Investigating Optimal Approaches for Energizations of DC Systems: The Stepwise Technique

Abstract

The increasing presence of HVDC (High Voltage Direct Current) links and MVDC (Medium Voltage Direct Current) connections in transmission networks needs an in-depth knowledge of the mechanisms that could affect life and reliability of electrical asset components. As for all electrical systems in general, a crucial factor affecting availability of a HVDC asset is the insulation system reliability. One of the most common causes of premature electrical failure of asset components is electrical insulation breakdown or loss of serviceability. MV/HV extrinsic accelerated aging of organic insulation is mainly due to partial discharges (PD). Their presence, even if not promoting immediate breakdown, is almost a certain cause of apparatus life not matching specifications. This paper focuses on energizations on HVDC insulation systems, and, in general, supply voltage variations, which could be affected by PD due to manufacturing, laying, interface, or structural defects (as butt gaps). To minimize PD inception risk, a theory-driven, stepwise voltage application concept is proposed. The basic idea arises from the consideration that the factor playing a key role in PD inception is the jump voltage, and the electric field distribution in insulation during voltage transients is driven by permittivity (capacitance). Thus, energizing an insulation system with a first step lower than the partial discharge inception voltage in AC (PDIVAC) and then growing voltage by smaller steps minimizing jump voltage would allow almost PD-free operation (apart from the risk of low-repetition PD at DC steady state if nominal voltage exceeds PDIVDC). The effectiveness of this approach is proved by means of tests carried out on a simple test object consisting of flat XLPE (Crossed Linked Polyethylene) specimens with electrodes designed to cause surface PD, having available an innovative PD detection system which performs automatically and effectively both during voltage transients and in DC steady-state conditions. The experimental results show that the stepwise voltage energization, compared to the single step energization, could decrease significantly the number and magnitude of detected PD during energization transients, leading therefore the reduction of one order of magnitude of PD associated energy and relevant damage.