一种提高虚拟同步机电流质量的电压-电流级联闭环控制方案

doi:10.19595/j.cnki.1000-6753.tces.222327

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摘要

当虚拟同步机（VSG）接入畸变电网时,其输出电流质量恶化。现有基于PCC点电压采样的各种谐波治理方案受限于线路阻抗的大小而抑制效果不佳。为此,该文提出一种电压-电流级联闭环控制策略实现了VSG基波电压和线路谐波电流的协调闭环控制。在基波频率下,VSG跟随功率控制回路的基波电压指令实现功率传输;在谐波频率下,通过单个一阶复矢量滤波器对谐波电流总和进行整体提取,并闭环输出谐波电压补偿的参考量,从而在物理实质上等效于增强了谐波电压补偿量的参考信号,大幅提升了VSG输出电流谐波的抑制效果。相比于现有方案,本方法克服了由于线路阻抗过低而抑制效果不佳的问题,能在不增加额外电感器件的条件下提高VSG在畸变电网下的输出电流质量。最后,该文搭建15kW的样机进行实验,并与现有的三类方法进行对比实验,较为全面地验证了本文方法的有效性及谐波抑制效果的提升。

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关键词 ：虚拟同步机, 谐波抑制, 阻抗重塑, 电能质量

Abstract：

With increasing amount of renewable energy sources and energy storage devices in the power grid, more power electronic inverters get tied to the grid as current sources. As a result, the overall grid rotary inertia and frequency stability will decrease. So Virtual Synchronous Generators (VSG) schemes are being implemented in grid-tied inverters. VSGs are inverters in voltage source control mode. Particularly, its output voltage vector instantaneous angle is obtained by rotor dynamics equation of the virtual synchronous generator, so that it provides inertia and frequency support to the grid just as the rotary synchronous generators. However, when VSG is connected to the grid, even slight amount of grid voltage distortion would induce significant harmonics currents. So that, various VSG harmonic current suppression schemes are introduced.
Existing VSG harmonic current suppression methods are mostly based on PCC voltage feedback, whose voltage harmonics get extracted and added to the VSG output voltage commands. Therefore, the line connecting VSG and PCC would have no net harmonic voltage across, so that no harmonic current would be induced. However, the existing methods based on PCC voltage feedback has practical limitations. As the line impedances are usually small, it only takes tiny amount of harmonic voltage difference to induce significant amount of harmonic currents. Slight noises and nonlinearity in the feedback path will introduce harmonic voltage difference and large harmonic current. Therefore, some paper further proposes adding extra inductors in series with the PCC connecting line, but extra volume and cost are incurred as a result.
To address the issues in the prior study, this paper introduces a VSG output harmonic current close loop to obtain the voltage harmonic signals feedback. Therefore, the noises and the nonlinearity in the previous open loop PCC harmonic voltage feedbacks can be eliminated, so that the VSG output voltage can accurately track the voltage harmonics at the PCC point. As a result, the output current harmonics suppression is more effective.
This paper first establishes the impedance model of VSG and analyzes the output current harmonics. To implement this close-loop control scheme, the sum of all orders of the harmonics is first extracted from the VSG output current feedbacks by a first-order complex vector filter, and fed through a proportional-resonant control loop to generate the VSG harmonic voltage reference. The enhancement of the harmonic loop impedance with the proposed method is analyzed in detail and compared with the existing methods. Moreover, this paper further analyzes the system stability of the proposed method for both fundamental frequency voltage control and harmonic current control, using the impedance-based stability criterion of the voltage-source inverter. Compared with the existing harmonic current suppression methods using virtual impedance, the proposed scheme can suppress the harmonic current with much larger loop gains than virtual impedance based method.
Finally, a 15kW VSG prototype is built for experimental validation. The effectiveness of the proposed method is experimentally verified by comparing with the conventional VSG, and the two harmonic current suppression methods based on PCC voltage feedback and the virtual impedance, respectively. Compared with the existing three schemes, the method proposed in this paper has significant improvement in VSG output current THD. Besides, it is noted that the harmonic suppression effect of the proposed method is much less susceptible to the low impedance value of the PCC connection line, compared to PCC voltage feedback based method. Also note that the comparison with the virtual impedance based method is made with its best suppression effect, i.e. when the highest possible virtual impedance before losing stability get used.

Key words： VSG harmonic suppression impedance reshaping power quality

收稿日期: 2022-12-17

PACS:

TM464

基金资助:

重庆市研究生科研创新资助项目（CYB21018）

通讯作者: 陆帅,男,1975年生,博士生导师,教授,研究方向为电力电子技术和电驱动、着重于高功率密度技术研究,应用于混合动力、纯电动汽车和可再生能源微电网系统。E-mail：Lushuai1975@gmail.com

作者简介: 徐菘,男,1997年生,硕士研究生,研究方向为新能源发电及电能质量控制的研究工作。E-mail：278398646@qq.com

引用本文:

徐菘, 杨博, 刘浩, 陆帅. 一种提高虚拟同步机电流质量的电压-电流级联闭环控制方案[J]. 电工技术学报, 0, (): 134-134. Xu Song, Yang Bo, Liu Hao, Lu Shuai. A Cascaded Harmonic Voltage and Current Closed-loop Control Method to Improve the Current Quality of VSG. Transactions of China Electrotechnical Society, 0, (): 134-134.

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