UBC Theses and Dissertations
Advanced control functionalities for photovoltaic and energy storage converters Serban, Emanuel
Power conversion systems including grid-connected photovoltaic (PV) and electrical energy storage (EES) stages open the prospects for new opportunities to improve the system’s performance in energy production and standards compliance. This dissertation proposes a completely revised state-of-the-art architecture with functionalities integrated within a unified system, which extracts more solar energy, provides safety compliance and grid stability. The first improvement of the power conversion system is focused on inverters' dc voltage extension range, which leads to increased PV energy harvesting. A proposed technique provides a lower voltage limit in the dc-bus utilization with the employment of the new voltage-reactive power control strategy accompanied with a modified zero-sequence modulation. Then, a higher dc-bus voltage limit is obtained by maximizing the utilization of power semiconductors. A graphical comparative analysis approach using I-V and P-V characteristics reflects remarkable PV-converter system behavior, which illustrates the advantages of the wide dc-bus range in 1500V systems. As a result, the maximum power point tracking (MPPT) dc voltage range is extended by an additional 30% improving the systems energy capture capabilities under extreme temperatures beyond the performance of traditional 1000V single-stage inverters. Furthermore, the single-stage conversion was extended to two-stages, with mini-boost rated for a fraction of the nominal power of the converter. Thus, the proposed design concept delivers significantly higher performance whilst reducing system cost at component level. The next proposed improvement of the system focuses on grid fault detection for standards compliance, using a search sequence function. This proposed technique is integrated within the active-reactive power control, MPPT algorithm, and phase-locked loop routine. In addition, the islanding search sequence is synchronized and incorporated within the MPPT (designed with an adaptive strategy to achieve system stability and minimum impact on power quality). Finally, the system’s control functionalities advances into grid support strategies, designed with frequency- and voltage-assist features for network stability. The change in active-reactive power flow is achieved using a responsive gradient to command the transitions between grid-feeding and grid-loading. The proposed system’s combined methods result in a cohesive PV/EES conversion architecture whose improved performance has been confirmed through electronic simulation and experimental results.
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