Space charge in XLPE under dc and ac electric stress

Abstract

There are growing evidences that space charge plays an important role in electrical performance of polymeric high voltage cables.  The main objectives of the present work were to evaluate the influence of various factors such as heat treatment (degassing), temperature, electric stress level and frequency on space charge characteristics.  Both the Laser Induced Pressure Pulse (LIPP) and the Pulsed Electroacoustic (PEA) methods were used to measure space charge in cross-linked polyethylene (XLPE) samples.

The effect of heat treatment on space charge in XLPE under dc electric stress was conducted.  The results showed that more volatile crosslinked by-products can be moved from the sample by increasing the length and temperature of degassing and that it is also easier to remove the by-products from thinner samples.

Degassing also has another effect on the material in that its morphological structure is changed.  Results from the different scanning calorimetry (DSC) and the transmission electron microscope (TEM) indicate changes in the morphological structure when degassing parameters change.  It was also found that the threshold stresses at which space charge initiates and space charge evolution are highly dependent on samples’ by-product concentrations while changes in the morphological structure have only minimal impact.

The testing temperature, apart from affecting the threshold stresses at which space charge initiates, also increases charge energy and mobility which generally result in less space charge trapped in the samples.  It was found that at elevated temperature positive charge propagation enhances which eventually result in the space distribution being dominated by positive charge and takes the same form regardless of whether the sample is degassed or as-received.

The experiment on XLPE samples subjected to ac electric stress shows that the amount of charge trapped in samples subjected to ac electric stress is always much smaller than when subjected to dc electric stress of the same magnitude.  It was also demonstrated that frequencies and applied stress of the ac electric stress affect the amount of charge trapped in a sample.

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