Photovoltaic direct current sending-out system and photovoltaic power generation equipment

Abstract

The invention discloses a photovoltaic direct current sending-out system and photovoltaic power generation equipment, which are applied to the field of photovoltaic power generation and comprise an inverter circuit, a multi-pulse transformer, a rectifying circuit and a first output filter circuit, the inverter circuit converts the photovoltaic direct current into low-voltage three-phase alternating current and regulates and controls the frequency and amplitude of the low-voltage three-phase alternating current; the multi-pulse transformer boosts the low-voltage three-phase alternating current and outputs multiple groups of high-voltage three-phase alternating current with phase differences to the rectifying circuit; the rectifying circuit rectifies the multiple groups of high-voltage three-phase alternating current into high-voltage direct current; the first output filter circuit filters the high-voltage direct current containing the alternating current component. According to the multi-pulse transformer, the frequency of the low-voltage three-phase alternating current is increased through the inverter circuit, so that the size and weight of the multi-pulse transformer are reduced, the frequency of alternating current components contained in the rectified high-voltage direct current is increased through the multiple sets of high-voltage three-phase alternating current with phase differences generated by the multi-pulse transformer, and the size of inductors and capacitors used for filtering is reduced; and the cost of the whole system is reduced.

Description

Technical Field

[0001] The present invention relates to the field of photovoltaic power generation, and in particular to a photovoltaic direct current transmission system and photovoltaic power generation equipment. Background Art

[0002] The electricity generated by photovoltaic power generation needs to be connected to the power grid and transmitted to the power consumption end through the power grid. The current power grid mainly has two forms: AC power grid and DC power grid. Among them, the AC power grid is relatively mature and well-built. With the completion of the construction of the flexible DC transmission network, the demand for photovoltaic power generation equipment that can be directly connected to the DC power grid is constantly increasing. However, the AC photovoltaic power generation system currently used does not have the ability to access the DC high-voltage power grid, and it is necessary to specially design equipment that can access the DC high-voltage power grid.

[0003] In response to the demand for photovoltaic DC transmission, the technical solution commonly proposed in the current technical field is the DC transformer solution, which relies on power electronic devices to achieve DC to DC energy conversion and generally adopts Modular Multilevel Converter (MMC) technology. As the DC bus voltage continues to increase, more and more power modules are required to be connected in series and parallel, and the cost also increases accordingly. Therefore, the implementation cost of this solution is significantly higher than the current AC grid-connected system, and it does not have engineering implementation value.

[0004] From the above, it can be concluded that how to achieve low-cost access of photovoltaic power to the DC high-voltage power grid is an urgent problem to be solved. Summary of the invention

[0005] In view of this, the purpose of the present invention is to provide a photovoltaic DC transmission system and photovoltaic power generation equipment to solve the problem of high cost of photovoltaic access to DC high-voltage power grid. The specific scheme is as follows:

[0006] In order to solve the above technical problems, the present application provides a photovoltaic DC transmission system, including: an inverter circuit, a multi-pulse transformer, a rectifier circuit, and a first output filter circuit;

[0007] The input end of the inverter circuit is connected to the photovoltaic direct current output end, and the output end of the inverter circuit is connected to the low-voltage three-phase input end of the multi-pulse transformer, for converting the photovoltaic direct current into low-voltage three-phase alternating current, and regulating the frequency and amplitude of the low-voltage three-phase alternating current;

[0008] The multiple groups of high-voltage three-phase output terminals of the multi-pulse transformer are connected to the multiple groups of input terminals of the rectifier circuit, so as to boost the low-voltage three-phase alternating current and output multiple groups of high-voltage three-phase alternating currents with phase differences to the rectifier circuit;

[0009] The output end of the rectifier circuit is connected to the input end of the first output filter circuit, and is used to rectify the multiple groups of high-voltage three-phase alternating current into high-voltage direct current;

[0010] The output end of the first output filter circuit is connected to a DC high voltage power grid and is used to filter the high voltage DC power containing an AC component.

[0011] As an optional solution, the inverter circuit includes: a control system, an inverter topology circuit, a second output filter circuit, and a three-phase switch;

[0012] The control system is connected to the inverter topology circuit and is used to control the inverter topology circuit to adjust the frequency and amplitude of the low-voltage three-phase alternating current;

[0013] The input end of the inverter topology circuit is connected to the photovoltaic direct current output end, and the output end of the inverter topology circuit is connected to the input end of the second output filter circuit;

[0014] A first end of the three-phase switch is connected to an output end of the second output filter circuit, and a second end of the three-phase switch is connected to a low-voltage three-phase input end of the multi-pulse transformer.

[0015] As an optional solution, the multi-pulse transformer is an m-pulse transformer, where m is 12, 18, 24, or 30; when m=12, a set of three-winding transformers is used;

[0016] The primary side of the three-winding transformer is composed of a group of low-voltage three-phase windings as the low-voltage three-phase input terminal, and the secondary side of the three-winding transformer is composed of m / 6 groups of high-voltage three-phase windings as the high-voltage three-phase output terminal;

[0017] The three-phase winding on the high-voltage side of the m / 6 group adopts a different wiring method.

[0018] As an optional solution, the rectifier circuit is an uncontrolled rectifier circuit, and the uncontrolled rectifier circuit is m / 6 three-phase uncontrolled rectifier bridges connected in series;

[0019] The three-phase uncontrolled rectifier bridge is composed of six diodes, which are divided into upper bridge arm diodes and lower bridge arm diodes, three in each group;

[0020] The cathode of the upper bridge arm diode is connected to the positive electrode of the first output filter circuit, and the anode of the upper bridge arm diode is respectively connected to different phases of the output of the same three-phase winding on the high-voltage side;

[0021] The anode of the lower bridge arm diode is connected to the cathode of the first output filter circuit, and the cathode of the lower bridge arm diode is respectively connected to different phases of the high-voltage side three-phase winding output that is the same as the upper bridge arm diode;

[0022] The m / 6 groups of the high-voltage side three-phase windings correspond one to one with the m / 6 three-phase uncontrolled rectifier bridges.

[0023] As an optional solution, the inverter circuit further includes: a first capacitor for filtering clutter in the photovoltaic direct current;

[0024] The first end of the first capacitor is connected to the positive electrode of the photovoltaic direct current output terminal, and the second end of the first capacitor is connected to the negative electrode of the photovoltaic direct current output terminal.

[0025] As an optional solution, the inverter topology circuit includes: a first switch tube, a second switch tube, a third switch tube, a fourth switch tube, a fifth switch tube, and a sixth switch tube;

[0026] The second end of the first switch tube is connected in series with the first end of the second switch tube, the second end of the third switch tube is connected in series with the first end of the fourth switch tube, and the second end of the fifth switch tube is connected in series with the first end of the sixth switch tube;

[0027] The first end of the first switch tube, the first end of the third switch tube, and the first end of the fifth switch tube are connected to the positive electrode of the photovoltaic direct current output end;

[0028] The second end of the second switch tube, the second end of the fourth switch tube, and the second end of the sixth switch tube are connected to the negative electrode of the photovoltaic direct current output end.

[0029] As an optional solution, the second output filter circuit includes: a second capacitor, a third capacitor, a fourth capacitor, a first inductor, a second inductor, and a third inductor;

[0030] The first end of the first inductor is connected to the common end of the fifth switch tube and the sixth switch tube, and the second end of the first inductor is connected to the second end of the fourth capacitor;

[0031] The first end of the second inductor is connected to the common end of the third switch tube and the fourth switch tube, and the second end of the second inductor is connected to the second end of the third capacitor;

[0032] A first end of the third inductor is connected to a common end of the first switch tube and the second switch tube, and a second end of the third inductor is connected to a second end of the second capacitor;

[0033] The first end of the second capacitor, the first end of the third capacitor and the first end of the fourth capacitor are connected to each other.

[0034] As an optional solution, the first output filter circuit includes: a fourth inductor and a fifth capacitor;

[0035] The first end of the fourth inductor is connected to the positive input end of the first output filter circuit, and the second end of the fourth inductor and the first end of the fifth capacitor are connected to the positive electrode of the DC high-voltage power grid as the positive output end of the first output filter circuit, and are connected to the positive electrode of the DC high-voltage power grid;

[0036] The second end of the fifth capacitor is connected to the negative input end of the first output filter circuit as the negative output end of the first output filter circuit, and is connected to the negative electrode of the DC high voltage grid.

[0037] As an optional solution, the frequency of the low-voltage three-phase alternating current is greater than or equal to the fundamental frequency of the alternating current grid.

[0038] In order to solve the above technical problems, the present application also provides a photovoltaic power generation device, including the above photovoltaic direct current transmission system.

[0039] The photovoltaic direct current transmission system provided by the present application includes: an inverter circuit, a multi-pulse transformer, a rectifier circuit, and a first output filter circuit; the inverter circuit is connected to the photovoltaic direct current output terminal, and can invert the received photovoltaic direct current into a low-voltage three-phase alternating current, and input the above-mentioned low-voltage three-phase alternating current into the low-voltage three-phase input terminal of the multi-pulse transformer through the output terminal of the inverter circuit, and the frequency and amplitude of the low-voltage three-phase alternating current are both regulated by the inverter circuit; the multi-pulse transformer can boost the above-mentioned low-voltage three-phase alternating current, and output multiple groups of high-voltage three-phase alternating currents with phase differences to the rectifier circuit, and the high-voltage three-phase output terminal of the multi-pulse transformer is connected one by one with the multiple groups of input terminals of the rectifier circuit; the rectifier circuit can rectify the multiple groups of high-voltage three-phase alternating currents inputted from its input terminal into high-voltage direct current, filter the AC component in the above-mentioned high-voltage direct current through the first output filter circuit, and input the filtered high-voltage direct current into the direct current high-voltage power grid. As can be seen from the above, the inverter circuit can control the frequency of the low-voltage three-phase AC power to be greater than or equal to the fundamental frequency of the AC power grid. Compared with the transformer using the power grid voltage frequency of 50HZ, increasing the frequency of the low-voltage three-phase AC power can greatly reduce the volume and weight of the transformer core, so the volume, weight and cost of the multi-pulse transformer can be greatly reduced. In addition, the use of a multi-pulse transformer can output multiple groups of high-voltage three-phase AC power, and the rectifier circuit can then rectify the multiple groups of high-voltage three-phase AC power into high-voltage DC power, which can increase the frequency of the AC component contained in the high-voltage DC power on the basis of the frequency of the low-voltage three-phase AC power, reduce the inductance and capacity of the inductor and capacitor in the first output filter circuit, and at the same time reduce the volume, weight and cost of the inductor and capacitor. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying creative work.

[0041] Figure 1 A schematic diagram of a photovoltaic direct current transmission system provided in an embodiment of the present application;

[0042] Figure 2 A circuit diagram of a specific photovoltaic DC transmission system provided in an embodiment of the present application;

[0043] Figure 3 A schematic diagram of a specific inverter circuit control method provided in an embodiment of the present application;

[0044] Figure 4 A specific multi-pulse transformer wiring method provided in an embodiment of the present application;

[0045] The reference numerals are as follows: 10 is an inverter circuit, 20 is a multi-pulse transformer, 30 is a rectifier circuit, and 40 is a first output filter circuit. DETAILED DESCRIPTION

[0046] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0047] The core of this application is to provide a photovoltaic direct current transmission system.

[0048] In order to enable those skilled in the art to better understand the present application, the present application is further described in detail below in conjunction with the accompanying drawings and specific implementation methods.

[0049] The electricity generated by photovoltaic power generation needs to be connected to the power grid and transmitted to the power consumption end through the power grid. The current power grid mainly has two forms: AC power grid and DC power grid. Among them, the AC power grid is relatively mature and well-built. With the completion of the construction of the flexible DC transmission network, the demand for photovoltaic power generation equipment that can be directly connected to the DC power grid is constantly increasing. However, the AC photovoltaic power generation system currently used does not have the ability to access the DC high-voltage power grid, and it is necessary to specially design equipment that can access the DC high-voltage power grid.

[0050] In response to the demand for photovoltaic DC transmission, the technical solution commonly proposed in the current technical field is the DC transformer solution, which relies on power electronic devices to achieve DC to DC energy conversion and generally adopts Modular Multilevel Converter (MMC) technology. As the DC bus voltage continues to increase, more and more power modules are required to be connected in series and parallel, and the cost also increases accordingly. Therefore, the implementation cost of this solution is significantly higher than the current AC grid-connected system, and it does not have engineering implementation value.

[0051] From the above, it can be concluded that how to achieve low-cost access of photovoltaic power to the DC high-voltage power grid is an urgent problem to be solved.

[0052] To solve the above problems, see Figure 1 As shown, the embodiment of the present invention discloses a photovoltaic direct current transmission system, comprising: an inverter circuit 10, a multi-pulse transformer 20, a rectifier circuit 30, and a first output filter circuit 40; the input end of the inverter circuit 10 is connected to the photovoltaic direct current output end, and the output end of the inverter circuit 10 is connected to the low-voltage three-phase input end of the multi-pulse transformer 20, for converting the photovoltaic direct current into a low-voltage three-phase alternating current, and regulating the frequency and amplitude of the low-voltage three-phase alternating current; multiple groups of high-voltage three-phase output ends of the multi-pulse transformer 20 are connected to multiple groups of input ends of the rectifier circuit 30, for boosting the low-voltage three-phase alternating current, and outputting multiple groups of high-voltage three-phase alternating currents with phase differences to the rectifier circuit 30; the output end of the rectifier circuit 30 is connected to the input end of the first output filter circuit 40, for rectifying the multiple groups of high-voltage three-phase alternating currents into high-voltage direct current; the output end of the first output filter circuit 40 is connected to the DC high-voltage power grid, for filtering the high-voltage direct current containing the AC component.

[0053] Figure 1 Schematic diagram of a photovoltaic DC transmission system provided in an embodiment of the present application, wherein DC+, DC- are the photovoltaic DC output terminals, U, V, W are the output terminals of the inverter circuit 10, A, B, C are the low-voltage three-phase input terminals of the multi-pulse transformer 20, a1, b1, c1 to a N ,b N ,c N The multiple high voltage three-phase output terminals of the multi-pulse transformer 20 are u1, v1, w1 to u N ,v N ,w N Bus+ and Bus- are the high-voltage three-phase input terminals of the rectifier circuit 30, IN+ and IN- are the input terminals of the first output filter circuit 40, and HVDC+ and HVDC- are the input terminals of the DC high-voltage power grid. It is the number of groups of high voltage three-phase alternating current output by the multi-pulse transformer 20.

[0054] In a specific embodiment, the inverter circuit 10 may be composed of a control system, an inverter topology circuit, a second output filter circuit and a three-phase switch, and the control system controls the inverter topology circuit to convert photovoltaic direct current to obtain low-voltage three-phase alternating current, and the frequency and amplitude of the low-voltage three-phase alternating current can be controlled by the control system. The inverter topology circuit may be a two-level topology or a three-level topology.

[0055] In another specific embodiment, the inverter circuit 10 may also be directly replaced by an existing photovoltaic inverter. It is understandable that directly replacing the inverter circuit 10 with an existing photovoltaic inverter can reduce the hardware development process and greatly reduce costs.

[0056] In this embodiment, the inverter circuit 10 outputs low-voltage three-phase AC power with a frequency of , the amplitude is The multi-pulse transformer 20 has its own transformer ratio. , the low-voltage three-phase alternating current input by the inverter circuit 10 is boosted into multiple sets of high-voltage three-phase alternating currents, the frequency of the high-voltage three-phase alternating currents is equal to the frequency of the above-mentioned low-voltage three-phase alternating currents, and the amplitudes of the multiple sets of high-voltage three-phase alternating currents are all , where the number of high voltage three-phase alternating current groups It is related to the number of pulses m of the multi-pulse transformer 20, where the value of m can be 12, 18, 24 or 30, and the specific formula is N=m / 6. It can be understood that the high-voltage three-phase AC generated by the multi-pulse transformer 20 needs to have a phase difference so that it can be rectified into a smooth high-voltage DC by the rectifier circuit 30.

[0057] In this embodiment, the rectifier circuit 30 can be a controlled rectifier circuit or an uncontrolled rectifier circuit. In a specific embodiment, the rectifier circuit 30 is composed of N three-phase uncontrolled rectifier bridges connected in series, which rectifies N groups of high-voltage three-phase alternating current into high-voltage direct current containing an alternating current component, and the amplitude of the high-voltage direct current is , the frequency of the above AC component is For example, when the multi-pulse transformer 20 is a 12-pulse transformer, the amplitude of the high-voltage direct current output by the rectifier circuit 30 is , the frequency of the AC component contained in the high voltage DC is .

[0058] It is understandable that in order to remove the AC component in the high-voltage DC power, it is also necessary to connect the first output filter circuit 40 between the rectifier circuit 30 and the DC high-voltage power grid. The first output filter circuit 40 may be as follows: Figure 3 The LC filter circuit shown, which is composed of the fourth inductor L4 and the fifth capacitor C5, can also be formed by using a separate filter capacitor.

[0059] It should be noted that the inverter circuit 10 can also control the power flow between the photovoltaic DC transmission system and the DC high-voltage power grid by adjusting the amplitude of the low-voltage three-phase AC power. Figure 2 As shown, first, the reference frequency of the inverter circuit 10 outputting the low-voltage three-phase AC power is set. With reference amplitude , the voltage reference value of the low-voltage three-phase alternating current output by the inverter circuit 10 is calculated according to the set value, and the specific formula is as follows:

[0060] ;

[0061] ;

[0062] ;

[0063] in, is the low voltage three-phase AC voltage reference value, Indicates time, is the reference amplitude of low voltage three-phase alternating current, It is the reference frequency of low voltage three-phase alternating current.

[0064] Next, the output voltage at the output end of the inverter circuit 10 is controlled , , , is the voltage of the DC high voltage grid, when Only when the power flows from the photovoltaic DC input terminal DC+, DC- to the DC high voltage grid input terminal HVDC+, HVDC-. Therefore, the inverter circuit 10 regulates The size of can control the power flow between the photovoltaic DC input terminal DC+, DC- and the DC high-voltage grid input terminal HVDC+, HVDC-.

[0065] Understandably, The reference amplitude of the low-voltage three-phase AC power set for the inverter circuit 10 is used to compare the voltage of the DC high-voltage power grid. By comparison, it is determined whether there is power flow between the photovoltaic DC input terminal and the input terminal of the DC high-voltage grid under the corresponding low-voltage three-phase AC amplitude, rather than the amplitude of the low-voltage three-phase AC actually output by the inverter circuit 10.

[0066] The photovoltaic direct current transmission system provided by the present application includes: an inverter circuit 10, a multi-pulse transformer 20, a rectifier circuit 30, and a first output filter circuit 40; the input end of the inverter circuit 10 is connected to the photovoltaic direct current output end, and the output end of the inverter circuit 10 is connected to the low-voltage three-phase input end of the multi-pulse transformer 20, which is used to convert the photovoltaic direct current into a low-voltage three-phase alternating current and regulate the frequency and amplitude of the low-voltage three-phase alternating current; multiple groups of high-voltage three-phase output ends of the multi-pulse transformer 20 are connected to multiple groups of input ends of the rectifier circuit 30, which are used to boost the low-voltage three-phase alternating current and output multiple groups of high-voltage three-phase alternating currents with phase differences to the rectifier circuit 30; the output end of the rectifier circuit 30 is connected to the input end of the first output filter circuit 40, which is used to rectify the multiple groups of high-voltage three-phase alternating currents into high-voltage direct current; the output end of the first output filter circuit 40 is connected to the direct current high-voltage power grid, which is used to filter the high-voltage direct current containing the alternating current component. As can be seen from the above, the inverter circuit 10 can control the frequency of the low-voltage three-phase AC power to be greater than or equal to the fundamental frequency of the AC power grid. Compared with the transformer using the power grid voltage frequency of 50HZ, increasing the frequency of the low-voltage three-phase AC power can greatly reduce the volume and weight of the transformer core, so the volume, weight and cost of the multi-pulse transformer 20 can be greatly reduced. In addition, the multi-pulse transformer 20 can output multiple groups of high-voltage three-phase AC power, and the rectifier circuit 30 can then rectify the multiple groups of high-voltage three-phase AC power into high-voltage DC power, which can increase the frequency of the AC component contained in the high-voltage DC power on the basis of the frequency of the low-voltage three-phase AC power, reduce the inductance and capacity of the inductor and capacitor in the first output filter circuit 40, and reduce the volume, weight and cost of the inductor and capacitor.

[0067] In order to convert photovoltaic direct current into low-voltage three-phase alternating current with a specific frequency and amplitude, according to the above embodiment, this embodiment provides a specific solution, wherein the inverter circuit 10 includes: a control system, an inverter topology circuit, a second output filter circuit, and a three-phase switch; the control system is connected to the inverter topology circuit, and is used to control the inverter topology circuit to adjust the frequency and amplitude of the low-voltage three-phase alternating current; the input end of the inverter topology circuit is connected to the photovoltaic direct current output end, and the output end of the inverter topology circuit is connected to the input end of the second output filter circuit; the first end of the three-phase switch is connected to the output end of the second output filter circuit, and the second end of the three-phase switch is connected to the low-voltage three-phase input end of the multi-pulse transformer 20.

[0068] like Figure 3As shown, the inverter circuit 10 can adopt a two-level topology or a three-level topology, and is composed of switch tubes. The photovoltaic DC power is input through the DC+ and DC- ports. The control system controls the inverter topology circuit composed of the switch tubes to invert the photovoltaic DC power into a low-voltage three-phase AC power. The three-phase low-voltage AC power is output from ports U, V, and W after passing through a three-phase AC filter composed of a first inductor L1, a second inductor L2, a third inductor L3, a second capacitor C2, a third capacitor C3, and a fourth capacitor C4, and a three-phase switch K.

[0069] As can be seen from the above, the inverter circuit 10 can control the frequency and amplitude of the output low-voltage three-phase alternating current by controlling the closing of the switch tube in the inverter topology circuit through the control system.

[0070] In order to output multiple groups of high-voltage three-phase alternating currents with phase differences, according to the above embodiment, this embodiment provides a specific solution, the multi-pulse transformer 20 is an m-pulse transformer, wherein m is 12, 18, 24 or 30; when m=12, a group of three-winding transformers is used; the primary side of the three-winding transformer is composed of a group of low-voltage side three-phase windings as a low-voltage three-phase input terminal, and the secondary side of the three-winding transformer is composed of m / 6 groups of high-voltage side three-phase windings as a high-voltage three-phase output terminal; the m / 6 groups of high-voltage side three-phase windings use different wiring methods.

[0071] In this embodiment, when the multi-pulse transformer 20 is a 12-pulse transformer, the transformer can be used as follows: Figure 3 A set of three-winding transformers with Y / Y / △ connection is shown, or a set of three-winding transformers with △ / Y / △ connection is used, wherein the three-phase windings on the low-voltage side are △-connected, and one set of three-phase windings on the high-voltage side are △-connected and the other set is Y-connected. It can be understood that, since the two sets of three-phase windings on the high-voltage side are connected in a non-through manner, the two sets of high-voltage three-phase alternating currents output by the multi-pulse transformer 20 have a phase difference of 30°.

[0072] In addition, the multi-pulse transformer 20 can also be realized by connecting two Y / Y, Y / △ or △ / Y, △ / △ double-winding transformers in parallel at the low-voltage side, wherein the two Y / Y, Y / △ double-winding transformers are connected in parallel. Figure 4 As shown. It can be understood that the price of selecting a set of three-winding transformers is lower than that of two sets of two-winding transformers.

[0073] In this embodiment, when the multi-pulse transformer 20 is an 18-pulse transformer, the three groups of high-voltage side windings can be connected in star, triangle and Z shapes, respectively. It should be noted that, as m continues to increase, only the three standard winding connection methods cannot meet the requirements of outputting multiple groups of high-voltage three-phase alternating currents with phase differences, and other customized non-standard winding connection methods can also be used to achieve different phase difference requirements.

[0074] As can be seen from the above, when the multi-pulse transformer 20 is a 12-pulse transformer, using a set of three-winding or two sets of double-winding transformers can ensure that the high-voltage side has two sets of windings, and one set is △-connected and the other set is Y-connected, ensuring that two sets of high-voltage three-phase alternating currents with phase differences can be output. Further, when the multi-pulse transformer 20 is an m-pulse transformer, where m is 12, 18, 24 or 30, as long as the multiple sets of high-voltage side windings of the multi-pulse transformer 20 are connected in different ways, multiple sets of high-voltage three-phase alternating currents with phase differences can be output.

[0075] In order to rectify multiple groups of high-voltage three-phase alternating current into high-voltage direct current, according to the above embodiment, as Figure 3 As shown, this embodiment provides a specific solution, the rectifier circuit 30 is an uncontrolled rectifier circuit, and the uncontrolled rectifier circuit is m / 6 three-phase uncontrolled rectifier bridges connected in series; the three-phase uncontrolled rectifier bridge is composed of six diodes, which are divided into an upper bridge arm diode and a lower bridge arm diode, with three in each group; the cathode of the upper bridge arm diode is connected to the positive electrode of the first output filter circuit 40, and the anode of the upper bridge arm diode is respectively connected to different phases of the same high-voltage side three-phase winding output; the anode of the lower bridge arm diode is connected to the negative electrode of the first output filter circuit 40, and the cathode of the lower bridge arm diode is respectively connected to different phases of the high-voltage side three-phase winding output that is the same as the upper bridge arm diode; the m / 6 groups of high-voltage side three-phase windings correspond one-to-one to the m / 6 three-phase uncontrolled rectifier bridges.

[0076] In this embodiment, the rectifier circuit 30 is an uncontrolled rectifier circuit composed of six diodes. Each phase of the high-voltage three-phase output end is respectively connected to the common end of a different upper bridge arm diode and a lower bridge arm diode, and the anode of the upper bridge arm diode is connected to the cathode of the lower bridge arm diode in a one-to-one correspondence, ensuring that m / 6 groups of high-voltage three-phase alternating current can enter the input end of the first output filter circuit 40 through different paths, and can be rectified into smooth high-voltage direct current due to the existence of the phase difference.

[0077] It should be noted that, considering the limited voltage tolerance and current capacity of a single diode, multiple diodes can be connected in series to increase the voltage tolerance, and diodes can be connected in parallel to increase the current carrying capacity, to equivalently replace a single diode to meet the system voltage level requirements.

[0078] From the above, it can be concluded that since the m / 6 groups of high-voltage three-phase alternating currents have a phase difference, the m / 6 groups of high-voltage three-phase alternating currents can be rectified into smooth high-voltage direct currents by an uncontrolled rectifier circuit.

[0079] In order to remove the possible clutter in photovoltaic direct current, according to the above embodiment, Figure 3As shown, this embodiment provides a specific solution, the inverter circuit 10 also includes: a first capacitor C1, used to filter the clutter in the photovoltaic direct current; the first end of the first capacitor C1 is connected to the positive electrode of the photovoltaic direct current output terminal, and the second end of the first capacitor C1 is connected to the negative electrode of the photovoltaic direct current output terminal.

[0080] It is understandable that there are noises in photovoltaic direct current. In order to avoid their influence on the generation of smooth and flat high-voltage direct current, it is necessary to use the charging and discharging characteristics, impedance characteristics, energy buffering effect, ripple suppression ability of the first capacitor C1 and the synergistic effect with the inductor to filter out the noises and high-frequency signals.

[0081] As can be seen from the above, the first capacitor C1 can utilize its own characteristics of storing and releasing charges to smooth out the clutter in the photovoltaic direct current.

[0082] In order to convert photovoltaic direct current into low-voltage alternating current, according to the above embodiment, this embodiment provides a specific solution, in which the inverter topology circuit includes: a first switch tube Q1, a second switch tube Q2, a third switch tube Q3, a fourth switch tube Q4, a fifth switch tube Q5, and a sixth switch tube Q6; the second end of the first switch tube Q1 is connected in series with the first end of the second switch tube Q2, the second end of the third switch tube Q3 is connected in series with the first end of the fourth switch tube Q4, and the second end of the fifth switch tube Q5 is connected in series with the first end of the sixth switch tube Q6; the first end of the first switch tube Q1, the first end of the third switch tube Q3, and the first end of the fifth switch tube Q5 are connected to the positive electrode of the photovoltaic direct current output end; the second end of the second switch tube Q2, the second end of the fourth switch tube Q4, and the second end of the sixth switch tube Q6 are connected to the negative electrode of the photovoltaic direct current output end.

[0083] like Figure 3 As shown, the inverter circuit 10 adopts a two-level topology and consists of six switch tubes. The photovoltaic DC power is input through the DC+ and DC- ports. The control system controls the inverter topology circuit composed of the first switch tube Q1 to the sixth switch tube Q6 to invert the photovoltaic DC power into a low-voltage three-phase AC power.

[0084] As can be seen from the above, the inverter circuit 10 can control the frequency and amplitude of the output low-voltage three-phase alternating current by controlling the closing of the first switch tube Q1 to the sixth switch tube Q6 through the control system.

[0085] In order to filter the AC component in the low-voltage three-phase AC power, according to the above embodiment, this embodiment provides a specific solution, and the second output filter circuit includes: a second capacitor C2, a third capacitor C3, a fourth capacitor C4, a first inductor L1, a second inductor L2, and a third inductor L3; a first end of the first inductor L1 is connected to the common end of the fifth switch tube Q5 and the sixth switch tube Q6, and a second end of the first inductor L1 is connected to the second end of the fourth capacitor C4; a first end of the second inductor L2 is connected to the common end of the third switch tube Q3 and the fourth switch tube Q4, and a second end of the second inductor L2 is connected to the second end of the third capacitor C3; a first end of the third inductor L3 is connected to the common end of the first switch tube Q1 and the second switch tube Q2, and a second end of the third inductor L3 is connected to the second end of the second capacitor C2; a first end of the second capacitor C2, a first end of the third capacitor C3, and a first end of the fourth capacitor C4 are connected to each other.

[0086] It can be understood that after the inverter topology circuit converts photovoltaic direct current into low-voltage three-phase alternating current, there is still an AC component. Therefore, the second capacitor C2, the third capacitor C3, the fourth capacitor C4, the first inductor L1, the second inductor L2, and the third inductor L3 can be connected as described above to form a three-phase AC filter to filter the AC component in the low-voltage three-phase alternating current.

[0087] From the above, it can be seen that the second capacitor C2, the third capacitor C3, the fourth capacitor C4, the first inductor L1, the second inductor L2, and the third inductor L3 can form a three-phase AC filter, and use the properties of capacitors and inductors to filter the AC component in the low-voltage three-phase AC power.

[0088] In order to filter out the AC component in the high voltage DC, according to the above embodiment, as Figure 3 As shown, this embodiment provides a specific solution, the first output filter circuit 40 includes: a fourth inductor L4 and a fifth capacitor C5; the first end of the fourth inductor L4 is connected to the positive input end of the first output filter circuit 40, the second end of the fourth inductor L4 and the first end of the fifth capacitor C5 are connected to the positive electrode of the DC high-voltage power grid as the positive output end of the first output filter circuit 40, and are connected to the positive electrode of the DC high-voltage power grid; the second end of the fifth capacitor C5 is connected to the negative input end of the first output filter circuit 40 as the negative output end of the first output filter circuit 40, and is connected to the negative electrode of the DC high-voltage power grid.

[0089] In this embodiment, the fourth inductor L4 and the fifth capacitor C5 are used to form an LC filter circuit, based on the impedance characteristics of capacitors and inductors to signals of different frequencies. The impedance of capacitors to alternating current decreases with increasing frequency, while the impedance of inductors to alternating current increases with increasing frequency. By reasonably combining these two components, selective passing or blocking of specific frequency signals can be achieved.

[0090] As can be seen from the above, the first output filter circuit 40 composed of the fourth inductor L4 and the fifth capacitor C5 can use the LC filtering method to filter the AC component in the high voltage DC power.

[0091] In order to reduce the volume, weight and cost of the multi-pulse transformer 20, this embodiment provides a specific solution, in which the frequency of the low-voltage three-phase AC power is greater than or equal to the fundamental frequency of the AC power grid.

[0092] It is understandable that the AC power frequency output by the traditional grid-connected system is equal to the AC grid fundamental frequency, but the AC voltage frequency of the photovoltaic DC transmission system is not limited by the grid frequency, so the frequency of the low-voltage three-phase AC power can be selected as ,and / 10≥ ≥50Hz. is the switching frequency of the switch tube in the inverter topology circuit. When the intermediate frequency is used, for example, the frequency of the low-voltage three-phase AC power The frequency of the transformer is 200Hz, which can greatly reduce the volume and weight of the transformer core compared to the transformer with a grid voltage frequency of 50Hz.

[0093] As can be seen from the above, by increasing the frequency of the low-voltage three-phase AC power to be greater than the AC grid fundamental frequency, the volume and weight of the transformer core can be greatly reduced, thereby reducing the volume, weight and cost of the multi-pulse transformer 20 .

[0094] Finally, this embodiment also provides a photovoltaic power generation device, including the above-mentioned photovoltaic direct current transmission system.

[0095] The photovoltaic direct current transmission system and photovoltaic power generation equipment provided by the present application are introduced in detail above. The various embodiments in the specification are described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same and similar parts between the embodiments can refer to each other. For the device disclosed in the embodiment, since it corresponds to the method disclosed in the embodiment, the description is relatively simple, and the relevant parts can refer to the method part description. It should be pointed out that for ordinary technicians in this technical field, without departing from the principles of the present application, several improvements and modifications can be made to the present application, and these improvements and modifications also fall within the scope of protection of the claims of the present application.

[0096] It should also be noted that, in this specification, 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 "comprise", "include" 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, an element defined by the statement "comprises a ..." does not exclude the presence of other identical elements in the process, method, article or device including the element.

Claims

1. A photovoltaic direct current transmission system, characterized in that: include: Inverter circuit, multi-pulse transformer, rectifier circuit, first output filter circuit; The input end of the inverter circuit is connected to the photovoltaic direct current output end, and the output end of the inverter circuit is connected to the low-voltage three-phase input end of the multi-pulse transformer, for converting the photovoltaic direct current into low-voltage three-phase alternating current, and regulating the frequency and amplitude of the low-voltage three-phase alternating current; The multiple groups of high-voltage three-phase output terminals of the multi-pulse transformer are connected to the multiple groups of input terminals of the rectifier circuit, so as to boost the low-voltage three-phase alternating current and output multiple groups of high-voltage three-phase alternating currents with phase differences to the rectifier circuit; The output end of the rectifier circuit is connected to the input end of the first output filter circuit, and is used to rectify the multiple groups of high-voltage three-phase alternating current into high-voltage direct current; The output end of the first output filter circuit is connected to a DC high voltage power grid and is used to filter the high voltage DC power containing an AC component.

2. The photovoltaic DC transmission system according to claim 1, characterized in that: The inverter circuit includes: a control system, an inverter topology circuit, a second output filter circuit, and a three-phase switch; The control system is connected to the inverter topology circuit and is used to control the inverter topology circuit to adjust the frequency and amplitude of the low-voltage three-phase alternating current; The input end of the inverter topology circuit is connected to the photovoltaic direct current output end, and the output end of the inverter topology circuit is connected to the input end of the second output filter circuit; A first end of the three-phase switch is connected to an output end of the second output filter circuit, and a second end of the three-phase switch is connected to a low-voltage three-phase input end of the multi-pulse transformer.

3. The photovoltaic DC transmission system according to claim 1, characterized in that: The multi-pulse transformer is an m-pulse transformer, wherein m is 12, 18, 24 or 30; when m=12, a three-winding transformer is used; The primary side of the three-winding transformer is composed of a group of low-voltage three-phase windings as the low-voltage three-phase input terminal, and the secondary side of the three-winding transformer is composed of m / 6 groups of high-voltage three-phase windings as the high-voltage three-phase output terminal; The three-phase winding on the high-voltage side of the m / 6 group adopts a different wiring method.

4. The photovoltaic DC transmission system according to claim 3, characterized in that: The rectifier circuit is an uncontrolled rectifier circuit, which is m / 6 three-phase uncontrolled rectifier bridges connected in series; The three-phase uncontrolled rectifier bridge is composed of six diodes, which are divided into upper bridge arm diodes and lower bridge arm diodes, three in each group; The cathode of the upper bridge arm diode is connected to the positive electrode of the first output filter circuit, and the anode of the upper bridge arm diode is respectively connected to different phases of the output of the same three-phase winding on the high-voltage side; The anode of the lower bridge arm diode is connected to the cathode of the first output filter circuit, and the cathode of the lower bridge arm diode is respectively connected to different phases of the high-voltage side three-phase winding output that is the same as the upper bridge arm diode; The m / 6 groups of the high-voltage side three-phase windings correspond one to one with the m / 6 three-phase uncontrolled rectifier bridges.

5. The photovoltaic DC transmission system according to claim 2, characterized in that: The inverter circuit further includes: a first capacitor for filtering clutter in the photovoltaic direct current; The first end of the first capacitor is connected to the positive electrode of the photovoltaic direct current output terminal, and the second end of the first capacitor is connected to the negative electrode of the photovoltaic direct current output terminal.

6. The photovoltaic DC transmission system according to claim 2, characterized in that: The inverter topology circuit includes: a first switch tube, a second switch tube, a third switch tube, a fourth switch tube, a fifth switch tube, and a sixth switch tube; The second end of the first switch tube is connected in series with the first end of the second switch tube, the second end of the third switch tube is connected in series with the first end of the fourth switch tube, and the second end of the fifth switch tube is connected in series with the first end of the sixth switch tube; The first end of the first switch tube, the first end of the third switch tube, and the first end of the fifth switch tube are connected to the positive electrode of the photovoltaic direct current output end; The second end of the second switch tube, the second end of the fourth switch tube, and the second end of the sixth switch tube are connected to the negative electrode of the photovoltaic direct current output end.

7. The photovoltaic DC transmission system according to claim 6, characterized in that: The second output filter circuit includes: a second capacitor, a third capacitor, a fourth capacitor, a first inductor, a second inductor, and a third inductor; The first end of the first inductor is connected to the common end of the fifth switch tube and the sixth switch tube, and the second end of the first inductor is connected to the second end of the fourth capacitor; The first end of the second inductor is connected to the common end of the third switch tube and the fourth switch tube, and the second end of the second inductor is connected to the second end of the third capacitor; A first end of the third inductor is connected to a common end of the first switch tube and the second switch tube, and a second end of the third inductor is connected to a second end of the second capacitor; The first end of the second capacitor, the first end of the third capacitor and the first end of the fourth capacitor are connected to each other.

8. The photovoltaic DC transmission system according to claim 1, characterized in that: The first output filter circuit includes: a fourth inductor and a fifth capacitor; The first end of the fourth inductor is connected to the positive input end of the first output filter circuit, and the second end of the fourth inductor and the first end of the fifth capacitor are connected to the positive electrode of the DC high-voltage power grid as the positive output end of the first output filter circuit, and are connected to the positive electrode of the DC high-voltage power grid; The second end of the fifth capacitor is connected to the negative input end of the first output filter circuit as the negative output end of the first output filter circuit, and is connected to the negative electrode of the DC high voltage grid.

9. The photovoltaic DC transmission system according to claim 2, characterized in that: The frequency of the low-voltage three-phase alternating current is greater than or equal to the fundamental frequency of the alternating current grid.

10. A photovoltaic power generation device, characterized in that: A photovoltaic direct current transmission system comprising any one of claims 1 to 9.

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