A control method for an ac / dc converter and a resonant ac / dc converter

Abstract

This application discloses a control method for an AC / DC converter and a resonant AC / DC converter, relating to the field of power electronics technology. The control method for the AC / DC converter includes the following steps: real-time acquisition of grid voltage, DC / DC converter output power, DC / AC converter output power, DC bus voltage, and grid-connected current to perform DC bus voltage ripple prediction calculation; frequency domain analysis of the grid-connected current signal based on discrete Fourier transform to extract odd harmonic components; conversion of odd harmonic components into even harmonic components; generation of a feedback signal based on the even harmonic components to correct the DC bus voltage ripple to eliminate odd harmonic components; and generation of a feedback signal based on the corrected DC bus voltage ripple to suppress the influence of DC bus voltage ripple on the control loop. This control strategy achieves grid-connected current suppression without adding unnecessary hardware circuitry, reducing cost and complexity.

Description

Technical Field

[0001] This application relates to the field of power electronics technology, and in particular to a control method for AC / DC converters and a resonant AC / DC converter. Background Technology

[0002] With the rapid development of distributed generation, AC / DC microgrids, and V2G (Vehicle-to-grid) systems, traditional two-stage bidirectional AC / DC converters typically use large-capacity electrolytic capacitors as intermediate bus capacitors. However, the short lifespan of electrolytic capacitors limits the overall lifespan of the converter.

[0003] Existing technologies use film capacitors instead of electrolytic capacitors to improve the lifespan of the converter; however, replacing electrolytic capacitors with film capacitors inevitably reduces the capacitance. This leads to a larger second-order harmonic component on the DC bus due to instantaneous power imbalance, affecting the AC / DC converter's control system. To suppress the impact of the second-order harmonic component in the DC bus voltage on the two-stage converter control system, existing technologies mainly propose some effective methods from the following two perspectives:

[0004] 1. Employ power decoupling technology to balance input and output power;

[0005] 2. Suppress the influence of double power frequency harmonic components on the two-stage converter from a control perspective.

[0006] Power decoupling techniques include pulsating port decoupling and second harmonic current compensators (SHCCs).

[0007] Pulsating port decoupling eliminates the impact of second-order power frequency harmonic components caused by power imbalance on the preceding and following converters by introducing a pulsating port to balance the instantaneous power difference between the input and output. However, this approach requires additional hardware support, increasing system complexity and cost. It is also closely related to the topology of the AC / DC converter and cannot be applied to all types of AC / DC converters.

[0008] The second harmonic current compensator absorbs or releases unbalanced power by connecting a second harmonic current compensator (SHCC) in parallel to the DC bus, thereby achieving power decoupling; however, the SHCC itself is difficult to implement soft switching operation, which limits the improvement of efficiency.

[0009] Control strategy optimization includes low-pass filter (LPF), feedforward control, notch filter control, and virtual impedance method.

[0010] LPF is used to suppress the second harmonic component of the DC bus voltage and reduce its impact on the control system; however, since the second harmonic frequency is low, the outer loop bandwidth of the DC bus voltage control is low, resulting in poor dynamic response capability of the system.

[0011] Feedforward control can improve the dynamic response performance of the system, but the optimal feedforward gain needs to be adjusted according to the changes in load or grid voltage, which increases the complexity of control parameter design and cannot fundamentally change the low bandwidth characteristics of the DC bus voltage outer loop.

[0012] Methods such as notch filters and series-parallel virtual impedance can be used to enhance the suppression of harmonics of the second power frequency voltage and improve the dynamic response performance of the system. However, they can affect the stability of the system, leading to additional oscillations or other instabilities. Furthermore, they require complex calculations or additional sensors, which reduces the power density and practicality of the system. Summary of the Invention

[0013] To address the aforementioned issues, this application discloses a control method for AC / DC converters, which improves the dynamic response performance of the converter and suppresses grid-connected current through control; it also eliminates the need for additional hardware circuitry, simplifying the control process; and proposes a resonant AC / DC converter to implement control methods for AC / DC converters under different conditions.

[0014] The first technical solution adopted in this application is: providing a control method for an AC / DC converter, including the following steps:

[0015] Real-time acquisition of grid voltage, DC / DC converter output power, DC / AC converter output power, DC bus voltage, and grid-connected current;

[0016] The DC bus voltage ripple prediction calculation is completed using the grid voltage, the grid-connected current, the DC / DC converter output power, the DC / AC converter output power, and the DC bus voltage input ripple prediction model; the odd harmonic components are extracted by frequency domain analysis of the grid-connected current signal based on discrete Fourier transform; and the odd harmonic components are converted into even harmonic components.

[0017] The even-order harmonic components are used to generate a feedback signal to correct the DC bus voltage ripple and eliminate the odd-order harmonic components; the corrected DC bus voltage ripple is then used to generate a feedback signal to suppress the influence of the DC bus voltage ripple on the control loop.

[0018] This also includes optimizing the average DC bus voltage based on the expression for DC bus voltage ripple, including the following steps:

[0019] The maximum and minimum values ​​of the DC bus voltage are obtained based on the expression for the DC bus voltage ripple.

[0020] Obtain the relative relationship between the average DC bus voltage and the maximum and minimum DC bus voltage;

[0021] The average DC bus voltage is adjusted based on the relative relationship to optimize the average DC bus voltage.

[0022] The DC bus voltage is positively correlated with the switching frequency of the AC / DC converter; the minimum DC bus voltage is obtained based on the set minimum switching frequency; and the average DC bus voltage under different power levels is obtained based on the minimum DC bus voltage.

[0023] Specifically, a feedback signal that is in phase with the DC bus voltage ripple is generated based on the even harmonic components, thereby canceling the even harmonic components in the DC bus voltage and suppressing the harmonic coupling phenomenon of the grid-connected current.

[0024] The even-order harmonic components generate a feedback signal that is in phase with the DC bus voltage ripple based on a phase adjustment factor.

[0025] This also includes a method for compensation based on capacitance value deviation, comprising the following steps:

[0026] The actual peak value of the DC bus voltage is monitored in real time, and the actual peak value is compared with the theoretically calculated peak value.

[0027] The parameters in the DC bus voltage ripple prediction model are adjusted based on the capacitance deviation coefficient, and a feedback signal is generated based on the adjusted prediction model to correct the DC bus voltage ripple.

[0028] The second technical solution adopted in this application is: a resonant AC / DC converter that applies the control method for AC / DC converters as described in any of the above claims, including a control system and a switching transistor assembly; the control system receives a feedback signal and then outputs a PWM signal to adjust the conduction state of the switching transistor assembly.

[0029] The third technical solution adopted in this application is: a resonant AC / DC converter is provided, including a DC / DC converter, a DC / AC converter and a DC bus capacitor;

[0030] The DC / DC converter includes an H-bridge, a transformer, a resonant inductor, and a half-bridge topology. The half-bridge topology includes a first half-bridge resonant capacitor and a second half-bridge resonant capacitor. The power flow is controlled by switching the H-bridge on and off. Voltage level conversion and isolation circuitry are implemented based on the transformer. Soft switching operation is achieved based on the resonance between the resonant inductor and the first and second half-bridge resonant capacitors.

[0031] The DC / AC converter includes a first switch, a second switch, a third switch, and a fourth switch; the first switch and the second switch are the high-frequency bridge arms of the DC / AC converter; the positive and negative half-cycle power frequency switching is realized based on the third switch and the fourth switch; the DC / AC converter realizes zero-voltage conduction of the high-frequency switch based on critical current modulation;

[0032] The DC bus capacitor is connected in parallel with the DC / DC converter and the DC / AC converter, and power decoupling is achieved based on the DC bus capacitor.

[0033] The DC / AC converter includes an AC power supply; a first terminal of the first switch is connected to a first terminal of the third switch; a second terminal of the first switch is connected to a first terminal of the second switch; a second terminal of the second switch is connected to a second terminal of the fourth switch; a first terminal of the fourth switch is connected to a second terminal of the third switch; one end of the AC power supply is connected to the second terminals of the first and third switches; the other end of the AC power supply is connected to the first terminal of the fourth switch; control terminals of the first, second, third, and fourth switches are connected to a control unit; the switching on and off is controlled by adjusting the potential of the control terminals based on the control unit.

[0034] The DC / AC converter further includes a capacitor, a first inductor, and a second inductor; one end of the first inductor is connected to one end of the second inductor; the other end of the first inductor is connected to an AC power supply; the other end of the second inductor is connected to the second end of the first switching transistor; one end of the capacitor is connected to the side of the first inductor closest to the second inductor; the other end of the capacitor is connected to the other end of the AC power supply; the first ends of the first switching transistor and the first ends of the third switching transistor are connected to one end of the DC bus capacitor, and the second ends of the second switching transistor and the second ends of the fourth switching transistor are connected to the other end of the DC bus capacitor.

[0035] The half-bridge topology further includes a fifth switch and a sixth switch; the second end of the fifth switch is connected to the first end of the sixth switch; the first half-bridge resonant capacitor and the second half-bridge resonant capacitor are connected; the first end of the fifth switch is connected to the side of the first half-bridge resonant capacitor away from the second half-bridge resonant capacitor; the second end of the sixth switch is connected to the side of the second half-bridge resonant capacitor away from the first half-bridge resonant capacitor; the second end of the fifth switch is connected to one end of the secondary side of the transformer through the resonant inductor; the other end of the secondary side of the transformer is connected to the first end of the sixth switch and the side of the second half-bridge resonant capacitor closest to the first half-bridge resonant capacitor.

[0036] The DC / DC converter includes a DC power supply; the H-bridge includes a seventh, eighth, ninth, and tenth switch transistors; the first terminal of the seventh switch transistor is connected to the first terminal of the ninth switch transistor; the second terminal of the seventh switch transistor is connected to the first terminal of the eighth switch transistor; the second terminal of the eighth switch transistor is connected to the second terminal of the tenth switch transistor; the first terminal of the tenth switch transistor is connected to the second terminal of the ninth switch transistor; the positive terminal of the DC power supply is connected to the first terminals of the seventh and ninth switch transistors; the negative terminal of the DC power supply is connected to the second terminals of the eighth and tenth switch transistors; the control terminals of the seventh, eighth, ninth, and tenth switch transistors are connected to a control unit; the control unit adjusts the potential of the control terminals to control the switching transistors' on / off states.

[0037] The DC bus capacitor is a thin-film capacitor; the lifespan of the resonant AC / DC converter is improved based on the thin-film capacitor.

[0038] Due to the adoption of the above technical solution, this application has at least one of the following beneficial effects compared with the prior art:

[0039] 1. Frequency domain analysis using discrete Fourier transform can quickly and accurately extract various harmonic components, improving dynamic response speed.

[0040] 2. The grid-connected current is suppressed based on the control strategy, eliminating the need to add extra hardware circuits, thus reducing cost and complexity.

[0041] 3. It is not dependent on a specific converter topology and can be applied in different types of AC / DC converters, exhibiting strong versatility and adaptability.

[0042] 4. Based on the critical current modulation mode, the AC / DC converter can achieve zero-voltage conduction on the high-frequency switching transistor, which greatly reduces switching losses, improves efficiency, and extends the service life of the switching devices.

[0043] 5. Replacing traditional electrolytic capacitors with thin-film capacitors can significantly improve the service life of resonant AC / DC converters. Attached Figure Description

[0044] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments 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. Wherein:

[0045] Figure 1 This is a schematic diagram showing the failure distribution of various components in an AC / DC converter.

[0046] Figure 2 A flowchart illustrating an embodiment of the control method for an AC / DC converter provided in this application;

[0047] Figure 3 This is a schematic diagram showing the curves of grid voltage, grid-connected current, DC / DC converter output power versus DC / AC converter output power and DC bus voltage.

[0048] Figure 4 A schematic diagram of a current harmonic suppression control scheme according to an embodiment of this application;

[0049] Figure 5 This is a schematic diagram of the framework of an embodiment of the DFT algorithm provided in this application;

[0050] Figure 6 A schematic diagram of the framework for generating DC bus voltage harmonic feedback signals provided in this application;

[0051] Figure 7 A schematic diagram of the current harmonic feedback control scheme provided in this application;

[0052] Figure 8 A schematic diagram of a DC bus voltage mean optimization control framework based on DC bus minimum limit provided in an embodiment of this application;

[0053] Figure 9 This is a schematic diagram illustrating the relationship between DC bus voltage and switching frequency provided in an embodiment of this application;

[0054] Figure 10 This is a schematic diagram showing the relationship between the capacitance decay of a thin-film capacitor and operating temperature and time.

[0055] Figure 11 A waveform diagram showing the tolerance of the DC bus capacitor;

[0056] Figure 12 This is a schematic diagram of an embodiment of the resonant AC / DC converter provided in this application. Detailed Implementation

[0057] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. It is understood that the specific embodiments described herein are only for explaining this application and not for limiting it. Furthermore, it should be noted that, for ease of description, only the parts related to this application are shown in the accompanying drawings, not all structures. All other embodiments obtained by those skilled in the art based on the embodiments of this application without creative effort are within the scope of protection of this application.

[0058] The terms "first," "second," etc., used in this application are used to distinguish different objects, not to describe a specific order. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion. For example, a process, method, system, product, or apparatus that includes a series of steps or units is not limited to the listed steps or units, but may optionally include steps or units not listed, or may optionally include other steps or units inherent to these processes, methods, products, or apparatuses.

[0059] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0060] AC / DC converters are widely used in many fields, especially suitable for scenarios requiring AC and DC conversion. Specific applications include, but are not limited to, photovoltaic micro-inverters and energy storage inverters. In solar photovoltaic power generation systems, AC / DC converters are used to convert the DC power generated by photovoltaic panels into AC power for grid compatibility or household use. Energy storage inverters are used in battery energy storage systems to convert AC power from the grid into DC power for storage in batteries, or to convert DC power from batteries back into AC power for use.

[0061] Electrolytic capacitors are widely used in AC / DC converters due to their low price, small size, and large capacitance. However, their short lifespan, poor stability, and inability to withstand high temperatures severely limit the lifespan of AC / DC converters. Figure 1 As shown, Figure 1This diagram illustrates the failure distribution of various components in an AC / DC converter. Among failed AC / DC converters, electrolytic capacitor failure accounts for 60% of the failures. To improve the lifespan of AC / DC converters, electrolytic capacitors become an unavoidable topic of discussion. The table below compares the performance of various types of capacitors.

[0062] Capacitor type capacity Withstand pressure High frequency characteristics Features Electrolytic capacitors Very large big Very bad Polarity, poor precision ceramic capacitors Small Small very good Nonpolarity Thin film capacitors smaller Very large good Nonpolarity

[0063] For the same capacitance, non-electrolytic capacitors are significantly larger than electrolytic capacitors. Therefore, eliminating electrolytic capacitors requires reducing the capacitance of the intermediate DC bus decoupling capacitor, allowing the use of film capacitors instead of electrolytic capacitors to improve the converter's lifespan and reduce the failure rate.

[0064] If film capacitors are used instead of electrolytic capacitors, the capacitance will inevitably decrease. This leads to a larger second-order harmonic component on the DC bus due to instantaneous power imbalance, affecting the AC / DC converter system. Existing technologies employ power decoupling techniques or control methods to suppress the impact of the second-order harmonic component on the AC / DC converter. However, both power decoupling and control methods have significant drawbacks. Therefore, this application discloses a control method for AC / DC converters that improves the converter's dynamic response performance and suppresses grid-connected current. Furthermore, it eliminates the need for additional hardware circuitry, simplifying the control process. Figure 2 As shown, Figure 2 A flowchart illustrating an embodiment of the control method for an AC / DC converter provided in this application includes the following steps:

[0065] Step S11: Real-time acquisition of grid voltage, DC / DC converter output power, DC / AC converter output power, DC bus voltage, and grid-connected current; such as Figure 3 As shown, Figure 3 This is a schematic diagram showing the curves of grid voltage, grid-connected current, DC / DC converter output power versus DC / AC converter output power and DC bus voltage; where... This is the grid voltage. For grid-connected current, For the output power of the DC / DC converter, For the output power of the DC / AC converter, This represents the maximum DC bus voltage. This is the minimum DC bus voltage. This is the average DC bus voltage. This represents the period of the grid voltage.

[0066] [0, 1 / 8] ]time, > Since the power absorbed by the DC / AC converter is less than the output power of the DC / DC converter, the excess power flows into the DC bus to charge the DC bus capacitor, causing the DC bus capacitor voltage to rise until it reaches 1 / 8 of its rated value. At this moment, the instantaneous power is equal, and the DC bus capacitor voltage is... It rises to the amplitude.

[0067] [1 / 8 1 / 4 ]time, < At this time, the power absorbed by the DC / AC converter is greater than the output power of the DC / DC converter. Therefore, in order to ensure the transmission characteristics of the output current, the DC / AC converter will still absorb energy in the DC bus capacitor, and thus discharge the DC bus capacitor.

[0068] [1 / 4 3 / 8 ]time, < At this point, the power absorbed by the DC / AC converter is still greater than the output power of the DC / DC converter, and the DC bus capacitor discharges; at 3 / 8 At that moment, the energy in the DC bus capacitor is finally discharged, and the DC bus capacitor voltage is... It reaches the minimum value.

[0069] [3 / 8 1 / 2 ]time, > At this point, the power absorbed by the DC / AC converter is less than the output power of the DC / DC converter. Therefore, the excess power is used to recharge the DC bus capacitor.

[0070] The DC bus capacitor acts as an energy storage medium. When the output power of the DC / DC converter exceeds that of the DC / AC converter, it stores the excess energy; when the output power of the DC / DC converter is less than that of the DC / AC converter, it releases its own energy. Power decoupling is achieved by absorbing the instantaneous power difference between the DC / DC converter and the DC / AC converter using the DC bus capacitor.

[0071] Compared to existing power balancing solutions that use power decoupling circuits, directly using DC bus capacitors for power decoupling simplifies the topology and reduces hardware costs and space requirements.

[0072] Step S12: Complete the DC bus voltage ripple prediction calculation using the grid voltage, grid-connected current, DC / DC converter output power, DC / AC converter output power, and DC bus voltage input ripple prediction model; the calculation principle of the DC bus voltage ripple prediction model is described in detail below:

[0073] DC bus capacitance at 1 / 4 The instantaneous power absorbed within a time period is given by the following equation (1):

[0074] .

[0075] Under steady-state conditions, the instantaneous power on the DC side is equal to half the peak power on the AC side, from which we can obtain equation (2):

[0076] .

[0077] at the same time, As shown in equation (3):

[0078] .

[0079] In equation (3), , These represent the DC bus voltage and the bus capacitance current, respectively. Expressed as the average DC bus voltage. This represents the DC bus voltage ripple. (Sampling) The DC bus voltage ripple is calculated directly, as shown in equation (4):

[0080] .

[0081] As shown in equation (4) above, the DC bus voltage ripple is related to the input and output power of the DC / DC converter and the DC / AC converter, the average DC bus voltage, the DC bus capacitance, etc. When the DC bus capacitance remains unchanged, the DC bus voltage ripple can be adjusted by adjusting the conduction state of the switching transistor through the output PWM signal of the control system, thereby improving the dynamic response capability of the system.

[0082] With explicit mathematical expressions, the amplitude and phase of ripple voltage can be directly calculated, thereby accurately evaluating the dynamic performance of the system. Based on the analytical expression of ripple voltage, the capacitor value or DC bus voltage can be adjusted to reduce the ripple amplitude and improve system stability.

[0083] like Figure 4 As shown, Figure 4This application provides a schematic diagram of a current harmonic suppression control scheme according to an embodiment. The principle of extracting feedback signals using grid-connected current harmonics is to extract the harmonic components in the AC side grid-connected current and generate feedback signals with opposite even-order DC bus voltage ripple components based on the extracted harmonic components. Feedback is sent to the DC bus voltage averaging control; feedback signal As shown in equation (5):

[0084] {v}_{k}\left ( {t} \right )=\sum _{k\in {N}^{+}} {{A}_{c\left ( {2k} \right )}}sin\left [ {\left ( {2k} \right ){w}_{ac}t+{\psi}_{2k}} \right ] .

[0085] In equation (5), The amplitude of the even-order DC bus voltage ripple component is given. The initial phase angle of the even-order DC bus voltage ripple component; based on control loop analysis, the signal... Convert to frequency domain signal analysis The equivalent is the following formula (6):

[0086] .

[0087] After passing through the outer loop control stage of the DC bus voltage, the final feedback signal of the DC bus voltage control is as follows (7):

[0088] .

[0089] In equation (7), The amplitude of the feedback signal, The gain of the PI controller in the outer loop system of the DC bus voltage. This represents the open-loop gain of the DC bus voltage control system.

[0090] The even-order DC bus voltage ripple component on the DC bus voltage, after being adjusted by the outer loop of the DC bus voltage, ultimately generates a harmonic extraction feedback signal. Then, after the harmonic extraction feedback signal and the even-order DC bus voltage ripple signal enter the current loop, the inner current loop is set to be equivalent to... After introducing the harmonic feedback signal, the odd harmonic components in the grid-connected current are as follows (8):

[0091] .

[0092] Substituting equation (7) into equation (8), we obtain equation (9):

[0093] .

[0094] Simplifying the above equation and converting it to the time domain, we get the following equation (10):

[0095] .

[0096] Equation (10) shows that if the feedback signal gain of the even-order DC bus voltage ripple in the DC bus voltage is infinitely large, the coupling between the even-order DC bus voltage ripple and the grid-connected current harmonics can be suppressed, ultimately eliminating the odd-order harmonic components on the AC side. Therefore, by adjusting the gain of the feedback signal, the content of odd-order harmonic components in the AC side grid-connected current can be further reduced. Finally, based on the cyclic coupling process between the even-order DC bus voltage ripple and the AC side grid-connected current in the power loop and control loop, further suppression of the even-order DC bus voltage ripple component in the DC bus voltage can be achieved.

[0097] This application employs a high-steady-state-accuracy Discrete Fourier Transform (DFT) algorithm for harmonic extraction, such as... Figure 5 As shown, Figure 5 This is a schematic diagram of the framework of an embodiment of the DFT algorithm provided in this application. The calculation process of the DFT algorithm in this embodiment is described in detail below:

[0098] set up as one For a series of vectors in a coordinate system, the vector DFT and its k-th inverse transform IDFT (Inverse Discrete Fourier Transform) are defined as follows (11):

[0099] .

[0100] Here, k is the extracted harmonic order, N is the number of sampling points within one fundamental period, and j is the imaginary unit; the obtained It is the k-th frequency spectrum at the nth sampling time. The following equation (12) is obtained by subtracting the DFT using a staggered method:

[0101] .

[0102] Based on the IDFT in equations (11) and (12), the results are restored. k-th frequency component .

[0103] Using the staggered subtraction method can reduce redundant calculations and improve computational efficiency; it only requires updating the most recent data point each time, making it suitable for real-time data processing and online analysis; and it can accurately extract specific frequency components through DFT and IDFT, facilitating subsequent harmonic analysis and filtering.

[0104] Frequency domain analysis of the grid-connected current signal based on Discrete Fourier Transform is used to extract odd harmonic components; these odd harmonic components are then converted into even harmonic components. The process of this step is described in detail below:

[0105] The odd harmonic component signal after DFT harmonic extraction is given by the following equation (13):

[0106] .

[0107] In equation (13), (n=3,5,7) represents the gain of the nth harmonic signal extracted by DFT. This represents the amplitude of the harmonic components of the grid-connected current.

[0108] The odd-order harmonic components extracted by DFT are converted into even-order harmonic components. The phase of the AC-side grid-connected voltage is sampled, and the sum-to-product formula of trigonometric functions is used to convert the phase of the AC-side grid-connected voltage into even-order harmonic components extracted by DFT. Finally, it can be represented as shown in equation (14):

[0109] .

[0110] like Figure 6-7 As shown, Figure 6 A schematic diagram of the framework for generating DC bus voltage harmonic feedback signals provided in this application; Figure 7 This is a schematic diagram of the current harmonic feedback control scheme provided in this application.

[0111] Step S13: Generate a feedback signal based on even-order harmonic components to correct the DC bus voltage ripple and eliminate odd-order harmonic components; the frequency of some even-order harmonic components in equation (14) is inconsistent with the phase of the corresponding harmonic components. To eliminate these components, according to the characteristics of trigonometric functions, both signals are delayed by 90 degrees before multiplication, and finally equation (15) can be obtained:

[0112] .

[0113] Adding equation (14) to equation (15) can cancel out the phase-error harmonic components, and finally obtain the even-order harmonic signal with the same phase and harmonic order, which can be used to compensate for the even-order DC bus voltage ripple on the DC bus voltage, as shown in equation (16):

[0114] .

[0115] The feedback signal generated by the grid-connected current harmonics has the same frequency and opposite phase as the corresponding DC bus voltage harmonic components, thereby reducing its impact on the AC side grid-connected current.

[0116] Feedback signals are generated based on the corrected DC bus voltage ripple to suppress its impact on the control loop. The corrected voltage ripple can prevent overvoltage or undervoltage caused by instantaneous power imbalance, thus protecting critical components in the converter from damage. Real-time adjustment using the feedback signal allows the system to respond more quickly to load changes, ensuring a stable output voltage even during sudden load changes. Precise compensation of voltage ripple can further improve the accuracy of the control loop, enabling the system to maintain efficient operation over a wider operating range.

[0117] In summary, the control method for AC / DC converters includes the following steps: acquiring grid voltage, DC / DC converter output power, DC / AC converter output power, DC bus voltage, and grid-connected current; performing DC bus voltage ripple prediction calculation based on grid voltage, grid-connected current, DC / DC converter output power, DC / AC converter output power, and DC bus voltage; extracting odd harmonic components from the grid-connected current signal using discrete Fourier transform frequency domain analysis; converting the odd harmonic components into even harmonic components; generating a feedback signal based on the even harmonic components to correct the DC bus voltage ripple and eliminate the odd harmonic components; and generating a feedback signal based on the corrected DC bus voltage ripple to suppress the impact of DC bus voltage ripple on the control loop. This control strategy achieves grid-connected current suppression without adding unnecessary hardware circuitry, reducing cost and complexity.

[0118] DC / AC converters require an input voltage higher than the AC output voltage to achieve a sinusoidal output via PWM modulation. Therefore, when the input DC bus voltage is low, the converter cannot operate normally, and the AC current may fail to rise, or even experience waveform clipping. Furthermore, if the input DC voltage is too high, the stress requirements on the converter's transistors become excessive.

[0119] like Figure 8 As shown, Figure 8 This application provides a schematic diagram of a DC bus voltage average value optimization control framework based on a minimum DC bus value limit. The control method for AC / DC converters further includes optimizing the DC bus voltage average value based on the expression for DC bus voltage ripple, including the following steps:

[0120] The maximum and minimum values ​​of the DC bus voltage are obtained based on the expression for DC bus voltage ripple; according to equation (4), the maximum value of the DC bus voltage can be calculated. Minimum value The expressions are as follows: (17) and (18):

[0121] ;

[0122] .

[0123] Obtain the relative relationship between the average DC bus voltage and the maximum and minimum DC bus voltage; based on the above equations (17) and (18), calculate the average DC bus voltage. Differentiating these equations yields equations (19) and (20):

[0124] ;

[0125] .

[0126] Always greater than 1, that is and Proportional; Let exist Greater than Under normal conditions, the average value of the DC bus voltage is always greater than 0. When the DC / AC converter is working properly, the smaller the average value of the DC bus voltage, the smaller its maximum and minimum values ​​will be.

[0127] The average DC bus voltage can be optimized by adjusting the relative relationship. In order to minimize the voltage stress of the power switch tubes and ensure the normal operation of the converter, the average DC bus voltage can be optimized based on different power levels.

[0128] like Figure 9 As shown, Figure 9 This is a schematic diagram illustrating the relationship between DC bus voltage and switching frequency according to an embodiment of this application. In this embodiment, the minimum frequency is 20kHz. In other embodiments, the minimum frequency may be taken as a different value, and no limitation is made thereto.

[0129] The DC bus voltage is positively correlated with the switching frequency of the AC / DC converter; the minimum DC bus voltage is obtained based on the set minimum switching frequency; to avoid the DC / AC converter switching frequency dropping too low to cross the resonant frequency of the LCL filter, which would cause distortion of the AC side grid current, a minimum frequency can be set; the minimum DC bus voltage is obtained based on the minimum frequency.

[0130] The DC bus voltage minimum value is used to obtain the average DC bus voltage under different power levels; the DC bus voltage minimum value is obtained based on the following formula (21) to obtain the average DC bus voltage under different power levels:

[0131] .

[0132] The average DC bus voltage is optimized based on equation (21) to obtain the average DC bus voltage. Maximum value Minimum value Relationship with power rating.

[0133] A lower DC bus voltage can reduce voltage stress on the switching transistors, thereby improving converter efficiency; by optimizing the voltage averaging, unnecessary energy loss can be reduced, especially under high-frequency operating conditions, which helps to improve overall energy efficiency.

[0134] In one embodiment, a feedback signal that is in phase with the DC bus voltage ripple is generated based on the even-order harmonic components, thereby canceling the even-order harmonic components in the DC bus voltage and suppressing the harmonic coupling phenomenon of the grid-connected current. By generating a feedback signal that is in phase with the DC bus voltage ripple, the even-order harmonic components in the DC bus voltage can be effectively canceled, thereby suppressing the harmonic coupling phenomenon of the grid-connected current, reducing the odd-order harmonic components in the grid-connected current, and improving the quality of the grid-connected current. By accurately compensating for the voltage ripple, overvoltage or undervoltage phenomena caused by instantaneous power imbalance can be avoided, protecting the key components in the converter from damage.

[0135] The even-order harmonic component generates a feedback signal that is in phase with the DC bus voltage ripple based on the phase adjustment factor; the phase difference between the even-order harmonic component and the DC bus voltage ripple is calculated, and the corresponding phase adjustment factor is generated based on the phase difference. The even-order harmonic component is multiplied by the phase adjustment factor to obtain the feedback signal that is in phase with the DC bus voltage ripple.

[0136] The generated feedback signal is introduced into the DC bus voltage control loop to form a closed-loop control system; the feedback signal is introduced into the voltage regulator to compensate for the ripple component in the DC bus voltage in real time; by adjusting the feedback gain, the dynamic response performance of the system is optimized to ensure that the system can maintain stable operation under different load conditions.

[0137] Film capacitors experience capacitance loss due to dielectric loss under high temperature and long-term operating conditions. Capacitance decreases with capacitor aging and increases in ambient temperature and humidity. The tolerance and capacitance deviation of the energy storage capacitor affect the compensation effect of secondary DC bus voltage ripple. For example... Figure 10 As shown, Figure 10 This is a schematic diagram showing the relationship between the capacitance decay of a thin-film capacitor and operating temperature and time.

[0138] Control methods for AC / DC converters also include methods based on capacitor value deviation compensation, comprising the following steps:

[0139] Real-time monitoring of the actual peak ripple of the DC bus voltage, and comparison of the actual peak ripple with the theoretically calculated peak ripple;

[0140] The parameters in the DC bus voltage ripple prediction model are adjusted based on the capacitance deviation coefficient, and a feedback signal is generated based on the adjusted prediction model to correct the DC bus voltage ripple.

[0141] like Figure 11 As shown, Figure 11 The waveform diagram shows the DC bus capacitance with tolerance. When the capacitance value decays, the DC bus voltage ripple calculated by the DC bus voltage ripple prediction is less than the actual DC bus voltage ripple. The DC bus waveform at this time is set as follows (22):

[0142] .

[0143] In equation (22), This represents the amplitude of the secondary DC bus voltage ripple. This represents the initial phase angle of the secondary DC bus voltage ripple. Voltage ripple prediction signal elimination. Then, assume that the DC bus capacitance tolerance is at this time. The capacitance deviation coefficient is times, that is, the capacitance deviation coefficient is At this time, the outer loop input of the DC bus voltage includes the secondary DC bus voltage ripple component as shown in the following formula:

[0144] ;

[0145] ;

[0146] .

[0147] This represents the processed DC bus voltage signal, with the addition of a capacitance deviation coefficient. This can compensate for the increased DC bus voltage ripple caused by capacitor value decay, ensuring stable operation of the system under different operating conditions.

[0148] This application provides a resonant AC / DC converter that can be used with the control method for AC / DC converters described in any of the above embodiments. The resonant AC / DC converter includes a control system and a switching transistor assembly. The control system receives a feedback signal and then outputs a PWM signal to adjust the conduction state of the switching transistor assembly.

[0149] This application also provides a resonant AC / DC converter, such as... Figure 12 As shown, Figure 12 This is a schematic diagram of an embodiment of the resonant AC / DC converter provided in this application, including a DC / DC converter, a DC / AC converter, and a DC bus capacitor. The DC / DC converter is a series resonant dual active bridge DC / DC converter, which uses an extended phase-shift + frequency modulation strategy as the power source to boost the output. The DC / AC full-bridge converter adopts a critical current modulation mode to achieve full-range soft switching. The output current of the grid-connected converter is controlled through the inner current loop, and the average value of the DC bus voltage is controlled through the outer voltage loop.

[0150] A DC / DC converter includes: an H-bridge, a transformer T, and a resonant inductor. and a half-bridge topology; the half-bridge topology includes a first half-bridge resonant capacitor. Second half-bridge resonant capacitor Resonance enables soft-switching operation.

[0151] The DC / AC converter includes a first switch Q1, a second switch Q2, a third switch Q3, and a fourth switch Q4; the first switch Q1 and the second switch Q2 are the high-frequency bridge arms of the AC / DC converter; the positive and negative half-cycle switching of the power frequency is realized based on the third switch Q3 and the fourth switch Q4; the DC / AC converter realizes zero-voltage conduction of the high-frequency switches based on critical current modulation.

[0152] The DC bus capacitor is connected in parallel with the DC / DC converter and the DC / AC converter to achieve power decoupling.

[0153] DC / AC converters include AC power supplies. The first terminal of the first switch Q1 is connected to the first terminal of the third switch Q3; the second terminal of the first switch Q1 is connected to the first terminal of the second switch Q2; the second terminal of the second switch Q2 is connected to the second terminal of the fourth switch Q4; the first terminal of the fourth switch Q4 is connected to the second terminal of the third switch Q3; AC power supply. One end is connected to the second terminal of the first switching transistor Q1 and the second terminal of the third switching transistor Q3; AC power supply. The other end is connected to the first end of the fourth switch Q4; the control terminals of the first switch Q1, the second switch Q2, the third switch Q3 and the fourth switch Q4 are connected to the control unit (not shown in the figure); the switch is turned on and off based on the potential of the control terminal adjusted by the control unit.

[0154] In this embodiment, the control unit generates a PWM signal to control the on-time and off-time of the switching transistor.

[0155] The DC / AC converter also includes a capacitor C, a first inductor L1, and a second inductor L2; one end of the first inductor L1 is connected to one end of the second inductor L2; the other end of the first inductor L1 is connected to the AC power supply; the other end of the second inductor L2 is connected to the second end of the first switching transistor Q1; one end of the capacitor C is connected to the side of the first inductor L1 closest to the second inductor L2; the other end of the capacitor C is connected to the other end of the AC power supply; the first end of the first switching transistor Q1 and the first end of the third switching transistor Q3 are connected to one end of the DC bus capacitor, and the second end of the second switching transistor Q2 and the second end of the fourth switching transistor Q4 are connected to the other end of the DC bus capacitor.

[0156] The half-bridge topology also includes a fifth switch Q5 and a sixth switch Q6; the second terminal of the fifth switch Q5 is connected to the first terminal of the sixth switch Q6; the first half-bridge resonant capacitor and the second half-bridge resonant capacitor are connected; the first terminal of the fifth switch Q5 is connected to the side of the first half-bridge resonant capacitor away from the second half-bridge resonant capacitor; the second terminal of the sixth switch Q6 is connected to the side of the second half-bridge resonant capacitor away from the first half-bridge resonant capacitor; the second terminal of the fifth switch Q5 is connected to one end of the secondary side of the transformer T through a resonant inductor; the other end of the secondary side of the transformer T is connected to the first terminal of the sixth switch Q6 and the side of the second half-bridge resonant capacitor closer to the first half-bridge resonant capacitor.

[0157] DC / DC converters include DC power supplies. The H-bridge includes a seventh switch Q7, an eighth switch Q8, a ninth switch Q9, and a tenth switch Q10. The first terminal of the seventh switch Q7 is connected to the first terminal of the ninth switch Q9. The second terminal of the seventh switch Q7 is connected to the first terminal of the eighth switch Q8. The second terminal of the eighth switch Q8 is connected to the second terminal of the tenth switch Q10. The first terminal of the tenth switch Q10 is connected to the second terminal of the ninth switch Q9. The positive terminal of the DC power supply is connected to the first terminals of the seventh switch Q7 and the ninth switch Q9. The negative terminal of the DC power supply is connected to the second terminals of the eighth switch Q8 and the tenth switch Q10. The control terminals of the seventh switch Q7, the eighth switch Q8, the ninth switch Q9, and the tenth switch Q10 are connected to a control unit. The control unit adjusts the potential of the control terminals to control the switching on and off of the switches.

[0158] In this embodiment, the switching transistors are all N-type MOS transistors, with the first terminal being the drain, the second terminal being the source, and the control terminal being the gate. In other embodiments, other types of switching elements can be selected, and no limitations are made here.

[0159] The DC bus capacitor is a film capacitor; the lifespan of the resonant AC / DC converter is improved by using film capacitors; unlike electrolytic capacitors, film capacitors are non-polarized, thus providing greater flexibility in circuit design and reducing the risk of failure due to misconnection; film capacitors can help achieve effective power decoupling, ensuring stable system operation even under instantaneous power imbalance, further improving system reliability.

[0160] In the several embodiments provided in this application, it should be understood that the disclosed methods and devices can be implemented in other ways. For example, the device embodiments described above are merely illustrative. For instance, the division of modules or units is only a logical functional division, and in actual implementation, there may be other division methods. For example, multiple units or components may be combined or integrated into another system, or some features may be ignored or not executed.

[0161] The units described as separate components may or may not be physically separate. The components shown as units may or may not be physical units; that is, they may be located in one place or distributed across multiple network units. Some or all of the units can be selected to achieve the purpose of this embodiment, depending on actual needs.

[0162] Furthermore, the functional units in the various embodiments of this application can be integrated into one processing unit, or each unit can exist physically separately, or two or more units can be integrated into one unit. The integrated unit can be implemented in hardware or as a software functional unit.

[0163] The above description is merely an embodiment of this application and does not limit the patent scope of this application. Any equivalent structural or procedural transformations made using the content of this application's specification and drawings, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of this application.

Claims

1. A control method for an AC / DC converter, characterized in that, Includes the following steps: Real-time acquisition of grid voltage, DC / DC converter output power, DC / AC converter output power, DC bus voltage, and grid-connected current; The DC bus voltage ripple prediction calculation is completed using the grid voltage, the grid-connected current, the DC / DC converter output power, the DC / AC converter output power, and the DC bus voltage input ripple prediction model; the odd harmonic components are extracted by frequency domain analysis of the grid-connected current signal based on discrete Fourier transform; and the odd harmonic components are converted into even harmonic components. The even-order harmonic components are used to generate a feedback signal to correct the DC bus voltage ripple and eliminate the odd-order harmonic components; the corrected DC bus voltage ripple is then used to generate a feedback signal to suppress the influence of the DC bus voltage ripple on the control loop.

2. The control method for an AC / DC converter according to claim 1, characterized in that, It also includes optimizing the DC bus voltage average based on the expression for DC bus voltage ripple, including the following steps: The maximum and minimum values ​​of the DC bus voltage are obtained based on the expression for the DC bus voltage ripple. Obtain the relative relationship between the average DC bus voltage and the maximum and minimum DC bus voltage; The average DC bus voltage is adjusted based on the relative relationship to optimize the average DC bus voltage.

3. The control method for an AC / DC converter according to claim 2, characterized in that, The DC bus voltage is positively correlated with the switching frequency of the AC / DC converter; the minimum DC bus voltage is obtained based on the set minimum switching frequency; the average DC bus voltage under different power levels is obtained based on the minimum DC bus voltage.

4. The control method for an AC / DC converter according to claim 1, characterized in that, Based on the even harmonic components, a feedback signal that is in phase with the DC bus voltage ripple is generated, thereby canceling the even harmonic components in the DC bus voltage and suppressing the harmonic coupling phenomenon of the grid-connected current.

5. The control method for an AC / DC converter according to claim 4, characterized in that, The even-order harmonic components generate a feedback signal that is in phase with the DC bus voltage ripple based on the phase adjustment factor.

6. The control method for an AC / DC converter according to claim 1, characterized in that, It also includes a method for compensation based on capacitance value deviation, which includes the following steps: The actual peak value of the DC bus voltage is monitored in real time, and the actual peak value is compared with the theoretically calculated peak value. The parameters in the DC bus voltage ripple prediction model are adjusted based on the capacitance deviation coefficient, and a feedback signal is generated based on the adjusted prediction model to correct the DC bus voltage ripple.

7. A resonant AC / DC converter, employing the control method for AC / DC converters as described in any one of claims 1-6, characterized in that, It includes a control system and a switching transistor assembly; the control system receives feedback signals and then outputs PWM signals to adjust the conduction state of the switching transistor assembly.

8. The resonant AC / DC converter according to claim 7, characterized in that, This includes DC / DC converters, DC / AC converters, and DC bus capacitors; The DC / DC converter includes an H-bridge, a transformer, a resonant inductor, and a half-bridge topology. The half-bridge topology includes a first half-bridge resonant capacitor and a second half-bridge resonant capacitor. The power flow is controlled by switching the H-bridge on and off. Voltage level conversion and isolation circuitry are implemented based on the transformer. Soft switching operation is achieved based on the resonance between the resonant inductor and the first and second half-bridge resonant capacitors. The DC / AC converter includes a first switch, a second switch, a third switch, and a fourth switch; the first switch and the second switch are the high-frequency bridge arms of the DC / AC converter; the positive and negative half-cycle power frequency switching is realized based on the third switch and the fourth switch; the DC / AC converter realizes zero-voltage conduction of the high-frequency switch based on critical current modulation; The DC bus capacitor is connected in parallel with the DC / DC converter and the DC / AC converter, and power decoupling is achieved based on the DC bus capacitor.

9. The resonant AC / DC converter according to claim 8, characterized in that, The DC / AC converter includes an AC power supply; a first terminal of the first switch is connected to a first terminal of the third switch; a second terminal of the first switch is connected to a first terminal of the second switch; a second terminal of the second switch is connected to a second terminal of the fourth switch; a first terminal of the fourth switch is connected to a second terminal of the third switch; one end of the AC power supply is connected to the second terminals of the first and third switches; the other end of the AC power supply is connected to the first terminal of the fourth switch; control terminals of the first, second, third, and fourth switches are connected to a control unit; the switching transistors are controlled by adjusting the potential of the control terminals based on the control unit.

10. The resonant AC / DC converter according to claim 9, characterized in that, The DC / AC converter further includes a capacitor, a first inductor, and a second inductor; one end of the first inductor is connected to one end of the second inductor; the other end of the first inductor is connected to an AC power supply; the other end of the second inductor is connected to the second end of the first switching transistor; one end of the capacitor is connected to the side of the first inductor closest to the second inductor; the other end of the capacitor is connected to the other end of the AC power supply; the first ends of the first switching transistor and the first ends of the third switching transistor are connected to one end of the DC bus capacitor, and the second ends of the second switching transistor and the second ends of the fourth switching transistor are connected to the other end of the DC bus capacitor.

11. The resonant AC / DC converter according to claim 8, characterized in that, The half-bridge topology further includes a fifth switch and a sixth switch; the second end of the fifth switch is connected to the first end of the sixth switch; the first half-bridge resonant capacitor and the second half-bridge resonant capacitor are connected; the first end of the fifth switch is connected to the side of the first half-bridge resonant capacitor away from the second half-bridge resonant capacitor; the second end of the sixth switch is connected to the side of the second half-bridge resonant capacitor away from the first half-bridge resonant capacitor; the second end of the fifth switch is connected to one end of the secondary side of the transformer through the resonant inductor; the other end of the secondary side of the transformer is connected to the first end of the sixth switch and the side of the second half-bridge resonant capacitor closest to the first half-bridge resonant capacitor.

12. The resonant AC / DC converter according to claim 11, characterized in that, The DC / DC converter includes a DC power supply; the H-bridge includes a seventh switch, an eighth switch, a ninth switch, and a tenth switch; the first terminal of the seventh switch is connected to the first terminal of the ninth switch; the second terminal of the seventh switch is connected to the first terminal of the eighth switch; the second terminal of the eighth switch is connected to the second terminal of the tenth switch; the first terminal of the tenth switch is connected to the second terminal of the ninth switch; the positive terminal of the DC power supply is connected to the first terminals of the seventh and ninth switches; the negative terminal of the DC power supply is connected to the second terminals of the eighth and tenth switches; the control terminals of the seventh, eighth, ninth, and tenth switches are connected to a control unit; the switching transistors are controlled by adjusting the potential of the control terminals based on the control unit.

13. The resonant AC / DC converter according to claim 8, characterized in that, The DC bus capacitor is a thin-film capacitor; the service life of the resonant AC / DC converter is improved based on the thin-film capacitor.

Images