Control of photovoltaic grid-connected quasi-z source inverter under abnormal grid conditions

Abstract

With the witnessed exponential growth of solar power installations over the past decade, the inverterinterfaced power conversion system has become highly significant. Photovoltaic (PV) systems have rapidly grown since 2012, and their significant impacts on power quality and voltage stability cannot be neglected. Many countries have improved their requirements for large-scale PV grid-connected inverter system, which requires low-voltage ride-through (LVRT) capability to maintain grid stability. Z source/quasi-Z source inverter (ZSI/ qZSI) has attracted engineers’ and researchers’ attention due to its promising single-stage buck/boost characteristic. Compared to the dual-stage voltage source converter topologies, the ZSI/ qZSI offers better reliability, higher efficiency, and lower cost of power converter system. Several linear control methods have been proposed for ZSI/ qZSIs in PV grid-tie systems. However, rare research focuses on the power quality and system stability issues on PV gridconnected ZSI/ qZSIs systems. Firstly, a novel control strategy for the qZSI applied in PV grid-connected systems during normal grid conditions is introduced to improve the power quality. The proposed control strategy employs a twostage DC-side and AC-side control. In addition, a novel constant boost control with 3rd harmonic injection method is proposed to replace the traditional Pulse Width Modulation (PWM) generation. The proposed strategy can achieve a unity power factor and better power quality from the PV array to the grid. Moreover, compared to the proportional-integral (PI) current controller, the proportional resonant (PR) current controller offers lower total harmonic distortion (THD) in the output current signal spectrum under normal grid or abnormal grid conditions. This thesis presents a comprehensive theoretical and experimental comparison between the PI and PR current controllers with the proposed control strategy. Under unbalanced grid conditions, the currents injected into the grid become non-sinusoidal and unbalanced. The unbalanced grid voltage and current also lead to double frequency oscillations in the active and reactive power from the inverter system. Therefore, this thesis proposes a flexible power control for PV grid-connected qZSI systems under unbalanced grid conditions to improve the system stability under unbalanced grid conditions. The proposed control strategy can inject the sinusoidal current and mitigate the double-frequency oscillation of the active or reactive power delivered to the network. Furthermore, an easy-to-implement current limitation strategy is introduced in the proposed control to protect the power converter but also to deliver maximum power to the grid. Furthermore, compared to the decoupled double synchronous reference dq frame (DDSRF) PI controllers, the PR controllers' simpler structure and lower computational burden make it very suitable for implementation in digital control. This is also investigated and demonstrated in this thesis. Simulation and experimental work were carried out to study the performance of the proposed control strategies of the PV grid-connected qZSI system. MATLAB/ Simulink platform is used in this thesis for simulating the PV grid-connected qZSI system and its control strategies. In addition, a downscaled prototype of the inverter is designed, built and tested. Finally, the comparisons between simulation and experimental results show excellent agreement between these results.