Oscillation suppression strategy for grid-connected systems based on phase-locked loop and current loop structure optimization

Abstract

The present invention discloses a grid-connected system oscillation suppression strategy based on phase-locked loop and current loop structure optimization, comprising the following steps: S1: optimizing the structure of a conventional phase-locked loop, and designing an improved phase-locked loop structure capable of eliminating the electrical angle disturbance Δθ; S2: introducing a feedback proportional link D and a decoupling proportional integral link H into the AC current loop. c (s), design an improved AC current loop structure that can improve the negative damping effect; S3: Based on the control structure after introducing the proposed oscillation suppression strategy, a multi-harmonic linearized output impedance model of the grid-connected converter is established; the present invention significantly reduces the grid-connected current distortion rate when there is harmonic disturbance in the grid voltage by eliminating the influence of voltage harmonic disturbance on the electrical angle and improving the negative damping effect, thereby achieving effective suppression of frequency-coupled oscillation.

Description

Technical Field

[0001] The present invention relates to the field of novel power system grid-connected stability, and in particular to a grid-connected system oscillation suppression strategy based on phase-locked loop and current loop structure optimization. Background Art

[0002] As a bridge between distributed power sources and the power grid, grid-connected converters have been widely used in new power systems with the rapid development of new energy distributed power sources, and have an important impact on the stability of the system. However, due to the differences in the control links of the grid-connected converter, the phase-locked loop is only arranged and controlled on the q-axis, the DC voltage loop is only arranged and controlled on the d-axis, and the AC current loop is arranged and controlled on the dq-axis, the control of the grid-connected converter has a variety of asymmetric cross-coupling situations on the dq-axis, which leads to the frequency coupling phenomenon of the grid-connected current showing a "single input and dual output" characteristic when there is a disturbance in the grid-connected point voltage. This frequency coupling phenomenon will introduce a negative damping effect, making it possible for the system to oscillate at the disturbance frequency and the coupling frequency, threatening the stability of the grid-connected system, thereby hindering the grid's ability to absorb new energy. In order to suppress frequency coupling oscillations, the present invention proposes a grid-connected system oscillation suppression strategy based on phase-locked loop and current loop structure optimization. Summary of the Invention

[0003] The present invention proposes a grid-connected system oscillation suppression strategy based on phase-locked loop and current loop structure optimization, which includes: S1: optimizing the structure of the traditional phase-locked loop and designing an improved phase-locked loop structure that can eliminate the electrical angle disturbance Δθ; S2: introducing a feedback proportional link D and a decoupling proportional integral link H into the AC current loop. c (s), design an improved AC current loop structure that can improve the negative damping effect; S3: model the improved AC current loop and establish its multi-harmonic linearization model.

[0004] To achieve the above objectives, the technical solutions provided by the present invention are:

[0005] In step S1, the specific steps of optimizing the structure of the conventional phase-locked loop and designing an improved phase-locked loop structure capable of eliminating the electrical angle disturbance Δθ are as follows:

[0006] S1: Improved phase-locked loop structure to eliminate electrical angle disturbance Δθ:

[0007] Since the phase-locked loop needs to control the grid voltage v abc The q-axis component V q To obtain the electrical angle θ, so when the grid voltage v abc When there is harmonic disturbance, the electrical angle disturbance Δθ will be derived from the electrical angle θ output by the phase-locked loop, and the disturbance will be transmitted to the entire system control through Park transformation; in order to eliminate the grid voltage v from the root abcThe influence of harmonic disturbance on the output electrical angle θ of the phase-locked loop is studied. An improved phase-locked loop structure is proposed to eliminate the electrical angle disturbance Δθ: the grid voltage v abc After Park transformation, the q-axis component V q After the phase-locked loop PI controller G p (s) The electrical angular velocity ω of the uneliminated disturbance is obtained PLL , and then the electrical angular velocity ω of the uneliminated disturbance PLL Subtract the fundamental frequency angular velocity ω1 to obtain the electrical angular velocity disturbance Δω at this time, and then subtract the electrical angular velocity ω that has not been eliminated from the disturbance PLL Subtracting the electrical angular velocity disturbance Δω can ensure that the output electrical angular velocity ω value is constant to the fundamental frequency angular velocity ω1, eliminating the grid connection point voltage v abc The influence of disturbance on the electrical angular velocity ω; then the electrical angle θ can be obtained by passing the electrical angular velocity ω through the integration link 1 / s;

[0008] By adopting the improved phase-locked loop structure, the harmonic disturbance of the grid-connected point voltage will no longer cause the generation of the electrical angle disturbance Δθ. The frequency domain expression of the electrical angle θ output by the improved phase-locked loop only contains the fundamental frequency θ1[±f1]. The grid-connected point voltage v abc , grid-connected current i abc The Parker transformation process is no longer affected by the electrical angle perturbation Δθ. The core of multi-harmonic linearization modeling is to establish the relationship between harmonic perturbations. Since the electrical angle θ no longer contains harmonic perturbations when the improved phase-locked loop structure is adopted, the final multi-harmonic linearization output impedance model of the grid-connected converter will not contain expressions related to the phase-locked loop, eliminating the influence of the asymmetric structure of the phase-locked loop on frequency coupling oscillation.

[0009] Furthermore, in step S2, a feedback proportional link D and a decoupling proportional integral link H are introduced into the AC current loop. c (s), the specific steps for designing an improved AC current loop structure that can improve the negative damping effect are as follows:

[0010] S2: Introducing the feedback proportional link D and the decoupling proportional integral link H c (s) Improved AC current loop structure:

[0011] In order to improve the negative damping effect caused by frequency coupling and reduce the possibility of system oscillation caused by frequency coupling, two control links are introduced into the AC current loop: feedback proportional link D and decoupling proportional integral link H. c (s); Taking the d-axis as an example, the improved AC current loop control structure is: the d-axis component of the grid-connected current I d After the feedback coefficient (1+D), the grid current d-axis component reference value I dr After the comparison step, the d-axis component difference ΔI is obtained d, and then the d-axis component difference ΔI d Through the current loop d-axis PI link G d (s) Get the d-axis control voltage V td , and then the d-axis control voltage V td The coupling term Q with the q-axis current d I q Subtract (Q d is the decoupling coefficient, I q is the q-axis component of the grid-connected current) to obtain the driving voltage V sd , driving voltage V sd Then, after the introduction of the decoupling proportional integral link H c (s) can get the d-axis modulation signal E d ;

[0012] Furthermore, in step S3, based on the control structure after introducing the proposed oscillation suppression strategy, the specific steps of establishing the multi-harmonic linearized output impedance model of the grid-connected converter are as follows:

[0013] Modeling idea: The core goal of establishing a multi-harmonic linearized output impedance model for a grid-connected converter is to establish the relationship between the positive and negative sequence harmonic disturbances of the grid-connected point voltage and the positive and negative sequence harmonic disturbances of the grid-connected current.

[0014] S3-1: Taking phase a as an example, modular modeling is performed on the DC voltage loop and AC current loop respectively, and the DC bus voltage V is established. dc Harmonic disturbance, modulation signal E a Harmonic disturbance and grid connection point voltage v a , grid-connected current i a The relational expression of positive and negative sequence harmonic disturbance quantities;

[0015] S3-2: The output voltage v of the converter port a phase can be obtained from the modulation loop transfer function. ia and DC bus voltage V dc , modulation signal E a The relationship between the three is that the DC bus voltage V obtained in step S3-1 dc Harmonic disturbance, modulation signal E a Harmonic disturbance quantity with respect to grid connection point voltage v a , grid-connected current i a Substitute the expression of positive and negative sequence harmonic disturbance into the output voltage v of phase a at the converter port ia , DC bus voltage V dc , modulation signal E a The frequency domain relationship between the three can be obtained as follows: ia The harmonic disturbance amount and grid connection point voltage v a , grid-connected current i aThe relational expression of positive and negative sequence harmonic disturbance quantities;

[0016] S3-3: According to the main circuit topology, the output voltage v of the converter port a phase can be obtained ia , grid-connected point a phase voltage v a , grid-connected a phase current i a The relationship between the three is: the output voltage v of the converter port a phase obtained in step S3-2 is ia The harmonic disturbance quantity is about the grid connection point voltage v a , grid-connected current i a Substitute the expression of positive and negative sequence harmonic disturbance into the output voltage v of phase a at the converter port ia , grid-connected point a phase voltage v a , grid-connected a-phase current i a In the frequency domain relationship between the three, the output voltage v of the converter port a phase can be eliminated. ia , get the grid-connected point a phase voltage v a The positive and negative sequence harmonic disturbances and the grid-connected a phase current i a The relationship between the positive and negative sequence harmonic disturbances is established to establish a multi-harmonic linear output impedance model of the grid-connected converter [Z con ] 2×2 :

[0017]

[0018] In formula (1), Z v11 、Z v12 、Z v21 、Z v22 Represent the voltage impedance coefficient matrix of subsystem 1, subsystem 2, subsystem 3, and subsystem 4 respectively; Z v11 , Z v12 、Z v21 、Z v22 The expression is:

[0019]

[0020] In formula (2), I L is the DC output current, s is the Laplace operator, C dc is the DC bus capacitance, V dc0 is the DC bus voltage given value, K m is the modulation coefficient, E1 represents the fundamental frequency component of the modulation signal, E1 * is the conjugate value of E1, I1 represents the fundamental frequency phasor of the grid-connected current, I1 * is the conjugate value of I1, G dc (s) is the PI link of the DC voltage loop;

[0021] In formula (1), Z i11、Z i12 、Z i21 、Z i22 Represent subsystem 1, subsystem 2, subsystem 3, and subsystem 4 of the current impedance coefficient matrix respectively; Z i11 、Z i12 、Z i21 、Z i22 The expression is:

[0022]

[0023] In formula (3), G q (s) is the PI link of the current loop q axis, j is an imaginary number, L f Represents the filter inductor, and V1 represents the fundamental frequency phasor of the grid-connected point voltage.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] The present invention proposes a grid-connected system oscillation suppression strategy based on phase-locked loop and current loop structure optimization. The improved phase-locked loop structure can make the output electrical angular velocity ω always equal to the fundamental frequency electrical angular velocity ω1, thereby avoiding the influence of the electrical angle disturbance Δθ caused by the grid-connected point voltage harmonic disturbance; the feedback proportional link D and the decoupling proportional integral link H are introduced. c The improved current loop (s) can improve the negative damping effect and enhance the robustness of the grid-connected system. The improved phase-locked loop and AC current loop work together to significantly reduce the grid-connected current distortion rate when harmonic disturbances are present in the grid voltage, effectively suppressing system oscillations. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 1 is a flowchart of the work flow for the proposed oscillation suppression strategy in an embodiment of the present invention;

[0027] Figure 2 The topology and control block diagram of the grid-connected system in an embodiment of the present invention;

[0028] Figure 3 Improve the control structure of the phase-locked loop in the embodiment of the present invention;

[0029] Figure 4 The control structure of the DC voltage loop and the improved current loop in the embodiment of the present invention;

[0030] Figure 5 This is the FFT analysis result of the grid-connected a-phase current under traditional control in an embodiment of the present invention;

[0031] Figure 6 This is the FFT analysis result of the grid-connected a-phase current under the proposed oscillation suppression strategy in an embodiment of the present invention. DETAILED DESCRIPTION

[0032] The present invention will be further described below in conjunction with specific embodiments:

[0033] Figure 1 The following is a workflow diagram for the proposed oscillation suppression strategy. Figure 2 The figure shows the topology and control block diagram of the grid-connected system. Figure 3 It is to improve the control structure of the phase-locked loop, Figure 4 The control structure of the DC voltage loop and the improved current loop is described. The grid-connected system oscillation suppression strategy based on the optimization of the phase-locked loop and the current loop structure described in this embodiment includes the following steps:

[0034] S1: Optimize the structure of the traditional phase-locked loop and design an improved phase-locked loop structure that can eliminate the electrical angle disturbance Δθ. The specific process is as follows:

[0035] Since the phase-locked loop needs to control the grid voltage v abc The q-axis component V q To obtain the electrical angle θ, so when the grid voltage v abc When there is harmonic disturbance, the electrical angle disturbance Δθ will be derived from the electrical angle θ output by the phase-locked loop, and the disturbance will be transmitted to the entire system control through Park transformation; in order to eliminate the grid voltage v from the root abc The influence of harmonic disturbance on the output electrical angle θ of the phase-locked loop is studied. An improved phase-locked loop structure is proposed to eliminate the electrical angle disturbance Δθ: the grid voltage v abc After Park transformation, the q-axis component V q After the phase-locked loop PI controller G p (s) The electrical angular velocity ω of the uneliminated disturbance is obtained PLL , and then the electrical angular velocity ω of the uneliminated disturbance PLL Subtract the fundamental frequency angular velocity ω1 to obtain the electrical angular velocity disturbance Δω at this time, and then subtract the electrical angular velocity ω that has not been eliminated from the disturbance PLL Subtracting the electrical angular velocity disturbance Δω can ensure that the output electrical angular velocity ω value is constant to the fundamental frequency angular velocity ω1, eliminating the grid connection point voltage v abc The influence of disturbance on the electrical angular velocity ω; then the electrical angle θ can be obtained by passing the electrical angular velocity ω through the integration link 1 / s;

[0036] By adopting the improved phase-locked loop structure, the harmonic disturbance of the grid-connected point voltage will no longer cause the generation of the electrical angle disturbance Δθ. The frequency domain expression of the electrical angle θ output by the improved phase-locked loop only contains the fundamental frequency θ1[±f1]. The grid-connected point voltage v abc , grid-connected current i abcThe Parker transformation process is no longer affected by the electrical angle perturbation Δθ. The core of multi-harmonic linearization modeling is to establish the relationship between harmonic perturbations. Since the electrical angle θ no longer contains harmonic perturbations when the improved phase-locked loop structure is adopted, the final multi-harmonic linearization output impedance model of the grid-connected converter will not contain expressions related to the phase-locked loop, eliminating the influence of the asymmetric structure of the phase-locked loop on frequency coupling oscillation.

[0037] S2: Introduce the feedback proportional link D and the decoupling proportional integral link H into the AC current loop c (s), design an improved AC current loop structure that can improve the negative damping effect; the specific process is as follows:

[0038] In order to improve the negative damping effect caused by frequency coupling and reduce the possibility of system oscillation caused by frequency coupling, two control links are introduced into the AC current loop: feedback proportional link D and decoupling proportional integral link H. c (s); Taking the d-axis as an example, the improved AC current loop control structure is: the d-axis component of the grid-connected current I d After the feedback coefficient (1+D), the grid current d-axis component reference value I dr After the comparison step, the d-axis component difference ΔI is obtained d , and then the d-axis component difference ΔI d Through the current loop d-axis PI link G d (s) Get the d-axis control voltage V td , and then the d-axis control voltage V td The coupling term Q with the q-axis current d I q Subtract (Q d is the decoupling coefficient, I q is the q-axis component of the grid-connected current) to obtain the driving voltage V sd , driving voltage V sd Then, after the introduction of the decoupling proportional integral link H c (s) can get the d-axis modulation signal E d ;

[0039] S3: Based on the control structure after introducing the proposed oscillation suppression strategy, a multi-harmonic linearized output impedance model of the grid-connected converter is established. The specific process is as follows:

[0040] Modeling idea: The core goal of establishing a multi-harmonic linearized output impedance model for a grid-connected converter is to establish the relationship between the positive and negative sequence harmonic disturbances of the grid-connected point voltage and the positive and negative sequence harmonic disturbances of the grid-connected current.

[0041] S3-1: Taking phase a as an example, modular modeling is performed on the DC voltage loop and AC current loop respectively, and the DC bus voltage V is established. dc Harmonic disturbance, modulation signal Ea Harmonic disturbance and grid connection point voltage v a , grid-connected current i a The relational expression of positive and negative sequence harmonic disturbance quantities;

[0042] S3-2: The output voltage v of the converter port a phase can be obtained from the modulation loop transfer function. ia and DC bus voltage V dc , modulation signal E a The relationship between the three is that the DC bus voltage V obtained in step S3-1 dc Harmonic disturbance, modulation signal E a Harmonic disturbance quantity with respect to grid connection point voltage v a , grid-connected current i a Substitute the expression of positive and negative sequence harmonic disturbance into the output voltage v of phase a at the converter port ia , DC bus voltage V dc , modulation signal E a The frequency domain relationship between the three can be obtained as follows: ia The harmonic disturbance amount and grid connection point voltage v a , grid-connected current i a The relational expression of positive and negative sequence harmonic disturbance quantities;

[0043] S3-3: According to the main circuit topology, the output voltage v of the converter port a phase can be obtained ia , grid-connected point a phase voltage v a , grid-connected a-phase current i a The relationship between the three is: the output voltage v of the converter port a phase obtained in step S3-2 is ia The harmonic disturbance quantity is about the grid connection point voltage v a , grid-connected current i a Substitute the expression of positive and negative sequence harmonic disturbance into the output voltage v of phase a at the converter port ia , grid-connected point a phase voltage v a , grid-connected a-phase current i a In the frequency domain relationship between the three, the output voltage v of the converter port a phase can be eliminated. ia , get the grid-connected point a phase voltage v a The positive and negative sequence harmonic disturbances and the grid-connected a phase current i a The relationship between the positive and negative sequence harmonic disturbances is established to establish a multi-harmonic linear output impedance model of the grid-connected converter [Z con ] 2×2 :

[0044]

[0045] In formula (4), Z v11 、Zv12 、Z v21 、Z v22 Represent the voltage impedance coefficient matrix of subsystem 1, subsystem 2, subsystem 3, and subsystem 4 respectively; Z v11 、Z v12 、Z v21 、Z v22 The expression is:

[0046]

[0047] In formula (5), I L is the DC output current, s is the Laplace operator, C dc is the DC bus capacitance, V dc0 is the DC bus voltage given value, K m is the modulation coefficient, E1 represents the fundamental frequency component of the modulation signal, E1 * is the conjugate value of E1, I1 represents the fundamental frequency phasor of the grid-connected current, I1 * is the conjugate value of I1, G dc (s) is the PI link of the DC voltage loop;

[0048] In formula (4), Z i11 、Z i12 、Z i21 、Z i22 Represent subsystem 1, subsystem 2, subsystem 3, and subsystem 4 of the current impedance coefficient matrix respectively; Z i11 、Z i12 、Z i21 、Z i22 The expression is:

[0049]

[0050] In formula (6), G q (s) is the PI link of the current loop q axis, j is an imaginary number, L f Represents the filter inductor, and V1 represents the fundamental frequency phasor of the grid-connected point voltage.

[0051] In order to verify the effectiveness of the proposed grid-connected converter oscillation suppression strategy based on phase-locked loop and current loop structure optimization, a simulation model was built in Matlab / Simulink software. Figure 2 The grid-connected system simulation model shown in the figure has the following main circuit parameters: system power P = 5kW, DC bus capacitance value C dc =15mF, filter inductor L f =0.42mH, grid voltage v g =690V, base frequency f1 = 50Hz, base frequency angular velocity ω1 = 2πf1; control loop parameters are: phase-locked loop PI controller G p(s)=0.0158+0.99 / s, DC bus voltage given value V dc0 =1150V, DC voltage loop PI link G dc (s) = 2.23 + 280.29 / s, new feedback proportional link D = 4, current loop d-axis PI link G d (s)=0.48+603.36 / s, current loop q-axis PI link G q (s) = 0.32 + 603.36 / s, decoupling coefficient K d =0.1319, newly added decoupling proportional integral link H c (s) = 10 + 200 / s, PWM modulation carrier frequency f s =10kHz.

[0052] When a 27.5Hz harmonic voltage with 20% fundamental frequency content is injected into the grid-connected point voltage, the simulation waveforms under different control are as follows: Figure 5 and Figure 6 As shown. Under the traditional control, the FFT analysis results of the grid-connected a phase current are as follows Figure 5 As shown. Figure 5 It can be seen that when there is a 27.5Hz harmonic voltage in the grid-connected point voltage, there is a 27.5Hz disturbance harmonic current with a content of 5.62% and a 72.5Hz coupling harmonic current with a content of 7.03% in the grid-connected current, which proves the existence of frequency coupling phenomenon. At this time, the total distortion rate of the grid-connected current is 7.81%, which does not meet the grid-connected current criterion of a distortion rate of less than 5%. When the proposed grid-connected system oscillation suppression strategy based on the phase-locked loop and current loop structure optimization is applied, the FFT analysis results of the grid-connected a phase current are as follows: Figure 6 As shown. Figure 6 It can be seen that when the proposed oscillation suppression strategy is applied, the 27.5Hz perturbation harmonic current content drops to 1.43%, a 74.55% decrease compared to traditional control; the 72.5Hz coupled harmonic current content drops to 1.40%, a 80.08% decrease compared to traditional control. Both the perturbation harmonic current and the coupled harmonic current decrease significantly, and the total grid-connected current distortion ratio is now 2.34%, meeting the 5% grid-connected standard requirement. This verifies the proposed grid-connected system oscillation suppression strategy based on phase-locked loop and current loop structure optimization effectively suppresses frequency-coupled oscillations.

[0053] The embodiments described above are only preferred embodiments of the present invention and are not intended to limit the scope of implementation of the present invention. Therefore, any changes made based on the shape and principle of the present invention should be included in the scope of protection of the present invention.

Claims

1. A grid-connected system oscillation suppression strategy based on phase-locked loop and current loop structure optimization, characterized in that: The following steps are involved: S1: Optimize the structure of the traditional phase-locked loop and design an improved phase-locked loop structure that can eliminate the electrical angle disturbance Δθ; S2: Introduce feedback proportional link D and decoupling proportional integral link H into the AC current loop c (s), designing an improved AC current loop structure that can improve the negative damping effect; S3: Based on the control structure after introducing the proposed oscillation suppression strategy, a multi-harmonic linearized output impedance model of the grid-connected converter is established; The specific steps of the improved phase-locked loop structure for eliminating the electrical angle disturbance Δθ in step S1 are as follows: Grid connection voltage v abc After Parker transformation, the q-axis component V q Through the phase-locked loop PI controller G p (s) The electrical angular velocity ω of the uneliminated disturbance is obtained PLL , and then the electrical angular velocity ω of the uneliminated disturbance PLL Subtract the fundamental frequency angular velocity ω1 to obtain the electrical angular velocity disturbance Δω at this time, and then subtract the electrical angular velocity ω that has not eliminated the disturbance PLL Subtracting the electrical angular velocity disturbance Δω can ensure that the output electrical angular velocity ω value is constant at the fundamental frequency angular velocity ω1, eliminating the grid connection point voltage v abc The influence of disturbance on the electrical angular velocity ω; then the electrical angle θ can be obtained by passing the electrical angular velocity ω through the integration link 1 / s; In step S2, a feedback proportional link D and a decoupling proportional integral link H are introduced. c The specific steps of improving the AC current loop structure of (s) are as follows: In order to improve the negative damping effect caused by frequency coupling and reduce the possibility of system oscillation caused by frequency coupling, two control links are introduced into the AC current loop: feedback proportional link D and decoupling proportional integral link H. c (s); The improved AC current loop control structure of the d-axis is: the d-axis component of the grid-connected current I d After the feedback coefficient (1+D), it is equal to the grid current d-axis component reference value I dr After the comparison step, the d-axis component difference ΔI is obtained d , and then the d-axis component difference ΔI d Through the current loop d-axis PI link G d (s) Get the d-axis control voltage V td , and then the d-axis control voltage V td The coupling term Q with the q-axis current d I q Subtract the driving voltage V sd , where Q d is the decoupling coefficient, I q is the q-axis component of the grid-connected current, and the driving voltage V sd After the introduction of the decoupled proportional integral link H c (s) can get the d-axis modulation signal E d ; In step S3, based on the control structure after the oscillation suppression strategy is introduced, the specific steps of establishing the multi-harmonic linearized output impedance model of the grid-connected converter are as follows: Modeling idea: The core goal of establishing the multi-harmonic linearized output impedance model of the grid-connected converter is to establish the relationship between the positive and negative sequence harmonic disturbances of the grid-connected point voltage and the positive and negative sequence harmonic disturbances of the grid-connected current; S3-1: Modularize the DC voltage loop and AC current loop, respectively, and establish the DC bus voltage V dc Harmonic disturbance, modulation signal E a Harmonic disturbance and grid connection point voltage v a , grid-connected current i a The relational expression of positive and negative sequence harmonic disturbance quantities; S3-2: The output voltage v of the converter port a phase can be obtained from the modulation loop transfer function ia With DC bus voltage V dc , modulation signal E a The relationship between the three is: the DC bus voltage V obtained in step S3-1 dc Harmonic disturbance, modulation signal E a Harmonic disturbance quantity with respect to grid connection point voltage v a , grid-connected current i a Substitute the expression of positive and negative sequence harmonic disturbance into the output voltage v of phase a at the converter port ia , DC bus voltage V dc , modulation signal E a The frequency domain relationship between the three can be obtained as follows: ia The harmonic disturbance and the grid connection point voltage v a , grid-connected current i a The relational expression of positive and negative sequence harmonic disturbance quantities; S3-3: According to the main circuit topology, the output voltage v of the converter port a phase can be obtained ia 、Grid connection point a phase voltage v a , grid-connected a phase current i a The relationship between the three is: the output voltage v of the converter port a phase obtained in step S3-2 is ia The harmonic disturbance quantity is about the grid connection point voltage v a , grid-connected current i a Substitute the expression of positive and negative sequence harmonic disturbance into the output voltage v of phase a at the converter port ia 、Grid connection point a phase voltage v a , grid-connected a phase current i a In the frequency domain relationship between the three, the output voltage v of phase a at the converter port can be eliminated. ia , get the grid-connected point a phase voltage v a The positive and negative sequence harmonic disturbances and the grid-connected a phase current i a The relationship between the positive and negative sequence harmonic disturbances is established to establish a multi-harmonic linear output impedance model of the grid-connected converter [Z con ] 2×2 : In formula (1), Z v11 , Z v12 , Z v21 , Z v22 Represent the subsystem 1, subsystem 2, subsystem 3, and subsystem 4 of the voltage impedance coefficient matrix respectively; Z v11 , Z v12 , Z v21 , Z v22 The expression is: In formula (2), I L is the DC output current, s is the Laplace operator, C dc is the DC bus capacitance, V dc0 is the given value of DC bus voltage, K m is the modulation coefficient, E1 represents the fundamental frequency component of the modulation signal, E1 * is the conjugate value of E1, I1 represents the fundamental frequency phasor of the grid-connected current, I1 * is the conjugate value of I1, G dc (s) is the PI link of the DC voltage loop; In formula (1), Z i11 , Z i12 , Z i21 , Z i22 Respectively represent subsystem 1, subsystem 2, subsystem 3, and subsystem 4 of the current impedance coefficient matrix; Z i11 , Z i12 , Z i21 , Z i22 The expression is: In formula (3), G q (s) is the PI link of the current loop q axis, j is an imaginary number, L f represents the filter inductor, and V1 represents the fundamental frequency phasor of the grid-connected point voltage.

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