Frequency-coupled grid-connected inverter stability enhancement method

Abstract

The application discloses a frequency coupling grid-connected inverter stability improvement method, which introduces the q-axis voltage and the q-axis current after the suppression link into the q-axis control in the form of positive feedback as the reference value of the q-axis current through the q-axis voltage loop compensation mode. The strategy can ingeniously utilize the sign difference of the frequency domain convolution itself in the positive sequence and the negative sequence, realize the increase of the amplitude of the main diagonal line subsystem in the admittance matrix of the grid-connected inverter, and greatly reduce the amplitude of the coupling subsystem, so that the coupling subsystem is negligible compared with the main diagonal line subsystem, and the frequency coupling oscillation is essentially suppressed and the stability of the system is improved.

Description

Technical Field

[0001] This invention relates to the field of grid-connected stability of novel power systems, and in particular to a method for improving the stability of frequency-coupled grid-connected inverters. Background Technology

[0002] In the process of building new power systems, the proportion of renewable energy continues to rise. Grid-connected inverters, as a key link for distributed power sources such as photovoltaic, wind power, and energy storage systems to connect to the grid, are increasingly favored. However, due to the interaction between grid-connected inverters and the grid, the system will oscillate in a wide frequency range from a few hertz to thousands of hertz, seriously threatening the stability of the system. Among them, the frequency coupling oscillation problem occurring near the fundamental frequency sideband has received increasing attention in recent years. Due to the influence of frequency coupling effect, the coupled subsystems in the grid-connected inverter output admittance model cannot be ignored, which complicates the system mechanism analysis and stability analysis. If the frequency coupling effect is ignored, the actual stable state of the power system cannot be accurately determined, and there is a risk of misjudgment. The frequency coupling effect can also induce subsynchronous oscillations, threatening system stability, and in severe cases, even causing generator disconnection. Suppressing frequency coupling oscillations is urgent. However, most existing methods for addressing the frequency coupling problem are proposed for phase-locked loops, and most of them are proposed from the perspective of system damping and stability, with the frequency coupling effect only reflected in the modeling process. Therefore, there is a lack of a suppression method specifically designed for DC voltage loops that addresses the essence of frequency coupling suppression. Summary of the Invention

[0003] To address the aforementioned technical problems, this invention provides a method for improving the stability of frequency-coupled grid-connected inverters.

[0004] To achieve the above objectives, the present invention is implemented according to the following technical solution:

[0005] A method for improving the stability of frequency-coupled grid-connected inverters includes the following steps:

[0006] 1) The DC voltage v is obtained by detecting the voltage across the DC bus capacitor. dc At the common coupling point, the three-phase voltages v of phases abc at the grid connection point are... abc and three-phase current i abc Sampling is performed, and then the three-phase voltages V of phases a, b, and c are... abc and three-phase current i abc The d-axis voltage v is obtained by performing Parker transformations respectively. d q-axis voltage v q d-axis current i d and q-axis current i q ;

[0007] 2) On the q-axis, the q-axis current i qMultiplying the power by the filter voltage coefficient X yields the power P of the suppressed filter voltage component. f Suppress coupling components related to the filter voltage; q-axis current i q Multiplying the fundamental frequency voltage coefficient Q yields the fundamental frequency voltage component suppression power P. V1 Suppress coupling components related to the fundamental frequency voltage; q-axis voltage v q Multiplying the fundamental frequency current coefficient Z by the fundamental frequency current component suppression power P yields the fundamental frequency current component suppression power P. I1 To suppress coupling components related to the fundamental frequency current; the expressions for the filter voltage coefficient X, fundamental frequency voltage coefficient Q, and fundamental frequency current coefficient Z are:

[0008]

[0009] In equation (1), s is the Laplace operator; L f V1 is the value of the filter inductance; V1 and I1 are the amplitudes of the fundamental frequency components of the grid-connected voltage and grid-connected current, respectively; φ i1 The initial phase angle of the grid-connected current;

[0010] 3) Reduce the power P of the filtered voltage component suppression f Fundamental frequency voltage component suppression power P V1 , fundamental frequency current component suppression power P I1 The sum of these three factors yields the total suppression power P. A Total suppression power P A After passing through DC voltage loop PI controller G dc (s) Obtain the q-axis control current I tq , control the q-axis current I tq Multiplying this by the DC voltage loop coefficient D yields the q-axis current reference value I. qr Its expression is:

[0011]

[0012] In equation (2), the expression for the DC voltage loop coefficient D is:

[0013]

[0014] In equation (3), I pv C is the DC side current. dc For DC bus capacitor; V dc0 The given value for DC voltage;

[0015] 4) Set the q-axis current reference value I qr With q-axis current i q The difference ΔI in the q-axis component is obtained through the comparison process. q Then the q-axis component difference ΔI q After passing through the current loop q-axis PI controller Hqi (s) Obtain the q-axis control voltage V tq Then adjust the q-axis control voltage V. tq K, the coupling term with the d-axis current d i d The summation yields the q-axis modulated signal m. q K d These are the decoupling coefficients;

[0016] 5) On the d-axis, the DC voltage v dc With DC voltage setpoint V dc0 The DC voltage difference ΔV is obtained through the comparison process. dc DC voltage difference ΔV dc After passing through DC voltage loop PI controller G dc (s) Obtain the d-axis current reference value I dr ; Set the d-axis current reference value I dr With d-axis current i d The difference ΔI between the d-axis components was obtained through the comparison process. d Then the difference ΔI between the d-axis components d After passing through the d-axis PI controller H of the current loop di (s) Obtain the d-axis control voltage V td Then adjust the d-axis control voltage V. td Subtract the q-axis current coupling term K d i q Obtain the d-axis modulated signal m d ;

[0017] 6) Modulate the d-axis signal m d and q-axis modulation signal m q After the Parker inverse transform, the phase a modulation signal m in the stationary coordinate system is obtained. a b-phase modulated signal m b c-phase modulation signal m c .

[0018] Furthermore, based on the frequency coupling-based method for improving the stability of grid-connected inverters, a multi-harmonic linearization model for the grid-connected inverter is established, including the following steps:

[0019] 1) Considering only the DC voltage disturbance and the disturbance derived through the control loop, establish the positive and negative sequence disturbances V of the grid-connected inverter port output voltage. iap V ian Regarding the positive and negative sequence harmonic disturbances V of the grid-connected voltage p V n Positive and negative sequence harmonic disturbances I of grid-connected current p I n The expression:

[0020]

[0021] In equation (4), [Y v ] 2×2 The voltage coefficient matrix before the positive and negative sequence harmonic disturbances of the grid-connected voltage, [Y i ] 2×2 The matrix represents the current coefficients before the positive and negative sequence harmonic disturbances of the grid-connected current. Both are matrices related to the DC voltage loop in the grid-connected inverter model.

[0022] In the voltage coefficient matrix [Y v ] 2×2 Middle, Y vpp It reflects V p For V iap The expression for the effect; Y vpn It reflects V n For V iap The expression for the effect; Y vnp It reflects V p For V ian The expression for the effect; Y vnn It reflects V n For V ian The expression for the effect is as follows:

[0023]

[0024] In equation (5), k m M1 is the modulation coefficient; M2 is the fundamental frequency component of the modulated signal. * M1 is the conjugate value; I1 is the fundamental frequency component of the grid-connected current. * Let I1 be the conjugate value; j be the imaginary number; K(s) be the controller element, whose expression is:

[0025] K(s)=k m V dc0 H di (s)G dc (s) (6)

[0026] In the current coefficient matrix [Y i ] 2×2 Middle, Y ipp It reflects I p For V iap The expression for the effect; Y ipn It reflects I n For V iap The expression for the effect; Y inp It reflects I p For V ian The expression for the effect; Y inn It reflects I n For V ianThe expression for the effect; the specific expression is:

[0027]

[0028] In equation (7), V1 is the fundamental frequency component of the grid-connected voltage;

[0029] 2) Based on equation (4), and combined with the phase-locked loop disturbance correlation matrix, the current loop disturbance correlation matrix and the main circuit relationship, a multi-harmonic linearization model of the grid-connected inverter after applying the frequency coupling grid-connected inverter stability improvement method is established.

[0030] Compared with existing technologies, the principles and advantages of this solution are as follows:

[0031] The frequency-coupled grid-connected inverter stability improvement method proposed in this invention utilizes DC voltage loop q-axis compensation. After passing through a suppression stage, the q-axis voltage and current are introduced into the q-axis control as a positive feedback, serving as a reference value for the q-axis current. This strategy cleverly leverages the sign difference between positive and negative sequences in frequency domain convolution to increase the amplitude of the main diagonal subsystem in the grid-connected inverter admittance matrix while significantly reducing the amplitude of its coupled subsystem. This makes the coupled subsystem negligible compared to the main diagonal subsystem, fundamentally suppressing frequency-coupled oscillations and improving system stability. Attached Figure Description

[0032] Figure 1 This is the main circuit topology diagram of the three-phase grid-connected inverter in an embodiment of the present invention;

[0033] Figure 2 This is the phase-locked loop control structure in the embodiment of the present invention;

[0034] Figure 3 This is the DC voltage loop and AC current loop control structure under the frequency-coupled grid-connected inverter stability improvement method in the embodiments of the present invention;

[0035] Figure 4 This is a Bode plot of the sequence admittance model of the grid-connected inverter in an embodiment of the present invention;

[0036] Figure 5 The three-phase current experimental waveforms under the conventional strategy in this embodiment of the invention are shown below.

[0037] Figure 6 This is the current FFT analysis result under the conventional strategy in this embodiment of the invention;

[0038] Figure 7 The three-phase current experimental waveforms under the proposed strategy in this embodiment of the invention are shown below.

[0039] Figure 8The results are the current FFT analysis results under the proposed strategy in the embodiments of the present invention. Detailed Implementation

[0040] The present invention will be further described below with reference to specific embodiments:

[0041] Figure 1 The diagram shown is the main circuit topology of a three-phase grid-connected inverter. Figure 2 The diagram shows the phase-locked loop (PLL) control structure. Figure 3 This is a DC voltage loop and AC current loop control structure under the frequency-coupled grid-connected inverter stability improvement method. The frequency-coupled grid-connected inverter stability improvement method described in this embodiment includes the following steps:

[0042] A method for improving the stability of frequency-coupled grid-connected inverters includes the following steps:

[0043] 1) The DC voltage v is obtained by detecting the voltage across the DC bus capacitor. dc At the common coupling point, the three-phase voltages v of phases abc at the grid connection point are... abc and three-phase current i abc Sampling is performed, and then the three-phase voltages V of phases a, b, and c are... abc and three-phase current i abc The d-axis voltage v is obtained by performing Parker transformations respectively. d q-axis voltage v q d-axis current i d and q-axis current i q The electrical angle θ required in the Parker transform is obtained through phase-locked loop (PLL) control. The PLL control structure is as follows: q-axis voltage v q First, it passes through the phase-locked loop PI controller H. PLL (s) The electric angular frequency ω is obtained, and the electric angle θ can be obtained by integrating the electric angular frequency ω through the 1 / s integral element.

[0044] 2) On the q-axis, the q-axis current i q Multiplying the power by the filter voltage coefficient X yields the power P of the suppressed filter voltage component. f Suppress coupling components related to the filter voltage; q-axis current i q Multiplying the fundamental frequency voltage coefficient Q yields the fundamental frequency voltage component suppression power P. V1 Suppress coupling components related to the fundamental frequency voltage; q-axis voltage v q Multiplying the fundamental frequency current coefficient Z by the fundamental frequency current component suppression power P yields the fundamental frequency current component suppression power P. I1 To suppress coupling components related to the fundamental frequency current; the expressions for the filter voltage coefficient X, fundamental frequency voltage coefficient Q, and fundamental frequency current coefficient Z are:

[0045]

[0046] In equation (8), s is the Laplace operator; L f V1 is the value of the filter inductance; V1 and I1 are the amplitudes of the fundamental frequency components of the grid-connected voltage and grid-connected current, respectively. The initial phase angle of the grid-connected current;

[0047] 3) Reduce the power P of the filtered voltage component suppression f Fundamental frequency voltage component suppression power P V1 , fundamental frequency current component suppression power P I1 The sum of these three factors yields the total suppression power P. A Total suppression power P A After passing through DC voltage loop PI controller G dc (s) Obtain the q-axis control current I tq , control the q-axis current I tq Multiplying this by the DC voltage loop coefficient D yields the q-axis current reference value I. qr Its expression is:

[0048]

[0049] In equation (9), the expression for the DC voltage loop coefficient D is:

[0050]

[0051] In equation (10), I pv C is the DC side current. dc For DC bus capacitor; V dc0 The given value for DC voltage;

[0052] 4) Set the q-axis current reference value I qr With q-axis current i q The difference ΔI in the q-axis component is obtained through the comparison process. q Then the q-axis component difference ΔI q After passing through the current loop q-axis PI controller H qi (s) Obtain the q-axis control voltage V tq Then adjust the q-axis control voltage V. tq K, the coupling term with the d-axis current d i d The summation yields the q-axis modulated signal m. q K d These are the decoupling coefficients;

[0053] 5) On the d-axis, the DC voltage v dc With DC voltage setpoint V dc0 The DC voltage difference ΔV is obtained through the comparison process. dc DC voltage difference ΔV dc After passing through DC voltage loop PI controller G dc(s) Obtain the d-axis current reference value I dr ; Set the d-axis current reference value I dr With d-axis current i d The difference ΔI between the d-axis components was obtained through the comparison process. d Then the difference ΔI between the d-axis components d After passing through the d-axis PI controller H of the current loop di (s) Obtain the d-axis control voltage V td Then adjust the d-axis control voltage V. td Subtract the q-axis current coupling term K d i q Obtain the d-axis modulated signal m d ;

[0054] 6) Modulate the d-axis signal m d and q-axis modulation signal m q After the Parker inverse transform, the phase a modulation signal m in the stationary coordinate system is obtained. a b-phase modulated signal m b c-phase modulation signal m c .

[0055] Furthermore, based on the frequency coupling-based method for improving the stability of grid-connected inverters, a multi-harmonic linearization model for the grid-connected inverter is established, including the following steps:

[0056] 1) Considering only the DC voltage disturbance and the disturbance derived through the control loop, establish the positive and negative sequence disturbances V of the grid-connected inverter port output voltage. iap V ian Regarding the positive and negative sequence harmonic disturbances V of the grid-connected voltage p V n Positive and negative sequence harmonic disturbances I of grid-connected current p I n The expression:

[0057]

[0058] In equation (11), [Y v ] 2×2 The voltage coefficient matrix before the positive and negative sequence harmonic disturbances of the grid-connected voltage, [Y i ] 2×2 The matrix represents the current coefficients before the positive and negative sequence harmonic disturbances of the grid-connected current. Both are matrices related to the DC voltage loop in the grid-connected inverter model.

[0059] In the voltage coefficient matrix [Y v ] 2×2 Middle, Y vpp It reflects V p For V iap The expression for the effect; Yvpn It reflects V n For V iap The expression for the effect; Y vnp It reflects V p For V ian The expression for the effect; Y vnn It reflects V n For V ian The expression for the effect is as follows:

[0060]

[0061] In equation (12), k m M1 is the modulation coefficient; M2 is the fundamental frequency component of the modulating signal. M1 is the conjugate value; I1 is the fundamental frequency component of the grid-connected current. Let I1 be the conjugate value; j be the imaginary number; K(s) be the controller element, whose expression is:

[0062] K(s)=k m V dc0 H di (s)G dc (s) (13)

[0063] In the current coefficient matrix [Y i ] 2×2 Middle, Y ipp It reflects I p For V iap The expression for the effect; Y ipn It reflects I n For V iap The expression for the effect; Y inp It reflects I p For V ian The expression for the effect; Y inn It reflects I n For V ian The expression for the effect; the specific expression is:

[0064]

[0065] In equation (14), V1 is the fundamental frequency component of the grid-connected voltage;

[0066] 2) Based on equation (11), and combined with the phase-locked loop disturbance correlation matrix, the current loop disturbance correlation matrix and the main circuit relationship, a multi-harmonic linearization model of the grid-connected inverter after applying the frequency coupling grid-connected inverter stability improvement method is established.

[0067] To verify the effectiveness of the proposed frequency-coupled grid-connected inverter stability improvement method, Bode plots were plotted using Matlab simulation software for theoretical analysis, and experimental verification was conducted on a hardware-in-the-loop experimental platform based on the RT-Lab real-time simulator. The main circuit parameters were: DC side current I... pv =75A, DC voltage setpoint V dc0 =700V, DC bus capacitor C dc =7mF, filter inductor L f =7mH, grid-connected voltage V1 = 311V; control loop parameters are: phase-locked loop PI controller H PLL (s)=0.0158+0.99 / s, DC voltage loop PI controller G dc (s)=2.23+280.29 / s, current loop d-axis PI controller H di (s)=0.48+603.36 / s, current loop q-axis PI controller H qi (s)=0.32+603.36 / s.

[0068] Bode plots of grid-connected inverter output admittance models using both traditional and proposed strategies are shown below. Figure 3 As shown. By Figure 3 It is known that under traditional control, the amplitude of the coupled subsystem is similar to that of the main diagonal subsystem in most frequency bands, especially in the low-frequency band where the impact on the system is greatest, indicating the existence of frequency coupling, which is not negligible. However, when the frequency coupling-based grid-connected inverter stability improvement method is applied, the amplitude of the coupled subsystem is significantly reduced across the entire frequency band, while the amplitude of the main diagonal subsystem remains unchanged or increases across the entire frequency band. At this point, the amplitude of the main diagonal subsystem is much larger than that of the coupled subsystem, which, from a theoretical analysis perspective, shows that the proposed control strategy essentially suppresses the frequency coupling effect.

[0069] In the experimental verification, the operating conditions were as follows: During the entire 0-12s period, 80Hz and 90Hz harmonic voltages with 20% fundamental frequency content were injected into the grid-connected voltage to simulate the background harmonics of the power grid in actual engineering. Under the traditional strategy, the grid was a strong grid from 0-6s, and the grid was switched from a strong grid to a weak grid at 6s. Under the proposed strategy, the grid was a strong grid from 0-8s, and the grid was switched from a strong grid to a weak grid at 8s. The experimental waveform diagram under the traditional strategy is shown below. Figure 4 and Figure 5 As shown, the experimental waveforms under the proposed strategy are as follows: Figure 6 and Figure 7As shown, under the traditional strategy, the 80Hz and 90Hz background harmonics not only generate current harmonics of the same frequency, but also induce coupled current harmonics at 20Hz and 10Hz, respectively, with coupled current harmonic contents of 3.73% and 3.25%, verifying the existence of frequency coupling. At 6 seconds, the grid transitions from a strong grid to a weak grid, and the system becomes unstable. After applying the proposed control strategy, during the 0-8s period under a strong grid, the proposed control method significantly reduces the coupled current harmonic contents of 10Hz and 20Hz to 0.03% and 0.02%, respectively. The frequency coupling effect is effectively suppressed. When the grid transitions from a strong grid to a weak grid at 8 seconds, the system remains stable using the proposed control strategy, and frequency coupling is still suppressed. The experimental results fully verify that the frequency-coupled grid inverter stability improvement method not only has the ability to suppress frequency coupling but also enhances stability.

[0070] The above-described embodiments are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Therefore, any changes made in accordance with the shape and principle of the present invention should be covered within the protection scope of the present invention.

Claims

1. A method for improving the stability of a frequency-coupled grid-connected inverter, characterized in that, Includes the following steps: 1) The DC voltage v is obtained by detecting the voltage across the DC bus capacitor. dc At the common coupling point, the three-phase voltages v of phases abc at the grid connection point are... abc and three-phase current i abc Sampling is performed, and then the three-phase voltages V of phases a, b, and c are... abc and three-phase current i abc The d-axis voltage v is obtained by performing Parker transformations respectively. d q-axis voltage v q d-axis current i d and q-axis current i q ; 2) On the q-axis, the q-axis current i q Multiplying the power by the filter voltage coefficient X yields the power P of the suppressed filter voltage component. f Suppress coupling components related to the filter voltage; q-axis current i q Multiplying the fundamental frequency voltage coefficient Q yields the fundamental frequency voltage component suppression power P. V1 Suppress coupling components related to the fundamental frequency voltage; q-axis voltage v q Multiplying the fundamental frequency current coefficient Z by the fundamental frequency current component suppression power P yields the fundamental frequency current component suppression power P. I1 To suppress coupling components related to the fundamental frequency current; the expressions for the filter voltage coefficient X, fundamental frequency voltage coefficient Q, and fundamental frequency current coefficient Z are: In equation (1), s is the Laplace operator; L f V1 is the value of the filter inductance; V1 and I1 are the amplitudes of the fundamental frequency components of the grid-connected voltage and grid-connected current, respectively. The initial phase angle of the grid-connected current; 3) Reduce the power P of the filtered voltage component suppression f Fundamental frequency voltage component suppression power P V1 , fundamental frequency current component suppression power P I1 The sum of these three factors yields the total suppression power P. A Total suppression power P A After passing through DC voltage loop PI controller G dc (s) Obtain the q-axis control current I tq , control the q-axis current I tq Multiplying this by the DC voltage loop coefficient D yields the q-axis current reference value I. qr Its expression is: In equation (2), the expression for the DC voltage loop coefficient D is: In equation (3), I pv C is the DC side current. dc For DC bus capacitor; V dc0 The given value for DC voltage; 4) Set the q-axis current reference value I qr With q-axis current i q The difference ΔI in the q-axis component is obtained through the comparison process. q Then the q-axis component difference ΔI q After passing through the current loop q-axis PI controller H qi (s) Obtain the q-axis control voltage V tq Then adjust the q-axis control voltage V. tq K, the coupling term with the d-axis current d i d The summation yields the q-axis modulated signal m. q K d These are the decoupling coefficients; 5) On the d-axis, the DC voltage v dc With DC voltage setpoint V dc0 The DC voltage difference ΔV is obtained through the comparison process. dc DC voltage difference ΔV dc After passing through DC voltage loop PI controller G dc (s) Obtain the d-axis current reference value I dr ; Set the d-axis current reference value I dr With d-axis current i d The difference ΔI between the d-axis components was obtained through the comparison process. d Then the difference ΔI between the d-axis components d After passing through the d-axis PI controller H of the current loop di (s) Obtain the d-axis control voltage V td Then adjust the d-axis control voltage V. td Subtract the q-axis current coupling term K d i q Obtain the d-axis modulated signal m d ; 6) Modulate the d-axis signal m d and q-axis modulation signal m q After the Parker inverse transform, the phase a modulation signal m in the stationary coordinate system is obtained. a b-phase modulation signal m b c-phase modulation signal m c .

2. The method for improving the stability of a frequency-coupled grid-connected inverter according to claim 1, wherein a multi-harmonic linearization model of the grid-connected inverter is established, characterized in that, Includes the following steps: 1) Considering only the DC voltage disturbance and the disturbance derived through the control loop, establish the positive and negative sequence disturbances V of the grid-connected inverter port output voltage. iap V ian Regarding the positive and negative sequence harmonic disturbances of grid-connected voltage Positive and negative sequence harmonic disturbances of grid-connected current The expression: In equation (4), [Y v ] 2×2 The voltage coefficient matrix before the positive and negative sequence harmonic disturbances of the grid-connected voltage, [Y i ] 2×2 The matrix represents the current coefficients before the positive and negative sequence harmonic disturbances of the grid-connected current. Both are matrices related to the DC voltage loop in the grid-connected inverter model. In the voltage coefficient matrix [Y v ] 2×2 Middle, Y vpp It reflects For V iap The expression for the effect; Y vpn It reflects For V iap The expression for the effect; Y vnp It reflects For V ian The expression for the effect; Y vnn It reflects For V ian The expression for the effect is as follows: In equation (5), k m The modulation coefficient; This is the fundamental frequency component of the modulated signal. for The conjugate value; This is the fundamental frequency component of the grid-connected current. for The conjugate value of ; j is an imaginary number; K(s) is the controller element, and its expression is: K(s)=k m V dc0 H di (s)G dc (s) (6) In the current coefficient matrix [Y i ] 2×2 Middle, Y ipp It reflects For V iap The expression for the effect; Y ipn It reflects For V iap The expression for the effect; Y inp It reflects For V ian The expression for the effect; Y inn It reflects For V ian The expression for the effect; the specific expression is: In equation (7), This is the fundamental frequency component of the grid-connected voltage; 2) Based on equation (4), and combined with the phase-locked loop disturbance correlation matrix, the current loop disturbance correlation matrix and the main circuit relationship, a multi-harmonic linearization model of the grid-connected inverter after applying the frequency coupling grid-connected inverter stability improvement method is established.

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