A method to improve grid impedance adaptability of grid-connected inverter

Abstract

The present invention discloses a method for improving the adaptability of grid impedance of grid-connected inverters, and belongs to the field of inverter control technology. The present invention first introduces a sampling coefficient in the proportional and second-order differential feedforward links in the traditional capacitor voltage full feedforward structure; secondly, a first-order low-pass filter is connected in series with the second-order differential feedforward link to improve the equivalent active damping effect; thirdly, a second-order low-pass filter is connected in series with the proportional feedforward link to broaden the positive damping range of the equivalent active damping and improve the system phase angle stability margin; finally, a grid-connected current prediction control based on a prediction factor is introduced in the current feedback channel to further improve the feasibility range of the system phase angle stability. The present invention shows good stable adaptability to the grid impedance and improves the robustness of the system to extremely weak grids.

Description

Technical Field

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

[0002] As a key device for connecting renewable energy sources to the grid, grid-connected inverters have long been a research hotspot in terms of control methods. The total harmonic distortion (THD) of the grid-connected current and the robustness to weak grid conditions are two key performance indicators for grid-connected inverters. Commonly used modulation methods are unipolar and bipolar SPWM modulation and single-stage frequency-doubled SPWM modulation. The output PWM voltage contains a large amount of switching harmonics, so the THD of the grid-connected current is largely determined by the performance of the passive filter. LCL filters have a smaller total inductance than L-type filters, making them popular in academia and industry in the inverter field. However, LCL filters are third-order systems and can produce resonance. Without appropriate damping, the inherent resonance in their frequency characteristics can seriously affect the overall operational stability of the grid-connected inverter, especially in weak grid environments.

[0003] 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 additional losses in the system, and the above-mentioned problems can only be improved to a certain extent. It is costly and increases 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 proportional feedforward of capacitor current and differential feedforward of capacitor voltage. In addition, the use of inverter side current feedback (ICF) or grid side current feedback (GCF) is also a single variable feedback method. Relevant scholars have studied the relationship between the stability of single variable feedback control LCL type grid-connected inverter and digital control delay, and obtained the stable time delay range of the ICF loop and GCF loop.

[0004] Due to the increasing penetration of distributed generation systems, longer transmission lines, and the extensive use of transformers, grid impedance is gradually exhibiting weak grid characteristics. This shift in grid impedance can cause the resonant frequency of LCL inverter systems to shift, leading to system instability in severe cases. Recent research has demonstrated that active damping methods using voltage feedforward as a state variable feedback method have a positive impact on addressing weak grid conditions. Professor Ruan Xinbo's team at Nanjing University of Aeronautics and Astronautics has made significant contributions in this area. Other researchers have attempted to reduce digital control delay by shifting the sampling point forward, but this approach is limited by hardware. Another approach uses double sampling to completely eliminate control delay, but this sacrifices the adjustable range of the duty cycle. Currently, the point of common coupling (PCC) feedforward method, based on grid-side current feedback, has been widely used to enhance the robustness of weak grids. Since the inductance of the LCL filter is inversely proportional to the rated capacity of the grid-connected inverter, the filter capacitor voltage can be equated to the PCC voltage. Furthermore, the capacitor voltage can serve as the input to the phase-locked loop (PLL), reducing the number of sensors and 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] To address the increasing penetration of power grids caused by the widespread use of power electronics and the large-scale integration of distributed power sources, this paper proposes a method for improving the grid impedance adaptability of grid-connected inverters. This method enables stable operation of LCL-type grid-connected inverters under extremely weak grid conditions. In practical applications, this method achieves low total harmonic distortion (THD) of the grid-connected current, high sinusoidality, and a high power factor, resulting in a robust system that can adapt to a wide range of grid impedance variations.

[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 according to 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 a first-order low-pass filter is connected 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 with 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 in the low-frequency band, where the damping effect is more obvious, 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 it to obtain the filter capacitor current signal, and using the capacitor current sampling coefficient k d Negative feedback is fed back to the output of the controller to suppress the active damping of the LCL filter resonance spike; the inverter output side current is obtained and fed back to the input of the controller.

[0015] Furthermore, step 1 introduces 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.

[0016] Furthermore, the specific process of step 1 includes: deducing 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 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, improving the positive damping range of the equivalent active damping.

[0017] Furthermore, the specific process of connecting a second-order low-pass filter in series with 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 with the proportional feedforward link to achieve (0~f s / 3) frequency band is all equivalent positive impedance, which broadens the positive damping range of equivalent active damping, making (0~f r ) frequency band, and improves the system phase stability margin.

[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 (n-1) moment, the grid-connected current value at the (n+1) moment 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 introduction of predictive control based on prediction factors increases the feasibility range of system phase angle stability and further improves the phase margin of the system.

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

[0020] Furthermore, it 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 between the grid-connected current and the sampling value is made, 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 the present invention for improving the grid impedance adaptability of a grid-connected inverter 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, thus reducing costs.

[0024] (2) The method of the present invention for improving the grid impedance adaptability of a grid-connected inverter does not add any additional control branches, 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 the 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 This 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 equivalent active damping of the traditional capacitor voltage full feedforward in the present invention;

[0029] Figure 4 The sampling coefficient H is introduced in the present invention i2 Parallel and series G LPF1 (s) Frequency characteristics of equivalent active damping;

[0030] Figure 5 The sampling coefficient H is introduced in the present invention i2 Parallel and series G LPF1 (s), GLPF2 (s) Frequency characteristics of equivalent active damping;

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

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

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

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

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

[0036] Figure 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] Figure 12 The sampling coefficient H is introduced in the present invention i2 Parallel and series G LPF1 (s) before and after different L g The following is the grid-connected current simulation waveform;

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

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

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

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

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

[0043] Figure 16 (b) is the addition of G under the background of harmonic injection in 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 with reference to 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 This is a topological diagram of a single-phase H6 bridge LCL type grid-connected inverter provided by an embodiment of the present invention. Figure 1 As shown, u dc The voltage source on the DC side is passed through the H6 bridge inverter structure with the effect of suppressing common mode voltage. S1 to S6 are switches, and D1 and D2 are freewheeling diodes. The output passes through the LCL filter, where L1 is the inverter side inductor, L2 is the grid side inductor, and C is the filter capacitor. The capacitor voltage u is collected. 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: Get the filter capacitor voltage signal and process it to get 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 control block diagram of LCL type grid-connected inverter 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. Based on 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 the figure below. Figure 3 As shown. Given that the capacitor voltage full feedforward is a set of positive feedback that easily affects the stability of the system, the original proportional second-order differential feedforward and proportional feedforward channels are set to 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) exhibits a negative resistance characteristic. This solution can effectively suppress the problem of the system's open-loop transfer function zeros and poles not being able to cancel each other due to the influence of digital control delay. It can also suppress the impact of the LCL filter's resonant frequency shift caused by increased grid impedance in weak power grids. This invention can greatly improve the system's robustness in weak power grid environments.

[0063] Step 3: The expression of the system output impedance 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 resonance peak is suppressed, and the phase characteristic curve no longer crosses the -180°±360° line at the resonance frequency.

[0070] See 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 resonant frequency. It can be seen that the series G 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 bilinear transformation discretization method (Tustin) is used to convert the above formula into an S-domain transfer function:

[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 8As can be seen, as δ increases, the feasible range of phase angle stability at low frequencies increases, and the phase margin also improves, but the peak value at high-frequency resonance also increases. Using this grid current predictive control method, the phase-frequency characteristic curve never crosses the -180° line, and the system remains stable. The value of δ can be selected as a compromise based on actual conditions, keeping it as small as possible within a desirable range, with a preferred value of 1.

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

[0082] When the grid impedance L g When it increases, it will cause the resonant frequency of the LCL filter to shift. If the system does not have enough stability margin, it will cause the grid current to become unstable, and in severe cases, it will damage the inverter device. Figure 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 the low frequency and be located in the stable region. g When the amplitude-phase characteristic curve increases to a certain extent, the change is almost minimal, which 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. Figure 11 In the if (s), the system's phase-frequency characteristic curve significantly improves, increasing the system's phase margin across a wider frequency band. When the grid has impedance, the phase-frequency characteristic curve shifts toward the lower frequency band, similarly parallel and amplified, and similarly improves the phase margin, demonstrating good adaptability to grid impedance and high system robustness.

[0083] In addition, the method also includes the step of using a quasi-resonant controller to suppress high-frequency switching harmonics and grid-connected current harmonics. ref The difference between the grid-connected current and the sampling value is made, 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: Figure 12 、 Figure 13 、 Figure 14 、 Figure 15 (a) Figure 16 As shown in (a). Figure 12 To introduce the sampling coefficient and then connect G LPF1(s) Grid-connected current simulation waveforms under different grid impedances compared with traditional capacitor voltage full feedforward; Figure 13 To introduce the sampling coefficient and then connect G LPF1 (s) in L g =Grid-connected current simulation waveform at 8.5mH; Figure 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; Figure 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 simulate the grid-connected current waveform. Figure 15 (b) Experimental diagram of the current waveform in this case; Figure 16 (a) To join G if (s) before and after adding the above-mentioned grid-connected current simulation waveforms under different grid impedances, Figure 16 (b) Add G to the above conditions if The experimental current waveform diagram under the condition of (s). From the simulation results, it can be concluded that the proposed method enables the system to operate stably under conditions with a wide range of grid impedance variations, with a high output current power factor and strong system robustness.

[0085] The above is a schematic description of the present invention and its embodiments, which 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 this and, without departing from the purpose of the present invention, designs a structure and embodiment similar to this technical solution without inventiveness, they shall fall within the scope of protection 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 in the capacitor voltage proportional feedforward and second-order differential feedforward links , and then a first-order low-pass filter is connected 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 with 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 grid impedance adaptability of a grid-connected inverter according to claim 1, wherein: The following steps are also included: Get the filter capacitor voltage signal and process it to get the filter capacitor current signal. Negative feedback is fed back to the output of the controller to suppress the active damping of the LCL filter resonance spike; the inverter output side current is obtained and fed back to the input of the controller.

3. The method for improving grid impedance adaptability of a grid-connected inverter according to claim 1, wherein: Step 1: Introduce the sampling coefficient in the capacitor voltage proportional feedforward and second-order differential feedforward links , 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 grid impedance adaptability of a grid-connected inverter according to claim 1, wherein: The specific process of step 1 includes: deducing the equivalent parallel resistance value of the filter capacitor based on the relationship between the capacitor voltage proportional feedforward, second-order differential feedforward and 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, the equivalent negative impedance is changed to the equivalent positive impedance, improving the positive damping range of the equivalent active damping.

5. The method for improving grid impedance adaptability of a grid-connected inverter according to claim 1, wherein: The specific process of connecting a second-order low-pass filter in series with 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 with the proportional feedforward link to achieve 0~ f s / 3 frequency band is all equivalent 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 grid impedance adaptability of a grid-connected inverter according to claim 1, wherein: The specific process of introducing predictive control based on prediction factors into the current closed-loop control loop includes: Indicates the At each cycle, by measuring n The sampled grid-connected current value at time -1, and n The sampled grid-connected current value at the moment is multiplied by the relevant prediction factor coefficient to obtain n The grid-connected current value at time +1 is used to predict the grid-connected current. According to the relationship between the grid-connected current negative feedback loop and the system controller input, the introduction of predictive control based on prediction factors increases the feasibility range of system phase angle stability and further improves the phase margin of the system.

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

8. The method for improving grid impedance adaptability of a grid-connected inverter according to any one of claims 1 to 7, characterized in that: It 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 to The difference between the grid-connected current and the sampling value is made, 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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