Self-synchronizing voltage source grid-connected system harmonic suppression method based on power grid voltage feedforward

Abstract

The invention provides a self-synchronizing voltage source grid-connected system harmonic suppression method based on power grid voltage feedforward, which comprises the following steps: step 1, carrying out topological structure analysis on a self-synchronizing voltage source grid-connected system, and constructing a mathematical model of the self-synchronizing voltage source grid-connected system; step 2, according to the mathematical model constructed in the step 1, analyzing the influence of the network voltage background harmonic on the grid-connected current of the self-synchronizing voltage source; step 3, improving an inductive current reference value of the self-synchronizing voltage source grid-connected system based on a bisecond-order generalized integrator; and step 4, improving a power grid voltage feed-forward control structure of the self-synchronizing voltage source grid-connected system, and completing harmonic suppression of the self-synchronizing voltage source grid-connected system based on power grid voltage feed-forward.

Description

Technical Field

[0001] The present invention relates to a method for suppressing harmonics in a voltage source grid-connected system, in particular to a method for suppressing harmonics in a self-synchronous voltage source grid-connected system based on grid voltage feedforward. Background Art

[0002] This section merely provides background information related to the present disclosure and is not necessarily prior art.

[0003] Faced with the dual pressures of the fossil energy crisis and environmental pollution, the development and utilization of renewable energy sources such as solar and wind power are continuously intensifying. New power systems with a large penetration of renewable energy generation have low virtual moments of inertia and small damping coefficients. Under sudden changes in operating conditions, traditional grid-connected inverters are unable to support grid voltage and frequency. The self-synchronous voltage source control strategy simulates the external characteristics of traditional synchronous generators to support grid voltage and frequency, providing a new approach for large-scale renewable energy generation grid integration. However, in weak grid scenarios, the grid impedance is difficult to ignore and grid voltage background harmonics are present. This results in a low grid stability margin when self-synchronous voltage sources are connected to weak grids, making harmonic resonance more likely to occur, which in turn reduces the current quality of the grid-connected system. To address the background harmonics issue in weak grid scenarios, grid voltage feedforward is an effective method to suppress grid voltage background harmonics.

[0004] Prior art strategies for suppressing grid voltage background harmonics in self-synchronous voltage sources primarily include two control methods: proportional resonant controllers and grid voltage feedforward. Proportional resonant controllers achieve infinite loop gain at the resonant frequency to control the target frequency. However, when the frequency of the grid voltage background harmonics has time-varying characteristics, the proportional resonant controller has difficulty accurately achieving infinite loop gain at the grid voltage background harmonic frequency. Furthermore, the phase-frequency characteristics of the loop gain may produce negative phase shifts.

[0005] It should be noted that the information disclosed in the above background technology section is only used to enhance the understanding of the background of the present disclosure, and therefore may include information that does not constitute prior art known to ordinary technicians in the field. Summary of the Invention

[0006] Purpose of the invention: The technical problem to be solved by the present invention is to address the deficiencies of the existing technology and provide a method for suppressing harmonics in a self-synchronous voltage source grid-connected system based on grid voltage feedforward.

[0007] In order to solve the above technical problems, the present invention discloses a method for suppressing harmonics in a self-synchronous voltage source grid-connected system based on grid voltage feedforward, comprising the following steps:

[0008] Step 1, performing a topological structure analysis on the self-synchronous voltage source grid-connected system and constructing a mathematical model of the self-synchronous voltage source grid-connected system;

[0009] Step 2: Analyze the influence of the grid voltage background harmonics on the grid-connected current of the self-synchronous voltage source based on the mathematical model constructed in step 1;

[0010] Step 3: improving the inductor current reference value of the self-synchronous voltage source grid-connected system based on a biquad generalized integrator;

[0011] Step 4: Improve the grid voltage feedforward control structure of the self-synchronous voltage source grid-connected system to complete the harmonic suppression of the self-synchronous voltage source grid-connected system based on grid voltage feedforward.

[0012] Furthermore, the mathematical model of the self-synchronous voltage source grid-connected system constructed in step 1 includes:

[0013] Step 1-1, establish the main circuit equation of the self-synchronous voltage source grid-connected system;

[0014] Step 1-2, calculate the instantaneous power of the self-synchronous voltage source grid-connected system;

[0015] Step 1-3, establish the active frequency control equation of the self-synchronous voltage source grid-connected system;

[0016] Steps 1-4, establishing reactive power and voltage control equations of the self-synchronous voltage source grid-connected system;

[0017] Step 1-5, establish the voltage loop equation of the self-synchronous voltage source grid-connected system;

[0018] Steps 1-6: Establish the current loop equation of the self-synchronous voltage source grid-connected system.

[0019] Furthermore, the main circuit equation for establishing the self-synchronous voltage source grid-connected system described in step 1-1 is as follows:

[0020]

[0021] Among them, e a 、e b 、e c are the A, B, and C three-phase output voltages of the three-phase inverter bridge in the self-synchronous voltage source grid-connected system, V a 、V b 、V c are the A, B, and C three-phase output voltages of the self-synchronous voltage source grid-connected system, L f is the filter inductor, I La , I Lb , I Lcare the three-phase inductor currents of the self-synchronous voltage source grid-connected system, I oa , I ob , I oc are the A, B, and C three-phase output currents of the self-synchronous voltage source grid-connected system; C f is the filter capacitor, and t is the time.

[0022] Furthermore, the instantaneous power of the self-synchronous voltage source grid-connected system is calculated as follows in steps 1-2:

[0023]

[0024] Among them, P e is the instantaneous value of output active power, Q e is the instantaneous value of output reactive power, V d is the d-axis component of the output voltage, V q is the q-axis component of the output voltage, I d is the d-axis component of the output current, I q is the q-axis component of the output current.

[0025] Furthermore, the active frequency control equation for the self-synchronous voltage source grid-connected system established in steps 1-3 is as follows:

[0026]

[0027] Among them, P ref is the output active power reference value, ω is the grid angular frequency, ω n is the rated angular frequency of the power grid, D p is the virtual damping coefficient, J is the virtual moment of inertia, and θ is the phase of the virtual internal potential.

[0028] Furthermore, the reactive power and voltage control equations for establishing the self-synchronous voltage source grid-connected system described in steps 1-4 are as follows:

[0029]

[0030] Among them, E m is the amplitude of the virtual internal potential, U is the output voltage, U n is the rated voltage, D q is the reactive voltage droop coefficient, Q ref is the output reactive power reference value, K is the excitation integral coefficient, and s is the complex frequency in Laplace transform.

[0031] Furthermore, the voltage loop equation for establishing the self-synchronous voltage source grid-connected system described in steps 1-5 is as follows:

[0032]

[0033] Among them, I dref is the d-axis component of the voltage loop output current, I qref is the q-axis component of the voltage loop output current, V dref is the d-axis component of the output voltage reference value, V qref is the q-axis component of the output voltage reference value, K pv , K iv are the proportional coefficient and integral coefficient of the voltage loop PI controller respectively;

[0034] The current loop equations for establishing the self-synchronous voltage source grid-connected system described in steps 1-6 are as follows:

[0035]

[0036] Among them, V id is the d-axis component of the current loop output voltage, V iq is the q-axis component of the current loop output voltage, I Ld is the d-axis component of the inductor current, I Lq is the q-axis component of the inductor current, K pi , K ii They are the proportional coefficient and integral coefficient of the current loop PI controller respectively.

[0037] Furthermore, the analysis of the impact of the grid voltage background harmonics on the grid-connected current of the self-synchronous voltage source described in step 2 includes:

[0038] Step 2-1, the self-synchronous voltage source grid-connected current i g The calculation method is as follows:

[0039]

[0040] Among them, L f is the filter inductor, C f is the filter capacitor, R c is the resistance value of the filter capacitor, i Lref is the inductor current reference value, u g is the grid voltage, s is the complex frequency in Laplace transform, where the function P(s) is expressed as follows:

[0041]

[0042] Among them, K pwm is the PWM modulation coefficient, is the voltage and current signal acquisition delay and PWM modulation delay transfer function, T s is the switching period of the inverter bridge circuit;

[0043] Step 2-2, inductor current reference value i Lref , the calculation method is as follows:

[0044]

[0045] Among them, e is the output voltage reference value of the sinusoidal wave voltage generation link.

[0046] Furthermore, the improvement of the inductor current reference value of the self-synchronous voltage source grid-connected system described in step 3 includes:

[0047] Step 3-1, establish the mathematical model of the second-order generalized integrator, as follows:

[0048]

[0049] Where v(s) is the input AC signal of the second-order generalized integrator; v′(s) and qv′(s) are the filtered output signal and the quadrature output signal of the second-order generalized integrator, respectively; k is the gain coefficient, ω′ is the positive-sequence fundamental angular frequency, D(s) is the transfer function between the filtered output signal v′(s) and the input AC signal v(s), and Q(s) is the transfer function between the quadrature output signal qv′(s) and the input AC signal v(s);

[0050] Step 3-3: Set the three-phase grid voltage v at the point of common coupling a 、v b 、v c After transformation, the α-axis component v in the αβ coordinate system is obtained α With the β-axis component v β ; The α-axis component v of the grid voltage α With the β-axis component v β After being input into the second-order generalized integrator respectively, the high-order harmonic components in the grid voltage v are filtered out to obtain the filtered output signal;

[0051] Step 3-4, after filtering out the high-order harmonics, the α-axis component v of the filtered output signal is α ′ and the β-axis component qv of the orthogonal output signal β ′, the β-axis component v of the filtered output signal β ′ and the α-axis component qv′ of the orthogonal output signal α Make a difference, filter out the fundamental negative sequence component of the grid voltage, and extract the fundamental positive sequence component u of the grid voltage g and is used to calculate the inductor current reference value.

[0052] Furthermore, the grid voltage feedforward control structure of the self-synchronous voltage source grid-connected system described in step 4 is improved, that is, a grid voltage feedforward path function A and a grid voltage feedforward path function B are introduced before and after the current loop in the self-synchronous voltage source grid-connected system, respectively, to eliminate the influence of the grid voltage background harmonics on the inductor current and the capacitor current, respectively. The specific method includes:

[0053] Grid-connected current i of self-synchronous voltage source with grid voltage feedforward g The mathematical expression is:

[0054]

[0055] Among them, the calculation method of function E(s) and function F(s) is:

[0056]

[0057] Let function E(s) and function F(s) be 0, and the feedforward path function A and feedforward path function B are obtained as follows:

[0058]

[0059] When background harmonics exist in the grid voltage, the grid-connected current after improving the grid voltage feedforward control structure of the self-synchronous voltage source grid-connected system is:

[0060]

[0061] The harmonic suppression of the self-synchronous voltage source grid-connected system based on grid voltage feedforward is completed.

[0062] Beneficial effects:

[0063] 1. The present invention uses DSOGI to extract the fundamental positive sequence component of the grid voltage at the point of common coupling (PCC) of the self-synchronous voltage source grid-connected system. Compared to the previous SOGI fundamental component extraction method, the DSOGI extraction method not only filters out the harmonic components of the grid voltage, but also filters out the fundamental negative sequence component of the grid voltage when the grid voltage is asymmetric, thus quickly and accurately extracting the fundamental positive sequence component of the grid voltage.

[0064] 2. The present invention improves the grid voltage feedforward control structure of the self-synchronous voltage source grid-connected system. Compared to the previous single-branch grid voltage feedforward method, the improved grid voltage feedforward control structure not only eliminates the impact of grid voltage background harmonics on the inductor current, but also eliminates the impact of grid voltage background harmonics on the filter capacitor current when an LC filter is used, thereby better suppressing the impact of grid voltage background harmonics on the self-synchronous voltage source grid-connected current.

[0065] 3. This invention improves the grid voltage feedforward control structure of the self-synchronous voltage source grid-connected system. Compared to the conventional proportional resonant controller approach, this improved grid voltage feedforward control structure not only eliminates the impact of grid voltage background harmonics on the self-synchronous voltage source grid-connected current, but also improves the self-synchronous voltage source's sequence impedance characteristics in the high-frequency range from capacitive to inductive, thus avoiding harmonic resonance in the self-synchronous voltage source grid-connected system in weak grid scenarios. BRIEF DESCRIPTION OF THE DRAWINGS

[0066] The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments, and the above and / or other advantages of the present invention will become more apparent.

[0067] Figure 1 It is a schematic diagram of the overall process of the present invention.

[0068] Figure 2 It is a schematic diagram of the topological structure of the self-synchronous voltage source grid-connected system of the present invention.

[0069] Figure 3 It is a control block diagram of the self-synchronous voltage source grid-connected system of the present invention.

[0070] Figure 4 It is a control block diagram of the self-synchronous voltage source grid-connected system containing DSOGI of the present invention.

[0071] Figure 5 It is a control structure block diagram of the biquad generalized integrator of the present invention.

[0072] Figure 6 This is a structural block diagram of the improved grid voltage feedforward control according to the present invention.

[0073] Figure 7 This is a schematic diagram of the grid-connected current waveform of the self-synchronous voltage source without grid voltage feedforward of the present invention.

[0074] Figure 8 It is a schematic diagram of the grid-connected current waveform of the self-synchronous voltage source with grid voltage feedforward of the present invention. DETAILED DESCRIPTION

[0075] The overall concept of this invention is as follows: a harmonic suppression strategy for a self-synchronous voltage source based on grid voltage feedforward is proposed, in order to suppress the background harmonics of the self-synchronous voltage source and improve the grid-connected stability, thereby improving the grid-connected current quality of the self-synchronous voltage source grid-connected system. The technical solution is as follows:

[0076] Step 1: Topological structure analysis and mathematical model construction of the self-synchronous voltage source grid-connected system;

[0077] S1-1: The main circuit equation of the self-synchronous voltage source grid-connected system is:

[0078]

[0079] In formula (1), e a 、e b 、e c are the A, B, and C three-phase output voltages of the three-phase inverter bridge, V a 、V b 、V c are the A, B, and C three-phase output voltages of the grid-connected system, L f is the filter inductor, I La , I Lb , I Lc are the three-phase inductor currents of the grid-connected system, I oa , I ob , I oc are the A, B, and C three-phase output currents of the grid-connected system; C f is the filter capacitor, and t is the time.

[0080] S1-2: The instantaneous power calculation equation of the self-synchronous voltage source grid-connected system is:

[0081]

[0082] In formula (2), P e is the instantaneous value of output active power, Q e is the instantaneous value of output reactive power, V d is the d-axis component of the output voltage, V q is the q-axis component of the output voltage, I d is the d-axis component of the output current, I q is the q-axis component of the output current.

[0083] S1-3: The PF control equation of the self-synchronous voltage source grid-connected system is:

[0084]

[0085] In formula (3), P ref is the output active power reference value, ω is the grid angular frequency, ω n is the rated angular frequency of the power grid, D p is the virtual damping coefficient, J is the virtual moment of inertia, and θ is the phase of the virtual internal potential.

[0086] S1-4: The QV control equation of the self-synchronous voltage source grid-connected system is:

[0087]

[0088] In formula (4), E m is the amplitude of the virtual internal potential, U is the output voltage, Un is the rated voltage, D q is the reactive voltage droop coefficient, Q ref is the output reactive power reference value, L is the excitation integral coefficient, and s is the complex frequency in Laplace transform.

[0089] S1-5: The voltage loop equation of the self-synchronous voltage source grid-connected system is:

[0090]

[0091] In formula (5), I dref is the d-axis component of the voltage loop output current, I qref is the q-axis component of the voltage loop output current, V dref is the d-axis component of the output voltage reference value, V qref is the q-axis component of the output voltage reference value, K pv , K iv are the proportional coefficient and integral coefficient of the voltage loop PI controller respectively.

[0092] S1-6: The current loop equation of the self-synchronous voltage source grid-connected system is:

[0093]

[0094] In formula (6), V id is the d-axis component of the current loop output voltage, V iq is the q-axis component of the current loop output voltage, I Ld is the d-axis component of the inductor current, I Lq is the q-axis component of the inductor current, K pi , K ii They are the proportional coefficient and integral coefficient of the current loop PI controller respectively.

[0095] Step 2: Analyze the impact mechanism of grid voltage background harmonics on the grid-connected current of the self-synchronous voltage source; the specific analysis steps are as follows:

[0096] S2-1: The mathematical expression of the grid-connected current of the self-synchronous voltage source is:

[0097]

[0098] In formula (7), L f is the filter inductor, C f is the filter capacitor, R c is the resistance value of the filter capacitor, i Lref is the inductor current reference value, u g is the grid voltage, i g is the grid-connected current, s is the complex frequency in Laplace transform, and the function P(s) can be expressed as:

[0099]

[0100] In formula (8), K pwm is the PWM modulation coefficient, is the voltage and current signal acquisition delay and PWM modulation delay transfer function, T s is the switching period of the inverter bridge circuit.

[0101] S2-2: Inductor current reference value i Lref The output grid voltage reference value e and grid voltage u g Expressed as:

[0102]

[0103] In formula (9), e is the output voltage reference value of the sinusoidal voltage generation link.

[0104] The grid-connected current i from the self-synchronous voltage source g The calculation formula of can reflect the influence of grid voltage background harmonics on the grid current of self-synchronous voltage source. The existence of background harmonics in grid voltage means u g There are harmonics in the grid, and the grid current y g The grid voltage component Affected by the background harmonics of the grid voltage, which also affects the grid-connected current y g ;Inductor current reference value Affected by the background harmonics of the grid voltage, the grid-connected current y g The inductor current reference value component Affected by the background harmonics of the grid voltage, it also affects the grid-connected current i g .

[0105] Step 3: A method for improving the inductor current reference value of a self-synchronous voltage source grid-connected system based on a DSOGI (Double Second-Order Generalized Integrator). The specific steps are as follows:

[0106] S3-1: The mathematical model of the second-order generalized integrator is:

[0107]

[0108] In formula (10), v(s) is the input AC signal of SOGI, that is, the grid voltage; v′(s) and qv′(s) are the filtered output signal and the quadrature output signal of SOGI, respectively; k is the gain coefficient, ω′ is the positive sequence fundamental angular frequency, D(s) is the transfer function between the filtered output signal v′(s) and the input AC signal v(s), and Q(s) is the transfer function between the quadrature output signal qv′(s) and the input AC signal v(s).

[0109] S3-2: The grid voltage v(s) is used as the input of a second-order generalized integrator to filter out the harmonic components of the grid voltage v(s) to extract the fundamental component of the grid voltage v(s).

[0110] S3-3: The three-phase grid voltage v at the common coupling point PCC a 、v b 、v c through After transformation, the α-axis component v in the αβ coordinate system can be obtained α With the β-axis component v β . The grid voltage α-axis component v α With the β-axis component v β After being input into the SOGI link respectively, the high-order harmonic components in the grid voltage v can be filtered out.

[0111] S3-4: After filtering out the high-order harmonics, the α-axis component v of the filtered output signal α ′ and the β-axis component qv of the orthogonal output signal β ′, the βγ axis component v of the filtered output signal β ′ and the α-axis component qv of the orthogonal output signal α ′, the fundamental negative sequence component of the grid voltage can be filtered out and the fundamental positive sequence component u of the grid voltage can be extracted. g and is used to calculate the inductor current reference value.

[0112] Among them, the fundamental positive sequence component u of the grid voltage is g is u in step s2-2 g , there will be higher harmonics, fundamental positive sequence components, and fundamental negative sequence components in the grid voltage. The above two steps filter out higher harmonics and fundamental negative sequence components respectively, and the final result is the fundamental positive sequence component, that is, u g . The e in the figure is the output voltage reference value that is not affected by the background harmonics of the grid voltage. g Similarly, the calculated inductor current reference value is correct only when the fundamental positive sequence component is present.

[0113] Step 4: Improvement method of grid voltage feedforward control structure of self-synchronous voltage source grid-connected system; the specific steps are as follows:

[0114] S4-1: Self-synchronous voltage source grid-connected systems often use LC filters. When there are background harmonics in the grid voltage, the filter inductor current and filter capacitor current in the LC filter are both affected by the background harmonics of the grid voltage.

[0115] S4-2: Introduce grid voltage feedforward path A and grid voltage feedforward path B before and after the current loop respectively to eliminate the influence of grid voltage background harmonics on inductor current and capacitor current respectively.

[0116] S4-3: Self-synchronous voltage source with grid voltage feedforward and grid-connected current i g The mathematical expression is:

[0117]

[0118] In formula (11), function E(s) and function F(s) are respectively:

[0119]

[0120] Let the function E(s) and the function F(s) be 0, and the feedforward function A and the feedforward function B are derived as follows:

[0121]

[0122] When there are background harmonics in the grid voltage, the grid-connected current of the self-synchronous voltage source with improved grid voltage feedforward is:

[0123]

[0124] Equation (14) shows that the grid voltage feedforward control can eliminate the influence of grid voltage background harmonics on the grid-connected current of the self-synchronous voltage source and improve the quality of the grid-connected current.

[0125] Example 1:

[0126] The present invention discloses a self-synchronous voltage source harmonic suppression strategy based on grid voltage feedforward, such as Figure 1 The specific steps are as follows:

[0127] Step 1: Topological structure analysis and mathematical model establishment of the self-synchronous voltage source grid-connected system. Figure 2 The topological structure of the self-synchronous voltage source grid-connected system is shown in the figure. The self-synchronous voltage source control strategy simulates the virtual rotational inertia and damping coefficient of the traditional synchronous generator to achieve the function of autonomously supporting the grid voltage and grid frequency. The self-synchronous voltage source grid-connected system includes the main circuit, instantaneous power calculation, virtual synchronous control, and voltage and current dual closed loops.

[0128] S1-1: The main circuit equation of the self-synchronous voltage source grid-connected system is:

[0129]

[0130] In formula (1), e a 、e b 、e c are the A, B, and C three-phase output voltages of the three-phase inverter bridge, V a 、V b 、V c are the A, B, and C three-phase output voltages of the grid-connected system, L f is the filter inductor, I La , I Lb , I Lc are the three-phase inductor currents of the grid-connected system, I oa , I ob , I oc are the A, B, and C three-phase output currents of the grid-connected system; C f is the filter capacitor, and t is the time.

[0131] S1-2: The instantaneous power calculation equation of the self-synchronous voltage source grid-connected system is:

[0132]

[0133] In formula (2), P e is the instantaneous value of output active power, Q e is the instantaneous value of output reactive power, V d is the d-axis component of the output voltage, V q is the q-axis component of the output voltage, I d is the d-axis component of the output current, I q is the q-axis component of the output current.

[0134] S1-3: The active frequency control equation of the self-synchronous voltage source grid-connected system is:

[0135]

[0136] In formula (3), P ref is the output active power reference value, ω is the grid angular frequency, ω n is the rated angular frequency of the power grid, D p is the virtual damping coefficient, J is the virtual moment of inertia, and θ is the phase of the virtual internal potential.

[0137] S1-4: The reactive voltage control equation of the self-synchronous voltage source grid-connected system is:

[0138]

[0139] In formula (4), Em is the amplitude of the virtual internal potential, U is the output voltage, U n is the rated voltage, D q is the reactive voltage droop coefficient, Q ref is the output reactive power reference value, K is the excitation integral coefficient, and s is the complex frequency in Laplace transform.

[0140] S1-5: The voltage loop equation of the self-synchronous voltage source grid-connected system is:

[0141]

[0142] In formula (5), I dref is the d-axis component of the voltage loop output current, I qref is the q-axis component of the voltage loop output current, V dref is the d-axis component of the output voltage reference value, V qref is the q-axis component of the output voltage reference value, K pv , K iv are the proportional coefficient and integral coefficient of the voltage loop PI controller respectively.

[0143] S1-6: The current loop equation of the self-synchronous voltage source grid-connected system is:

[0144]

[0145] In formula (6), V id is the d-axis component of the current loop output voltage, V iq is the q-axis component of the current loop output voltage, I Ld is the d-axis component of the inductor current, I Lq is the q-axis component of the inductor current, K pi , K ii They are the proportional coefficient and integral coefficient of the current loop PI controller respectively.

[0146] Step 2: Analyze the impact mechanism of grid voltage background harmonics on the grid-connected current of the self-synchronous voltage source. Based on the analysis of the topology of the self-synchronous voltage source grid-connected system and the establishment of the mathematical model in step 1, the control block diagram of the self-synchronous voltage source grid-connected system can be drawn as follows: Figure 3 shown.

[0147] S2-1: The mathematical expression of the grid-connected current of the self-synchronous voltage source is:

[0148]

[0149] From formula (7), we can know that the grid-connected current i of the self-synchronous voltage source grid-connected system is g The inductor current reference value i Lref Component and grid voltage u gThe components are composed of two parts. f is the filter inductor, C f is the filter capacitor, R c is the resistance value of the filter capacitor, i Lref is the inductor current reference value, u g is the grid voltage, i g is the grid-connected current. Function P(s) is:

[0150]

[0151] In formula (8), K pwm is the PWM modulation coefficient, is the voltage and current signal acquisition delay and PWM modulation delay transfer function, T s is the switching period of the inverter bridge circuit.

[0152] S2-2: According to Figure 3 From the control block diagram of the self-synchronous voltage source grid-connected system, it can be seen that the inductor current reference value i Lref The output grid voltage reference value e and grid voltage u g Expressed as:

[0153]

[0154] In formula (9), e is the output voltage reference value of the sine wave voltage generation link. From formula (9), we can know that the inductor current reference value i Lref With the grid voltage u g Directly related, affected by the background harmonics of the grid voltage. In order to eliminate the grid voltage u g The influence of background harmonics on the grid current needs to be calculated for the inductor current reference value i Lref Implement improvements.

[0155] Step 3: Improvement method of inductor current reference value of self-synchronous voltage source grid-connected system based on DSOGI. From formula (9), we can see that the grid voltage u in the weak grid scenario is g It can be decomposed into fundamental component and harmonic component, while the output grid voltage reference value e contains only the fundamental positive sequence voltage component, so a biquad generalized integrator is required to extract the grid voltage u g The positive sequence component of the fundamental wave.

[0156] S3-1: The mathematical model of the second-order generalized integrator is:

[0157]

[0158] In formula (10), v(s) is the input AC signal of SOGI, that is, the grid voltage; v′(s) and qv′(s) are the filtered output signal and the quadrature output signal of SOGI, respectively; k is the gain coefficient, ω′ is the positive sequence fundamental angular frequency, D(s) is the transfer function between the filtered output signal v′(s) and the input AC signal v(s), and Q(s) is the transfer function between the quadrature output signal qv′(s) and the input AC signal v(s).

[0159] S2-2: Combination Figure 5 The control block diagram of the biquad generalized integrator is used to convert the three-phase grid voltage v at the common coupling point PCC into a 、v b 、v c through After transformation, the α-axis component v in the αβ coordinate system can be obtained α With the β-axis component v β . The grid voltage α-axis component v α With the β-axis component v β After inputting SOGI respectively, the output voltage α-axis component v′ without high-order harmonic components can be obtained α and the output voltage β-axis component v′ β .

[0160] S2-3: Combination Figure 5 The control block diagram of the biquad generalized integrator. After filtering out the high-order harmonics, the α-axis component v′ of the filtered output signal is α The β-axis component qv of the quadrature output signal β ′, the β-axis component v′ of the filtered output signal β The α-axis component qv′ of the quadrature output signal α By making a difference, the fundamental negative sequence component of the grid voltage can be filtered out, and the fundamental positive sequence component of the grid voltage can be extracted. Figure 4 The inductor current reference value is calculated by means of the control block diagram of the self-synchronous voltage source grid-connected system containing DSOGI.

[0161] Step 4: Improvement method of grid voltage feedforward control structure of self-synchronous voltage source grid-connected system.

[0162] S3-1: Traditional grid-connected inverters mostly use L-type filters and LCL-type filters. When there are background harmonics in the grid voltage, only the inductor current is affected by the background harmonics of the grid voltage. In the LC-type filter of the self-synchronous voltage source, both the filter inductor current and the filter capacitor current are affected by the background harmonics of the grid voltage.

[0163] S3-2: According to Figure 6From the block diagram of the improved grid voltage feedforward control structure, it can be seen that grid voltage feedforward path A and grid voltage feedforward path B are introduced before and after the current loop to eliminate the influence of grid voltage background harmonics on inductor current and capacitor current respectively.

[0164] S3-3: Derivation of mathematical expressions of feedforward function A and feedforward function B: According to the improved grid voltage feedforward control structure block diagram, the self-synchronous voltage source grid-connected current u g The mathematical expression is:

[0165]

[0166] In formula (11), the function W(s) and the function F(s) are respectively:

[0167]

[0168] In order to eliminate the influence of the background harmonics of the grid voltage on the grid-connected current of the self-synchronous voltage source, the function E(s) and the function F(s) are both set to 0, and the mathematical expressions of the feedforward function A and the feedforward function B are derived respectively:

[0169]

[0170] The mathematical expression of the grid-connected current of the self-synchronous voltage source with improved grid voltage feedforward is:

[0171]

[0172] Equation (14) shows that the grid voltage feedforward control can eliminate the influence of grid voltage background harmonics on the grid-connected current of the self-synchronous voltage source and improve the quality of the grid-connected current.

[0173] Step 5: Simulation verification of the self-synchronous voltage source resonance suppression strategy based on grid voltage feedforward.

[0174] Example 2:

[0175] According to the resonance suppression strategy proposed above, a self-synchronous voltage source grid-connected system simulation model was built based on the Matlab / Simulink simulation experimental platform. The simulation waveforms of the self-synchronous voltage source grid-connected current before and after the introduction of grid voltage feedforward control are shown as follows: Figure 7 and Figure 8 As shown in Figure 3, the simulation results show that when there are background harmonics in the grid voltage, the control strategy based on grid voltage feedforward ensures that the grid-connected current quality of the self-synchronous voltage source meets the grid-connected requirements.

[0176] In order to verify the effectiveness of the proposed harmonic suppression strategy, a simulation model of the self-synchronous voltage source grid-connected system was built in the Matlab / Simulink simulation experiment platform, and the grid voltage u gThe 5th positive sequence harmonic component, the 7th negative sequence harmonic component and the 11th positive sequence harmonic component are injected into the power grid to simulate the grid voltage background harmonics in the actual power grid.

[0177] The parameters of the self-synchronous voltage source grid-connected system are as follows: DC voltage V dc is 800V, the rated voltage RMS value is U n The voltage is 220V, and the virtual moment of inertia J is 0.5kg.m 2 , damping coefficient D p is 10N·m·s / rad, the filter inductance L f is 0.8mH, the filter capacitor C f The resistance value of the filter capacitor is 100uF. c is 1Ω, the proportional adjustment coefficient K pv With K pi are 5 and 100 respectively, and the integral time constant K iv With K ii are 50 and 500 respectively, the excitation integral coefficient is 3.33, and the output active power reference value P ref The output reactive power reference value is 100kW, Q ref The mathematical model parameters of the second-order generalized integrator are set as follows: the gain coefficient k is 1, and the positive sequence fundamental angular frequency ω′ is 314 rad / s.

[0178] Based on the mathematical model of the self-synchronous voltage source established in step 1, a grid-connected system simulation model is built. In the case of background harmonics in the grid voltage, the simulation results before the grid voltage feedforward control strategy in steps 3 and 4 is introduced are as follows: Figure 7 As shown in the figure, the grid-connected current quality of the self-synchronous voltage source cannot meet the requirements; the simulation results after introducing the grid voltage feedforward control strategy in steps 3 and 4 are as follows Figure 8 As shown in Figure 3, the grid-connected current quality of the self-synchronous voltage source meets the requirements.

[0179] In a specific implementation, the present application provides a computer storage medium and a corresponding data processing unit, wherein the computer storage medium is capable of storing a computer program that, when executed by the data processing unit, executes the invention disclosure of a method for harmonic suppression in a self-synchronous voltage source grid-connected system based on grid voltage feedforward provided by the present invention, as well as some or all of the steps in each embodiment. The storage medium may be a magnetic disk, an optical disk, a read-only memory (ROM), or a random access memory (RAM).

[0180] Those skilled in the art will clearly understand that the technical solutions in the embodiments of the present invention can be implemented by means of computer programs and their corresponding general hardware platforms. Based on this understanding, the technical solutions in the embodiments of the present invention, in essence or in other words, the part that contributes to the prior art, can be embodied in the form of a computer program, i.e., a software product. The computer program software product can be stored in a storage medium and includes a number of instructions for enabling a device including a data processing unit (which can be a personal computer, server, single-chip microcomputer, MCU, or network device, etc.) to execute the methods described in various embodiments of the present invention or certain parts of the embodiments.

[0181] The present invention provides a concept and method for harmonic suppression in a self-synchronous voltage source grid-connected system based on grid voltage feedforward. There are numerous methods and approaches for implementing this technical solution. The foregoing is merely a preferred embodiment of the present invention. It should be noted that those skilled in the art may make various improvements and modifications without departing from the principles of the present invention, and such improvements and modifications are also within the scope of protection of the present invention. Any components not specified in this embodiment may be implemented using existing technologies.

Claims

1. A method for suppressing harmonics in a self-synchronous voltage source grid-connected system based on grid voltage feedforward, characterized in that: The following steps are involved: Step 1, performing a topological structure analysis on the self-synchronous voltage source grid-connected system and constructing a mathematical model of the self-synchronous voltage source grid-connected system; Step 2: Analyze the influence of the grid voltage background harmonics on the grid-connected current of the self-synchronous voltage source based on the mathematical model constructed in step 1; Step 3: improving the inductor current reference value of the self-synchronous voltage source grid-connected system based on a biquad generalized integrator; Step 4: Improve the grid voltage feedforward control structure of the self-synchronous voltage source grid-connected system to complete the harmonic suppression of the self-synchronous voltage source grid-connected system based on grid voltage feedforward.

2. The method for suppressing harmonics in a self-synchronous voltage source grid-connected system based on grid voltage feedforward according to claim 1, characterized in that: The mathematical model of the self-synchronous voltage source grid-connected system described in step 1 includes: Step 1-1, establish the main circuit equation of the self-synchronous voltage source grid-connected system; Step 1-2, calculate the instantaneous power of the self-synchronous voltage source grid-connected system; Step 1-3, establish the active frequency control equation of the self-synchronous voltage source grid-connected system; Steps 1-4, establishing reactive power and voltage control equations of the self-synchronous voltage source grid-connected system; Step 1-5, establish the voltage loop equation of the self-synchronous voltage source grid-connected system; Steps 1-6: Establish the current loop equation of the self-synchronous voltage source grid-connected system.

3. The method for suppressing harmonics in a self-synchronous voltage source grid-connected system based on grid voltage feedforward according to claim 2, characterized in that: The main circuit equations for establishing the self-synchronous voltage source grid-connected system described in step 1-1 are as follows: Among them, e a 、e b 、e c are the A, B, and C three-phase output voltages of the three-phase inverter bridge in the self-synchronous voltage source grid-connected system, V a 、V b 、V c are the A, B, and C three-phase output voltages of the self-synchronous voltage source grid-connected system, L f is the filter inductor, I La , I Lb , I Lc are the three-phase inductor currents of the self-synchronous voltage source grid-connected system, I oa , I ob , I oc are the A, B, and C three-phase output currents of the self-synchronous voltage source grid-connected system; C f is the filter capacitor, and t is the time.

4. The method for suppressing harmonics in a self-synchronous voltage source grid-connected system based on grid voltage feedforward according to claim 3, characterized in that: The calculation of the instantaneous power of the self-synchronous voltage source grid-connected system described in steps 1-2 is as follows: Among them, P e is the instantaneous value of output active power, Q e is the instantaneous value of output reactive power, V d is the d-axis component of the output voltage, V q is the q-axis component of the output voltage, I d is the d-axis component of the output current, I q is the q-axis component of the output current.

5. The method for suppressing harmonics in a self-synchronous voltage source grid-connected system based on grid voltage feedforward according to claim 4, characterized in that: The active frequency control equations for the self-synchronous voltage source grid-connected system established in steps 1-3 are as follows: Among them, P ref is the output active power reference value, ω is the grid angular frequency, ω n is the rated angular frequency of the power grid, D p is the virtual damping coefficient, J is the virtual moment of inertia, and θ is the phase of the virtual internal potential.

6. The method for suppressing harmonics in a self-synchronous voltage source grid-connected system based on grid voltage feedforward according to claim 5, characterized in that: The reactive power and voltage control equations for the self-synchronous voltage source grid-connected system described in steps 1-4 are as follows: Among them, E m is the amplitude of the virtual internal potential, U is the output voltage, U n is the rated voltage, D q is the reactive voltage droop coefficient, Q ref is the output reactive power reference value, K is the excitation integral coefficient, and s is the complex frequency in Laplace transform.

7. The method for suppressing harmonics in a self-synchronous voltage source grid-connected system based on grid voltage feedforward according to claim 6, characterized in that: The voltage loop equations for establishing the self-synchronous voltage source grid-connected system described in steps 1-5 are as follows: Among them, I dref is the d-axis component of the voltage loop output current, I qref is the q-axis component of the voltage loop output current, V dref is the d-axis component of the output voltage reference value, V qref is the q-axis component of the output voltage reference value, K pv , K iv are the proportional coefficient and integral coefficient of the voltage loop PI controller respectively; The current loop equations for establishing the self-synchronous voltage source grid-connected system described in steps 1-6 are as follows: Among them, V id is the d-axis component of the current loop output voltage, V iq is the q-axis component of the current loop output voltage, I Ld is the d-axis component of the inductor current, I Lq is the q-axis component of the inductor current, K pi , K ii They are the proportional coefficient and integral coefficient of the current loop PI controller respectively.

8. The method for suppressing harmonics in a self-synchronous voltage source grid-connected system based on grid voltage feedforward according to claim 7, characterized in that: The analysis of the impact of grid voltage background harmonics on the grid-connected current of the self-synchronous voltage source described in step 2 includes: Step 2-1, the self-synchronous voltage source grid-connected current i g The calculation method is as follows: Among them, L f is the filter inductor, C f is the filter capacitor, R c is the resistance value of the filter capacitor, i Lref is the inductor current reference value, u g is the grid voltage, s is the complex frequency in Laplace transform, where the function P(s) is expressed as follows: Among them, K pwm is the PWM modulation coefficient, is the voltage and current signal acquisition delay and PWM modulation delay transfer function, T s is the switching period of the inverter bridge circuit; Step 2-2, inductor current reference value i Lref , the calculation method is as follows: Among them, e is the output voltage reference value of the sinusoidal wave voltage generation link.

9. The method for suppressing harmonics in a self-synchronous voltage source grid-connected system based on grid voltage feedforward according to claim 8, characterized in that: The step 3 of improving the inductor current reference value of the self-synchronous voltage source grid-connected system includes: Step 3-1, establish the mathematical model of the second-order generalized integrator, as follows: Where v(s) is the input AC signal of the second-order generalized integrator; v′(s) and qv′(s) are the filtered output signal and the quadrature output signal of the second-order generalized integrator, respectively; k is the gain coefficient; ω′ is the positive sequence fundamental angular frequency; and D(s) is the filtered output signal v ′ (s) is the transfer function between the input AC signal v(s), Q(s) is the transfer function between the orthogonal output signal qv′(s) and the input AC signal v(s); Step 3-3: Set the three-phase grid voltage v at the point of common coupling a 、v b 、v c After transformation, the α-axis component v in the αβ coordinate system is obtained α With the β-axis component v β ; The α-axis component v of the grid voltage α With the β-axis component v β After being input into the second-order generalized integrator respectively, the high-order harmonic components in the grid voltage v are filtered out to obtain the filtered output signal; Step 3-4, after filtering out the high-order harmonics, the α-axis component v of the filtered output signal is α ′ The β-axis component qv′ of the quadrature output signal β Make a difference, and filter the β-axis component v′ of the output signal β The α-axis component qv′ of the quadrature output signal α Make a difference, filter out the fundamental negative sequence component of the grid voltage, and extract the fundamental positive sequence component u of the grid voltage g and is used to calculate the inductor current reference value.

10. The method for suppressing harmonics in a self-synchronous voltage source grid-connected system based on grid voltage feedforward according to claim 9, characterized in that: The grid voltage feedforward control structure of the self-synchronous voltage source grid-connected system is improved in step 4, that is, grid voltage feedforward path function A and grid voltage feedforward path function B are introduced before and after the current loop of the self-synchronous voltage source grid-connected system, respectively, to eliminate the influence of grid voltage background harmonics on inductor current and capacitor current, respectively. The specific method includes: Grid-connected current i of self-synchronous voltage source with grid voltage feedforward g The mathematical expression is: Among them, the calculation method of function E(s) and function F(s) is: Let function E(s) and function F(s) be 0, and the feedforward path function A and feedforward path function B are obtained as follows: When background harmonics exist in the grid voltage, the grid-connected current after improving the grid voltage feedforward control structure of the self-synchronous voltage source grid-connected system is: The harmonic suppression of the self-synchronous voltage source grid-connected system based on grid voltage feedforward is completed.

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