弱电网下应对复杂稳定性问题的并网逆变器改进电压前馈通路研究

doi:10.19595/j.cnki.1000-6753.tces.230986

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摘要

在并网逆变器中,锁相环和电网电压前馈是提高并网电流质量和输出性能的环节。然而在弱电网下,这两个环节均有可能对并网系统的稳定性产生影响,同时两者的影响范围存在重叠,导致稳定性问题变得更为复杂。该文针对这一复杂稳定性问题展开研究,分析弱电网下两个环节导致系统不稳定的原因,并进一步提出优化方案。首先考虑加入加权系数,讨论其对于电网电压前馈以及锁相环的影响,兼顾这两种因素,需要对加权系数进行权衡取值;然后考虑在更弱电网下,只加入加权系数对于系统稳定性提升效果是有限的,因此需要增加一条用于改善输出导纳相位的电压前馈通路;最后通过实验证明,加入改进方案后的并网逆变器,在阻抗变化较大的弱电网下依然保持良好的鲁棒性。

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关键词 ：并网逆变器, 弱电网, 鲁棒性, 锁相环, 电网电压前馈

Abstract：

Phase-locked loop (PLL) and the grid voltage feedforward (GVF) are commonly adopted in grid-connected inverters. However, both of these two links may have impact on the stability of the system in weak grid. From the perspective of impedance, considering PLL and the GVF is equivalent to two additional admittances Y_PLL(s) and Y_ff(s) in parallel on top of the original output admittance. The output admittance Bode plots considering the PLL and grid voltage feed-forward are given in Fig.A1, which shows that the output characteristics in the middle and low frequency bands are influenced by the two factors, resulting in a joint-action frequency band.

Fig.A1 Bode plots of output admittance of inverter considering dual factors
As the grid impedance increases, the admittance intersection frequency point enters the GVF-dominant frequency range of firstly. Accordingly, the primary consideration is to improve the GVF by introducing weighting coefficient to ensure the stability of the inverter as well as maintain a certain level of harmonic suppression capability of the system. With the increasing of the grid impedance, the bandwidth of the current loop decreases rapidly, approaching the frequency range influenced by the PLL bandwidth. This results in coupling between the current loop and the PLL. The introduction of the weighting coefficient into the GVF can enhance the bandwidth of the current loop, thereby improving the poor robustness induced by PLL.
However, for weaker grids with smaller SCR, the presence of the PLL would bring about a less or even negative phase margin, leading to the system unstable. Consequently, the improving effect of the weighting-coefficient-based GVF is limited in this case. Therefore, as shown in Fig.A2, a voltage feedforward path is added to improve the phase introduced by the phase-locked loop, allowing the system to adapt to a wider range of varying grid impedance. Specifically, an all-pass filter (APF) is added in this path, and then phase compensation is applied to it, which is followed by the design of its parameters at last.

Fig.A2 Control block diagram with an improved feedforward path
Finally, a prototype was constructed, and experimental waveforms verified the theoretical analysis mentioned above. The results indicate that, with the addition of the improved voltage feedforward path to the grid-connected inverter, the system can maintain a certain level of robustness even when the grid impedance varies significantly. The contribution of this article lies in following aspects: (1) The frequency-band division considering dual unstable factors is carried out. Then, the three regions including the PLL-dominant region, joint-action region and GVF-dominant region are acquired. (2) It is found that, the GVF with weighted coefficient has the merit of enhancing the robustness issue caused by the PLL to a certain extent. While, the role of improvement is somewhat limited since the phase margin is not big enough. (3) In joint-action frequency region, on the basis of the weighted coefficient, the improved feedforward path is ultimately introduced to ensure a good stability of the system in weak grid.

Key words： Grid-connected inverter weak grid robustness phase-locked loop grid voltage feedforward

收稿日期: 2023-06-25

PACS:

TM464

基金资助:

国家自然科学基金（52077102）和江苏省自然科学基金（BK20201299）资助项目

通讯作者: 方天治男,1977年生,教授,博士生导师,研究方向为逆变器及电力电子系统集成等。E-mail：fangtianzhi@126.com

作者简介: 刘昊男,2000年生,硕士研究生,研究方向为逆变器等。E-mail：lh2018@nuaa.edu.cn

引用本文:

刘昊, 方天治, 张惠丽, 朱延涛. 弱电网下应对复杂稳定性问题的并网逆变器改进电压前馈通路研究[J]. 电工技术学报, 2024, 39(16): 4955-4967. Liu Hao, Fang Tianzhi, Zhang Huili, Zhu Yantao. Research on an Improved Voltage Feedforward Path of Grid-Connected Inverter Coping with Complex Stability Issues in Weak Grid. Transactions of China Electrotechnical Society, 2024, 39(16): 4955-4967.

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