Showing papers on "Partial discharge published in 2001"

Pulse electrical discharges in water and their applications

Abstract: Partial electrical discharges in a water solution as a function of conductivity have been studied experimentally. Using needle-plate electrodes it has been demonstrated that the discharge evolves in two phases. During the first streamer-like phase, the discharge propagated with a velocity of 106 cm/s, while during the second arc-like phase the length of the discharge remained almost constant although the current still increased. Higher solution conductivity resulted in the generation of shorter channels, in larger discharge current, and in a higher plasma electron density. Degradation of phenol by the discharge has also been demonstrated. A special metallic electrode covered by a thin layer of porous ceramic has been developed and used for generation of a multichannel discharge. At comparable solution conductivity the ceramic-coated electrode produced plasma with very similar parameters as the needle-plate electrode configuration. Generation of strong focused shock waves by the multichannel discharge in a...

The effect of voltage distortion on ageing acceleration of insulation systems under partial discharge activity

Abstract: This paper focuses on the features of harmonic distortion which may affect significantly the reliability of typical ac-power network equipment, such as low-voltage self-healing capacitors used for reactive power and harmonic compensation. Moreover, the effect of high-frequency pulse-like voltage generated by Adjustable Speed Drives (ASD) on electrical machine insulation is also investigated, resorting to life tests carried out on different insulating materials of the standard and "corona resistant" type, at electrical field levels able to incept partial discharges (PD).

Partial discharge theory and technologies related to medium-voltage electrical equipment

Abstract: Partial discharge (PD) monitoring is an effective online predictive maintenance test for medium-voltage (MV) motors, MV generators, and MV switchgear at 4160 V 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 PD, 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 PD technology, as well as various implementation and measurement techniques that have evolved in the industry. Data interpretation, corrective actions, and application to electrical distribution equipment is also reviewed.

Optical fiber sensor-based detection of partial discharges in power transformers

Abstract: In this paper, a fiber optic acoustic sensor system is designed and tested for on-line detection of the partial discharges inside high voltage power transformers The fiber optic sensor uses a silica diaphragm and a single mode optical fiber encapsulated in a silica glass tube to form an extrinsic Fabry–Perot interferometer Test results indicate that the developed fiber optic sensors are capable of detecting the acoustic signals propagating inside the transformer oil with high resolution and high frequency

Dielectric windows for UHF partial discharge detection

Abstract: Increasing the permittivity of the propagation medium inside a hollow tube lowers its cut-off frequency, allowing it to transmit a wider range of signal frequencies. This principle is applied to the design of dielectric windows that can be used to facilitate partial discharge (PD) detection at ultra high frequency (UHF) in a HV metalclad plant such as gas-insulated substation (GIS), power transformers and circuit breakers. External UHF couplers mounted on such windows can be removed or replaced without compromising the integrity of the insulation system. By increasing the signal power level incident on a window-mounted external UHF coupler, higher sensitivity to PD can be achieved. The problem is analyzed using waveguide theory and, using GIS as an example, improvements of /spl ap/6 dB are demonstrated by means of coupler sensitivity measurements and experiments using PD sources.

A study of partial discharges from water droplets on a silicone rubber insulating surface

Abstract: This paper presents measurements of partial discharge (PD) from water droplets on a silicone rubber insulating surface in an ac field. Tests are done by placing droplets on the surface using a micro-pipette, and by condensing water in a controlled humidity chamber. The field enhancement factor due to droplets placed on the insulation surface is experimentally and numerically determined. Values in the range of 3.8 to 6.3 are obtained depending on the size and number of droplets. The time resolved analysis of PD from water condensation indicates a threshold field, between 3.0 and 3.5 kV/cm, above which the PD activity increases with time, and brings the insulation to a surface breakdown. The corresponding field enhancement factor obtained for the condensation experiments is between 5.8 and 6.8. The results therefore suggest that the PD activity on the hydrophobic surface can be used effectively to learn about the insulation surface conditions.

Aging acceleration of insulating materials for electrical machine windings supplied by PWM in the presence and in the absence of partial discharges

Abstract: The speed control of ac-electric motors can be carried out efficiently by adjustable speed drives (ASD), which are constituted by an AC/DC conversion block, a DC filter and a DC/AC inverter representing the core of the whole power converter. A new generation of DC/AC converters for adjustable speed drives, the so-called PWM (Pulse Width Modulation) inverters, has been developed in recent years. It has significant advantages for motor speed control, but has been proved to overstress electrical insulation with respect to the sinusoidal supply. The increase of partial discharge (PD) activity seems to be mainly responsible for the degradation acceleration of motors fed by ASD. In order to investigate the aging behavior, the results of life tests carried out on twisted pairs subjected to 50 Hz and 10 kHz sinusoidal voltage, as well as unipolar and bipolar square pulses, are presented.

Discharge plasma generating method, discharge plasma generating apparatus, semiconductor device fabrication method, and semiconductor device fabrication apparatus

Abstract: A discharge plasma generating method includes (a) opposing a discharge electrode having a substantially plane discharge portion to a substrate to be processed in a vacuum reaction vessel such that the discharge electrode and the substrate are substantially parallel to each other, (b) evacuating the vacuum reaction vessel and supplying a process gas to a space between the discharge electrode and the substrate, and (c) applying HF power to the discharge electrode such that an envelope representing the spatial distribution of a HF voltage φ on the discharge electrode in a split second changes in accordance with a function including time as a parameter, thereby generating a discharge plasma of the process gas between the discharge electrode and the substrate, with substantially no standing wave of the HF voltage φ generated on the discharge electrode.

Simulation of partial discharges in conducting and non-conducting electrical tree structures

Abstract: Electrical treeing is of interest to the electrical generation, transmission and distribution industries as it is one of the causes of insulation failure in electrical machines, switchgear and transformer bushings. Previous experimental investigations of electrical treeing in epoxy resins have found evidence that the tree structures formed were either electrically conducting or non-conducting, depending on whether the epoxy resin was in a flexible state (above its glass transition temperature) or in the glassy state (below its glass transition temperature). In this paper we extend an existing model, of partial discharges within an arbitrarily defined non-conducting electrical tree structure, to the case of electrical conducting trees. With the inclusion of tree channel conductivity, the partial discharge model could simulate successfully the experimentally observed partial discharge activity occurring in trees grown in both the flexible and glassy epoxy resins. This modelling highlights a fundamental difference in the mechanism of electrical tree growth in flexible and glassy epoxy resins. The much lower resistivities of the tree channels grown in the glassy epoxy resins may be due to conducting decomposition (carbonized) products condensing on the side walls of the existing channels, whereas, in the case of non-conducting tree channels, subsequent discharges within the main branches lead to side-wall erosion and a consequent widening of the tubules. The differing electrical characteristics of the tree tubules also have consequences for the development of diagnostic tools for the early detection of pre-breakdown phenomena.

The influence of dielectric surface charge distribution upon the partial discharge behavior in short air gaps

Abstract: A two-dimensional breakdown model based upon the continuity equations for electrons and ions and the Poisson equation is employed to examine the influence of surface charges upon the partial discharge behavior of a short metallic-dielectric plane-parallel electrode gap of 0.05 cm in an air-like mixture at atmospheric pressure. The form of charge distribution at the dielectric electrode surface is found to significantly affect a number of important discharge parameters, such as the uniformity of the electric field, the charge density and its radial distribution within the gap, as well as the discharge propagation time.

Partial Discharges Studied with Variable Frequency of the Applied Voltage

Abstract: This thesis concerns partial discharge (PD) diagnostics withvariable frequency of applied voltage in the frequency range 1mHz - 400 Hz. The exploration of a new type of spectroscopythat combines pa ...

Feature of new standard iec 60270 for partial discharge measurement and its influence on pd measuring activity

Abstract: The new standard IEC 60270/FDIS[2000] for partial discharge(PD) measurement has been published in September 2000, which builds up an integral quality assurance system for PD measurement based on the IEC 60060-2[1994] It takes traceability of quantity as its core in order to ensure accuracy of measuring results, comparability and consistency of different instruments and different measuring methods Furthermore it treats digital PD measuring instruments as important as the analogue one except that some additional test items are required This paper discusses the feature of the new standard as well as its influence on PD measuring activity

Peculiarities of dielectric barrier discharges

Abstract: The dielectric barrier discharge (DBD) is a highly transient, non-thermal discharge form, which exists in a broad pressure range. It occurs in arrangements, where a dielectric layer covers at least one electrode. The dielectric quenches the current and distributes the discharge over the whole surface. Depending on the geometrical conditions three basic types of DBD arrangements are distinguished. In arrangements with a gas gap a filamentary or a homogeneous-diffuse discharge mode appears. The (stable) filamentary mode consists of a multitude of microdischarges, which in some extent can be rather easily tailored for e. g. plasma-chemical applications. In arrangements with a long electrode (or several in parallel) on a dielectric surface and a plane counter-electrode on the reverse side of the dielectric, pure surface discharges can be observed. They are characterised by low ignition voltages. The extension of the discharge on the surface depends on the voltage amplitude. If pairs of long electrodes are within the bulk of a dielectric, discharge phenomena appear on the surface of the dielectric. As these devices can be produced with small and precise electrode gaps, high mean field strengths in the discharge region can be realised. The properties of the discharges in these arrangements as well as their dynamics are described in detail and compared with one another. The advantages of each type are highlighted. Some aspects, which may be of interest for plasma-chemical reactions on surfaces and in the gas space are discussed.

Measurement and simulation of electrical tree growth and partial discharge activity in epoxy resin

Abstract: Electrical treeing in an epoxy resin under an ac voltage for a point-to-plane gap configuration has been investigated using a phase-resolved partial discharge measurement system and shadow survey and light emission registration techniques. A self-consistent model based on the concepts of energy and electric field has been developed and used for quantitative description the temporal-spatial and electric characteristics of electrical treeing obtained in experiment. The electrical treeing is considered to be a phenomenon of self-organized criticality.

IGBT module technology with high partial discharge resistance

Abstract: The high operating voltages of 6.5 kV IGBT modules place additional demands on the insulation and partial-discharge resistance. The most important component affected here is the metallized aluminum nitride ceramic which is embedded in a silicone gel. A high electric field strength can cause a local electric discharge in the silicone gel known as partial discharge, leading ultimately to electric insulation failure and reducing the reliability of the IGBT module. For a 6.5 kV IGBT module, the insulation test must be performed up to a voltage of 10.5 kV rms. Technological steps have been carried out to reduce the maximum electric field strength along the edge of the copper metallization. The edge of the ceramic was coated with a high-impedance layer of doped amorphous silicon. The electric current along the edge of the ceramic homogenizes the electric field strength. The partial discharge was determined up to 11 kV and a considerable reduction was observed compared to standard modules. Without an a-Si:H coating, the partial discharge already increases strongly at low voltages of 3-4 kV. At high voltages, the interface between the silicone gel and the substrate is a major source of partial discharge. The a-Si:H coating reduces electric field peaks and the partial discharge does not exceed 10 pC up to a voltage of 10 kV.

Characteristics of PD pulses in electrical trees and interfaces in extruded cables

Abstract: The characteristics of partial discharge (PD) occurring within electrical trees in extruded cable insulation, and also at model interfaces between a cable and its accessories, have been investigated. A wideband PD detection system was used that enabled the true shape of the PD pulses to be measured. The rise- and fall-times, pulse width, amplitude and frequency of the pulses were recorded as electrical trees grew through the insulation, and also as the tracking progressed along a model cable/accessory interface. The PD pulse characteristics in electrical trees varied with time of voltage application and also with the electrical stress in the insulation. Differences in the PD characteristics of electrical trees in the bulk of a polymer and at interfaces between polymers were observed.

Re-locatable partial discharge transducer head

Abstract: An apparatus (10) for detecting the occurence of partial discharge in electrical equipment, such as high voltage transformers. The apparatus comprises a detecting means including an ultrasonic transducer (11) mounted at one end of an electrically insulating elongate rod (13). The ultrasonic transducer (11) is adapted to detect ultrasonic pulses or waves generated by the occurence of a partial discharge in the electrical equipment. A radio frequency (RF) transducer (17) is also utilised to detect radio frequency (RF) pulses or waves also generated by the partial discharge. The rod (13) can be mounted in the wall of the electrical equipment and so provide a mechanism for adjusting the position of the detecting means within the electrical equipment. The apparatus is also removable from the electrical equipment and re-locatable to other equipment.

A Novel Technique for Detecting Electromagnetic Waves Caused by Partial Discharge in GIS

Abstract: For the insulation diagnosis of gas-insulated switchgear (GIS), it is necessary to detect partial discharge (PD) under noisy conditions and to specify the direction of electromagnetic wave radiation due to PD. We propose the "phase gate control method" for discriminating the electromagnetic waves caused by PD in SF6 gas from external noises. The proposed method enables us to discriminate PD of about 100 pC in SF6 gas from external noises of more than 10,000 pC in air. We also compare polarization characteristics of electromagnetic waves radiated from PD with those from a half-wave dipole antenna and estimate the radiation length of electromagnetic waves caused by PD. The results derived in this paper will contribute greatly to the insulation diagnosis of GIS on site.

PD characteristics as a stochastic process and its integral equation under sinusoidal voltage

Abstract: This paper proposes an integral equation to describe the stochastic fluctuation of partial discharge (PD) occurrence under sinusoidal waveform voltage conditions based on a simple PD model (an equivalent circuit model). The stochastic behavior of PD fluctuation is assumed to arise from the fluctuation of PD delay time after the discharge inception voltage is built up across a discharge gap. The distribution of delay time is assumed to obey an exponential distribution and the dependence of the delay time on the overvoltage is assumed to be expressed by an exponential function of the overvoltage. The integral equation has been solved numerically under the low voltage condition in which only one PD can occur in a half AC voltage cycle. It is found that the proposed integral equation can provide the basic characteristics such as PD pulse repetition rate vs. applied voltage phase angle.

A neuro-fuzzy approach for the detection of partial discharge

Abstract: Dielectric surfaces exposed to partial discharges (PD) undergo aging, which is reflected by changes in the discharge pulse form. An approach is described in which fuzzy logic and neural networks are used in conjunction with the wavelet transform to identify the parameters in the PD pulse form for the purpose of classifying the aging phenomena due to partial discharge degradation.

Insulation optimization by electrode contour modification based on breakdown area/volume effects

Abstract: This paper shows how to determine the electrode contour with the best insulation performance on the basis of an area effect and a volume effect in the breakdown field strength. Previous electric field optimization techniques have provided us only with an optimum electric field distribution, For the design of power apparatus insulation, we have to consider the dielectric breakdown characteristics rather than the electric field distribution. As a first step, we developed a new optimization technique which enables us to obtain an optimum electrode contour with the highest breakdown strength while taking into account the area and volume effects of breakdown strength of insulating media. From the results, we have confirmed that the proposed optimization method improved the breakdown voltage more than did the electric field optimization. This leads to an effective insulation design of electric power apparatus.

Acoustic Measurements of Partial Discharge Signals

Abstract: The acoustic pressure waves associated with partial discharges in HV power transformers can be detected with external piezoelectric sensors. The propagation time and the waveshape of the received signals are affected by factors such as the sensor position, internal barriers and sensor type. These are investigated in this paper. In particular, it is shown that the commonly-used resonant sensor gives a cleaner signal. On the other hand, the wideband sensor enables comparison between low and high frequency components. It was found that as the sensor is closer to the normal, the higher frequency components become more dominant. This can be utilised as a new diagnostic method for discharge location. The wavelet transform is used to show changes in the frequency distribution via the time-scale plot and also to de-noise the signal.

Method and device for detecting a partial discharge in an electrical device

Abstract: The present invention provides a method and device for detecting a discharge in an electrical device comprising, providing a bushing having a bottom lower flange and a top upper flange, providing a shield inside the bushing extending from the bottom lower flange to a position at least below the top upper flange and providing an antenna outside the bushing at least above the position for measuring an output signal from the device for detecting the discharge.

Seasonal changes in partial discharge activity on hydraulic generators

Abstract: Six hydraulic generators in two separate power stations were equipped with partial discharge cable type couplers for long term PD monitoring. The stator windings used four different insulation systems from three different manufacturers. Partial discharge activity was monitored monthly on each generator for periods of time ranging from 2 to 9 years. PD data was acquired in the frequency range of 40 to 350 MHz. External noise pulses were filtered out using the differential method with two cable couplers per phase. The present paper is a case study showing how PD activity varies dramatically depending on the time of the year the online testing was performed. A ratio higher than 100 was recorded between winter and summer readings. Winter readings, (January-March) were always higher than summer readings (July-September). A strong correlation was found between temperature, atmospheric absolute humidity levels (water partial pressure in mm Hg) and the well-known PD basic quantities such as Q/sub max/ and NQN values. An interpretation of the monthly-acquired data is offered based on both external and internal PD activity.

Influence of thermal cycling on the cable-joint interfacial pressure

Abstract: Cable joints are generally known to be the weak points of underground distribution lines. One of the most common failure modes of premolded cable joints is linked to the occurrence of dielectric breakdown at the interface. Many parameters may influence the dielectric strength of a cable-joint interface: surface roughness, the presence of dielectric grease, and the length of the interface to name a few. Nevertheless, interfacial pressure is known to be a key factor that sets the cable-joint interfacial breakdown strength. AC dielectric breakdown strength is proportional to interfacial pressure up to a threshold pressure. A high interfacial pressure leads to good dielectric breakdown strength whereas a lower pressure leads to a poor long-term performance. Interfacial pressure decreases with the number of thermal cycles. Moreover, partial discharge (PD) monitoring during thermal cycles with thermal gradients in the insulation, following IEEE 404 standard protocol, did reveal a maximum PD intensity at the end of the temperature decrease for each cycle. This could be linked to a decrease in the interfacial pressure. We study the behavior of interfacial pressure when a cable-joint assembly is thermally cycled and identify the worse case for cycles at 75, 90 and 130/spl deg/C with thermal gradients in the bulk of the insulation. The cycling protocol is based on existing IEEE 404 standard for cable joints for use with extruded dielectric cable rated 5-138 kV.

On-line versus off-line partial discharge testing in power cables

Abstract: Two approaches are available to detect partial discharge (PD) from power cable insulation-online and offline detection systems. The online system is based on detecting PD at the system operating voltage, while elevated voltages are used to initiate PD in the offline approach. This paper presents the fundamentals of PD measurements. It also discusses the advantages and disadvantages of each approach.

Fundamental aspects of on-line PD measurements on distribution power cables

Abstract: In this paper the fundamental aspects of online partial discharge detection on distribution power cables as a tool for insulation condition assessment are discussed. In particular, based on laboratory and field tests, the online PD diagnostics is presented to be a good PD detection diagnostic.

Conduction and partial discharge activity in HVDC cable insulation of lapped polypropylene films

Abstract: The purpose of the work presented here is to characterise polymeric HVDC cable insulation consisting of lapped thin film polypropylene with respect to DC current conduction and partial discharge activity in air filled butt gaps. The PD activity was found to strongly increase with increasing temperature and applied electric stress. The discharge frequency decreased with time after voltage application and the effect of the void size was found to be of minor importance. These observations agree very well with the proposed theory stating that the discharge frequency will be proportional to the current flowing through the insulation.