Partial Discharge Events Research Articles

The influence of spherical cavity surface charge distribution on the sequence of partial discharge events

In this work, a model representing partial discharge (PD) behaviour of a spherical cavity within a homogeneous dielectric material has been developed to study the influence of cavity surface charge distribution on the electric field distribution in both the cavity and the material itself. The charge accumulation on the cavity surface after a PD event and charge movement along the cavity wall under the influence of electric field magnitude and direction has been found to affect the electric field distribution in the whole cavity and in the material. This in turn affects the likelihood of any subsequent PD activity in the cavity and the whole sequence of PD events. The model parameters influencing cavity surface charge distribution can be readily identified; they are the cavity surface conductivity, the inception field and the extinction field. Comparison of measurement and simulation results has been undertaken to validate the model.

A frequency-based RF partial discharge detector for low-power wireless sensing

Partial discharge (PD) monitoring has been the subject of significant research in recent years, which has given rise to a range of well-established PD detection and measurement techniques, such as acoustic and RF, on which condition monitoring systems for highvoltage equipment have been based. This paper presents a novel approach to partial discharge monitoring by using a low-cost, low-power RF detector. The detector employs a frequency-based technique that can distinguish between multiple partial discharge events and other impulsive noise sources within a substation, tracking defect severity over time and providing information pertaining to plant health. The detector is designed to operate as part of a wireless condition monitoring network, removing the need for additional wiring to be installed into substations whilst still gaining the benefits of the RF technique. This novel approach to PD detection not only provides a low-cost solution to on-line partial discharge monitoring, but also presents a means to deploy wide-scale RF monitoring without the associated costs of wide-band monitoring systems.

A Feasibility Study on Autonomous Online Condition Monitoring of High-Voltage Overhead Power Lines

For electric power transmission, high-voltage overhead power lines play an important role, as the costs for power transmission are comparatively low. However, the sag of the conductors (e.g., due to temperature variations, aging, or icing of conductors as a result of extreme weather conditions) may increase safety margins and limit the operability of these power lines. Furthermore, heavy loads due to icing or vibrations excited by wind streams increase the risk of line breakage. With online condition monitoring of power lines, critical states can be detected early, and appropriate countermeasures can be applied. In this paper, we investigate possibilities for monitoring devices that are directly mounted onto a conductor. The feasibility of powering the device from the electric field, protection of electronic circuitry from strong electric fields, and data transmission by means of a wireless link is demonstrated, as well as its operability during the presence of strong magnetic fields due to high currents and transient signals due to partial and spark discharge events.

A Simple Metric for Assessing the Severity of Partial Discharge Activity Based on Time-Sequence-Analysis-Discharge Level Patterns

This paper introduces a partial discharge (PD) severity metric, S, based on the evaluation of time-sequence PD data capture and resulting Time-Sequence-Analysis Discharge (TSAD) level distributions. Basically based on an IEC60270 measurement technique, each PD event is time stamped and the discharge level noted. By evaluating the time differences between a previous and subsequent discharge, a 3D plot of time-sequence activity and discharge levels can be produced. From these parameters a measurement of severity, which takes into account dynamic or instantaneous variations in both the time of occurrence and the level of discharge, rather than using standard repetition rate techniques, can be formulated. The idea is to provide a measure of the severity of PD activity for potentially measuring the state of insulation within an item of plant. This severity measure is evaluated for a simple point-plane geometry in $SF_{6}$ as a function of gap distance and applied high voltage. The results show that as the partial discharge activity increases, the severity measure also increases. The importance of future investigations, quantifications and evaluations of the robustness, sensitivity and importance of such a severity measurement, as well as comparing it with typical repetition rate assessment techniques, and other monitoring techniques, are also very briefly discussed.

Classification of partial discharge events in gas-insulated substations using wavelet packet transform and neural network approaches

To ensure the safe and reliable operation of a gas-insulated substation (GIS), it is crucial to quickly identify partial discharge (PD) sources to prevent the occurrance of breakdowns. A method based on wavelet packet transform techniques is developed to meet this requirement. The proposed method extracts is able to extract features from ultra-high frequency resonance signals measured from a test GIS section. These features are subsequently used to train a neural network that is then able to quickly and reliably diagnose PD events. A quality-assurance scheme is developed that ensures the robustness of the PD classification to changes in the background noise level and the location of the PD event within the test GIS section.

Partial discharge pulse sequence patterns and cavity development times in transformer oils under ac conditions

Discrete partial discharge (PD) pulse occurrence times within the PD pulse bursts in transformer oils were found to extend from approximately 0.42 to 2.9 /spl mu/s, with the discrete pulse separation times evincing a weak dependence on the inverse of the oil viscosity. Many PD pulse burst patterns exhibited substantial deviation from the classical behavior, which is normally characterized by successive discrete pulses of ascending amplitude with well defined increases in the pulse separation times. The average elapsed time from the incipient formation for the cavity to the onset of the first partial discharge event extended downwards from 0.62 to 0.42 /spl mu/s with increasing oil viscosity.

Online calibration of high-frequency partial discharge signals in three-phase belted power cables

Partial discharge (PD) magnitudes from classical detection techniques are expressed in terms of apparent charges. Signals from HF/VHF/UHF techniques on substation components are often hard to express in this quantity because of complex signal excitation and propagation channels. A method to calibrate PD signals obtained during online inductive detection in medium voltage belted cables is described. online inductive detection implies that the impedances of components present in substations essentially determine the detected PD signal magnitude. The use of belted cables means, that the coupling of a PD event to the conductors not only depends on the PD site within the cross-section of the cable or cable accessory, but also becomes dependent on the momentary phase angle. In addition, during signal propagation the signal magnitude may alter according to the propagation modes of a multi-conductor cable. These aspects are studied quantitatively by the use of theoretical modelling in addition to offline and online experiments. PD diagnostic equipment including pulse injection capability allows online calibration with sufficient accuracy, irrespective of the actual substation arrangement.

PD data analysis and evaluation of partial discharge patterns for uniform characterisation

A modern partial discharge (PD) analyser provides a complete record of PD events obtained during the time of observation. A variety of analysis techniques can be applied to the data to extract characteristic information. An analysis technique that would support instrument-independent characterisation needs to have a method of identifying characteristic parameters that measure identically with all PD detector–analyser systems. The same parameters can be used for PD representation. The paper introduces a new compositional approach based on PD sequence properties for characterisation. The same approach is used for evaluation of PD patterns with a detector–analyser having limited resolution. From the study a representation that can be measured identically is identified and recommended for characterisation.

Impulse testing of power transformers – a model reference approach

An objective classification of faults that can occur during impulse tests on power transformers is proposed. It includes nonlinear elements, breakdown, and partial discharge events. An analysis of a layer winding, in the time and frequency domains, with these faults shows that current assessment methods must be used with caution. A model reference approach is proposed to distinctly improve recognition in such cases. The method is immune to changes of wave shape and the instant of fault is available by observation.

Pulse Sequence Analysis - a diagnostic tool based on the physics behind partial discharges

The analysis of partial discharge signals is a well-known tool to monitor the degradation of electric equipment. In contrast to the common practice of the analysis of data sets (phase angles of occurrence and discharge magnitudes) accumulated during a predefined time interval, the Pulse Sequence Analysis (PSA) is based on the evaluation of data sets in which also the individual sequence or partial discharge events are registered.The classical analyses are mainly based on the frequency distributions of the pulse heights of discharge events and the phase angles of the applied voltage at which they occur. Detailed analyses of partial discharge sequences reveal that - if liquid or solid dielectrics are involved - the phase angle of the externally applied voltage is not a meaningful parameter in the partial discharge process. The ignition of the partial discharge is determined by the local electric field at the defect site and not by the absolute value or the phase angle of the externally applied voltage.Experiments with different model defects as well as measurements of commercial apparatuses show that there are characteristic sequencesof partial discharge events. In many cases a systematic shift of the phase angles of occurrence occurs, which produces an apparently `statistic scatter' of the phase angles unless the correlation between consecutive discharges is taken into account. Sequence correlated parameters such as the voltage differences of the applied voltage or time differences between consecutive discharges are far more decisive parameters in the process. By means of the pulse sequence analysis, local defects can be characterized more precisely and single and multiple discharge sites can be clearly distinguished.A physically based model to describe discharge phenomena will be discussed and experimental results will be compared with results of numerical simulations of partial discharge processes.

Importance of bandwidth in PD measurement in operating motors and generators

It is well known that partial discharge (PD) events produce electromagnetic radiation with frequencies from de to sometimes hundreds of megahertz. In addition, in complex structures such as stator windings, the initial pulses are significantly distorted and attenuated as they propagate through a winding. The detection of the PD pulses needs to account for these effects. In particular, the bandwidth of the detection system will influence the sensitivity of the PD measurement, as well as in what portions of the winding PD can be detected. This paper summarizes over 20 years of research into these effects in stator windings, and discusses the implications for off-line and on-line PD testing, and the calibration of the PD quantities into apparent charge.

Simplified model of the discharge path in electrical devices by an iterative FEM procedure

A simple FEM procedure is proposed to predict approximately the electrostatic discharge path in a complex configuration. The method is based on the FEM solution of the Laplace equation in order to find the maximum electrical field in the examined domain. When the calculated local electric field is greater than the electric strength of the considered material, a partial discharge event is imposed in the numerical procedure and modelled by an equipotential segment. The procedure continues iteratively to solve the Laplace equation modifying adequately the boundary conditions at each iteration to approximate the breakdown discharge path. Details of the FEM implementation and numerical results of the proposed iterative procedure are given.