Transient state control method for DAB micro inverter

Abstract

The invention discloses a DAB micro inverter transient state control method, which comprises the following steps that: a controller acquires an external power instruction in real time, determines that a power instruction at a previous moment and a power instruction at a next moment are, and a preset external power instruction abrupt change threshold is; at the moment, judging that the external power instruction is suddenly changed, and entering a transient control mode; in the transient state control mode, the driving duty ratio of the direct current side switching device is kept to be Dm, the adjusting duty ratio Dt is preset, and Dt is smaller than Dm; adjusting the working state of an AC side switching device according to the AC voltage polarity of the AC power supply; reducing the driving duty ratio from Dm to Dt; the duty ratio adjusting state is maintained for n PWM driving periods, after resonant cavity current transient oscillation is suppressed, the driving duty ratio of the switching device corresponding to the alternating current side is recovered to Dm, and the transient control mode is quitted. By adjusting the duty ratio of the alternating current side switching device, the secondary side voltage phase can be directly changed, and the suppression effect opposite to the resonant cavity current transient component is generated.

Description

TECHNICAL FIELD

[0001] The present application relates to a DAB micro-inverter transient control method, belonging to the technical field of inverter control. BACKGROUND

[0002] In the field of photovoltaic inverters, the Dual Active Bridge (DAB) topology is widely used in micro-inverters due to its isolation and high efficiency.

[0003] However, during the actual operation of the DAB topology micro-inverter, when the external power command changes abruptly, the resonant cavity current will produce a large amplitude transient oscillation. If this transient oscillation is not suppressed, it may trigger the overvoltage or overcurrent risk of internal components in the inverter, greatly affecting the safety performance and service life of the equipment.

[0004] In the prior art, although theoretical research has proposed to construct a pulse response with the same frequency and opposite amplitude as the transient component of the resonant cavity current to cancel the transient component and thus suppress the transient oscillation, this method has obvious defects. On the one hand, it is complex and tedious to operate, and requires high computing power of the control system. On the other hand, since the switching frequency and the external phase angle change in real time with the phase of the alternating voltage, the constructed pulse response deviates greatly from the ideal value, making it difficult to be widely applied in practical engineering. Therefore, there is an urgent need for a simple and effective DAB micro-inverter transient control method that is easy to implement in engineering and can reliably suppress transient oscillation. SUMMARY

[0005] The purpose of the present application is to solve the above-mentioned problems of the prior art. In view of the problem that the resonant cavity current of the traditional DAB topology micro-inverter is prone to large amplitude transient oscillation when the external power command changes abruptly, and further causes the risk of overvoltage and overcurrent of components, a DAB micro-inverter transient control method is proposed.

[0006] To achieve the above purpose, the technical solution adopted by the present application is as follows: A DAB micro-inverter transient control method, the topology circuit of the DAB micro-inverter includes a direct current power supply DC, a direct current side filter capacitor Cpv, a direct current side switching device, an isolation transformer T, an equivalent resistor , a resonant inductor , an alternating current side switching device, a resonant capacitor Cr, a grid side filter inductor Lg, and an alternating current power supply AC. The DC power supply DC voltage forms a primary voltage through the direct current side switching device, and the alternating current power supply AC voltage forms a secondary voltage through the alternating current side switching device. The direct current side switching device comprises a switching tube S1, a switching tube S2, a switching tube S3 and a switching tube S4; the alternating current side switching device comprises a switching tube S5, a switching tube S6, a switching tube S7 and a switching tube S8, wherein the switching tube S1 and the switching tube S2 are complementarily driven, the switching tube S3 and the switching tube S4 are complementarily driven, the switching tube S6 and the switching tube S8 are always turned on when the AC voltage is positive, and the switching tube S5 and the switching tube S7 are complementarily turned on; when the AC voltage is negative, the switching tube S5 and the switching tube S7 are always turned on, and the switching tube S6 and the switching tube S8 are complementarily turned on. The controller collects the external power instruction in real time, determines that the power instruction at the last moment is , and the power instruction at the next moment is The preset external power instruction mutation threshold is . When , it is determined that the external power instruction has a mutation, and the transient control mode is entered. In the transient control mode, the driving duty cycle of the direct current side switching device is kept as Dm, and the preset adjustment duty cycle Dt is less than Dm. According to the polarity of the AC power supply AC voltage, the working state of the alternating current side switching device is adjusted: If the AC voltage is in the positive half cycle, the driving duty cycle of the switching tube S5 is reduced from Dm to Dt. If the AC voltage is in the negative half cycle, the driving duty cycle of the switching tube S6 is reduced from Dm to Dt. The duty cycle adjustment state is maintained for n PWM driving periods, and after the transient oscillation of the resonant cavity current is suppressed, the driving duty cycle of the corresponding switching device of the alternating current side is restored to Dm, and the transient control mode is exited.

[0007] Preferably, the setting of the external power instruction mutation threshold is determined according to the rated power of the DAB micro-inverter, the voltage resistance value and the current resistance value of the components.

[0008] Preferably, the driving duty cycle Dm of the direct current side switching device is 0.5. The preset of the adjustment duty cycle Dt is adaptively adjusted according to the resonant cavity impedance parameters, the switching frequency and the power mutation amplitude, and the adjustment principle is that the greater the power mutation amplitude, the smaller the preset value of the duty cycle, and the falling edge of the secondary side voltage needs to be advanced to offset the transient component of the resonant cavity current.

[0009] Preferably, the duty cycle adjustment state is maintained for n PWM driving periods until the resonant cavity current oscillation amplitude is reduced to within ±5% of the steady state value.

[0010] The beneficial effects of the present application mainly include: 1. By adjusting the duty cycle of the AC side switching device, the secondary side voltage phase can be directly changed, and the suppression effect opposite to the transient component of the resonant cavity current is generated. After using this method, the resonant cavity current oscillation amplitude is significantly reduced, the AC output current oscillation is effectively suppressed, and the risk of overvoltage and overcurrent of components is avoided.

[0011] 2. Simple control logic and less resource occupation: without complex external phase angle compensation value calculation and closed loop control, the control target can be achieved only by adjusting the duty cycle, which greatly reduces the calculation burden of the controller and saves the system hardware cost and software development cost.

[0012] 3. Strong adaptability and high engineering degree: the preset value of the duty cycle and the number n of PWM periods can be flexibly adjusted according to the rated power of the inverter, power mutation amplitude and other working conditions, and the control delay is small, which can adapt to fast power mutation scenes and is convenient for mass production and practical application. BRIEF DESCRIPTION OF DRAWINGS

[0013] Other features, objects and advantages of the present application will become more apparent from the following detailed description of non-limiting embodiments made with reference to the accompanying drawings: Figure 1 is a flowchart of a DAB micro-inverter transient control method of the present application.

[0014] Figure 2 is a schematic diagram of the topology circuit of the DAB micro-inverter in the present application.

[0015] Figure 3 is a schematic diagram of the drive waveform and the primary side and secondary side voltage in the present application.

[0016] Figure 4 is a schematic diagram of the equivalent circuit of the topology circuit of the DAB micro-inverter in the present application.

[0017] Figure 5 is a schematic diagram of the transient process drive control of a DAB micro-inverter transient control method of the present application.

[0018] Figure 6 is a schematic diagram of the transient oscillation suppression result of a DAB micro-inverter transient control method of the present application. DETAILED DESCRIPTION

[0019] In order to make the purpose, technical scheme and advantages of the embodiments of the present application more clear, the technical scheme in the embodiments of the present application will be described clearly and completely below with reference to the drawings of the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, not all. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present application.

[0020] The present application will now be described in further detail with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and not intended to limit it. Furthermore, it should be noted that, for ease of description, only the parts relevant to the invention are shown in the accompanying drawings. It should be noted that, unless otherwise specified, the embodiments and features described in the present application can be combined with each other.

[0021] like Figure 2 As shown, the single-stage DAB microinverter topology consists of a DC power supply DC, a DC-side filter capacitor Cpv, DC-side switching devices S1-S4, an isolation transformer T, and an equivalent resistor. Resonant inductor AC-side switching devices S5-S8, resonant capacitor The circuit consists of a grid-side filter inductor Lg and an AC power supply. Here, the DC power supply is the battery, and the AC power supply is the power grid. The battery voltage is converted into the primary voltage via DC-side switching devices S1-S4. The grid voltage is converted into the secondary voltage via AC-side switching devices S5-S8. For DC-side switching devices, S1 and S2 are complementary drives, and S3 and S4 are complementary drives. For AC-side switching devices, when the AC voltage is positive, S6 and S8 are normally on, and S5 and S7 are complementary conductions; when the AC voltage is negative, S5 and S7 are normally on, and S6 and S8 are complementary conductions.

[0022] The operating frequency of the switching transistor drive is angular frequency The cycle is Duty cycle is Define the inward phase angle. The phase difference between transistors S1 and S4, and the outward phase shift angle. Primary voltage With secondary voltage Midpoint phase difference. Assume the instantaneous input voltage value is... The instantaneous value of the output voltage is The transformer turns ratio is The voltage gain is ,and Resonant cavity impedance ,in, For the instantaneous power on the AC side, the switching frequency is... Let the phase angle shift inward. First outward phase angle Second outward phase angle The outer phase angle weighting coefficient is defined as follows: Then the phase angle shifts outward. , .

[0023] Combining Figure 3 , the inner moving phase angle in radian is , the outer moving phase angle in radian is , the inner moving phase angle coefficient is , and the outer moving phase angle coefficient is , the working time sequence of each switch tube can be determined. The turn-on time of the switch tube S1 is 0, the duty ratio is , and the turn-off time is ; S2 and S1 are complementary to turn on; the turn-on time of the switch tube S4 is , the duty ratio is , and the turn-off time is ; S3 and S4 are complementary to turn on; during the positive half cycle of the AC voltage, S6 and S8 are always on; the turn-on time of the switch tube S5 is , the duty ratio is , and the turn-off time is ; S7 and S5 are complementary to turn on.

[0024] Let the external power instruction be , when mutates, the instantaneous power of the AC side mutates, the switching frequency mutates, and the resonant cavity current will produce a large amplitude transient oscillation, which seriously triggers the overvoltage or overcurrent risk of the components and devices, affecting the safety performance of the equipment.

[0025] In combination with the above problems, the transient process caused by the mutation of is analyzed. The equivalent circuit of the DAB topology is shown in Figure 4 , the primary side voltage is , and the secondary side voltage is . Let the voltage difference between the primary side and the secondary side be , , wherein . Let the capacitor voltage be , the inductor voltage be , and the inductor current be . The transient process of the equivalent circuit of the DAB topology can be decomposed into a steady-state component and a transient component, which is expressed as . Among them, , are the steady-state components of the inductor current and the capacitor voltage, respectively; , are the transient components of the inductor current and the capacitor voltage, respectively.

[0026] According to the equivalent circuit of the DAB topology, the steady-state components of the inductor current and the capacitor voltage are derived as , . Let , ,in, Capacitor voltage Amplitude, Inductor current Amplitude, Capacitor voltage Inductor current Phase.

[0027] Assuming the steady-state resonant cavity current before the power surge is: After the power surge, the steady-state resonant cavity current is ,and , .in, , , , These are the switching frequencies at the moment before the power surge and the switching frequencies at the moment after the power surge, respectively.

[0028] Assuming the steady-state resonant capacitor voltage before the power surge is: After the power surge, the steady-state resonant capacitor voltage is ,and , .in, , .

[0029] definition ,in This refers to the moment when a power surge occurs. Based on the DAB topological equivalent circuit, it can be derived that... At that time, the transient component of the resonant cavity current ,in, , , , , ;when hour, ,when hour, , This is the damped oscillation frequency. Therefore, after a power surge, the transient process resonant cavity current... .

[0030] When the external power command changes abruptly, the resonant cavity current will generate a large-amplitude transient oscillation, which may trigger overvoltage or overcurrent risks in components and affect the safety performance of the equipment.

[0031] Theoretical research shows that, based on the above analysis of transient oscillation problems and mathematical derivation of transient components, an external phase shift compensation value can be constructed. For the original outward shift phase angle Compensation was performed, and ultimately an outward phase shift was adopted. for Used for switching transistor drive control, thereby controlling the secondary voltage. This generates transient components of the resonant cavity current. Pulse responses with the same frequency but opposite amplitudes cancel out transient components to zero, thereby suppressing transient oscillations.

[0032] However, the outward phase angle compensation value The solution process is complex and cumbersome, and consumes a lot of controller resources, making it difficult to apply in practical engineering. Therefore, this invention provides a simple and effective transient control method that suppresses transient oscillations by adjusting the duty cycle of the switching transistor.

[0033] This invention provides a transient control method for a DAB microinverter. The topology of the DAB microinverter includes a DC power supply DC, a DC-side filter capacitor Cpv, DC-side switching devices, an isolation transformer T, and an equivalent resistor. Resonant inductor AC-side switching devices, resonant capacitor Cr, grid-side filter inductor Lg, AC power supply; The DC voltage from the DC power supply is used to form the primary voltage via DC-side switching devices. The AC power supply voltage is used to form the secondary voltage through the AC-side switching devices. ; The DC-side switching devices include switching transistors S1, S2, S3, and S4; the AC-side switching devices include switching transistors S5, S6, S7, and S8. Switches S1 and S2 are complementary drives, as are switches S3 and S4. When the AC voltage is positive, switches S6 and S8 are normally on, and switches S5 and S7 are complementary conductions. When the AC voltage is negative, switches S5 and S7 are normally on, and switches S6 and S8 are complementary conductions.

[0034] like Figure 1 As shown, the controller collects external power commands in real time and determines the power command from the previous moment. The power command for the next moment is The preset external power command mutation threshold is ; when When a sudden change in external power command is detected, the system enters transient control mode. In transient control mode, the drive duty cycle of the DC-side switching device is kept at Dm, and the preset adjustment duty cycle is Dt, where Dt < Dm; Adjust the operating state of the AC-side switching devices according to the polarity of the AC power supply voltage: If the AC voltage is in the positive half-cycle, reduce the drive duty cycle of the switching transistor S5 from Dm to Dt; If the AC voltage is in the negative half-cycle, reduce the drive duty cycle of the switching transistor S6 from Dm to Dt; The duty cycle adjustment state is maintained for n PWM drive cycles. After the transient oscillation of the resonant cavity current is suppressed, the drive duty cycle of the corresponding switching device on the AC side is restored to Dm, and the transient control mode is exited.

[0035] That is, when there is a sudden change in external power, the secondary voltage can be controlled by adjusting the duty cycle of the AC-side switching transistor. This allows for control of the resonant cavity current during transient processes. By reducing the driving duty cycle, transient oscillations of the resonant cavity current can be suppressed.

[0036] In one specific embodiment, the external power command sudden change threshold The settings are determined based on the rated power, component withstand voltage, and withstand current of the DAB micro-inverter. The drive duty cycle of the DC-side switching devices is Dm = 0.5. The preset duty cycle Dt is adjusted adaptively based on the resonant cavity impedance parameters, switching frequency, and power fluctuation amplitude. The adjustment principle is: the larger the power fluctuation amplitude, the smaller the preset duty cycle value, and the secondary voltage fall-off advance must be able to offset the transient component of the resonant cavity current.

[0037] Combination Figure 5 To illustrate, taking the transient process occurring during the positive half-cycle of the AC voltage as an example, transistors S1 and S2 conduct complementaryly, S3 and S4 conduct complementaryly, S6 and S8 are normally on, and S5 and S7 conduct complementaryly. Before the external power command abruptly changes, the duty cycle of transistors S1-S8 is 0.5. After the external power command abruptly changes, during the transient process, the duty cycle of transistors S1-S4 remains at 0.5, S6 and S8 are normally on, and S7 and S5 conduct complementaryly. By reducing the duty cycle of transistor S5 from 0.5 to 0.2 and continuing this reduction for n PWM drive cycles (n should be appropriately selected based on the actual operating conditions), the secondary voltage can be reduced. The falling edge is brought forward, effectively reducing the primary voltage. With secondary voltage Midpoint phase difference, i.e., outward phase shift angle This reduces the resonant cavity current and suppresses transient oscillations in the resonant cavity current. After the transient oscillations are effectively suppressed, the duty cycle of transistor S5 is restored to 0.5.

[0038] In one specific embodiment, the duty cycle adjustment state is maintained for n PWM drive cycles until the resonant cavity current oscillation amplitude drops to within ±5% of the steady-state value. The adjustment n can be set as desired based on the steady-state value.

[0039] like Figure 6 As shown, at 0.026s, the power command changes abruptly, with the active power increasing abruptly from 100W to 500W, and the switching frequency... Sudden change, resonant cavity inductor current Transient oscillations occur, and the AC output current also experiences transient oscillations. When a power surge occurs and the transient oscillations are not suppressed (i.e., the duty cycle of the S5 transistor remains at 0.5), the transient oscillation amplitude of the resonant cavity current is relatively large. If, after the power surge, the duty cycle of the S5 transistor is reduced to 0.2 and maintained for n PWM cycles (where n is 60, which can be selected according to the actual operating conditions), and then the duty cycle of the S5 transistor returns to 0.5, the transient oscillation amplitudes of the resonant cavity current and the AC output current are significantly reduced. This indicates that the proposed control method can effectively suppress transient oscillations, thereby avoiding overvoltage and overcurrent of components and improving the safety performance of the equipment.

[0040] As described above, adjusting the duty cycle of the AC-side switching devices directly alters the secondary voltage phase, generating a suppression effect opposite to the transient component of the resonant cavity current. Using this method significantly reduces the amplitude of the resonant cavity current oscillation, effectively suppressing AC output current oscillation and avoiding the risks of overvoltage and overcurrent in components. The control logic is simple and resource-efficient: No complex external phase angle compensation calculations or closed-loop control are required; the control objective can be achieved simply by adjusting the duty cycle, greatly reducing the controller's computational burden and saving system hardware and software development costs. It is highly adaptable and engineering-grade: the preset duty cycle value and the number of PWM cycles (n) can be flexibly adjusted according to the inverter's rated power, power surge amplitude, and other operating conditions, with minimal control delay, adapting to rapid power surge scenarios and facilitating mass production and practical applications.

[0041] The term "comprising" or any other similar term is intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus / device that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent in such process, method, article, or apparatus / device.

[0042] The technical solution of the present invention has been described above with reference to the preferred embodiments shown in the accompanying drawings. However, it will be readily understood by those skilled in the art that the scope of protection of the present invention is obviously not limited to these specific embodiments. Without departing from the principles of the present invention, those skilled in the art can make equivalent changes or substitutions to the relevant technical features, and the technical solutions after such changes or substitutions will all fall within the scope of protection of the present invention.

Claims

1. A transient control method for a DAB microinverter, wherein the topology of the DAB microinverter includes a DC power supply DC, a DC-side filter capacitor Cpv, DC-side switching devices, an isolation transformer T, and an equivalent resistor. Resonant inductor AC-side switching devices, resonant capacitor Cr, grid-side filter inductor Lg, AC power supply; The DC voltage from the DC power supply is used to form the primary voltage via DC-side switching devices. The AC power supply voltage is used to form the secondary voltage through the AC-side switching devices. ; The DC-side switching devices include switching transistors S1, S2, S3, and S4; the AC-side switching devices include switching transistors S5, S6, S7, and S8, wherein... Switches S1 and S2 are complementary drives, and switches S3 and S4 are complementary drives. When the AC voltage is positive, switches S6 and S8 are normally on, and switches S5 and S7 are complementary conductions. When the AC voltage is negative, switches S5 and S7 are normally on, and switches S6 and S8 are complementary conductions. Its features are: The controller collects external power commands in real time and determines the power command from the previous moment. The power command for the next moment is The preset external power command mutation threshold is ; when When a sudden change in external power command is detected, the system enters transient control mode. In transient control mode, the drive duty cycle of the DC-side switching device is kept at Dm, and the preset adjustment duty cycle is Dt, where Dt < Dm; Adjust the operating state of the AC-side switching devices according to the polarity of the AC power supply voltage: If the AC voltage is in the positive half-cycle, reduce the drive duty cycle of the switching transistor S5 from Dm to Dt; If the AC voltage is in the negative half-cycle, reduce the drive duty cycle of the switching transistor S6 from Dm to Dt; The duty cycle adjustment state is maintained for n PWM drive cycles. After the transient oscillation of the resonant cavity current is suppressed, the drive duty cycle of the corresponding switching device on the AC side is restored to Dm, and the transient control mode is exited.

2. The transient control method for a DAB micro-inverter according to claim 1, characterized in that: The external power command sudden change threshold The settings are determined based on the rated power, component withstand voltage, and withstand current of the DAB micro inverter.

3. The transient control method for a DAB micro-inverter according to claim 1, characterized in that: The drive duty cycle of the DC-side switching device is Dm = 0.5; The preset duty cycle Dt is adjusted adaptively based on the resonant cavity impedance parameters, switching frequency, and power fluctuation amplitude. The adjustment principle is: the larger the power fluctuation amplitude, the smaller the preset duty cycle value, and the secondary voltage fall-off advance must be able to offset the transient component of the resonant cavity current.

4. The transient control method for a DAB micro-inverter according to claim 1, characterized in that: The duty cycle adjustment state is maintained for n PWM drive cycles until the resonant cavity current oscillation amplitude drops to within ±5% of the steady-state value.

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