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 (PDIVAC). 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 PDIVAC is generally smaller than DC PD inception voltage (PDIVDC). Hence

energising a DC cable by an initial step lower than PDIVAC

and then increasing the voltage in steps smaller than PDIVAC

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 PDIVDC.