Partial Discharge Extinction Voltage Research Articles

Derivation of Ambient Enhancement Factors of Impregnated Twisted Pairs for Partial Discharge Risk Evaluation

Partial discharge (PD) is the main issue challenging the electrical machine (EM) insulation system for more electric aircraft (MEA) applications. Indeed, the trend of adopting higher DC link voltage and fast switching device further boost the risk of PD. Partial discharge inception voltage (PDIV) is mostly dominated by ambient conditions (i.e., temperature, pressure, and humidity), and their influence is accounted for through experimentally determined ambient enhancement factors. For non-impregnated windings, ambient enhancement factors are suggested by both IEC 60034-18-41 and recent literature. However, in many applications, EMs’ windings are often impregnated with resin to improve thermal performance, mechanical resistance, and electrical insulation. In this case, no indications of the enhancement factors for PD risk assessment are given. The ambient enhancement factors are experimentally determined in this paper to enable the PD risk evaluation for impregnated windings. These factors are obtained relying on an extensive PD investigation, where PDIV and partial discharge extinction voltage are measured under variable temperature, pressure, and humidity conditions. Finally, the tests are performed on different wire types and motorette samples to validate the applicability of the derived factors.

Determination of Hotspot Temperature Margin for Rectangular Wire Windings Considering Insulation Thermal Degradation and Partial Discharge

Increasing current density is a common way to improve the power density of electrical machines (EMs) which also boosts the winding temperature. Higher winding temperature accelerates the thermal degradation of insulation and also increases the risk of partial discharge (PD). Commonly, the winding hotspot temperature constraint is decided based on an over-engineering approach (i.e., the winding hotspot temperature is 30°C below the wire thermal class) constraining the EM’s performance. In this paper, the margin of winding hotspot temperature is precisely determined focusing mainly on the insulation degradation issues through experimental investigation. The partial discharge inception voltage and partial discharge extinction voltage have been measured under different temperature conditions for samples with different thermal aging statuses. Rectangular wires which find application in automotive motors are selected as test samples. The results link the temperature with the PD risk and the thermal lifetime of the insulation, and the winding hotspot temperature margin is accordingly determined. An automotive case study is presented to prove the applicability of the investigation outcomes.

On the Frequency Dependence of the PDIV in Twisted Pair Magnet Wire Analogy in Dry Air

This paper presents an experimental verification of a PDIV model for twisted-pair magnet wires (TPMW). The model is physically rooted in an inception criterion based on secondary electron feedback and parametrizes the PDIV of wedge-shaped gas gaps (such as TPMW) solely in terms of its reduced coating thickness—the ratio of coating thickness to its relative permittivity. According to the literature, the coating’s permittivity and thickness should play a significant role in determining the PDIV. Still, no explicit quantification of their concurrent effects is often given, and they are frequently treated as decoupled parameters. Therefore, extensive measurements of PDIV of samples analogous to TPMW coated with dielectric materials of different permittivity (sealed and unsealed alumina, silicone polyester, and perfluoroalkoxy alkane) and thickness (≈45 to 80 μm) are performed. The PDIV of the test samples is determined under sinusoidal test voltage in the frequency between 10 to 60 000 Hz. It is shown that the PDIV can be predicted based on the relative permittivity and thickness of the coating, and the frequency dependence is due only to the change of permittivity with frequency. Temperature and pressure are kept approximately constant at their ambient values (ca. 23 °C & 970 mbar), and only dry air is used to exclude the effect of humidity on the insulation’s permittivity. Ultimately, the role of surface charge is discussed qualitatively by comparing the PDIV with the partial discharge extinction voltage (PDEV) and their repetitive counterparts (RPDIV, RPDEV).

Deep Learning and Long-Duration PRPD Analysis to Uncover Weak Partial Discharge Signals for Defect Identification

This study focuses on improving the defect recognition accuracy under weak partial discharges (PDs) in epoxy resin through phase-resolved partial discharge (PRPD) analysis. The method is to refine the data rather than enhance the algorithm. Two measurement conditions are compared until PRPD pattern saturation is achieved: one-minute and one-hour durations. The PD data specifically target three void types located at different positions within the epoxy material. The aim is to evaluate how the presence of weak PDs at the PD extinction voltage (PDEV) influences defect recognition accuracy. This research sheds light on the potential implications of neglecting the significance of weak PD signals in defect detection. A convolutional neural network (CNN) model is trained using PRPD data recorded at the repetitive PD inception voltage (RPDIV) and tested using the new PRPD data from both conditions recorded from a lower PDIV to a PDEV. The trained CNN model achieves a defect recognition accuracy of 100% for a one-hour duration, highlighting the importance of not neglecting weak PD signals. This emphasizes the significance of extended measurement duration and pattern saturation in capturing and analyzing weak PD signals for an improved defect recognition. This study contributes to the advancement of practical applications by understanding the behavior of the epoxy material and enhancing defect detection techniques.

Partial discharge and decomposition characteristics of environmental insulating gas C5F10O under different background gases

SF6 is widely developed and applied in gas insulated electrical equipment due to its excellent arc extinguishing performance and insulation performance. However, SF6 is very destructive to the ozone layer and has a strong cumulative effect on the greenhouse effect of global warming. Therefore, the inevitable trend of the development of power grid equipment in the future is to reduce or even prohibit the use of SF6 gas in the power system, power industry and electrical equipment, or to find insulation media that can replace SF6. As a result, the research on environment-friendly insulating gas has become popular. In this paper, power frequency partial discharge platform and GC–MS​ detection platform are used to systematically study the partial discharge and decomposition characteristics of C5F10O under N2 and CO2 background gases. The partial discharge extinction voltage (PDIV) and partial discharge statistical characteristics of C5F10O under two different background gases, as well as the variety and concentration change of mixed gas decomposition products are theoretically analyzed and compared with SF6. It is found that, similar to SF6, the PDIV of C5F10O under two kinds of background gases increases linearly with the increase of air pressure. The PDIV under N2 background is smaller than that under CO2 background under the same conditions, and its insulation performance is slightly better than that under CO2 background. The partial discharge decomposition of C5F10O mixed gas mainly produces fluorocarbons such as CF4, C2F4, C2F6, C3F8, C3F6, and a small amount of CO, COF2, C3HF7, and CHF3. The increase of air pressure inhibits the partial discharge decomposition of mixed gas.

Improving the Integrated Fabrication of Insulation Systems in Electric Drives by Injection Molding of Thermosets Due to Processing Conditions and Slot Design

The expanding demand for electro mobility in general and specifically for electrified vehicles requires the expansion of electro mobility technology with respect to variations in the requirements of the process and the application. Within the stator, the electrical insulation system has a high impact on the application properties. So far, limitations, such as the identification of suitable materials for the stator insulation or high costs in the processes, have hindered the implementation of new applications. Therefore, a new technology that allows integrated fabrication via the injection molding of thermosets is founded in order to expand the applications of stators. The possibility of the integrated fabrication of insulation systems to meet the demands of the application can be improved by the processing conditions and the slot design. Within this paper, two epoxy (EP) types with different fillers are investigated to show the impact of the fabrication process in terms of different parameters; these include the holding pressure or the temperature setup, as well as the slot design and with that the flow conditions. To evaluate the improvement in the insulation system of electric drives, a single slot sample, consisting of two parallel copper wires, was used. Then, the two parameters of the average partial discharge (PD) and the partial discharge extinction voltage (PDEV), as well as the full encapsulation detected by microscopy images, were analyzed. It was shown that both characteristics (electric properties-PD and PDEV; full encapsulation) could be improved in terms of an increase in the holding pressure (up to 600 bar) or a reduction in the heating time (around 40 s), as well as the injection speed (down to 15 mm/s). Further, an improvement in the properties can be reached by increasing the space between the wires, as well as the wire and the stack, due to a higher slot depth or by implementing flow-improving grooves that have a positive effect on the flow conditions. With that, the optimization of the integrated fabrication of insulation systems in electric drives via the injection molding of thermosets was enabled with respect to the process conditions and the slot design.

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.

A Contribution to Thermal Ageing Assessment of Glass Fibre Insulated Wire Based on Partial Discharges Activity

This paper aims to evaluate a Type-II insulated wire (i.e., Glass fibre insulated wire) as a function of thermal ageing time based on partial discharges (PD) activity. Partial discharge inception voltage (PDIV), partial discharge extinction voltage (PDEV), and repetitive partial discharge inception voltage (RPDIV) measurements have been performed on both unaged and thermally aged specimens. Each specimen modelled the turn-to-turn insulation; hence it was made by a pair of Glass fibre wires wrapped in PTFE tape for a mechanical purpose. The PD tests have been carried out under positive and negative unipolar square waveform excitations using different rise times: 80, 400 and 800 ns. The trend of PD quantities vs exposure time demonstrates that the effect of space charge accumulation under unipolar excitation must be taken into account when unaged and thermally aged specimens are compared. Results revealed that although sometimes thermally aged samples might show a temporary superiority in the short run, unaged ones always present a better performance after space charge accumulation, as expected. Following the statistic postprocessing of the collected data, shape/slope parameters of the Weibull distribution relevant to PDIV can be used as an ageing indicator for Glass fibre insulated wire. Finally, the permittivity evaluation vs thermal and electrical stresses have been performed, relying on the measured capacitance of each specimen just after every PD test. Such analysis revealed that thermal ageing increases permittivity in the long run. However, electrical stress has different impacts on permittivity depending on the rise time, polarity, and magnitude of the applied voltage.

Influences of low temperature on internal gas-filled cavity discharges in EPR cable termination and its feature factors analysis

Ethylene–propylene rubber (EPR) cable is the most commonly used type in high-speed railway. Several flashover accidents take place in cable termination in an extremely cold environment. In this study, EPR cable terminations were tested in a simulated low-temperature system with varied applied voltages. The results showed that the differences in elastic modulus and free space volume between EPR insulation and stress control tube (SCT) in a frigid environment would lead to structural defects between them. Damages on the EPR insulation occurred in the form of flashover point when the ambient temperature declined to −30°C and below, with the growth rate of discharge and resulting extent of damage across the EPR/SCT interface being non-linearly related to ambient temperature. Besides, the partial discharge (PD) initial voltage and PD extinction voltage under low-temperature conditions reduced by >40% compared with those at 20°C. The flashover probability of EPR terminations increased significantly, and the shape characteristics of PD patterns have also changed obviously at low temperatures. It can be verified that under the high-voltage and low-temperature conditions, the EPR/SCT interface is prone to organisational mismatch, which will lead to the PD phenomenon and seriously affect the stable operation of EPR cable terminations.

Partial discharge characteristics of C6F12O/CO2 mixed gas at power frequency AC voltage

The greenhouse effect of SF6 has made its application in various types of gas-insulated equipment increasingly restricted. C6F12O combines the advantages of insulation strength, safety and environmental friendliness and is considered to have potential for use in medium and low voltage equipment when mixed with buffer gas. In this paper, an experimental study on the partial discharge characteristics of C6F12O/CO2 mixed gas was carried out. The partial discharge initial voltage and the partial discharge extinction voltage of the mixed gas under the power frequency voltage are measured, and compared with the breakdown voltage. It was found that the partial discharge initiation voltage and the extinction voltage of the mixed gas gradually increased with the mixing ratio, and the influence of the gas pressure on the high mixing ratio was large. Compared with pure CO2, the difference between the partial discharge starting voltage and the breakdown voltage of the mixed gas is large. The research in this paper can provide guidance for the operation and maintenance of C6F12O mixed gas in the application of medium and low voltage equipment.

Modelling and simulation of PD characteristics in non-conductive electrical trees

Electric tree growth is a key ageing mechanism leading to breakdown of high voltage electrical insulation. Partial discharges (PDs) are invariably associated with electrical tree inception and propagation. In turn, the physical structure of an electrical tree influences the characteristics of partial discharge activity. Interpretation of PD patterns is therefore central to developing an understanding of the tree propagation process, and also to the use of PD patterns as an asset management tool. Our previous research indicates that the phase resolved PD (PRPD) patterns and pulse sequence analysis (PSA) patterns evolve with tree propagation. A method was proposed to estimate the point-on-wave inception and extinction voltages of PDs in tree channels within each power cycle. It was shown that the evolution of PD patterns is a consequence of changes to PD inception and extinction voltages as a tree develops. This paper provides a deterministic model of partial discharge in tree channels. Simulations of PDs in a straight non-conductive tree channel are based on experimental PD inception, extinction and residual voltages. The quantitative simulations reproduce almost all the characteristics of observed PRPD and PSA patterns. It is concluded that PD events are determined by five key parameters: tree structure, applied voltage, PD inception voltage, PD extinction voltage and PD residual voltage. Key parameters estimated by the method, and the models proposed explain PD activity in non-conductive trees. It is suggested that the PSA and PRPD patterns should be discussed together to fully understand the PD events. This model forms a platform for generating robust information for asset managers using PD measurements from high voltage equipment in service.

Simulating partial discharge activity in a cylindrical void using a model of plasma dynamics

Partial discharge (PD) activity models typically use simplified descriptions of individual discharges to develop a model of discharge activity. This approach neglects the plasma dynamics of the discharge, and requires the use of multiple assumptions. In this work, plasma dynamic simulations of individual PDs are used to inform a PD activity model for discharges within a cylindrical cavity bounded by low density polyethylene. Specifically, by considering the plasma dynamics of the discharges it is possible to determine: surface charge density distributions, apparent charges, the inception electric field and the residual electric field. The resulting PD activity model had only a single adjustable parameter, relating to the availability of seed charges, and was able to produce phase resolved PD patterns that were comparable with experimental data. Good agreement was also observed between the measured and simulated PD extinction voltage. The shortcomings of PD activity models are discussed including the poor understanding of the seed charge generation rate. Nevertheless, the model does allow robust conclusions on the PD dynamics in the experiment. The main contribution of this work is to show how simulations of plasma dynamics can be used to provide additional insight PD activity.

Effects of DC prestressing on partial discharge in oil-impregnated pressboard insulation

The AC (50 Hz) partial discharge (PD) characteristics of oil-impregnated pressboard (OIP) insulation are severely influenced by the DC voltage component in the converter transformer. Therefore, this study investigates the effect of different DC prestressing voltage (DCPV) amplitudes, prestressing time, and repetitions times of the applied AC voltage on the OIP insulation AC PD characteristics. Meanwhile, the space charge characteristics of distribution and dissipation are measured by pulsed electro-acoustic method. The experimental results show that the PD inception voltage (PDIV) and PD extinction voltage (PDEV) after DCPV were lower than that under AC voltage without DCPV. The PDIV decrease with higher DCPV at prestressing time from 30 to 60 min. But it increase with higher DCPV at prestressing time from 90 to 120 min. However, PDIV initially increase then decrease with the increasing repetition times of the applied AC voltage. The space charge distribution in 1 mm OIP insulation takes 60 min to reach a balanced condition. The residual space charge is relatively higher at a longer prestressing voltage time and higher DC voltage when the DC voltage was turn off. The PDIV and PDEV after DCPV is improved by Joule heating with longer prestressing time and higher DCPV. The space charge had negative effect on them.

Study on Characteristics of Partial Discharge in XLPE Cable under Damped AC Voltage at Different Frequencies

Damped AC (DAC) voltage method is widely used in partial discharge (PD) detection of power cable. The frequency of DAC voltage is changed due to different length of testing cable. To study the PD characteristics in the heat-shrinkable cable accessories under DAC voltage at different frequencies, four different cases of artificial defects in cross-linked polyethylene (XLPE) cables were investigated. These defects include termination without stress tube, incorrect use of insulation tape as the semi-conductive adhesive tape in the middle joint, void and metal particles on the surface of XLPE insulation. Furthermore, some characteristic PD parameters including the partial discharge inception voltage (PDIV), the partial discharge extinction voltage (PDEV), discharge magnitude, discharge times and so on were compared and analyzed. The results show that PD characteristics are different as the DAC frequency changes, which provides guidance for the practical application.

Partial-Discharge Magnitude Estimation With Three-Dimensional Finite-Element Methods

Prediction of partial-discharge (PD) magnitude for ellipsoidal and spheroidal cavities in bulk insulation is easily addressed using axisymmetric 2-D finite-element methods (FEM), with the cavity on-axis. The introduction of an arbitrary defect even within an axisymmetric device breaks the 2-D symmetry, which makes prediction of PD magnitudes impossible without the use of 3-D FEM. A separable connector with tracks along various interfaces was modeled using 3-D FEM, and the apparent PD magnitude was estimated by multiplying the change in capacitance during discharges and the PD extinction voltage. Experimental PD measurements for the separable connector show good agreement with predictions. Since failure of separable connectors can cause significant damage, knowledge of the PD magnitude as a function of defect severity is useful to investigate diagnostics for defective connectors.

Influence of rise time on partial discharge extinction voltage at semi-square voltage waveforms

This work presents measurements of the partial discharge (PD) extinction voltage in three different types of test objects, using semi-square voltages with 2 ?s and 100 ?s rise time. A needle creating corona discharges, a twisted pair specimen commonly used for testing motor insulation and a paper/oil test object modelling the turn-to-turn insulation of a transformer winding were investigated, presenting extinction voltages between approximately 1 kV and 8 kV. For the twisted pair specimen the PD extinction voltage was significantly lower at the shorter rise time, whereas for the other test objects, just a small or no difference could be detected. The conclusion is that depending on what range of rise times and what insulation system that are studied, there may be an influence of the rise time of the applied voltage on the PD extinction voltage.

Some Problems of the Partial Discharge Burning Time

The problem of the partial discharge (PD) extinction is investigated. The transient process takes place in a small spherical inclusion which is located in the dielectric. Both the losses caused by polarization and ohmic losses as the dielectric parameters are taken into account. From the inclusion standpoint the dielectric is considered as an active two-pole element (equivalent generator) and inclusion represents by own current-voltage curve. PD extinction voltage was shown to depend on the polarization loss tangent.