A main circuit and method for suppressing low-frequency ripple of a two-stage single-phase inverter

Abstract

The present invention discloses a main circuit and method for suppressing low-frequency ripple in a two-stage single-phase inverter. The main circuit includes a bidirectional DC-DC converter and a single-phase inverter, with the input side connected in parallel and the output side connected in series. The input side of the bidirectional DC-DC converter is connected in parallel to an input power supply, and the output side is respectively connected in parallel to an output capacitor, and the two output capacitors are connected in series. The output side of the bidirectional DC-DC converter is connected in parallel to a single-phase inverter, and a filter inductor is connected in series at the output end of the single-phase inverter. The two series output capacitors of the bidirectional DC-DC converter have different capacitance values ​​and different DC operating points. The purpose of the present invention is to address the problem of suppressing low-frequency ripple in a two-stage single-phase inverter. The main circuit is designed to differentiate the capacitance value and DC operating point of the output series capacitors, so that the circuit itself has active power decoupling capability, significantly reducing the busbar capacitance requirement without introducing additional circuits, and achieving small capacitance, low ripple, and low cost for the system.

Description

Technical Field

[0001] The present invention belongs to the field of distributed power generation such as new energy photovoltaics and fuel cells, and particularly relates to a main circuit and method for suppressing low-frequency ripple of a two-stage single-phase inverter. Background Art

[0002] Compared to coal-fired power generation, renewable energy sources produce less pollution, have higher reserves, and offer broad application prospects. Renewable energy sources such as photovoltaics and fuel cells generate direct current (DC) electricity, which requires an inverter to connect to the grid. Two-stage single-phase inverters are suitable for small and medium power levels, such as distributed renewable energy systems like household photovoltaics. However, the inherent low-frequency ripple of single-phase inverters can cause bus voltage fluctuations, reduce system efficiency, and shorten the service life of renewable energy sources. Traditional solutions require the use of large-capacity electrolytic capacitors in parallel to absorb this ripple energy, significantly increasing system size and cost. Furthermore, electrolytic capacitors have a lifespan of only one to two years, and their lifespan is halved for every 10°C increase in temperature. This reduces the operational reliability and significantly shortens the service life of single-phase inverters. Therefore, low-capacitance, low-ripple solutions suitable for two-stage single-phase inverters have become a hot topic of research for scholars both domestically and internationally.

[0003] Existing solutions can be divided into two categories: passive power decoupling and active power decoupling. Passive power decoupling uses an LC filter with a resonant frequency at 2ω0 to buffer the ripple power, which can reduce the system capacitance requirements. However, the power density of the magnetic components is low, and the hysteresis loss and eddy current loss are high, which cannot meet the requirements of high efficiency and high power density. Active power decoupling solutions can be further divided into topology type and control algorithm type. The topology type uses auxiliary converters composed of topologies such as Buck, Boost, and H-bridge to buffer the ripple power and transfer the ripple power to non-bus capacitors. The non-bus capacitor voltage does not need to be maintained constant, so the capacitance required for the topology-type active power decoupling solution is significantly reduced compared to the traditional solution. However, the additional active power circuit added to this solution inevitably increases the cost and power loss of the device. Active power decoupling technology based on control algorithms reduces the impact of double-frequency ripple energy on inductance or capacitance by optimizing the control algorithm without changing the original circuit. The double-frequency ripple power still flows in the main circuit. However, this type of technology is highly dependent on the DC converter topology, the control algorithm is complex, and it requires more energy storage elements and control degrees of freedom.

[0004] In summary, a low-frequency ripple suppression solution with wide applicability, simple control logic, low cost, small size and light weight is urgently needed in single-phase inverters. Summary of the Invention

[0005] The present invention aims to address the problem of low-frequency ripple suppression in a two-stage single-phase inverter and propose a main circuit and method for suppressing low-frequency ripple in a two-stage single-phase inverter. The main circuit differentially designs the capacitance and DC operating point of the output series capacitor, so that the circuit itself has active power decoupling capability, significantly reducing the bus capacitance requirement without introducing additional circuits, thereby achieving small capacitance, low ripple, and low cost for the system.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions:

[0007] A main circuit for suppressing low-frequency ripple of a two-stage single-phase inverter includes a bidirectional DC-DC converter and a single-phase inverter, with the input side connected in parallel and the output side connected in series. The input side of the bidirectional DC-DC converter is connected in parallel to an input power supply, and the output side is connected in parallel to an output capacitor, and the two output capacitors are connected in series. The output side of the bidirectional DC-DC converter is connected in parallel to the single-phase inverter, and a filter inductor is connected in series to the output end of the single-phase inverter.

[0008] The two series-connected output capacitors of the bidirectional DC-DC converter have different capacitance values ​​and different DC operating points.

[0009] As a further improvement of the present invention, the capacitance of the series output capacitor of the bidirectional DC-DC converter and the DC operating point meet the following conditions:

[0010] The ratio of the bidirectional DC-DC converter's maximum output current to the single-phase inverter's DC bus current rating is determined to be m+1. Initially, the larger of the two series-connected output capacitors is selected as C2, and the smaller as C1, satisfying the equation C1 = 0.3 × C2. The parameter k is determined as follows:

[0011]

[0012] Where ω0 is the fundamental angular frequency of the power grid; V bus is the rated voltage of the DC bus of the single-phase inverter; I bus is the DC bus rated current;

[0013] Based on the parameters k, C1, and C2, determine the voltage across the series output capacitors C1 and C2:

[0014]

[0015]

[0016] If v C1 The minimum value of v is greater than zero or C2 If the maximum value is less than the rated voltage of the DC bus, reduce C2 and recalculate the parameter k and the voltage of the series output capacitor until v C1 The minimum value is approximately 0 or v C2The maximum value is approximately the bus rated voltage; if v C1 The minimum value of v is less than zero or C2 If the maximum value is greater than the DC bus rated voltage, increase C2 and recalculate the parameter k and the voltage of the series output capacitor until v C1 The minimum value is approximately 0 or v C2 The maximum value is approximately the rated voltage of the DC bus;

[0017] The series output capacitor C1 and capacitor C2 finally determined based on the above parameter iteration process are the minimum capacitance values ​​that can completely suppress the low-frequency ripple of the two-stage single-phase inverter; the DC operating point of the series output capacitor C1 and capacitor C2 is v C1 and v C2 The DC component of the FFT decomposition.

[0018] A control method for the main circuit of the two-stage single-phase inverter for suppressing low-frequency ripple is disclosed. Closed-loop control is used to ensure that capacitor voltage ripples are complementary and the DC bus voltage is constant. Specifically, a voltage-sharing outer loop using a PI controller and a capacitor voltage waveform control loop using a proportional-integral controller and a resonant controller are employed.

[0019] As a further improvement of the present invention, the bus voltage outer loop regulator G v It is a proportional-integral controller used to stabilize the DC bus voltage near the rated value: when the bus voltage is lower than the rated value, the proportional-integral controller output value increases, which increases the duty cycle of the PWM wave output by the controller and increases the bus voltage; when the bus voltage is lower than the rated value, the proportional-integral controller output value decreases, which reduces the duty cycle of the PWM wave output by the controller and reduces the bus voltage;

[0020] Capacitor voltage inner loop G c It is a proportional-integral controller + resonant controller, where the PI controller is a proportional-integral controller used to control the steady-state operating point of the capacitor voltage. The resonant frequency point of the resonant controller PR1 to PR8 is 2hω0, h=1,2…8, used to control the ripple shape of the capacitor voltage.

[0021] As a further improvement of the present invention, the resonant controller transfer function is shown as follows:

[0022]

[0023] Among them, K r is the resonant gain, w c is the resonance bandwidth, w0 is the grid fundamental angular frequency, and h is the harmonic order.

[0024] A distributed power supply system includes photovoltaic cells or fuel cells. The photovoltaic cells or fuel cells adopt the main circuit for suppressing low-frequency ripple of a two-stage single-phase inverter.

[0025] Compared with the prior art, the present invention has the following advantages:

[0026] The main circuit of the present invention has the ability to decouple active power by differentially designing the capacitance and DC operating point of the output series capacitor, which greatly reduces the bus capacitance requirement without introducing additional circuits, thereby achieving small capacitance, low ripple and low cost for the system. By differentially designing the capacitance and DC operating point of the output series capacitor of the DC-DC circuit, the DC-DC circuit itself has the ability to suppress low-frequency ripple without the need to introduce additional auxiliary circuits. The single-phase inverter system designed based on this method can completely eliminate DC bus voltage ripple and input current ripple, and the bus capacitance requirement is reduced by 90% compared to traditional design solutions. At the same time, the present invention proposes a method for designing the capacitance and DC operating point parameters of the output series capacitor, and a method for controlling the capacitor voltage waveform that can be applied to any DC-DC circuit. Compared with other low-frequency ripple suppression methods, this method has a smaller bus capacitance requirement, a higher device power density, a simpler and more effective control algorithm, a wider range of applications, and can be functionally upgraded based on the existing circuit structure and controller. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is a main circuit for suppressing low-frequency ripple of a two-stage single-phase inverter of the present invention;

[0028] Figure 2 This is a typical working waveform diagram of a traditional design that does not adopt the design method of the present invention;

[0029] Figure 3 The following is a typical working waveform diagram of the design method of the present invention;

[0030] Figure 4 This is a structural diagram of a controller for suppressing low-frequency ripple in a two-stage single-phase inverter main circuit according to the present invention;

[0031] Figure 5 This is the structure diagram of the capacitor voltage inner loop regulator;

[0032] Figure 6 This is an experimental waveform diagram using the design method of the present invention;

[0033] Among them, (a) is the output voltage and output current waveform of the single-phase inverter; (b) is the DC bus voltage waveform; (c) is the output series capacitor voltage waveform; (d) is the input current waveform;

[0034] Figure 7 This is a test waveform diagram without adopting the design method of the present invention;

[0035] Among them, (a) is the output voltage and output current waveform of the single-phase inverter; (b) is the DC bus voltage waveform; (c) is the output series capacitor voltage waveform; (d) is the input current waveform. DETAILED DESCRIPTION

[0036] In order to enable those skilled in the art to better understand the technical solutions of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings of the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts should fall within the scope of protection of the present invention.

[0037] It should be noted that when an element is referred to as being "disposed on" another element, it may be directly on the other element or there may be an element centered thereon. When an element is referred to as being "connected to" another element, it may be directly connected to the other element or there may be an element centered thereon. The terms "vertical," "horizontal," "left," "right," and similar expressions used herein are for illustrative purposes only and do not represent the only embodiments.

[0038] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which this invention pertains. The terms used herein in the specification of the present invention are for the purpose of describing specific embodiments only and are not intended to limit the present invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0039] In order to enable those skilled in the art to better understand the solutions of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the embodiments described are only embodiments of a part of the present invention, not all embodiments, and are not intended to limit the scope of the present invention. In addition, in the following description, descriptions of well-known structures and technologies are omitted to avoid unnecessary confusion of the concepts disclosed in the present invention. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work should fall within the scope of protection of the present invention.

[0040] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments:

[0041] See also Figure 1 , Figure 1This is the main circuit for suppressing low-frequency ripple in a two-stage single-phase inverter. In a conventional design, the output series capacitors C1 and C2 have the same capacitance and DC operating point, and the first DC-DC module 1 and the second DC-DC module 2 operate in the same state. When the subsequent single-phase inverter operates at rated operating conditions, the output series capacitor voltage is expressed as:

[0042]

[0043] The bus voltage fluctuation is the sum of the capacitor voltage fluctuations, so the bus voltage ripple peak-to-peak value is:

[0044]

[0045] As can be seen from the above formula, in the traditional design scheme, the bus voltage ripple cannot be completely eliminated by increasing the capacitance value.

[0046] Figure 2 The operating waveform of the system using the traditional design scheme is given, where the rated operating point of the main circuit is V in =200V,I bus =5A,V bus =400V,V C1 =200V,V C2 =200V,V ac =310V,ω0=100πrad / s.

[0047] To achieve the requirement that the bus voltage ripple peak-to-peak value is less than 2%, the bus capacitor value required by the traditional design can be obtained according to formula (3):

[0048]

[0049] To meet this capacitance requirement, traditional designs must use multiple large-capacitance electrolytic capacitors in parallel, which increases the system volume and reduces system operation reliability.

[0050] In order to make the system have active power decoupling capability, the present invention proposes a differentiated design method for output series capacitors, that is, C1≠C2, and V C1 ≠V C2 The voltage waveform of the output series capacitor is controlled as follows:

[0051]

[0052] The ripple power absorbed by the output series capacitor can be expressed as:

[0053]

[0054] That is, when the output series capacitor is designed differently, the power absorbed by the capacitor is equal to the ripple power transmitted by the DC bus, achieving complete suppression of the low-frequency ripple energy of the single-phase inverter.

[0055] Figure 3 The system operating waveform using the differentiated design method is given, where the rated operating point of the main circuit is Figure 2 Same, but C1 <C2,V C1 <V C2 Different from the traditional design scheme, the capacitor voltage ripple of the differentiated design method is complementary, ensuring the DC bus voltage is constant. From the perspective of power flow: when the bus current i bus Exceeding the average value I bus When the output series capacitor needs to provide energy to the single-phase inverter, when the voltage fluctuation is the same, the energy fluctuation of the capacitor with large capacitance and high DC working point is greater, so the capacitor C2 releases energy and the voltage drops. To maintain the bus voltage constant, the capacitor C1 absorbs energy and the voltage rises. When the bus current i bus When the voltage is lower than the average value, the output series capacitor needs to absorb the excess energy of the previous DC-DC converter. The capacitor C2 with large capacitance and high DC working point stores energy, and the voltage rises. The capacitor C1 with small capacitance and low DC working point releases energy, and the voltage drops. C2 Rise and v C1 If the drop is the same, the bus voltage remains unchanged.

[0056] The present invention also discloses a main circuit parameter design method for suppressing low-frequency ripple of a two-stage single-phase inverter, so that the above-mentioned differentiated design method can be applied to any bidirectional DC-DC converter circuit, comprising the following steps:

[0057] First, analyze the operating principle of the bidirectional DC-DC converter and determine that the ratio of its maximum output current to the rated DC bus current of the single-phase inverter is m+1. Preliminary selections are made for the larger of the two series-connected output capacitors: C2 and C1, with C1 = 0.3C2. Determine the parameter k:

[0058]

[0059] Where ω0 is the fundamental angular frequency of the power grid; V bus is the rated voltage of the inverter DC bus; I bus is the DC bus rated current.

[0060] Next, determine the voltage across the series output capacitors C1 and C2 based on the parameters k, C1, and C2:

[0061]

[0062]

[0063] If v C1 The minimum value of v is greater than zero or C2 If the maximum value is less than the rated voltage of the DC bus, reduce C2 and recalculate the parameter k and the voltage of the series output capacitor until v C1 The minimum value is approximately 0 or v C2 The maximum value is approximately the bus rated voltage; if v C1 The minimum value of v is less than zero or C2 If the maximum value is greater than the DC bus rated voltage, increase C2 and recalculate the parameter k and the voltage of the series output capacitor until v C1 The minimum value is approximately 0 or v C2 The maximum value is approximately the rated DC bus voltage.

[0064] The series output capacitor C1 and capacitor C2 finally determined based on the above parameter iteration process are the minimum capacitance values ​​that can completely suppress the low-frequency ripple of the single-phase inverter; the DC operating point of the series output capacitor C1 and capacitor C2 is v C1 and v C2 The FFT decomposition of the DC component ensures that the DC-DC output current i out1 and i out2 The circuit can operate stably and normally as long as it does not exceed its maximum output current range, the voltage of the series output capacitor at the low DC operating point is not lower than 0, and the voltage of the series output capacitor at the high DC operating point is not higher than the rated voltage of the DC bus.

[0065] The present invention also discloses a control method for suppressing the low-frequency ripple main circuit of a two-stage single-phase inverter, using closed-loop control to ensure that capacitor voltage ripples are complementary and the DC bus voltage is constant. Figure 4 The flow chart of the closed-loop control is given, which consists of the bus voltage outer loop and the capacitor voltage inner loop. v It is a proportional-integral controller used to stabilize the DC bus voltage near the rated value: when the bus voltage is lower than the rated value, the proportional-integral controller output value increases, which increases the duty cycle of the PWM wave output by the controller and increases the bus voltage; when the bus voltage is lower than the rated value, the proportional-integral controller output value decreases, which reduces the duty cycle of the PWM wave output by the controller and reduces the bus voltage. c It is a proportional integral controller + resonant controller, and its structure is as follows Figure 5 As shown, PI is a proportional-integral controller used to control the steady-state operating point of the capacitor voltage. PR1 to PR8 are resonant controllers with a resonant frequency of 2hω0 (h = 1, 2…8) used to control the ripple shape of the capacitor voltage. The resonant controller transfer function is shown below:

[0066]

[0067] Among them, K r is the resonant gain, ω c is the resonant bandwidth, ω0 is the grid fundamental angular frequency, and h is the harmonic order. The resonant controller designed based on Equation (7) ignores the 20th and above harmonic components of the series capacitor voltage shown in Equation (5), reducing the control complexity while achieving rapid tracking of the capacitor voltage harmonic components and ensuring the active power decoupling effect.

[0068] The present invention relates to low-frequency ripple suppression of a single-phase inverter in a distributed power supply system powered by new energy sources such as photovoltaic cells and fuel cells.

[0069] Therefore, the present application provides a distributed power supply system, including photovoltaic cells or fuel cells, which adopt the main circuit for suppressing low-frequency ripple of the two-stage single-phase inverter.

[0070] In particular, a main circuit and method for suppressing low-frequency ripple in single-phase inverters are proposed. By differentially designing the capacitance and DC operating point of the series output capacitors of the bidirectional DC-DC converter, the circuit itself possesses active power decoupling capabilities. Matlab system simulations verify that this method effectively reduces the required bus capacitor value and significantly reduces input current ripple and bus voltage ripple.

[0071] Example

[0072] In order to verify the above theoretical analysis, the present invention provides a practical design example. The front-stage bidirectional DC-DC converter is selected as a dual active bridge converter, and the main circuit parameters are as follows: Input voltage V in =200V, bus voltage V bus =400V, bus current I bus =5A, input power P in =2kW, grid voltage V ac =310V, grid angular frequency w0=100πrad / s switching frequency f s =50kHz, dual active bridge circuit resonant inductor L k =20uH. According to the main circuit parameter design method proposed in the present invention, first, based on the operating principle of the dual active bridge circuit, its maximum output current is determined to be 16.75A, which is 3.35 times the rated current of the DC bus, that is, m+1=3.35; the capacitance of the larger capacitance and high DC operating point capacitor of the two series output capacitors is initially selected as C2=50uF, C1=0.3×C2=15uF, and the parameter k is calculated as:

[0073]

[0074] Secondly, the parameters k, C1 and C2 are substituted into formula (5), and the minimum voltage of the series output capacitor C1 is less than zero, and the maximum voltage of the capacitor C1 is higher than the rated voltage of the DC bus. Increase C2 and recalculate the parameter k and the voltage of the series output capacitor. After repeated iterations, when v C1 When the minimum value is approximately 0, C2=120uF, C1=36uF. Considering the suppression of high-frequency ripple, the capacitance value is appropriately increased, and C2=200uF, C1=60uF is selected. At this time, v C1 The DC operating point is 145.2V, v C2 The DC operating point is 254.8V.

[0075] According to the main circuit parameters obtained above and the control method proposed by the present invention, the simulation model main circuit and control system are built. The simulation results are as follows: Figure 6 As shown in (a)(b)(c)(d). Figure 6 (a) shows the output voltage and current waveforms of the single-phase inverter designed by the present invention, with a total harmonic distortion rate of only 0.13%; Figure 6 (b) is the DC bus voltage waveform under the design method of the present invention, and its peak-to-peak ripple is 4V; Figure 6 (c) The waveform of the series capacitor output under the design method of the present invention has complementary ripples and the DC operating point is stable at the set value; Figure 6 (d) is the input current waveform under the design method of the present invention, and its ripple peak-to-peak value is 0.5A.

[0076] As a comparison, Figure 7 (a)(b)(c)(d) are when the total capacitance is the same (C1=130uF, C2=130uF, V C1 =200V, V C2 =200V) Under the traditional design scheme, the simulation waveform shows that the total harmonic distortion rate of the single-phase inverter output current increases to 14.3%; the DC bus voltage ripple increases to 228V, an increase of 57 times; and the input current ripple increases to 5.5A, an increase of 11 times.

[0077] Clearly, the design method of the present invention effectively suppresses input current ripple and bus voltage ripple, significantly improving the output waveform quality of the single-phase inverter, while maintaining the same total capacitance of the series output capacitors. Conversely, to achieve the same ripple suppression effect as the design method of the present invention, the bus capacitor capacitance of the conventional design would need to be increased by 57 times, significantly increasing system size and cost. Compared to other low-frequency ripple suppression solutions, the present invention does not introduce any additional auxiliary circuitry, but instead utilizes a differentiated design to provide the circuit with active power decoupling capabilities.

[0078] The above content is only for explaining the technical idea of ​​the present invention and cannot be used to limit the protection scope of the present invention. Any changes made on the basis of the technical solution in accordance with the technical idea proposed by the present invention shall fall within the protection scope of the claims of the present invention.

Claims

1. A main circuit for suppressing low-frequency ripple in a two-stage single-phase inverter, characterized by: It includes a bidirectional DC-DC converter and a single-phase inverter with the input side connected in parallel and the output side connected in series. The input side of the bidirectional DC-DC converter is connected in parallel with the input power supply, the output side is connected in parallel with an output capacitor, the two output capacitors are connected in series, the output side of the bidirectional DC-DC converter is connected in parallel with the single-phase inverter, and the output end of the single-phase inverter is connected in series with a filter inductor; The two series output capacitors of the bidirectional DC-DC converter have different capacitance values ​​and different DC operating points; The capacitance of the series output capacitor of the bidirectional DC-DC converter and the DC operating point meet the following conditions: The ratio of the maximum output current of the bidirectional DC-DC converter to the rated DC bus current of the single-phase inverter is determined to be m+1; the capacitance of the larger capacitor in the two series output capacitors is initially selected to be C 2. The capacitance of small capacitor is C 1, and satisfy C 1 =0.3×C 2. Determine parameters k : in, ω 0 is the fundamental angular frequency of the power grid; V bus is the rated voltage of the DC bus of the single-phase inverter; I bus is the DC bus rated current; According to the parameters k 、 C 1 and C 2. Determine the series output capacitor C 1 and C 2. Voltage: if v C1 The minimum value is greater than zero or v C2 If the maximum value is less than the DC bus rated voltage, reduce C 2. Recalculate parameters k and the voltage of the series output capacitor until v C1 The minimum value is approximately 0 or v C2 The maximum value is approximately the bus rated voltage; if v C1 The minimum value is less than zero or v C2 If the maximum value is greater than the DC bus rated voltage, increase C 2. Recalculate parameters k and the voltage of the series output capacitor until v C1 The minimum value is approximately 0 or v C2 The maximum value is approximately the rated voltage of the DC bus; The series output capacitor is finally determined based on the above parameter iteration process. C 1 and capacitor C 2 is the minimum capacitance value to completely suppress the low-frequency ripple of the two-stage single-phase inverter; the output capacitor in series C 1 and capacitor C The DC operating point of 2 is v C1 and v C2 The DC component of the FFT decomposition.

2. A control method for a main circuit for suppressing low-frequency ripple of a two-stage single-phase inverter according to claim 1, characterized in that: Closed-loop control is used to ensure that the capacitor voltage ripple is complementary and the DC bus voltage is constant. Specifically, a DC bus voltage outer loop using a PI controller and a capacitor voltage waveform control loop using a proportional-integral controller and a resonant controller are used.

3. The control method for suppressing the low-frequency ripple of the main circuit of a two-stage single-phase inverter according to claim 2, characterized in that: Bus voltage outer loop regulator G v It is a proportional-integral controller used to stabilize the DC bus voltage near the rated value: when the bus voltage is lower than the rated value, the proportional-integral controller output value increases, which increases the duty cycle of the PWM wave output by the controller and increases the bus voltage; when the bus voltage is higher than the rated value, the proportional-integral controller output value decreases, which reduces the duty cycle of the PWM wave output by the controller and reduces the bus voltage; Capacitor voltage inner loop G c It is a proportional integral controller + resonant controller, where the PI controller is a proportional integral controller used to control the steady-state operating point of the capacitor voltage. The resonant frequency point of the resonant controller PR1~PR8 is 2 hω 0, h =1,2…8, used to control the ripple shape of the capacitor voltage.

4. The control method for suppressing the low-frequency ripple of the main circuit of the two-stage single-phase inverter according to claim 3, characterized in that: The resonant controller transfer function is shown below: in, K r is the resonant gain, w c is the resonant bandwidth, w 0 is the grid fundamental angular frequency, h is the harmonic order.

5. A distributed power supply system, characterized in that: The photovoltaic cell or fuel cell comprises a photovoltaic cell or a fuel cell, and the photovoltaic cell or fuel cell adopts the main circuit for suppressing low-frequency ripple of a two-stage single-phase inverter according to claim 1.

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