Low-harmonic strong-stability control method and system for photovoltaic inverter under weak power grid working condition

Abstract

The invention provides a low-harmonic strong-stability control method and system for a photovoltaic inverter under a weak power grid working condition, and the method comprises the steps: obtaining a grid-connected current of a grid-connected inverter system grid-connected point, obtaining a grid-connected current reference value, and enabling a controller to generate grid-connected current control information according to the grid-connected current and the grid-connected current reference value; and the grid-connected current is multiplied by the impedance remodeling link and then fed back to the output end of the controller, so that the output impedance of the inverter is connected with a virtual impedance in series, and the virtual impedance is utilized to generate a virtual resistance characteristic at the resonant frequency so as to suppress low-frequency harmonic waves in the grid-connected current. According to the invention, the virtual impedance is connected in series with the output impedance of the inverter, so that impedance remodeling is realized, the influence of background harmonics and low-frequency harmonics caused by small capacitance on the grid-connected current can be reduced, and the low-frequency harmonics in the grid-connected current can be effectively suppressed.

Description

Technical Field

[0001] The present invention belongs to the field of stability control, and in particular relates to a method and system for controlling low harmonics and strong stability of a photovoltaic inverter under weak power grid conditions. Background Art

[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 access of new energy to the grid, its penetration rate continues to increase. For remote areas at the end of the grid, the internal impedance of the grid is large, showing a weak grid characteristic with a low short-circuit ratio. Under a weak grid, the background harmonics in the grid voltage will interact with the grid impedance, increasing the harmonic content in the grid-connected current. In addition, in practice, in order to reduce system costs, the DC side capacitor value of the three-level inverter is usually small, especially when discontinuous pulse width modulation (DPWM) is used, the midpoint voltage has severe fluctuations. The midpoint voltage fluctuation will also cause low-frequency harmonics in the grid-connected current. However, the impedance of the LCL filter in the low-frequency band is low, and the ability to suppress low-frequency harmonics in the grid-connected current is weak. In addition to the current harmonic problem, the grid-connected inverter system is also affected by the negative impedance characteristics of the phase-locked loop, which greatly reduces the stability margin of the system.

[0004] In summary, LCL-type DC-side small-capacitor inverters under weak power grids face two major problems: current harmonics and stability. To address the current harmonic problem, the method of grid connection point (PCC) voltage feedforward has been widely used. However, the positive feedback loop introduced by the PCC voltage feedforward will couple with the grid impedance, introducing a phase lag link, which greatly reduces the phase margin of the system, and the PCC voltage feedforward has no inhibitory effect on the low-frequency harmonics caused by small capacitors. In response to the problem of reduced system stability margin caused by the phase-locked loop, some literature has proposed the use of virtual impedance reshaping, which reshapes the inverter output impedance, increases the amplitude and phase angle of the inverter equivalent output impedance, and improves the stability margin of the system. However, this method is insufficient to suppress grid-connected current harmonics.

[0005] Therefore, in view of the current harmonics and stability problems faced by LCL type DC side small capacitor inverters under weak power grids, the existing literature only solves one of the problems, and it is difficult to solve the current harmonics and stability problems at the same time. Therefore, it is of great significance to study a low harmonic strong stability control method for photovoltaic inverters under weak power grid conditions. Summary of the invention

[0006] In order to solve the above problems, the present invention proposes a low-harmonic strong stability control method and system for photovoltaic inverters under weak power grid conditions. The present invention realizes impedance reshaping by connecting a virtual impedance in series with the inverter output impedance, which can reduce the impact of background harmonics and low-frequency harmonics caused by small capacitors on the grid-connected current, and effectively suppress the low-frequency harmonics in the grid-connected current.

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

[0008] A method for controlling low harmonics and strong stability of a photovoltaic inverter under weak power grid conditions comprises the following steps:

[0009] Obtaining the grid-connected current of the grid-connected inverter system grid-connected point, obtaining the grid-connected current reference value, and the controller generates grid-connected current control information according to the grid-connected current and its reference value;

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

[0011] As an optional implementation, a phase-locked loop is used to collect the grid-connected point voltage and obtain synchronous phase information to obtain a grid-connected current reference value.

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

[0013] As an optional implementation, the virtual impedance is formed by a resistor R connected in series with an LC parallel circuit, and the resistor R presents a positive resistance characteristic, which is used to increase the amplitude and phase angle of the inverter equivalent output impedance and improve the stability margin of the system.

[0014] As a further feature, the LC parallel circuit is connected in series with the inverter output impedance to generate a virtual resistance characteristic at the resonant frequency, so that the inverter equivalent output impedance becomes infinite.

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

[0016] As an optional 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 terminal.

[0017] A photovoltaic inverter low harmonic strong stability control system under weak power grid conditions, comprising:

[0018] A data acquisition module is used to obtain the grid-connected current of the grid-connected inverter system grid-connected point and obtain a grid-connected current reference value;

[0019] A controller, used for generating grid-connected current control information according to the grid-connected current and its reference value;

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

[0021] An electronic device comprises a memory and a processor, and computer instructions stored in the memory and executed on the processor. When the computer instructions are executed by the processor, the steps in the above method are completed.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] The present invention adopts virtual impedance to achieve harmonic suppression, and can still ensure the quality of grid-connected current when the DC side capacitance is small, without adding additional damping resistance, thereby reducing system energy loss and saving system hardware cost.

[0024] Compared with the traditional impedance reshaping method of introducing virtual resistance, the present invention introduces a virtual LC parallel loop on the basis of virtual resistance, which improves the system stability and effectively suppresses the 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. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The accompanying drawings in the specification, which constitute a part of the present invention, are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.

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

[0028] Figure 2 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 is a block diagram of a system control structure according to an embodiment of the present invention;

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

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

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

[0033] FIG. 7( a ) is a diagram of RLC series-parallel virtual impedance according to one embodiment of the present invention;

[0034] FIG7( b ) is a frequency response diagram of virtual impedance according to one embodiment of the present invention;

[0035] FIG8( a ) is a control block diagram of a series virtual impedance of a grid-connected inverter system according to an embodiment of the present invention;

[0036] FIG8( b ) is a control block diagram of a series virtual impedance of a grid-connected inverter system after equivalent transformation according to an embodiment of the present invention;

[0037] FIG9( a ) is a current waveform diagram of an inverter without impedance reshaping according to an embodiment of the present invention;

[0038] FIG9( b ) is a grid-side current waveform diagram without impedance reshaping according to one embodiment of the present invention;

[0039] FIG9( c ) is a FFT analysis diagram of the grid-connected current without impedance reshaping according to one embodiment of the present invention;

[0040] FIG9( d ) is a current waveform diagram of the inverter side after impedance reshaping according to one embodiment of the present invention;

[0041] FIG9( e ) is a grid-side current waveform diagram after impedance reshaping according to an embodiment of the present invention;

[0042] FIG9( f ) is an FFT analysis diagram of the grid-connected current after impedance reshaping according to one embodiment of the present invention. DETAILED DESCRIPTION

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

[0044] It should be noted that the following detailed descriptions are all 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 terms used herein are only for describing specific embodiments and are not intended to limit exemplary embodiments according to the present invention. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprising" and / or "including" are used in this specification, it indicates the presence of features, steps, operations, devices, components and / or combinations thereof.

[0046] In the absence of conflict, the embodiments in this application and the features in the embodiments may be combined with each other.

[0047] Figure 1 The block diagram of the LCL type three-phase three-level grid-connected inverter is shown in Figure 2. L1 and L2 are the inverter side inductor and the grid side inductor, respectively. f Represents the filter capacitor, and the three constitute an LCL filter. g Indicates the grid impedance. To simulate the worst case, the grid impedance is considered to be pure inductive reactance. K d is the capacitor voltage active damping coefficient, which can suppress the LCL inherent resonance peak. pcc is the voltage at the common coupling point, which is collected by a phase-locked loop (PLL) to obtain synchronous phase information and obtain the grid-connected current reference value i gref .

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

[0049]

[0050] In the formula, u p 、u n are the upper and lower capacitor voltages, I np is the average midpoint current, which is related to the modulation mode, ω is the angular frequency, C dc =C p =C n is the DC side capacitance.

[0051] From formula 1, we can see that 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 actual projects, in order to save costs, the DC side capacitor is usually relatively small. Especially when the DPWM strategy is adopted, it will cause a large midpoint fluctuation. Such a large midpoint fluctuation will cause the grid-connected current to contain abundant 5th and 7th harmonics, affecting the grid-connected quality.

[0052] In addition, under weak grid conditions, the PCC terminal voltage of the inverter usually contains low-order background harmonics such as 5, 7, 11, and 13. The presence of background harmonics will cause serious distortion of the grid-connected current, making it unable to meet the grid-connected requirements. However, the impedance of the low-frequency band of the LCL filter is low, and the ability to suppress background harmonics in the grid voltage is weak. In short, affected by the small capacitance on the DC side and the background harmonics under weak grid conditions, the grid-connected current will contain low-frequency harmonics such as 5, 7, 11, and 13, especially the 5th and 7th.

[0053] In addition to the grid-connected current harmonic problem, the three-phase inverter in a weak grid also faces stability problems. Due to the strong coupling between the PLL and the grid impedance, the negative phase shift generated by the PLL has a significant impact on the inverter output impedance phase-frequency characteristics, further reducing the stability of the grid-connected inverter control system. Figure 2 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 of the corresponding variable, u α and u β represents the PCC voltage in the stationary α-β coordinate system, u q represents the PCC voltage on the q axis after Park transformation, k ppll and k ipll represents the proportional and integral coefficients of the phase-locked loop PI controller, U pcc and I m Indicates the amplitude of PCC voltage and grid-connected current, i gref represents the grid-connected current reference value, θ0 represents the phase of the PCC voltage, and φ represents the power factor angle.

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

[0059]

[0060] Where ω0 represents the PCC voltage angular frequency.

[0061] From formula (4), we can get the equivalent transfer function G of the phase-locked loop: pll (s)

[0062]

[0063] After considering PLL, the overall control block diagram of the grid-connected inverter system is as follows: Figure 3 As shown, combined with the Norton equivalent, the output impedance model of the grid-connected inverter taking into account the influence of the phase-locked loop can be obtained, as shown in Figure 4 shown.

[0064] In the figure, Z o (s) is the inverter output impedance, Z pll (s) is the equivalent impedance of the phase-locked loop. The two can be expressed as:

[0065]

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

[0067]

[0068] According to the impedance stability criterion, if the inverter system is to remain stable, two conditions must be met. First, the inverter can remain stable under strong grid conditions, which is easy to meet; second, the inverter can remain stable under weak grid conditions. out (s) / Z g The value of (s) complies with 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 curve has no intersection or is at the intersection f s The phase margin at is positive, so the phase margin that meets 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 Indicates 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 curve.

[0071] Since the grid impedance only considers the inductive component, that is, ∠Z g (jw s )=90°, so 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 The Bode diagram of the inverter output impedance before and after considering the phase-locked loop is given. Figure 5 It can be seen that the characteristics of the inverter output impedance in the low frequency band have changed significantly after considering the phase-locked loop. The system output impedance Z in the low frequency band out The amplitude and phase margin of (s) are greatly reduced. When the grid impedance increases, the inverter output impedance Z out The cutoff frequency of (s) moves to the low frequency band, which means that the response speed of the system slows down, the dynamic performance deteriorates, and the phase margin of the system continues to decrease, thereby deteriorating the steady-state and dynamic performance of the system, and even causing system oscillation and instability.

[0074] From the above analysis, it can be seen that the small capacitor inverter system considering the phase-locked loop under weak power grid faces two major problems: one is the grid-connected current harmonic problem caused by small capacitors and grid voltage background harmonics, and the other is the stability problem of the grid-connected inverter system caused by the larger grid impedance. However, the existing literature only solves one of the problems, and does not solve the current harmonic and stability problems at the same time.

[0075] The present invention is based on impedance reshaping and introduces an RLC series-parallel virtual impedance Z s1 (s) is connected in series with the inverter equivalent output impedance. While increasing the amplitude and phase angle of the inverter equivalent output impedance 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 A series virtual impedance equivalent loop 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 can be seen that after considering the phase-locked loop, the output impedance of the inverter exhibits a negative resistance characteristic in the low-frequency band. The introduction of a series virtual resistor can effectively compensate for the negative resistance and achieve an improvement in the stability margin. However, in addition to improving the stability margin of the inverter system, low-frequency harmonics also need to be suppressed. Therefore, the present invention proposes a method for reshaping the virtual impedance of an RLC series-parallel circuit. A virtual LC parallel loop is connected in series on the basis of the virtual resistor R, and the resonant characteristics of the LC parallel loop are used to achieve suppression of 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, which can not only ensure that the virtual impedance has sufficient positive resistance characteristics in the full frequency band, thereby ensuring that the inverter system has sufficient stability margin, but also generate a sufficiently large virtual impedance at the harmonic frequency to meet the requirements for low-frequency harmonic suppression. In Figure 7(b), the LC parallel circuit is at f res Resonance occurs at the point where the circuit is equivalent to a pure resistance link 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] From the above analysis, it can be seen that the low-frequency harmonics in the grid-connected current are mainly concentrated in the 5th and 7th harmonics. In addition, it can be seen from [3] that the low-frequency 5th and 7th harmonics are characterized as 6th 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 the inverter system with virtual impedance reshaping. In order to achieve virtual impedance reshaping, the control block diagram needs to be equivalently transformed. Figure 6 It can be seen that after the virtual impedance is connected in series, the PCC voltage becomes:

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

[0084] The virtual impedance is fed back from the grid current to the PCC voltage, as shown in Figure 8(a), but the feedback loop controller cannot implement it. Therefore, the virtual impedance is fed back to the controller output through equivalent transformation, as shown in Figure 8(b), and the transfer function of the feedback link can be expressed as:

[0085]

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

[0087]

[0088] In order to verify the effect of the method of the present invention, experimental simulation was carried out. In order to simulate the 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) are the current waveforms of the inverter side and the grid side when the impedance reshaping is not performed. Affected by the midpoint fluctuation of the small capacitor and the background harmonics of the grid voltage, the grid-connected current is seriously distorted. It can be seen from the grid-connected current FFT analysis diagram in Figure 9(c) that the grid-connected current contains abundant 5th and 7th harmonics, which cannot meet the grid-connected 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 only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made by those skilled in the art within the spirit and principle of the present invention without creative labor shall be included in the protection scope of the present invention.

Claims

1. A low harmonic strong stability control method for photovoltaic inverters under weak power grid conditions, characterized in that: The following steps are involved: Obtaining the grid-connected current of the grid-connected inverter system grid-connected point, obtaining the grid-connected current reference value, and the controller generates grid-connected current control information according to the grid-connected current and its reference value; The grid-connected current is multiplied by the impedance reshaping link and then fed back to the output end of the controller, so that a virtual impedance is connected in series with the inverter output impedance. The virtual impedance is used to generate a virtual resistance characteristic at the resonant frequency to suppress low-frequency harmonics in the grid-connected current.

2. A method for controlling low harmonics and strong stability of photovoltaic inverters under weak power grid conditions as claimed in claim 1, characterized in that: The phase-locked loop is used to collect the grid-connected point voltage and obtain the synchronous phase information to obtain the grid-connected current reference value.

3. The method for controlling low harmonics and strong stability of photovoltaic inverters under weak power grid conditions as claimed in claim 1 is characterized in that: The controller is a PI controller.

4. The method for controlling low harmonics and strong stability of photovoltaic inverters under weak power grid conditions as claimed in claim 1 is characterized in that: The virtual impedance is formed by connecting a resistor R in series with an LC parallel circuit. The resistor R is used to increase the amplitude and phase angle of the inverter equivalent output impedance and improve the stability margin of the system.

5. A method for controlling low harmonics and strong stability of photovoltaic inverters under weak power grid conditions as claimed in claim 4, characterized in that: The resistor R exhibits positive resistance characteristics.

6. A method for controlling low harmonics and strong stability of photovoltaic inverters under weak power grid conditions as claimed in claim 4, characterized in that: The LC parallel circuit is connected in series with the inverter output impedance to generate a virtual resistance characteristic at the resonant frequency, so that the inverter equivalent output impedance becomes infinite.

7. A method for controlling low harmonics and strong stability of a photovoltaic inverter under weak power grid conditions as claimed in claim 4 or 5, characterized in that: The resonant frequency of the LC parallel circuit is set to 6 times the fundamental frequency.

8. A method for controlling low harmonics and strong stability of photovoltaic inverters under weak power grid conditions as claimed in claim 4, 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 output end of the controller.

9. A photovoltaic inverter low harmonic strong stability control system under weak power grid conditions, characterized in that: include: A data acquisition module is used to obtain the grid-connected current of the grid-connected inverter system grid-connected point and obtain a grid-connected current reference value; A controller, used for generating grid-connected current control information according to the grid-connected current and its reference value; The feedback link is used to multiply the grid-connected current by the impedance reshaping link and then feed it back to the output end of the controller, so that a virtual impedance is connected in series with the inverter output impedance, and the virtual impedance is used to generate a virtual resistance characteristic at the resonant frequency to suppress low-frequency harmonics in the grid-connected current.

10. An electronic device, characterized in that: The method comprises a memory and a processor, and computer instructions stored in the memory and executed on the processor, wherein when the computer instructions are executed by the processor, the steps in the method according to any one of claims 1 to 8 are completed.

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