Impact of DC Voltage Enhancement on Partial Discharges in Medium Voltage Cables—An Empirical Study with Defects at Semicon-Dielectric Interface

Abstract

Abstract:A scientific consensus is emerging on the benefits of direct current (DC) distribution in medium voltage (MV) power systems of ships and cities. At least 50% space savings and increased power transfer capacity are estimated with enhanced voltage DC operation of electric cables. The goal of this research is to contribute to developing the empirical knowledge on the insulation performance in order to validate the feasibility of such anticipated gains of DC versus alternating current (AC), and to determine the comparative impact of different operational conditions from a component engineering point of view. The partial discharge (PD) activity in cables is measured under AC and DC conditions as an indicator of insulation performance. Specifically, PDs in defects at the semicon-insulation interface are studied in terms of inception voltage, repetition rate and discharge magnitude. Empirical understanding is drawn for operating voltage and frequency dependence of the discharge behavior in such voids in the range of 10 to 20 kV and 0 to 0.1 Hz, respectively. The change in PD activity with void evolution post temperature-induced ageing process is explored.

Reliability, Efficiency and Cost Trade-offs for Medium Voltage Distribution Network Expansion using Refurbished AC-DC Reconfigurable Links

“…The key assumption contributing to this number is the operating voltage ensuring similar or better insulation performance of the system over its lifetime. Some empirical evidence for supporting an assumption of operating dc voltage rating equal to peak of the rated cable ac voltage is offered in [10]. In [11] however, it was shown that this enhancement factor is achievable only under normal operating conditions.…”

“…A comparison on the solutions for capacity enhancement with overhead lines is illustrated in [13]. Even though we have conservatively considered dc capacity enhancement factor at 1.5 in our work [9], primarily based on the empirical evidence of cable insulation performance under ac and dc conditions [10], other studies consider higher capacity gains for underground cables [14]. The reasoning behind this anticipation is, possibly, lower partial discharges under dc operation as compared to ac for the same cable insulation [15].…”

“…Based on qualitative understanding, the detrimental effect on insulation lifetime in case of frequent and sudden polarity reversal may encourage minimization of dc to ac reconfiguration during (n-1) contingencies. However, many aspects need further investigation, such as: (i) magnitude of accumulated space charges at medium voltage levels (translated to imposed electric fields on the insulation) [22] (ii) time and temperature dependence of space charges when dc voltage is imposed or removed [23] (iii) insulation performance under the expected space charge accumulation at different locations in cable insulation system over its entire operational lifetime [10] (iv) understanding of impact on insulation with dc to ac reconfiguration based on a) reconfiguration frequency b) timelag after each such reconfiguration. On one hand it is difficult to measure the accumulated space charges during on-site operation; On the other, it is even harder to quantify their precise effects on the insulation lifetime.…”

Modular Multilevel Converter Performance with Dynamic MVDC Distribution Link Voltage Rating

“…The quantification of the expected capacity and efficiency enhancement is provided in [4], wherein, better infrastructure utilization with dc operation is combined with superior performance of modular multilevel converters (MMC) [6], [7]. The consequences of dc voltage enhancement on the cable insulation performance is empirically studied in detail [8]. Furthermore, in [9], the concept of system reconfigurability is introduced to maintain capacity enhancement during (n-1) contingencies.…”

“…The 1 p.u dc link voltage V d is decided based on the cable dc voltage rating, which is 2 √ 2 times the rms phase voltage at the ac side of the MMC. More information on deciding the dc voltage rating of cable based on its performance under ac and dc operating conditions can be found in [8]. Case I considers rated current I d at 1 p.u when 1 p.u active power is transferred by the dc link.…”