Fixed-frequency control method and equipment for single-stage dual-active-bridge micro-inverter

Abstract

The invention discloses a fixed frequency control method and equipment for a single-stage dual-active bridge micro-inverter. The single-stage dual-active bridge micro-inverter adopts a half-bridge circuit consisting of two-way switches as a network side bridge; the fixed-frequency control method of the single-stage dual-active-bridge micro-inverter comprises the following two steps: step one, an output current is controlled by using a known control method to obtain a known control method block diagram, and the known control method is that an output current equation can be obtained by taking the output current as a controlled object; the switching frequency is set to be equal to the product of the linear function expression of the phase shift angle and the highest switching frequency, so that a new output current equation is obtained, and the output current and the phase shift angle are in a linear relation; and step 2, updating a frequency phase shift angle module in a known control method block diagram, specifically, enabling the right sides of two output current equations to be equal, and solving a quadratic equation to obtain a new phase shift angle under a fixed frequency set value.

Description

Technical Field

[0001] The present invention relates to the technical field of power electronic converters, and in particular to a fixed-frequency control method and device for a single-stage dual-active-bridge micro-inverter, which is applicable to high-efficiency power conversion scenarios of renewable energy systems and electric vehicles. Background Art

[0002] Traditional single-stage dual-active-bridge micro-inverters have problems such as large current stress and limited efficiency caused by reactive power circulation. Phase-shift modulation strategies often face challenges such as complex non-linear control relationships, high switching losses, and difficulties in designing closed-loop controllers. For example, the non-linear relationship between the output current and the phase-shift angle significantly increases the difficulty of controller design. The article "Highly Efficient Single-Stage DAB Microinverter Using a Novel Modulation Strategy to Minimize Reactive Power" was published in the IEEE Journal of Emerging and Selected Topics in Power Electronics in 2022, and this reference solved the above problems well.

[0003] Of the reference Figure 2 Is a dual-active-bridge (DAB) micro-inverter that uses a half-bridge circuit composed of bidirectional switches as the grid-side bridge. The primary side of the transformer T uses an H-bridge circuit composed of four switching tubes S1 - S4. The grid-side bridge arm uses a half-bridge circuit with bidirectional switches to withstand the AC grid voltage vg. This circuit includes two bidirectional switch groups S5, S6 and S7, S8, and two capacitors C1 and C2; the turns ratio n of the transformer is the secondary turns divided by the primary turns, and the primary and secondary leakage inductances are concentrated and equivalent to the Ll_k parameter on the secondary side of the transformer. The H-bridge switches S1 - S4 operate in a phase-shift mode with a fixed duty cycle of 50%. During the positive half-cycle of the grid voltage, the switching tubes S5 and S7 also perform phase-shift operations with a fixed duty cycle of 50% relative to S3, S4, while the switching tubes S6 and S8 always remain conducting; during the negative half-cycle of the grid voltage, the S6 and S8 switch groups are mainly driven, and at this time, the S5 and S7 switching tubes remain conducting continuously.

[0004] Where, ip is the primary current, is is the secondary current, io is the output current, Vpv represents the output voltage of the photovoltaic panel, Vp is the primary voltage, Vs is the secondary voltage, and Vg is the grid voltage.

[0005] Through derivation, it can be obtained that Io = (n * Vpv / (4 * Llk * fs)) * φ * (1 - m * φ), and this equation is called Equation A1 here. Here, taking the output current as the controlled object, an output current equation can be obtained. The left side of the equation is the output current, and the right side is proportional to the phase shift angle, proportional to a linear function expression of a phase shift angle, and inversely proportional to the switching frequency.

[0006] Among them, m = 2 * (2 * k * k - 2 * k + 1), and this equation is called Equation A2 here; φ_outer = k * φ, and this equation is called Equation A3 here; fs is the switching frequency, φ is the primary side switch phase shift angle, usually called the inner phase shift angle, φ_outer is the switch phase shift angle between the primary and secondary H-bridges, usually called the outer phase shift angle, and k is the phase shift index, which is determined by the zero-voltage switching condition.

[0007] Let: fs(φ) = fs,max * (1 - m * φ), where fs,max is the designed highest switching frequency, and this equation is called Equation A4 here.

[0008] Then: Io = (n * Vpv / (4 * Llk * fs,max)) * φ, and this equation is called Equation A5 here. In Equation A5, Io and φ have a linear relationship.

[0009] Here, by setting the switching frequency to be equal to the product of the above linear function expression of the phase shift angle and the highest switching frequency, a new output current equation is obtained. The left side of the equation is the output current, and the right side is proportional to the phase shift angle and inversely proportional to the highest switching frequency. Since the highest switching frequency is a fixed value, the output current in the new output current equation has a linear relationship with the phase shift angle.

[0010] Thus, a linear regulator such as a PI regulator can be used to make Io track the given current. The input of the PI regulator is the error between the feedback value of Io and the given current, and the output is the phase shift angle φ, realizing the closed-loop regulation of Io.

[0011] Figure 9 of the reference gives the block diagram of the control algorithm, which adopts the combination of feedforward control based on the nominal phase shift and PI regulator feedback control. The feedforward part calculates the nominal phase shift of φ (the value of φ obtained according to Equation A5) according to the instantaneous operating conditions (such as photovoltaic voltage, grid voltage), reducing the burden of feedback control; the feedback part corrects the error through the PI regulator to ensure that the output current accurately tracks the target value.

[0012] Here, by using the linear relationship between the output current and the phase shift angle, the design of the closed-loop controller is simplified, and the accurate tracking of the sine trajectory is realized.

[0013] The maximum value of the phase-shift angle φ is 0.5. The minimum switching frequency fs,min of the switching frequency fs can be obtained from Equation A4, and the parameter value of the minimum switching frequency can be used for the design of the filter.

[0014] The core of the reference lies in proposing an analytical frequency conversion modulation strategy to achieve efficiency improvement and control simplification through the following innovation points. 1. Reactive power minimization: By dynamically adjusting the phase-shift index k, the time when the secondary-side current is opposite to the primary-side voltage direction is minimized, significantly reducing current stress and power loss. Experiments show that this strategy enables the highest efficiency of the 330W prototype to reach 96.87%. 2. Zero-voltage switching (ZVS) guarantee: Optimize the phase-shift angle φ and the switching frequency fs within the switching period to ensure that all switching devices (S1-S8) achieve ZVS turn-on, reducing switching losses. 3. Linearized control: Adopt a variable-frequency control strategy to convert the non-linear relationship between the output current Io and the phase-shift angle φ into a linear relationship, simplifying the design of the closed-loop feedback controller; at the same time, combine feed-forward control to further improve the tracking accuracy. 4. Analytical parameter calculation: The phase-shift angle φ and the switching frequency fs are directly generated in an analytical form without complex iterative calculations, facilitating the implementation on a real-time digital signal processor (DSP).

[0015] However, fixed switching frequency still has many advantages compared to variable frequency.

[0016] Fixed switching frequency can simplify the design of magnetic components such as transformers and inductors, without considering the design difficulties caused by changes in parameters such as magnetic field strength and loss due to changes in switching frequency; fixed switching frequency is also beneficial to the design of EMC (electromagnetic compatibility) filters, enabling the filter to be designed for a specific frequency band, reducing the order and volume of filter inductors and capacitors; fixed switching frequency makes the switching period fixed, facilitating the program design of the DSP. For example, by determining the switching period as the period of the main interrupt of the program, the program control of the DSP can be conveniently realized.

[0017] On the basis of maintaining the advantages of the analytical frequency conversion modulation strategy in the reference, converting variable frequency to fixed frequency is a research topic worthy of study. Summary of the Invention

[0018] This application is made in consideration of the above problems.

[0019] The single-stage dual-active-bridge micro-inverter of this application uses a half-bridge circuit composed of bidirectional switches as the grid-side bridge. The fixed-frequency control method of the single-stage dual-active-bridge micro-inverter includes:

[0020] Step 1: Control the output current using a known control method to obtain a block diagram of the known control method;

[0021] The known control method takes the output current as the controlled object, and an output current equation can be obtained. The left side of the equation is the output current, and the right side is proportional to the phase shift angle, proportional to a linear function expression of a phase shift angle, and inversely proportional to the switching frequency.

[0022] By setting the switching frequency equal to the product of the linear function expression of the above phase shift angle and the highest switching frequency, a new output current equation is obtained. The left side of the equation is the output current, and the right side is proportional to the phase shift angle and inversely proportional to the highest switching frequency. Since the highest switching frequency is a fixed value, the output current in the new output current equation has a linear relationship with the phase shift angle.

[0023] Utilize the linear relationship between the output current and the phase shift angle to design a closed-loop controller and achieve precise tracking of the sine trajectory. The known control method block diagram includes a frequency-phase shift angle module, that is, a module that obtains the switching frequency for generating the switching tube drive signal and the required phase shift angle according to the phase shift angle and the phase shift index.

[0024] Step 2: Update the frequency-phase shift angle module in the known control method block diagram.

[0025] According to the above closed-loop controller, the phase shift angle can be obtained. Substituting this phase shift angle into the above new output current equation, the output current can be obtained.

[0026] At the same time, according to the above output current equation, that is, the left side of the equation is the output current, and the right side is proportional to the phase shift angle, proportional to a linear function expression of a phase shift angle, and inversely proportional to the switching frequency. Let the switching frequency be the fixed-frequency set value, then the phase shift angle in this output current equation is the new phase shift angle at the fixed-frequency set value of the switching frequency.

[0027] And let the right sides of the above new output current equation and the output current equation at the fixed-frequency set value of the switching frequency be equal, solve this quadratic equation, and select the real root that conforms to the variable domain among the two real roots as the new phase shift angle at the fixed-frequency set value of the switching frequency.

[0028] Thus, the switching frequency and the new phase shift angle equal to the fixed-frequency set value can be deduced, and this is the new frequency-phase shift angle module. The so-called update of the frequency-phase shift angle module means replacing the above frequency-phase shift angle module with the new frequency-phase shift angle module.

[0029] The fixed-frequency set value of the switching frequency is the lowest switching frequency.

[0030] The new phase shift angle is the smaller one of the two real roots of the above quadratic equation.

[0031] A computer device includes a memory, a processor, and a computer program stored in the memory and executable on the processor. When the processor executes the computer program, the steps of the fixed-frequency control method for the single-stage dual-active-bridge micro-inverter as described above are implemented.

[0032] The fixed-frequency control method for the single-stage dual-active-bridge micro-inverter of the present application, while retaining the advantages of the analytical frequency modulation strategy of the known control method, mainly retaining the closed-loop controller, converts frequency conversion to fixed frequency; thus, the performance of the single-stage dual-active-bridge micro-inverter is improved.

[0033] The fixed-frequency control method for the single-stage dual-active-bridge micro-inverter of the present application is based on the linearized frequency modulation strategy, and customized parameters for the next switching period are given with the goal of keeping the output current unchanged; then, if the switching frequency of the next period is a certain specified frequency, a new phase-shift angle can be obtained. Therefore, the entire micro-inverter system can still maintain linearized control, but the switching frequency of the next switching period is continuously changed to a new frequency, a fixed-frequency set value; in this way, the switching frequency of the micro-inverter is actually a fixed frequency.

[0034] At the same time, since in the new frequency phase-shift angle module, the phase-shift index k remains unchanged, the advantages of the original frequency modulation strategy, including minimum reactive power, zero-voltage switching (ZVS), etc., are retained. Moreover, all algorithms are directly generated in an analytical form without complex iterative calculations, which is convenient for implementation on a real-time digital signal processor (DSP). Description of the Drawings

[0035] The advantages of the present application will become clearer and more understandable from the following detailed description in conjunction with the drawings, where:

[0036] Figure 1 Schematic diagram of the circuit structure of the dual-active-bridge micro-inverter for reference.

[0037] Figure 2 Voltage waveform diagram of the dual-active-bridge micro-inverter for reference.

[0038] Figure 3 Schematic diagram of the algorithm and control of the dual-active-bridge micro-inverter for reference.

[0039] Figure 4 Block diagram of the new frequency phase-shift angle module of the dual-active-bridge micro-inverter of the present application. Detailed Description of the Embodiment

[0040] The present application will be described in detail below in conjunction with the drawings.

[0041] The reference literature is in English. Two lowercase forms of the capital Greek letter Φ are used to represent two phase-shifting angles respectively. To avoid confusion in this application, the writing forms of these two phase-shifting angles are redefined. Among them, φ is the primary-side switch phase-shifting angle, which is usually called the inner phase-shifting angle, and φ_outer is the switch phase-shifting angle between the primary and secondary H-bridges, which is usually called the outer phase-shifting angle. It can be seen that φ as the inner phase-shifting angle is the same in the reference literature and this application, while the writing form of the outer phase-shifting angle has changed.

[0042] The reference literature of this application is "Highly Efficient Single-Stage DAB Microinverter Using a Novel Modulation Strategy to Minimize Reactive Power" published in IEEE Journal of Emerging and Selected Topics in Power Electronics in 2022.

[0043] Figure 1 It is a schematic circuit diagram of the dual-active-bridge microinverter in the reference literature. The primary side of the transformer T uses an H-bridge circuit composed of four switching tubes S1 - S4. The grid-side bridge arm uses a half-bridge circuit with bidirectional switches to withstand the AC grid voltage vg. This circuit includes two bidirectional switch groups S5, S6 and S7, S8, and two capacitors C1 and C2; the turns ratio n of the transformer is the secondary turns divided by the primary turns, and the primary and secondary leakage inductances are concentrated and equivalent to the LlK parameter on the secondary side of the transformer. The H-bridge switches S1 - S4 operate in the phase-shifting mode with a fixed duty cycle of 50%. During the positive half-cycle of the grid voltage, the switching tubes S5 and S7 also perform phase-shifting operations with a fixed duty cycle of 50% relative to S3, S4, while the switching tubes S6 and S8 always remain conducting; during the negative half-cycle of the grid voltage, the S6 and S8 switch groups are mainly driven, and at this time the S5 and S7 switching tubes remain conducting continuously.

[0044] Among them, ip is the primary-side current, is is the secondary-side current, io is the output current, Vpv represents the output voltage of the photovoltaic panel, Vp is the primary-side voltage, Vs is the secondary-side voltage, and Vg is the grid voltage.

[0045] Figure 2 It is the voltage waveform diagram of the dual-active-bridge microinverter in the reference literature. The upper 4 waveforms in the figure are the driving waveforms of the four switching tubes S1 - S4 on the primary side, and the lower 4 waveforms are the driving waveforms of the four switching tubes S5 - S8 on the secondary side. In the middle of the primary-side driving waveform and the secondary-side driving waveform are the voltage waveforms of the primary side and the secondary side of the transformer.

[0046] The single-stage dual-active-bridge micro-inverter of this application uses a half-bridge circuit composed of bidirectional switches as the grid-side bridge. The fixed-frequency control method of the single-stage dual-active-bridge micro-inverter includes:

[0047] Step 1: Control the output current using a known control method to obtain a block diagram of the known control method;

[0048] The known control method takes the output current as the controlled object, and an output current equation can be obtained. The left side of the equation is the output current, and the right side is proportional to the phase-shift angle, proportional to a first-order function expression of a phase-shift angle, and inversely proportional to the switching frequency; that is, Io = (n * Vpv / (4 * Llk * fs)) * φ * (1 - m * φ). This equation is called Equation A1 here;

[0049] Among them, m = 2 * (2 * k * k - 2 * k + 1). This equation is called Equation A2 here;

[0050] φ_outer = k * φ. This equation is called Equation A3 here;

[0051] fs is the switching frequency, φ is the phase-shift angle of the primary-side switch, usually called the inner phase-shift angle, φ_outer is the phase-shift angle between the two H-bridges on the primary and secondary sides, usually called the outer phase-shift angle, and k is the phase-shift index, which is determined by the zero-voltage switching condition.

[0052] By setting the switching frequency to be equal to the product of the first-order function expression of the above phase-shift angle and the highest switching frequency, that is, let: fs(φ) = fs,max * (1 - m * φ), where fs,max is the designed highest switching frequency. This equation is called Equation A4 here;

[0053] Thus, a new output current equation is obtained. The left side of the equation is the output current, and the right side is proportional to the phase-shift angle and inversely proportional to the highest switching frequency. Since the highest switching frequency is a fixed value, the output current in the new output current equation has a linear relationship with the phase-shift angle; that is: Io = (n * Vpv / (4 * Llk * fs,max)) * φ. This equation is called Equation A5 here;

[0054] In Equation A5, Io and φ have a linear relationship. Therefore, a linear regulator such as a PI regulator can be used to make Io track the given current. The input of the PI regulator is the error between the feedback value of Io and the given current, and the output is the phase-shift angle φ, realizing the closed-loop regulation of Io. That is, using the linear relationship between the output current and the phase-shift angle, a closed-loop controller is designed and the accurate tracking of the sine trajectory is realized.

[0055] Figure 3Schematic diagram of the algorithm and control of a dual-active-bridge micro-inverter for reference. A method combining feedforward control based on nominal phase shift and feedback control with a PI regulator is adopted. The feedforward part calculates the nominal phase shift φ (the value of φ obtained according to Equation A5) based on instantaneous operating conditions (such as PV voltage, grid voltage), reducing the burden on the feedback control; the feedback part corrects the error through a PI regulator to ensure that the output current accurately tracks the target value.

[0056] The maximum value of the phase shift angle φ is 0.5. The minimum switching frequency fs,min of the switching frequency fs can be obtained from Equation A4, and the parameter values of the minimum switching frequency can be used for the design of the filter.

[0057] Step 2: Update the frequency phase shift angle module in the known control method block diagram;

[0058] According to the above closed-loop controller, the phase shift angle can be obtained. Substituting this phase shift angle into the above new output current equation, the output current can be obtained;

[0059] At the same time, according to the above output current equation, that is, the left side of the equation is the output current, and the right side is proportional to the phase shift angle, proportional to a linear function expression of a phase shift angle, and inversely proportional to the switching frequency. Let the switching frequency be a fixed-frequency set value, then the phase shift angle in this output current equation is the new phase shift angle at the fixed-frequency set value of the switching frequency;

[0060] And let the right side of the above new output current equation be equal to the right side of the output current equation at the fixed-frequency set value of the above switching frequency, and we can get

[0061] (n*Vpv / (4*Llk*fs,max))*φ=(n*Vpv / (4*Llk*fs,set))*φnew*(1 - m*φnew), this equation is called Equation A6 here. Further simplifying, we can get

[0062] (fs,set / fs,max)*φ=φnew*(1 - m*φnew), this equation is called Equation A7 here; where fs,set is the fixed-frequency set value, and φnew is the new phase shift angle at the fixed-frequency set value of the switching frequency; the left side of Equation A7 is a known number, and the right side is a quadratic function of φnew. Solving this quadratic equation to get φnew; select the real root that meets the variable domain among the two real roots as the new phase shift angle at the fixed-frequency set value of the switching frequency; thus, the switching frequency equal to the fixed-frequency set value and the new phase shift angle can be deduced, and this is the new frequency phase shift angle module; the so-called update of the frequency phase shift angle module is to replace the above frequency phase shift angle module with the new frequency phase shift angle module.

[0063] The fixed-frequency set value of the switching frequency is the minimum switching frequency, that is: fs,set = fs,min.

[0064] The new phase-shifting angle is the smaller of the two real roots of the quadratic equation, because the maximum value of the phase-shifting angle is 0.5.

[0065] φnew = 0.5 * (1 - sqrt(abs(1 - 4 * m * (fs,min / fs,max) * φ))) / m, and this equation is called Equation A8 here;

[0066] φnew_outer = k * φnew, and this equation is called Equation A9 here;

[0067] Where, φnew is the new internal phase-shifting angle, and φnew_outer is the new external phase-shifting angle.

[0068] Figure 3 is a schematic diagram of the algorithm and control of the dual-active-bridge micro-inverter of the reference, that is: the known control method block diagram; the known control method block diagram includes a frequency phase-shifting angle module, that is, a module that obtains the switching frequency and the required phase-shifting angle for generating the switching tube drive signal according to the phase-shifting angle and the phase-shift index. Figure 3 The input of the block diagram of the frequency phase-shifting angle module in is φ, k, and the switching frequency and the external phase-shifting angle are obtained through calculation.

[0069] Figure 4 is the block diagram of the new frequency phase-shifting angle module of the dual-active-bridge micro-inverter of the present application; the so-called updated frequency phase-shifting angle module is to use Figure 4 the new frequency phase-shifting angle module in to replace Figure 3 the frequency phase-shifting angle module in, the inputs are still φ, k, and the outputs are two new phase-shifting angles φ_new, φ_new_outer; fs,min is the new switching frequency, and the duty cycle D = 0.5 remains unchanged, so that the drive signals of all 8 switching devices (S1 - S8) can be obtained.

Claims

1. A fixed-frequency control method and device for a single-stage dual-active-bridge micro-inverter, wherein the single-stage dual-active-bridge micro-inverter uses a half-bridge circuit composed of bidirectional switches as the grid-side bridge, and is characterized in that, The fixed-frequency control method of the single-stage dual-active-bridge micro-inverter includes: Step 1: Control the output current using a known control method to obtain a block diagram of the known control method; The known control method takes the output current as the controlled object, and an output current equation can be obtained. The left side of the equation is the output current, and the right side is proportional to the phase-shift angle, proportional to a linear function expression of the phase-shift angle, and inversely proportional to the switching frequency; By setting the switching frequency equal to the product of the linear function expression of the phase-shift angle and the highest switching frequency, a new output current equation is obtained. The left side of the equation is the output current, and the right side is proportional to the phase-shift angle and inversely proportional to the highest switching frequency. Since the highest switching frequency is a fixed value, the output current in the new output current equation has a linear relationship with the phase-shift angle; Utilize the linear relationship between the output current and the phase-shift angle to design a closed-loop controller and achieve precise tracking of the sine trajectory. The block diagram of the known control method includes a frequency-phase-shift angle module, that is, a module that obtains the switching frequency for generating the switch tube drive signal and the required phase-shift angle according to the phase-shift angle and the phase-shift index. Step 2: Update the frequency-phase-shift angle module in the block diagram of the known control method; According to the above closed-loop controller, the phase-shift angle can be obtained. Substitute this phase-shift angle into the above new output current equation to obtain the output current; At the same time, according to the above output current equation, that is, the left side of the equation is the output current, and the right side is proportional to the phase-shift angle, proportional to a linear function expression of the phase-shift angle, and inversely proportional to the switching frequency. Let the switching frequency be the fixed-frequency set value, then the phase-shift angle in this output current equation is the new phase-shift angle at the fixed-frequency set value of the switching frequency; And let the right sides of the above new output current equation and the output current equation at the fixed-frequency set value of the switching frequency be equal, solve this quadratic equation, and select the real root that conforms to the variable domain among the two real roots as the new phase-shift angle at the fixed-frequency set value of the switching frequency; Thus, the switching frequency equal to the fixed-frequency set value and the new phase-shift angle can be deduced, which is the new frequency-phase-shift angle module; the so-called update of the frequency-phase-shift angle module means replacing the above frequency-phase-shift angle module with the new frequency-phase-shift angle module.

2. The fixed-frequency control method for a single-stage dual-active-bridge micro-inverter according to claim 1, wherein, The fixed-frequency set value of the switching frequency is the lowest switching frequency.

3. The fixed-frequency control method for a single-stage dual-active-bridge micro-inverter according to claim 1, wherein, The new phase-shift angle is the smaller one of the two real roots of the quadratic equation.

4. A computer device, comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, characterized in that, When the processor executes the computer program, it realizes the steps of the fixed-frequency control method for the single-stage dual-active-bridge micro-inverter as described in any one of claims 1-3.

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