VSG grid-connected current quality improvement method based on bridge-side quasi-harmonic current feedforward

Abstract

The method for improving the grid-connected current quality of VSG based on bridge-side quasi-harmonic current feedforward belongs to the field of inverter grid connection technology. It solves the problem of how to suppress harmonics in the grid-connected current of virtual synchronous generators and improve the grid-connected current quality. This invention uses the reference voltage signal output by the VSG power loop to calculate the ideal grid-connected current signal and the ideal capacitor branch current signal. Then, it combines the actual sampled signal of the bridge-side current to obtain the quasi-harmonic current signal. After passing through a high-pass filter to achieve a 90° phase shift, it simulates the harmonic voltage signal on the inductive impedance and feeds it forward to the VSG control loop, which effectively improves the grid-connected current quality of VSG. Moreover, the bridge-side quasi-harmonic current feedforward is simple to implement, improves the reliability of the system, and is convenient for engineering practice.

Description

Technical Field

[0001] This invention belongs to the field of inverter grid connection technology and relates to a method for improving the grid-connected current quality of VSG based on bridge-side quasi-harmonic current feedforward. Background Technology

[0002] With the rapid integration of renewable energy generation, power systems that were once dominated by centralized synchronous generators are transitioning to distributed generation systems connected by converters, most of which are designed to operate in parallel with traditional power systems. However, as the penetration rate of distributed photovoltaics increases, the strength of the power system will decrease, which may pose stability challenges.

[0003] Virtual synchronous generators (VSGs), as a new type of power device that simulates the characteristics of traditional synchronous generators using power electronics technology, can provide voltage and frequency support for the power grid and have attracted widespread attention. As a means to promote the use of environmentally friendly renewable energy, VSGs play an even more important role in injecting high-quality power into the grid. Existing technologies, such as the invention patent application CN117811088A, disclose a VSG control method for smooth grid connection of inverters. This method adds a first-order differential feedforward and a first-order differential negative feedback loop to the traditional VSG active-frequency control to achieve joint compensation, effectively reducing active power oscillations during grid frequency fluctuations.

[0004] However, distributed power sources based on power electronics are mainly located in remote areas where the power grid is easily distorted by increased nonlinear loads / generation penetration. In this situation, factors such as grid voltage and local nonlinear load currents can cause power quality problems for VSGs. Similar to traditional current-controlled inverters (TCCI) or voltage-controlled inverters (VCI) based on droop control, both grid current and PCC voltage can be severely distorted, compromising the adaptability of VSGs to distorted grids. VSGs integrated into weak grids are susceptible to grid background harmonics, leading to VSG grid-connected current distortion. Therefore, VSG control must consider power quality under grid distortion conditions, thus hindering the development of VSG technology in the renewable energy generation sector. Summary of the Invention

[0005] The technical problem to be solved by this invention is how to suppress the harmonics of the grid-connected current of the virtual synchronous generator and improve the quality of the grid-connected current.

[0006] The present invention solves the above-mentioned technical problems through the following technical solutions:

[0007] The method for improving the grid-connected current quality of VSG based on bridge-side quasi-harmonic current feedforward includes the following steps: S1, Determine the parameters of the filter capacitor branch of the grid-connected converter, including at least the filter capacitor. C f Damping resistor R cf ; S2, obtain the ideal voltage signal at the common coupling point based on the port voltage signal output from the VSG power outer loop. v iabc With ideal grid-connected current signal i iabc Then, combined with the filter capacitor branch parameters C f and R cf Calculate the ideal bridge-side current signal i iLabc ; S3, Sample and obtain the actual value of the bridge-side current. i Labc and the ideal bridge-side current signal i iLabc The difference is used as the bridge-side quasi-harmonic current signal. i Labc - i iLabc ; S4 inputs the bridge-side quasi-harmonic current signal to a preset high-pass filter to achieve a 90° phase shift in the operating frequency band of the bridge-side quasi-harmonic current signal, thereby simulating the current harmonics acting on the inductor to generate the power grid background harmonic signal, and outputs the harmonic voltage compensation signal Δe. abc .

[0008] Furthermore, the method also includes: S5, the harmonic voltage compensation signal Δe abc Feedforward to the VSG control circuit to suppress harmonic components in the grid-connected current.

[0009] Furthermore, in S2, the ideal bridge-side current signal is calculated. i iLabc Specifically: First, the port voltage signal output by the VSG power loop. v abc To obtain the phase information of the ideal grid-connected current signal Based on the effective value of rated voltage V n With phase information Solving for the common coupling point to process the desired voltage signal v iabc ; Next, based on instantaneous active power P e With the rated voltage RMS valueV n Solve for the rated grid-connected current. I i Based on phase information With grid-connected current rating I i To obtain the ideal grid-connected current signal i iabc ; Then, based on the effective value of the rated voltage V n With filter capacitor branch parameters C f and R cf Solve for the current signal of the ideal filter capacitor branch. I c ; Finally, the ideal grid-connected current signal i iabc With the current signal of the ideal filter capacitor branch I c The sum of these values ​​serves as the ideal bridge-side current signal. i iLabc .

[0010] Furthermore, in S2, the common coupling point is processed using the following logic to represent the desired voltage signal. v iabc :

[0011] in, v ia , v ib , v ic The voltage signals of phases A through C are processed at the common coupling point. V n This is the effective value of the rated voltage. The VSG outputs an electrical angle.

[0012] Furthermore, the ideal grid-connected current signal is represented in S2 using the following logic. i iabc :

[0013] in, i ia , i ib , i ic The grid-connected current signals of phases A through C are processed at the common coupling point. The VSG outputs an electrical angle.

[0014] Furthermore, in S2, the rated value of the grid-connected current is solved using the following logic. I i :

[0015] in, P e Instantaneous active power V n This is the effective value of the rated voltage.

[0016] Furthermore, the ideal filter capacitor branch current signal is represented in S2 using the following logic. I c :

[0017] in, R cf For damping resistor, C f For filtering capacitors, V n is the effective value of the rated voltage, and s is the Laplace operator.

[0018] Furthermore, the ideal bridge-side current signal is represented in S2 using the following logic. i iLabc :

[0019] in, i iLa , i iLb , i iLc These are the ideal bridge-side current signals of phases A through C at the common coupling point.

[0020] Furthermore, the transfer function of the high-pass filter described in S4 Using the following logical representation:

[0021] Where s is the Laplace operator, k is the filter gain, and k i This is the cutoff angular frequency.

[0022] The present invention also provides a grid-connected inverter that is connected to the grid based on the above-mentioned VSG grid-connected current quality improvement method with bridge-side quasi-harmonic current feedforward.

[0023] The advantages of this invention are: In weak grid environments, VSGs are easily affected by grid background harmonics, resulting in poor grid-connected current quality. The core idea of ​​this invention is to introduce bridge-side quasi-harmonic current feedforward control into the VSG control loop, extract and control the bridge-side quasi-harmonic current signal, and construct a virtual harmonic impedance, thereby improving the equivalent output impedance of the VSG and suppressing current harmonics.

[0024] Specifically, this invention utilizes the reference voltage signal output from the VSG power loop to calculate the ideal grid-connected current signal and the ideal capacitor branch current signal. Combined with the actual sampled signal of the bridge-side current, the quasi-harmonic current signal is solved. Then, a 90° phase shift is achieved through a high-pass filter to simulate the harmonic voltage signal on the inductive impedance and feed it forward to the VSG control loop. In the process of suppressing harmonics, there is no need to sample the grid-connected voltage in real time, which effectively improves the quality of the VSG grid-connected current. Furthermore, the bridge-side quasi-harmonic current feedforward is simple to implement, which improves the reliability of the system. The high-pass filter involved has a simple structure, is easy to design, and is convenient for engineering practice. Attached Figure Description

[0025] Figure 1 This is a topology diagram of the grid-connected converter main circuit and VSG control circuit according to Embodiment 1 of the present invention; Figure 2 This is a topology diagram of bridge-side quasi-harmonic current feedforward control according to Embodiment 1 of the present invention; Figure 3 This is a waveform diagram of the grid-connected current controlled by a conventional VSG when the grid voltage contains 4% 5th and 3% 7th background harmonics, according to Embodiment 1 of the present invention. Figure 4 This is a grid current waveform diagram of the first embodiment of the present invention when the grid voltage contains 4% 5th and 3% 7th background harmonics, using bridge-side quasi-harmonic current feedforward control. Detailed Implementation

[0026] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0027] The technical solution of the present invention will be further described below with reference to the accompanying drawings and specific embodiments: Example 1 like Figure 1 , Figure 2As shown, specifically, this invention discloses a method for improving the grid-connected current quality of VSG based on bridge-side quasi-harmonic current feedforward, including the following steps: S1. Determine the basic parameters of the grid-connected converter: Based on the VSG's requirements for grid-connected current harmonics, determine the parameters of the filter capacitor branch of the grid-connected converter, including at least the filter capacitor. C f Damping resistor R cf .

[0028] like Figure 1 As shown, the main circuit of the grid-connected converter in this embodiment includes a DC side voltage. V dc Three-phase inverter bridge, LC filter and grid impedance Z g0 The LC filter consists of bridge-side inductors. L f Filter capacitor C f and with C f series damping resistors R cf Composition. In the main circuit, e abc This is the internal potential of the VSG, which is also the output voltage of the three-phase inverter bridge; v abc This refers to the VSG port voltage, which is also the voltage across the filter capacitor in the LC filter. C f Voltage at both ends; i Labc For bridge-side current, including i La , i Lb and i Lc That is, the current flows through the bridge-side inductance L f The current; i abc This refers to the grid-side current, which is the current injected into the power grid.

[0029] Before implementing the bridge-side quasi-harmonic current feedforward control strategy, it is first necessary to determine the basic electrical parameters of the grid-connected converter. In this embodiment, based on the system rated power and grid-connected current harmonic suppression requirements, the parameters of the filter capacitor branch of the grid-connected converter are determined, including at least the filter capacitor. C f Damping resistor R cf .

[0030] Furthermore, the VSG control circuit includes a power outer loop and a voltage and current inner loop; wherein, the power outer loop is used to simulate the rotor motion equation and reactive power-voltage droop characteristics of the synchronous generator, and the amplitude and phase of the voltage reference signal at the output port; the voltage and current inner loop is used to track the reference signal through a PI regulator and generate a PWM drive signal to control the inverter switching; the VSG control circuit participates in the frequency and voltage regulation of the power grid by simulating the characteristics of a synchronous machine.

[0031] like Figure 1 The following explains the basic parameters involved in the power outer loop: , These are the VSG output angular frequency and electrical angle, respectively. ω n The rated angular frequency, J, D p 、 These are the moment of inertia, damping coefficient, and primary frequency modulation coefficient, respectively. P ref For a given active power, P e Instantaneous active power; This is the effective value of the VSG output voltage. V n This is the effective value of the rated voltage. D q This is the reactive power droop factor. Q ref Given reactive power, Q e This refers to instantaneous reactive power.

[0032] S2, Determine the ideal bridge-side current signal: Obtain the ideal voltage signal at the point of common coupling (hereinafter referred to as PCC) based on the port voltage signal output from the VSG power outer loop. v iabc With ideal grid-connected current signal i iabc Then, combined with the filter capacitor branch parameters C f and R cf Calculate the ideal bridge-side current signal i iLabc ;like Figure 2 As shown, it includes the following: First, the port voltage signal output by the VSG power loop. v abc To obtain the phase information of the ideal grid-connected current signal Based on the effective value of rated voltage V n With phase information Solve the PCC processing of the desired voltage signal. v iabc ; In this embodiment, the PCC point (Point of Common Coupling) refers to the connection point between the VSG grid-connected converter and the power grid, such as... Figures 1-2 As shown, in this embodiment, the PCC point is selected from the filter capacitor in the LC filter circuit. C f The connection point between the PCC and the grid side, at this point the voltage is... v abc It is also used as the VSG port voltage.

[0033] In this embodiment, considering that the reactive power of VSG grid connection is normally zero, it is set as follows: Q ref =0, therefore, the component of the current orthogonal to the voltage is considered to be 0, and only the component in phase with the voltage exists. The VSG port voltage is in phase with the grid current, and the VSG operates in unity power factor mode. The following logic represents the PCC processing of the desired voltage signal. v iabc :

[0034] in, v ia , v ib , v ic The PCC processes the voltage signals of phases A through C. V n This is the effective value of the rated voltage. The VSG outputs an electrical angle.

[0035] Next, based on instantaneous active power P e With the rated voltage RMS value V n Solve for the rated grid-connected current. I i Based on phase information With grid-connected current rating I i To obtain the ideal grid-connected current signal i iabc The ideal grid-connected current signal can be represented using the following logic. i iabc :

[0036] in, i ia ,i ib , i ic The PCC processes the grid-connected current signals of phases A through C. VSG output electrical angle, grid-connected current rating I i Solve using the following formula:

[0037] in, P e Instantaneous active power V n This is the effective value of the rated voltage.

[0038] Then, based on the effective value of the rated voltage V n With filter capacitor branch parameters C f and R cf Solve for the current signal of the ideal filter capacitor branch. I c The solution process can be represented by the following logic: According to Kirchhoff's laws, we can obtain... After sorting, we can get ,in, R cf For damping resistor, C f For filtering capacitors, V n This is the effective value of the rated voltage. s This is the Laplace operator; since the ideal signal, i.e., the signal at the fundamental frequency, is known at the fundamental frequency. as well as Where j is the imaginary unit, substituting it into the above formula yields:

[0039] Finally, the ideal grid-connected current signal i iabc With the current signal of the ideal filter capacitor branch I c The sum of these values ​​serves as the ideal bridge-side current signal. i iLabc This can be represented using the following logic:

[0040] in, i iLa , i iLb , i iLcThese are the ideal bridge-side current signals of phases A to C at PCC, respectively.

[0041] S3, Determine the bridge-side quasi-harmonic current signal: Sample and acquire the actual value of the bridge-side current. i Labc and the ideal bridge-side current signal i iLabc The difference is used as the bridge-side quasi-harmonic current signal. i Labc - i iLabc ,like Figure 2 As shown.

[0042] S4, Design a high-pass filter: Input the bridge-side quasi-harmonic current signal to a preset high-pass filter to achieve a 90° phase shift in the operating frequency band of the bridge-side quasi-harmonic current signal, thereby simulating the current harmonics acting on the inductor to generate the power grid background harmonic signal, and outputting the harmonic voltage compensation signal Δe. abc .

[0043] In this embodiment, the high-pass filter operates at a frequency of 5 GHz. th ~19 th The high-pass filter's transfer function is expressed using the following logic: [Frequency band], and a 90° phase shift is achieved for the bridge-side harmonic current signal. The phase-shifted bridge-side harmonic current signal (i.e., the harmonic voltage compensation signal) is then fed forward into the VSG control loop. :

[0044] Where s is the Laplace operator, k is the filter gain, and k i This is the cutoff angular frequency.

[0045] In a preferred embodiment, to ensure that the phase of the signal within the operating frequency band is close to 90° after passing through the filter, the parameters k=600 and k0 of the high-pass filter are set through theoretical analysis and simulation debugging. i =100000.

[0046] S5, the harmonic voltage compensation signal Δe abc Feedforward to the VSG control circuit to suppress harmonic components in the grid-connected current.

[0047] In this embodiment, the output signal Δe of the high-pass filter is used... abcThe voltage reference value is fed forward into the VSG control loop, forming a new voltage reference value that has been corrected for harmonic suppression. This reference value is then fed into the subsequent voltage-current inner loop control loop. Through this feedforward control, the VSG output voltage actively includes a voltage component that acts opposite to the harmonic current, thereby effectively suppressing grid-connected current harmonics caused by grid background harmonics. From the perspective of the system equivalent model, this control strategy is equivalent to increasing the equivalent output impedance of the VSG system, thus enhancing its ability to suppress harmonic currents.

[0048] Furthermore, considering the system's stability, disturbance rejection, and current harmonic suppression effect, k and k can be adjusted. i Until the best control effect is achieved.

[0049] To further verify the effectiveness of the grid-connected current quality improvement method provided by this invention, a VSG grid-connected simulation model was built based on the Matlab / Simulink platform.

[0050] The simulation conditions are: 4% of the 5th harmonic and 3% of the 7th harmonic are injected into the grid voltage.

[0051] Figure 3 The grid-connected current waveform when using traditional VSG control (i.e. without adding the bridge-side quasi-harmonic current feedforward proposed in this invention) shows that the total harmonic distortion (THD) of the current is 0.63% before harmonics are injected 2 seconds ago; after harmonics are injected 2 seconds ago, the current THD increases to 6.47%, and the waveform is significantly distorted.

[0052] Figure 4 The grid-connected current waveform when using the bridge-side quasi-harmonic current feedforward control strategy proposed in this invention is shown. Under the same harmonic injection conditions, the THD of the grid-connected current after 2 seconds is only 3.86%. The lower the THD, the better the power quality. It can be seen that compared with traditional VSG control, Figure 4 The waveform distortion was significantly improved. Simulation results show that the bridge-side quasi-harmonic current feedforward has a good harmonic suppression effect and improves the grid-connected current quality.

[0053] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for improving the grid-connected current quality of VSG based on bridge-side quasi-harmonic current feedforward, characterized in that, Includes the following steps: S1, Determine the parameters of the filter capacitor branch of the grid-connected converter, including at least the filter capacitor. C f Damping resistor R cf ; S2, obtain the ideal voltage signal at the common coupling point based on the port voltage signal output from the VSG power outer loop. v iabc With ideal grid-connected current signal i iabc Then, combined with the filter capacitor branch parameters C f and R cf Calculate the ideal bridge-side current signal i iLabc ; S3, Sample and obtain the actual value of the bridge-side current. i Labc and the ideal bridge-side current signal i iLabc The difference is used as the bridge-side quasi-harmonic current signal. i Labc - i iLabc ; S4 inputs the bridge-side quasi-harmonic current signal to a preset high-pass filter to achieve a 90° phase shift in the operating frequency band of the bridge-side quasi-harmonic current signal, thereby simulating the current harmonics acting on the inductor to generate the power grid background harmonic signal, and outputs the harmonic voltage compensation signal Δe. abc .

2. The method for improving VSG grid-connected current quality based on bridge-side quasi-harmonic current feedforward according to claim 1, characterized in that, The method further includes: S5, the harmonic voltage compensation signal Δe abc Feedforward to the VSG control circuit to suppress harmonic components in the grid-connected current.

3. The method for improving VSG grid-connected current quality based on bridge-side quasi-harmonic current feedforward according to claim 1, characterized in that, The ideal bridge-side current signal is calculated in S2. i iLabc Specifically: First, the port voltage signal output by the VSG power loop. v abc To obtain the phase information of the ideal grid-connected current signal Based on the effective value of rated voltage V n With phase information Solving for the common coupling point to process the desired voltage signal v iabc ; Next, based on instantaneous active power P e With the rated voltage RMS value V n Solve for the rated grid-connected current. I i Based on phase information With grid-connected current rating I i To obtain the ideal grid-connected current signal i iabc ; Then, based on the effective value of the rated voltage V n With filter capacitor branch parameters C f and R cf Solve for the current signal of the ideal filter capacitor branch. I c ; Finally, the ideal grid-connected current signal i iabc With the current signal of the ideal filter capacitor branch I c The sum of these values ​​serves as the ideal bridge-side current signal. i iLabc .

4. The method for improving VSG grid-connected current quality based on bridge-side quasi-harmonic current feedforward according to claim 3, characterized in that, In S2, the common coupling point is used to process the desired voltage signal using the following logic representation. v iabc : in, v ia , v ib , v ic The voltage signals of phases A through C are processed at the common coupling point. V n This is the effective value of the rated voltage. The VSG outputs an electrical angle.

5. The method for improving VSG grid-connected current quality based on bridge-side quasi-harmonic current feedforward according to claim 3, characterized in that, The ideal grid-connected current signal is represented in S2 using the following logic. i iabc : in, i ia , i ib , i ic The grid-connected current signals of phases A through C are processed at the common coupling point. The VSG outputs an electrical angle.

6. The method for improving VSG grid-connected current quality based on bridge-side quasi-harmonic current feedforward according to claim 5, characterized in that, The rated value of the grid-connected current is solved using the following logic in S2. I i : in, P e Instantaneous active power V n This is the effective value of the rated voltage.

7. The method for improving VSG grid-connected current quality based on bridge-side quasi-harmonic current feedforward according to claim 3, characterized in that, The ideal filter capacitor branch current signal is represented in S2 using the following logic. I c : in, R cf For damping resistor, C f For filtering capacitors, V n is the effective value of the rated voltage, and s is the Laplace operator.

8. The method for improving VSG grid-connected current quality based on bridge-side quasi-harmonic current feedforward according to claim 1, characterized in that, The ideal bridge-side current signal is represented in S2 using the following logic. i iLabc : in, i iLa , i iLb , i iLc These are the ideal bridge-side current signals of phases A through C at the common coupling point.

9. The method for improving VSG grid-connected current quality based on bridge-side quasi-harmonic current feedforward according to claim 1, characterized in that, The transfer function of the high-pass filter described in S4 Using the following logical representation: Where s is the Laplace operator, k is the filter gain, and k i This is the cutoff angular frequency.

10. A grid-connected inverter, characterized in that, The grid connection is carried out based on the VSG grid-connected current quality improvement method based on bridge-side quasi-harmonic current feedforward as described in any one of claims 1-9.

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