Multi-output mode single-phase photovoltaic inverter and control method for adaptive light conditions

Abstract

The present invention provides a multi-output mode single-phase photovoltaic inverter and a control method for adaptive lighting conditions, and relates to the technical field of photovoltaic power generation. The photovoltaic inverter includes a DC / DC conversion unit and a DC / AC conversion unit; the DC / DC conversion unit includes an input capacitor, a three-phase LLC topology, a high-frequency transformer, a bridge rectifier topology and an output capacitor, and the DC / DC conversion unit has a strong light operation mode and a weak light operation mode, and high-efficiency electric energy conversion can be achieved under all lighting conditions; the DC / AC conversion unit includes a full-bridge inverter topology and a grid-connected inductor; the control method is divided into a normal output mode and a reactive output mode. In the normal output mode, the photovoltaic inverter realizes the maximum output of active power and the on-demand optimized output of reactive power. In the reactive output mode, the reactive support capacity of the photovoltaic inverter is improved first, and the active power is output according to the real-time residual capacity under the premise of ensuring low voltage ride-through, so as to realize the high utilization rate of the photovoltaic inverter capacity.

Description

Technical Field

[0001] The present invention relates to the technical field of photovoltaic power generation, and in particular to a multi-output mode single-phase photovoltaic inverter that is adaptive to lighting conditions and a control method thereof. Background Art

[0002] As fossil energy is gradually depleted, photovoltaic power generation, as a technology that converts solar energy into electrical energy, has developed rapidly. Compared with centralized photovoltaic power generation, distributed photovoltaic power generation has become a research hotspot due to its advantages of decentralized power generation, flexible site, and local consumption. Distributed photovoltaic inverters, as key devices connecting distributed photovoltaic power generation systems with power grids, have also been widely studied and have a broad application market.

[0003] The development trend of distributed photovoltaic inverters is miniaturization and household use. In recent years, with the strong support of the government, distributed photovoltaic inverters have begun to flourish and occupy an important share of the inverter market. This phenomenon has enabled more and more distributed photovoltaic power generation systems to be connected to the power grid, which has achieved efficient utilization of renewable energy to a certain extent, but has also brought some problems to the stable operation of the power grid. On the one hand, since most distributed photovoltaic inverters on the market are currently connected to the grid with the goal of achieving unity power factor output, without considering the reactive power demand of local loads, the reactive pressure of the power grid has been greatly increased, resulting in a decrease in power quality and transmission efficiency. Many distributed photovoltaic power generation systems use the solution of installing SVG on site to solve this problem, but the installation cost and maintenance cost are increased, and the reactive output capacity of the photovoltaic inverter itself is not fully utilized; on the other hand, with a high proportion of distributed photovoltaic power generation systems connected to the power grid, when the voltage drops due to a fault in the power grid, the removal of the photovoltaic inverter will further increase the disturbance of the power system, thereby affecting the normal recovery of the power grid. Therefore, the low voltage ride-through capability of the distributed photovoltaic inverter is also very important. In addition, since distributed photovoltaic inverters are often used in situations where the input end is low voltage and high current, the capacity of the distributed photovoltaic power generation system is limited by the maximum current capability of the power electronic devices, making it difficult to achieve an increase in the power level. In addition, affected by lighting conditions, the photovoltaic output is random, and photovoltaic inverters often operate under light load conditions. The high loss of traditional photovoltaic inverters under light load conditions results in low power conversion efficiency. Summary of the invention

[0004] The technical problem to be solved by the present invention is to provide a multi-output mode single-phase photovoltaic inverter and a control method with adaptive lighting conditions in view of the deficiencies of the above-mentioned prior art. The single-phase photovoltaic inverter can realize electrical isolation between photovoltaic components and the power grid, has a higher power level under low-voltage and high-current conditions, and has corresponding operating modes under different lighting conditions, thereby improving the conversion efficiency of photovoltaic energy; the control method can enable the photovoltaic inverter to operate in normal output mode and reactive output mode according to the lighting conditions; in the normal output mode, the photovoltaic inverter realizes the maximum output of active power and the on-demand optimized output of reactive power. In the reactive output mode, the reactive support capacity of the photovoltaic inverter is preferentially improved. Under the premise of ensuring low voltage ride-through, the active power is output according to the real-time remaining capacity, thereby realizing high utilization of the photovoltaic inverter capacity.

[0005] In order to solve the above technical problems, the technical solution adopted by the present invention is: on the one hand, the present invention provides a multi-output mode single-phase photovoltaic inverter with adaptive lighting conditions, which adopts a bipolar structure, with an isolated DC / DC conversion unit in the front stage and a DC / AC conversion unit in the rear stage; the input end of the isolated DC / DC conversion unit is connected to the output end of the photovoltaic module; the input end of the DC / AC conversion unit is connected to the output end of the isolated DC / DC conversion unit; the output end of the DC / AC conversion unit is connected to the single-phase AC power grid; the isolated DC / DC conversion unit includes a high-frequency transformer and a three-phase LLC topology on the primary side of the high-frequency transformer and a bridge rectifier topology on the secondary side of the high-frequency transformer; the DC / AC conversion unit includes a full-bridge inverter topology and a grid-connected inductor.

[0006] Further, the three-phase LLC topology includes an input capacitor, a first bridge arm, a second bridge arm, a third bridge arm, and a resonant capacitor, a resonant inductor, and an excitation inductor respectively matched with each bridge arm connected in parallel; the input capacitor, the first bridge arm, the second bridge arm, and the third bridge arm are sequentially connected in parallel at both ends of the photovoltaic module; the midpoints of the first bridge arm, the second bridge arm, and the third bridge arm are respectively connected in series with the resonant capacitor, the resonant inductor, and the excitation inductor matched with each bridge arm, and the other ends of the excitation inductors matched with each bridge arm are connected to each other;

[0007] The first bridge arm, the second bridge arm and the third bridge arm each include two fully-controlled devices connected in series.

[0008] Furthermore, the bridge rectifier topology includes a fourth bridge arm, a fifth bridge arm, a sixth bridge arm and an output capacitor connected in parallel in sequence; the fourth bridge arm, the fifth bridge arm, the sixth bridge arm and the secondary winding of the high-frequency transformer are connected in a star connection manner.

[0009] Further, the bridge rectification topology adopts any form of diode rectification or synchronous rectification; if diode rectification is adopted, the fourth bridge arm, the fifth bridge arm, and the sixth bridge arm each include two diodes connected in series; if synchronous rectification is adopted, the fourth bridge arm, the fifth bridge arm, and the sixth bridge arm each include two fully controlled devices connected in series;

[0010] Furthermore, the isolated DC / DC conversion unit is used to achieve a boost function, so that the low-voltage direct current output by the photovoltaic module is converted into high-voltage direct current suitable for the subsequent DC / AC conversion unit. The isolated DC / DC conversion unit has four operating modes and adopts pulse frequency modulation control.

[0011] Further, the four operating modes of the isolated DC / DC conversion unit are respectively S1 operating mode, S2 operating mode, S3 operating mode and S4 operating mode;

[0012] Among them, the S1 operating mode is: the first bridge arm, the second bridge arm, the third bridge arm, the fourth bridge arm, the fifth bridge arm, the sixth bridge arm and the resonant capacitor, the resonant inductor and the excitation inductor respectively matched with the first bridge arm, the second bridge arm and the third bridge arm all participate in power conversion; the control signals of the fully controlled devices located in the same bridge arm in the first bridge arm, the second bridge arm and the third bridge arm are complementary, and the control signals of the fully controlled devices at adjacent positions between the first bridge arm, the second bridge arm and the third bridge arm have the same amplitude, and the phases differ by 120° respectively;

[0013] The S2 operation mode is: the first bridge arm, the second bridge arm, the fourth bridge arm, the fifth bridge arm and the resonant capacitor, resonant inductor and excitation inductor respectively matched with the first bridge arm and the second bridge arm participate in power conversion; the control signals of the fully-controlled devices in the first bridge arm and the second bridge arm located in the same bridge arm are complementary, the control signals of the fully-controlled devices in adjacent positions between the first bridge arm and the second bridge arm have the same amplitude and a phase difference of 180°, and the fully-controlled devices in the third bridge arm are turned off;

[0014] The S3 operation mode is: the second bridge arm, the third bridge arm, the fifth bridge arm, the sixth bridge arm and the resonant capacitor, resonant inductor and excitation inductor respectively matched with the second bridge arm and the third bridge arm participate in power conversion; the control signals of the fully controlled devices in the second bridge arm and the third bridge arm located in the same bridge arm are complementary, the control signal amplitudes of the fully controlled devices in adjacent positions between the second bridge arm and the third bridge arm are the same, the phase difference is 180°, and the fully controlled devices in the first bridge arm are turned off;

[0015] The S4 operating mode is: the first bridge arm, the third bridge arm, the fourth bridge arm, the sixth bridge arm and the resonant capacitor, resonant inductor and excitation inductor respectively matched with the first bridge arm and the third bridge arm participate in power conversion; the control signals of the fully-controlled devices located in the same bridge arm in the first bridge arm and the third bridge arm are complementary, the control signals of the fully-controlled devices at adjacent positions between the second bridge arm and the third bridge arm have the same amplitude and a phase difference of 180°, and the fully-controlled devices in the second bridge arm are turned off.

[0016] Furthermore, the DC / AC conversion unit is used to convert the direct current output by the isolated DC / DC conversion unit into single-phase alternating current suitable for grid connection, and the DC / AC conversion unit adopts SPWM control; the DC / AC conversion unit adopts a single-phase full-bridge inverter topology, including a seventh bridge arm, an eighth bridge arm and a grid-connected inductor; the seventh bridge arm and the eighth bridge arm each include two fully-controlled devices connected in series; the seventh bridge arm and the eighth bridge arm are sequentially connected in parallel at both ends of the bridge rectifier topology output capacitor; the midpoint of the seventh bridge arm is connected to the single-phase AC power grid by connecting the grid-connected inductor; the midpoint of the eighth bridge arm is directly connected to the single-phase AC power grid.

[0017] Furthermore, the fully-controlled device adopts any one of IGBT and MOSFET; the fully-controlled device adopts a power electronic device with SiC as the substrate.

[0018] On the other hand, the present invention provides a control method for a multi-output mode single-phase photovoltaic inverter with adaptive lighting conditions, which is used to control the above-mentioned multi-output mode single-phase photovoltaic inverter, specifically: when the lighting conditions are different, the isolated DC / DC conversion unit is controlled to operate in a strong light operation mode or a weak light operation mode; when the output power of the photovoltaic component is greater than the set strong light threshold power, the isolated DC / DC conversion unit is operated in the strong light operation mode S1; when the output power of the photovoltaic component is less than the set strong light threshold power, the operation mode of the isolated DC / DC conversion unit is switched in a certain order in the weak light operation mode. The single-phase photovoltaic inverter is switched between modes S2, S3, and S4 in the order of S2-S3-S4-S2. The single-phase photovoltaic inverter is set to have a normal output mode and a reactive output mode. When the single-phase AC grid voltage is greater than the set normal output mode threshold voltage, the single-phase photovoltaic inverter is controlled to switch to the normal output mode. When the single-phase AC grid voltage is less than the set reactive output mode threshold voltage, the single-phase photovoltaic inverter is controlled to switch to the reactive output mode. When the single-phase AC grid voltage is less than the normal output mode threshold voltage and greater than the reactive output mode threshold voltage, the single-phase photovoltaic inverter is controlled to maintain the current output mode unchanged.

[0019] Furthermore, the normal output mode controls the isolated DC / DC conversion unit to implement maximum power point tracking control of the photovoltaic module, so that the photovoltaic inverter outputs active power that is the real-time maximum output power of the photovoltaic module and outputs reactive power according to the reactive current demand of the load;

[0020] According to the instantaneous reactive power theory, the load reactive current demand is obtained based on the generalized second-order integrator, specifically: Based on the single-phase phase-locked loop to detect the single-phase AC grid voltage V g (t) = V p sin(w1t), load current Detecting the output current signal of the generalized second-order integrator Obtain the load reactive current demand I qh (t) = {I α (t)cos(w1t)+I β (t)sin(w1t)}cos(w1t); where V p Grid peak voltage, I p is the peak current of the power grid, w1 is the power grid frequency, is the power factor angle of the load current, I α (t) and I β (t) are two mutually orthogonal current signals generated by the generalized second-order integrator at time t;

[0021] When the single-phase photovoltaic inverter operates in normal output mode, the output current command I ref =I refPV +αI q , where I refPV is the real-time equivalent active current of the photovoltaic module, I q is the reactive current output by the photovoltaic inverter, and α is the reactive current compensation coefficient. It is limited by the rated capacity S of the photovoltaic inverter, and α needs to meet the following conditions:

[0022]

[0023] Obtain the reactive current compensation coefficient

[0024] When the reactive current compensation coefficient α is less than 1, all the remaining capacity of the photovoltaic inverter is used for reactive compensation. When the reactive current compensation coefficient α is greater than 1, it is limited to 1, and the photovoltaic inverter realizes reactive compensation according to load demand.

[0025] When the single-phase photovoltaic inverter works in the reactive output mode, according to the actual voltage of the single-phase AC power grid, the photovoltaic inverter is preferentially guaranteed to output the reactive current I q The following conditions must be met:

[0026]

[0027] Among them, I N is the rated current of the photovoltaic inverter, ΔU is the difference between the rated voltage of the single-phase AC grid and the current output voltage of the photovoltaic inverter, U N is the rated voltage of the single-phase AC power grid;

[0028] Obtain the maximum active current of the photovoltaic inverter in real time Further obtain the phase angle difference between the maximum active current output current command of the reactive output mode and the single-phase AC grid voltage

[0029] The single-phase photovoltaic inverter reactive output mode controls the phase angle difference between the photovoltaic inverter output current and the single-phase AC grid voltage according to the single-phase AC grid voltage in different ranges to achieve reactive support. The phase angle difference θ between the output current command and the single-phase AC grid voltage in the reactive output mode is:

[0030]

[0031] The beneficial effects of the above technical solution are as follows: the present invention provides a multi-output mode single-phase photovoltaic inverter and control method for adaptive lighting conditions, (1) the multi-output mode single-phase photovoltaic inverter for adaptive lighting conditions is a bipolar structure, the input current range of the front stage is wide, and the maximum current capacity of the power electronic device is less limited, which is suitable for the operating conditions of the input end of the distributed photovoltaic power generation system with low voltage and high current, thereby improving the potential of distributed photovoltaic inverters for medium and high power occasions; under the condition of low photovoltaic output, the front stage of the multi-output mode single-phase photovoltaic inverter for adaptive lighting conditions adopts a single-phase operation mode, thereby reducing the conduction loss of the photovoltaic inverter and improving the photovoltaic energy conversion efficiency; the multi-output mode single-phase photovoltaic inverter for adaptive lighting conditions proposed by the present invention has a wide operating temperature range, low requirements on the working environment, and has an electrical isolation function, thereby improving the stability of the distributed photovoltaic power generation system and the safety of grid connection. Compared with the traditional bipolar photovoltaic inverter, the single-phase photovoltaic inverter proposed by the present invention has the advantages of high switching frequency, high power conversion efficiency, high power density, low EMI, etc., which is in line with the development trend of high frequency and miniaturization of photovoltaic inverters.

[0032] (2) The multi-output mode single-phase photovoltaic inverter control method for adaptive lighting conditions proposed in the present invention can make the photovoltaic inverter operate in the corresponding mode according to the lighting conditions, and realize adaptive high-efficiency energy conversion under full lighting conditions. Under sufficient lighting conditions, the photovoltaic inverter can realize maximum power control of output active power and on-demand optimization control of reactive power. Under insufficient lighting conditions such as at night, the photovoltaic inverter can output reactive power on demand to realize on-site compensation of reactive power, which saves the access of SVG and improves the capacity utilization of the photovoltaic inverter.

[0033] (3) The multi-output mode single-phase photovoltaic inverter control method with adaptive lighting conditions proposed in the present invention can realize the automatic switching of the single-phase photovoltaic inverter between the normal output mode and the reactive output mode. When the grid voltage decreases, considering the maximum current capacity of the photovoltaic inverter bridge arm, the photovoltaic inverter is controlled to provide reactive support to the grid first, and assist the grid to resume normal operation. Under the premise of achieving low voltage ride-through, the photovoltaic inverter is controlled to maximize the output active power according to the remaining capacity of the photovoltaic inverter, thereby improving the response speed of the photovoltaic inverter to resume normal output after the grid fault is eliminated. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 A structural block diagram of a single-phase photovoltaic inverter with multiple output modes and adaptive lighting conditions provided by an embodiment of the present invention;

[0035] Figure 2 A circuit schematic diagram of a single-phase photovoltaic inverter with multiple output modes that is adaptive to lighting conditions provided by an embodiment of the present invention;

[0036] Figure 3 A schematic diagram of the operating mode circuit of the isolated DC / DC conversion unit S1 provided in an embodiment of the present invention;

[0037] Figure 4 A schematic diagram of the operating mode circuit of the isolated DC / DC conversion unit S2 provided in an embodiment of the present invention;

[0038] Figure 5 A schematic diagram of the operating mode circuit of the isolated DC / DC conversion unit S3 provided in an embodiment of the present invention;

[0039] Figure 6 A schematic diagram of the operating mode circuit of the isolated DC / DC conversion unit S4 provided in an embodiment of the present invention;

[0040] Figure 7 The present invention provides an overall flow chart of a method for controlling a single-phase photovoltaic inverter with multiple output modes and adaptive lighting conditions according to an embodiment of the present invention. DETAILED DESCRIPTION

[0041] The specific implementation of the present invention is further described in detail below in conjunction with the accompanying drawings and examples. The following examples are used to illustrate the present invention, but are not intended to limit the scope of the present invention.

[0042] In this embodiment, a multi-output mode single-phase photovoltaic inverter that adapts to light conditions is provided. Figure 1As shown, a bipolar structure is adopted, the front stage is an isolated DC / DC conversion unit, the rear stage is a DC / AC conversion unit, the input end of the isolated DC / DC conversion unit is connected to the output end of the photovoltaic module, the output end of the isolated DC / DC conversion unit is connected to the DC / AC input end, and the output end of the DC / AC conversion unit is connected to the single-phase AC grid; in this embodiment, the rated voltage of the photovoltaic module is 40V. In this embodiment, the circuit schematic diagram of the multi-output mode single-phase photovoltaic inverter with adaptive lighting conditions is as follows Figure 2 As shown;

[0043] The isolated DC / DC conversion unit includes a high-frequency transformer, a three-phase LLC topology on the primary side of the high-frequency transformer, and a bridge rectifier topology on the secondary side of the high-frequency transformer; the primary side and the secondary side of the high-frequency transformer are respectively composed of three windings;

[0044] The three-phase LLC topology includes an input capacitor Cin, a first bridge arm A1, a second bridge arm B1, a third bridge arm C1 connected in parallel, and resonant capacitors Cr1, Cr2, Cr3, resonant inductors Lr1, Lr2, Lr3 and excitation inductors Lm1, Lm2, Lm3 respectively matched with each bridge arm;

[0045] The first bridge arm, the second bridge arm and the third bridge arm each include two fully-controlled devices connected in series;

[0046] The input capacitor, the first bridge arm, the second bridge arm, and the third bridge arm are sequentially connected in parallel at both ends of the photovoltaic module; the midpoints of the first bridge arm, the second bridge arm, and the third bridge arm are respectively connected in series with the resonant capacitor, the resonant inductor, and the excitation inductor of each bridge arm, and the other ends of the excitation inductors of each bridge arm are connected to each other;

[0047] In this embodiment, the first bridge arm A1 is composed of fully controlled devices Q1 and Q2, the source of Q1 is connected to the drain of Q2; the source of Q1 is connected in series with the resonant capacitor Cr1, the resonant inductor Lr1 and the excitation inductor Lm1 in sequence;

[0048] The second bridge arm B1 is composed of fully controlled devices Q3 and Q4, the source of Q3 is connected to the drain of Q4; the source of Q3 is connected in series with the resonant capacitor Cr2, the resonant inductor Lr2 and the excitation inductor Lm2 in sequence;

[0049] The third bridge arm C1 is composed of fully controlled devices Q5 and Q6, the source of Q5 is connected to the drain of Q6; the source of Q5 is connected in series with the resonant capacitor Cr3, the resonant inductor Lr3 and the excitation inductor Lm3 in sequence;

[0050] The excitation inductors Lm1, Lm2, and Lm3 are respectively connected in parallel with the primary winding of the high-frequency transformer; the other ends of the excitation inductors Lm1, Lm2, and Lm3 are connected to each other;

[0051] The output end of the photovoltaic module is connected in parallel with the input capacitor Cin, the first bridge arm A1, the second bridge arm B1, and the third bridge arm C1 in sequence;

[0052] The bridge rectifier topology includes a fourth bridge arm, a fifth bridge arm, a sixth bridge arm and an output capacitor connected in parallel in sequence;

[0053] The bridge rectification topology adopts any form of diode rectification or synchronous rectification; if diode rectification is adopted, the fourth bridge arm, the fifth bridge arm and the sixth bridge arm each include two diodes connected in series; if synchronous rectification is adopted, the fourth bridge arm, the fifth bridge arm and the sixth bridge arm each include two fully controlled devices connected in series;

[0054] The fourth bridge arm, the fifth bridge arm, the sixth bridge arm and the secondary winding of the high-frequency transformer are connected in a star connection manner;

[0055] In this embodiment, the bridge rectifier topology is composed of a fourth bridge arm A2, a fifth bridge arm B2, a sixth bridge arm C2 and an output capacitor Cout; wherein the fourth bridge arm A2 is composed of rectifier diodes D1 and D2, and the anode of D1 is connected to the cathode of D2; the fifth bridge arm B2 phase bridge arm is composed of rectifier diodes D3 and D4, and the anode of D3 is connected to the cathode of D4; the sixth bridge arm C2 phase bridge arm is composed of rectifier diodes D5 and D6, and the anode of D5 is connected to the cathode of D6;

[0056] The fourth bridge arm A2, the fifth bridge arm B2, the sixth bridge arm C2 and the output capacitor Cout are sequentially connected in parallel to the input end of the DC / AC conversion unit; the fourth bridge arm A2, the fifth bridge arm B2, the sixth bridge arm C2 and the secondary winding of the high-frequency transformer are connected in a star connection mode;

[0057] The isolated DC / DC conversion unit is used to realize the boost function, so that the low-voltage direct current output by the photovoltaic module is converted into high-voltage direct current suitable for the subsequent DC / AC conversion unit. Ignoring the dead time, the duty cycle of the control signal of the fully controlled device is 50%, and the voltage control of the input and output ends of the isolated DC / DC conversion unit is realized by adjusting the switching frequency of the fully controlled device;

[0058] The isolated DC / DC conversion unit has four operating modes, namely S1, S2, S3 and S4, and adopts pulse frequency modulation (PFM) control;

[0059] Among them, the S1 operating mode is: the first bridge arm, the second bridge arm, the third bridge arm, the fourth bridge arm, the fifth bridge arm, the sixth bridge arm and the resonant capacitor, the resonant inductor and the excitation inductor respectively matched with the first bridge arm, the second bridge arm and the third bridge arm all participate in power conversion; the control signals of the fully controlled devices located in the same bridge arm in the first bridge arm, the second bridge arm and the third bridge arm are complementary, and the control signals of the fully controlled devices at adjacent positions between the first bridge arm, the second bridge arm and the third bridge arm have the same amplitude, and the phases differ by 120° respectively;

[0060] The S2 operation mode is: the first bridge arm, the second bridge arm, the fourth bridge arm, the fifth bridge arm and the resonant capacitor, resonant inductor and excitation inductor respectively matched with the first bridge arm and the second bridge arm participate in power conversion; the control signals of the fully-controlled devices in the first bridge arm and the second bridge arm located in the same bridge arm are complementary, the control signals of the fully-controlled devices in adjacent positions between the first bridge arm and the second bridge arm have the same amplitude and a phase difference of 180°, and the fully-controlled devices in the third bridge arm are turned off;

[0061] The S3 operation mode is: the second bridge arm, the third bridge arm, the fifth bridge arm, the sixth bridge arm and the resonant capacitor, resonant inductor and excitation inductor respectively matched with the second bridge arm and the third bridge arm participate in power conversion; the control signals of the fully controlled devices in the second bridge arm and the third bridge arm located in the same bridge arm are complementary, the control signal amplitudes of the fully controlled devices in adjacent positions between the second bridge arm and the third bridge arm are the same, the phase difference is 180°, and the fully controlled devices in the first bridge arm are turned off;

[0062] The S4 operating mode is: the first bridge arm, the third bridge arm, the fourth bridge arm, the sixth bridge arm and the resonant capacitor, resonant inductor and excitation inductor respectively matched with the first bridge arm and the third bridge arm participate in power conversion; the control signals of the fully-controlled devices located in the same bridge arm in the first bridge arm and the third bridge arm are complementary, the control signals of the fully-controlled devices at adjacent positions between the second bridge arm and the third bridge arm have the same amplitude and a phase difference of 180°, and the fully-controlled devices in the second bridge arm are turned off.

[0063] In this embodiment, the circuit schematic diagram of the S1 operating mode is as follows: Figure 3 As shown, the fully controlled devices Q1, Q2, Q3, Q4, Q5, Q6, the rectifier diodes D1, D2, D3, D4, D5, D6, the resonant capacitors Cr1, Cr2, Cr3, the resonant inductors Lr1, Lr2, Lr3 and the excitation inductors Lm1, Lm2, Lm3 all participate in power conversion; the control signals of the fully controlled devices Q1 and Q2, Q3 and Q4, Q5 and Q6 located in the same bridge arm are complementary, the control signals of the fully controlled devices Q1, Q3, Q5 have the same amplitude, and the phases differ by 120° respectively, and the control signals of the fully controlled devices Q2, Q4, Q6 have the same amplitude, and the phases differ by 120° respectively;

[0064] The circuit schematic diagram of S2 operation mode is as follows Figure 4As shown, the fully controlled devices Q1, Q2, Q3, Q4, the rectifier diodes D1, D2, D3, D4, the resonant capacitors Cr1, Cr2, the resonant inductors Lr1, Lr2 and the excitation inductors Lm1, Lm2 participate in power conversion; the control signals of the fully controlled devices Q1 and Q2, Q3 and Q4 located in the same bridge arm are complementary, the control signals of the fully controlled devices Q1 and Q3 have the same amplitude and a phase difference of 180°, the control signals of the fully controlled devices Q2 and Q4 have the same amplitude and a phase difference of 180°, and the fully controlled devices Q5 and Q6 are turned off;

[0065] The circuit schematic diagram of S3 operation mode is as follows Figure 5 As shown, the fully controlled devices Q3, Q4, Q5, Q6, the rectifier diodes D3, D4, D5, D6, the resonant capacitors Cr2, Cr3, the resonant inductors Lr2, Lr3 and the excitation inductors Lm2, Lm3 participate in power conversion; the control signals of the fully controlled devices Q3 and Q4, Q5 and Q6 located in the same bridge arm are complementary, the control signals of the fully controlled devices Q3 and Q5 have the same amplitude and a phase difference of 180°, the control signals of the fully controlled devices Q4 and Q6 have the same amplitude and a phase difference of 180°, and the fully controlled devices Q1 and Q2 are turned off;

[0066] The circuit schematic diagram of S4 operation mode is as follows Figure 6 As shown, the fully controlled devices Q1, Q2, Q5, Q6, rectifier diodes D1, D2, D5, D6, resonant capacitors Cr1, Cr3, resonant inductors Lr1, Lr3 and excitation inductors Lm1, Lm3 participate in power conversion; the control signals of the fully controlled devices Q1 and Q2, Q5 and Q6 located in the same bridge arm are complementary, the control signals of the fully controlled devices Q1 and Q5 have the same amplitude and a phase difference of 180°, the control signals of the fully controlled devices Q2 and Q6 have the same amplitude and a phase difference of 180°, and the fully controlled devices Q3 and Q4 are turned off.

[0067] The DC / AC conversion unit adopts a single-phase full-bridge inverter topology, including a seventh bridge arm, an eighth bridge arm and a grid-connected inductor;

[0068] The seventh bridge arm and the eighth bridge arm each include two fully-controlled devices connected in series;

[0069] The seventh bridge arm and the eighth bridge arm are connected in parallel at both ends of the bridge rectifier topology output capacitor in sequence; the midpoint of the seventh bridge arm is connected to the single-phase AC power grid by connecting the grid-connected inductor; the midpoint of the eighth bridge arm is directly connected to the single-phase AC power grid;

[0070] The DC / AC conversion unit is used to convert the direct current output by the isolated DC / DC conversion unit into single-phase alternating current suitable for grid connection, and the DC / AC conversion unit adopts SPWM control;

[0071] In this embodiment, the input end of the DC / AC conversion unit is connected to the output end of the isolated DC / DC unit, and the output end of the DC / AC conversion unit is connected to the single-phase power grid; the seventh bridge arm is composed of fully controlled devices Q7 and Q8, and the source of Q7 is connected to the drain of Q8; the eighth bridge arm is composed of fully controlled devices Q9 and Q10, and the source of Q9 is connected to the drain of Q10; the source of the fully controlled device Q7 is connected to one end of the grid-connected inductor L, and the other end of the grid-connected inductor L is connected to one end of the single-phase power grid, and the other end of the single-phase power grid is connected to the source of the fully controlled device Q9; the output capacitor Cout is connected in parallel with the seventh bridge arm and the eighth bridge arm in sequence;

[0072] The DC / AC conversion unit detects the voltage and current at the output end of the isolated DC / DC unit and the voltage, frequency and phase of the single-phase AC power grid, and uses SPWM technology to modulate the control signals of the fully controlled devices Q7, Q8, Q9 and Q10 that constitute the full-bridge topology to control the amplitude, frequency and phase of the output current of the full-bridge topology, thereby realizing the control of the active power and reactive power output to the single-phase AC power grid by the multi-output mode single-phase photovoltaic inverter with adaptive lighting conditions.

[0073] The fully-controlled device adopts any one of IGBT and MOSFET. In this embodiment, the fully-controlled device adopts MOSFET with SiC as substrate.

[0074] A control method for a single-phase photovoltaic inverter with multiple output modes that is adaptive to light conditions, such as Figure 7 As shown, when the illumination conditions are different, the isolated DC / DC conversion unit is controlled to operate in a strong illumination operation mode or a weak illumination operation mode; when the output power of the photovoltaic module is greater than the set strong illumination threshold power, the isolated DC / DC conversion unit is operated in a strong illumination operation mode S1; when the output power of the photovoltaic module is less than the set strong illumination threshold power, the operation mode of the isolated DC / DC conversion unit is switched among the weak illumination operation modes S2, S3, and S4 in a certain order, and the switching order is: S2-S3-S4-S2; the single-phase photovoltaic inverter is set to have a normal output mode and a reactive output mode; when the single-phase AC grid voltage is greater than the set normal output mode threshold voltage, the single-phase photovoltaic inverter is controlled to switch to the normal output mode; when the single-phase AC grid voltage is less than the set reactive output mode threshold voltage, the single-phase photovoltaic inverter is controlled to switch to the reactive output mode; when the single-phase AC grid voltage is less than the normal output mode threshold voltage and greater than the reactive output mode threshold voltage, the single-phase photovoltaic inverter is controlled to maintain the current output mode unchanged.

[0075] Under different lighting conditions, the isolated DC / DC conversion unit operates in different operating modes. When the output power of the photovoltaic module is greater than the set strong light threshold power, the isolated DC / DC conversion unit operates in S1 mode. When the output power of the photovoltaic module is less than the set strong light threshold power, the operating mode of the isolated DC / DC conversion unit switches between S2, S3, and S4 modes in a certain order. The switching order is: the output power of the photovoltaic module decreases from greater than the set strong light threshold power to less than the set strong light threshold power, the isolated DC / DC conversion unit switches from S1 operating mode to S2, the output power of the photovoltaic module increases from less than the set strong light threshold power to greater than the set strong light threshold power, and the isolated DC / DC conversion unit switches from S2 operating mode to S1; the output power of the photovoltaic module decreases from greater than the set strong light threshold power to less than the set strong light threshold power, the isolated DC / DC conversion unit switches from S1 operating mode to S3, and the photovoltaic module output The power increases from less than the set strong light threshold power to greater than the set strong light threshold power, and the isolated DC / DC conversion unit switches from S3 operation mode to S1; the output power of the photovoltaic module decreases from greater than the set strong light threshold power to less than the set strong light threshold power, the isolated DC / DC conversion unit switches from S1 operation mode to S4, the output power of the photovoltaic module increases from less than the set strong light threshold power to greater than the set strong light threshold power, and the isolated DC / DC conversion unit switches from S4 operation mode to S1; the output power of the photovoltaic module decreases from greater than the set strong light threshold power to less than the set strong light threshold power, the isolated DC / DC conversion unit switches from S1 operation mode to S2, the output power of the photovoltaic module increases from less than the set strong light threshold power to greater than the set strong light threshold power, and the isolated DC / DC conversion unit switches from S2 operation mode to S1; the operation mode of the isolated DC / DC conversion unit is converted according to the above switching sequence.

[0076] When the multi-output mode single-phase photovoltaic inverter with adaptive light conditions works in normal output mode, the isolated DC / DC conversion unit is controlled to realize maximum power point tracking control of the photovoltaic module, so that the photovoltaic inverter outputs active power equal to the real-time maximum output power of the photovoltaic module and outputs reactive power according to the reactive current demand of the load;

[0077] The input end of the isolated DC / DC conversion unit is connected to the output end of the photovoltaic module. By detecting the voltage and current at the output end of the photovoltaic module, the conductivity increment method is used. According to the characteristic that the slope of the power-voltage curve of the photovoltaic module is zero when the photovoltaic module operates at the maximum power point, the isolated DC / DC conversion unit is subjected to PFM control to adjust the input impedance so that the output voltage and output current of the photovoltaic module satisfy the following formula, thereby realizing the maximum power point tracking control of the photovoltaic module.

[0078]

[0079] Among them, I PV is the output current of the photovoltaic module, U PV is the output voltage of the PV module.

[0080] According to the instantaneous reactive power theory, the load reactive current demand is obtained based on the generalized second-order integrator, specifically: Based on the single-phase phase-locked loop to detect the single-phase AC grid voltage V g (t) = V p sin(w1t), load current Detecting the output current signal of the generalized second-order integrator Obtain the load reactive current demand I qh (t) = {I α (t)cos(w1t)+I β (t)sin(w1t)}cos(w1t); where V p Grid peak voltage, I p is the peak current of the power grid, w1 is the power grid frequency, is the power factor angle of the load current, I α (t) and I β (t) are two mutually orthogonal current signals generated by the generalized second-order integrator at time t;

[0081] When the multi-output mode single-phase photovoltaic inverter with adaptive light conditions works in normal output mode, the isolated DC / DC conversion unit is controlled to realize maximum power point tracking control of the photovoltaic module, so that the photovoltaic inverter outputs active power equal to the real-time maximum output power of the photovoltaic module and outputs reactive power according to the reactive current demand of the load;

[0082] When the single-phase photovoltaic inverter works in normal output mode, the output current command I ref =I refPV +αI q , where I refPV is the real-time equivalent active current of the photovoltaic module, I q is the reactive current output by the photovoltaic inverter, and α is the reactive current compensation coefficient. It is limited by the rated capacity S of the photovoltaic inverter, and α needs to meet the following conditions:

[0083]

[0084] Obtain the reactive current compensation coefficient

[0085] When the reactive current compensation coefficient α is less than 1, all the remaining capacity of the photovoltaic inverter is used for reactive compensation. When the reactive current compensation coefficient α is greater than 1, it is limited to 1, and the photovoltaic inverter realizes reactive compensation according to load demand.

[0086] When the single-phase photovoltaic inverter works in the reactive output mode, according to the actual voltage of the single-phase AC grid, the photovoltaic inverter output reactive current I q The following conditions must be met:

[0087]

[0088] Among them, I N is the rated current of the photovoltaic inverter, ΔU is the difference between the rated voltage of the single-phase AC grid and the current output voltage of the photovoltaic inverter, U N is the rated voltage of the single-phase AC power grid;

[0089] Obtain the maximum active current of the photovoltaic inverter in real time Further obtain the phase angle difference between the maximum active current output current command of the reactive output mode and the single-phase AC grid voltage

[0090] The reactive output mode of the single-phase photovoltaic inverter controls the phase angle difference between the photovoltaic inverter output current and the single-phase AC grid voltage according to the single-phase AC grid voltage in different ranges to achieve reactive support. The phase angle difference θ between the output current command and the single-phase AC grid voltage in the reactive output mode is:

[0091]

[0092] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some or all of the technical features therein. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope defined by the claims of the present invention.

Claims

1. A multi-output mode single-phase photovoltaic inverter that is adaptive to light conditions, characterized in that: A bipolar structure is adopted, with an isolated DC / DC conversion unit in the front stage and a DC / AC conversion unit in the rear stage; the input end of the isolated DC / DC conversion unit is connected to the output end of the photovoltaic module; the input end of the DC / AC conversion unit is connected to the output end of the isolated DC / DC conversion unit; the output end of the DC / AC conversion unit is connected to a single-phase AC power grid; the isolated DC / DC conversion unit includes a high-frequency transformer and a three-phase LLC topology on the primary side of the high-frequency transformer and a bridge rectifier topology on the secondary side of the high-frequency transformer; the DC / AC conversion unit includes a full-bridge inverter topology and a grid-connected inductor; The isolated DC / DC conversion unit is used to realize the boost function, so that the low-voltage direct current output by the photovoltaic module is converted into high-voltage direct current suitable for the subsequent DC / AC conversion unit. The isolated DC / DC conversion unit has four operating modes and adopts pulse frequency modulation control; The four operating modes of the isolated DC / DC conversion unit are respectively S1 operating mode, S2 operating mode, S3 operating mode and S4 operating mode; Among them, the S1 operating mode is: the first bridge arm, the second bridge arm, the third bridge arm, the fourth bridge arm, the fifth bridge arm, the sixth bridge arm and the resonant capacitor, the resonant inductor and the excitation inductor respectively matched with the first bridge arm, the second bridge arm and the third bridge arm all participate in power conversion; the control signals of the fully controlled devices located in the same bridge arm in the first bridge arm, the second bridge arm and the third bridge arm are complementary, and the control signals of the fully controlled devices at adjacent positions between the first bridge arm, the second bridge arm and the third bridge arm have the same amplitude, and the phases differ by 120° respectively; The S2 operation mode is: the first bridge arm, the second bridge arm, the fourth bridge arm, the fifth bridge arm and the resonant capacitor, resonant inductor and excitation inductor respectively matched with the first bridge arm and the second bridge arm participate in power conversion; the control signals of the fully-controlled devices in the first bridge arm and the second bridge arm located in the same bridge arm are complementary, the control signals of the fully-controlled devices in adjacent positions between the first bridge arm and the second bridge arm have the same amplitude and a phase difference of 180°, and the fully-controlled devices in the third bridge arm are turned off; The S3 operation mode is: the second bridge arm, the third bridge arm, the fifth bridge arm, the sixth bridge arm and the resonant capacitor, resonant inductor and excitation inductor respectively matched with the second bridge arm and the third bridge arm participate in power conversion; the control signals of the fully controlled devices in the second bridge arm and the third bridge arm located in the same bridge arm are complementary, the control signal amplitudes of the fully controlled devices in adjacent positions between the second bridge arm and the third bridge arm are the same, the phase difference is 180°, and the fully controlled devices in the first bridge arm are turned off; The S4 operation mode is: the first bridge arm, the third bridge arm, the fourth bridge arm, the sixth bridge arm and the resonant capacitor, the resonant inductor and the excitation inductor respectively matched with the first bridge arm and the third bridge arm participate in power conversion; the control signals of the fully controlled devices in the same bridge arm of the first bridge arm and the third bridge arm are complementary, the control signals of the fully controlled devices in adjacent positions between the second bridge arm and the third bridge arm have the same amplitude and a phase difference of 180°, and the fully controlled devices in the second bridge arm are turned off; When the illumination conditions are different, the isolated DC / DC conversion unit is controlled to operate in a strong illumination operation mode or a weak illumination operation mode; when the output power of the photovoltaic module is greater than the set strong illumination threshold power, the isolated DC / DC conversion unit is operated in the strong illumination operation mode S1; when the output power of the photovoltaic module is less than the set strong illumination threshold power, the operation mode of the isolated DC / DC conversion unit is switched among the weak illumination operation modes S2, S3, and S4 in a certain order, and the switching order is: S2-S3-S4-S2; the single-phase photovoltaic inverter is set to have a normal output mode and a reactive output mode; when the single-phase AC grid voltage is greater than the set normal output mode threshold voltage, the single-phase photovoltaic inverter is controlled to switch to the normal output mode; when the single-phase AC grid voltage is less than the set reactive output mode threshold voltage, the single-phase photovoltaic inverter is controlled to switch to the reactive output mode; when the single-phase AC grid voltage is less than the normal output mode threshold voltage and greater than the reactive output mode threshold voltage, the single-phase photovoltaic inverter is controlled to maintain the current output mode unchanged.

2. The multi-output mode single-phase photovoltaic inverter according to claim 1, characterized in that: The three-phase LLC topology includes an input capacitor, a first bridge arm, a second bridge arm, a third bridge arm, and a resonant capacitor, a resonant inductor, and an excitation inductor respectively matched with each bridge arm connected in parallel; the input capacitor, the first bridge arm, the second bridge arm, and the third bridge arm are sequentially connected in parallel at both ends of the photovoltaic module; the midpoints of the first bridge arm, the second bridge arm, and the third bridge arm are respectively connected in series with the resonant capacitor, the resonant inductor, and the excitation inductor matched with each bridge arm, and the other ends of the excitation inductors matched with each bridge arm are connected to each other; The first bridge arm, the second bridge arm and the third bridge arm each include two fully-controlled devices connected in series.

3. The multi-output mode single-phase photovoltaic inverter according to claim 2, characterized in that: The bridge rectifier topology includes a fourth bridge arm, a fifth bridge arm, a sixth bridge arm and an output capacitor connected in parallel in sequence; the fourth bridge arm, the fifth bridge arm, the sixth bridge arm and the secondary winding of the high-frequency transformer are connected in a star connection manner.

4. The multi-output mode single-phase photovoltaic inverter according to claim 3, characterized in that: The bridge rectification topology adopts either diode rectification or synchronous rectification; if diode rectification is adopted, the fourth bridge arm, the fifth bridge arm and the sixth bridge arm each include two diodes connected in series; if synchronous rectification is adopted, the fourth bridge arm, the fifth bridge arm and the sixth bridge arm each include two fully-controlled devices connected in series.

5. The multi-output mode single-phase photovoltaic inverter according to claim 4, characterized in that: The DC / AC conversion unit is used to convert the direct current output by the isolated DC / DC conversion unit into single-phase alternating current suitable for grid connection, and the DC / AC conversion unit adopts SPWM control; the DC / AC conversion unit adopts a single-phase full-bridge inverter topology, including a seventh bridge arm, an eighth bridge arm and a grid-connected inductor; the seventh bridge arm and the eighth bridge arm each include two fully controlled devices connected in series; the seventh bridge arm and the eighth bridge arm are sequentially connected in parallel at both ends of the bridge rectifier topology output capacitor; the midpoint of the seventh bridge arm is connected to the single-phase AC power grid by connecting the grid-connected inductor; the midpoint of the eighth bridge arm is directly connected to the single-phase AC power grid.

6. The multi-output mode single-phase photovoltaic inverter according to claim 5, characterized in that: The fully-controlled device adopts any one of IGBT and MOSFET; the fully-controlled device adopts a power electronic device with SiC as the substrate.

7. A control method for a multi-output mode single-phase photovoltaic inverter that is adaptive to light conditions, used to control the multi-output mode single-phase photovoltaic inverter according to claim 1, characterized in that: When the illumination conditions are different, the isolated DC / DC conversion unit is controlled to operate in a strong illumination operation mode or a weak illumination operation mode; when the output power of the photovoltaic module is greater than the set strong illumination threshold power, the isolated DC / DC conversion unit is operated in the strong illumination operation mode S1; when the output power of the photovoltaic module is less than the set strong illumination threshold power, the operation mode of the isolated DC / DC conversion unit is switched among the weak illumination operation modes S2, S3, and S4 in a certain order, and the switching order is: S2-S3-S4-S2; the single-phase photovoltaic inverter is set to have a normal output mode and a reactive output mode; when the single-phase AC grid voltage is greater than the set normal output mode threshold voltage, the single-phase photovoltaic inverter is controlled to switch to the normal output mode; when the single-phase AC grid voltage is less than the set reactive output mode threshold voltage, the single-phase photovoltaic inverter is controlled to switch to the reactive output mode; when the single-phase AC grid voltage is less than the normal output mode threshold voltage and greater than the reactive output mode threshold voltage, the single-phase photovoltaic inverter is controlled to maintain the current output mode unchanged.

8. The control method of the multi-output mode single-phase photovoltaic inverter according to claim 7, characterized in that: The normal output mode controls the isolated DC / DC conversion unit to achieve maximum power point tracking control of the photovoltaic module, so that the photovoltaic inverter outputs active power that is the real-time maximum output power of the photovoltaic module and outputs reactive power according to the reactive current demand of the load; According to the instantaneous reactive power theory, the load reactive current demand is obtained based on the generalized second-order integrator, specifically: Based on the single-phase phase-locked loop to detect the single-phase AC grid voltage V g (t) = V p sin(w1t), load current Detecting the output current signal of the generalized second-order integrator Obtain the load reactive current demand I qh (t) = {I α (t)cos(w1t)+I β (t)sin(w1t)}cos(w1t); Among them, V p Grid peak voltage, I p is the peak current of the power grid, w1 is the power grid frequency, is the power factor angle of the load current, I α (t) and I β (t) are two mutually orthogonal current signals generated by the generalized second-order integrator at time t; When the single-phase photovoltaic inverter operates in normal output mode, the output current command I ref =I refPV +αI q , where I refPV is the real-time equivalent active current of the photovoltaic module, I q is the reactive current output by the photovoltaic inverter, and α is the reactive current compensation coefficient. It is limited by the rated capacity S of the photovoltaic inverter, and α needs to meet the following conditions: Obtain the reactive current compensation coefficient When the reactive current compensation coefficient α is less than 1, all the remaining capacity of the photovoltaic inverter is used for reactive compensation. When the reactive current compensation coefficient α is greater than 1, it is limited to 1, and the photovoltaic inverter realizes reactive compensation according to load demand. When the single-phase photovoltaic inverter works in the reactive output mode, according to the actual voltage of the single-phase AC power grid, the photovoltaic inverter is preferentially guaranteed to output the reactive current I q The following conditions must be met: Among them, I N is the rated current of the photovoltaic inverter, △U is the difference between the rated voltage of the single-phase AC grid and the current output voltage of the photovoltaic inverter, U N is the rated voltage of the single-phase AC power grid; Obtain the maximum active current of the photovoltaic inverter in real time Further obtain the phase angle difference between the maximum active current output current command of the reactive output mode and the single-phase AC grid voltage The single-phase photovoltaic inverter reactive output mode controls the phase angle difference between the photovoltaic inverter output current and the single-phase AC grid voltage according to the single-phase AC grid voltage in different ranges to achieve reactive support. The phase angle difference θ between the output current command and the single-phase AC grid voltage in the reactive output mode is:

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