Power semiconductor based grid-connected five-level inverter, modulation method and system

Abstract

The application discloses a grid-connected five-level inverter based on a power semiconductor, a modulation method and a system, and the inverter comprises first to third circuits; the first circuit comprises a first inductor, a first switch, a second inductor and a second switch which are connected in series, one end of the first circuit close to the first inductor is a first output end, and the other end of the first circuit is a first input end; the second circuit comprises a third inductor and a first diode which are connected in series, one end of the third inductor is a first end of the second circuit, and the negative electrode of the first diode is electrically connected with the connection end of the first switch and the third inductor; the third circuit comprises a first capacitor and a second diode which are connected in series, one end of the third circuit is electrically connected with the negative electrode of the first diode, and the other end of the third circuit is a second input end; the first and second input ends are electrically connected with two ends of a photovoltaic output side, and the first output end and the second input end are electrically connected with two ends of a load. The application can reduce the number of switches, reduce the product cost and improve the output current quality.

Description

Technical Field

[0001] This invention relates to the fields of photovoltaic systems and inverter technology, and in particular to a grid-connected five-level inverter based on power semiconductors, a modulation method and system. Background Technology

[0002] Multilevel inverters have become key power conversion devices in photovoltaic grid-connected power generation systems due to their significant advantages in improving output voltage quality and reducing filtering requirements. However, traditional multilevel inverters typically use power frequency transformers for electrical isolation, resulting in problems such as large system size, increased weight, and high cost.

[0003] To overcome the aforementioned drawbacks, transformerless photovoltaic inverter topologies have been widely researched and applied. However, after eliminating the transformer, these topologies generally suffer from leakage current problems caused by high-frequency common-mode voltage due to the lack of reliable electrical isolation. This not only affects the system's conversion efficiency but may also pose a threat to personal safety and lead to excessive electromagnetic interference. In recent years, to address the leakage current problem, some common-ground inverter topologies based on a three-level architecture have been proposed. These topologies clamp the common-mode voltage through specific modulation strategies or circuit structures, thereby effectively suppressing leakage current.

[0004] Existing three-level common-ground inverter solutions often require a large number of passive components such as power switching devices, diodes, or capacitors, resulting in a relatively complex topology, high manufacturing costs, and a still high total harmonic distortion rate of the output voltage, which restricts further improvement of the overall system performance.

[0005] The above background information is provided only to assist in understanding the inventive concept and technical solution of this invention. It does not necessarily belong to the prior art of this application, nor does it necessarily provide technical teaching. In the absence of clear evidence that the above information was disclosed before the filing date of this application, the above background information should not be used to evaluate the novelty and inventiveness of this application. Summary of the Invention

[0006] The purpose of this invention is to provide a grid-connected five-level inverter, modulation method and system based on power semiconductors, which can reduce the number of switches, reduce product costs and improve output current quality.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0008] A photovoltaic grid-connected five-level inverter based on power semiconductors includes a first circuit, a second circuit, and a third circuit; wherein, the first circuit includes a first inductor, a first switch, a second inductor, and a second switch connected in series, the end of the first circuit closest to the first inductor is configured as a first output terminal, and the other end of the first circuit is configured as a first input terminal;

[0009] The second circuit includes a third inductor and a first diode connected in series. One end of the third inductor is configured as the first terminal of the second circuit and is electrically connected to the end of the first inductor near the first input terminal. The cathode of the first diode is configured as the second terminal of the second circuit and is electrically connected to the connection terminal of the first switch and the third inductor.

[0010] The third circuit includes a first capacitor and a second diode connected in series. The first terminal of the third circuit is electrically connected to the negative terminal of the first diode. The second terminal of the third circuit is configured as a second input terminal. The positive terminal of the second diode is closer to the first terminal of the third circuit than its negative terminal.

[0011] The first input terminal and the second input terminal are configured to be electrically connected to both ends of the photovoltaic output side, and the first output terminal and the second input terminal are configured to be electrically connected to both ends of the load.

[0012] Furthermore, following any one or a combination of the aforementioned technical solutions, a third diode is also included, wherein the negative terminal of the third diode is electrically connected to the connection terminal of the first inductor and the first switch, and the positive terminal of the third diode is electrically connected to the connection terminal of the first capacitor and the second diode.

[0013] Furthermore, following any one or a combination of the aforementioned technical solutions, a third switch is also included, one end of which is electrically connected to the connection terminal of the third inductor and the first diode, and the other end of which is electrically connected to the connection terminal of the first capacitor and the second diode.

[0014] Furthermore, following any one or a combination of the aforementioned technical solutions, the operating states of the power semiconductor-based photovoltaic grid-connected five-level inverter include:

[0015] The first and second switches are configured to be closed, and the third switch is configured to be open;

[0016] The first switch, the second switch, and the third switch are all configured to be closed;

[0017] The first switch and the third switch are configured to be open, and the second switch is configured to be closed;

[0018] The first switch is configured to be open, and the second and third switches are configured to be closed.

[0019] Furthermore, following any or a combination of the aforementioned technical solutions, a fourth switch is also included, one end of which is electrically connected to the connection terminal of the second inductor and the second switch, and the other end of which is electrically connected to the second input terminal.

[0020] The operating states of the photovoltaic grid-connected five-level inverter based on power semiconductors include: the first switch and the second switch are configured to be open, and the fourth switch is configured to be closed.

[0021] Furthermore, following any or a combination of the aforementioned technical solutions, the system further includes a fourth switch, one end of which is electrically connected to the connection end of the second inductor and the second switch, the first input end is electrically connected to the first end of the photovoltaic output side, and the other end of the fourth switch is electrically connected to the second end of the photovoltaic output side.

[0022] The operating states of the photovoltaic grid-connected five-level inverter based on power semiconductors include: the first switch and the fourth switch are configured to be closed, and the second switch is configured to be open.

[0023] Furthermore, in accordance with any or a combination of the aforementioned technical solutions, a third switch is also included, one end of which is electrically connected to the connection terminal of the second inductor and the first diode, and the other end of which is electrically connected to the connection terminal of the first capacitor and the second diode.

[0024] The operating states of the photovoltaic grid-connected five-level inverter based on power semiconductors include: the first switch, the third switch, and the fourth switch are configured to be closed, and the second switch is configured to be open.

[0025] Furthermore, based on any or a combination of the aforementioned technical solutions, the first switch and the fourth switch are high-frequency switches, and the second switch and the third switch are low-frequency switches.

[0026] Furthermore, in accordance with any or a combination of the aforementioned technical solutions, a third diode is also included, wherein the negative terminal of the third diode is electrically connected to the connection terminal of the first inductor and the second inductor, and the positive terminal of the third diode is electrically connected to the connection terminal of the first capacitor and the second diode.

[0027] The operating states of the photovoltaic grid-connected five-level inverter based on power semiconductors include: the first and second switches are configured to be open, and the third and fourth switches are configured to be closed.

[0028] Furthermore, following any one or a combination of the aforementioned technical solutions, the system further includes a second capacitor, one end of which is electrically connected to the first terminal of the photovoltaic output side, and the other end of which is electrically connected to the second terminal of the photovoltaic output side. Additionally, the other end of the second capacitor, the second terminal of the photovoltaic output side, and the second terminal of the load are all grounded; and / or,

[0029] One end of the first inductor and one end of the third inductor are electrically connected, and the first inductor and the third inductor are configured as magnetically coupled inductors; and / or,

[0030] It also includes a fourth inductor, one end of which is connected in series with the first inductor, and the other end of which is configured as the first output terminal.

[0031] According to another aspect of the present invention, the present invention provides a modulation method for a power semiconductor-based photovoltaic grid-connected five-level inverter, based on any or a combination of the above-described technical solutions, the modulation method comprising the following steps:

[0032] Collect grid voltage Vg and actual grid-connected current And obtain the instantaneous voltage phase angle θ of the grid connection;

[0033] Determine the effective grid voltage Vgrms based on the grid voltage Vg.

[0034] Based on the target active power output of the inverter Target reactive power output of the inverter The grid-connected reference current is determined by the phase angle of the grid voltage. ;

[0035] Based on the actual grid-connected current and grid reference current Calculate the current difference to obtain the current offset, and convert the grid reference current into the switching duty cycle required by the inverter;

[0036] The inverter is controlled by generating modulation waveforms for each switch based on the switch duty cycle and current offset.

[0037] According to another aspect of the present invention, a photovoltaic inverter system is provided, comprising a power semiconductor-based photovoltaic grid-connected five-level inverter as described in any of the above technical solutions or a combination of multiple technical solutions, wherein the input terminal of the power semiconductor-based photovoltaic grid-connected five-level inverter is configured to be electrically connected to the output terminal on the photovoltaic side, and the output terminal of the power semiconductor-based photovoltaic grid-connected five-level inverter is configured to be electrically connected to the load.

[0038] The beneficial effects of the technical solution provided by this invention are as follows:

[0039] a. The five-level common-ground inverter proposed in this invention supports complete bus voltage utilization. It does not require an inverter and only requires four power semiconductor switches, three diodes and two inductors to achieve an output voltage range of ±Vdc. This effectively reduces the cost of the inverter and greatly reduces its size. It also eliminates the need for additional sensors and control circuits to control the transformer, further reducing costs and increasing efficiency.

[0040] b. By employing a magnetically coupled inductor, this invention ensures that the current does not surge abruptly when the first to fourth switches are closed and turned off, but rather flows smoothly, reducing current stress. This is particularly valuable for high-frequency device switches, enabling the introduction of a natural current commutation path during switching, thus mitigating the sudden current surge experienced by the device during commutation, thereby reducing stress and losses. Furthermore, the switch-interleaving operation based on the magnetically coupled inductor can improve the quality of the output current, ensuring that the main harmonic clusters are located at approximately twice the switching frequency.

[0041] c. Compared with traditional NPC three-level or partially low-gain topologies, the inverter provided by this invention has an output capability that is not limited by the midpoint voltage, avoiding DC voltage waste and improving system energy efficiency and power density. Attached Figure Description

[0042] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0043] Figure 1 A circuit diagram of a power semiconductor-based photovoltaic grid-connected five-level inverter is provided as an exemplary embodiment of the present invention.

[0044] Figure 2 A schematic diagram of the current path of an inverter in state A, provided as an exemplary embodiment of the present invention;

[0045] Figure 3 A schematic diagram of the current path of an inverter in state B, provided as an exemplary embodiment of the present invention;

[0046] Figure 4 A schematic diagram of the current path of an inverter in state C, provided as an exemplary embodiment of the present invention;

[0047] Figure 5A schematic diagram of the current path of an inverter in state D, provided as an exemplary embodiment of the present invention;

[0048] Figure 6 A schematic diagram of the current path of an inverter in state E, provided as an exemplary embodiment of the present invention;

[0049] Figure 7 A schematic diagram of the current path of an inverter in state F, provided as an exemplary embodiment of the present invention;

[0050] Figure 8 A schematic diagram of the current path of an inverter in state G, provided as an exemplary embodiment of the present invention;

[0051] Figure 9 A schematic diagram of the current path of an inverter in state H, provided as an exemplary embodiment of the present invention;

[0052] Figure 10 A schematic diagram of the A-to-H state switching of an inverter provided as an exemplary embodiment of the present invention;

[0053] Figure 11 A schematic diagram of a first modulation method for an inverter provided as an exemplary embodiment of the present invention;

[0054] Figure 12 A schematic diagram of a second modulation method for an inverter provided as an exemplary embodiment of the present invention;

[0055] Figure 13 A schematic diagram of a third modulation method for an inverter provided as an exemplary embodiment of the present invention;

[0056] Figure 14 A schematic diagram of a fourth modulation method for an inverter provided as an exemplary embodiment of the present invention;

[0057] Figure 15 A schematic diagram of a photovoltaic inverter system provided as an exemplary embodiment of the present invention. Detailed Implementation

[0058] To enable those skilled in the art to better understand the present invention, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of the present invention.

[0059] It should be noted that the terms "first," "second," etc., in the specification, claims, and accompanying drawings of this invention are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that the embodiments of the invention described herein can be implemented in orders other than those illustrated or described herein. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover a non-exclusive inclusion; for example, a process, method, apparatus, product, or device that comprises a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or devices.

[0060] In view of the shortcomings of the prior art, the present invention aims to provide a transformerless photovoltaic grid-connected five-level common-ground inverter that uses fewer power semiconductor devices. By reducing the number of power switches and adopting a common-ground design, the inverter proposed in this application has a simple structure, fewer power semiconductor components, and excellent output power quality. The new inverter topology proposed in this application can solve the leakage current problem in the prior art, while maintaining high voltage utilization and output current quality.

[0061] In one embodiment of the present invention, a photovoltaic grid-connected five-level inverter based on power semiconductors is provided, such as... Figure 1 As shown, the inverter includes a first circuit, a second circuit, and a third circuit.

[0062] The first circuit includes a first inductor and a first switch connected in series. Figures 1 to 9 In S 1 ), second inductor ( Figures 1 to 9 In L r ) and the second switch ( Figures 1 to 9 In S 3 The end of the first circuit closest to the first inductor is configured as the first output terminal, and the other end of the first circuit is configured as the first input terminal.

[0063] The second circuit includes a third inductor and a first diode connected in series. Figures 1 to 9 In D 1 One end of the third inductor is configured as the first end of the second circuit and electrically connected to the end of the first inductor near the first input end. The negative terminal of the first diode is configured as the second end of the second circuit and electrically connected to the connection end of the first switch and the third inductor.

[0064] The third circuit includes a first capacitor connected in series ( Figures 1 to 9 In C 1 ) and second diode ( Figures 1 to 9 In D 3 The first terminal of the third circuit, the first capacitor, the second diode, and the second terminal of the third circuit are connected in sequence. The first terminal of the third circuit is electrically connected to the negative terminal of the first diode, and the second terminal of the third circuit is configured as the second input terminal. The positive terminal of the second diode is closer to the first terminal of the third circuit than its negative terminal.

[0065] The first input terminal and the second input terminal are configured to connect to the photovoltaic output side ( Figure 1 The two ends of PV shown are electrically connected, and the first output terminal and the second input terminal are configured to connect to a load (for outputting voltage Vg to the load, such as...). Figure 1 The two ends of the transformer (as shown) are electrically connected. It should be noted that in this application, the two input terminals of the transformer are configured as a first input terminal and a second input terminal, and the two output terminals of the transformer are configured as a first output terminal and a second output terminal. The first input terminal of the transformer is configured to be electrically connected to the first terminal of the photovoltaic output side, the second input terminal of the transformer is configured to be electrically connected to the second terminal of the photovoltaic output side, and the first and second output terminals of the transformer are configured to be electrically connected to the two ends of the load. The configuration of the first output terminal and the second input terminal to be electrically connected to the two ends of the load is achieved by grounding the second output terminal and the second input terminal of the transformer together.

[0066] In one embodiment of the invention, the inverter further includes a third diode ( Figures 1 to 9 In D 2 ), third switch ( Figures 1 to 9 In S 2 ) and the fourth switch ( Figures 1 to 9 In S 4 The negative terminal of the third diode is electrically connected to the connection terminal of the first inductor and the first switch, and the positive terminal of the third diode is electrically connected to the connection terminal of the first capacitor and the second diode. One end of the third switch is electrically connected to the connection terminal of the third inductor and the first diode, and the other end of the third switch is electrically connected to the connection terminal of the first capacitor and the second diode. One end of the fourth switch is electrically connected to the connection terminal of the second inductor and the second switch, the first input terminal is electrically connected to the first terminal of the photovoltaic output side, and the other end of the fourth switch is electrically connected to the second input terminal.

[0067] Preferably, the first switch S1 and the fourth switch S 4 Both are high-frequency switches, operating at PWM frequency; high-frequency switches ( S 1 , S 4 The second switch undertakes the main PWM modulation task and is preferably a GaN device with faster switching speed and lower switching losses. S 3 and the third switch S 2 These are low-frequency switches, operating at the mains frequency (50 / 60Hz). The second and third switches have low switching losses and low speed requirements, allowing for the use of lower-cost silicon MOSFET or IGBT switches, thus reducing system costs. This invention achieves both engineering-level efficiency and cost optimization by using high-frequency switches for the first and fourth switches and low-frequency switches for the second and third switches.

[0068] More preferably, the first inductor and the third inductor are configured as magnetically coupled inductors. Figures 1 to 9 In L T The magnetically coupled inductor L T The middle terminal is configured as the first output terminal, and the magnetically coupled inductor L T One end is connected to the first switch S 1 One end is electrically connected to the magnetically coupled inductor. L T The other end is electrically connected to the positive terminal of the first diode D1.

[0069] This invention employs magnetically coupled inductors to ensure that the current flows smoothly rather than abruptly during the closing and closing of the first to fourth switches. This reduces current stress, which is particularly valuable for high-frequency devices (such as S1 / S4). It introduces a natural current commutation path during switching, mitigating the sudden current surges experienced by the devices during commutation and thus reducing stress and losses. The switch-interleaving operation based on magnetically coupled inductors improves the quality of the output current, ensuring that the main harmonic clusters are located at approximately twice the switching frequency.

[0070] In one embodiment of the invention, the inverter further includes a fourth inductor ( Figures 1 to 9 In L f One end of the fourth inductor is connected in series with the first inductor, and the other end of the fourth inductor is configured as the first output terminal.

[0071] Referring to Table 1, the inverter includes 8 operating states. In Table 1, "1" indicates that the switch / diode is on, "0" indicates that the switch / diode is off, "↑" indicates that the capacitor is charging, "↓" indicates that the capacitor is discharging, and "-" indicates that the capacitor is neither charging nor discharging.

[0072] Table 1

[0073]

[0074] See Figures 2 to 10 The operating states of the photovoltaic grid-connected five-level inverter based on power semiconductors include the following eight states: state A to state H.

[0075] like Figure 2 The inverter is shown in state A: First switch closed. S 1 Second switch S 3 Disconnect the third switch S 2 and the fourth switch S 4 At this time, the first diode D 1 Second diode D 3 On, third diode D 2 The inverter's output voltage is not conducting. Vdc, the first capacitor C 1 Charging, Vdc is the output voltage on the photovoltaic output side.

[0076] like Figure 3 The inverter's state B is shown: First switch closed. S 1 Second switch S 3 and the third switch S 2 Disconnect the fourth switch S 4 At this time, the second diode D 3 The first diode is conducting. D 1 and the third diode D 2 The inverter output is not conducting. The voltage is 0.5Vdc, and the voltage across the magnetically coupled inductor is... Vdc, the first capacitor C 1Charging, Vdc is the output voltage on the photovoltaic output side.

[0077] like Figure 4 The inverter's state C shown is: First switch off. S 1 Third switch S 2 And the fourth switch S4, close the second switch. S 3 At this time, the first diode D 1 Second diode D 3 and the third diode D 2 All are on, the inverter's output voltage The voltage across the magnetically coupled inductor is 0.5Vdc. -Vdc, the first capacitor C 1 Charging, Vdc is the output voltage on the photovoltaic output side.

[0078] like Figure 5 The inverter's state D is shown as: First switch closed. S 1 and the fourth switch S 4 Disconnect the second switch S 3 and the third switch S 2 At this time, the first diode D 1 The second diode is turned on. D 3 and the third diode D 2 The inverter output is not conducting. The voltage is 0, and the first capacitor... C 1 It neither charges nor discharges.

[0079] like Figure 6 The inverter's state E is shown: the second switch is closed. S 3 and the third switch S 2 Disconnect the first switch S 1 and the fourth switch S 4 At this time, the first diode D 1 The second diode is not conducting. D 3 and the third diode D2 When the inverter is turned on, its output voltage... The value is 0, the first capacitor C 1 Charge.

[0080] like Figure 7 The inverter's state F shown is: First switch closed. S 1 Third switch S 2 and the fourth switch S 4 Disconnect the second switch S 3 At this time, the first diode D 1 Second diode D 3 and the third diode D 2 Neither of them is conducting, and the inverter's output voltage is... The voltage across the magnetically coupled inductor is -0.5Vdc. Vdc, the first capacitor C 1 Discharge, Vdc is the output voltage on the photovoltaic output side.

[0081] like Figure 8 The inverter's state G shown is: First switch disconnected. S 1 Second switch S 3 and the third switch S 2 Close the fourth switch S 4 At this time, the first diode D 1 Third diode D 2 The second diode is conducting. D 3 The inverter's output voltage is not conducting. The voltage across the magnetically coupled inductor is -0.5Vdc. -Vdc, the first capacitor C 1 Discharge, Vdc is the output voltage on the photovoltaic output side.

[0082] like Figure 9 The inverter's state H shown is: First switch off. S 1 Second switch S 3 Close the third switch S 2 and the fourth switchS 4 At this time, the third diode D 2 The first diode is conducting. D 1 Second diode D 3 The inverter's output voltage is not conducting. -Vdc, the first capacitor C 1 Discharge, Vdc is the output voltage on the photovoltaic output side.

[0083] As can be seen from the above, through states A to H of the inverter, the positive and negative peak voltages at the inverter output can both reach Vdc, achieving complete utilization of the DC bus voltage. The five-level common-ground inverter proposed in this application supports complete bus voltage utilization, and its output voltage range is ±Vdc. Compared with traditional NPC three-level or partially low-gain topologies, its output capability is not limited by the midpoint voltage, avoiding DC voltage waste, improving system energy efficiency and power density, and requiring only 4 switches, 3 diodes, and 2 inductors, effectively reducing the cost of the inverter.

[0084] Furthermore, in this application, the first capacitor is provided on the third circuit. C 1 Based on the self-balancing capability of the first capacitor, there is no need to use additional sensors and control circuits to control the transformer, resulting in low cost and high efficiency.

[0085] Preferably, the inverter further includes a second capacitor ( Figure 1 In C in One end of the second capacitor is electrically connected to the first end of the photovoltaic output side, and the other end of the second capacitor is electrically connected to the second end of the photovoltaic output side. The other end of the second capacitor, the second end of the photovoltaic output side, and the second end of the load are all grounded to form a common grounding point. The negative terminal (PV-) of the photovoltaic output side, i.e., the PV source, is connected to the common grounding point, which is the neutral point N of the power grid, via a DC link. This structure allows the negative terminal of the PV array to be directly electrically connected to the neutral point of the power grid. By connecting the negative terminal of the PV source to the neutral point of the power grid, the system maintains a stable common-mode voltage and suppresses the common-mode leakage current of the PV source to ground, thereby meeting the grid connection safety requirements under the condition of transformer isolation removal.

[0086] In one embodiment of the present invention, a modulation method suitable for the power semiconductor-based photovoltaic grid-connected five-level inverter described in the above embodiments is provided, such as... Figures 10 to 14 As shown, the modulation method includes modulating the inverter based on three-phase carrier interleaved PWM. Based on this modulation method, the following can be achieved: Figure 10 The diagram illustrates the switching between eight states, from state A to state H, of the inverter described in the above embodiment. R1-R4 represent the voltage range required for the inverter to switch between states A and H. R1 is the voltage range of -Vdc to -0.5Vdc, R2 is the voltage range of -0.5Vdc to 0Vdc, R3 is the voltage range of 0Vdc to 0.5Vdcc, and R4 is the voltage range of 0.5Vdc to Vdc.

[0087] Specifically, the modulation method is implemented based on a preset control unit, which includes a modulation generation module, a switch driving module, and a feedback control module. The control unit generates a five-level modulation waveform based on the output reference signal and the power grid synchronization signal, driving the first to fourth switches. S 1 ~ S 4 The inverter function is implemented by turning the circuit on and off according to a predetermined strategy, realizing the inverter function corresponding to any state from state A to state H.

[0088] like Figure 15 As shown, the modulation method for the photovoltaic grid-connected five-level inverter based on power semiconductors is as follows.

[0089] Collect grid signal (voltage) Vg and actual grid-connected current The phase angle θ of the instantaneous grid-connected voltage is obtained using a phase-locked loop (PLL), where θ = ωt, t represents time, and ω represents the angular frequency. The effective grid voltage Vgrms is determined based on the grid voltage Vg. The target active power output of the inverter is then used to determine the voltage. Target reactive power output of the inverter The grid-connected reference current is calculated from the instantaneous voltage phase angle. .

[0090] The grid-connected reference current and actual grid-connected current The calculation formula is as follows:

[0091]

[0092] Vg /

[0093] in, It is the target active power output of the inverter, used to ensure the efficient transmission of energy to the grid. It is the target reactive power output of the inverter, which is used to maintain the voltage stability of the power grid.

[0094] Based on the actual grid-connected current Grid-connected reference current The current difference is calculated to obtain the current offset aM, which is then sent to the controller. The ideal sinusoidal reference current is converted into the switching duty cycle d required by the inverter. Based on the switching duty cycle d and the current offset aM, the modulation waveforms d of each switch S1~S4 of the inverter are generated. S1 ~d S4 To control the operation of the inverter. This is achieved through a carrier wave (triangular wave). The modulation waveforms d of S1~S4 S1 ~d S4 Compare and confirm the required output signal (level) of switches S1~S4.

[0095] Preferably, the actual active power P output by the inverter to the grid and the actual reactive power Q related to the grid voltage phase angle are also collected. The grid-connected current is determined based on the actual active power P, the actual reactive power Q output by the inverter to the grid, and the grid voltage phase angle. Using the calculation of grid-connected current For actual grid-connected current Compensation is performed on the inverter's control system, including by calculating the grid-connected current. Relative to actual grid-connected current The difference determines the voltage drop, which is then superimposed on the output angle of the phase-locked loop (PLL) or the outer voltage loop reference to automatically compensate for the phase and amplitude of the output voltage.

[0096] by Figure 11 Taking this example, the horizontal axis represents the normalized time (k–1)t / Ts → kt / Ts, that is, one switching cycle Ts is expanded. The vertical axis represents the states of the three switches, the inverter's output voltage, and the output current. Voltage across the magnetically coupled inductor , through current The changes.

[0097] See Figure 1 , Figure 10 and Figure 11 The voltage is in the range of -Vdc to -0.5Vdc, that is... Figure 10 In region R1, S1, S2, and S3 represent the PWM signals driving the upper arm of the inverter, respectively. The PWM signals are modulated by the wave d. S1 ~d S3 and triangular carrier The modulated wave d is output from the comparator. S1 ~d S4 Greater than the corresponding triangular carrier When the signal is in the specified state, the PWM signal is "1"; otherwise, it is "0". Thus, the switching signals for each state can be obtained: F state: S1=1, S2=1, S3=0, S4=1 (S4 and S3 are complementary); H state: S1=0, S2=1, S3=0, S4=1; G state: S1=0, S2=0, S3=0, S4=1.

[0098] Figure 12 Corresponding to Figure 10 In the R2 region, the inverter's output voltage is in the range of -0.5Vdc to -0. Figure 13 Corresponding to Figure 10 In the R3 region, the inverter's output voltage is in the range of 0~+0.5Vdc; Figure 14 Corresponding to Figure 10 In the R4 region, the inverter's output voltage is in the range of 0.5Vdc to 1Vdc. Figures 12 to 14 modulation principle and Figure 11 The same applies, so I won't repeat myself.

[0099] In one embodiment of the present invention, a photovoltaic inverter system is provided, including a power semiconductor-based photovoltaic grid-connected five-level inverter as described in any of the above embodiments, wherein the input terminal of the power semiconductor-based photovoltaic grid-connected five-level inverter is configured to be electrically connected to the output terminal on the photovoltaic side, and the output terminal of the power semiconductor-based photovoltaic grid-connected five-level inverter is configured to be electrically connected to the load.

[0100] The photovoltaic inverter system includes a DC power supply (PV) input, a power conversion unit, a filter and grid connection interface, and a control unit, the specific structure of which is described below. The DC power supply (such as a photovoltaic module): provides an input DC voltage Vdc, with its negative terminal grounded or connected to the grid neutral point to achieve a common-ground structure, suppressing common-mode voltage variations at the source and reducing leakage current.

[0101] The photovoltaic grid-connected five-level inverter based on power semiconductors consists of four power switching devices (S1~S4), two capacitors (voltage divider to form the midpoint level), two freewheeling diodes (D1, D2), and an output filter inductor. This circuit structure can achieve five output voltage levels: ±Vdc, ±0.5Vdc, and 0, effectively improving bus voltage utilization and output voltage quality.

[0102] The filtering and grid connection interface specifically refers to the filter inductor and grid connection synchronization interface set on the output side, which can ensure that the output current waveform is smooth and meets the grid harmonic requirements.

[0103] The control unit includes a modulation generation module, a switch driving module, and a feedback control module. Based on the output reference signal and the power grid synchronization signal, the control unit generates a five-level modulation waveform to drive the first to fourth switches. S1 ~ S 4 The inverter operates by switching on and off according to a predetermined strategy, enabling the inverter function corresponding to any state from A to H. The system block diagram visually illustrates the overall design of the inverter structure and control of this invention, demonstrating its technical advantages in device optimization, bus voltage utilization, and common-mode voltage control.

[0104] It should be noted that the above-mentioned modulation method and photovoltaic inverter system for a power semiconductor-based photovoltaic grid-connected five-level inverter belong to the same inventive concept as the embodiment of the power semiconductor-based photovoltaic grid-connected five-level inverter. By reference, all contents of the embodiment of the power semiconductor-based photovoltaic grid-connected five-level inverter are incorporated into the embodiment of the modulation method and photovoltaic inverter system for a power semiconductor-based photovoltaic grid-connected five-level inverter.

[0105] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0106] The above description is only a specific embodiment of this application. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of this application, and these improvements and modifications should also be considered within the scope of protection of this application.

Claims

1. A photovoltaic grid-connected five-level inverter based on power semiconductors, characterized in that, It includes a first circuit, a second circuit, and a third circuit; wherein, the first circuit includes a first inductor, a first switch, a second inductor, and a second switch connected in series, the end of the first circuit near the first inductor is configured as a first output terminal, and the other end of the first circuit is configured as a first input terminal; The second circuit includes a third inductor and a first diode connected in series. One end of the third inductor is configured as the first terminal of the second circuit and is electrically connected to the end of the first inductor near the first input terminal. The cathode of the first diode is configured as the second terminal of the second circuit and is electrically connected to the connection terminal of the first switch and the third inductor. The third circuit includes a first capacitor and a second diode connected in series. The first terminal of the third circuit is electrically connected to the negative terminal of the first diode. The second terminal of the third circuit is configured as a second input terminal. The positive terminal of the second diode is closer to the first terminal of the third circuit than its negative terminal. The first input terminal and the second input terminal are configured to be electrically connected to both ends of the photovoltaic output side, and the first output terminal and the second input terminal are configured to be electrically connected to both ends of the load.

2. The photovoltaic grid-connected five-level inverter based on power semiconductors according to claim 1, characterized in that, It also includes a third diode, the negative terminal of which is electrically connected to the connection terminal of the first inductor and the first switch, and the positive terminal of which is electrically connected to the connection terminal of the first capacitor and the second diode.

3. The photovoltaic grid-connected five-level inverter based on power semiconductors according to claim 2, characterized in that, It also includes a third switch, one end of which is electrically connected to the connection terminal of the third inductor and the first diode, and the other end of which is electrically connected to the connection terminal of the first capacitor and the second diode.

4. The photovoltaic grid-connected five-level inverter based on power semiconductors according to claim 3, characterized in that, The operating states of the photovoltaic grid-connected five-level inverter based on power semiconductors include: The first and second switches are configured to be closed, and the third switch is configured to be open; The first switch, the second switch, and the third switch are all configured to be closed; The first switch and the third switch are configured to be open, and the second switch is configured to be closed; The first switch is configured to be open, and the second and third switches are configured to be closed.

5. The photovoltaic grid-connected five-level inverter based on power semiconductors according to claim 2, characterized in that, It also includes a fourth switch, one end of which is electrically connected to the connection terminal of the second inductor and the second switch, and the other end of which is electrically connected to the second input terminal; The operating states of the photovoltaic grid-connected five-level inverter based on power semiconductors include: the first switch and the second switch are configured to be open, and the fourth switch is configured to be closed.

6. The photovoltaic grid-connected five-level inverter based on power semiconductors according to claim 1, characterized in that, It also includes a fourth switch, one end of which is electrically connected to the connection end of the second inductor and the second switch, the first input end is electrically connected to the first end of the photovoltaic output side, and the other end of the fourth switch is electrically connected to the second end of the photovoltaic output side; The operating states of the photovoltaic grid-connected five-level inverter based on power semiconductors include: the first switch and the fourth switch are configured to be closed, and the second switch is configured to be open.

7. The photovoltaic grid-connected five-level inverter based on power semiconductors according to claim 6, characterized in that, It also includes a third switch, one end of which is electrically connected to the connection terminal of the second inductor and the first diode, and the other end of which is electrically connected to the connection terminal of the first capacitor and the second diode; The operating states of the photovoltaic grid-connected five-level inverter based on power semiconductors include: the first switch, the third switch, and the fourth switch are configured to be closed, and the second switch is configured to be open.

8. The photovoltaic grid-connected five-level inverter based on power semiconductors according to claim 7, characterized in that, The first switch and the fourth switch are high-frequency switches, and the second switch and the third switch are low-frequency switches.

9. The photovoltaic grid-connected five-level inverter based on power semiconductors according to claim 7, characterized in that, It also includes a third diode, the negative terminal of which is electrically connected to the connection terminal of the first inductor and the second inductor, and the positive terminal of which is electrically connected to the connection terminal of the first capacitor and the second diode. The operating states of the photovoltaic grid-connected five-level inverter based on power semiconductors include: the first and second switches are configured to be open, and the third and fourth switches are configured to be closed.

10. The photovoltaic grid-connected five-level inverter based on power semiconductors according to claim 1, characterized in that, It also includes a second capacitor, one end of which is electrically connected to the first terminal of the photovoltaic output side, and the other end of which is electrically connected to the second terminal of the photovoltaic output side. Furthermore, the other end of the second capacitor, the second terminal of the photovoltaic output side, and the second terminal of the load are all grounded; and / or, One end of the first inductor and one end of the third inductor are electrically connected, and the first inductor and the third inductor are configured as magnetically coupled inductors; and / or, It also includes a fourth inductor, one end of which is connected in series with the first inductor, and the other end of which is configured as the first output terminal.

11. A modulation method for a photovoltaic grid-connected five-level inverter based on power semiconductors, characterized in that, Based on the power semiconductor-based photovoltaic grid-connected five-level inverter as described in claim 1, the modulation method includes the following steps: Collect grid voltage Vg and actual grid-connected current And obtain the instantaneous voltage phase angle θ of the grid connection; Determine the effective grid voltage Vgrms based on the grid voltage Vg. Based on the target active power output of the inverter Target reactive power output of the inverter The grid-connected reference current is determined by the phase angle of the grid voltage. ; Based on the actual grid-connected current and grid reference current Calculate the current difference to obtain the current offset, and convert the grid reference current into the switching duty cycle required by the inverter; The inverter is controlled by generating modulation waveforms for each switch based on the switch duty cycle and current offset.

12. A photovoltaic inverter system, characterized in that, Includes a power semiconductor-based photovoltaic grid-connected five-level inverter as described in any one of claims 1 to 10, wherein the input terminal of the power semiconductor-based photovoltaic grid-connected five-level inverter is configured to be electrically connected to the output terminal on the photovoltaic side, and the output terminal of the power semiconductor-based photovoltaic grid-connected five-level inverter is configured to be electrically connected to the load.

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