An Improved Control System for Modular Multilevel Converters with New Modulation Strategy and Voltage Balancing Control

Modular multilevel converter (MMC) has become one of the most promising converter topologies for future high-power applications. A challenging issue of the MMC is the voltage balancing among arm capacitors. A good overall control system is also vital for the MMC, which should be based on sound mathematical model, readily adaptable for different applications, and capable of high performance. This paper presents a general control structure for MMC inverters, which is suitable for both voltage-based and energy-based control methods, and includes voltage balancing between the upper and lower arms. A new method for voltage balancing among arm capacitors, which is based on an improved pulse-width modulation, is also presented. The proposed method avoids some major disadvantages found in present voltage balancing methods, such as dependence on computation-intensive voltage sorting algorithms, extra switching actions, interference with output voltage, etc. Furthermore, all switching actions are evenly distributed among power devices. The proposed control system as a whole can serve as a promising solution for practical applications, especially when the number of submodules is fairly high. Simulation and experimental results verify the effectiveness of the proposed methods.

I. Introduction

Recently, multilevel converters have attracted growing attentions and found themselves in increasing market of high power and high/medium voltage applications [1] [2], such as high-voltage dc transmission (HVDC), flexible ac transmission systems (FACTS), industrial motor drives, utility-scale renewable energy systems, and so on. Among various multilevel converters, the diode-clamped or neutral-point clamped (NPC), flying capacitor (FC), and cascaded H-bridge (CHB) are the most studied topologies [3]–[7]. However, for the applications requiring more than four or five levels, the NPC and FC topologies become less attractive due to significantly increased number of clamping diodes or FCs, higher power losses, and difficulty to balance the capacitor voltages [3]–[6]. The CHB topology then seems more suitable due to its modular structure. However, it requires a large number of isolated dc sources, which are often provided by a bulky, multiwinding transformer together with a series of rectifiers [7].