Design and control of modular multilevel alternate arm converter (AAC) with Zero Current Switching of director switches

Abstract

Alternate Arm Converter (AAC) is considered as a new breed of hybrid multilevel voltage source converters. In this paper, modeling, design and control of the Alternate Arm Converter (AAC) are presented and operation principles of the AAC is discussed. One of the issues regarding to the AAC is the switching between the upper and lower arms which provides hard switching of the so called Director Switches (DS) of the AAC and makes high voltage stress on the devices due to the fast di/dt changes of the current in the turn-off time of the DS. To eliminate the voltage spike, Zero Current Switching (ZCS) operation of the director switches is presented. Moreover, the mathematical analysis is carried out to investigate on the capacitor voltage waveforms in different load power factors and the essence of having overlap time between upper and lower arm modules for balancing of the energy between the arm modules are described. To find out the advantages and disadvantages of the AAC comparing to its counterpart Half-Bridge Modular Multilevel Converter (MMC), the design analysis and comparison have been done for a specific case study. The simulation analysis has been carried out using Simulink/ MATLAB. Results show that the AAC can reduce the size of the arm inductor and the power cell capacitor as well as it has quite smaller semiconductor losses; however, the relatively high power losses of the director switches will be the main issue.