Three-phase four-bridge-arm multi-direct-current-source multi-level inverter and control method thereof

Abstract

The invention discloses a three-phase four-bridge-arm multi-direct-current-source multi-level inverter and a control method thereof.The inverter comprises a multi-source unit comprising a plurality of direct-current sources, the direct-current sources are batteries with different powers, the batteries with the different powers can provide direct-current link voltages with different voltage values for a back-stage inverter circuit, and the direct-current link voltages with the different voltage values can be supplied to the back-stage inverter circuit. Therefore, the AC voltage output by the inverter has different level combinations to adapt to the working characteristics of different loads, and the working efficiency is improved. According to the topological structure of the multi-source three-phase four-bridge-arm inverter, generated voltage values of different direct-current links are combined with changing voltage levels so as to work under different loads, and therefore the purpose of reducing switching loss is achieved. In addition, by connecting the fourth bridge arm to the load neutral point, effective control under the unbalanced load condition is achieved, and stable operation of the system is kept.

Description

Technical Field

[0001] The present invention belongs to the technical field of power electronics, and in particular relates to a three-phase four-bridge-arm multi-DC source multi-level inverter and a control method thereof. Background Art

[0002] In the development of modern power electronics technology, inverters, as key equipment for power conversion, play a core role in many fields such as industrial automation, renewable energy generation, electric vehicles, etc. With the continuous improvement of power system requirements for power quality and the increasing complexity of application scenarios, inverters are facing many challenges.

[0003] In the industrial field, a large number of motor-driven equipment require stable, efficient inverters that can adapt to different working conditions. The ability to handle unbalanced loads has become one of the important indicators for measuring inverter performance, because in actual industrial production, unbalanced loads often occur, such as when the motor is starting, braking or running, the three-phase current or voltage is unbalanced due to mechanical structure, uneven load distribution and other reasons. If the inverter cannot effectively respond, it will cause motor torque pulsation, increase heat generation, reduce the service life of the equipment, and even affect the stability and efficiency of the entire production process; in terms of renewable energy generation, especially photovoltaic and wind power generation systems, their output voltage and power are intermittent and volatile. The inverter needs to convert these unstable DC power into AC power that meets the requirements of the power grid, and when the grid voltage fluctuates or fails, it can maintain normal operation to ensure the reliability and safety of the power system. This requires the inverter to have good control performance and the ability to adapt to complex power grid environments.

[0004] The rapid development of electric vehicles has also put forward higher requirements for inverter technology. The power system of electric vehicles requires an inverter to convert the DC power of the battery into AC power to drive the motor to achieve the driving of the vehicle. On the one hand, in order to improve the range of electric vehicles, the inverter needs to have high efficiency and reduce energy loss; on the other hand, under different working conditions such as vehicle acceleration and climbing, the motor load changes greatly, and the inverter must be able to respond quickly and provide stable power output, while also considering the battery life and charge and discharge management.

[0005] Traditional voltage source inverter (VSI): For example, the standard two-level VSI widely used in many industrial motor drive systems has a relatively simple power circuit structure consisting of six power switch devices. In electric vehicle applications, it directly connects the battery and the motor, uses pulse width modulation (PWM) technology to control the on and off of the power switch, and converts the DC voltage to AC voltage to drive the motor. However, it has obvious limitations. Due to the wide range of motor performance and high power requirements, VSI efficiency is low at low loads, and motor performance is limited by the fixed battery voltage. For example, when an electric vehicle is driving at low speed and light load, the motor requires less power, but the VSI still converts power at a fixed battery voltage, resulting in energy waste and reduced system efficiency.

[0006] DC-DC converter and VSI combination structure: Some electric vehicle manufacturers adopt this solution, adding a DC-DC converter between the battery and the VSI. The DC-DC converter can adjust the DC voltage input to the VSI according to the operating status of the motor, thereby improving the overall performance of the motor and inverter. For example, when the motor is running at high speed or overloaded, the DC-DC converter can increase the voltage to ensure that the motor obtains sufficient driving power; when the load is low, the voltage is reduced to reduce energy loss. However, this structure increases the cost and complexity of the system, and the power level of the DC-DC converter needs to match the battery pack. As the power of the battery pack increases, the difficulty of system design and cost control also increases accordingly.

[0007] Multilevel inverter: In high-voltage application scenarios, multilevel inverters such as the neutral point clamped (NPC) type and the flying capacitor type have received some attention. Taking the NPC multilevel inverter as an example, it can output multiple levels of AC voltage by using multiple capacitors to divide the voltage on the DC side or using different combinations of multiple power switching devices on the AC side. Compared with traditional two-level inverters, it has lower total harmonic distortion (THD) and power loss. It is used in some large industrial motor speed control systems and high-voltage DC transmission converter stations. However, multilevel inverters usually require a large number of power switching devices and complex control strategies, resulting in high system cost, large size, high control difficulty, and more stringent performance requirements for the controller.

[0008] Existing multi-source inverter (MSI) structure: Based on the dual DC source MSI structure, this type of MSI uses two DC sources on the input side. According to the load requirements, a high-voltage battery pack or a low-voltage battery pack can be selected to power the load. It is essentially a two-level natural diode clamped inverter or T-type inverter. It can produce three different working modes: V dc1 、V dc2 and V dc2 -V dc1However, the disadvantage is that the two DC sources cannot be connected in series to obtain a higher DC link voltage. In electric vehicle applications, this will result in a higher capacity battery pack, increasing cost, volume and weight. At the same time, under high current load, the current extracted from the battery pack does not change compared to when the battery and load are directly connected through a rigid DC link inverter, which cannot effectively reduce the burden on the battery.

[0009] Traditional voltage source inverters have low efficiency under low load conditions, which affects motor performance; inverters that combine DC-DC and VSI have high costs, complex systems, and difficult designs; existing multi-source inverters cannot generate a DC link voltage that is the sum of two DC sources, and cannot meet high load working conditions. Summary of the invention

[0010] In order to optimize the configuration of power switch tubes and the combination of DC sources so that the inverter can adapt to the working efficiency under different load power conditions, reduce the waste of electric energy and increase the voltage of the DC link, so that the inverter can adapt to the working conditions of unbalanced loads, a new three-phase four-bridge arm multi-DC source inverter topology is invented.

[0011] The present invention provides a three-phase four-bridge-arm multi-DC source multi-level inverter, comprising:

[0012] A first power switch tube, a second power switch tube, a third power switch tube, a fourth power switch tube, a fifth power switch tube, a first DC power supply, and a second DC power supply; wherein,

[0013] The emitter of the first power switch tube is connected to the collector of the second power switch tube, and the collector of the first power switch tube is connected to the positive electrode of the first DC power supply;

[0014] The emitter of the second power switch tube is connected to the negative electrode of the first DC power supply;

[0015] The emitter of the third power switch tube is connected to the positive electrode of the first DC power supply, and the collector of the third power switch tube is connected to the positive electrode of the second DC power supply;

[0016] The emitter of the fourth power switch tube is connected to the negative electrode of the second DC power supply, and the collector of the fourth power switch tube is connected to the positive electrode of the first DC power supply;

[0017] The fifth power switch tube is composed of two switch tubes whose emitters are connected to each other, and whose two collectors are respectively connected to the negative electrode of the first DC power supply and the negative electrode of the second DC power supply.

[0018] Furthermore, the first power switch tube, the second power switch tube, the third power switch tube, and the fourth power switch tube are all unidirectional insulated gate bipolar transistors with parallel diodes.

[0019] Furthermore, the fifth power switch tube is a bidirectional power switch composed of two IGBTs and two parallel diodes.

[0020] Furthermore, the positive lead wire of the second DC power supply is used as point o, and the lead wire of the collector of the second power switch tube is used as point n to form a DC link voltage of the inverter.

[0021] Furthermore, after the point o and the point n, a four-bridge arm circuit is further connected.

[0022] Furthermore, the four-bridge arm circuit includes an A-phase bridge arm, a B-phase bridge arm, a C-phase bridge arm and a D-phase bridge arm connected in parallel, and each bridge arm includes two switching tubes connected in series.

[0023] Furthermore, at least one DC power supply unit is further connected in series between the positive and negative electrodes of the first DC power supply, and each DC power supply unit includes four power switch tubes and a DC power supply.

[0024] According to another aspect of the present invention, there is also provided a control method according to the three-phase four-bridge-arm multi-DC source multi-level inverter, comprising:

[0025] Establishing a continuous model of the inverter and obtaining a prediction model through discretization processing;

[0026] Measure the grid voltage, inverter output current, and DC side voltage at time k in real time, and transmit the measurement results to the prediction model;

[0027] Predicting the predicted current at time k+1 by using the prediction model, and constructing a cost function based on the predicted current and the reference current;

[0028] By selecting the minimum cost function, it is used as the trigger signal of the inverter.

[0029] Furthermore, the prediction model is:

[0030]

[0031] Among them, R f is the filter resistor, L f is the filter inductor, T s is the sampling period, x=α,β,γ,i x and V x are the components of current and voltage in the αβγ coordinate system respectively.

[0032] Furthermore, the cost function is:

[0033]

[0034] in, It is the reference component in the αβγ coordinate system at time k+1.

[0035] The advantages of the present invention are: the topology of the multi-source three-phase four-bridge-arm inverter generates different DC link voltage values ​​and variable voltage levels to work under different loads, thereby achieving the purpose of reducing switching losses. In addition, by connecting the fourth bridge arm to the load neutral point, effective control is achieved under unbalanced load conditions to maintain stable system operation. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Various other advantages and benefits will become apparent to those of ordinary skill in the art by reading the detailed description of the preferred embodiments below. The accompanying drawings are only for the purpose of illustrating the preferred embodiments and are not to be considered as limiting the present invention. Moreover, the same reference symbols are used throughout the accompanying drawings to represent the same components. In the accompanying drawings:

[0037] Figure 1 A diagram showing a multi-source unit topology with n power supplies according to an embodiment of the present invention is shown.

[0038] Figure 2 A topological structure diagram of a dual-power traction three-phase four-bridge-leg inverter according to an embodiment of the present invention is shown.

[0039] Figure 3 A block diagram of a predictive current control system according to an embodiment of the present invention is shown.

[0040] Figure 4 shows the V of the operating mode according to the embodiment of the present invention. on and V abc Schematic diagram.

[0041] Figure 5 shows V in operation mode 2 according to an embodiment of the present invention. on and V abc Schematic diagram.

[0042] Figure 6 A schematic diagram of the current id on the fourth bridge arm according to an embodiment of the present invention is shown. DETAILED DESCRIPTION

[0043] The exemplary embodiments of the present disclosure will be described in more detail below with reference to the accompanying drawings. Although the exemplary embodiments of the present disclosure are shown in the accompanying drawings, it should be understood that the present disclosure can be implemented in various forms and should not be limited by the embodiments described herein. On the contrary, these embodiments are provided in order to enable a more thorough understanding of the present disclosure and to fully convey the scope of the present disclosure to those skilled in the art.

[0044] Terminology explanation:

[0045] Three-phase four-bridge-arm energy storage converter: a power electronic converter with four bridge arms that outputs three-phase voltage.

[0046] Multisource Inverter (MSI): An inverter that contains multiple DC power sources on the input side. It can combine these DC sources of different voltage levels to adapt to different load requirements, reduce battery capacity requirements in applications such as electric vehicles, and provide multiple operating modes, such as series, parallel or combined power supply of different voltage sources, which has higher flexibility and efficiency potential than traditional inverters.

[0047] Voltage Source Inverter (VSI): A type of power electronic converter, whose DC side is usually powered by a voltage source (such as a battery), and converts DC voltage into AC voltage output by controlling the on and off of power switching devices. It is more common in applications such as electric vehicle traction systems, and has advantages such as low cost, high power density and simple control, but has disadvantages such as limited performance at low loads and motor performance being constrained by fixed battery voltage.

[0048] The present invention designs a circuit topology structure of a multi-source inverter, and the voltage of the DC link has four different working modes, namely: V dc1 、V dc2 -V dc1 、V dc2 and V dc1 +V dc2 It can be combined with a variable voltage level to work under different loads, thereby achieving the purpose of reducing switching losses. At the same time, the fourth bridge arm of the energy storage converter is connected to the load neutral point to effectively control the operation of the energy storage converter under unbalanced loads.

[0049] The multi-source unit topology structure with n power supplies proposed by the present invention is as follows: Figure 1 As shown, it consists of a DC source V dc The DC source is composed of multiple source units. Each DC source here is a battery with different power. These batteries with different power can provide DC link voltages with different voltage values ​​for the subsequent inverter circuit, so that the AC voltage output by the inverter has different level combinations, adapting to the working characteristics of different loads and improving working efficiency. In order to obtain the maximum working mode, the voltage value of the DC source is selected according to the following formula:

[0050] V dc，1 =V dc

[0051] V dc，2 =3V dc

[0052] V dc·3 =9V dc

[0053] V dc，j =3 j-1 V dc , where j = 1, 2....n.

[0054] The topological structure of a multi-source unit with n DC sources is given above. Next, in order to simplify the analysis and simulation, the present invention adopts n=2, and the multi-source unit has two DC sources as the DC input of the three-phase four-leg inverter. The specific topological structure is as follows: Figure 2 Given.

[0055] exist Figure 2 In the figure, except for switch S5 which is a bidirectional power switch composed of two IGBTs and two parallel diodes, all other power switch tubes are unidirectional insulated gate bipolar transistors with parallel diodes. The topological structure of the multi-source unit is analyzed in detail below: the emitter of switch S1 is connected to the collector of S2, and the collector is connected to the positive electrode of the DC power supply Vdc1; the emitter of switch S2 is connected to the negative electrode of the power supply Vdc1; the emitter of switch S3 is connected to the positive electrode of the power supply Vdc1, and the collector is connected to the positive electrode of the power supply Vdc2; the emitter of switch S4 is connected to the negative electrode of the power supply Vdc2, and the collector is connected to the positive electrode of the power supply Vdc1; switch S5 is composed of two switch tubes with their emitters connected to each other, and the other two collectors are connected to the negative electrode of the power supply Vdc1 and the negative electrode of the power supply Vdc2 respectively. The positive lead wire of the power supply Vdc2 is taken as point o, and the lead wire of the collector of switch tube S2 is taken as point n. Von is the output voltage value of the multi-source unit, forming the DC link voltage of the inverter.

[0056] After the point o and the point n, a four-bridge arm circuit is further connected.

[0057] The four-bridge arm circuit includes an A-phase bridge arm, a B-phase bridge arm, a C-phase bridge arm and a D-phase bridge arm connected in parallel, and each bridge arm includes two switching tubes connected in series.

[0058] In order to suppress excessive fluctuations in the circuit, the inverter also includes a filter circuit, and the A-phase bridge arm, the B-phase bridge arm, the C-phase bridge arm and the D-phase bridge arm are all connected to the three-phase loads R1, R2, and R3 through the filter circuit.

[0059] The filter circuit includes filter inductors L1, L2, L3 and filter capacitors C1, C2, C3.

[0060] Since the maximum voltage combination that can be generated by the two DC sources is 4 combinations, four different DC link voltages can be provided for the three-phase four-bridge-leg inverter according to the different conduction and shutdown of the five switch tubes. Based on the possible switching states of the switch tubes, all operation modes are given in Table 1:

[0061] Operation Mode S1 S2 S3 S4 S5 <![CDATA[V on ]]> one OFF ON ON OFF OFF <![CDATA[V dc1 ]]> two ON OFF OFF OFF ON <![CDATA[V dc2 -V dc1 ]]> three OFF ON OFF OFF ON <![CDATA[V dc2 ]]> three ON OFF OFF ON OFF <![CDATA[V dc2 ]]> Four OFF ON OFF ON OFF <![CDATA[V dc1 +V dc2 ]]>

[0062] Table 1: Four different operating modes based on DC voltage source

[0063] Since both different switching modes can generate the same DC link voltage, they are both classified as mode three.

[0064] The control algorithm used in this experiment is based on the model predictive control of the three-phase four-leg inverter, which is mainly achieved through three steps:

[0065] (1) Measurement module: It measures the grid voltage, inverter output current, and DC side voltage in real time and transmits the information to the system's prediction model.

[0066] (2) Prediction model: The control system design is based on the optimal algorithm and can be easily implemented in hardware circuits. Therefore, after establishing the continuous model of the system, it needs to be discretized to obtain:

[0067]

[0068] Among them, R f is the filter resistor, L f is the filter inductor, T s is the sampling period, x=α,β,γ,i x and V x are the components of current and voltage in the αβγ coordinate system respectively.

[0069] (3) Cost function: The cost function is constructed using the predicted current and reference current at time k+1:

[0070]

[0071] in, It is the reference component in the αβγ coordinate system at time k+1.

[0072] The minimum cost function is selected as the trigger signal of the inverter.

[0073] The system block diagram of the present invention is as follows Figure 3 shown.

[0074] Next, the three-phase four-bridge-arm inverter model of the present invention is built in MATLAB to test the DC link voltage V under different working modes.on and the inverter output voltage V abc First, verify the feasibility of the simulation, set the load to an asymmetric three-phase load, and the parameters in the circuit are: V dc =30V, L=6mH, C=30μF, Ra=10Ω, Rb=50Ω, Rc=100Ω.

[0075] Depend on Figure 4 It can be seen that in the case of operation mode 1, V on =30V, when the inverter output is unbalanced in load, due to the existence of the fourth bridge arm in the circuit, it can guide the zero-sequence component in the unbalanced three-phase load current to the neutral line, i.e. the fourth bridge arm, so that the three-phase output voltage can still maintain a relative balance under the condition of unbalanced load, ensuring the normal power supply of the system and the stable operation of the load.

[0076] Next, operation mode 2 is simulated and verified, and the parameters of the components in the circuit remain unchanged.

[0077] Depend on Figure 5 It can be seen that in operation mode 2, the voltage V on The output voltage of the inverter has also changed. Although the load is an unbalanced load, the inverter can quickly adapt to the working conditions of the unbalanced load due to the existence of the fourth bridge arm.

[0078] Depend on Figure 6 It can be seen from the current of the fourth bridge arm on the inverter that, in the case of unbalanced load, the zero-sequence current mainly flows back through the fourth bridge arm, so that the inverter can work normally.

[0079] The third and fourth operation modes produce the same effects as the above two operation modes, except that different switch combinations are used to make the voltage value of the DC link different, which will not be described in detail here.

[0080] The above is only a preferred specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by a person skilled in the art within the technical scope disclosed by the present invention should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention shall be based on the protection scope of the claims.

Claims

1. A three-phase four-bridge-arm multi-DC source multi-level inverter, characterized in that: include: A first power switch tube, a second power switch tube, a third power switch tube, a fourth power switch tube, a fifth power switch tube, a first DC power supply, and a second DC power supply; wherein, The emitter of the first power switch tube is connected to the collector of the second power switch tube, and the collector of the first power switch tube is connected to the positive electrode of the first DC power supply; The emitter of the second power switch tube is connected to the negative electrode of the first DC power supply; The emitter of the third power switch tube is connected to the positive electrode of the first DC power supply, and the collector of the third power switch tube is connected to the positive electrode of the second DC power supply; The emitter of the fourth power switch tube is connected to the negative electrode of the second DC power supply, and the collector of the fourth power switch tube is connected to the positive electrode of the first DC power supply; The fifth power switch tube is composed of two switch tubes whose emitters are connected to each other, and whose two collectors are respectively connected to the negative electrode of the first DC power supply and the negative electrode of the second DC power supply.

2. The three-phase four-bridge-arm multi-DC source multi-level inverter according to claim 1, characterized in that: The first power switch tube, the second power switch tube, the third power switch tube, and the fourth power switch tube are all unidirectional insulated gate bipolar transistors with parallel diodes.

3. The three-phase four-bridge-arm multi-DC source multi-level inverter according to claim 1, characterized in that: The fifth power switch tube is a bidirectional power switch composed of two IGBTs and two parallel diodes.

4. The three-phase four-bridge-arm multi-DC source multi-level inverter according to claim 1, characterized in that: The positive lead wire of the second DC power supply is used as point o, and the lead wire of the collector of the second power switch tube is used as point n to form a DC link voltage of the inverter.

5. The three-phase four-bridge-arm multi-DC source multi-level inverter according to claim 4, characterized in that: After the point o and the point n, a four-bridge arm circuit is further connected.

6. The three-phase four-bridge-arm multi-DC source multi-level inverter according to claim 5, characterized in that: The four-bridge arm circuit includes an A-phase bridge arm, a B-phase bridge arm, a C-phase bridge arm and a D-phase bridge arm connected in parallel, and each bridge arm includes two switching tubes connected in series.

7. The three-phase four-bridge-arm multi-DC source multi-level inverter according to any one of claims 1 to 6, characterized in that: At least one DC power supply unit is further connected in series between the positive and negative electrodes of the first DC power supply, and each DC power supply unit includes four power switch tubes and a DC power supply.

8. A control method for a three-phase four-bridge-arm multi-DC source multi-level inverter according to any one of claims 1 to 7, characterized in that: include: Establishing a continuous model of the inverter and obtaining a prediction model through discretization processing; Measure the grid voltage, inverter output current, and DC side voltage at time k in real time, and transmit the measurement results to the prediction model; Predicting the predicted current at time k+1 by using the prediction model, and constructing a cost function based on the predicted current and the reference current; The minimum cost function is selected as the trigger signal of the inverter.

9. The control method according to claim 8, characterized in that: The prediction model is: Among them, R f is the filter resistor, L f is the filter inductor, T s is the sampling period, x=α,β,γ,i x and V x are the components of current and voltage in the αβγ coordinate system respectively.

10. The control method according to claim 9, characterized in that: The cost function is: in, It is the reference component in the αβγ coordinate system at time k+1.

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