Method for improving power grid impedance adaptivity of grid-connected inverter

Abstract

The invention discloses a method for improving power grid impedance adaptivity of a grid-connected inverter, and belongs to the technical field of inverter control. The method comprises the following steps: firstly, introducing a sampling coefficient in a proportion and second-order differential feed-forward link in a traditional capacitor voltage full feed-forward structure; secondly, connecting a first-order low-pass filter in series in a second-order differential feed-forward link to improve an equivalent active damping effect; thirdly, second-order low-pass filtering is connected in series in a proportional feed-forward link to broaden the positive damping range of equivalent active damping, and the phase angle stability margin of the system is improved; and finally, grid-connected current predictive control based on predictive factors is introduced into a current feedback channel, so that the feasibility range of phase angle stability of the system is further expanded. The method shows good stable self-adaptability to power grid impedance, and the robustness of the system to an extremely weak power grid is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of grid-connected control, and more specifically to a method for improving the adaptability of grid impedance of a grid-connected inverter. Background Art

[0002] As a key device for connecting new energy to the power grid, the control method of the grid-connected inverter has always been a hot topic of research. The total harmonic distortion (THD) of the grid-connected current and the robustness of the weak power grid are two key indicators of the performance of the grid-connected inverter. The commonly used modulation methods are unipolar, bipolar SPWM modulation and single-stage frequency-doubled SPWM modulation. The output PWM voltage contains a large number of switching harmonics, so the THD of the grid-connected current is largely determined by the performance of the passive filter. The LCL filter has a smaller total inductance than the L-type filter, which makes the LCL filter favored by academia and industry in the field of inverters. However, the LCL filter belongs to a third-order system and will produce resonance. If there is no appropriate damping, the inherent resonance in its frequency characteristics will seriously affect the overall operating stability of the grid-connected inverter, especially in a weak power grid environment.

[0003] The research on LCL filter resonance damping methods can be roughly divided into two types: passive and active. The passive damping method is simple to implement, but it will increase the system's additional losses, and the above problems can only be improved to a certain extent, with high costs and increased system complexity. The active damping method is to add additional control methods at the software level to achieve the purpose of providing virtual damping for the system. The commonly used method is the state variable feedback method, which uses the voltage or current on the filter capacitor as a state variable for feedback to suppress resonance, such as the proportional feedforward of the capacitor current and the differential feedforward of the capacitor voltage. In addition, the use of inverter current feedback (ICF) or grid current feedback (GCF) is also a single variable feedback method. Relevant scholars have studied the relationship between the stability of the single variable feedback control LCL type grid-connected inverter and the digital control delay, and obtained the stable time delay range of the ICF loop and the GCF loop.

[0004] Due to the increasing penetration rate of distributed generation systems, long transmission lines, and the extensive use of transformers, the grid impedance gradually presents the characteristics of a weak grid. The transformation of the grid impedance will cause the resonant frequency of the LCL inverter system to shift, and in severe cases, it will cause the system to operate unstably. In recent years, many studies have shown that the active damping method of voltage feedforward as a feedback of state variables has a certain positive effect in dealing with weak grids. Among them, Professor Ruan Xinbo's team at Nanjing University of Aeronautics and Astronautics has made many achievements in this regard. In addition, some scholars have reduced the digital control delay by moving the sampling point forward, but this method is limited by hardware. There is also a method of using double sampling to ignore all control delays, but it requires sacrificing the adjustable range of the duty cycle. At present, the point of common coupling (PCC) feedforward method proposed on the basis of grid-side current feedback has been widely used in the robustness of weak grids. Since the inductance value of the LCL filter is inversely proportional to the rated capacity of the grid-connected inverter, the filter capacitor voltage can be equivalent to the PCC voltage. And the capacitor voltage can be used as the input of the phase-locked loop, reducing the use of sensors and reducing costs. And by taking certain measures on the feedback branch, the system can be made more robust to weak power grids. Summary of the invention

[0005] 1. Technical problem to be solved by the invention

[0006] In view of the problems such as the widespread use of power electronic equipment and the large-scale access of distributed power sources to the power grid causing the power grid to gradually show high penetration, the present invention proposes a method for improving the adaptability of the grid impedance of the grid-connected inverter, which can realize the stable operation of the LCL type grid-connected inverter under extremely weak grid conditions. In practical applications, the total harmonic distortion rate of the grid-connected current is low, the sinusoidality is high, the power factor is high, the system is robust, and it can adapt to the wide range of changes in the grid impedance.

[0007] 2. Technical solution

[0008] In order to achieve the above object, the technical solution provided by the present invention is:

[0009] A method for improving the grid impedance adaptability of a grid-connected inverter of the present invention comprises the following steps:

[0010] Step 1: Introduce the sampling coefficient H in the capacitor voltage proportional feedforward and second-order differential feedforward links i2 , and then connect a first-order low-pass filter in series in the second-order differential feedforward link to improve the equivalent active damping effect of the system;

[0011] Step 2: Based on step 1, a second-order low-pass filter is connected in series in the proportional feedforward link to broaden the positive damping range of the equivalent active power and improve the stability margin of the phase angle;

[0012] Step 3: Introduce predictive control based on prediction factors into the current closed-loop control loop to improve the feasibility range of system phase angle stability.

[0013] Through the above scheme, the positive damping range of the capacitor voltage full feedforward equivalent active damping can be improved and widened, especially the damping effect is more obvious in the low frequency band, and the system's adaptability to wide range changes in grid impedance under extremely weak power grids is improved.

[0014] Furthermore, the method further includes the following steps: obtaining the filter capacitor voltage signal, and processing the filter capacitor current signal, and using the capacitor current sampling coefficient k d Negative feedback is provided to the output end of the controller to suppress the active damping of the LCL filter resonance peak; the inverter output side current is obtained and fed back to the input end of the controller.

[0015] Furthermore, step 1 introduces a sampling coefficient H in the capacitor voltage proportional feedforward and second-order differential feedforward links. i2 , and then before the second-order differential feedforward link is connected in series with the first-order low-pass filter, the following steps are also included: the capacitor voltage is positively fed back to the controller input after proportional, first differential, and second differential.

[0016] Furthermore, the specific process of step 1 includes: deriving the equivalent parallel resistance value of the filter capacitor according to the relationship between the capacitor voltage proportional feedforward, the second-order differential feedforward and the digital control delay function of the mathematical model of the LCL type grid-connected inverter, and realizing it by connecting a single first-order low-pass filter in series with the second-order differential link (f r ~f s / 3) changes from an equivalent negative impedance to an equivalent positive impedance, thereby improving the positive damping range of the equivalent active damping.

[0017] Furthermore, the specific process of connecting the second-order low-pass filter in series in the proportional feedforward link includes: according to the relationship between the proportional feedforward link and the equivalent active damping, a single second-order low-pass filter is connected in series in the proportional feedforward link to achieve (0~f s / 3) frequency band is all equivalent positive impedance, broadening the positive damping range of equivalent active damping, making (0~f r ) The positive damping effect in the frequency band is obvious, and the system phase stability margin is improved.

[0018] Furthermore, the specific process of introducing predictive control based on prediction factors into the current closed-loop control loop includes: n represents the nth cycle moment, by measuring the sampled grid-connected current value at the moment (n-1), the grid-connected current value at the moment (n+1) is obtained, and multiplied by the relevant prediction factor coefficient to realize the prediction of the grid-connected current. According to the relationship between the grid-connected current negative feedback loop and the system controller input, the introduced predictive control based on prediction factors increases the feasibility range of the system phase angle stability, and the phase margin of the system is further improved.

[0019] Furthermore, the size of the prediction factor δ should not be too large within the desirable range and should take the minimum value of 1.

[0020] Furthermore, the invention also includes using a quasi-resonant controller to suppress high-frequency switching harmonics and grid-connected current harmonics, specifically: setting the grid-connected current reference value i ref The difference is made with the grid-connected current sampling value, and the output is sent to the quasi-proportional resonant controller to achieve zero-error tracking of the grid-connected current and suppress harmonics using the frequency characteristics of the quasi-proportional resonant controller.

[0021] 3. Beneficial effects

[0022] Compared with the existing known technologies, the technical solution provided by the present invention has the following significant effects:

[0023] (1) The method of improving the grid impedance adaptability of a grid-connected inverter of the present invention only requires collecting the filter capacitor voltage and the grid-connected current to realize active damping feedback of the filter capacitor current, full feedforward of the filter capacitor voltage, and closed-loop control of the grid-connected current. No additional hardware is required, thereby reducing costs.

[0024] (2) The method of improving the grid impedance adaptability of a grid-connected inverter of the present invention does not add an additional control branch, but greatly enhances the robustness of the system and the adaptability to the grid impedance compared to the traditional capacitor voltage full feedforward control.

[0025] (3) The method of improving the grid impedance adaptability of a grid-connected inverter of the present invention does not require adjustment of the sampling frequency and sampling mode of the controller, does not increase the computational burden of the digital control system, and has high practical value. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 This is a circuit topology diagram of a single-phase H6 bridge LCL type grid-connected inverter in the present invention;

[0027] Figure 2 It is a control block diagram of the LCL type grid-connected inverter in the present invention;

[0028] Figure 3 This is a frequency characteristic diagram of the traditional capacitor voltage full feedforward equivalent active damping in the present invention;

[0029] Figure 4 The sampling coefficient H is introduced in the present invention i2 Parallel series G LPF1 (s) Equivalent active damping frequency characteristic diagram;

[0030] Figure 5 The sampling coefficient H is introduced in the present invention i2 Parallel series G LPF1 (s), GLPF2 (s) Equivalent active damping frequency characteristic diagram;

[0031] Figure 6 The sampling coefficient H is introduced in the present invention i2 Parallel series G LPF1 (s) Frequency characteristics of system output impedance before and after;

[0032] Figure 7 The series connection G LPF2 (s) Frequency characteristics of system output impedance before and after;

[0033] Figure 8 is the influence of the prediction factor δ on the system open-loop transfer function in the present invention;

[0034] Fig. 9 In the present invention, G is added if (s) Impact on the system output impedance frequency characteristics;

[0035] Fig.10 is the grid impedance L in the present invention g The sampling factor H is introduced i2 Parallel series G LPF1 (s), G LPF2 (s) The impact of the system open-loop transfer function;

[0036] Fig.11 is the grid impedance L in the present invention g Join G if (s) The impact of the open-loop transfer function of the front and rear systems;

[0037] Fig.12 The sampling coefficient H is introduced in the present invention i2 Parallel series G LPF1 (s) before and after different L g The following is the grid-connected current simulation waveform;

[0038] Fig.13 The sampling coefficient H is introduced in the present invention i2 Parallel series G LPF1 (s) in L g =Grid-connected current simulation waveform at 8.5mH;

[0039] Fig.14 The sampling coefficient H is introduced in the present invention i2 Parallel series G LPF1 (s), G LPF2 (s) before and after different L g The following is the grid-connected current simulation waveform;

[0040] Fig.15 (a) is the sampling coefficient H introduced in the present invention. i2Parallel series G LPF1 (s), G LPF2 (s) in L g =20mH injected harmonic grid-connected current simulation waveform;

[0041] Fig.15 (b) is the sampling coefficient H introduced in the present invention. i2 Parallel series G LPF1 (s), G LPF2 (s) in L g =20mH harmonic injection grid-connected current experimental waveform;

[0042] Fig.16 (a) is the addition of G in the harmonic background of the present invention. if (s) before and after different L g The following is the grid-connected current simulation waveform;

[0043] Fig.16 (b) is the addition of G in the harmonic background of the present invention. if (s) in L g =Grid-connected current experimental waveform at 30mH. DETAILED DESCRIPTION

[0044] In order to further understand the content of the present invention, the present invention is described in detail in conjunction with the accompanying drawings and embodiments.

[0045] Example 1

[0046] A method for improving the grid impedance adaptability of a grid-connected inverter in this embodiment is as follows: Figure 1 The topological structure diagram of the single-phase H6 bridge LCL type grid-connected inverter provided by the embodiment of the present invention. Figure 1 As shown, u dc is the DC side voltage source; it passes through the H6 bridge inverter structure with the effect of suppressing common mode voltage, S1~S6 are switch tubes, D1 and D2 are freewheeling diodes; the output passes through the LCL filter, L1 is the inverter side inductor, L2 is the grid side inductor, and C is the filter capacitor. Collect capacitor voltage u c Perform full feedforward and use for phase locking, transforming into i c As active damping; u g is the grid voltage, L g Represents the grid impedance. The specific steps include:

[0047] Step 1: Obtain the filter capacitor voltage signal and process it to obtain the filter capacitor current signal. dNegative feedback is fed back to the output of the controller to suppress the active damping effect of the LCL filter resonant peak; the inverter output current is obtained and fed back to the input of the controller. The relationship between the filter capacitor voltage and current is:

[0048] i c (s) = u c (s)·C·s (1)

[0049] Among them, i c (s) is the filter capacitor current, u c (s) is the filter capacitor current, and C is the filter capacitor value.

[0050] The capacitor voltage is fed back to the controller input after proportional, first differential and second differential, so as to suppress the grid-connected current harmonics caused by the grid voltage background harmonics.

[0051] According to the LCL type grid-connected inverter control block diagram Figure 2 , where K PWM is the pulse width modulation gain, k d is the active damping coefficient of the capacitor current, G d (s) is the digital control delay function, G i (s) is the current controller. According to the relationship between the capacitor voltage proportional feedforward, second-order differential feedforward and digital control delay function of the LCL type grid-connected inverter, its equivalent resistance value in parallel with the filter capacitor is derived. Its frequency characteristic curve is shown in Figure 3 As shown. Based on the premise that the capacitor voltage full feedforward is a set of positive feedback that is easy to affect the stability of the system, the original proportional second-order differential feedforward and proportional feedforward channels are set with a sampling coefficient H i2 , set to 0.1. And by connecting a single first-order low-pass filter in series with the second-order differential link, (f r ~f s / 3) changes from equivalent negative impedance to equivalent positive impedance, improving the positive damping range of equivalent active damping, such as Figure 4 As shown. Among them, f r is the system resonant frequency, f s is the system sampling frequency.

[0052] The expression of a series-connected first-order low-pass filter is:

[0053]

[0054] Where: f c is the cutoff frequency of the first-order low-pass filter.

[0055] Series G LPF1 (s) The expression of equivalent virtual resistance is:

[0056]

[0057] Step 2: Combine with Figure 5 , a second-order low-pass filter G is connected in series in the proportional feedforward link LPF2 (s), whose expression is:

[0058]

[0059] Where: ξ1 is the damping coefficient, ω n is the turning frequency.

[0060] Virtual resistance R eq (ω) is changed to:

[0061]

[0062] After the proportional feedforward link is connected in series with a second-order low-pass filter, R eq (ω) in (0~f s / 3) shows positive resistance characteristics, and the damping effect is more obvious in the low frequency band. s / 3~f s / 2) presents negative resistance characteristics, this solution can effectively suppress the problem that the zero poles of the system open-loop transfer function cannot be offset due to the influence of digital control delay, and can suppress the influence of the LCL filter resonant frequency offset caused by the increase of grid impedance under weak power grid. The present invention can greatly improve the robustness of the system in a weak power grid environment.

[0063] Step 3: The expression of the output impedance of the system without considering the influence of the phase-locked loop is:

[0064]

[0065] Series G LPF1 After (s), the output impedance is:

[0066]

[0067] Series G LPF2 After (s), the output impedance is:

[0068]

[0069] Figure 6 Middle Z out_1 (s) is only connected in series with G LPF1 (s) system output impedance, it can be seen that at f s / 3, because the change of virtual equivalent impedance will cause Z outThe phase of (s) crosses -180° and enters the active region, and there is a resonance peak at high frequency. This phenomenon will cause the system to have a right half plane pole, making the system unstable when the grid impedance is large. LPF1 (s) After that, the resonant peak is suppressed and the phase characteristic curve no longer crosses the -180°±360° line at the resonant frequency.

[0070] See also Figure 7 , a second-order low-pass filter G is connected in series LPF2 (s), the amplitude-frequency characteristic resonance peak of the system output impedance is further suppressed, and the phase characteristic curve no longer crosses the -180°±360° line at the resonance frequency. LPF2 (s) not only broadens the range of the positive resistance characteristics of the virtual impedance, but also improves the phase margin in the mid-frequency band in the passive area, and the stability of the system is improved to a certain extent.

[0071] Step 4: In order to make the system operate stably under extremely weak power grid, predictive control based on prediction factors is introduced into the current closed-loop control loop to improve the feasibility range of system phase angle stability. This prediction scheme can be expressed as:

[0072]

[0073] Where n represents the nth cycle time, i g n+1 It represents the predicted value of the grid-connected current in the next cycle. By measuring the sampled grid-connected current value at time (n-1), the grid-connected current value at time (n+1) is obtained.

[0074] In the Z domain it can be expressed as:

[0075]

[0076] The above formula is converted into an S-domain transfer function using the bilinear transformation discretization method (Tustin):

[0077]

[0078] The open-loop transfer function of the system using this prediction scheme is:

[0079]

[0080] The value of the prediction factor δ determines the stability of the system. Figure 8It can be seen that as δ increases, the feasibility range of phase angle stability at low frequency is improved, and the phase margin is also improved, but the peak value at high-frequency resonance is also increased. With this grid current prediction control method, the phase-frequency characteristic curve does not cross the -180° line, and the system is always in a stable state. The value of δ can be selected as a compromise according to the actual situation. It should be as small as possible within the desirable range, and δ is preferably 1.

[0081] Join G if (s) The frequency characteristic curve of the system output impedance is as follows Fig. 9 As shown in the figure, we can see that the phase margin of the system in the passive area is improved, the phase-frequency characteristic curve also rises in the high frequency band, and the phase characteristic remains unchanged in the base frequency band, further enhancing the system phase stability margin.

[0082] When the grid impedance L g When it increases, the resonant frequency of the LCL filter will shift. If the system does not have sufficient stability margin, the grid-connected current will become unstable, and in severe cases, the inverter will be damaged. Fig.10 It can be seen that when the grid impedance increases, the resonant frequency at high frequencies will gradually decrease, but the phase-frequency characteristic will improve and move to low frequencies in the stable region. g When the amplitude-phase characteristic curve increases to a certain extent, the change is almost small. This greatly broadens the range of system phase angle stability, and the system still has good stability when the grid impedance changes in a wide range. Fig.11 In, add G if (s), the system phase-frequency characteristic curve has a significant improvement, and the system phase margin is increased in a wider frequency band. When there is impedance in the power grid, the phase-frequency characteristic curve moves to the low frequency band in a similar and amplified manner, and the phase margin is also improved, showing good adaptability to the power grid impedance, and the system has high robustness.

[0083] In addition, the method also includes the step of suppressing high-frequency switching harmonics and grid-connected current harmonics by using a quasi-resonant controller. ref The difference is made with the grid-connected current sampling value, and the output is sent to the quasi-proportional resonant controller to achieve zero-error tracking of the grid-connected current and suppress harmonics using the frequency characteristics of the quasi-proportional resonant controller.

[0084] In order to verify the effect of the control method of this embodiment, an experimental simulation is carried out. The simulation results are as follows: Fig.12 , Fig.13 , Fig.14 , Fig.15 (a) Fig.16 (a) shown. Fig.12 To introduce the sampling coefficient and then connect G LPF1(s) Grid-connected current simulation waveform under different grid impedances compared with traditional capacitor voltage full feedforward; Fig.13 To introduce the sampling coefficient and then connect G LPF1 (s) in L g =Grid-connected current simulation waveform at 8.5mH; Fig.14 For series connection only G LPF1 (s) and G in series LPF1 (s), G LPF2 (s) Grid-connected current simulation waveform under different grid impedances; Fig.15 (a) is L g =20mH, then introduce the sampling coefficient and connect G in series LPF1 (s), G LPF2 (s) and inject 3% of the 3rd harmonic, 2% of the 5th and 7th harmonics, and 1% of the 11th and 13th harmonics into the system to obtain the grid-connected current simulation waveform. Fig.15 (b) The current waveform experimental diagram for this case; Fig.16 (a) To join G if (s) before and after adding the above harmonics under different grid impedances, the grid-connected current simulation waveform, Fig.16 (b) Add G to the above conditions if (s) The current waveform experimental diagram under the condition of s. From the simulation result diagram, it can be concluded that the proposed method enables the system to operate stably under the condition of wide range of grid impedance changes, with high output current power factor and strong system robustness.

[0085] The present invention and its embodiments are described schematically above, and the description is not restrictive. The drawings show only one embodiment of the present invention, and the actual structure is not limited thereto. Therefore, if a person skilled in the art is inspired by it and designs a structural method and an embodiment similar to the technical solution without creativity without departing from the purpose of the invention, they shall all fall within the protection scope of the present invention.

Claims

1. A method for improving the grid impedance adaptability of a grid-connected inverter, characterized in that: The steps are: Step 1: Introduce the sampling coefficient H in the capacitor voltage proportional feedforward and second-order differential feedforward links i2 , and then connect a first-order low-pass filter in series in the second-order differential feedforward link to improve the equivalent active damping effect of the system; Step 2: Based on step 1, a second-order low-pass filter is connected in series in the proportional feedforward link to broaden the positive damping range of the equivalent active power and improve the stability margin of the phase angle; Step 3: Introduce predictive control based on prediction factors into the current closed-loop control loop to improve the feasibility range of system phase angle stability.

2. The method for improving the grid impedance adaptability of a grid-connected inverter according to claim 1, characterized in that: The following steps are also included: Obtain the filter capacitor voltage signal and process it to obtain the filter capacitor current signal. d Negative feedback is provided to the output end of the controller to suppress the active damping of the LCL filter resonance peak; the inverter output side current is obtained and fed back to the input end of the controller.

3. The method for improving the grid impedance adaptability of a grid-connected inverter according to claim 1, characterized in that: Step 1: Introduce the sampling coefficient H in the capacitor voltage proportional feedforward and second-order differential feedforward links. i2 , and then before the second-order differential feedforward link is connected in series with the first-order low-pass filter, the following steps are also included: the capacitor voltage is positively fed back to the controller input after proportional, first differential, and second differential.

4. The method for improving the grid impedance adaptability of a grid-connected inverter according to claim 1, characterized in that: The specific process of step 1 includes: deriving the equivalent resistance value of the filter capacitor in parallel according to the relationship between the capacitor voltage proportional feedforward, the second-order differential feedforward and the digital control delay function of the mathematical model of the LCL type grid-connected inverter, and realizing f by connecting a single first-order low-pass filter in series with the second-order differential link. r ~f s / 3, the equivalent negative impedance is changed into the equivalent positive impedance, thus improving the positive damping range of the equivalent active damping.

5. The method for improving the grid impedance adaptability of a grid-connected inverter according to claim 1, characterized in that: The specific process of connecting a second-order low-pass filter in series in the proportional feedforward link includes: according to the relationship between the proportional feedforward link and the equivalent active damping, a single second-order low-pass filter is connected in series in the proportional feedforward link to achieve 0~f s / 3 The whole frequency band is equivalent to positive impedance, which broadens the positive damping range of equivalent active damping, making 0~f r The positive damping effect within the frequency band is obvious and improves the system phase stability margin.

6. The method for improving the grid impedance adaptability of a grid-connected inverter according to claim 1, characterized in that: The specific process of introducing predictive control based on prediction factors into the current closed-loop control loop includes: n represents the nth cycle moment, and the grid-connected current value at the moment n+1 is obtained by measuring the sampled grid-connected current value at the moment n-1, and multiplying it by the relevant prediction factor coefficient to realize the prediction of the grid-connected current. According to the relationship between the grid-connected current negative feedback loop and the system controller input, the introduced predictive control based on prediction factors increases the feasibility range of the system phase angle stability and further improves the phase margin of the system.

7. A method for improving the grid impedance adaptability of a grid-connected inverter according to claim 6, characterized in that: The value of the prediction factor δ is 1.

8. A method for improving the grid impedance adaptability of a grid-connected inverter according to any one of claims 1 to 7, characterized in that: The invention also includes using a quasi-resonant controller to suppress high-frequency switching harmonics and grid-connected current harmonics, specifically: setting the grid-connected current reference value i ref The difference is made with the grid-connected current sampling value, and the output is sent to the quasi-proportional resonant controller to achieve zero-error tracking of the grid-connected current and suppress harmonics using the frequency characteristics of the quasi-proportional resonant controller.

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