The uses of flexible alternating current transmission system (FACTS) controllers in next generation smart grids are encouraged by the increased uses of decentralized and highly meshed grid structures that may affect the stability of power systems. Voltage source converter (VSC) based FACTS devices have reduced footprint and offer increased control flexibility, extended range and faster reaction time than line commutated thyristor based equivalent solutions. The performance of commonly used FACTS devices that employ a two-level converter is summarized. Then, multilevel converters and direct AC-AC converters which are viable for FACTS applications are reviewed. The outcomes of the literature surveys are refined to identify new features that may be critical for future centralised and decentralized smart grids such as: control range extension, improved efficiency and power density at reduced hardware cost. To pursue these features, three novel VSC topologies are proposed and analysed: An AC voltage-doubled (ACVD) topology with an internal inverting buck-boost cell in each phase-leg, is able to synthesize twice the output voltage of a conventional two-level VSC for the same dc link voltage, is proposed.A number of new modulation and control strategies that aim to further increase DC utilization of the ACVD converter and to manage its internal dynamic interaction to prevent the appearance of low-order harmonics in the output currents, are presented. With its high DC-rail utilization and sophisticated control strategies, the ACVD converter offers an extended power control range, which is increasingly important for shunt and series type FACTS devices. The controlled transition full-bridge hybrid multilevel converter (CTFB-HMC) with chain-links of full-bridge cells is proposed to combine the advantages of improved wave-shaping ability, reduced footprint and high efficiency, which promote its applications in medium and high voltage FACTS devices. An AC hexagonal chopper using heterodyne modulation to decouple the control of AC voltage amplitude from that of the phase-angle is proposed. For scalability to medium and high voltage, a modular multilevel AC hexagonal chopper (M2AHC) is developed. With adoption of a quasi-two-level transitional mode for reduced cell number and minimized footprint, dv/dt is limited and reliability is improved. Simulation and experimentation are used to validate the modulation, control and FACTS implementation of the three proposed converters.
|Date of Award||1 Nov 2014|
- University Of Strathclyde
|Sponsors||Mitsubishi & University of Strathclyde|
|Supervisor||Derrick Holliday (Supervisor) & Barry Williams (Supervisor)|