Voltage feedforward lag compensation control method based on impedance self-adaption under weak power grid

Abstract

The invention discloses a grid voltage feedforward lag compensation control method based on grid impedance self-adaption under a weak power grid. The method realizes grid voltage feedforward lag compensation control based on grid impedance self-adaption by identifying grid impedance for the problem of stability of a grid-connected inverter due to conventional grid voltage direct feedforward control under the weak power grid, that is, when grid impedance is large, adding a low-pass filter, the cutoff frequency of which is low, to a grid voltage feedforward channel, and when the grid impedance is small, adding a low-pass filter, the cutoff frequency of which is high, to the grid voltage feedforward channel. The method not only keeps the capability of the conventional grid voltage feedforwardcontrol in suppressing power grid background harmonics, but also greatly increases stability of the grid-connected inverter under the condition of great change of the power grid impedance, and improves grid-connected current quality of the grid-connected inverter.

Description

technical field [0001] The invention relates to a control method of a grid-connected inverter system, in particular to a voltage feedforward lag compensation control method based on impedance self-adaptation under a weak grid. Background technique [0002] As a power conversion device, the three-phase grid-connected inverter plays an increasingly critical role in the process of connecting distributed power sources such as wind power generation and photovoltaics with the AC grid. Considering the spatial distribution characteristics and cost of renewable energy, most of these distributed power sources are located in remote areas such as deserts and islands. Due to the influence of long transmission lines and transformers, the power grid presents a high-impedance weak grid characteristic. In this case, a dynamic interconnection system will be formed between the inverter and the grid, resulting in a decrease in the bandwidth of the inverter control system and affecting its contr...

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Problems solved by technology

[0004] 1) Xu Jinming, Xie Shaojun and Tang Ting published "Adaptive Current Control of LCL Filter Grid-connected Inverter under Weak Power Grid" on August 25, 2014 in "Proceedings of the Chinese Society for Electrical Engineering" Volume 34 Issue 24, which pointed out that Under the weak grid, the grid voltage proportional feedforward will form a positive feedback channel, which will greatly reduce the stability margin of the grid-connected inverter, and even lead to system instability. This paper proposes an adaptive current control method based on grid impedance measurement

However, this method relies on accurate grid impedance values, which makes the algorithm more complicated. In addition, the proposed stability compensation scheme contains a differential link, which will face the problem of noise interference in engineering applications.

[0005] 2) Xu Fei, Tang Yu and Gu Wei published "Resonance Feedforward Control Strategy of LCL Grid-connected Inverter under Weak Grid Conditions" published in "Proceedings of the Chinese Society for Electrical Engineering" on September 20, 2016, Volume 36, Issue 18 , this paper proposes an improved feed-forward control method using the second-order generalized integrator to realize the band-pass filter characteristics, so as to improve the stability of the system under weak grid. However, this method will greatly increase the current command step of the grid-connected inverter, etc. The overshoot in the dynamic process deteriorates the dynamic performance of the grid-connected system

[0006] 3) Qian Qiang, Xie Shaojun and Ji Lin published "A Current Control Strategy to Improve the Adaptability of the Inverter to the Power Grid" on November 20, 2016 in the "Proceedings of the Chinese Society for Electrical Engineering", Volume 36, Issue 22. This paper improves the stability margin of the grid-connected inverter by reducing the feed-forward gain of the grid voltage to improve the stability of the grid-connected inverter to a certain extent, but this method will make the fundamental wave gain of the grid-connected inverter It is greatly reduced, which is not conducive to the tracking of the current fundamental wave command, and increases the steady-state error between the grid-connected current feedback value and the command value; moreover, this scheme will also reduce the dynamic performance of the grid-connected inverter, which is not suitable for dynamic performance demanding occasions

[0007] To sum up, in view of the problem that the stability of the grid-connected inverter is greatly reduced by the direct feed-forward control of the traditional grid voltage in the case of a weak grid, the existing technology has the following problems:

[0008] (1) The existing adaptive current control method based on grid impedance measurement relies on accurate grid impedance values, resulting in a more complex algorithm, and because the stability compensation scheme contains a differential link, it will face the problem of noise interference in engineering applications;

[0009] (2) The existing improved feed-forward control method using the second-order generalized integrator to realize the band-pass filter characteristics will greatly increase the overshoot in the dynamic process such as the current command step of the grid-connected inverter, and deteriorate the dynamics of the grid-connected system performance;

[0010] (3) The existing method of reducing the feed-forward gain of the grid voltage will greatly reduce the fundamental wave gain of the grid-connected inverter, which is not conducive to the tracking of the current fundamental wave command, and increases the stability between the grid-connected current feedback value and the command value. State error, and reduce the dynamic performance of the grid-connected inverter, not suitable for occasions with high dynamic performance requirements;

[0011] (4) None of the schemes proposed in the existing literature involve the realization of grid voltage feed-forward lag compensation control based on grid impedance adaptation by identifying the grid impedance, that is, when the grid impedance is large, the grid voltage feed-forward A low-pass filter with a lower cut-off frequency is added to the channel, and a low-pass filter with a higher cut-off frequency is added to the grid voltage feedforward channel when the grid impedance is small, so as to realize that the inverter can adapt to a wide range of grid impedance changes. Grid stability issues

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