A multi-modal current stress optimized modulation strategy for fault-tolerant DAB converter

Abstract

The application relates to a multi-modal current stress optimization modulation strategy for fault tolerance of a DAB converter, which optimizes the current stress of an S-DAB converter degraded from the DAB converter when an open circuit fault occurs in the DAB converter, all working modes of the converter are analyzed, and the optimal working modes under different voltage transformation ratios are screened out; each mode under low power and high power is selected, a KKT equation is applied, and the optimal phase shift ratio under different modes is solved by taking the minimization of the current stress as an optimization target. In order to realize the optimal performance in the full power range, a multi-modal closed-loop control strategy is further designed; according to the operation of a PI controller, an independent variable V is selected to select the corresponding optimal operation mode, the corresponding duty cycle is obtained, and finally a PWM signal is generated to drive a switch tube, and the strategy ensures that the converter can automatically switch to the mode with the optimal current stress under different power working conditions, so that the overall efficiency and reliability of the system under the fault degradation state are effectively improved.

Description

Technical Field

[0001] This invention belongs to the field of power electronics technology, specifically referring to a fault-tolerant multi-mode current stress optimization modulation strategy for DAB converters. Background Technology

[0002] Solid-state transformers (SSTs), a high-frequency power conversion technology based on power electronics, can provide additional functions such as bidirectional power flow, reactive power compensation, voltage regulation, and harmonic suppression while achieving voltage transformation and electrical isolation. They also possess significant advantages such as small size, light weight, and high power density. Therefore, SSTs are considered core equipment for future applications such as smart grids, renewable energy grid integration, electric vehicle fast charging stations, and DC microgrids.

[0003] Dual Active Bridge (DAB) converters achieve electrical isolation between the primary and secondary sides through high-frequency transformers. Because their operating frequency is much higher than the power frequency, the size and weight of the transformer core can be significantly reduced. Based on this characteristic, DAB converters are widely used in the DC / DC stage of SST (Supply Chain Relay). The DAB topology itself has a symmetrical structure, naturally supporting bidirectional power transmission. This characteristic allows SST to flexibly adapt to the bidirectional energy flow requirements of distributed generation and energy storage systems.

[0004] While DAB (Digital-Analog Converter) is an ideal choice for SST (Short-Switch Converter), existing DAB technologies still face several challenges when meeting the high-performance and high-reliability requirements of SST applications. Traditional single-phase-shift (SPS) modulation, under voltage mismatch or light load conditions, leads to problems such as high current stress, high circulating current loss, and limited soft-switching range, severely restricting the efficiency of SST over a wide voltage and load range. To improve these issues, researchers have proposed modulation strategies such as dual-phase-shift (DPS), extended-phase-shift (EPS), and triple-phase-shift (TPS). However, most existing optimized modulation strategies are designed for normal operating conditions of DAB converters, with insufficient attention paid to optimizing current stress under fault conditions.

[0005] In DAB converters, open-circuit faults (OCFs) are common in switching transistors. Faults in the transistor's drive circuit can lead to OCFs, resulting in a loss of topological symmetry in the DAB system, distortion of high-frequency voltage and current, and DC bias, negatively impacting system reliability and long-term operational stability. A common fault-tolerant approach is topology reconfiguration, downgrading the original full-bridge DAB structure to a semi-dual active bridge (S-DAB) with asymmetrical bridge arms, or a simpler topology. However, fault downgrading disrupts the converter's original soft-switching characteristics and voltage matching. Continuing with the original single-phase-shift modulation strategy leads to a sharp increase in current stress, a significant reduction in transmission power and efficiency, and a corresponding decrease in control degrees of freedom. Therefore, a thorough analysis of the S-DAB converter resulting from fault downgrading is necessary to avoid excessive current stress caused by topology reconfiguration.

[0006] Similar to DAB converters, S-DAB converters formed through topology reconstruction can also employ classic modulation strategies such as single-phase shift (SPS) and dual-phase shift (DPS). However, under traditional SPS or DPS modulation, the converter may face high current stress during operation, leading to higher conduction and switching losses, reducing overall system efficiency, and affecting converter performance. Existing research on fault-tolerant operation of DAB converters mainly focuses on fault diagnosis and topology reconstruction, while systematic research on high-performance fault modulation strategies after topology reconstruction is lacking.

[0007] In summary, existing fault-tolerant technologies for DAB converters have the following shortcomings: (1) When using traditional modulation strategies after fault degradation, the current stress of the S-DAB converter is too large, resulting in a significant increase in conduction and switching losses; (2) There is a lack of differentiated modulation strategies for different fault degradation topology modes of the S-DAB converter, making it impossible to effectively optimize the current stress during fault-tolerant operation; (3) Existing current stress optimization methods are mainly designed for normal operating conditions and do not fully consider the constraints of topology changes after fault degradation on the optimization objective and control degrees of freedom. Therefore, it is urgent to propose a strategy that can perform multi-mode current stress optimization modulation based on different fault degradation modes of the S-DAB converter to improve the overall performance and reliability of the DAB converter under fault-tolerant operation. Summary of the Invention

[0008] The present invention aims to provide a fault-tolerant multi-mode current stress optimization modulation strategy for DAB converters, which can effectively improve the overall performance and reliability of DAB converters under fault-tolerant operation.

[0009] To achieve the above objectives, this invention proposes a fault-tolerant multi-mode current stress optimization modulation strategy for DAB converters. Based on the operating modes of the S-DAB converter degenerating into an open-circuit converter when a DAB converter experiences an open-circuit fault, optimal operating modes for different voltage turns ratios are selected. Among the modes that meet the requirements, based on the benchmark that the voltage and current at both ends of the transformer are in the same direction, two modes are determined for each of the increasing and decreasing voltage turns ratio conditions. These two modes serve as low-power and high-power operating states under different voltage turns ratios, respectively.

[0010] As a further aspect of the present invention: the S-DAB converter consists of an active H-bridge and a semi-active H-bridge, wherein the active H-bridge is the primary side, and the primary side has two arms, each arm containing two series-connected fully controlled switches, namely S1, S2 and S3, S4. The semi-active H-bridge is the secondary side, one arm of the secondary side consists of two diodes connected in series, namely D5 and D6, and the other arm consists of two series-connected fully controlled switches, namely S7 and S8. The control signals of the fully controlled switches on the same arm of the converter are complementary, the transformer ratio is n:1, and the system voltage ratio is M=nU2 / U1, wherein the voltage increase ratio is M>1, and the voltage decrease ratio is M<1, where U1 is the input voltage on the primary side, U2 is the output voltage on the secondary side, L is the high-frequency inductor, C1 and C2 are the input capacitor and output capacitor respectively, R is the output resistor, and v p v s These are the output voltages of the H-bridges on both sides, i L It is the inductor current;

[0011] Define two shift ratios as D0 and D1, where D0 and D1 are both half-cycle shift ratios, where D0 is the shift ratio between S1 and S8, and D1 is the shift ratio between S1 and S4.

[0012] As a further aspect of the present invention: the optimal working mode includes mode 1.1 and mode 1.2, and the specific formulas are as follows:

[0013]

[0014] Wherein, β is defined as the ratio of the time from the zero point of a switching cycle to the time when the inductor current first becomes zero to half a cycle.

[0015] As a further aspect of the present invention: the moment when the inductor current of mode 1.1 first reaches 0 is greater than 0 and less than D1T. hs At that moment, this mode is the low-power mode under two voltage turns ratios; the moment when the inductor current in mode 1.2 first reaches 0 is greater than D1T. hs And less than D0T hs At time T, this mode is a high-power mode under two voltage turns ratios; where, T hsHalf the switching cycle; the per-unit power P of the S-DAB converter * Per unit value of current stress I * The formulas for low-power mode and high-power mode under different voltage turns ratios are as follows:

[0016] Low power mode under increased voltage ratio:

[0017]

[0018]

[0019] High-power mode under increased voltage ratio:

[0020]

[0021]

[0022] Low power mode under reduced voltage ratio:

[0023]

[0024]

[0025] High-power mode under reduced voltage ratio:

[0026]

[0027] in, , These represent the per-unit values ​​of transmission power and current stress in the low-power mode under increased voltage ratio, respectively. , These are the per-unit values ​​of transmission power and current stress in the high-power mode under increased voltage ratio, respectively. , These represent the per-unit values ​​of transmission power and current stress in the low-power mode under reduced voltage ratio, respectively. , These represent the per-unit values ​​of transmission power and current stress in the high-power mode under reduced voltage ratio, respectively, and β. 1L For the low power mode under increased voltage ratio, β, β 1H For the high power mode under increased voltage ratio, β, β 2L For the low power mode under reduced voltage ratio, β, β 2H β is the high-power mode under reduced voltage turns ratio.

[0028] As a further aspect of this invention: The parameters for different power ranges are standardized using per-unit calculations. The maximum transmission power of the DAB converter under SPS modulation and the input current at that time are taken as the reference values, and the per-unit transmission power P is... * Per unit value of current stress I * The calculation formula is as follows:

[0029]

[0030]

[0031] Where P N I N These are the reference values ​​for transmission power and current, respectively; P and I are the actual values ​​of transmission power and current stress, respectively; f s Where L is the switching frequency and L is the inductance value.

[0032] As a further aspect of this invention: Current stress optimization for two operating modes under different voltage turns ratios is achieved by minimizing current stress. A KKT objective function is established, with the relationship between transmission power and the shift ratio as constraints. The KKT conditions for current stress optimization are expressed as follows:

[0033]

[0034] Where X is the optimal solution for the shift ratio. The per-unit value of current stress for the desired mode is... To represent the per-unit value of the transmission power corresponding to the shift ratio, α is the Lagrange multiplier of the transmission power equation. α≠0 indicates that the constraint activation requires precise transmission power. For the inequality constraints formed by the ratio of the shifts, β j For the j-th inequality constraint The corresponding Lagrange multipliers indicate the degree to which the inequality constraint hinders the optimal solution. This indicates whether the constraint is in effect, with j ranging from 1 to k representing different shifts compared to X. j This represents the optimal solution point on the boundary of the j-th inequality constraint.

[0035] As a further aspect of this invention: the optimal shift ratio expression relationship for each power range under different voltage ratio ranges is obtained by solving the KKT condition equations, as follows:

[0036] The relationship between the phase shift ratios D1 and D0 in the low-power mode under increased voltage ratio is expressed as follows:

[0037]

[0038] The phase shift ratio D1 of the high-power mode under the increased voltage transformer ratio is expressed as follows:

[0039]

[0040] The relationship between the phase shift ratios D1 and D0 in the low-power mode under reduced voltage turns ratio is as follows:

[0041]

[0042] The relationship between the phase shift ratios D1 and D0 in the high-power mode under reduced voltage turns ratio is as follows:

[0043] .

[0044] As a further aspect of the present invention: a conversion function is constructed based on the relationship between the shift ratios corresponding to different modes and the functional relationship between the shift ratio and the transmission power. The shift ratio expression is obtained through the independent variable V. The range of the independent variables of the conversion function under different modes and the corresponding shift ratio can be expressed as follows:

[0045] The optimized phase shift ratios D1 and D0 for the low-power mode under the increased voltage transformer ratio are expressed as follows:

[0046]

[0047]

[0048] The optimized phase shift ratios D1 and D0 for the high-power mode under the increased voltage transformer ratio are expressed as follows:

[0049]

[0050]

[0051] The optimized phase shift ratios D1 and D0 for the low-power mode under reduced voltage turns ratio are expressed as follows:

[0052]

[0053]

[0054] The optimized phase shift ratios D1 and D0 of the high-power mode under reduced voltage turns ratio are expressed as follows:

[0055]

[0056]

[0057] Among them, V min V represents the minimum value of the independent variable in the corresponding mode. max This represents the maximum value of the independent variable within the corresponding mode.

[0058] When M=1, a single-phase-shift modulation strategy is adopted, i.e., D1=0 with only one variable shift ratio D0. A conversion function is constructed to examine the relationship between the transmission power and the shift ratio under single-phase-shift modulation. This yields the range of independent variables under single-phase-shift modulation, and the corresponding expression for D0 is:

[0059]

[0060] .

[0061] The above strategy employs a multi-modal control closed-loop control system to implement the current stress optimization modulation strategy. The complete operation process of the closed-loop control system includes the following steps:

[0062] Step 1: Sample the input voltage U1 and output voltage U2 of the S-DAB converter and calculate the voltage transformation ratio M;

[0063] Step 2: Subtract the output reference voltage from the output voltage input subtractor to obtain the error quantity, input it to the PI controller, and obtain the independent variable V through corresponding calculations;

[0064] Step 3: Determine the power limit for mode switching based on the voltage ratio M, and dynamically select the appropriate operating mode based on the range of the independent variable V obtained from the PI controller calculation;

[0065] Step 4: Generate the corresponding PWM signal based on the optimal shift ratio expression of the mode to drive the corresponding switch to turn on and off.

[0066] Compared with the prior art, the beneficial effects of the present invention are as follows: First, by analyzing the operating modes of the S-DAB converter, selecting the two optimal operating modes, and optimizing the current stress by combining the value function, the current stress is reduced while ensuring the transmission power; Second, based on the voltage ratio of the converter, the mode switching range is determined, and a closed-loop control system based on multi-mode control can be implemented to dynamically switch the operating mode corresponding to the required power range according to the system operating conditions, so that the converter always operates in the optimal state in different power ranges, significantly improving the power transmission efficiency of the system, thereby effectively improving the overall performance and reliability of the DAB converter under fault-tolerant operation. Attached Figure Description

[0067] Figure 1 This is the main topology diagram of the S-DAB converter in this embodiment of the invention.

[0068] Figure 2 These are the drive signal and voltage / current waveforms of the S-DAB converter in the embodiments of the present invention under low-power mode and high-power mode under different voltage changes, where (a) and (b) are respectively under increased voltage ratio. , (c) and (d) are respectively the voltage transformer ratio at the reduced point. , .

[0069] Figure 3 (a) and (b) are, respectively, comparisons of the current stress of the S-DAB converter after optimization by the proposed optimization method and the current stress under SPS modulation in embodiments of the present invention when M=0.5 and M=1.5. * -I * Relationship diagram.

[0070] Figure 4 This is a schematic diagram of a closed-loop controller system according to an embodiment of the present invention.

[0071] Figure 5 This is a simulation waveform diagram of the dynamic response of the output voltage and inductor current of the S-DAB converter under the modulation of the present invention. Detailed Implementation

[0072] The invention will now be further described with reference to the accompanying drawings.

[0073] This invention presents a fault-tolerant multi-mode current stress optimization modulation strategy for DAB converters. It studies the degradation of a DAB converter into an S-DAB converter when an open-circuit fault occurs. The system analyzes all operating modes of the converter and selects the optimal operating modes for different voltage turns ratios, denoted by M. Based on the benchmark that the voltages at both ends of the transformer are in the same direction, two modes are determined: one with increasing voltage turns ratio (M>1) and the other with decreasing voltage turns ratio (M<1), representing low-power and high-power operating states under different voltage turns ratios. Furthermore, this invention employs a closed-loop control system with multi-mode control to ensure that the converter automatically switches to the optimal current stress mode under different power conditions, thereby effectively improving the overall efficiency and reliability of the system under fault degradation conditions.

[0074] like Figure 1 As shown, the S-DAB converter consists of an active H-bridge and a semi-active H-bridge. The active H-bridge is the primary side, with two arms. Each arm contains two series-connected fully controlled switches, namely S1, S2 and S3, S4. The semi-active H-bridge is the secondary side. One arm of the secondary side consists of two diodes connected in series, namely D5 and D6, while the other arm consists of two series-connected fully controlled switches, namely S7 and S8. The control signals of the fully controlled switches on the same arm are complementary. The transformer turns ratio is n:1, and the system voltage ratio is M = nU2 / U1, where U1 is the primary side input voltage, U2 is the secondary side output voltage, L is the high-frequency inductor, C1 and C2 are the input and output capacitors, respectively, R is the output resistance, and vp v s These are the output voltages of the H-bridges on both sides, i L This represents the inductor current.

[0075] Define two shift ratios as D0 and D1, where D0 and D1 are both half-cycle shift ratios, where D0 is the shift ratio between S1 and S8, and D1 is the shift ratio between S1 and S4.

[0076] Based on the relationship between the shift ratios of the S-DAB converter and D0 and D1, all existing operating modes are classified as follows:

[0077]

[0078] Here, β is defined as the ratio of the time from the zero point of a switching cycle to the moment when the inductor current first becomes zero to half a cycle. Considering the feasibility of the modes and whether the voltage transformation ratio condition can be achieved, only five operating modes remain under the voltage transformation ratio: Mode 1.1, Mode 1.2, Mode 2.1, Mode 2.2, and Mode 3.1. A specific analysis is conducted on the inductor current and the voltage across the transformer for these five operating modes, based on the benchmark that the voltages across the transformer are in the same direction, and the zero-voltage switching time of the switching transistors in each operating mode. Using the ZVS condition as the evaluation criterion, the final selected operating modes are Mode 1.1 and Mode 1.2. According to the same selection principle, the modes retained under reduced voltage ratio are Mode 1.1, Mode 1.2, and Mode 4.1. The power and current expressions of Mode 4.1 and Mode 1.1 are the same. Considering the degree of control freedom and complexity, Mode 1.1 is selected. Using the same evaluation criterion, the operating modes under reduced voltage ratio are selected, and the final selected operating modes are still Mode 1.1 and Mode 1.2.

[0079] according to Figure 2 The inductor current waveform is analyzed using piecewise linearization. Due to the rotational symmetry of the inductor current about half a period, the specific current value at each inflection point can be obtained by solving for the sum of the maximum and minimum inductor current values ​​being zero. This yields the expression for the inductor current, and further, the transmission power of the S-DAB converter in both modes is derived. The formula for calculating the transmission power is:

[0080]

[0081] In the formula, P is the actual value of the transmission power, and T is the actual value of the transmission power. s The switching cycle.

[0082] Furthermore, according to Figure 2 From (a) and (b), it can be found that the current stress in modes 1.1 and 1.2 under the increased voltage ratio both occur at time t3. According to Figure 2From (c) and (d), it can be found that the current stress in modes 1.1 and 1.2 under reduced voltage ratio both occur at time t4.

[0083] The moment when the inductor current in mode 1.1 first reaches 0 is greater than 0 and less than D1T. hs At that moment, this mode is the low-power mode under two voltage turns ratios; the moment when the inductor current in mode 1.2 first reaches 0 is greater than D1T. hs And less than D0T hs At time T, this mode is a high-power mode under two voltage turns ratios; where, T hs It is half of the switching cycle; the per-unit values ​​of the transmission power and current stress of the S-DAB converter under different voltage ratios for low-power and high-power modes are as follows:

[0084] Low power mode under increased voltage ratio:

[0085]

[0086]

[0087] High-power mode under increased voltage ratio:

[0088]

[0089]

[0090] Low power mode under reduced voltage ratio:

[0091]

[0092]

[0093] High-power mode under reduced voltage ratio:

[0094]

[0095] in, , These represent the per-unit values ​​of transmission power and current stress in the low-power mode under increased voltage ratio, respectively. , These are the per-unit values ​​of transmission power and current stress in the high-power mode under increased voltage ratio, respectively. , These represent the per-unit values ​​of transmission power and current stress in the low-power mode under reduced voltage ratio, respectively. , These represent the per-unit values ​​of transmission power and current stress in the high-power mode under reduced voltage ratio, respectively, and β.1L For the low power mode under increased voltage ratio, β, β 1H For the high power mode under increased voltage ratio, β, β 2L For the low power mode under reduced voltage ratio, β, β 2H β represents the high-power mode under reduced voltage turns ratio.

[0096] Figure 3 (a) and (b) are the P values ​​of the current stress of the S-DAB converter after optimization by the optimization method proposed in this invention and the current stress under SPS modulation, respectively, when M=0.5 and M=1.5. * -I * The relationship diagram shows that the optimization method proposed in this invention significantly optimizes the current stress compared to traditional SPS modulation under different voltage ratios, thereby significantly improving the transmission efficiency of the system and reducing system losses.

[0097] Figure 4 This invention employs a multimodal control closed-loop control system, the complete operation flow of which mainly includes the following steps:

[0098] Step 1: Sample the input voltage U1 and output voltage U2 of the S-DAB converter and calculate the voltage transformation ratio M = nU2 / U1.

[0099] Step 2: Subtract the output reference voltage from the output voltage input subtractor to obtain the error value, which is then input to the PI controller and processed to obtain V.

[0100] Step 3: Determine the power limit for mode switching based on the voltage ratio M, and dynamically select the appropriate operating mode based on the range of V obtained from the PI controller calculation.

[0101] Step 4: Generate the corresponding PWM signal based on the optimal shift ratio expression of the mode to drive the corresponding switch to turn on and off.

[0102] Using per-unit values ​​allows for the standardization of parameters across different power ranges, facilitating comparison and optimization under varying operating conditions. The maximum transmission power of the DAB converter under SPS modulation and the input current at that point are taken as the baseline values, and the per-unit transmission power value P is used. * Per unit value of current stress I * The calculation formula is as follows:

[0103]

[0104]

[0105] Where P N I NThese are the reference values ​​for transmission power and current, respectively; P and I are the actual values ​​of transmission power and current stress, respectively; f s Where L is the switching frequency and L is the inductance value. , These are the per-unit values ​​for transmission power and current stress, respectively.

[0106] With the goal of minimizing current stress, a KKT objective function is established, using the relationship between transmission power and shift ratio as constraints. The KKT conditions for current stress optimization are expressed as follows:

[0107]

[0108] Where X is the optimal solution for the shift ratio. The per-unit value of current stress for the desired mode is... This corresponds to the per-unit value of transmission power under the shift ratio. Let α be the per-unit value of the required transmission power of the system, and α be the Lagrange multiplier of the transmission power equation. α≠0 indicates that the constraint activation requires precise transmission power. For the inequality constraints formed by the ratio of the shifts, β j For the j-th inequality constraint The corresponding Lagrange multipliers indicate the degree to which the inequality constraint hinders the optimal solution. This indicates whether the constraint is in effect, with j ranging from 1 to k representing different shifts compared to X. j This represents the optimal solution point on the boundary of the j-th inequality constraint.

[0109] The optimal shift ratio expressions for each power range under different voltage ratio ranges are obtained by solving the KKT condition equations, as follows:

[0110] The relationship between the phase shift ratios D1 and D0 in the low-power mode under increased voltage ratio is expressed as follows:

[0111]

[0112] The phase shift ratio D1 of the high-power mode under the increased voltage transformer ratio is expressed as follows:

[0113]

[0114] The relationship between the phase shift ratios D1 and D0 in the low-power mode under reduced voltage turns ratio is as follows:

[0115]

[0116] The relationship between the phase shift ratios D1 and D0 in the high-power mode under reduced voltage turns ratio is as follows:

[0117]

[0118] Based on the relationship between the shift ratios corresponding to different modes and the functional relationship between the shift ratio and the increase / decrease of transmission power, a simpler conversion function is constructed. A simpler expression for the shift ratio is obtained through the independent variable V, replacing the complex expression for the shift ratio with respect to the per-unit value of transmission power. The range of independent variables and the corresponding shift ratio of the conversion function under different modes can be expressed as follows:

[0119] The optimized phase shift ratios D1 and D0 for the low-power mode under the increased voltage transformer ratio are expressed as follows:

[0120]

[0121]

[0122] The optimized phase shift ratios D1 and D0 for the high-power mode under the increased voltage transformer ratio are expressed as follows:

[0123]

[0124]

[0125] The optimized phase shift ratios D1 and D0 for the low-power mode under reduced voltage turns ratio are expressed as follows:

[0126]

[0127]

[0128] The optimized phase shift ratios D1 and D0 of the high-power mode under reduced voltage turns ratio are expressed as follows:

[0129]

[0130]

[0131] Among them, V min V represents the minimum value of the independent variable in the corresponding mode. max This represents the maximum value of an independent variable within the corresponding mode. The maximum and minimum values ​​of an independent variable in different modes are obtained through the transmission power range of that mode and the constructed transformation function.

[0132] When M=1, a single-phase-shift modulation strategy is adopted, i.e., D1=0 with only one variable shift ratio D0. A conversion function is constructed to examine the relationship between the transmission power and the shift ratio under single-phase-shift modulation. This yields the range of independent variables under single-phase-shift modulation, and the corresponding expression for D0 is:

[0133]

[0134]

[0135] Figure 5 The figure shows the simulation waveforms of the output voltage and inductor current dynamic response of the S-DAB converter under the modulation of this invention. At 0.15s, a load change occurs, with the load changing from 50Ω to 25Ω. Before the load change, the system was operating in a low-power state, and after the load change, the system was operating in a high-power state. The inductor current in the figure represents the current waveforms of the power inductor under two different modes. At the same time, the output voltage drops briefly when the load changes, and recovers to the target value in a short time.

[0136] In summary, the multi-mode control-based current stress optimization modulation strategy for S-DAB converters proposed in this invention optimizes the operating mode of S-DAB that degenerates due to open-circuit faults in traditional DAB converters. Compared to the traditional single-phase shift strategy, the proposed multi-mode control-based current stress optimization modulation strategy not only broadens the range of transmission power but also effectively reduces current stress. Furthermore, it dynamically selects the appropriate power range of the operating mode based on the system's required transmission power. Therefore, this invention significantly improves converter performance and reduces system losses.

Claims

1. A fault-tolerant multi-mode current stress optimization modulation strategy for DAB converters, characterized in that, Based on all operating modes of the S-DAB converter degraded to when the DAB converter experiences an open-circuit fault, the optimal operating modes with different voltage turns ratios are selected. Among the modes that can meet the conditions, based on the benchmark that the voltages at both ends of the transformer are in the same direction, two modes are determined for each of the voltage increase and voltage decrease conditions. These two modes are respectively used as the low-power and high-power operating states under different voltage turns ratios.

2. The fault-tolerant multi-mode current stress optimization modulation strategy for DAB converters according to claim 1, characterized in that, The S-DAB converter consists of an active H-bridge and a semi-active H-bridge. The active H-bridge is the primary side, with two arms. Each arm contains two series-connected fully controlled switches, S1, S2 and S3, S4. The semi-active H-bridge is the secondary side. One arm of the secondary side consists of two series-connected diodes, D5 and D6, while the other arm consists of two series-connected fully controlled switches, S7 and S8. The control signals of the upper and lower fully controlled switches on the same arm are complementary. The transformer turns ratio is n:1, and the system voltage turns ratio is M = nU2 / U1. The voltage increase ratio is M > 1, and the voltage decrease ratio is M < 1. U1 is the primary side input voltage, U2 is the secondary side output voltage, L is the high-frequency inductor, C1 and C2 are the input and output capacitors, respectively, R is the output resistance, and v p v s These are the output voltages of the H-bridges on both sides, i L It is the inductor current; Define two shift ratios as D0 and D1, where D0 and D1 are both half-cycle shift ratios, where D0 is the shift ratio between S1 and S8, and D1 is the shift ratio between S1 and S4.

3. The fault-tolerant multi-mode current stress optimization modulation strategy for DAB converters according to claim 2, characterized in that, The optimal operating modes include mode 1.1 and mode 1.2, and the specific formulas are as follows: Wherein, β is defined as the ratio of the time from the zero point of a switching cycle to the time when the inductor current first becomes zero to half a cycle.

4. The fault-tolerant multi-mode current stress optimization modulation strategy for DAB converters according to claim 3, characterized in that, The moment when the inductor current in mode 1.1 first reaches 0 is greater than 0 and less than D1T. hs At that moment, this mode is the low-power mode under two voltage turns ratios; the moment when the inductor current in mode 1.2 first reaches 0 is greater than D1T. hs And less than D0T hs At time T, this mode is a high-power mode under two voltage turns ratios; where, T hs Half the switching cycle; the per-unit power P of the S-DAB converter * Per unit value of current stress I * The formulas for low-power mode and high-power mode under different voltage turns ratios are as follows: Low power mode under increased voltage ratio: High-power mode under increased voltage ratio: Low power mode under reduced voltage ratio: High-power mode under reduced voltage ratio: in, , These represent the per-unit values ​​of transmission power and current stress in the low-power mode under increased voltage ratio, respectively. , These are the per-unit values ​​of transmission power and current stress in the high-power mode under increased voltage ratio, respectively. , These represent the per-unit values ​​of transmission power and current stress in the low-power mode under reduced voltage ratio, respectively. , These represent the per-unit values ​​of transmission power and current stress in the high-power mode under reduced voltage ratio, respectively, and β. 1L For the low power mode under increased voltage ratio, β, β 1H For the high power mode under increased voltage ratio, β, β 2L For the low power mode under reduced voltage ratio, β, β 2H β is the high-power mode under reduced voltage turns ratio.

5. The fault-tolerant multi-mode current stress optimization modulation strategy for DAB converters according to claim 4, characterized in that, The parameters for different power ranges are standardized using per-unit calculations. The maximum transmission power of the DAB converter under SPS modulation and the input current at that time are taken as the reference values, and the per-unit transmission power P is calculated. * Per unit value of current stress I * The calculation formula is as follows: Where P N I N These are the transmission power reference value and the current reference value, respectively. P and I are the actual values ​​of transmission power and current stress, respectively. s Where L is the switching frequency and L is the inductance value.

6. The fault-tolerant multi-mode current stress optimization modulation strategy for DAB converters according to claim 3, characterized in that, For the current stress optimization of two optimal operating modes under different voltage turns ratios, with the goal of minimizing current stress, a KKT objective function is established, using the relationship between transmission power and the shift ratio as constraints. The KKT conditions for current stress optimization are expressed as follows: Where X is the optimal solution for the shift ratio. The per-unit value of current stress for the desired mode is... To represent the per-unit value of the transmission power corresponding to the shift ratio, α is the Lagrange multiplier of the transmission power equation. α≠0 indicates that the constraint activation requires precise transmission power. For the inequality constraints formed by the ratio of the shifts, β j For the j-th inequality constraint The corresponding Lagrange multipliers indicate the degree to which the inequality constraint hinders the optimal solution. This indicates whether the constraint is in effect, with j ranging from 1 to k representing different shifts compared to X. j This represents the optimal solution point on the boundary of the j-th inequality constraint.

7. The fault-tolerant multi-mode current stress optimization modulation strategy for DAB converters according to claim 6, characterized in that, The optimal shift ratio expressions for each power range under different voltage ratio ranges are obtained by solving the KKT condition equations, as follows: The relationship between the phase shift ratios D1 and D0 in the low-power mode under increased voltage ratio is expressed as follows: The phase shift ratio D1 of the high-power mode under the increased voltage transformer ratio is expressed as follows: The relationship between the phase shift ratios D1 and D0 in the low-power mode under reduced voltage turns ratio is as follows: The relationship between the phase shift ratios D1 and D0 in the high-power mode under reduced voltage turns ratio is as follows: 。 8. The fault-tolerant multi-mode current stress optimization modulation strategy for DAB converters according to claim 7, characterized in that, Based on the relationship between the shift ratios corresponding to different modes and the functional relationship between the shift ratio and the transmission power, a conversion function is constructed. The shift ratio expression is obtained through the independent variable V. The range of independent variables of the conversion function under different modes and the corresponding shift ratio can be expressed as follows: The optimized phase shift ratios D1 and D0 for the low-power mode under the increased voltage transformer ratio are expressed as follows: The optimized phase shift ratios D1 and D0 for the high-power mode under the increased voltage transformer ratio are expressed as follows: The optimized phase shift ratios D1 and D0 for the low-power mode under reduced voltage turns ratio are expressed as follows: The optimized phase shift ratios D1 and D0 of the high-power mode under reduced voltage turns ratio are expressed as follows: Among them, V min V represents the minimum value of the independent variable in the corresponding mode. max This represents the maximum value of the independent variable within the corresponding mode. When M=1, a single-phase-shift modulation strategy is adopted, i.e., D1=0 with only one variable shift ratio D0. A conversion function is constructed to examine the relationship between the transmission power and the shift ratio under single-phase-shift modulation. This yields the range of independent variables under single-phase-shift modulation, and the corresponding expression for D0 is: 。 9. The fault-tolerant multi-mode current stress optimization modulation strategy for DAB converters according to any one of claims 1 to 8, characterized in that, A closed-loop control system employing multi-modal control is used to implement a current stress optimization modulation strategy. The complete operation process of the closed-loop control system includes the following steps: Step 1: Sample the input voltage U1 and output voltage U2 of the S-DAB converter and calculate the voltage transformation ratio M; Step 2: Subtract the output reference voltage from the output voltage input subtractor to obtain the error quantity, input it to the PI controller, and obtain the independent variable V through corresponding calculations; Step 3: Determine the power limit for mode switching based on the voltage ratio M, and dynamically select the appropriate operating mode based on the range of the independent variable V obtained from the PI controller calculation; Step 4: Generate the corresponding PWM signal based on the optimal shift ratio expression of the mode to drive the corresponding switch to turn on and off.

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