Analysis of Transmission Distance of the MMC-HVDC Connecting Islanded Renewable Energy Generation System

Unlike the conventional Line-Commuted Converter High Voltage Direct Current (LCC-HVDC) system, the Modular Multilevel Converter High Voltage Direct Current (MMC-HVDC) system experiences no commutation failures. It enables the transmission of islanded renewable energy generations (REGs) to the power grid. The Short-Circuit Ratio (SCR) is traditionally used to evaluate the strength of power systems with REGs. It is significant to develop the SCR to reflect the supportive role of the MMC-HVDC in providing voltage support to the REG island. This paper proposes an assessment method of the strength of the MMC-HVDC connecting islanded REG system. It is helpful to analyze the transmission distance of the REG island bordered by the MMC-HVDC during the planning period. First, it explored the voltage support mechanism provided by the MMC-HVDC in the steady-state control mode. Then, a model for studying the strength of islanded systems is established, and it is consistent with the strength analysis of the REGs connected to the power system by AC transmission lines. Further, the Multiple Renewable Short-Circuit Ratio (MRSCR) indicator suitable for assessing the strength of the islanded system of REGs through the MMC-HVDC system is established. The MRSCR reflected the complex interactions among various REGs that are the main constraint of the transmitted ability of REGs. The effectiveness of this indicator is demonstrated through simulation examples.