An inverter and control method

Abstract

This application provides an inverter and its control method. The inverter includes a Heric inverter bridge, a first freewheeling unit, a second freewheeling unit, and a control module. During the freewheeling phase of the inverter, the first, second, third, and fourth high-frequency switches are turned off, and the first or second freewheeling unit is turned on to form a freewheeling loop. During the power phase of the inverter, the first, second, third, and fourth high-frequency switches are enabled, and the first or second freewheeling unit is also enabled to synchronously switch its conduction direction, thereby achieving power transfer in the inverter. The inverter and control method provided in this application, through a division of labor modulation strategy, effectively solves the common-mode current and system reliability problems while maintaining the high efficiency advantages of the Heric inverter topology.

Description

Technical Field

[0001] This application relates to the field of power electronics technology, and in particular to an inverter and a control method thereof. Background Technology

[0002] Heric (Highly Efficient and Reliable Inverter Concept) inverter topologies are widely used in photovoltaic grid-connected systems due to their high efficiency and low leakage current characteristics. Traditional Heric control methods typically employ high-frequency switching transistors for power regulation, with freewheeling transistors operating at power frequency for freewheeling and isolation. However, traditional methods suffer from issues such as high switching losses, insufficient common-mode current suppression, waveform quality deficiencies, and challenges in thermal management, particularly limiting performance under reactive power regulation and extreme operating conditions. Summary of the Invention

[0003] In view of this, embodiments of this application provide an inverter and control method, which are optimized based on the traditional Heric inverter structure, aiming to solve the technical problems of large switching losses and insufficient common-mode current suppression in inverters.

[0004] A first aspect of this application provides an inverter, the inverter including a Heric inverter bridge, a first freewheeling unit, a second freewheeling unit, and a control module, the input terminal of the inverter being used to connect to a DC power supply, and the output terminal of the inverter being used to connect to the power grid; the Heric inverter bridge includes a first high-frequency switch, a second high-frequency switch, a third high-frequency switch, and a fourth high-frequency switch;

[0005] One end of the first freewheeling unit is connected to the negative terminal of the DC power supply, and the other end of the first freewheeling unit is connected to the first output terminal of the Heric inverter bridge. One end of the second freewheeling unit is connected to the positive terminal of the DC power supply, and the other end of the second freewheeling unit is connected to the second output terminal of the Heric inverter bridge.

[0006] The control module is used for:

[0007] During the freewheeling phase of the inverter, the first high-frequency switch, the second high-frequency switch, the third high-frequency switch, and the fourth high-frequency switch are turned off, and the first freewheeling unit or the second freewheeling unit is turned on to form a freewheeling circuit.

[0008] During the power phase of the inverter, the first high-frequency switch, the second high-frequency switch, the third high-frequency switch, and the fourth high-frequency switch are enabled, and the first freewheeling unit or the second freewheeling unit is enabled to synchronously switch the conduction direction to realize the power transfer of the inverter.

[0009] In one embodiment, the first freewheeling unit includes a first power frequency freewheeling tube and a first freewheeling diode, and the second freewheeling unit includes a second power frequency freewheeling tube and a second freewheeling diode. The first power frequency freewheeling tube and the first freewheeling diode are connected in reverse parallel and then connected in series between the negative terminal of the DC power supply and the first output terminal of the Heric inverter bridge. The second power frequency freewheeling tube and the second freewheeling diode are connected in reverse parallel and then connected in series between the positive terminal of the DC power supply and the second output terminal of the Heric inverter bridge.

[0010] In one embodiment, the commutation of the first freewheeling unit and the second freewheeling unit is synchronized with the zero-crossing point of the power grid.

[0011] In one embodiment, the control module is used to:

[0012] During the freewheeling phase of the inverter and during the positive half-cycle of the grid voltage, the first power frequency freewheeling diode is turned on and the second power frequency freewheeling diode is turned off.

[0013] During the freewheeling phase of the inverter and during the negative half-cycle of the grid voltage, the second power frequency freewheeling diode is turned on, and the first power frequency freewheeling diode is turned off.

[0014] In one embodiment, the control module is used to:

[0015] During the power phase of the inverter and in the positive half-cycle of the grid voltage, the first high-frequency switch, the fourth high-frequency switch, and the first power frequency freewheeling diode are turned on, or the second high-frequency switch, the third high-frequency switch, and the first power frequency freewheeling diode are turned on.

[0016] During the power phase of the inverter and in the negative half-cycle of the grid voltage, the first high-frequency switch, the fourth high-frequency switch, and the second power frequency freewheeling diode are turned on, or the second high-frequency switch, the third high-frequency switch, and the second power frequency freewheeling diode are turned on.

[0017] In one embodiment, the inverter further includes a filter circuit, which includes a first DC-side inductor, a second DC-side inductor, a first filter capacitor, a first grid-side inductor, and a second grid-side inductor.

[0018] The first DC-side inductor and the first grid-side inductor are connected in series between the first output terminal of the Heric inverter bridge and the power grid. The second DC-side inductor and the second grid-side inductor are connected in series between the second output terminal of the Heric inverter bridge and the power grid. The first filter capacitor is connected between the series node of the first DC-side inductor and the first grid-side inductor and the series node of the second DC-side inductor and the second grid-side inductor.

[0019] In one embodiment, the control module is used to:

[0020] Determine whether the output of the inverter meets the grid connection requirements; if the grid connection requirements are met, control the inverter to complete the grid connection.

[0021] The grid connection requirements are total harmonic distortion (THD) < 2% and leakage current < 50mA.

[0022] In one embodiment, the control module is further configured to:

[0023] At the start of the freewheeling phase of the inverter, the conduction combination of the first power frequency freewheeling tube and the second power frequency freewheeling tube is determined according to the direction of the output current of the inverter;

[0024] When the output current direction is positive, the first power frequency freewheeling diode is turned on and the second power frequency freewheeling diode is turned off, and a freewheeling circuit is established using the first freewheeling diode;

[0025] When the output current direction is negative, the second power frequency freewheeling diode is turned on and the first power frequency freewheeling diode is turned off, and a freewheeling circuit is established using the second freewheeling diode;

[0026] as well as

[0027] During the freewheeling phase, the common-mode voltage of the inverter is continuously monitored, and the deviation between the common-mode voltage and half of the DC bus voltage is maintained to be less than or equal to 5V.

[0028] In one embodiment, the control module is further configured to:

[0029] When the commutation operation of the first freewheeling unit and the second freewheeling unit is switched, the first power frequency freewheeling tube or the second power frequency freewheeling tube is controlled to be pre-turned off. The pre-turn-off time window is two high-frequency switching cycles. The duration of the freewheeling phase accounts for 40% to 60% of the high-frequency switching cycle.

[0030] The high-frequency switching period is 50μs, and the pre-turn-off time window is 100μs;

[0031] The duration of the freewheeling phase is 20~30μs.

[0032] A second aspect of this application provides a control method applied to the inverter described above. The control method includes: during the freewheeling phase of the inverter, turning off the first high-frequency switch, the second high-frequency switch, the third high-frequency switch, and the fourth high-frequency switch, and turning on the first freewheeling unit or the second freewheeling unit to form a freewheeling loop.

[0033] During the power phase of the inverter, the first high-frequency switch, the second high-frequency switch, the third high-frequency switch, and the fourth high-frequency switch are enabled, and the first freewheeling unit or the second freewheeling unit is enabled to synchronously switch the conduction direction to realize the power transfer of the inverter.

[0034] The beneficial effects of the embodiments of this application are as follows:

[0035] The control module coordinates the Heric inverter bridge, the first freewheeling unit, and the second freewheeling unit. Through a division of labor modulation strategy, all high-frequency switches are turned off during the freewheeling phase, and only the freewheeling unit operating at the power frequency maintains the current. This greatly reduces the number of high-frequency switches and the corresponding switching losses, and efficiently transmits energy during the power phase, thereby significantly improving the overall system efficiency. During the freewheeling phase, by turning off the high-frequency switches and turning on specific freewheeling units, a physical isolation channel is formed between the DC power supply and the grid, effectively blocking the common-mode current path caused by common-mode voltage fluctuations. This results in the system leakage current being far below the grid connection standard requirements. The inverter and control method provided in this application effectively solve the common-mode current and system reliability problems while maintaining the high efficiency advantages of the Heric inverter topology. Attached Figure Description

[0036] To more clearly illustrate the technical solutions in the embodiments of this application, 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 of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0037] Figure 1 This is a schematic diagram of the structure of an inverter provided in one embodiment of this application;

[0038] Figure 2 A flowchart illustrating a control method for an inverter provided in an embodiment of this application;

[0039] Figure 3 A flowchart of an inverter control method provided in another embodiment of this application. Detailed Implementation

[0040] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0041] It should be noted that when a component is referred to as being "fixed to" or "set on" another component, it can be directly on or indirectly on that other component. When a component is referred to as being "connected to" another component, it can be directly connected to or indirectly connected to that other component.

[0042] It should be understood that the terms "length", "width", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this application.

[0043] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this application, "multiple" means two or more, unless otherwise explicitly specified.

[0044] Please see Figure 1 As shown, this application embodiment provides an inverter 100, which includes a Heric inverter bridge 110, a first freewheeling unit 120, a second freewheeling unit 130, and a control module 140. The input terminal of the inverter 100 is used to connect to a DC power supply V. DC The output terminal of inverter 100 is used to connect to the power grid. Heric inverter bridge 110 includes a first high-frequency switch Q1, a second high-frequency switch Q2, a third high-frequency switch Q3, and a fourth high-frequency switch Q4. One end of the first freewheeling unit 120 is connected to the negative terminal of the DC power supply Vdc, and the other end of the first freewheeling unit 120 is connected to the first output terminal, point A, of Heric inverter bridge 110. One end of the second freewheeling unit 130 is connected to the DC power supply V DC The positive terminal of the second freewheeling unit 130 is connected to the positive terminal of the Heric inverter bridge 110, i.e., point B.

[0045] The control module 140 is used to turn off the first high-frequency switch Q1, the second high-frequency switch Q2, the third high-frequency switch Q3 and the fourth high-frequency switch Q4 during the freewheeling phase of the inverter 100, and turn on the first freewheeling unit 120 or the second freewheeling unit 130 to form a freewheeling circuit.

[0046] During the power phase of the inverter 100, the first high-frequency switch Q1, the second high-frequency switch Q2, the third high-frequency switch Q3 and the fourth high-frequency switch Q4 are enabled, and the first freewheeling unit 120 or the second freewheeling unit 130 is enabled to synchronously switch the conduction direction to realize the power transfer of the inverter 100.

[0047] The inverter 100 provided in this application embodiment has a control module 140 that coordinates the Heric inverter bridge 110, the first freewheeling unit 120, and the second freewheeling unit 130. Through a division of labor modulation strategy, all high-frequency switching transistors are turned off during the freewheeling phase, and the current is maintained only by the freewheeling unit operating at the power frequency. This greatly reduces the number of high-frequency switching operations and the corresponding switching losses. Energy is efficiently transmitted during the power phase, significantly improving the overall system efficiency. During the freewheeling phase, by turning off the high-frequency switching transistors and turning on specific freewheeling units, a physical isolation channel is formed between the DC power supply and the grid, effectively blocking the common-mode current path caused by common-mode voltage fluctuations. This makes the system leakage current much lower than the grid connection standard requirements, effectively solving the common-mode current and system reliability problems.

[0048] Please see Figure 1 As shown, in one embodiment, the first freewheeling unit 120 includes a first power frequency freewheeling transistor IGBT1 and a first freewheeling diode D1, and the second freewheeling unit 130 includes a second power frequency freewheeling transistor IGBT2 and a second freewheeling diode D2. The first power frequency freewheeling transistor IGBT1 and the first freewheeling diode D1 are connected in reverse parallel and then in series between the negative terminal of the DC power supply Vdc and the first output terminal of the Heric inverter bridge 110. The second power frequency freewheeling transistor IGBT2 and the second freewheeling diode D2 are connected in reverse parallel and then in series between the DC power supply Vdc and the first output terminal of the Heric inverter bridge 110. DC The positive terminal is between the positive terminal and the second output terminal of the Heric inverter bridge 110.

[0049] In one embodiment, the first freewheeling unit 120 and the second freewheeling unit 130 adopt a hybrid device structure. The main switching unit, namely the first power frequency freewheeling diode IGBT1 and the second power frequency freewheeling diode IGBT2, is a silicon-based IGBT with a rated voltage of 1200V and a rated current of 30A. The anti-parallel freewheeling circuit, namely the first freewheeling diode D1 and the second freewheeling diode D2, uses silicon carbide Schottky diodes with a rated voltage of 650V and a rated current of 15A. Silicon carbide Schottky diodes have zero reverse recovery charge characteristics, almost eliminating the reverse recovery process during the freewheeling stage. Compared with traditional silicon-based fast recovery diodes, their conduction losses are lower. In this embodiment, the first freewheeling unit 120 and the second freewheeling unit 130 effectively improve the efficiency and thermal stability of the freewheeling circuit while maintaining power frequency conduction stability, ensuring that the Heric inverter bridge 110 achieves high efficiency, low loss, and low leakage current operating goals during grid-connected operation.

[0050] Please see Figure 1 As shown, in one embodiment, the commutation operation of the first freewheeling unit 120 and the second freewheeling unit 130 is synchronized with the zero-crossing point of the grid voltage. It can be understood that the commutation operation here refers to the switching of the conduction combination of the first freewheeling unit 120 and the second freewheeling unit 130. At the zero-crossing point of the grid voltage, the instantaneous voltage value is close to zero. Even if the commutation operations of the two IGBTs have an extremely small overlap, causing them to conduct simultaneously momentarily, since there is almost no voltage difference between points A and B, no large inrush current will be generated, significantly reducing switching losses and stress, and improving efficiency and reliability.

[0051] Please see Figure 1 As shown, in one embodiment, the control module 140 is configured to: turn on the first power frequency freewheeling diode IGBT1 and turn off the second power frequency freewheeling diode IGBT2 during the freewheeling phase of the inverter 100 and during the positive half-cycle of the grid voltage; and turn on the second power frequency freewheeling diode IGBT2 and turn off the first power frequency freewheeling diode IGBT1 during the freewheeling phase of the inverter 100 and during the negative half-cycle of the grid voltage.

[0052] Please see Figure 1 As shown, in one embodiment, the control module 140 is used to turn on the first high-frequency switch Q1, the fourth high-frequency switch Q4 and the first power frequency freewheeling diode IGBT1 during the power phase of the inverter 100 and during the positive half-cycle of the grid voltage, or to turn on the second high-frequency switch Q2, the third high-frequency switch Q3 and the first power frequency freewheeling diode IGBT1.

[0053] During the power phase of inverter 100 and during the negative half-cycle of grid voltage, the first high-frequency switch Q1, the fourth high-frequency switch Q4, and the second power frequency freewheeling diode IGBT2 are turned on, or the second high-frequency switch Q2, the third high-frequency switch Q3, and the second power frequency freewheeling diode IGBT2 are turned on.

[0054] Please see Figure 1 As shown, in one embodiment, the inverter 100 further includes a filter circuit 150. The filter circuit 150 includes a first DC-side inductor L1, a second DC-side inductor L2, a first filter capacitor C2, a first grid-side inductor L3, and a second grid-side inductor L4. The first DC-side inductor L1 and the first grid-side inductor L3 are connected in series between the first output terminal of the Heric inverter bridge 110 and the power grid. The second DC-side inductor L2 and the second grid-side inductor L4 are connected in series between the second output terminal of the Heric inverter bridge 110 and the power grid. The first filter capacitor C2 is connected between the series connection node of the first DC-side inductor L1 and the first grid-side inductor L3 and the series connection node of the second DC-side inductor L2 and the second grid-side inductor L4. The filter circuit 150 can effectively filter out switching frequency harmonics at and near 20kHz, preventing high-frequency noise generated by the inverter 100. The bus capacitor C1 is connected to the DC power supply V. DCParallel connection is used to absorb ripple current, provide a stable DC operating voltage, and prevent drastic fluctuations in bus voltage.

[0055] Please see Figure 1 As shown, in one embodiment, the control module 140 is used to determine whether the output of the inverter 100 meets the grid connection requirements. If the grid connection requirements are met, the control module 140 controls the inverter 100 to complete the grid connection. The grid connection requirements are that the total harmonic distortion (THD) is less than 2% and the leakage current is less than 50mA.

[0056] Please see Figure 1 As shown, in one embodiment, the control module 140 is further configured to determine the conduction combination of the first power frequency freewheeling transistor IGBT1 and the second power frequency freewheeling transistor IGBT2 according to the output current direction of the inverter 100 at the start of the freewheeling phase of the inverter 100.

[0057] When the output current direction is positive, the first power frequency freewheeling diode IGBT1 is turned on and the second power frequency freewheeling diode IGBT2 is turned off, and a freewheeling circuit is established using the first freewheeling diode D1.

[0058] When the output current is negative, the second power frequency freewheeling diode IGBT2 is turned on and the first power frequency freewheeling diode IGBT1 is turned off, and a freewheeling circuit is established using the second freewheeling diode D2.

[0059] During the freewheeling phase, the common-mode voltage of inverter 100 is continuously monitored, and the deviation between the common-mode voltage and half of the DC bus voltage is maintained at less than or equal to 5V.

[0060] Please see Figure 1 As shown, in one embodiment, the control module 140 is further configured to control the first power frequency freewheeling tube IGBT1 or the second power frequency freewheeling tube IGBT2 to pre-turn off when the commutation action of the first freewheeling unit 120 and the second freewheeling unit 130 is switched. The pre-turn-off time window is two high-frequency switching cycles, and the duration of the freewheeling phase accounts for 40% to 60% of the high-frequency switching cycle.

[0061] The high-frequency switching period is 50 μs, and the pre-turn-off time window is 100 μs. The duration of the freewheeling phase is 20~30 μs.

[0062] Please see Figure 1 As shown, in one embodiment, the control module 140 is also used to collect the DC bus voltage Vdc, grid voltage phase and output current Io of the inverter 100 in real time, and obtain the grid synchronization angular frequency θ. Specifically, the control module 140 extracts the grid synchronization angular frequency θ through a second-order generalized integral phase-locked loop (SOGI-PLL).

[0063] The control module 140 further generates SPWM modulation signals for four high-frequency switching transistors (Q1~Q4) based on the active power command P* and reactive power command Q*, combined with the grid synchronization angular frequency θ, using a direct power control algorithm. The switching frequency of these transistors is 20kHz. The module also calculates the conduction timing intervals of the first power frequency freewheeling transistor IGBT1 and the second power frequency freewheeling transistor IGBT2.

[0064] Specifically, in one embodiment, the direct power control algorithm first establishes a synchronous rotating coordinate system based on the grid synchronization angular frequency θ, and then transforms the acquired instantaneous grid voltage and output current values ​​to the dq coordinate system through Park transformation to obtain the grid voltage components. , and output current component , By calculating the active power error ΔP and reactive power error ΔQ in real time, where:

[0065] Where P* is the active power command set by the power control loop, and Q* is the reactive power command. This represents the current actual active power. Given the current actual reactive power, the direct power controller unit of control module 140 uses proportional-integral regulation to adjust the modulation ratio in real time according to the following formula:

[0066] ;in, This is the proportionality coefficient, which determines the instantaneous effect of error changes on the output modulation ratio. These are integral coefficients used to eliminate steady-state errors and maintain long-term power accuracy. The modulation ratio M is the time integral of the active power error. The space vector modulation (SVPWM) module of the control module 140 is coupled to generate bipolar SPWM drive signals for high-frequency switching transistors Q1 to Q4. The switching frequency is maintained at 20kHz to reduce high-frequency harmonics in the output current.

[0067] The conduction intervals of the first and second freewheeling diodes IGBT1 and IGBT2 are determined based on the instantaneous polarity of the grid voltage. During the positive half-cycle of the grid voltage, IGBT1 is on and IGBT2 is off; during the negative half-cycle, IGBT2 is on and IGBT1 is off. The switching action is strictly completed within a 5ms time window before and after the grid voltage crosses zero, ensuring that the conduction direction of the freewheeling diodes is consistent with the grid voltage polarity and avoiding inrush current caused by zero-point switching. The entire control process achieves dual closed-loop regulation of active and reactive power within one fundamental cycle, while coordinating with the freewheeling and commutation processes of inverter 100 to ensure that inverter 100 maintains high efficiency and low leakage current characteristics during grid-connected operation.

[0068] Specifically, in one embodiment, the high-frequency switching transistors Q1~Q4 employ a bipolar modulation method, alternating the polarity of the bridge arm output by comparing the polarity of the carrier wave and the modulating wave. The carrier signal is a symmetrical triangular wave with a frequency of 20kHz, consistent with the switching frequency of 20kHz, ensuring timing synchronization between the control loop and the modulation loop. The modulating wave is defined as:

[0069] {V}_{m}={V}_{m}\left [ {sin(2\pi \cdot 50t)+0.15sin(6\pi \cdot 50t)} \right ] ;in, To modulate the amplitude, when the DC bus voltage At 550V, It is approximately equal to 311V, which means that the peak value of the fundamental component accounts for about 80% of the peak voltage on the DC side. sin(2π·50t) is a sinusoidal component with the same frequency as the fundamental wave of the grid, which is responsible for controlling the main frequency component of the output current to ensure that it is consistent with the phase of the grid. 0.15sin(6π·50t) is the third harmonic component, whose amplitude accounts for 15% of the peak value of the fundamental wave. It is used to smooth the nonlinear clipping phenomenon at the top of the modulated wave waveform, improve the modulation efficiency and reduce the DC component injection of the bridge arm, so that the total harmonic distortion of the output waveform after the action of the filter circuit 150 is kept within the required THD < 2%.

[0070] Bipolar modulation switches the output of the inverter bridge arm between positive and negative bus voltages during each carrier cycle. While the switching losses are slightly higher than those of unipolar modulation, the strategy of high-frequency commutation only at zero crossings effectively controls the loss level. At the same time, physical isolation between the DC power supply and the grid is achieved during the freewheeling phase, ensuring that the output waveform remains consistent with the common-mode voltage control target, i.e., the deviation between the common-mode voltage and half of the DC bus voltage is less than or equal to 5V. This balances grid connection stability and electromagnetic compatibility.

[0071] Specifically, in one embodiment, the time allocation of the freewheeling isolation operation within the high-frequency switching cycle is strictly limited, when the high-frequency SPWM cycle T... s When the current-carrying phase lasts for 50 μs, the duration T of the freewheeling phase is... k It needs to account for 40% to 60% of the entire cycle and must meet T. k ≥0.4T s The lower limit constraint is set to ensure that the DC power supply and the grid form effective physical isolation during the freewheeling period.

[0072] At the start of the freewheeling phase, the control module 140 determines the output current I based on the freewheeling phase. o The direction, i.e., the sign of the instantaneous value of the output current, determines the conduction combination of the freewheeling diode. When I o When I > 0, the first power frequency freewheeling diode IGBT1 is turned on and a freewheeling path is established using the first freewheeling diode D1. o When the current is less than 0, the second power frequency freewheeling diode IGBT2 is turned on and the freewheeling path is established using the second freewheeling diode D2. This polarity-corresponding conduction strategy ensures that the freewheeling path is consistent with the direction of the output current, thereby avoiding reverse impact current.

[0073] Furthermore, the common-mode voltage of inverter 100 is continuously monitored during the freewheeling phase. :

[0074] ;in, and These are the instantaneous voltages at points A and B of the inverter bridge arm relative to the negative terminal of the DC bus. If detected... That is, common-mode voltage When the deviation from half of the DC bus voltage exceeds 10V, the control module 140 will dynamically fine-tune the on and off times of the power frequency freewheeling diode in subsequent cycles to maintain... , of which 275V is =550V is the theoretical common-mode voltage target value. Through the above closed-loop regulation process, the common-mode voltage during the freewheeling stage is ensured to be stable within a narrow fluctuation range, effectively blocking the common-mode current path caused by distributed capacitance, thereby reducing leakage current and improving grid connection safety.

[0075] Specifically, in one embodiment, in the power frequency freewheeling diode driving logic, the control module 140 is based on the zero-crossing detection of the mains voltage and the delay angle. The synchronization timing, δ, reflects the phase lag corresponding to the ratio of reactive power to active power. Pre-turn-off is implemented 100μs ahead of the planned polarity switching instant. Specifically, this involves turning off the gate of the currently conducting IGBT and switching it into the freewheeling path of its anti-parallel diode, thus completing a shockless transition of the current-carrying path before the actual zero-crossing point. This pre-turn-off window is two high-frequency switching cycles long, because the switching frequency is 20kHz and the period T... s =50μs, which is greater than the set dead time, to ensure that there is no risk of shoot-through in the upper and lower bridge arms and to fully release the IGBT tail current, so that the freewheeling current decays naturally in one direction in D1 or D2 and is aligned with the freewheeling isolation interval.

[0076] Furthermore, in one embodiment, to suppress voltage spikes and radiated interference during pre-turn-off and re-turn-on, an RC snubber circuit is connected in parallel across the first power frequency freewheeling diode IGBT1 and the second power frequency freewheeling diode IGBT2. The RC snubber circuit is configured with R=100Ω, C=10nF, and the snubber circuit time constant τ=RC=100ns to perform first-order broadening of the switching edge, limiting the junction temperature transient overshoot and reducing the injection of high-frequency components into the filter circuit 150. The energy E stored in the snubber capacitor under the DC bus voltage is... s Approximately 1.51 mJ:

[0077] Under power frequency switching, two charge-discharge cycles are completed per grid frequency cycle. The average power absorption of a single freewheeling branch is approximately 0.151W, which matches the power frequency operating characteristics and has sufficient heat load. The average power absorption P s : ;in, At the fundamental frequency of the power grid, this timing sequence, in conjunction with absorption, transfers the device voltage rise rate and reverse recovery related stress to the Schottky diode path near the zero crossing. Combined with the maintenance of the common-mode voltage closed loop, the excitation source of the common-mode current is weakened, the energy back-off during the freewheeling phase is smoothed, and the transient surges and EMI during commutation are suppressed, further supporting the grid connection constraints of THD less than 2% and leakage current less than 50mA.

[0078] Specifically, in one embodiment, the implementation of the cooperative control first involves adding a 1μs dead time to the SPWM drive signal of the high-frequency switching transistors Q1~Q4. The dead time is the delay interval between adjacent upper and lower bridge arm devices when they are switched off simultaneously, which is used to avoid bridge arm shoot-through faults. Its setting value accounts for 2% of the period corresponding to the switching frequency of 20kHz, which ensures a reliable turn-off margin without significantly affecting the quality of the modulation waveform.

[0079] Specifically, in one embodiment, thermal management is performed on the first freewheeling unit 120 and the second freewheeling unit 130, and their losses consist of conduction losses and switching losses. The conduction losses are calculated according to the following formula:

[0080] ;

[0081] in, R is the equivalent DC current value for thermal effects. ds_on(T) This is the resistance between the drain and source of the IGBT when it is turned on.

[0082] The switching loss is calculated using the following formula: ;in, The peak current flowing at the instant the switch is activated. The time required for the current to rise from 10% to 90%. The time required for the current to drop from 90% to 10%. This refers to the switching frequency. To ensure... =Junction temperature margin at 550V and 20kHz switching stress. The heat dissipation path requires the equivalent thermal resistance from junction to case to meet R < 0.5℃ / W, thereby limiting the junction-to-case temperature rise to a small range. Together with the strategy of zero-crossing commutation operating only at the current zero point, it reduces switching losses and makes the thermal design meet reliability requirements.

[0083] Specifically, in one embodiment, the first power frequency freewheeling diode IGBT1 and the second power frequency freewheeling diode IGBT2 are silicon-based IGBTs, and the first freewheeling diode D1 and the second freewheeling diode D2 are silicon carbide Schottky diodes. The conduction loss of the first freewheeling unit 120 and the second freewheeling unit 130 is calculated as follows:

[0084] Among them, V CE(sat) This is the voltage difference between the collector and emitter of the IGBT when it is fully turned on. This represents the average current flowing through the IGBT during one switching cycle.

[0085] The conduction loss of a diode is calculated as follows:

[0086] ;in, This is the forward voltage drop of the diode.

[0087] Under the constraints of zero-crossing pre-turn-off and RC snubber circuit, the reverse recovery related losses are suppressed by the zero reverse recovery characteristics of the silicon carbide Schottky diode. In one embodiment, to reduce the additional junction temperature surge caused by inter-device thermal coupling, the geometric spacing between the silicon carbide Schottky diode and the IGBT chip in the freewheeling channel is set to be greater than or equal to 2 mm to reduce the junction-to-junction coupling thermal resistance. This arrangement, together with the heat dissipation path, forms a multi-branch low thermal resistance network from the junction to the environment, simultaneously constraining the transient heat accumulation of the high-frequency switching transistors Q1~Q4 in the high-frequency band and the average thermal load of the freewheeling branch in the power frequency band, maintaining stable device junction temperature and meeting thermal reliability requirements.

[0088] In the simulation verification, an equivalent model of the inverter provided in the embodiment of this application was constructed using Simulink. It was confirmed that the high-frequency ripple amplitude does not exceed 2.63V and is isolated from the frequency band of the LCL resonant point of about 8.2kHz. The leakage current recorded in the final steady-state plateau stage is less than or equal to 35mA, which meets the common-mode current suppression target.

[0089] Furthermore, the working effect of the inverter provided in the embodiments of this application is verified in specific scenarios.

[0090] Scenario 1: Rated power grid-connected operation:

[0091] DC bus voltage V DC Under the conditions of 550V and grid voltage of 230V / 50Hz, the grid phase is extracted through a second-order generalized integral phase-locked loop, and the fundamental frequency of 50Hz is locked. According to the active power command P*=5kW and reactive power command Q*=0, a 20kHz SPWM modulation signal is generated. The conduction timing interval of the freewheeling diode is calculated. During the freewheeling phase, IGBT1 or IGBT2 is turned on, the common-mode voltage is stabilized at 275V±5V, and the common-mode current is blocked. High-frequency transistors Q1~Q4 switch at the current zero-crossing point, and the freewheeling diode switches at the grid zero-crossing point. The dead time is 1μs to avoid shoot-through. After filtering, the output AC power has THD<2% and leakage current<35mA, which meets the grid connection requirements.

[0092] Scenario 2: Adaptive operation in reactive power mode:

[0093] When the system receives a reactive power command, it dynamically adjusts the phase of the SPWM modulation signal and the freewheeling diode delay angle δ. The freewheeling diode is pre-turned off 100μs before the zero crossing point. The RC absorption circuit suppresses voltage spikes, and the common-mode voltage remains at 275V±5V. The effective value of the common-mode current is <35mA. The phase difference between the output current and the grid voltage adapts to the reactive power command, and the system operates stably.

[0094] Please see Figure 1 and Figure 2 This application provides a control method applied to the inverter 100 provided in this application embodiment. The control method includes:

[0095] S201. During the freewheeling phase of the inverter 100, the first high-frequency switch Q1, the second high-frequency switch Q2, the third high-frequency switch Q3 and the fourth high-frequency switch Q4 are turned off, and the first freewheeling unit 120 or the second freewheeling unit 130 is turned on to form a freewheeling circuit.

[0096] S202. During the power phase of the inverter 100, the first high-frequency switch Q1, the second high-frequency switch Q2, the third high-frequency switch Q3 and the fourth high-frequency switch Q4 are enabled, and the first freewheeling unit 120 or the second freewheeling unit 130 is enabled to synchronously switch the conduction direction, so as to realize the power transfer of the inverter 100.

[0097] Please see Figure 1 and Figure 3 In one embodiment, the control method further includes:

[0098] S301. During the freewheeling phase of inverter 100 and during the positive half-cycle of the grid voltage, the first power frequency freewheeling diode IGBT1 is turned on, and the second power frequency freewheeling diode IGBT2 is turned off.

[0099] S302. During the freewheeling phase of inverter 100 and during the negative half-cycle of the grid voltage, the second power frequency freewheeling diode IGBT2 is turned on, and the first power frequency freewheeling diode IGBT1 is turned off.

[0100] S303. During the power phase of inverter 100 and during the positive half-cycle of grid voltage, turn on the first high-frequency switch Q1, the fourth high-frequency switch Q4 and the first power frequency freewheeling diode IGBT1, or turn on the second high-frequency switch Q2, the third high-frequency switch Q3 and the first power frequency freewheeling diode IGBT1.

[0101] S304. During the power phase of inverter 100 and during the negative half-cycle of grid voltage, turn on the first high-frequency switch Q1, the fourth high-frequency switch Q4, and the second power frequency freewheeling diode IGBT2, or turn on the second high-frequency switch Q2, the third high-frequency switch Q3, and the second power frequency freewheeling diode IGBT2.

[0102] The above-described embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of this application, and should all be included within the protection scope of this application.

Claims

1. An inverter, characterized in that, The inverter includes a Heric inverter bridge, a first freewheeling unit, a second freewheeling unit, and a control module. The input terminal of the inverter is used to connect to a DC power supply, and the output terminal of the inverter is used to connect to the power grid. The Heric inverter bridge includes a first high-frequency switch, a second high-frequency switch, a third high-frequency switch, and a fourth high-frequency switch. The first freewheeling unit includes a first power frequency freewheeling diode, and the second freewheeling unit includes a second power frequency freewheeling diode. One end of the first freewheeling unit is connected to the negative terminal of the DC power supply, and the other end of the first freewheeling unit is connected to the first output terminal of the Heric inverter bridge. One end of the second freewheeling unit is connected to the positive terminal of the DC power supply, and the other end of the second freewheeling unit is connected to the second output terminal of the Heric inverter bridge. The control module is used for: During the freewheeling phase of the inverter, the first high-frequency switch, the second high-frequency switch, the third high-frequency switch, and the fourth high-frequency switch are turned off, and the first freewheeling unit or the second freewheeling unit is turned on to form a freewheeling circuit. During the power phase of the inverter, the first high-frequency switch, the second high-frequency switch, the third high-frequency switch, and the fourth high-frequency switch are enabled, and the first freewheeling unit or the second freewheeling unit is enabled to synchronously switch the conduction direction to realize the power transfer of the inverter. The commutation operation of the first freewheeling unit and the second freewheeling unit is synchronized with the voltage zero-crossing point of the power grid; The control module is also used for: When the commutation operation of the first freewheeling unit and the second freewheeling unit is switched, the first power frequency freewheeling tube or the second power frequency freewheeling tube is controlled to be pre-turned off. The pre-turn-off time window is two high-frequency switching cycles. The duration of the freewheeling phase accounts for 40% to 60% of the high-frequency switching cycle. The high-frequency switching period is 50μs, and the pre-turn-off time window is 100μs; The duration of the follow current phase is 20~30μs; The control module is also used to: generate SPWM modulation signals for four high-frequency switching transistors based on the active power command P* and reactive power command Q*, combined with the grid synchronization angular frequency θ, using a direct power control algorithm; The inverter operates in the following scenarios: Scenario 1: Rated power grid-connected operation, with DC bus voltage V DC =550V, under the condition of grid voltage 230V / 50Hz, the grid phase is extracted by the second-order generalized integral phase-locked loop, the fundamental frequency is locked at 50Hz, and a 20kHz SPWM modulation signal is generated according to the active power command P*=5kW and the reactive power command Q*=0. The conduction timing interval of the first power frequency freewheeling tube and the second power frequency freewheeling tube is calculated. Scenario 2: Adaptive operation in reactive power mode. When the system receives a reactive power command, it dynamically adjusts the phase of the SPWM modulation signal and the freewheeling diode delay angle δ. The first or second power frequency freewheeling diode is pre-turned off 100μs before the zero-crossing point. The control module is based on the grid voltage zero-crossing detection and the delay angle. The synchronization timing, δ, reflects the phase lag corresponding to the ratio of reactive power to active power, and the pre-shutdown is implemented 100μs forward from the planned polarity switching instant.

2. The inverter as described in claim 1, characterized in that, The first freewheeling unit further includes a first freewheeling diode, and the second freewheeling unit further includes a second freewheeling diode. The first power frequency freewheeling diode and the first freewheeling diode are connected in reverse parallel and then connected in series between the negative terminal of the DC power supply and the first output terminal of the Heric inverter bridge. The second power frequency freewheeling diode and the second freewheeling diode are connected in reverse parallel and then connected in series between the positive terminal of the DC power supply and the second output terminal of the Heric inverter bridge.

3. The inverter as described in claim 1, characterized in that, The control module is used for: During the freewheeling phase of the inverter and during the positive half-cycle of the grid voltage, the first power frequency freewheeling diode is turned on and the second power frequency freewheeling diode is turned off. During the freewheeling phase of the inverter and in the negative half-cycle of the grid voltage, the second power frequency freewheeling diode is turned on and the first power frequency freewheeling diode is turned off.

4. The inverter as described in claim 3, characterized in that, The control module is used for: During the power phase of the inverter and in the positive half-cycle of the grid voltage, the first high-frequency switch, the fourth high-frequency switch, and the first power frequency freewheeling diode are turned on, or the second high-frequency switch, the third high-frequency switch, and the first power frequency freewheeling diode are turned on. During the power phase of the inverter and in the negative half-cycle of the grid voltage, the first high-frequency switch, the fourth high-frequency switch, and the second power frequency freewheeling diode are turned on, or the second high-frequency switch, the third high-frequency switch, and the second power frequency freewheeling diode are turned on.

5. The inverter as described in claim 1, characterized in that, The inverter also includes a filter circuit, which includes a first DC-side inductor, a second DC-side inductor, a first filter capacitor, a first grid-side inductor, and a second grid-side inductor. The first DC-side inductor and the first grid-side inductor are connected in series between the first output terminal of the Heric inverter bridge and the power grid. The second DC-side inductor and the second grid-side inductor are connected in series between the second output terminal of the Heric inverter bridge and the power grid. The first filter capacitor is connected between the series node of the first DC-side inductor and the first grid-side inductor and the series node of the second DC-side inductor and the second grid-side inductor.

6. The inverter as described in claim 2, characterized in that, The control module is used for: Determine whether the output of the inverter meets the grid connection requirements; if the grid connection requirements are met, control the inverter to complete the grid connection. The grid connection requirements are total harmonic distortion (THD) < 2% and leakage current < 50mA.

7. The inverter as described in claim 2, characterized in that, The control module is also used for: At the start of the freewheeling phase of the inverter, the conduction combination of the first power frequency freewheeling tube and the second power frequency freewheeling tube is determined according to the direction of the output current of the inverter; When the output current direction is positive, the first power frequency freewheeling diode is turned on and the second power frequency freewheeling diode is turned off, and a freewheeling circuit is established using the first freewheeling diode; When the output current direction is negative, the second power frequency freewheeling diode is turned on and the first power frequency freewheeling diode is turned off, and a freewheeling circuit is established using the second freewheeling diode; as well as During the freewheeling phase, the common-mode voltage of the inverter is continuously monitored, and the deviation between the common-mode voltage and half of the DC bus voltage is maintained to be less than or equal to 5V.

8. A control method applied to the inverter according to any one of claims 1 to 7, characterized in that, include: During the freewheeling phase of the inverter, the first high-frequency switch, the second high-frequency switch, the third high-frequency switch, and the fourth high-frequency switch are turned off, and the first freewheeling unit or the second freewheeling unit is turned on to form a freewheeling circuit. During the power phase of the inverter, the first high-frequency switch, the second high-frequency switch, the third high-frequency switch, and the fourth high-frequency switch are enabled, and the first freewheeling unit or the second freewheeling unit is enabled to synchronously switch the conduction direction to realize the power transfer of the inverter. The commutation operation of the first freewheeling unit and the second freewheeling unit is synchronized with the voltage zero-crossing point of the power grid; When the commutation operation of the first freewheeling unit and the second freewheeling unit is switched, the first power frequency freewheeling tube or the second power frequency freewheeling tube is controlled to be pre-turned off. The pre-turn-off time window is two high-frequency switching cycles. The duration of the freewheeling phase accounts for 40% to 60% of the high-frequency switching cycle. The high-frequency switching period is 50μs, and the pre-turn-off time window is 100μs; The duration of the follow current phase is 20~30μs; The control module is also used to: generate SPWM modulation signals for four high-frequency switching transistors based on the active power command P* and reactive power command Q*, combined with the grid synchronization angular frequency θ, using a direct power control algorithm; The inverter operates in the following scenarios: Scenario 1: Rated power grid-connected operation, with DC bus voltage V DC =550V, under the condition of grid voltage 230V / 50Hz, the grid phase is extracted by the second-order generalized integral phase-locked loop, the fundamental frequency is locked at 50Hz, and a 20kHz SPWM modulation signal is generated according to the active power command P*=5kW and the reactive power command Q*=0. The conduction timing interval of the first power frequency freewheeling tube and the second power frequency freewheeling tube is calculated. Scenario 2: Adaptive operation in reactive power mode. When the system receives a reactive power command, it dynamically adjusts the phase of the SPWM modulation signal and the freewheeling diode delay angle δ. The first or second power frequency freewheeling diode is pre-turned off 100μs before the zero-crossing point. The control module is based on the grid voltage zero-crossing detection and the delay angle. The synchronization timing, δ, reflects the phase lag corresponding to the ratio of reactive power to active power, and the pre-shutdown is implemented 100μs forward from the planned polarity switching instant.

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