Low harmonic strong stability control method and system for photovoltaic inverter under weak grid conditions

Abstract

This invention provides a method and system for controlling low-harmonic strong stability of photovoltaic inverters under weak grid conditions. The method involves acquiring the grid-connected current at the grid connection point of the grid-connected inverter system and obtaining a reference value for the grid-connected current. The controller generates grid-connected current control information based on the grid-connected current and its reference value. The grid-connected current is multiplied by an impedance reshaping stage and fed back to the controller's output, causing a virtual impedance to be connected in series with the inverter's output impedance. This virtual impedance generates virtual resistance at the resonant frequency to suppress low-frequency harmonics in the grid-connected current. By connecting a virtual impedance in series with the inverter's output impedance, this invention achieves impedance reshaping, reducing the impact of background harmonics and low-frequency harmonics caused by small capacitors on the grid-connected current, effectively suppressing low-frequency harmonics in the grid-connected current.

Description

Technical Field

[0001] This invention belongs to the field of stability control, specifically relating to a method and system for controlling the low harmonics and strong stability of a photovoltaic inverter under weak grid conditions. Background Technology

[0002] The statements in this section merely provide background information related to the present invention and do not necessarily constitute prior art.

[0003] With the large-scale integration of new energy sources into the power grid and their increasing penetration rate, remote areas at the end of the grid experience higher grid impedance, exhibiting weak grid characteristics with low short-circuit ratios. Under weak grid conditions, background harmonics in the grid voltage interact with the grid impedance, increasing the harmonic content in the grid-connected current. Furthermore, in practice, to reduce system costs, the DC-side capacitor value of three-level inverters is typically small, especially when using discontinuous pulse width modulation (DPWM), resulting in severe fluctuations in the midpoint voltage. These midpoint voltage fluctuations also contribute to low-frequency harmonics in the grid-connected current. However, LCL filters have low impedance in the low-frequency range, making them less effective at suppressing low-frequency harmonics in the grid-connected current. In addition to current harmonic issues, the grid-connected inverter system is also affected by the negative impedance characteristics of the phase-locked loop (PLL), leading to a significant reduction in the system's stability margin.

[0004] In summary, LCL-type DC-side small-capacitor inverters in weak grid environments face two major challenges: current harmonics and stability. To address the current harmonic problem, point-of-connection (PCC) voltage feedforward is widely used. However, the positive feedback loop introduced by PCC voltage feedforward couples with the grid impedance, introducing a phase lag element that significantly reduces the system's phase margin. Furthermore, PCC voltage feedforward does not suppress low-frequency harmonics induced by the small capacitor. To address the reduced system stability margin caused by the phase-locked loop (PLL), some literature proposes virtual impedance reshaping. By reshaping the inverter's output impedance, the amplitude and phase angle of the inverter's equivalent output impedance are increased, thereby improving the system's stability margin. However, this method has insufficient suppression capability for grid-connected current harmonics.

[0005] Therefore, existing literature only addresses one of the current harmonic and stability issues faced by LCL-type DC-side small capacitor inverters under weak grid conditions, making it difficult to solve both simultaneously. Thus, researching a low-harmonic, high-stability control method for photovoltaic inverters under weak grid conditions is of great significance. Summary of the Invention

[0006] To address the aforementioned problems, this invention proposes a method and system for controlling low-harmonic strong stability of photovoltaic inverters under weak grid conditions. This invention achieves impedance reshaping by connecting a virtual impedance in series with the inverter's output impedance, thereby reducing the impact of background harmonics and low-frequency harmonics caused by small capacitors on the grid-connected current and effectively suppressing low-frequency harmonics in the grid-connected current.

[0007] According to some embodiments, the present invention adopts the following technical solution:

[0008] A method for controlling the low harmonic distortion and strong stability of a photovoltaic inverter under weak grid conditions includes the following steps:

[0009] The grid-connected inverter system obtains the grid-connected current at the grid connection point and obtains the grid-connected current reference value. The controller generates grid-connected current control information based on the grid-connected current and its reference value.

[0010] The grid-connected current is multiplied by the impedance reshaping stage and fed back to the output of the controller, so that a virtual impedance is connected in series with the output impedance of the inverter. The virtual impedance is used to generate virtual resistance at the resonant frequency to suppress low-frequency harmonics in the grid-connected current.

[0011] As an alternative implementation method, a phase-locked loop is used to collect the grid connection point voltage and obtain synchronization phase information to obtain a grid connection current reference value.

[0012] As an alternative implementation, the controller is a PI controller.

[0013] As an alternative implementation, the virtual impedance is formed by a resistor R connected in series with an LC parallel circuit. The resistor R exhibits positive resistance characteristics and is used to increase the amplitude and phase angle of the inverter's equivalent output impedance, thereby improving the system's stability margin.

[0014] As a further step, the LC parallel circuit is connected in series with the inverter output impedance, generating a virtual resistance characteristic at the resonant frequency, making the inverter's equivalent output impedance infinite.

[0015] As a further step, the resonant frequency of the LC parallel circuit is set to 6 times the fundamental frequency.

[0016] As an alternative implementation, the product of the virtual impedance and the grid-connected current is added to the grid-connected point voltage, and the feedback point of the product of the grid-connected current and the virtual impedance is moved from the grid-connected point voltage to the controller output.

[0017] A low-harmonic, high-stability control system for a photovoltaic inverter under weak grid conditions includes:

[0018] The data acquisition module is used to acquire the grid-connected current at the grid connection point of the grid-connected inverter system and to acquire the reference value of the grid-connected current.

[0019] The controller is used to generate grid-connected current control information based on the grid-connected current and its reference value;

[0020] The feedback loop is used to multiply the grid-connected current by the impedance reshaping loop and feed it back to the output of the controller, so that a virtual impedance is connected in series with the output impedance of the inverter. The virtual impedance generates a virtual resistance characteristic at the resonant frequency to suppress low-frequency harmonics in the grid-connected current.

[0021] An electronic device includes a memory and a processor, as well as computer instructions stored in the memory and running on the processor, wherein the computer instructions, when executed by the processor, perform the steps in the method described above.

[0022] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0023] This invention uses virtual impedance to achieve harmonic suppression, which can still ensure grid-connected current quality even with a small DC-side capacitor. It does not require the addition of an extra damping resistor, thus reducing system energy loss and saving system hardware costs.

[0024] Compared with the traditional impedance reshaping method that introduces virtual resistance, this invention introduces a virtual LC parallel loop on the basis of virtual resistance, which not only improves system stability but also effectively suppresses grid-connected current harmonics.

[0025] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, preferred embodiments are given below and described in detail with reference to the accompanying drawings. Attached Figure Description

[0026] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0027] Figure 1 This is a general block diagram of a three-phase LCL grid-connected inverter according to an embodiment of the present invention;

[0028] Figure 2 This is a structural diagram of a phase-locked loop based on a synchronous rotating coordinate system according to an embodiment of the present invention;

[0029] Figure 3 This is a system control structure block diagram according to an embodiment of the present invention;

[0030] Figure 4 This is an equivalent output impedance diagram of an inverter considering a phase-locked loop according to an embodiment of the present invention.

[0031] Figure 5 This is a Bode plot of inverter output impedance and grid impedance according to an embodiment of the present invention;

[0032] Figure 6 This is an equivalent output impedance diagram of an inverter after series virtual impedance according to an embodiment of the present invention;

[0033] Figure 7(a) is a virtual impedance diagram of RLC series and parallel connection according to an embodiment of the present invention;

[0034] Figure 7(b) is a virtual impedance frequency response diagram according to an embodiment of the present invention;

[0035] Figure 8(a) is a control block diagram of the series virtual impedance of a grid-connected inverter system according to an embodiment of the present invention;

[0036] Figure 8(b) is a control block diagram of the series virtual impedance of a grid-connected inverter system after equivalent transformation according to an embodiment of the present invention;

[0037] Figure 9(a) is a waveform diagram of inverter-side current without impedance reshaping according to an embodiment of the present invention;

[0038] Figure 9(b) is a waveform diagram of the grid-side current without impedance reshaping according to an embodiment of the present invention;

[0039] Figure 9(c) is an FFT analysis diagram of the grid-connected current without impedance reshaping according to an embodiment of the present invention;

[0040] Figure 9(d) is a waveform diagram of the inverter-side current after impedance reshaping according to an embodiment of the present invention;

[0041] Figure 9(e) is a waveform diagram of the grid-side current after impedance reshaping according to an embodiment of the present invention;

[0042] Figure 9(f) is an FFT analysis diagram of the grid-connected current after impedance reshaping according to an embodiment of the present invention. Detailed Implementation

[0043] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0044] It should be noted that the following detailed descriptions are illustrative and intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meanings as those commonly understood by those skilled in the art to which the present invention belongs.

[0045] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the scope of exemplary embodiments according to the invention. As used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise. Furthermore, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0046] Where there is no conflict, the embodiments and features described in this application may be combined with each other.

[0047] Figure 1 This is a block diagram of an LCL-type three-phase three-level grid-connected inverter. L1 and L2 are the inverter-side inductance and the grid-side inductance, respectively. C f This represents the filter capacitor; the three components together form an LCL-type filter. g K represents the grid impedance. To simulate the worst-case scenario, the grid impedance is assumed to be purely inductive reactance. d This is the active damping coefficient of the capacitor voltage, which can suppress the inherent resonant spike of the LCL. pcc The voltage at the common coupling point is acquired by a phase-locked loop (PLL) to obtain synchronization phase information and obtain the grid-connected current reference value i. gref .

[0048] C p C n The upper and lower capacitors are on the DC side, and O is the midpoint of the upper and lower capacitors. The voltage at this point is the midpoint voltage u. np Defined as the voltage difference between the upper and lower capacitors, fluctuations in the midpoint voltage have a significant impact on the grid-connected current, which can be expressed as:

[0049]

[0050] Where u p 、u n These are the voltages of the upper and lower capacitors, I. np The average midpoint current is related to the modulation method, ω is the angular frequency, and C is the average midpoint current. dc =C p =C n This is the capacitance value of the DC side capacitor.

[0051] As shown in Equation 1, the midpoint voltage is affected by the capacitance of the DC-side capacitor. The smaller the DC-side capacitor, the greater the midpoint voltage fluctuation. In practical engineering, to save costs, the DC-side capacitor is usually relatively small. Especially when using the DPWM strategy, this will lead to a large midpoint fluctuation. Such a large midpoint fluctuation will result in the grid-connected current containing abundant 5th and 7th harmonics, affecting the grid connection quality.

[0052] Furthermore, under weak grid conditions, the inverter's PCC terminal voltage typically contains low-order background harmonics such as the 5th, 7th, 11th, and 13th orders. The presence of these background harmonics severely distorts the grid-connected current, making it unable to meet grid connection requirements. However, LCL-type filters have low impedance in the low-frequency range, resulting in weak suppression of background harmonics in the grid voltage. In summary, due to the small DC-side capacitor and the influence of background harmonics under weak grid conditions, the grid-connected current will contain low-frequency harmonics such as the 5th, 7th, 11th, and 13th orders, especially the 5th and 7th orders.

[0053] Besides grid-connected current harmonics, three-phase inverters in weak grids also face stability issues. Due to the strong coupling between the PLL and the grid impedance, the negative phase shift generated by the PLL significantly affects the phase frequency characteristics of the inverter's output impedance, further reducing the stability of the grid-connected inverter control system. A phase-locked loop control circuit based on the synchronous rotating coordinate system method is shown below. Figure 2 As shown. The corresponding small-signal model is

[0054]

[0055] Δu q =Δu β cosθ0-Δu α sinθ0-U pcc Δθ0(3)

[0056]

[0057] In the formula, Δx represents the small-signal variable corresponding to the variable, u α and u β The voltage u of the PCC in the stationary α-β coordinate system represents the voltage of the PCC. q This represents the PCC voltage on the q-axis after the Park transformation, k ppll and k ipll U represents the proportional and integral coefficients of the phase-locked loop PI controller. pcc and I m Indicates the magnitude of PCC voltage and grid-connected current, i gref θ0 represents the reference value of the grid-connected current, φ represents the phase of the PCC voltage, and φ represents the power factor angle.

[0058] Combining (2)-(4), and setting the power factor angle φ to 0, we can obtain

[0059]

[0060] In the formula, ω0 represents the angular frequency of the PCC voltage.

[0061] From equation (4), the equivalent transfer function G of the phase-locked loop can be obtained. pll (s) is

[0062]

[0063] Considering the PLL, the overall control block diagram of the grid-connected inverter system is as follows: Figure 3 As shown, combining the Norton equivalent, a grid-connected inverter output impedance model considering the effects of the phase-locked loop can be obtained, such as... Figure 4 As shown.

[0064] In the diagram, Z o (s) represents the inverter output impedance, Z pll (s) represents the equivalent impedance of the phase-locked loop. Both can be expressed as:

[0065]

[0066] Therefore, according to Figure 4 The equivalent output impedance of a grid-connected inverter system considering a phase-locked loop is:

[0067]

[0068] According to the impedance stability criterion, two conditions must be met for the inverter system to remain stable. First, the inverter must remain stable under strong grid conditions, which is easily satisfied. Second, under weak grid conditions, Z... out (s) / Z g The value of (s) satisfies the Nyquist criterion, that is, the equivalent output impedance Z of the grid-connected inverter system considering the phase-locked loop. out (s) and grid impedance Z g (s) The amplitude-frequency curves have no intersection or are at an intersection point f s The phase margin at point is positive, therefore the phase margin that satisfies Nyquist stability can be expressed as:

[0069] PM = 180° - ((∠Z) g (jω s )-∠Z out (jω s ))>0° (10)

[0070] In the formula, ω s This represents the equivalent output impedance Z of the inverter considering the phase-locked loop. out (s) and grid impedance Z g (s) Angular frequency at the intersection of the amplitude-frequency curves.

[0071] Since the grid impedance only considers the inductive component, i.e., ∠Z g (jw s Since 90°, the inverter output impedance Z can be obtained when the system is stable. out (s) must satisfy the phase relationship

[0072] ∠Z out (jωs -90° (11)

[0073] Figure 5 Bode plots of the inverter output impedance before and after considering the phase-locked loop are given. Figure 5 It can be seen that the inverter output impedance characteristics in the low-frequency range change significantly after considering the phase-locked loop (PLL). The system output impedance Z in the low-frequency range... out The amplitude and phase margin of (s) decrease significantly when the grid impedance increases, which is related to the inverter output impedance Z. out The intersection frequency of (s) shifts to a lower frequency band, which means that the system response speed is slower, the dynamic performance is worse, and the phase margin of the system is also decreasing, thereby deteriorating the steady-state and dynamic performance of the system, and even causing system oscillation and instability.

[0074] The above analysis reveals two major problems for small-capacitor inverter systems with phase-locked loops (PLLs) in weak grid environments: firstly, grid-connected current harmonics caused by the small capacitor and grid voltage background harmonics; and secondly, stability issues caused by the large grid impedance. Existing literature only addresses one of these problems, failing to simultaneously solve both current harmonics and stability issues.

[0075] This invention is based on impedance reshaping and introduces a virtual RLC series-parallel impedance Z. s1 (s) is connected in series with the equivalent output impedance of the inverter. While increasing the amplitude and phase angle of the equivalent output impedance of the inverter and improving the stability margin of the system, it also generates an infinite virtual resistance at the resonant frequency, reducing the impact of background harmonics and low-frequency harmonics caused by small capacitors on the grid-connected current, and effectively suppressing low-frequency harmonics in the grid-connected current. Figure 6 The equivalent loop of the series virtual impedance based on impedance reshaping is given, and the total equivalent output impedance of the inverter becomes:

[0076] Z out1 (s)=Z out (s)+Z s1 (s)(12)

[0077] Depend on Figure 5It is known that, considering the phase-locked loop (PLL), the inverter output impedance exhibits negative resistance characteristics in the low-frequency range. Introducing a series virtual resistor can effectively compensate for this negative resistance, thereby improving the stability margin. However, in addition to improving the stability margin of the inverter system, it is also necessary to suppress low-frequency harmonics. Therefore, this invention proposes a virtual impedance reshaping method for an RLC series-parallel circuit. A virtual LC parallel loop is connected in series with the virtual resistor R, and the resonant characteristics of the LC parallel loop are used to suppress low-frequency harmonics. The RLC series-parallel circuit is shown in Figure 7(a), and its impedance variation characteristics with frequency are shown in Figure 7(b). As can be seen from Figure 7(a), the series virtual impedance Z... s1 (s) can be expressed as:

[0078]

[0079] The resistor R is connected in series with the LC parallel loop. This ensures that the virtual impedance has sufficient positive resistance characteristics across the entire frequency range, thus guaranteeing sufficient stability margin for the inverter system. Simultaneously, it generates a sufficiently large virtual impedance at harmonic frequencies, thereby meeting the requirements for low-frequency harmonic suppression. In Figure 7(b), the LC parallel circuit at f... res When resonance occurs at a certain point, the circuit is equivalent to a pure resistive element with infinite impedance, thereby reducing the impact of background harmonics and low-frequency harmonics caused by small capacitors on the grid-connected current and effectively suppressing low-frequency harmonics in the grid-connected current.

[0080] As can be seen from the above analysis, the low-frequency harmonics in the grid-connected current are mainly concentrated in the 5th and 7th harmonics. In addition, as can be seen from [3], the low-frequency 5th and 7th harmonics are represented as the 6th harmonic in the dq coordinate system. Therefore, the resonant frequency of the LC parallel circuit should be selected as 6 times the fundamental frequency, that is:

[0081]

[0082] Figure 8 shows the equivalent control block diagram of an inverter system with virtual impedance reshaping. To achieve virtual impedance reshaping, the control block diagram needs to be transformed equivalently. Figure 6 It can be seen that after connecting the virtual impedance in series, the PCC voltage becomes:

[0083] u' pccx (s)=u pccx (s)+i g (s)Z s1 (s) (15)

[0084] This virtual impedance is fed back to the PCC voltage from the grid-connected current, as shown in Figure 8(a). However, the controller cannot realize this feedback loop. Therefore, the virtual impedance is fed back to the controller output through an equivalent transformation, as shown in Figure 8(b). The transfer function of the feedback loop can be expressed as:

[0085]

[0086] Since the lead element 1 / Gd(s) is difficult to achieve in practice, the actual feedback element can be represented as:

[0087]

[0088] To verify the effectiveness of the method of this invention, experimental simulations were conducted. To simulate background harmonics in the power grid, 5% of the 5th harmonic and 3% of the 7th harmonic were injected into the grid voltage. The simulation results are shown in Figures 9(a), 9(b), 9(c), 9(d), 9(e), and 9(f). Figures 9(a) and 9(b) show the inverter-side and grid-side current waveforms without impedance reshaping. Due to the fluctuations at the midpoint of the small capacitor and the background harmonics of the grid voltage, the grid-connected current exhibits severe distortion. As can be seen from the FFT analysis of the grid-connected current in Figure 9(c), the grid-connected current contains abundant 5th and 7th harmonics, failing to meet grid connection requirements. After adopting the proposed impedance reshaping method, the simulation results are shown in Figures 9(d), 9(e), and 9(f). It can be seen that after adopting the impedance reshaping method, the 5th and 7th harmonics in the grid-connected current are effectively suppressed, and the quality of the grid-connected current is significantly improved.

[0089] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made by those skilled in the art without creative effort within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for controlling the low harmonic distortion and strong stability of a photovoltaic inverter under weak grid conditions, characterized in that, Includes the following steps: The grid-connected inverter system obtains the grid-connected current at the grid connection point and obtains the grid-connected current reference value. The controller generates grid-connected current control information based on the grid-connected current and its reference value. The grid-connected current is multiplied by the impedance reshaping stage and fed back to the output of the controller, so that a virtual impedance is connected in series with the output impedance of the inverter. The virtual impedance generates a virtual resistance characteristic at the resonant frequency to suppress low-frequency harmonics in the grid-connected current. A phase-locked loop (PLL) is used to collect the grid-connected point voltage and obtain synchronous phase information to obtain the grid-connected current reference value. The equivalent transfer function G of the PLL is... pll (s) is: After considering the phase-locked loop (PLL), and combining it with Norton's equivalent model, we obtain a grid-connected inverter output impedance model that takes the PLL's influence into account, which in turn represents the inverter output impedance Z. o (s) Equivalent impedance Z of the phase-locked loop pll (s) are respectively: The equivalent output impedance of a grid-connected inverter system considering a phase-locked loop is: Where L1 and L2 are the inverter-side inductance and grid-side inductance, respectively, and Cf represents the filter capacitor. These three components constitute an LCL-type filter, and kd is the active damping coefficient of the capacitor voltage, which can suppress the inherent resonant spikes of the LCL. ppll and k ipll U represents the proportional and integral coefficients of the phase-locked loop PI controller. pcc and I m Indicates the magnitude of PCC voltage and grid-connected current, i gref This represents the grid-connected current reference value, ω0 represents the PCC voltage angular frequency, and u α G represents the PCC voltage on the α-axis in the stationary α-β coordinate system. c For the transfer function of the current controller, G d For the transfer function that controls the delay, k pwm This is the gain of the PWM modulation.

2. The method for controlling low harmonic distortion and strong stability of a photovoltaic inverter under weak grid conditions as described in claim 1, characterized in that, The controller is a PI controller.

3. The method for controlling low harmonic distortion and strong stability of a photovoltaic inverter under weak grid conditions as described in claim 1, characterized in that, The virtual impedance is formed by a resistor R connected in series with an LC parallel circuit. The resistor R is used to increase the amplitude and phase angle of the inverter's equivalent output impedance, thereby improving the system's stability margin.

4. The method for controlling low harmonic distortion and strong stability of a photovoltaic inverter under weak grid conditions as described in claim 3, characterized in that, The resistor R exhibits positive resistance characteristics.

5. The method for controlling low harmonic distortion and strong stability of a photovoltaic inverter under weak grid conditions as described in claim 3, characterized in that, The LC parallel circuit is connected in series with the inverter output impedance, generating a virtual resistance characteristic at the resonant frequency, making the inverter's equivalent output impedance infinite.

6. A method for controlling low harmonic distortion and strong stability of a photovoltaic inverter under weak grid conditions as described in claim 3 or 4, characterized in that, The resonant frequency of the LC parallel circuit is set to 6 times the fundamental frequency.

7. The method for controlling low harmonic distortion and strong stability of a photovoltaic inverter under weak grid conditions as described in claim 3, characterized in that, The product of the virtual impedance and the grid-connected current is added to the grid-connected point voltage, and the feedback point of the product of the grid-connected current and the virtual impedance is moved from the grid-connected point voltage to the controller output.

8. A low-harmonic, high-stability control system for a photovoltaic inverter under weak grid conditions, employing the method described in claim 1, characterized in that... include: The data acquisition module is used to acquire the grid-connected current at the grid connection point of the grid-connected inverter system and to acquire the reference value of the grid-connected current. The controller is used to generate grid-connected current control information based on the grid-connected current and its reference value; The feedback loop is used to multiply the grid-connected current by the impedance reshaping loop and feed it back to the output of the controller, so that a virtual impedance is connected in series with the output impedance of the inverter. The virtual impedance generates a virtual resistance characteristic at the resonant frequency to suppress low-frequency harmonics in the grid-connected current.

9. An electronic device, characterized in that, It includes a memory and a processor, as well as computer instructions stored in the memory and running on the processor, which, when executed by the processor, perform the steps of the method according to any one of claims 1-7.

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