A current controlled technique of the Modular Multilevel Converter

Abstract

At the beginning of this century, many researchers are focusing their studies on the power electronics converters to improve the advantages of the power electronics converters such as high efficiency, less maintenance, fast dynamic response, low thermal dissipation and compact size. With all these advantages, power electronics converters occupy a great place at the industrial applications, like tractions industry, high voltage DC transmission systems (HVDC), and power conditioners applications, drive control system for electrical motor, power quality industry and electrical vehicles industry. Moreover, smart grid and renewable energy interfacing systems are using these converters greatly to achieve their required functions with high performance and less loss. These converters have many topologies and different types, each type is compatible with several applications. Multi-level converters are the most popular types used at last years, because of high efficiency and ease maintenance. This thesis studies two types of multi-level converters, the modular multi-level converter (MMC) and the Granular Multilevel Converter (GMC). This thesis proposes a current controlled GMC based on the Ramp Current Control (RCC) technique which is characterized by simplicity and fixed switching frequency. Besides controlling the output currents, the proposed control system for the GMC regulates the sub-modules capacitors voltages at their desired settings. A proper switching state is selected to keep the sub-module capacitors voltages based on hysteresis capacitor voltages controllers, the required phase voltage level, and the sign of the output current. The reference phase voltages are determined using the RCC to control the d- and q- current components of the GMC in the synchronous frame. These current components are set using two Proportional-Integral (PI) controllers to manage the active and reactive powers fed to grid. The same current controller is applied for the MMC. Simulations results, using EMTDC/ PSCAD, are presented to evaluate the dynamic performance of the proposed systems under different operating conditions. Different test scenarios are conducted under different conditions to evaluate the dynamic behavior of the proposed system. Simulation results show fast dynamic response and accurate performance of the proposed control systems.

Index Terms: Granular Multilevel Converter, Capacitor Voltage Control, Ramp Current Controller, PI Controller.