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能量存储系统的分布式能源（DER）单元正被越来越多地部署以提高效率、增加可靠性并降低能量成本。DER单元通常通过电压源转换器（VSC）与主电网连接，因为它们提供高可控性、能量转换灵活性和快速动态响应。尽管有其优点，但基于VSC的DER产生了一定数量的问题，例如，电能质量和稳定性下降，电网不平衡引起的VSC直流侧电压纹波。施加在主机电网上的电压不平衡是不可避免的，由于存在单相负载/发电机，部分换位的传输线等电压纹波的存在下，直流侧的VSC会导致应力和加速电容器的退化，并减少其寿命。为了减少纹波，广泛使用寿命有限的大型电解电容器。直流纹波也可以导致注入的非特征谐波的VSC交流侧，这反过来又导致额外的功率损耗，故障的保护系统，Transformer饱和等本项目的目的是开发和原型的即插即用转换器的三相和单相DER系统，以解决上述电能质量问题。其核心思想是根据电网不平衡情况，提供一个小的、受控的不平衡电流，以保证直流侧电压无纹波，交流侧电流无谐波。与现有的VSC技术相比，所提出的技术提供了以下独特的创新特征：（i）无纹波直流侧：因此实现了延长的寿命、减小的直流电容器尺寸、更具成本竞争力、改善的稳定性。(ii)无谐波交流侧电流：保证降低功率损耗，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.