R. Bodega

Polymeric HVDC Cable Design and Space Charge Accumulation. Part 1: Insulation/Semicon Interface

From theory and experiments, it can be deduced that materials for DC applications should not accumulate a large amount of space charge if accelerated degradation of the insulation system is to be avoided. Therefore, the characterization of DC insulation must take into account the evaluation of space charge accumulation. This cannot be done exhaustively without taking a system approach considering both the semiconductive material and the insulation, in particular, the properties of the semicon/insulation interface. The latter interface, in fact, plays a major role in space charge injection/accumulation in the insulation bulk. Having analyzed different semiconductive and insulating materials candidate for HVDC cable applications, the best solution to be exploited for HVDC cable design would be the combination showing a high threshold for space charge accumulation, a small rate of charge accumulation as a function of electric field and a small activation energy, i.e., a space charge amount less dependent on temperature. Therefore, space charge measurements will provide important information to cable material manufacturers with the aim of tailoring insulation and semicon specifically for HVDC application and, thus, improving the reliability of polymeric cables.

Feature article - Polymeric HVDC cable design and space charge accumulation. Part 2: insulation interfaces

The electric field distribution of cable insulation systems under HVDC can be affected significantly at interfaces due to space charge build-up. In this article, the second part of a three-article series, face and space charge accumulation are analyzed first in terms of macroscopic physics, then through approximate mathematical models that will be used to fit experimental data obtained for model cables having two insulation layers and constituting cylindrical interfaces.

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.

HVDC Cable Design and Space Charge Accumulation. Part 3: Effect of Temperature Gradient

The aim of this article is to investigate the extent of space charge accumulation due to a temperature gradient in comparison with other charge-supply mechanisms, particularly injection from the electrodes. For this purpose, space-charge measurements were carried out on HVDC cable models under application of different temperature gradients across the cable insulation, above and close to the threshold field for space-charge accumulation. The main results, consisting of space-charge patterns and extracted quantities, are discussed here.

PD recurrence in cavities at different energizing methods

The measurement of partial discharge (PD) activity has become an invaluable tool for monitoring the insulation condition of high-voltage components in service. In particular, it is important for factories and for the utilities to get an indication of the time to breakdown. In this paper, the authors describe a study on the use of a damped ac voltage (DAC) method and a very low frequency (VLF) method to stress insulation and to detect partial discharge activity. The main goal of this study was to find out whether the PD patterns obtained at 50 (60)-Hz voltage correspond to those obtained at either lower frequency voltage (VLF) or higher frequency voltage (DAC). For this purpose, a combined theoretical and experimental approach for dielectric bounded cavities was used. The effect of the frequency and the shape of the voltage (DAC) on the PD phase-resolved pattern were studied in the laboratory. Tests were performed on dielectric bounded cavities in polyester with the DAC method, the VLF method (0.1 Hz), and the 50 (60)-Hz method. Moreover, the PD phenomenon was theoretically analyzed at different stress frequencies, and a comparison was made between theory and measurement.

The effect of voltage frequency on partial discharge activity

The knowledge of partial discharge (PD) as a physical phenomenon has made considerable progress in the last decade(s). These efforts have led to a validated statistical-mathematical model able to simulate the PD behavior in dielectric bounded spheroidal cavities. Recent studies demonstrated that an approach based on simulations can give both qualitative information on the PD process and a good estimation of the main PD quantities (PD magnitude, PD inception/extinction voltage, PD pattern). This theoretical operating procedure was until now restricted to discharges ignited at the power stress frequency of 50 (60) Hz. In this paper the authors describe the use of a procedure to simulate partial discharges in dielectric bounded spheroidal cavities at frequencies in the range of 0.1 Hz....1000 Hz. Information on the effect of voltage frequency on the PD process has been derived from the simulation results. The results were validated by measurements at a range of test frequencies on test specimens containing spherical cavities.

Conduction current measurements on XLPE and EPR insulation

Electrical conduction of crosslinked polyethylene (XLPE) and ethylene propylene rubber (EPR) is studied by means of DC charging current measurements. The threshold for space charge accumulation and the DC conductivity of XLPE and EPR have been investigated at different electric fields and at different temperatures. Current-voltage characteristics show a transition from ohmic behavior to high field behavior, above a threshold value of the applied electric field. An Arrhenius-type dependence of both XLPE and EPR conductivity with temperature was found. Measurements were performed also on two-layer XLPE/EPR combinations. Based on conductivity values obtained by measurements on a single layer sample, the electric field distribution of the two-layer combination is discussed.

Dielectric Interfaces in DC Constructions: Space Charge and Polarization Phenomena

Interfaces between dielectrics are considered one of the weakest parts of an insulation system but their behavior under electrical stress is not yet completely understood. In particular, when a dc voltage is applied, the electric field distribution at the interface is quite difficult to predict. This is mainly due to the accumulation of internal charge, which distorts the initial Laplacian field. To shed some new light on this topic, space charge measurements were performed on two types of coaxial XLPE-EPR interfaces. A numerical procedure, based on the Maxwell-Wagner theory for interfacial polarization, was developed for the estimation of the dynamic charge build-up at the interface. Experimentally obtained space charge profiles were compared to the calculated profiles. The limits of the Maxwell-Wagner model were assessed and the main parameters, which affect the interfacial polarization but which are not included in the model, were identified.

The Effect of Temperature Gradient on Space Charge and Electric Field Distribution of HVDC Cable Models

This paper deals with the analysis of the effect of temperature gradient applied to polymeric cables on space charge accumulation inside insulation bulk. In particular, experimental measurements on HVDC cable models showed that space charge due to temperature gradient is generally negligible with respect to excess charge injected from the electrodes at electrical fields larger than the threshold for space charge accumulation. The effect of temperature gradient can be revealed only at fields close to the threshold, i.e. 5 to 10 kV/mm, leading, however, to a small extent of charge accumulation, of the order of tens of mC/m