DESIGN OF HYBRID ALTERNATE ARM CONVERTER USING SI AND SIC

Abstract

The Alternate Arm Converter (AAC) is emerging as a viable alternative to the Modular Multilevel Converter (MMC) for HVDC applications due to its lower energy storage requirements and DC fault tolerance. This project report details the modeling, design, and control of the AAC, along with a discussion on its operational principles. To achieve energy balancing across a wide range of modulation indices, a dedicated controller is needed to redistribute unbalanced energy between the upper and lower arms of each phase leg, ensuring the balancing of submodule (SM) capacitor voltages. The control scheme presented in this report for the AAC focuses on balancing SM capacitor voltages. This scheme relies on a constant overlap time, with the current magnitude during the overlap time varying based on the operating modulation index and power factor. By maintaining a constant overlap time, the proposed control scheme allows the AAC to operate at higher modulation ranges without increasing the number of SMs per arm. Additionally, a mathematical analysis is conducted to examine capacitor voltage waveforms under various load power factors, highlighting the importance of overlap time for energy balancing between arm modules. A comparative design analysis between the AAC and its counterpart, the Half-Bridge MMC, is performed for a specific case study to identify the advantages and disadvantages of each. Simulation analysis using Simulink/MATLAB reveals that the AAC can reduce the size of the arm inductor and power cell capacitor, and it has lower semiconductor losses. However, the relatively high power losses of the director switches remain a significant concern.