Showing papers on "Partial discharge published in 1998"

AC (60 Hz) and impulse breakdown strength of a colloidal fluid based on transformer oil and magnetite nanoparticles

Abstract: A new class of colloidal dielectric fluids was developed by modifying the mineral oil based ferrofluids to enhance their dielectric performance. At its optimum composition, the colloidal insulation has AC (60 Hz) breakdown strength close to that of the carrier oil, while its impulse voltage withstand in needle to sphere geometry is improved: for the needle positive, the impulse breakdown voltage increased up to 50% as compared to the dry and degassed mineral oil (Exxon Univolt 60) while for the needle negative the breakdown value remains close to that of the oil carrier, so that the two values are practically equal. The PD inception voltage (AC, 60 Hz, 500 V/sec rise, needle to sphere geometry) showed and increase of up to 30%. The new colloidal insulation showed little change in electrical resistivity and kinematic viscosity after accelerated aging at 185/spl deg/C.

On-line partial discharge detection in cables

Abstract: This paper describes an on-line PD (partial discharge) detection technique applicable to power cables. The method can be applied to both wire screen and solid shielded cables. In wire screen shielded cables, PD components with frequencies <100 MHz can be detected at distances over 150 m from the source, PD activity with levels as low as 20 pC can be detected within 100 m. However, in the case of solid shielded cables, PD levels of /spl ges/50 pC can be measured within 60 m in the frequency range <50 MHz. This method also can be utilized to check cable accessories such as splices and terminations. PD pulses with frequencies to /spl sim/300 MHz can be measured near the joints and terminations. The suitability and the sensitivity of the VHF technique were checked against two other alternative PD detecting techniques (pulse phase analyzer and high speed digital oscilloscope) by performing PD measurements in standard air gap and defects induced in EPR-insulated cable under laboratory-controlled conditions. The suitability and sensitivity of the VHF method were as good as the two alternative PD detection techniques.

Control and performance of a pulse-density-modulated series-resonant inverter for corona discharge processes

Abstract: This paper presents the control and performance of a pulse-density-modulated (PDM) series-resonant voltage-source inverter developed for corona discharge processes. The PDM inverter produces either a square-wave AC-voltage state or a zero-voltage state at its AC terminals to control the average output voltage under constant DC voltage and operating frequency. This results in a wide range of power control from 0.5% to 100%, even in the corona discharge load with a strong nonlinear characteristic. A 30 kHz 6 kW surface treatment system consisting of a voltage-source PDM inverter, a step-up transformer, and a corona discharge treater shows the establishment of a stable corona discharge in an extremely wide range of power control and, therefore, succeeds in performing both strong and weak surface treatment processes for film.

Stator winding monitoring

Abstract: Partial discharge tests can determine which motor and generator stator windings are experiencing insulation problems. A deteriorated winding has a PD activity which can be 30 times or more higher than a winding in good condition. This great difference in PD activity enables even nonspecialized maintenance personnel to identify the few motors or generators in a company which need further investigation and/or maintenance. There are several advantages to on-line partial discharge tests. Machines in good condition require less attention. The overall effect is lower maintenance costs. After implementing on-line partial discharge tests, as well as other monitoring such as flux probing or current harmonic analysis and improved temperature sensing, companies can often confidently extend the outage between major machine inspections. This saves on outage costs and reduces the risk of a stator fault due to human error during maintenance. Finally, on older machines, if there has been no increase in partial discharge activity over time, then the life of the stator winding can be confidently extended, saving a considerable capital expenditure.

Experimental study of atmospheric pressure glow discharge

Abstract: The atmospheric pressure glow discharge (APGD) is a new type of the discharge discovered by OKAZAKI et al. [1] recently. The APGD can be used in practical application (e.g. ozone generation), but the phenomena occurring in the discharge are not fully understood. We studied the APGD driven at 50 Hz, applied voltage up to 5 kV and burning in helium, argon and nitrogen. We used a planar configuration of discharge cell: metal electrode - dielectric plate (1 mm thick) - discharge gap (1.5 mm thick) - dielectric plate - metal electrode. Between the metal electrode and the dielectric plate a fine metal mesh was placed. For each gas we recorded the time dependence of power supply voltage and the time dependence of charge transferred through the discharge. We recorded also the discharges by the CCD camera with a video recorder. The same measurements were done in case of the discharge cells without the metal mesh. The metal mesh changes the discharge significantly. In the case of discharge cell with the mesh we can observe a homogeneous discharge and only one peak in the discharge current per half cycle of applied voltage while in the case of the discharge without mesh we observed individual micro-discharges and many pulses in the current per half cycle. This effect is most obvious in helium. In order to explain the observed phenomena we calculated the electric field in the discharge cell for APGD and propose a possible explanation of APGD. It has been found that the mesh does not influence the electric field in the discharge gap. Thus we propose that the mesh could influence the discharge by its electrical resistance, which is higher than the resistance of the metallic electrode.

Failure analysis of composite dielectric of power capacitors in distribution systems

Abstract: This paper describes the study of the reliability of capacitor units installed and operated in a distribution system during the period 1980 through 1990. Failures of capacitor units in distribution substations can be very costly to the supply of reliable power to consumers. To enhance utility reliability, failure analysis and rates, failure origin and physical damage causes were performed for these capacitor units. Two approaches, statistical and physical, were utilized in this study. In the statistical area, failure modes, reliability levels and failure causes are analyzed. The physical study mainly deals with the mechanism of deterioration of the composite dielectric, The results of the study on 2912 capacitor banks including 8736 capacitors installed at 153 distribution substations showed that the failure mode of capacitor units may be represented by a Weibull distribution and each capacitor manufactures has different failure rate. Analysis showed that partial discharge properties are a critical indicator for capacitor failure. Useful conclusions are presented both for power system operators and manufacturers. The methodology used in this study also applies to other equipment in the distribution system such as oil switches, transformers and insulators.

Statistical pattern analysis methods of partial discharge measurments in high voltage insulation

Abstract: Statistical methods of partial discharge analysis utilizing histogram similarity measures are provided by quality assessment and condition monitoring methods of evaluating high voltage electrical insulation. The quality assessment method is utilized to evaluate the quality of insulation within the electrical equipment. The condition monitoring method is utilized during normal operation of the equipment to identify degradation in the insulation and to predict catastrophic insulation failures.

Electromagnetic noise spectrum caused by partial discharge in air at high voltage substations

Abstract: The authors investigated the characteristics of the electromagnetic spectrum caused by partial discharges (PDs) in air at three different types of high-voltage substations. From the measured results, they characterized the electromagnetic noise spectrum depending on the type of substation. They also measured the long-term change of the average gain of the noise spectrum and discussed the influence of atmospheric conditions on the results. Moreover, they introduced the "equivalent charge (q/sub e/)" which was derived by converting the electric field strength estimated from the average gain into the charge magnitude of PD. They also proposed a "phase gate control method" for better understanding the electromagnetic noise spectrum characteristics and the mechanism causing electromagnetic waves from PDs.

Low-pressure hollow-cathode glow discharge plasma for broad beam gaseous ion source

Abstract: We have developed a hollow-cathode glow discharge plasma for a dc broad beam ion source. For a broad beam ion source, it is hard to obtain adequate pressure drop between the discharge space and the extraction region. The high-current low-voltage mode of discharge is limited to a pressure about 10−3 Torr, but for stable extraction of ions without breakdown the pressure needs to be at least one order of magnitude lower. To decrease the limited operation pressure for the high-current mode of the hollow-cathode glow, an external electron beam was used. These electrons were generated in a “keep-alive” discharge and are accelerated in the cathode layer of the primary discharge. In this way we successfully decreased the operation pressure to 10−4–3×10−5 Torr, as well as reaching a value of the discharge voltage as low as 80 V. The characteristics of this discharge system are presented, and the influence of external electrons on the discharge parameters is discussed.

An approach to the modelling of partial discharges in electrical trees

Abstract: A partial discharge model has been developed to simulate experimental partial discharge activity within electrical tree structures grown in polymeric based insulation. A pin plane electrode arrangement and tree were defined on a grid representing an area 2 mm by 2.1 mm. A sinusoidal 50 Hz voltage was applied to the pin tip and 3600 time steps per cycle were used to give a phase resolution of . Partial discharges were simulated at each time step by adding one or more dipoles of charge onto the tree structure to reduce the potential difference between adjacent points in the tree from to . Each dipole of charge represents a local electron avalanche occurring over a distance equal to the grid spacing. The model parameters, and , represent the discharge properties of the decomposition gas. Partial discharge current pulses in the external circuit and the induced image charge were calculated as a function of time. The spatial distribution of the emitted light in the tree structure over one cycle of the applied voltage was also calculated. The close agreement between the calculated and experimental data suggests that the underlying assumptions used in the construction of the partial discharge model are appropriate for the case of electrical trees grown in the flexible epoxy resin.

Aging diagnosis of insulation systems by PD measurements. Extraction of partial discharge features in electrical treeing

Abstract: The charge height values associated with PD measured during electrical tree inception and growth are processed according to the Weibull distribution. Test arrangements, based on the needle-plane electrode system, consider specimens having either a cavity at the tip or an intimate contact with insulation. It is shown that the Weibull-function parameters have close relation to discharges occurring in cavity and tree channels, and that an indication of inception of electrical trees can be obtained.

System and method for diagnosing and measuring partial discharge

Abstract: A method and system diagnose and measure partial discharge on-line in a power transmission system. A first wideband directional clamp-on detector detects pulses corresponding to partial discharge transmitted via a transmission line. A second wideband directional detector detects pulses corresponding to partial discharge output from at least one bushing tap of a high voltage device. The first and second pulses are nulled and then added and/or subtracted, and a diagnostic and measurement system analyzes the results to determine whether the high voltage device and/or an external source produces partial discharge, determine the type of discharge, and measure the level of discharge. For analysis of internal partial discharge, external partial discharges are rejected. The diagnostic and measurement system simultaneously analyzes multiple phases of the output of the high voltage device, while the device is energized. A method and system also diagnose and measure partial discharge off-line in a high voltage device. An output from at least one bushing tap of the high voltage device is split into AC and high frequency components which are synchrod in a diagnostic and measurement system. The diagnostic and measurement system simultaneously analyzes multiple phases of the output of the high voltage device.

On-site partial discharge measurements on premoulded cross-bonding joints of 170 kV XLPE and EPR cables

Abstract: A total number of 15 premoulded joints of three 170 kV cable circuits (XLPE and EPR) were equipped with partial discharge (PD) sensors. The developed sensors are based on inductive high frequency coupling of partial discharge pulses occurring in the joints. The PD activity of each joint was monitored during an after laying AC acceptance test. The test voltage of 190 kV was generated by means of a frequency-tuned series-resonance test set. The signals of the PD sensors were assessed with a spectrum analyser in a frequency range from 15 to 50 MHz, allowing a good separation of noise from PD signals under high interference levels. The investigation demonstrated the feasibility of on-site PD measurements on joints during after laying tests.

Partial discharge detector

Abstract: PROBLEM TO BE SOLVED: To reduce a cost for a partial discharge detector by actualizing detection on whether a failure exists or not by adding thereto a small amount of hardware. SOLUTION: This partial discharge detector is equipped with a sensor for detecting partial discharge to generate a current signal, a matching resistor for the sensor for converting the current signal into a voltage signal, first and second input resistors connected to a power supply for determining a reset input terminal voltage together with the matching resistor for the sensor, a dummy pulse generator for generating dummy pulses at fixed time intervals when the terminal voltage is a normal voltage lower than a prescribed voltage and interrupting the generation of the pulse voltage when the terminal voltage is an abnormal voltage higher than the prescribed voltage, an amplifier for amplifying a voltage signal owing to the dummy pulses from the pulse generator or owing to partial discharge, and a partial discharge determination/self-diagnosis circuit for performing determination on whether partial discharge has occurred or self-diagnosis by using a signal from the amplifier. COPYRIGHT: (C)2004,JPO

On-line detection of partial discharge in electrical power systems

Abstract: A method and apparatus for on-line detection of partial discharge events in an a.c. power system, in which high frequency electromagnetic pulses generated by partial discharge events are detected and analyzed in the frequency domain and the time domain to determine the type and location of the partial discharge event. The phase relationship between the partial discharge events and the on-line power signal is also examined to help indicate severity of the insulation anomaly giving rise to the partial discharge events.

Partial discharge behaviour of power electronic packaging insulation

Abstract: The partial discharge behaviour of the insulation between power electronic devices, e.g. IGBT's (Insulated Gate Bipolar Transistors), and its heat sink has been investigated. In conventional IGBT packages this insulation consists of a ceramic layer which is metallized on both sides embedded in silicon gel. Voids at the interface between ceramic and copper due to delamination as well as gas filled bubbles in silicon gel located at the border of metallisation were revealed as major sources of partial discharges. Especially discharges starting at the border of metallisation will deteriorate the insulation. Furthermore a significant decrease of the PD inception voltage with an increase of temperature could be observed.

Discharges from moving particles in GIS [gas-insulated switchgear]

Abstract: The study of the motion of moving metallic particles in GIS requires a good knowledge of the carried charge of the particle. In this paper, measurements of the charging process of a moving particle, performed in a realistic single-phase GIS, are presented. By measuring the charge impulses and the voltage phase at consecutive impacts, it is possible to estimate the carried charge and then compare the results with the existing models of the particle charge. Also, besides the discharges from in-flight corona reported by several authors, it is shown that, for higher voltage levels, several discharges between the particle and the enclosure (outer electrode) can occur as the particle impacts with the enclosure.

Metalclad switchgear assembly with partial discharge detection

Abstract: A metalclad switchgear assembly for high or medium voltage, having a cladding, includes at least one sensor arranged in an interior space of the cladding for detecting a condition variable within the cladding, the sensor having an antenna; and a transmitting and receiving antenna directed towards the interior space, the transmitting and receiving antenna serving for performing a wireless information interchange with the sensor, and for simultaneously detecting electromagnetically partial discharges in the interior space.

High bandwidth measurement of partial discharge current pulses

Abstract: This paper presents the first measurements of partial discharge current pulses captured using both a high analogue bandwidth and an extremely fast sampling rate. These pulses are of interest because when they occur in gas insulated substations they generate UHF signals that can be used to detect internal defects. A special test cell was designed to enable direct measurement of particle generated pulses under conditions similar to those present in high voltage gas insulated apparatus. The analogue bandwidth of the measurement system is in excess of 4.0 GHz and the single-shot sampling rate is 102.4/spl times/10/sup 9/ samples s/sup -1/. A signal processing scheme used to compensate the measured pulses is described. Results presented show individual current pulses from moving particles in SF/sub 6/. The measurements demonstrate that the pulse widths and risetimes are shorter than has previously been established.

Locating defects in high voltage transmission lines

Abstract: Methods of locating defects in overhead high voltage and extra-high voltage transmission lines are described and evaluated. Included is a technology which utilizes corona partial discharge detection to find faulty conditions while simultaneously performing routine visual inspections from a helicopter. The corona inspection system proved to be very useful in locating broken strands, loose spacers, insulators with internal or external discharges, and other faulty line conditions.

Mechanisms causing nonlinear dielectric response of water treed XLPE cables

Abstract: The nonlinear dielectric response of water tree degraded XLPE power cable insulation has been found to be an important additional diagnostic criterion. It has been experimentally shown that in case of long water trees the dielectric response increase more than proportional with increasing test voltage above a certain voltage level. In this paper we present possible explanations for the formation of nonlinear dielectric response based upon the mechanical damage theory of water treeing. In order to verify the proposed hypothesis, we have performed electric field computations, water content and microscopy treeing analysis.

Inference of PD in electrical insulation by charge-height probability distribution. Diagnosis of insulation system degradation

Abstract: This paper deals with application of stochastic procedures for the analysis of PDHD (partial discharge height distributions) detected during life of insulation systems. Experiments of accelerated aging under electrical stress were carried out on both an artificial flat cavity and a 23 kV epoxy-resin insulator, where PD are already active at the beginning of aging. A correlation between aging time and PDHD features is observed during long-term voltage application, until breakdown, on both objects. In particular, the PDHD analysis is approached by means of the Weibull function. It is shown that the time evolution of shape and scale parameters can provide diagnostic indications on aging amount, incoming failure and thus on the reliability of insulation systems.

Diagnosis of medium voltage XLPE cables by high voltage dielectric spectroscopy

Abstract: This paper summarize results from work on diagnosis of medium voltage (MV) XLPE cables with respect to water treeing performed at KTH, Stockholm The work, performed both in laboratory and in the field, has shown that high voltage dielectric spectroscopy is a good tool for diagnosis of MV XLPE cables and that it is able to differentiate between different stages of water tree ageing

A simulation model for propagation of partial discharge pulses in transformers

Abstract: Partial discharge (PD) measurement is widely accepted as a quality control test for power transformers due to its ability to detect incipient insulation faults. For further development of PD testing as a diagnostic tool, it is important to study the propagation of PD pulses in transformers. A general simulation model is necessary which should include the different construction of transformer windings. In this paper, a simulation model is introduced which is constructed in the frequency domain and which takes account of the frequency dependent characteristics of transformer windings. A description of parameter calculation methods and system solutions is given. Some simulation results are presented. The promising application of this model in the simulation study of PD's in power transformers is outlined.

Practical experiences of on-line partial discharge measurements on a variety of medium voltage electrical equipment

Abstract: There has been a considerable amount of discussion concerning partial discharge (PD) measurements on medium voltage motors and generators. If one considers the total plant electrical distribution system and production equipment, motors comprise only a small part of the plant electrical equipment. If a motor should fail, part of production will likely be shut down and there will be lost revenue. However, there is a wide variety of other equipment in a facility besides a motor that can fail and can have a much more devastating effect on the plant's production. Additional items to consider for partial discharge monitoring are the plant incoming transformers (they may be owned by the plant if the plant is purchasing primary power), the incoming switchgear, cables, bus duct, other remote substation switchgear and power center transformers. Methods have been developed for monitoring partial discharges in all types of medium voltage equipment. This paper discusses application of sensors, methods of measurement and data interpretation and presents several case studies.

Study of propagation phenomena of partial discharge pulses in gas insulated substation

Abstract: This paper describes the results of tests and discussions carried out to develop a system for monitoring partial discharges generated in gas-insulated substations (GIS). They include the propagation characteristics and the mechanisms of partial discharge pulses in the GIS, the results of computer simulation, and the results of comparisons with previous tests to verify the partial discharge locating method.