Global optimization method for reducing backflow power of energy storage converter

Abstract

The invention discloses a global optimization method for reducing the backflow power of an energy storage converter, and belongs to the technical field of direct-current converter control. Aiming at the problem of minimum backflow power optimization of an energy storage converter in a full-working mode and a full-power range, mathematical models of transmission power and backflow power corresponding to different working modes are established based on a backflow power generation mechanism under dual-phase-shift control of a dual-active-bridge direct-current converter; in combination with a power limit value, analyzing specific characteristics of the backflow power in each working mode to obtain an optimal phase shift angle of each power partition, and obtaining an optimized path corresponding to the minimum backflow power; the optimization results of all the working modes are compared, and a global optimization result is obtained; according to the basic condition of zero-voltage conduction of a power switch tube of the dual-active bridge converter, an inductive current constraint condition of the converter under dual-phase-shift control is obtained in combination with an inductive current waveform, global optimization is completed, and the transmission efficiency of the energy storage converter is improved.

Description

Technical Field

[0001] The invention belongs to the technical field of direct current converter control, and in particular relates to a global optimization method for reducing reflux power of an energy storage converter. Background Art

[0002] In a DC microgrid, energy storage devices are key equipment for maintaining system energy balance and bus voltage stability, and the core unit of the energy storage device is a DC-DC converter that can transmit energy in both directions. At present, dual active bridge (DAB) DC-DC converters are widely used due to their symmetrical topology, wide voltage conversion gain range, high power density, electrical isolation, and easy soft switching zero voltage switching (ZVS). As a bridge for energy interaction, DAB converters are an important direction of current DAB research to improve efficiency by reducing reflux power.

[0003] Dual active bridge converters usually use phase shift control. The traditional single phase shift (SPS) control has a large reflux power under light load, which will increase the power loss of the converter and reduce the power factor and transmission efficiency of the system. In view of the limitations of SPS control, existing research has improved the transmission efficiency by increasing the control freedom, including extended phase shift (EPS), dual phase shift (DPS) and triple phase shift (TPS) control methods. Compared with other control methods, DPS control takes into account both control freedom and dynamic performance, and has a wider adjustment range than EPS, and has broad application prospects. At the same time, DPS control has significant advantages in terms of flexibility in output power regulation, reduction of reflux power, and improvement of work efficiency. However, DPS control has four working modes, and the reflux power characteristics of the converter in each working mode are different. In addition, when the input and output voltages do not match, the reflux power of the converter will increase. Summary of the invention

[0004] Aiming at the problem of optimizing the minimum reflux power of an energy storage converter in full working mode and full power range, the present invention provides a global optimization method for reducing the reflux power of an energy storage converter.

[0005] In order to achieve the above object, the present invention adopts the following technical solutions:

[0006] A global optimization method for reducing the reflux power of an energy storage converter comprises the following steps:

[0007] Step 1: Based on the mechanism of the circulating power generation under the dual active bridge DC converter's dual phase-shift control, establish the mathematical models of the transmitted power and the circulating power corresponding to different operating modes;

[0008] Step 2: According to the mathematical models of the transmitted power and the circulating power corresponding to different operating modes, considering the power limits of different operating modes, partition their powers to obtain multiple different power partitions;

[0009] Step 3: According to the characteristics that different transmitted powers correspond to multiple operating modes, analyze the specific characteristics of the circulating power under each operating mode to obtain the optimal phase-shift angles of each power partition, and finally obtain the optimization path of the minimum circulating power corresponding to different operating modes;

[0010] Step 4: According to the optimization path of the minimum circulating power corresponding to different operating modes, obtain the relationship curve between the transmitted power and the circulating power, and then compare the optimization results of each operating mode to obtain the global optimization result;

[0011] Step 5: According to the basic conditions for the power switch tubes of the dual active bridge converter to achieve zero voltage switching (ZVS), and combining with the inductor current waveforms under different operating modes, obtain the inductor current constraint conditions of the converter under dual phase-shift control to complete the global optimization.

[0012] Step 6: Based on the above research steps, conduct experimental verification on the above theoretical analysis.

[0013] Furthermore, the operating state of the dual active bridge DC converter is power forward transmission / operating in Buck mode, the transmitted power P>0, and the voltage regulation ratio k≥1;

[0014] The dual phase-shift control adopted by the converter includes two phase-shift freedoms, namely the internal phase-shift ratio D1 between the primary side and the secondary side bridges and the external phase-shift ratio D2 between the primary side and the secondary side bridges;

[0015] According to different free combination forms of the phase-shift ratios, there are 4 operating modes in total: Mode 1, D1<D2, D1+D2>1; Mode 2, D1<D2, D1+D2<1; Mode 3, D1>D2, D1+D2<1; Mode 4, D1>D2, D1+D2>1.

[0016] Furthermore, the specific operation of establishing the mathematical models of the transmitted power and the circulating power corresponding to different operating modes in Step 1 is as follows:

[0017] Based on the circulating power calculation formula within a single interval, calculate the normalized circulating power P of the dual active bridge DC converter in the four operating modes bf * Expression:

[0018]

[0019] The expression of the per-unit transmission power P* under four working modes:

[0020]

[0021] Among them, P bf represents the reflux power value of different working modes; P represents the transmission power value of different working modes; P N is the transmission power reference value, P N =(nU1U2) / (8fL); n represents the turns ratio of the high-frequency transformer; U1 and U2 are the input and output voltages of the dual-active-bridge converter; f represents the switching frequency; L represents the series inductor; k is the voltage regulation ratio, k = U1 / (nU2); D1 is the inner-phase-shift ratio between the primary side and the secondary side bridges; D2 is the outer-phase-shift ratio between the primary side and the secondary side bridges.

[0022] Furthermore, the specific operation of step 2 to partition the power according to the mathematical models of the transmission power and the reflux power of different working modes and considering the power limits of different working modes is as follows:

[0023] Taking into account the constraint relationship between the inner and outer phase-shift angles, considering the power transmission direction, taking into account the upper and lower limits of the transmission power, and based on the power limit division method for the full working modes, the power of different working modes is partitioned. The range of the per-unit value of the transmission power under each working mode is:

[0024] Working mode 1: When the outer-phase-shift ratio is 0 ≤ D2 ≤ 0.5, there is no corresponding power lower limit P * min and power upper limit P * max ; when the outer-phase-shift ratio is 0.5 < D2 ≤ 1, the power lower limit P * min is 2(1 - D2) 2 , and the power upper limit P * max is 2(1 - D2)(2D2 - 1);

[0025] Working mode 2: When the outer-phase-shift ratio is 0 ≤ D2 ≤ 0.5, the power lower limit P * min is 2D2(2 - 3D2), and the power upper limit P * max is 4D2(1 - D2); when the outer-phase-shift ratio is 0.5 < D2 ≤ 1, the power lower limit P * min is 2(1 - D2)(3D2 - 1), and the power upper limit P * max is 4D2(1 - D2);

[0026] Operating mode 3: When the external shift ratio is 0 ≤ D2 ≤ 0.5, the lower power limit P * min is 2D2 2 , and the upper power limit P * max is 2D2(2 - 3D2); when the external shift ratio is 0.5 < D2 ≤ 1, there is no corresponding lower power limit P * min and upper power limit P * max ;;

[0027] Operating mode 4: When the external shift ratio is 0 ≤ D2 ≤ 0.5, the lower power limit P * min is 0, and the upper power limit P * max is 2D2 2 ; when the external shift ratio is 0.5 < D2 ≤ 1, the lower power limit P * min is 0, and the upper power limit P * max is 2(1 - D2) 2 ;

[0028] Under different operating modes, the relationship characteristics between the reflux power and the transmission power of the dual-active-bridge DC converter are different. According to different voltage regulation ratios k, when the transmission power P is constant, find the optimal phase-shift angle combination, that is, D1 and D2, so that the reflux power of the converter is minimized at this time; based on this, the 4 operating modes are divided into 3 power regions:

[0029] Power region 1, 0 ≤ P* ≤ 1 / 2;

[0030] Power region 2, 1 / 2 < P* ≤ 2 / 3;

[0031] Power region 3, 2 / 3 < P* ≤ 1.

[0032] Furthermore, in step 3, according to the characteristics that multiple operating modes correspond to different transmission powers, analyze the specific characteristics of the reflux power under each operating mode, obtain the optimal phase-shift angles of each power partition, and finally the specific operation of obtaining the optimization path of the minimum reflux power corresponding to different operating modes is:

[0033] Step 3.1, based on the characteristics of the dual-active-bridge DC converter under different phase-shift ratio combinations of each operating mode, obtain the reflux power expression of each operating mode;

[0034] Step 3.2, according to the reflux power expressions of each operating mode, obtain the change curve of the reflux power with the external shift ratio under different transmission powers P;

[0035] Step 3.3, deriving the reflux power expression of each working mode, and obtaining the optimal solution expression of the minimum reflux power corresponding to the external phase shift in each working mode;

[0036] Step 3.4, and then obtain the optimal path of the minimum return power of the dual active bridge DC converter in each working mode.

[0037] Furthermore, the step 4 obtains the relationship curve between the transmission power and the return power according to the optimization path of the minimum return power corresponding to different working modes, and then compares the optimization results of each working mode to obtain the global optimization result. The specific operation is:

[0038] Step 4.1, according to the optimized paths of different transmission power zones in each working mode, the relationship curve between transmission power and return power is obtained; when the transmission power P * In power zone 1, the DAB converter can operate in modes 1, 2, 3, and 4. When the transmission power P * In power zone 2, the DAB converter can operate in modes 1, 2, and 3; when the transmission power P * When in power zone 3, the DAB converter can only operate in mode 2;

[0039] Step 4.2, compare the optimization results of each working mode to obtain the global optimization result.

[0040] Furthermore, in step 5, based on the basic conditions for the power switch tube of the dual active bridge converter to achieve zero voltage conduction, combined with the inductor current waveforms under different working modes, the inductor current constraint conditions of the converter under dual phase shift control are obtained, and the specific operation of completing the global optimization is: when the phase shift ratio (D1, D2) combination is in the region of mode 1 and mode 2, all switch tubes of the DAB converter can achieve zero voltage conduction; the entire transmission power feasible domain of the minimum return power is mode 1 and mode 2, so under the minimum return power global optimization strategy proposed in this article, all switch tubes in mode 1 and mode 2 can achieve zero voltage conduction.

[0041] Compared with the prior art, the present invention has the following advantages:

[0042] The present invention first establishes the transmission power and return power mathematical models of different working modes according to the return power generation mechanism. Then, taking into account the power limits of different working modes, the transmission power of the energy storage converter phase shift control is divided into different power zones. Finally, by analyzing the optimization paths under different working modes, the optimal phase shift angle of each power zone is obtained, and the soft switching characteristics under the minimum return power are analyzed, thereby improving the transmission efficiency of the energy storage converter. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 It is a flow chart of the present invention;

[0044] Figure 2 Schematic diagram of the main circuit topology structure of the dual active bridge converter of the present invention;

[0045] Figure 3 The four-mode principle waveforms of the dual active bridge converter DPS control are shown below;

[0046] Figure 4 Power characteristics of dual active bridge converter DPS control;

[0047] Figure 5 is the relationship curve between the return power and the external shift phase angle;

[0048] Figure 6 Taking mode 2 as an example, the minimum return power optimization path is shown;

[0049] Figure 7 Optimize the path for minimum return power in the feasible region of mode 2;

[0050] Figure 8 is the relationship curve between return power and transmission power;

[0051] Fig. 9 Minimum reflux power optimization curves for 4 modes;

[0052] Fig.10 It is the global optimal curve of the return power when the shift ratio k is 1.5;

[0053] Fig.11 Optimize the path for minimum return power at different k values;

[0054] Fig.12 It is the feasible region under full ZVS conditions;

[0055] Fig.13 (a) is the efficiency curve of different control schemes in power zone 1; (b) is the efficiency curve of different control schemes in power zone 2; (c) is the efficiency curve of different control schemes in power zone 3. DETAILED DESCRIPTION

[0056] In order to gain a deeper understanding of the present invention, we will provide a comprehensive and detailed description of the present invention. However, the present invention has multiple implementations and is not limited to the specific examples listed herein. The presentation of these examples is intended to deepen the comprehensive understanding of the disclosure of the present invention.

[0057] Example 1

[0058] See also Figure 1 , a global optimization method for reducing the reflux power of an energy storage converter, comprising the following steps:

[0059] Step 1: Based on the reflux power generation mechanism under dual phase-shift control of the dual active bridge DC converter, a mathematical model of the transmission power and reflux power corresponding to different working modes is established;

[0060] like Figure 2 As shown in FIG. 1 , a dual active bridge converter is mainly composed of a high-frequency transformer T, an inductor L, and two symmetrical H bridges. In the figure: S1~S4 and Q1~Q4 are the switches of the full bridge on the primary side and the secondary side respectively; n is the high-frequency transformer ratio; U1 and U2 are the input and output voltages of the DAB converter; u h1 is the output square wave voltage of H1 bridge, u h2 is the input square wave voltage of H2 bridge; C1 and C2 are the DC side capacitors of H1 and H2 respectively.

[0061] By controlling the inner and outer phase shift angles, the forward and reverse power transmission can be realized. According to the size of the transmission power P and the voltage regulation ratio k, the DAB converter can work in the following four different scenarios.

[0062] Table 1 Working scenarios of dual active bridge converter

[0063]

[0064] DPS control includes two phase shift degrees of freedom, namely the inward phase shift between the primary and secondary sides D1 and the outward phase shift between the two sides D2. There are four working modes according to different free combinations of phase shifts:

[0065] Working mode 1, D1<D2,D1+D2> 1;

[0066] Working mode 2, D1 <D2,D1+D2<1;

[0067] Working mode 3, D1>D2, D1+D2<1;

[0068] Working mode 4, D1>D2, D1+D2>1.

[0069] See also Figure 3 As shown, the schematic diagram corresponding to the four working modes involved in the present invention, the yellow marked part is the interval where the inductor current is in the opposite direction to the voltage it receives, that is, the reflux power generation interval.

[0070] Based on the calculation formula of return power in a single interval, the normalized return power and transmission power expressions of the DAB converter in four working modes are calculated.

[0071]

[0072] According to 4 different working modes, draw Figure 3The power distribution diagram under DPS control is shown.

[0073] Step 2: Based on the mathematical models of transmission power and return power corresponding to different working modes, the power limits of different working modes are considered and the power is partitioned to obtain multiple different power partitions;

[0074] Taking into account the constraint relationship between the internal and external phase shift angles, the power transmission direction, and the upper and lower limits of the transmission power, the power is partitioned for different working modes based on the power limit division method of all working modes. The range of the transmission power per unit value in each working mode is shown in Table 2.

[0075] Table 2 Power distribution of four working modes.

[0076]

[0077]

[0078] In different working modes, the relationship between the return power and the transmission power of the DAB converter is also different. According to different voltage regulation ratios k, in the transmission power P * The optimal phase shift angle combination, namely D1 and D2, is found at a certain time, so that the reflux power of the converter is minimized. Figure 3 And Table 2, the four working modes are divided into three power zones: Power zone 1, 0≤P * ≤1 / 2; Power zone 2, 1 / 2 <P * ≤2 / 3; Power zone 3, 2 / 3 <P * ≤1.

[0079] Step 3: According to the characteristics of different transmission powers corresponding to multiple working modes, the specific characteristics of the return power in each working mode are analyzed to obtain the optimal phase shift angle of each power partition, and finally the optimal path corresponding to the minimum return power of different working modes is obtained;

[0080] The optimal phase shift ratio parameters are obtained with the minimum reflux power as the optimization goal, and the operating mode 2 in Table 2 is taken as an example to study the characteristics of the DAB converter under different phase shift ratio combinations (D1, D2).

[0081] The normalized return power expression is calculated as follows:

[0082]

[0083] According to the return power expression of working mode 2, the system can be obtained under different transmission powers P * Under, the reflux power P bf * As D2 changes, Figure 5 Shown

[0084] Figure 5 In the middle, as the outward movement is compared with D 2 As the transmission power increases, the return power changes nonlinearly, and the return power also varies under different transmission powers. * =0.4, that is, when the converter works in power zone 1, the return power is shown in the two red curves. The return power in the first section is significantly smaller than that in the second section. In addition, in the first section, the return power increases with D 2 decreases with the increase of .

[0085] Furthermore, the derivative of the return power expression of working mode 2 is obtained, that is, dP bf * / dD2=0, the optimal solution of the minimum reflux power corresponding to the external phase shift in working mode 2 is obtained.

[0086]

[0087] Since the transmission power of working mode 4 is only related to D1, it is not analyzed. Similarly, the optimal solution of the minimum return power corresponding to working modes 1 and 3 is:

[0088]

[0089] Based on the optimal solution expression of the minimum return power corresponding to the external phase shift in working mode 2, the optimal path of the minimum return power of the DAB converter in mode 2 can be drawn, such as Figure 6 shown.

[0090] Depend on Figure 6 It can be seen that the optimized path is outside the feasible control domain in the low power segment and does not meet the shift ratio value range of working mode 2. Figure 5 By analyzing, the optimal path of the DAB converter under the equality constraint and inequality constraint of the minimum return power in the feasible domain of working mode 2 is obtained, as shown in Figure 7 shown.

[0091] Step 4: According to the optimization path of the minimum return power corresponding to different working modes, the relationship curve between the transmission power and the return power is obtained, and then the optimization results of each working mode are compared to obtain the global optimization result;

[0092] According to the optimized paths of different transmission power zones in working mode 2, the relationship curve between return power and transmission power is obtained, such as Figure 8 shown.

[0093] Analyze the remaining three modes. Fig. 9 The optimization curves of the return power of four modes under DPS control are given. *In power zone 1, the DAB converter can operate in modes 1, 2, 3, and 4. When the transmission power P * In power zone 2, the DAB converter can operate in modes 1, 2, and 3; when the transmission power P * When in power zone 3, the DAB converter can only operate in mode 2.

[0094] The optimization results of the four modes are compared to obtain the global optimization results, such as Fig.10 shown.

[0095] According to the above analysis, the present invention optimizes the DPS control and the DPS optimal control minimizes the reflux power in the full power range and full working mode.

[0096] Through the optimal solution of the minimum return power corresponding to the external phase shift in working mode 2, the expression of the optimal path of the minimum return power in working mode 2 is obtained.

[0097]

[0098] Compared with the power upper limit expression P of mode 3 in Table 2 * max =2D2(2-3D2), we can get Figure 7 The outward displacement of the converter at point O is compared with D o2 and transmission power P o * .

[0099]

[0100] When the voltage regulation ratio k is 1, that is, the input voltage and output voltage of the DAB converter match, P o * is 2 / 3 of the upper boundary value of power zone 2. As k increases, that is, the mismatch between the input voltage and the output voltage becomes larger and larger, the intersection O gradually moves to the right, but its ordinate transmission power P o * is always greater than 1 / 2, such as Fig.11 Shown as the red curve in the middle.

[0101] analyze Fig.11 It can be concluded that when k>1, power area 2 is divided into two feasible regions, and the feasible region is 1 / 2 <P * ≤P o * and P o * <P * ≤2 / 3.

[0102] Combined with the optimization control analysis of minimum reflux power, the optimal control mode and optimal shift ratio combination of each power zone under DPS optimal control can be obtained, as shown in Table 3.

[0103] Table 3 DPS optimal control reflux power optimization results.

[0104]

[0105] After analysis, it can be seen that under the same transmission power, when the voltage regulation ratio k is constant, the return power of DPS optimal control is significantly less than that of SPS control. Under a certain transmission power, for SPS control, as the voltage regulation ratio k increases, the return power also increases, especially in the low power segment. As the voltage mismatch increases, the return power growth will accelerate; when the voltage regulation ratio k is close to 3, in the entire transmission power range, the return power is greater than the transmission power, which greatly reduces the transmission efficiency of the DAB converter. For DPS optimal control, as the voltage regulation ratio k increases, the return power changes very little and always maintains the optimal state.

[0106] Compared with SPS control, the proposed DPS optimal control greatly reduces the return power of the DAB converter in the full power range, especially in the low transmission power section. The return power after optimized phase shift control is much smaller than the return power under SPS control. In addition, with the increase of voltage regulation ratio k, the optimization effect of return power is more obvious.

[0107] Step 5: Based on the basic conditions for the dual active bridge converter power switch to achieve zero voltage conduction, combined with the inductor current waveforms under different working modes, the inductor current constraint conditions of the converter under dual phase shift control are obtained to complete the global optimization.

[0108] According to the basic conditions of DAB converter power switch tube to achieve ZVS, combined with Figure 3 The inductor current waveforms under different working modes can be used to obtain the inductor current constraints of the converter under DPS control, as shown in Table 4.

[0109] Table 4 Constraints for the switch to achieve ZVS

[0110]

[0111]

[0112] According to the constraints of the inductor current in Table 4, the feasible domain of all switches of the converter to achieve ZVS is obtained, as follows: Fig.12 shown.

[0113] See Fig.12, it can be seen that when the phase shift ratio (D1, D2) combination is in the regions of operating mode 1 and operating mode 2, all the switching tubes of the DAB converter can achieve ZVS. Further analysis according to Table 4 shows that the entire transmission power feasible region of the minimum reflux power is operating mode 1 and operating mode 2. Therefore, under the global optimization strategy of the minimum reflux power proposed in the present invention, all the switching tubes in operating mode 1 and operating mode 2 can achieve ZVS.

[0114] Embodiment 2 conducts experimental verification based on the above theoretical analysis

[0115] Table 5 Related parameters of the system.

[0116]

[0117] The transmission power range of power region 1 is 0 ≤ P* ≤ 1 / 2. According to Table 3, in this power region, the DPS optimal control operates in mode 1. Combining Fig.10 , the DPS control and SPS control in mode 2, mode 3, and mode 4 are selected as a comparison to verify the reflux power of the DAB converter. The specific experimental parameters are shown in Table 6.

[0118] Table 6 Comparison of reflux power when the per-unit value of transmission power P* = 0.4.

[0119]

[0120] The transmission power range of power region 2 is 1 / 2 < P* ≤ 2 / 3. According to Table 3, in this power region, the DPS optimal control operates in mode 2. Combining Fig.10 , the DPS control and SPS control in mode 1 and mode 3 are selected as a comparison to verify the reflux power of the DAB converter. The specific experimental parameters are shown in Table 7.

[0121] Table 7 Comparison of reflux power when the per-unit value of transmission power P * = 0.6.

[0122]

[0123] The transmission power range of power region 3 is 2 / 3 < P* ≤ 1. According to Table 3, in this power region, the DPS optimal control operates in mode 2. Combining Figure 5 and Fig.10 it can be known that in power region 3, there is only one operating mode, mode 2. Therefore, in this power region, the control effects of DPS control and DPS optimal control are the same. The DPS control in mode 2 and the SPS control are selected as a comparison to verify the reflux power of the DAB converter. The specific experimental parameters are shown in Table 8.

[0124] Table 8 Comparison of reflux power when the per-unit value of transmission power P * = 0.8.

[0125]

[0126] Fig.13 The efficiency comparison curves of various working modes under different power zones and different voltage regulation ratios k are shown in the figure. It can be seen from the figure that when the input and output voltages of different control schemes match, that is, when k = 1, the transmission efficiency of the DAB converter is not much different and is above 80%. As the k value increases, the SPS control efficiency decreases significantly in different power zones, especially in the low power segment, with the maximum efficiency drop of 27.9%; as the transmission power increases, the DPS optimal control is always in the optimal state compared to other working modes. In addition, by comparison, it can be seen that as the voltage regulation ratio k value increases, the DPS optimal control improves the transmission efficiency of the DAB converter, and the optimization effect is more obvious.

[0127] The contents not described in detail in the specification of the present invention belong to the prior art known to the professional and technical personnel in the field. Although the illustrative specific embodiments of the present invention are described above to facilitate the understanding of the present invention by the technical personnel in the field, it should be clear that the present invention is not limited to the scope of the specific embodiments. For the ordinary technical personnel in the field, as long as various changes are within the spirit and scope of the present invention defined and determined by the attached claims, these changes are obvious, and all inventions and creations using the concept of the present invention are protected.

Claims

1. A global optimization method for reducing the reflux power of an energy storage converter, characterized in that: It includes the following steps: Step 1: Based on the mechanism of the reflux power generation under the dual-phase-shift control of the dual-active-bridge DC converter, establish the mathematical models of the transmission power and the reflux power corresponding to different working modes; Step 2: According to the mathematical models of the transmission power and the reflux power corresponding to different working modes, considering the power limits of different working modes, partition their powers to obtain multiple different power partitions; Step 3: According to the characteristics that different transmission powers correspond to multiple working modes, analyze the specific characteristics of the reflux power under each working mode to obtain the optimal phase-shift angles of each power partition, and finally obtain the optimization path of the minimum reflux power corresponding to different working modes; Step 4: According to the optimization path of the minimum reflux power corresponding to different working modes, obtain the relationship curve between the transmission power and the reflux power, and then compare the optimization results of each working mode to obtain the global optimization result; Step 5: According to the basic conditions for the power switch tubes of the dual-active-bridge converter to achieve zero-voltage turn-on, and combining with the inductor current waveforms under different working modes, obtain the inductor current constraint conditions of the converter under the dual-phase-shift control to complete the global optimization.

2. A global optimization method for reducing the reflux power of an energy storage converter according to claim 1, characterized in that: The working state of the dual-active-bridge DC converter is operating in the power forward transmission / Buck mode, the transmission power P>0, and the voltage regulation ratio k≥1; the dual-phase-shift control adopted by the converter includes two phase-shift freedoms, namely the internal phase-shift ratio D1 within the primary-side and secondary-side bridges and the external phase-shift ratio D2 between the primary-side and secondary-side bridges; there are 4 working modes according to different free combination forms of the phase-shift ratios: Working mode 1, D1<D2, D1+D2>1; Working mode 2, D1<D2, D1+D2<1; Working mode 3, D1>D2, D1+D2<1; Working mode 4, D1>D2, D1+D2>1.

3. A global optimization method for reducing the reflux power of an energy storage converter according to claim 2, characterized in that: The specific operation of establishing the mathematical models of the transmission power and the reflux power corresponding to different working modes in the above Step 1 is: Based on the calculation formula of the return power in a single interval, the normalized return power P of the dual active bridge DC converter in four working modes is calculated. bf * expression: The expressions of the per-unit transmission power P* under the four working modes: Among them, P bf Indicates the return power value of different working modes; P indicates the transmission power value of different working modes; P N is the transmission power reference value, P N =(nU1U2) / (8fL); n represents the high-frequency transformer ratio; U1 and U2 are the input and output voltages of the dual active bridge converter; f represents the switching frequency; L represents the series inductance; k is the voltage regulation ratio, k=U1 / (nU2); D1 is the ratio of the inward movement of the primary side and the secondary side bridge; D2 is the ratio of the outward movement of the primary side and the secondary side bridge.

4. A global optimization method for reducing the reflux power of an energy storage converter according to claim 3, characterized in that: The specific operation of partitioning the powers according to the mathematical models of the transmission power and the reflux power of different working modes and considering the power limits of different working modes in the above Step 2 is: Taking into account the constraint relationship between the internal and external phase-shift angles, considering the power transmission direction, taking into account the upper and lower limits of the transmission power, and based on the power limit division method of the full working mode, partition the powers of different working modes. The range of the per-unit value of the transmission power under each working mode is: Working mode 1: When the external movement ratio is 0 ≤ D2 ≤ 0.5, there is no corresponding lower power limit P * min and upper power limit P * max ; When the external movement ratio is 0.5 < D2 ≤ 1, the lower power limit P * min is 2(1 - D2) 2 and the upper power limit P * max is 2(1 - D2)(2D2 - 1); Operating mode 2: When the external shift ratio is 0 ≤ D2 ≤ 0.5, the lower power limit P * min is 2D2(2 - 3D2), and the upper power limit P * max is 4D2(1 - D2); When the external shift ratio is 0.5 < D2 ≤ 1, the lower power limit P * min is 2(1 - D2)(3D2 - 1), and the upper power limit P * max is 4D2(1 - D2); Operating mode 3: When the outward movement ratio is 0 ≤ D2 ≤ 0.5, the lower power limit P * min is 2D2 2 , and the upper power limit P * max is 2D2(2 - 3D2); When the outward movement ratio is 0.5 < D2 ≤ 1, there is no corresponding lower power limit P * min and upper power limit P * max ;; Operating mode 4: When the external shift ratio is 0 ≤ D2 ≤ 0.5, the lower power limit P * min is 0, and the upper power limit P * max is 2D2 2 ; When the external shift ratio is 0.5 < D2 ≤ 1, the lower power limit P * min is 0, and the upper power limit P * max is 2(1 - D2) 2 ; Under different working modes, the relationship characteristics between the reflux power and the transmission power of the dual-active-bridge DC converter are different. According to different voltage regulation ratios k, when the transmission power P is constant, find the optimal phase-shift angle combination, that is, D1 and D2, so that the reflux power of the converter is the smallest at this time; based on this, divide the 4 working modes into 3 power regions: Power region 1, 0≤P*≤1 / 2; Power region 2, 1 / 2<P*≤2 / 3; Power region 3, 2 / 3<P*≤1.

5. A global optimization method for reducing the reflux power of an energy storage converter according to claim 4, characterized in that: The specific operation of step 3, according to the characteristics of different transmission powers corresponding to multiple working modes, analyzing the specific characteristics of the return power in each working mode, obtaining the optimal phase shift angle of each power partition, and finally obtaining the optimized path of the minimum return power corresponding to different working modes is: Step 3.1, combining the characteristics of the dual active bridge DC converter under different shift ratios of each working mode, and deriving the reflux power expression of each working mode; Step 3.2, according to the return power expression of each working mode, the change curve of the return power under different transmission powers P with respect to the external shift ratio is obtained; Step 3.3, deriving the reflux power expression of each working mode, and obtaining the optimal solution expression of the minimum reflux power corresponding to the external phase shift in each working mode; Step 3.4, and then obtain the optimal path of the minimum return power of the dual active bridge DC converter in each working mode.

6. A global optimization method for reducing the reflux power of an energy storage converter according to claim 5, characterized in that: In step 3.3, the specific method of deriving the reflux power expression of each working mode to obtain the optimal solution expression of the minimum reflux power corresponding to the external phase shift in each working mode is: Working mode 2, the characteristics of the dual active bridge converter under the shift phase (D1, D2) combination, corresponding to the normalized return power expression: Derivative of the above formula, that is, dP bf * / dD2=0, the optimal solution for the minimum reflux power in working mode 2 is: According to D 22min The expression of the minimum reflux power optimization path and the corresponding phase shift angle of working mode 2 are obtained; Since the transmission power of working mode 4 is only related to D1, it is not analyzed. Similarly, the optimal solution of the minimum return power corresponding to working modes 1 and 3 is:

7. A global optimization method for reducing the reflux power of an energy storage converter according to claim 6, characterized in that: The specific operation of step 4 is to obtain the relationship curve between transmission power and return power according to the optimization path of the minimum return power corresponding to different working modes, and then compare the optimization results of each working mode to obtain the global optimization result: Step 4.1, according to the optimized paths of different transmission power zones in each working mode, the relationship curve between transmission power and return power is obtained; when the transmission power P * In power zone 1, the DAB converter can operate in modes 1, 2, 3, and 4. When the transmission power P * In power zone 2, the DAB converter can operate in modes 1, 2, and 3; when the transmission power P * When in power zone 3, the DAB converter can only operate in mode 2; Step 4.2, compare the optimization results of each working mode to obtain the global optimization result.

8. A global optimization method for reducing the reflux power of an energy storage converter according to claim 7, characterized in that: In step 5, based on the basic conditions for the power switch tube of the dual active bridge converter to achieve zero voltage conduction, combined with the inductor current waveforms under different working modes, the inductor current constraint conditions of the converter under dual phase shift control are obtained, and the specific operation to complete the global optimization is: when the phase shift ratio (D1, D2) combination is in the area of ​​mode 1 and mode 2, all switch tubes of the DAB converter can achieve zero voltage conduction; the entire transmission power feasible domain of the minimum return power is mode 1 and mode 2, so under the minimum return power global optimization strategy proposed in this article, all switch tubes in mode 1 and mode 2 can achieve zero voltage conduction.

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