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

包括分布式发电机和储能系统在内的分布式能源（DER）单元正越来越多地用于提高效率、提高可靠性和降低能源成本。DER单元通常通过电压源转换器（VSCs）连接到主电网，因为它们具有高可控性，能量转换灵活性和快速动态响应。尽管基于VSC的der具有优势，但也产生了一些问题，例如电能质量和稳定性下降，电网不平衡导致的VSC直流侧电压波动。由于单相负载/发电机、部分转置的输电线路等的存在，主机电网上的电压不平衡是不可避免的。电压波动的存在在直流侧的VSC造成应力和加速退化的电容器，并减少其寿命。为了减小波纹，广泛使用寿命有限的大型电解电容器。直流纹波还会导致非特征谐波注入VSC交流侧，进而导致额外的功率损失、保护系统故障、变压器饱和等。该项目的目的是开发一个即插即用转换器的原型，用于三相和单相DER系统，以解决上述电能质量问题。其核心思想是提供一个小的、可控的不平衡电流，以响应不平衡电网，从而保证无纹波的直流侧电压和无谐波的交流侧电流。与现有的VSC技术相比，该技术具有以下独特的创新特点：(i)无纹波直流侧：延长了使用寿命，减小了直流电容器尺寸，更具成本竞争力，从而提高了稳定性。（ii）无谐波交流侧电流：保证减少功率损耗，提高电能质量，减少交流侧电网激发的不稳定性。

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.