A Highly Modular Grid Interface For Utility Scale Renewables: Mmc With Isolated High Frequency Link Sub-Modules

Abstract

A novel isolated Modular Multilevel Converter that allows low voltage DC (LVDC) to medium voltage ac (MVAC) power transmission is proposed in this dissertation. The proposed topology is composed of a primary inverter and a collection of sub-modules. Each sub-module is composed of a high-frequency transformer, a diode bridge rectifier, a capacitor, and a half-bridge cell. The sub-modules are arrange in a three phase MMC configuration where the output of each sub-module is the half-bridge cell. The input of all sub-modules are connected to the primary inverter via a high-frequency bus bar. Each sub-module is capable of generating its own isolated DC voltage by tapping into the high-frequency bus bar. The galvanic isolation between the MMC side and the input of the inverter, and the unidirectional power flow, is ideal for use with PV panels. The proposed solution allows for scalability with the possibility of reaching MVAC, making it an ideal converter for large scale PV power plants. With the use of high frequency transformers the proposed topology can be built small and lightweight, allowing it for other applications. One example is as a converter for wind energy systems where the reduce size and weight can allow the converter to be located in the hub of smaller wind turbines. This strategy allows the turbine to transmit high voltage power at lower current levels, hence reducing the cable thickness and the conduction losses associated with it. In this dissertation, a detail analysis of the proposed topology, simulations and experimental results are shown. Simulations were created using the PLECS tool set in Matlab/Simulink. The hardware prototype is a proof of concept design to operate at 1kW, with an input of 100 V, and a 9-level output voltage with a ± 200 V limit. A total of 24 sub-modules were created, 8 for each phase of this three phase converter. Two alternatives for the MMC cell are proposed, a half-bridge solutions and a full-bridge solution, although only the first is developed in hardware. It is shown through simulation and experimentation that the average values of the capacitor voltages are self-balanced, and no additional balancing algorithm is needed. The simulation and experimental results confirm the overall intended operation of the proposed topology.