An important tool for improving the reliability of HV insulation systems are partial discharge (PD) measurements. The interpretation of such measurements aims at extracting from the measured data information about insulation defects which then are used for estimating the risk of insulation failure of the equipment. Because the physical understanding of PD has made substantial progress in the last decade, it can now be exploited to support interpretation. In this paper a concept is presented which merges the available physical knowledge about various PD types into a generalized model which can be applied to arbitrary insulation defects. This approach will be restricted to PD of the streamer type in gases and at gas-insulator interfaces which cover a large fraction of the cases encountered in technical insulation systems. The generalized model allows us to derive approximate relations between defect characteristics, insulation design parameters and test conditions on one side, and measurable PD characteristics on the other. The inversion of these relations yields rules for extracting defect information from the PD data. The application of the generalized model is illustrated by two simple examples, namely, spherical voids in an insulator and electrode protrusions in SF/sub 6/. >

A generalized approach to partial discharge modeling

The review begins with a very brief history of electrets and of piezo and pyroelectricity in nonuniform space charge electrets. The development of cellular propylene as an electret-transducer material since the mid-eighties is described, and the accompanying research into the origins of (quasi-)ferro-, pyro- and piezoelectric effects in voided polymer electrets is discussed in some detail. In this context, the most relevant charging and measuring techniques for piezo and pyroelectric polymer-foam electrets are introduced. Porous fluoropolymer electrets and their investigation as single or multiple-layer piezoelectrics are considered in view of their better thermal and temporal stabilities. Finally, some proposed or demonstrated applications of voided polymer electrets are briefly mentioned.

Less can be more. Holes in polymers lead to a new paradigm of piezoelectric materials for electret transducers

The authors present a review article of the properties of XLPE and an examination of the origins of space charge and how it is measured. The space charge in the insulation and the conduction current can affect the electric stress distribution in the insulation. The effect of morphology on the electrical properties of polyethylene is discussed in detail.

A general review of polymeric insulation for use in HVDC cables

Condition monitoring of engineering plant has increased in importance as engineering processes are automated and manpower is reduced. However, electrical machinery receives attention only at infrequent intervals when plant is shut down and the application of protective relays to machines has also reduced operator surveillance. A first edition of Condition Monitoring of Electrical Machines, written by Tavner and Penman, was published in 1987. The economics of industry have now changed, as a result of the privatisation and deregulation of the energy industry, placing emphasis on the importance of reliable operation of plant, throughout the whole life cycle, regardless of first cost. The availability of advanced electronics and software in powerful instrumentation, computers, and digital signal processors (DSP) has simplified our ability to instrument and analyse machinery. As a result condition monitoring is now being applied to a wider range of systems, from fault-tolerant drives of a few hundred watts in the aerospace industry, to machinery of a few hundred megawatts in major capital plant. In this new book the original authors have been joined by Ran, an expert in power electronics and control, and Sedding, an expert in the monitoring of electrical insulation systems. Together the authors have revised and expanded the earlier book, merging their own experience with that of machine analysts to bring it up to date. The book is aimed at professional engineers in the energy, process engineering and manufacturing industries, plus research workers and students.

Condition Monitoring of Rotating Electrical Machines

The decay of surface charges on polymeric insulators in air and SF6 is studied. Surface charges decay by three mechanisms, by volume and surface resistivity of the insulator as well as by neutralization from the gas. These mechanisms are investigated. In particular the mechanism of gas neutralization is explained in detail. It is shown that within a certain volume ions generated by natural radiation lead to a neutralization of surface charges on specimens and can contribute significantly to surface charge decay. Relevant gas parameters like ion pair generation, ion mobility and ion-ion recombination coefficient are given for air and SF6. The decay of surface charges generated by corona discharge is simulated by taking the three mechanisms into consideration. The intention of the simulation is to quantify the effect of the individual mechanisms of the decay of surface charges and to provide the patterns needed to interpret the experimental result given in part II of this study.

Surface charge decay on insulators in air and sulfurhexafluorid - part I: simulation

A theoretical approach to partial discharge transients is presented. This approach is based on the relationship between the charge induced on the measurement electrode by those created in the interelectrode volume during partial discharge activity. The primary sources for these induced charges are the charges which, as a result of partial discharge activity, are distributed within the voids of the insulation medium. These charge relationships are derived, and their application to the measured transients associated with the time dependence of the induced charge is presented. The application to multiple electrode systems of practical interest is illustrated. A discussion of the salient features and practical aspects of the theory is included. >

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The theory and measurement of partial discharge transients

Analytical expressions are deduced for the quantitative evaluation of the charge induced on an electrode by a partial discharge (PD) within a dielectric body. Two approaches have been employed to effect this evaluation. These are characterized by the /spl lambda/ function and the /spl phi/ function, and the essential relationships to be fulfilled by /spl lambda/ and /spl phi/ are derived. The principal difference between these functions is how the dielectric polarization is accounted for, either implicitly (/spl lambda/) or explicitly (/spl phi/). This difference is brought clearly into focus. Thereafter the application of these functions to PD studies is considered. It is demonstrated quantitatively, that perturbations in the dielectric polarization, brought about by the PD, can contribute significantly to the magnitude of the induced charge. As a consequence, the currently accepted approach to PD energy considerations should be re-examined. >

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The functional relation between partial discharges and induced charge

The phenomenon of electricification by electrostatic induction is introduced. This fundamental approach allows the concepts of Poissonian and Laplacian induced charge to be clearly developed. These concepts form the necessary basis for partial discharge detection analysis. Thereafter a quantitative theory is developed. This field-theoretical approach furnishes the correct input parameters for the subsequent study of the detection-circuit response, in which the electrodes of the field analysis serve as the input terminals in the circuit analysis. Subsequently the response of a basic partial-discharge detection circuit is examined with reference to the induced-charge signal. It is concluded that the maximum value of this signal will be most accurately determined if a detection circuit with a longtime constant is employed.

Partial discharge detection: theoretical and practical aspects

After partial discharge activity within a gaseous void, charges accumulate on the wall of the void. The decay of such charges due to surface currents at the void wall is studied analytically, and the factors affecting this decay are indicated. The results show that in terms of the basic time constant, the decay can take a considerable amount of time. The decay rate is significantly reduced by an increase in the permittivity of the bulk medium. The dominating influence of this permittivity is likewise reflected in the increased duration and thereby prolonged inhomogeneity of the electric field sustained in the void. However, the absolute value of this field is reduced with an increase in bulk permittivity. It is concluded that the present choice of a point charge to simulate the wall charge has the disadvantage that such a source is associated with a field singularity, and thus it is not possible to represent the maximum field at the void wall in a realistic manner. >

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Decay of charge deposited on the wall of gaseous void

The accumulation of space charge in solid dielectrics is examined from the macroscopic point of view using electromagnetic field theory. For practical dielectrics, it is shown that the occurrence of such charges is an inherent consequence of a non-uniform conductivity. The influence of both temperature and electric field strength upon this space charge formation is elucidated. Thereafter the theory is applied to DC cables. >

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I.W. McAllister

Papers

The theory and measurement of partial discharge transients

The functional relation between partial discharges and induced charge

Partial discharge detection: theoretical and practical aspects

Decay of charge deposited on the wall of gaseous void

Charge accumulation in DC cables: a macroscopic approach