Voltage-Sourced Converters in Power Systems

6

Three-Level, Three-Phase, Neutral-Point Clamped, Voltage-Sourced Converter

6.1 INTRODUCTION

Chapter 5 introduced the two-level, three-phase voltage-sourced converter (VSC) (Fig. 5.1) as the composition of three half-bridge converters. The two-level VSC is the dominant building block for a wide range of apparatus in medium/high-power applications. Figure 5.1 (and Fig. 2.13 for the half-bridge converter) illustrates that each switch cell in the two-level VSC must withstand the whole DC-side voltage in off state. Therefore, if the two-level VSC is employed for a high-power/high-voltage application, the switch cells must be rated for a high DC voltage level. If we consider a high voltage level for a specific application, then to implement the two-level VSC, probably the highest voltage switches that fulfill the voltage rating requirement are selected. Such switches are often the state of the art and typically expensive. Other than the cost and availability issues, beyond a voltage rating, even the highest voltage available switches may not fulfill the voltage requirements. The voltage ratings of the state-of-the-art switches, currently and most probably in the future, would not meet the voltage requirements of most utility applications.

One approach to achieving high-voltage switch cells is to connect a number of lower voltage switches in series. This approach has been extensively practiced in high-power/high-voltage applications such as HVDC converter systems. The main issue in ...