Frequency coupling grid-connected inverter stability improving method

Abstract

The invention discloses a frequency-coupled grid-connected inverter stability improvement method, which comprises the following steps of: introducing q-axis voltage and q-axis current into q-axis control in a positive feedback form after passing through a suppression link in a direct-current voltage loop q-axis compensation mode, and taking the q-axis voltage and q-axis current as reference values of the q-axis current; according to the strategy, the symbol difference of the frequency domain convolution on the positive sequence and the negative sequence can be ingeniously utilized, the amplitude of a coupling subsystem is greatly reduced while the amplitude of a main diagonal subsystem in a grid-connected inverter admittance matrix is increased, so that the coupling subsystem has negligibility compared with the main diagonal subsystem, and the coupling efficiency of the grid-connected inverter is improved. The frequency coupling oscillation is inhibited essentially; and the stability of the system is improved.

Description

Technical Field

[0001] The present invention relates to the field of grid-connected stability of a novel power system, and in particular to a method for improving the stability of a frequency-coupled grid-connected inverter. Background Art

[0002] In the process of building new power systems, the proportion of renewable energy continues to rise. Grid-connected inverters are increasingly favored as the key link for distributed power sources such as photovoltaics, wind power, and energy storage systems to access the power grid. However, due to the interaction between the grid-connected inverter and the power grid, the system will oscillate in a wide frequency domain from a few hertz to thousands of hertz, seriously threatening the stability of the system. Among them, the problem of frequency coupling oscillation occurring near the fundamental frequency sideband has received increasing attention in recent years. Due to the influence of the frequency coupling effect, the coupling subsystem in the output admittance model of the grid-connected inverter 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 judged, and there is a risk of misjudgment. The frequency coupling effect will also cause subsynchronous oscillations, threatening the stability of the system. In severe cases, it will also cause the generator to be decoupled. It is urgent to suppress the frequency coupling oscillation. However, for the frequency coupling problem, most of the existing methods are proposed for phase-locked loops, and most of them are proposed from the aspects of system damping and stability. The frequency coupling effect is only reflected in the modeling process. Therefore, there is a lack of a suppression method for the DC voltage loop that starts from the essence of suppressing frequency coupling. Summary of the invention

[0003] In order to solve the above technical problems, the present invention provides a method for improving the stability of a frequency-coupled grid-connected inverter.

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

[0005] The method for improving the stability of a frequency-coupled grid-connected inverter comprises the following steps:

[0006] 1) Detect the voltage across the DC bus capacitor to obtain the DC voltage v dc ; At the common coupling point, the three-phase voltage v of abc at the grid connection point abc and three-phase current i abc Sampling is performed, and then the abc three-phase voltage v abc and three-phase current i abc After Park transformation, the d-axis voltage v is obtained. 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 with the filter voltage coefficient X gives the filter voltage component suppression power P f , suppressing the coupling component related to the filter voltage; q-axis current i q Multiplying by the fundamental frequency voltage coefficient Q, we get the fundamental frequency voltage component suppression power P V1 , suppressing the coupling component related to the fundamental frequency voltage; q-axis voltage v q Multiplying by the fundamental frequency current coefficient Z, we get the fundamental frequency current component suppression power P I1 , suppress the coupling component related to the fundamental frequency current; the expressions of the filter voltage coefficient X, fundamental frequency voltage coefficient Q, and fundamental frequency current coefficient Z are:

[0008]

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

[0010] 3) Suppress the power P of the filtered voltage component f , fundamental frequency voltage component suppression power P V1 , fundamental frequency current component suppression power P I1 The total suppression power P is obtained by adding the three A ; Total suppression power P A After the DC voltage loop PI controller G dc (s) to obtain the q-axis control current I tq , the q-axis control current I tq Multiplying by the DC voltage loop coefficient D, we get the q-axis current reference value I qr , whose expression is:

[0011]

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

[0013]

[0014] In formula (3), I pv is the DC side current; C dc is the DC bus capacitance; V dc0 is the given value of DC voltage;

[0015] 4) Set the q-axis current reference value I qr and q-axis current i q After the comparison link, the q-axis component difference ΔI is obtained q , and then the q-axis component difference ΔI q Through the current loop q-axis PI controller Hqi (s) Get the q-axis control voltage V tq , and then the q-axis control voltage V tq The coupling term K with the d-axis current d i d Add to get the q-axis modulation signal m q , where K d is the decoupling coefficient;

[0016] 5) On the d-axis, the DC voltage v dc With the DC voltage given value V dc0 After the comparison link, the DC voltage difference ΔV is obtained dc , DC voltage difference ΔV dc After the DC voltage loop PI controller G dc (s) Get the d-axis current reference value I dr ; Set the d-axis current reference value I dr and d-axis current i d 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 controller H di (s) Get the d-axis control voltage V td , and then the d-axis control voltage V td Subtract the q-axis current coupling term K d i q Get the d-axis modulation signal m d ;

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

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

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

[0020]

[0021] In formula (4), [Y v ] 2×2 is the voltage coefficient matrix before the positive and negative sequence harmonic disturbance of the grid-connected voltage, [Y i ] 2×2 is the current coefficient matrix before the positive and negative sequence harmonic disturbance of the grid-connected current, and 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 Is a reflection of V p V iap The expression of influence; Y vpn Is a reflection of V n V iap The expression of influence; Y vnp Is a reflection of V p V ian The expression of influence; Y vnn Is a reflection of V n V ian The affected expression is:

[0023]

[0024] In formula (5), k m is the modulation coefficient; M1 is the fundamental frequency component of the modulation signal, M1 * is the conjugate value of M1; I1 is the fundamental frequency component of the grid-connected current, I1 * is the conjugate value of I1; j is an imaginary number; K(s) is the controller link, and its 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 p V iap The expression of influence; Y ipn It reflects n V iap The expression of influence; Y inp It reflects p V ian The expression of influence; Y inn It reflects n V ianThe expression affected; the specific expression is:

[0027]

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

[0029] 2) According to formula (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 is established after applying the frequency-coupled grid-connected inverter stability improvement method.

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

[0031] The frequency-coupled grid-connected inverter stability improvement method proposed in the present invention uses a DC voltage loop q-axis compensation method to introduce the q-axis voltage and q-axis current into the q-axis control in the form of positive feedback after passing through the suppression link, as a reference value of the q-axis current. This strategy can cleverly utilize the sign difference of the frequency domain convolution itself in the positive and negative sequences to achieve the increase of the amplitude of the main diagonal subsystem in the admittance matrix of the grid-connected inverter while greatly reducing the amplitude of its coupled subsystem, so that its coupled subsystem is negligible compared to the main diagonal subsystem, which essentially suppresses the frequency coupling oscillation and improves the stability of the system. BRIEF DESCRIPTION OF THE DRAWINGS

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

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

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

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

[0036] Figure 5 The three-phase current experimental waveform in the embodiment of the present invention under the traditional strategy;

[0037] Figure 6 The current FFT analysis result under the traditional strategy in the embodiment of the present invention;

[0038] Figure 7 : is the three-phase current experimental waveform under the proposed strategy in an embodiment of the present invention;

[0039] Figure 8This is the current FFT analysis result under the proposed strategy in an embodiment of the present invention. DETAILED DESCRIPTION

[0040] The present invention will be further described below in conjunction with specific embodiments:

[0041] Figure 1 The main circuit topology diagram of the three-phase grid-connected inverter is shown in Figure 1. Figure 2 The phase-locked loop control structure is shown. Figure 3 The invention is a DC voltage loop and an 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] The method for improving the stability of a frequency-coupled grid-connected inverter comprises the following steps:

[0043] 1) Detect the voltage across the DC bus capacitor to obtain the DC voltage v dc ; At the common coupling point, the three-phase voltage v of abc at the grid connection point abc and three-phase current i abc Sampling is performed, and then the abc three-phase voltage v abc and three-phase current i abc After Park transformation, the d-axis voltage v is obtained. d , q-axis voltage v q , d-axis current i d and q-axis current i q The electrical angle θ required in the Park transformation is obtained through phase-locked loop control. The phase-locked loop control structure is: q-axis voltage v q First pass through the phase-locked loop PI controller H PLL (s) to obtain the electrical angular frequency ω, and the electrical angular frequency ω can be further integrated through the integral link 1 / s to obtain the electrical angle θ;

[0044] 2) On the q-axis, the q-axis current i q Multiplying with the filter voltage coefficient X gives the filter voltage component suppression power P f , suppressing the coupling component related to the filter voltage; q-axis current i q Multiplying by the fundamental frequency voltage coefficient Q, we get the fundamental frequency voltage component suppression power P V1 , suppressing the coupling component related to the fundamental frequency voltage; q-axis voltage v q Multiplying by the fundamental frequency current coefficient Z, we get the fundamental frequency current component suppression power P I1 , suppress the coupling component related to the fundamental frequency current; the expressions of the filter voltage coefficient X, fundamental frequency voltage coefficient Q, and fundamental frequency current coefficient Z are:

[0045]

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

[0047] 3) Suppress the power P of the filtered voltage component f , fundamental frequency voltage component suppression power P V1 , fundamental frequency current component suppression power P I1 The total suppression power P is obtained by adding the three A ; Total suppression power P A After the DC voltage loop PI controller G dc (s) to obtain the q-axis control current I tq , the q-axis control current I tq Multiplying by the DC voltage loop coefficient D, we get the q-axis current reference value I qr , whose expression is:

[0048]

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

[0050]

[0051] In formula (10), I pv is the DC side current; C dc is the DC bus capacitance; V dc0 is the given value of DC voltage;

[0052] 4) Set the q-axis current reference value I qr and q-axis current i q After the comparison link, the q-axis component difference ΔI is obtained q , and then the q-axis component difference ΔI q Through the current loop q-axis PI controller H qi (s) Get the q-axis control voltage V tq , and then the q-axis control voltage V tq The coupling term K with the d-axis current d i d Add to get the q-axis modulation signal m q , where K d is the decoupling coefficient;

[0053] 5) On the d-axis, the DC voltage v dc With the DC voltage given value V dc0 After the comparison link, the DC voltage difference ΔV is obtained dc , DC voltage difference ΔV dc After the DC voltage loop PI controller G dc(s) Get the d-axis current reference value I dr ; Set the d-axis current reference value I dr and d-axis current i d 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 controller H di (s) Get the d-axis control voltage V td , and then the d-axis control voltage V td Subtract the q-axis current coupling term K d i q Get the d-axis modulation signal m d ;

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

[0055] Furthermore, based on the frequency-coupled grid-connected inverter stability improvement method, a multi-harmonic linearization model of the grid-connected inverter is established, which includes the following steps:

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

[0057]

[0058] In formula (11), [Y v ] 2×2 is the voltage coefficient matrix before the positive and negative sequence harmonic disturbance of the grid-connected voltage, [Y i ] 2×2 is the current coefficient matrix before the positive and negative sequence harmonic disturbance of the grid-connected current, and 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 Is a reflection of V p V iap The expression of influence; Yvpn Is a reflection of V n V iap The expression of influence; Y vnp Is a reflection of V p V ian The expression of influence; Y vnn Is a reflection of V n V ian The affected expression is:

[0060]

[0061] In formula (12), k m is the modulation coefficient; M1 is the fundamental frequency component of the modulation signal, is the conjugate value of M1; I1 is the fundamental frequency component of the grid-connected current, is the conjugate value of I1; j is an imaginary number; K(s) is the controller link, and its 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 p V iap The expression of influence; Y ipn It reflects n V iap The expression of influence; Y inp It reflects p V ian The expression of influence; Y inn It reflects n V ian The expression affected; the specific expression is:

[0064]

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

[0066] 2) According to formula (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 is established after applying the frequency-coupled grid-connected inverter stability improvement method.

[0067] In order to verify the effectiveness of the proposed method for improving the stability of grid-connected inverters based on frequency coupling, a Bode diagram was drawn in Matlab simulation software for theoretical analysis, and an experimental verification was carried out on a hardware-in-the-loop experimental platform based on RT-Lab real-time simulator. The main circuit parameters are: DC side current I pv =75A, DC voltage given value V dc0 =700V, DC bus capacitance C dc =7mF, filter inductor L f =7mH, grid 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] The Bode diagram of the output admittance model of the grid-connected inverter using the traditional strategy and the proposed strategy is shown in the figure below. Figure 3 As shown. Figure 3 It can be seen that under traditional control, the amplitude of the coupling subsystem is similar to that of the main diagonal subsystem in most frequency bands, especially in the low-frequency band that has the greatest impact on the system, indicating the existence of frequency coupling and it cannot be ignored. However, when the frequency coupling grid-connected inverter stability improvement method is applied, the amplitude of the coupling subsystem is greatly reduced in the whole frequency band, and the amplitude of the main diagonal subsystem remains unchanged or increases in the whole frequency band. At this time, the amplitude of the main diagonal subsystem is much larger than that of the coupling subsystem, which shows from the theoretical analysis level that the proposed control strategy essentially achieves the suppression of the frequency coupling effect.

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

[0070] 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, all changes made according to the shape and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A frequency-coupled grid-connected inverter stability improvement method, characterized in that: The following steps are involved: 1) Detect the voltage across the DC bus capacitor to obtain the DC voltage v dc ; At the common coupling point, the three-phase voltage v of abc at the grid connection point abc and three-phase current i abc Sampling is performed, and then the abc three-phase voltage v abc and three-phase current i abc After Park transformation, the d-axis voltage v is obtained. 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 with the filter voltage coefficient X gives the filter voltage component suppression power P f , suppressing the coupling component related to the filter voltage; q-axis current i q Multiplying by the fundamental frequency voltage coefficient Q, we get the fundamental frequency voltage component suppression power P V1 , suppressing the coupling component related to the fundamental frequency voltage; q-axis voltage v q Multiplying by the fundamental frequency current coefficient Z, we get the fundamental frequency current component suppression power P I1 , suppress the coupling component related to the fundamental frequency current; the expressions of the filter voltage coefficient X, fundamental frequency voltage coefficient Q, and fundamental frequency current coefficient Z are: In formula (1), s is the Laplace operator; L f is the filter inductance value; V1 and I1 are the amplitudes of the fundamental frequency components of the grid-connected voltage and grid-connected current respectively; is the initial phase angle of the grid-connected current; 3) Suppress the power P of the filtered voltage component f , fundamental frequency voltage component suppression power P V1 , fundamental frequency current component suppression power P I1 The total suppression power P is obtained by adding the three A ; Total suppression power P A After the DC voltage loop PI controller G dc (s) to obtain the q-axis control current I tq , the q-axis control current I tq Multiplying by the DC voltage loop coefficient D, we get the q-axis current reference value I qr , whose expression is: In formula (2), the expression of DC voltage loop coefficient D is: In formula (3), I pv is the DC side current; C dc is the DC bus capacitance; V dc0 is the given value of DC voltage; 4) Set the q-axis current reference value I qr and q-axis current i q After the comparison link, the q-axis component difference ΔI is obtained q , and then the q-axis component difference ΔI q Through the current loop q-axis PI controller H qi (s) Get the q-axis control voltage V tq , and then the q-axis control voltage V tq The coupling term K with the d-axis current d i d Add to get the q-axis modulation signal m q , where K d is the decoupling coefficient; 5) On the d-axis, the DC voltage v dc With the DC voltage given value V dc0 After the comparison link, the DC voltage difference ΔV is obtained dc , DC voltage difference ΔV dc After the DC voltage loop PI controller G dc (s) Get the d-axis current reference value I dr ; Set the d-axis current reference value I dr and d-axis current i d 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 controller H di (s) Get the d-axis control voltage V td , and then the d-axis control voltage V td Subtract the q-axis current coupling term K d i q Get the d-axis modulation signal m d ; 6) The d-axis modulation signal m d and the q-axis modulation signal m q After the Park inverse transform, the a-phase modulation signal m in the stationary coordinate system is obtained. a , b-phase modulation signal m b , c-phase modulation signal m c .

2. According to the frequency-coupled grid-connected inverter stability improvement method of claim 1, a multi-harmonic linearization model of the grid-connected inverter is established, characterized in that: The following steps are involved: 1) Considering only the DC voltage disturbance and its derived disturbance through the control link, the positive and negative sequence disturbances V of the output voltage of the grid-connected inverter port are established. iap 、V ian Regarding the positive and negative sequence harmonic disturbances V of the grid-connected voltage p 、V n , the positive and negative sequence harmonic disturbances of the grid-connected current I p ,I n The expression is: In formula (4), [Y v ] 2×2 is the voltage coefficient matrix before the positive and negative sequence harmonic disturbance of the grid-connected voltage, [Y i ] 2×2 is the current coefficient matrix before the positive and negative sequence harmonic disturbance of the grid-connected current, and 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 Is a reflection of V p V iap The expression of influence; Y vpn Is a reflection of V n V iap The expression of influence; Y vnp Is a reflection of V p V ian The expression of influence; Y vnn Is a reflection of V n V ian The expression affected is: In formula (5), k m is the modulation coefficient; M1 is the fundamental frequency component of the modulation signal, is the conjugate value of M1; I1 is the fundamental frequency component of the grid-connected current, is the conjugate value of I1; j is an imaginary number; K(s) is the controller link, 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 p V iap The expression of influence; Y ipn It reflects n V iap The expression of influence; Y inp It reflects p V ian The expression of influence; Y inn It reflects n V ian The expression affected; the specific expression is: In formula (7), V1 is the fundamental frequency component of the grid-connected voltage; 2) According to formula (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 is established after applying the frequency-coupled grid-connected inverter stability improvement method.

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