Partial Discharge Behavior Research Articles

Analysis and evaluation of dielectric parameters for design verification and calibration of a newly developed diagnostic system for MV power cables

For the purpose of a condition-based asset management, in-depth knowledge of the equipment condition is needed. In medium-voltage power networks, paper-insulated lead-covered (PILC) cables are to see as special critical assets. In order to evaluate the most appropriate diagnostic parameters and their characteristic values, multiple PILC cables have been artificially aged at power frequency during a long-lasting laboratory ageing experiment. Over this process, the relevant parameters like dissipation factor and partial discharge behaviour have been monitored and recorded and a fundamental database was built up. As a next step, field measurements of these values need to follow. Therefore, a mobile measurement system has been furtherly developed and integrated in a cable test van. In order to get improved diagnostic criteria the mobile test system facilitates, that the dielectric properties of the PILC cables are determined at 0.1 and 50 Hz test voltage frequency. Here, the development, the calibration and the adjustment of the measurement system as well as influences on field measurements will be described and discussed.

Partial discharge behavior under operational and anomalous conditions in HVDC systems

Power cables undergo various types of overstressing conditions during their operation that influence the integrity of their insulation systems. This causes accelerated ageing and might lead to their premature failure in severe cases. This paper presents an investigation of the impacts of various dynamic electric fields produced by ripples, polarity reversal and transient switching impulses on partial discharge (PD) activity within solid dielectrics with the aim of considering such phenomena in high voltage direct current (HVDC) cable systems. Appropriate terminal voltages of a generic HVDC converter were reproduced — with different harmonic contaminations — and applied to the test samples. The effects of systematic operational polarity reversal and superimposed switching impulses with the possibility of transient polarity reversal were also studied in this investigation. The experimental results show that the PD is greatly affected by the dynamic changes of electric field represented by polarity reversal, ripples and switching. The findings of this study will assist in understanding the behaviour of PDs under HVDC conditions and would be of interest to asset managers considering the effects of such conditions on the insulation diagnostics.

Partial Discharge Behaviour within Palm Oil-based Fe2O3 Nanofluids under AC Voltage

In this work, comparison studies of partial discharge (PD) behaviours within palm oil-based Fe2O3 nanofluids under AC voltage were performed. The samples were prepared with different weight percentage of Fe2O3 nanoparticles; LC (Low Concentration), MC (Medium Concentration) and HC (High Concentration) in order to investigate the „electron trapping“ effectiveness, their discharge withstand capability, discharge numbers, magnitude and Phase Resolved Partial Discharge (PRPD) patterns under high voltage stress. The results indicate that the presence of Fe2O3 nanoparticles with LC enhances the withstand characteristics of palm oil against PD.

Influence of void defects on partial discharge behavior of superconducting busbar insulation

For a magnetic confinement fusion device, the superconducting magnets and busbars need to be insulated with one layer of solid insulation to isolate the high voltage potential from the ground. The insulation layer commonly consists of several interleaved layers of epoxy resin-impregnated glass fiber tapes and polyimide films. The traditional electrical inspection methods for such solidified insulation on the magnet and busbar are a DC voltage test or a Paschen test. These tests measure the quality of the insulation based on the value of leakage currents. However, even if there is a larger quantity of high dielectric strength material implemented, if there are some microcavities or delaminations in the insulation system, the leakage current may be limited to microampere levels under testing levels over dozens of kilovolts. Therefore, it is difficult to judge the insulation quality just by the magnitudes of leakage current. Under long-term operation, such imperceptible defects will worsen and finally completely break down the insulation because of partial discharge (PD) incidents. Therefore, a PD detection test is an important complement to the DC voltage and Paschen tests for magnet and busbar insulations in the field of fusion.It is known that the PD detection test is a mature technique in the electric power industry. In this paper, the PD characteristics of samples containing glass fiber-reinforced composite insulations for use with the superconducting busbar were presented and discussed. Various samples with different void contents were prepared and the PD behaviors were tested.

Analysis of HVDC superimposed harmonic voltage effects on partial discharge behavior in solid dielectric media

This paper presents an investigation of effects of high voltage direct current (HVDC) superimposed voltage harmonics on partial discharge (PD) behavior. Firstly, a single air filled void embedded in a slab of polymeric insulation was modeled using finite element methods (FEM). PD phenomena activated under the influence of rippled DC voltages were simulated using an interlinked environment of MATLAB and multiphysics simulation package COMSOL. Secondly, the results of the modeling were backed up by experimental tests, where HVDC superimposed harmonics were applied to artificial void samples in the lab and trends in the resultant PD characterized with varying harmonic orders and amplitudes. The results of both modeling and experiment are generally in agreement showing that as the harmonic orders and levels increase, the repetition rate of PD pulses rises in comparison to a purely DC applied voltage having the same peak amplitude. The findings of this study will assist in understanding of PD behavior under HVDC conditions where harmonic rippling occurs on the voltage waveform.

Comparison of oil-impregnated papers with SiO<inf>2</inf> and ZnO nanoparticles or high lignin content, for the effect of superimposed impulse voltage on AC surface PD

Surface discharge behavior of modified oil-impregnated paper (OIP) with nanoparticles (NPs), has been investigated under AC voltage with superimposed impulses. Surface Partial Discharges (PD) can develop at an oil-paper interface and lead to its degradation. Modified paper, made from fibers with adsorbed nanoparticles, can affect the partial discharge behavior of a paper in combination with oil at the interface between oil and fibers. Papers with two different concentrations (2 wt% and 6 wt%) of silica (SiO 2 ), and paper with silanized zinc oxide (ZnO) nanoparticles (1 wt%) have been studied. Papers with SiO 2 NPs showed lower impulse-induced surface PD activity. However, thorough purification during the production of SiO 2 filled papers was necessary to achieve a good performance. With less purification, paper with 2 wt% of SiO 2 did not show such significant improvements. Paper with 6 wt% of SiO 2 NPs showed a large number of AC surface PDs, but low influence of impulse voltage on subsequent PD. Papers containing 1 wt% of silanized ZnO showed reduced relative permittivity, but no significant difference in surface PD behavior. The effect of high lignin content in Kraft paper has also been studied. Paper with higher lignin content showed better surface PD characteristics under the impulse. Paper with low concentrations of pure SiO 2 NPs, and paper with high lignin content thus appear good candidates for further studies to improve the surface PD behavior of OIP.

Influences of temperature on partial discharge behavior in oil-paper bounded gas cavity under pulsating DC voltage

The oil-paper insulation of valve-side windings in a high voltage direct current (HVDC) converter transformer is subjected to both complicated voltage and severe thermal stresses. Partial discharge (PD) occurring in oil-paper bounded cavities is influenced by voltage stress type as well as temperature. This paper presents PD behaviors in an oil-paper bounded gas cavity under pulsating DC voltages. The voltage across the gas cavity and PD inception voltages at different test conditions are analyzed. Influences of temperature on statistical PD distributions and parameters are also presented. A numerical model is used to interpret PD behaviors under pulsating DC voltages and various temperatures. Experimental results and theoretical statements show that the temperature exhibits a great influence on the behaviors of cavity discharge under pulsating DC voltages. Volume resistivity, gas pressure, charge decay time constant, and electron generation rate are the primary parameters influencing PD behaviors at different temperatures.

Effect of direct fluorination on partial discharge characteristics of polyimide film used as magnet wire insulation of generator

Polyimide (PI) film is widely used as the insulation between coils in generator, where partial discharge (PD) may occur to cause the insulation breakdown. Fluorination is considered as an effective method for surface modification of polymer materials to change its electrical properties. In this study, to study the effect of fluorination on the PD behaviour of PI film used as magnet wire insulation, the samples of magnet wire were covered by PI film. Samples were fluorinated in a reaction kettle using the F2/N2 mixture for different time. The non-fluorinated and fluorinated samples were subjected to partial charges under alternating electric field. A wideband detection system was used to detect PD waveform and record discharge phenomenon. The experimental results indicate that the direct fluorination has great effects on the surface charge accumulation and the decrease in PD intensity. The magnet wire fluorinated for 45 min shows the highest PD inception voltage, the lowest intensity of successive discharges and the smallest number of discharge pulses.

Evaluation of aging in nanofilled polypropylene by surface discharges

Literature indicates conflicting results on improvement/degradation of electrical characteristics due to introduction of nanofillers into polymers. Since polypropylene has superior electrical, mechanical and thermal properties and has significant utility in electrical industry, this work investigates the effect of nanofiller content on the partial discharge resistance and breakdown strength of the polypropylene films with 0%, 2% and 6% organoclay nanofillers. Since the surface discharges are random in nature, the overall partial discharge behavior is quantified in terms of temporal behavior of the Weibull parameters of the partial discharge pulse amplitude spectra obtained during sample aging and analyzed within the framework of time series analysis. Experiments indicate that the partial discharge characteristics are affected by the percentage of nanofiller content. Microscopic examinations indicate that on exposure to aging by partial discharges, the defect volume decreased in nanofilled polymer and was found to be least in samples where nanofiller content was below the percolation threshold. Profilometric studies indicate that the partial discharge resistance increases with increase in filler content. Results indicate that in fresh unaged samples, the breakdown strength improves with nanofiller content. However, the reduction in breakdown strength due to aging in nanofilled samples is noticeably higher as compared to unfilled samples.

Effect of superimposed impulses on AC partial discharge characteristics of oil-impregnated paper

The effect of HV impulses on surface and cavity partial discharges (PD) of oil-impregnated paper (OIP) is investigated in this work, by subjecting specimens to impulse voltages superimposed on a continuous AC voltage. The superposition of transient and AC voltage is relevant to insulation systems in power-system equipment, which is exposed to transients such as lightning and switching impulses during its operation. For several different PD-generating defects in oil-impregnated paper, the PD behaviors before and after the impulse are compared. The influence of moisture content is studied by using OIP with moisture content <0.5%, 3.0% and 5.5%. The results show a distinctive behavior of PD due to the impulse for each defect and insulation condition, either by change in the PD rate and magnitude or by causing PD inception or extinction. Recordings of the voltage signals and associated PD measurements during and after the incidence of a high voltage transient are of potential use for condition assessment of the insulation of power transformers and their bushings, as a form of online diagnostics.

Chopped partial discharge sequence

In order to investigate partial discharge behavior, its mechanism and dynamics, a novel methodology – chopped partial discharges – has been elaborated. Typically, either during laboratory measurements or at on-site field testing, partial discharges are detected by applying a continuous sinusoidal excitation, most regularly 50/60 Hz or other frequency according to certain standards. A chopped partial discharge sequence consists of a number of packets of a base waveform, separated by a variable time delay. The base waveform may be sinusoidal, square-like, trapezoidal or with the different shapes reflecting various switching phenomena and stresses occurring in insulation systems. In this paper, a Chopped Partial Discharge approach is applied in the investigation of the internal mechanism inside voids, especially with respect to void surface conditions and charge decay phenomena. Two types of voids were investigated, the first having a semiconductive surface and the second embedded in epoxy resin. In order to properly understand the mechanism of partial discharge in a void, it is necessary to understand the field created by surface charges which remain after previous discharge events. Such deposited charges create an internal field, which interplays with superimposed external electric field in terms of the discharge inception conditions. The decay of the field created by charges results in a higher inception voltage, as this field component is in phase with the external background field immediately after the reversal of the polarity of the voltage. As a result of such compensation, partial discharge inception occurs at a different phase angle of the external background field relative to the equivalent inception angle for a continuous sequence. The chopped partial discharge method creates potential for various investigations in high voltage insulating systems.

Influence of temperature on dielectric properties of EPR and partial discharge behavior of spherical cavity in EPR insulation

Ethylene propylene rubber (EPR) with outstanding high temperature performance has been widely used as cable insulation. To understand the influence of temperature on the feature of EPR, a series of study has been conducted including the dielectric measurements, the breakdown voltage and PD behavior of cavities. In the experiments, the dependency of permittivity, conductivity and electric modulus on frequency and temperature has been investigated firstly. Then, the experiments of breakdown voltage have been performed under the applied voltage in the temperature range from 25 °C to 200 °C. Finally, PD behavior caused by artificial spherical cavities in EPR has been examined at the temperature of 25 °C, 100 °C and 150 °C, respectively. It can be concluded by the experiments that the relaxation processes of EPR similar to other polymer materials can be featured by a rapid decrease of the real part of complex permittivity and a slow decrease of dissipation factor with increasing frequency at higher temperature. The dielectric dispersion phenomena can be observed on the spectroscope of the electric modulus which follows the Havriliak/Negami model. The phase resolved PD distribution, such as ϕ-q-n plots and ϕ-n plots, caused by cavities can be characterized as the multi-stem-like distribution. The repetition rate, the average discharge power and the average discharge current have shown a more strong dependency on the temperature.

Partial discharge measurement and analysis at standard oscillating switching and lightning impulses on a GIS with artificial protrusion defects

Partial discharges (PDs) induced by artificial fixed protrusions inside an SF6 gasinsulated switchgear (GIS) test object under oscillating switching and lightning impulse voltages (OIVs) were investigated experimentally. The features of PD current and light pulses (sequences) under impulses were analyzed. The characteristics of PD behaviors under oscillating lightning impulse (OLI), oscillating switching impulse (OSI) and 50 Hz AC voltages were studied according to their statistical distributions. The experimental data were quantitatively analyzed to compare the effectiveness of AC and OIV voltages in exciting the PDs in terms of PD inception and PD activity. Most PDs occurred successively around the peaks of the oscillation period of OIVs and periodical PD sequences were formed. Compared to positive point PDs, negatives occurred more frequently, but with smaller magnitudes. The positive OSI was found to have relative high efficiency in exciting PDs from fixed protrusion comparing to other types of voltage investigated. The occurrence frequency of PD under negative OLI was one or more orders of magnitude greater than that under AC voltage. The average PD magnitudes under positive OIVs were much greater than the others. Both the average PD magnitude and the occurrence frequency were greater at 0.2 MPa, than at 0.4 MPa. This indicates that the AC PD test is not very sensitive to local enhanced field defects, such as fixed protrusions on conductors, because of the limited PD measurement sensitivity and the corona stabilization under AC voltage.

Role of type and content of nanoparticles on certain dielectric characteristics of polypropylene nanocomposites

In practical power apparatus, the insulation under high electrical stress is exposed to recurrent partial discharge pulses, and the utility of the insulation is largely determined by its resistance to aging by partial discharges. The occurrence of partial discharges is largely governed by a combination of the applied field and the space charges that are generated due to these discharges. Since the space charge profile generated by a time-varying voltage and, thus, the resultant field would be much different from that of a constant ac voltage, this work investigates how the partial discharge characteristics, the resulting erosion, and remnant breakdown strength change when samples are aged with a step voltage profile. Since incorporation of nanofillers in controlled quantities is known to alter the space charge formation and transportation, this work also focuses on understanding the effect of incorporation of different quantities of natural (cloisite 20 A° natural montmorillonite clay) and synthetic (tetrasilisic fluro mica) nanofillers into a pure polypropylene matrix on its partial discharge characteristics and the resulting effect when aged under the application of step voltages. Isotactic polypropylene films with different filler concentrations of natural and synthetic nanofillers were aged. The cumulative effect of the partial discharge behavior was investigated through analysis of the erosion depth produced by these pulses on sample surface and remnant breakdown strengths of aged samples. Results indicate that maximum partial discharge magnitudes become prominently lower in nanofilled samples as compared to that of base polypropylene only when the applied voltage is beyond a certain level. Up to that voltage, the average discharge magnitude is significantly lower in samples whose filler content is below a percolation threshold, but above that voltage, the average magnitude depends on the aspect ratio of the nanofillers. Partial discharge resistance and characteristic breakdown strength increases with increase in filler content in both types of nanofilled samples, but excessive filler content is not effective in enhancing these parameters.

Partial discharge behavior of mineral oil based nanofluids

A previous study showed that both mineral oil based nanofluids with 0.01% silica mass fraction and with 0.1% fullerene mass fraction have a higher AC breakdown strength than mineral oil. Breakdown occurs following discharge initiation and propagation in the oil. The breakdown strength value alone provides little information on the discharge process. Therefore, it is important to investigate the details of the discharge mechanisms in mineral oil and in nanofluids. Hence, this study focuses on the partial discharge (PD) behavior of mineral oil, silica and fullerene nanofluids. The total charge, voltage and pulse shape were recorded with the help of a high bandwidth PD measuring system. The discharge mechanism in mineral oil appeared to depend strongly on the polarity of the applied DC voltage. Under positive DC voltage, the silica and the fullerene nanofluids show increased inception voltage and a reduction of the total discharge magnitude compared to the reference mineral oil. Under negative polarity, inception voltage and discharge magnitude of the nanofluids and the reference mineral oil are virtually the same.

Influence of fast rise voltage and pressure on partial discharges in liquid embedded power electronics

Initiation of partial discharges at the highly stressed regions of an Insulated Gate Bipolar Transistor (IGBT) can lead to degradation of the insulation and eventual total breakdown of the system. In this work, an experimental setup has been designed for the study of partial discharges (PDs) under different voltage waveforms. PD behavior of IGBT insulation was investigated using conventional and optical techniques. Influence of pressure and voltage wave shape is documented. The test environment was first characterized with point-plane geometry under sinusoidal and slow rise square voltage of up to 20 kVpeak and fast rise square voltage of up to +50 kV. The measured electrical and optical PDs showed good correlation, revealing that optical PDs can be relied on for the characterization of PD phenomena. High slew rate of the square voltage reduced the inception voltage and increased magnitude. The PD pattern from the trench shows the existence of space charges. The PDs which occurred within the triple point region are most likely attracted along the board interface and become surface discharges. Pressure suppresses the initiation and propagation of the discharge.

Characteristics and aging of PCB embedded power electronics

The decrease of battery prices makes the electrification of the power train an irresistible development. Also the introduction of a 48 V on-board grid is decided to supply large auxiliaries. For such applications cost is always an important factor.PCB embedded power electronics offers improvements compared to power electronics bonded and soldered on Al2O3 as dielectric material. These improvements are a reduction of height, lightweight construction, cost efficiency of the applied materials and a reduction of the parasitic inductance. Sinter layers offer an increase of the melting temperature from 220 to 935°C, compared to solder, and the thermal resistance between semiconductor and cooling water is low due to a thin dielectric layer of 150μm and the heat spreading inside the copper leadframe, together with PCB materials having a thermal conductivity of 1.5W/mK. A thermal resistance of 0.5K/W per IGBT can be achieved. This paper describes the challenges of the production process, the 5μm copper metallization of the semiconductor and the sinter process. Production failures were detected by curve tracer measurements, lock-in thermography and cross sections. The application potential of the embedding technology is demonstrated by measurements of the thermal impedance and temperature dependent partial discharge measurements. The effect of aging of PCB material by thermal cycling on the partial discharge behavior could be demonstrated. First results of an ongoing power cycling measurement are described as well. Finally the aspect of new PCB material without glass-fiber enforcement offering higher thermal conductivity and higher glass-transition-temperature is shown.

Nonlinear time series analysis of partial discharges in electrical trees of XLPE cable insulation samples

This paper presents an investigation into the nonlinear dynamics characteristics of the partial discharge (PD) sequence measured during the propagation of typical electrical trees in XLPE cable insulation. Calculation of the correlation dimension and the largest Lyapunov exponent from the PD sequence is used to illustrate the nonlinear characteristics of the PD behavior in the electrical tree growth. The results provide strong evidence for the existence of deterministic chaos in the PD phenomenon of electrical trees, where the correlation dimension and the largest Lyapunov exponent are related with the PD activity of the tree growth, rather than a presence of random stochastic process. The fluctuating correlation dimension of the PD sequence during the branch-pine tree growth is associated with the transition of PD patterns and the change of the channel conductivity of the tree structure. It is also found that the nonlinear dynamics characteristics of the PD sequence are influenced by the applied voltage, the channel conductivity and the electrical tree shape. A possible mechanism is presented for the formation of different chaotic extents of typical electrical trees in XLPE cable insulation.

The Effect of Magnitude and Shape of Applied Voltage on the Partial Discharge Behavior in Cylindrical Cavity within a Dielectric Material

One of the important phenomena which is important in equipment when they are under high voltage stress is Partial discharge (PD). Partial discharge measurement is commonly used to evaluate the performance of the insulation in high voltage system. Modeling of the discharge in insulation can give a better understanding of this important phenomenon. A cavity within a dielectric material is a highly stressed area and the ideal place for starting partial discharge and breakdown. The applied voltage is one of the important factors that PD activity is influenced by that. Previous studies have considered the behavior of PD activity in a spherical cavity at 50Hz of sinusoidal applied voltage. However, still not so much work on the simulation of the PD activity in the cavity under different condition. Also, there is little data are available on the effect of parameters on the PD activity. This paper describes the behavior of PD activity as a function of applied voltage in a cylindrical cavity within a homogeneous dielectric. COMSOL and MATLAB software were used to carry out the simulation based on the finite element method (FEM). The number of PDs per cycle changed with different applied voltage so the highest number of PD at 20kV and the lowest value at 8kV. The maximum PD magnitude is raised from 1200pC at 8kV to 2400pC at 20kV applied voltage. The number of PD is most prevalent for sinusoidal followed by triangular waveform and then sawtooth. Comparison of the simulation results and experimental results are in good agreement. The result can help the researcher to understand more about the characteristics and behavior of partial discharge. Which may help the insulation diagnostic testing or condition monitoring.

Partial discharge behaviours within a void‐dielectric system under square waveform applied voltage stress

There are many factors, which influence the behaviours of partial discharge (PD) within a void located in a solid dielectric material. One of the important factors is the waveform of the applied voltage on the electrode of a system, such as its amplitude and frequency. In this study, a two-dimensional axial-symmetric model of a cylindrical void in polyethylene layers has been developed using finite element analysis software. The model was used to simulate PD activity in the void under square waveform applied voltage under different amplitudes of the voltage. The model has considered the effect of charge propagation along the void surface during a PD occurrence. The obtained simulation results were compared with the measurement results reported from previous literature to validate the PD model that has been developed in this work. It was found that the simulation and measurement results are within reasonable agreement with only slight disagreement. From the comparison, critical parameters from the model that affect PD behaviours under square waveform applied voltage were found to be the electron generation rate and the inception, extinction and critical charge propagation fields. These findings may increase an understanding of PD behaviours within a void in a dielectric material under square waveform applied voltage, which is important for insulation diagnosis.