A virtual reactance inverter grid-connected method based on droop control

Abstract

The application discloses a virtual reactance inverter grid-connected method based on droop control, which is based on the reference voltage generated by single-loop droop control and the dq-axis current, and adds virtual impedance control to generate virtual voltage to correct the reference voltage. The implementation effect of the method is equivalent to the increase of the internal inductance of the power grid, and the effect of weakening the strong grid inertia connected is achieved, so that the stability of the single-loop droop control inverter connected with the strong power grid is strengthened. The application is applied to the grid-structured single-loop droop control inverter, can realize the function of stable interconnection of the single-loop droop control inverter and the strong power grid, and has the characteristics of simple control method, less input devices and the like.

Description

Technical Field

[0001] This invention belongs to the field of new energy power generation grid connection technology, specifically relating to a grid connection method for a virtual reactor inverter based on droop control. Background Technology

[0002] Grid-based control methods for inverters, such as droop control, aim to autonomously construct voltage amplitude and frequency to support the grid. Therefore, grid-based inverters typically exhibit high-inertia voltage sources. When a grid-based inverter is connected to a high-inertia, low-impedance grid, it is prone to instability phenomena such as subsynchronous oscillations, leading to a deterioration in system stability margin. For converters involved in high-power applications, the requirements for control accuracy and speed can be relaxed in exchange for stability margin. Common droop methods employ dual-loop voltage and current controllers, but their small-signal stability margin is poor. In contrast, single-loop droop methods have slightly lower control accuracy and speed, but the control structure is simpler and the small-signal stability margin is larger.

[0003] To address the issue of degraded stability margin when droop-controlled inverters are connected to a strong grid, virtual impedance technology, as an effective damping enhancement method, has been widely studied in grid control technology. However, current research on methods combining single-loop droop control and virtual impedance is insufficient. Therefore, to achieve higher stability margin for droop controllers when connected to a strong grid, finding a method combining virtual impedance technology and single-loop droop control technology has significant theoretical and practical value.

[0004] In the prior art, Chinese invention patent application CN121238681A discloses a method for wide grid strength adaptive control of grid-connected inverters. This method adds virtual impedance to increase the equivalent internal resistance of the inverter to achieve impedance matching based on the dual-loop voltage and current droop control. However, its model derivation and parameter tuning are difficult, and the engineering implementation cost is high. Chinese invention patent application CN121192781A discloses a method for optimizing and adjusting grid-connected power supply parameters to improve the stability of strong grids. This method requires establishing a system state-space model, determining the dominant oscillation mode based on eigenvalues, and using a single-variable perturbation method and particle swarm optimization algorithm for parameter optimization. The control structure is complex, and the requirements for computing power and device cost are high. Summary of the Invention

[0005] In view of the above, the present invention provides a virtual reactance inverter grid connection method based on droop control, which improves the small-signal stability margin of the system by increasing the equivalent internal resistance of the grid and weakening the strength of the external grid to achieve impedance matching. The control logic is simple and easy to implement in engineering.

[0006] A grid-connection method for a virtual reactor inverter based on droop control includes the following steps: (1) For grid-connected inverter systems, collect the three-phase voltage and current signals at the PCC (point of common coupling) to calculate the active and reactive power output of the inverter; (2) Based on the active and reactive power output of the inverter, a preliminary voltage reference value and voltage reference angle are generated using a droop algorithm; (3) Input the dq-axis current signal at PCC to the virtual impedance, and output the virtual dq-axis voltage through the virtual impedance; (4) Generate dq axis reference voltage based on preliminary voltage reference value and virtual dq axis voltage, then use voltage reference angle to transform it into three-phase reference voltage, and then generate PWM signal through modulation to control the inverter.

[0007] Furthermore, after acquiring the three-phase voltage and current signals at the PCC in step (1), the dq-axis voltage and current signals are obtained through Park transformation. Then, the active and reactive power outputs of the inverter are calculated and filtered based on the dq-axis voltage and current signals, as shown in the following specific expressions: in: P The output active power of the inverter. Q The output reactive power of the inverter. and These are the d-axis voltage and q-axis voltage at PCC, respectively. and These are the d-axis current and q-axis current at PCC, respectively. This is the cutoff angular frequency of the low-pass filter. s For the Laplace operator.

[0008] Furthermore, the calculation expression for the voltage reference angle in step (2) is as follows: in: For voltage reference angle, ω This is the reference value for angular frequency. The system's rated angular frequency, m This is the active power droop coefficient. P The output active power of the inverter. This refers to the rated output active power of the inverter. s For the Laplace operator.

[0009] Furthermore, the calculation expression for the preliminary voltage reference value in step (2) is as follows: in: This is a preliminary voltage reference value. The rated voltage amplitude at PCC. E The voltage amplitude is the actual measured and filtered value at the PCC. n This is the reactive power droop factor. This is a correction amount for the voltage reference value. Q The output reactive power of the inverter. This refers to the rated output reactive power of the inverter. and These are the proportional and integral coefficients of the PI (proportional-integral) controller, respectively. s For the Laplace operator.

[0010] Furthermore, the calculation expression for the virtual dq-axis voltage in step (3) is as follows: in: For virtual d-axis voltage, For virtual q-axis voltage, For virtual resistance, For virtual inductance, The system's rated angular frequency, and These are the d-axis current and q-axis current at PCC, respectively.

[0011] Furthermore, the calculation expression for the dq-axis reference voltage in step (4) is as follows: in: and These are the d-axis reference voltage and the q-axis reference voltage, respectively. This is a preliminary voltage reference value. For virtual d-axis voltage, This is the virtual q-axis voltage.

[0012] A computer device includes a memory and a processor, wherein the memory stores a computer program and the processor executes the computer program to implement the above-described method for grid connection of a virtual reactor inverter based on droop control.

[0013] A computer-readable storage medium storing a computer program, which, when executed by a processor, implements the above-described method for grid-connecting a virtual reactor inverter based on droop control.

[0014] This invention addresses the instability issues of droop-controlled inverters when connected to high-inertia power grids, such as low stability margins and susceptibility to subsynchronous oscillations. The method measures both voltage and current signals, ignoring the influence of filter capacitors. After droop control and the generation of reference voltage amplitude and angle signals via a proportional-integral (PI) or integrator, the reference voltage amplitude signal is adjusted using virtual impedance control based on the current signal. Finally, the inverter output voltage is directly controlled. This invention, while achieving single-loop droop inverter control, increases the equivalent line impedance, effectively increasing the inductance within the high-inertia grid. This reduces the inertia of the connected grid, making the high-inertia grid equivalent to a weak grid for the inverter. This achieves stable connection between the grid-connected single-loop droop control and the high-inertia grid, increasing the system's stability margin. Furthermore, the voltage control stage based on a PPI introduced after the reactive power droop control ensures steady-state accuracy of the inverter's voltage amplitude and provides rapid adjustment capabilities in the event of disturbances. Attached Figure Description

[0015] Figure 1 This is a schematic diagram of the grid connection method for a virtual reactor inverter based on droop control according to the present invention.

[0016] Figure 2 This is a block diagram of the grid-connected control system for a virtual reactor inverter based on droop control according to the present invention. Detailed Implementation

[0017] To describe the present invention in more detail, the technical solution of the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0018] This invention provides an inverter control method combining single-loop droop and virtual reactance, applicable to grid-connected inverters connected to high-inertia, high-voltage power grids. This improves the stability margin of the single-loop droop control method when the inverter interacts with the high-voltage power grid. The specific steps of this method are as follows: Figure 1 As shown: (1) Obtain the three-phase voltage and current information at the PCC common connection point of the power grid branch, obtain the voltage and current information in the dq coordinate system through Park coordinate transformation, and then calculate and filter the active and reactive power output of the inverter to the power grid based on the dq axis voltage and current signal.

[0019] (2) The reference voltage amplitude and voltage angle signals are initially generated using the droop algorithm. The active power obtained by measurement and calculation is compared with the active power setpoint. The difference in active power is processed by the active power droop coefficient and added to the rated frequency to obtain the frequency reference value generated by the droop element. The frequency reference value is then input to the integrator to obtain the voltage signal angle information generated by the droop control. The reactive power obtained by measurement and calculation is compared with the reactive power setpoint. The difference in reactive power is processed by the reactive power droop coefficient and added to the difference between the rated voltage amplitude and the measured voltage amplitude (which is the amplitude of the dq voltage vector obtained through a low-pass filter) to obtain the correction amount of the voltage reference value. This correction amount is added to the rated voltage amplitude through a proportional integrator to obtain the preliminary voltage reference value.

[0020] (3) Input the dq axis current signal to the virtual impedance, and the signal output by the virtual impedance is the virtual dq axis voltage; the virtual impedance is used to simulate the internal impedance of the connected power grid in order to weaken the strength of the connected power grid.

[0021] (4) The voltage reference value is subtracted from the virtual d-axis voltage value to obtain the d-axis voltage reference value of the PWM generator. Generally, the q-axis voltage reference value is set to 0, so the negative of the virtual q-axis voltage value is directly used as the q-axis voltage reference value of the PWM generator. The angle information obtained from the active power droop is used to perform a Park inverse transformation on the dq-axis voltage reference value to obtain the three-phase voltage reference value. Finally, the PWM generator generates a PWM signal to drive the three-phase circuit to control the voltage amplitude and angle at the inverter output point.

[0022] Example Figure 2 The diagram illustrates the physical and control structures of one embodiment of the present invention, wherein the inverter is connected to a DC power supply and, driven by a PWM signal, converts the DC signal into an AC signal and feeds the power into the power grid. The grid voltage in the diagram is... The internal inductance is Short-circuit ratio .

[0023] In this embodiment, the inverter control method combining single-loop droop and virtual reactance is applied to the inverter grid-connected system. The specific implementation process is as follows: Step 1: Measure and calculate the dq axis voltage and current information, and calculate the active and reactive power.

[0024] Step 1.1: First, it is necessary to obtain voltage and current information. This is due to the filter capacitor... Cf The impact on the fundamental frequency signal is minimal and can be ignored in the control design. Three-phase voltage and current information is collected at the PCC, and the voltage and current signals are transformed to obtain the voltage and current parameters in the dq coordinate system for later use. , These are the voltage components along the d-axis and q-axis, respectively. , These are the d-axis and q-axis current components, respectively.

[0025] Step 1.2: The active and reactive power output of the inverter needs to be calculated. To filter out high-frequency ripple and measurement noise in the instantaneous power, a first-order low-pass filter is generally introduced after the instantaneous power calculation. At this point, under the equal amplitude Park coordinate transformation, the active power in the dq coordinate system... reactive power The calculation formula is: in, This is the cutoff angular frequency of the low-pass filter, which can generally be selected as the angular frequency corresponding to the power frequency to filter out high-frequency noise.

[0026] Step 2: Use the droop method to preliminarily calculate the reference values ​​for voltage amplitude and voltage angle.

[0027] Step 2.1: Obtain the angular frequency generated by droop control using the active droop control method. The calculation formula is as follows: in, The system's rated angular frequency, The inverter outputs rated active power. Let be the active power droop coefficient. Expanding at the steady-state operating point and using the Δ prefix to represent small-signal physical quantities, we obtain the following relationship: From this, we can derive the relationship between small signals: Integrating across the angular frequency yields the voltage angle reference value generated by the active power droop control and the corresponding small-signal model: Step 2.2: Obtain the initial reference voltage correction value using the reactive power droop control method. The calculation formula is as follows: in: The rated voltage amplitude at the PCC point. To calculate and filter the PCC point voltage amplitude using actual measured values, This is the reactive power droop factor. Given the inverter's rated reactive power output, expanding the equation at the steady-state operating point yields the following relationship: From this, we can derive the relationship between small signals: Step 2.3: The voltage error signal obtained from the reactive power droop is processed by a proportional-integral voltage control circuit and added to the rated voltage amplitude to generate a voltage amplitude reference value. The calculation formula and corresponding small-signal model are as follows: in: and These are the proportional coefficient and integral coefficient of the proportional-integral converter, respectively; the calculation steps involved in this sub-step can realize the rapid adjustment of the voltage amplitude at the PCC point and eliminate its steady-state error.

[0028] Step 3: Generate virtual d-q axis voltages using virtual impedance. Input the d-q axis current signal calculated in Step 1.1 into the virtual impedance; the signal output by the virtual impedance is the virtual d- and q-axis voltage. , .according to Figure 2 The control block diagram of the virtual reactor module and the specific calculation formula for the virtual dq axis voltage are as follows: in: For virtual resistance, For virtual inductance, and This is its virtual reactance at power frequency.

[0029] Step 4: Synthesize the final dq axis voltage reference signal to drive the inverter.

[0030] Step 4.1: Subtract the virtual d-axis voltage calculated in Step 3 from the voltage amplitude reference value calculated in Step 2 to obtain the final d-axis voltage reference value. Since the q-axis voltage is generally required to be 0, the negative of the virtual q-axis voltage calculated in Step 3 can be used as the final q-axis voltage reference value. The specific calculation formula is as follows: in: , These are the final reference values ​​for the d-axis and q-axis voltages.

[0031] At this point, the inductance strength inside the power grid connected to the inverter is equivalent to... The internal resistance is equivalent to The equivalent short-circuit ratio of the power grid is: Step 4.2: Using the final dq voltage reference value obtained in Step 4.1 and the reference angle signal calculated using the active power droop in Step 2, perform Park inverse transformation to generate the PWM signal of the three-phase bridge arm switching transistors to drive the inverter to work.

[0032] The above description of the embodiments is provided to enable those skilled in the art to understand and apply the present invention. Those skilled in the art can readily make various modifications to the above embodiments and apply the general principles described herein to other embodiments without creative effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made to the present invention by those skilled in the art based on the disclosure thereof should be within the scope of protection of the present invention.

Claims

1. A grid-connection method for a virtual reactance inverter based on droop control, characterized in that, Includes the following steps: (1) For grid-connected inverter systems, collect the three-phase voltage and current signals at the PCC to calculate the active and reactive power output of the inverter; (2) Based on the active and reactive power output of the inverter, a preliminary voltage reference value and voltage reference angle are generated using a droop algorithm; (3) Input the dq-axis current signal at PCC to the virtual impedance, and output the virtual dq-axis voltage through the virtual impedance; (4) Generate dq axis reference voltage based on preliminary voltage reference value and virtual dq axis voltage, then use voltage reference angle to transform it into three-phase reference voltage, and then generate PWM signal through modulation to control the inverter.

2. The grid-connection method for virtual reactance inverters based on droop control according to claim 1, characterized in that: In step (1), after acquiring the three-phase voltage and current signals at the PCC, the dq-axis voltage and current signals are obtained through Park transformation. Then, the active and reactive power outputs of the inverter are calculated and filtered based on the dq-axis voltage and current signals. The specific expressions are as follows: in: P The output active power of the inverter. Q The output reactive power of the inverter. and These are the d-axis voltage and q-axis voltage at PCC, respectively. and These are the d-axis current and q-axis current at PCC, respectively. This is the cutoff angular frequency of the low-pass filter. s For the Laplace operator.

3. The grid-connection method for virtual reactance inverters based on droop control according to claim 1, characterized in that, The expression for calculating the voltage reference angle in step (2) is as follows: in: For voltage reference angle, ω This is the reference value for angular frequency. The system's rated angular frequency, m This is the active power droop coefficient. P The output active power of the inverter. This refers to the rated output active power of the inverter. s For the Laplace operator.

4. The grid-connection method for a virtual reactance inverter based on droop control according to claim 1, characterized in that, The calculation expression for the preliminary voltage reference value in step (2) is as follows: in: This is a preliminary voltage reference value. The rated voltage amplitude at PCC. E The voltage amplitude is the actual measured and filtered value at the PCC. n This is the reactive power droop factor. This is the correction amount for the voltage reference value. Q The output reactive power of the inverter. This refers to the rated output reactive power of the inverter. and These are the proportional and integral coefficients of the PI controller, respectively. s For the Laplace operator.

5. The grid-connection method for a virtual reactance inverter based on droop control according to claim 1, characterized in that, The calculation expression for the virtual dq axis voltage in step (3) is as follows: in: For virtual d-axis voltage, For virtual q-axis voltage, For virtual resistance, For virtual inductance, The system's rated angular frequency, and These are the d-axis current and q-axis current at PCC, respectively.

6. The grid-connection method for a virtual reactance inverter based on droop control according to claim 1, characterized in that, The calculation expression for the dq-axis reference voltage in step (4) is as follows: in: and These are the d-axis reference voltage and the q-axis reference voltage, respectively. This is a preliminary voltage reference value. For virtual d-axis voltage, This is the virtual q-axis voltage.

7. A computer device comprising a memory and a processor, wherein the memory stores a computer program, characterized in that: The processor is used to execute the computer program to implement the grid connection method for virtual reactor inverters based on droop control as described in any one of claims 1 to 6.

8. A computer-readable storage medium storing a computer program, characterized in that: When the computer program is executed by the processor, it implements the grid connection method for a virtual reactor inverter based on droop control as described in any one of claims 1 to 6.

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