阅读说明：本技术 一种三相三电平dab变换器 (Three-phase three-level DAB converter ) 是由 刘金路 陈文洁 杨旭 马鑫 闫瑞涛 张茹 周永兴 于 2021-08-13 设计创作，主要内容包括：本发明公开了一种三相三电平DAB变换器,属于电力电子研究领域,变换器两侧结构对称,分别由三个飞跨电容桥臂组成,两侧经变压器连接,电气隔离,能够实现能量双向流动,这种桥臂结构,能够在同样的电压场景下应用耐压较低、性能较好,开关速度更快、导通损耗更小的开关管,三相结构适用于大功率工况,同等功率下,每相分担的电流更小；本发明通过对此变换器的建模和分析,设计提出调制策略。灵活的桥臂状态切换使变换器工作在宽范围调压状态时,效率更高,性能更好,实验证明了该拓扑和调制策略的有效性。(The invention discloses a three-phase three-level DAB converter, which belongs to the field of power electronic research, wherein the two side structures of the converter are symmetrical and respectively consist of three flying capacitor bridge arms, the two sides of the converter are connected through a transformer and are electrically isolated, and energy bidirectional flow can be realized; the invention designs and provides a modulation strategy through modeling and analyzing the converter. When the converter works in a wide-range voltage regulation state due to flexible bridge arm state switching, the efficiency is higher, the performance is better, and the effectiveness of the topology and the modulation strategy is proved by experiments.)

1. A three-phase three-level DAB converter, characterized in that said circuit comprises:

the converter is symmetrical in structure at two sides and respectively consists of three flying capacitor bridge arms, the two sides are connected through a transformer and are electrically isolated, energy can flow in two directions, the bridge arm structure can be applied to a switching tube which is low in voltage resistance, good in performance, high in switching speed and low in conduction loss under the same voltage scene, a three-phase structure is suitable for a high-power working condition, and the current shared by each phase is smaller under the same power;

structure of flying capacitor bridge arm (taking primary side phase a as an example): the source electrode of the switch tube S1 is connected with the upper bus, the drain electrode of the switch tube S1 is connected with the source electrode of the switch tube S2, the source electrode of the switch tube S2 is connected with the drain electrode of the switch tube S3, the drain electrode of the switch tube S3 is connected with the source electrode of the switch tube S4, the source electrode of the switch tube S4 is connected with the lower bus, the drain electrode of the switch tube S2 is connected with the source electrode of the switch tube S3 and the flying capacitor, and the two nodes are respectively connected with the cathode of the diode D1 and the anode of the diode D2. The node of the cathode of the diode D1 and the cathode of the diode D2 is connected with the middle point of two capacitors of the bus, and the structures of the other bridge arms are consistent with the primary side A.

2. The circuit of claim 1, wherein three flying capacitor arms on the primary side are connected in parallel, the drain of the upper end S1, the drain of S5 and the drain of S9 of each arm are connected to each other by an upper bus, the source of the lower end S4, the source of S8 and the source of S12 of each arm are connected to each other by a lower bus, the node at which the diode of each arm is connected, the node at which the anode of diode D1 is connected to the cathode of diode D2, the node at which the anode of diode D3 is connected to diode D4, the node at which the anode of diode D5 is connected to the cathode of diode D6 are connected to each other and to the midpoint of the bus capacitors, the two sides are symmetrical, and the secondary side and the primary side are the same.

3. The circuit of claim 1, wherein the source of the output point S2, the source of S6, and the source of S10 of the primary leg are connected to one end of the primary side of three transformers via inductors, respectively, and the other ends of the primary side are connected to each other, and the output point of the secondary leg is connected to one end of the secondary side of three transformers, the same-name end of the primary side input, and the other ends are connected to each other in a YY connection.

4. The circuit of claim 1, wherein the flying capacitor bridge arm can operate in different states through different switch combinations to output a positive level, a negative level, and a zero level, respectively, and the duty cycle of the primary or secondary bridge arm voltage is adjusted to adjust the operating state of the converter in view of the input and output voltages.

5. The circuit of claim 1, wherein the flying capacitor bridge arm has an automatic voltage grading function, and the flying capacitor bridge arm can be charged through a clamping diode when the flying capacitor voltage is low.

6. The circuit of claim 1, wherein the converter is modeled and analyzed by a frequency domain analysis method, power is taken as a constraint condition, an effective value of current is taken as an optimization target for optimization analysis, and the converter is enabled to work in a state of the minimum effective value of current by adjusting a duty ratio of a zero level of a bridge arm, so that efficiency is optimized.

Technical Field

The invention belongs to the field of power electronic research, and discloses a three-phase three-level DAB topology which is suitable for a wide-range voltage regulation application scene.

Background

In recent years, new energy technologies have been developed. The new energy is connected to an alternating current power grid, and an inverter is generally needed to convert direct current into alternating current. Compared with an alternating current power grid, the direct current power grid does not need an inverter, and the access of new energy is facilitated. In addition, the direct current power grid has no problems of frequency synchronization, reactive power and the like, and has higher efficiency.

New energy such as photovoltaic, wind-powered electricity generation receive external environment's influence comparatively obvious, have great volatility, if do not control, can influence the electric wire netting safety and stability operation. Energy storage devices in the dc power grid can absorb energy or feed back energy to the grid when necessary. The battery capacity of the electric automobile is large, and the time for accessing the power grid when the electric automobile is idle is long, so that the electric automobile is a good energy storage device. And reasonably controlling the energy interaction between the electric automobile and the power grid is beneficial to the stable operation of the power grid. The voltage variation range of the power battery of the electric automobile is large in the charging and discharging processes, and a converter with good performance under the condition of voltage regulation is required, so that the energy interaction between the electric automobile and a direct-current power grid is realized.

The three-level bridge arm structure can enable each transistor to bear lower voltage, and compared with the two-level bridge arm structure, the three-level bridge arm structure can use the transistors with lower voltage levels under the same voltage level. Such a transistor has a faster switching speed and a smaller on-resistance. And the switch combination with the flexible three-level bridge arm structure is suitable for application scenes of large-range voltage regulation.

A three-phase three-level DAB (Dual Active bridge) converter topology is presented. The method is suitable for energy interaction between the electric automobile and the direct current power grid.

Disclosure of Invention

In order to optimize energy interaction between an electric automobile and a power grid, the invention designs a three-phase three-level DAB converter topology.

The content of the invention comprises:

a three-phase three-level DAB converter topology is provided;

analyzing the working state of a flying capacitor bridge arm;

a modulation strategy for minimizing the effective value of the inductor current is provided

Experiments are carried out on the proposed three-phase three-level DAB converter, a prototype is built, and the correctness of the proposed topological structure and the correctness of the modulation strategy are verified.

The technical scheme adopted by the invention is realized as follows:

structure of flying capacitor bridge arm (taking primary side phase a as an example): the source of the switch tube S1 is connected to the upper bus, and the drain of the switch tube S1 is connected to the source of the switch tube S2. The source of S2 is connected to the drain of S3, and the drain of S3 is connected to the source of S4. The source of switch tube S4 is connected to the lower bus. The drain of the switch tube S2 is connected to the source of the switch tube S3 and the flying capacitor, and these two nodes are also connected to the cathode of the diode D1 and the anode of the diode D2, respectively. The node connecting the cathode of the diode D1 and the cathode of the diode D2 is connected with the middle point of the two capacitors of the bus. The structures of the other bridge arms are consistent with the primary side A.

The connection mode between the bridge arms is as follows: the three flying capacitor arms on the primary side are connected in parallel, the upper ends of each arm (the drain of S1, the drain of S5, and the drain of S9) are connected to each other by an upper bus, and the lower ends of each arm (the source of S4, the source of S8, and the source of S12) are connected to each other by a lower bus. The diode-connected nodes of each leg (the node connecting the anode of diode D1 and the cathode of diode D2, the node connecting the anode of diode D3 and diode D4, and the node connecting the anode of diode D5 and the cathode of diode D6) are connected to each other and to the midpoint of the bus capacitor. The two sides are symmetrical structures, and the secondary side is the same as the primary side.

And the connection mode of the transformer: the output points (source of S2, source of S6 and source of S10) of the primary side bridge arm are respectively connected with one end of the primary sides of the three transformers through inductors, and the other ends of the primary sides are connected with each other. The output points (sources of Q2, Q6 and Q10) of the secondary side bridge arms are respectively connected with one ends (same-name ends of primary side inputs) of the secondary sides of the three transformers, and the other ends of the secondary side bridge arms are mutually connected and are YY-shaped.

The flying capacitor bridge arm structure can use a switching device with lower withstand voltage and lower loss in a high-voltage scene. The three-phase structure helps that under the working condition that the topology is applied to high power, compared with a single-phase topology, each phase shares less current. And the flying capacitor bridge arm has various switch states, which is beneficial to large-scale voltage regulation. The topology is suitable for the charge-discharge power module of the electric automobile, and contributes to miniaturization and high efficiency of the power module.

In the modulation mode, the modulation strategy with the minimum effective current value is provided through frequency domain analysis, and the duty ratio can be reasonably adjusted through the combination of different switch states of a bridge arm under the scene of a large voltage change range, so that the effective current value is kept in a small state, and the efficiency is improved.

Drawings

The following detailed description is made in conjunction with the accompanying drawings and embodiments implemented by the inventors.

Figure 1 is a three-phase three-level DAB topology of the present invention.

FIG. 2 illustrates the operation of the flying capacitor bridge arm of the present invention.

FIG. 3 is a flying capacitor charging path according to the present invention.

FIG. 4 shows the switching sequence for each leg of the present invention

FIG. 5 is a schematic diagram of bridge arm output voltage according to the present invention

FIG. 6 is a simplified diagram of the topology of the present invention

FIG. 7 is a flow chart of a control strategy according to the present invention

FIG. 8 is a comparative graph of the experiment of the present invention

FIG. 9 is a graph comparing efficiency curves of the present invention

Detailed Description

The three-phase three-level DAB converter topology of the present invention is shown in fig. 1. The two sides of the converter are of symmetrical structures, which is beneficial to the bidirectional flow of energy. The primary side and the secondary side are respectively composed of three flying capacitor bridge arms, namely an A phase, a B phase and a C phase. The bridge arms have the same structure, and each flying capacitor bridge arm is composed of four MOSFETs, two diodes and one flying capacitor.

Taking the left-side A-phase flying capacitor bridge arm as an example, the switches S1, S2,S3 and S4 are connected in series. The cathode of the diode D1 is connected to the source of the switch S1, and the anode of the diode D2 is connected to the drain of the switch S4. The anode of the diode D1 is connected to the cathode of the diode D2. The flying capacitor is connected in parallel with two diodes. V₁And V₂Which are the dc voltages on both sides, respectively.

The flying capacitor bridge arm has various switch combinations and can be divided into four different states, namely a P state, an N state and an O state₁State and O₂Status. As shown in fig. 2. When the bridge arm is in the P state, the switches S1 and S2 are turned on, S3 and S4 are turned off, and V_AO0.5V 1. When the bridge arm is in the N state, the switches S3 and S4 are turned on, S1 and S2 are turned off, and V_AO＝-0.5V₁. When the bridge arm is at O₁In the state, the switches S1 and S3 are turned on, S2 and S4 are turned off, and the voltage on the flying capacitor is V₁Half of (A), V_AO0. When the bridge arm is at O₂In this state, switches S2 and S4 are on, S1 and S3 are off, and V is on_AO＝0。

Under the condition of reasonable switching sequence, the voltage on the flying capacitor can be kept at 0.5V₁. When the voltage of the flying capacitor is less than 0.5V₁At this time, the flying capacitor is charged through a path, as shown in fig. 3. For example, when the bridge arm is at O₁In this state, the upper bus capacitor charges the flying capacitor via switch S1 and diode D2. If the bridge arm switching sequence is not reasonable, the flying capacitor voltage may be charged all the time to cause the voltage to be too high, and the reasonable switching sequence is shown in fig. 4.

FIG. 5 is a schematic diagram of the waveform of phase A, U_A、U_BAnd U_CHas the same angle of zero level, which is D₁(ii) a Subsidiary edge U_X、U_YAnd U_ZHas the same angle of duty ratio as D₂. The duty ratio of the B phase and the C phase is consistent with that of the A phase, and the phase lags or leads the A phase by 120 degrees.

The magnitude and the loss of the current effective value are basically in positive correlation, the minimum current effective value is used as the optimization target of the duty ratio of the DAB converter, the optimal duty ratio is solved, and therefore the efficiency is improved. A simplified model of a three-phase three-level topology is shown in fig. 6, based on which the optimal duty cycle is found.

The current in the inductor is determined by the voltage on both sides (e.g. phase A current is determined by phase V_AMAnd V_XNDecision), but the voltage-fragmentation function is too cumbersome if traditional time-domain analysis methods are used, due to the interplay between the three phases. In the invention, the optimal duty ratio is solved by using a frequency domain analysis method.

VA, VB and VC are represented using fourier series:

the voltage of the transformer primary side neutral point M is as follows:

voltage V of point A relative to neutral point of primary side transformer_AMComprises the following steps:

also the voltage between points X and N can be expressed as:

the inductive voltage is:

V_LA(t)＝V_AM(t)-NV_XN(t) (7)

depending on the nature of the inductor, an inductor current may be obtained.

Considering the symmetry of the inductor current, there are

Can obtain

Wherein

Due to the three-phase symmetric modulation scheme, the power of the converter can be expressed as:

the power expression of the converter can be obtained by combining the formulas (13), (18) and (21)

Setting the power reference value as:

the effective value of the current is:

the power transmitted by the analyzed fundamental wave is basically equal to the total power transmitted by the converter, and the effective value of the fundamental wave current is basically equal to the effective value of the inductive current. Therefore, the power transmitted by the fundamental wave is taken as a constraint condition, and the effective value of the fundamental wave current is taken as the effective value

Through solving, the optimized condition can be obtained

When V is₁>NV₂Time of flight

When V is₁<NV₂Time of flight

The modulation strategy diagram is shown in fig. 7, and the duty ratio of the primary side or the secondary side voltage is adjusted according to the difference between the input voltage and the output voltage, so that the problem of efficiency reduction caused by large inductive current of the three-phase three-level converter when the voltages are not matched is solved. The converter can keep good performance under the condition of wide-range voltage regulation.

FIG. 8 is a waveform diagram of an experiment, in which U is shown from top to bottom_A、U_XAnd i_LA. The input voltage is 150V and the power is 200W. The experimental waveforms under the same working conditions can be seenThe algorithm provided by the invention effectively reduces the magnitude of the inductive current. The efficiency curves of fig. 9 demonstrate the effectiveness of this control strategy for efficiency improvement.