A photovoltaic power generation system

Abstract

A photovoltaic power generation system includes: a bus capacitor, a grid-connected relay, a photovoltaic inverter, an uncontrolled rectifier circuit, and a control unit. The bus capacitor has a first terminal connected to a photovoltaic module and a second terminal connected to the photovoltaic inverter. The grid-connected relay has a first terminal connected to the photovoltaic inverter, a second terminal connected to the control unit, and a third terminal connected to the power grid, used to connect or disconnect the photovoltaic inverter and the power grid. The uncontrolled rectifier circuit has a first terminal connected to the power grid, a second terminal connected to the bus capacitor, and a third terminal connected to the control unit, used to provide a rectified voltage. The control unit has a first terminal connected to the first terminal of the bus capacitor, a second terminal connected to the second terminal of the bus capacitor, and a third terminal connected to the grid-connected relay. The control unit detects the charging voltage at the first terminal of the bus capacitor and the rectified voltage at the second terminal of the bus capacitor, and controls the grid-connected relay to close when the charging voltage is greater than the rectified voltage. This invention protects the bus capacitor by eliminating the bias voltage.

Description

Technical Field

[0001] This invention belongs to the field of photovoltaic power generation technology, and specifically relates to a photovoltaic power generation system. Background Technology

[0002] Photovoltaic (PV) power generation is a technology that converts light energy into electrical energy using the photovoltaic effect at semiconductor interfaces. A grid-connected PV power generation system includes solar panels and a grid-connected PV inverter unit. The PV inverter converts the direct current (DC) obtained from the PV modules into alternating current (AC) and connects it to the grid. However, before the inverter actually outputs power, the grid voltage already acts as a bias voltage. In this situation, grid connection presents a dual risk: firstly, if the bus voltage is lower than the grid peak value, grid energy will generate an uncontrolled surge charging current on the bus capacitor through the anti-parallel diodes of the inverter bridge; secondly, the abrupt voltage change (high dU / dt) at the moment of grid connection will generate a strong differential effect on the bus capacitor, triggering extremely high-frequency transient spike currents. The superposition of these two effects poses a risk of breakdown of power devices and damage to the metallization film of the bus capacitor. Summary of the Invention

[0003] To address the aforementioned technical problems, the first aspect of this invention provides a photovoltaic power generation system, comprising: a bus capacitor, a grid-connected relay, a photovoltaic inverter, an uncontrolled rectifier circuit, and a control unit; the first terminal of the bus capacitor is connected to a photovoltaic module, and the second terminal is connected to the photovoltaic inverter; the first terminal of the grid-connected relay is connected to the photovoltaic inverter, the second terminal is connected to the control unit, and the third terminal is connected to the power grid, for connecting or disconnecting the photovoltaic inverter and the power grid; the first terminal of the uncontrolled rectifier circuit is connected to the power grid, the second terminal is connected to the bus capacitor, and the third terminal is connected to the control unit, the uncontrolled rectifier circuit being used to provide a rectified voltage; the first terminal of the control unit is connected to the first terminal of the bus capacitor, the second terminal of the control unit is connected to the second terminal of the bus capacitor, and the third terminal of the control unit is connected to the grid-connected relay; the control unit is used to detect the charging voltage at the first terminal of the bus capacitor and the rectified voltage at the second terminal of the bus capacitor, and when the charging voltage is greater than the rectified voltage, it controls the grid-connected relay to close.

[0004] The beneficial effects of the present invention are as follows: by optimizing the control logic and further setting the charging parameters, the present invention eliminates the bias voltage, thereby achieving the purpose of protecting the bus capacitor.

[0005] Further beneficial effects of the present invention are as follows: not only does it eliminate the surge charging current on the bus capacitor through pre-biased charging, but it also introduces a differential suppression circuit and a zero-differential point grid connection control strategy. By connecting to the grid at the peak or trough point where the grid voltage change rate approaches zero, it suppresses the differential effect caused by voltage step from both hardware circuit and control timing aspects, extends the service life of the bus capacitor, and achieves smooth grid connection without impact. Attached Figure Description

[0006] Figure 1 A circuit schematic diagram provided in an embodiment of the present invention;

[0007] Figure 2 Another circuit schematic provided in this embodiment of the invention;

[0008] Figure 3 Another circuit schematic diagram provided in this embodiment of the invention;

[0009] Figure 4 An embodiment of the present invention provides a control flowchart. Detailed Implementation

[0010] The following embodiments further illustrate the content of the present invention, but should not be construed as limiting the present invention. Any modifications or substitutions made to the methods, steps, or conditions of the present invention without departing from the spirit and essence of the invention are within the scope of the present invention.

[0011] In embodiments of the present invention, such as Figure 1 As shown, a photovoltaic power generation system includes: a bus capacitor, a grid-connected relay, a photovoltaic inverter, an uncontrolled rectifier circuit, and a control unit;

[0012] The first end of the bus capacitor is connected to the photovoltaic module, and the second end is connected to the photovoltaic inverter.

[0013] The grid-connected relay is connected to the photovoltaic inverter at its first end, to the control unit at its second end, and to the power grid at its third end, and is used to connect or disconnect the photovoltaic inverter and the power grid.

[0014] The uncontrolled rectifier circuit is connected to the power grid at its first end, to the bus capacitor at its second end, and to the control unit at its third end. The uncontrolled rectifier circuit is used to provide rectified voltage.

[0015] The first terminal of the control unit is connected to the first terminal of the bus capacitor, the second terminal of the control unit is connected to the second terminal of the bus capacitor, and the third terminal of the control unit is connected to the grid-connected relay. The control unit is used to detect the charging voltage of the first terminal of the bus capacitor and the rectified voltage of the second terminal of the bus capacitor. When the charging voltage is greater than the rectified voltage, the control unit controls the grid-connected relay to close.

[0016] In this embodiment, the inverter bus capacitor is a crucial component of the inverter circuit. Its function is to store electrical energy, smooth voltage fluctuations, and ensure the stability and reliability of the inverter's output voltage. Therefore, the capacity and type of the inverter bus capacitor must be carefully selected during the design and manufacturing of the inverter to guarantee its performance and reliability.

[0017] The maximum ripple current of the bus support capacitor under extreme conditions of a photovoltaic inverter; a thin-film capacitor can be selected as the bus support capacitor.

[0018] In a preferred embodiment of the present invention, to meet the reliability requirements of an industrial-grade photovoltaic grid-connected system, the selection and parameter configuration of the system's core components are as follows:

[0019] The bus capacitor is preferably a metallized polypropylene film capacitor, which has a higher ripple current withstand capability and a longer service life compared to electrolytic capacitors. Its rated withstand voltage is preferably 1100VDC~1500VDC, and its total capacitance is preferably 1mF~10mF.

[0020] The grid-connected relay is preferably a three-phase AC contactor or a solid-state relay with auxiliary contacts. Its AC rated operating voltage is usually 400VAC or 800VAC, and the rated current is selected according to the inverter power level, usually 50A to 1000A.

[0021] The uncontrolled rectifier unit in the uncontrolled rectifier circuit is preferably a three-phase diode rectifier bridge module, with a reverse withstand voltage of 1600V and a rated current of 30A to 100A.

[0022] The control unit preferably uses a digital signal processor with high-precision analog-to-digital conversion and rich PWM peripherals, such as the TMS320F28335 or TMS320F28377 series. Alternatively, a microcontroller based on the ARM Cortex-M4 or Cortex-M7 core can be used to meet the requirements of high-frequency sampling and complex logic operations.

[0023] The control unit and the monitoring unit are powered by an auxiliary power supply, which can be obtained by drawing power from the power grid through the AC auxiliary circuit and then converting it, or by using an independent standby power supply. This invention does not limit the specific power supply to this.

[0024] In embodiments of the present invention, such as Figure 2 As shown, the photovoltaic power generation system further includes: an AC auxiliary circuit; the first end of the AC auxiliary circuit is connected to the power grid, and the second end is connected to the photovoltaic inverter and the control unit through a monitoring unit, for transmitting energy from the power grid to the photovoltaic inverter to charge the bus capacitor.

[0025] In this embodiment, the photovoltaic inverter needs a pre-bias start-up process to start inversion after closing the grid-connected relay. This is because the output terminal is already under the influence of the grid voltage before the actual output of the inverter, i.e., there is a bias voltage. The DC bus needs to be charged to a level greater than the uncontrolled rectified voltage of the bus capacitor by the grid voltage before the relay is closed.

[0026] Upon receiving the start command, it draws power from the AC mains or from the AC auxiliary power supply.

[0027] After power is drawn, it is fed back to the bus capacitor at the input end of the inverter grid-connected unit in the photovoltaic grid-connected power generation system to enable the inverter grid-connected unit to start.

[0028] When drawing power from the AC grid, the obtained power is AC. Therefore, the obtained AC power needs to be converted into DC power of a preset amplitude and then fed back to the bus capacitor at the input end of the inverter grid-connected unit.

[0029] When drawing power from the AC auxiliary power supply, power can be drawn from either the DC bus of the AC auxiliary power supply or from its DC output terminal. Both draws are DC. The DC power drawn from the AC auxiliary power supply is converted to a preset amplitude and then fed back to the bus capacitor at the input terminal of the inverter grid-connected unit.

[0030] It is important to note that the DC voltage input to the bus capacitor must be greater than the grid connection voltage of the photovoltaic grid-connected power generation system so that the output voltage of the photovoltaic inverter grid-connected unit can be input into the AC grid.

[0031] In embodiments of the present invention, such as Figure 3 As shown, the uncontrolled rectifier circuit of the photovoltaic power generation system includes: a DC / DC voltage regulation unit and an uncontrolled rectifier unit;

[0032] The first terminal of the DC / DC voltage regulating unit supplies rectified voltage to the second terminal of the bus capacitor, the second terminal is connected to the uncontrolled rectifier unit, and the third terminal is connected to the control unit; it is used to receive adjustment commands from the control unit and convert the voltage after uncontrolled rectification into rectified voltage.

[0033] It should be noted that, Figure 2 The “DC / DC voltage regulation circuit” marked in the text refers to the DC / DC voltage regulation unit described herein and in the claims. Figure 2 The "uncontrolled rectifier circuit" block diagram marked in the text is the uncontrolled rectifier unit described in this article; the two together constitute... Figure 1 The uncontrolled rectifier circuit is shown as a whole in the middle; Figure 3 The "DC / DC voltage regulation circuit" marked in the text also refers to the aforementioned DC / DC voltage regulation unit. Figure 3 The uncontrolled rectifier unit is schematically shown in the figure as a three-phase uncontrolled rectifier circuit consisting of six diodes. In this invention, the "DC / DC voltage regulation circuit" in the figure and the "DC / DC voltage regulation unit" in the text refer to the same functional module. Meanwhile, Figure 2 This is a simplified schematic diagram of the overall system control architecture. To highlight the core control logic, the AC auxiliary circuit is not fully shown. The connection relationship between the AC auxiliary circuit and the power grid and monitoring unit can be found in the diagram. Figure 3 A shared understanding.

[0034] It should also be noted that the inverter bridge of the photovoltaic inverter itself contains an anti-parallel diode. After the grid-connected relay is closed, if the bus voltage is lower than the grid peak value, the anti-parallel diode will form an uncontrolled surge charging path, which is the target of the present invention. The uncontrolled rectifier circuit is independent of the grid-connected relay. When the grid-connected relay is open, it pre-charges the bus capacitor. The two are independent of each other and have different functions.

[0035] In this embodiment, a three-phase half-wave circuit is selected as the uncontrolled rectifier unit: a three-phase half-wave circuit refers to a circuit in which three diodes conduct alternately within one power supply cycle, thus obtaining a three-pulse rectified voltage. The advantage of this circuit is its simple wiring, but the conduction angle of the transformer secondary winding is only 120°, resulting in low winding utilization. Furthermore, the current is unidirectional, and its DC component forms a DC ampere-turn magnetomotive force and generates a large leakage flux, thus requiring an increase in the cross-sectional area of ​​the transformer core and incurring additional losses. Therefore, this type of circuit is mostly used in medium to small-sized equipment.

[0036] The most widely used uncontrolled rectifier units are single-phase bridge circuits and three-phase bridge circuits. A bridge circuit is characterized by two sets of rectifier bridges: a common-cathode set and a common-anode set, both connected in series with the load. Therefore, it is suitable for operation under high voltage and low current conditions. If the power supply size is appropriate, a transformer is not required. For operation under low voltage and high current conditions, two sets of three-phase half-wave circuits should be connected in parallel. After parallel connection, by utilizing the appropriate connection of the transformer windings, DC magnetization is eliminated, and each set provides half of the load current. To solve the current balance problem between the two sets, a balancing reactor is specially designed, thus leveraging the advantage of fewer components while eliminating the disadvantages of the three-phase half-wave circuit.

[0037] It should also be noted that, Figure 3 The inductor, capacitor, and other component symbols drawn in the "DC / DC voltage regulation circuit" block diagram are for schematic purposes only, indicating that the DC / DC voltage regulation unit includes energy storage, filtering, and power conversion stages. They are not drawn according to the actual device type, polarity, direction, and complete connection relationship. The function of the DC / DC voltage regulation unit is as described in the text of this specification, namely, receiving the adjustment command from the control unit and converting the uncontrolled rectified voltage into the required rectified voltage. Its specific circuit structure is not limited to the schematic form shown in the attached drawings.

[0038] It is understood that the DC / DC voltage regulation unit can be implemented using various DC-DC converter circuits that can convert the uncontrolled rectified voltage into the required rectified voltage under the adjustment command of the control unit, such as flyback converter circuits and other isolated or non-isolated DC-DC converter circuits.

[0039] In embodiments of the present invention, such as Figure 4As shown, the control unit of the photovoltaic power generation system is configured as follows:

[0040] Receive the charging voltage and rectified voltage ;

[0041] Calculate the charging voltage function and rectified voltage function ;

[0042] If the charging voltage function value Greater than the rectified voltage function value (Corresponding to the YES branch of step S2 in Figure 4), then control the grid-connected relay to close and start the inverter (corresponding to step S301 in Figure 4).

[0043] If the charging voltage function value H(u1) is less than or equal to the rectified voltage function value G(u2), the grid-connected relay is kept open and the inverter does not work.

[0044] Charging voltage function rectified voltage function Proceed according to the following formula:

[0045] (I)

[0046] (II)

[0047] In the formula, Indicates the charging voltage. This represents the rectified voltage, and k represents the charging coefficient. This represents the rectification factor.

[0048] In practical engineering applications, when the control unit uses the charging voltage function H(u1) and the rectified voltage function G(u2) to determine the closing of the grid-connected relay, it can also comprehensively consider the following control factors and quantitative relationships:

[0049] Firstly, sampling error and hysteresis control. To prevent frequent operation of the grid-connected relay due to small fluctuations in the charging voltage near the critical point, a hysteresis voltage range ΔU can be introduced into the control logic. h (Preferred voltage is 5V to 15V), only when H(u1) > G(u2) + ΔU h The closing command is issued only at that time.

[0050] Secondly, compensation for the mechanical action delay of the relay. There is a mechanical delay T between receiving the closing command and the contacts fully closing in the grid-connected relay. delay (Typically 20ms to 50ms), the control unit can perform feedforward prediction by incorporating the rate of change of the grid voltage during calculation to ensure that within the delay time T...delay After completion, the bus capacitor voltage still meets the condition of being greater than the bias voltage.

[0051] Third, grid synchronization factors. Before controlling the grid-connected relay to close, the control unit can also extract the phase θ and frequency f of the grid voltage in real time through a software phase-locked loop. Only when the voltage amplitude condition H(u1)>G(u2) is met, the grid frequency is within the range of 50Hz±0.5Hz, and the phase locking error is less than the preset threshold (such as ±2°), will the closing command be finally issued, thereby eliminating the circulating current impact at the moment of grid connection.

[0052] In this embodiment of the invention, the control unit of the photovoltaic power generation system is further configured to:

[0053] Receive the charging voltage and rectified voltage ;

[0054] Calculate the charging voltage and rectified voltage The difference ;

[0055] like If the value is greater than 0, then the rectified voltage output by the DC / DC voltage regulation unit remains unchanged.

[0056] like Then, the DC / DC voltage regulating unit is controlled to output rectified voltage. ;

[0057] (III)

[0058] In the formula: Indicates the charging voltage. Indicates the rectified voltage. This represents the overpressure constant.

[0059] The physical significance of setting the overvoltage constant σ lies in achieving shock-free grid connection of photovoltaic inverters. At the moment of grid connection of the photovoltaic inverter, if the bus capacitor voltage is lower than the peak value of the grid AC voltage, the grid energy will form an uncontrolled surge charging current to the bus capacitor through the anti-parallel diodes inside the inverter bridge, which can easily cause diode breakdown or overcurrent damage to the bus capacitor.

[0060] In a further embodiment of the present invention, to address the strong differential effect (i.e., capacitor current I) generated on the bus capacitor during the instantaneous voltage step at grid connection... c=C·(dU / dt) causes transient high-frequency spike current). This system also includes a differential suppression circuit. This differential suppression circuit can use a microhenry-level high-frequency buffer inductor (such as a saturated reactor) connected in series at the front end of the bus capacitor, supplemented by a parallel RC damping absorption network: when the grid-connected relay is closed, the buffer inductor limits the rate of change of current di / dt, and the RC damping absorption network absorbs the high-frequency ringing voltage, thereby buffering the differential effect in the hardware circuit.

[0061] More preferably, the control unit in this embodiment adds zero-derivative grid-connected control logic to the software algorithm. The grid voltage can be expressed as u. grid (t)=U m ·sin(ωt), its voltage change rate du grid / dt=U m ·ω·cos(ωt); the rate of change of voltage du at the peak (phase π / 2) or trough (phase 3π / 2) of the grid voltage. grid / dt approaches zero. Therefore, the control unit does not immediately connect to the grid after satisfying the amplitude condition H(u1)>G(u2), but instead calculates |du in real time. grid / dt|, when |du is detected grid When / dt|<ε (ε is a preset threshold close to zero, and in practical engineering applications, its value can be determined in combination with the grid voltage level and sampling rate, with a preferred range of 0.5 V / ms to 5 V / ms), that is, when the grid voltage is near the peak point, the grid-connected relay is controlled to close.

[0062] This control logic forms a closed loop with the aforementioned overvoltage constant σ: the bus capacitor is pre-charged to slightly above the grid peak value (U0). m The grid-connected relay closes when the grid voltage reaches its peak and the rate of change approaches zero. At this time, the voltage difference is the smallest (approximately σ), and the voltage change rate is almost zero, thereby simultaneously suppressing the surge charging current and the differential effect peak current, achieving smooth soft grid connection.

[0063] It should be noted that at the positive peak (phase π / 2) and negative peak (phase 3π / 2) of the grid voltage, the rate of voltage change du grid Since / dt approaches zero, the differential effect peak current caused by the voltage step can be minimized at both grid connection points. The difference lies in the pre-charge voltage (U) of the bus capacitor at the positive peak value of π / 2. mThe difference between the +σ) and the instantaneous grid voltage is minimal (approximately σ), at which point both surge charging current and differential effect peak current can be simultaneously suppressed, representing the optimal grid connection time. However, at the negative peak value of 3π / 2, the instantaneous grid voltage is negative, and the difference between it and the positive bus voltage is significant, making surge suppression less effective. This point primarily suppresses the differential effect peak current at the moment of grid connection. Therefore, this embodiment preferably connects to the grid near the positive peak value of π / 2 to balance the suppression of both effects; in cases where only the differential effect peak current needs to be suppressed, grid connection can also be chosen near 3π / 2.

[0064] Therefore, the ideal grid connection condition is to ensure that the charging voltage of the bus capacitor is just slightly higher than the peak value of the grid voltage, so that all the anti-parallel diodes in the inverter bridge are in a reverse-biased cutoff state. The ideal value of the overvoltage constant σ is preferably 10V~30V (or 2%~5% of the peak grid voltage): on the one hand, this margin is sufficient to offset the transient harmonic spikes, voltage fluctuations, and analog-to-digital conversion sampling errors of the grid voltage, ensuring reliable diode cutoff and avoiding backflow current from the grid to the bus; on the other hand, this margin is not too large, avoiding excessive active current surges to the grid due to excessively high bus voltage at the moment of inverter startup, thereby achieving the smoothest grid connection transition and maximizing the protection of the bus capacitor and power devices.

[0065] It should also be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0066] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments. The same or similar parts between the various embodiments can be referred to each other.

[0067] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments described herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

[0068] Although the present invention has been described in detail above with general descriptions, specific embodiments, and experiments, modifications or improvements can be made to it, which will be obvious to those skilled in the art. Therefore, all such modifications or improvements made without departing from the spirit of the present invention fall within the scope of protection claimed by the present invention.

Claims

1. A photovoltaic power generation system, characterized in that, include: Bus capacitors, grid-connected relays, photovoltaic inverters, uncontrolled rectifier circuits and control units; The first end of the bus capacitor is connected to the photovoltaic module, and the second end is connected to the photovoltaic inverter. The grid-connected relay is connected to the photovoltaic inverter at its first end, to the control unit at its second end, and to the power grid at its third end, and is used to connect or disconnect the photovoltaic inverter and the power grid. The uncontrolled rectifier circuit is connected to the power grid at its first end, to the bus capacitor at its second end, and to the control unit at its third end. The uncontrolled rectifier circuit is used to provide rectified voltage. The first terminal of the control unit is connected to the first terminal of the bus capacitor, the second terminal of the control unit is connected to the second terminal of the bus capacitor, and the third terminal of the control unit is connected to the grid-connected relay. The control unit is used to detect the charging voltage of the first terminal of the bus capacitor and the rectified voltage of the second terminal of the bus capacitor. When the charging voltage is greater than the rectified voltage, the control unit controls the grid-connected relay to close.

2. The photovoltaic power generation system according to claim 1, characterized in that, Also includes: An AC auxiliary circuit; the first end of the AC auxiliary circuit is connected to the power grid, and the second end is connected to the photovoltaic inverter and the control unit through a monitoring unit, for transmitting energy from the power grid to the photovoltaic inverter to charge the bus capacitor.

3. The photovoltaic power generation system according to claim 1, characterized in that, The uncontrolled rectifier circuit includes: a DC / DC voltage regulation unit and an uncontrolled rectifier unit; The first terminal of the DC / DC voltage regulating unit supplies rectified voltage to the second terminal of the bus capacitor, the second terminal is connected to the uncontrolled rectifier unit, and the third terminal is connected to the control unit; it is used to receive adjustment commands from the control unit and convert the voltage after uncontrolled rectification into rectified voltage.

4. The photovoltaic power generation system according to claim 1, characterized in that, The control unit is configured as follows: Receive the charging voltage at the first terminal of the bus capacitor Rectified voltage at the second terminal of the bus capacitor ; Calculate the charging voltage function and rectified voltage function ; If the charging voltage function value Greater than the rectified voltage function value If the grid voltage amplitude, phase, and frequency are normal, then the grid-connected relay is closed to start the inverter. If the charging voltage function value H(u1) is less than or equal to the rectified voltage function value G(u2), the grid-connected relay is kept open and the inverter does not work. Charging voltage function rectified voltage function Proceed according to the following formula: (I) (II) In the formula, Indicates the charging voltage. This represents the rectified voltage, and k represents the charging coefficient. This represents the rectification factor.

5. The photovoltaic power generation system according to claim 3, characterized in that, The control unit is also configured to: Receive the charging voltage and rectified voltage ; Calculate the charging voltage and rectified voltage The difference ; like If the value is greater than 0, then the rectified voltage output by the DC / DC voltage regulation unit remains unchanged. like Then, the DC / DC voltage regulating unit is controlled to output rectified voltage. ; (III) In the formula: Indicates the charging voltage. Indicates the rectified voltage. This represents the overpressure constant.

6. The photovoltaic power generation system according to claim 1 or 2, characterized in that, It also includes a differential suppression circuit; The differential suppression circuit is set in the loop between the grid-connected relay and the bus capacitor to limit the instantaneous rate of change of the voltage across the bus capacitor at the moment the grid-connected relay is closed, so as to eliminate the differential effect spike current generated on the bus capacitor by the sudden change of the grid bias voltage.

7. The photovoltaic power generation system according to claim 6, characterized in that, The differential suppression circuit includes: a high-frequency buffer inductor connected in series in the positive and negative circuits of the bus capacitor, and an RC damping absorption network connected in parallel with the bus capacitor; the high-frequency buffer inductor is used to suppress the current surge rate at the moment of grid connection, and the RC damping absorption network is used to absorb the high-frequency harmonics generated by the voltage step.

8. The photovoltaic power generation system according to claim 4, characterized in that, The control unit is also configured to perform zero-derivative-point grid-connected control: real-time acquisition of grid voltage and calculation of the instantaneous rate of change of grid voltage, du. grid / dt; Under the condition that the charging voltage function value H(u1) is greater than the rectified voltage function value G(u2), further determine the absolute value of the instantaneous rate of change of the grid voltage |du grid If / dt| is less than the preset rate of change threshold ε, then the grid-connected relay is controlled to close at this moment.

9. The photovoltaic power generation system according to claim 8, characterized in that, The specific logic for the control unit to perform zero-derivative point grid-connected control is as follows: Since the AC voltage of the power grid exhibits a sinusoidal change, its instantaneous rate of change approaches zero at voltage peaks or troughs. The control unit locks the grid voltage phase through a phase-locked loop. When the grid voltage phase is detected to be near π / 2 or 3π / 2, and |du| is satisfied... grid When / dt|<ε, a closing command is issued; combined with the setting of the overvoltage constant σ, which is a preset voltage margin to make the bus capacitor charging voltage higher than the peak voltage of the grid, the grid-connected relay closes at the moment when the grid voltage is at its peak and the rate of change is the lowest, thereby simultaneously suppressing the surge charging current and the differential effect peak current at the positive peak π / 2, and suppressing the differential effect peak current at the negative peak 3π / 2.

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