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Title: Control of Multi-Terminal Direct Current Grids
Committee:
Dr. Maryam Saeedifard, ECE, Chair, Advisor
Dr. , Co-Advisor
Dr. Deepak Divan, ECE
Dr. Sakis Meliopoulos, ECE
Dr. Lukas Graber, ECE
Dr. Chaneyop Park, Missisipi State
Abstract: The objective of this research is to address some of the main challenges in control of the Multi-Terminal DC (MTDC) grids. The control of MTDC grids needs to be enhanced from two percpectives. i) Sophisticated supplementary controllers should be introduced to exploit the MTDC grids to provide ancillary services for their surrounding AC grids; ii) Medium- and high-voltage DC-DC converters should be used to control power flow and voltage in the MTDC grids. From the first perspective, two challenges are addressed in this research: i) A Model Predictive Control (MPC)-based method is proposed to use the MTDC converter stations along with the AC grid generators for AC and DC transmission line overload alleviation. The proposed controller regularly computes the setpoints of the MTDC converter stations and AC grid generators based on a cost function; ii) A control method for frequency support of the AC grids through the MTDC grids is proposed. The proposed control method enables the MTDC grids to use the offshore wind farms to provide frequency support for the AC grids. The proposed controller has two unique features: a) It is able to adjust the maximum additional power that the wind turbines temporarily provide for frequency support through MTDC grid; b) It is capable of adjusting the time interval that the offshore wind farms provide additional temporary power to support the disturbed AC grid interconnected to the MTDC grid. From the second perspective, two important challenges with one of the promising converter topologies, i.e., Isolated Modular Multielvel DC-DC (IMM DC-DC) converter are discussed. First, the possibility of using an LCL filter to reduce the conduction losses of the converter is investigated. A detailed method is presented for the design of the LCL filter. Simulation and analytical results are discussed to investigate the impact of the LCL filter on the converter performance. Second, the impact of the magnetizing inductance on the performance of the converter is discussed. Simulation and experimental results are presented to verify the impact of the magnetizing inductance on the converter performance.
