Abstract：With the 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 the rotor dynamics equation of the virtual synchronous generator to provide inertia and frequency support to the grid, just as the rotary synchronous generators. However, when VSG is connected to the grid, even a slight amount of grid voltage distortion would induce significant harmonics currents. Therefore, various VSG harmonic current suppression schemes are introduced. Existing VSG harmonic current suppression methods are mainly 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, and no harmonic current would be induced. However, the existing methods based on PCC voltage feedback have practical limitations. As the line impedances are usually small, it only takes a tiny amount of harmonic voltage difference to induce a 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 proposes adding extra inductors in series with the PCC connecting line, but extra volume and cost are incurred. This paper introduces a VSG output harmonic current closed loop to receive the voltage harmonic signal feedback. Therefore, the noises and the nonlinearity in the previous open-loop PCC harmonic voltage feedbacks can be eliminated, and 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 closed-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 the virtual impedance-based method. Finally, a 15 kW VSG prototype is built to verify the proposed method compared with the conventional VSG and two harmonic current suppression methods based on PCC voltage feedback and the virtual impedance, respectively. The method proposed in this paper significantly improves 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 than the PCC voltage feedback-based method. In addition, the best suppression effect can be realized using the maximum virtual impedance value without instability.
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, 2024, 39(6): 1871-1885.
Xu Song, Yang Bo, Liu Hao, Lu Shuai. A Cascaded Harmonic Voltage and Current Closed-Loop Control Method to Improve the Current Quality of Virtual Synchronous Generators. Transactions of China Electrotechnical Society, 2024, 39(6): 1871-1885.
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2024, Vol. 39 
