J.J. Smit

Partial discharges at DC voltage: their mechanism, detection and analysis

A concise review is given of the progress made in the field of partial discharges (PD) at DC voltage. Although ample reference will be made to work of other authors in this field, the paper will concentrate on the progress that was made at Delft University of Technology over a period of 14 years in three PhD projects. In the first project, a start was made with the analysis of the physics of partial discharges at DC voltage and different types of PD were characterized based on parameters like time interval between PD and PD magnitude. In a second project, PD analysis was applied to HVDC apparatus and different means of classification of PD at DC voltage were proposed. In the third project, PD analysis was applied to HVDC mass-impregnated cables and test specifications were proposed. In this paper the work performed in the above three Ph.D. projects is summarized with ample reference to papers of other workers in this field. Attention is given to the mechanism of PD at DC voltage as compared to AC voltage and techniques for measurement and analysis of DC PD patterns. Examples of practical application of DC PD testing are given. Finally, some thoughts on future work are presented.

Monitoring and Diagnostic of Transformer Solid Insulation

Liberalization of the energy market has put increasing pressure on both electricity producers and distributors for lower costs. Since maintenance is a major expense account, such companies will be inclined to reduce maintenance budgets. At the same time, increased liability for nondelivered energy increases the costs of sudden failure of a component. Transformers are such a component; they are often an essential link in the distribution network. In order to reconcile both decreasing maintenance spending and reliable service, condition-based maintenance (CBM) is often proposed. The basis of a successful application of CBM lies in obtaining information on transformers so that on the one hand a critical condition will be noted early enough to take measures; on the other hand so that only minimal maintenance is being applied to transformers still in good condition. This paper will review a series of often-mentioned techniques in order to assess what the value of this technique will be for CBM, and whether it can be used for condition monitoring.

Advanced partial discharge diagnostic of MV power cable system using oscillating wave test system

To obtain a sensitive picture of discharging faults in power cables the PD should be ignited, detected, and located at power frequencies that are comparable to operating conditions at 50 or 60 Hz. In this way, realistic magnitudes in [pC] and reproducible patterns of discharges in a power cable can be obtained. PD measurements during service as well as on-site continuous energizing at 50 (60) Hz of MV cables are not always economically realistic for on-site inspections. Different energizing methods have been introduced and employed during recent years. Therefore, based on the assumption that sensitive detection of critical PD sites occurs by a method mostly similar to 50 Hz energizing conditions, a method as introduced for on-site PD diagnosis of MV cables is discussed: oscillating wave test system (OWTS).

Anomalous behaviour of the dielectric spectroscopy response of nanocomposites

A study on the dielectric spectroscopy of epoxy-based nanocomposites filled with different types of particles, such as Al2O3, AlN, MgO, SiO2 and BN, is presented. The surface of the nanoparticles was modified with a silane coupling agent, in order to make them compatible with the organic host and create a system with homogeneously dispersed filler material. Morphological characterizations of individual particles and fabricated composites were performed by means of transmission and scanning electron microscopy. The present research addresses an analysis of the complex permittivity. The relative permittivity of nanocomposites shows an unusual behaviour. Introduction of a low percentage of high permittivity filler results in a decrease of the permittivity of the bulk polymer material. We propose a qualitative explanation for the reduction of the relative permittivity, compared to the reference samples. The interface layer of surface modified particles plays a more important role than the nature of the particles themselves. The immobilization caused by the surface treatment of the nanoparticles seems to be the main factor determining the relative permittivity of the composites with fillgrade below 5 wt.%. The imaginary part of the complex permittivity, which represents the dielectric losses in the system, does not change significantly with addition of nanofiller up to 5 wt.%.

Modelling of the thermal conductivity in polymer nanocomposites and the impact of the interface between filler and matrix

In this paper the thermal conductivity of epoxy-based composite materials is analysed. Two and three-phase Lewis–Nielsen models are proposed for fitting the experimental values of the thermal conductivity of epoxy-based polymer composites. Various inorganic nano- and microparticles were used, namely aluminium oxide, aluminium nitride, magnesium oxide and silicon dioxide with average particle size between 20 nm and 20?m. It is shown that the filler–matrix interface plays a dominant role in the thermal conduction process of the nanocomposites. The two-phase model was proposed as an initial step for describing systems containing 2 constituents, i.e. an epoxy matrix and an inorganic filler. The three-phase model was introduced to specifically address the properties of the interfacial zone between the host polymer and the surface modified nanoparticles.

Pattern analysis of partial discharges in SF/sub 6/ GIS

The measurement of partial discharge(PD) of several faults in gas-insulated system (GIS) is discussed. Phase-resolved PD patterns have been measured using three different PD detection measuring systems: according to the IEC 270 recommendations, a VHF/UHF measuring system with narrow band filtering, and the UHF measuring system with wide band filtering. PD patterns are compared using computer-based discrimination tools. The influence of the selected center frequency on the PD patterns is discussed for the narrow band VHF/UHF measuring system. The influence of the number and type of GIS components between the discharging defect and the capacitive coupler on the shape of the PD patterns is analyzed. For several GIS components the signal reduction is studied. It was found that the shape of PD patterns is independent on the used PD detection circuit and the propagation path of the PD signals. As a result, discrimination and classification of PD distributions of several studied defects are possible using digital tools.

On-site testing and PD diagnosis of high voltage power cables

In addition to after-laying of new-installed high voltage (HV) power cables the use of on-site non-destructive on-site testing and diagnosis of service aged power cables is becoming an important issue to determine the actual condition of the cable systems and to determine the future performances. In this paper based on field experience an overview is presented on on-site testing and partial discharge diagnosis of HV power cables with regard to on-site testing methods: energizing, diagnostic aspects, possibilities and implications for new and service aged power cables.

Space charge measurements on multi-dielectrics by means of the pulsed electroacoustic method

The pulsed electroacoustic (PEA) method is now widely accepted as one of the most simple and effective techniques for the measurement of the dynamic space charge distribution in solid dielectrics. Recently, the PEA method has been applied also to laminar test objects composed of two or more layers of different dielectrics (multi-dielectrics). However, when a multi-dielectric is tested by means of the PEA method, the different acoustic and electric properties of the materials affect the detected space charge signal. In this paper, the principle of the PEA technique is reviewed in case the test object is a multi-dielectric. The generation, transmission and reflection of electrically-induced acoustic waves are described. Based on the proposed approach, results of PEA measurements performed on various kinds of multi-dielectrics are presented and discussed.

PD measurements on-site using oscillating wave test system

This paper discusses results of non-destructive dielectric tests using oscillating voltages. In particular, using comparison to discharge detection on HV cables at the conventional power frequency of 50 (60)Hz, the sensitivity and the reproducibility of this new method for on-site testing is discussed. Based on laboratory and on-site tests the usefulness of this PD measuring technique for practical applications is presented.

Thermal conductivity of nano-filled epoxy systems

Epoxy resin systems are used in high voltage transformers, cable terminations, bushings, power apparatus, or insulation for X-ray tubes. They could be used more widely, but low thermal conductivity limits their applications. Polymers used as insulating materials generally lack the ability to dissipate excess heat efficiently. The aim of this study is to investigate the influence of different types and filler loadings of electrically insulating — but thermally conductive -nanosized particles on the thermal conductivity of epoxy resin. The concentration of the filler is varied from 0.5 wt.% to 5 wt.%. A combination of ultrasonic processing and high shear force stirring is used to obtain an even dispersion of the corresponding filler in the base material. A silane coupling agent is used for surface functionalization of the nanoparticles. The application of the coupling agent improves the compatibility of the particles with the base polymer. Morphological characteristics of the samples are determined by using transmission electron microscope observation. The composites of epoxy resin containing nanoscale particles showed improved thermal conductivity values compared to epoxy resin without particles. The experimentally measured thermal conductivity results are compared with theoretical and empirical models for two component systems.