Control Method for Double Y-connection Modular Multilevel Converter

Abstract

Recently, voltage source converter (VSC) HVDC systems employing a modular multilevel converter (MMC) topology have been introduced in power systems because of the low loss and the low harmonic characteristics. An MMC consists of half bridge cells or full bridge cells connected in series. The double Y-connection MMC is a type of MMC family that can handle active power and is expected to be used for HVDC and BTB systems.
In this study, we proposed a transformation matrix for double Y-connection MMCs. The voltage and current of the double Y-connection MMC are separated into AC, loop and DC components in the transformation. We write state equations for the double Y-connection MMC circuit and prove the independence of the three components with the transformation. By using the transformation, we can control the voltage and current of the AC, DC, and loop circuits for the DC capacitor voltage balance control independently. In addition, we calculated the DC voltage amplitude that the double Y-connection MMC could output, and analyzed the voltage ripple of the cell capacitor when a second order current was superimposed on the arm current of the MMC. The utilization ratio of the semiconductor devices is improved and the amplitude of the cell voltage ripple becomes large by second order superimposition.