不平衡电网下虚拟同步机的多模式协调策略

doi:10.19595/j.cnki.1000-6753.tces.L10082

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摘要

新能源并网换流器采用虚拟同步机（VSG）控制有利于提升电网惯量和阻尼水平,但在电网电压不平衡时存在输出电流畸变和瞬时功率脉动的问题。为灵活调控不平衡电网下VSG输出电流质量与功率纹波,该文提出一种VSG多模式协调策略。首先,研究不平衡电网下的VSG瞬时功率脉动抑制机理,在功率恒定模式并网电流谐波特征分析的基础上,利用VSG虚拟功率计算电流参考指令并引入协调系数对其修正;其次,在多谐振电流内环控制下实现VSG电流平衡-功率恒定多模式协调,并研究协调系数影响及其取值区间选取方法;最后,通过仿真与实验结果验证了该文理论分析与所提控制策略的有效性。

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关键词 ：虚拟同步机, 不平衡电网, 电流质量, 功率纹波, 协调控制

Abstract：

Virtual synchronous generator control has been widely studied in the field of renewable energy generation to provide equivalent inertia support for the power grid. However, current distortion and power oscillation occur severely when the grid voltage becomes unbalanced, which significantly aggravate the power supply quality of renewable energy generation. The existing methods generally take the fundamental-frequency negative-sequence component as VSG control object to solve the above issues. It is obvious that positive-negative sequence separation increases the calculation burden and active and reactive power oscillations cannot be suppressed simultaneously. In addition, the mutual influence between optimization targets is rarely considered, so that the realization of one optimization target may lead to the deterioration of the others. Therefore, this paper proposes a multi-mode coordination control of VSG and quantitatively analyzes the restrictive relation of VSG current distortion and power fluctuation under unbalanced power grid.
This paper first analyzes the VSG power oscillation mechanism under unbalanced power grid, and then further studies the harmonic components of VSG grid-connected current in constant-power mode. The results indicates that VSG grid-connected current will appear low-order odd harmonic components and the proportion of which can determine both of double-line frequency active and reactive power oscillation. On this basis, the initial current reference is obtained from VSG virtual power calculation in order to maintain the operating characteristics of synchronous machine. Then a multi-resonance current inner loop is used to control not only fundamental frequency current but also the harmonic components. It should be noted that the coordination coefficient is introduced to modify the current reference of the harmonic controller, so as to realize the VSG multi-mode coordination control. Finally, the influence of coordination coefficient on VSG current quality and power ripple is quantified and the calculation method of selection range is given.
In order to validate the effectiveness of the proposed method, modular multilevel converter(MMC) is selected as the grid-connected AC/DC topology, and the simulation and experiment are respectively performed on Matlab/Simulink platform and on a three-phase five-level MMC prototype. In simulation, the power step performance of the proposed method is well verified under balanced-current and constant-power modes. After that, VSG multi-mode coordination control and the corresponding coordination coefficient selection method are simulated, the results of which show the effectiveness of proposed method. As for experiment, the verification is carried out when the b-phase voltage drops to 0.6pu. The experiment waveforms and the components of VSG current under the two typical modes further verified the validity of the aforementioned theoretical analysis and the control strategy. In the process of VSG multi-mode coordination, the data of experimental results show that the trends of both current total harmonic distortion and the power ripple are consistent with the theory calculation.
The following conclusions can be drawn from this paper: (1) Under unbalanced power grid, the VSG grid-connected current appears low-order odd harmonic components in constant-power mode, the amplitude of which is positively related to the voltage imbalance degree. (2) The proposed method can realize the multi-mode coordinated control between balanced-current and constant-power modes, so as to regulate the VSG output current quality and power ripple flexibly. (3) This method simplifies the control and reduces the calculation burden with no need for positive-negative sequence separation and double dq transformation.

Key words： Virtual synchronous generator unbalanced power grid current quality power ripple coordinated control

收稿日期: 2019-12-08

PACS:

TM46

基金资助:

新能源与储能运行控制国家重点实验室(中国电力科学研究院有限公司)开放基金资助项目(DGB51202101307)

通讯作者: 杨晓峰男,1980年生,副教授,博士生导师,研究方向为多电平变换器、柔性直流输电、宽禁带半导体及电力电子技术在轨道交通中的应用等。E-mail：xfyang@bjtu.edu.cn

作者简介: 潘子迅男,1998年生,硕士研究生,研究方向为模块化多电平换流器控制技术与应用。E-mail：20121478@bjtu.edu.cn

引用本文:

潘子迅, 杨晓峰, 赵锐, 崔晨阳, 闫涛. 不平衡电网下虚拟同步机的多模式协调策略[J]. 电工技术学报, 0, (): 150-150. Pan Zixun, Yang Xiaofeng, Zhao Rui, Cui Chenyang, Yan Tao. Multi-mode Coordination Control of Virtual Synchronous Generator Under Unbalanced Power Grid. Transactions of China Electrotechnical Society, 0, (): 150-150.

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