Four-degree-of-freedom modulation method for minimizing current stress of 3 / 2 LNPC-DAB direct-current converter

Abstract

The invention discloses a four-degree-of-freedom modulation method for minimizing current stress of a 3 / 2LNPC-DAB direct-current converter, and the method comprises the steps: building a unified model of current stress and transmission power through an equivalent wave modeling method on the basis of a four-degree-of-freedom modulation model of the 3 / 2LNPC-DAB direct-current converter, deducing five optimization principles for minimizing the current stress in combination with the graphical features of the unified model, and obtaining a four-degree-of-freedom modulation model of the 3 / 2LNPC-DAB direct-current converter. The method comprises the following steps of: quickly screening out an optimal mode of current stress minimization of the direct-current converter, obtaining a first-order equality constraint corresponding to each optimal mode, and finally solving an analytical solution of current stress minimization modulation in a full-power range through a KKT condition by utilizing the obtained equality constraints. According to the method, the implementation process is simple and visual, the problem of four-degree-of-freedom modulation optimization of the direct-current converter is solved, and the conduction loss in a power loop is reduced by realizing the minimization of the current stress, so that the efficient and reliable operation of the converter is ensured.

Description

Technical Field

[0001] The present invention belongs to the field of power electronics technology, and particularly relates to a four-degree-of-freedom modulation strategy for minimizing the current stress of a multilevel DAB DC converter. Background Technique

[0002] The 3 / 2LNPC-DAB DC converter, as a key interface connecting medium-voltage DC systems and low-voltage DC systems, has broad application prospects in fields such as energy storage systems, large-scale photovoltaic power plants, electric vehicles, and medium-voltage DC architecture microgrids. This topology not only inherits the advantages of traditional two-level DAB converters, such as high power density, high efficiency, electrical isolation, soft-switching ability, and bidirectional power transmission, but also its three-level neutral-point clamped (NPC) topology has higher voltage withstand capacity and voltage boost ratio, which can effectively reduce the voltage stress and switching losses of power switching devices, and is conducive to improving the efficiency of DAB converters under wide-range voltage variations. The article "Analysis and Optimal Modulation for 2 / 3-Level DAB Converters to Minimize Current Stress With Five-Level Control" published in IEEE Transactions on Power Electronics in 2023 pointed out that compared with the combined IPOS DC converter composed of two-level DAB converters, the 3 / 3LNPC-DAB DC converter avoids problems such as uneven power distribution and circulating current between modules, thus simplifying the control system.

[0003] Single-phase shift (SPS) control is widely used in DAB converters because of its simple control. However, when the input and output voltages of the converter are mismatched or the converter operates under light load conditions, the peak inductor current and reactive power of the converter will increase significantly, thus reducing the efficiency of the converter. To solve this problem, various phase-shift control methods have been proposed to improve the performance of DAB converters, such as dual-phase shift (DPS) control, extended phase shift (EPS) control, three-phase shift (TPS) control, and five-level control. However, the optimization modulation method based on numerical solutions can only be implemented through a look-up table in a microcontroller, and its calculation results are affected by changes in circuit parameters. Therefore, the optimization modulation method based on numerical solutions has certain limitations in practical applications. The optimization modulation method based on analytical solutions ensures good dynamic characteristics of the converter when the transmitted power changes compared with numerical solutions. Moreover, as the number of control freedoms increases, the difficulty of solving the analytical solution of its optimization modulation increases sharply. The symmetric phase-shift modulation of multilevel converters with different structures has at most different numbers of control freedoms. For example, the symmetric phase-shift modulation of a two-level DAB DC converter has at most 3 control freedoms, and the symmetric phase-shift modulation of a 3 / 2LNPC-DAB DC converter has at most 4 control freedoms. In the paper "Optimal Phase-Shift Control to Minimize Reactive Power for a Dual Active Bridge DC–DC Converter" published in IEEE Transactions on Power Electronics in 2019, a TPS optimization modulation strategy based on analytical solutions to minimize the current stress of a two-level DAB converter was proposed. However, this strategy only realizes the minimization of the current stress of a two-level DAB converter under 3 control freedoms.

[0004] Glossary: KKT conditions: The Karush-Kuhn-Tucker Conditions (original English name: Karush-Kuhn-Tucker Conditions, common aliases: Kuhn-Tucker, KKT conditions, Karush-Kuhn-Tucker optimization conditions, Karush-Kuhn-Tucker conditions, Kuhn-Tucker optimization conditions, Kuhn-Tucker conditions) are a necessary and sufficient condition for a nonlinear programming problem to have an optimal solution under some regular conditions. This is a generalization of the Lagrange multiplier result. Summary of the Invention

[0005] To solve the above problems, the present invention discloses a four-degree-of-freedom modulation method for minimizing the current stress of a 3 / 2LNPC-DAB DC converter.

[0006] To achieve the above object, the technical solution of the present invention is as follows: A four-degree-of-freedom modulation method for minimizing the current stress of a 3 / 2LNPC-DAB DC converter, comprising the following steps: Step 1, establish a modulation model of the 3 / 2LNPC-DAB DC converter under four degrees of freedom; Step 2, according to the modulation model, establish a unified model of the current stress and transmission power of the 3 / 2LNPC-DAB DC converter by the equivalent wave modeling method; Step 3, obtain five optimization principles for minimizing the current stress according to the graphical characteristics of the unified model of the current stress and transmission power: 1) The rising edge of the small level is not lagging behind the rising edge; 2) The falling edge of the small level is not lagging behind the falling edge; 3) For the falling edge of while keeping the central axis unchanged the falling edges of the small level and the large level can be translated backward; 4) For the rising edge of if is greater than while keeping the central axis unchanged the rising edge can be translated forward; 5) The falling edge of the large level is not ahead of the rising edge; wherein, is the voltage difference between the midpoints of the two bridge arms of the primary full bridge of the 3 / 2LNPC-DAB DC converter, is the primary voltage of the 3 / 2LNPC-DAB DC converter; Step 4, obtain the first-order equality constraint corresponding to the optimal mode for minimizing the current stress of the 3 / 2LNPC-DAB DC converter through the five optimization principles for minimizing the current stress; Step 5, obtain the first-order equality constraint of each optimal mode, combine the KKT conditions to solve the analytical solution of the modulation for minimizing the current stress of the 3 / 2LNPC-DAB DC converter, and perform the modulation of the 3 / 2LNPC-DAB DC converter according to the analytical solution.

[0007] For further improvement, the modulation model in Step 1 is as follows: Define the voltage difference between the midpoints of the two arms of the primary full-bridge of the 3 / 2LNPC-DAB DC converter as , and define the voltage difference between the midpoints of the two arms of the secondary full-bridge as , define The voltage converted to the primary side of the transformer through an ideal transformer is , define , , The levels are used to describe the working waveform; among them, The level is the unilateral large level of , and the level is the unilateral small level of is the DC voltage on the primary side; Define four control degrees of freedom and for controlling the 3 / 2LNPC-DAB DC converter; among them, is the duty cycle of the unilateral small level, is the duty cycle of the unilateral large level, is the duty cycle of the high level, is the phase shift ratio between the rising edge of the small level and the rising edge of the high level; is half of the switching period.

[0008] For further improvement, in the second step, the unified model of current stress and transmission power is as follows: 2.1) Define as a square wave with an amplitude of 0.5 and a period of at the current time with a 50% duty cycle, is the integral of is the turns ratio of the transformer; 2.2) According to the square wave , and are respectively represented by the superposition of four voltage sources and two voltage sources, as shown in the following formula:

[0009] Among them, is the value of at the current time , is the value of at the current time , is the DC voltage on the secondary side, is the current moment a square wave with an amplitude of 0.5, a period of , and an initial phase of at a 50% duty cycle, is the current moment a square wave with an amplitude of 0.5, a period of , and an initial phase of at a 50% duty cycle; Among them, , is the current moment a square wave with an amplitude of 0.5, a period of , and an initial phase of at a 50% duty cycle, is the current moment a square wave with an amplitude of 0.5, a period of , and an initial phase of at a 50% duty cycle, is the current moment a square wave with an amplitude of 0.5, a period of , and an initial phase of at a 50% duty cycle; 2.3) Determine the power transfer characteristics based on the voltage outputs of the full bridges on both sides of the DAB converter. The inductor current changes as shown in the following formula:

[0010] Among them, is the inductor current, is the leakage inductance of the transformer; represents taking the derivative; 2.4) The inductor current decomposition model is: ; Among them, ; 2.5) According to the inductor current decomposition model, by integrating and , the input power , the output power are:

[0011] Among them, is the rising edge moment of the level, is the falling edge moment of the level, is the falling edge moment of the level, is the rising edge time of the level, is the falling edge time of the level.

[0012] For further improvement, in step three, first, three ideas for minimizing current stress are obtained based on the graphical features, namely: reducing current stress while keeping the transmission power constant; increasing the transmission power while keeping the current stress constant; increasing the transmission power while reducing the current stress; and then, based on the three ideas, the five optimization principles for minimizing current stress are obtained.

[0013] For further improvement, in step four, the first-order equality constraint corresponding to the optimal mode of minimizing current stress is as follows: .

[0014] For further improvement, the process of solving the analytical solution in step five is described as follows:

[0015] In the formula, represents the constructed Lagrangian function, and represent the Lagrange multipliers, represents the optimal set of control degree-of-freedom variables and needs to satisfy the equality constraint and inequality constraint conditions, represents the set of four control degrees of freedom, represents the per-unit peak current, represents the per-unit transmission power, represents the per-unit transmission power corresponding to the optimal set of control degree-of-freedom variables that satisfy the equality constraint and inequality constraint conditions, represents the per-unit target transmission power, represents the number of inequality constraint conditions, represents the inequality constraint condition of the control variable, represents taking the partial derivative.

[0016] For further improvement, the analytical solution in step five is as follows: When :

[0017] When :

[0018] In the formula,

[0019] Among them, denotes the maximum transmission power corresponding to the first operating condition when denotes the maximum transmission power corresponding to the second operating condition when denotes the maximum transmission power corresponding to the third operating condition when is the per-unit transmission power magnitude of the optimal solution for minimizing the current stress under the current voltage transmission ratio ; When : .

[0020] Advantages of the present invention: Compared with the prior art, in order to solve the problem of difficult solution of the optimal modulation analytical solution of the 3 / 2LNPC-DAB DC converter as the number of control degrees of freedom increases, the present invention provides a four-degree-of-freedom modulation strategy for minimizing the current stress of the 3 / 2LNPC-DAB DC converter. Based on the four-degree-of-freedom modulation model of the 3 / 2LNPC-DAB DC converter, the present invention establishes a unified model of current stress and transmission power through the equivalent wave modeling method, and derives five optimization principles for minimizing current stress in combination with its graphical characteristics. Based on these five optimization principles, the optimal modes for minimizing the current stress of the 3 / 2LNPC-DAB DC converter are quickly screened out, and the first-order equality constraints corresponding to each optimal mode are obtained. Finally, the analytical solution of the modulation for minimizing the current stress in the full power range is solved by using the obtained equality constraints through the KKT conditions. The implementation process of the present invention is simple and intuitive, solves the problem of four-degree-of-freedom modulation optimization of the 3 / 2LNPC-DAB DC converter, reduces the conduction loss in the power loop by minimizing the current stress, and thus ensures the efficient and reliable operation of the converter. Description of the Drawings

[0021] Figure 1 is the circuit topology diagram of the 3 / 2LNPC-DAB DC converter in the embodiment of the present invention; Figure 2 is the modulation model diagram of the converter in the embodiment of the present invention; Figure 3 is the equivalent circuit model diagram of the converter in the embodiment of the present invention; Figure 4 is the optimization flow chart of the best mode of the 3 / 2LNPC-DAB DC converter in the embodiment of the invention; Figure 5 is in the embodiment of the present invention k ≤ 1 the optimal mode diagram of the converter under the operating condition; where (a) is mode 1 and (b) is mode 2; Figure 6is the optimal mode diagram of the converter under the condition of 1< k ≤ 2 working conditions; where (a) is mode 3, (b) is mode (4), and (c) is mode 5; Figure 7 is in the embodiment of the present invention k > 2 working conditions of the optimal mode diagram of the converter; where, (a) is mode 6, (b) is mode 7, and (c) is mode 8; Figure 8 is in the simulation example working condition of the converter corresponding to different power levels of the working waveform diagram; Figure 9 is in the simulation example working condition of the converter corresponding to different power levels of the working waveform diagram; Figure 10 is in the simulation example working condition of the converter corresponding to different power levels of the working waveform diagram. Specific embodiments

[0022] The present invention will be further described below with reference to the accompanying drawings and embodiments. Embodiment

[0023] The present invention provides a four-degree-of-freedom modulation strategy for minimizing the current stress of a 3 / 2LNPC-DAB DC converter. As Figure 1 is the circuit topology of the DC converter, which consists of a primary-side three-level neutral-point clamped (NPC) bridge, a secondary-side two-level H bridge, and a high-frequency transformer. The primary side is composed of eight power switch devices P1-P8 and four diodes D1-D4. Two bridge arms form a full-bridge circuit, and the midpoints of the two bridge arms are points a and b respectively. The secondary side is composed of four power switch devices S1-S4. Two bridge arms form a full-bridge circuit, and the midpoints of the two bridge arms are points x and y respectively. Points x and y are connected to the secondary-side winding of the high-frequency transformer. is the turns ratio of the transformer; is the leakage inductance of the transformer or an externally added inductance; , are the input-side neutral-point capacitors; is the output-side capacitor. is the voltage difference between points a and b; is the voltage difference between points x and y; The voltage converted to the primary side of the transformer through an ideal transformer is ; is the inductor current; is the DC voltage on the primary side; is the DC voltage on the secondary side.

[0024] The modulation model of the 3 / 2LNPC-DAB DC converter is as follows Figure 2 shown. Ignoring the dead time, the driving signals of power switch devices P1 and P3, P2 and P4, P5 and P7, P6 and P8, S1 and S2, S3 and S4 are complementary respectively. Among them, the duty cycles of P1 - P8 are not fixed at 50%, while the duty cycles of S1 - S4 are fixed at 50%. Define the level as the small level, level as the large level, and represent the duty cycles of the single-sided small level and the single-sided large level respectively. Define the level as the high level, represents the duty cycle of the high level, represents the phase shift ratio between the rising edge of the small level and the rising edge of the high level. In addition, represents half of the switching period.

[0025] Define as a square wave with an amplitude of 0.5 and a duty cycle of 50% with a period of , and define as the integral of . Decompose into four equivalent voltage sources by the equivalent wave modeling method, and decompose Figure 3 into two equivalent voltage sources. Figure 3 This is the equivalent circuit model established for the converter in the embodiment of the present invention. Through this model, a mathematical model of the inductor current is further established as shown in the following equations (1) and (2): ; (1)

[0026] ; (2)

[0027] Based on equations (1) and (2), integrate and to obtain the input power , output power as follows: (3) In equation (3), is the rising edge time of the level, is the falling edge time of the level, is the falling edge time of the level, is​​​​​​​​​​​​​​​ The rising edge moment of the level, is the falling edge moment of the level.

[0028] Analyze the moment when the current peak appears in the 3 / 2LNPC-DAB DC converter from the graphical characteristics of Equation (3), and obtain three ideas for minimizing the current stress of the 3 / 2LNPC-DAB DC converter, namely: reducing the current stress while keeping the transmission power unchanged; increasing the transmission power while keeping the current stress unchanged; increasing the transmission power while reducing the current stress.

[0029] Based on the above three ideas, five optimization principles for minimizing current stress (FOP-MCS) are analyzed and summarized, namely: 1. The rising edge of the small level does not lag behind the rising edge; 2. The falling edge of the small level does not lag behind the falling edge; 3. For the falling edge of, while keeping the central axis unchanged, the falling edges of the small level and the large level can be translated backward; 4. For the rising edge of, if is greater than , then while keeping the central axis unchanged, the rising edge can be translated forward; 5. The falling edge of the large level does not lead the rising edge.

[0030] Based on the above first, second, and fifth optimization principles FOP-MCS, all modes beneficial to minimizing the current stress of the 3 / 2LNPC-DAB DC converter are screened. Under different voltage transfer ratios conditions, all modes of the converter screened by the above third and fourth optimization principles FOP-MCS are further optimized. The optimization flow chart of the best mode of the 3 / 2LNPC-DAB DC converter is as Figure 4 shown. When , two optimal modes of the 3 / 2LNPC-DAB DC converter are as Figure 5 shown in (a) and (b) of. When , three optimal modes of the 3 / 2LNPC-DAB DC converter are as Figure 6 shown in (a), (b), and (c) of. When , three optimal modes of the 3 / 2LNPC-DAB DC converter are as Figure 7As shown in (a), (b), and (c) therein. The first-order equality constraints corresponding to the optimal mode of the 3 / 2LNPC-DAB DC converter are as follows: (4) Combined with the KKT conditions, the problem of minimizing the current stress of the 3 / 2LNPC-DAB DC converter is solved from the first-order equality constraint conditions of the optimal mode in Equation (4), which can be described as follows: (5) In Equation (5), represents the constructed Lagrangian function, and represent the Lagrange multipliers, represents an optimal set of control variables and needs to satisfy the equality constraints and inequality constraint conditions, represents four control variables, represents the per-unit peak current, represents the per-unit transmission power, represents the per-unit target transmission power, represents the number of inequality constraint conditions, represents the inequality constraint conditions of the control variables, represents taking the partial derivative.

[0031] From the solution method in Equation (5), when , the optimal solution for minimizing the converter current stress is represented by Table 1; when , the optimal solution for minimizing the converter current stress is represented by Table 2; when , the optimal solution for minimizing the converter current stress is represented by Table 3, is the per-unit transmission power magnitude of the optimal solution for minimizing the current stress at the current voltage transfer ratio .

[0032] Table 1 Optimal solution for minimizing current stress when

[0033] Table 2 Optimal solution for minimizing current stress when

[0034] Table 3 Optimal solution for minimizing current stress when

[0035] Build the simulation model for minimizing the current stress of the 3 / 2LNPC-DAB DC converter on the PLECS simulation platform, and the key simulation parameters are shown in Table 4.

[0036] Table 4 Main parameters of the simulation model

[0037] When the 3 / 2LNPC-DAB DC converter operates in the following conditions, the main operating waveforms of the converter are as Figure 8 shown. In two power intervals, the input voltage and the output voltage are 150V and 200V respectively, and the voltage transfer ratio is 0.61, is constantly 0. In the first power interval, the inductor current is a discontinuous triangular wave, and the converter operates in Mode 1. In the second power interval, is constantly 0, the converter operates in Mode 2, and the four control degrees of freedom of the 3 / 2LNPC-DAB DC converter are reduced to two control degrees of freedom, namely DPS modulation.

[0038] When the 3 / 2LNPC-DAB DC converter operates in the following conditions, the main operating waveforms of the converter are as Figure 9 shown. The input voltage and the output voltage are 200V and 100V respectively, and the voltage transfer ratio is 1.62. In the first power interval, , the converter operates in Mode 3. When is at 0 level, the inductor current is 0, and the peak value of the inductor positive current of the converter will appear twice within one switching period. In the remaining three power intervals, , that is does not contain 0 level. Among them, in the second and third power intervals, the converter operates in Mode 4. In the second power interval, the peak value of the inductor positive current of the converter will also appear twice within one switching period, but the rising edge of the high level will precede the rising edge of the large level. In the fourth power interval, the converter operates in Mode 5.

[0039] When the 3 / 2LNPC-DAB DC converter operates in the following conditions, the main operating waveforms of the converter are as Figure 10 shown. The input voltage and the output voltage are 240V and 80V respectively, and the voltage transfer ratio is 2.42. In the first two power intervals, is constantly 0, that is It consists entirely of low levels and 0 levels. In the latter two power intervals, it is constantly 1. Among them, in the first power interval, the converter operates in Mode 6, and its inductor current is a discontinuous triangular wave. In the second and third power intervals, the converter operates in Mode 7. In the fourth power interval, the converter operates in Mode 8.

[0040] Although the embodiments of the present invention have been disclosed as above, they are not limited to the applications listed in the specification and embodiments. It can be fully applied to various fields suitable for the present invention. For those familiar with the field, additional modifications can be easily implemented. Therefore, without departing from the general concept defined by the claims and the equivalent scope, the present invention is not limited to the specific details and those shown here.

Claims

1. A four-degree-of-freedom modulation method for minimizing the current stress of a 3 / 2LNPC-DAB DC converter, characterized in that It includes the following steps: Step 1: Establish a modulation model of the 3 / 2LNPC-DAB DC converter under four degrees of freedom; Step 2: According to the modulation model, establish a unified model of the current stress and transmission power of the 3 / 2LNPC-DAB DC converter by the equivalent wave modeling method; Step 3: Obtain five optimization principles for minimizing current stress according to the graphical characteristics of the unified model of current stress and transmission power: 1) The rising edge of the small level does not lag behind the rising edge; 2) The falling edge of the small level does not lag behind the falling edge; 3) For 's falling edge, while keeping the central axis unchanged, the falling edges of the low level and the high level can be translated backward; 4) For 's rising edge, if is greater than , then while keeping the central axis unchanged, the rising edge can be translated forward; 5) The falling edge of the large level does not lead the rising edge; Among them, is the voltage difference between the midpoints of the two bridge arms of the full-bridge on the primary side of the 3 / 2LNPC-DAB DC converter, is the primary-side voltage of the 3 / 2LNPC-DAB DC converter; Step 4: Obtain the first-order equality constraints corresponding to the optimal mode of minimizing the current stress of the 3 / 2LNPC-DAB DC converter through the five optimization principles for minimizing current stress; Step 5: Combine the first-order equality constraints of each obtained optimal mode, solve the analytical solution of the modulation for minimizing the current stress of the 3 / 2LNPC-DAB DC converter according to the KKT conditions, and perform the modulation of the 3 / 2LNPC-DAB DC converter according to the analytical solution.

2. The four-degree-of-freedom modulation method for minimizing the current stress of the 3 / 2LNPC-DAB DC converter as described in claim 1, wherein, The modulation model in Step 1 is as follows: Define the voltage difference between the midpoints of the two bridge arms of the primary full bridge of the 3 / 2LNPC-DAB DC converter as , define the voltage difference between the midpoints of the two bridge arms of the secondary full bridge as , define the voltage converted to the primary side of the transformer through an ideal transformer as , define , , levels are used to describe the working waveform of ; among them, level is the unilateral large level of , level is the unilateral small level of ; is the DC voltage on the primary side; Define four control degrees of freedom and are used to control the 3 / 2LNPC-DAB DC converter; where is the duty ratio of the single-sided small level, is the duty ratio of the single-sided large level, is the duty ratio of the high level, is the phase shift ratio between the rising edge of the small level and the rising edge of the high level; is half of the switching period.

3. The four-degree-of-freedom modulation method for minimizing the current stress of the 3 / 2LNPC-DAB DC converter according to claim 1, characterized in that, In Step 2, the unified model of current stress and transmission power is as follows: 2.1) Definition is the current moment when the amplitude is 0.5 and the period is a square wave with a 50% duty cycle, is the integral of is the turns ratio of the transformer; 2.2) According to the square wave , and are respectively represented by the superposition of four voltage sources and two voltage sources, as shown in the following formula: ； wherein, is the value at the current moment when ; is the value at the current moment when , is the DC voltage on the secondary side; is a square wave with an amplitude of 0.5, a period of and an initial phase of at the current moment with a 50% duty cycle; is a square wave with an amplitude of 0.5, a period of and an initial phase of at the current moment with a 50% duty cycle; Among them, , is a square wave with an amplitude of 0.5, a period of at the current time , and an initial phase of with a 50% duty cycle, is a square wave with an amplitude of 0.5, a period of at the current time , and an initial phase of with a 50% duty cycle, is a square wave with an amplitude of 0.5, a period of at the current time , and an initial phase of with a 50% duty cycle; 2.3) Determine the power transmission characteristics based on the voltage outputs of the full bridges on both sides of the DAB converter, and the inductor current changes as shown in the following formula: ； Among them, is the inductor current, is the leakage inductance of the transformer; represents taking the derivative; 2.4) The inductor current decomposition model is: Among them, ; 2.5) According to the inductor current decomposition model, by integrating and , the input power and the output power are obtained as follows: ； Among them, is the rising edge time of the level, is the falling edge time of the level, is the falling edge time of the level, is the rising edge time of the level, is the falling edge time of the level.

4. The four-degree-of-freedom modulation method for minimizing the current stress of the 3 / 2LNPC-DAB DC converter according to claim 1, wherein In Step 3, first obtain three ideas for minimizing current stress according to the graphical characteristics, namely: reducing current stress while keeping the transmission power unchanged; increasing the transmission power while keeping the current stress unchanged; increasing the transmission power while reducing the current stress; then obtain the five optimization principles for minimizing current stress according to the three ideas.

5. The four-degree-of-freedom modulation method for minimizing the current stress of the 3 / 2LNPC-DAB DC converter according to claim 1, characterized in that, In Step 4, the first-order equality constraints corresponding to the optimal mode of minimizing current stress are as follows: 。 6. The four-degree-of-freedom modulation method for minimizing the current stress of the 3 / 2LNPC-DAB DC converter as described in claim 1, wherein The process of solving the analytical solution in Step 5 is described as follows: ； In the formula, represents the constructed Lagrangian function, and represent the Lagrange multipliers, represents the optimal set of control degree-of-freedom variables and needs to satisfy the equality constraints and inequality constraint conditions, represents the set of four control degrees of freedom, represents the per-unit peak current, represents the per-unit transmission power, represents the per-unit transmission power corresponding to the optimal set of control degree-of-freedom variables that satisfy the equality constraints and inequality constraint conditions, represents the per-unit target transmission power, represents the number of inequality constraint conditions, represents the inequality constraint conditions of the control variables, represents taking the partial derivative.

7. The four-degree-of-freedom modulation method for minimizing the current stress of the 3 / 2LNPC-DAB DC converter according to any one of claims 1-6, characterized in that, In Step 5, the analytical solution is as follows: When : ； When : ； In the formula, ； Among them, represents the maximum transmission power corresponding to the first operating condition at represents the maximum transmission power corresponding to the second operating condition at represents the maximum transmission power corresponding to the third operating condition at is the per-unit transmission power magnitude of the optimal solution for minimizing the current stress under the current voltage transmission ratio ; When : 。

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