Soft Start-up Scheme for Weak AC Grid Dominated MTDC Networks

• Published in: IECON 2020 The 46th Annual Conference of the IEEE Industrial Electronics Society pp. 4194 - 4199
• Main Authors: Isik, Semih, Bhattacharya, Subhashish
• Format: Conference Proceeding
• Language: English
• Published: IEEE 18-10-2020
• Subjects: Capacitors; Frequency control; HVDC transmission; Mathematical model; MMC; MTDC; Resistors; Soft Start-up; Surges; Voltage control; VSC-HVDC
• ISSN: 2577-1647
• DOI: 10.1109/IECON43393.2020.9254777

HVDC transmission is considered as an economical alternative for HVAC transmission at higher voltage levels for long distances due to bulk power carrying capability of cost-effective DC cables. The developments in the AC/DC converters, such as MMC, have made a remarkable contribution to the growth of the HVDC systems. This paper investigates soft start-up strategies for the MTDC networks, consisting of weak (SCR<3) or very weak AC grids (SCR<2). Conventional energization of an MTDC grid through pre-insertion resistors causes high surge energy and power losses. Thus, this paper adopts a two-level converter and frequency controller to provide soft energization to the DC grid, preventing high inrush current and high transients in the weak terminals. Theoretical analysis of an average large-signal model of each converter, along with its control structure, is depicted in detail. The proposed soft start-up scheme and the dynamic characteristics of the MTDC network are carried out on the RTDS.