Photovoltaic grid-connected box control method and system suitable for active harmonic suppression

Abstract

The invention relates to the technical field of photovoltaic grid-connected control, and discloses a photovoltaic grid-connected box control method and system suitable for active harmonic suppression, and the system comprises a photovoltaic assembly module, a grid-connected inverter and a composite repetitive control module. According to the system, solar energy is converted into direct current through a photovoltaic assembly module, then the direct current is converted into alternating current through a grid-connected inverter to be connected into a power grid, a composite repetitive control module is used for detecting grid-connected current harmonics, generating a total harmonic distortion rate and judging whether a photovoltaic grid-connected box meets grid-connected conditions or not, and if the total harmonic distortion rate is smaller than or equal to 5%, the grid-connected conditions are met; and if the total harmonic distortion rate is greater than 5%, a harmonic suppression measure should be taken in time, so that compared with a traditional control system, the photovoltaic grid-connected box has better harmonic suppression capability, realizes active suppression of the photovoltaic grid-connected box on harmonic current caused by power grid background harmonic waves and nonlinear loads, and has great significance in guaranteeing the electric energy quality in the photovoltaic grid-connected process. And the method has a positive effect on promoting distributed power generation friendly grid connection and the like.

Description

Technical Field

[0001] This invention relates to the field of photovoltaic grid-connected control technology, specifically to a photovoltaic grid-connected box control method and system suitable for active harmonic suppression. Background Technology

[0002] With the continuous expansion of installed capacity of distributed new energy sources such as photovoltaics, photovoltaic grid-connected boxes, as a medium for transmission between renewable energy and electrical energy, have been widely used in distribution networks. According to the technical requirements of photovoltaic grid-connected inverters, the total harmonic distortion (THD) of the grid-connected current in conventional photovoltaic grid-connected boxes should be kept below 5%. Therefore, designing a control method for photovoltaic grid-connected boxes suitable for active harmonic suppression, and achieving active suppression of grid-connected harmonic current, is of great significance and a practical necessity for photovoltaic grid-connected boxes to meet grid connection standards.

[0003] Currently, traditional photovoltaic grid-connected box control methods and systems only consider that photovoltaic modules convert DC power into AC power and feed it into the grid via grid-connected inverters, and are equipped with certain protection devices. However, they do not have corresponding configurations for active harmonic suppression. With the penetration of nonlinear loads such as rectifiers and frequency converters in the system, coupled with the influence of background harmonics in the power grid, photovoltaic grid-connected boxes face complex and severe harmonic threats, affecting the power quality of grid-connected current. Summary of the Invention

[0004] (a) Technical problems to be solved To address the shortcomings of existing technologies, this invention provides a photovoltaic grid-connected box control method and system suitable for active harmonic suppression. It has advantages such as high dynamic response control accuracy and excellent active suppression current quality, solving the problems of traditional photovoltaic grid-connected box control methods and systems lacking active harmonic suppression function and having poor grid-connected current quality.

[0005] (II) Technical Solution To achieve the above objectives, the present invention provides the following technical solution: a photovoltaic grid-connected box control method suitable for active harmonic suppression, comprising the following steps: Step 1: Obtain the reference current of the grid-connected inverter through the photovoltaic grid-connected box, and control the grid-connected inverter to generate the grid-connected current, as well as the error signal between the reference current and the grid-connected current; Step 2: Based on the error signal between the reference current and the grid-connected current, perform forward processing and control adjustment, and output the corresponding feedback signal; Step 3: Calculate the total harmonic distortion rate of the grid-connected current based on the feedback signal. And determine whether the photovoltaic grid-connected box meets the grid connection conditions. If the total harmonic distortion rate is... If the total harmonic distortion is ≤5%, then the grid connection standard is met and successful grid connection is allowed. If the harmonic distortion is greater than 5%, the grid connection conditions are not met, and harmonic suppression measures should be implemented in a timely manner.

[0006] Preferably, in step one, the reference current of the grid-connected inverter is denoted as... Then, an error signal is generated through an adder. , These are the operational variables of the control system.

[0007] Preferably, in step two, a delay unit for repetitive control is set. Perform forward processing to generate intermediate signals. ,in, For the delay length, , Indicates the system sampling frequency. Represents the power grid frequency, delay unit Internal model coefficient is set. Used to adjust the frequency and phase of a signal, intermediate signal To error signal A transfer function is set between them. As shown in equation (1): (1) Preferably, in step two, after completing the forward processing flow, an advance correction step is performed. Low-pass filter Proportional-integral controller Controlled objects and disturbance signals Control and regulation of output current Among them, the advance correction stage Used to compensate for phase lag caused by delay elements in composite repetitive control. Represents the lead compensation coefficient, low-pass filter Used to attenuate high-frequency signals, the cutoff frequency is set to 1kHz, as shown in equation (2): (2) Preferably, in step two, the proportional-integral controller To improve dynamic response performance, as shown in equation (3): (3) In the formula, Represents the proportionality coefficient. Represents the integral coefficient. Indicates the sampling period.

[0008] Preferably, in step two, the controlled object The transfer function from the output voltage to the grid-connected current of the photovoltaic grid-connected box, configured as an LCL type filter. As shown in equations (4) and (5): (4) (5) In the formula, This indicates the output inductance of the grid-connected inverter. This indicates the interface inductance connected to the power grid. This represents the filter capacitor.

[0009] Preferably, in step three, the total harmonic distortion rate... As shown in equation (6): (6) In the formula, Indicates the effective value of the fundamental current. express Effective value of the second harmonic current.

[0010] Preferably, in step three, the total harmonic distortion rate... <5% indicates that the photovoltaic grid-connected box meets the grid connection standards and is allowed to be successfully connected to the grid.

[0011] Preferably, in step three, the total harmonic distortion rate... A value greater than 5% indicates that the photovoltaic grid-connected box does not meet the grid connection requirements, and harmonic suppression measures should be implemented in a timely manner.

[0012] A photovoltaic grid-connected box control system suitable for active harmonic suppression includes a photovoltaic module module, a grid-connected inverter, and a composite repetitive control module; The photovoltaic module is used to convert solar energy into direct current. The grid-connected inverter is used to convert direct current into alternating current and then connect it to the power grid; The composite repetitive control module is used to detect the harmonics of the grid-connected current after conversion by the grid-connected inverter and calculate the corresponding total harmonic distortion rate. The composite repetitive control module is based on the total harmonic distortion rate. To determine whether the photovoltaic grid-connected box meets the grid connection conditions, if the total harmonic distortion rate is... If the total harmonic distortion (THD) is less than 5%, then the grid connection standard is met and successful grid connection is allowed. If the harmonic distortion is greater than 5%, the grid connection conditions are not met, and harmonic suppression measures should be implemented in a timely manner.

[0013] Compared with the prior art, the present invention provides a photovoltaic grid-connected box control method and system suitable for active harmonic suppression, which has the following beneficial effects: 1. This invention obtains the reference current of the grid-connected inverter through a photovoltaic grid-connected box, controls the grid-connected inverter to generate the grid-connected current, and obtains the error signal between the reference current and the grid-connected current. A delay unit for repetitive control is set up to perform forward processing on the error signal and generate an intermediate signal. intermediate signal To error signal A transfer function is set between them. This enables the tracking of the grid frequency signal and the suppression of harmonic interference at integer multiples of the grid frequency, thereby reducing the impact of grid background harmonics on the grid-connected current distortion rate. After completing the forward processing flow, it passes through the advance correction stage. Low-pass filter to compensate for phase lag caused by delay element in composite repetitive control Attenuation of high-frequency signals, proportional-integral controller To improve the overall dynamic response performance of the control system and the controlled object and disturbance signals Control and regulation of output current It has high precision in dynamic response control.

[0014] 2. This invention calculates the total harmonic distortion rate of the grid-connected current. To determine whether the photovoltaic grid-connected box meets the grid connection conditions, if the total harmonic distortion rate is... If the total harmonic distortion is ≤5%, then the grid connection standard is met and successful grid connection is allowed. If the harmonic distortion is greater than 5%, the grid connection conditions are not met, and harmonic suppression measures should be implemented promptly. Compared with traditional proportional-integral control, this effectively suppresses the total harmonic distortion rate of the grid current. It solves the problem of current distortion under nonlinear loads and actively suppresses current quality. Attached Figure Description

[0015] Figure 1 This is a diagram illustrating the steps of the method of the present invention; Figure 2 This is a system flowchart of the present invention; Figure 3 This is a diagram of the composite repetitive control structure of the present invention; Figure 4 This is a Bode plot of the transfer function between the intermediate signal and the error signal in this invention. Figure 5 This is a schematic diagram of the frequency characteristics of the equivalent controlled object of the present invention; Figure 6 This is a flowchart illustrating the overall control process of the photovoltaic grid-connected box with active harmonic suppression function according to the present invention. Figure 7 This is a schematic diagram of the grid-connected current waveform without active harmonic suppression in this invention; Figure 8This is a schematic diagram illustrating the total harmonic distortion rate of the grid-connected current without the use of active harmonic suppression in this invention. Figure 9 This is a schematic diagram of the grid-connected current waveform after active harmonic suppression in this invention; Figure 10 This is a schematic diagram of the total harmonic distortion rate of the grid-connected current after active harmonic suppression in this invention. Detailed Implementation

[0016] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0017] Example 1 Please see Figure 1 and Figure 3 This invention provides a photovoltaic grid-connected box control method suitable for active harmonic suppression, comprising the following steps: Step 1: Obtain the reference current of the grid-connected inverter through the photovoltaic grid-connected box. Control the grid-connected inverter to generate grid-connected current The error signal between the reference current and the grid-connected current is obtained. , These are the operational variables of the control system; Step 2: Based on the error signal between the reference current and the grid-connected current It performs forward processing and control adjustment, and outputs corresponding feedback signals; The forward processing flow is as follows: Set the delay unit for repetitive control Perform forward processing to generate intermediate signals. ,in, For the delay length, , Indicates the system sampling frequency. The value is 10kHz. Indicates the power grid frequency. The value is 50Hz, therefore, It is 200; Due to delay unit The poles are all located on the unit circle, which can easily lead to unstable states. Therefore, the internal modulus coefficient is increased. Used to adjust the frequency and phase of a signal. Setting it to a constant of 0.98 improves system stability by sacrificing some steady-state error, ensuring the control performance of the compound repetitive control, and the intermediate signal... To error signal A transfer function is set between them. As shown in equation (1): (1) Please see Figure 4 transfer function In the Bode plot, the composite repetitive control has a large gain at the grid frequency of 50 Hz and its integer multiples, such as 100 Hz and 150 Hz, with an amplitude gain of about 40 dB. Therefore, based on the internal mode principle, it is possible to track the grid frequency signal and suppress the interference of harmonic signals at integer multiples of the grid frequency, thereby reducing the impact of grid background harmonics on the grid-connected current distortion rate. The control and adjustment process is as follows: After completing the forward processing flow, the advanced correction stage is performed. Low-pass filter Proportional-integral controller Controlled objects and disturbance signals Control and regulation of output current Among them, the advance correction stage The lead compensation coefficient is used to compensate for phase lag caused by the delay element in composite repetitive control. Set to 5, low-pass filter To attenuate high-frequency signals, a Butterworth filter with a cutoff frequency of 1kHz is selected, as shown in equation (2): (2) Proportional-integral controller This is used to improve the overall dynamic response performance of the control system, as shown in equation (3): (3) In the formula, Represents the proportionality coefficient. The value is 10. Represents the integral coefficient. The value is 200. Indicates the sampling period. The value is 0.0001; Control object By configuring the filter as an LCL type, and since the additional resistance of the LCL filter is small, the transfer function from the output voltage to the grid-connected current of the photovoltaic grid-connected box can be obtained by neglecting the effect of resistance. As shown in equations (4) and (5): (4) (5) In the formula, This indicates the output inductance of the grid-connected inverter. The value is 4.2mH. This indicates the interface inductance connected to the power grid. The value is 0.2mH. Indicates the filter capacitor. The value is 3.5μF. Combined with the above parameters, Equation (5) can be discretized to obtain Equation (4); Please see Figure 5 According to the proportional-integral controller Controlled objects and low-pass filter The frequency characteristics of the equivalent controlled object of the composite repetitive control structure are obtained. The amplitude of the equivalent controlled object is close to 0 in the range of 1kHz, and the phase characteristic has a certain phase shift in this range. Overall, it is within -90 degrees. At the same time, the lead compensation can be performed by using the lead compensation link mentioned above, so that the overall phase characteristic is close to zero, showing good controlled object characteristics, which is conducive to the composite repetitive control to play its role in suppressing harmonics. Step 3: Calculate the total harmonic distortion rate of the grid-connected current based on the feedback signal. And determine whether the photovoltaic grid-connected box meets the grid connection conditions. If the total harmonic distortion rate is... If the total harmonic distortion is ≤5%, then the grid connection standard is met and successful grid connection is allowed. If the harmonic distortion is greater than 5%, the grid connection conditions are not met, and harmonic suppression measures should be implemented promptly. Please see Figure 6 The overall control process of a photovoltaic grid-connected box with active harmonic suppression function is as follows: First, the three-phase voltage signals of the power grid are acquired. , and ,Will , and After passing through the phase-locked loop (PLL) stage, a power grid reference phase is generated. Then the reference current and Multiplication serves as the command input signal for the photovoltaic grid-connected box control structure, while simultaneously incorporating the grid-connected current. , and As a feedback signal, , , The error signal is input into the composite repetitive control structure CRC by subtracting the input signal from the command signal. Finally, the controller outputs to the PWM module and generates corresponding drive signals to control the operation of the inverter and other modules in the photovoltaic grid-connected box to generate the corresponding grid-connected current. The output grid-connected current of the photovoltaic grid-connected box is collected and the total harmonic distortion rate is calculated. Total Harmonic Distortion As shown in equation (6): (6) In the formula, Indicates the effective value of the fundamental current. express RMS value of subharmonic current The value is a positive integer between 2 and 20.

[0018] Example 2 Please see Figure 2 and Figures 7-10 A photovoltaic grid-connected box control system suitable for active harmonic suppression includes a photovoltaic module module, a grid-connected inverter, and a composite repetitive control module; Photovoltaic module modules are used to convert solar energy into direct current. Grid-connected inverters are used to convert direct current (DC) into alternating current (AC) before connecting it to the power grid. The composite repetitive control module is used to detect the harmonics of the grid-connected current after conversion by the grid-connected inverter and calculate the corresponding total harmonic distortion (THD). The composite repetitive control module is based on the total harmonic distortion rate. To determine whether the photovoltaic grid-connected box meets the grid connection conditions, if the total harmonic distortion rate is... If the total harmonic distortion is ≤5%, then the grid connection standard is met and successful grid connection is allowed. If the harmonic distortion is greater than 5%, the grid connection conditions are not met, and harmonic suppression measures should be implemented promptly. First, the peak value of the grid fundamental voltage is set at 311V. Relative to the fundamental voltage, the background harmonics are set as follows: the 3rd harmonic accounts for 10%, the 5th and 7th harmonics each account for 5%, the 9th and 11th harmonics each account for 3%, and the 13th and 15th harmonics each account for 1%. When using traditional proportional-integral control, the grid-connected current of the photovoltaic grid-connected box is as follows: Figure 7 As shown, the harmonic content and total harmonic distortion rate like Figure 8 As shown, under the influence of background harmonics, the grid-connected current exhibits a certain degree of distortion. The total harmonic distortion (THD) is 5.32%, exceeding 5% and failing to meet grid-connected current standards. After implementing composite repetitive control with active harmonic suppression capabilities, the grid-connected current and total harmonic distortion (THD) will decrease. Each as Figure 9 and Figure 10 As shown, after adopting the active harmonic suppression strategy, the grid-connected current waveform is improved, and the total harmonic current distortion rate is reduced. The harmonic current of the photovoltaic grid-connected box was reduced to 3.07%, which did not exceed 5%, and met the grid-connected harmonic current standard of the photovoltaic grid-connected box, effectively improving the power quality of the grid-connected current of the photovoltaic grid-connected box system.

[0019] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A control method for a photovoltaic grid-connected box suitable for active harmonic suppression, characterized in that, Includes the following steps: Step 1: Obtain the reference current of the grid-connected inverter through the photovoltaic grid-connected box, control the grid-connected inverter to generate the grid-connected current, and obtain the error signal between the reference current and the grid-connected current; Step 2: Based on the error signal between the reference current and the grid-connected current, perform forward processing and control adjustment, and output the corresponding feedback signal; Step 3: Calculate the total harmonic distortion rate of the grid-connected current based on the feedback signal. And determine whether the photovoltaic grid-connected box meets the grid connection conditions. If the total harmonic distortion rate is... If the total harmonic distortion is ≤5%, then the grid connection standard is met and successful grid connection is allowed. If the harmonic distortion is greater than 5%, the grid connection conditions are not met, and harmonic suppression measures should be implemented in a timely manner.

2. The photovoltaic grid-connected box control method for active harmonic suppression according to claim 1, characterized in that: In step one, the reference current of the grid-connected inverter is denoted as... Then, an error signal is generated through an adder. , These are the operational variables of the control system.

3. The photovoltaic grid-connected box control method for active harmonic suppression according to claim 2, characterized in that: In step two, a delay unit for repetitive control is set. Perform forward processing to generate intermediate signals. ,in, For the delay length, , Indicates the system sampling frequency. Represents the power grid frequency, delay unit Internal model coefficient is set. Used to adjust the frequency and phase of a signal, intermediate signal To error signal A transfer function is set between them. As shown in equation (1): （1）。 4. A photovoltaic grid-connected box control method for active harmonic suppression according to claim 3, characterized in that: In step two, after completing the forward processing flow, the advanced correction stage is followed. Low-pass filter Proportional-integral controller Controlled objects and disturbance signals Control and regulation of output current Among them, the advance correction stage Used to compensate for phase lag caused by delay elements in composite repetitive control. Represents the lead compensation coefficient, low-pass filter Used to attenuate high-frequency signals, the cutoff frequency is set to 1kHz, as shown in equation (2): （2）。 5. A photovoltaic grid-connected box control method for active harmonic suppression according to claim 4, characterized in that: In step two, the proportional-integral controller To improve dynamic response performance, as shown in equation (3): （3） In the formula, Represents the proportionality coefficient. Represents the integral coefficient. Indicates the sampling period.

6. A photovoltaic grid-connected box control method for active harmonic suppression according to claim 5, characterized in that: In step two, the controlled object The transfer function from the output voltage to the grid-connected current of the photovoltaic grid-connected box, configured as an LCL type filter. As shown in equations (4) and (5): （4） （5） In the formula, This indicates the output inductance of the grid-connected inverter. This indicates the interface inductance connected to the power grid. This represents the filter capacitor.

7. A photovoltaic grid-connected box control method for active harmonic suppression according to claim 6, characterized in that: In step three, the total harmonic distortion rate As shown in equation (6): （6） In the formula, Indicates the effective value of the fundamental current. express Effective value of the second harmonic current.

8. A photovoltaic grid-connected box control method for active harmonic suppression according to claim 7, characterized in that: In step three, the total harmonic distortion rate <5% indicates that the photovoltaic grid-connected box meets the grid connection standards and is allowed to be successfully connected to the grid.

9. A photovoltaic grid-connected box control method for active harmonic suppression according to claim 8, characterized in that: In step three, the total harmonic distortion rate A value greater than 5% indicates that the photovoltaic grid-connected box does not meet the grid connection requirements, and harmonic suppression measures should be implemented in a timely manner.

10. A photovoltaic grid-connected box control system suitable for active harmonic suppression, applied to the photovoltaic grid-connected box control method for active harmonic suppression as described in any one of claims 1-9, characterized in that: Includes photovoltaic module, grid-connected inverter and composite repetitive control module; The photovoltaic module is used to convert solar energy into direct current. The grid-connected inverter is used to convert direct current into alternating current and then connect it to the power grid; The composite repetitive control module is used to detect the harmonics of the grid-connected current after conversion by the grid-connected inverter and calculate the corresponding total harmonic distortion rate. The composite repetitive control module is based on the total harmonic distortion rate. To determine whether the photovoltaic grid-connected box meets the grid connection conditions, if the total harmonic distortion rate is... If the total harmonic distortion (THD) is less than 5%, then the grid connection standard is met and successful grid connection is allowed. If the harmonic distortion is greater than 5%, the grid connection conditions are not met, and harmonic suppression measures should be implemented in a timely manner.

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