Showing papers on "Partial discharge published in 2016"

Partial discharge phenomena within an artificial void in cable insulation geometry: experimental validation and simulation

Abstract: In the presence of void in cable insulation, repetition of partial discharge (PD) occurrences is one of the main sources of insulation degradation, which may lead to complete breakdown. Therefore, it is important to monitor the condition of cable insulation through PD measurement. Replicating PD measurement on a test object with the presence of a void within cable insulation may help a better understanding of PD characteristics within cable insulation to be achieved. Therefore, in this work, test samples of cable insulation geometry containing an artificial void were prepared in the laboratory for PD experiment. The PD measurements were done as a function of void size and applied voltage. A physical model of PD activities within an artificial void in cable insulation geometry was also designed using finite element analysis method. The model was developed by considering PD occurrences within non-uniform electric field distribution in the void and charge movement along the void surface. The model was applied for simulation of PDs within the void in cable insulation geometry to increase the understanding on PD physical phenomena. This includes the impact of distribution of void surface charge on the distribution of the electric field within the void through comparison with the captured measurement results.

A clustering technique for partial discharge and noise sources identification in power cables by means of waveform parameters

Abstract: On-line partial discharge (PD) measurements have become a common technique for assessing the insulation condition of installed high voltage (HV) insulated cables. When on-line tests are performed in noisy environments, or when more than one source of pulse-shaped signals are present in a cable system, it is difficult to perform accurate diagnoses. In these cases, an adequate selection of the non-conventional measuring technique and the implementation of effective signal processing tools are essential for a correct evaluation of the insulation degradation. Once a specific noise rejection filter is applied, many signals can be identified as potential PD pulses, therefore, a classification tool to discriminate the PD sources involved is required. This paper proposes an efficient method for the classification of PD signals and pulse-type noise interferences measured in power cables with HFCT sensors. By using a signal feature generation algorithm, representative parameters associated to the waveform of each pulse acquired are calculated so that they can be separated in different clusters. The efficiency of the clustering technique proposed is demonstrated through an example with three different PD sources and several pulse-shaped interferences measured simultaneously in a cable system with a high frequency current transformer (HFCT).

Partial Discharge Monitoring in Power Transformers Using Low-Cost Piezoelectric Sensors

Abstract: Power transformers are crucial in an electric power system. Failures in transformers can affect the quality and cause interruptions in the power supply. Partial discharges are a phenomenon that can cause failures in the transformers if not properly monitored. Typically, the monitoring requires high-cost corrective maintenance or even interruptions of the power system. Therefore, the development of online non-invasive monitoring systems to detect partial discharges in power transformers has great relevance since it can reduce significant maintenance costs. Although commercial acoustic emission sensors have been used to monitor partial discharges in power transformers, they still represent a significant cost. In order to overcome this drawback, this paper presents a study of the feasibility of low-cost piezoelectric sensors to identify partial discharges in mineral insulating oil of power transformers. The analysis of the feasibility of the proposed low-cost sensor is performed by its comparison with a commercial acoustic emission sensor commonly used to detect partial discharges. The comparison between the responses in the time and frequency domain of both sensors was carried out and the experimental results indicate that the proposed piezoelectric sensors have great potential in the detection of acoustic waves generated by partial discharges in insulation oil, contributing for the popularization of this noninvasive technique.

A Novel Bias Detection Technique for Partial Discharge Localization in Oil Insulation System

Abstract: This paper presents a sequential fault detection and identification algorithm for detecting a bias fault in the measurements of partial discharge (PD) in a transformer insulation system using acoustic signals. The algorithm identifies a bias fault in any of the ultrasonic sensors by computing the probability of having that bias fault given a carefully constructed measurement residual. The constructed measurement residual is a function of the measurement noise and the possible measurement fault. A set of bias hypotheses is assumed and initially given equal alarm probability. It is assumed that only one sensor will acquire a bias at any given time. Once the probability of a hypothesis approaches 1, that hypothesis is declared as the correct hypothesis and the bias associated with the hypothesis is removed from the sensors’ reading. Therefore, this will enable high-accuracy estimation of the location of PD. First, the algorithm is verified in a simulation environment. Subsequently, the accuracy of the proposed algorithm is verified using the experimental results.

Creepage discharge of oil-pressboard Insulation in AC-DC composite field: phenomenon and characteristics

Abstract: Converter transformers use oil-pressboard as its main dielectric medium and are operated mainly in AC-DC composite electric field. Creepage discharge is one of the common defects for oil-pressboard insulation, especially in converter transformers. Oil-pressboard insulation shows distinctive flashover behaviors and surface electric strengths in AC-DC composite electric field from that in pure AC or DC electric field. Compared to conventional power transformers, there remain bigger challenges to realize insulation assessment and fault diagnosis for the converter transformers. Adequate knowledge of the phenomenon and characteristics of oil-pressboard insulation creepage discharge is critical for fault diagnosis of converter transformers. This paper attempts to fill the existing research gap by studying the dynamic process and phenomenon of creepage discharge in the composite field so as to provide criteria for insulation diagnosis and assessment. With the established experimental platform, the present paper observed the phenomenon of oil-pressboard insulation creepage discharge from its initiation till final flashover under composite AC-DC voltages, identified the trends of partial discharge and dissolved gas throughout the entire process, and summarized accordingly the criteria for insulation diagnosis and assessment. Results indicate that 1) for creepage discharge of oil-pressboard insulation under AC-DC superimposed voltages, abrupt changes were observed in terms of discharge repetition rate and magnitude. Discharge phases kept enlarging during the whole discharge evolution process; 2) based on the observed discharge characteristics, the evolution process of creepage discharge could be classified into three stages which imply three severity levels of the discharge; 3) compared to that in AC electric field, the creepage discharge in AC-DC composite field witnessed much less amount of and different composition of dissolved gas in oil. More C2H6 and C2H2 were observed in the AC-DC composite electric field whereas CH4 and C2H2 dominated the AC electric field. The research results indicate that as much as both PD-identification and DGA methods are recognized as effective means of fault diagnosis for converter transformers, the referential assessment criteria should be enriched and adjusted for diagnosis of creep discharge under AC-DC composite voltages, taking into full consideration of the distinctive characteristics of both discharge and gases dissolved in oil.

Partial discharge detection in medium voltage and high voltage cables: maximum distance for detection, length of cable, and some answers

Abstract: Partial discharge (PD) measurement is becoming a common procedure in the commissioning and off-line diagnostic testing of cable systems. On-line PD cable monitoring is beginning to show growing interest as well. Arising from this is an important question being asked by testing companies, maintenance personnel, and asset managers: "What are the maximum lengths of high voltage (HV) and medium voltage (MV) cables that can be scanned with sufficient sensitivity for PD activity by present day PD detectors?" In other words, what is the maximum distance a PD measurement point can be from the PD source and still detect the PD. The answer can be complex, or not satisfactorily understood, being related to the measurement sensitivity, voltage level, type of insulation, and cable construction. Detection also depends on the bandwidth of the measurement circuit. The signal-to-noise ratio must be large enough to enable PD detection at a level compatible with the reliability of the cable, that is, terminating maintenance or commissioning diagnostic tests before cable breakdown. Indeed, the capability of detecting pulses with an apparent charge below 5 pC is a common requirement for HV polymeric cable systems [1], whereas a much lower sensitivity, i.e., 5 nC, for mass-impregnated cables is generally accepted. The meaning of such limits in cable systems is doubtful as PD measurements are most often done from the cable termination because localized sensors are often unavailable, and since not in close proximity to the source, a lumped-parameter network representing the cable network for picocoulomb calibration is invalid. In a cable system PD pulses travel on the conductor and metal sheath, and the cable is viewed as a distributed parameter network; thus, the PD quantity measured is typically volts or millivolts. As the PD pulses travel along the cable, they lose frequency content as a consequence of skin effect or ohmic loss at high frequencies, and semicon and insulation dielectric losses [2], [3].

An overview on the current status of partial discharge measurements on AC high voltage cable accessories

Abstract: High voltage power cables and their accessories are vital components of the electrical power network. Because of this, reliable operation over their lifetime is essential to minimize failures during service. As in-service failures most often incur very high costs, condition monitoring of high voltage cable accessories is becoming increasingly of interest and is being applied. Monitoring of partial discharge (PD) is currently the most common method implemented. This article discusses the fundamental aspects of PD and its measurement on high voltage power cable accessories. The article begins with discussing the various PD measurement techniques, which are explained and compared. Finally, the results of typical measurements on high voltage cable accessories are shown, and their consequences in operation are discussed.

Experience with on-line partial discharge measurement in high voltage inverter fed motors

Abstract: Partial discharge (PD) testing has long been an important tool for assessing the condition of the high voltage insulation in motor and generator stator windings. In the past several years, many motors have been powered from inverters which facilitate variable speed motor operation. The most common drive used today is the voltage-source, pulse width modulation (VS-PWM) type. VS-PWM drives rated up to 13.8 kV are becoming more common in natural gas processing plants, as well as in other petrochemical facilities. Such drives generate high voltage impulses in the kV range with risetimes in the sub-microsecond range. These impulses are a form of severe electrical interference that can make the on-line detection of partial discharge (with magnitudes 1000 times smaller) difficult due to the overlapping frequency content in PD and in the impulses. Thus, PD detection on medium voltage VS-PWM systems has been a challenge in spite of the serious stator winding insulation aging that such drives may cause to these motors. This paper discusses the stator winding failure mechanisms which produce PD, including the insulation problems that VS-PWM drives can accelerate. A research project that lasted several years is reviewed. It culminated in a prototype on-line PD monitoring system suitable for motors fed by VS-PWM drives.

Multiple partial discharge source discrimination with multiclass support vector machines

Abstract: Different types of partial discharges are created with test objects in laboratory.Their frequency content depends on the type of discharge and other external factors.An SVM extracts characteristics from the power spectral density of the pulses.Noise, corona, internal and surface discharges have different characteristics.The differences are used to classify discharges and separate them from noise. The costs of decommissioning high-voltage equipment due to insulation breakdown are associated to the substitution of the asset and to the interruption of service. They can reach millions of dollars in new equipment purchases, fines and civil lawsuits, aggravated by the negative perception of the grid utility. Thus, condition based maintenance techniques are widely applied to have information about the status of the machine or power cable readily available. Partial discharge (PD) measurements are an important tool in the diagnosis of power systems equipment. The presence of PD can accelerate the local degradation of insulation systems and generate premature failures. Conventionally, PD classification is carried out using the phase resolved partial discharge (PRPD) pattern of pulses. The PRPD is a two dimensional representation of pulses that enables visual inspection but lacks discriminative power in common scenarios found in industrial environments, such as many simultaneous PD sources and low magnitude events that can be hidden below noise. The literature shows several works that complement PRPD with machine learning detectors (neural networks and support vector machines) and with more sophisticated signal representations, like statistics captured in several modalities, wavelets and other transforms, etc. These methods improve the classification accuracy but obscure the interpretation of the results. In this paper, the use of a support vector machine (SVM) operating on the power spectrum density of signals is proposed to identify different pulses what could be used in an online tool in the maintenance decision-making of the utility. Particularly, the approach is based on an SVM endowed with a special kernel that operates in the frequency domain. The SVM is previously trained with pulses of different PD types (internal, surface and corona) and noise that are obtained with several test objects in the laboratory. The experimental results demonstrate that this technique is highly effective in identifying PD for cases where several sources are active or when the noise level is high. Thus, the early identification of critical events with this approach during normal operation of the equipment will help in the decision of decommissioning the asset with reduced costs and low impact to the grid reliability.

A Fiber-Optic Sensor for Acoustic Emission Detection in a High Voltage Cable System.

Abstract: We have proposed and demonstrated a Michelson interferometer-based fiber sensor for detecting acoustic emission generated from the partial discharge (PD) of the accessories of a high-voltage cable system. The developed sensor head is integrated with a compact and relatively high sensitivity cylindrical elastomer. Such a sensor has a broadband frequency response and a relatively high sensitivity in a harsh environment under a high-voltage electric field. The design and fabrication of the sensor head integrated with the cylindrical elastomer is described, and a series of experiments was conducted to evaluate the sensing performance. The experimental results demonstrate that the sensitivity of our developed sensor for acoustic detection of partial discharges is 1.7 rad / ( m ⋅ Pa ) . A high frequency response up to 150 kHz is achieved. Moreover, the relatively high sensitivity for the detection of PD is verified in both the laboratory environment and gas insulated switchgear. The obtained results show the great potential application of a Michelson interferometer-based fiber sensor integrated with a cylindrical elastomer for in-situ monitoring high-voltage cable accessories for safety work.

Moisture Migration in an Oil-Paper Insulation System in Relation to Online Partial Discharge Monitoring of Power Transformers

Abstract: Most power transformers operating in a power system possess oil-paper insulation. A serious defect of this type of insulation, which is associated with long operation time, is an increase in the moisture content. Moisture introduces a number of threats to proper operation of the transformer, e.g., ignition of partial discharges (PDs). Due to the varying temperature of the insulation system during the unit’s normal operation, a dynamic change (migration of water) takes place, precipitating the oil-paper system from a state of hydrodynamic equilibrium. This causes the PDs to be variable in time, and they may intensify or extinguish. Studies on model objects have been conducted to determine the conditions (temperature, humidity, time) that will have an impact on the ignition and intensity of the observed phenomenon of PDs. The conclusions of this study will have a practical application in the evaluation of measurements conducted in the field, especially in relation to the registration of an online PD monitoring system.

Statistical Analysis of Partial Discharges in SF6 Gas via Optical Detection in Various Spectral Ranges

Abstract: Partial discharge (PD) detection is essential to the operation of high-voltage systems. In this context, we investigate the basic characteristics of light emission during PDs in SF6 gas from the perspective of insulation diagnosis. A synchronous system is constructed using three optical photoelectric instruments with separate wavelength responses in the ultraviolet (UV, 189–352 nm), visible (VIS, 381–675 nm), and near-infrared (NIR, 737–920 nm) spectral ranges and a wide-band PD current pulse detector with a response of 1 pC. The results indicate that light emission depends upon the type of insulation defect and discharge energy. An increase in PD charge gives rise to more components in the spectral range from UV to VIS, and the presence of an insulator surface in discharges yields a more complex VIS-to-NIR spectrum. The phase-resolved partial discharge pattern (PRPD) of UV light pulses can reasonably reflect the electroluminescence process in the presence of the insulator surface and weak corona at negative voltage points. The PRPD of VIS light describes the features of the actual PD pattern in most cases. In comparison with the other two spectral ranges, light intensity in the VIS range is more sensitive to changes in gas-pressure-normalized voltage (Vrms/p). The linear fitting analysis of the relationships between the light intensity and PD charge shows that UV light detection has a greater sensitivity to the PD charge and that UV detection exhibits a greater degree of linearity. NIR detection is applicable only to severe PDs. We believe that our findings can significantly aid in application of optical PD diagnosis in SF6 gas insulated systems.

Needle-array to Plate DBD Plasma Using Sine AC and Nanosecond Pulse Excitations for Purpose of Improving Indoor Air Quality.

Abstract: In this study, needle-array to plate electrode configuration was employed to generate an atmospheric air diffuse discharge using both nanosecond pulse and sine AC voltage as excitation voltage for the purpose of improving indoor air quality. Different types of voltage sources and electrode configurations are employed to optimize electrical field distribution and improve discharge stability. Discharge images, electrical characteristics, optical emission spectra, and plasma gas temperatures in both sine AC discharge and nanosecond pulse discharge were compared and the discharge stability during long operating time were discussed. Compared with the discharge excited by sine AC voltage, the nanosecond pulsed discharge is more homogenous and stable, besides, the plasma gas temperature of nanosecond pulse discharge is much lower. Using packed-bed structure, where γ- Al2O3 pellets are filled in the electrode gap, has obvious efficacy in the production of homogenous discharge. Furthermore, both sine AC discharge and nanosecond pulse discharge were used for removing formaldehyde from flowing air. It shows that nanosecond pulse discharge has a significant advantage in energy cost. And the main physiochemical processes for the generation of active species and the degradation of formaldehyde were discussed.

A Wireless and Passive Online Temperature Monitoring System for GIS Based on Surface-Acoustic-Wave Sensor

Abstract: A novel temperature monitoring system for gas-insulated switchgears (GISs) based on surface-acoustic-wave (SAW) sensing is proposed in this paper. Compared with the existing systems, this system has several unique advantages, such as being wireless, passive, and at a proper cost with continuous monitoring. First, an accurate SAW temperature sensor is designed for the GIS conductor, where epoxy encapsulating and loop antennas are adopted in the SAW sensor to prohibit the potential partial discharge of the sensor itself. After that, a multifunctional reader antenna is developed, which can measure the conductor temperature and partial discharge by the time-division multiplexing technique. Experiments are carried out to examine the accuracy and response speed of the SAW sensors, and the results indicate that the uncertainty of the contact temperature measurement is smaller than 10 $^{\circ}$ C. Moreover, the insulation level and anti-interference ability are demonstrated based on experiments carried out on a 126-kV GIS. At last, temperature monitoring performance of the developed system is investigated by changing the current in the 126-kV GIS conductor. The experiment results prove that the developed system can meet the requirement of temperature monitoring in GIS.

Optimization of a High-Frequency Current Transformer Sensor for Partial Discharge Detection Using Finite-Element Analysis

Abstract: High-frequency current transformer (HFCT) sensors are widely used for partial discharge detection due to their versatility, high sensitivity, and wide bandwidth. This paper reports on a finite-element analysis (FEA) methodology that can be employed to optimize HFCT performance. The FEA model consists of accurate 3D representations of the sensor components. Two different FEA software modules were used in order to cover the wide operating frequency range of the sensor. The simulation computes the frequency response of the sensor in the range 0.3–50 MHz for various HFCT geometric and material parameters, specifically the number of winding turns, spacer thickness, aperture size, and core material. A prototype HFCT was constructed and the measured response compared with that of the simulation. The shapes of the responses were similar, with the simulated sensitivity being higher than the measured sensitivity by 1 dB on average. The measured low-frequency cutoff of the sensor was found to be only 0.05 MHz lower than that of the simulation.

Partial discharge signals separation using cumulative energy function and mathematical morphology gradient

Abstract: Partial discharge (PD) measurement and interpretation have become a powerful tool for condition monitoring and failure risk assessment of high voltage power equipment insulation. The occurrence of multiple discharge sources affects interpretation accuracy. This paper presents a PD signal separation algorithm using cumulative energy (CE) function parameters clustering technique. The waveform of PD signals are acquired by digital detection instruments with high sampling rate. Cumulative energy functions in time domain (TCE) and frequency domain (FCE) are calculated from PD waveforms and their FFT spectrums, respectively. Mathematical morphology gradient (MMG) operation is applied to the TCE and FCE to describe their variation characteristics. The feature parameters including width, sharpness and gravity are extracted from CEs and MMGs in both time and frequency domain, and compose a six-dimension feature space. The improved density-based spatial clustering of applications with noise (IDBSCAN) clustering algorithm is adopted to discover clusters in the feature space. The proposed separation algorithm is examined with mixed current impulse signals acquired from PD experiments on artificial multi-defect models and an on-site transformer. The separation results indicate that the proposed algorithm is effective for separating mixed PD signals initiated from multiple sources.

The dynamic character of partial discharge in epoxy resin at different stages of treeing

Abstract: Simultaneous imaging of electrical tree growth and partial discharge measurement have allowed 5 stages of tree growth to be identified. Partial discharge (PD) characteristics have been collected from tree growth in needle-plane epoxy resin samples under 15 kV peak AC voltage, and have been shown to behave differently at different stages of tree growth. Initial slow growth is accompanied by large discharges (about 10 pC). Thereafter, growth of fine channels is associated with smaller PDs (about 0.3 pC). It was observed that the tree propagation is further driven by even smaller PDs below the equipment sensitivity. However, when the tree approaches the counter electrode, the PD magnitude increases again and a more substantial tree grows with a low propagation rate. Reverse tree channels then occur from the planar electrode towards the needle, together with higher magnitude PDs characterized by thick and dark branches before breakdown.

A Review on Nanocomposite Based Electrical Insulations

Abstract: The potential of nanocomposites have been drawing the intention of the researchers from energy storage to electrical insulation applications. Nanocomposites are known to improve dielectric properties, such as the increase in dielectric breakdown strength, suppressing the partial discharge (PD) as well as space charge, and prolonging the treeing, etc. In this review, different theories have been established to explain the reactions at the interaction zone of polymer matrix and nanofiller; the characterization methods of nanocomposites are also presented. Furthermore, the remarkable findings in the fields of epoxy, cross-linked polyethylene (XLPE), polypropylene and polyvinyl chloride (PVC) nanocomposites are reviewed. In this study, it was observed that there is lack of comparison between results of lab scale specimens and actual field aged cables. Also, non-standardization of the preparation methods and processing parameters lead to changes in the polymer structure and its surface degradation. However, on the positive side, recent attempt of 250 kV XLPE nanocomposite HVDC cables in service may deliver a promising performance in the coming years. Moreover, materials such as self-healing polymer nanocomposites may emerge as substitutes to traditional insulations.

Toward High-Accuracy Estimation of Partial Discharge Location

Abstract: In this paper, high-accuracy estimation of partial discharge (PD) location in an oil insulation system is addressed. The study aims at forming a fault-tolerant PD localization system. Initially, the algorithm periodically and probabilistically checks for possible bias in the acoustic emission sensors’ measurements. Once the detected bias is removed from the sensors’ measurements, multiple-model extended Kalman filters are used to estimate the location of the PD. The proposed multiple-model approach is intended to compensate for increased measurement noise statistics that result from sensor aging or ambient noise. The accuracy of the proposed algorithm is verified using multiple experimental results when the PD is initiated in different locations within the oil-filled transformer tank under different working conditions.

Influence of pressure on partial discharge spectra

Abstract: In the near future, the aircrafts will become more compact and lighter, since hydraulic, pneumatic or mechanical systems will be replaced by electrical ones. The network voltage is expected to increase far above Paschen's minimum, thus increasing the risk of partial discharges. These discharges, also known as silent enemies, may affect the reliability of the systems. Moreover, some electrical equipments are located in unpressurized areas so their operating conditions (temperature, pressure) may change in a large range. These environmental parameters (and their cycling) have a large impact on discharge inception voltage and may even modify its nature. This paper deals with the influence of the pressure on partial discharges and more particularly on the modification of partial discharges spectra with pressure. Twisted pairs of enamel wires were subjected to repetitive square voltage waveform and sinusoidal voltage at various pressures.

Challenges in calibration of the measurement of partial discharges at ultrahigh frequencies in power transformers

Abstract: The detection and measurement of partial discharge (PD) is a well-known method for evaluating the insulation performance of power transformers. According to the IEC 60270 standard, the apparent charge is measured in a frequency range below 1 MHz using an electrical circuit which quantifies only a portion of the discharge energy conducted to the PD detector through the conductors. However, it is also well known that the PD energy is not only conducted but also transmitted via electromagnetic (EM) and optic waves, acoustic waves, and thermal energy.

Design considerations for higher electrical power system voltages in aerospace vehicles

Abstract: Electrical and electronic equipment used in aerospace applications must be designed to operate over a wide range of environmental conditions that include variations in pressure, temperature, and humidity. Electrical power systems for advanced aircraft utilize voltages well above the traditional levels of 12 to 42 VDC and 115/200 VAC, 400 Hz. Current airborne systems can contain 270 VDC, and bipolar systems with a 540 V differential are appearing in certain applications. Higher potentials create increased probability of arcing and flashover compared to the risks associated with traditional ac or low-voltage dc. The low pressures of high altitude environments only serve only to worsen such concerns. This paper summarizes the development of a guideline document containing methods of managing higher voltages in aerospace vehicles. Based upon both current and archival work, the design guidance (1) provides a basis for identifying high voltage design risks, (2) defines areas of concern as a function of environment, and (3) illustrates potential risk mitigation methods and test and evaluation techniques. The document is focused on electrical discharge mechanisms including partial discharge and does not address personnel safety. Some of the key areas of concern are power conversion devices, electrical machines, connectors and cabling/wiring, as well as interactions between components and subsystems. The document is intended for application to high voltage systems used in aerospace vehicles operating to a maximum altitude of 30,000 m. (approximately 100,000 ft.), and maximum operating voltages of below 1500 Vrms. Fundamental issues addressing some of the key areas will be presented and discussed.

Impact of velocity on partial discharge characteristics of moving metal particles in transformer oil using UHF technique

Abstract: Metal particle contamination can cause partial discharge (PD) and seriously degrade the withstand voltage of transformer oil. The moving metal particles, which display PD characteristics that differ from those of free particles, perform complex movements within the transformer oil duct under the force of a submerged pump. This study analyzed the Φ-U-N diagram and measured the inception voltage, number, and amplitude of PD under different supply voltages. The influence of flow velocity on the characteristic parameters of PD was examined. The forces of charged metal particles were analyzed, and the trajectories were simulated in an oil duct under different flow velocities. As the supply voltage was increased, the number and amplitude of PD increased, whereas the shape of the Φ-U-N diagram remained the same. As the flow velocity was increased, the inception voltage of PD increased, but the number and amplitude of PD decreased. The ΦU-N diagram demonstrated slight changes under different flow velocities. Overall, an increase in flow velocity reduces PD possibly because of particle oscillation within the system.

PD characteristics of oil-pressboard insulation under AC and DC mixed voltage

Abstract: The insulation problem of converter transformers has become one of the core issues in HVDC system, which is receiving more and more attention. A converter transformer has complex structures and requires high insulation level. The voltage waveform which the converter transformer insulation withstands is different from AC transformers. In the valve side of the converter transformer, the insulation endures a mixed voltage of AC voltage, DC voltage, harmonics and the pulsating component etc. Partial discharge (PD) occurring in oil-pressboard insulation is the one of the main causes of insulation breakdown, and it is the main factor of electrical aging of materials. Even though PD characteristics under pure AC or DC voltage waveform have been studied by many researchers, little research has been done with the mixed voltage. In this paper, the AC + DC mixed voltage experiment system is set up to study the PD characteristics under mixed voltage waveforms. The study results indicate that the pressboard withstands most of the DC component and void withstands most of the AC component when applied mixed voltage. For PD inception voltage, it is decided by both the AC component and DC component. For PD development, AC component has more significant influence than DC component. However, the DC component is found to promote PD activity under AC half cycle with the same DC polarity and inhibit PD activity under AC half cycle with the opposite DC polarity. The results in this paper provide an experimental foundation for the converter transformer insulation breakdown mechanism and analysis.

Study on Descriptors of Acoustic Emission Signals Generated by Partial Discharges under Laboratory Conditions and in On-Site Electrical Power Transformer

Abstract: An acoustic emission method (AE) is widespread and often applied for partial discharge (PD) diagnostics, mainly due to its ease of application as well as noninvasiveness and relatively high sensitivity. This paper presents comparative analysis of AE signals measurement results archived under laboratory conditions as well as on-site actual AE signals generated by inside PDs in electrical power transformer during its normal service. Three different PD model sources are applied for laboratory research: point to point, multipoint to plate and surface type. A typical measuring set up commonly used for on-site transformer PD diagnostics is provided for the laboratory tasks: piezoelectric joint transducer, preamplifier, amplifier and measuring PC interface. During the on-site research there are three measuring tracks applied simultaneously. Time domain, time-frequency domain and statistical tools are used for registered AE signals analysis. A number of descriptors are proposed as a result of the analysis. In the paper, attempt of AE signals descriptors, archived under laboratory condition application possibilities for on-site PD diagnostics of power transformers during normal service is made.

Finite-difference time-domain simulation of partial discharges in a gas insulated switchgear

Abstract: It is important to estimate the current magnitude and charge amount of a partial discharge (PD) in a gas-insulated switchgear (GIS). In this study, electromagnetic fields generated by a PD in an 84 kV-class three-phase GIS for testing have been measured with a voltage probe installed on the outer surface of an insulation spacer of the GIS tank, and the corresponding simulations have been carried out using the three-dimensional finite-difference time-domain (FDTD) method. FDTD-computed time-domain voltages across spacers agree reasonably well with the corresponding measured ones, and the current magnitude of PD and its charge amount have been estimated.

Study on micro bridge impurities in oil-paper insulation at DC voltage: their generation, growth and interaction with partial discharge

Abstract: To investigate the characteristics of micro bridges and their interactions with partial discharge (PD) in oil-paper insulation under DC voltage, an experimental platform with two-copper-electrode models is established. Generation and growth of micro bridge impurities are observed. PDs in experimental process are detected and analyzed. The result shows that the generation and growth of micro bridges have three stages based on their quantity and shape. At first stage PD accelerates the generation of micro bridges. At second stage the grown bridges trigger stronger discharges. At the last stage a so-called "hazardous bridge" that triggers highly intensive PDs emerges, through which breakdown happens. Therefore, bridges and PDs promote each other to cause insulation failure. In addition, impurities at the second stage are observed to transform into pile-like, bridge-like or particle-like shapes. The transformation process is considered to relate with the change of composition force on fiber bundles. Besides, a "cluster discharging" phenomenon is observed at the second stage, which is associated with the generation and disappearance of micro bridge impurities.

An UWB Printed Antenna for Partial Discharge UHF Detection in High Voltage Switchgears

Abstract: As important fundamental equipment, high voltage switchgears are widely used in electric power systems and directly relative to the power reliability and quality. Partial discharge (PD) online monitoring is one of the most effective methods used for insulation testing and diagnosis in high voltage switchgears and power systems. This paper proposes a unique ultra-wide-band (UWB) antenna with high performance for PD ultra-high-frequency (UHF) detection in high voltage switchgears. Actual PD experiments were carried out, and the designed antenna was used for PD measurements. The measured results demonstrate that the proposed antenna has wide work frequency band, good omnidirectional radiation patterns and appreciable gain, which indicate that the proposed antenna is suitable for UHF online monitoring of PDs in high voltage switchgears.