Dielectric spectroscopy in the time or frequency domain offers new opportunities for an off-line, on-site insulation condition assessment of HV electric power equipment and its predictive maintenance.

Applications of dielectric spectroscopy in time and frequency domain for HV power equipment

This paper proposes a method for comparing the effect of different maintenance strategies on system reliability and cost. This method relates reliability theory with the experience gained from statistics and practical knowledge of component failures and maintenance measures. The approach has been applied to rural and urban distribution systems. In particular, a functional relationship between failure rate and maintenance measures has been developed for a cable component. The results show the value of using a systematic quantitative approach for investigating the effect of different maintenance strategies.

A reliability-centered asset maintenance method for assessing the impact of maintenance in power distribution systems

This paper describes briefly the main ageing and failure mechanisms and will indicate the advantages and limitations of the diagnostic tests available for the different insulation systems used in distribution and transmission cable systems. SC21 of CIGRE has addressed the subject of ageing factors and diagnostics of cable systems and has published three reports covering both fluid-filled and extruded cables. The reports describe ageing factors and several diagnostic methods, their purpose, and guidelines for use.

Ageing mechanisms and diagnostics for power cables - an overview

An HV dielectric spectroscopy system has been developed for diagnostics of water tree deteriorated extruded medium voltage cables. The technique is based on the measurement of nonlinear dielectric response in the frequency domain. Today's commercially available systems are capable of resolving low loss and small variations of permittivity as a function of frequency and voltage. Experience from more than 200 field measurements was combined with laboratory investigations. Small samples were used in an accelerated aging test to elucidate the correlation between water tree growth and dielectric response. Furthermore, field aged cables were investigated in the laboratory. It has been shown that the dielectric response of water tree deteriorated crosslinked polyethylene (XLPE) cables can be recognized and classified into different types of responses related to the aging status and breakdown strength. The influence of termination and artifacts such as surface currents was investigated. The measurement method enables us to separate the response of the cable from the influence of accessories. Finally, two different field studies of the implementation of the diagnostic method are presented. The field studies show that the fault rate decreased significantly when replacement strategy was based on the diagnostic criteria formulated.

Dielectric spectroscopy for diagnosis of water tree deterioration in XLPE cables

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On-line detection and location of partial discharges in medium-voltage power cables

This paper summarize results from work on diagnosis of medium voltage (MV) XLPE cables with respect to water treeing performed at KTH, Stockholm The work, performed both in laboratory and in the field, has shown that high voltage dielectric spectroscopy is a good tool for diagnosis of MV XLPE cables and that it is able to differentiate between different stages of water tree ageing

Diagnosis of medium voltage XLPE cables by high voltage dielectric spectroscopy

The authors report on a high-voltage, frequency-domain measurement system capable of resolving losses in unaged XLPE (cross-linked polyethylene). The system allows analyses of harmonics as well as of the fundamental frequency. The highest operating voltage is 20 kV and the frequency range is 0.0001 Hz to 1000 Hz. Different XLPE cable samples (new, artificially aged, field aged) were measured in the frequency domain using 4 kV peak voltage at a temperature of 22/spl deg/C. >

High voltage dielectric response analyser for cable diagnostics

A research project was started in 1992 with the aim to develop a useful nondestructive method to determine the condition of water tree deteriorated XLPE cables in service. Since 1995 an extensive laboratory investigation has been made on cables aged in service and a large number of field measurements have been performed on cables in service. The purpose of this report is to present these results and the further development of the diagnostic procedures. This report starts with presentation of the measurement procedure. The procedure is applied both to laboratory and field measurements. In the part on laboratory investigations the ability to detect deteriorated cables is verified and measured dielectric response is related to breakdown voltages. The application of the method on cables in service is then presented. The field investigation part also includes practical considerations on the measurement procedure and on the influence of accessories and terminations.

U. Gafvert

Papers

Dielectric spectroscopy for diagnosis of water tree deterioration in XLPE cables

Diagnosis of medium voltage XLPE cables by high voltage dielectric spectroscopy

High voltage dielectric response analyser for cable diagnostics

Dielectric measurements for diagnosis of XPLE cable insulation