Control method of multi-module DAB converter

Abstract

The invention discloses a control method of a multi-module DAB converter. The control method comprises the following steps: S1, constructing a transmission power calculation model and a current stress calculation model of the DAB converter in an extended phase-shift control mode; s2, combining the two models through a Lagrangian function and then deriving, and determining a relational expression among a phase shift ratio, a voltage conversion ratio and transmission power of the DAB converter under the minimum current stress; s3, acquiring voltage and current information of the DAB converter, and calculating actual transmission power of each DAB module according to a direct power control method; and S4, solving the relational expression among the phase shift ratio, the voltage conversion ratio and the transmission power to obtain the optimal solution of the phase shift ratio of each DAB module, and sending the optimal solution of each phase shift ratio into an extended phase shift control circuit to realize control of each DAB module. While the power balance is realized, the inductive current stress is reduced, the system loss is reduced, and the overall operation efficiency is improved.

Description

Technical Field

[0001] The invention relates to the technical field of power electronic converters, and in particular to a control method for a multi-module DAB converter. Background Art

[0002] The dual-active-bridge (DAB) converter has attracted extensive attention from experts and scholars at home and abroad in recent years due to its advantages such as electrical isolation, simple modular expansion, high-ratio buck-boost, and bidirectional power transmission. Among the different types of DAB converters, the power level of multi-module DAB converters is higher than that of single DAB converters. Multi-module DAB converters have advantages that single DAB converters do not have, but they also have problems such as excessive current stress and large reflux power. In addition, multi-module DAB converters also have some unique problems, namely, the power imbalance between each module causes the system to be unable to operate stably. Nowadays, domestic and foreign experts have proposed many methods to solve the power imbalance of each module, each with its own advantages and disadvantages, and different in terms of dynamic characteristics, degree of modularization, number of sensors, etc.

[0003] For modular combination systems, if the circuit parameters of each module are completely matched, then the common duty cycle control is the simplest and most effective method. Its essence is to control the total output voltage through a voltage closed loop, and the output generates a duty cycle signal through a pulse width modulator to jointly control multiple groups of converters. In theory, the parameters between the modules are the same. As long as the driving signals of each switch tube are consistent, the input and output voltage and current parameters can be guaranteed to be the same, and the power balance between the modules can be automatically achieved. However, in actual circuits, the circuit parameters of each module cannot be exactly the same, and there will always be different impacts on the hardware equipment during production, assembly, and use. Therefore, the power imbalance phenomenon cannot be avoided. The traditional solution is mainly to add additional control circuits to achieve power balance, such as the three-loop control power balance strategy, which adds a current inner loop and has high system dynamic characteristics, but the number of system sensors is large and the power balancing circuit is complex; some topologies use non-interconnected upturn control to improve the degree of modularization, but this method has poor load adjustment and a response speed of about 100ms; in order to reduce the cost of the device, sensorless current sharing control can achieve power balance without additional sensors, but the control method is complex and the dynamic performance is poor. Therefore, how to more simply achieve power balance, reduce current stress, and improve system operation reliability is an issue that needs to be urgently addressed in the industry. Summary of the invention

[0004] The technical problem to be solved by the present invention is to provide a control method for a multi-module DAB converter, which can reduce inductor current stress, reduce system loss, and improve overall operating efficiency while achieving power balance, and is simple and effective.

[0005] In order to solve the above technical problems, the present invention adopts the following technical method: a control method of a multi-module DAB converter, comprising:

[0006] Step S1, constructing a transmission power calculation model and a current stress calculation model of a DAB converter under an extended phase shift control mode;

[0007] Step S2, combining the two models and then deriving them through Lagrangian function, to determine the relationship between the shift ratio, voltage conversion ratio and transmission power of the DAB converter under minimum current stress;

[0008] Step S3, collecting voltage and current information of the DAB converter, and according to the direct power control method, taking the output of the DAB converter output voltage PI controller as the total transmission power reference value, and taking the output of each DAB module input voltage PI controller of the DAB converter as the dynamic power component of each DAB module, calculating the actual transmission power of each DAB module;

[0009] Step S4, solving the relationship between the phase shift ratio, voltage conversion ratio and transmission power to obtain the optimal phase shift ratio of each DAB module, and sending the optimal phase shift ratio solution to the extended phase shift control circuit to achieve control of each DAB module.

[0010] Further, the DAB converter is an ISOP type DAB converter, the H-bridge switch tubes on the primary side of the transformer of the ISOP type DAB converter are S1, S2, S3, and S4, the H-bridge switch tubes on the secondary side of the transformer are Q1, Q2, Q3, and Q4, the switch tubes S1, S2, S3, and S4 are all turned on and off with a duty cycle of 50%, and the switch tubes S1 and S2, S3 and S4 are complementarily turned on, the shift phase ratio between the switch tubes S1 and S3 is an inner shift phase ratio D1, and the shift phase ratio between the switch tubes S1 and Q1 is an outer shift phase ratio D2; within a switching cycle, the switching state in which the switch tubes S1, S3, Q1, and Q4 are turned off and the switch tubes S2, S4, Q2, and Q3 are turned on is the starting time t0, and t1 and t3 are the change times of the drive signal of the switch tube Q1.

[0011] Furthermore, according to the size of the inner shift phase D1 and the outer shift phase D2 of the DAB converter, the working mode of the DAB converter is divided into mode 1 and mode 2. When 0≤D1≤D2≤1, the DAB converter operates in mode 1, and when 0≤D2<D1≤1, the DAB converter operates in mode 2.

[0012] In step S1, a transmission power calculation model is constructed according to the average power calculation method of the DAB converter under the extended phase shift control mode, and the transmission power is normalized as follows:

[0013]

[0014] Where p is the per-unit value of the transmission power; P is the actual transmission power; P N is the maximum transmission power, and its value is:

[0015]

[0016] Where n is the transformation ratio of the DAB converter transformer; V1 is the DAB converter input voltage; V2 is the DAB converter output voltage; f is the DAB converter switching frequency; L is the DAB converter inductance;

[0017] In step S1, according to the change of the inductor current of the DAB converter under the extended phase shift control mode, a current stress calculation model is constructed, and the current stress is normalized as follows:

[0018]

[0019] Where k is the voltage conversion ratio of the DAB converter; i p is the per unit value of current stress; i max is the peak current of the DAB converter in steady-state operation; i N is the current reference value, and its value is calculated using the following formula.

[0020]

[0021] Furthermore, in step S2, the power calculation model of the DAB converter is combined with the current stress calculation model through the Lagrangian function, that is:

[0022]

[0023] Where, i = 1, 2, ..., m, m is the number of DAB modules in the DAB converter; E i represents the Lagrangian function; i pi is the per-unit value of the current stress of the i-th DAB module of the DAB converter; λ is the Lagrange multiplier; is the average transmission power per unit value of the DAB converter; is the given transmission power per unit value of the i-th DAB module of the DAB converter;

[0024] Derivative of the Lagrangian function, that is:

[0025]

[0026] Where D i1 is the internal shift ratio of the i-th DAB module of the DAB converter; D i2 is the outward shift comparison of the i-th DAB module of the DAB converter;

[0027] Combining formulas (1), (2), (5), and (6), we can obtain the relationship between the phase shift ratio, voltage conversion ratio, and transmission power under the minimum current stress of the DAB converter in two operating modes, as follows:

[0028] When the DAB converter operates in mode 1, the relationship between the phase shift ratio, voltage conversion ratio and transmission power under minimum current stress is:

[0029]

[0030] In the formula, k i is the voltage conversion ratio of the i-th DAB module of the DAB converter;

[0031] When the DAB converter operates in mode 2, there is a first operating condition: the current at time t3 is not less than the current at time t1. At this time, the relationship between the transfer ratio under the minimum current stress and the voltage conversion ratio and the transmission power is:

[0032]

[0033] When the DAB converter operates in mode 2, there is a second operating condition: when the current at time t1 is not less than the current at time t3, the relationship between the phase shift ratio under the minimum current stress and the voltage conversion ratio and the transmission power is as follows.

[0034]

[0035] Furthermore, in step S3, according to the direct power control method, the output of the DAB converter output voltage PI controller is used as the total transmission power reference value P v The output of the PI controller of the input voltage of each DAB module of the DAB converter is used as the dynamic power component of each DAB module. The actual transmission power of each DAB module is expressed as:

[0036]

[0037] Where P i is the actual transmission power of the ith DAB module of the DAB converter; P v is the total transmission power reference value of the DAB converter; P di is the dynamic power component of the i-th DAB module of the DAB converter;

[0038] The actual transmission power of each DAB module is normalized to obtain:

[0039] The actual transmission power of the m-1 DAB modules before the DAB converter is calculated as follows:

[0040]

[0041] Where, j = 1, 2, ..., m-1; p j is the actual transmission power per unit value of the jth DAB module of the DAB converter; P Nj is the maximum transmission power of the jth DAB module of the DAB converter; P dj is the dynamic power component of the jth DAB module of the DAB converter; L j is the inductance value of the jth DAB module of the DAB converter; U dcj is the input voltage of the jth DAB module of the DAB converter;

[0042] The actual transmission power of the mth DAB module of the DAB converter is calculated as follows:

[0043]

[0044] In the formula, p m is the actual transmission power per unit value of the mth DAB module of the DAB converter; L m is the inductance value of the mth DAB module of the DAB converter; P dm-1 is the dynamic power component of the m-1th DAB module of the DAB converter; U dcm is the input voltage of the mth DAB module of the DAB converter.

[0045] Furthermore, in step S3, the average value U of the input voltage of each DAB module is used. av Replace the input voltage sampling value of each DAB module.

[0046] Furthermore, the calculation formula of the voltage conversion ratio is as follows.

[0047]

[0048] Preferably, step S4, solving the relationship between the phase shift ratio, the voltage conversion ratio and the transmission power under the condition of minimum current stress and soft switching, obtaining the optimal solution of the phase shift ratio of each DAB module, and sending the optimal solution of each phase shift ratio to the extended phase shift control circuit to control each DAB module respectively;

[0049] The soft switching constraints of the DAB converter when operating in mode 1 are:

[0050]

[0051] The soft switching constraints of the DAB converter when operating in mode 2 are:

[0052]

[0053] Substituting formula (7) into formula (15), the relationship between the voltage conversion ratio and the transmission power that satisfies both the minimum current stress and the soft switching conditions under mode 1 is obtained as follows:

[0054]

[0055] Substituting formula (8) into formula (17), the relationship between the voltage conversion ratio and the transmission power that satisfies both the minimum current stress and the soft switching conditions under the first working condition of mode 2 is obtained as follows:

[0056]

[0057] Substituting formulas (9) and (10) into formula (17), the relationship between the voltage conversion ratio and the transmission power that satisfies both the minimum current stress and the soft switching conditions under the second working condition of mode 2 is obtained as follows.

[0058]

[0059] Compared with the traditional control method, the control method of the multi-module DAB converter proposed in the present invention is simple and effective. By finely adjusting the shift ratio, the power balance between each DAB module in the multi-module DAB converter can be achieved. In the traditional control method, due to the inconsistency of hardware parameters, the system is often difficult to restore the power balance state after being disturbed. The control method proposed in the present invention effectively solves the problem of imbalance between modules by dynamically adjusting the shift ratio parameters, and improves the stability and reliability of the system. In addition, while achieving power balance, the present invention focuses on reducing the return power to improve the efficiency of the converter, finds the best shift ratio combination through the optimization algorithm, reduces the inductor current stress, reduces the system loss, realizes soft switching optimization, and improves the overall operating efficiency. Finally, the present invention also considers the dynamic response performance of the system, introduces a direct power control method, so that the control process can quickly respond to changes in input voltage and load, and realizes rapid dynamic adjustment. In practical applications, the control method proposed in the present invention has a wide range of applications, and is suitable for a variety of scenarios such as high-voltage DC microgrids, medium-voltage DC microgrids, and AC / DC microgrids with basic solid-state transformers. BRIEF DESCRIPTION OF THE DRAWINGS

[0060] Figure 1 The circuit diagram of the ISOP type DAB converter involved in the present invention;

[0061] Figure 2 A flowchart of a control method for a multi-module DAB converter according to the present invention;

[0062] Figure 3 The waveform diagram of each switch tube signal and main parameters of the ISOP type DAB converter involved in the present invention under the control of the traditional EPS;

[0063] Figure 4 is a waveform diagram of the inductor current in the simulation experiment of the present invention;

[0064] Figure 5 This is an output voltage waveform diagram when the reference voltage of the DAB module changes suddenly from 100V to 60V in the simulation experiment of the present invention;

[0065] Figure 6 The waveform diagram of the output voltage and output current of the DAB module when the load resistance suddenly changes from 40Ω to 30Ω in the simulation experiment of the present invention. DETAILED DESCRIPTION

[0066] In order to facilitate the understanding of those skilled in the art, the present invention is further described below in conjunction with embodiments and drawings. The contents mentioned in the implementation modes are not intended to limit the present invention.

[0067] There are four typical structures of modular DAB, among which the two structures of output parallel connection are the most common. This embodiment takes the input-series output-parallel (ISOP) structure of multiple DAB modules as an example to elaborate the control method of the present invention.

[0068] like Figure 1 The ISOP type DAB converter shown in the figure, V1, V2 are the converter input and output voltages, V h1 is the equivalent AC output voltage of the H1 full bridge on the V1 side of the converter, V h2 is the equivalent AC output voltage of the H2 full bridge on the V1 side of the converter, V L ,I L are the voltage and current of the inductor L respectively. Figure 2 In the converter, there are three DAB modules, and the number of modules can be further expanded or reduced according to the situation. Each DAB module has a total of 8 switches. Specifically, the H-bridge switches on the primary side of the converter transformer are S1, S2, S3, and S4, and the H-bridge switches on the secondary side of the converter transformer are Q1, Q2, Q3, and Q4. The switches on the same bridge arm cannot be turned on at the same time, otherwise a short circuit will occur. The switches S1, S2, S3, and S4 are all turned on and off with a duty cycle of 50%, and the switches S1 and S2, S3 and S4 are turned on complementarily. The shift ratio between the switches S1 and S3 is the inner shift ratio D1, and the shift ratio between the switches S1 and Q1 is the outer shift ratio D2.

[0069] During the operation of the ISOP type DAB converter, the input voltage of each DAB module full bridge on the V1 side should be distributed equally, and the current on the V2 side should be distributed equally, so that the power between each DAB module is balanced. Power balance is the basis for stable operation of the equipment. The converter should also have a high efficiency, so it is necessary to reduce the inductor current stress or reflux power. The present invention optimizes the traditional extended phase shift control (hereinafter referred to as EPS) and proposes a control method for a multi-module DAB converter.

[0070] like Figure 2 As shown, a control method for a multi-module DAB converter includes:

[0071] Step S1, constructing a transmission power calculation model and a current stress calculation model of a DAB converter under an extended phase shift control mode.

[0072] The size of the reflux power is related to the inductor current. To reduce the reflux power, the inductor current at each moment needs to be analyzed, such as Figure 3 As shown, T hs is half a switching cycle, P in For instantaneous power transmission, within a switching cycle, in this embodiment, the switching state in which the switches S1, S3, Q1, and Q4 are turned off and the switches S2, S4, Q2, and Q3 are turned on is taken as the starting time t0, t1 and t3 are the time when the driving signal of the switch Q1 (or Q4) changes, and the expression of the inductive force can be expressed in 6 moments:

[0073] In the time period from t0 to t1, the inductor current is:

[0074]

[0075] In the time period from t1 to t1', the inductor current is:

[0076]

[0077] In the time period t1'~t2, the inductor current is:

[0078]

[0079] In the time period from t2 to t3, the inductor current is:

[0080]

[0081] In the time period from t3 to t4, the inductor current is:

[0082]

[0083] In the time period from t4 to t4', the inductor current is:

[0084]

[0085] In the time period t4'~t5, the inductor current is:

[0086]

[0087] During the time period from t5 to t6, the inductor current is:

[0088]

[0089] In a switching cycle, the average current of the inductor is 0, and we can get Figure 3 The inductor current expression at each time point is:

[0090]

[0091] In formulas (20)-(31), n ​​is the transformation ratio of the DAB converter transformer; V1 is the DAB converter input voltage; V2 is the DAB converter output voltage; f is the DAB converter switching frequency; and L is the DAB converter inductance.

[0092] Since the size of the inner and outer phase shifts will affect the inductor current waveform, the converter can be divided into two modes for current stress optimization analysis. When 0≤D1≤D2≤1, the DAB converter operates in mode 1, and when 0≤D2<D1≤1, the DAB converter operates in mode 2. The transmission power calculation model is constructed based on the average power calculation method of the DAB converter under the extended phase shift control mode, and the transmission power is normalized as follows:

[0093]

[0094] Where p is the per-unit value of the transmission power; P is the actual transmission power; P N is the maximum transmission power, and its value is:

[0095]

[0096] Current stress is the peak current of the DAB converter during steady-state operation, which has an important impact on circuit loss, device selection, etc. For current stress, when the DAB converter operates in mode 1, according to the change law of the inductor current, the peak current occurs at time t3, while when it operates in mode 2, its maximum current may occur not only at time t3, but also at time t1. Therefore, the normalized current stress under EPS control is:

[0097]

[0098] Where k is the voltage conversion ratio of the DAB converter; i p is the per unit value of current stress; i max is the peak current of the DAB converter in steady-state operation; i N is the current reference value, and its value is calculated using the following formula.

[0099]

[0100] Step S2, combining the two models through Lagrangian function and then deriving them, to determine the relationship between the shift ratio, voltage conversion ratio and transmission power of the DAB converter under minimum current stress.

[0101] In order to select a suitable combination of phase shifts to minimize the current stress of the DAB converter under extended phase shift control, it is an effective solution to combine the power model of the converter with the current stress for derivative calculation. However, the calculation process of this solution method is relatively cumbersome. Finding the optimal combination of phase shifts to minimize the current stress of the converter under a given transmission power is an optimization problem under equality constraints from a mathematical point of view. The present invention adopts the Lagrange multiplier method to solve this problem. This method can be used to balance the trade-offs between different objectives and find the optimal compromise solution. It has wide applicability, a simple solution process, and accurate results.

[0102] Firstly, the power calculation model of the DAB converter is combined with the current stress calculation model by defining the Lagrangian function, namely:

[0103]

[0104] Where, i = 1, 2, ..., m, m is the number of DAB modules in the DAB converter; E i represents the Lagrangian function; i pi is the per unit value of the current stress of the i-th DAB module of the DAB converter; λ is the Lagrange multiplier; p Ti is the average transmission power per unit value of the DAB converter; is the given transmission power per unit value of the i-th DAB module of the DAB converter.

[0105] Derivative of the Lagrangian function, that is:

[0106]

[0107] Where D i1 is the internal shift ratio of the i-th DAB module of the DAB converter; D i2 is compared with the outward shift of the i-th DAB module of the DAB converter.

[0108] Combining formulas (1), (2), (5), and (6), we can obtain the relationship between the phase shift ratio, voltage conversion ratio, and transmission power under the minimum current stress of the DAB converter in two operating modes, as follows:

[0109] When the DAB converter operates in mode 1, the relationship between the phase shift ratio, voltage conversion ratio and transmission power under minimum current stress is:

[0110]

[0111] In the formula, k i is the voltage conversion ratio of the i-th DAB module of the DAB converter; p i is the transmission power per unit value of the i-th DAB module of the DAB converter. It is worth mentioning here that the present invention constructs an output power calculation model based on the average power calculation method. For each DAB module, its transmission power per unit value p i That is, it is equal to the average transmission power per unit value p Ti .

[0112] When the DAB converter operates in mode 2, there is a first operating condition: the current at time t3 is not less than the current at time t1. At this time, the relationship between the transfer ratio under the minimum current stress and the voltage conversion ratio and the transmission power is:

[0113]

[0114] When the DAB converter operates in mode 2, there is a second operating condition: when the current at time t1 is not less than the current at time t3, the relationship between the phase shift ratio under the minimum current stress and the voltage conversion ratio and the transmission power is as follows.

[0115]

[0116]

[0117] Since the Lagrange multiplier method can be used to solve the optimal solution of current stress within the full power range, it is more efficient than the traditional EPS, and the design of inductors and transformers is relatively simpler and more reliable. This method can be compatible with other target optimizations such as power balance and soft switching while achieving current stress, and has stronger scalability.

[0118] In terms of soft switching optimization:

[0119] When the DAB converter operates in mode 1, when the inductor current satisfies i L When (t1)≤0, the H-bridge switch tube on the primary side of the transformer is in the soft switching state. When the inductor current satisfies i LWhen (t2)≥0, the H-bridge switch tube on the secondary side of the transformer is in the soft switching state. The conditions for achieving soft switching of all switch tubes are:

[0120]

[0121] When the DAB converter operates in mode 2, in order to ensure that the H-bridge switch tube on the primary side of the transformer is in a soft switching state, i L (t2)≤0, to ensure that the H-bridge switch tube on the secondary side of the transformer is in a soft switching state, i L (t1)≥0, the conditions for achieving soft switching of all switches are:

[0122]

[0123] Substituting formula (7) into formula (15), the relationship between the voltage conversion ratio and the transmission power that satisfies both the minimum current stress and the soft switching conditions under mode 1 is obtained as follows:

[0124]

[0125] In formula (17), the transmission power is proportional to the value on the left side of the expression. When the minimum value is taken in the transmission power range of mode 1, formula (17) still holds. The same can be obtained in mode 2. Specifically:

[0126] Substituting formula (8) into formula (17), the relationship between the voltage conversion ratio and the transmission power that satisfies both the minimum current stress and the soft switching conditions under the first working condition of mode 2 is obtained as follows:

[0127]

[0128] Substituting formulas (9) and (10) into formula (17), the relationship between the voltage conversion ratio and the transmission power that satisfies both the minimum current stress and the soft switching conditions under the second working condition of mode 2 is obtained as follows:

[0129]

[0130] Step S3, collecting voltage and current information of the DAB converter, and according to the direct power control method, using the output of the DAB converter output voltage PI controller as the total transmission power reference value, and using the output of each DAB module input voltage PI controller of the DAB converter as the dynamic power component of each DAB module, calculating the actual transmission power of each DAB module.

[0131] Depend on Figure 1It can be seen that for a multi-module DAB converter with serial input and parallel output, the input current of each DAB module is the same and the output voltage is V2. Assuming that the efficiency of each DAB module is consistent and the loss is ignored, theoretically, the power balance of each DAB module, the voltage balance on the series side of the converter, and the current balance on the parallel side are equivalent. If one of them is achieved, the other two are automatically achieved. Therefore, to make the transmission power of each DAB module equal, the input voltage or output current of each DAB module can be controlled to be equal. Taking the input voltage equal to achieve power balance as an example, it satisfies:

[0132] U dc1 =U dc2 =…=U dcm (32)

[0133] Where U dcm is the input voltage of the mth DAB module of the DAB converter.

[0134] Due to factors such as circuit parasitic parameters and switch tube voltage drop, the power loss of the converter cannot be ignored, so the actual transmission power of the converter cannot be equal to the output power. In order to compensate for the power loss of the converter, the present invention adopts a direct power control method, using the output of the DAB converter output voltage PI controller as the total transmission power reference value P v , then the reference transmission power component of each DAB module can be expressed as P v / m, and in order to achieve input voltage balance of each DAB module, the output of the DAB converter's DAB module input voltage PI controller is used as the dynamic power component P of each DAB module. di , then the actual transmission power P of each DAB module i It is expressed as:

[0135]

[0136] The actual transmission power of each DAB module is normalized to obtain:

[0137] The actual transmission power of the m-1 DAB modules before the DAB converter is calculated as follows:

[0138]

[0139] Where, j = 1, 2, ..., m-1; p j is the actual transmission power per unit value of the jth DAB module of the DAB converter; P Nj is the maximum transmission power of the jth DAB module of the DAB converter; P dj is the dynamic power component of the jth DAB module of the DAB converter; L j is the inductance value of the jth DAB module of the DAB converter; U dcjis the input voltage of the jth DAB module of the DAB converter.

[0140] For the multi-module DAB converter with serial input and parallel output, the sum of the total dynamic power components should be zero, that is, it satisfies:

[0141] P d1 +P d2 +P d3 +…+P dm =0 (33)

[0142] Combining equation (12) and equation (33), the actual transmission power of the mth DAB module of the DAB converter can be calculated as:

[0143]

[0144] In the formula, p m is the actual transmission power per unit value of the mth DAB module of the DAB converter; L m is the inductance value of the mth DAB module of the DAB converter; P dm-1 is the dynamic power component of the m-1th DAB module of the DAB converter; U dcm is the input voltage of the mth DAB module of the DAB converter.

[0145] It is worth noting that in the control process of a single DAB converter, the input voltage parameters of each module introduced in the control system constitute its negative feedback control of the input voltage (see equation (12) and the calculation of the voltage conversion ratio), so as to control the stability of the output voltage of the converter when the input voltage changes. In the control process of a multi-module DAB converter with series input and parallel output, the negative feedback control of the input voltage in the control system of a single DAB module will become positive feedback, which will cause the converter to fail to work properly. Therefore, in order to avoid positive feedback in the control system, the average value U of the input voltage of each DAB module is used in the calculation formula of the transmission power per unit value represented by equations (12) and (13). av Instead of the input voltage sampling value of each DAB module, let U av For U dci , the dynamic power component P of each DAB module di The input voltage U of each DAB module is sampled dci And the input average voltage U av The difference is obtained through the output of the PI controller. At the same time, the voltage conversion ratio of each DAB module is calculated as follows:

[0146]

[0147] The average value of the input voltage U avIt is expressed as:

[0148]

[0149] Step S4, substitute the actual transmission power calculated according to formulas (12) and (13) into the relationship between the phase shift ratio, voltage conversion ratio and transmission power under the corresponding working conditions, that is, formula (7) or formula (8) or formula (9), and solve to obtain the optimal solution of the phase shift ratio of each DAB module. Send the optimal solution of each phase shift ratio to the extended phase shift control circuit to realize the control of each DAB module.

[0150] Next, this embodiment verifies the effectiveness of the present invention under different working conditions through simulation. Figure 1 The converter topology is constructed, and the main simulation parameters involved are shown in Table 1.

[0151] Table 1 Main parameters of ISOP converter

[0152] <![CDATA[Input voltage V1]]> 200V <![CDATA[Output voltage V2]]> 100V inductance 180uH Filter capacitor <![CDATA[2.5e -6 F]]> Load resistance 40Ω

[0153] The results of this simulation experiment can be found in Figure 4-6 :

[0154] like Figure 4 As shown, at time t1 the inductor current is less than 0, and at time t2 the inductor current is greater than 0, which can meet the conditions required for soft switching, and the maximum value of the inductor current is 10.8A.

[0155] like Figure 5 As shown, when the reference voltage suddenly changes from 100V to 60V, the transmission power can be changed immediately due to the direct power control, and the output voltages of the three DAB modules can be kept consistent. The adjustment time is within 0.002S, making the circuit stable immediately. Figure 5 In FIG. 1 , Vm1 , Vm5 , and Vm8 represent the output voltages of the first, second, and third DAB modules, respectively.

[0156] like Figure 6 As shown in the figure, when the load changes from 40Ω to 30Ω, the input voltage can return to stability within 0.2S. Since the transmission power does not change suddenly and the input power becomes smaller, the transmission power is less than the output power when the sudden change occurs, causing the input voltage to decrease. In this experiment, the output current is sampled and the transmission power is changed immediately, so the response speed is fast. Figure 6 In FIG. 1 , Vm1 and Am1 represent the output voltage and output current of the first DAB module respectively.

[0157] Through this simulation experiment verification, it can be known that the present invention can optimize current stress, realize soft switching of each switch tube, and realize power balance of each module, and has good dynamic response characteristics.

[0158] The above embodiments are preferred implementation schemes of the present invention. In addition, the present invention may also be implemented in other ways. Any obvious replacement without departing from the concept of the present technical solution is within the protection scope of the present invention.

[0159] In order to make it easier for ordinary technicians in the field to understand the improvements of the present invention over the prior art, some drawings and descriptions of the present invention have been simplified, and for the sake of clarity, some other elements are omitted in this application document. Ordinary technicians in the field should realize that these omitted elements may also constitute the content of the present invention.

Claims

1. A control method for a multi-module DAB converter, characterized in that: include: Step S1, constructing a transmission power calculation model and a current stress calculation model of a DAB converter under an extended phase shift control mode; Step S2, combining the two models and then deriving them through Lagrangian function, to determine the relationship between the shift ratio, voltage conversion ratio and transmission power of the DAB converter under minimum current stress; Step S3, collecting voltage and current information of the DAB converter, and according to the direct power control method, taking the output of the DAB converter output voltage PI controller as the total transmission power reference value, and taking the output of each DAB module input voltage PI controller of the DAB converter as the dynamic power component of each DAB module, calculating the actual transmission power of each DAB module; Step S4, solving the relationship between the phase shift ratio, voltage conversion ratio and transmission power to obtain the optimal phase shift ratio of each DAB module, and sending the optimal phase shift ratio solution to the extended phase shift control circuit to achieve control of each DAB module.

2. The control method of the multi-module DAB converter according to claim 1, characterized in that: The DAB converter is an ISOP type DAB converter, the H-bridge switch tubes on the primary side of the transformer of the ISOP type DAB converter are S1, S2, S3, and S4, the H-bridge switch tubes on the secondary side of the transformer are Q1, Q2, Q3, and Q4, the switch tubes S1, S2, S3, and S4 are all turned on and off with a duty cycle of 50%, and the switch tubes S1 and S2, S3 and S4 are complementarily turned on, the shift phase between the switch tubes S1 and S3 is an inner shift phase D1, and the shift phase between the switch tubes S1 and Q1 is an outer shift phase D2; in one switching cycle, the switching state in which the switch tubes S1, S3, Q1, and Q4 are turned off and the switch tubes S2, S4, Q2, and Q3 are turned on is the starting time t0, and t1 and t3 are the change times of the drive signal of the switch tube Q1.

3. The control method of the multi-module DAB converter according to claim 2, characterized in that: According to the size of the inner shift phase D1 and the outer shift phase D2 of the DAB converter, the working mode of the DAB converter is divided into mode 1 and mode 2. When 0≤D1≤D2≤1, the DAB converter operates in mode 1, and when 0≤D2<D1≤1, the DAB converter operates in mode 2. In step S1, a transmission power calculation model is constructed according to the average power calculation method of the DAB converter under the extended phase shift control mode, and the transmission power is normalized as follows: Where p is the per-unit value of the transmission power; P is the actual transmission power; P N is the maximum transmission power, and its value is: Where n is the transformation ratio of the DAB converter transformer; V1 is the DAB converter input voltage; V2 is the DAB converter output voltage; f is the DAB converter switching frequency; L is the DAB converter inductance; In step S1, according to the change of the inductor current of the DAB converter under the extended phase shift control mode, a current stress calculation model is constructed, and the current stress is normalized as follows: Where k is the voltage conversion ratio of the DAB converter; i p is the per unit value of current stress; i max is the peak current of the DAB converter in steady-state operation; i N is the current reference value, and its value is calculated using the following formula.

4. The control method of the multi-module DAB converter according to claim 3, characterized in that: In step S2, the power calculation model of the DAB converter is combined with the current stress calculation model through the Lagrangian function, that is: Where, i = 1, 2, ..., m, m is the number of DAB modules in the DAB converter; E i represents the Lagrangian function; i pi is the per unit value of the current stress of the i-th DAB module of the DAB converter; λ is the Lagrange multiplier; p Ti is the average transmission power per unit value of the DAB converter; is the given transmission power per unit value of the i-th DAB module of the DAB converter; Derivative of the Lagrangian function, that is: Where D i1 is the internal shift ratio of the i-th DAB module of the DAB converter; D i2 is the outward shift comparison of the i-th DAB module of the DAB converter; Combining formulas (1), (2), (5), and (6), we can obtain the relationship between the phase shift ratio, voltage conversion ratio, and transmission power under the minimum current stress of the DAB converter in two operating modes, as follows: When the DAB converter operates in mode 1, the relationship between the phase shift ratio, voltage conversion ratio and transmission power under minimum current stress is: In the formula, k i is the voltage conversion ratio of the i-th DAB module of the DAB converter; When the DAB converter operates in mode 2, there is a first operating condition: the current at time t3 is not less than the current at time t1. At this time, the relationship between the transfer ratio under the minimum current stress and the voltage conversion ratio and the transmission power is: When the DAB converter operates in mode 2, there is a second operating condition: when the current at time t1 is not less than the current at time t3, the relationship between the phase shift ratio under the minimum current stress and the voltage conversion ratio and the transmission power is as follows.

5. The control method of the multi-module DAB converter according to claim 4, characterized in that: In step S3, according to the direct power control method, the output of the DAB converter output voltage PI controller is used as the total transmission power reference value P v The output of the PI controller of the input voltage of each DAB module of the DAB converter is used as the dynamic power component of each DAB module. The actual transmission power of each DAB module is expressed as: Where P i is the actual transmission power of the ith DAB module of the DAB converter; P v is the total transmission power reference value of the DAB converter; P di is the dynamic power component of the i-th DAB module of the DAB converter; The actual transmission power of each DAB module is normalized to obtain: The actual transmission power of the m-1 DAB modules before the DAB converter is calculated as follows: Where, j = 1, 2, ..., m-1; p j is the actual transmission power per unit value of the jth DAB module of the DAB converter; P Nj is the maximum transmission power of the jth DAB module of the DAB converter; P dj is the dynamic power component of the jth DAB module of the DAB converter; L j is the inductance value of the jth DAB module of the DAB converter; U dcj is the input voltage of the jth DAB module of the DAB converter; The actual transmission power of the mth DAB module of the DAB converter is calculated as follows: In the formula, p m is the actual transmission power per unit value of the mth DAB module of the DAB converter; L m is the inductance value of the mth DAB module of the DAB converter; P dm-1 is the dynamic power component of the m-1th DAB module of the DAB converter; U dcm is the input voltage of the mth DAB module of the DAB converter.

6. The control method of the multi-module DAB converter according to claim 5, characterized in that: In step S3, the average value U of the input voltage of each DAB module is used. av Replace the input voltage sampling value of each DAB module.

7. The control method of the multi-module DAB converter according to claim 6, characterized in that: The calculation formula of the voltage conversion ratio is as follows.

8. The control method of the multi-module DAB converter according to claim 7, characterized in that: Step S4, solving the relationship between the phase shift ratio, the voltage conversion ratio and the transmission power under the conditions of minimum current stress and soft switching, obtaining the optimal solution of the phase shift ratio of each DAB module, and sending the optimal solution of each phase shift ratio to the extended phase shift control circuit to control each DAB module respectively; The soft switching constraints of the DAB converter when operating in mode 1 are: The soft switching constraints of the DAB converter when operating in mode 2 are: Substituting formula (7) into formula (15), the relationship between the voltage conversion ratio and the transmission power that satisfies both the minimum current stress and the soft switching conditions under mode 1 is obtained as follows: Substituting formula (8) into formula (17), the relationship between the voltage conversion ratio and the transmission power that satisfies both the minimum current stress and the soft switching conditions under the first working condition of mode 2 is obtained as follows: Substituting formulas (9) and (10) into formula (17), the relationship between the voltage conversion ratio and the transmission power that satisfies both the minimum current stress and the soft switching conditions under the second working condition of mode 2 is obtained as follows.

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