Investigating New Approaches for Optimal Energisations of DC Cables: The Stepwise Energisation Technique

Abstract

This paper focuses on the energisation of high voltage DC (HVDC) and medium voltage DC (MVDC) insulation systems, referring mainly to cables for both theoretical development and validation testing. Cable system energisation can be frequent during its lifetime, and it can possibly be affected by partial discharges (PD), because of manufacturing, laying, ageing, interfaces or structural cavities (as butt gaps). A theory-driven and measurement-based procedure is presented in this paper, having the purpose to minimise PD inception risk. This procedure is based on stepwise voltage application during cable energisation. The fundamental idea behind the proposed approach stems from considering that the jump voltage is the trigger of PD occurrence. Indeed, the jump voltage, and the consequent electric field variation, directly relates to AC PD inception voltage (PDIV_AC). In addition, the electric field distribution in an insulation system is driven by insulation permittivity (capacitance) during voltage transients, and by conductivity in DC, thus the PDIV_AC is generally smaller than DC PD inception voltage (PDIV_DC). Hence, energising a DC cable by an initial step lower than PDIV_AC, and then increasing the voltage in steps smaller than PDIV_AC, would minimise the risk of PD inception during transients and the relevant degradation rate. However, this does not change, the risk of occurrence (if any) of low-repetition partial discharges at DC steady state. Effectiveness of the proposed technique is proved by the help of tests performed on cables with artificial surface and internal defects. It is shown that compared with the conventional energisation consisting of rapidly increasing voltage, the stepwise approach can reduce the risk of PD inception and related extrinsic ageing, even for the steady state voltages larger than PDIV_DC.