A Three-Phase LCL-DAB DC Converter Topology and Its Control Method

Abstract

The present invention discloses a three-phase LCL-DAB DC converter topology and its control method. The three-phase LCL-DAB DC converter includes a primary side, a secondary side, and an intermediate magnetic component. Both the primary side and the secondary side are three-phase bridge circuits, and the intermediate magnetic component is composed of a star connection of three-phase high-frequency transformers. A three-phase LCL resonant cavity circuit is connected between the three-phase bridge circuit on the primary side and the three-phase high-frequency transformer. The three-phase bridge circuit on the primary side is connected in parallel with an input capacitor C1 as the input port, and the three-phase bridge circuit on the secondary side is connected in parallel with an output capacitor C2 as the output port. This DC converter topology can significantly reduce the effective value of the current. On the premise of retaining the characteristics of isolation and high power density, it also ensures high power conversion efficiency and relatively low hardware costs.

Description

Technical Field

[0001] The present invention belongs to the technical field of power electronics and relates to a three-phase LCL-DAB DC converter topology and its control method. Background Art

[0002] All along, the high-frequency bidirectional isolated DC converter (IBDC) has been widely concerned as a research hotspot in the field of power electronics. It is widely used in high-voltage ratio, high-power industrial fields with bidirectional energy flow, such as important industrial application fields like power electronic transformers, multi-electric aircraft, household electric vehicles, and distributed DC microgrids. As a very promising topology in IBDC, the three-phase DAB topology can simply achieve multiple functions such as high voltage ratio, high power density, low device stress, and soft switching of switching devices. However, at the same time, in the case of high voltage ratio, the three-phase DAB has defects such as high reflux power and excessive current effective value. Summary of the Invention

[0003] The purpose of the present invention is to overcome the technical defect of excessive current effective value in the existing DAB converter under the condition of high voltage ratio, and propose a three-phase LCL-DAB DC converter topology and its control method. This DC converter topology can significantly reduce the current effective value, and on the premise of retaining the characteristics of isolation and high power density, it also ensures high power conversion efficiency and relatively low hardware cost.

[0004] The purpose of the present invention is achieved by the following technical solutions:

[0005] A three-phase LCL-DAB DC converter includes a primary side, a secondary side, and an intermediate magnetic component;

[0006] Both the primary side and the secondary side are three-phase bridge circuits, and the intermediate magnetic component is composed of a star connection of three-phase high-frequency transformers; a three-phase LCL resonant cavity circuit is connected between the three-phase bridge circuit on the primary side and the three-phase high-frequency transformer; the three-phase bridge circuit on the primary side is connected in parallel with an input capacitor C1 as the input port, and the three-phase bridge circuit on the secondary side is connected in parallel with an output capacitor as the output port.

[0007] As a further improvement of the present invention, each phase of the three-phase LCL resonant cavity circuit is connected in series with the primary winding of the three-phase high-frequency transformer; the three-phase LCL resonant cavity is connected in a star shape to the neutral point O3, and the three-phase high-frequency transformer is connected in a star shape to the neutral point O1 of the primary winding and the neutral point O2 of the secondary winding.

[0008] As a further improvement of the present invention, the three-phase bridge circuit on the primary side is a three-phase bridge inverter circuit composed of six switching tubes; the six switching tubes are connected in series across the input voltage, and the source electrode of the switching tube of the switching tube is connected to the drain. The midpoints of the three-phase bridge arms are connected to the transformer through a three-phase LCL resonant cavity. The midpoint of the A-phase bridge arm is connected to one end of the left resonant inductor. The left resonant inductor is connected to the right resonant inductor, and the intersection node is connected to one end of the resonant capacitor. The three-phase resonant capacitors are commonly connected to the neutral point ; the other end of the right resonant inductor is connected to the primary side winding of the transformer.

[0009] As a further improvement of the present invention, the secondary side three-phase bridge circuit is a three-phase bridge rectifier circuit composed of six switching tubes.

[0010] As a further improvement of the present invention, both the primary side and the secondary side three-phase bridge circuits adopt three-phase symmetric control, and the switching signals of each phase bridge arm are out of phase with each other by 120 degrees.

[0011] As a further improvement of the present invention, the parameters of the three-phase LCL resonant cavity circuit satisfy the following relationship:

[0012]

[0013] In the formula, is the switching frequency, is the resonant inductor, is the resonant capacitor.

[0014] A control method for a three-phase LCL-DAB DC converter includes the following steps:

[0015] The external phase shift angle between the primary side and the secondary side three-phase bridge circuits always remains at the maximum phase shift angle unchanged, and the effective value of the primary and secondary side bridge port voltages is changed by a modulation method to change the power transmission.

[0016] As a further improvement of the present invention, the current effective value expression and power expression of the three-phase LCL-DAB are:

[0017]

[0018] Among them, and are the per-unit values of the primary side current effective value and the secondary side current effective value respectively; by adjusting the duty cycle variables and , the three-phase LCL-DAB converter minimizes the current effective value under the given transmission power condition;

[0019] According to the Lagrange multiplier method, when the duty cycle variables and satisfy the following relationship, the current effective value can obtain the minimum value:

[0020]

[0021] By controlling the magnitudes of the duty cycle variables of the primary and secondary three-phase bridges, the reflux power is reduced, and on the basis of reducing the effective value of the working current, the transmitted power is regulated.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] The three-phase LCL-DAB topology proposed by the present invention, compared with the traditional three-phase DAB converter, has a three-phase bridge circuit on both the primary side and the secondary side, and the intermediate magnetic component is composed of a star connection of a three-phase high-frequency transformer; a three-phase LCL resonant cavity circuit is connected between the primary three-phase bridge circuit and the three-phase high-frequency transformer. By changing the topology structure and control method, it can achieve unity power factor operation. Under the condition of transmitting the same power, it can significantly reduce the reflux power generated during the operation of the converter, reduce the effective value of the current, greatly improve the working efficiency of the converter, and is suitable for industrial application occasions with high power and high voltage ratio. On the premise of retaining the characteristics of isolation and high power density, it also ensures high electrical energy conversion efficiency and relatively low hardware cost. Compared with the traditional three-phase DAB converter, the present invention can greatly reduce the reflux power, reduce the effective value of the current, and improve the working efficiency of the converter under the condition of high voltage ratio. Brief Description of the Drawings

[0024] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0025] Figure 1 Topology of three-phase LCL-DAB DC converter;

[0026] Figure 2 A-phase equivalent circuit of three-phase LCL-DAB;

[0027] Figure 3 Voltage and current phasor diagram;

[0028] Figure 4 Topology of three-phase LCL-DAB converter;

[0029] Figure 5 Phase current waveforms of the primary and secondary sides of the LCL-DAB converter (from top to bottom are the primary side and the secondary side respectively);

[0030] Figure 6 Phase current waveforms of the traditional three-phase DAB converter. Detailed implementation manners

[0031] To make the objectives and technical solutions of the present invention clearer and easier to understand, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. The specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0032] In the description of the present invention, it should be understood that the orientation or positional relationships indicated by the terms "center", "longitudinal", "transverse", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc. are based on the orientation or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and thus should not be construed as limiting the present invention. In addition, the terms "first" and "second" are only used for descriptive purposes and cannot be understood as indicating or implying relative importance or implicitly specifying the quantity of the indicated technical features. Thus, the features defined with "first" and "second" may explicitly or implicitly include one or more of such features. In the description of the present invention, unless otherwise stated, the meaning of "a plurality" is two or more. In the description of the present invention, it should be noted that unless otherwise clearly specified and limited, the terms "mounted", "connected" and "connected" should be understood in a broad sense. For example, it may be a fixed connection, a detachable connection, or an integral connection; it may be a mechanical connection or an electrical connection; it may be directly connected, or indirectly connected through an intermediate medium, and it may be the communication inside two elements. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific situations.

[0033] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings and specific embodiments. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the scope of the present invention.

[0034] The present invention provides an isolated DC converter applicable to battery charging and renewable energy power generation, including a primary side, a secondary side and an intermediate magnetic component;

[0035] Both its primary side and secondary side are traditional three-phase bridge circuits, and the intermediate magnetic component is composed of a star connection of three-phase high-frequency transformers. The three-phase bridge circuit on the primary side is connected to the three-phase high-frequency transformer by a three-phase LCL resonant cavity circuit. The specific structural diagram is as Figure 1 .

[0036] The switching tubes on the primary side of the converter Form a three-phase bridge inverter circuit and a three-phase LCL resonant cavity It is connected in series with the primary winding of the three-phase high-frequency transformer. And the three-phase LCL resonant cavity is connected in star, with the neutral point defined as point O3. The three-phase high-frequency transformer is also connected in star, with the neutral point of the primary winding defined as point O1 and the neutral point of the secondary winding defined as point O2. On the secondary side, the switching tubes Form a three-phase bridge rectifier circuit and are connected in parallel with the output capacitor As the output port. The three-phase bridge circuits on the primary and secondary sides both adopt three-phase symmetric control, and the switching signals of each phase bridge arm have a phase difference of 120 degrees from each other.

[0037] The present invention defines Figure 1 in as the primary three-phase phase current, and in the figure as the secondary three-phase phase current. Different from the traditional structure of directly connecting the three-phase DAB three-phase inductors to the three-phase high-frequency transformer, the external phase shift angle between the primary and secondary three-phase bridges of the present invention always remains at the maximum phase shift angle unchanged, and the power transmission is changed by modulating the effective values of the primary and secondary bridge port voltages.

[0038] The present invention uses a three-phase LCL resonant cavity to replace the three-phase inductor. This design is to utilize the phase shift characteristic of the LCL to make the primary and secondary voltages and currents in the same phase, ensure that the converter has a unity power factor of 1, minimize the reflux power as much as possible, and reduce the effective value of the current during the operation of the converter.

[0039] A three-phase LCL-DAB topology structure proposed by the present invention realizes the unity power factor operation of the three-phase LCL-DAB converter through changing the topology structure of the resonant cavity and special phase shift duty cycle hybrid control.

[0040] For in-depth explanation, the equivalent circuit diagram of the three-phase LCL-DAB is given below. Due to the consistency of the three-phase circuit, the three-phase DC converter of the present invention can be equivalent to three independent single-phase circuits. The voltage and current waveforms of each phase are the same, and the phases are 120 degrees different from each other. For the sake of simplicity in analysis, only the A-phase circuit is taken as an example for analysis here. The waveforms of the other two-phase circuits can be obtained in the same way. The equivalent circuit diagram of the A-phase circuit is Figure 2 .

[0041] Figure 2 In and are the A-phase bridge port voltages of the primary and secondary three-phase bridge circuits, and constantly have a phase shift angle of unchanged, and are the LCL resonant cavity, is the phase current of the primary side A-phase, is the phase current of the secondary side A-phase.

[0042] In this new topology of three-phase LCL-DAB, the parameters of the LCL designed by the present invention satisfy the following relationship:

[0043] (1)

[0044] In the formula is the switching frequency. After determining the LC parameters of the resonant cavity, the switching frequency of this topology is also uniquely determined. When the fixed phase shift of the three-phase active bridges on the primary and secondary sides is constantly set to , the present invention can make a voltage-current phasor diagram based on the Figure 2 equivalent circuit as Figure 3 .

[0045] From the phasor diagram Figure 3 it can be seen that when the phase shift angle between the primary and secondary sides is constantly , the primary side voltage phasor and the primary side current phasor are in the same phase, and the secondary side voltage phasor and the secondary side current phasor are in the same phase. From the phasor diagram, it can be concluded that the topology of this three-phase LCL-DAB can achieve unity power factor, minimize the reflux power, and reduce the effective value of the current during operation. Thus, the efficiency of the converter is improved.

[0046] Since in order to achieve unity power factor, the external phase shift angle between the primary and secondary sides of the three-phase LCL-DAB is constantly , this restricts the power regulation ability of the three-phase LCL-DAB. In order to make the power transmission of this topology more flexible, the present invention introduces a duty cycle variable into the three-phase bridge circuits on the primary and secondary sides, where the duty cycle of each phase arm of the primary three-phase bridge is the same, and the duty cycle of each phase arm is defined as , the duty cycle of each phase arm of the secondary three-phase bridge is the same, and the duty cycle of each phase arm is defined as . By controlling the variable duty cycles of the three-phase bridges on the primary and secondary sides, the present invention can conveniently achieve flexible control of the transmitted power.

[0047] In order to make the conclusion universal and general, the present invention performs per-unit processing on each expression. The present invention defines the base values of power and current as follows:

[0048] (2)

[0049] Through calculation, the present invention can obtain the expressions of the effective value of the current and the power of the three-phase LCL-DAB after per-unit processing.

[0050] (4)

[0051] Among them, and are respectively the per-unit values of the effective values of the primary-side current and the secondary-side current. By reasonably designing and adjusting the duty-cycle variables and , the three-phase LCL-DAB converter can minimize the effective value of the current under the given transmission power condition. According to the Lagrange multiplier method, the present invention can obtain that when the duty-cycle variables and satisfy the following relationship, the minimum value of the effective current can be obtained.

[0052] (5)

[0053] In digital control, the present invention only needs to control the magnitudes of the duty-cycle variables of the primary and secondary three-phase bridges, and can flexibly adjust the transmitted power on the basis of significantly reducing the reflux power and the effective value of the working current, so as to meet various load conditions.

[0054] The three-phase LCL-DAB DC converter of the present invention will be described in detail below in conjunction with specific embodiments and parameter verification.

[0055] Embodiment

[0056] The present invention provides a topological structure of a three-phase LCL-DAB DC converter as Figure 4 .

[0057] The primary side forms a three-phase bridge inverter circuit, and 6 switching devices are connected in series across the input voltage, that is, the source of is connected to the drain of , , Similarly, the midpoints of the three-phase bridge arms are connected to the transformer through a three-phase LCL resonant cavity, that is, the midpoint of the A-phase bridge arm is connected to one end of the left resonant inductor, the left resonant inductor is connected to the right resonant inductor, the intersection node is connected to one end of the resonant capacitor, and the three resonant capacitors are commonly connected to the neutral point . The other end of the right resonant inductor is connected to the primary-side winding of the transformer. Among them, the parameters of the three-phase LCL resonant cavity are the same, the resonant inductor is defined as , and the resonant capacitor is defined as . The secondary side is also a three-phase bridge inverter circuit. A three-phase bridge inverter is formed, and a large capacitor is connected in parallel on the DC side as DC support. The three-phase AC voltage output by the three-phase high-frequency transformer is rectified into the required DC output voltage. Among them, the switching signals of the upper and lower switching tubes of each bridge arm in the three-phase active bridges on the primary and secondary sides are complementary, and the phase shift angle between each phase bridge arm is 120 degrees. At the same time, the external phase shift angle between the three-phase active bridges on the primary and secondary sides is constantly maintained at unchanged.

[0058] The switching frequency of the driving signals of the primary and secondary side switching tubes remains fixed. The fixed switching frequency is related to the parameters of the three-phase LCL resonant cavity , and is uniquely determined by the following formula. In the proposed novel three-phase LCL-DAB topology, the switching frequency needs to satisfy the following relationship:

[0059] (6)

[0060] After meeting the above conditions, the proposed three-phase LCL-DAB topology can achieve unity power factor operation, greatly reduce the reflux power, and reduce the effective value of the current at the working moment. However, due to the fact that the external phase shift angle between the primary and secondary sides of the three-phase LCL-DAB is constantly 90 degrees, which restricts the power transmission of the three-phase LCL-DAB. In the present invention, a duty cycle variable is introduced into the three-phase active bridges on the primary and secondary sides. By reasonably adjusting the duty cycles of the three-phase bridges on the primary and secondary sides, it is possible to significantly reduce the effective value of the working current and improve the converter efficiency under the given power condition.

[0061] To quantify the superiority of the proposed three-phase LCL-DAB DC converter, the present invention compares the effective value of the working current of the traditional three-phase DAB topology with the same circuit parameters under the same transmission power condition with that of the proposed three-phase LCL-DAB topology. The converter parameters are shown in Table 1:

[0062] Table 1 Specific parameters of the converter

[0063]

[0064] By using MATLAB / Simulink to simulate the three-phase LCL-DAB and three-phase DAB converters respectively, the relationship between the effective value of the working current and the transmission power is compared under the condition of a voltage ratio of 0.6. As follows Figure 5 shown, Figure 5 Waveforms of the phase currents on the primary and secondary sides of the LCL-DAB converter (from top to bottom are the primary side and the secondary side respectively).

[0065] Figure 5 Among them, and are the primary side current and secondary side current of the three-phase LCL-DAB converter respectively. It can be seen from the figure that under the working conditions of an input voltage of 1000V, an output voltage of 300V, a transmission power of 4.5kW, and a voltage ratio of 0.3, the three-phase current waveforms of the three-phase LCL-DAB converter proposed by the present invention are symmetrical, and the effective values of the three-phase currents are equal. Among them, the effective value of the current can be calculated to obtain the effective value of the primary side current , and the effective value of the secondary side current . Similarly, the present invention lists the current waveforms of the traditional three-phase DAB under the same working conditions, as shown in Figure 6 .

[0066] Figure 6 . Among them, represents the phase current waveforms of the three phases of the traditional three-phase DAB. It can be seen from the figure that under the working conditions of an input voltage of 1000V, an output voltage of 300V, a transmission power of 4.5kW, and a voltage ratio of 0.3, the three-phase current waveforms of the traditional three-phase DAB are symmetrical, and the effective values of the three-phase currents are equal. Among them, the effective value of the current can be calculated to obtain the effective value of each phase current . By comparing Figure 5 and Figure 6 , the present invention can clearly see that although the effective value of the secondary side current of the three-phase LCL-DAB converter proposed by the present invention is slightly smaller than the effective value of the phase current of the traditional three-phase DAB, the effective value of the primary side current can be significantly reduced to about half of the traditional three-phase DAB. Thus, the working loss of the primary side circuit is significantly reduced, and the overall working efficiency of the converter is significantly improved. It can also be concluded that the present invention is a new type of three-phase LCL dual active bridge DC converter that can significantly reduce the effective value of the current. On the premise of retaining the characteristics of isolation and high power density, it also ensures high power conversion efficiency and relatively low hardware cost.

[0067] The above is only a preferred embodiment of the present invention, and does not impose any limitations on the present invention. Any simple modifications, changes, and equivalent structural changes made to the above embodiments according to the technical essence of the present invention still fall within the protection scope of the technical solution of the present invention.

[0068] The above embodiments are only used to illustrate the technical solution of the present invention and do not limit it. Although the present invention has been described in detail with reference to the above embodiments, those of ordinary skill in the art can still modify the specific implementation of the present invention or make equivalent replacements, and any modifications or equivalent replacements that do not depart from the spirit and scope of the present invention are within the protection scope of the claims of the present invention.

Claims

1. A control method for a three-phase LCL-DAB DC converter, characterized in that, It includes a primary side, a secondary side and an intermediate magnetic component; The primary side and the secondary side are both three-phase bridge circuits, and the intermediate magnetic component is composed of a star connection of three-phase high-frequency transformers; a three-phase LCL resonant cavity circuit is connected between the primary side three-phase bridge circuit and the three-phase high-frequency transformer; the primary side three-phase bridge circuit is connected in parallel with an input capacitor C1 as an input port, and the secondary side three-phase bridge circuit is connected in parallel with an output capacitor as an output port; The control method includes the following steps: The external phase shift angle between the primary and secondary three-phase bridge circuits always remains at the maximum phase shift angle unchanged, and the power transmission is changed by modulating the effective values of the voltages at the primary and secondary bridge ports The effective current expressions and power expressions of the three-phase LCL-DAB are as follows: Among them, and are respectively the per-unit value of the effective value of the primary side current and the per-unit value of the effective value of the secondary side current; by adjusting the duty cycle variables D1 and D2, the three-phase LCL-DAB converter minimizes the effective value of the current under the given transmission power condition. According to the Lagrange multiplier method, when the duty ratio variables D1 and D2 satisfy the following relationship, the minimum value of the effective current can be obtained: D1 = D2 By controlling the magnitudes of the duty ratio variables of the primary and secondary three-phase bridges, the reflux power is reduced, and on the basis of reducing the effective value of the working current, the transmitted power is adjusted.

2. The control method for a three-phase LCL-DAB DC converter according to claim 1, characterized in that, Each phase of the three-phase LCL resonant cavity circuit is connected in series with the primary winding of the three-phase high-frequency transformer; the three-phase LCL resonant cavity is star-connected to the neutral point O3, and the three-phase high-frequency transformer is star-connected to the primary winding neutral point O1 and the secondary winding neutral point O2.

3. The control method for a three-phase LCL-DAB DC converter according to claim 2, characterized in that, The primary three-phase bridge circuit is a three-phase bridge inverter circuit composed of six switching tubes; the six switching tubes are connected in series across the input voltage, and the source electrode of the switching tube is connected to the drain electrode of the switching tube . The midpoints of the three-phase bridge arms are connected to the transformer through a three-phase LCL resonant cavity. The midpoint of the A-phase bridge arm is connected to one end of the left resonant inductor. The other end of the left resonant inductor is connected to the right resonant inductor, and the intersection node is connected to one end of the resonant capacitor. The three-phase resonant capacitors are commonly connected to the neutral point ; the other end of the right resonant inductor is connected to the primary side winding of the transformer.

4. The control method for a three-phase LCL-DAB DC converter according to claim 1, characterized in that, The secondary side three-phase bridge circuit is a three-phase bridge rectifier circuit composed of six switching tubes.

5. The control method for a three-phase LCL-DAB DC converter according to claim 1, characterized in that, Both the primary side and secondary side three-phase bridge circuits adopt three-phase symmetric control, and the switching signals of each phase bridge arm have a phase difference of 120 degrees from each other.

6. The control method for a three-phase LCL-DAB DC converter according to claim 1, characterized in that, The parameters of the three-phase LCL resonant cavity circuit satisfy the following relationship: Wherein, is the switching frequency, is the resonant inductor, is the resonant capacitor.

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