolved analysis method. The fault gases produced were extracted and analysed by standard gas chromatography methods. Test results indicate that the PD phase resolved patterns are, in general, similar for the two oil types and thus existing PD pattern interpretations can be used to distinguish different types of PD faults, e.g. corona versus surface discharges. However, the values of various discharge quantities (PD magnitudes, repetition rate, current, etc) are very different under the same test condition. The quantities and the trend of dissolved gases for faults in biodegradable oil are substantially different compared with mineral oil. For the PD fault, biodegradable oil was found to release only a limited number of gases. For arcing and overheating faults, the main key gas produced is different between the two oils. Also, the presence of cellulosic materials such as pressboard influences the extent of hydrocarbon gases dissolved in the oil. It was found that for biodegradable oil, fewer amounts of gases are produced as the oil gets wetter.

Epoxy/paper insulated materials, with the merits of great insulation performance, excellent mechanical strength, are widely used in tubular busbars playing an important role in safe and steady operation of national grid. In this work, the partial discharge (PD) in epoxy/paper insulated tubular busbars with two types of artificial defects and different aging time were measured. The aging voltage imposed on the samples was 18 kV, and the aging temperature was 428 K. PD of samples at varied aging stage were measured. Sample with defect of cavity and metallic impurity (sample B) showed larger apparent charge magnitude, compared with the sample with defect of cavity (sample A). For example, the measured largest apparent charges of sample B was 8337 pC, much larger than the value of 2266 pC measured in sample A when aged for 700h. In addition, the value of PD inception voltage of sample A was fluctuated around the 16 kV, while PD inception voltage of sample B varied from 10 kV to 8.4 kV. And we analyzed the results in the article. To figure out the influence of defects on the PD activity, MATLAB Simulink tools were used to simulate the PD of samples with different defects. The results showed that the defect of cavity and metallic impurity discharged more severely than the defect of cavity. Moreover, finite element analysis (FEA) method was applied to obtain the electrical field distribution, which revealed that the defect of cavity and metallic impurity caused local electrical field distortion whose maximal value was 1.8 times of the defect of cavity.

Among the factors influencing partial discharge (PD) activity in insulation systems, deposition of electric charges on dielectrics significantly impact the subsequent behaviour. This work presents a measurement technique that allows studying the charge released during PD events by measuring the resulting excess current. The method, being similar to the dielectric response analysis, measures the current flowing through a test object before, during and after a PD event. By comparing the dielectric (capacitive) and the total currents, the charging process as well as its decay can be explored. The applicability of the method is demonstrated when analysing PD activity in a twisted-pair test object exposed to a square voltage waveforms.

Previous observations on electrical tree growth in epoxy resins has shown that different types of tree growth structure, electrically conducting and non-conducting, can occur dependent on the state, glassy or flexible, of the epoxy resin. In this current study, the partial discharge characteristics were characterized experimentally at a temperature of 20°C within two different epoxy resins systems having glass transition temperatures of 0°C and 50°C. The partial discharge activity (determined from apparent charge measurements) was characterized in terms of φ~q~n patterns using statistical tools. The aim was to compare the apparent charge measurements obtained from conducting and non-conducting electrical tree structures with computer simulations of the partial discharge activity in both conducting and non-conducting electrical trees. The results show that there is a significant relationship between the local extent of the partial discharge phenomena, as determined by the conductivity of the tree channels, and the apparent charge, as shown by the experimental and simulated partial discharge patterns. The implications of this work for partial discharge detection as well as for condition monitoring in real insulating systems are discussed.

Partial Discharge (PD) measurements are very promising for insulation condition monitoring in Gas Insulated Switchgears (GISs). However, for accurate PD detection and diagnosis, there is a need for better understanding of physical mechanisms behind PD activity. Accordingly, we have been investigating the PD characteristics and associated mechanisms for electrode/epoxy delamination of GIS spacers as one of the most serious defects in GIS. The gas pressure inside delamination usually changes from sub-atmospheric pressure, just after delamination initiates, to filled SF 6 gas pressure in the GIS tank, after SF 6 gas permeates into the delamination area. Therefore, in this paper, the PD activity at different gas pressures, from 0.02 MPa to 0.4 MPa, is acquired. The relative PD generation rate and its phase appearance are analyzed at different pressures and then discussed from the viewpoint of PD mechanism and PD detection. Experimental results show that, with increasing pressure, the peak of the PD generation rate shifts from around zero crossing like void-type discharge to around a peak of ac voltage, before PD pulses vanish at high pressure for the applied electric field stress. The obtained results could be useful for delamination diagnosis in GIS spacers.

This paper examines the influence of cavity size and location in the insulation paper on the Partial Discharge (PD) activities through Finite Element Method (FEM). The model consisted of a conductor wrapped with insulation paper. Two different locations of the spherical cavities were introduced in this study, namely Location 1 (L1) and Location 2 (L2), located at the center and left corner of the insulation paper. The model introduced two different sizes of cavities with diameters of 0.5 mm and 0.8 mm. An AC voltage source of 17 kV, 50 Hz, was applied at the conductor while the bottom of the insulation paper was grounded. The real and apparent PDs were obtained by integrating the current flowing through the cavity and ground electrode with the respective surface area. The simulation was carried out for 100 cycles. The resultant model was used to study the PD occurrence, magnitude, and Phase Resolved Partial Discharge (PRPD) within the insulation paper. It is found that the large cavity size produces a lower number of PD occurrences per cycle than the small cavity size. The large cavity size produces a higher charge magnitude as compared with the small cavity size. The PD occurrence per cycle and charge magnitude are higher for the cavity location at L1 compared to L2. The PRPD yields the same pattern for cavity location at L1 and L2, whereby the differences are only on the charge magnitude and PD occurrence per cycle.

PD characteristics and degradation in PET insulation with vented and unvented internal voids

Space charge accumulation is theorized to be a contributor to partial discharge activity. The analysis of partial discharge (PD) activity is a method of predicting electrical breakdown in dielectric materials. Rapid aging tests of an acrylic dielectric elastomer using a partial discharge analyzer (PDA) has shown that under DC voltage application consisting of a 1 minute period of constant voltage followed by an increase in voltage results in bursts of partial discharge activity. This paper will explore the potential cause of this phenomenon. It is theorized that space charge accumulation during the constant voltage period followed by rapid charge injection during the increase of voltage results in an impulse like stress of the elastomer causing PDs.

Dry-type transformers require little maintenance and are more environmentally friendly with respect to traditional oil-and-paper design. However, partial discharges (PDs) and thermal stress may lead to aging and—eventually—failure of the insulation. Diagnostics of their conditions—often performed by means of PD measurement—can be tricky, as PD activity depends on many parameters. In this work, a cast-resin power transformer is aged by means of 28 heating/cooling thermal cycles, to reproduce the thermal–electrical stress it may experience in operation. PD activity is monitored during the cooling branch of each cycle using a high-frequency current transformer (HFCT) to measure the current bled to the ground by the transformer shield, whose results are compared to the apparent charge measured by an IEC 60270-compliant commercial system installed on the medium-voltage (MV) winding. The effect of temperature and aging (number of cycles) is then investigated. The results show good agreement between the two measuring systems, in terms of both the phase-resolved partial discharge (PRPD) patterns and the average intensity trend over time. According to the intensity dependence on temperature, two PD series were identified: one series shows a loose dependence on temperature, and the other one a strong dependence. It was speculated that two types of defects may exist, whose geometry (and hence PD activity) may be affected by temperature to different extents (e.g., due to their position). Finally, a decrease in PD rate in time was observed, without a significant reduction of the average discharge intensity, which is supposedly due to different aging rates of different defects.