KR-102963204-B1 - Grid forming inverter device and method of operating the same

KR102963204B1KR 102963204 B1KR102963204 B1KR 102963204B1KR-102963204-B1

Abstract

The present invention discloses a grid forming inverter device and a method of operation thereof, comprising a grid forming inverter connected to a grid, an inverter controller for controlling the output voltage of the grid forming inverter, and a current sensor unit for sensing the output current of the grid forming inverter, wherein the inverter controller calculates the output voltage of the grid forming inverter based on the output current of the grid forming inverter, and if the output voltage changes to a level below the normal operating range of the grid forming inverter, determines that a voltage drop accident has occurred, calculates the virtual impedance of the grid forming inverter, applies the calculated virtual impedance to recalculate the output voltage command of the grid forming inverter to calculate the final output voltage command, and transmits the final output voltage command to the grid forming inverter to control the output voltage of the grid forming inverter.

Inventors

양형규
장푸름
김석원
박준성
최준혁
김진홍
현병조
노용수
주동명
박상민
황대연

Assignees

한국전자기술연구원

Dates

Publication Date: 20260511
Application Date: 20250901

Claims (12)

In a grid forming inverter device, Grid forming inverter connected to the grid; An inverter controller that controls the output voltage of the grid forming inverter; and A current sensor unit for sensing the output current of the grid forming inverter; is included, The above inverter controller is, Calculate the output voltage of the grid forming inverter based on the output current of the grid forming inverter, and If the above output voltage changes to a level below the normal operating range of the grid forming inverter, it is determined that a voltage drop accident has occurred, and the virtual impedance of the grid forming inverter is calculated, and The above normal operating voltage is set to be determined as the occurrence of a voltage drop accident if it changes to less than 0.9 pu (per unit) of the system nominal voltage of the above system, and The above virtual impedance includes virtual reactance and virtual resistance, and The above inverter controller is, In relation to the calculation of the virtual reactance above, the virtual reactance is calculated by subtracting the actual grid reactance value of the grid from the value obtained by dividing the grid nominal voltage of the grid by the maximum allowable current of the grid forming inverter, and In relation to the calculation of the virtual resistance above, the virtual resistance value is calculated by dividing the virtual reactance value by the virtual reactance ratio for the preset virtual resistance. The virtual reactance ratio for the above virtual resistance is characterized by being preset to a value between 0.1 and 10, and The above-calculated virtual impedance is applied to recalculate the output voltage command of the grid forming inverter to calculate the final output voltage command, and A grid forming inverter device characterized by being configured to control the output voltage of the grid forming inverter by transmitting the above final output voltage command to the grid forming inverter.
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In paragraph 1, The above inverter controller The d-axis reference voltage of the grid forming inverter is set to the grid nominal voltage, and the q-axis reference voltage of the grid forming inverter is set to 0. In relation to the calculation of the d-axis final output voltage command of the grid forming inverter, the d-axis final output voltage command is recalculated by subtracting the value obtained by multiplying the d-axis current value sensed by the current sensor unit by the virtual resistance from the d-axis reference voltage, and adding the value obtained by multiplying the q-axis current value sensed by the current sensor unit by the virtual reactance. A grid forming inverter device characterized by being configured to recalculate the q-axis final output voltage command by subtracting the value obtained by multiplying the q-axis current value by the virtual resistance and the value obtained by multiplying the d-axis current value by the virtual reactance from the q-axis reference voltage in relation to the calculation of the q-axis final output voltage command of the grid forming inverter.
In paragraph 1, The above inverter controller Monitor the output voltage status of the grid forming inverter to check whether the voltage drop fault is cleared, and A grid forming inverter device characterized by being configured to control the grid forming inverter based on a preset reactive power droop control algorithm when the above voltage drop fault is released.
In paragraph 5, The above inverter controller A grid forming inverter device characterized by being configured to determine that the voltage drop fault is released when the magnitude of the output voltage of the grid forming inverter changes to 0.9 pu or more of the grid nominal voltage of the grid and the changed state is maintained for a set time or longer.
In a method of operating a grid forming inverter device connected to a grid, An inverter controller that controls the output voltage of the above-mentioned grid forming inverter, A step of sensing the output current through the current sensor unit of the grid forming inverter; A step of calculating the output voltage of the grid forming inverter based on the output current of the grid forming inverter; A step of determining that a voltage drop accident has occurred if the output voltage changes below the normal operating range of the grid forming inverter; The step of determining the occurrence of the above voltage drop accident is, A step of determining that a voltage drop accident has occurred when the output voltage of the grid forming inverter changes to less than 0.9 pu (per unit) of the grid nominal voltage of the grid; and If it is determined that the above voltage drop accident has occurred, the method includes the step of calculating the virtual impedance of the grid forming inverter; The above virtual impedance includes virtual reactance and virtual resistance, and The step of calculating the virtual impedance above is, A step of calculating the virtual reactance by subtracting the actual grid reactance value of the grid from the value obtained by dividing the grid nominal voltage of the grid by the maximum allowable current of the grid forming inverter in relation to the calculation of the virtual reactance; and A step of calculating the virtual resistance value by dividing the virtual reactance value by the virtual reactance ratio for a preset virtual resistance in relation to the calculation of the virtual resistance above; A step of pre-setting the virtual reactance ratio for the above virtual resistance to a value between 0.1 and 10; A step of applying the above-calculated virtual impedance to recalculate the output voltage command of the grid forming inverter to calculate the final output voltage command; and A step of transmitting the above final output voltage command to the grid forming inverter to control the output voltage of the grid forming inverter; A method of operating a grid forming inverter device characterized by including
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In Paragraph 7, The step of calculating the above final output voltage command is, A step of setting the d-axis reference voltage of the grid forming inverter to the grid nominal voltage and setting the q-axis reference voltage of the grid forming inverter to 0; A step of recalculating the d-axis final output voltage command in relation to the calculation of the d-axis final output voltage command of the grid forming inverter by subtracting the value obtained by multiplying the d-axis current value sensed by the current sensor unit by the virtual resistance from the d-axis reference voltage, and adding the value obtained by multiplying the q-axis current value sensed by the current sensor unit by the virtual reactance; and A method for operating a grid forming inverter device, characterized by including the step of recalculating the q-axis final output voltage command by subtracting the value obtained by multiplying the q-axis current value by the virtual resistance and the value obtained by multiplying the d-axis current value by the virtual reactance from the q-axis reference voltage in relation to the calculation of the q-axis final output voltage command of the grid forming inverter.
In Paragraph 7, A step of monitoring the output voltage status of the grid forming inverter to determine whether the voltage drop fault is cleared; A method for operating a grid forming inverter device, further comprising the step of controlling the grid forming inverter based on a preset reactive power droop control algorithm when the above voltage drop accident is released.
In Paragraph 11, The step of checking whether the above voltage drop fault has been released is, A method of operating a grid forming inverter device characterized by including the step of checking whether the change state is maintained for a set time or longer while the magnitude of the output voltage of the grid forming inverter is changed to 0.9 pu or more of the grid nominal voltage of the grid.

Description

Grid forming inverter device and method of operating the same The present invention relates to a grid forming inverter device, and more specifically, to a grid forming inverter device that supports a reactive power limit supply and a method of operating the same. Grid forming is a term used in power systems to refer to the operational method of power conversion systems designed to maintain the stability and reliability of the power grid. Conventional power grids primarily operate using a grid feeding method. In this method, power suppliers reliably supply power to the grid, and consumers receive the necessary power from the grid. Meanwhile, renewable energy possesses variability that makes it difficult to guarantee a consistent power supply depending on environmental conditions. This characteristic of renewable energy poses a problem in that it is difficult to integrate with conventional grid-feeding systems, and due to this issue, new power conversion methods such as grid forming are attracting attention. Grid forming refers to a mode of operation in which power demand is driven within the power grid. By effectively integrating renewable energy sources and storage systems, grid forming maintains grid stability and enables the balanced regulation of power supply and demand. Grid forming plays a crucial role in providing stable voltage and frequency to the power grid and ensuring continued grid operation even in the event of failures. A grid-forming inverter is a power converter that controls the output voltage to connect with the power grid as a voltage source. Meanwhile, a grid experiencing a voltage drop requires reactive power to restore the voltage, and the grid-forming inverter can immediately supply this reactive power to the grid in response to such faults. However, grid-forming inverters connected to the grid as a voltage source pose a risk of overcurrent flow due to the potential difference between the output voltage and the grid voltage if the grid voltage drop is excessive. This can damage the power semiconductors of the grid-forming inverter, leading to hardware failure and potentially degrading grid connection stability. The information described above, disclosed in the background technology of this invention, is intended only to enhance understanding of the background of this disclosure and may therefore include information that does not constitute prior art. FIG. 1 is a drawing showing an example of a grid forming inverter system according to one embodiment. FIG. 2 is a diagram showing an example of an inverter controller configuration according to an embodiment of the present invention. FIG. 3 is a diagram showing an example of a control method for a grid forming inverter device according to an embodiment of the present invention. Figure 4 is a diagram showing the simulation results without the application of virtual impedance compensation during a voltage drop fault in a typical grid forming inverter system. FIG. 5 is a diagram showing the simulation results according to virtual impedance compensation in the event of a voltage drop accident in a grid forming inverter system according to one embodiment. It should be noted that in the following description, only the parts necessary for understanding the embodiments of the present invention are described, and the description of other parts will be omitted to the extent that it does not detract from the gist of the present invention. The grid forming inverter device and the method of operation thereof according to the embodiment of the present invention described below provide a reactive power limit supply technique that can supply the maximum amount of reactive power to the grid while protecting the grid forming inverter even in fault situations, in order to suppress a situation where power semiconductor devices of the grid forming inverter are damaged and lead to hardware failure if the reactive current increases rapidly when a grid voltage drop fault occurs. For example, if a method is used in which virtual resistance and virtual reactance are calculated based on the error value between the reactive current and the maximum allowable current after the conventional reactive current exceeds the maximum allowable current, and the output voltage command is adjusted using these values, the current limiting effect can only be achieved when the current error exceeds a certain level. Therefore, there is a limitation in that the actual reactive current inevitably still exceeds the maximum allowable current even during reactive current limiting, and there is a risk of hardware damage due to control instability caused by excessive adjustment of the output voltage command depending on the coefficient setting. In this regard, the present invention supports the supply of reactive power up to the limit by recalculating the output voltage command so that the reactive current is output exactly 1 pu when the system voltage drops to 0 pu, in order to use a meth