Electrical treeing and tracking in polymeric interfaces in cable joints in DC networks

High voltage systems are ubiquitous in the modern world and are now an integral part of all electricity power networks. They are critical in the transmission of renewable energy and intra-system connections. The use of high voltage systems is now becoming more prevalent in mass-transit systems and large electric vehicles, notably ships and aircraft. HV DC links are also becoming more important both for aspirational intercontinental networks, and for point-to-point links of which some have now reached +/-800 kV, stretching across China for example. Recently the UK government announced a plan for 30% of electricity production to come from offshore windfarms by 2030, which will also increase dependence on HVDC sub-sea cables and place additional importance on their reliability.

The project described here underpins the need for HVDC connections and their reliability and so contributes to EPSRC's research themes of: Energy Networks, Materials for Energy Applications, and Infrastructure and Urban Systems.

A key mechanism for failure of high voltage polymeric cables and their fittings (joints and terminations) is electrical tree growth. Such degradation takes the form of bifurcated, tubules which resemble botanical trees. The growth of such trees across the insulation leads to catastrophic failure and has been widely studied for AC systems. However much less is known concerning treeing in DC systems. In particular, there is little work concerning interfaces and the vulnerability to these areas in joints to treeing/tracking. The importance of power quality in DC ageing is well-established, but not quantified. Similarly the use of partial discharge analysis for understanding progression of AC tree growth is becoming better understood, but that is not the case for DC tree growth. The research question asked here then is, 'what is the role of steady state power quality on tree growth in DC polymeric insulation?'.

This project will study tree growth and inception under DC conditions with controlled power quality. This work will be largely experimental building on the expertise already available in the HV voltage laboratory. A key feature of this work is that higher DC voltages will be used in the laboratory than has been achieved previously in this context. This is challenging, but will add substantially to our ability to take laboratory work and apply it to real applications. In particular, tree initiation at interfaces will be considered in detail. Polyethylene and silicone systems will be studied. PD measurements, both optical and electrical, will be used to understand the role of discharges in the progression of DC trees. Consequently, the use of PD as an asset management tool will be considered.