Developing a Robust and Universal Single-phase Three-phase DC/AC Converter with Small-size, Ripple-free DC Side and Harmonic-free AC Side

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Distributed energy resource (DER) units including distributed generators and energy storage systems are being increasingly deployed to improve efficiency, increase reliability, and decrease energy costs. The DER units are often interfaced to the host grid through voltage-sourced converters (VSCs) as they offer high controllability, energy conversion flexibility, and fast dynamic response. Despite their advantages, the VSC-based DERs create certain number of concerns, e.g., power quality and stability degradation, voltage ripples on the VSC dc-side caused by unbalanced grid. The voltage imbalance imposed on the host grid is inevitable due to the presence of single-phase loads/generators, partially transposed transmission lines, etc. The presence of voltage ripples on the dc-side of the VSC causes stress and accelerates the degradation of the capacitor, and reduces its life-time. To reduce the ripples, large electrolytic capacitors are widely used which have a limited life span. The dc ripples can also cause injection of non-characteristic harmonics in the VSC ac-side which in turn lead to additional power loss, malfunctioning of protection systems, transformer saturation, etc. The aim of this project is to develop and prototype a plug-and-play converter for three-phase and single-phase DER systems to tackle the above power quality issues. The central idea is to feed a small, controlled level of unbalanced current in response to the unbalanced grid such that the ripple-free dc-side voltage and harmonic-free ac-side currents are guaranteed. Comparing to the existing VSC technologies, the proposed technology provides the following unique innovative features: (i) Ripple-free dc-side: Increased lifetime, reduced dc capacitor size, more cost competitive, improved stability are thus achieved. (ii) Harmonic-free ac-side current: Reduced power loss, increased power quality, reduced instabilities excited from ac-side grid are guaranteed.