Effect of Voltage Slew Rate on Partial Discharge Phenomenology During Voltage Transient in HVDC Insulation: The Case of Polymeric Cables

In this article, the partial discharge (PD) behavior inside a cavity embedded in a dc cable, upon application of a voltage step from transient till steady-state dc, is analyzed as a function of voltage slew rate. The variation of PD characteristics, namely PD charge amplitude and repetition rate, is modeled and fitted from the beginning of the voltage transient to dc steady state, and the extent of aging associated with PD at different slew rates is evaluated through a cumulative damage concept. Two possible scenarios for the electric field transient and PD repetition rate are presented, differing for the ratio of conductivity and permittivity of dielectric and cavity medium. Having demonstrated that the proposed PD repetition rate model can predict reasonably the experimental results, it is shown that slowing down voltage rise time during, e.g., cable energization or voltage polarity inversion, could be beneficial to reduce the accelerated aging extent associated with voltage transients.

I. Introduction

HVdc and MVdc cable insulation design is a challenge for several reasons. The most important one is that in steady state, the electric field is ruled by conductivity, varying with temperature (load), while during voltage transients, as energization or voltage polarity reversals, the electric field is driven initially by permittivity [1]–[3]. Such dynamically changing electric field distribution may result in different contributions to both intrinsic and extrinsic aging rates [4]. In particular, referring to extrinsic accelerated aging, partial discharges (PDs), phenomenology can change considerably from transient to steady-state conditions. This is due to electric field distribution that can incept PD in different defects or at different field levels depending on defect location because the amplitude of PD pulses and repetition rate can differ from the initial part of a transient when the electric field is rising rapidly, to the steady-state dc, when the field is constant. On the other hand, dc insulation, and particularly dc cables, may experience a large number of energizations or voltage polarity inversions during life. Although the latter is going to be prevented somehow using the modern converter technologies, the former is increasing considerably in modern grids and assets tending toward enhanced interconnections, hybrid grid concepts, and renewable integration [5].