09_DEIS_Space_thick_ - F E A T U R E A R T I C L E Electric Field And Space Charge Measurements in Thick Power Cable Insulation Key Words space

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30 IEEE Electrical Insulation Magazine F E A T U R E A R T I C L E The evolution of electric field and space charge in high voltage ac and dc cable polymeric insulation, fol- lowed using the thermal step method, is described. 0883-7554/07/$25/©2009IEEE Electric Field And Space Charge Measurements in Thick Power Cable Insulation Key Words: space charge, electric field, polymeric insulation, thermal step method, high voltage ac, high voltage dc Introduction Q uantitative evaluation of the state of power cable insulation is still a challenge, involving much academic and indus- trial work. Build-up of space charge within the insulation under both ac and dc conditions is one of the aging indicators used by the electrical engineering community. However, well-proven and sensitive techniques are required to obtain accurate and re- producible results. This paper deals with measurement of space charge profiles in power cable insulation under high-voltage ac (HVAC) and dc (HVDC) electrical stress (Figure 1). Understanding the fundamental mechanisms involved in the degradation with time of the dielectric properties of cross-linked polyethylene (XLPE) cable insulation, and thus identifying the factors governing its electrical and thermal aging, are essential for predicting the long-term behaviour of new and installed high voltage cables. On the basis of correlations which have been made in the last ten years between degradation of XLPE and electric charge accumulation in its bulk (space charge) [1], [2], a European project involving industry and academic researchers was launched, with the goal of developing a diagnosis system for high voltage ac power cables [3]. The aim of the first part of the present work was to investigate space charge evolution in the insulation of cables submitted to long-term thermal and electrical ac conditioning, in order to identify potential aging markers. This investigation was carried out using the thermal step method (TSM), a non-destructive space charge measurement technique. It involves applying a temperature step to a short- circuited insulating sample and measuring the capacitive cur- rent response (of order 10 -12 to 10 -9 A). The response reflects the amount of charge trapped within the insulation; and from it the internal charge distribution can be calculated. The TSM can be applied to flat specimens [2], short pieces of cable or cable loops several meters long [4], [5]. Two variants of the TSM can be used: the inner heating tech- nique (IHT) and the outer cooling technique (OCT). The IHT, which does not require any specific preparation of the cable prior to the measurements, follows the evolution of the mean electrical state of the whole cable insulation (several meters long) in relation to applied aging electro-thermal stresses. Mea- surements are performed after ac aging and/or after low dc field poling (sometimes the application of a low dc field is necessary to reveal defects within the insulation). The OCT aims at a local analysis of the cable (20 to 40 cm lengths). A cooling exchanger
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This note was uploaded on 06/11/2011 for the course ELECTRICAL 124 taught by Professor Ghjk during the Spring '11 term at Institute of Technology.

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09_DEIS_Space_thick_ - F E A T U R E A R T I C L E Electric Field And Space Charge Measurements in Thick Power Cable Insulation Key Words space

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