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

Abstract

This invention discloses a four-degree-of-freedom modulation method for minimizing current stress in a 3 / 2LNPC-DAB DC converter. Based on the four-degree-of-freedom modulation model of the 3 / 2LNPC-DAB DC converter, a unified model of current stress and transmission power is established using the equivalent wave modeling method. Combining its graphical features, five optimization principles for minimizing current stress are derived. The optimal modes for minimizing current stress in the DC converter are quickly screened, and first-order equality constraints corresponding to each optimal mode are obtained. Finally, the obtained equality constraints are used to obtain an analytical solution for current stress minimization modulation over the full power range using the KKT condition. The invention has a simple and intuitive implementation process, solves the difficult problem of optimizing four-degree-of-freedom modulation in DC converters, and reduces conduction losses in the power circuit by minimizing current stress, thereby ensuring efficient and reliable operation of the converter.

Description

Technical Field

[0001] The present invention belongs to the technical field of power electronics, and in particular relates to a four-degree-of-freedom modulation strategy for minimizing current stress of a multi-level DAB direct current converter. Background Art

[0002] As a key interface between medium-voltage and low-voltage DC systems, the 3 / 2LNPC-DAB DC converter holds broad application prospects in energy storage systems, large-scale photovoltaic power plants, electric vehicles, and medium-voltage DC microgrids. This topology inherits the advantages of traditional two-level DAB converters, such as high power density, high efficiency, electrical isolation, soft switching capability, and bidirectional power transmission. Its three-level neutral-point-clamped (NPC) topology offers higher voltage resistance and step-up ratio, effectively reducing voltage stress and switching losses in power switching devices and improving DAB converter efficiency over a wide voltage range. 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, notes that compared to a combined IPOS DC converter consisting of two-level DAB converters, the 3 / 3LNPC-DAB DC converter avoids issues such as uneven power distribution and circulating current between modules, thereby simplifying the control system.

[0003] Single-phase shift (SPS) control is widely used in DAB converters due to its simplicity. However, when the converter's input and output voltages are mismatched or the converter operates under light load conditions, the converter's peak inductor current and reactive power increase significantly, reducing converter efficiency. To address this issue, various phase-shift control methods have been proposed to improve DAB converter performance, such as dual-phase shift (DPS) control, extended phase shift (EPS) control, three-phase shift (TPS) control, and five-level control. However, numerical optimization modulation methods can only be implemented using lookup tables in a microcontroller, and their calculation results are affected by variations in circuit parameters. Therefore, numerical optimization modulation methods have certain limitations in practical applications. Compared to numerical solutions, analytical optimization modulation methods ensure good dynamic characteristics of the converter when the transmitted power varies. Furthermore, as the number of control degrees of freedom increases, the difficulty of solving the analytical solution for the optimal modulation increases dramatically. Symmetrical phase-shift modulation in multilevel converters of different structures has a maximum number of control degrees of freedom. For example, the symmetrical phase-shift modulation of a two-level DAB DC converter has a maximum of three degrees of freedom, while the symmetrical phase-shift modulation of a 3 / 2LNPC-DAB DC converter has a maximum of four degrees of freedom. In "Optimal Phase-Shift Control to Minimize Reactive Power for a DualActive Bridge DC–DC Converter," published in IEEE Transactions on Power Electronics in 2019, a TPS optimization modulation strategy for minimizing the current stress of a two-level DAB converter based on an analytical solution was proposed. However, this strategy only minimizes the current stress of the two-level DAB converter with three degrees of freedom.

[0004] Glossary:

[0005] KKT Condition: Karush-Kuhn-Tucker Condition (common aliases: Kuhn-Tucker, KKT Condition, Karush-Kuhn-Tucker Optimization Condition, Karush-Kuhn-Tucker Condition, Kuhn-Tucker Optimization Condition, Kuhn-Tucker Condition) is a necessary and sufficient condition for a nonlinear programming problem to have an optimal solution under certain regular conditions. This is the result of a generalized Lagrange multiplier. Summary of the Invention

[0006] In order 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.

[0007] To achieve the above object, the technical solution of the present invention is:

[0008] A four-degree-of-freedom modulation method for minimizing current stress of a 3 / 2LNPC-DAB DC converter comprises the following steps:

[0009] Step 1: Establish a modulation model of the 3 / 2LNPC-DAB DC converter under four degrees of freedom;

[0010] Step 2: Based on the modulation model, a unified model of current stress and transmission power of the 3 / 2LNPC-DAB DC converter is established by using the equivalent wave modeling method;

[0011] Step 3: Based on the graphical characteristics of the unified model of current stress and transmission power, five optimization principles for minimizing current stress are obtained:

[0012] 1) The rising edge of the small level does not lag behind rising edge;

[0013] 2) The falling edge of the small level does not lag behind Falling edge;

[0014] 3) Yes For the falling edge of When the central axis remains unchanged The falling edges of small and large levels can be shifted backward;

[0015] 4) Yes For the rising edge of Greater than , then keep When the central axis remains unchanged The rising edge can be translated forward;

[0016] 5) The falling edge of the high level does not precede rising edge;

[0017] in, 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;

[0018] 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;

[0019] Step 5: The first-order equality constraints of each optimal mode are obtained, and the KKT condition is combined to solve the analytical solution for minimizing the current stress modulation of the 3 / 2LNPC-DAB DC converter, and the 3 / 2LNPC-DAB DC converter is modulated according to the analytical solution.

[0020] As a further improvement, the modulation model in step 1 is as follows:

[0021] The voltage difference between the midpoints of the two bridge arms of the primary full bridge of the 3 / 2LNPC-DAB DC converter is defined as , define the voltage difference between the midpoints of the two bridge arms of the secondary full bridge as ,definition The voltage converted to the primary side of the transformer by the ideal transformer is ,definition 、 、 Level is used to describe The working waveform of Level is The unilateral high level, Level is Unilateral small level; is the DC voltage on the primary side;

[0022] Define the four control degrees of freedom and Used to control 3 / 2LNPC-DAB DC converter; yes The duty cycle of unilateral small level, yes The duty cycle of unilateral large level, yes The duty cycle of the high level, yes Small level rising edge and Phase shift ratio between high level rising edges; is half a switching cycle.

[0023] As a further improvement, in step 2, the unified model of current stress and transmission power is as follows:

[0024] 2.1) Definition It is the current moment When the amplitude is 0.5 and the period is A square wave with a 50% duty cycle, for The points, is the transformation ratio of the transformer;

[0025] 2.2) According to the square wave , and They are represented by the superposition of four voltage sources and two voltage sources, as shown in the following formula:

[0026]

[0027] in, For the current moment hour The value of For the current moment hour , is the DC voltage on the secondary side, For the current moment When the amplitude is 0.5 and the period is The initial phase is A square wave with a 50% duty cycle, For the current moment When the amplitude is 0.5 and the period is The initial phase is A square wave with a 50% duty cycle;

[0028] in, , For the current moment When the amplitude is 0.5 and the period is The initial phase is A square wave with a 50% duty cycle, For the current moment When the amplitude is 0.5 and the period is The initial phase is A square wave with a 50% duty cycle, For the current moment When the amplitude is 0.5 and the period is The initial phase is A square wave with a 50% duty cycle;

[0029] 2.3) The power transfer characteristics are determined based on the voltage output of the full-bridges on both sides of the DAB converter. The inductor current changes are shown in the following equation:

[0030]

[0031] in, is the inductor current, is the leakage inductance of the transformer; represents the derivation;

[0032] 2.4) The inductor current decomposition model is:

[0033] ;

[0034] in, ;

[0035] 2.5) According to the inductor current decomposition model, and Integrate to get the input power , output power for:

[0036]

[0037] in, for The rising edge time of the level, for The falling edge time of the level, for The falling edge time of the level, for The rising edge time of the level, for The falling edge moment of the level.

[0038] As a further improvement, in step three, three ideas for minimizing current stress are first obtained based on the graphical features, namely: reducing current stress while keeping the transmission power unchanged; increasing transmission power while keeping the current stress unchanged; and increasing transmission power while reducing current stress; and then the five optimization principles for minimizing current stress are obtained based on the three ideas.

[0039] As a further improvement, in step 4, the first-order equality constraint corresponding to the optimal mode for minimizing current stress is as follows:

[0040] .

[0041] As a further improvement, the process of obtaining the analytical solution in step 5 is described as follows:

[0042]

[0043] Where, represents the constructed Lagrangian function, and represents the Lagrange multiplier, represents the optimal set of control degrees of freedom variables and needs to satisfy equality and inequality constraints. represents the set of four control degrees of freedom, Indicates 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 and inequality constraints, represents the per-unit target transmission power, represents the number of inequality constraints, represents the inequality constraints on the control variables, It means partial derivative.

[0044] For further improvement, in step 5, the analytical solution is as follows:

[0045] when hour:

[0046]

[0047] when hour:

[0048]

[0049] Where,

[0050]

[0051] in, express The maximum transmission power corresponding to the first working condition is express The maximum transmission power corresponding to the second working condition is express The maximum transmission power corresponding to the third working condition is: is the current voltage transfer ratio The per-unit transmission power of the optimal solution for minimizing the current stress is:

[0052] when hour:

[0053] .

[0054] Advantages of the present invention:

[0055] Compared to existing technologies, the present invention addresses the difficulty in finding an analytical solution for optimizing modulation of a 3 / 2LNPC-DAB DC converter as the number of control degrees of freedom increases. This strategy provides a four-degree-of-freedom modulation strategy for minimizing current stress in a 3 / 2LNPC-DAB DC converter. Based on the four-degree-of-freedom modulation model of the 3 / 2LNPC-DAB DC converter, this strategy establishes a unified model of current stress and transmission power using equivalent wave modeling. Combining this model with graphical features, five optimization principles for minimizing current stress are derived. Based on these five optimization principles, the optimal modes for minimizing current stress in the 3 / 2LNPC-DAB DC converter are quickly identified, and first-order equality constraints corresponding to each optimal mode are derived. Finally, the resulting equality constraints are used to obtain an analytical solution for minimizing current stress over the full power range using KKT conditions. The present invention's implementation process is concise and intuitive, addressing the challenges of optimizing four-degree-of-freedom modulation in a 3 / 2LNPC-DAB DC converter. By minimizing current stress, conduction losses in the power circuit are reduced, thereby ensuring efficient and reliable operation of the converter. BRIEF DESCRIPTION OF THE DRAWINGS

[0056] Figure 1 1 is a circuit topology diagram of a 3 / 2LNPC-DAB DC converter according to an embodiment of the present invention;

[0057] Figure 2 is a modulation model diagram of a converter in an embodiment of the present invention;

[0058] Figure 3 is an equivalent circuit model diagram of a converter according to an embodiment of the present invention;

[0059] Figure 4 This is a flow chart for optimizing the optimal mode of a 3 / 2LNPC-DAB DC converter according to an embodiment of the invention;

[0060] Figure 5 In the embodiment of the present invention, k Optimal modal diagram of the converter under the condition of ≤1; (a) is mode 1, (b) is mode 2;

[0061] Figure 6 In the embodiment of the present invention, k Optimal modal diagram of the converter under ≤2 working conditions; (a) is mode 3, (b) is mode (4), and (c) is mode 5;

[0062] Figure 7 In the embodiment of the present invention, k >2 optimal modal diagram of the converter; (a) is mode 6, (b) is mode 7, and (c) is mode 8;

[0063] Figure 8 In the simulation example Working waveforms corresponding to different power levels of the converter under working conditions;

[0064] Figure 9 In the simulation example Working waveforms corresponding to different power levels of the converter under working conditions;

[0065] Figure 10 In the simulation example The working waveforms of the converter corresponding to different power levels under working conditions. DETAILED DESCRIPTION

[0066] The present invention will be further described below with reference to the accompanying drawings and embodiments. Example

[0067] The present invention provides a four-degree-of-freedom modulation strategy for minimizing the current stress of a 3 / 2LNPC-DAB DC converter. Figure 1 This is the circuit topology of the DC converter, consisting of a three-level neutral-point-clamped (NPC) bridge on the primary side, a two-level H-bridge on the secondary side, and a high-frequency transformer. The primary side consists of eight power switches P1-P8 and four diodes D1-D4, with two bridge arms forming a full-bridge circuit, with their midpoints at points a and b. The secondary side consists of four power switches S1-S4, with two bridge arms forming a full-bridge circuit, with their midpoints at points x and y, respectively. These points are connected to the secondary winding of the high-frequency transformer. is the transformation ratio of the transformer; It is the transformer leakage inductance or external inductance; 、 is the midpoint capacitance on the input side; is the output side capacitor. is the voltage difference between point a and point b; is the voltage difference between point x and point y; The voltage converted to the primary side of the transformer by the ideal transformer is ; is the inductor current; is the DC voltage on the primary side; is the DC voltage on the secondary side.

[0068] The modulation model of 3 / 2LNPC-DAB DC converter is as follows: Figure 2 As shown. Ignoring the dead time, the driving signals of the power switch devices P1 and P3, P2 and P4, P5 and P7, P6 and P8, S1 and S2, S3 and S4 are complementary. Among them, the duty cycle of P1-P8 is not fixed to 50%, while the duty cycle of S1-S4 is fixed to 50%. of The level is defined as a small level, The level is defined as the maximum level, 、 Respectively The duty cycle of unilateral small level and unilateral large level. of The level is defined as high level, express The duty cycle of the high level, express Small level rising edge and The phase shift ratio between the high level rising edges. In addition, Represents half a switching cycle.

[0069] Will It is defined as an amplitude of 0.5 and a period of A 50% duty cycle square wave, Defined as The integral of . Using the equivalent wave modeling method, Decompose into four equivalent voltage sources, Decomposed into two equivalent voltage sources. Figure 3 This is the equivalent circuit model established for the converter in the embodiment of the present invention. Based on this model, the mathematical model of the inductor current is further established, as shown in the following equations (1) and (2):

[0070] ; (1)

[0071] ; (2)

[0072] Based on formula (1) and (2), and Integrate to get the input power , output power for:

[0073] (3)

[0074] In formula (3), for The rising edge time of the level, for The falling edge time of the level, for The falling edge time of the level, for The rising edge time of the level, for The falling edge moment of the level.

[0075] By analyzing the time when the current peak of the 3 / 2LNPC-DAB DC converter occurs based on the graphical characteristics of formula (3), three ideas for minimizing the current stress of the 3 / 2LNPC-DAB DC converter are obtained, namely: reducing the current stress while keeping the transmission power unchanged; increasing the transmission power while keeping the current stress unchanged; and increasing the transmission power while reducing the current stress.

[0076] 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 Rising edge; 2. The falling edge of the small level does not lag behind Falling edge; 3. For the falling edge of When the central axis remains unchanged The falling edge of small level and large level can be shifted backward; 4. For the rising edge of Greater than , then keep When the central axis remains unchanged The rising edge can be translated forward; 5. The falling edge of the high level does not precede rising edge.

[0077] Based on the first, second and fifth optimization principles FOP-MCS, all modes that are beneficial to minimizing the current stress of the 3 / 2LNPC-DAB DC converter are screened. In this case, all the modes after the converter screening are further optimized by the third and fourth optimization principles FOP-MCS. The optimization flow chart of the best mode of 3 / 2LNPC-DAB DC converter is as follows: Figure 4 As shown. When , the two optimal modes of 3 / 2LNPC-DAB DC converter are as follows Figure 5 As shown in (a) and (b) in the figure. When , the three optimal modes of 3 / 2LNPC-DAB DC converter are as follows Figure 6 As shown in (a), (b), and (c). When , the three optimal modes of 3 / 2LNPC-DAB DC converter are as follows Figure 7 The first-order equality constraint corresponding to the optimal mode of the 3 / 2LNPC-DAB DC converter is shown in (a), (b), and (c).

[0078] (4)

[0079] Based on the optimal modal first-order equality constraint of formula (4), combined with the KKT condition, the current stress minimization problem of the 3 / 2LNPC-DAB DC converter is solved, which can be described as follows:

[0080] (5)

[0081] In formula (5), represents the constructed Lagrangian function, and represents the Lagrange multiplier, represents the optimal set of control variables and needs to satisfy equality and inequality constraints. represents four control variables, Indicates per-unit peak current, represents the per-unit transmission power, represents the per-unit target transmission power, represents the number of inequality constraints, represents the inequality constraints on the control variables, It means partial derivative.

[0082] According to the solution in formula (5), when The optimal solution for minimizing the converter current stress is shown in Table 1. The optimal solution for minimizing the converter current stress is shown in Table 2. When , the optimal solution for minimizing the converter current stress is shown in Table 3. is the current voltage transfer ratio The per-unit transmission power of the optimal solution for minimizing current stress is shown in Figure 2.

[0083] Table 1 The optimal solution to minimize current stress

[0084]

[0085] Table 2 The optimal solution to minimize current stress when

[0086]

[0087] Table 3 The optimal solution to minimize current stress when

[0088]

[0089] The current stress minimization simulation model of the 3 / 2LNPC-DAB DC converter was built on the PLECS simulation platform. The key simulation parameters are shown in Table 4.

[0090] Table 4 Main parameters of simulation model

[0091]

[0092] When the 3 / 2LNPC-DAB DC converter works In this case, the main working waveforms of the converter are as follows: Figure 8 In the two power ranges, the input voltage and output voltage 150V and 200V respectively, the voltage transfer ratio is 0.61, Constant at 0. In the first power range, the inductor current It is a discontinuous triangle wave, and the converter works in mode 1. In the second power range, is constant to 0, the converter works in mode 2, and the four control degrees of freedom of the 3 / 2LNPC-DAB DC converter are simplified to two control degrees of freedom, namely DPS modulation.

[0093] When the 3 / 2LNPC-DAB DC converter works In this case, the main working waveforms of the converter are as follows: Figure 9 Input voltage and output voltage 200V and 100V respectively, the voltage transfer ratio is 1.62. In the first power range, , the converter works in mode 3, when When the level is 0, the inductor current is 0, and the converter's inductor forward current peak will appear twice in one switching cycle. In the other three power ranges, ,Right now It does not contain 0 level. In the second and third power ranges, the converter operates in mode 4. In the second power range, the converter's inductor forward current peak also appears twice in one switching cycle, but The rising edge of the high level will precede In the fourth power range, the converter operates in mode 5.

[0094] When the 3 / 2LNPC-DAB DC converter works In this case, the main working waveforms of the converter are as follows: Figure 10 Input voltage and output voltage 240V and 80V respectively, the voltage transfer ratio is 2.42. In the first two power ranges, Constantly equal to 0, that is It is composed entirely of small levels and 0 levels. In the last two power intervals, Constant at 1. In the first power range, the converter operates in mode 6, and its inductor current is a discontinuous triangle 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.

[0095] Although the embodiments of the present invention have been disclosed above, they are not limited to the applications listed in the description and the embodiments. They can be fully applied to various fields suitable for the present invention. For those familiar with the art, additional modifications can be easily implemented. Therefore, without departing from the general concept defined by the claims and the scope of equivalents, the present invention is not limited to the specific details and shown here.

Claims

1. A four-degree-of-freedom modulation method for minimizing current stress of a 3 / 2LNPC-DAB DC converter, characterized in that: The steps include: Step 1: Establish a modulation model of the 3 / 2LNPC-DAB DC converter under four degrees of freedom; Step 2: Based on the modulation model, a unified model of current stress and transmission power of the 3 / 2LNPC-DAB DC converter is established by using the equivalent wave modeling method; Step 3: Based on the graphical characteristics of the unified model of current stress and transmission power, five optimization principles for minimizing current stress are obtained: 1) The rising edge of the small level does not lag behind rising edge; 2) The falling edge of the small level does not lag behind Falling edge; 3) Yes For the falling edge of When the central axis remains unchanged The falling edges of small and large levels can be shifted backward; 4) Yes For the rising edge of Greater than , then keep When the central axis remains unchanged The rising edge can be translated forward; 5) The falling edge of the high level does not precede rising edge; in, 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: The first-order equality constraints of each optimal mode are obtained, and the KKT condition is combined to solve the analytical solution for minimizing the current stress modulation of the 3 / 2LNPC-DAB DC converter, and the 3 / 2LNPC-DAB DC converter is modulated according to the analytical solution.

2. The four-degree-of-freedom modulation method for minimizing current stress of a 3 / 2LNPC-DAB DC converter according to claim 1, characterized in that: The modulation model in step 1 is as follows: The voltage difference between the midpoints of the two bridge arms of the primary full bridge of the 3 / 2LNPC-DAB DC converter is defined as , define the voltage difference between the midpoints of the two bridge arms of the secondary full bridge as ,definition The voltage converted to the primary side of the transformer by the ideal transformer is ,definition 、 、 Level is used to describe The working waveform of Level is The unilateral high level, Level is Unilateral small level; is the DC voltage on the primary side; Define the four control degrees of freedom and Used to control 3 / 2LNPC-DAB DC converter; yes The duty cycle of unilateral small level, yes The duty cycle of unilateral large level, yes The duty cycle of the high level, yes Small level rising edge and Phase shift ratio between high level rising edges; is half a switching cycle.

3. The four-degree-of-freedom modulation method for minimizing current stress of a 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 It is the current moment When the amplitude is 0.5 and the period is A square wave with a 50% duty cycle, for The points, is the transformation ratio of the transformer; 2.2) According to the square wave , and They are represented by the superposition of four voltage sources and two voltage sources, as shown in the following formula: ； in, For the current moment hour The value of For the current moment hour , is the DC voltage on the secondary side, For the current moment When the amplitude is 0.5 and the period is The initial phase is A square wave with a 50% duty cycle, For the current moment When the amplitude is 0.5 and the period is The initial phase is A square wave with a 50% duty cycle; in, , For the current moment When the amplitude is 0.5 and the period is The initial phase is A square wave with a 50% duty cycle, For the current moment When the amplitude is 0.5 and the period is The initial phase is A square wave with a 50% duty cycle, For the current moment When the amplitude is 0.5 and the period is The initial phase is A square wave with a 50% duty cycle; 2.3) The power transfer characteristics are determined based on the voltage output of the full-bridges on both sides of the DAB converter. The inductor current changes are shown in the following equation: ； in, is the inductor current, is the leakage inductance of the transformer; represents the derivation; 2.4) The inductor current decomposition model is: in, ; 2.5) According to the inductor current decomposition model, and Integrate to get the input power , output power for: ； in, for The rising edge time of the level, for The falling edge time of the level, for The falling edge time of the level, for The rising edge time of the level, for The falling edge moment of the level.

4. The four-degree-of-freedom modulation method for minimizing current stress of a 3 / 2LNPC-DAB DC converter according to claim 1, characterized in that: In the step three, three ideas for minimizing current stress are first obtained based on the graphical features, namely: reducing current stress while keeping the transmission power unchanged; increasing transmission power while keeping the current stress unchanged; and increasing transmission power while reducing current stress; then, the five optimization principles for minimizing current stress are obtained based on the three ideas.

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

7. The four-degree-of-freedom modulation method for minimizing current stress of a 3 / 2LNPC-DAB DC converter according to any one of claims 1 to 6, characterized in that: In step 5, the analytical solution is as follows: when hour: ； when hour: ； Where, ； in, express The maximum transmission power corresponding to the first working condition is express The maximum transmission power corresponding to the second working condition is express The maximum transmission power corresponding to the third working condition is: is the current voltage transfer ratio The per-unit transmission power of the optimal solution for minimizing the current stress is: when hour: 。

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