A power converter and a method for harmonic suppression of grid current

Abstract

This application discloses a power converter and a method for harmonic suppression of grid-connected current. The power converter includes a current sampling circuit, a power conversion circuit, and a controller. The output of the power conversion circuit is used to connect to the power grid through a grid-connected filter. The current sampling circuit is used to collect the grid-connected current. The controller is used to obtain the fundamental amplitude and the amplitude of the nth harmonic based on the grid-connected current, where the nth harmonic is the harmonic to be suppressed, and n is an integer. The distortion coefficient is obtained based on the amplitude of the nth harmonic and the fundamental amplitude. When the power converter is in resonance at the frequency of the nth harmonic based on the distortion coefficient, the gain coefficient of the nth harmonic control loop is gradually reduced to make the power converter exit the frequency resonance of the nth harmonic. When the distortion coefficient is large, the adjustment frequency of the harmonic control loop coincides with or is approximately the resonant frequency of the grid-connected filter, resulting in resonance. The effect of the harmonic control loop weakens or disappears and becomes ineffective, thus suppressing the harmonic current.

Description

Technical Field

[0001] This application relates to the field of power electronics technology, specifically to a power converter and a method for harmonic suppression of grid-connected current. Background Technology

[0002] In wind and solar power generation scenarios, power converters connect renewable energy generation to the grid. The grid-connected current of the power converter needs to meet harmonic requirements, outputting a high-quality, stable grid-connected current. Because renewable energy output fluctuates significantly, and these large-scale fluctuations cause fluctuations in grid impedance, the power converter should be capable of suppressing grid-connected current harmonics under conditions of wide-range grid impedance fluctuations.

[0003] To address the harmonic problem of grid-connected current, power converters currently often employ the regulator suppression method, which involves adding multiple resonant regulators or proportional-integral (PI) regulators in a rotating coordinate system to suppress the harmonics that need to be suppressed in the grid-connected current.

[0004] However, when the grid impedance fluctuates over a wide range, the grid impedance couples with the grid-connected filter of the power converter, causing the resonant frequency of the grid-connected filter to also fluctuate over a wide range. This can easily cover the 5th, 7th, 11th, and 13th harmonic frequency ranges that the regulator operates on. When the resonant frequency of the grid-connected filter approaches or equals the regulation frequency of a certain harmonic control loop that operates on the regulator, it will trigger the resonance of the grid current at that frequency, which will amplify the amplitude of that current harmonic and may even cause the entire grid-connected system to become unstable. Summary of the Invention

[0005] In view of this, this application provides a power converter and a method for harmonic suppression of grid-connected current, which can suppress harmonics of grid-connected current under conditions of wide range of grid impedance fluctuations.

[0006] This application provides a power converter, including: a current sampling circuit, a power conversion circuit, and a controller; the output terminal of the power conversion circuit is used to connect to the power grid through a grid-connected filter;

[0007] Current sampling circuit, used to collect grid-connected current;

[0008] The controller is used to obtain the fundamental amplitude and the amplitude of the nth harmonic based on the grid-connected current. The nth harmonic is the harmonic to be suppressed, and n is an integer. The distortion coefficient is obtained based on the amplitude of the nth harmonic and the fundamental amplitude. Based on the distortion coefficient, it is determined that when the power converter is in frequency resonance of the nth harmonic, the gain coefficient of the nth harmonic control loop is gradually reduced so that the power converter exits the frequency resonance of the nth harmonic.

[0009] Preferably, the controller is specifically configured to obtain the distortion coefficient based on the ratio of the amplitude of the nth harmonic to the amplitude of the fundamental wave.

[0010] Preferably, the controller is specifically used to determine that the power converter has a frequency resonance of the nth harmonic when the distortion coefficient is greater than or equal to a first preset threshold, and gradually reduce the gain coefficient of the nth harmonic control loop.

[0011] Preferably, the controller is specifically configured to determine whether the distortion coefficient has decreased after each reduction of the gain coefficient of the nth harmonic control loop. If it has, the controller continues to reduce the gain coefficient of the nth harmonic control loop; otherwise, the controller stops reducing the gain coefficient of the nth harmonic control loop.

[0012] Preferably, the controller is further configured to gradually increase the gain coefficient of the nth harmonic control loop when the distortion coefficient is less than a first preset threshold and greater than a second preset threshold; the first preset threshold is greater than the second preset threshold.

[0013] Preferably, the controller is specifically configured to, after each increase in the gain coefficient of the nth harmonic control loop, determine whether the distortion coefficient decreases and whether the gain coefficient of the nth harmonic control loop is less than a preset maximum threshold. If so, the controller continues to increase the gain coefficient of the nth harmonic control loop; otherwise, it stops increasing the gain coefficient of the nth harmonic control loop.

[0014] Preferably, the first preset threshold is different when n takes different values;

[0015] The second preset threshold varies depending on the value of n.

[0016] Preferably, the controller is further configured to obtain a harmonic current control value based on the gain coefficient of the adjusted nth harmonic control loop, obtain a modulation voltage from the harmonic current control value and the fundamental current control value, and use the modulation voltage to generate a drive signal for the power conversion circuit.

[0017] This application provides a method for harmonic suppression of grid-connected current in a power converter, including:

[0018] Collect the grid-connected current of the power converter;

[0019] The fundamental amplitude and the amplitude of the nth harmonic are obtained from the grid-connected current. The nth harmonic is the harmonic to be suppressed, and n is an integer.

[0020] The distortion coefficient is obtained by comparing the amplitude of the nth harmonic with the amplitude of the fundamental wave.

[0021] Based on the distortion coefficient, when the power converter reaches the frequency resonance of the nth harmonic, the gain coefficient of the nth harmonic control loop is reduced.

[0022] Preferably, the distortion coefficient is obtained based on the amplitude of the nth harmonic and the fundamental amplitude, specifically including:

[0023] The distortion coefficient is obtained by the ratio of the amplitude of the nth harmonic to the amplitude of the fundamental wave.

[0024] Preferably, when the frequency of the power converter resonating at the nth harmonic is determined based on the distortion coefficient, the gain coefficient of the nth harmonic control loop is reduced, specifically including:

[0025] When the distortion coefficient is greater than or equal to the first preset threshold, it is determined that the power converter has the frequency resonance of the nth harmonic, and the gain coefficient of the nth harmonic control loop is gradually reduced.

[0026] Preferably, the gain coefficient of the nth harmonic control loop is gradually reduced, specifically including:

[0027] After each reduction of the gain coefficient of the nth harmonic control loop, it is determined whether the distortion coefficient has decreased. If it has, the gain coefficient of the nth harmonic control loop is further reduced; otherwise, the reduction of the gain coefficient of the nth harmonic control loop is stopped.

[0028] Preferably, it further includes:

[0029] When the distortion coefficient is less than the first preset threshold and greater than the second preset threshold, the gain coefficient of the nth harmonic control loop is gradually increased; the first preset threshold is greater than the second preset threshold.

[0030] Preferably, the gain coefficient of the nth harmonic control loop is gradually increased, specifically including:

[0031] After each increase in the gain coefficient of the nth harmonic control loop, it is determined whether the distortion coefficient decreases and whether the gain coefficient of the nth harmonic control loop is less than the preset maximum threshold. If so, the gain coefficient of the nth harmonic control loop is increased further; otherwise, the increase in the gain coefficient of the nth harmonic control loop is stopped.

[0032] Preferably, the first preset threshold is different when n takes different values;

[0033] The second preset threshold varies depending on the value of n.

[0034] Preferably, it further includes:

[0035] The harmonic current control value is obtained based on the gain coefficient of the adjusted nth harmonic control loop.

[0036] The modulation voltage is obtained from the harmonic current control value and the basic current control value;

[0037] The driving signal for the power conversion circuit is generated by using the modulated voltage.

[0038] Therefore, this application has the following beneficial effects:

[0039] The power converter provided in this application uses a controller to obtain the fundamental amplitude and the amplitude of the nth harmonic based on the grid-connected current. It then obtains the distortion coefficient based on the amplitude of the nth harmonic and the fundamental amplitude. Based on the distortion coefficient, it determines when the power converter resonates at the frequency of the nth harmonic and reduces the gain coefficient of the nth harmonic control loop. When the distortion coefficient is large, it indicates that the adjustment frequency of the harmonic control loop coincides with or is approximately the resonant frequency of the grid-connected filter, indicating resonance. Therefore, harmonic suppression is necessary. This requires weakening or eliminating the effect of the harmonic control loop. Specifically, this can be achieved by reducing the gain coefficient of the harmonic control loop, thus preventing it from having a counterproductive effect and preventing it from increasing the amplitude of the harmonic current, thereby suppressing the harmonic current. Attached Figure Description

[0040] Figure 1 A schematic diagram of a grid-connected system provided in an embodiment of this application;

[0041] Figure 2 A schematic diagram of a power converter provided in an embodiment of this application;

[0042] Figure 3 A schematic diagram of another power converter provided in an embodiment of this application;

[0043] Figure 4 A control principle diagram of a power converter provided in an embodiment of this application;

[0044] Figure 5 A flowchart illustrating a harmonic suppression method for grid-connected current of a power converter, provided in an embodiment of this application;

[0045] Figure 6 A flowchart illustrating another method for harmonic suppression of grid-connected current in a power converter, provided in an embodiment of this application;

[0046] Figure 7 A flowchart illustrating another method for suppressing harmonics in the grid-connected current of a power converter, provided in an embodiment of this application. Detailed Implementation

[0047] To make the technical solutions provided in the embodiments of this application easier to understand, the application scenarios will be introduced below with reference to the accompanying drawings.

[0048] See Figure 1 The figure is a schematic diagram of a grid-connected system provided in an embodiment of this application.

[0049] Wind power or photovoltaic power generation needs to pass through a power converter 1000 to transmit electrical energy to the power grid.

[0050] The power converter 1000 includes a power conversion circuit 100, a controller 200, and a grid-connected filter 300;

[0051] The power conversion circuit 100 converts electrical energy. A parallel filter 300 is connected to the output of the power conversion circuit 100 to filter the current and voltage output by the power conversion circuit 100. The output of the grid-connected filter 300 is connected to the power grid.

[0052] The grid side includes grid impedance.

[0053] The controller 200 is used to control the power conversion circuit 100 according to the grid connection parameters. Specifically, the controller 200 can send a wave to the switching transistor in the power conversion circuit 100, that is, send a drive signal, such as a PWM signal.

[0054] Due to the presence of the grid-connected filter, when the resonant frequency of the grid-connected filter approaches or equals a certain harmonic control loop of the regulator, it will trigger the resonance of the grid-connected current at that frequency, which will amplify the amplitude of the harmonic of that current and even cause instability of the entire power grid system.

[0055] Therefore, in order to solve the above technical problems, the technical solution provided in this application adjusts the parameters of the harmonic control loop and changes the frequency of the harmonic control loop when the resonant frequency of the grid-connected filter is detected to be close to or equal to the frequency of the harmonic control loop, so as not to cause the grid-connected current resonance at that frequency, thereby suppressing the harmonic current.

[0056] To make the above-mentioned objectives, features and advantages of this application more apparent and understandable, the embodiments of this application will be further described in detail below with reference to the accompanying drawings and specific implementation methods.

[0057] See Figure 2 This figure is a schematic diagram of a power converter provided in an embodiment of this application.

[0058] The power converter provided in this application embodiment includes: a current sampling circuit 400, a power conversion circuit 100, and a controller 200; the output terminal of the power conversion circuit 100 is used to connect to the power grid through a grid-connected filter 300;

[0059] The current sampling circuit 400 is used to collect grid-connected current. It should be understood that the current sampling circuit 400 can collect the current output by the power conversion circuit 100 or the current output by the grid-connected filter 300. Figure 2 The example given is that the current sampling circuit 400 can collect the output current of the power conversion circuit 100. In addition to harmonic control, the collected current can also detect whether there is an overcurrent in the output current of the power conversion circuit 100. Figure 3The current sampling circuit 400 collects the output current of the grid-connected filter 300.

[0060] The controller 200 is used to obtain the fundamental amplitude and the amplitude of the nth harmonic based on the grid-connected current, where the nth harmonic is the harmonic to be suppressed and n is an integer; to obtain the distortion coefficient based on the amplitude of the nth harmonic and the fundamental amplitude; to determine when the power converter is in frequency resonance of the nth harmonic based on the distortion coefficient; and to gradually reduce the gain coefficient of the nth harmonic control loop so that the power converter exits the frequency resonance of the nth harmonic.

[0061] It should be understood that the distortion coefficient is used to characterize whether the harmonics are severe, and thus to determine whether the frequency of the harmonic control loop is close to the resonant frequency of the grid-connected filter.

[0062] One specific implementation involves a controller that obtains the distortion coefficient based on the ratio of the amplitude of the nth harmonic to the amplitude of the fundamental wave.

[0063] When the distortion coefficient is large, it indicates that the harmonic frequency coincides with or is close to the resonant frequency of the grid-connected filter, resulting in resonance. Therefore, it is necessary to suppress the harmonics. At this time, it is necessary to weaken or eliminate the effect of the harmonic control loop. Specifically, the effect of the harmonic control loop can be weakened by reducing the gain coefficient of the harmonic control loop, so that the operating frequency of the harmonic control loop avoids the resonant frequency. In this way, it will not have a counterproductive effect, that is, it will not increase the amplitude of the harmonic current, thus playing a role in harmonic suppression.

[0064] The technical solution provided in this application embodiment can be applied to a wide range of grid impedance fluctuations without detecting grid impedance, thus achieving stable grid-connected operation of the power converter. Because the grid-connected current is detected in real time, the gain coefficient of the harmonic control loop can be adaptively adjusted, effectively suppressing grid current harmonics when the grid impedance fluctuates over a wide range.

[0065] Performing a Fast Fourier Transform (FFT) on the grid-connected current allows for real-time acquisition of the fundamental amplitude and the amplitude of the nth harmonic. It should be understood that the nth harmonic (n) is the harmonic that needs to be suppressed. n can take multiple values; for example, n can be at least one of 5, 7, 11, and 13. When n is 5, 7, 11, or 13, the controller can suppress the 5th harmonic current, the 7th harmonic current, the 11th harmonic current, and the 13th harmonic current, thus suppressing multiple harmonic currents. The gain coefficient and adjustment range of the harmonic control loop corresponding to different harmonic currents can be different. It should be understood that n can take other values ​​besides those mentioned above, corresponding to other harmonics, which will not be listed here.

[0066] The embodiments of this application do not specifically limit the number of phases of the power grid. It can also be a single-phase power grid or a three-phase power grid. The technical solutions provided in the embodiments of this application can be used to achieve harmonic suppression of the grid-connected current.

[0067] In addition, when the grid current oscillates, causing harmonic amplification, the grid voltage will also experience amplification of specific harmonics due to the existence of grid impedance. Therefore, identifying the specific harmonic distortion of the grid voltage can be indirectly used as a criterion for adjusting the gain coefficient of the harmonic control loop.

[0068] For example, a first preset threshold can be set. The controller is specifically used to determine the frequency resonance of the nth harmonic of the power converter when the distortion coefficient is greater than or equal to the first preset threshold, and gradually reduce the gain coefficient of the nth harmonic control loop. That is, the grid-connected current oscillates at the nth harmonic, and the gain of the control loop for that harmonic needs to be reduced.

[0069] It should be understood that when n takes different values, the corresponding first preset threshold is different; for example, the first preset threshold corresponding to the 5th harmonic can be different from the first preset threshold corresponding to the 7th harmonic. Each harmonic can be judged whether it needs to be suppressed using the corresponding first preset threshold.

[0070] The controller, specifically, determines whether the distortion coefficient decreases after each reduction of the gain coefficient of the nth harmonic control loop. If it does, the controller continues to reduce the gain coefficient of the nth harmonic control loop; otherwise, it stops reducing the gain coefficient. For example, if resonance has been eliminated and the harmonics are within the allowable range, the original gain coefficient adjustment mode can be restored. For instance, if reducing the gain coefficient of the nth harmonic control loop causes the distortion coefficient to increase, then the adjustment of the gain coefficient of the nth harmonic control loop should be stopped. Furthermore, the step size of each reduction in the gain coefficient can be varied to determine if the distortion coefficient can be further reduced. This application does not specifically limit the process of reducing the gain coefficient, as long as the general trend is to reduce the gain coefficient and thus lower the distortion coefficient.

[0071] In addition, when the power converter stops resonating, that is, the oscillation is restored and the harmonics are reduced or eliminated, but with the change of the grid impedance, it may be necessary to restore the previous gain of the harmonic control loop, that is, increase the gain coefficient. Therefore, a second preset threshold can also be set.

[0072] The controller is also used to gradually increase the gain coefficient of the nth harmonic control loop to a preset threshold when the distortion coefficient is less than a first preset threshold and greater than a second preset threshold; the first preset threshold is greater than the second preset threshold.

[0073] The controller, specifically, determines whether the distortion coefficient decreases and the gain coefficient of the nth harmonic control loop is less than a preset maximum threshold after each increase in the gain coefficient of the nth harmonic control loop. If so, it continues to increase the gain coefficient of the nth harmonic control loop; otherwise, it stops increasing the gain coefficient. For example, if the distortion coefficient increases instead of decreasing after increasing the gain coefficient of the nth harmonic control loop, it stops increasing the gain coefficient. Alternatively, the step size of each increase in the gain coefficient can be varied to see if the distortion coefficient can be further reduced. This application does not specifically limit the process of increasing the gain coefficient, as long as the general trend is to increase the gain coefficient and reduce the distortion coefficient.

[0074] When the distortion coefficient is between the first preset threshold and the second preset threshold, the gain coefficient of the harmonic control loop can be increased, indicating that harmonic suppression has been completed and the previous gain coefficient can be restored.

[0075] Similar to the first preset threshold, the second preset threshold varies depending on the value of n. For example, the second preset threshold for the 5th harmonic can be different from the second preset threshold for the 7th harmonic. Each harmonic can be evaluated using its corresponding second preset threshold to determine whether the gain of the harmonic control loop needs to be adjusted.

[0076] In addition, when the distortion coefficient is less than the second preset threshold, it means that the harmonic current meets the standard and no gain adjustment of the harmonic control loop is required.

[0077] Since the controller ultimately needs to control the switching transistors in the power conversion circuit, i.e., adjust the drive signals of the switching transistors, and thus adjust the output current of the power conversion circuit, the controller is also used to obtain the harmonic current control value based on the gain coefficient of the adjusted nth harmonic control loop, obtain the modulation voltage from the harmonic current control value and the basic current control value, and use the modulation voltage to generate the drive signal for the power conversion circuit.

[0078] In this embodiment, the adjustment of the gain coefficient of the harmonic control loop is incorporated into the adjustment of the drive signal of the switching transistor.

[0079] The control principle of a power converter provided in the embodiments of this application is described below.

[0080] See Figure 4 This figure is a control principle diagram of a power converter provided in an embodiment of this application.

[0081] Figure 4 The current sampled by the medium current sampling circuit 400 is iL_abc, and the distortion coefficient is obtained by the oscillation analysis unit.

[0082] The adaptive current harmonic control unit obtains the harmonic current control value Uharm_out based on the distortion coefficient and the dq axis current iL_dq;

[0083] The fundamental current control unit obtains the fundamental current control value Udq_out based on iL_dq and the dq-axis current reference value iL_dq_ref output by the power control unit.

[0084] The modulation voltage calculation unit outputs the modulation voltage Um_dq based on Uharm_out and Udq_out. Then, the two-phase rotation to three-phase stationary coordinate transformation unit obtains the three-phase modulation voltage um_abc, and finally, the PWM modulation unit outputs the PWM drive signal.

[0085] It should be understood that the controls described above are all located in the controller 200 and implemented by the controller 200.

[0086] Based on the power converter provided in the above embodiments, this application also provides a method for harmonic suppression of the grid-connected current of the power converter, which will be described in detail below with reference to the accompanying drawings.

[0087] See Figure 5 The figure is a flowchart of a harmonic suppression method for grid-connected current of a power converter provided in an embodiment of this application.

[0088] The harmonic suppression method for grid-connected current of the power converter provided in this application includes:

[0089] S501: Collects the grid-connected current output from the power conversion circuit;

[0090] S502: The fundamental amplitude and the amplitude of the nth harmonic are obtained from the grid-connected current. The nth harmonic is the harmonic to be suppressed, and n is an integer. Specifically, the fundamental amplitude and the amplitude of the nth harmonic can be obtained by means of fast Fourier transform or bandpass filter.

[0091] S503: Obtain the distortion coefficient based on the amplitude of the nth harmonic and the fundamental frequency amplitude;

[0092] The distortion coefficient is obtained based on the amplitude of the nth harmonic and the fundamental frequency amplitude, specifically including:

[0093] The distortion coefficient is obtained by the ratio of the amplitude of the nth harmonic to the amplitude of the fundamental wave.

[0094] S504: Based on the distortion coefficient, when the power converter reaches the frequency resonance of the nth harmonic, reduce the gain coefficient of the nth harmonic control loop.

[0095] It should be understood that the distortion coefficient is used to characterize whether the harmonics are severe, and thus to determine whether the frequency of the harmonic control loop is close to the resonant frequency of the grid-connected filter.

[0096] One specific implementation involves a controller that obtains the distortion coefficient based on the ratio of the amplitude of the nth harmonic to the amplitude of the fundamental wave.

[0097] When the distortion coefficient is large, it indicates that the harmonic frequency coincides with or is close to the resonant frequency of the grid-connected filter, resulting in resonance. Therefore, it is necessary to suppress the harmonics. At this time, it is necessary to weaken or eliminate the effect of the harmonic control loop. Specifically, the effect of the harmonic control loop can be weakened by reducing the gain coefficient of the harmonic control loop, so that the operating frequency of the harmonic control loop avoids the resonant frequency. In this way, it will not have a counterproductive effect, that is, it will not increase the amplitude of the harmonic current, thus playing a role in harmonic suppression.

[0098] The technical solution provided in this application embodiment can be applied to a wide range of grid impedance fluctuations without detecting grid impedance, thus achieving stable grid-connected operation of the power converter. Because the grid-connected current is detected in real time, the gain coefficient of the harmonic control loop can be adaptively adjusted, effectively suppressing grid current harmonics when the grid impedance fluctuates over a wide range.

[0099] By performing Fast Fourier Transform (FFT) or bandpass filtering on the grid-connected current, the fundamental amplitude and the amplitude of the nth harmonic can be obtained in real time. It should be understood that the nth harmonic (n) is the harmonic that needs to be suppressed. n can take multiple values; for example, n can be at least one of 5, 7, 11, and 13. When n is 5, 7, 11, and 13, the controller can suppress the 5th harmonic current, the 7th harmonic current, the 11th harmonic current, and the 13th harmonic current, thus suppressing multiple harmonic currents. The gain coefficient and adjustment range of the harmonic control loop corresponding to different harmonic currents can be different.

[0100] The following is a detailed description with reference to the accompanying drawings.

[0101] See Figure 6 The figure is a flowchart of another method for suppressing harmonics in the grid-connected current of a power converter provided in an embodiment of this application.

[0102] S601: Sample the grid-connected current and obtain the distortion coefficient based on the fundamental amplitude and the nth harmonic amplitude in the grid-connected current;

[0103] The distortion coefficient represents the proportion of harmonics.

[0104] S602: Determine whether the distortion coefficient is greater than or equal to the first preset threshold. If yes, execute S603; otherwise, maintain the original gain coefficient adjustment mode.

[0105] S603: Gradually decrease the gain coefficient of the nth harmonic control loop;

[0106] S604: Determine whether the distortion coefficient has decreased and the gain coefficient of the nth harmonic control loop is greater than zero. If not, execute S605; if yes, execute S603.

[0107] S605: Stop the gain coefficient decreasing adjustment mode. That is, enter the non-decreasing adjustment mode.

[0108] The following describes the adjustment strategy when the grid impedance changes in the non-decreasing adjustment mode.

[0109] See Figure 7 The figure is a flowchart of another harmonic suppression method for grid-connected current of a power converter provided in an embodiment of this application.

[0110] S701: Determine whether the distortion coefficient is less than the first preset threshold and greater than the second preset threshold; if yes, execute S702; otherwise, execute S704; the first preset threshold is greater than the second preset threshold.

[0111] S702: Gradually increase the gain coefficient of the nth harmonic control loop;

[0112] S703: Determine whether the distortion coefficient has decreased and the gain coefficient of the nth harmonic control loop is less than the preset maximum threshold. If not, execute S704; if yes, execute S702.

[0113] S704: Stop adjusting the gain coefficient of the nth harmonic control loop.

[0114] The harmonic suppression method for grid-connected current provided in this application embodiment further includes:

[0115] The harmonic current control value is obtained based on the gain coefficient of the adjusted nth harmonic control loop.

[0116] The modulation voltage is obtained from the harmonic current control value and the fundamental current control value;

[0117] The driving signal for the power conversion circuit is generated by using the modulated voltage.

[0118] Since the controller ultimately needs to control the switching transistors in the power conversion circuit, i.e., adjust the drive signals of the switching transistors, and thus adjust the output current of the power conversion circuit, the controller is also used to obtain the harmonic current control value based on the gain coefficient of the adjusted nth harmonic control loop, obtain the modulation voltage from the harmonic current control value and the fundamental current control value, and use the modulation voltage to generate the drive signal for the power conversion circuit.

[0119] It should be noted that the various embodiments in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments. Similar or identical parts between embodiments can be referred to interchangeably. For the systems or apparatus disclosed in the embodiments, since they correspond to the methods disclosed in the embodiments, the descriptions are relatively simple, and relevant parts can be referred to the method section.

[0120] The above description of the disclosed embodiments enables those skilled in the art to make or use this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A power converter, characterized in that, include: The system includes a current sampling circuit, a power conversion circuit, and a controller; the output of the power conversion circuit is used to connect to the power grid through a grid-connected filter. The current sampling circuit is used to collect grid-connected current; The controller is configured to obtain the fundamental amplitude and the amplitude of the nth harmonic based on the grid-connected current, wherein the nth harmonic is a harmonic to be suppressed, and n is an integer; obtain the distortion coefficient based on the amplitude of the nth harmonic and the fundamental amplitude; determine, based on the distortion coefficient, when the power converter is in frequency resonance of the nth harmonic, gradually reduce the gain coefficient of the nth harmonic control loop to make the power converter exit the frequency resonance of the nth harmonic.

2. The power converter according to claim 1, characterized in that, The controller is specifically used to obtain the distortion coefficient based on the ratio of the amplitude of the nth harmonic to the amplitude of the fundamental wave.

3. The power converter according to claim 1, characterized in that, Specifically, when the distortion coefficient is greater than or equal to a first preset threshold, the controller determines that the power converter has an nth harmonic frequency resonance and gradually reduces the gain coefficient of the nth harmonic control loop.

4. The power converter according to claim 1, characterized in that, The controller is specifically configured to determine whether the distortion coefficient has decreased after each reduction of the gain coefficient of the nth harmonic control loop. If it has, the controller continues to reduce the gain coefficient of the nth harmonic control loop; otherwise, the controller stops reducing the gain coefficient of the nth harmonic control loop.

5. The power converter according to claim 3, characterized in that, The controller is further configured to gradually increase the gain coefficient of the nth harmonic control loop when the distortion coefficient is less than the first preset threshold and greater than the second preset threshold; the first preset threshold is greater than the second preset threshold.

6. The power converter according to claim 5, characterized in that, The controller is specifically configured to determine whether the distortion coefficient decreases and the gain coefficient of the nth harmonic control loop is less than a preset maximum threshold after each increase in the gain coefficient of the nth harmonic control loop. If so, the controller continues to increase the gain coefficient of the nth harmonic control loop; otherwise, it stops increasing the gain coefficient of the nth harmonic control loop.

7. The power converter according to claim 5, characterized in that, When n takes different values, the corresponding first preset threshold is different; When n takes different values, the corresponding second preset threshold is different.

8. The power converter according to any one of claims 1-7, characterized in that, The controller is further configured to obtain a harmonic current control value based on the adjusted gain coefficient of the nth harmonic control loop, obtain a modulation voltage from the harmonic current control value and the fundamental current control value, and generate a drive signal for the power conversion circuit using the modulation voltage.

9. A method for harmonic suppression of grid-connected current in a power converter, characterized in that, include: Collect the grid-connected current of the power converter; The fundamental amplitude and the amplitude of the nth harmonic are obtained from the grid-connected current, where the nth harmonic is the harmonic to be suppressed and n is an integer. The distortion coefficient is obtained based on the amplitude of the nth harmonic and the amplitude of the fundamental wave. When the power converter reaches frequency resonance of the nth harmonic based on the distortion coefficient, the gain coefficient of the nth harmonic control loop is reduced.

10. The method according to claim 9, characterized in that, The process of obtaining the distortion coefficient based on the amplitude of the nth harmonic and the fundamental frequency amplitude specifically includes: The distortion coefficient is obtained by the ratio of the amplitude of the nth harmonic to the amplitude of the fundamental wave.

11. The method according to claim 9, characterized in that, When the frequency resonance of the power converter's nth harmonic is determined based on the distortion coefficient, the gain coefficient of the nth harmonic control loop is reduced, specifically including: When the distortion coefficient is greater than or equal to the first preset threshold, it is determined that the power converter has a frequency resonance of the nth harmonic, and the gain coefficient of the nth harmonic control loop is gradually reduced.

12. The method according to claim 11, characterized in that, The gradual reduction of the gain coefficient of the nth harmonic control loop specifically includes: After each reduction of the gain coefficient of the nth harmonic control loop, it is determined whether the distortion coefficient has decreased. If it has, the gain coefficient of the nth harmonic control loop is further reduced; otherwise, the reduction of the gain coefficient of the nth harmonic control loop is stopped.

13. The method according to claim 11, characterized in that, Also includes: When the distortion coefficient is less than the first preset threshold and greater than the second preset threshold, the gain coefficient of the nth harmonic control loop is gradually increased. The first preset threshold is greater than the second preset threshold.

14. The method according to claim 13, characterized in that, The gradual increase of the gain coefficient of the nth harmonic control loop specifically includes: After each increase in the gain coefficient of the nth harmonic control loop, it is determined whether the distortion coefficient decreases and whether the gain coefficient of the nth harmonic control loop is less than a preset maximum threshold. If so, the gain coefficient of the nth harmonic control loop is increased further; otherwise, the increase in the gain coefficient of the nth harmonic control loop is stopped.

15. The method according to claim 13, characterized in that, When n takes different values, the corresponding first preset threshold is different; When n takes different values, the corresponding second preset threshold is different.

16. The method according to any one of claims 9-15, characterized in that, Also includes: The harmonic current control value is obtained based on the adjusted gain coefficient of the nth harmonic control loop; The modulation voltage is obtained from the harmonic current control value and the basic current control value; The driving signal for the power conversion circuit is generated using the modulation voltage.

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