Multilevel inverters and their applications in power system

A power system composes three interconnected subsystems: generation, transmission, and distribution. With the ongoing expansion and deregulation, there are also three major trends to reform the power systems: (1) to utilize distributed generation (DG) to meet the increasing power demands; (2) to utilize Flexible AC Transmission devices (FACTs) to improve the transmission and distribution systems; and (3) to utilize Custom Power devices to improve the power quality to the end users at the distribution level.; Power electronics inverters are enabling parts of DG and core parts of FACTS and Custom Power devices. In the power electronics inverter family, multilevel inverters are the newest breed. Compared with other inverters, multilevel inverters have two distinguished features that make them more suitable for the aforementioned power system applications: (1) the multilevel structures enable the utilization of low voltage rating power semiconductor devices in high voltage applications; (2) and the almost near sinusoidal output voltage waveforms from the multilevel inverters make it possible to eliminate the output filters and zigzag transformers, which are often connected after traditional inverters. Three major issues involved with multilevel inverters in power systems are DC bank voltage maintenance, balance of DC banks, and harmonics optimization of the inverters' output voltage.; This dissertation first summarizes the previous works on FACTS, Custom Power, DG and multilevel inverters, and discusses the major application issues for cascade multilevel inverters. A new harmonics elimination method for multilevel inverters is proposed. Then, in the main part of the dissertation, two applications of cascade multilevel inverters in FACTS devices---a new Unified Power Flow Controller (UPFC) and a Universal Static Synchronous Compensator (U-STATCOM)---are proposed. Unlike conventional UPFC, in the proposed UPFC, cascade multilevel inverters can be used without compromising power flow controllability. The proposed U-STATCOM can be used to compensate not only reactive power and harmonics but also load current imbalance. The circuit structures, control methods, comparisons with conventional solutions, and simulation results are shown for both cases. Furthermore, new applications of multilevel inverters in Custom Power and DG are presented as the extended applications of cascade multilevel inverters in power systems.; In the end, the realization of the control unit for a seventeen level cascade multilevel inverter is described. Experimental results based on the inverter are shown to prove the validity of proposed harmonics elimination method.