Showing papers on "Partial discharge published in 2020"

10-kV SiC MOSFET Power Module With Reduced Common-Mode Noise and Electric Field

Abstract: The advancement of silicon carbide (SiC) power devices with voltage ratings exceeding 10 kV is expected to revolutionize medium- and high-voltage systems. However, present power module packages are limiting the performance of these unique switches. The objective of this research is to push the boundaries of high-density, high-speed, 10-kV power module packaging. The proposed package addresses the well-known electromagnetic and thermal challenges, as well as the prominent electrostatic and electromagnetic interference (EMI) issues associated with high-speed, 10-kV devices. The high-speed switching and high voltage rating of these devices causes significant EMI and high electric fields. Existing power module packages are unable to address these challenges, resulting in detrimental EMI and partial discharge that limit the converter operation. This article presents the design and testing of a 10-kV SiC mosfet power module that switches at a record 250 V/ns without compromising the signal and ground integrity due to an integrated screen reduces the common-mode current by ten times. This screen connection simultaneously increases the partial discharge inception voltage by more than 50%. With the integrated cooling system, the power module prototype achieves a power density of 4 W/mm3.

Understanding Partial Discharge Behavior from the Memory Effect Induced by Residual Charges: A Review

Abstract: Residual charges produced in previous partial discharges (PDs) are considered as one aspect of memory effects that affect subsequent discharge behavior. In this paper, we review past studies about memory effects induced by residual charges during successive PDs. At first, surface charge dynamics for cavity and needle-to-dielectric discharges are presented. Subsequently, three methods used to unravel the memory effects are described, e.g. conditional distribution, pulse sequence analysis (PSA) and chopped PD (CPD). Then effects of residual charges on PD inception, discharge propagation and collective behavior are summarized. In addition, contributions of residual charges to the transitions between discharge modes and discharge patterns are presented. At last, some problems relevant to the memory effects are discussed, followed by suggestions for future work.

Design and Experimental Validation of a Wire-Bond-Less 10-kV SiC MOSFET Power Module

Abstract: Wide bandgap (WBG) power devices with voltage ratings exceeding 10 kV have the potential to revolutionize medium- and high-voltage systems due to their high-speed switching and lower ON-state losses. However, the present power module packages are limiting the performance of these unique switches. The objective of this article is to push the boundaries of high-density, high-speed, 10-kV power module packaging. The proposed package addresses the well-known electromagnetic and thermal challenges, as well as the more recent and prominent electrostatic and electromagnetic interference (EMI) issues associated with high-speed, 10-kV devices. The module achieves low and balanced parasitic inductances, resulting in a record switching speed of 250 V/ns with negligible ringing and voltage overshoot. An integrated screen reduces the common-mode (CM) current that is generated by these fast voltage transients by ten times. This screen connection simultaneously increases the partial discharge inception voltage (PDIV) by more than 50%. A compact, medium-voltage termination and system interface design is also proposed in this article. With the integrated jet-impingement cooler, the power module prototype achieves a power density of 4 W/mm3. This article presents the design, prototyping, and testing of this optimized package for 10-kV SiC MOSFETs.

Combined geometrical techniques and applying nonlinear field dependent conductivity layers to address the high electric field stress issue in high voltage high-density wide bandgap power modules

Abstract: Wide bandgap (WBG) power modules made from materials such as SiC and GaN (and soon Ga 2 O 3 and diamond), which can tolerate higher voltages and currents than Si-based modules, are the most promising solution for reducing the size and weight of power electronics systems. In addition to the higher blocking voltages of WBG power modules, their volume has been targeted to be several times smaller than that of Si-based modules. This translates into higher electric stress within the module and, in turn, a higher risk for unacceptable partial discharge (PD) activities, leading to aging and degradation of both the ceramic substrate and the silicone gel. Due to the small dimensions of power module geometry, in the mm or μm (for protrusions) range, and due to its extremely non-uniform electric field geometry, conventional high voltage testing electrode geometries cannot simulate real conditions. On the other hand, university-based laboratories often cannot provide testing samples under manufacturing/factory conditions and with high-quality materials. Thus, it is difficult to determine the efficacy of electric field and PD control methods. To address this issue, an electric field criterion based on precise dimensions of a power module and its PD measurement is introduced. Then, combined geometrical techniques and the application of nonlinear field-dependent conductivity (FDC) layers are proposed, for the first time, to address the high electric field issue in an envisaged 25 kV high-density WBG power module. Electric field modeling and simulations are carried in COMSOL Multiphysics where various electric field reduction methods proposed in this paper can be used as a guideline and reference to design the insulation system for next-generation WBG power modules, meeting both the one-minute insulation and PD tests based on IEC 61287-1.

Surface charge accumulation and pre-flashover characteristics induced by metal particles on the insulator surfaces of 1100 kV GILs under AC voltage

Abstract: With the increase in the voltage and capacitance of gas-insulated transmission lines (GILs), the insulation failure of GIL has attracted more and more attention. Further understanding of the partial discharge (PD) and flashover characteristics of metal particles on the insulator surface, as well as the flashover mechanism, is necessary to reduce the failure rate and improve the reliability of the equipment. In this study, an ultra-high voltage (UHV) AC insulator PD test and measurement system based on the cross-reference pulse current (PC) and ultra-high frequency (UHF) methods were established. The PD development and flashover characteristics of 5 mm-long metal particles at different positions on the surface of a UHV AC insulator, as well as the surface charge accumulation, were studied. The results show that the discharge of millimetre-scale metal particles on the insulator surface under PD test conditions is relatively low (generally lower than 2PC), and it is difficult to be detected by the conventional PC and UHF methods due to their insufficient sensitivity. Moreover, it is found that the weak PDs of the millimetre-scale metal particles will result in a charge accumulation on the insulator surface under AC voltage, which eventually will lead to the insulator flashover. The PDs of the UHV AC insulator before flashover are generally small, which will make it difficult for online monitoring systems to give an effective alarm before flashover.

Adsorption behaviour of SO2 and SOF2 gas on Rh-doped BNNT: a DFT study

Abstract: As the insulating medium, SF6 is widely used in gas insulation equipment. Partial discharge and local overheating can cause the decomposition of SF6, resulting in a decrease in insulation strength ...

Noise Rejection and Detection of Partial Discharges Under Repetitive Impulse Supply Voltage

Abstract: Partial discharge (PD) detection is a fundamental tool for the design and diagnosis of insulation systems, particularly organic materials that must operate throughout their life without PD. While the methods for PD measurements under sinusoidal supply voltage are well-established, the same does not hold under repetitive-pulse voltage waveforms, such as those provided by inverters. Electromagnetic noise generated by electronic switch commutation in inverters can overlap with PD pulses, making it difficult to measure, separate from noise, and identify PD. This is particularly more cumbersome when fast rise-time switches are employed. To address these issues, this paper investigates methods to detect PD pulses on enameled wires under repetitive impulsive voltages with different rise times of 60, 500, and 1000 ns, proposing techniques which allow separation of PD from noise even when they overlap in time and frequency domains. This paper employs a time-domain approach using standard deviation curve acquired from a set of measurements to discriminate PD from repetitive commutation disturbance. These techniques can be implemented to achieve unsupervised noise rejection, which would be an important goal for both offline and online PD testing. The proposed approach is validated successfully through tests carried out on a low-voltage motor.

Partial discharge behavior of protrusion on high voltage conductor in GIS/GIL under high voltage direct current: Comparison of SF 6 and SF 6 alternative gases

Abstract: Recent studies have demonstrated that fluoronitrile (FN) Novec™ 4710 and fluoroketone (FK) Novec™ 5110 show higher dielectric strength than SF 6 . These gases can be mixed with a buffer gas such as CO 2 and technical air to have suitable dielectric properties for high voltage insulation applications. With the rapid growth of HVDC networks, these gases may replace SF 6 in future HVDC apparatus such as GIS/GIL. This paper focuses on the study of the partial discharge (PD) characteristics of SF 6 and SF6 alternative gases with a protrusion defect on the high voltage conductor with DC voltage. The results show that, for calculated equivalent SF 6 dielectric strength under homogeneous electric field, a difference in terms of partial discharge inception voltage was noted for gas mixtures compared to SF 6 . This difference may be due to the influence of gas pressure under highly inhomogeneous electric field. FN-CO 2 and FK-Air mixtures showed the highest PD amplitude under positive polarity while it was higher under negative polarity for SF 6 . Moreover, PD repetition frequency is higher for SF 6 investigated alternative gases. The results can help for the design of SF 6 free HVDC GIS partial discharge monitoring system.

Characterization of Nonlinear Field-Dependent Conductivity Layer Coupled With Protruding Substrate to Address High Electric Field Issue Within High-Voltage High-Density Wide Bandgap Power Modules

Abstract: In addition to higher blocking voltages of wide bandgap (WBG) power modules, their volume has been targeted to be several times smaller than that of Si-based modules. This translates into higher electric stress within the module and, in turn, a higher risk for unacceptable partial discharge (PD) activities, leading to aging and degradation of both the ceramic substrate and the silicone gel. Due to the small dimensions of power module geometry, in the mm- or $\mu \text{m}$ (for protrusions)-range, and due to its extremely non-uniform electric field geometry, conventional high-voltage testing electrode geometries cannot simulate real conditions. On the other hand, university-based laboratories often cannot provide manufacturing/factory conditions for testing samples and for high-quality materials. Thus, it is difficult to determine the efficacy of electric field control methods through experiments. In these situations, numerical electric field calculation is the only feasible way to evaluate different electrical insulation designs. To this end, the finite-element method (FEM) models of the electrical insulation system used in WBG power modules are developed in COMSOL Multiphysics. It is shown that the current geometrical techniques alone cannot address the high-electric field issue within high-density WBG modules. To address this issue, for the first time, nonlinear field-dependent conductivity (FDC) materials applied to high-electric stress regions in combination with a recently introduced geometrical technique known as the protruding substrate is proposed. In this regard, the nonlinear FDC layer is characterized and various designs to reduce the electric field are evaluated. Moreover, the effect of the operating frequency on the performance of the solution mentioned above will be studied.

A Denoising Algorithm for Partial Discharge Measurement Based on the Combination of Wavelet Threshold and Total Variation Theory

Abstract: In electrical engineering, partial discharge (PD) measurement is frequently employed to detect insulation defects and judge insulation conditions of high-voltage electrical apparatus. However, it is easily corrupted by white noises in the field. In this article, a joint algorithm is proposed, in which the wavelet threshold and total variation (TV) denoising methods are combined by the convex optimization theory to denoise ultrahigh frequency (UHF) PD signals corrupted by white noises. Since the two respective methods are incorporated into the joint algorithm, it is with high potential to reduce oscillation error introduced by the wavelet threshold method and eliminate stair error introduced by the TV denoising method. In order to validate the effect of the proposed algorithm, a numerical simulation is carried out to compare it with several existing methods. Indicators of their performance are computed, and the results verify that the proposed algorithm outperforms all the other methods.

Recent Advances in Polymer Nanocomposites Based on Polyethylene and Polyvinylchloride for Power Cables

Abstract: Polymer nanocomposites used in underground cables have been of great interest to researchers over the past 10 years. Their preparation and the dispersion of the nanoparticles through the polymer host matrix are the key factors leading to their enhanced dielectric properties. Their important dielectric properties are breakdown strength, permittivity, conductivity, dielectric loss, space charge accumulation, tracking, and erosion, and partial discharge. An overview of recent advances in polymer nanocomposites based on LDPE, HDPE, XLPE, and PVC is presented, focusing on their preparation and electrical properties.

A Dual-Slot Barrier Sensor for Partial Discharge Detection in Gas-Insulated Equipment

Abstract: This paper reports on the development and testing of a novel barrier sensor for UHF Partial Discharge (PD) detection in gas-insulated equipment. The sensor features a unique dual-slot planar antenna backed by an air-filled cavity. The dual-slot arrangement allows different parts of the antenna to resonate at different frequency ranges in the UHF spectrum. As a result, the sensor exhibits broader bandwidth and higher sensitivity than other barrier sensors. Finite Element Analysis simulations have been used to optimize the sensor design. Furthermore, testing using a specially made PD test rig, GTEM cell testing and testing on a gas-insulated line section in the high voltage laboratory, have validated the simulation results and the capabilities of the sensor. The Dual-Slot Barrier (DSB) sensor exhibits a bandwidth of 0.3 – 2.0 GHz with a mean effective height of 13 mm, and an effective height above 2 mm for 90% of the frequency range. The sensor can be used with both wideband instruments, such as oscilloscopes, and narrow band instruments such as frequency downconverters. Additionally, its optimized dimensions and unique replaceable sealing attachment ensure maximum compatibility for retrofitting on a wide range of equipment.

Feasibility Study on Simultaneous Detection of Partial Discharge and Axial Displacement of HV Transformer Winding Using Electromagnetic Waves

Abstract: Recently, axial displacement detection of transformer winding has been performed using synthetic-aperture radar (SAR) imaging, as an online method, in ultra-wideband frequency band. Another important problem in power transformers is partial discharge (PD), which emits signal in the ultra-high-frequency (UHF) frequency band. If detection of axial displacement using SAR imaging can be applied to the UHF band, both defects can be detected using only one set of antennas and application of these methods can be more economic. However, as shown in this paper, using the same frequency band for detection of both defects leads to wrong conclusions about winding conditions, if PD is occurred during the application of SAR imaging. As a solution, UHF stepped-frequency imaging method and generalized likelihood ratio test method are proposed for detection of axial displacement. Finally, on a three-phase real transformer both defects are simultaneously detected using designed monitoring system.

Design of Archimedes Spiral Antenna to Optimize for Partial Discharge Detection of Inverter Fed Motor Insulation

Abstract: The partial discharge (PD) in inverter-fed motors generated from high frequency and short rise time impulsive voltage are more complex than traditional sinusoidal voltage operated motors due to the noise initiated from fast switching power electronics devices. Available PD sensors and related technology for DC and sinusoidal voltage cannot be used at repetitive impulsive voltage conditions. This work aims to report the design of antenna with optimized geometry and satisfied parameters to detect PD for impulsive voltage. The antenna is verified and the influence of both impulse voltage parameters and test configuration on PD features are discussed. The envelope detection technique after the filter in the frequency domain was proposed to reduce the complexity and cost of PD test hardware under impulsive voltages with fast rise times. The frequency domain energy of PD from motor insulation is mainly distributed in the range of 0.6-1.8 GHz. By increasing the amplitude and frequency of input impulsive voltage, the range of PD energy spectrum is enlarged to (1.5-1.8 GHz) and by decreasing input amplitude the PD signal strength decays abruptly. Moreover, the high-frequency distribution of PD energy attenuates more severely with the distance and the electromagnetic energy of PD decays nonlinearly with the increase of propagation distance. Therefore, under the premise of satisfying the safety test, selecting the test distance below 15 cm can ensure that the test results have a high signal-to-noise ratio and signal integrity. The experience reported in this study could provide suggestions for off-line PD measurements for inverter-fed motor insulation evaluations for manufacturers.

Experimental Study of Visual Corona under Aeronautic Pressure Conditions Using Low-Cost Imaging Sensors

Abstract: Visual corona tests have been broadly applied for identifying the critical corona points of diverse high-voltage devices, although other approaches based on partial discharge or radio interference voltage measurements are also widely applied to detect corona activity. Nevertheless, these two techniques must be applied in screened laboratories, which are scarce and expensive, require sophisticated instrumentation, and typically do not allow location of the discharge points. This paper describes the detection of the visual corona and location of the critical corona points of a sphere-plane gap configurations under different pressure conditions ranging from 100 to 20 kPa, covering the pressures typically found in aeronautic environments. The corona detection is made with a low-cost CMOS imaging sensor from both the visible and ultraviolet (UV) spectrum, which allows detection of the discharge points and their locations, thus significantly reducing the complexity and costs of the instrumentation required while preserving the sensitivity and accuracy of the measurements. The approach proposed in this paper can be applied in aerospace applications to prevent the arc tracking phenomenon, which can lead to catastrophic consequences since there is not a clear protection solution, due to the low levels of leakage current involved in the pre-arc phenomenon.

Partial Discharge Localization Using Time Reversal: Application to Power Transformers.

Abstract: In this work, we present a novel technique to locate partial discharge (PD) sources based on the concept of time reversal. The localization of the PD sources is of interest for numerous applications, including the monitoring of power transformers, Gas Insulated Substations, electric motors, super capacitors, or any other device or system that can suffer from PDs. To the best of the authors' knowledge, this is the first time that the concept of time reversal is applied to localize PD sources. Partial discharges emit both electromagnetic and acoustic waves. The proposed method can be used to localize PD sources using either electromagnetic or acoustic waves. As a proof of concept, we present only the results for the electromagnetic case. The proposed method consists of three general steps: (1) recording of the waves from the PD source(s) via proper sensor(s), (2) the time-reversal and back-propagation of the recorded signal(s) into the medium using numerical simulations, and (3) the localization of focal spots. We demonstrate that, unlike the conventional techniques based on the time difference of arrival, the proposed time reversal method can accurately localize PD sources using only one sensor. As a result, the proposed method is much more cost effective compared to existing techniques. The performance of the proposed method is tested considering practical scenarios in which none of the former developed methods can provide reasonable results. Moreover, the proposed method has the unique advantage of being able to locate multiple simultaneous PD sources and doing so with a single sensor. The efficiency of the method against the variation in the polarization of the PDs, their length, and against environmental noise is also investigated. Finally, the validity of the proposed procedure is tested against experimental observations.

Modeling partial discharge phenomena

Abstract: Measurements of partial discharge (PD) activity are a commonly used technique to diagnose the health of insulation material within high voltage plant. The definition of PD is relatively broad, it is generally defined as a localized or confined discharge within an insulating medium [1]. Due to the high electric breakdown strength of insulation materials, PDs typically occur within low density defects in the system. One family of defects which has received significant research attention is air-filled voids, typically spherical or cylindrical, surrounded by solid dielectric material. Experiments have typically used a ‘sandwich’ technique to fabricate cylindrical voids, using three slabs of material with a circular hole drilled into the central slab. The slabs are then pressed together and placed between parallel plate electrodes. A common dielectric material used to fabricate cylindrical voids is LDPE [2]. By injecting air through a syringe before the curing process spherical voids have been fabricated in epoxy resins [3,4] and silicone rubber [5].

Classification of Partial Discharge Images Using Deep Convolutional Neural Networks

Abstract: Artificial intelligence-based solutions and applications have great potential in various fields of electrical power engineering. The problem of the electrical reliability of power equipment directly refers to the immunity of high-voltage (HV) insulation systems to operating stresses, overvoltages and other stresses—in particular, those involving strong electric fields. Therefore, tracing material degradation processes in insulation systems requires dedicated diagnostics; one of the most reliable quality indicators of high-voltage insulation systems is partial discharge (PD) measurement. In this paper, an example of the application of a neural network to partial discharge images is presented, which is based on the convolutional neural network (CNN) architecture, and used to recognize the stages of the aging of high-voltage electrical insulation based on PD images. Partial discharge images refer to phase-resolved patterns revealing various discharge stages and forms. The test specimens were aged under high electric stress, and the measurement results were saved continuously within a predefined time period. The four distinguishable classes of the electrical insulation degradation process were defined, mimicking the changes that occurred within the electrical insulation in the specimens (i.e., start, middle, end and noise/disturbance), with the goal of properly recognizing these stages in the untrained image samples. The results reflect the exemplary performance of the CNN and its resilience to manipulations of the network architecture and values of the hyperparameters. Convolutional neural networks seem to be a promising component of future autonomous PD expert systems.

Partial Discharge Investigation of Form-Wound Electric Machine Winding for Electric Aircraft Propulsion

Abstract: Electric machines with high specific power are crucial for the more electric aircraft propulsion. However, the harsh operating environment such as low air pressure and high temperature will challenge the motor’s insulation system and cause partial discharge (PD), which threatens the reliability of the machine. This article proposes a PD-free stator winding design regarding an air-core high-frequency permanent magnet synchronous machine for aircraft propulsion. The experiment and simulation are performed under sinusoidal voltage due to the power quality requirement by the standard for aircraft. PD inception voltage (PDIV) with respect to air pressure and temperature was obtained experimentally for form-wound windings and was analyzed to derive the PD-free criteria. A 3-D stator model was built and used to simulate the electric field and temperature distributions with a finite-element method. In addition, the effect of voids and delamination on electric field distortion was also taken into consideration to allow for a safety margin and meet the PD-free requirement. To achieve high-specific-power requirement and PD-free requirement simultaneously, a multiobjective genetic algorithm was adopted to perform the optimization and obtain the Pareto fronts at various air pressures. Finally, the PDIV value of the designed winding was measured to verify the PD-free design. The main purpose is to combine the experimental results and the simulation tool to obtain an acceptable PD-free design in the meantime achieving high power density, which may provide helpful guidance on motor insulation design for the more electric aircraft propulsion.

A Finite Element Analysis Model for Partial Discharges in Silicone Gel under a High Slew Rate, High-Frequency Square Wave Voltage in Low-Pressure Conditions

Abstract: Wide bandgap (WBG) devices made from materials such as SiC, GaN, Ga2O3 and diamond, which can tolerate higher voltages and currents compared to silicon-based devices, are the most promising approach for reducing the size and weight of power management and conversion systems. Silicone gel, which is the existing commercial option for encapsulation of power modules, is susceptible to partial discharges (PDs). PDs often occur in air-filled cavities located in high electric field regions around the sharp edges of metallization in the gel. This study focuses on the modeling of PD phenomenon in an air filled-cavity in silicone gel for the combination of (1) a fast, high-frequency square wave voltage and (2) low-pressure conditions. The low-pressure condition is common in the aviation industry where pressure can go as low as 4 psi. To integrate the pressure impact into PD model, in the first place, the model parameters are adjusted with the experimental results reported in the literature and in the second place, the dependencies of various PD characteristics such as dielectric constant and inception electric field on pressure are examined. Finally, the reflections of these changes in PD intensity, duration and inception time are investigated. The results imply that the low pressure at high altitudes can considerably affect the PD inception and extinction criterion, also the transient state conditions during PD events. These changes result in the prolongation of PD events and more intense ones. As the PD model is strongly dependent upon the accurate estimation electric field estimation of the system, a finite-element analysis (FEA) model developed in COMSOL Multiphysics linked with MATLAB is employed that numerically calculates the electric field distribution.

Sensor Comparison for Corona Discharge Detection Under Low Pressure Conditions

Abstract: Low pressure environments, situate insulation systems in a challenging position since partial discharges (PDs), corona and arc tracking are more likely to develop. Therefore, specific solutions are required to detect such harmful phenomena before major failure occurrence. This paper deals with three low-cost and small-size sensing methods, i.e., a single loop antenna, a visible-UV imaging sensor and the measurement of the leakage current to detect corona in the early stage, thus anticipating the appearance of severer effects such as arc tracking or disruptive breakdown. The three studied methods can be applied for an on-line monitoring of corona activity under low pressure environments, thus being compatible with predictive maintenance approaches. This on-line monitoring can be used to develop improved electrical protection devices able to detect such effects in an initial stage, thus improving current solutions which are unable to do so. All three studied sensors give consistent linear responses within the studied pressure range, i.e., 10-100 kPa, with almost no drift. The sensitivity of the visible-UV imaging sensor is slightly lower than that of the others, but it has the advantage of directly locating the discharge points. Results presented in this paper can be very useful for the more electrical aircraft (MEA), which is forcing electrical distribution systems to operate at higher voltage levels. Due to the little experience and scarcity of published data, the experimental results presented in this paper can be valuable for a better understanding of the combined action of high voltage and low pressure environments.

Modelling of supply voltage frequency effect on partial discharge repetition rate and charge amplitude from AC to DC at room temperature

Abstract: This paper has the purpose to derive an analytical model ("continuum" model) able to describe the behavior of partial discharge (PD) repetition rate and amplitude, occurring in a cavity embedded in polymeric insulation, as a function of the frequency of the supply voltage, going from AC power supply frequency, 50–60 Hz, to DC. In the range between DC and 50–60 Hz focus is made on data coming from tests under AC sinewave with very low frequency (VLF) such as 0.1 Hz and 0.01 Hz, which are commonly used for cable testing. It is shown that the proposed "continuum" model can provide reasonably good fit to the experimental results obtained in the range DC to 60 Hz, regarding PD repetition rate and amplitude. To reach such result, the equivalent circuit is modified from that commonly used and made by fully-capacitive or resistive components, in order to take into account the change of polarization mechanisms which, depending on dielectric material, may play a non-negligible role to establish the repetition rate from low frequency to DC power supply. In addition, the residual voltage after a PD event has to vary with frequency to reach good fitting. Also, it is shown that PD amplitude under DC and VLF can be lower than under AC 50–60 Hz due to the delay time of the firing electron, thus experimental PD amplitude varies with frequency depending on material and defect typology and location.

Partial Discharge Localization Using Electromagnetic Time Reversal: A Performance Analysis

Abstract: In this study, first, a comparison on the application of electromagnetic time reversal (EMTR) and time difference of arrival (TDoA) in partial discharge localization in power transformers is presented. A two-dimensional finite-difference time-domain simulation is used to calculate the signal recorded by the sensors. Results show that, in a transformer tank excluding its windings, both methods yield similar results in terms of location accuracy, although the EMTR method only needs one sensor to localize the partial discharge (PD) source while the TDoA method needs at least three sensors in the 2D localization problem. However, the presence of transformer windings leads to a degradation of the performance of the TDoA method if the line of sight from the source to the sensor is blocked by any of the winding blocks. On the other hand, the presence of the transformer windings has an effect on the localization of PD sources that occur between two adjacent phase windings when the distance between the outer winding distances is shorter than the minimum wavelength, λ min . The degradation is directly caused by the diffraction limit. It is shown that, if the distance between two adjacent phase windings is greater than λ min , the EMTR process can locate PD sources occurring between two adjacent phase windings with acceptable accuracy. A case of occurrence of PDs in close proximity (less than λ min /2) to a single metallic object is analyzed both numerically and experimentally. The analysis reveals that although a degradation in the accuracy of the localization is observed compared to the case of longer distances between the PD source and the metallic object, a reasonable localization error of 10 mm (corresponding to λ min /10) is obtained.

Ageing and reliability of electrical insulation: the risk of hybrid AC/DC grids

Abstract: With an impetuous growth of DC links and future grids that will be often hybrid, providing AC and DC supply to nearby or the same customer, an important element is often neglected, that is, electrical insulation. There are no doubts that overhead lines will step back compared to insulated cables, for known reasons of environmental impact, right of way and reliability, especially in an urban or harsh environment. On the other hand, while AC insulation systems have been investigated and used for decades, there is no analogue experience for DC polymeric insulation, nor for insulation systems subjected to power electronics supply. This study aims at enhancing the attention on this topic, focusing on insulation ageing mechanisms and models of relevance for hybrid AC/DC assets and grids. Ageing and life models for AC, DC and power electronics supply are discussed and compared to experimental data, focusing on partial discharge and space charge. It is shown how these two phenomena affect lifetime for different AC, DC and transient operating conditions. The main drivers for insulation design and material selection are discussed, showing that insulation reliability can be achieved only if operating stresses, ageing mechanisms and life models are known and accounted for properly by insulation designers.