Conversion system based on series connection of small household fans

Abstract

The invention provides a conversion system based on series connection of small household fans, and relates to the technical field of wind power generation. The system comprises n small fan generators, n AC / DC rectifiers and a DC / AC inverter, according to the small fan series conversion system, each small fan generator is connected with an AC / DC rectifier, the outputs of the n AC / DC rectifiers are connected in series and then connected to an inverter, and the output of the inverter is connected to a power grid. Compared with a fan grid connection scheme of a traditional scheme, the small fan series conversion system has the advantage that the wind energy utilization rate of each small fan is remarkably improved. A small fan often cannot output direct-current voltage meeting the grid-connected amplitude requirement under the working condition of low wind speed, and the conversion benefit of wind energy captured by the fan is greatly reduced, direct-current voltage output ends of a plurality of fans are connected in series and then electric energy is utilized, so that the conversion benefit of the wind energy captured by the fan is greatly reduced under the working condition of small direct-current voltage obtained after rectification of a single small fan. And the part of low direct current voltage can still be effectively utilized to participate in large system grid connection, and the electric energy conversion efficiency of the fan is utilized to the maximum extent.

Description

Technical Field

[0001] The invention relates to the technical field of wind power generation, and in particular to a conversion system based on series connection of small household wind turbines. Background Art

[0002] As a new energy technology, wind power generation has an increasingly wide range of applications. Grid-connected wind farms can be compensated and supported by large power grids, and more fully developing available wind resources is the main development direction of wind power generation at home and abroad. Common wind turbine grid-connected solutions generally use permanent magnet synchronous generators and doubly fed generators for power generation. After the AC-DC-AC conversion method, the AC power that changes with wind speed is converted into AC power that meets the grid connection requirements, and the wind turbine is connected to the grid using a quasi-synchronous grid-connected method.

[0003] like Figure 1 As shown in the figure, taking the permanent magnet synchronous generator as an example, the AC-DC-AC conversion circuit is currently the most widely used solution for wind power grid connection. In this system, the wind wheel is directly connected to the permanent magnet synchronous generator without the need for a speed-up gearbox. First, the wind energy is converted into AC power with varying frequency and amplitude, which is then converted into DC power after rectification, and then converted into three-phase AC power with constant frequency through a three-phase inverter and connected to the grid. This system adopts an AC-DC-AC structure and is connected to the grid through full-power conversion; in household small wind turbine systems, an AC-DC-load (battery, etc.) structure is also often used.

[0004] However, in a working environment with fluctuating wind speeds, the speed of household small fans is greatly limited by wind speed, and the fan voltage may be insufficient. In the case of a single fan connected to the grid, when the voltage of the small fan decreases, the above scheme cannot provide stable boosting measures to achieve grid connection. Therefore, the working conditions of small fans are relatively weak, and they can often only be used to drive small-power household loads, which greatly reduces the utilization rate of power generation and transmission equipment, while generating a large amount of additional losses, and has poor adaptability to complex working conditions. That is, in the case of a single fan connected to the grid, the existing scheme will have the technical problem of being unable to provide stable boosting measures to achieve grid connection when the voltage decreases. Summary of the invention

[0005] 1. Technical issues to be resolved

[0006] In view of the deficiencies in the prior art, the present invention provides a conversion system based on the series connection of small household fans, which solves the technical problem that the existing small household fan system will have insufficient fan voltage in a working environment with fluctuating wind speed, and cannot provide stable voltage boosting measures to achieve grid connection when a single fan is connected to the grid.

[0007] (II) Technical solution

[0008] To achieve the above objectives, the present invention is implemented through the following technical solutions:

[0009] The present invention provides a conversion system based on the series connection of household small wind turbines, comprising n small wind turbine generators, n AC / DC rectifiers and DC / AC inverters, where n is a positive integer and n≥2;

[0010] Each small wind turbine generator is respectively connected to an AC / DC rectifier, and the outputs of n AC / DC rectifiers are connected in series to a DC / AC inverter, and the output of the DC / AC inverter is connected to the power grid.

[0011] Preferably, the n AC / DC rectifiers adopt a single rectification method or a mixed rectification method;

[0012] The single rectification method is that the n AC / DC rectifiers all use the same type of rectifier;

[0013] The hybrid rectification method is that the n AC / DC rectifiers include at least two different types of rectifiers.

[0014] Preferably, the n AC / DC rectifiers all adopt full-bridge rectification, and each AC / DC rectifier includes 6 switching tubes and a voltage stabilizing capacitor;

[0015] The first ends of the first, second and third switch tubes are connected as the first output end of the AC / DC rectifier, the second ends of the first, second and third switch tubes are connected to the first ends of the fourth, fifth and sixth switch tubes respectively, and the second ends of the fourth, fifth and sixth switch tubes are connected as the second output end of the AC / DC rectifier; a voltage stabilizing capacitor is connected in parallel to the two output ends;

[0016] The output ends of the n AC / DC rectifiers are sequentially connected in series in a manner that the second output end of the previous AC / DC rectifier is connected to the first output end of the next AC / DC rectifier, wherein the first output end of the first AC / DC rectifier is connected to the first input end of the DC / AC inverter; and the second output end of the last AC / DC rectifier is connected to the second input end of the DC / AC inverter;

[0017] The three-phase output of the small fan is respectively connected to the common end of the first and fourth switch tubes, the common end of the second and fifth switch tubes, and the common end of the third and sixth switch tubes.

[0018] Preferably, the n AC / DC rectifiers all adopt uncontrolled diode rectification, and each AC / DC rectifier includes 7 diodes, a capacitor, an inductor, a switch tube and a voltage stabilizing capacitor;

[0019] The cathodes of the first, second and third diodes are connected, and their common ends are connected to the anode of the seventh diode via the inductor, and the cathode of the seventh diode serves as the first output end of the AC / DC rectifier; the anodes of the first, second and third diodes are connected to the cathodes of the fourth, fifth and sixth diodes respectively, and the anodes of the fourth, fifth and sixth diodes are connected together, and their common ends are connected to the first end of the capacitor and the second end of the switch tube, serving as the second output end of the AC / DC rectifier; a voltage stabilizing capacitor is connected in parallel to the two output ends;

[0020] The first end of the capacitor is connected to the connection end of the inductor and the cathodes of the first, second and third diodes, and the first end of the switch tube is connected to the common end of the inductor and the seventh diode;

[0021] The output ends of the n AC / DC rectifiers are sequentially connected in series in a manner that the second output end of the previous AC / DC rectifier is connected to the first output end of the next AC / DC rectifier, wherein the first output end of the first AC / DC rectifier is connected to the first input end of the DC / AC inverter; and the second output end of the last AC / DC rectifier is connected to the second input end of the DC / AC inverter;

[0022] The three-phase output of the small fan is respectively connected to the common end of the first and fourth diodes, the common end of the second and fifth diodes, and the common end of the third and sixth diodes.

[0023] Preferably, some of the n AC / DC rectifiers adopt full-bridge rectification, and some adopt uncontrolled diode rectification;

[0024] Among them, in the full-bridge rectification, each AC / DC rectifier includes 6 switch tubes and a first voltage-stabilizing capacitor; the first ends of the first, second and third switch tubes are connected as the first output end of the AC / DC rectifier, the second ends of the first, second and third switch tubes are respectively connected to the first ends of the fourth, fifth and sixth switch tubes, and the second ends of the fourth, fifth and sixth switch tubes are connected as the second output end of the AC / DC rectifier; a voltage-stabilizing capacitor is connected in parallel to the two output ends; the three-phase output of each small fan is respectively connected to the common end of the first and fourth switch tubes, the common end of the second and fifth switch tubes, and the common end of the third and sixth switch tubes; the three-phase output of the small fan is respectively connected to the common end of the first and fourth switch tubes, the common end of the second and fifth switch tubes, and the common end of the third and sixth switch tubes;

[0025] In uncontrolled diode rectification, each AC / DC rectifier includes 7 diodes, capacitors, inductors, a switch tube and a second voltage stabilizing capacitor;

[0026] Among them, the cathodes of the first, second and third diodes are connected, and their common ends are connected to the anode of the seventh diode via the inductor, and the cathode of the seventh diode serves as the first output end of the AC / DC rectifier; the anodes of the first, second and third diodes are respectively connected to the cathodes of the fourth, fifth and sixth diodes, and the anodes of the fourth, fifth and sixth diodes are connected together, and their common ends are connected to the first end of the capacitor and the second end of the switch tube as the second output end of the AC / DC rectifier; the second voltage-stabilizing capacitor is connected in parallel to the two output ends; the first end of the capacitor is connected to the connecting end of the inductor and the cathodes of the first, second and third diodes, and the first end of the switch tube is connected to the common end of the inductor and the seventh diode; the three-phase output of the small fan is respectively connected to the common end of the first and fourth diodes, the common end of the second and fifth diodes, and the common end of the third and sixth diodes;

[0027] The output ends of n AC / DC rectifiers are connected in series in sequence in a manner that the second output end of the previous AC / DC rectifier is connected to the first output end of the next AC / DC rectifier, wherein the first output end of the first AC / DC rectifier is connected to the first input end of the DC / AC inverter; and the second output end of the last AC / DC rectifier is connected to the second input end of the DC / AC inverter.

[0028] Preferably, the DC / AC inverter is selected according to voltage conditions, and the selection range includes: T-type three-level inverter, I-type three-level inverter and two-level inverter.

[0029] Preferably, the two-level inverter includes 6 switch tubes, the first ends of the seventh, eighth and ninth switch tubes are connected as the first input end of the two-level inverter, the second ends of the seventh, eighth and ninth switch tubes are connected to the first ends of the tenth, eleventh and twelfth switch tubes, and the second ends of the tenth, eleventh and twelfth switch tubes are connected as the second input end of the two-level inverter;

[0030] The connection ends of the seventh and tenth switch tubes, the connection ends of the eighth and eleventh switch tubes, and the connection ends of the ninth and twelfth switch tubes are respectively used as the three-phase output ends of the U phase, V phase, and W phase of the two-level inverter, and the three-phase output ends are respectively connected to the three-phase power grid via an LC filter.

[0031] Preferably, the DC / AC inverter is controlled by SVPWM modulation wave.

[0032] Preferably, the output ends of the n AC / DC rectifiers are connected in series and then connected to a DC load.

[0033] Preferably, the control strategy of the conversion system based on the series connection of household small fans includes:

[0034] With the goal of obtaining a sufficiently stable high total DC voltage, the optimal speed of the fan, and the optimal power output of the system, tasks are assigned to the rectifier side of different household small fans, the maximum torque current ratio control method is used to optimize control, and the power distribution of each fan in the conversion system is coordinated.

[0035] (III) Beneficial effects

[0036] The present invention provides a conversion system based on the series connection of household small fans. Compared with the prior art, it has the following beneficial effects:

[0037] The small wind turbine series conversion system proposed by the present invention significantly improves the wind energy utilization rate of each wind turbine, especially the small wind turbine, compared with the traditional wind turbine grid-connected scheme. When a small wind turbine faces a low wind speed working condition, it is often unable to output a DC voltage that meets the grid-connected amplitude requirements, which greatly reduces the conversion efficiency of the wind energy captured by the wind turbine. The present invention connects the DC voltage output ends of multiple wind turbines in series for electrical energy utilization. Therefore, when faced with a working condition where the DC voltage obtained after rectification of a single small wind turbine is small, this part of the low DC voltage can still be effectively utilized to participate in the large system grid connection, maximizing the efficiency of wind turbine electrical energy conversion. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] 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 some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0039] Figure 1 The present invention is an architecture diagram of an existing direct-drive permanent magnet synchronous wind turbine generator system;

[0040] Figure 2 The circuit diagram of the existing direct-drive permanent magnet synchronous wind turbine generator system using uncontrolled rectification + Boost on the machine side;

[0041] Figure 3 A circuit diagram of a direct-drive permanent magnet synchronous wind turbine generator system with an existing machine-side rectifier;

[0042] Figure 4 This is an architecture diagram of the conversion system based on the series connection of household small wind turbines in Example 1;

[0043] Figure 5 The circuit diagram of the conversion system based on the series connection of household small fans in Example 2;

[0044] Figure 6 The circuit diagram of the conversion system based on the series connection of household small fans in Example 3;

[0045] Figure 7 The circuit diagram of the conversion system based on the series connection of household small fans in Example 4;

[0046] Figure 8 for Figure 7 The circuit diagram of the conversion system based on the series connection of household small wind turbines when the DC / AC inverter is a three-phase two-level inverter;

[0047] Fig. 9 This is a control flow chart of the conversion system based on the series connection of household small fans in Example 5. DETAILED DESCRIPTION

[0048] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention are clearly and completely described. Obviously, the described embodiments are 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.

[0049] It should be noted that, for the convenience of description, the switch IGBT is used to represent the controllable (on and off) switch tube in the embodiment of the present invention, but the switch tube in the present invention is not limited to IGBT. Take IGBT as an example for explanation. The first end of the IGBT refers to the collector, the second end refers to the emitter, and the control end refers to the gate. A driving control signal is applied to the control end of each switch tube in the embodiment of the present invention. For the sake of brevity, it will not be repeated later. The power switch tube in the embodiment of the present invention can also be implemented by other controllable switch tube devices other than IGBT, such as MOSFET. At the same time, in order to ensure the normal operation of each switch device in the embodiment of the present invention, a freewheeling diode needs to be connected in parallel to each switch device. The parallel direction of the freewheeling diode is related to the type of the switch device. The technician can set it according to the type of the switch device, which is not limited here. If not specified, the switch device defaults to including a freewheeling diode, which will be pointed out in this embodiment in special cases.

[0050] The embodiment of the present application provides a conversion system based on the series connection of small household fans, which solves the problem that the fan voltage of the existing small household fan system is insufficient in a working environment with fluctuating wind speed, and when a single fan is connected to the grid, it is impossible to provide stable boosting measures to achieve grid connection. The small fan can still participate in the grid connection link when the voltage is low. After the small fan is rectified in series, it supplies power to the DC side load or adopts a two-level inverter or a three-level grid connection, thereby solving the problem of insufficient boosting of the small fan, improving the power utilization rate of the small fan, and reducing the power loss.

[0051] The technical solution in the embodiment of the present application is to solve the above technical problems, and the overall idea is as follows:

[0052] As 2, Figure 3 There are two common AC / DC+DC / AC architectures. Figure 2 The DC voltage obtained by the full-bridge diode rectification is then Boost-converted to achieve DC side voltage stability, and then connected to the grid through a PWM inverter composed of fully controlled power switching devices. Figure 3 The system uses a dual PWM back-to-back conversion circuit. The system uses two PWM converters. The rectifier part uses PWM three-phase rectification to achieve MPPT (maximum power point tracking) of the generator power. The inverter part uses PWM three-phase inverter to stabilize the DC bus voltage and control the grid-side power factor.

[0053] However, for the above scheme, in a working environment with fluctuating wind speed, the speed of household small fans is greatly limited by the wind speed, and the fan voltage may be insufficient. In the case of a single fan connected to the grid, when the voltage of the small fan decreases, the above scheme cannot provide a stable boosting measure to achieve grid connection. Therefore, the working condition of the small fan is relatively weak, and it can often only be used to drive small-power household loads, which greatly reduces the utilization rate of power generation and transmission equipment, and generates a large amount of additional losses, and has poor adaptability to complex working conditions.

[0054] The voltage level of a single wind turbine after rectification is limited by wind speed, wind turbine body and rectifier, with low control flexibility and low voltage level. Therefore, a transformer is often required to step up the voltage when connected to the grid, and the cost of the entire solution is relatively high.

[0055] When a single wind turbine is facing insufficient wind speed, the front-stage rectification link cannot take into account both MPPT and voltage stabilization at the same time. For DC loads that require stable voltage, the utilization of wind energy cannot be maximized at this time; or it cannot provide a higher DC voltage and cannot meet the DC voltage requirements of the subsequent inverter.

[0056] To solve the above problems, the embodiment of the present invention provides a conversion system based on the series connection of household small wind turbines, which mobilizes multiple wind turbines to generate electricity in series. The front stage adopts the traditional AC / DC link, and after rectification, the small wind turbine DC side is connected in series to obtain a higher DC voltage, so that the small wind turbine can still participate in the grid connection link when the voltage is low. After the small wind turbine is rectified in series, it supplies power to the DC side load or adopts a two-level inverter or a three-level grid connection, thereby solving the problem of insufficient boosting of the small wind turbine, improving the power utilization rate of the small wind turbine, and reducing the loss of electric energy. For the conversion system of the household small wind turbine in series, the task allocation of the front-stage rectification link can be realized for different wind turbines in the system, such as fast MPPT with variable step tracking for some wind turbines, and dynamic adjustment of the DC side voltage for some wind turbines, so as to simultaneously achieve the stability of the total voltage on the DC side of the system and maximize the utilization of the wind energy of the system. When the total voltage level on the DC side is increased, the AC output voltage of the rear-stage inverter can also be increased, and the transformer in some grid-connected conditions can be omitted, reducing the grid-connected cost of the system.

[0057] In order to better understand the above technical solution, the above technical solution will be described in detail below in conjunction with the accompanying drawings and specific implementation methods.

[0058] Embodiment 1:

[0059] like Figure 4 As shown, the conversion system based on the series connection of household small wind turbines in this embodiment includes n small wind turbine generators (in the embodiment of the present invention, the small wind turbine generator is an AC generator, the embodiment of the present invention takes a permanent magnet synchronous generator PMSG as an example, and the number of small wind turbine generators in the accompanying drawings is 3 as an example. In the specific implementation process, more small wind turbine generators can be connected in series), n AC / DC rectifiers, and DC / AC inverters;

[0060] Each small wind turbine generator is connected to an AC / DC rectifier, and the outputs of n AC / DC rectifiers are connected in series to a DC / AC inverter, and the output of the DC / AC inverter is connected to the power grid. n is a positive integer, and n≥2.

[0061] In the embodiment of the present invention, the DC / AC inverter outputs three-phase electricity, and the three-phase lines of the DC / AC inverter are respectively connected to the three-phase power grid via an inductor.

[0062] The output ends of n AC / DC rectifiers are connected in series and directly connected to the DC load.

[0063] The n AC / DC rectifiers are connected to a number of control terminals, and are suitable for allocating tasks to the rectifier sides of different fans and outputting different DC voltages under the control of the control signals connected to the control terminals; the DC / AC inverters are connected to a number of control terminals, and are suitable for outputting different voltage levels under the control of the control signals connected to the control terminals.

[0064] It should be noted that, in the specific implementation process, the AC / DC rectifier can adopt common schemes such as full-bridge rectification, uncontrolled diode rectification, three-phase bridge full-controlled rectification, three-phase bridge uncontrolled rectification + buck / boost, etc. The following embodiments are described in detail with the schemes of full-bridge rectification and uncontrolled diode rectification.

[0065] The DC / AC inverters applicable to the system include three-phase two-level, three-level, three-phase four-leg inverters and the like.

[0066] Embodiment 2:

[0067] like Figure 5 As shown, the AC / DC rectifier in the conversion system based on the series connection of household small wind turbines adopts full-bridge rectification, and each AC / DC rectifier includes 6 switching tubes, wherein the first ends (collectors) of the first, second and third switching tubes are connected as the first output end of the AC / DC rectifier, the second ends (emitters) of the first, second and third switching tubes are respectively connected to the first ends of the fourth, fifth and sixth switching tubes, and the second ends of the fourth, fifth and sixth switching tubes are connected as the second output end of the AC / DC rectifier.

[0068] The three-phase output of each PMSG is respectively connected to the common end of the first and fourth switch tubes, the common end of the second and fifth switch tubes, and the common end of the third and sixth switch tubes.

[0069] The first output end of the first AC / DC rectifier is connected to the first input end of the DC / AC inverter; the second output end of the first AC / DC rectifier is connected to the first output end of the second AC / DC rectifier, the second output end of the second AC / DC rectifier is connected to the first output end of the third AC / DC rectifier, and so on. The outputs of multiple AC / DC rectifiers are connected in series, and the second output end of the last AC / DC rectifier is connected to the second input end of the DC / AC inverter.

[0070] In a specific implementation process, a voltage stabilizing capacitor is connected in parallel to the two output ends of each AC / DC rectifier to stabilize each DC side output voltage.

[0071] Embodiment 3:

[0072] like Figure 6 As shown, the AC / DC rectifier in the conversion system based on the series connection of household small wind turbines adopts uncontrolled diode rectification, and each AC / DC rectifier includes 7 diodes, capacitors, inductors and a switch tube.

[0073] The cathodes of the first, second and third diodes are connected, and their common end is connected to the anode of the seventh diode via an inductor, and the cathode of the seventh diode serves as the first output end of the AC / DC rectifier.

[0074] The anodes of the first, second and third diodes are connected to the cathodes of the fourth, fifth and sixth diodes respectively. The anodes of the fourth, fifth and sixth diodes are connected together, and their common end is connected to the first end of the capacitor and the second end of the switch tube, serving as the second output end of the AC / DC rectifier. The first end of the capacitor is connected to the connection end of the inductor and the cathodes of the first, second and third diodes, and the first end of the switch tube is connected to the common end of the inductor and the seventh diode.

[0075] The first output end of the first AC / DC rectifier is connected to the first input end of the DC / AC inverter; the second output end of the first AC / DC rectifier is connected to the first output end of the second AC / DC rectifier, the second output end of the second AC / DC rectifier is connected to the first output end of the third AC / DC rectifier, and so on. The outputs of multiple AC / DC rectifiers are connected in series, and the second output end of the last AC / DC rectifier is connected to the second input end of the DC / AC inverter.

[0076] In a specific implementation process, a voltage stabilizing capacitor is connected in parallel to the two output ends of each AC / DC rectifier to stabilize each DC side output voltage.

[0077] The DC / AC inverter and the connection method between the DC / AC inverter and the power grid are the same as those in Example 2 and will not be described in detail here.

[0078] Embodiment 4:

[0079] like Figure 7 As shown, a hybrid rectification method is adopted in the conversion system based on the series connection of household small fans, that is, multiple AC / DC rectifiers include both uncontrolled diode rectifiers and full-bridge rectifiers, and their connection methods are similar to those in Example 2 and Example 4, which will not be repeated here. In the specific implementation process, the hybrid rectification method can also include more than two different forms of rectifiers. Figure 8 for Figure 7 Circuit diagram when the DC / AC inverter in is a three-phase two-level inverter.

[0080] The DC / AC inverter can be selected according to the voltage operating conditions. This embodiment takes a two-level inverter as an example, which includes 6 switching tubes. The first ends (collectors) of the seventh, eighth, and ninth switching tubes are connected as the first input end of the DC / AC inverter, the second ends (emitters) of the seventh, eighth, and ninth switching tubes are connected to the first ends of the tenth, eleventh, and twelfth switching tubes, and the second ends of the tenth, eleventh, and twelfth switching tubes are connected as the second input end of the DC / AC inverter.

[0081] The connection ends of the seventh and tenth switch tubes, the connection ends of the eighth and eleventh switch tubes, and the connection ends of the ninth and twelfth switch tubes are respectively used as the three-phase output ends of the U phase, V phase, and W phase of the DC / AC inverter, and the three-phase output ends are respectively connected to the three-phase power grid via an LC filter. The LC filter is composed of a filter inductor and a filter capacitor.

[0082] In this embodiment, by adjusting the SVPWM modulation wave of the DC / AC inverter, a high-efficiency grid-connected controller with adjustable power factor and a power system adaptable to different voltage levels can be obtained.

[0083] It should be noted that in the small wind turbine series conversion system, the DC side output voltage of each wind turbine is not fixed, and the optimal DC voltage solution of the system wind turbine can be allocated according to factors such as the change in the wind speed of the working environment, so as to mobilize wind turbines of various models with the maximum benefit and make the best use of wind energy. In the face of different levels of voltage conditions, the grid-side inverter can also select T-type three-level, I-type three-level, two-level and other inverters with mature topologies to achieve industrial requirements such as improving the utilization ratio of the series DC voltage, reducing voltage and current harmonics, and improving the power factor. Examples 2 to 4 take the two-level inverter as an example.

[0084] Embodiment 5:

[0085] From the perspective of system control, a control scheme is proposed with the primary goal of obtaining a sufficiently stable high total DC voltage, optimal fan speed, and optimal system power output. Tasks are assigned to the rectifier side of different fans, and the maximum torque current ratio control method is used to optimize control to coordinate the power distribution of each fan in the system. The specific process is as follows Fig. 9 shown.

[0086] For charging or grid connection, the DC side voltage has minimum value and stability requirements, but the stability of output voltage and wind speed utilization of different wind turbines are different.

[0087] When the wind speed is low, for small wind turbines with high wind speed utilization, the control target can be set to maximum power (i.e., optimal utilization of wind speed), giving up the stability of its voltage; for the remaining small wind turbines in the system, the control target is set to the stable output of the total DC voltage after series connection, and the DC voltage of these small wind turbines is dynamically adjusted to meet the minimum DC voltage output requirement.

[0088] When the wind speed is too high, the system stability can be set as the primary goal, that is, the DC voltage is stable, the wind turbine blade speed is not too high, etc. In this way, the wind speed utilization range can be improved for the system. For example, the original wind speed utilization range is 6m / s-15m / s, and after the system is upgraded, it can be expanded to 3m / s-20m / s.

[0089] In summary, compared with the prior art, the present invention has the following beneficial effects:

[0090] 1. The small wind turbine series conversion system proposed in the embodiment of the present invention significantly improves the wind energy utilization rate of each wind turbine, especially the small wind turbine, compared with the traditional wind turbine grid-connected scheme. When a small wind turbine faces a low wind speed working condition, it is often unable to output a DC voltage that meets the grid-connected amplitude requirements, which greatly reduces the conversion efficiency of the wind energy captured by the wind turbine. The embodiment of the present invention connects the DC voltage output terminals of multiple wind turbines in series for electric energy utilization. Therefore, when faced with a small DC voltage obtained after rectification of a single small wind turbine, this part of the low DC voltage can still be effectively utilized to participate in the large system grid connection, maximizing the efficiency of wind turbine power conversion. When the wind energy is sufficient, the system is tracked for the maximum power point with variable steps, which can achieve the optimal conversion of the system to capture wind energy. Ideally, when the DC voltage level after series connection is sufficient, the transformer on the subsequent grid-connected side can be omitted, and the grid-side voltage level, active power, reactive power and other requirements can be met through the control of the grid-connected inverter.

[0091] 2. The small wind turbine series conversion system can not only utilize the higher stable DC voltage generated by the small wind turbine combination when wind resources are scarce, but also adjust the power and voltage distribution of each wind turbine in real time according to different working conditions when facing changes in wind energy, thereby enhancing the system's adaptability to complex working conditions, improving the utilization rate of wind energy, and being more conducive to achieving high-quality conversion of wind energy into electrical energy, thereby achieving the industrial goal of reducing costs and increasing efficiency.

[0092] 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 presence of other identical elements in the process, method, article or device including the elements.

[0093] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that the technical solutions described in the aforementioned embodiments may still be modified, or some of the technical features may be replaced by equivalents. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A conversion system based on series connection of household small fans, characterized in that: It includes n small wind turbine generators, n AC / DC rectifiers and DC / AC inverters, where n is a positive integer and n≥2; Each small wind turbine generator is respectively connected to an AC / DC rectifier, and the outputs of n AC / DC rectifiers are connected in series to a DC / AC inverter, and the output of the DC / AC inverter is connected to the power grid.

2. The conversion system based on series connection of household small fans as claimed in claim 1 is characterized in that: The n AC / DC rectifiers adopt a single rectification method or a mixed rectification method; The single rectification method is that the n AC / DC rectifiers all use the same type of rectifier; The hybrid rectification method is that the n AC / DC rectifiers include at least two different types of rectifiers.

3. The conversion system based on series connection of household small wind turbines as claimed in claim 2 is characterized in that: The n AC / DC rectifiers all adopt full-bridge rectification, and each AC / DC rectifier includes 6 switching tubes and a voltage stabilizing capacitor; The first ends of the first, second and third switch tubes are connected as the first output end of the AC / DC rectifier, the second ends of the first, second and third switch tubes are connected to the first ends of the fourth, fifth and sixth switch tubes respectively, and the second ends of the fourth, fifth and sixth switch tubes are connected as the second output end of the AC / DC rectifier; a voltage stabilizing capacitor is connected in parallel to the two output ends; The output ends of the n AC / DC rectifiers are sequentially connected in series in a manner that the second output end of the previous AC / DC rectifier is connected to the first output end of the next AC / DC rectifier, wherein the first output end of the first AC / DC rectifier is connected to the first input end of the DC / AC inverter; and the second output end of the last AC / DC rectifier is connected to the second input end of the DC / AC inverter; The three-phase output of the small fan is respectively connected to the common end of the first and fourth switch tubes, the common end of the second and fifth switch tubes, and the common end of the third and sixth switch tubes.

4. The conversion system based on the series connection of household small wind turbines as claimed in claim 2 is characterized in that: The n AC / DC rectifiers all adopt uncontrolled diode rectification, and each AC / DC rectifier includes 7 diodes, a capacitor, an inductor, a switch tube and a voltage stabilizing capacitor; The cathodes of the first, second and third diodes are connected, and their common ends are connected to the anode of the seventh diode via the inductor, and the cathode of the seventh diode serves as the first output end of the AC / DC rectifier; the anodes of the first, second and third diodes are connected to the cathodes of the fourth, fifth and sixth diodes respectively, and the anodes of the fourth, fifth and sixth diodes are connected together, and their common ends are connected to the first end of the capacitor and the second end of the switch tube, serving as the second output end of the AC / DC rectifier; a voltage stabilizing capacitor is connected in parallel to the two output ends; The first end of the capacitor is connected to the connection end of the inductor and the cathodes of the first, second and third diodes, and the first end of the switch tube is connected to the common end of the inductor and the seventh diode; The output ends of the n AC / DC rectifiers are sequentially connected in series in a manner that the second output end of the previous AC / DC rectifier is connected to the first output end of the next AC / DC rectifier, wherein the first output end of the first AC / DC rectifier is connected to the first input end of the DC / AC inverter; and the second output end of the last AC / DC rectifier is connected to the second input end of the DC / AC inverter; The three-phase output of the small fan is respectively connected to the common end of the first and fourth diodes, the common end of the second and fifth diodes, and the common end of the third and sixth diodes.

5. The conversion system based on series connection of household small wind turbines as claimed in claim 2 is characterized in that: The n AC / DC rectifiers partially adopt full-bridge rectification and partially adopt uncontrolled diode rectification; Among them, in the full-bridge rectification, each AC / DC rectifier includes 6 switch tubes and a first voltage-stabilizing capacitor; the first ends of the first, second and third switch tubes are connected as the first output end of the AC / DC rectifier, the second ends of the first, second and third switch tubes are respectively connected to the first ends of the fourth, fifth and sixth switch tubes, and the second ends of the fourth, fifth and sixth switch tubes are connected as the second output end of the AC / DC rectifier; a voltage-stabilizing capacitor is connected in parallel to the two output ends; the three-phase output of each small fan is respectively connected to the common end of the first and fourth switch tubes, the common end of the second and fifth switch tubes, and the common end of the third and sixth switch tubes; the three-phase output of the small fan is respectively connected to the common end of the first and fourth switch tubes, the common end of the second and fifth switch tubes, and the common end of the third and sixth switch tubes; In uncontrolled diode rectification, each AC / DC rectifier includes 7 diodes, capacitors, inductors, a switch tube and a second voltage stabilizing capacitor; Among them, the cathodes of the first, second and third diodes are connected, and their common ends are connected to the anode of the seventh diode via the inductor, and the cathode of the seventh diode serves as the first output end of the AC / DC rectifier; the anodes of the first, second and third diodes are respectively connected to the cathodes of the fourth, fifth and sixth diodes, and the anodes of the fourth, fifth and sixth diodes are connected together, and their common ends are connected to the first end of the capacitor and the second end of the switch tube as the second output end of the AC / DC rectifier; the second voltage-stabilizing capacitor is connected in parallel to the two output ends; the first end of the capacitor is connected to the connecting end of the inductor and the cathodes of the first, second and third diodes, and the first end of the switch tube is connected to the common end of the inductor and the seventh diode; the three-phase output of the small fan is respectively connected to the common end of the first and fourth diodes, the common end of the second and fifth diodes, and the common end of the third and sixth diodes; The output ends of n AC / DC rectifiers are connected in series in sequence in a manner that the second output end of the previous AC / DC rectifier is connected to the first output end of the next AC / DC rectifier, wherein the first output end of the first AC / DC rectifier is connected to the first input end of the DC / AC inverter; and the second output end of the last AC / DC rectifier is connected to the second input end of the DC / AC inverter.

6. The conversion system based on series connection of household small wind turbines according to any one of claims 1 to 5, characterized in that: The DC / AC inverter is selected according to the voltage working condition, and the selection range includes: T-type three-level inverter, I-type three-level inverter and two-level inverter.

7. The conversion system based on series connection of household small wind turbines as claimed in claim 6 is characterized in that: The two-level inverter includes 6 switch tubes, the first ends of the seventh, eighth and ninth switch tubes are connected as the first input end of the two-level inverter, the second ends of the seventh, eighth and ninth switch tubes are connected to the first ends of the tenth, eleventh and twelfth switch tubes, and the second ends of the tenth, eleventh and twelfth switch tubes are connected as the second input end of the two-level inverter; The connection ends of the seventh and tenth switch tubes, the connection ends of the eighth and eleventh switch tubes, and the connection ends of the ninth and twelfth switch tubes are respectively used as the three-phase output ends of the U phase, V phase, and W phase of the two-level inverter, and the three-phase output ends are respectively connected to the three-phase power grid via an LC filter.

8. The conversion system based on series connection of household small wind turbines as claimed in claim 6 is characterized in that: The DC / AC inverter is controlled by SVPWM modulation wave.

9. The conversion system based on series connection of household small wind turbines according to any one of claims 1 to 5, characterized in that: The output ends of the n AC / DC rectifiers are connected in series and then connected to a DC load.

10. The conversion system based on series connection of household small wind turbines according to any one of claims 1 to 5, characterized in that: The control strategy of the conversion system based on the series connection of household small fans includes: With the goal of obtaining a sufficiently stable high total DC voltage, the optimal speed of the fan, and the optimal power output of the system, tasks are assigned to the rectifier side of different household small fans, the maximum torque current ratio control method is used to optimize control, and the power distribution of each fan in the conversion system is coordinated.

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