Design and Control of Modular Multilevel Converters

Abstract

An increasing awareness of energy efficiency has led to the development of several improved semiconductor devices, power converter topologies and control schemes within the field of power electronics. Recent advances in multilevel converters, especially Modular Multilevel Converters (M2LCs), have improved upon existing power conversion technology in several aspects, including efficiency, power quality, modularity and reliability. There are, however, several challenges associated with the M2LC topology, which include capacitor voltage variations, voltage balancing, circulating currents and increased complexity of the overall control scheme. This thesis contributes to the ongoing research on the M2LC topology by proposing the following solutions to the aforementioned challenges. Typically, schemes with cascaded control loops have been used to control M2LCs and these schemes could affect the performance of the converter. Model Predictive Direct Current Control (MPDCC), which has a single loop, is proposed to keep load currents within tight bounds, while minimising both capacitor voltage variations and circulating currents. Moreover, the width of the current bounds sets the level of Total Harmonic Distortion (THD) of the currents. Simulated and experimental results for a 860-VA prototype M2LC are presented to demonstrate the effectiveness of the MPDCC scheme. A modified M2LC topology that employs a full-bridge module in each arm is proposed to minimise both circulating currents and capacitor voltage variations. The modified topology allows for decoupled control of the load and circulating currents, where the load and circulating currents are controlled by the half-bridge and full-bridge modules, respectively. A comparative investigation with respect to a conventional topology, using theoretical as well as experimental results for a 800-VA prototype converter, reveals that the modified topology offers superior performance. Capacitor voltage variations are difficult to control within the conventional M2LC, because the variations are coupled to arm currents and load currents. An alternative M2LC topology that uses Inductive Power Transfer (IPT) technology is proposed to simplify the control of the capacitor voltages. The IPT system keeps the voltage variations within bounds irrespective of the operating conditions of the converter. The feasibility of the concept and improvements achieved with the alternative topology are demonstrated using simulations of a 800-VA converter.