Showing papers on "Partial discharge published in 1999"

Effective methods of assessment of insulation system conditions in power transformers: a view based on practical experience

Abstract: Condition-based monitoring of power transformer insulation should center on the prediction of a substantial drop in the dielectric safety margin under the impact of moisture, oil by-products, contaminating particles, paper insulation aging and partial discharge activity. A functional failure model of power transformer insulation and possible effective methods of the insulation condition assessment are discussed based on practical experience.

Localisation of electrical-insulation- and partial-discharge failures of IGBT modules

Abstract: The partial discharge (PD) and insulation resistance is very important in view of the increasing operating voltages of insulated gate bipolar transistor modules. PD spectroscopy showed that the PDs from metallization edges and interfaces in silicone gel were the main sources of PD at high voltages. It also allows these types of PDs to be clearly distinguished. As the PDs from interfaces in silicone gel increase strongly at high voltages, it is especially important for the silicone gel to adhere well to the ceramic.

Partial discharge theory and applications to electrical systems

Abstract: Partial discharge monitoring is an effective on-line predictive maintenance test for motors and generators at 4160 volt and above, as well as other electrical distribution equipment. The benefits of online testing allow for equipment analysis and diagnostics during normal production. Corrective actions can be planned and implemented, resulting in reduced unscheduled downtime. An understanding of the theory related to partial discharge, and the relationship to early detection of insulation deterioration is required to properly evaluate this predictive maintenance tool. This paper presents a theory to promote the understanding of partial discharge technology, as well as various implementation and measurement techniques that have evolved in the industry. Data interpretation and corrective actions are reviewed, in conjunction with comprehensive predictive maintenance practices that employ partial discharge testing and analysis.

Discrimination between internal PD and other pulses using directional coupling sensors on HV cable systems

Abstract: On-site partial discharge (PD) measurement is required to ensure proper installation of extra high voltage (EHV) cable systems accessories. To achieve high sensitivity and good localization, two problems have to be overcome. First, the strong high frequency in long XLPE cables requires that the sensors be located along the cable, preferably directly at the accessories. Secondly, the detection system must be able to distinguish internal PD from other pulses. This paper describes a solution based on directional coupling sensors and a data visualization system, which displays phase-amplitude diagrams for individual PD sources which are identified by the direction of pulse propagation. It has been applied to on-site measurements, type and routine testing of HV cable joints and stress cones. Due to the reliable discrimination between internal PD from the accessory measured and from other pulses, testing can be done in unshielded rooms even using terminations with internal PD and corona. The method works independently well on line voltage, resonance sources, oscillating voltages and 0.1 Hz cosine-square voltage. It has been used to verify the cable accessories installed in the 6.3 km long 380 kV cable system in Berlin, Germany.

Transformer monitoring using the UHF technique

Abstract: The viability of applying UHF partial discharge monitoring techniques to an in-service oil-filled power transformer is established. Discharges in oil are shown to radiate the high frequency signals necessary to allow detection. On-site pulse injection is used to test the operation of a UHF coupler fitted to the transformer. The effects of on-load tap changing operations are also investigated.

The effect of pulsed voltages on dielectric material properties

Abstract: The problem of premature failure of rotating machines fed by inverters appears to be of crucial importance. Understanding of the phenomena caused by this new type of supply on the stator insulation of motors provides enhanced reliability of the inverter/motor set. After identifying the stresses created by this type of device, a study of aging of the materials used as models, and subjected to similar stresses, is undertaken. For the voltage, rise time and frequency under study, the intrinsic part of the aging is studied. The results are presented and discussed according to the nature of the interaction existing between the electrical stress and the material properties.

A generalized model for partial discharge processes based on a stochastic process approach

Abstract: A general framework for the physical description of partial discharge (PD) processes is presented that holds for different types of PD causing defects A PD process is treated as a stochastic process consisting of short duration discharges (point-like in time) and charge carrier drift/recombination intervals between these discharges It is determined by few basic physical parameters and, in a stochastic process framework, can be described in a closed form by a master equation Since usually only the fast discharges can be measured as PD signals, a restricted possibility of observing a PD process results The link between the stochastic process and observable quantities is derived A specific type of measurements is reported, the so-called phase-resolved partial discharge (PRPD) patterns Here the total charge transferred during a discharge and the time or alternating current phase at which the discharge occurs are measured Thus each discharge event is described by the two quantities, charge and phase angle The modelling of the observation process is explicitly derived for this case However, the used method can easily be generalized to other types of PD measurements The proposed approach yields new possibilities for the interpretation and analysis of PD patterns Features of PD patterns can be derived analytically from the process parameters Conversely, quantitative information about the discharge physics can be gained from measured patterns Some limiting cases of model parameter values leading to typical pattern features are discussed explicitly Examples are presented that demonstrate the applicability of the model for three different discharge types (internal discharge in a gas-filled void, surface discharge in oil, corona in air)

On-line monitoring of power transformers and components: a review of key parameters

Abstract: The IEEE Transformer On-Line Monitoring Task Force is developing a guide for the on-line monitoring of power transformers, its components, and accessories. Publication of the guide is expected in mid 2000. This paper gives an overview of the key parameters that can be monitored on-line (dissolved gas-in-oil analysis, moisture-in-oil, partial discharge, oil temperature, winding temperature, load current and voltage, insulation power factor and pump/fan operation). The techniques to monitor instrument transformers are then discussed (tan /spl delta/ and relative capacitance measurements, gas analysis, PD measurement, and pressure measurement). The paper also discusses measurements of bushings and load tap changers.

Continuous on-line partial discharge monitoring of generator stator windings

Abstract: The use of on-line partial discharge (PD) measurements as an indicator of stator winding insulation condition, is well known. Trending results from on-line PD measurements over time, often allows maintenance personnel to become aware of developing stator insulation problems in motors and generators, and take corrective measures before failure occurs. However, partial discharge measurements on operating rotating machines can be influenced by conditions such as humidity, temperature, terminal voltage, and load. Thus, to provide reliable data which can be trended, it is important to take readings under similar machine operating conditions. In remote plants, or on pump storage units which are frequently cycled, it can be both time consuming and costly to ensure that personnel are available to take PD readings under ideal conditions occur. This paper describes the development of an automatic partial discharge monitoring system which is permanently connected to relevant machine sensors and can be configured to collect PD data when suitable conditions are met. The operating principles of the system are described, and some results are presented from data collected to date on 30 hydrogenerators.

Stator winding failures: contamination, surface discharge, tracking

Abstract: Stator winding failures are generally attributed to groundwall or turn insulation failures. The failure mechanism involves the gradual development of a weakness in the insulation. This weakness is usually caused by a trapped void that is continuously enlarging due to ever-increasing corona or partial discharge activity. The failure mode is from the inside out. Recent studies of failures in actual machines, and of coils on voltage endurance testing, indicate that failures can occur from the outside inward. Surface contamination leads to intense surface discharge and tracking. This surface discharge can lead to rapid winding failure following the inward motion of the corona activity. This paper looks at tracking as one failure mechanism and describes a series of tests on standard insulation materials aimed at comparing the antitracking capability of the materials. The tests show that combinations of insulation materials can reduce the antitracking capacity of a rather robust insulation system and predispose it to failure.

Novel characterization of PD signals by real-time measurement of pulse parameters

Abstract: Measurements of partial discharge (PD) are used to examine the insulation characteristics of HV equipment. This test method is also suitable for low-cost components in mass production. To monitor the quality of prefabricated parts, products and production processes, short test cycles and automatic classification are indispensable. In addition, it is desirable to acquire more information about the PD signals for classification. This can be achieved with an ultra wide band (UWB) measurement system. In this paper a new test method, based on analog capture of PD parameters which describe the PD waveform, in addition to the usual parameters, is proposed. The analog values are further processed employing a digital signal processing hardware and a PC. Measurements of frequently encountered PD pulse shapes and their frequency spectra are used to discuss problems which arise from the integration in the frequency domain. These problems are avoided using integration in the time domain, which is employed to determine the apparent charge with the described measurement system. Using the PD pulse duration as an example, measurements on artificially created test samples are presented, which show the influence of the defect dimensions on the PD pulse parameters. This is feasible for test circuit dimensions not extending to a certain value, which depends on the used frequency range of the measurement system.

Partial discharge site location system for determining the position of faults in a high voltage cable

Abstract: A pulse discharge site location (PDSL) system (12) is provided which captures pulses on a conductor without requiring the triggering functions of an oscilloscope. The PDSL system (12) comprises a pulse discharge measurement (PDM) system and is programmed to store the data captured by the PDM system in a reference buffer (46) corresponding in size to the propagation time of a pulse along the length of the conductor. Samples of the captured pulses are scanned to locate the peaks of pulses above a selected noise level. These pulses are stored into a temperary working buffer, along with a selected number of samples, normalized and then added to the reference buffer (46). The reference buffer (46) provides a statistical average of pulse activity. Fault site distances from the end of the cable can be determined due to the proportional dimension of the reference buffer (46) with respect to the length of the cable and cable propagation time.

On-line partial discharge monitoring of HV components

Abstract: Partial discharge (PD) measurements are an important tool to assess the condition of the insulation of high-voltage systems. Preferably, these measurements are done on-line during regular operation. This paper gives an overview of developments and experiences with online PD measurements in The Netherlands on turbine generators, transformers and switchgear.

Partial Discharge Theory and Applications to Electrical Systems

Abstract: Partial discharge monitoring is an effective on-line predictive maintenance test for motors and generators at 4160 volt and above, as well as other electrical distribution equipment. The benefits of on- line testing allow for equipment analysis and diagnostics during normal production. Corrective actions can be planned and implemented, resulting in reduced unscheduled downtime. An understanding of the theory related to partial discharge, and the relationship to early detection of insulation deterioration is required to properly evaluate this predictive maintenance tool. This paper will present a theory to promote the understanding of partial discharge technology, as well as various implementation and measurement techniques that have evolved in the industry. Data interpretation and corrective actions will be reviewed, in conjunction with comprehensive predictive maintenance practices that employ partial discharge testing and analysis.

New tools to determine the vulnerability of stator windings to voltage surges from IFDs

Abstract: It is well known that repetitive voltage surges from modern inverter fed drives (IFD) may age the insulation of motor stator windings, even on motors rated 440 V, leading to premature failure. Insulation aging seems to be correlated to the partial discharge (PD) activity of surges with very fast risetimes and high magnitudes. The magnitude and the risetime of the surges for a particular motor are determined by a host of factors, including power cable length, cable insulation, grounding, IFD type, and motor surge impedance, Hence, the nature of the surges can not be estimated without direct measurement. This paper describes a portable instrument which can accurately measure the characteristics of a surge, even those with a risetime as fast as 50 ns. Measurements with this instrument can be carried out both economically and with minimal training. Also discussed is a new device which permits the easy measurement of PD during 100 ns risetime surges. The latter can be used to determine the discharge inception voltage of stators under actual steep fronted surge conditions.

Change in partial discharge properties of a void in LDPE

Abstract: Partial Discharges (PD) cause the degradation of insulating materials and govern the lifetime of high-voltage power apparatus and cables. Therefore, it is important to understand the PD behaviors in the aging process. In this paper, partial discharges and their current shapes of low-density polyethylene (LDPE) with a void were measured. The PD /spl phi/-q-n pattern and the current shape changed remarkably with time and were related to each other. They depended not only upon the size and shape of voids but also on the degree of PD degradation. This suggests the possibility of insulation diagnosis using PD characteristics.

The behavior of water in XLPE and EPR cables and its influence on the electric characteristics of insulation

Abstract: This paper presents the latest results of continuous investigations of cable insulations degradation of crosslinked polyethylene (XPLE) and ethylene-propylene with rubber based formulation (EPR) when subjected to electric stress and heating in the presence of water or water vapour. The paper deals with water absorption and diffusion in two kinds of crosslinked polyethylene insulation-dry-cured and steam-cured, and steam-cured EPR insulation. The aim of this investigation is to present the results of the influence of changing of water or water vapour pressure in the conductors of XLPE and EPR cables in different service conditions on the electric characteristics of XLPE and EPR insulations-breakdown voltage (AC BDV), dissipation factor (tan /spl delta/) and rata of partial discharge (RPD). In this paper, RPD is defined as, the maximum electrical field when the beginning partial discharge in the cable insulation and partial discharge were measured in accordance with the IEC standard. This paper also shows the relation between AC BDV and water content, and AC BDV and tan /spl delta/ in XLPE and EPR insulations. In this testing the tap water was put in the cable conductors and the ends were properly closed by terminal boxes. The results indicate that the combined effects of water or water vapour, pressure, moisture, electric field and temperature will greatly accelerate deterioration of XLPE and EPR insulations.

Discharges in combined transformer oil/paper insulation

Abstract: Discharge signatures and breakdown levels have been recorded for combined transformer oil/impregnated paper insulation subjected to AC high voltage. Different electrode geometries have been studied, varying from the point-plane gap to a semi-homogeneous sphere-plane configuration. At high moisture levels discharge activity was monitored at an average electric field below 1 kV/mm, with breakdown occurring 6 kV/mm. Characteristic discharge patterns were identified for corona, wedge, void and creeping discharges, but depend strongly on applied voltage, time and moisture content of the insulation.

Measuring system for phase-resolved partial discharge detection at low frequencies

Abstract: This paper addresses the measurement of partial discharges (PD) for a range of low frequencies of applied voltage and explores the frequency dependence of PD patterns for a few objects. The aim is to determine the existence of frequency-dependent trends in PD patterns and how these trends can be expressed through use of statistical parameters calculated from the PD patterns. An experimental setup was developed to measure phase-resolved PD patterns for a range of low frequencies, from 1 mHz to 100 Hz. Tests were performed on a few PD-infested objects, including a corona gap, a needle-film object, and a generator stator bar, to demonstrate the measuring system and analysis methods. Statistical operators were used to quantify the PD patterns, and the resulting parameters were then considered as functions of frequency.

Partial discharge relay and monitoring device

Abstract: A portable partial discharge detecting device for measuring partial discharge in energized electrical systems encloses a relay circuit and a DC power source. A pair of sensor circuits are enclosed within electrically coupling clamping mechanisms and are coupled to the relay circuit at sensor inputs by cabling. The clamping mechanisms engage the ground leads of the electrical system. Trip and alarm networks of the relay circuit continuously compare the picocoulomb values of the partial discharge pulses against user set threshold values. Relays coupled to the trip and alarm networks respond if the threshold values are exceeded. The alarm setting is always set to a pick-up value equal to or less than the trip setting. The trip network relays are coupled to a control circuit of the electrical system and can take the system off-line. The alarm network relays are coupled to an alarm circuit and can warn the user that a certain level of partial discharge has been reached within the electrical system. The preferred system to monitor is a three phase power transformer.

Light source using dielectric barrier discharge lamp, and power supply

Abstract: A dielectric barrier discharge lamp apparatus comprises a high-voltage A.C. power supply for an electrode that cooperates with discharge gas and an interleaving dielectric to produce an excimer by dielectric barrier discharge. The voltage waveform applied to the lamp includes a slow-varying period of time before reaching the firing potential toward another discharge after one complete discharge. The following condition is satisfied: 0.3 y x x y< = VF-VA.VA : the voltage at the end of one discharge, VB : the voltage at the end of the next discharge, and VF : the voltage at the end of the slow-varying period.

Nonlinear dielectric response of vented water treed XLPE insulation

Abstract: This paper describes experiments performed on laboratory aged Rogowski test objects. Results from measurements of residual AC breakdown strength, degree of water treeing and the dielectric response are presented. The main reason for performing laboratory ageing was to facilitate diagnostic testing as a function of ageing time of samples containing mainly vented water trees. A new method, based upon inserting NaCl particles at the interface between the upper semiconductive screen and the insulation, was found to successfully enhance initiation and growth of vented water trees. The results show that vented water treeing was found to cause both the time and frequency domain dielectric responses to become nonlinear at a low test voltage, and to reduce the breakdown strength of the test objects. However, the dielectric response in the frequency domain was found to be more nonlinear than in the time domain.

Transfer functions for UHF partial discharge signals in GIS

Abstract: The principles of electromagnetic radiation by partial discharges (PD) in gas insulated substations (GIS) are described. Calculated transfer functions are presented, showing that transverse electric modes are predominant for radial PD currents in the coaxial geometry. By using actual PD current pulses from a free moving particle together with the measured sensitivity of a UHF coupler, the entire transfer function for the PD detection problem is modelled. The relationship between UHF signal energy at the coupler output and the electronic component of the PD pulse is investigated using the model, revealing a significant degree of correlation.

On-site PD diagnostics of power cables using oscillating wave test system

Abstract: This paper discusses the results of partial discharge measurements in medium voltage power cables losing oscillating test voltages. In particular, the sensitivity and the reproducibility of this new method for on-site testing are discussed. Based on laboratory and on-site tests the usefulness of this PD measuring technique for practical applications is presented.

Partial discharge detection method

Abstract: An electromagnetic waveform developing in synchronism with a system power supply by a partial discharge of electric equipment such as a power distribution device is received by an antenna 1. From the electromagnetic waveform of measurement frequency included in the received signal, there are extracted a component changing according to the frequency of the system power supply and a component changing at the frequency twice that of the system power supply. The presence or absence of discharge is determined from the magnitude of each of the extracted components, thereby detecting a partial discharge.

Dielectric barrier discharge lamp apparatus

Abstract: A dielectric barrier discharge lamp apparatus comprises dielectric barrier discharge lamps including a discharge plasma space filled with discharge gas that can produce excimers by dielectric barrier discharge. A dielectric lies between at least one of the discharge electrodes and the discharge gases. The dielectric barrier discharge lamp apparatus also comprises power supply means for applying high voltage to the electrodes. The power supply means is divided into front and back stages, and the back stages are corresponding to the lamps, respectively. The front stage of the power supply means supplies a common DC voltage to the back stages, and each of the back stage converts the DC voltage into a high AC voltage using switching elements and a booster transformer and applies it between the electrodes. Each of the back stages can independently adjust the frequency of the high AC voltage, and the front stage can adjust the common DC supply voltage.

Simulation of transformer PD pulse propagation and monitoring for a 500 kV substation

Abstract: This paper discusses the development of a simulation model to study the propagation of partial discharge (PD) pulses of transformers in a 500 kV substation. The results will be used to design an on-line PD monitoring system for transformers. The impedance matrix of a 500 kV, single-phase transformer is computed based on geometry data and its external coupling network (often referred to as coupling impedance) is calculated with electromagnetic transients program (EMTP). It was found that the frequency characteristic of the coupling network is complex and that the traditional lumped coupling capacitance representation is only feasible at <60 kHz. The entire substation network was included to calculate the PD propagation of the transformer. PD responses are obtained at the tank grounding and 500 kV bushing tap grounding with a simulated PD pulse injected at the 500 kV terminal and 220 kV terminal of the transformer, respectively. Simulation shows that, using the resonant frequency of the substation network as monitoring frequency, one effectively can increase the monitoring sensitivity. The off-line and on-line PD calibration can also be made based on the developed simulation model.

Modeling PD inception voltage of epoxy resin post insulators using an adaptive neural network

Abstract: One of the parameters used to characterize the partial discharge (PD) behavior is its inception voltage. The partial discharge inception voltage (PDIV) should always be higher than the operating voltage to ensure that PD does not occur at or near operating voltage. Under these circumstances, other PD parameters such as apparent charge, energy dissipation need not be considered. This paper deals with modeling of PDIV of epoxy-resin post insulators using a neural network (NN). The PDIV is obtained experimentally for various shapes and sizes of post insulators, with a working voltage in the range of 3.3 to 33 kV. The electrode spacing d and creepage length l are the key parameters employed for the present modeling. An adaptively trained multilayer NN is employed for the modeling. Detailed studies are carried out to optimize the NN parameters for minimum error. The model results obtained closely follow the experimental data indicating the effectiveness of NN as an efficient tool in estimation of PDIV of epoxy-resin postinsulators.