Partial Discharge Degradation Research Articles

Informed machine learning for image-data-driven diagnostics of hydrogenerators

Visual inspection is one of the most common and reliable methods used by human experts to perform diagnostics in the industry. However, it requires costly, specific expertise that could benefit from being automated. Such specific tasks are precisely the type of narrow knowledge that machine learning algorithms are best at learning. Yet, data-driven diagnostics from images are lagging behind compared to that from sensor data. This is due to sparse image data, as human experts only need a few photographs to accurately diagnose machine degradation. This paper presents a methodology to incorporate expert knowledge into the development of a data-driven diagnostic model for hydrogenerators based on visual inspection of the presence of partial discharge degradation products. The proposed methodology is validated using real industrial images. It emphasizes on the integration of human knowledge to address multiple challenges such as data sparsity, knowledge conformity, and human interpretability.

Surface Degradation of Oil-Immersed Nomex Paper Caused by Partial Discharge of High-Frequency Voltage

The oil-paper insulation of high-frequency (HF) transformers frequently withstands high-frequency over-voltages at frequencies of several kHz and above. This leads to the occurrence of partial discharge (PD), which causes premature insulation failure in HF transformers. In order to investigate the effect of PD under high-frequency stresses on oil-impregnated Nomex paper, this paper analyzes the surface morphology, bond-broken types of the molecular chain, and product types formed for oil paper after PD degradation. Then, the damage mechanism of oil-immersed Nomex paper between high-frequency and AC stress is explored. The experimental results show that the branches of creepage do not exist in the oil-paper insulation during the entire discharge process under high-frequency stresses, and that their damage degree is higher than that of AC stress. This is mainly because the benzene ring of oil-impregnated Nomex paper is destroyed and opened caused by high-energy particles, the heating effect in HF discharge, and the bulk effect. These results help to improve the design theory of insulation structures and to develop PD-resistant insulation materials in HF transformers.

Effect of Aging Degree on Partial Discharge Degradation of Oil-paper Insulation

This study conducts thermal aging treatment to study the effect of thermal aging on the PD degradation process of the oil-paper insulation. The partial discharge inception voltages, extinction voltages, carbon mark, and microscopic morphology of samples are analyzed. The results show that thermal aging does not reduce the partial discharge inception voltages of oil-paper but can make the spark produced in the oil-paper more difficult to extinguish. At the initial stage, thermal aging has little effect on the development of partial discharge in normal direction of samples, while in later stages, the effect begins to appear; thermal aging reduces the insulation performance between layers of oil impregnated paper and accelerates the carbon trace growth rate in the horizontal direction. In this model, the first layer is most severely bombarded by electrons, and this layer is most obviously affected by the electric field in the normal direction at the contact point between the electrode and the insulating paper. The lower layer is affected by the horizontal and vertical electric fields; therefore, a large area of fiber looseness occurs on the surface of the oil impregnated paper. The degree of deterioration is proportional to the endurance time and aging time, and the damage degree of oil impregnated paper decreases with the increase of the number of layers.

Study on the characteristics of elongation at break and tensile strength of photovoltaic insulating backsheets subjected to partial discharge degradation

As widely distributed vast reserves of emerging green energy, solar energy has gained widespread attention in recent years. Photovoltaic (PV) power generation is one of the most scientific ways to use solar energy. In PV systems, polyethylene terephthalate (PET) is the chief material of PV backsheets. It is well known that PV backsheets will encounter various challenges, among which partial discharge (PD) is one of these that cause deterioration in insulating and mechanical properties of PET. It has severely affected the service life of PV backsheets. In this paper, the PD characteristic of PET samples at different PD durations and temperatures was studied through phase-resolved PD patterns. In addition, the change mechanism of mechanical properties including elongation at break and tensile strength of PET after PD in 30, 60, and 120 min at 30 and 55 °C was also studied. It can be found that the insulating and mechanical properties of PET are reduced after PD degradation at different aging durations and temperatures. This study can provide a reference for the performance improvement of PV PET backsheets, which will also help to evaluate the service life of the PV module and PV power generation systems.

Charge migration of multilayer oil paper on the process of partial discharge under AC voltage

As one of the main reasons for the deterioration of Nomex paper and charge migration, partial discharge (PD) plays a critical role during PD degradation. In this study, the PD characteristics of multilayer oil paper are focused on. In order to explore the characteristics of charge migration, the experiments of the relative permittivity, conductivity, isothermal surface potential decay, scanning electron microscopy and Fourier infrared spectroscopy have been carried out. The results show that the discharge branches of the second layer are two and three times longer than those in the first layer. In addition, the trap level of the first layer for PD degradation samples increases with the degree of PD degradation. The charges captured by deep traps provide seed charges for PD on the surface of the first layer. The charge transportation will accelerate the formation of shallow traps. The charge distribution and shallow traps contribute to the expansion of discharge branches.

Optical Sensing of Partial Discharge in More Electric Aircraft

The aviation sector is one of the main contributors to greenhouse gas emissions and there is a need to develop more-electric aircraft (MEA) that are operationally efficient. In traditional aircraft, the limited number of electrical systems operate at low voltage. The next generation of MEA will operate at voltages of above 1 kV to facilitate the transfer of significant levels of electrical power in an aircraft. However, the higher voltage and frequency levels will lead to an increased chance of partial discharge (PD) degradation in the insulation systems. Therefore, it is critical to have an accurate method of PD detection available for use in these new aircraft. Conventional PD detection methods are optimized for 50/60 Hz applications in the power industry and can be disturbed by harmonics generated via power electronic devices operating at high frequency. This paper explores a bespoke PD detection technique developed using a Sagnac optical fiber sensor capable of measuring the frequency-dependent PD information. Tests were conducted in the 50–1000 Hz range in which existing variable frequency AC power systems are being used in MEA. The results demonstrate that the increasing frequency will have an adverse effect on PD and must be considered in MEA design.

Study on the Correlation between Partial Discharge Energy and SF6 Decomposition Gas Generation

In this paper, the amount of SF6 decomposition gases due to the partial discharge (PD) was studied in the SF6 gas-insulated transformer. The long-term PD degradation experiment was performed while controlling the discharge magnitude using the surface discharge, and the gas generation amount was measured by using gas chromatography for SO2F2, SOF2, SO2, CO, and CF4. In addition, to investigate the relationship between the partial discharge energy and the decomposed gas generation amount, partial discharge energy was calculated by a data processing program and converted to the unit of joule per mole. With the finite element method (FEM), the electric field distribution and SF6 gas decomposition mechanism were explained for the partial discharge energy effect on the gas generation. This study helps understand the relationship between the partial discharge energy and the decomposed gas generation ratio with the experimental results and can be used for the diagnosis of PD and maintenance process for the gas-insulated transformers.

A Contribution to Everlasting Electrical Insulation for DC Voltage: PD-Phobic Materials

This paper focuses on the design of insulating materials candidate from DC-supply applications, proposing viable solutions that may increase the defect-tolerance of an insulation system and avoid the presence of highly energetic phenomena, specifically partial discharges, which can cause premature insulation breakdown. It is shown that, in principle, polymeric materials for DC insulation can be structured, possibly through nanotechnology, to avoid partial discharge inception in steady state even in UHVDC cables and high-field design insulation systems, which would exclude partial discharge degradation from the aging factors that can affect insulation reliability and life. This approach may provide basic tools to design DC insulation having electro-thermal life and reliability of virtually unlimited extent, thus of so-called PD-phobic materials.

The Evolution of Trapping Parameters on Three-Layer Oil-Paper of Partial Discharge Degradation for On-Board Traction Transformers

Trap plays an important role in the charge transportation of three-layer oil-paper and the investigation of trap parameters at different degradation degree is of great significance for further research of the partial discharge (PD) mechanism. This article considers the turn-to-turn structure of on-board traction transformer. A defect model of three-layer oil-paper insulation was designed for PD test. The PD degradation samples are selected from the inception of PD to the breakdown of PD to study the characteristics of surface potential decay on degradation samples by using isothermal surface potential decay method. Given that, the trap distribution at different degradation stages was calculated and its influence on PD characteristics was investigated. The result shows that AC fluctuation voltages caused by inter-harmonic has a great influence on PD distribution, surface charge distribution and trap distribution. With the increase of degradation, the decay rate of surface potential increases and the decay rate of AC fluctuation voltages is larger than that pure AC. And also, the number of shallow trap with AC fluctuation voltages is larger than that with pure AC. The rate of free charge increases with the number of shallow trap increasing. More free electrons involves in the process of PD, accelerating the aging rate of three-layer oil paper samples. Therefore, it can be concluded that surface charge and trap distribution are helpful to clarify PD mechanism.

Influence of temperature on partial discharge of insulating PET backsheet in photovoltaic module

Solar energy has been widely used as a kind of renewable power generation in recent years. In a photovoltaic (PV) system, polyethylene terephthalate (PET) is the core component of backsheet and affects the reliability of the system. In this paper, the effect of temperature on partial discharge (PD) of PET is studied. As shown by the PD measurement system, the amplitude of PD of PET films decreases significantly with temperature increasing. The mechanism of PD degradation, which is closely related to the operational temperature and the chemical change of the insulation backsheet, has been investigated.

Assessment of Surface Degradation of Silicone Rubber Caused by Partial Discharge

This paper reports experimental and analytical results of partial discharge degradation of silicone rubber sheets in accordance with proposed procedures. Considering the actual usage condition of silicone rubber as an insulating material of polymer insulators, an experimental procedure is established to evaluate long-term surface erosion caused only by partial discharge. Silicone rubber is subjected to partial discharge for 8 h using an electrode system with air gap. Voltage application is stopped for subsequent 16 h for recovery of hydrophobicity. The 24 h cycle is repeated 50 or 100 times. Deterioration of sample surface is evaluated in terms of contact angle and surface roughness. It is confirmed the proposed experimental procedure has advantage of no arc discharge occurrence, good repeatability of results, and possible acceleration of erosion. Surface erosion of silicone rubber progresses gradually and finally breakdown of silicone rubber occurs. Alumina trihydrate (ATH), an additive to avoid tracking and erosion by discharge, is not necessarily effective to prevent breakdown caused by partial discharge when localized electric field in air is enhanced by adding ATH. In such a situation, lower permittivity and higher resistance of silicone rubber seem dominant factors to prevent partial discharge breakdown and a careful insulation design should be required.

Filler and additive effects on partial discharge degradation of PET films used in PV devices

A series of poly(ethylene terephthalate) (PET) films with different additives were subjected to identical electrical stresses to investigate their partial discharge (PD) degradation behaviour. Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) were used to study the surface chemistry changes of the degraded samples. The filled samples showed markedly better PD resistance and lifetime compared to the unfilled PET. A filler ‘pile-up’ effect has been directly observed for the filled samples and is proposed as the mechanism underlying the enhanced stability. PD-induced breakdown results also revealed that TiO2 filled PET has a superior PD lifetime to BaSO4 filled PET, which could be attributed to both the higher permittivity of the TiO2 fillers and the voids that are created around the BaSO4 particles during the film orientation process. Further improvements to PD resistance and PD lifetime, through the reduction of surface oxidation, are observed for a BaSO4-filled sample additionally containing the UV stabilizer Tinuvin 1577.

The effects of pressure and temperature on partial discharge degradation of silicone conformai coatings

The partial discharge damage rates for silicone-coated printed circuit boards have been quantified in a series of experiments at pressures and temperatures relevant to the aerospace industry (down to 116 mbar, −55°C to +70°C) and up to 6 kV. Surface cracking was observed, and damage magnitude was found to be non-linear with coating thickness, with thinner coatings experiencing relatively greater damage rates. This is attributed to higher surface electric fields for a given energisation voltage. Increasing temperature or reducing pressure increased the rate of damage. For coating thicknesses less than 100 μm, reducing pressure to 116 mbar (1 mbar = 100 Pa) increased the relative crack growth rate by nearly an order of magnitude. Temperature change had the most profound influence on damage; low temperatures were observed to substantially reduce damage rate, with very little or no damage observed, whereas higher temperatures substantially increased damage rate, with the resulting magnitude of surface damage too large to quantify. Silicone coatings of thickness greater than 250 μm showed no appreciable damage from partial discharge when aged at either low pressure or high temperature at voltages up to 6 kV. Corresponding damage-free surface electric fields are computed. No samples were observed to fail, indicating the robustness of high quality silicone coatings. Possible causes of crack formation in silicone are discussed.

Artificial Neural Network Application for Partial Discharge Recognition: Survey and Future Directions

In order to investigate how artificial neural networks (ANNs) have been applied for partial discharge (PD) pattern recognition, this paper reviews recent progress made on ANN development for PD classification by a literature survey. Contributions from several authors have been presented and discussed. High recognition rate has been recorded for several PD faults, but there are still many factors that hinder correct recognition of PD by the ANN, such as high-amplitude noise or wide spectral content typical from industrial environments, trial and error approaches in determining an optimum ANN, multiple PD sources acting simultaneously, lack of comprehensive and up to date databank of PD faults, and the appropriate selection of the characteristics that allow a correct recognition of the type of source which are currently being addressed by researchers. Several suggestions for improvement are proposed by the authors include: (1) determining the optimum weights in training the ANN; (2) using PD data captured over long stressing period in training the ANN; (3) ANN recognizing different PD degradation levels; (4) using the same resolution sizes of the PD patterns when training and testing the ANN with different PD dataset; (5) understanding the characteristics of multiple concurrent PD faults and effectively recognizing them; and (6) developing techniques in order to shorten the training time for the ANN as applied for PD recognition Finally, this paper critically assesses the suitability of ANNs for both online and offline PD detections outlining the advantages to the practitioners in the field. It is possible for the ANNs to determine the stage of degradation of the PD, thereby giving an indication of the seriousness of the fault.

OFケーブル油浸紙-絶縁油複合絶縁系の設計電界下における部分放電推移と劣化進展様相

OFケーブル油浸紙-絶縁油複合絶縁系の設計電界下における部分放電推移と劣化進展様相

PD characteristics and degradation in PET insulation with vented and unvented internal voids

Voids in solid insulation give rise to partial discharge (PD). Factors affecting PD activity in internal voids include the type of gas contained, gas pressure and surface properties of the void. Identifying the presence of PD and understanding the measurable parameters may allow improved management of industrial assets. This paper reports on quantifiable and measurable differences in PD activity and material degradation among voids which have vented and unvented channels. To investigate changes in PD activity with void characteristics experiments are carried out using artificial voids created from layered sections of poly(ethylene terephthalate) (PET) polymer. One set of samples has a confined internal void; a second set of samples has a void with a channel having a closed end and a third set of samples has a vented channel connecting the void to the outer atmosphere. The results show measurable differences in PD activity and subsequent material degradation in the various voids.

Comparison of partial discharge characteristics and degradation in several polymeric insulators

Polymeric insulating materials with excellent electrical properties are widely used in electrical power equipment. These materials are degraded, however, when they are subjected to partial discharge (PD) with different polymeric materials possessing different levels of PD resistance. In this study the PD characteristics and degradation caused by PD are analysed and compared for four different polymers – poly(ether sulphone) (PES), poly(ethylene terephthalate) (PET), poly(propylene) (PP) and low-density poly(ethylene) (LDPE). An artificial void is created in each of the polymer samples and electrically stressed above PD inception voltage (PDIV) for 4 h. The simulated electric field distribution inside the void for each of the polymer materials at the respective PDIVs is modelled using COMSOL to validate the experimental results. Little work has correlated void PD characteristics with material degradation induced by PD across a range of different polymers and subsequently categorised them according to their PD resistances. This study therefore presents a comparative study of the N–Q patterns of PD pulses (a plot of the number of discharges against the measure of apparent charge), PD repetition rate and the chemical and morphological changes occurring on the void surfaces for each type of polymer. The results show that the PD resistance of these four polymers can be categorised in a decreasing order according to the following sequence: LDPE, PP, PET and PES with LDPE being most resistant to PD and PES the least resistant.

Study on the characteristics of partial discharges in voids under square voltage by detecting light emission intensity

In order to quantitatively study partial discharge (PD) behavior in voids under square voltage, the feasibility of PD measurement by detection of light emission intensity was investigated at first. Experiments were carried out to study the influences of void size on the correlation between light emission intensity and PD magnitude and its transition with PD degradation. It is confirmed that a good correlation existed between light emission intensity and PD magnitude in a void with diameter larger than 3mm and the correlation did not change in the PD degradation process. Then, the effect of square voltage rise rate on PD characteristics under square voltage was investigated by detecting light emission intensity. In the PD degradation process, an abnormal phenomenon that the maximum PD magnitude decreased with the increase of voltage rise rate was observed. This was interpreted in terms of the change of surface condition due to PD degradation.

Partial discharge degradation of several biodegradable polymers

AbstractPartial discharge (PD) resistance was examined by applying a constant voltage to four different biodegradable polymers—poly‐L‐lactic acid (PLLA), polyethylene terephthalate succinate (PETS), poly ε‐caprolactone butylene succinate (PCL‐BS), and polybutylene succinate (PBS)—and the results were compared with those of low‐density polyethylene (LDPE) and crosslinked low‐density polyethylene (XLPE). The PD resistance is determined by the erosion depth and the surface roughness caused by PDs, and is ranked as LDPE≈XLPE>PLLA≈PETS>PBS>PCL−BS. This means that the sample with a lower permittivity has better PD resistance. Furthermore, examination of the sample surfaces by a polarization microscope and a laser confocal microscope reveals that the PD resistance of crystalline regions with spherulites is higher than that of amorphous regions. Therefore, good PD resistance can be achieved by the sample with a high crystallinity and a low permittivity. © 2009 Wiley Periodicals, Inc. Electr Eng Jpn, 168(2): 1–10, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20807

Effects of discharge area and surface conductivity on partial discharge behavior in voids under square voltages

The transitions of partial discharge (PD) behavior with aging in a void under ac voltages with sine wave and square wave were observed. The transition of the PD magnitude with aging under square wave showed the same tendency as that under sine wave. Moreover, the experimental results also showed some distinctive characteristics of PDs under square voltage (e.g. more than two pulses after the voltage rise time and their transition with aging). These phenomena were explained in terms of the effect of discharge area and the change in surface condition due to PD degradation