Partial Discharge Inception Voltage Research Articles

Hydroxyl-group decreased dielectric loss coupled with 3D-BN network enhanced high thermal conductivity epoxy composite for high voltage-high frequency conditions

The rapid development of electrical and electronics industry puts forward new requirements for high thermal conductivity and low dielectric loss of insulating materials. However, the traditional strategies to improve thermal conductivity of insulating materials with high thermal additives quantity usually accompany with the huge increase in dielectric loss, which will eventually lead to thermal breakdown and cannot meet the real demand under high voltage-high frequency conditions. Herein, in-situ bubble-induced self-assembled strategy was used to fabricate 3D boron nitride (BN) network/epoxy composites. The cellulose with rich hydroxyl groups could both crosslink with epoxy group and form hydrogen bonding with the hydroxyl groups on BN, which restricts the polymer chain movement and ensures the low dielectric loss. Improved thermal conductivity (1.62 W/(m·K), 8 times higher than that of EP) and decreased dielectric loss (0.00597, 18.1% lower than that of EP at 46.4 kHz, 20 °C) of the as-prepared composites contribute to the significant extension of the thermal breakdown time (411.4% higher than that of EP at 44 kHz). In addition, the partial discharge inception voltage of the composites is 8.9% higher than that of EP. These advantages make it attractive to high reliability of thermal management applications in the electrical and electronics industry.

Influence of Dielectric Liquid Type on Partial-Discharge Inception Voltage in Oil-Wedge-Type Insulating System under AC Stress

This article describes the results of laboratory tests on an oil-wedge-type electrode system, which were supplemented by FEM (finite element method) simulations. The studies were focused on the comparison of the partial-discharge inception voltage (PDIV) in the abovementioned system when immersed in different liquid dielectrics, namely inhibited mineral oil, uninhibited mineral oil, synthetic ester, and natural ester. In addition, the electric field stress obtained from the simulations was used in each case to determine the safe level for the actual transformer insulation. The studies were performed under AC voltage. Both electrical and optical detection methods were applied in order to properly determine the discharge inception. The statistical analysis of the results obtained from the laboratory measurements was carried out using Weibull distribution. We found that both mineral oils demonstrated better properties than the ester liquids in terms of resistance against partial-discharge appearance under the conditions of the oil-wedge-type electrode model. Therefore, for all considered cases, the inception electric field stress obtained from the FEM-based simulations corresponding to the partial-discharge inception voltage was found to be significantly higher than the commonly accepted safe design level, which is in the range of 10–12 kV/mm. This proved the good electrical strength of all liquids under test.

Prediction of Partial Discharge Inception Voltage for Electric Vehicle Motor Insulation Using Deep Learning

Monitoring the health of electric motor insulation by measuring the early breakdown phenomenon such as Partial discharge (PD) is required for the safer operation of electric vehicles (EVs). The prediction of partial discharge inception voltage (PDIV) is of great significance to evaluate the condition of motor insulation. This paper presents a method to predict the PDIV of electric motor magnetic wires winding. The data is driven by constructing a single point discharge model of turn-to-turn insulation of two magnetic wires. The model is simulated by varying the input parameters of magnetic wire configuration such as diameter, insulation thickness, winding temperature and insulation permittivity to calculate the PDIV based on Townsend discharge theory. The calculated PDIVs are used to train the convolution neural network (CNN)and extract the nonlinear relationship between characteristic variables and PDIV to test it for large data sets of magnetic wire. The predicted value is compared with the measured value, which proves the superiority and accuracy of the method in this paper. Based on the calculation results of the mean impact value algorithm, a method to improve the PDIV of the turn-to-turn insulation is given.

A Partial Discharge Inception Voltage Modeling Approach

A partial discharge inception voltage modeling approach promoting design-for-reliability considerations in advanced power modules is presented. As power modules are being operated at higher voltages and become more compact to meet power density demands, there is an increased risk of partial discharge, the silent precursor to electrical breakdown that degrades insulation material. The trade-off between voltage class and module compaction must be quantified. This work presents a methodology to model the tradeoff for any substrate and encapsulant material. A surface charge density-based partial discharge inception voltage (PDIV) model was developed to overcome the challenges in electric field-based models. The model consists of a closed-form equation that accepts finite element simulation results as input and produces PDIV as output. The model was experimentally validated for various trace gaps in a 12/25/12 mil alumina DBC using Dow Corning 3-6635 dielectric gel. An expected trend of diminishing returns was observed between PDIV benefit and trace-gap. This approach quantifies this diminishing returns point. To design for reliability, the maximum operating voltage must be limited to the PDIV. For a material set and manufacturing process, this methodology can be used to determine the voltage class and trace gap design rules.

Characterization of PDIV, PDEV, and RPDIV in Insulated Wires Under Unipolar Repetitive Square Wave Excitations for Inverter-Fed Motors

The behaviour of partial discharge inception voltage (PDIV), partial discharge extinction voltage (PDEV) and repetitive partial discharge inception voltage (RPDIV) as a function of exposure/poling time under unipolar repetitive square wave excitations at atmospheric pressure (1013 mbar) is experimentally measured and investigated. The partial discharge (PD) tests are performed on two commercially available insulated wires: Glass fibre as Type II and Polyether Ether Ketone (PEEK) as Type I insulated wires used in inverter-fed motors. The impact of excitation polarity, pulse repetition frequency and rise time on the PD quantities (i.e., PDIV, PDEV and RPDIV) is studied as a function of stressing duration. In addition, the trend of RPDIV and PDIV dispersion levels are compared as a function of poling time under positive and negative unipolar excitations with two rise times (i.e., 80 and 800 ns) and two pulse repetition frequencies (i.e., 50 Hz and 2.5 kHz), relying on the shape/slope parameter of the Weibull distribution. The collected data shows that the electric field induced by the interface charge deposited by PD occurring during consecutive pulses and the space charge accumulation in the insulation bulk plays a decisive role in PD quantities’ variations under unipolar repetitive square wave excitations.

Partial Discharge Activity in Rotating Machine Type II Insulation Systems as a Function of Temperature and AC Supply Frequency

The main purpose of this study was to investigate if the application of very low frequencies for PD measurements in type II generator bar insulation is relevant when compared to applying power line frequency at different test temperatures. Laboratory experiments were performed on two stator bar types rated 6.4 kV and 7.4 kV, labeled as Group A and Group B. The stator bars were taken from service and spare part storage (pristine). The samples were preconditioned at the maximum test voltage of 1.5 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">U</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> before sweeping the AC high voltage supply from 0.1 Hz and 50 Hz. The test temperature was varied between 20 °C and 155 °C. Investigation of cross sections of the generator bars was done to qualitatively describe the voids. The main results showed that the partial discharge inception voltage (PDIV) for Group A was temperature independent at 50 Hz and decreased by 40 % at 0.1 Hz with increased temperatures, whereas the PDIV for Group B was frequency independent and increased by 70 % with temperature. Similarly, the PD magnitude and total apparent charge decreased by 90 % for Group B with increased temperature, whereas it more than doubled for Group A with temperature. The tested bars from Group A showed significantly larger voids, located close to the copper conductors, compared to Group B, where voids were mainly located within the mainwall insulation. Results indicated that tests performed at 0.1 Hz cannot directly represent results obtained at 50 Hz. Using a wide range of frequencies and test temperatures broadens the understanding of the behavior of the voids inside the mainwall insulation.

Increasing Partial Discharge Inception Voltage at High Temperatures Using Parylene-HT Electrets

Increasing Partial Discharge Inception Voltage at High Temperatures Using Parylene-HT Electrets

Influence of Bridging between Insulating Materials on the Partial Discharge Inception Voltage in Insulating Material-Air Gap-Insulating Material Electrode Systems

A minute air gap may result in a successive partial discharge in an insulation system. Here, the partial discharge inception voltage (PDIV) in a half-pipe-type insulating material-air gap-insulating material electrode system with or without bridging between insulating materials was measured to investigate the influence of the bridging between insulating materials on the PDIV. The PDIV under the DC ramp voltage (DC-PDIV) with bridging between insulating materials using Al tape is higher than that without bridging. The DC-PDIV after cutting the bridging Al tape between insulating materials also corresponds to that without bridging. In addition, the PDIV under a surge voltage application (Surge-PDIV) is almost the same despite the bridging change. These results indicate that the inverse field due to the charge traveling via the Al tape for bridging between insulating materials relaxes the external field.

Optical Detection Method for Partial Discharge of Printed Circuit Boards in Electrified Aircraft Under Various Pressures and Voltages

Realization of an electrical aircraft demands a low-weight electric distribution and propulsion system. The use of high voltage and power electronics operated at high switching frequencies is essential to achieve this objective. However, printed circuit boards (PCBs) in electrified aircraft are in a harsh working environment, which can make PCBs more susceptible to generate partial discharges (PDs). The current PD detection technology has poor immunity to the electromagnetic interference and acoustic interference in the operating environment of aircraft. Therefore, this article proposes an optical-based PD detection method for PCBs, which is effectively immune to electromagnetic and acoustic interference. This method uses fluorescent fiber as a PD optical signal sensor and then collects the optical signal by the avalanche photodiode (APD). Experiments have verified that the detection sensitivity, sensing range, and anti-interference performance of this method are well satisfied with the PD detection. In addition, single PD pulse, optical phase resolved PD (PRPD) patterns, and PD inception voltage (PDIV) under different air pressure and voltage conditions are investigated. Finally, the relationship between the optical signal and PD amplitude is found to be proportional, which proves that the severity of the PD on PCBs can be effectively detected and evaluated by this method.

Experimental investigation of partial discharges activities in palm kernel oil methyl ester and mineral oil by using a Rogowski coil

This paper deals with the influence of the characteristics of a Rogowski coil compared to a shunt sensor in the detection of partial discharges (PDs) in insulating oils. An experimental investigation is conducted on the activity of PDs and their inception voltage in mineral oil (MO) and palm kernel oil methyl ester (PKOME). This is done by using the analysis of the phase-resolved partial discharge patterns model and the Weibull distribution. The experimental set-up consists of a 220 V/50 kV high voltage transformer, a control box and a test cell containing four electrode configurations. The tests are conducted according to the IEC 61,294 for the determination of partial discharge inception voltage. The experimental results show that the characteristics of a Rogowski coil have a considerable effect on the waveform and the number of occurrences of detected discharges. The number of occurrences of discharges detected with the shunt sensor is reduced by almost 50% compared to those of a Rogowski sensor. The parameters obtained from the Weibull distribution show that PKOME has a higher resistance to discharge inception than MO.

Preventing Space Charge Accumulation by Incorporating Electrets

AbstractSpace charge accumulates near electrodes in insulating materials under high voltage direct current. Electric stress created by charge accumulation distorts electric field, reduces the remaining useful life of insulating materials, and decreases partial discharge inception voltage that are critical for ensuring the dielectric integrity of power applications. In this article, a novel approach that mitigates space charge accumulation using electrets is presented. Electrets are used to reduce electric fields that cause charge injection and space charge accumulation in dielectric materials. The validity of using electrets for preventing space charge accumulation is numerically and experimentally demonstrated. Bipolar charge transport model is developed to simulate space charge injection and accumulation in silicone rubber without and with the incorporation of electret. Space charge experiments are performed to confirm the numerical results. The results show that electrets can effectively mitigate space charge accumulation in dielectric materials. The results also show that the incorrect use of electret such as reversing the polarity can lead to more substantial space charge accumulation.

Packaging of a 10-kV Double-Side Cooled Silicon Carbide Diode Module With Thin Substrates Coated by a Nonlinear Resistive Polymer-Nanoparticle Composite

Medium-voltage silicon carbide (SiC) power modules are a critical component in grid-bound power conversion systems, and the packaging of these modules dictates the performance and reliability of the systems. One of the key issues for packaging the modules is managing the tradeoff between heat dissipation and insulation. To improve thermal performance without sacrificing insulation, a 10-kV SiC full-wave diode rectifier was designed and fabricated by incorporating double-sided cooling and wirebond-less interconnection and by utilizing thin alumina direct-bond copper substrates. To ensure that the substrates met insulation requirement, triple points on the substrates were coated by a nonlinear resistive polymer-nanoparticle composite to reduce electric field concentration. The nonlinear resistive coating increased the partial discharge inception voltage of the substrate with 0.5-mm thick alumina to 17.3 kV, an 84% improvement over that of the substrate without the coating. Electrical and thermal simulations of the module showed a low power loop inductance of 3.51 nH and a low junction-to-case thermal resistance of 0.114°C/W. The feasibility of the packaging techniques was demonstrated from successful fabrication and functional testing of the packagedmodule.

Insulation degradation and fracture cracks of mechanical impacted thermal‐ageing oil‐impregnated papers

In order to elucidate mechanical impacts affecting the insulation strength of thermal-ageing oil-impregnated pressboards, we perform mechanical degradation experiments of impacting forces that are represented as the winding-suffered stresses on thermal-ageing oil-paper insulation in transient short-circuit of transformer. Water content, degree of polymerisation (DP), partial discharge inception voltage and AC breakdown field strength are tested and analysed for diverse thermal-ageing levels. An analytical model of mechanical degradation is established by relating dielectric breakdown strength to mechanical impact. Mechanical impacts result in less water content in thermal-aging oil-impregnated pressboards while causing reduction and increment of DP, respectively, in mildly and severely aged oil-impregnated pressboards. Partial discharge inception voltage and dielectric breakdown strength vary significantly with the numbers of impact cycles, showing a maximum value under low-intensity impact and decreasing monotonously under high-intensity impact. High-intensity mechanical impacts will evidently exacerbate the thermal deterioration process of oil-impregnated pressboards, which is primarily attributed to cellulose fibre fractures in pressboard cracks produced by equivalent tensile stress of mechanical impacts. It is acceptable for mildly aged oil-impregnated pressboards to suffer low-intensity mechanical impacts that will alleviate insulation degradation by restraining partial discharge in smaller oil-gaps and prolonging breakdown discharge pathways along impact-caused cracks.

Partial Discharge Characteristics of C3F7CN Gas Mixture Using the UHF Method

Manufacturing or assembly defects in gas-insulated equipment can introduce field enhancements that could lead to partial discharge (PD). This paper examines the PD characteristics of SF6 alternatives considered for potential application to retro-filling existing SF6-designed equipment. The PD performance of the C3F7CN/CO2 gas mixture and SF6 were characterised adopting the ultra-high frequency (UHF) method and investigated for different defect configurations, pressures, and gas mediums. Hemispherical rod-plane and plane-to-plane configurations with needle on the high-voltage (HV) and ground electrodes were used to mimic conductor and enclosure protrusion defects, respectively. The results demonstrate that with a needle length of 15 mm, the 20% C3F7CN/80% CO2 gas mixture had almost half the partial discharge inception and extinction voltages (PDIV/EV) of SF6. For less divergent fields, the 20% C3F7CN/80% CO2 gas mixture demonstrated a comparable PDIV/EV performance as SF6. The phase-resolved PD patterns of the 20% C3F7CN/80% CO2 gas mixture demonstrated a 3-stage transition phase that was not observed with SF6, which could be due to the discharge mechanism of the weakly attaching CO2 gas used within the mixture.

Partial Discharge Detection in Gas-Insulated Switchgears Using Sensors Integrated With UHF and Optical Sensing Methods

Partial discharge (PD) detection from protrusion defects is performed in an actual gas-insulated switchgears (GISs) based on a built-in sensor integrated with fluorescent fiber and ultrahigh frequency (UHF) antenna. The characteristics of both optical and UHF signals are analyzed in terms of inception voltage, pulse distribution, and statistical characteristics including the phase-resolved partial discharge (PRPD) pattern and PD number. In the experiment, protrusions with different curvatures are adopted to investigate the reason for the differences in the characteristics of UHF and optical signals. The results show that UHF and optical methods have different detection sensitivity for different discharge types owing to the different energies of electromagnetic waves and optical signals radiated in different discharge types. The optical method can easily detect avalanche discharge, while the UHF method can easily detect streamer discharge. This difference leads to the difference of the PD characteristics obtained by the two partial methods. For example, for the PD of small curvature protrusions, the PD inception voltage (PDIV) of the optical method is lower than that of the UHF method, because the defect triggers avalanche discharge. Moreover, the optical and UHF signal pulses are not always simultaneous in the time domain because they detect different discharge types. The PRPD patterns obtained by the optical method have a wider phase distribution than the UHF method because the optical method can detect avalanche discharge under low voltage.

Influence of Voltage Harmonics on Partial Discharge Diagnostics in Electric Motors Fed by Variable-Frequency Drives

Typically, in partial discharges (PD) diagnostics tests carried out in a laboratory, harmonic content in a voltage waveform is ignored. However, during the operation of variable-frequency drives (VFD)-fed electric motors, the distorted voltage waveform is produced due to the addition of harmonics components in the sinusoidal voltage waveform. The distortion level increases when the VFD is operated at a lower speed ratio. The additional spectral components in the applied voltage waveform can have a significant impact on the PD behavior in the insulation of the electric motor. Therefore, during online PD monitoring, overlooking the harmonic regime in the applied voltage may lead to a false interpretation of PD data. This article investigates the influence of voltage harmonics distortion produced during the variable-speed operation of VFD on PD characteristics. For this purpose, online PD measurements have been carried out by performing a series of experiments on eight VFD-fed motors under variable operating conditions. The PD severity at different harmonic compositions is experimentally investigated by determining various PD characteristic parameters including PD inception voltage, accumulated apparent charge, average discharge current, discharge power, and quadratic rate. Also, two terms Gaussian models have been mathematically developed using 288 data samples to quantify the variations in PD characteristic parameters against distortion parameters. The article proposes a framework for properly evaluating harmonics impact in PD for electric motors and correctly estimating stator insulation degradation.

Investigation on combined effect of humidity–temperature on partial discharge through dielectric performance evaluation

The humidity role in the partial discharge (PD) inception mechanism is quite challenging, especially when considering the environmental temperature. Indeed, there is no general rule to explain the humidity effect on the PD phenomenon. In this paper, the PD activity in inter-turn insulation is experimentally investigated for different relative humidity (RH) conditions at three different ambient temperatures, that is, 30°C, 60°C, and 90°C. Partial discharge inception voltage (PDIV) is directly measured through a photomultiplier tube (PMT), whereas the tip-up tests are performed aiming at monitoring both dissipation factor (tanδ) and insulation capacitance (IC). These extra measurements (diagnostic dielectric markers) allow better assessing the insulation status. The adoption of the tip-up test enables the insulation properties measurement. Based on the tip-up tests’ findings, the interfacial polarization process starts at 75% RH under 60°C, while the high conductivity area is already formed at 75% RH when the ambient temperature is 90°C. The water film formation deduced from the tip-up test is then used to explain the trend of PDIV, and the validity is further proved by finite element analysis (FEA).

Comparison between the PD Characteristics of g3 and Dry Air for Gas-Insulated Switchgears

This paper presents a comparison between the partial discharge (PD) characteristics of g3 and dry air for gas-insulated switchgears. PD signals were measured with a conventional method according to IEC 60270 and an ultra-high frequency (UHF) method. The partial discharge inception voltages (PDIVs) of g3 and dry air are about 74% to 84% and 58% to 72%, respectively, in the protrusion on conductor (POC) system, and 90% to 96% and 80% to 93%, respectively, in the free moving particle (FMP) system, depending on the gas pressure of 0.1 MPa to 0.5 MPa. The single PD pulse in time and frequency domains are not distinguished according to gas type. The PRPD patterns have different phase angles depending on the gas type in the POC, while the phase angle is 0–360° in the FMP, regardless of the gas type. Lastly, the correlation was analyzed, showing that the output voltage in mV of the UHF sensor increases linearly in accordance with the apparent discharge in pC, regardless of the gas type. The experimental results in this paper are important as a fundamental database for the application of UHF monitoring systems in an eco-friendly GIS.

Multiple detections of insulation defects partial discharge in gas-insulated equipment

Gas-insulated equipment is an important part of the power system owing to its advantages of small footprint, large transmission capacity, and high reliability. Detection of insulation defects partial discharge (PD) is important for its operation and maintenance, and the combination of multiple detection methods can greatly improve the accuracy of PD detection. In this study, the three kinds of insulation defects, namely, floating defect, void defect, and surface defect, are set up on the PD simulation platform and detected using the ultra-high frequency (UHF) method, high-frequency current (HFCT) method, and acoustic emission (AE) method. The partial discharge inception voltage (PDIV) and phase-resolved partial discharge (PRPD) spectrum of defects measured by these three methods are recorded in the experiments. The experimental results show that the UHF method is effective for the detection of the three defects. The HFCT method is sensitive to floating defects but not to void and surface defects. Discharge signals of these two defects can be detected by HFCT when the discharge amplitude is large. The AE method can detect floating defects effectively but cannot detect void defects and surface defects.

Experimental Study on Partial Discharge Inception Voltage of Pressboard-Enclosed Void at Different Frequencies From the Perspective of the First Partial Discharge Time Lag

Partial discharge inception voltage (PDIV) at different frequencies is of great interest for PD tests conducted at nonpower frequency. However, in the existing literature, there is no agreement on PDIV characteristics with respect to frequency. There is a lack of comprehensive analysis of frequency-dependent PDIV characteristics either. In this article, the PDIV of a cylindrical void at different frequencies (from 0.5 to 300 Hz) is investigated from the perspective of the first PD time lag. First, an appropriate time interval between consecutive measurements is experimentally determined. Then, PDIV measurements at different frequencies are conducted by increasing voltage at a fixed ramp rate. Furthermore, the PDIVs defined by peak voltage and instantaneous voltage are compared. After that, a bipolar square wave is applied to measure the first PD time lag. Finally, based on the first PD time-lag distribution with respect to voltage at different frequencies, the cumulative probability of the first PD under sinusoidal voltage is computed and analyzed. The results suggest that the first PD time lag has a significant influence on the measured PDIV at frequencies above 10 Hz. This is because the mean time lag is nearly half the time period of half voltage cycle at low voltages. Thus, high voltages (HVs) are needed to obtain a high probability of the first PD, i.e., a large measured PDIV.