阅读说明：本技术 一种六开关五电平整流器及其控制方法 (Six-switch five-level rectifier and control method thereof ) 是由 刘兆伟 杨栋 于 2020-06-24 设计创作，主要内容包括：本发明公开了一种六开关五电平整流器及其控制方法,拓扑AC-DC包括三相桥臂,每相桥臂包括多组二极管电路,多组二极管电路顺次通过连接节点连接于AC-DC的正负输出端之间,第一节点与第四节点之间并联有两个串接的飞跨电容,第二节点与第三节点之间并联有两个串接的开关,两个开关的连接点与两个飞跨电容的连接点相连接,第二节点、第三节点分别通过一组二极管电路与AC-DC输出端中点相连接。新拓扑输入侧并网电流波形THD较小,开关数量少,输出侧电压灵活可控。本发明还通过价值函数及开关状态选择实现了直流侧电容电压稳定控制以及飞跨电容电压波动抑制。新拓扑及其控制方法实用性强,可广泛应用于电动汽车充电系统。(The invention discloses a six-switch five-level rectifier and a control method thereof, wherein a topology AC-DC comprises three-phase bridge arms, each phase of bridge arm comprises a plurality of groups of diode circuits, the plurality of groups of diode circuits are sequentially connected between a positive output end and a negative output end of the AC-DC through connecting nodes, two flying capacitors connected in series are connected in parallel between a first node and a fourth node, two switches connected in series are connected in parallel between a second node and a third node, the connecting points of the two switches are connected with the connecting points of the two flying capacitors, and the second node and the third node are respectively connected with the midpoint of the output end of the AC-DC through a group of diode circuits. The grid-connected current waveform THD of the input side of the new topology is small, the number of switches is small, and the voltage of the output side is flexible and controllable. The invention also realizes the voltage stability control of the direct current side capacitor and the voltage fluctuation suppression of the flying capacitor through the value function and the switch state selection. The novel topology and the control method thereof have strong practicability and can be widely applied to an electric automobile charging system.)

1. A six-switch five-level rectifier comprises a front-end AC-DC part (3) and a rear-end DC-DC part (4), wherein the grid side of the AC-DC part (3) is connected into a power grid (1) through a filter (2), the positive output end and the negative output end of the AC-DC part (3) are respectively connected with the positive input end and the negative input end of the DC-DC part (4), 2 series-connected direct-current side capacitors are connected between the positive output end and the negative output end of the AC-DC part (3), and the output end of the DC-DC part (4) is connected with a load, and the six-switch five-level rectifier is characterized in that:

the AC-DC part (3) comprises three-phase bridge arms, each phase of bridge arm comprises a plurality of groups of diode circuits and nodes, a first diode circuit is connected between each phase of positive output end and the first node (3-3), and the conduction direction of the first diode circuit is from the first node (3-3) to the positive output end; a second diode circuit is connected between the first node (3-3) and the second node (3-4), and the conduction direction of the second diode circuit is from the second node (3-4) to the first node (3-3); a third diode circuit is connected between the second node (3-4) and the input end, and the conduction direction of the third diode circuit is from the input end to the second node (3-4); a fourth diode circuit is connected between the input end and the third node (3-5), and the conduction direction of the fourth diode circuit is from the third node (3-5) to the input end; a fifth diode circuit is connected between the third node (3-5) and the fourth node (3-6), and the conduction direction of the fifth diode circuit is from the fourth node (3-6) to the third node (3-5); a sixth diode circuit is connected between the fourth node (3-6) and the negative output end, and the conduction direction of the sixth diode circuit is from the negative output end to the fourth node (3-6); an upper flying capacitor (3-1) and a lower flying capacitor (3-8) which are connected in series are connected in parallel between the first node (3-3) and the fourth node (3-6), an upper switch (3-2) and a lower switch (3-7) which are connected in series are connected in parallel between the second node (3-4) and the third node (3-5), and the connecting point of the upper switch and the lower switch is connected with the connecting point of the upper flying capacitor and the lower flying capacitor;

the second node (3-4) is connected with the midpoint of the output end through a seventh diode circuit, and the conduction direction of the seventh diode circuit is from the midpoint of the output end to the second node (3-4); the third node (3-5) is connected with the midpoint of the output end through an eighth diode circuit, and the conduction direction of the eighth diode circuit is from the third node (3-5) to the midpoint of the output end;

the positive output end of each phase of bridge arm is connected with the positive output end of the AC-DC part (3), and the negative output end of each phase of bridge arm is connected with the negative output end of the AC-DC part (3);

the DC-DC part (4) comprises two parallel circuit branches, each parallel circuit branch comprises 1 IGBT, 1 inductor and 1 diode, the IGBT is connected with the inductor in series, a connecting point of the IGBT and the inductor is connected with a diode through a grounding terminal, and the conduction direction of the diode is from the grounding terminal to the connecting point of the IGBT and the inductor.

2. The six-switch five-level rectifier of claim 1, wherein: the circuit also comprises a sampling circuit (5), a signal conditioning circuit (6), a protection circuit (7), a control circuit (8) and a main circuit driving circuit (9); the sampling circuit (5), the signal conditioning circuit (6), the control circuit (8) and the main circuit driving circuit (9) are sequentially and unidirectionally electrically connected, and the protection circuit (7) is electrically connected with the control circuit (8);

the sampling circuit (5) samples direct current side capacitor voltage, three-phase input current and three-phase power grid voltage, the signal conditioning circuit (6) converts the sampled voltage and current into voltage which can be controlled by the control circuit (8), and the control circuit (8) sends control signals to the main circuit driving circuit (9), so that the on-off of each switch is completed.

3. The six-switch five-level rectifier of claim 2, wherein: the control circuit (8) is a DSP control circuit.

4. The six-switch five-level rectifier of claim 1, wherein: the filter (2) is a reactor.

5. A control method for controlling a six-switch five-level rectifier according to any of claims 1 to 4, characterized in that: converting voltage and current components of an abc axis into alpha and beta axis voltage and current, calculating through a model to obtain a voltage vector, and sending the voltage vector and the direct-current side capacitor voltage into a cost function to obtain an optimal vector; and controlling the redundant states of 2 switches of each phase of bridge arm according to the optimal vector.

6. The control method according to claim 5, characterized in that: the cost function is g₂(k)＝|u^* _αj(k+1)-u_αj(k+1)|+|u^* _βj(k+1)-u_βj(k+1)|+λ|V_P-V_NL, where u^* _αj(k +1) and u^* _βj(k +1) is a reference voltage vector, u_αj(k +1) and u_βj(k +1) is a component of the voltage vector in αβ coordinate system, VP and VN are dc side capacitor voltages, and λ is a weight coefficient.

7. The control method according to claim 5, characterized in that: sending the voltage vector and the direct-current side capacitor voltage into a cost function, respectively substituting 125 reference voltage vectors of the five-level rectifier into the cost function, and selecting the reference voltage vector with the minimum cost function value as an optimal vector;

the selection of the redundant state of 2 switches of each phase of bridge arm is determined according to the network side input current, the single-phase output voltage, the upper flying capacitor voltage, the lower flying capacitor voltage and the optimal vector, if the requirement of the output voltage of one phase in the optimal vector is V/4, then: when the phase input current on the network side is positive, if the voltage of the upper flying capacitor (3-1) is greater than the voltage of the lower flying capacitor (3-8), the upper switch (3-2) is selected to be in a turn-off state of the lower switch (3-7) to enable the upper flying capacitor (3-1) to discharge, and if the voltage of the upper flying capacitor (3-1) is less than the voltage of the lower flying capacitor (3-8), the upper switch (3-2) is selected to be in a turn-off state of the lower switch (3-7) to enable the upper flying capacitor (3-1) to charge; when the input current of the phase on the network side is negative, if the voltage of the upper flying capacitor (3-1) is greater than that of the lower flying capacitor (3-8), the upper switch (3-2) is selected to turn off the lower switch (3-7) to charge the lower flying capacitor, and if the voltage of the upper flying capacitor (3-1) is less than that of the lower flying capacitor (3-8), the upper switch (3-2) is selected to turn on the lower switch (3-7) to discharge the lower flying capacitor.

Technical Field

The invention relates to the technical field of rectifiers, in particular to a six-switch five-level rectifier and a control method thereof.

Background

In recent years, five-level rectifiers have been widely used in wind power generation, electric vehicle charging, and other applications. Compared with a traditional two-level rectifier, the five-level rectifier has obvious advantages in low voltage stress, low harmonic waves and low EMI. At present, five-level rectifier topologies comprise ANPC, NPC and the like, but the topologies have the problems of more switching devices, so that the topologies have lower reliability and higher cost. Therefore, a topological circuit with low switching devices becomes an ideal scheme for a multi-level rectifier.

Furthermore, multilevel, despite the advantages described above, the switching state also increases exponentially. And the multi-level rectifier has the problem of voltage balance between the flying capacitor and the DC side capacitor. If the control is not good, the current on the multi-level network side is distorted, and the output voltage on the direct current side is unstable.

Therefore, aiming at a five-level rectifier system, the invention of the topology with the advantages of less switches, small grid-connected current distortion at the grid side, flexible and controllable output voltage and the corresponding control method has important significance.

Disclosure of Invention

The invention provides a six-switch five-level rectifier and a control method thereof, aiming at: the number of switches is reduced, grid-connected current distortion on the grid side is small, output voltage is flexible and controllable, and stable control of direct-current side capacitor voltage and suppression of flying capacitor voltage fluctuation are achieved.

The technical scheme of the invention is as follows:

a six-switch five-level rectifier comprises a front-end AC-DC part and a rear-end DC-DC part, wherein the network side of the AC-DC part is connected into a power grid through a filter, the positive output end and the negative output end of the AC-DC part are respectively connected with the positive input end and the negative input end of the DC-DC part, 2 direct-current side capacitors connected in series are connected between the positive output end and the negative output end of the AC-DC part, and the output end of the DC-DC part is connected with a load.

The AC-DC part comprises three-phase bridge arms, each phase of bridge arm comprises a plurality of groups of diode circuits and nodes, a first diode circuit is connected between each phase of positive output end and a first node, and the conduction direction of the first diode circuit is from the first node to the positive output end; a second diode circuit is connected between the first node and the second node, and the conduction direction of the second diode circuit is from the second node to the first node; a third diode circuit is connected between the second node and the input end, and the conduction direction of the third diode circuit is from the input end to the second node; a fourth diode circuit is connected between the input end and the third node, and the conduction direction of the fourth diode circuit is from the third node to the input end; a fifth diode circuit is connected between the third node and the fourth node, and the conduction direction of the fifth diode circuit is from the fourth node to the third node; and a sixth diode circuit is connected between the fourth node and the negative output end, and the conduction direction of the sixth diode circuit is from the negative output end to the fourth node.

The upper flying capacitor and the lower flying capacitor which are connected in series are connected in parallel between the first node and the fourth node, the upper switch and the lower switch which are connected in series are connected in parallel between the second node and the third node, and the connecting point of the upper switch and the lower switch is connected with the connecting point of the upper flying capacitor and the lower flying capacitor.

The second node is connected with the midpoint of the output end through a seventh diode circuit, and the conduction direction of the seventh diode circuit is from the midpoint of the output end to the second node; the third node is connected with the midpoint of the output end through an eighth diode circuit, and the conduction direction of the eighth diode circuit is from the third node to the midpoint of the output end.

The positive output end of each phase of bridge arm is connected with the positive output end of the AC-DC part, and the negative output end of each phase of bridge arm is connected with the negative output end of the AC-DC part.

The DC-DC part comprises two parallel circuit branches, each parallel circuit branch comprises 1 IGBT, 1 inductor and 1 diode, the IGBTs are connected with the inductors in series, the connecting point of the IGBTs and the inductors and a grounding end are connected with the diodes, and the conduction direction of the diodes is from the grounding end to the connecting point of the IGBTs and the inductors.

Furthermore, the six-switch five-level rectifier further comprises a sampling circuit, a signal conditioning circuit, a protection circuit, a control circuit and a main circuit driving circuit, wherein the sampling circuit, the signal conditioning circuit, the control circuit and the main circuit driving circuit are sequentially and unidirectionally electrically connected, and the protection circuit is electrically connected with the control circuit.

The sampling circuit samples the direct current side capacitor voltage, the three-phase input current and the three-phase power grid voltage, the signal conditioning circuit converts the sampled voltage and current into voltage which can be controlled by the control circuit, and the control circuit sends a control signal to the main circuit driving circuit, so that the on and off of each switch are completed.

Preferably, the control circuit is a DSP control circuit.

Preferably, the filter is a reactor.

The invention also provides a control method for the six-switch five-level rectifier, which comprises the following steps: converting voltage and current components of an abc axis into alpha and beta axis voltages and currents, obtaining a voltage vector through model calculation, sending the voltage vector and the direct-current side capacitor voltage into a cost function to obtain an optimal vector, and controlling the redundant states of 2 switches of each phase of bridge arm according to the optimal vector.

Further, the cost function is g₂(k)＝|u^* _αj(k+1)-u_αj(k+1)|+|u^* _βj(k+1)-u_βj(k+1)|+λ|V_P-V_NL, where u^* _αj(k +1) and u^* _βj(k +1) is a reference voltage vector, u_αj(k +1) and u_βj(k +1) is a component of the voltage vector in αβ coordinate system, VP and VN are dc side capacitor voltages, and λ is a weight coefficient.

Further, the control method sends the voltage vector and the direct-current side capacitor voltage into the cost function, then the 125 reference voltage vectors of the five-level rectifier are respectively substituted into the cost function, and the reference voltage vector with the minimum cost function value is selected as the optimal vector.

The selection of the redundant state of 2 switches of each phase of bridge arm is determined according to the network side input current, the single-phase output voltage, the upper flying capacitor voltage, the lower flying capacitor voltage and the optimal vector, if the requirement of the output voltage of one phase in the optimal vector is V/4, then: when the phase input current on the network side is positive, if the voltage of the upper flying capacitor is greater than the voltage of the lower flying capacitor, the upper switch is selected to be switched on, and the lower switch is switched off, so that the upper flying capacitor is discharged, and if the voltage of the upper flying capacitor is less than the voltage of the lower flying capacitor, the upper switch is selected to be switched off, and the lower switch is switched on, so that the upper flying capacitor is charged; when the input current of the phase on the network side is negative, if the voltage of the upper flying capacitor is greater than that of the lower flying capacitor, the upper switch is selected to turn off the lower switch, so that the lower flying capacitor is charged, and if the voltage of the upper flying capacitor is less than that of the lower flying capacitor, the upper switch is selected to turn on the lower switch, so that the lower flying capacitor is discharged.

Compared with the prior art, the invention has the following beneficial effects:

(1) the six-switch five-level rectifier provided by the invention has the advantages that the grid-connected current waveform THD at the topological input side of the rectifier is smaller, the number of switches is small, the output voltage of the rectifier is changed by selecting different voltage vectors, the voltage at the output side of the rectifier is flexible and controllable, and therefore, the six-switch five-level rectifier can be connected with electric automobiles with different voltage grades and various loads;

(2) the rectifier control method provided by the invention realizes current tracking and suppression of capacitor voltage offset at the direct current side by introducing a value function of capacitor voltage at the direct current side; and further, by selecting the redundant state of each phase of switch, the suppression of flying capacitor voltage fluctuation is realized.

Drawings

FIG. 1 is a block diagram of the system of the present invention;

FIG. 2 is a block diagram of a control architecture of the present invention;

FIG. 3 is a control circuit diagram of the present invention.

Detailed Description

The technical scheme of the invention is explained in detail in the following with the accompanying drawings:

as shown in fig. 1, a six-switch five-level rectifier includes a front-end AC-DC part 3 and a rear-end DC-DC part 4, the network side of the AC-DC part 3 is connected to a power grid 1 through a filter 2, the positive and negative output ends of the AC-DC part 3 are respectively connected to the positive and negative input ends of the DC-DC part 4, 2 series-connected DC-side capacitors are connected between the positive and negative output ends of the AC-DC part 3, and the output end of the DC-DC part 4 is connected to a load. The filter 2 in this embodiment is a reactor.

The AC-DC part 3 comprises three-phase bridge arms, each phase of bridge arm comprises a plurality of groups of diode circuits, a first diode circuit is connected between each phase of positive output end and a first node 3-3, and the conduction direction of the first diode circuit is from the first node 3-3 to the positive output end; a second diode circuit is connected between the first node 3-3 and the second node 3-4, and the conduction direction of the second diode circuit is from the second node 3-4 to the first node 3-3; a third diode circuit is connected between the second node 3-4 and the input end, and the conduction direction of the third diode circuit is from the input end to the second node 3-4; a fourth diode circuit is connected between the input end and the third node 3-5, and the conduction direction of the fourth diode circuit is from the third node 3-5 to the input end; a fifth diode circuit is connected between the third node 3-5 and the fourth node 3-6, and the conduction direction of the fifth diode circuit is from the fourth node 3-6 to the third node 3-5; and a sixth diode circuit is connected between the fourth node 3-6 and the negative output end, and the conduction direction of the sixth diode circuit is from the negative output end to the fourth node 3-6.

An upper flying capacitor 3-1 and a lower flying capacitor 3-8 which are connected in series are connected in parallel between the first node 3-3 and the fourth node 3-6, an upper switch 3-2 and a lower switch 3-7 which are connected in series are connected in parallel between the second node 3-4 and the third node 3-5, and the connecting point of the upper switch and the lower switch is connected with the connecting point of the upper flying capacitor and the lower flying capacitor.

The second node 3-4 is connected with the midpoint of the output end through a seventh diode circuit, and the conduction direction of the seventh diode circuit is from the midpoint of the output end to the second node 3-4; the third node 3-5 is connected with the midpoint of the output end through an eighth diode circuit, and the conduction direction of the eighth diode circuit is from the third node 3-5 to the midpoint of the output end.

The DC-DC part 4 comprises two parallel circuit branches, each parallel circuit branch comprises 1 IGBT, 1 inductor and 1 diode, the IGBTs are connected with the inductors in series, the connecting point of the IGBTs and the inductors is connected with a diode through a grounding terminal, and the conducting direction of the diodes is from the grounding terminal to the connecting point of the IGBTs and the inductors.

As shown in fig. 3, the six-switch five-level rectifier of the present invention further includes a sampling circuit 5, a signal conditioning circuit 6, a protection circuit 7, a control circuit 8, and a main circuit driving circuit 9; the sampling circuit 5, the signal conditioning circuit 6, the control circuit 8 and the main circuit driving circuit 9 are sequentially and unidirectionally electrically connected, and the protection circuit 7 is electrically connected with the control circuit 8. The control circuit 8 in this embodiment is a DSP control circuit.

The sampling circuit 5 samples the direct-current side capacitor voltages VP and VN, the three-phase input currents ia, ib and ic and the three-phase power grid voltages ea, eb and ec, the signal conditioning circuit 6 converts the sampling voltage and current into voltages which can be controlled by the DSP control circuit, and the DSP control circuit sends control signals to the main circuit driving circuit 9, so that the on and off of the switches are completed.

As shown in fig. 2, the method for controlling a six-switch five-level rectifier according to the present invention converts voltage and current components of an abc axis into α β axis voltage and current, analyzes the cause of dc-side capacitor voltage imbalance, obtains a voltage vector from a reference current i × abc through model calculation according to the dc-side capacitor voltage imbalance and vector correlation, sends the voltage vector and the dc-side capacitor voltage to a cost function 2, has 125 space vectors in total for five levels, compares the voltage vector with the 125 reference voltage vectors according to the cost function 2, and selects a reference voltage vector having the smallest cost function value (i.e., closest to the voltage vector) as an optimal vector for realizing current tracking and capacitor voltage offset suppression.

Cost function 1: g₁(k)＝|u(k+1)-u^*|

Cost function 2: g₂(k)＝|u^* _αj(k+1)-u_αj(k+1)|+|u^* _βj(k+1)-u_βj(k+1)|+λ|V_P-V_N|

Wherein u is^* _αj(k +1) and u^* _βj(k +1) is a reference voltage vector, u_αj(k +1) and u_βj(k +1) is a component of the voltage vector in the αβ coordinate system, VP and VN are upper and lower dc-side capacitor voltages, and λ is a weight coefficient.

If the requirement of the output voltage of a certain phase in the optimal vector is V/4, two switching signals (10) and (01) can realize the output of V/4, namely, a redundant state can be selected, and at the moment, the flying capacitor voltage fluctuation suppression is realized by specifically selecting (01) or (10) according to the direction of the network side input current ix and the upper and lower flying capacitor voltages Vcx1 and Vcx2 in the table 1.

TABLE 1 AC-DC FIVE-LEVEL RECTIFIER SYSTEM VECTOR STATE TABLE

Table 1 shows the relationship between the single-phase output voltage Vxo and the flying capacitor voltage of the AC-DC five-level rectifier under different grid-side input current ix and different switching states of each phase, where Vxo is the voltage between the x phase and the o point, Cx1 and Cx2 are three-phase flying capacitors, and x is a, b, and c.

When ix current is positive (flowing from the power grid 1 to the rectifier), the state (10) can discharge the upper flying capacitor 3-1, and the state (01) is charged, at the moment, if the voltage of the upper flying capacitor 3-1 is larger than the voltage of the lower flying capacitor 3-8, the state (10) is selected, otherwise, the state (01) is selected; when ix current is negative (flowing from the rectifier to the grid 1), the lower flying capacitor 3-8 is discharged by the state (10), the state (01) is charged, at this time, if the voltage of the upper flying capacitor 3-1 is larger than that of the lower flying capacitor 3-8, the state (01) is selected, and otherwise, the state (10) is selected.

The DC-DC part 4 firstly models the DC-DC topology, then provides a model predictive controller, and realizes DC-DC voltage reduction output through a cost function 1.

According to the six-switch five-level rectifier and the prediction control method thereof, the new topological switching value is small, the output voltage of the rectifier is changed by selecting different voltage vectors, and meanwhile, the input side voltage is unchanged, and the current is changed along with the input side voltage, so that the output side voltage is flexible and controllable, and the THD of the input side grid-connected current is reduced; the grid-connected current tracking and the direct-current side capacitor voltage balance control are realized by introducing a value function of direct-current side capacitor voltage, compared with the traditional PI, a voltage vector required at the next moment can be directly obtained through model prediction calculation, the pre-error comparison is realized, and the control speed and the accuracy are improved; flying capacitor voltage ripple suppression is achieved by selecting either state (10) or (01).

The novel topology and the control method thereof have strong practicability, can be widely applied to an electric automobile charging system, and have important significance for the rapid development of electric automobiles.