Recent research and industrial accomplishment has revealed the advantages of cascaded smart converter PV system over traditional centralized and string PV system. However, even by adopting the cascaded smart converter, it is not always possible to track maximum power point (MPP) for all the panels under heavy shading condition, and a central converter is still required to track the peak power point of PV array. Based on the analysis of system configurations for smart converter PV system, an alternative PV system configuration is introduced which can extract peak power from all the panels under different mismatch condition and connect PV array to 380V DC bus without central converter. Based on this alternative PV system configuration, a multi-phase smart converter with single controller is proposed as a low cost panel-level MPPT solution. This proposal can largely reduce cost by saving MPPT controllers, current and voltage sensors without sacrificing energy production. The effectiveness of the proposal has been verified by both simulation and experiment results. / Master of Science

A new concept of current sensor reduction technique called multi-branch current sensing technique (MCST) is proposed in this dissertation. In the proposed current sensing method, one more branch currents are simultaneously measured several times in a single switching cycle by using a single current sensor. After that, the current reconstruction algorithm is applied to obtain all phase currents information. Compared to traditional single current sensor techniques (SCSTs), the proposed method samples the output of the current sensor regularly, and the current sensing dead-zone is dramatically reduced. Since the current sampling is performed periodically, its implementation using a digital controller is extremely simple. Moreover, the periodical dead-zone and the dead-zone near the origin of the voltage vector space which have been a big problem in the existing methods can be completely eliminated. Accordingly, there is no need to have a complicated vector reconfiguration or current estimation algorithm. The proposed MCST also takes the advantages of a SCST such as reduced cost and elimination of the sensor gain discrepancy problem in the multiple current sensor method. The fundamental concept, implementation issues, and limitation of the proposed MCST are described based on three-phase systems first. After that, the proposed MCST is adopted to two-phase inverters and multi-phase dc-dc converters with little modifications. Computer simulations and hardware experiments have been conducted for a three-phase boost converter, a three-phase motor drive system, a two-phase two-leg inverter, a two-phase four-leg inverter with bipolar modulation, a two-phase four-leg inverter with unipolar modulation, and a four-phase dc-dc converter applications. From the simulations and the experimental results, the feasibilities of the proposed method mentioned above are fully verified. / Ph. D.

This dissertation studies modeling and control issues of parallel three-phase pulse-width modulated (PWM) converters. The converters include three-phase boost rectifiers, voltage source inverters, buck rectifiers and current source inverters. The averaging of the parallel converters is performed based on a generic functional switching unit, which is called a phase leg in boost rectifiers and voltage source inverters, and a rail arm in buck rectifiers and current source inverters. Based on phase-leg and rail-arm averaging, the developed models are not only equivalent to the conventional three-phase converter models that are based on phase-to-phase averaging, but they also preserve common-mode information, which is critical in the analysis of the parallel converters. The models reveal such parallel dynamics as reactive power circulation and small-signal interaction. A unique feature of the parallel three-phase converters is a zero-sequence circulating current. This work proposes a novel zero-sequence control concept that uses variable zero-vectors in the space-vector modulation (SVM) of the converters. The control can be implemented within an individual converter and is independent from the other control loops for the converter. Therefore, it greatly facilitates the design and expansion of a parallel system. Proper operation of the parallel converters requires an explicit load-sharing mechanism. In order to have a modular design, a droop method is recommended. Traditionally, however, a droop method has to compromise between voltage regulation and load sharing. After parametric analysis, a novel droop method using a gain-scheduling technique is proposed. The numeric analysis shows that the proposed droop method can achieve both good voltage regulation and good load sharing. An interleaving technique is often used in parallel converter systems in order to reduce current ripples. Because of its symmetrical circuit structure, the parallel three-phase converter system can reduce both differential-mode and common-mode noise with a center-aligned symmetrical SVM. Based on the concept that a symmetrical circuit can reduce common-mode dv/dt noise, a conventional three-phase, four-leg inverter is modified so that its fourth leg is symmetrical to the other three legs. The common-mode dv/dt noise can be practically eliminated with a new modulation strategy. Meanwhile, with a modified control design, the new four-leg inverter still can handle low-frequency common-mode components that occur due to unbalanced and nonlinear load. / Ph. D.