Stability control method for multi-inverter system based on mode adaptation in weak grid

Abstract

The invention discloses a multi-inverter system stability control method based on mode self-adaption in a weak power grid. The invention provides the multi-inverter system stability control method based on mode self-adaption for the stability of multiple inverters operating in a current source mode under the weak power grid. The method comprises the steps that the equivalent power grid impedance of the common coupling point of a grid-connected inverter in a multi-inverter system is acquired through a power grid impedance identification algorithm; if the value of the equivalent power grid impedance is greater than a set equivalent power grid impedance boundary value, other grid-connected inverters in the multi-inverter system are switched to operate in a voltage source mode one by one untilthe identified equivalent power grid impedance value is smaller than the set equivalent power grid impedance boundary value; and the grid-connected stability of the entire multi-inverter system is improved. According to the invention, the method is simple to implement; the stability margin of the multi-inverter system under the weak power grid is greatly improved; and the power grid adaptabilityof the multi-inverter system is improved.

Description

Technical field [0001] The invention relates to a control method for grid connection of a multi-inverter system, in particular to a stable control method of a multi-inverter system based on mode adaptation in a weak grid. Background technique [0002] With the rapid development of distributed power generation systems, grid-connected inverters have been widely used. Due to the existence of long-distance transmission lines and a large number of transformer devices in remote distributed power generation systems, the power grid presents a non-negligible equivalent impedance, which makes the power grid weak grid characteristics. At this time, a multi-inverter system consisting of multiple grid-connected inverters will form a dynamic interconnection system with the grid when it is connected to the grid. The system will exist at its point of common coupling (PCC). The grid impedance will cause the stability of the grid-connected multi-inverter system to decrease and cause the output gr...

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Problems solved by technology

However, this document considers the problem of stability degradation caused by the LCL output filter of the multi-inverter system itself, and does not involve the multi-inverter system caused by the presence of grid impedance when the multi-inverter system is connected to a weak grid. System resonance, and because the schemes proposed in the two documents require additional resonance suppression devices, there are problems of high cost and large space occupation

Similarly, the multi-inverter system described in this document only considers that all grid-connected inverters in the system operate in single current source mode, and does not consider switching the inverters to voltage source mode to improve system stability

[0011] 3) Chinese patent document CN 106026194 A published on October 12, 2016 "Methods for Improving the Robustness of Grid-connected Systems with Multi-Inverter Parallel Grid-connected Systems" is based on a passive network method, that is, switching capacitors method and parallel capacitor-resistance method to improve the robustness of the grid-connected inverter to the grid impedance, however, the scheme proposed in this paper needs to add additional passive devices such as capacitors and resistors, which also has the problems of high cost and large space occupation , and also introduces unnecessary loss

Similarly, the multi-inverter system described in this document only considers the grid-connected inverter operating in a single current source mode, and does not address the issue of improving system stability through mode adaptive switching

[0012] 4) Chinese patent document CN 105207219 A published on December 30, 2015 "Multiple Resonance Suppression Method for Multiple Inverters Connected to Weak Power Grid in Parallel", is an active trap filter with capacitor voltage feedback to realize multi-inverter The suppression of the resonance peak of the inverter system under the weak power grid, but this paper only considers the suppression of the resonance of the current control loop of a single inverter, but it cannot guarantee that the resonance of the multi-inverter system is suppressed, that is, it does not consider the proposed Can the scheme suppress the resonance problem caused by the weak grid in the entire multi-inverter system; and the scheme proposed in this paper needs to add a corresponding control algorithm for each inverter, which increases the complexity of the multi-inverter control system

[0014] (1) The existing literature on switching between current source and voltage source modes includes the switching between current source and voltage source modes in the case of grid-connected and islanded, and there are also reports based on whether the grid frequency fluctuates violently as the current source and voltage source mode. The basis for switching between voltage source modes, and the switching between inverter current source and voltage source grid-connected modes through grid impedance identification. However, the above-mentioned mode switching methods are all based on a single grid-connected inverter. A multi-inverter system consisting of multiple grid-connected inverters is not designed;

[0015] (2) Aiming at the multi-inverter system resonance problem under the condition of weak power grid, the existing literature adopts the method of installing resonance suppression device or connecting passive devices in the system to realize the suppression of multi-inverter system resonance, which increases the system The cost and volume, and increase the loss of the system;

[0016] (3) The existing literature focuses on the active damping suppression method of the multi-inverter system under the weak grid, only considering the method of suppressing the resonance of each grid-connected inverter to achieve the purpose of system resonance suppression, which cannot guarantee the multi-inverter The suppression of resonance at the system level of the inverter, and the corresponding control algorithm needs to be added to each grid-connected inverter, which increases the complexity of the multi-grid inverter control system;

[0017] (4) None of the existing literature involves obtaining the equivalent grid impedance of a grid-connected inverter common coupling point in a multi-inverter system through the grid impedance identification algorithm. When its value is greater than the set equivalent grid impedance boundary value , switch the rest of the grid-connected inverters in the multi-inverter system to the voltage source mode one by one until the identified equivalent grid impedance value is less than the set equivalent grid impedance boundary value, thereby improving the efficiency of the entire multi-inverter system. The problem of grid-connected stability of the system

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