Partial Discharge Inception Research Articles

Investigating the progression of insulation degradation in power cable based on partial discharge measurements

Assessments of aging and insulation degradation in power components are important to observe and track their condition during operation. Partial discharge (PD) diagnostics is an efficient method for health estimation of the components’ insulation. PD is initiated when the dielectric strength of localized insulation part is decreased. The intensity of PD increases with time, which may lead to insulation failure. This paper investigates the influence of the electrical stresses on the behavior of PD activity in medium voltage (MV) cable containing insulation defects. For this purpose, insulation defects such as internal, surface, and corona are developed in the MV cable environment under laboratory conditions. PD behavior is studied in these defects at gradually increased voltage levels by observing different PD characteristic features including PD inception voltage (PDIV), pulse repetition rate, accumulated apparent charge, average discharge current, discharge power, and quadratic rate. The Tobit regression model is developed for these progressing defects to identify the trends and to predict the behavior based on PD characteristic features. The presented work proposes a framework for the evaluation of the progression of insulation degradation due to increased electrical stress. The study is carried out on XLPE cables, however, the developed approach can be equally implemented on other types of dielectrics and components.

Comparative Study on Partial Discharge Characteristics of Seven Accessory Defects Under Four On-Site Testing Voltages of a 10-kV Cable Line

Due to the increase in voltage levels and line lengths of power cables, the commonly used alternating current resonance (ACR) voltage is challenging to meet the growth rate of the capacitance of ultra-high-voltage cables for the on-site partial discharge (PD) tests. PD research on power-frequency-like voltages such as damped alternating current (DAC) voltage, very low-frequency sine wave (VLF-Sine) voltage, and very low-frequency cosine rectangular wave (VLF-CR) voltage, has important engineering value for evaluating the cable insulation conditions. In this paper, a PD test and measurement system of a 10 kV XLPE cable line with a 2 km length is established. Seven typical accessory defects are made artificially, including insulation hole, insulation screen tip, ring-cut tool mark, and stress cone installation dislocation defects at the terminals, and connector tip, conductive impurity, and stress cone installation dislocation defects at the joints. PD tests under four kinds of testing voltages of ACR, DAC, VLF-Sine, and VLF-CR are carried out to compare the PD inception voltages (PDIV), phase-resolved PD (PRPD) patterns, and PD location results of various defects. PD equivalence between power-frequency-like voltages and ACR is characterized by PDIV equivalent coefficient <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">PDIV</sub> , PRPD pattern characteristic quantities and discharge amplitudes. The results show that DAC and VLF-Sine have an outstanding ability to detect and characterize the severity of PD. DAC and VLF-CR have a good ability to recognize the type of PD. And DAC, VLF-Sine and VLF-CR can accurately locate PD. This study provides an important reference for on-site cable defect diagnosis and PD assessment.

Statistical analysis of the impact of FeO3 and ZnO nanoparticles on the physicochemical and dielectric performance of monoester-based nanofluids

This article deals with a comparative study of the physicochemical and electrical properties of monoesters of castor oil compared with their counterparts based on FeO3 and ZnO nanoparticles. The results are also compared with those in the literature on triesters, and also with the recommendations of the IEEE C 57.14 standard. The data is analysed statistically using a goodness-of-fit test. The analysis of the viscosity data at 40 °C shows an increase in viscosity. For concentrations of 0.10 wt%, 0.15 wt% and 0.20 wt% these are respectively 5.4%, 9.69%, 12.9% for FeO3 NFs and 7.6%, 9.91% and 12.7% for ZnO NFs. For the same concentrations, the increase in acid number is respectively 3.2%, 2.9%, 2.5% for FeO3 samples and 3.18%, 2.0%, 1.2% for ZnO samples. For the same concentrations, the fire point shows an increment of 4%, 3% and 2% for FeO3 samples and a regression of 8.75%, 6.88% and 5.63% for ZnO samples. As for the breakdown voltage, for the same concentrations we observe respectively an increment of 43%, 27%, 34% for the FeO3. The results show an improvement on partial discharge inception voltage with FeO3 of 24%, 8.13% and 15.21% respectively for the concentrations 0.10 wt%, 0.15 wt% and 0.20 wt%.

Impact of air pressure variations on electrical vehicle motor insulation

AbstractVariation in air pressure severely affects the insulation of electric vehicle (EV) motors, hence weakening the reliability of EVs for safe operation. Nomex‐polyimide‐Nomex, a typical insulation material for EV motors, was used to investigate the motor insulation performance under different air pressures. The results show that the partial discharge inception voltage is significantly reduced for EV motors operated at lower air pressures, and the probability of partial discharge (PD) occurrence is increased. The macroscopic results reveal that the active area of the PD expands at low pressure, while the non‐corroded ring appears in the centre. Additionally, although the number and amplitude of the PD increase significantly with decrease in air pressure, the active area of the PD expands and electrical stress on the insulation per unit area increases slowly. Therefore, when the pressure decreased from 60 to 40 kPa, the endurance life does not show a significant downward trend. Furthermore, the dielectric constant and loss of the low‐pressure samples significantly change during the ageing process, which further indicates critical degradation of the insulation. The aforementioned investigations reveal that the air pressure at different altitudes has a significant impact on the performance of insulation materials.

Influence of Water Ingress on Surface Discharges Occurring on the Silicone Gel Encapsulated Printed Circuit Boards Developed for Deep-Sea Applications

This paper discusses the influence of water ingress on the electrical discharges that arise on the surface of printed circuit boards that are developed for deep sea applications. The primary concern is the electrical discharges arising on the surface of the PCB bridging localized spots/areas between conductive traces, high voltage terminal, mounting hole options, and so on. The current literature focuses on electrical discharges arising on the surface of PCB at low-pressure environments emulating aircraft and space applications. Extending the same approach to deep-sea environments is not feasible since the pressure is very high and the temperatures are very low. In all, a meager attempt has been made to investigate the possible application of such gel-encapsulated PCBs in adverse high-pressure, salt/sea water, low-temperature, deep-sea environments. This experimental study focuses on studying the influence of deionized and sea water on electrical discharges arising on the surface, between conductive tracks of PCB. A group of PCBs with different gaps between the conductive tracks was produced and immersed in deionized and seawater for a specific duration at standard pressure. Afterward, the discharge characteristics were measured the using partial discharge (PD) test method and the respective phase-resolved PD pattern was studied and analyzed. Pertinent experiments revealed that the PD process and eventual failure manifests as a typical and substantial pattern. The apparent charge measured during the PD inception and near-by failure condition, influenced by deionized and seawater, reveals a regular trend. Naturally, a simple observation of the PD pattern might help to identify the intricacy and to initiate a preventive measure well before the complete system suffers a premature failure. Also, based on these results, making slight structural modifications on the PCB at crucial locations might help in retaining the dielectric integrity of the material.

High-Temperature Partial Discharge Characteristics of Power Module Packaging Insulation Under Square Pulse With High Dv/Dt Based on Down-Mixing Method

Partial discharge (PD) in the high voltage packaging insulation under square pulse with a high slew rate (SR or dV&#x002F;dt) is one of the main causes of the power module failure, especially for wide bandgap (WBG) devices. But PD under fast square pulse at high temperatures is rarely studied and one of the critical reasons is the lack of an effective PD detection method under fast square pulses. In addition, the existing PD standard defines the PD criterion under sinusoidal alternating current (AC) situations, which is not the actual working condition of the power module. To address that, this paper proposes a down-mixing PD detection method and explores the influence of temperature on the PD characteristics of the power module packaging insulation. The results show that the partial discharge inception voltage (PDIV) decreases as the temperature increases. The PDIV under square pulse is always higher than that under AC voltage. The less the SR and the greater the pulse width (PW) of the square pulse, the lower the PDIV. Physical-based models for PDIV estimation concerning SR, PW, and temperature are proposed for industrial reference. Moreover, statistical analysis of PD magnitude, number, and occurrence phase under square pulse indicates that PD number decreases but PD magnitude increases with the increase in temperature. Higher temperature leads to PDs with large magnitudes, which may accelerate the degradation of packaging insulation.

Investigation and Evaluation of High-Temperature Encapsulation Materials for Power Module Applications

With the advent of ultra-wide bandgap semiconductor materials, such as gallium oxide (Ga2O3) and aluminum nitride (AlN), higher temperature and higher voltage operation of power devices are becoming realizable. However, conventional polymeric and organic encapsulant materials are typically limited to operating temperatures of 200 degrees C and below. In this work, six materials were identified and evaluated as candidates for use as encapsulants for operation and high-voltage insulation at and above 250 degrees C. High-temperature silicone gel was used as a reference material and was compared with five novel encapsulants including an epoxy resin, a hydro-set cement, two low-melting point glass compounds, and a ceramic potting compound. Gas pycnometry was utilized to evaluate the voiding concentration to avoid partial discharge. Each material was then processed onto a direct-bonded-aluminum substrate test coupon to evaluate compatibility with a commonly used metal-ceramic substrate and processability for use in a power module. The insulation capability of each material was evaluated by testing the partial discharge inception voltage (PDIV) across a 1-mm gap etched in the substrate. The dielectric stability was then tested by soaking the materials in air at 250 degrees C for various intervals and observing the degradation of their PDIVs and appearances. The results of each test were compared, and conclusions were drawn about each material’s feasibility for use as a dielectric encapsulation material for a power module operating at temperatures exceeding 200 degrees C.

Automated Testing for Early Identification of PD in the Stator Insulation of Low Voltage VFD Motors

Electrical stresses in industrial variable frequency drive (VFD) motors are increasing with the higher voltage and dv/dt levels, and with the advent of wide bandgap power devices. This increases the likelihood of partial discharge (PD) in VFD motors, and is expected to increase the risk of stator insulation failures in low voltage (LV) motors for which the insulation is not resistant to PD. To ensure that PD does not occur during operation, LV VFD motors are qualified for PD-free operation in the design or manufacturing stages. However, the PD inception voltage of qualified LV motors decreases with insulation aging exposing them to the risk of PD-induced insulation failure. In this paper, an automated off-line test method for VFD-embedded PD testing is proposed. The main idea is to perform PD testing whenever the motor is stopped at a voltage higher than the operating voltage to identify PD activity in the insulation before it occurs during operation. A series impulse and common mode voltage tests are proposed for stressing the different components of insulation for early identification of PD in terminal-/neutral-end ground, phase, and turn insulation. Testing on a LV, VFD motor verifies that PD can be identified whenever the motor is stopped to provide early warning of insulation failure.

A new approach to the design of surface subsystems of polymeric insulators for HV and MV apparatus under AC voltage

AbstractThe design of high voltage and medium voltage AC and DC standoff insulators, and, in general, of insulation systems where surface and interface properties are predominant factors, is based on specifications and standards that rely upon long‐term experiences and lab testing. Surface insulating properties depend on electrical and thermal stresses, besides environmental factors, and the inception of extrinsic ageing phenomena, such as partial discharges, can be already a cause of reduced reliability and premature failure (for organic insulating materials). An innovative approach, based on field simulation, discharge modelling, and partial discharge inception measurements is presented by the authors, which can establish a solid basis to optimise insulation design for any type of supply voltage waveform. This method, the three‐leg approach, is applied here to a cable system under AC sinusoidal voltage, but it can work for any type of insulation system, such as bushings, switch gears, transformers, rotating machines, and power electronics boards.

Influence of mixed-frequency medium-voltage and environmental stress on the aging of epoxy

Recent developments in power electronic technologies lead to new challenges for insulation systems. This contribution aims to clarify the influence of a broad range of mixed-frequency (MF) medium-voltage and environmental stress parameters on the aging of epoxy insulation. For this purpose, test samples are stressed with an AC (50 Hz) or a DC voltage, superimposed with a pulse-width-modulated (PWM) voltage (kHz range). An analysis of the samples’ health state is carried out after the aging by the evaluation of potential aging markers (AC breakdown strength, dielectric permittivity, glass transition temperature, Fourier-transform infrared spectroscopy spectra). Although the main focus of this work is on aging below the inception of partial discharges (PDs), it was first confirmed that PD-related aging depends mainly on the peak voltage stress. In contrast, the results obtained by aging below PD inception suggests a dependence on the root-mean-square of the applied voltage stress, and consequently on the energy dissipation. Aging in the PD-free regime was only observed at alternating electric field stress and high relative humidity or elevated temperatures. No influences of space charge and of the slew rate of the PWM voltage were observed. Remarkably, higher PWM frequencies lead to less insulation aging. This might be attributed to the increasing hindrance of polymer side chain movement at higher frequencies, as observed by dielectric spectroscopy. In addition, it is indicated that the aging mechanisms under MF voltage stress result from superimposed single-frequency aging mechanisms and that aging is activated after a latency period. Of the investigated potential aging markers, only the residual breakdown strength revealed aging effects, which correlates with lifetime observations in the PD-free voltage stress regime. It is hypothesized that the aging mechanism is associated with a rearrangement of the free volume in the polymer, followed by a localized breaking of van der Waals bonds.

Investigation of air breakdown and plasma evolution on microstrip antenna surface using the particle-in-cell method

This paper investigates the breakdown of two microstrip antennas through experiments and particle-in-cell simulations. The breakdown thresholds of a microstrip dipole antenna and a double-layer patch microstrip antenna are investigated experimentally and found to be 15.6 and 30.8 kW, respectively. Ablation is observed on the surface of the antennas, indicating that the local electric field is particularly intense. To further understand the breakdown process, the particle-in-cell and Monte Carlo collision methods are combined to investigate the inception and development of partial discharge on the surface of the antennas. Under the criterion of continuous electron density growth, the breakdown thresholds of the dipole and double-layer patch antennas are estimated to be 19.4 and 52.9 kW in our simulations. The simulation results are in reasonable agreement with the experimental measurements. In the microstrip dipole antenna, the plasma evolution of the partial discharge is initiated near the tip, while the electric field close to the tip is distorted and a positive streamer directed toward the tip is observed. In the double-layer patch microstrip antenna, a more uniform discharge is observed at the edge of the circular patch in the simulations.

Medium-Voltage Isolated Auxiliary Power Supply Design for High Insulation Capability, Ultra-Low Coupling Capacitance, and Small Size

To fully utilize the benefits of MV SiC-based converter and achieve high power density, the isolated auxiliary power supply (IAPS) needs to be as small as possible. However, the high insulation and low coupling capacitance requirements, considering the high voltage and high dv/dt, are constraints of size reduction. This paper proposes a MV IAPS design for high insulation capability, ultra-low coupling capacitance, and small size. The topology selection, efficiency, load regulation, coupling capacitance, and size reduction considerations are discussed. A 2.5 W MV IAPS is designed and built for a 10 kV SiC MOSFET-based 13.8 kV/100 kW three-phase converter, achieving a partial discharge inception voltage of higher than 15 kV RMS, ultra-low coupling capacitance of 1.03 pF, and smaller size than that of the commercial products as well as the state-of-art research prototypes. Experimental results of partial discharge (PD) test, 10 kV SiC MOSFET-based half-bridge test, and 13.8 kV/100 kVA three-phase converter full rating test are conducted for design validation.

Influence of Pressure on the PD and Induced Aging Behavior of Polyimide Insulation Under Repetitive Pulse Voltage

Reduction of air pressure can influence insulation material’s behavior with respect to resisting partial discharge (PD) generation and its induced aging process. This may bring threat to the insulation reliability of medium frequency transformers working under low pressure condition, considering they are exposed to high probability of PD inception caused by PWM voltage. Aiming at investigating the performance of insulation of medium frequency transformer being operated in an environment with a decreasing pressure, this paper introduces a test system that can conduct partial discharge and endurance lifetime tests under repetitive pulse voltage and controlled pressure. Using this test system and focusing on polyimide film, which is one of the main insulation materials used by medium frequency transformers, partial discharge and endurance lifetime tests are conducted under different pressures. The results show that partial discharge inception voltage decreases with decreasing pressure. However, the changing of partial discharge magnitude, time lag and its induced insulation lifetime do not change monotonously. Surface of the failed insulation samples show that the erosion area caused by partial discharge is larger under lower pressure. The mechanisms behind the changing of partial discharge time lag, discharge magnitude, endurance lifetime, along with the discharge eroded area with pressure are discussed in detail.

Investigation of the Stability and Insulating Properties of Mineral Oil-Based Surface Modified Silicon Nanofluid

The requirement for high voltage is growing extensively, which has obligated to enhance the dielectric and thermal properties of the insulators. Nanofluids are a new insulating material class that exhibits exceptional thermal and electrical characteristics. The relative permittivity and conductivity of Si nanoparticles (nSi) of diameter 10-30 nm synthesized from the p-type wafer are ideal values for free charge trapping; hence the mixture of mineral oil and nSi has the potential to replace the standard insulating oil. During the pre-breakdown phenomena of the insulator, the nSi in the base fluid act as a charge trap, capable of capturing about 45 electrons with a significantly shorter relaxation time of 1.42 × 10-14 s relative to the streamer propagation time. Oleic acid, an oil-soluble dispersant, improved the dispersion and stability of the nanofluids. The FTIR analysis determined the type of interaction as chelating bidentate between the oleic acid and the nSi surface. The impulse breakdown voltage of the nanofluids revealed the effect of trapping the free charges generated by the molecular ionization on enhancing the impulse breakdown strength. The relative permittivity measurement of the nanofluid followed the Clausius-Mossotti polarization model of a mixed dielectric. Due to increased nanosized pores (with the number of nanoparticles), partial discharge inception voltage and the AC breakdown voltage were reduced with concentration.

A data-driven approach for exploring partial discharge inception voltage of turn-to-turn insulation in inverter-fed motors

A data-driven approach for exploring partial discharge inception voltage of turn-to-turn insulation in inverter-fed motors

Experimental verification of the inhibiting effect of hydraulic pressure on point discharge in oil-paper insulation under constant AC voltage

This paper reports on experimental studies of point discharge in oil-paper insulation under different hydraulic pressures. The point partial discharge of oil-paper insulation under different hydraulic pressure was measured for the first time. With the help of a commercial partial discharge detector, five discharge parameters under constant AC voltage were recorded and plotted as curves. These show that the partial discharge under very low pressure developed very quickly after it was initiated. So, care should be taken with oil-paper equipment operating at high altitudes, because once the discharge is initiated, it would develop into a catastrophe in a relatively short time. The partial discharge inception voltage and breakdown voltage increased with pressure, but the rising rates decreased with the increase of pressure. The discharge parameters decreased with pressure but the difference between the adjacent curves became smaller and smaller, which is called the saturation characteristic of the hydraulic pressure’s effect on discharge. Therefore, very high hydraulic pressure is not necessary for weakening the partial discharge in oil paper insulation. It is believed that the hydraulic pressure influences the discharge in the oil-paper insulation by changing the state of the gas in this compound insulation system. This paper provides a reference for the operation and maintenance of oil-paper insulation equipment running in high-altitude areas. Adjusting hydraulic pressure might be an effective and economical way to reduce the size and weight of oil-paper insulation equipment.

Design of a Rectangular Wave High Voltage Generator for the Evaluation of Inverter-Fed Motor Insulation

The urgent need for the safer operation of inverter-fed motors is leading research to the qualification tests under high frequency rectangular wave voltages. Consequently, the generation of rectangular wave voltages with adjustable parameters has become a major concern for manufacturers and researchers. This paper presents a design of high frequency and high voltage pulse generator able to provide unipolar or bipolar pulses up to 20 kV peak-to-peak with variable rise and fall time that can be tuned down to 40 ns. Such a generator is performant enough for testing a large part of motors following standards and with more stressful conditions for research of new winding technologies. Resorting to high voltage push-pull solid-state switches and dielectrics liquid &amp; air-cooling technique, the tested capacitance load can reach up to 3000 pF. Also, the over-current protection circuit is carefully designed and verified by using real-time logic control hardware. The proposed generator has been successfully used for endurance tests and partial discharge inception voltage (PDIV) detection at repetitive impulsive voltages for capacitive, inductive insulation structures and windings during the qualification evaluation of inverter-fed motors.

A Surface Discharge Model for the Design of Surface Components of Insulation Systems Under AC Stress in Industrial Electronics Environments

Industrial electronics is trending towards higher levels of voltage supply and specific power, with a broad diversity of applications. Adequate criteria for the design of insulation systems, subjected to types and extent of stresses non-comparable with conventional low voltage, frequency, and temperature design practice, need to be developed. Therefore, modelling the most significant mechanisms that can cause design failure in providing the specified reliability is sought after. Since one of the most harmful mechanisms for accelerated aging is partial discharges, this paper focuses of achieving a generalization of a well-known, approximate partial discharge inception model, proposed by Niemeyer, to calculate the partial discharge inception voltage of an insulation system for surface discharges, that may have a significant impact on components used in industrial electronics environments. The validation of the proposed model is performed on simple test objects.

The Effects of PWM With High dv/dt on Partial Discharge and Lifetime of Medium-Frequency Transformer for Medium-Voltage (MV) Solid State Transformer Applications

Medium-frequency (MF) transformer in a phase-shift modulated dc solid state transformer (SST) is prone to partial discharge (PD) that impacts the system reliability and lifetime. Tightly packed MF transformer windings carrying PWM voltages create favorable conditions for PD inception. A peak potential condition is created between adjacent posterior windings as a result of phase-shifted modulation. This along with fast alternating electric fields incepts PD at lower than expected operating voltages. Early inception of PD lead to premature transformer failure leading to unanticipated DC SST down-time. The effects of PWM voltages containing high dv/dt switching transients on PD and lifetime characteristics of MF transformer in a dc SST is not yet understood, which is investigated in this paper. A non-intrusive PD detection strategy for MF PWM ac voltage is presented using high speed optical sensors. PD characterization is performed on Kapton polyimide insulated copper foil windings for a foil-type MF transformer. Winding samples are stress tested with steady state PWM voltages upto 2 kV and frequencies upto 50 kHz with high dv/dt upto 60 V/ns while maintaining the transformers geometric profile. Accelerated dielectric lifetime characterization is performed and novel lifetime prediction models are derived containing both frequency and dv/dt dependent variables.

Influence of Humidity and Salinity on the Discharge Characteristics of a Short Rod-Plane Gap Under Negative DC Voltage

In this article, the effects of air humidity and salinity on the discharge characteristics of a short rod-plane gap under negative dc voltage are investigated experimentally. The discharge of the rod-plane gap was recorded using a measuring resistance, photomultiplier tube, and ultraviolet imager. The experimental results show that corona discharge of rod electrode can be observed in the rod-plane gap when the relative humidity is lower than 70%, and furthermore, partial discharge of water molecules in the air can also be observed in the rod-plane gap when the relative humidity is higher than 70%. The partial discharge inception voltage (PDIV) decreases with the increase of air humidity and salinity, and the corona discharge inception voltage (CDIV) increases slightly with the increase of humidity. The average discharge current increases with the increase of applied voltage and humidity. The repetition rate of the optical pulse and the Trichel pulse increases with the increase of applied voltage. The discharge repetition rate of water molecules in the air can be estimated from the repetition rate difference between the optical pulse and the Trichel pulse. The amplitude of the Trichel pulse in clean air is higher than that in the salty air, while the repetition rate of the Trichel pulse in clean air is lower than that in the salty air, which is due to the charging of NaCl particles during discharge.