Implementation of On-line Partial Discharge Measurements in Medium Voltage Cable Network
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The reliability of power supply has become an increasingly significant factor in modern society. The aging of the existing network, the increased quality requirements of electric supply and the risk of major interruptions due to storms are factors that are driving towards the cabling of the distribution network. It is important to develop a condition management process for medium voltage cables to be able to optimize the construction and total life cycle costs of such of a cable network. Due to its hidden structure the condition of a cable network can be evaluated only by means of measurements. It is widely acknowledged that a clear dependence exists between the deterioration of the cable and partial discharges. For years, partial discharge measurements have been used as part of condition management in case of single expensive electrical equipment like generator etc. and it is proofed to be a working solution. The goal of this work is to bring out the main principles of on-line partial discharge measurements and the current situation in case of extruded medium voltage cables. In addition, aging and other phenomena leading to faults and partial discharges related to these phenomena in extruded medium voltage cables have been studied. Finally, guidelines about how on-line partial discharge measurements could be used as part of condition monitoring of medium voltage cables have been created. The nature of this work is a literature survey and the source material mainly consists of international publications. The temperature, voltage stress and moisture are the main factors that accelerate the aging of solid insulation. Thus overloading should be avoided, voltage stresses should be minimized and water proof components should be used. Cable joints and terminations seem to be the most fault prone components in the medium voltage cable network and in most cases partial discharges can be spotted before the breakdown. It is also noted that the quality of workmanship in the installation of cable accessories plays an important role while the reliability of these components is concerned. This work concludes that on-line partial discharge measurements are the most suitable for detecting problems in the cable joints, terminations or metallic shield. The evaluation of the general condition of a cable system is very challenging with current on-line partial discharge measurement systems because voltage above normal operation voltage can’t be used during measurements. In addition, in many cases also the measurement sensitivity is not good enough. With continuous measurements, reliable time to next breakdown estimations can be made. The biggest challenges for continuous measurements are related to their cost-effectiveness. The future will show whether these kinds of measurement increase or not.