Three-phase modular multilevel converter based unified power flow controller

The power system needs to be flexible to cater for the increasing demand by its consumers. Flexible AC Transmission System (FACTS) devices use power electronic devices to achieve this by controlling some power system parameters. The Unified Power Flow Controller (UPFC) is the most versatile and flexible of FACTS devices. Modeling of UPFCs is not an easy task especially with the recent introduction of modular multilevel converters (MMC) as a new type of voltage source converter (VSC). Most models are average or equivalent circuit models which do not really depict the situation when UPFCs are implemented practically. This thesis provides a detailed model of an MMC based UPFC made up of half-bridge converter submodules. An MMC has been designed and tested in a simulation environment before being connected in back to back mode with one end in series and the other in parallel with a transmission line (UPFC configuration). A DQ real and reactive power flow control scheme has been proposed and results obtained from the numerous simulations will be presented. UPFC is able to control real and reactive power flows on the chosen line and at the same time, it can control the bus voltage dynamically while the power flow control loops are active. Key Words: Flexible AC Transmission Systems (FACTS), Unified Power Flow Controller (UPFC), Voltage Source Converter (VSC), Modular Multilevel Converter (MMC), Half Bridge Sub-module, Real and Reactive Power Flow Control and Bus Voltage control.