AC / DC converter for microgrid distributed power supply access and hybrid optimization control method

Abstract

The invention provides an AC / DC converter for microgrid distributed power supply access and a hybrid optimization control method. The method comprises the following steps: presetting the maximum switching frequency fsmax of a DAB converter; when the real-time switching frequency fs of the DAB converter is smaller than the preset maximum switching frequency fsmax, modulating the DAB converter by using the frequency conversion SPS; when the switching frequency fs reaches a preset maximum switching frequency fsmax, modulating the DAB converter by using a fixed-frequency EPS, and enabling fs to be equal to fsmax; when the fixed-frequency EPS is used for modulation, if Iacsin (omega t) is smaller than or equal to nVdc (K1) / (8fsLkK2), the DAB converter is in an internal mode; otherwise, enabling the DAB converter to be in an external mode. According to the method disclosed by the invention, all the switching tubes can realize zero-voltage switching in a full-load range, and the converter can keep high PF and low THD values.

Description

Technical Field

[0001] The present invention belongs to the field of AC / DC converters, and particularly relates to an AC / DC converter for accessing distributed power sources in a microgrid and a hybrid optimization control method. Background Art

[0002] In recent years, high-frequency isolated AC-DC converters have become key conversion devices for interconnected microgrid systems, battery energy storage systems, and V2G systems. According to different structures, high-frequency isolated AC-DC converters are divided into single-stage and two-stage types. For the two-stage structure, the front-stage AC-DC circuit is responsible for converting the AC voltage into a stable DC bus voltage and completing power factor correction (PFC). The rear-stage DC-DC converter provides electrical isolation and voltage control. However, this topology requires the use of large-capacity electrolytic capacitors to stabilize the intermediate DC bus voltage, resulting in a decrease in power density and conversion efficiency.

[0003] Compared with the two-stage structure, the single-stage AC-DC converter uses a small-capacity filter capacitor to replace the large electrolytic capacitor in the middle of the two stages, thereby achieving higher power density and lower cost. Single-stage AC-DC conversion can be achieved by using a dual active bridge (DAB) converter after the power frequency rectifier. By controlling the phase shift ratio between the high-frequency switching tubes in the DAB converter, the input current and the DC output voltage can be adjusted simultaneously. The single-stage DAB-type AC-DC converter is a derivative topology of the DAB-type DC-DC converter, and has characteristics such as bidirectional wide-voltage conversion, high conversion efficiency, and long service life.

[0004] The basis of DAB optimal control is three degrees of freedom of phase shift. Single phase shift (SPS) has only one degree of freedom and the method is relatively simple. However, when the voltage amplitudes on both sides of the transformer do not match, the current stress will increase significantly and the soft-switching range is relatively narrow. To overcome the limitations of SPS modulation, an internal mode using extended phase shift (EPS) modulation is adopted, enabling all primary-side switching devices to achieve zero current switching (ZCS) and ensuring that all secondary-side switching devices can achieve zero voltage switching (ZVS). There are two different EPS modes used to achieve power transfer, broadening the ZVS turn-on range and ensuring smooth switching between modes. There is also a triple phase shift (TPS) modulation operating mode with three different types adopted, broadening the operating range of zero voltage soft switching. However, under light load conditions, the primary-side switching devices still cannot achieve ZVS turn-on. Some scholars classified and analyzed 12 operating modes under TPS modulation and further explored the power transfer characteristics of the DAB converter under different combinations of switching devices. In fact, while TPS modulation provides the maximum control freedom, it also significantly increases the control complexity.

[0005] To improve the control flexibility, some scholars proposed adding variable frequency control to the phase shift modulation. Reference [3] proposed a modulation method combining single phase shift and variable frequency, achieving ZVS turn-on throughout the operating range, but the turn-off current of the switching devices in this reference is relatively large. Aiming at this problem, Reference [6] proposed a dual-mode variable frequency phase shift modulation strategy to reduce the turn-off current of the switching devices. However, under light load conditions, the switching frequency of this strategy is close to 400 kHz. For variable frequency control, the quality of the current waveform of the converter under light load conditions is easily affected by the switching frequency. According to the modulation scheme in Reference [4], all the switching devices in the article can achieve ZVS turn-on. However, this scheme does not fully consider the impact of the switching frequency on the system, resulting in a relatively high distortion rate of the alternating current during light load operation. To improve the performance of the converter under light load conditions, Reference [7] proposed a hybrid modulation strategy of variable frequency and fixed frequency. However, under light load conditions, the switching frequency is fixed at a relatively high 500 kHz in this article, which will lead to an increase in the conduction loss of the switching devices and the loss of magnetic components.

[0006] Reference [1]: Liu Meng, Sun Xiaofeng, Wang Honglong, et al. Research on a novel single-stage isolated AC-DC converter [J]. Proceedings of the CSEE, 2023, 43(7): 2802-2814; Reference [2]: Guo Dongxin, Jia Yanbing, Ren Chunguang, et al. Minimum current stress control strategy for isolated bidirectional AC / DC converter based on virtual frequency [J]. High Voltage Engineering, 2021, 47(8): 2914-2922; Reference [3]: Wang Zhangyi, Lu Daorong, Li Xiang, et al. Critical current modulation strategy for single-stage bidirectional AC-DC converter based on phase-shift and frequency modulation [J]. Transactions of China Electrotechnical Society, 2023, 38(14): 3888-3897; Reference [4]: Lu Juncheng, Liu Guanliang, Bai Hua, et al. Applying variable-switching-frequency variable-phase-shift control and E-Mode GaN HEMTs to an indirect matrix converter-based EV battery charger [J]. IEEE Transactions on Transportation Electrification, 2017, 3(3): 554-564; Reference [5]: Jauch F, Biela J. Combined phase-shift and frequency modulation of a dual-active-bridge AC-DC converter with PFC [J]. IEEE Transactions on Power Electronics, 2016, 31(12): 8387-8397; Reference [6]: Wang Yaoqiang, Li Hao, Li Xiang, et al. Single-stage dual-active-bridge DC-AC converter based on dual-mode variable-frequency phase-shift modulation [J]. Transactions of China Electrotechnical Society, 2024, 39(21): 6865-6876; Document [7]: Tian Qi, Huang A Q, Bai Hua, et al. A novel light load performance enhanced variable-switching-frequency and hybrid single-dual-phase-shift control for single stage dual-active-bridge based AC / DC converter[C] / / IECON 2016-42nd Annual Conference of the IEEE Industrial Electronics Society, Florence, Italy, 2016: 1227-1232。 Summary of the Invention

[0007] The object of the present invention is to overcome the deficiencies of the prior art and provide an AC / DC converter and a hybrid optimization control method for accessing microgrid distributed power sources, which can effectively reduce the turn-off current of the switching tube, limit the excessive switching frequency, and achieve zero-voltage turn-on within the full range, so as to solve the problems of large turn-off current and high switching frequency of the single-stage DAB type AC-DC converter.

[0008] In the first aspect, the present invention provides a hybrid optimization control method based on variable-fixed frequency, which is applied to a single-stage DAB type AC-DC converter and includes: Presetting the maximum switching frequency of the DAB converter f smax ; When the real-time switching frequency of the DAB converter f s is less than the preset maximum switching frequency f smax , modulate the DAB converter using variable-frequency SPS; When the switching frequency f s reaches the preset maximum switching frequency f smax , modulate the DAB converter using fixed-frequency EPS, and let f s = f smax ; When modulating using fixed-frequency EPS, if the condition I ac |sin( ωt )| ≤ nV dc (K −1) / (8 f s L k K 2 ), make the DAB converter in the internal mode; Otherwise, make the DAB converter in the external mode; Define D f as v p and v s compared with the outward shift between, D 1 is the inward shift comparison between the secondary side switching tubes S3 and S6 of the DAB converter; The method to make the DAB converter in the internal mode is: Calculate according to the following formula D f and D 1, and perform extended phase shift modulation according to the calculated D f and D 1;

[0009] The method to make the DAB converter in the external mode is: Calculate according to the following formula D f and D 1, and perform extended phase shift modulation according to the calculated D f and D 1;

[0010] Among them, K is the voltage conversion ratio, K =2n V dc / | v ac |; I ac |sin( ωt )| is the rectifier bridge output current of the single-stage DAB type AC-DC converter, i ac = I ac sin( ωt ); n is the turns ratio of the high-frequency transformer; V dc is the DC voltage of the single-stage DAB type AC-DC converter; vac is the AC voltage of the single-stage DAB-type AC-DC converter; i ac is the AC current of the single-stage DAB-type AC-DC converter; L k represents the leakage inductance of the high-frequency transformer.

[0011] In a second aspect, the present invention provides a variable-fixed frequency hybrid optimization controller for controlling a single-stage DAB-type AC-DC converter, including: A switching frequency acquisition module for real-time acquisition of the switching frequency f s ; A preset module for storing the maximum switching frequency of the preset DAB converter f smax ; A first modulation module, connected to the switching frequency acquisition module and the preset module, for when the real-time switching frequency of the DAB converter f s is less than the preset maximum switching frequency f smax , modulating the DAB converter using variable-frequency SPS; A second modulation module, connected to the switching frequency acquisition module and the preset module, for when the switching frequency f s reaches the preset maximum switching frequency f smax , modulating the DAB converter using fixed-frequency EPS and making f s = f smax ; It is also used for when modulating using fixed-frequency EPS, if I ac |sin( ωt )| ≤ nV dc ( K −1) / (8 f s L k K 2 )), making the DAB converter in the internal mode; Otherwise, making the DAB converter in the external mode; Define D f as v p and v s compared with the external shift betweenD 1 is the internal shift comparison between the secondary side switching transistors S3 and S6 of the DAB converter; The method for making the DAB converter in the internal mode is as follows: Calculate according to the following formula D f and D 1, and perform extended phase shift modulation according to the obtained D f and D 1;

[0012] The method for making the DAB converter in the external mode is as follows: Calculate according to the following formula D f and D 1, and perform extended phase shift modulation according to the obtained D f and D 1;

[0013] Wherein, K is the voltage conversion ratio, K = 2n V dc / | v ac |; I ac |sin( ωt )| is the rectifier bridge output current of the single-stage DAB type AC-DC converter, i ac = I ac sin( ωt ); n is the turns ratio of the high-frequency transformer; V dc is the DC voltage of the single-stage DAB type AC-DC converter; v ac is the AC voltage of the single-stage DAB type AC-DC converter; i ac is the AC current of the single-stage DAB type AC-DC converter; L k represents the leakage inductance of the high-frequency transformer.

[0014] In the third aspect, an AC / DC converter for microgrid distributed power access includes a single-stage DAB type AC-DC converter and a controller; The controller is controlled and connected to the single-stage DAB type AC-DC converter; The method by which the controller controls the single-stage DAB type AC-DC converter adopts the aforementioned hybrid optimization control method based on variable-fixed frequency.

[0015] In a fourth aspect, the present invention provides a computer device, including a memory, a processor, and a computer program stored on the memory, where the processor executes the computer program to implement the aforementioned hybrid optimization control method based on variable-fixed frequency.

[0016] In a fifth aspect, the present invention provides a computer-readable storage medium, on which a computer program / instructions are stored. It is characterized in that when the computer program / instructions are executed by a processor, the aforementioned hybrid optimization control method based on variable-fixed frequency is implemented.

[0017] In a sixth aspect, the present invention provides a computer program product, including a computer program / instructions. It is characterized in that when the computer program / instructions are executed by a processor, the aforementioned hybrid optimization control method based on variable-fixed frequency is implemented.

[0018] The present invention has prominent substantial features and remarkable progress compared with the prior art. Specifically: 1) By reducing the soft-switching constant current value I B , the magnitude of the turn-off current of the switching tube can be effectively reduced.

[0019] 2) In order to limit the switching frequency under light load conditions, by presetting a saturated switching frequency (maximum switching frequency) under light load conditions, when the switching frequency reaches the saturated switching frequency, the modulation strategy is switched from variable-frequency single-phase shift to fixed-frequency extended phase shift, and thus the excessively high switching frequency can be effectively limited.

[0020] 3) The proposed modulation strategy enables all switching tubes to achieve zero-voltage turn-on within the full load range, and enables the converter to maintain high PF (power factor) and low THD (total harmonic distortion) values. Description of the Drawings

[0021] Figure 1 is the topology diagram of a single-stage dual-active-bridge type AC-DC converter.

[0022] Figure 2 is the equivalent circuit diagram of the DAB converter.

[0023] Figure 3 is the main working waveform diagram of single-phase shift modulation.

[0024] Figure 4 is the waveform diagram of the leakage inductance current and the primary-side bridge arm current.

[0025] Figure 5 is different I BGraph of the change in switching frequency at a certain value.

[0026] Figure 6 It is a schematic diagram of fixed-frequency and variable-frequency hybrid control.

[0027] Figure 7 It is the main working waveform diagram of the internal mode.

[0028] Figure 8 It is the main working waveform diagram of the external mode.

[0029] Figure 9 It is the flowchart of the method of the present invention.

[0030] Figure 10 It is the graph of the change in the phase shift ratio of the converter.

[0031] Figure 11 It is the per-unit value of the transmitted power in different modes.

[0032] Figure 12 It is the comparison diagram of the turn-off current.

[0033] Figure 13 It is the working waveform diagram under fixed-frequency and variable-frequency hybrid modulation.

[0034] Figure 14 It is v p , nv s , i Lk Simulation waveform diagram.

[0035] Figure 15 It is the simulation diagram of the current of switch S1. Detailed implementation manner

[0036] In order to make the purpose, technical solutions and advantages of this application clearer, the following further details this application in combination with the accompanying drawings and embodiments. It should be understood that the specific embodiments described here are only used to explain this application and are not used to limit this application.

[0037] The terms "including" and "having" in the specification and claims of this application and any variations thereof are intended to cover non-exclusive inclusion. For example, a process, method, system that includes a series of steps or units is not limited to the listed steps or units, but may include steps or units not listed.

[0038] To facilitate understanding of the technical solutions provided by this application, the single-stage DAB type AC-DC converter involved in the embodiments of this application is explained below.

[0039] Figure 1It is the topology of a single-stage DAB-type AC-DC converter. The power-frequency switching transistors Q1−Q4 form a rectifier circuit. In the DAB converter, the high-frequency switching transistors S1 and S2 form the half-bridge structure on the primary side of the DAB converter, and S3−S6 form the full-bridge structure on the secondary side. The high-frequency transformer T is used to connect the primary side and the secondary side of the DAB converter, L k represents the leakage inductance of the transformer. L ac , C ac1 and C ac2 together form a filter circuit.

[0040] Figure 2 presents the equivalent model of the DAB converter. Among them, v p and v s respectively represent the square-wave voltages generated on the primary side and the secondary side of the DAB, and their amplitudes are |v ac | / 2 and V dc . Define K as the voltage conversion ratio, K =2 nV dc / | v ac . When v p 's phase leads nv s , the converter realizes forward power transmission.

[0041] Embodiment 1 As Figure 9 shown, this embodiment proposes a variable-fixed frequency hybrid optimization control method applied to a single-stage DAB-type AC-DC converter, including: Step 1: Sample the converter parameters: the DC voltage V dc of the single-stage DAB-type AC-DC converter; the AC voltage v ac of the single-stage DAB-type AC-DC converter; the AC current i ac of the single-stage DAB-type AC-DC converter.

[0042] Preset the maximum switching frequency f smax of the DAB converter.

[0043] Step 2: Calculate the switching frequency f s ; Step 3: Determine the real-time switching frequency of the DAB converter f s Is it less than the preset maximum switching frequency f smax ; When the real-time switching frequency f s is less than the preset maximum switching frequency f smax , go to Step 4; When the switching frequency f s reaches the preset maximum switching frequency f smax , go to Step 5.

[0044] Step 4: Modulate the DAB converter using variable-frequency SPS.

[0045] Step 5: Modulate the DAB converter using fixed-frequency EPS and let f s = f smax ; When modulating with fixed-frequency EPS, if the condition I ac |sin( ωt )|≤ nV dc ( K −1) / (8 f s L k K 2 ) is satisfied, the DAB converter is in the internal mode; Otherwise, the DAB converter is in the external mode.

[0046] Specifically, the switching frequency f s is calculated by the following formula:

[0047] where A0, B0, and C0 are:

[0048]

[0049] where, I B is the conditional value for the switch of the DAB converter to achieve ZVS turn-on.

[0050] Preferably, set IB = 1A, the maximum switching frequency f smax = 175 kHz.

[0051] The method to make the DAB converter in the internal mode is as follows: Calculate according to the following formula D f and D 1, and perform extended phase-shift modulation according to the calculated D f and D 1;

[0052] The method to make the DAB converter in the external mode is as follows: Calculate according to the following formula D f and D 1, and perform extended phase-shift modulation according to the calculated D f and D 1;

[0053] Among them, K is the voltage conversion ratio, K = 2n V dc / | v ac |; I ac |sin( ωt )| is the rectifier bridge output current of the single-stage DAB type AC-DC converter, i ac = I ac sin( ωt ); n is the turns ratio of the high-frequency transformer; L k represents the leakage inductance of the high-frequency transformer.

[0054] 1. Variable-frequency SPS modulation 1.1. Basic principle of single-phase-shift modulation Under SPS modulation, the working waveform of the converter can be expressed as Figure 3 . In the figure, D represents the phase-shift ratio between S1 and S3, T hs represents half of the switching period.

[0055] Ignoring the influence of the switching dead time, according to Figure 3 it can be obtained that at different time periodsi Lk The time-domain expression is: (1) According to the negative symmetry characteristic of the inductor current waveform, that is i Lk ( t 0)=− i Lk ( t 0+ T hs ), combined with Equation (1), t 0 and t the i Lk expression at time 1 can be obtained respectively as: (2) In the formula, f s represents the switching frequency, that is f s =1 / 2 T hs .

[0056] For half of the switching period, the average value of the input current i 1 of the DAB converter can be expressed as: (3) The per-unit value of the transmitted power can be expressed as: (4) Among them, the transmitted power reference value P N is: (5) 1.2. Analysis of the turn-off current and switching frequency According to Figure 3 the working waveforms of the SPS modulation in (6) In Figure 3 , due to v p < nv s , in t 1− t the inductor current i Lk shows a downward trend at time 2, that is, it satisfies i Lk ( t 1)> i Lk( t )。Combined with i Lk ( t 0) = − i Lk ( t ), if the constraint i Lk ( t 0) < 0, then i Lk ( t (1) naturally satisfies being greater than 0.

[0057] The present invention satisfies the condition for realizing full - range soft - switching by constraining i Lk ( t 0) to be a constant value of − I B (where I B > 0), that is, the condition for realizing full - range soft - switching can be met. Its expression is as follows: (7) At this time, the current waveform of the primary - side bridge arm of the DAB can be expressed as Figure 4 .

[0058] For Figure 1 the topology shown, in order to achieve PFC, the average input current of the DAB converter should be equal to the output current i rec of the rectifier bridge, as shown in the following formula: (8) Combining formulas (3), (7) and (8), the phase - shift ratio D and the switching frequency f s can be obtained as the following expressions: D = Af s + B (9) (10) where A , B , A 0, B 0 and C 0 are: (11) (12) Observing formulas (10) - (12), it can be seen that the switching frequency is affected by the constant current I BThe influence. According to the converter parameters listed in Table 1, Figure 5 shows that within half of the power frequency cycle, the switching frequency follows I B the changing curve.

[0059] In reference [3], in order to achieve soft switching in the full range, it selects I B = 5A. For Figure 1 the topology, the leakage inductance current i Lk determines the current flowing through the switching device. In the above analysis, i Lk ( t 0) is constrained to a constant current I B , and this value determines the turn-off current of the primary-side switching device of the DAB.

[0060] Table 1 Converter parameters Technical Parameters Value <![CDATA[AC voltage v ac / V]]> 220 <![CDATA[DC voltage V dc / V]]> 48 Rated Power W 500 <![CDATA[Leakage inductance L k / μH]]> 100 Transformer Turns Ratio: 1 4:1 <![CDATA[Filter capacitor C rec / μF]]> 3 AC Voltage Frequency / Hz 50 Therefore, the present invention proposes to set I B = 1A to significantly reduce the turn-off current of the primary-side switching device. According to Figure 5 , when I B = 1A, the maximum switching frequency of the converter will exceed 1000 kHz. Although high-frequency operation can reduce the volume of magnetic components, the efficiency needs to be weighed. For medium and small power applications, a frequency of 100 - 200 kHz can already significantly reduce the size of the transformer, and further increasing the frequency has limited improvement in volume and will cause an increase in the losses of the converter.

[0061] Based on the above analysis, in order to limit the influence of the too high switching frequency on the converter under light load conditions, a variable-fixed frequency hybrid modulation strategy is proposed. By presetting the maximum switching frequency f smax , fixed-frequency extended phase-shifted modulation is used under light load conditions; while variable-frequency single phase-shifted modulation is used under heavy load conditions, as Figure 6 shown.

[0062] 2. Fixed-frequency EPS modulation Under EPS modulation, based on the v p and n vsAccording to the different rising and falling edge inflection point positions, it can be divided into three working modes [Yang Qiqing, Li Rui, Xu Jun. Optimized modulation strategy of dual-active-bridge micro-inverter based on mode switching [J]. Proceedings of the CSEE, 2023, 43(23): 9273-9284.]. In this invention, two working modes with relatively small root mean square values of the transformer inductor current are selected for analysis. For the convenience of analysis, they are respectively defined as the internal mode and the external mode in this invention.

[0063] 2.1. Internal Mode The working waveforms of the converter in the internal mode are as Figure 3 shown. This invention uses a new extended definition of the phase shift angle: an external phase shift ratio v p and nv s is introduced between D f , D where 1 represents the internal phase shift ratio between the secondary side switching tubes S3 and S6 of the DAB converter.

[0064] Let t 0 = 0. According to the phase relationship shown in Figure 3 , the turn-on moments of each switch are: t 1 = (0.5 - 0.5 D 1 + D f ) T hs , t 2 = (0.5 + 0.5 D 1 + D f ) T hs , t 3 = T hs . By means of an analysis method similar to the aforementioned SPS modulation, the t 0, t 1 and t expressions of i Lk at 2 moments can be deduced as: (13) The average value i of the input current 1 of the DAB converter and the per-unit value of the transmission power expression can be respectively expressed as: (14) (15) 2.2. External Mode The working waveforms of the external mode are as follows Figure 8 shown. Let t 0 = 0. According to the phase relationship in the figure, it can be known that t 1 = (-0.5 + 0.5 D 1 + D f ) T hs , t 2 = (0.5 - 0.5 D 1 + D f ) T hs , t 3 = T hs .

[0065] Similarly, in this mode, the expressions of the leakage inductance current i Lk at moments t 0,[[]] t 1 and t 2 can be derived as follows (16) The average value of the input current i 1 of the DAB converter and the per-unit value of the transmission power expression can be expressed respectively as (17) (18) The above discussion on the internal and external modes of extended phase shift lays the foundation for analyzing the full-range ZVS turn-on characteristics and provides theoretical support for the seamless switching between modes

[0066] 2.3. Full-range soft-switching analysis In a single-stage DAB-type AC-DC converter, achieving ZVS turn-on of high-frequency switching devices helps reduce their losses, and the realization of ZVS is closely related to i Lk . Specifically, the ZVS turn-on conditions follow the following rules: For switches S1, S4, and S5, when triggered by the rising edge of the driving signal, if i Lk < 0, ZVS turn-on can be achieved; similarly, for switches S2, S3, and S6, when triggered by the rising edge of the corresponding driving signal, if i Lk > 0, ZVS turn-on is achieved. Based on the above analysis, the soft-switching realization conditions in the two modes are shown in Table 2

[0067] Table 2 ZVS Turn-on Conditions under Different Modes

[0068] According to Figure 7 and Figure 8 , the boundary between the extended phase-shifted internal mode and the external mode is D f +0.5 D 1 = 0.5. When D f +0.5 D 1 < 0.5, the converter operates in the internal mode; otherwise, it operates in the external mode. To obtain the maximum ZVS range for the EPS internal mode and the external mode, the traditional modulation strategy defines D 1 as the reciprocal of the voltage conversion ratio

[10] , and its expression is as follows: (19) (1)Internal Mode To determine the leakage inductance current i Lk magnitude at the turn-on moment of each switch in this mode, substitute Equation (19) into the expression of the leakage inductance current (13), and combine it with the phase-shift ratio constraint condition D f +0.5 D 1 < 0.5, we can obtain: (20) Analyzing Equation (20), it can be inferred that in the internal mode, the secondary-side switches S3 - S6 satisfy the ZVS turn-on conditions listed in Table 2. However, since the primary-side switches S1 and S2 are i Lk 0 at the turn-on moment, they cannot achieve ZVS turn-on and can only achieve ZCS turn-off. However, Ref. [4] shows that the energy consumed when the switch turns on is much greater than the energy required for turn-off. Therefore, in terms of reducing switching losses, the ZVS turn-on characteristic is superior to the ZCS turn-off characteristic.

[0069] To broaden the ZVS range of the switches S1 and S2 in the internal mode, in Figure 7 , it is set that the leakage inductance current satisfies i Lk ( t 0) = i Lk ( t 2). This constraint condition provides a necessary condition for S1 and S2 to achieve ZVS. Combining Equation (13), a new ZVS constraint equation can be derived: (21) The above formula can be simplified as: (22) Similarly, in order to achieve PFC, the following equation must be satisfied: (23) Combining Equation (23) with the ZVS constraint equation (22) of the internal mode, we can obtain D The expression of 1 is as follows: (24) It is easy to know that by substituting Equation (24) into Equation (22), we can deduce D f The expression of.

[0070] (2)External mode Similarly, substituting Equation (19) into the leakage inductance current expression (16) of the external mode and combining the mode constraint conditions D f +0.5 D 1>0.5, the instantaneous value of each switch-on moment i Lk in this mode can be obtained: (25) It can be inferred from Equation (25) that in the external mode, the switch tubes S1−S2 and S3−S6 both meet the ZVS turn-on conditions listed in Table 2.

[0071] To ensure that the converter achieves PFC in the external mode, it is necessary to satisfy: (26) By combining Equation (26) with the ZVS constraint equation (19) of the external mode, the expressions of 1 and D 1 and D f in this mode are as follows: (27) 3. Seamless mode switching and modulation strategy advantages 3.1 Seamless mode switching analysis To ensure that Equations (24) and (27) have solutions and combining the above analysis, the present invention defines f smax =175 kHz. Figure 10 The variation range of each phase shift ratio within half of the power frequency cycle is plotted. In the figure, D , D 1 and D fThe amplitudes are all within the range of [0, 1]. At the moment of switching between the internal EPS mode and the external EPS mode, D 1 and D f satisfy the condition D f +0.5 D 1 = 0.5, which is consistent with the theoretical analysis above.

[0072] To ensure that equations (24) and (27) have solutions and combining the above analysis, it is preset that f smax = 175 kHz. Figure 10 The variation ranges of the phase shift ratios in half a power frequency cycle are plotted. In the figure, D , D 1 and D f The amplitudes are all within the range of [0, 1]. At the moment of switching between the internal mode and the external mode, D 1 and D f satisfy the condition D f +0.5 D 1 = 0.5, which is consistent with the theoretical analysis above.

[0073] According to the per-unit value expressions of the transmission power in equations (4), (15) and (18), and combining with Figure 10 the variation range of the phase shift ratio in Figure 11 it shows the per-unit value curve of the transmission power. It can be seen that the transmission power can vary continuously at the moment of mode switching without sudden changes.

[0074] 3.2. Analysis of the Advantages of the Modulation Strategy Some literature has studied different modulation strategies for the single-stage DAB-type AC-DC converter topology. Table 3 shows the comparison between the modulation strategy in this paper and the schemes in other literature. Among them, in [1], the totem-pole bridgeless PFC and the DAB circuit are multiplexed in the bridge arms, but the bus capacitor in this topology needs to bear twice the input voltage, resulting in a large voltage stress on the AC-side switching devices. In [2], no optimization is carried out for soft switching, resulting in large switching losses. In [4], under light load conditions, due to the high switching frequency, the distortion degree of the AC current increases significantly. The optimized modulation strategy in [5] uses the look-up table method, which stores the control variables at different AC phases. Iterative calculations are carried out according to different operating states to achieve specific power transmission, and the control method is relatively complex.

[0075] Table 3 Comparison with Other Works Comparison Items The Present Invention Literature [1] Literature [2] Literature [3] Literature [4] Literature [5] Whether Full - Range Soft - Switching Yes Yes No Yes Yes Yes Difficulty of Modulation Strategy Implementation Low Low Low Low Medium High Number of High - Frequency Switching Tubes 6 8 8 6 8 6 Degree of Current Distortion Low Low Low Medium High Low Voltage Stress of Switching Tubes Low High Low Low Low Low Maximum Switching Frequency / kHz 175 100 100 350 500 140 In addition, to verify the effectiveness of the present invention in reducing the turn-off current of the primary-side switching device, Figure 12 under the same conditions, the turn-off current values of the primary-side switching device under the modulation strategies of the present invention and the literature [3] were compared. It can be seen that the modulation strategy proposed by the present invention can significantly reduce the turn-off current of the primary-side switching device.

[0076] 4. Simulation verification To verify the effectiveness of the variable-fixed frequency hybrid optimization control strategy proposed by the present invention, using the parameters listed in Table 1, simulation verification was carried out using PSIM software. Figure 13 The simulation results are shown. It can be seen that the changing trend of the switching frequency is consistent with the theoretical analysis, v ac and i ac the phases are basically the same, and the power factor is 0.9975. In addition, the total harmonic distortion rate of the alternating current i ac is 1.305%.

[0077] Figure 14 Shows v p 、nv s 、i Lk the simulation waveforms. In the figure, T 1, T 2 and T 3 correspond to the EPS internal mode, the EPS external mode, and the single-phase shift mode respectively at different times. It can be seen that i Lk there are no obvious current spikes, and seamless switching between each mode can be achieved.

[0078] Figure 15 Shows the simulation diagram of the current flowing through the switching device S1. It can be seen from the figure that the turn-off current of S1 in the literature [3] is constant at 5 A; while the turn-off current of S1 in this paper is 1 A within a wide range, thus effectively reducing the turn-off current value of the switching device. This is consistent with Figure 12 the theoretical analysis results.

[0079] Embodiment 2 Based on the same inventive concept, the embodiment of the present application also provides a variable-fixed frequency hybrid optimization controller. The implementation solutions provided by the variable-fixed frequency hybrid optimization controller to solve problems are similar to the implementation solutions described in the method of Embodiment 1. Therefore, the specific limitations in one or more embodiments of the variable-fixed frequency hybrid optimization controller provided below can refer to the limitations on the method in Embodiment 1, and will not be elaborated here.

[0080] A variable-fixed frequency hybrid optimization controller for controlling a single-stage DAB type AC-DC converter, comprising: A switching frequency acquisition module for acquiring the switching frequency in real time f s ; A preset module for storing the maximum switching frequency of the preset DAB converter f smax ; A first modulation module, connected to the switching frequency acquisition module and the preset module, for when the real-time switching frequency of the DAB converter f s is less than the preset maximum switching frequency f smax , modulating the DAB converter using variable frequency SPS; A second modulation module, connected to the switching frequency acquisition module and the preset module, for when the switching frequency f s reaches the preset maximum switching frequency f smax , modulating the DAB converter using fixed frequency EPS and making f s = f smax ; It is also used for when modulating using fixed frequency EPS, if I ac |sin( ωt )|≤ nV dc ( K −1) / (8 f s L k K 2 ), making the DAB converter in the internal mode; Otherwise, making the DAB converter in the external mode; Define D f as v p and v s compared with the external shift between, D 1 is the internal shift ratio between the secondary side switching tubes S3 and S6 of the DAB converter; The method for making the DAB converter in the internal mode is: Calculate D f and D 1 according to the following formula, and according to the calculated Df and D 1 is used for extended phase-shift modulation;

[0081] The method to make the DAB converter in the external mode is as follows: Calculate according to the following formula D f and D 1, and according to the obtained D f and D 1 is used for extended phase-shift modulation;

[0082] wherein, K is the voltage conversion ratio, K =2n V dc / | v ac |; I ac |sin( ωt )| is the rectifier bridge output current of the single-stage DAB type AC-DC converter; n is the turns ratio of the high-frequency transformer; V dc is the DC voltage of the single-stage DAB type AC-DC converter; v ac is the AC voltage of the single-stage DAB type AC-DC converter; L k represents the leakage inductance of the high-frequency transformer.

[0083] Embodiment 3 This embodiment provides an AC / DC converter for microgrid distributed power access, which is characterized in that it includes a single-stage DAB type AC-DC converter and a controller; The controller is controlled and connected to the single-stage DAB type AC-DC converter; The method for the controller to control the single-stage DAB type AC-DC converter adopts the variable-fixed frequency based hybrid optimization control method described in Embodiment 1.

[0084] Embodiment 4 In an exemplary embodiment, a computer device is provided, and the computer device may be a terminal. The computer device further includes a processor, a memory, an input / output interface, a communication interface, a display unit, and an input device. Among them, the processor, the memory, and the input / output interface are connected through a system bus, and the communication interface, the display unit, and the input device are connected to the system bus through the input / output interface. The memory of the computer device includes a non-volatile storage medium and an internal memory. The non-volatile storage medium stores an operating system and a computer program. The internal memory provides an environment for the operation of the operating system and the computer program in the non-volatile storage medium. The input / output interface of the computer device is used for the processor to exchange information with external devices. The communication interface of the computer device is used for wired or wireless communication with an external terminal, and the wireless mode can be implemented through WIFI, a mobile cellular network, NFC (Near Field Communication), or other technologies. When the computer program is executed by the processor, it realizes the steps of the variable-fixed frequency-based hybrid optimization control method disclosed in Embodiment 1 of the present application. The display unit of the computer device is used to form a visually visible picture, which may be a display screen, a projection device, or a virtual reality imaging device. The display screen may be a liquid crystal display screen or an electronic ink display screen. The input device of the computer device may be a touch layer covering the display screen, or a button, a trackball, or a touchpad provided on the housing of the computer device, or an external keyboard, touchpad, or mouse, etc.

[0085] Those skilled in the art can understand that the structure of the above computer device is only a partial structure related to the solution of the present application, and does not constitute a limitation on the computer device to which the solution of the present application is applied. The specific computer device may include more or fewer components, or combine certain components, or have different component arrangements.

[0086] In an exemplary embodiment, a computer-readable storage medium is provided, on which a computer program is stored, and when the computer program is executed by a processor, it realizes the steps of the variable-fixed frequency-based hybrid optimization control method disclosed in Embodiment 1 of the present application.

[0087] The embodiment of the present application further provides a computer program product, including a computer program / instructions, and when the computer program / instructions are executed by a processor, it realizes the steps of the variable-fixed frequency-based hybrid optimization control method disclosed in Embodiment 1 of the present application.

[0088] Each embodiment in this specification is described in a progressive manner. Each embodiment focuses on the differences from other embodiments. The same or similar parts among the embodiments can be referred to each other.

[0089] Those skilled in the art should understand that the embodiments of the present application can be provided as a method, a device, or a computer program product. Therefore, the embodiments of the present application can take the form of a complete hardware embodiment, a complete software embodiment, or an embodiment combining software and hardware aspects. Moreover, the embodiments of the present application can take the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0090] The embodiments of the present application are described with reference to the flowcharts and / or block diagrams of the method, device, storage medium, and program product according to the embodiments of the present application. It should be understood that each flow and / or block in the flowchart and / or block diagram, as well as the combination of flows and / or blocks in the flowchart and / or block diagram, can be implemented by computer program instructions. These computer program instructions can be provided to the processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing terminal devices to generate a machine, such that the instructions executed by the processor of the computer or other programmable data processing terminal devices generate means for implementing the specified functions in Figure 1 one flow or multiple flows and / or blocks Figure 1 one block or multiple blocks.

[0091] The above has introduced the method, device, medium, and product provided by the present application in detail. Specific examples are used in this article to elaborate on the principle and implementation manner of the present application. The description of the above embodiments is only used to help understand the method and its core idea of the present application; at the same time, for those of ordinary skill in the art, according to the idea of the present application, there will be changes in the specific implementation manner and application scope. In summary, the content of this specification should not be construed as a limitation to the present application.

Claims

1. A variable-fixed frequency based hybrid optimization control method, applied to a single-stage DAB type AC-DC converter, characterized in that, including: Preset the maximum switching frequency of the DAB converter f smax ; When the real-time switching frequency of the DAB converter f s is less than the preset maximum switching frequency f smax the DAB converter is modulated using variable-frequency SPS; When the switching frequency f s reaches the preset maximum switching frequency f smax , the DAB converter is modulated using a fixed-frequency EPS, and let f s = f smax ; When modulating with a fixed-frequency EPS, if the following conditions are met I ac |sin( ωt )|≤ nV dc ( K −1) / (8 f s L k K 2 ), the DAB converter is in the internal mode; Otherwise, put the DAB converter in the external mode; Definition D f is v p and v s Compared with the outward movement between D 1 is the inward movement between the secondary side switching transistors S3 and S6 of the DAB converter; The method for putting the DAB converter in the internal mode is: Calculate according to the following formula D f and D 1, and perform extended phase-shift modulation based on the calculated D f and D 1 The method for putting the DAB converter in the external mode is: Calculate according to the following formula D f and D 1, and based on the calculated D f and D 1 to perform extended phase shift modulation; Among them, K is the voltage conversion ratio, K = 2n V dc / | v ac |; I ac |sin( ωt )| is the output current of the rectifier bridge of the single-stage DAB type AC-DC converter, i ac = I ac sin( ωt ); n is the turns ratio of the high-frequency transformer; V dc is the DC voltage of the single-stage DAB type AC-DC converter; v ac is the AC voltage of the single-stage DAB type AC-DC converter; i ac is the AC current of the single-stage DAB type AC-DC converter; L k represents the leakage inductance of the high-frequency transformer.

2. The hybrid optimization control method based on variable-fixed frequency according to claim 1, wherein Switching frequency f s Obtained by calculation according to the following formula: wherein, A0, B0, and C0 are: Among them, I B is the conditional value for the switching transistor of the DAB converter to achieve ZVS turn-on.

3. The variable-fixed frequency based hybrid optimization control method according to claim 2, wherein Setting I B = 1A, maximum switching frequency f smax = 175 kHz.

4. A variable-fixed frequency based hybrid optimization controller for controlling a single-stage DAB type AC-DC converter, characterized in that, including: A switching frequency acquisition module for acquiring the switching frequency in real time f s ; Preset module, used to store the maximum switching frequency of a preset DAB converter f smax ; The first modulation module, connected to the switching frequency acquisition module and the preset module, is configured to use variable-frequency SPS to modulate the DAB converter when the real-time switching frequency of the DAB converter f s is less than the preset maximum switching frequency f smax ; The second modulation module, connected to the switching frequency acquisition module and the preset module, is configured to use a fixed-frequency EPS to modulate the DAB converter when the switching frequency f s reaches the preset maximum switching frequency f smax and make f s = f smax ; It is also used when modulating with a fixed-frequency EPS. If the following conditions are met I ac |sin( ωt )|≤ nV dc ( K −1) / (8 f s L k K 2 ), the DAB converter is in the internal mode; Otherwise, put the DAB converter in the external mode; Definition D f is v p and v s compared with the outward shift between, D 1 is the inward shift between the secondary side switching transistors S3 and S6 of the DAB converter; The method for putting the DAB converter in the internal mode is: Calculate according to the following formula D f and D 1, and perform extended phase-shift modulation based on the calculated D f and D 1 The method for putting the DAB converter in the external mode is: Calculate according to the following formula D f and D 1, and perform extended phase-shift modulation based on the calculated D f and D 1 Among them, K is the voltage conversion ratio, K = 2n V dc / | v ac |; I ac |sin( ωt )| is the output current of the rectifier bridge of the single-stage DAB type AC-DC converter, i ac = I ac sin( ωt ); n is the turns ratio of the high-frequency transformer; V dc is the DC voltage of the single-stage DAB type AC-DC converter; v ac is the AC voltage of the single-stage DAB type AC-DC converter; i ac is the AC current of the single-stage DAB type AC-DC converter; L k represents the leakage inductance of the high-frequency transformer.

5. The variable-fixed frequency based hybrid optimization controller according to claim 4, wherein Switching frequency f s Obtained by calculation according to the following formula: wherein, A0, B0, and C0 are: Among them, I B is the conditional value for the switching transistor of the DAB converter to achieve ZVS turn-on.

6. The variable-fixed frequency based hybrid optimization controller according to claim 5, wherein Setting I B = 1A, maximum switching frequency f smax = 175 kHz.

7. An AC / DC converter for microgrid distributed power access, characterized in that, including a single-stage DAB type AC-DC converter and a controller; The controller is controlledly connected to the single-stage DAB type AC-DC converter; The method for the controller to control the single-stage DAB type AC-DC converter adopts the variable-fixed frequency based hybrid optimization control method described in any one of claims 1 to 3.

8. A computer device, comprising a memory, a processor, and a computer program stored on the memory, characterized in that, The processor executes the computer program to implement the variable-fixed frequency based hybrid optimization control method described in any one of claims 1 to 3.

9. A computer-readable storage medium having computer programs / instructions stored thereon, characterized in that, When the computer program / instructions are executed by the processor, the variable-fixed frequency based hybrid optimization control method described in any one of claims 1 to 3 is implemented.

10. A computer program product, comprising a computer program / instructions, characterized in that, When the computer program / instructions are executed by the processor, the variable-fixed frequency based hybrid optimization control method described in any one of claims 1 to 3 is implemented.

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