Portable installation for space charge measurements on full-size high voltage cable loops based on a multicell power converter

The development and the qualification of polymer-insulated high voltage dc cables require the measurement of the space charge in order to ensure that charge accumulation does not result in high internal fields leading to breakdown. The aim of this work is to set up a portable installation to be used for space charge measurements on long cable loops (up to 100 m) and high conductor sections (up to 2500 mm²). Its principle is based on a transient heating of the cable core or of the outer conducting screen by Joule effect for generating a thermal pulse of several degrees, which, by crossing the insulation, generate a capacitive current measured by a current amplifier (inner heating thermal method). This requires the ability to obtain and to control a high current (up to 12 kA) during a given period of time. The use of ac current heating requiring significant apparent power, it can hardly be used for such a goal. We propose a set-up based on a coupled inductor parallel multicell power buck converter, with a topology especially designed for generating and controlling high currents during short periods of time. The design and the manufacturing of the converter are described. The validation of a base cell delivering 600 A through tests on power cable is presented.