I. Introduction
Pulsewidth-Modulation (PWM)-based voltage-source converter (VSC) techniques have been widely used in grid-connected applications, such as adjustable-speed drives (ASDs) with PWM rectifiers, power quality improvement, wind turbines, etc. [1]. Thanks to the gradually increased rating and reduced costs, they have also been applied for high-voltage dc (HVDC) transmission in recent years [2]. Compared to the conventional thyristor-based HVDC, VSC-HVDC has a number of technical merits: reactive-power support to the ac system, possibility to connect to very weak ac systems, black-start capability, and lower cable cost, just to name a few.