Behaviour of HVDC Cables Under Variable Load Conditions: PD Detection and Recognition

HVDC technology is widely used today in the field of energy transmission over long distances and with underground or submarine cable lines. The advantages of these applications lie in the absence of stability and transport capacity problems which characterize an equivalent HVAC line as the distance increases. However, new challenges arise in the transition from HVAC to HVDC technology. The accumulation of space charge plays a fundamental role in the ageing process of the component and in the possible triggering of partial discharges. Furthermore, the dependence of the conductivity of the dielectric material on the temperature causes the load condition, whereby a radial thermal gradient is established, to influence this charge distribution and therefore on the electric field profile. The aim of this paper is to analyze the effect of the load conditions on the behaviour of an HVDC cable evaluated in terms of partial discharge activity. The test object of this work is an extruded HVDC model cable with an artificial insulation defect. With a transformer-like device a current was induced to simulate the operating conditions. The acquired data concern two tests carried out with and without load current. Following the measurements, the pulse classification was performed with a new algorithm based on the cross-correlation operation. The results show that in the absence of a thermal gradient the partial discharge activity is very low and most of the acquired pulses are identified as noise. Instead, in the presence of a thermal gradient, a more intense activity is observed.