A Method for Resonance Suppression of Photovoltaic Inverter Clusters Based on Capacitor Voltage Feedforward

Abstract

The present invention discloses a method for suppressing resonance in a photovoltaic inverter cluster based on capacitor voltage feedforward, including: establishing a control structure and a mathematical model of an original grid-connected system of a photovoltaic inverter cluster; introducing a capacitor voltage feedforward loop into the control structure of the original grid-connected system of the photovoltaic inverter cluster, and introducing a high-pass filter into the capacitor voltage feedforward loop to obtain an improved control structure of the grid-connected system of the photovoltaic inverter cluster; determining the open-loop and closed-loop transfer functions of the improved control structure of the grid-connected system of the photovoltaic inverter cluster in the discrete domain; selecting the parameters involved in the high-pass filter based on the pole diagrams of the open-loop and closed-loop transfer functions to reduce the influence of the high-pass filter on high-frequency components and suppress low-frequency components in the capacitor voltage feedforward loop. The present invention can simply and effectively suppress grid voltage harmonics and inverter self-harmonics.

Description

Technical Field

[0001] The present invention relates to the technical field of grid-connected systems for photovoltaic inverter clusters, and more specifically, to a method for suppressing resonance in a photovoltaic inverter cluster based on capacitor voltage feedforward. Background Art

[0002] Due to the convenience of renewable energy generation and the flexibility of grid connection, its proportion in the total national power generation is increasing continuously, including relatively dispersed small-scale distributed generation and highly efficient and centralized medium and large-scale power plants. Since the locations of renewable energy generation are relatively dispersed, long transmission lines are required to connect the system to the public grid. Grid-connected inverters are the main equipment for connecting renewable energy to the grid. However, in order to attenuate the high-frequency harmonics caused by the inverter, a filter is usually required. Compared with a simple L filter, since the LCL filter is smaller in size and has stronger high-frequency harmonic attenuation ability, the latter is more advantageous when considering cost and performance comprehensively. However, without proper damping, the self-resonance problem of the LCL filter will threaten the system stability. Existing methods do not consider the situation of inverter clusters, and the feedback of multiple variables increases the cost of the measurement sensor part in the system.

[0003] Therefore, how to simply and effectively suppress the grid voltage harmonics and the self-resonance of the inverter in the grid-connected system of a photovoltaic inverter cluster is an urgent problem to be solved by those skilled in the art. Summary of the Invention

[0004] In view of this, the present invention provides a method for suppressing resonance in a photovoltaic inverter cluster based on capacitor voltage feedforward, which can simply and effectively suppress the grid voltage harmonics and the self-harmonics of the inverter.

[0005] In order to achieve the above object, the present invention adopts the following technical solutions:

[0006] A method for suppressing resonance in a photovoltaic inverter cluster based on capacitor voltage feedforward includes:

[0007] Establishing the control structure and mathematical model of the original grid-connected system of a photovoltaic inverter cluster;

[0008] Introducing a capacitor voltage feedforward loop into the control structure of the original grid-connected system of a photovoltaic inverter cluster, and introducing a high-pass filter into the capacitor voltage feedforward loop to obtain an improved control structure of the grid-connected system of a photovoltaic inverter cluster;

[0009] Determining the open-loop and closed-loop transfer functions of the improved control structure of the grid-connected system of a photovoltaic inverter cluster in the discrete domain;

[0010] Select the parameters involved in the high-pass filter based on the pole diagrams of the open-loop and closed-loop transfer functions to reduce the influence of the high-pass filter on high-frequency components and suppress the low-frequency components in the capacitor voltage feed-forward loop.

[0011] Further, in the above method for suppressing resonance in a photovoltaic inverter cluster based on capacitor voltage feed-forward, the construction process of the mathematical model of the original photovoltaic inverter cluster grid-connected system is as follows:

[0012] Determine the mathematical model of the original photovoltaic inverter cluster grid-connected system in the frequency domain after neglecting parasitic inductance:

[0013] v i_abc (s) = sL1i i_abc (s) + v C_abc (s)

[0014] v C_abc (s) = sL x i g_abc (s) + v g_abc (s)

[0015] i i_abc (s) = sC f v C_abc (s) + i g_abc (s)

[0016] L x = L2 + L g

[0017] Where, v i_abc (S), i i_abc (S) are the output phase voltage and current of the inverter; v g_abc (S), i g_abc (S) are the grid-side phase voltage and current; v C_abc (S) is the voltage of the filter capacitor C f ; L2 is the grid-side inductor of the LCL filter in the photovoltaic inverter cluster grid-connected system; L g is the grid-side impedance;

[0018] Obtain the mathematical model of the original photovoltaic inverter cluster grid-connected system in the αβ coordinate system through Clark transformation:

[0019]

[0020] Further, in the above method for suppressing resonance in a photovoltaic inverter cluster based on capacitor voltage feed-forward, the process of introducing a high-pass filter into the capacitor voltage feed-forward loop includes:

[0021] In the capacitor voltage feed-forward loop, introduce a high-pass filter in the frequency domain;

[0022] The high-pass filter after discretization is obtained by the Tustin discretization method.

[0023] Furthermore, in the above method for suppressing resonance in a photovoltaic inverter cluster based on capacitor voltage feedforward, the expression of the high-pass filter in the frequency domain is:

[0024]

[0025] where H is the gain coefficient of the high-pass filter, ω c is the cut-off frequency of the high-pass filter, and s is the variable symbol of the function in the frequency domain.

[0026] Furthermore, in the above method for suppressing resonance in a photovoltaic inverter cluster based on capacitor voltage feedforward, the expression of the high-pass filter after discretization is:

[0027]

[0028] where T s is the control period, and z represents the variable symbol of the function in the discrete domain.

[0029] Furthermore, in the above method for suppressing resonance in a photovoltaic inverter cluster based on capacitor voltage feedforward, the value range of the cut-off frequency ω c of the high-pass filter is:

[0030] 0.5ω res_min ≤ ω c ≤ 0.7ω res_min ;

[0031] where ω res_min is the minimum resonance frequency of the LCL filter considering the grid-side inductor L g .

[0032] Furthermore, in the above method for suppressing resonance in a photovoltaic inverter cluster based on capacitor voltage feedforward, the gain coefficient H of the high-pass filter is determined by the pole diagram and its value is 0.5.

[0033] Furthermore, in the above method for suppressing resonance in a photovoltaic inverter cluster based on capacitor voltage feedforward, the expressions of the open-loop and closed-loop transfer functions of the improved control structure of the photovoltaic inverter cluster grid-connected system are:

[0034]

[0035]

[0036] where, ω resis the resonant angular frequency of the LCL filter in the grid-connected system of the photovoltaic inverter cluster, and its expression is:

[0037] As can be seen from the above technical solutions, compared with the prior art, the present invention discloses a method for suppressing resonance of a photovoltaic inverter cluster based on capacitor voltage feedforward. First, the system mathematical model and control structure are described in the discrete domain; then, in view of the problem that the inverter-side current is difficult to control and causes resonance, a capacitor voltage feedforward loop is introduced, and stability analysis is carried out through the pole diagram. Finally, a high-pass filter is introduced into the capacitor voltage feedforward loop, and two parameters ωc and H in the high-pass filter are selected by constructing the open-loop and closed-loop transfer functions of the system, so as to reduce the influence of the high-pass filter on high-frequency components while suppressing the low-frequency components in the capacitor voltage feedforward. Finally, the grid voltage harmonics and the resonance of the inverter itself are effectively suppressed. Description of the Drawings

[0038] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are only the embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained according to the provided drawings without creative efforts.

[0039] Figure 1 is the grid-connected system of the photovoltaic inverter cluster;

[0040] Figure 2 is the control block diagram of the grid-connected system after introducing the capacitor voltage feedforward loop and the high-pass filter;

[0041] Figure 3 is the simplified control block diagram of the grid-connected system in the discrete domain;

[0042] Figure 4 is the pole diagram of the ICF control closed-loop transfer function; (a) is when the CVF is not added, and (b) is when the CVF is added; ICF is the inverter-side current feedback; CVF is the capacitor voltage feedforward;

[0043] Figure 5 is the pole diagram of the ICF control closed-loop transfer function with the HPF and CVF added; (a) H = 1, (b) H = 0.75, (c) H = 0.5, (d) H = 0.25; HPF is the high-pass filter;

[0044] Figure 6 is Iref Transfer function diagram of the improved grid-connected system of photovoltaic inverters in the frequency domain when = 0;

[0045] Figure 7 is L g Bode diagrams of G(s) under different CVF forms when = 0 μH and 800 μH; vgig (s);

[0046] Figure 8 Pole diagram of the closed-loop transfer function of the ICF control; (a) is adding CVF; (b) is adding HPF and CVF;

[0047] Figure 9 Control structure of the simulation model;

[0048] Figure 10 is L g Starting current waveform of the inverter when = 0 μH; (a) is the capacitor voltage using only high-pass filtering, (b) is the capacitor voltage using high-pass filtering and the fundamental frequency component of the capacitor voltage for compensation;

[0049] Figure 11 Current waveforms on the inverter side and grid side without adding CVF; in (a) and (b), L g = 0 μH, in (c) and (d), L g = 800 μH;

[0050] Figure 12 is L g Grid-side current simulation waveform when = 800 μH; (a) is the unit CVF used in the ICF control, (b) is the HPF and CVF used in the ICF control;

[0051] Figure 13 is L g Grid-side current waveform under the condition of grid voltage distortion when = 800 μH; (a) is the unit CVF used in the ICF control, (b) is the HPF and CVF used in the ICF control;

[0052] Figure 14 is L g FFT analysis of the simulation waveform under the condition of grid voltage distortion when = 800 μH; (a) is the unit CVF used in the ICF control, (b) is the HPF and CVF used in the ICF control. Specific implementation manners

[0053] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0054] An embodiment of the present invention discloses a method for suppressing resonance in a photovoltaic inverter cluster based on capacitor voltage feedforward, including:

[0055] S1. Establish the control structure and mathematical model of the original photovoltaic inverter cluster grid-connected system;

[0056] S2. Introduce a capacitor voltage feedforward loop into the control structure of the original photovoltaic inverter cluster grid-connected system, and introduce a high-pass filter into the capacitor voltage feedforward loop to obtain an improved control structure of the photovoltaic inverter cluster grid-connected system;

[0057] S3. Determine the open-loop and closed-loop transfer functions of the improved control structure of the photovoltaic inverter cluster grid-connected system in the discrete domain;

[0058] S4. Select the parameters involved in the high-pass filter based on the pole diagrams of the open-loop and closed-loop transfer functions to reduce the influence of the high-pass filter on high-frequency components and suppress the low-frequency components in the capacitor voltage feedforward loop.

[0059] The above steps will be further described below.

[0060] In S1, as Figure 1 shown, the original photovoltaic inverter cluster grid-connected system is a grid-connected system in which a single inverter is connected to an LCL filter, where L1, L2, C f , L g are the inverter-side inductor, grid-side inductor, grid-side capacitor, and grid-side impedance of the LCL filter respectively. The resistive component in a weak grid can increase the damping of the system and improve the system stability. To verify the suppression ability of the suppression method of the present invention in the most severe case of system resonance, it is assumed that the weak grid is purely inductive, that is, the impedance only contains L g . Therefore, ignoring the parasitic inductance and applying Kirchhoff's law and Laplace transform to Figure 1 , the frequency-domain mathematical model of the system can be expressed as:

[0061] v i_abc (s) = sL1i i_abc (s) + v C_abc (s)

[0062] v C_abc (s) = sL x i g_abc(s) + v g_abc (s)

[0063] i i_abc (s) = sC f v C_abc (s) + i g_abc (s)

[0064] L x = L2 + L g

[0065] In the formula, v i_abc , i i_abc are the output phase voltage and current of the inverter; v g_abc , i g_abc are the grid-side phase voltage and current; v c_abc , i c_abc are respectively the voltage and current of the filter capacitor C f .

[0066] The present invention adopts a stationary αβ coordinate system, and the original mathematical model of the grid-connected system of the photovoltaic inverter cluster in its coordinate system obtained through Clarke transformation is:

[0067] v i_αβ (s) = sL1i i_αβ (s) + v C_αβ (s)

[0068] v C_αβ (s) = sL x i g_αβ (s) + v g_αβ (s)

[0069] i i_αβ (s) = sC f v C_αβ (s) + i g_αβ (s).

[0070] In S2, as Figure 2 shown, it is the control block diagram of the grid-connected system after introducing the capacitor voltage feed-forward loop and the high-pass filter, that is, the improved control block diagram of the photovoltaic inverter cluster grid-connected system. Among them, G c (z) is the discrete current controller on the inverter side. G vf (z) is the function of the CVF part; z -1 is the digital delay for one-cycle calculation; the zero-order holder (Zero-Order Holder, ZOH) is PWM modulation. In addition, Figure 2 the grid voltage v g may contain low-frequency harmonics in addition to the fundamental frequency component.

[0071] The most common form of CVF is unit voltage feedforward, where G vf (z) = 1. However, the dynamic performance of the unit CVF in a weak grid and its ability to suppress low-frequency grid voltage harmonics are poor. Therefore, the present invention proposes adding an HPF to the CVF loop, and the expression of the HPF in the frequency domain is:

[0072]

[0073] where H and ω c are the gain coefficient and cut-off frequency of the HPF.

[0074] According to the Tustin discretization method, the discretized expression of the HPF can be obtained as:

[0075]

[0076] where T s is the control period, and z represents the variable symbol of the function in the discrete domain.

[0077] In S3, since the resonance frequency of the LCL filter is usually very high, the stability of the ICF control at the resonance frequency is very sensitive to digital delay. To ensure the correctness of the stability analysis, the accuracy of the system modeling in the high-frequency region must be guaranteed. Therefore, the Figure 2 transfer function model of the system in should be converted to the discrete domain as shown in Figure 3 , where the grid voltage v g can be temporarily ignored.

[0078] Through the z-transform method, the continuous part in the hybrid system based on ZOH can be accurately discretized. Therefore, the discrete transfer functions G Figure 3 (z) and G ii (z) and G vc (z) in can be derived as:

[0079]

[0080]

[0081] In the formula, Gii(z) and Gvc(z) are obtained through Figure 2 for simplification only and have no practical significance; ω res is the resonance angular frequency of the LCL filter, and its expression is:

[0082]

[0083] The open-loop and closed-loop transfer functions of the improved grid-connected system control structure of the photovoltaic inverter cluster in the discrete domain are respectively derived as:

[0084]

[0085]

[0086] In S4, in order to select the parameters involved in the high-pass filter, first, in Figure 4 the polar plots of the closed-loop transfer function T cl (z) of two different CVF forms are plotted, where G c (z) is defined as a constant (G c (z) = 1.85), and the gate inductor L g varies from 0 to 2000 μH in sequence. To represent the grid inductor per unit change, the inverter-side inductor of the LCL filter is selected as the reference, so that the grid inductor per unit represented by L g can be represented by the ratio of L g_pu to L1. In Figure 4 , (a) represents the case without adding CVF (G vf (z) = 0), and (b) represents the case of adding a unit CVF (G vf (z) = 1), where L g_pu increases from 0 to 5.

[0087] Figure 4 In (a), without CVF, it is very difficult for the ICF control loop to be stable unless the grid inductor is very large. When a unit CVF is adopted in Figure 4 (b), the stability of the system is greatly improved, and all poles are inside the unit circle. However, as can be observed from Figure 4 (b), there are high-frequency and low-frequency poles in the pole plot of the closed-loop transfer function. In the case of a unit CVF, although the high-frequency poles are inside the unit circle, as the grid inductor increases, the low-frequency poles gradually move towards the edge of the unit circle. That is to say, as the value of the grid inductor increases, the stability of the system in the low-frequency range becomes worse, although the high-frequency stability becomes better. Therefore, it can be seen that the unit CVF exhibits good active damping performance, but when the grid inductor is large, it is necessary to take measures to avoid low-frequency oscillations.

[0088] Actually, voltage feedforward in the case of a weak grid is very likely to cause low-frequency oscillations in the output current of the inverter. As shown in Figure 4 (a), although it is unstable at high frequencies, the low frequency is in a very safe position. Therefore, in order to maintain the advantages of the CVF in the high-frequency range and eliminate the disadvantages in the low-frequency range, an HPF is added to the CVF loop.

[0089] The HPF has two parameters ω cand H. To reduce the influence of the HPF on high-frequency components and simultaneously attempt to completely suppress the low-frequency components in the CVF, the cut-off frequency ω of the HPF can be designed as: c Designed as:

[0090] 0.5ω res_min ≤ω c ≤0.7ω res_min

[0091] where ω res_min represents the minimum resonance frequency of the LCL filter considering the grid inductance L g , and ω res_min can be calculated by the following formula:

[0092]

[0093] Assuming L g_max = 800 μH, ω c can be determined as 6280 rad / s.

[0094] Regarding the parameter H, too high an H will cause low-frequency oscillations, while too small an H will result in insufficient suppression of high-frequency LCL resonance. Therefore, the final value of H can be determined by the pole diagram under actual parameters. When the HPF and CVF are adopted, the pole diagrams of T cl (z) with different H are as shown in Figure 5 . It can be seen from Figure 5 (a) to (d) that the optimal value of H is 0.5, which can ensure good stability margins for the system in both low-frequency and high-frequency ranges. The design process of the HPF parameters can be achieved simply by observing the Matlab diagrams.

[0095] Finally, comparing Figure 4 and Figure 5 (c), it is obvious that adding the HPF and CVF can not only ensure effective resonance suppression at the resonance frequency of the LCL filter, but also avoid low-frequency oscillations in a weak grid.

[0096] To further verify the resonance suppression effect of the present invention, the following experiment is carried out.

[0097] In actual three-phase applications, the grid voltage will contain some low-order harmonics, such as the 5th, 7th, 11th, 13th, 17th, and 19th harmonics. These harmonics cannot be ignored, especially for grid-connected inverters controlled by ICF. Since the control objective of the ICF is the inverter-side current, the harmonic currents generated by the grid voltage harmonics can freely flow through the capacitor in the LCL, resulting in serious harmonic pollution of the grid-side current.

[0098] The method proposed by the present invention studies the suppression of power grid voltage harmonics, and the main purpose of the study is the low-order harmonics in the power grid voltage. Therefore, the analysis process is not very sensitive to digital delay. Of course, discrete-domain analysis can also be applied to this part, but the disadvantage of the discrete-domain analysis method is that the discretization process of the transfer function is very complex. To reduce the complex derivation process, the corresponding analysis is carried out in the frequency domain. Therefore, Figure 2 can be simplified to Figure 6 . Assume I ref = 0, and Figure 6 The transfer function G d (s) in represents the calculation and ZOH delay and cannot be ignored. Otherwise, derivation errors will occur. G d (s) can be expressed according to the Padé approximation as:

[0099]

[0100] where T d = 1.5T s .

[0101] Based on Figure 6 , the transfer function G g from v g to i vgig (s) can be derived as:

[0102]

[0103] Before discussing the suppression ability of G vgig (s), the form of the current controller G c (s) needs to be explained first.

[0104] Since a fixed αβ reference frame is adopted, the quasi-proportional-resonant current controller is preferred. The design of the QPR controller is independent because the QPR controller is close to the fundamental frequency and hardly affects the damping of the LCL resonance. The QPR controller function for controlling the inverter-side current is:

[0105]

[0106] where K p is the overall gain of G c (s); K r is the fundamental-frequency gain; ω0 is the power grid angular frequency. In fact, the power grid frequency f i usually fluctuates within 0.5 Hz relative to the standard value. Therefore, ω i = 2πf i . h is the harmonic order to be suppressed; K rh is the gain at the harmonic frequency; is the phase angle to be compensated. In Gc (s) The reason for adding the 5th and 7th harmonic controllers is to suppress the 5th and 7th harmonics in the grid voltage and compensate for the zero-crossing distortion of the current waveform caused by the PWM dead zone.

[0107] Ideally: If G vf (s) meets the conditions of the following formula, the harmonics of the grid voltage can be completely suppressed. However, the conditions in the following formula include the first derivative and the second derivative, which are impractical in a digital control system.

[0108]

[0109] The influence of grid voltage harmonics can also be effectively reduced by adding an HPF in the CVF loop. The principle of this kind of method is simple, but it has good performance. To clarify the effectiveness of the proposed method, the Bode plot of G vgig (s) is as Figure 7 shown.

[0110] In Figure 7 , two different L g values (L g = 0 and 800 μH) are considered, and three different CVF forms are compared, which are unit CVF, HPF & CVF, and CVF. When L g = 0 μH, the unit CVF method shows good grid voltage suppression in the low-frequency range below 350 Hz, but the suppression ability becomes poor near 950 Hz, and there is an obvious resonance peak at 950 Hz. Generally, the maximum value of this resonance peak is still below the 0 dB line, so it still has a certain inhibitory effect on the grid voltage harmonics. For HPF & CVF, the resonance peak of G vgig (s) at 950 Hz can be completely eliminated. Therefore, compared with the unit CVF, HPF & CVF has a better inhibitory effect on the 5th to 19th grid voltage harmonics. However, in the frequency band below 350 Hz, the ability of HPF & CVF to suppress the grid voltage harmonics is not very strong, but the 5th and 7th harmonic controllers in G c (s) can make up for this deficiency.

[0111] When L g = 800 μH, when using the unit CVF method, the inhibitory ability of G vgig (s) on the grid voltage harmonics becomes weak. It can be seen from Figure 7 that G vgigThe resonant peak of (s) drops to 550 Hz. More seriously, the ability of the unit CVF to suppress grid voltage harmonics is basically non-existent, and it may even amplify grid voltage harmonics. However, even in the case of a large grid inductance, the method based on HPF and CVF also exhibits a stable ability to suppress grid voltage harmonics.

[0112] In Figure 8 the pole diagrams of the closed-loop transfer function of the ICF control under varying grid inductances for HPF and CVF, and CVF were compared, where the first-order differential in CVF was discretized by the backward Euler method. As the grid inductance increases, the poles of the CVF method gradually approach the edge of the unit circle, indicating that the stability of the ICF control gradually weakens. Therefore, the method proposed in the present invention, which introduces HPF and CVF, is more suitable for weak grids.

[0113] The present invention also uses a three-phase grid-connected two-stage inverter simulation model based on Matlab / Simulink to verify the controller design of the method of the present invention. The control structure of the simulation model is as Figure 9 shown. In Figure 9 another important function of the voltage feedforward is to prevent inrush current during the start-up of the inverter PWM. If the HPF and CVF methods are directly adopted, the low-frequency components in the capacitor voltage will be overly suppressed, resulting in a small amplitude of the feedforward voltage, which will lead to inrush current. Therefore, additional compensation measures must be taken to prevent inrush current during inverter start-up. Figure 9 The method adopted in

[0114] is to divide HPF and CVF into two parts. The first part is the capacitor voltage after high-pass filtering, and the second part is the fundamental frequency component of the capacitor voltage. Figure 9 The fundamental frequency component can be obtained by DDSRF-PLL. DDSRF-PLL can be regarded as a PLL plus a pre-filter, as Figure 9 shown. The capacitor voltage filtered by the pre-filter can be approximately regarded as a pure fundamental frequency component, which is only used to prevent inrush current and does not affect other characteristics of the ICF control. Therefore, according to

[0115] the CVF structure in

[0116] When SW is in position 1, it means there is no CVF. When SW is in position 2, it means HPF and CVF; when SW is in position 3, it means unit CVF. Figure 9 In the simulation, the peak value of the grid phase voltage is 155 V, and the DC side voltage of the inverter is 320 V. When the peak value of the current reference command rises from 0 A to 28 A, the dynamic process of the waveform will be recorded. Figure 10 To further illustrate Figure 10(a) indicates the use of only the high-pass filtered capacitor voltage, while Figure 10 (b) indicates the use of the high-pass filtered capacitor voltage and the fundamental frequency component compensation of the capacitor voltage.

[0117] In Figure 10 , at 0.02 s, the inverter control starts and the current reference command is 0 A. If only the high-pass filtered capacitor voltage is applied, there will be a huge inrush current (close to 80 A) during startup, which is very dangerous. However, if the voltage feedforward structure in Figure 9 (SW is in position 2) is adopted, the inrush current can be almost completely eliminated, as shown in Figure 10 (b). In Figure 11 , when the grid inductor is 0 or 800 μH, the ICF control without CVF is tested.

[0118] It can be seen from Figure 11 that when CVF is not added, the ICF control cannot maintain stability regardless of the grid inductor value. However, when L g = 800 μH, the current waveform is more stable, which is consistent with the analysis in Figure 4 (a).

[0119] In Figure 12 (a), the unit CVF is used in the ICF control. Comparing Figure 11 (c) and Figure 12 (a), when L g = 800 μH, the unit CVF significantly improves the stability of the grid-side current. However, the unit CVF also brings another problem. When the inverter controller starts working at 0.05 s. In Figure 12 (a), there is an obvious oscillation process in the virtual coil. Because as shown in Figure 4 (b), the voltage feedforward causes low-frequency oscillations in the weak grid. To solve this problem, HPFCVF is adopted in Figure 12 (b). It can be clearly seen from Figure 12 (a) and 12(b) that HPF CVF can effectively reduce the oscillation during the grid-side current step process, which is beneficial to the stability of the system.

[0120] To verify the grid voltage harmonic suppression ability of the proposed method, in Figure 13 , 1% of the 5th and 11th harmonic voltages are added to the grid voltage v g .

[0121] Obviously, when the unit CVF is used, there is serious harmonic pollution in the grid-side current in Figure 13 (a), but when the HPF and CVF methods are used, the total harmonic distortion current on the grid side is greatly reduced. In addition, in Figure 14In Figure 13 the grid-side current simulation waveform was subjected to fast Fourier transform analysis. Figure 14 (a) is the FFT result of the unit CVF method, Figure 14 (b) is the FFT result of the HPF and CVF methods.

[0122] It can be seen that by using the method of introducing HPF and CVF for resonance suppression, the THD of the grid-side current can be reduced from 5.55% to 1.74%, and the suppression of the 11th harmonic is particularly obvious. Therefore, the method of the present invention can improve the grid voltage harmonic suppression ability of ICF control.

[0123] In this specification, the various embodiments are described in a progressive manner. Each embodiment focuses on the differences from other embodiments. For the same or similar parts among the various embodiments, reference can be made to each other. For the devices disclosed in the embodiments, since they correspond to the methods disclosed in the embodiments, the description is relatively simple. For the relevant parts, reference can be made to the description in the method part.

[0124] The above description of the disclosed embodiments enables those skilled in the art to implement or use the present invention. Various modifications to these embodiments will be obvious to those skilled in the art. The general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to these embodiments shown herein, but rather will be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for suppressing resonance in a photovoltaic inverter cluster based on capacitor voltage feedforward, characterized in that Including: Establish the control structure and mathematical model of the original photovoltaic inverter cluster grid-connected system; Introduce a capacitor voltage feedforward loop into the control structure of the original photovoltaic inverter cluster grid-connected system, and introduce a high-pass filter into the capacitor voltage feedforward loop to obtain an improved control structure of the photovoltaic inverter cluster grid-connected system; Determine the open-loop and closed-loop transfer functions of the improved control structure of the photovoltaic inverter cluster grid-connected system in the discrete domain; Select the parameters involved in the high-pass filter based on the pole diagrams of the open-loop and closed-loop transfer functions to reduce the influence of the high-pass filter on high-frequency components and suppress low-frequency components in the capacitor voltage feedforward loop; The original photovoltaic inverter cluster grid-connected system is a grid-connected system where a single inverter is connected to an LCL filter. The construction process of its mathematical model is as follows: Determine the mathematical model of the original photovoltaic inverter cluster grid-connected system in the frequency domain after ignoring parasitic inductance: v i_abc (s) = sL1i i_abc (s) + v C_abc (s) v C_abc (s) = sL x i g_abc (s) + v g_abc (s) i i_abc (s) = sC f v C_abc (s) + i g_abc (s) L x = L2 + L g Among them, v i_abc (S), i i_abc (S) are the output phase voltage and current of the inverter; v g_abc (S), i g_abc (S) are the grid-side phase voltage and current; v C_abc (S) is the voltage of the filter capacitor C f ; L2 is the grid-side inductor of the LCL filter in the PV inverter cluster grid-connected system; L g is the grid-side impedance; Obtain the mathematical model of the original photovoltaic inverter cluster grid-connected system in the αβ coordinate system through Clark transformation:

2. A method for suppressing resonance of a photovoltaic inverter cluster based on capacitor voltage feedforward according to claim 1, characterized in that, The process of introducing a high-pass filter into the capacitor voltage feedforward loop includes: Introduce a high-pass filter in the frequency domain into the capacitor voltage feedforward loop; Obtain the discretized high-pass filter through the Tustin discretization method.

3. A method for suppressing resonance of a photovoltaic inverter cluster based on capacitor voltage feedforward according to claim 2, characterized in that The expression of the high-pass filter in the frequency domain is: where H is the gain coefficient of the high-pass filter, ω c is the cut-off frequency of the high-pass filter, and s is the variable symbol of the function in the frequency domain.

4. A method for suppressing resonance in a photovoltaic inverter cluster based on capacitor voltage feedforward according to claim 3, characterized in that, The expression of the discretized high-pass filter is: Among them, T s is the control period, and z represents the variable symbol of the function in the discrete domain.

5. A method for suppressing resonance in a photovoltaic inverter cluster based on capacitor voltage feedforward according to claim 4, characterized in that, The cut-off frequency ω of the high-pass filter c has a value range of: 0.5ω res_min ≤ω c ≤0.7ω res_min ; Among them, ω res_min is the minimum resonance frequency of the LCL filter considering the grid-side inductor L g .

6. A method for suppressing resonance of a photovoltaic inverter cluster based on capacitor voltage feedforward according to claim 4, characterized in that, The gain coefficient H of the high-pass filter is determined by the pole diagram and takes a value of 0.

5.

7. A method for suppressing resonance in a photovoltaic inverter cluster based on capacitor voltage feedforward according to claim 4, characterized in that, The expressions of the open-loop and closed-loop transfer functions of the improved control structure of the photovoltaic inverter cluster grid-connected system are: Among them, ω res is the resonant angular frequency of the LCL filter in the grid-connected system of a photovoltaic inverter cluster, and its expression is:

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