Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Electrical and Computer Engineering

Supervisor

Dr. Amirnaser Yazdani

Abstract

This thesis is mainly focused on (i) modeling of large-scale PV systems in order to study the factors that influence the capacity, efficiency, power quality and safety in connection with the power grid and (ii) proposing system- and circuit-level solutions and control strategies/techniques to improve the aforementioned factors. To that end, a model for PV array is developed to study the mismatch power loss in different PV array interconnection methods, especially during the partial shading condition. Further, a two-MPPT structure is proposed to reduce the mismatch power loss in centrally structured PV systems. Then, a single-stage VSC-based system is proposed to utilize the two-MPPT structure for better efficiency. The proposed system also doubles the DC-link voltage while respecting the safety standards, which in turn, increases the capacity and the efficiency of the central inverter. To further improve capacity and efficiency, a two-stage system is proposed in which the variable MPP voltage of smaller sub-arrays are regulated, by dedicated DC-DC boost converters, at the inverter DC-side terminals. This lets the inverter use the full DC voltage permissible rating and reduces the ohmic loss in DC and AC wirings and in the transformer. The system employs a three-level NPC inverter which generates output with better power quality, facilitates the adoption of two-MPPT structure, and permits utilizing low-voltage (half-rated) switches. The system also promotes the modular/distributed structure which improves the efficiency under partial shading and enables the possibility of utilizing PV modules of different types, ratings, and alignments. Finally, a mitigation technique is proposed in the inverter and grid-interface structures to prevent formation of damaging temporary overvoltages, which sometimes are produced in power systems by distributed generations including PV. The technique utilizes a fourleg inverter connected to a grid through a Y/YG isolation transformer. The effectiveness of the proposed techniques and control strategies are demonstrated through time-domain simulation studies conducted in the PSCAD/EMTDC software environment.


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