Unidirectional multi-string photovoltaic inverter circuit control device and control method

Abstract

The present invention discloses a unidirectional multi-string photovoltaic inverter circuit control device and control method, the device includes: a unidirectional multi-string photovoltaic inverter circuit and a switching regulator, wherein the unidirectional multi-string photovoltaic inverter circuit includes a solar cell array component, a high-frequency AC link and a three-phase AC inverter, and the switching regulator is used to control the input circuit switch tube in the solar cell array component according to the input reference current output after the MPPT operation of several solar cell array strings in the solar cell array component, and is used to control the output circuit switch tube in the three-phase AC inverter according to the output reference current of the three-phase AC inverter. The present invention can reduce the voltage stress on the power electronic switch in the inverter circuit, and can improve the conversion efficiency and power density of the photovoltaic system, and can make the solar cell array string work under different operating conditions and realize the isolation of different working modes.

Description

Technical Field

[0001] The present invention relates to the technical field of power conversion, and in particular to a unidirectional multi-string photovoltaic inverter circuit control device and a control method. Background Art

[0002] The inverter is the core of the photovoltaic system design. In traditional photovoltaic systems, such as Figure 1 As shown in FIG. 1 , an isolated DC-DC (direct current-to-direct current) converter is embedded between the solar cell array and the inverter to control the level of the DC-Link (direct current link) voltage so that the multi-stage power converter can absorb the maximum power from the solar cell. However, since this structure uses large DC-Link capacitors with a short life, this structure is inefficient and unreliable. In addition, since this structure needs to use large passive components such as inductors, capacitors, transformers, etc., it is easy to reduce the power density of the system.

[0003] like Figure 2 As shown in FIG. 1 , a new converter technology solution is adopted, namely, a high-frequency AC-Link (alternating current link), which completes the operation tasks of the traditional photovoltaic system at one time through the high-frequency AC-Link, thereby increasing the power-to-weight ratio and improving the reliability of the converter. However, in a bidirectional three-phase inverter structure with high-frequency AC-Link and soft switching capability and a unidirectional switch on the output side, the high-frequency AC-Link link voltage has a large negative peak value, which imposes a large stress on the power electronic switch.

[0004] In addition, there are many structures of photovoltaic inverters, such as centralized, string, multi-string and micro-inverter. At present, through the analysis of various structures, it can be concluded that the multi-string structure has the ability to perform MPPT (Maximum Power Point Tracking) for each string separately, which is better than other structures. Figure 3 As shown, the multi-string bidirectional three-phase inverter equipped with a high-frequency AC link for photovoltaic applications still has the problems of large voltage stress on the power electronic switches, low inverter efficiency, and low power density of the photovoltaic system. Summary of the invention

[0005] The present invention aims to solve at least one of the technical problems in the related art to a certain extent. To this end, the first object of the present invention is to provide a unidirectional multi-string photovoltaic inverter circuit control device, which can reduce the voltage stress on the power electronic switch in the inverter circuit, and can improve the conversion efficiency and power density of the photovoltaic system, and can make the solar cell array string work under different operating conditions and realize the isolation of different working modes.

[0006] To achieve the above object, the present invention is implemented through the following technical solutions:

[0007] A unidirectional multi-string photovoltaic inverter circuit control device comprises: a unidirectional multi-string photovoltaic inverter circuit and a switching regulator, wherein the unidirectional multi-string photovoltaic inverter circuit is connected to the switching regulator, and the unidirectional multi-string photovoltaic inverter circuit comprises:

[0008] A solar cell array assembly, wherein the solar cell array assembly includes a plurality of solar cell array strings, the solar cell array strings include a plurality of input circuit switch tubes, and the solar cell array assembly is used to output a first direct current;

[0009] A high-frequency AC link, the high-frequency AC link is connected to the solar cell array assembly, the high-frequency AC link includes an energy storage inductor and a resonant capacitor, the solar cell array assembly charges the energy storage inductor through the first direct current, so that the unidirectional multi-string photovoltaic inverter circuit operates in a charging mode, wherein the high-frequency AC link and the solar cell array assembly form an input loop;

[0010] A three-phase AC inverter is connected to the high-frequency AC link. The three-phase AC inverter includes a plurality of output circuit switch tubes. The energy storage inductor and the three-phase AC inverter form an output circuit. The energy storage inductor discharges to the three-phase AC inverter, so that the unidirectional multi-string photovoltaic inverter circuit works in a discharge mode. The three-phase AC inverter is used to invert the second DC power obtained by discharging the energy storage inductor into AC power and input it into the power grid;

[0011] The switching regulator is used to control the input circuit switch tube in the solar cell array assembly according to the input reference current output after the MPPT operation of several solar cell array strings, and is used to control the output circuit switch tube in the three-phase AC inverter according to the output reference current of the three-phase AC inverter, wherein the output reference current is obtained according to the output power output after the MPPT operation of several solar cell array strings.

[0012] Optionally, each of the solar cell array strings comprises:

[0013] Solar arrays;

[0014] An input loop diode and an input loop switch tube, wherein the anode of the input loop diode is connected to the positive electrode of the solar cell array, and the first end of the input loop switch tube is connected to the cathode of the input loop diode;

[0015] Among them, the negative pole of each solar cell array in the solar cell array string is connected to a first node, the second end of each input circuit switch tube is connected to a second node, the third end of each input circuit switch tube is connected to the switching regulator, and the switching regulator controls the corresponding input circuit switch tube through the third end.

[0016] Optionally, the three-phase AC inverter includes:

[0017] A first switch tube to a sixth switch tube, wherein the second ends of the first switch tube, the third switch tube and the fifth switch tube are connected to the second node, wherein the first switch tube to the sixth switch tube are the output loop switch tubes;

[0018] The first diode to the sixth diode, the anodes of the second diode, the fourth diode and the sixth diode are connected to the first node, the first ends of the first switch tube to the sixth switch tube are respectively connected to the cathodes of the corresponding first diode to the sixth diode, wherein the anode of the first diode is connected to the second end of the second switch tube and has a third node, the anode of the third diode is connected to the second end of the fourth switch tube and has a fourth node, and the anode of the fifth diode is connected to the second end of the sixth switch tube and has a fifth node;

[0019] A first inductor to a third inductor, one end of the first inductor is connected to the third node, the other end of the first inductor is connected to the a-phase input terminal of the power grid, one end of the second inductor is connected to the fourth node, the other end of the second inductor is connected to the b-phase input terminal of the power grid, one end of the third inductor is connected to the fifth node, the other end of the third inductor is connected to the c-phase input terminal of the power grid;

[0020] The first capacitor to the third capacitor, one end of the first capacitor, the second capacitor, and the third capacitor are respectively connected to the third node, the fourth node, and the fifth node, and the other ends of the first capacitor, the second capacitor, and the third capacitor are connected and grounded.

[0021] Optionally, one end of the energy storage inductor and the resonant capacitor connected in parallel is connected to the first node, and the other end is connected to the second node. After the energy storage inductor is charged and discharged, it resonates with the resonant capacitor, so that the unidirectional multi-string photovoltaic inverter circuit operates in a resonant mode.

[0022] Optionally, the device further comprises: a plurality of MPPT operation units, each of which is connected to a corresponding solar cell array, and the MPPT operation unit is used to obtain an output voltage and an output current of a corresponding solar cell array, and obtain a corresponding input reference current and an output power according to the output voltage and the output current;

[0023] An AC link unit, connected to the high-frequency AC link, and used to collect the voltage across the energy storage inductor and the current of the branch where the energy storage inductor is located;

[0024] The switching regulator is also used to obtain the output voltage and output current of each solar cell array, the voltage across the energy storage inductor and the current of the branch where the energy storage inductor is located, and based on the input reference current, according to the output voltage and output current of each solar cell array, the voltage across the energy storage inductor and the current of the branch where the energy storage inductor is located, control the corresponding input circuit switch tube in the solar cell array component.

[0025] Optionally, the device further comprises:

[0026] A power accumulation unit connected to each of the MPPT operation units, the power accumulation unit being used to accumulate the output powers output by each of the MPPT operation units to obtain accumulated power;

[0027] A power loss unit, connected to the power accumulation unit, and configured to obtain a corresponding power loss according to the accumulated power calculation;

[0028] A phase angle calculation unit is connected to the output end of the three-phase AC inverter, and is used to calculate the corresponding phase angle according to the output voltage of each phase of the three-phase AC inverter;

[0029] an output reference current calculation unit, connected to the power accumulation unit, the power loss unit, the phase angle calculation unit and the output end of the three-phase AC inverter respectively, and used for calculating the output reference current outputted by each phase of the three-phase AC inverter according to the accumulated power, the power loss, the phase angle and the output voltage of each phase of the three-phase AC inverter;

[0030] The switching regulator is also used to obtain the actual output current of each phase output of the three-phase AC inverter, and based on the output reference current, control the corresponding output circuit switch tube according to the actual output current.

[0031] Optionally, the device further comprises:

[0032] A controller is connected to the output reference current calculation unit and the output end of the three-phase AC inverter, respectively, and is used to obtain the output reference current of each phase and the output voltage of each phase of the three-phase AC inverter, and determine the first phase quadrant and the second phase quadrant according to the output reference current of each phase, and determine the corresponding first line voltage and second line voltage according to the first phase quadrant, the second phase quadrant and the output voltage of each phase, and determine the first phase quadrant corresponding to the line voltage with a smaller amplitude between the first line voltage and the second line voltage, so as to control the energy storage inductor to work in a first discharge mode of discharging to the first phase quadrant when discharging according to the first phase quadrant, and determine the second phase quadrant corresponding to the line voltage with a larger amplitude between the first line voltage and the second line voltage, so as to control the energy storage inductor to work in a second discharge mode of discharging to the second phase quadrant when discharging according to the second phase quadrant.

[0033] Optionally, the high-frequency AC link is used to isolate the charging mode and the discharging mode of the unidirectional multi-string photovoltaic inverter circuit.

[0034] Optionally, each output circuit switch tube is connected to a corresponding diode to form a reverse blocking switch.

[0035] To achieve the above object, the second aspect of the present invention provides a unidirectional multi-string photovoltaic inverter circuit control method, which is applied to the unidirectional multi-string photovoltaic inverter circuit control device described above, and the method comprises:

[0036] Obtaining the output voltage, output current, input reference current of each solar cell array, as well as the voltage across the energy storage inductor and the current of the branch where the energy storage inductor is located;

[0037] Controlling the input circuit switch tubes corresponding to each solar cell array according to the output voltage, the output current, the input reference current, the voltage across the energy storage inductor, and the current of the branch where the energy storage inductor is located;

[0038] Obtaining the actual output current and output reference current of each phase output of the three-phase AC inverter;

[0039] The output circuit switch tube in the three-phase AC inverter is controlled according to the actual output current and the output reference current.

[0040] The present invention has at least the following technical effects:

[0041] (1) The present invention performs separate MPPT operations on different solar cell array strings, so that the solar cell array strings can operate under different operating conditions. The present invention obtains the output voltage, output current, input reference current, voltage across the energy storage inductor, current in the branch where the energy storage inductor is located, actual output current and output reference current of each phase of the three-phase AC inverter, and accurately controls the input circuit switch tube corresponding to each solar cell array and the output circuit switch tube in the three-phase AC inverter according to the above parameters, thereby effectively reducing the voltage stress on the power electronic switch, i.e., each switch tube, and improving the conversion efficiency and power density of the system.

[0042] (2) The present invention can also achieve isolation between the charging mode and the discharging mode of the unidirectional multi-string photovoltaic inverter circuit through the design of a high-frequency AC link, and in the discharge mode, the controller design can further divide the discharge mode to achieve more precise control of the output circuit switch tube; in addition, each output circuit switch tube is connected to the corresponding diode to form a unidirectional switch, which can also make the number of switches less than the number of switches in the structure with bidirectional switches, so that the control device and the inverter circuit can meet medium and low power applications.

[0043] Additional aspects and advantages of the present invention will be given in part in the following description and in part will be obvious from the following description, or will be learned through practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 This is a block diagram of the traditional photovoltaic system structure;

[0045] Figure 2 This is the structural block diagram of the improved photovoltaic system;

[0046] Figure 3 A schematic diagram of a circuit topology of a multi-string bidirectional three-phase inverter equipped with a high-frequency AC link;

[0047] Figure 4 A structural block diagram of a unidirectional multi-string photovoltaic inverter circuit control device provided by an embodiment of the present invention;

[0048] Figure 5 A schematic diagram of a unidirectional multi-string photovoltaic inverter circuit topology provided in one embodiment of the present invention;

[0049] Figure 6 A working principle diagram of a unidirectional multi-string photovoltaic inverter circuit control device provided by an embodiment of the present invention;

[0050] Figure 7 A flow chart of a unidirectional multi-string photovoltaic inverter circuit control method provided by an embodiment of the present invention;

[0051] Figure 8-Figure 12 A diagram of different working modes of a unidirectional multi-string photovoltaic inverter circuit provided by an embodiment of the present invention;

[0052] Fig.13 This is a working timing diagram of a unidirectional multi-string photovoltaic inverter circuit provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0053] The present embodiment is described in detail below, and examples of the embodiment are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are intended to be used to explain the present invention, and should not be construed as limiting the present invention.

[0054] The unidirectional multi-string photovoltaic inverter circuit control device and control method of this embodiment are described below with reference to the accompanying drawings.

[0055] Figure 4 This is a structural block diagram of a unidirectional multi-string photovoltaic inverter circuit control device provided by an embodiment of the present invention. Figure 4 As shown, the unidirectional multi-string photovoltaic inverter circuit control device 100 includes: a unidirectional multi-string photovoltaic inverter circuit 10 and a switching regulator 20, and the unidirectional multi-string photovoltaic inverter circuit 10 is connected to the switching regulator 20. Figure 5 As shown, the unidirectional multi-string photovoltaic inverter circuit 10 includes: a solar cell array component 11, a high-frequency AC link 12 and a three-phase AC inverter 13.

[0056] The solar cell array assembly 11 includes a plurality of solar cell array strings, and the solar cell array strings include a plurality of input circuit switch tubes such as S7, S8, ... N The solar cell array assembly 11 is used to output a first direct current; the high-frequency AC link 12 is connected to the solar cell array assembly 11, and the high-frequency AC link 12 includes an energy storage inductor L1 and a resonant capacitor C1. The solar cell array assembly 11 charges the energy storage inductor L1 through the first direct current, so that the unidirectional multi-string photovoltaic inverter circuit 10 works in a charging mode, wherein the high-frequency AC link 12 and the solar cell array assembly 11 form an input loop; the three-phase AC inverter 13 is connected to the high-frequency AC link 12, and the three-phase AC inverter 13 includes a plurality of output loop switch tubes S1-S6, the energy storage inductor L1 and the three-phase AC inverter 13 form an output loop, and the energy storage inductor L1 discharges to the three-phase AC inverter 13, so that the unidirectional multi-string photovoltaic inverter circuit 10 works in a discharging mode, and the three-phase AC inverter 13 is used to invert the second direct current obtained by discharging the energy storage inductor L1 into alternating current and input it into the power grid.

[0057] The switch regulator 20 is connected to the control end of the input circuit switch tube and the output circuit switch tube ( Figure 5The switching regulator 20 is used to control the input circuit switch tube in the solar cell array assembly 11 according to the input reference current output after the MPPT operation of the plurality of solar cell array strings, and is used to control the output circuit switch tube in the three-phase AC inverter 13 according to the output reference current of the three-phase AC inverter 13, wherein the output reference current is obtained by calculating the output power output after the MPPT operation of the plurality of solar cell array strings.

[0058] In this embodiment, by performing MPPT operations on different solar cell array strings, and obtaining input reference currents and output reference currents, and then accurately controlling each switch tube through the input reference currents and output reference currents, the solar cell array strings can be made to work under different operating conditions, and the voltage stress on each switch tube can be reduced. In addition, in this embodiment, solar cell array strings with different light radiation or different voltages and power levels can be selected as inputs of the inverter circuit. When a separate MPPT operation is performed for each solar cell array string with different voltages and power levels, the conversion efficiency and power density of the entire photovoltaic system can be improved.

[0059] like Figure 5 As shown, each solar cell array string includes: a solar cell array (such as PV string 1, PV string 2), an input circuit diode (such as D7...DN) and an input circuit switch tube (such as S7...S N ), the anode of the input loop diode is connected to the positive electrode of the solar cell array, and the first end of the input loop switch tube is connected to the cathode of the input loop diode; wherein, the negative electrode of each solar cell array in the solar cell array string is connected to a first node, the second end of each input loop switch tube is connected to a second node, and the third end of each input loop switch tube is connected to the switch regulator 20, and the switch regulator 20 controls the corresponding input loop switch tube through the third end. In addition, each solar cell array is also connected in parallel with a filter capacitor, such as PV string 1 is connected in parallel with a filter capacitor C5. In this embodiment, the signal output by PV string 1 can be filtered by the filter capacitor C5.

[0060] Please continue to refer to Figure 5 , the three-phase AC inverter 13 comprises:

[0061] The first to sixth switch tubes are S1-S6, and the second ends of the first switch tube S1, the third switch tube S3 and the fifth switch tube S5 are connected to the second node, wherein the first to sixth switch tube S1 to S6 are output loop switch tubes.

[0062] The anodes of the first diode D1 to the sixth diode D6, the second diode D2, the fourth diode D4 and the sixth diode D6 are connected to the first node, and the first ends of the first switching tubes S1 to the sixth switching tubes S6 are respectively connected to the cathodes of the corresponding first diodes D1 to the sixth diodes D6, wherein the anode of the first diode D1 is connected to the second end of the second switching tube S2 and has a third node, the anode of the third diode D3 is connected to the second end of the fourth switching tube S4 and has a fourth node, and the anode of the fifth diode D5 is connected to the second end of the sixth switching tube S6 and has a fifth node.

[0063] The first inductor L2 to the third inductor L4, one end of the first inductor L2 is connected to the third node, the other end of the first inductor L2 is connected to the a-phase input terminal of the power grid, one end of the second inductor L3 is connected to the fourth node, the other end of the second inductor L3 is connected to the b-phase input terminal of the power grid, one end of the third inductor L4 is connected to the fifth node, and the other end of the third inductor L4 is connected to the c-phase input terminal of the power grid.

[0064] The first capacitor C2 to the third capacitor C4, one end of the first capacitor C2, the second capacitor C3, and the third capacitor C4 are respectively connected to the third node, the fourth node, and the fifth node, and the other ends of the first capacitor C2, the second capacitor C3, and the third capacitor C4 are connected and grounded.

[0065] Among them, one end of the energy storage inductor L1 and the resonant capacitor C1 connected in parallel is connected to the first node, and the other end is connected to the second node. After the energy storage inductor L1 is charged and discharged, it resonates with the resonant capacitor C1, so that the unidirectional multi-string photovoltaic inverter circuit 10 operates in the resonant mode.

[0066] In this embodiment, each input PV string can charge the energy storage inductor L1 in a separate mode and discharge it in a separate mode. Among them, the charging and discharging of the high-frequency AC link 12 are all performed at high frequency. In this inverter circuit, the energy storage inductor L1 is the main component that transmits power from the input side to the output side, and the resonant capacitor C1 is mainly used to participate in the partial resonance between each charging / discharging mode, so that the inverter circuit works in the resonant mode, and all input loops and output loop switches achieve ZVS (zero voltage switching), thereby improving the inverter efficiency. Compared with the components of the traditional DC link, the high-frequency AC link 12 in this embodiment is more compact in the energy storage inductor L1 and the resonant capacitor C1. Therefore, in this embodiment, by setting the high-frequency AC link 12, there is always a resonant stage between the input side charging the link and the output side discharging the link, so that the inherent isolation between the input and the output can be achieved, so that the charging and discharging modes will not be connected to the link at the same time. Among them, the input side refers to the side where the PV string charges the energy storage inductor L1, and the output side refers to the side where the energy storage inductor L1 discharges to the three-phase AC inverter 13.

[0067] It should be noted that all partial resonant modes of the inverter circuit occur in soft switching operation, so that the inverter circuit has soft switching capability. Specifically, in this way, the appropriate switch of the next charging / discharging mode can be activated in the partial resonant mode. Since the voltage across the high-frequency AC link 12 is reverse biased, the appropriate switch is not turned on. Subsequently, all input loop and output loop switches are turned on in ZVS. In addition, due to the use of link capacitors and precise switch tube control methods, all inverter switches will be turned off in ZVS, and after the shutdown operation, the switch voltage will drop / rise from zero.

[0068] In addition, the output circuit switch tube is connected to the diode to form a unidirectional RB (reverse blocking) switch, so that the inverter switch has a reverse blocking capability. In this embodiment, by using the unidirectional RB switch, the three-phase AC inverter 13 can have a lower total on-state voltage. Of course, in order to suppress switching harmonics, a small low-pass filter can also be used on the output side. In this embodiment, since a unidirectional switch is used, the number of switches is less than the number of switches in the inverter structure with a bidirectional switch, so that the inverter circuit can adapt to medium and low power applications.

[0069] like Figure 6 As shown, the device also includes: a plurality of MPPT operation units and an AC link unit.

[0070] Each MPPT operation unit is connected to a corresponding solar cell array, and the MPPT operation unit is used to obtain the output voltage (such as V PV1 ) and output current (such as I PV1 ), and obtain the corresponding input reference current (such as I*1) and output power (such as P*1) according to the output voltage and output current calculation; the AC link unit is connected to the high-frequency AC link 12, and the AC link unit is used to collect the voltage VL across the energy storage inductor L1 and the current IL of the branch where the energy storage inductor L1 is located. Figure 6 As shown, the switching regulator 20 is also used to obtain the output voltage and output current of each solar cell array, as well as the voltage VL across the energy storage inductor L1 and the current IL of the branch where the energy storage inductor L1 is located, and based on the input reference current, according to the output voltage and output current of each solar cell array and the voltage VL across the energy storage inductor L1 and the current IL of the branch where the energy storage inductor L1 is located, the corresponding input circuit switch tube in the solar cell array component is controlled.

[0071] Please continue to refer to Figure 6 The device also includes: a power accumulation unit, a power loss unit, a phase angle calculation unit and an output reference current calculation unit.

[0072] Among them, the power accumulation unit is connected to each MPPT operation unit, and the power accumulation unit is used to accumulate the output power (such as P*1 and P*2) output by each MPPT operation unit to obtain the accumulated power Pin; the power loss unit is connected to the power accumulation unit, and the power loss unit is used to calculate the corresponding power loss according to the accumulated power Pin; the phase angle calculation unit is connected to the output end of the three-phase AC inverter 13, and the phase angle calculation unit is used to calculate the corresponding phase angle θ according to the output voltage Va, Vb and Vc of each phase of the three-phase AC inverter 13, wherein the angle needs to be selected in such a way that the output power factor is equal to unity; the output reference current calculation unit is respectively connected to the power accumulation unit, the power loss unit, the phase angle calculation unit and the output end of the three-phase AC inverter 13, and the output reference current calculation unit is used to calculate the output reference current I*a, I*b and I*c of each phase output of the three-phase AC inverter 13 according to the accumulated power Pin, the power loss, the phase angle θ and the output voltage Va, Vb and Vc of each phase of the three-phase AC inverter 13. Figure 6 As shown, the switching regulator 20 is also used to obtain the actual output currents Ia, Ib and Ic output by each phase of the three-phase AC inverter 13, and control the corresponding output circuit switch tube according to the actual output current based on the output reference current.

[0073] It should be noted that the output reference current in this embodiment is an output reference current with a unity power factor, and the output reference current with a unity power factor is expressed by the following formula:

[0074]

[0075] Where, I*a is the output reference current of phase a, i*a is its effective value, f0 is the frequency of the public grid, Pout is the output power delivered to the public grid, V line It is the effective value of the line grid voltage.

[0076] In one embodiment of the present invention, the device further includes: a controller. The controller is connected to the output reference current calculation unit and the output end of the three-phase AC inverter 13, respectively, and is used to obtain the output reference currents I*a, I*b and I*c of each phase and the output voltages Va, Vb and Vc of each phase of the three-phase AC inverter 13, and determine the first phase quadrant and the second phase quadrant according to the output reference currents I*a, I*b and I*c of each phase, and determine the corresponding first line voltage and second line voltage according to the first phase quadrant, the second phase quadrant and the output voltages Va, Vb and Vc of each phase, and determine the first phase quadrant corresponding to the line voltage with a smaller amplitude in the first line voltage and the second line voltage, so as to control the energy storage inductor L1 to work in the first discharge mode of discharging to the first phase quadrant according to the first phase quadrant, and determine the second phase quadrant corresponding to the line voltage with a larger amplitude in the first line voltage and the second line voltage, so as to control the energy storage inductor L1 to work in the second discharge mode of discharging to the second phase quadrant according to the second phase quadrant.

[0077] Specifically, since the sum of each output reference current is zero. Therefore, one of the output reference currents has the highest amplitude and a certain polarity, and the other two output reference currents have a lower amplitude and an opposite polarity. In this way, there are two pairs that can discharge the link. In the first discharge mode, the controller can select the phase with the highest and second highest output reference current amplitudes as the phase, and in the second discharge mode, the controller can select the phase with the highest and lowest amplitudes as the phase. Between these two pairs, the phase with the smaller line voltage amplitude is first selected. For example, if the instantaneous value of the output reference current is I*a=1A, I*b=-3A, I*c=2A, then the relative ba quadrant and bc quadrant are selected by the controller. If the instantaneous output line voltages of these pairs are Vba=-180V and Vbc=-240V at this time, the relative ba quadrant is first selected, and the inverter circuit is operated in the first discharge mode, and then the relative bc quadrant is selected to operate the inverter circuit in the second discharge mode.

[0078] Figure 7 Flow chart of a unidirectional multi-string photovoltaic inverter circuit control method provided by an embodiment of the present invention. Figure 7 As shown, the unidirectional multi-string photovoltaic inverter circuit control method includes:

[0079] Step S1: obtaining the output voltage, output current, input reference current of each solar cell array, the voltage across the energy storage inductor, and the current of the branch where the energy storage inductor is located.

[0080] Step S2: Control the input circuit switch tubes corresponding to each solar cell array according to the output voltage, output current, input reference current of each solar cell array, the voltage across the energy storage inductor and the current of the branch where the energy storage inductor is located.

[0081] Step S3: obtaining the actual output current and the output reference current of each phase output of the three-phase AC inverter.

[0082] Step S4: Control the output circuit switch tube in the three-phase AC inverter according to the actual output current and the output reference current.

[0083] It should be noted that the specific implementation of the unidirectional multi-string photovoltaic inverter circuit control method of this embodiment can refer to the specific implementation of the unidirectional multi-string photovoltaic inverter circuit control device mentioned above, and will not be repeated here to avoid redundancy.

[0084] The following takes two PV strings as an example and combines Figure 8-Figure 12 Various working mode diagrams and Fig.13 The working timing diagram of the embodiment is used to describe in detail the unidirectional multi-string photovoltaic inverter circuit control method of the present embodiment.

[0085] Working mode 1 (link charging mode):

[0086] like Figure 8 As shown, V PV1 >V PV2 、V b <0 and Vc>Va>0, the input side switch S7 is at the highest voltage level, that is, the output voltage V PV1 The switching regulator turns on under the action of the PV string 1 and connects the solar cell array PV string 1 to the high-frequency AC link. At this time, the energy storage inductor L1 on the link is charged under the action of the PV string 1, and then the output current of the PV string 1 is obtained. When the switching regulator detects that the average current of the PV string 1 reaches its input reference current I*1, the input side switch S7 corresponding to the PV string 1 is controlled to turn off. After the link is charged, the inverter circuit enters the partial resonance mode. In this embodiment, the switching regulator can also control the corresponding input side switch tube according to the current of the branch where the energy storage inductor is located, such as controlling the input side switch tube to turn off when the charging current reaches the preset value.

[0087] Working mode 2 (partial resonance mode):

[0088] like Fig. 9 As shown, V PV1 >V PV2 、V b <0 and Vc>Va>0, both the output and input switches are turned off, the energy storage inductor L1 and the resonant capacitor C1 on the link resonate, and cause the link voltage to drop from the PV string 1 voltage level (highest voltage) to the PV string 2 voltage level (second highest voltage). When the voltage across the energy storage inductor L1 drops to the PV string 2 voltage level, the switch S8 corresponding to the PV string 2 is valid in this mode (its gate signal is ready) but is in a reverse biased state, so it will not open.

[0089] Working mode 3 (link charging mode):

[0090] like Fig.10 As shown, V PV1 >V PV2 、V b <0 and Vc>Va>0, the switch S8 corresponding to PV string 2 is forward biased and turned on, and connects PV string 2 to the high-frequency AC link. The energy storage inductor L1 on the link is charged again under the action of the input side PV string 2 until the switching regulator detects that the average current of PV string 2 reaches its input reference current I*2, and controls its corresponding switch S8 to close.

[0091] It should be noted that if one of the PV strings has zero power due to a fault or insufficient light radiation, the proposed inverter has the ability to respond correctly and perform well in this case. For example, if PV string 1 fails and its power becomes zero, the MPPT algorithm sets its input reference current I*1 to zero. Since the output current of PV string 1 is also zero, the inverter circuit will jump from operating mode 1 and immediately enter operating mode 2.

[0092] Working mode 4 (partial resonance mode):

[0093] like Fig. 9 As shown, in this mode, all input-side and output-side switches are turned off and the high-frequency AC link begins to resonate.

[0094] In order to have good control over the output current and minimize the output and input harmonics, the link discharge mode can be divided into the first discharge mode and the second discharge mode. In this mode, the link voltage, i.e., the voltage across the energy storage inductor L1, is reduced to a negative voltage level, which is equal to the line voltage level of the selected relative phase. When the link voltage becomes negative in operation mode 4, the controller can select two output phase pairs and execute the first discharge mode and the second discharge mode in sequence.

[0095] Working mode 5 (first discharge mode):

[0096] like Fig.11 As shown, at the beginning of this mode, if the controller detects that the link voltage of the resonance in the working mode 4 reaches the voltage level of the selected output relative (for example, the ba quadrant), the appropriate switches S2 and S3 are forward biased and opened through the switching regulator. At this time, the charged high-frequency AC link is discharged into the first relative ba quadrant corresponding to the first relative ba quadrant, until the actual current of the a phase, that is, the average current, reaches its output reference current, the switching regulator controls the switch S2 to close, and the link enters the next partial resonance mode.

[0097] Working mode 6 (partial resonance mode):

[0098] like Fig. 9 As shown, in this mode, the link voltage is allowed to swing to the other relative line voltage (eg, bc quadrant) selected during Mode 4 of Operation.

[0099] Working mode 7 (second discharge mode):

[0100] like Fig.12 As shown, at the beginning of this mode, the link voltage is equal to the second relative line voltage Vbc. At this time, S3 and S6 are forward biased, and the high-frequency AC link is discharged to the second relative bc quadrant corresponding to the second relative bc, until the actual current of the c phase, that is, the average current, reaches its output reference current. At the end of working mode 7, the switching regulator controls switches S3 and S6 to close.

[0101] Working mode 8 (partial resonance mode):

[0102] In this case, the link voltage and current enter a swing state. In this mode, the link voltage reaches negative and positive peaks, respectively. After that, the link voltage reaches the appropriate PV string voltage level, that is, the string selected by the controller. In this mode, the controller can send a gate signal to the correct switch that must be turned on in operation mode 1. However, before the start of operation mode 1, the switch is reverse biased and will not turn on.

[0103] In summary, the present invention can make solar cell array strings work under different operating conditions by performing separate MPPT operations on different solar cell array strings, and can obtain the output voltage, output current, input reference current, voltage across the energy storage inductor, current of the branch where the energy storage inductor is located, actual output current and output reference current of each phase output of the three-phase AC inverter by obtaining the output voltage, output current, input circuit switch tube corresponding to each solar cell array and output circuit switch tube in the three-phase AC inverter according to the above parameters, so as to effectively reduce the voltage stress on the power electronic switch, i.e., each switch tube, and improve the conversion efficiency and power density of the system; in addition, the present invention can also achieve the isolation of the charging mode and the discharging mode of the unidirectional multi-string photovoltaic inverter circuit through the design of the high-frequency AC link, and in the discharge mode, the design of the controller can further divide the discharge mode to achieve more accurate control of the output circuit switch tube; in addition, each output circuit switch tube is connected to the corresponding diode to form a unidirectional switch, and the number of switches can be less than the number of switches in the structure with a bidirectional switch, so that the control device and the inverter circuit can meet medium and low power applications.

[0104] It should be noted that, in this article, relational terms such as first and second, etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device. In the absence of further restrictions, the elements defined by the sentence "comprise a ..." do not exclude the existence of other identical elements in the process, method, article or device including the elements.

[0105] Although the content of the present invention has been described in detail through the above preferred embodiments, it should be appreciated that the above description should not be considered as a limitation of the present invention. After reading the above content, it will be apparent to those skilled in the art that various modifications and substitutions of the present invention will occur. Therefore, the protection scope of the present invention should be limited by the appended claims.

Claims

1. A unidirectional multi-string photovoltaic inverter circuit control device, characterized in that: include: A unidirectional multi-string photovoltaic inverter circuit and a switching regulator, wherein the unidirectional multi-string photovoltaic inverter circuit is connected to the switching regulator, and the unidirectional multi-string photovoltaic inverter circuit comprises: A solar cell array assembly, wherein the solar cell array assembly includes a plurality of solar cell array strings, the solar cell array strings include a plurality of input circuit switch tubes, and the solar cell array assembly is used to output a first direct current; A high-frequency AC link, the high-frequency AC link is connected to the solar cell array assembly, the high-frequency AC link includes an energy storage inductor and a resonant capacitor, the solar cell array assembly charges the energy storage inductor through the first direct current, so that the unidirectional multi-string photovoltaic inverter circuit operates in a charging mode, wherein the high-frequency AC link and the solar cell array assembly form an input loop; A three-phase AC inverter is connected to the high-frequency AC link. The three-phase AC inverter includes a plurality of output circuit switch tubes. The energy storage inductor and the three-phase AC inverter form an output circuit. The energy storage inductor discharges to the three-phase AC inverter, so that the unidirectional multi-string photovoltaic inverter circuit works in a discharge mode. The three-phase AC inverter is used to invert the second DC power obtained by discharging the energy storage inductor into AC power and input it into the power grid; The switching regulator is used to control the input circuit switch tube in the solar cell array assembly according to the input reference current output after the MPPT operation of a plurality of solar cell array strings, and is used to control the output circuit switch tube in the three-phase AC inverter according to the output reference current of the three-phase AC inverter, wherein the output reference current is obtained by calculating the output power output after the MPPT operation of a plurality of solar cell array strings; Also includes: A power accumulation unit connected to each MPPT operation unit, the power accumulation unit is used to accumulate the output power output by each of the MPPT operation units to obtain accumulated power; A power loss unit, connected to the power accumulation unit, and configured to obtain a corresponding power loss according to the accumulated power calculation; A phase angle calculation unit is connected to the output end of the three-phase AC inverter, and is used to calculate the corresponding phase angle according to the output voltage of each phase of the three-phase AC inverter; an output reference current calculation unit, connected to the power accumulation unit, the power loss unit, the phase angle calculation unit and the output end of the three-phase AC inverter respectively, and used for calculating the output reference current outputted by each phase of the three-phase AC inverter according to the accumulated power, the power loss, the phase angle and the output voltage of each phase of the three-phase AC inverter; The switching regulator is also used to obtain the actual output current of each phase output of the three-phase AC inverter, and based on the output reference current, control the corresponding output circuit switch tube according to the actual output current; Also includes: A controller is connected to the output reference current calculation unit and the output end of the three-phase AC inverter, respectively, and is used to obtain the output reference current of each phase and the output voltage of each phase of the three-phase AC inverter, and determine the first phase quadrant and the second phase quadrant according to the output reference current of each phase, and determine the corresponding first line voltage and second line voltage according to the first phase quadrant, the second phase quadrant and the output voltage of each phase, and determine the first phase quadrant corresponding to the line voltage with a smaller amplitude between the first line voltage and the second line voltage, so as to control the energy storage inductor to work in a first discharge mode of discharging to the first phase quadrant when discharging according to the first phase quadrant, and determine the second phase quadrant corresponding to the line voltage with a larger amplitude between the first line voltage and the second line voltage, so as to control the energy storage inductor to work in a second discharge mode of discharging to the second phase quadrant when discharging according to the second phase quadrant.

2. The unidirectional multi-string photovoltaic inverter circuit control device according to claim 1, characterized in that: Each of the solar cell array strings comprises: Solar arrays; An input loop diode and an input loop switch tube, wherein the anode of the input loop diode is connected to the positive electrode of the solar cell array, and the first end of the input loop switch tube is connected to the cathode of the input loop diode; Among them, the negative pole of each solar cell array in the solar cell array string is connected to a first node, the second end of each input circuit switch tube is connected to a second node, the third end of each input circuit switch tube is connected to the switching regulator, and the switching regulator controls the corresponding input circuit switch tube through the third end.

3. The unidirectional multi-string photovoltaic inverter circuit control device according to claim 2, characterized in that: The three-phase AC inverter comprises: A first switch tube to a sixth switch tube, wherein the second ends of the first switch tube, the third switch tube and the fifth switch tube are connected to the second node, wherein the first switch tube to the sixth switch tube are the output loop switch tubes; The anodes of the first diode to the sixth diode, the second diode, the fourth diode and the sixth diode are connected to the first node, the first ends of the first switch tube to the sixth switch tube are respectively connected to the cathodes of the corresponding first diode to the sixth diode, wherein the anode of the first diode is connected to the second end of the second switch tube and has a third node, the anode of the third diode is connected to the second end of the fourth switch tube and has a fourth node, and the anode of the fifth diode is connected to the second end of the sixth switch tube and has a fifth node; A first inductor to a third inductor, one end of the first inductor is connected to the third node, the other end of the first inductor is connected to the a-phase input terminal of the power grid, one end of the second inductor is connected to the fourth node, the other end of the second inductor is connected to the b-phase input terminal of the power grid, one end of the third inductor is connected to the fifth node, the other end of the third inductor is connected to the c-phase input terminal of the power grid; The first capacitor to the third capacitor, one end of the first capacitor, the second capacitor, and the third capacitor are respectively connected to the third node, the fourth node, and the fifth node, and the other ends of the first capacitor, the second capacitor, and the third capacitor are connected and grounded.

4. The unidirectional multi-string photovoltaic inverter circuit control device according to claim 3, characterized in that: One end of the energy storage inductor and the resonant capacitor connected in parallel is connected to the first node, and the other end is connected to the second node. After the energy storage inductor is charged and discharged, it resonates with the resonant capacitor, so that the unidirectional multi-string photovoltaic inverter circuit operates in a resonant mode.

5. The unidirectional multi-string photovoltaic inverter circuit control device according to claim 4, characterized in that: Also includes: A plurality of MPPT operation units, each of which is connected to a corresponding solar cell array, and is used to obtain an output voltage and an output current of the corresponding solar cell array, and obtain a corresponding input reference current and an output power according to the output voltage and the output current; An AC link unit, connected to the high-frequency AC link, and used to collect the voltage across the energy storage inductor and the current of the branch where the energy storage inductor is located; The switching regulator is also used to obtain the output voltage and output current of each solar cell array, the voltage across the energy storage inductor and the current of the branch where the energy storage inductor is located, and based on the input reference current, according to the output voltage and output current of each solar cell array, the voltage across the energy storage inductor and the current of the branch where the energy storage inductor is located, control the corresponding input circuit switch tube in the solar cell array component.

6. The unidirectional multi-string photovoltaic inverter circuit control device according to claim 5, characterized in that: The high frequency AC link is used to isolate the charging mode and the discharging mode of the unidirectional multi-string photovoltaic inverter circuit.

7. The unidirectional multi-string photovoltaic inverter circuit control device according to claim 6, characterized in that: After each output circuit switch tube is connected to the corresponding diode, a reverse blocking switch is formed.

8. A unidirectional multi-string photovoltaic inverter circuit control method, applied to a unidirectional multi-string photovoltaic inverter circuit control device as described in any one of claims 1 to 7, characterized in that: include: Obtaining the output voltage, output current, input reference current of each solar cell array, as well as the voltage across the energy storage inductor and the current of the branch where the energy storage inductor is located; Controlling the input circuit switch tubes corresponding to each solar cell array according to the output voltage, the output current, the input reference current, the voltage across the energy storage inductor, and the current of the branch where the energy storage inductor is located; Obtaining the actual output current and output reference current of each phase output of the three-phase AC inverter; The output circuit switch tube in the three-phase AC inverter is controlled according to the actual output current and the output reference current.

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