Analysis of Lightning Transient Characteristics of Short-Length Mixed MMC-MVDC Transmission System

Abstract

Medium voltage direct current (MVDC) transmission system are growing due to their assistive quality in conventional grid and compatibility with renewable power network. MVDC distribution links with “Mixed” overhead (OH) & underground (UG) sections could be devised based on urban planning. UG Cables or substations are indirectly exposed to lightning strikes due to adjacent tower sections. In case of MVDC converter or cable, present researchers do not specify lightning voltage impulse level for related system voltage. Therefore, preluding electromagnetic (EM) transient investigation are required to determine the maximum lightning overvoltages for system peripherals i.e. cable & Modular Multilevel Converter (MMC) substation. This research focuses on analyzing lightning performance of OH transmission towers-cable junction & tower-substation link in case of a shielding failure (SF) and back flashover (BF) for a ±35kV short-length mixed MMC-MVDC transmission scheme. This article provides broad-band modeling method for MMC substation for lightning investigation. In addition, based on a detailed time-domain parametric evaluation in PSCAD/EMTDC program, lightning impulse voltage across the transmission line’s pole insulator and embedded cable section are estimated along with numerical validation relying on travelling wave theory. Effect of project parameters such as tower grounding resistance, riser section surge impedance (which connects cable & OH line) and cable length on lightning overvoltage impacting the cable and connected tower section has been demonstrated.

Keywords

• MVDC transmission
• modular multilevel converter
• maximum shielding failure
• insulation flashover
• PSCAD/EMTDC
• sheath grounding