Current control method and three-phase grid-connected inverter

Abstract

The application provides a current control method and a three-phase grid-connected inverter. The current control method comprises the following steps: obtaining three-phase direct current components of three-phase grid-connected currents; converting the three-phase direct current components into first direct current components and second direct current components in a two-phase stationary coordinate system through Clarke transformation; inputting the first direct current components and the second direct current components into two independent proportional integral controllers respectively, and outputting first compensation signals and second compensation signals in the two-phase stationary coordinate system; converting the first compensation signals and the second compensation signals into three-phase compensation signals in a three-phase stationary coordinate system through Clarke inverse transformation; determining three-phase current signals after compensation based on the three-phase compensation signals and three-phase current sampling signals of the three-phase grid-connected inverter; and controlling the three-phase grid-connected inverter according to the three-phase current signals after compensation, so as to output grid-connected currents. The application aims to reduce the coupling of the three-phase direct current components and improve the precision and accuracy of the three-phase direct current component control.

Description

Technical Field

[0002] This application relates to the field of inverters, specifically to a current control method. Background Technology

[0003] Currently, for grid-connected inverters, the traditional method is to independently control the DC components of the three-phase current in a three-phase coordinate system. This leads to mutual coupling and interference between the three-phase DC components, thereby affecting the stability and accuracy of the three-phase DC component control. Summary of the Invention

[0004] The main objective of this application is to propose a current control method and a three-phase grid-connected inverter, which aims to reduce the coupling of the three-phase DC components and improve the precision and accuracy of the three-phase DC component control.

[0005] This application proposes a current control method applied to a three-phase grid-connected inverter, the current control method comprising: Obtain the three-phase DC components of the three-phase grid-connected current; The three-phase DC components are transformed into the first and second DC components in a two-phase stationary coordinate system using Clarke transformation. The first DC component and the second DC component are respectively input to two independent proportional-integral controllers, and the first compensation signal and the second compensation signal in the two-phase stationary coordinate system are output. The first compensation signal and the second compensation signal are converted into three-phase compensation signals in a three-phase stationary coordinate system by Clarke inverse transform; The compensated three-phase current signal is determined based on the three-phase compensation signal and the three-phase current sampling signal of the three-phase grid-connected inverter. The three-phase grid-connected inverter is current-controlled based on the compensated three-phase current signal to output grid-connected current.

[0006] Optionally, the Clarke transform employs a constant amplitude transform method; The positive transform coefficients of the Clarke transform are two-thirds; The inverse transform coefficients of the Clarke transform are three-half.

[0007] Optionally, obtaining the three-phase DC component of the three-phase grid-connected current includes: The three-phase grid-connected current sampling signal is subjected to low-pass filtering to extract the three-phase DC components.

[0008] Optionally, the step of inputting the first DC component and the second DC component to two independent proportional-integral controllers respectively, and outputting the first compensation signal and the second compensation signal in the two-phase stationary coordinate system includes: The difference between the first DC component and the preset reference value is taken as the first error value. The first error value is input to the first proportional-integral controller, and the first compensation signal is output. The difference between the second DC component and the preset reference value is used as the second error value. The second error value is input to the second proportional-integral controller, and the second compensation signal is output.

[0009] Optionally, the preset reference value is zero.

[0010] Optionally, determining the compensated three-phase current signal based on the three-phase compensation signal and the three-phase current sampling signal of the three-phase grid-connected inverter includes: The three-phase current sampling signal is superimposed with the three-phase compensation signal to obtain the compensated three-phase current signal.

[0011] Optionally, the step of controlling the current of the three-phase grid-connected inverter based on the compensated three-phase current signal to output grid-connected current includes: The compensated three-phase current signal is converted into a first current component and a second current component in a rotating coordinate system through Park transformation. The three-phase grid-connected inverter is current-controlled based on the first current component and the second current component to output grid-connected current.

[0012] Optionally, the step of performing current control on the three-phase grid-connected inverter based on the first current component and the second current component to output grid-connected current includes: The first current component is compared with the first current command to obtain the first current error, and the second current component is compared with the second current command to obtain the second current error. The three-phase grid-connected inverter is current-controlled based on the first current error and the second current error to output grid-connected current.

[0013] Optionally, the step of performing current control on the three-phase grid-connected inverter based on the first current error and the second current error to output grid-connected current includes: The first current error and the second current error are input into the current controller to generate a control signal, which controls the three-phase grid-connected inverter to output the grid-connected current.

[0014] This application also proposes a three-phase grid-connected inverter, the three-phase grid-connected inverter including a control module configured to perform the current control method as described in any of the preceding claims.

[0015] This application proposes a current control method applied to a three-phase grid-connected inverter. The current control method includes: acquiring the three-phase DC components of the three-phase grid-connected current; converting the three-phase DC components into a first DC component and a second DC component in a two-phase stationary coordinate system using Clarke transform; inputting the first DC component and the second DC component to two independent proportional-integral (PI) controllers respectively, and outputting a first compensation signal and a second compensation signal in the two-phase stationary coordinate system; converting the first compensation signal and the second compensation signal into a three-phase compensation signal in a three-phase stationary coordinate system using inverse Clarke transform; determining the compensated three-phase current signal based on the three-phase compensation signal and the three-phase current sampling signal of the three-phase grid-connected inverter; and performing current control on the three-phase grid-connected inverter according to the compensated three-phase current signal to output the grid-connected current. Through the above setup, DC component control is performed in the α-β two-phase stationary coordinate system. Utilizing the natural orthogonality and mathematical decoupling of the α-axis and β-axis, there is no cross-coupling path between the two PI controllers, reducing coupling interference from the control structure perspective and improving the accuracy and precision of the three-phase DC component control. Attached Figure Description

[0016] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0017] Figure 1 This is a schematic flowchart of an embodiment of the current control method and three-phase grid-connected inverter of this application. Figure 2 This is a schematic diagram of the current control method and the method for obtaining the three-phase DC component of the three-phase grid-connected current in the three-phase grid-connected inverter of this application. Figure 3 This is a schematic flowchart of the current control method and the method for outputting the first compensation signal and the second compensation signal in a three-phase grid-connected inverter of this application. Figure 4 This is a schematic diagram of the current control method and the method for controlling the current of a three-phase grid-connected inverter in this application. Figure 5 This is a schematic diagram of the current control method and the method for controlling the current of a three-phase grid-connected inverter based on a first current component and a second current component, as described in this application. Figure 6This is a schematic diagram illustrating the current control method and the method for controlling the current of a three-phase grid-connected inverter using a current controller, as described in this application.

[0018] The realization of the purpose, functional features and advantages of this application will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0019] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of the embodiments. Based on the embodiments of this application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of this application.

[0020] It should be noted that if the embodiments of this application involve directional indicators (such as up, down, left, right, front, back, etc.), the directional indicators are only used to explain the relative positional relationship and movement of the components in a specific posture. If the specific posture changes, the directional indicators will also change accordingly.

[0021] Furthermore, if the embodiments of this application involve descriptions such as "first" or "second," these descriptions are for descriptive purposes only and should not be construed as indicating or implying their relative importance or implicitly specifying the number of technical features indicated. Therefore, a feature defined with "first" or "second" may explicitly or implicitly include at least one of those features. Additionally, the use of "and / or" or "and / or" throughout the text includes three parallel solutions. For example, "A and / or B" includes solution A, solution B, or a solution that simultaneously satisfies A and B. Furthermore, the technical solutions of the various embodiments can be combined with each other, but this must be based on the ability of those skilled in the art to implement them. When the combination of technical solutions is contradictory or impossible to implement, it should be considered that such a combination of technical solutions does not exist and is not within the scope of protection claimed in this application.

[0022] Currently, for grid-connected inverters, the traditional method is to independently control the DC components of the three-phase current in a three-phase coordinate system. This leads to mutual coupling and interference between the three-phase DC components, thereby affecting the stability and accuracy of the three-phase DC component control.

[0023] Therefore, this application proposes a current control method and a three-phase grid-connected inverter, aiming to reduce the coupling of the three-phase DC components and improve the precision and accuracy of the three-phase DC component control.

[0024] This application proposes a current control method applied to a three-phase grid-connected inverter, referencing... Figure 1In one embodiment of this application, the current control method includes: S100: Obtain the three-phase DC component of the three-phase grid-connected current; S200. The three-phase DC components are converted into the first and second DC components in a two-phase stationary coordinate system through Clarke transformation. S300: Input the first DC component and the second DC component to two independent proportional-integral controllers respectively, and output the first compensation signal and the second compensation signal in the two-phase stationary coordinate system; S400: Convert the first compensation signal and the second compensation signal into a three-phase compensation signal in a three-phase stationary coordinate system through Clarke inverse transformation; S500: Determine the compensated three-phase current signal based on the three-phase compensation signal and the three-phase current sampling signal of the three-phase grid-connected inverter; S600 performs current control on the three-phase grid-connected inverter based on the compensated three-phase current signal to output grid-connected current.

[0025] In this embodiment, optionally, the three-phase grid-connected inverter includes a control module, which can be implemented using a main controller, such as an MCU, PLC, DSP (Digital Signal Processor), SOC (System on Chip), FPGA (Field Programmable Gate Array), dual-core BLE5.0 microcontroller chip, etc. This control module is used to execute current control methods.

[0026] Optionally, the control module can sample the three-phase grid-connected current using its built-in analog-to-digital converter (ADC) to obtain instantaneous current sampling signals for phases A, B, and C, respectively. Alternatively, the control module can directly acquire the three-phase grid-connected current from the inverter output side sampled by a Hall sensor or current transformer and convert it into a digital signal using the ADC. Subsequently, the control module uses a moving average filtering method or a low-pass filtering method to extract the DC bias component from the sampled three-phase grid-connected current, denoted as the three-phase DC components Idc_a, Idc_b, and Idc_c. These three-phase DC components reflect the DC bias introduced into the grid-connected current due to factors such as sampling zero drift, device asymmetry, or control errors.

[0027] Optionally, the three-phase DC components Idc_a, Idc_b, and Idc_c obtained in S100 are substituted into the Clarke transformation matrix for coordinate transformation. The Clarke transformation is a mathematical transformation that converts the components in a three-phase stationary coordinate system (abc) into components in a two-phase stationary coordinate system (α-β). Through the above calculation, the first DC component Idc_α (corresponding to the α-axis) and the second DC component Idc_β (corresponding to the β-axis) in the two-phase stationary coordinate system are obtained. This transformation decouples the three-phase coupled DC components into orthogonal two-phase components, facilitating subsequent independent adjustment.

[0028] Optionally, the first DC component Idc_α obtained in S200 is used as the input signal of the first proportional-integral controller (PI controller), and the second DC component Idc_β is used as the input signal of the second PI controller. The two PI controllers operate independently, each performing proportional-integral calculations and outputting a first compensation signal and a second compensation signal, respectively. Their physical meaning is the voltage compensation amount injected in the two-phase stationary coordinate system to offset the DC component. By setting appropriate PI parameters, the compensation signal can track the changes in the DC component and tend to converge.

[0029] Optionally, the compensation signals obtained from the two-phase stationary coordinate system in S300 are used to calculate three-phase compensation signals Idc_offset_a, Idc_offset_b, and Idc_offset_c. These three-phase compensation signals are the compensation amounts used to correct the original current sampling values.

[0030] Optionally, the control module acquires the raw sampling signals of the three-phase grid-connected current directly measured by the sampling circuit within the current cycle, denoted as I_a, I_b, and I_c. These raw sampling signals may contain DC bias errors. The three-phase compensation signals Idc_offset_a, Idc_offset_b, and Idc_offset_c obtained from S400 are superimposed with these three-phase current sampling signals to obtain the compensated three-phase current signals. This compensated signal has removed the influence of DC bias and more accurately reflects the AC fundamental current output by the inverter.

[0031] Optionally, the compensated three-phase current signal obtained in S500 is used as the feedback value for the current closed-loop control and fed into the current inner-loop controller of the three-phase grid-connected inverter. This current inner-loop controller can employ existing proportional resonant control or feedforward decoupling PI control strategies to generate corresponding modulation wave signals. After PWM modulation, this modulation wave signal generates a drive signal to control the switching of the inverter's power switching devices, such as IGBTs or SiC MOSFETs, thereby driving the inverter to output grid-connected current.

[0032] With the above settings, the DC component is controlled in the α-β two-phase stationary coordinate system. By utilizing the natural orthogonality and mathematical decoupling between the α and β axes, there is no cross-coupling path between the two PI controllers. This reduces coupling interference from the control structure and improves the precision and accuracy of the three-phase DC component control.

[0033] According to some embodiments of this application, optionally, in one embodiment of this application, the Clarke transform adopts an equal-amplitude transform method; The positive transform coefficients of the Clarke transform are two-thirds; The inverse transform coefficients of the Clarke transform are three-half.

[0034] In this embodiment, optionally, in actual three-phase grid-connected inverters, Clarke conversion has two common conversion methods: equal power conversion and equal amplitude conversion. This embodiment adopts the equal amplitude conversion method, so that the three-phase variables before and after the conversion are consistent with the two-phase variables in terms of amplitude, which facilitates the subsequent parameter setting and limiting processing of the PI controller.

[0035] Optionally, the positive transformation coefficient before the Clarke transformation matrix is ​​2 / 3. Through this equal-amplitude transformation, if the three-phase DC components Idc_a, Idc_b, and Idc_c are three-phase balanced quantities with equal amplitudes, then the amplitudes of the transformed Idc_α and Idc_β are equal to the amplitudes of these three-phase quantities. For example, if all three-phase DC components are DC biases with an amplitude of A, i.e., Idc_a = A, Idc_b = A, Idc_c = A, then after the above transformation, the amplitudes of Idc_α and Idc_β remain unchanged at A.

[0036] Optionally, the inverse transform coefficients before the Clarke transform matrix are 3 / 2. These inverse transform coefficients are reciprocals of the forward transform coefficients (2 / 3), ensuring the amplitude consistency of the entire coordinate transformation link—from three-phase to two-phase and then back from two-phase to three-phase. In other words, performing the inverse transform after the forward transform can recover the original three-phase signal amplitude to the greatest extent.

[0037] By setting the parameters as described above, the Clarke transform is limited to an equal-amplitude transformation method, with the forward transform coefficient set to 2 / 3 and the inverse transform coefficient to 3 / 2, ensuring that the three-phase DC components maintain the same amplitude as the two-phase DC components. This amplitude consistency gives the physical quantities of the control system a clear amplitude correspondence. When tuning the PI controller parameters, engineers can design according to the actual amplitude of the current loop without needing to perform additional coordinate transformation coefficient calculations, thus simplifying the engineering debugging process of the control system.

[0038] According to some embodiments of this application, optionally, in one embodiment of this application, reference is made to... Figure 2 To obtain the three-phase DC components of the three-phase grid-connected current, including: S110. Perform low-pass filtering on the three-phase grid-connected current sampling signal to extract the three-phase DC components.

[0039] In this embodiment, it is important to understand that in a real three-phase grid-connected inverter, the three-phase grid-connected current is obtained by sampling using current sensors, such as Hall effect sensors or current transformers. Since the grid-connected current is mainly a 50Hz or 60Hz power frequency AC signal, the DC component contained therein is usually smaller in amplitude than the fundamental AC component. However, if the average value of the sampled signal is directly calculated, it will be strongly interfered with by the fundamental AC component, making it impossible to accurately extract the DC component.

[0040] Optionally, after the control module acquires the raw sampling signals of the three-phase grid-connected current, each phase current sampling signal is sent to an independent low-pass filter (LPF) for filtering. This low-pass filter can be a first-order low-pass filter or a moving average filter. After the above low-pass filtering, the AC component in each phase current sampling signal is significantly attenuated, while the DC bias component is retained, thus obtaining the three-phase DC components, denoted as Idc_a, Idc_b, and Idc_c.

[0041] With the above settings, the low-pass filter can effectively separate the DC component and the AC fundamental component in the frequency domain. This method is simple to implement, requires little computation, and does not require complex signal processing algorithms, making it suitable for embedded implementation in grid-connected inverter control systems with high real-time requirements.

[0042] According to some embodiments of this application, optionally, in one embodiment of this application, reference is made to... Figure 3 The first DC component and the second DC component are respectively input to two independent proportional-integral controllers, and the output first compensation signal and second compensation signal in the two-phase stationary coordinate system include: S310. The difference between the first DC component and the preset reference value is taken as the first error value. The first error value is input to the first proportional-integral controller and the first compensation signal is output. S320. The difference between the second DC component and the preset reference value is taken as the second error value. The second error value is input to the second proportional-integral controller and the second compensation signal is output.

[0043] In this embodiment, optionally, a preset reference value is first set, which is used to characterize the desired DC component target value.

[0044] In one example, the preset reference value is zero because the ideal grid-connected current of a three-phase grid-connected inverter should be a pure AC sine wave without any DC bias component. Therefore, the target values ​​of the DC components in the two-phase stationary coordinate system are both expected to be zero. When the first DC component Idc_α is positive, the first error is negative, and the first PI controller outputs a negative compensation signal. This negative compensation signal, after being superimposed with the current sampling signal, will cancel out the positive DC bias.

[0045] Optionally, the first DC component Idc_α is subtracted from a preset reference value to obtain a first error, and the second DC component Idc_β is subtracted from the preset reference value to obtain a second error.

[0046] Optionally, the time-domain expression of the PI controller is: K p Ki is the proportional coefficient of the PI controller, e(t) represents the first error or the second error, and V(t) represents the first compensation signal or the second compensation signal obtained.

[0047] In another example, if the grid connection standard allows for a non-zero DC bias, or if the system has special requirements that necessitate the injection of a specific DC component, the preset reference value can also be set to a non-zero value.

[0048] With the above setup, the two PI controllers are completely independent, and the control of the α-axis and β-axis does not affect each other. This decoupling characteristic simplifies the controller design and parameter tuning process. Engineers can independently debug the parameters of the two PI controllers without considering the effects of inter-axis coupling, reducing debugging difficulty and shortening the development cycle.

[0049] According to some embodiments of this application, optionally, in one embodiment of this application, the preset reference value is zero.

[0050] In this embodiment, optionally, when it is necessary to calculate the error, the control system obtains the value stored in the preset reference value register, which is configured as 0 in this embodiment.

[0051] It's important to understand that setting the preset reference value to zero has the physical meaning that the ideal grid-connected current should be a pure AC sine wave with zero DC component. In a three-phase stationary coordinate system (abc), the three-phase DC components of the ideal grid-connected current are all zero; correspondingly, after the Clarke transformation, the DC components Idc_α and Idc_β in the two-phase stationary coordinate system (α-β) are also zero. Therefore, setting the preset reference value to zero, i.e., using a sine wave without DC bias as the control target, aims to eliminate the extracted actual DC component through the closed-loop adjustment of the PI controller.

[0052] Optionally, when the preset reference value is zero, the input to the PI controller is actually the negative value of the DC component itself, and the entire control loop constitutes a typical negative feedback closed-loop system. The setpoint of this closed-loop system is zero, and the feedback value is the extracted DC component. The PI controller adjusts the output compensation signal to make the feedback value track the given zero value.

[0053] By setting the preset reference value to zero, the ideal control objective of zero DC bias in the grid-connected current is directly achieved. This setting ensures that the control objective of the entire control system is clear and unambiguous: to suppress the DC component of the grid-connected current to the greatest extent possible.

[0054] According to some embodiments of this application, optionally, in one embodiment of this application, determining the compensated three-phase current signal based on the three-phase compensation signal and the three-phase current sampling signal of the three-phase grid-connected inverter includes: The three-phase current sampling signal is superimposed with the three-phase compensation signal to obtain the compensated three-phase current signal.

[0055] In this embodiment, optionally, after obtaining the three-phase compensation signal through Clarke inverse transform in S400, the control module acquires the original sampling signal of the three-phase grid-connected current within the current control cycle. This original sampling signal of the three-phase current includes the actual AC fundamental current component output by the inverter, and also includes DC bias error introduced by factors such as zero drift of the sampling circuit, sensor bias, asymmetry of the drive signal, or inconsistent on-state voltage drop of the power devices.

[0056] In one example, assuming that at a certain moment, the three-phase compensation signals obtained through steps S10 to S40 are +0.05A, -0.03A, and -0.02A, while the original sampled signals are I_a=5.05A, I_b=4.97A, and I_c=4.98A, then the compensated three-phase current signals obtained after superposition are Icorr_a=5.10A, Icorr_b=4.94A, and Icorr_c=4.96A. After this compensated current signal is subsequently sent as feedback value to the current loop controller, the current loop will adjust the modulation waveform accordingly, so that the positive DC bias of phase a in the actual grid-connected current output by the inverter is canceled.

[0057] It is important to understand that superposition can be either arithmetic addition or equivalently subtraction. For example, when the compensation signal is negative, addition is essentially equivalent to subtracting a positive value. In this embodiment, since the three-phase compensation signal itself contains sign information, a positive value indicates positive compensation and a negative value indicates reverse compensation. Therefore, the general addition expression covers all cases.

[0058] By superimposing the three-phase compensation signal with the three-phase current sampling signal using the above settings, the original sampling signal is corrected. This superposition method has a clear physical meaning and simple mathematical operations, requiring only three addition operations in each control cycle of the digital controller. Its computational complexity is low, making it suitable for grid-connected inverter control with high-frequency switching.

[0059] According to some embodiments of this application, optionally, in one embodiment of this application, reference is made to... Figure 4 The three-phase grid-connected inverter is current-controlled based on the compensated three-phase current signal to output the grid-connected current, including: S610. The compensated three-phase current signal is converted into the first current component and the second current component in the rotating coordinate system through Park transformation. S620 performs current control on the three-phase grid-connected inverter based on the first current component and the second current component to output grid-connected current.

[0060] In this embodiment, optionally, the Park transformation is a coordinate transformation that converts the AC signal in a three-phase stationary coordinate system (abc) into a DC signal in a two-phase rotating coordinate system (dq). Through the Park transformation, the compensated three-phase current signal is converted into a first current component and a second current component, which facilitates subsequent tracking control using a PI controller.

[0061] It should be noted that since the DC bias component in the compensated three-phase current signal has been effectively canceled or significantly attenuated in the aforementioned steps, the second harmonic fluctuation component caused by DC bias in the current signal obtained after Park transformation will be reduced, making the feedback signal of the current loop more stable.

[0062] Optionally, the first and second current components obtained in sub-step S61 are used as feedback values ​​for the d-axis current loop and q-axis current loop, respectively, and sent to the inner current loop controller to obtain the d-axis voltage command and q-axis voltage command. Subsequently, the d-axis voltage command and q-axis voltage command are transformed back from the rotating coordinate system to the two-phase stationary coordinate system through Park inverse transformation, and then subjected to space vector pulse width modulation or sinusoidal pulse width modulation to generate a three-phase modulated wave signal. After being amplified by the drive circuit, this three-phase modulated wave signal controls the switching of power switching devices in the three-phase grid-connected inverter, such as IGBTs, SiC MOSFETs, or GaN HEMTs, thereby driving the inverter to output the desired grid-connected current.

[0063] It is important to understand that, because the DC bias in the compensated three-phase current signal, which serves as the feedback input, is reduced, the feedback value received by the current loop controller more accurately reflects the fundamental current component of the inverter's actual output. Therefore, the additional component used to offset the DC bias in the voltage command output by the current loop is also reduced, and the asymmetric modulation or overmodulation that occurs in PWM modulation due to the need for additional compensation for the DC component is also reduced.

[0064] With the above settings, the compensated three-phase current signal is converted into a DC component in a rotating coordinate system using the Park transform, allowing the current loop to use a traditional PI controller to track and control the AC fundamental current. The PI controller has a greater gain for DC signals, enabling the grid-connected current to accurately track the current command value, achieving high-precision active and reactive power control.

[0065] According to some embodiments of this application, optionally, in one embodiment of this application, reference is made to... Figure 5 The three-phase grid-connected inverter is current-controlled based on the first and second current components to output grid-connected current, including: S621. Compare the first current component with the first current command to obtain the first current error, and compare the second current component with the second current command to obtain the second current error; S622. Based on the first current error and the second current error, the three-phase grid-connected inverter is current controlled to output grid-connected current.

[0066] In this embodiment, optionally, the first current command is a d-axis current command value, typically calculated by the PI controller of the DC bus voltage outer loop based on the deviation between the given and feedback values ​​of the DC bus voltage. This value is used to control the active power output of the inverter and maintain the stability of the DC bus voltage. The second current command is a q-axis current command value, typically calculated from a reactive power command or a power factor angle command. This value is used to control the reactive power output of the inverter, achieving reactive power compensation or power factor regulation for the power grid. These two command values ​​are well-known control parameters in the art, and their specific acquisition methods are not improvements of this invention and will not be elaborated upon here.

[0067] It should be noted that since the first and second current components input to the error calculation stage are obtained by Park transformation of the three-phase current signals after DC bias compensation, there are fewer second harmonic fluctuation components caused by DC bias in the first and second current components. This makes the input signals of the d-axis and q-axis PI controllers more stable, resulting in lower harmonic content in the grid-connected current output of the final inverter and effective suppression of the DC component.

[0068] By comparing the d-axis and q-axis current components with their respective current commands to form two independent current error signals, decoupled and independent control of active and reactive current is achieved. This dq-axis decoupled control architecture allows the inverter to independently adjust active and reactive power, meeting the functional requirements of grid-connected inverters for adjustable power factor and reactive power support.

[0069] According to some embodiments of this application, optionally, in one embodiment of this application, reference is made to... Figure 6 The current control of the three-phase grid-connected inverter is based on the first current error and the second current error, so that the output grid-connected current includes: S6221. Input the first current error and the second current error into the current controller to generate a control signal, and control the three-phase grid-connected inverter to output the grid-connected current.

[0070] In this embodiment, optionally, the current controller is the core control unit in the three-phase grid-connected inverter control system. Its physical implementation can be a software control algorithm module running in a digital signal processor (DSP), microcontroller (MCU), or field-programmable gate array (FPGA). The current controller receives a first current error and a second current error as inputs, performs internal adjustment calculations, and outputs corresponding control signals.

[0071] In one example, the current controller includes a first current regulator and a second current regulator. The first current regulator receives a first current error and performs proportional-integral (PI) calculations, while the second current regulator receives a second current error and performs the same calculations. The two regulators operate independently and in parallel, outputting d-axis and q-axis voltage commands. These commands are then inversely transformed using Park and input to a pulse width modulation (PWM) module. This PWM module can be a space vector PWM module or a sinusoidal PWM module. Based on the input voltage commands, the PWM module generates six or more PWM drive signals. These signals are amplified and isolated by a drive circuit and then used to control the gates of various power switching devices (such as IGBTs or SiC MOSFETs) in the three-phase grid-connected inverter, thereby controlling the on-time of the power switching devices and controlling the amplitude, frequency, and phase of the inverter's output voltage.

[0072] With the above settings, the error signal input to the current controller is calculated by Park transforming the compensated three-phase current signal obtained after processing such as DC component extraction, PI compensation, and compensation signal superposition in the aforementioned steps. Therefore, when the current controller generates control signals based on these error signals, these control signals already contain the adjustment component required to eliminate DC bias, so that the DC bias component in the final output grid-connected current is effectively suppressed.

[0073] This application also proposes a three-phase grid-connected inverter, which includes a control module configured to perform the current control method as described in any of the preceding claims.

[0074] In this embodiment, the method and technical effects performed by the control module have been described in detail in the above embodiments, and will not be repeated here.

[0075] The above description is merely an exemplary embodiment of this application and does not limit the patent scope of this application. Any equivalent structural transformations made based on the technical concept of this application and the contents of the specification and drawings of this application, or direct / indirect applications in other related technical fields, are included within the patent protection scope of this application.

Claims

1. A current control method applied to a three-phase grid-connected inverter, characterized in that, The current control method includes: Obtain the three-phase DC components of the three-phase grid-connected current; The three-phase DC components are transformed into the first and second DC components in a two-phase stationary coordinate system using Clarke transformation. The first DC component and the second DC component are respectively input to two independent proportional-integral controllers, and the first compensation signal and the second compensation signal in the two-phase stationary coordinate system are output. The first compensation signal and the second compensation signal are converted into three-phase compensation signals in a three-phase stationary coordinate system by Clarke inverse transform; The compensated three-phase current signal is determined based on the three-phase compensation signal and the three-phase current sampling signal of the three-phase grid-connected inverter. The three-phase grid-connected inverter is current-controlled based on the compensated three-phase current signal to output grid-connected current.

2. The current control method according to claim 1, characterized in that, The Clarke transform uses a constant amplitude transform method; The positive transform coefficients of the Clarke transform are two-thirds; The inverse transform coefficients of the Clarke transform are three-half.

3. The current control method according to claim 1, characterized in that, The acquisition of the three-phase DC component of the three-phase grid-connected current includes: The three-phase grid-connected current sampling signal is subjected to low-pass filtering to extract the three-phase DC components.

4. The current control method according to claim 1, characterized in that, The step of inputting the first DC component and the second DC component to two independent proportional-integral controllers respectively, and outputting the first compensation signal and the second compensation signal in the two-phase stationary coordinate system includes: The difference between the first DC component and the preset reference value is taken as the first error value. The first error value is input to the first proportional-integral controller, and the first compensation signal is output. The difference between the second DC component and the preset reference value is used as the second error value. The second error value is input to the second proportional-integral controller, and the second compensation signal is output.

5. The current control method according to claim 4, characterized in that, The preset reference value is zero.

6. The current control method according to claim 1, characterized in that, The determination of the compensated three-phase current signal based on the three-phase compensation signal and the three-phase current sampling signal of the three-phase grid-connected inverter includes: The three-phase current sampling signal is superimposed with the three-phase compensation signal to obtain the compensated three-phase current signal.

7. The current control method according to claim 1, characterized in that, The step of controlling the current of the three-phase grid-connected inverter based on the compensated three-phase current signal to output grid-connected current includes: The compensated three-phase current signal is converted into a first current component and a second current component in a rotating coordinate system through Park transformation. The three-phase grid-connected inverter is current-controlled based on the first current component and the second current component to output grid-connected current.

8. The current control method according to claim 7, characterized in that, The current control of the three-phase grid-connected inverter based on the first current component and the second current component to output grid-connected current includes: The first current component is compared with the first current command to obtain the first current error, and the second current component is compared with the second current command to obtain the second current error. The three-phase grid-connected inverter is current-controlled based on the first current error and the second current error to output grid-connected current.

9. The current control method according to claim 8, characterized in that, The current control of the three-phase grid-connected inverter based on the first current error and the second current error to output grid-connected current includes: The first current error and the second current error are input into the current controller to generate a control signal, which controls the three-phase grid-connected inverter to output the grid-connected current.

10. A three-phase grid-connected inverter, characterized in that, The three-phase grid-connected inverter includes a control module configured to perform the current control method as described in any one of claims 1 to 9.

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