This article reviews the different components, conductors, semiconductive shields, insulation, sheaths, and jackets, that make up extruded, high-voltage cables. The different types of conductors are described as are the different insulation systems that are available for medium and HV cables. There is a comparison of the two main insulators used in extruded HV cables, EPR, and XLPE. Each material has some properties that are better than those of the other material.

Polymer insulated high voltage cables

Partial discharges are symptomatic of many degradation phenomena in power cables and may cause further deterioration of the insulation in many cases. Electrical im- pulses, generated by partial discharges, travel towards the cable ends, and can there be detected using appropriate sensors. Continuous monitoring of the insulation con- dition can be achieved by on-line detection and location of partial discharge (PD) signals. An important aspect of such a diagnostic is the analysis of on-line measure- ments. The research reported in this thesis is aimed at analysis of PD signals from on-line measurements and location of discharge sites. Signal analysis depends on knowledge of both signals and disturbances that are to be expected. To that end, characteristics of PD signals in medium voltage cables are studied in this thesis. The result of this study is a signal model of the propagation path between the discharge site and the sensors. The model accounts for cable sections with di®erent properties, and incorporates the propagation channel load impedances, i.e. the equipment to which a cable is terminated in an on-line situation. The exact propagation properties and load impedances depend on the speci¯c cable connection under test, and are unknown a priori. For this reason, research is conducted on meth- ods that enable experimental characterization of the parameters, by evaluating the response of the cable to applied transients. The presented methods rely on the ex- traction of pulses that are re°ected on impedance transitions within the cable system under test. On-line ¯eld measurements are corrupted by noise and interference, which impede PD signal detection and location. Generally, narrowband interferences resulting from radio broadcasts dominate the measurements, thus prohibiting data-acquisition trig- gered by PD signals. Broadband background noise is present within the entire PD signal bandwidth, and therefore poses a fundamental limit on PD signal analysis. Generally, existing extraction techniques for PD signals only partially exploit a priori knowledge of both signals and interference. In this thesis, matched ¯lters are ap- plied that are derived from the signal model, and are optimally adapted to the signals that can be expected. Besides signal extraction, matched ¯lters provide a means to estimate the PD magnitude and the signal arrival time. Likewise, discharge location methods based on the signal model are proposed, resulting in optimal location esti- mators. Computer simulations illustrate the e®ectiveness of the proposed algorithms and show that the attainable accuracy can be speci¯ed by theoretical bounds. Accurate PD location relies on estimation of the di®erence in arrival times of signals originating from the same discharge. In case of on-line detection, the cable is connected to the grid, and signals are not necessarily re°ected at the cable ends. Therefore signal detection at both sides is generally required for the purpose of dis- charge location. Synchronization of the measurement equipment is achieved using pulses that are injected into the cable connection. Finite-energy disturbances, such as PD signals that originate outside the cable connection under test, frequently occur in on-line situations. Since measurements are synchronously conducted at both cable ends, pulses originating within and outside the cable can be distinguished by examining the di®erence in time of arrival. Moreover, in many situations, the signal direction of arrival can be determined by detecting pulses in two di®erent current paths at a cable termination. This method is applied as an additional technique to discriminate PD signals and disturbances. Based on the results of research, a measurement system is proposed, which enables automated on-line PD detection and location in medium voltage cable connections. The conceptual design is validated by experiments, and the results demonstrate that the practical application is promising.

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On-line signal analysis of partial discharges in medium-voltage power cables

The electrical, thermal, and multiple-stress (thermal and electrical) aging of cross-linked polyethylene (XLPE) cable models has been investigated in order to get information on aging effects and mechanisms. After endurance tests, XLPE cable models have been subjected to chemical, physical, electrical, and microstructural characterization. Thermal aging results reveal that bulk degradation occurs in the cables at temperatures higher than the melting point. Multiple-stress aging data emphasize a synergistic effect of electric field and temperature. Significant microstructural changes detected in the cables aged under multiple stress are evidence of this effect and can partly explain the time behavior of the electric strength. >

A contribution to the study of aging of XLPE insulated cables

As a result of experimental and theoretical progress over the last decade, our understanding of water treeing is approaching the molecular level at which the fundamental chemical nature of the phenomenon will be revealed. Further effort is required to complete the picture. The analytical techniques employed to date indicate that the water tree region contains a wide range of chemical species, with various forms of carbonyl and metal ions and water being dominant. Among carbonyl species, carboxylate ions are dominant, followed by trace amounts of esters and ketones. Present oxidation theories of water treeing are based on these observations and provide the best and most detailed description of the process. >

The chemical nature of water treeing: theories and evidence

A new and unique measuring system is presented for the online acquisition and analysis of partial discharges (PD-s) in medium-voltage power cables. The system is able to locate the origin(s) of PD-s by using two inductive sensors, each at one cable end. The measuring system is called PDOL, which stands for PD detection On-line with Localization. A patented solution is used for the time synchronization of the data intake at both cable ends and for the on-line calibration. One computer at a remote location collects data from various PDOL measuring systems for final presentation and interpretation. This paper discusses the basics of PDOL and some first measurement results.

/pdf/on-line-partial-discharge-detection-of-mv-cables-with-defect-324i6ixgeu.pdf

On-line partial discharge detection of MV cables with defect localisation (PDOL) based on two time synchronised sensors

This paper describes a partial discharge propagation model and a location estimate which uses digital signal processing techniques. The model and the estimate are developed for an instrument which is capable of locating partial discharge sites in power cables non-destructively and in situ. Since the model utilizes actual cable response to partial discharge signals, it provides an excellent waveform match between simulations and measurements. The estimator can locate partial discharge sites in noisy environments with an error not exceeding 2 m. Applications of this model to field test data are also discussed. >

Partial discharge propagation model and location estimate

The resolution and robustness of a cable PD locating instrument, based on time-domain reflectrometry (TDR), are critically affected by noise present in a field environment. The permanent background noise encountered during field tests has been shown to be mainly due to amplitude modulation (AM) radio signals. This paper describes a self-learning digital filter aimed at mitigating this noise. The filter, in conjunction with a signal averaging routine, has been successfully used to reduce noise and enhance the PD signal obtained from a cable in a field environment. Examples are provided to demonstrate the ability of the filter to significantly improve the signal-to-noise ratio (SNR) and effectively extract PD signals from field recordings. >

http://ieeexplore.ieee.org/iel3/3187/9053/00401520.pdf

Self-learning digital filter for the field location of partial discharge in cables

A clean, crosslinked polyethylene insulating material was sandwiched between two parallel layers of semiconducting shield compound in test cells which simulated extruded medium-voltage cables. Four commercial and four experimental semiconducting compounds were used. Cells were aged at 60 Hz, with a stress of 2.6 kV/mm (65 V/mil) or 3.4 kV/mm (85 V/mil), with one shielding layer exposed to air and the other exposed to distilled, deionized water. The effects of water soluble impurities in the semiconducting compounds on the number, size, and location of water trees developed in the insulation, at the semiconducting shield interfaces, are discussed. The movement of soluble impurities is also addressed. >

Role of semiconducting compounds in the premature aging of XLPE cable insulation

High static voltages can be generated as a result of the flow of a low conductivity fluid over a solid surface. While past work has concentrated mainly on the properties and conditions of the fluid component, the fundamental charge generation process was studied in this research emphasizing the role of solid insulating materials. Streaming potential was measured by means of a streaming potential apparatus, under well controlled moisture and temperature conditions as well as oil charging properties. Characterization of solid insulating materials was performed using image analysis, dc conductivity measurement, and time-domain dielectric spectroscopy (TDDS). The results showed a linear relationship between streaming potential and pressure drop as predicted by theory. Statistical analysis indicated that the surface roughness significantly affects the streaming potential in the laminar flow regime, but the dielectric constant has no effect. >

M.S. Mashikian

Papers

Partial discharge propagation model and location estimate

Self-learning digital filter for the field location of partial discharge in cables

Role of semiconducting compounds in the premature aging of XLPE cable insulation

Role of solid insulating materials in the flow electrification of power apparatus