A variable level converter and control method thereof

Abstract

The present application proposes a variable level converter and a control method thereof. The variable level converter is composed of three DC bus capacitors and a three-phase variable level bridge arm. Each DC bus capacitor is connected in series in sequence. Each phase variable level bridge arm includes six switching devices and a bypass switch. When the total voltage of the DC bus is high, the bypass switch is disconnected, and the variable level converter works in a four-level mode. The carrier overlap PWM modulation strategy is used to control the voltage of the three DC bus capacitors, reduce the voltage of the second DC bus capacitor, and control the voltage of the first and third DC bus capacitors to maintain the rated value unchanged. When the voltage of the second DC bus capacitor is close to 0, the bypass switch is closed, and the variable level converter works in a three-level mode. A hybrid modulation strategy is used to control the high-voltage switch device at the fundamental frequency. The present application has the advantages of fewer switching devices and high system efficiency, and is suitable for photovoltaic, energy storage grid-connected inverters and other occasions.

Description

Technical Field

[0001] The present application relates to the field of power electronic multi-level converter topology, and in particular to a variable level converter and a control method thereof. Background Art

[0002] In the fields of photovoltaics and energy storage, in order to improve efficiency and reduce costs, the capacity of a single machine is gradually increasing, and the voltage level is getting higher and higher. At present, photovoltaic strings and battery clusters in the fields of photovoltaics and energy storage widely use 1500V DC voltage, and the corresponding grid-connected inverter usually adopts a three-level topology: including diode clamped three-level, active midpoint clamped three-level and other topological structures, generally using 1200V power devices. However, when the voltage level of photovoltaic strings or battery clusters is increased to 2000V, if the traditional three-level topology is still used, 1700V switching devices must be used. However, 1700V power devices have slow switching speed, large switching losses, and high costs, which will seriously affect system efficiency and cost. On the other hand, the DC output voltage of photovoltaic strings and battery clusters will vary over a large range with different lighting conditions, temperature, and battery SOC. The number of output levels of traditional multi-level converters and the voltage of each level remain unchanged, making it difficult to achieve efficiency optimization within the full voltage range. Therefore, it is necessary to optimize the topological structure of photovoltaic and energy storage grid-connected inverters with higher voltage levels. Summary of the invention

[0003] The present application aims to solve one of the technical problems in the related art at least to some extent.

[0004] To this end, the first objective of the present application is to provide a control method for a variable level converter.

[0005] A second objective of the present application is to provide a control device for a variable level converter.

[0006] To achieve the above-mentioned purpose, the first aspect of the present application provides a control method for a variable level converter, the method being applied to a variable level converter, the variable level converter comprising three DC bus capacitors and a three-phase variable level bridge arm, the DC bus capacitors being connected in series in sequence, each phase variable level bridge arm comprising six switch devices and a bypass switch, the method comprising:

[0007] Determining the state of the bypass switch according to a comparison result of the total DC bus voltage and the rated voltage of the DC bus capacitor;

[0008] In response to the bypass switch being disconnected, determining that the variable level converter operates in a four-level mode, using a carrier overlap PWM modulation strategy to control the voltages of the three DC bus capacitors, reducing only the voltage of the second DC bus capacitor when the total DC bus voltage decreases, and controlling the voltages of the first and third DC bus capacitors to maintain a rated value unchanged, and closing the bypass switch when the voltage of the second DC bus capacitor approaches 0;

[0009] In response to the bypass switch being closed, it is determined that the variable level converter operates in a three-level mode, and a hybrid modulation strategy is adopted to control the high voltage switch device at a fundamental frequency.

[0010] Optional, three DC bus capacitors C d1 , C d2 and C d3 Sequentially connected in series, each phase variable level bridge arm includes switch device S1, switch device S2, switch device S3, switch device S1', switch device S2', switch device S3', and a total of four DC input ports X1, X2, X3, X4 and one AC output port X5 are derived, wherein,

[0011] The emitter of the switching device S1 is connected to the collector of the switching device S1';

[0012] The emitter of the switch device S3 is connected to the collector of the switch device S3';

[0013] The collector of the switching device S2 is connected to the emitter of the switching device S1;

[0014] The emitter of the switch device S2' is connected to the collector of the switch device S3';

[0015] The emitter of the switch device S2 is connected to the collector of the switch device S2' and serves as the AC output terminal X5 of the variable level bridge arm;

[0016] The collector of the switch device S1 is connected to the DC input port X1;

[0017] The emitter of the switch device S1' is connected to the DC input port X2;

[0018] The collector of the switch device S3 is connected to the DC input port X3;

[0019] The emitter of the switch device S3' is connected to the DC input port X4;

[0020] The bypass switch K is connected in parallel between the DC input ports X2 and X3;

[0021] The DC input ports X1 of the variable level bridge arms of all phases are connected in parallel and connected to the first DC bus capacitor C d1 The positive electrode;

[0022] The DC input ports X2 of the variable level bridge arms of all phases are connected in parallel and connected to the first DC bus capacitor C d1 The negative electrode of the second DC bus capacitor C d2 The positive electrode;

[0023] The DC input ports X3 of the variable level bridge arms of all phases are connected in parallel and connected to the second DC bus capacitor C d2 The negative electrode of the third DC bus capacitor C d3 The positive electrode;

[0024] The DC input ports X4 of the variable level bridge arms of all phases are connected in parallel and connected to the third DC bus capacitor C d3 of the negative electrode.

[0025] Optionally, when the variable level converter operates in a four-level mode, it has four switch states capable of outputting four different levels, including:

[0026] When the switch devices S1, S2, and S3 are turned on and the other switch devices are turned off, the output voltage of the variable level converter is:

[0027] U o =U dc =U d1 +U d2 +U d3

[0028] Among them, U o It represents the output voltage of the variable level converter with the negative pole of the DC bus as reference ground potential, U dc Indicates the total DC bus voltage, U d1 Represents the first DC bus capacitor C d1 Voltage, U d2 Represents the second DC bus capacitor C d2 Voltage, U d3 Represents the third DC bus capacitor C d3 Voltage;

[0029] When the switch devices S1', S2, and S3 are turned on and the other switch devices are turned off, the output voltage of the variable level converter is:

[0030] U o =U d2 +U d3

[0031] When the switch devices S1', S2', and S3 are turned on and the other switch devices are turned off, the output voltage of the variable level converter is:

[0032] U o =U d3

[0033] When the switch devices S1', S2', and S3' are turned on and the other switch devices are turned off, the output voltage of the variable level converter is:

[0034] U o =0

[0035] Optionally, when the variable level converter operates in the three-level mode, it has four switching states capable of outputting three different levels, including:

[0036] When the switch devices S1, S2, and S3 are turned on and the other switch devices are turned off, the output voltage of the variable level converter is:

[0037] U o =U dc =U d1 +U d3

[0038] When the switch devices S1', S2, and S3' are turned on and the other switch devices are turned off, the output voltage of the variable level converter is:

[0039] U o =U d3

[0040] When the switch devices S1, S2', and S3 are turned on and the other switch devices are turned off, the output voltage of the variable level converter is:

[0041] U o =U d3

[0042] When the switch devices S1', S2', and S3' are turned on and the other switch devices are turned off, the output voltage of the variable level converter is:

[0043] U o =0

[0044] Optionally, determining the state of the bypass switch according to a comparison result of the DC bus total voltage and the rated voltage of the first DC bus capacitor includes:

[0045] If U dc >2E, the bypass switch K is disconnected, the variable level converter operates in the four-level mode, and E represents the first DC bus capacitor C d1 Or the third DC bus capacitor C d3Rated voltage;

[0046] If U dc ≤2E, the bypass switch K is closed, and the variable level converter operates in a three-level mode.

[0047] Optionally, also include:

[0048] According to the different voltage levels of the DC bus, select different types of switching devices with corresponding voltage levels;

[0049] Depending on the voltage and current levels, choose different types of bypass switches with corresponding voltage and current levels.

[0050] To achieve the above-mentioned purpose, the second aspect of the present application provides a control device for a variable level converter, the device is applied to a variable level converter, the variable level converter is composed of three DC bus capacitors and a three-phase variable level bridge arm, the DC bus capacitors are connected in series in sequence, each phase variable level bridge arm includes six switch devices and a bypass switch, the device includes:

[0051] A bypass switch state determination module, used to determine the state of the bypass switch according to a comparison result between the total DC bus voltage and the rated voltage of the first DC bus capacitor;

[0052] a four-level mode operation module, in response to the bypass switch being disconnected, for determining that the variable level converter operates in the four-level mode, adopting a carrier overlap PWM modulation strategy to control the voltages of the three DC bus capacitors, reducing only the voltage of the second DC bus capacitor when the total DC bus voltage decreases, and controlling the voltages of the first and third DC bus capacitors to maintain the rated values ​​unchanged, and closing the bypass switch when the voltage of the second DC bus capacitor approaches 0;

[0053] The three-level mode operation module is used to determine that the variable level converter operates in the three-level mode in response to the bypass switch being closed, and to control the high-voltage switch device at the fundamental frequency using a hybrid modulation strategy.

[0054] The technical solution provided by the embodiments of the present application brings at least the following beneficial effects:

[0055] A novel variable level converter is proposed. The variable level converter consists of three DC bus capacitors and a three-phase variable level bridge arm. The DC bus capacitors are connected in series in sequence. Each phase variable level bridge arm includes six switching devices and a bypass switch. On the one hand, by mixing and using switching devices of different voltage levels, the number of devices can be reduced and the cost can be reduced; on the other hand, the output level voltage can be changed as the DC bus voltage changes, and the switching loss can be reduced by reducing the working voltage of the high-voltage device. When the DC bus voltage drops to a certain level, it can directly switch from a four-level working mode to a three-level working mode. At this time, the high-voltage device directly works at the fundamental frequency, which greatly improves the system efficiency. The application has the advantages of fewer switching devices and high system efficiency, and is suitable for occasions such as photovoltaic and energy storage grid-connected inverters.

[0056] Additional aspects and advantages of the present application will be given in part in the description below, and in part will become apparent from the description below, or will be learned through the practice of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0057] The above and / or additional aspects and advantages of the present application will become apparent and easily understood from the following description of the embodiments in conjunction with the accompanying drawings, in which:

[0058] Figure 1 It is a structural diagram of a single-phase bridge arm of a diode-clamped three-level converter in the prior art;

[0059] Figure 2 It is a structural diagram of a single-phase bridge arm of an active midpoint clamped three-level converter in the prior art;

[0060] Figure 3 is a structural diagram of a variable level converter according to an embodiment of the present application;

[0061] Figure 4 is a flow chart of a control method of a variable level converter according to an embodiment of the present application;

[0062] Figure 5 is a schematic diagram of carrier overlap PWM used in a four-level mode according to an embodiment of the present application;

[0063] Figure 6 is a schematic diagram of hybrid modulation adopted in a three-level mode according to an embodiment of the present application;

[0064] Figure 7 It is a block diagram of a control device of a variable level converter according to an embodiment of the present application. DETAILED DESCRIPTION

[0065] The embodiments of the present application are described in detail below, and examples of the embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are intended to be used to explain the present application, and should not be construed as limiting the present application.

[0066] The variable level converter and its control method and control device according to the embodiments of the present application are described below with reference to the accompanying drawings.

[0067] In the embodiment of the present application, the variable level converter is composed of three DC bus capacitors and three-phase variable level bridge arms, the DC bus capacitors are connected in series in sequence, and each phase variable level bridge arm includes six switching devices and a bypass switch.

[0068] Specifically, if Figure 3 As shown, the three DC bus capacitors C d1 , C d2 and C d3 Connected in series in sequence, each phase variable level bridge arm includes switch device S1, switch device S2, switch device S3, switch device S1', switch device S2', switch device S3', and a total of four DC input ports X1, X2, X3, X4 and one AC output port X5 are derived, wherein:

[0069] The emitter of the switching device S1 is connected to the collector of the switching device S1';

[0070] The emitter of the switching device S3 is connected to the collector of the switching device S3';

[0071] The collector of the switching device S2 is connected to the emitter of the switching device S1;

[0072] The emitter of the switching device S2' is connected to the collector of the switching device S3';

[0073] The emitter of the switch device S2 is connected to the collector of the switch device S2' and serves as the AC output terminal X5 of the variable level bridge arm;

[0074] The collector of the switch device S1 is connected to the DC input port X1;

[0075] The emitter of the switch device S1' is connected to the DC input port X2;

[0076] The collector of the switch device S3 is connected to the DC input port X3;

[0077] The emitter of the switch device S3' is connected to the DC input port X4;

[0078] The bypass switch K is connected in parallel between the DC input ports X2 and X3;

[0079] The DC input ports X1 of the variable level bridge arms of all phases are connected in parallel and connected to the first DC bus capacitor C d1 The positive electrode;

[0080] The DC input ports X2 of the variable level bridge arms of all phases are connected in parallel and connected to the first DC bus capacitor C d1 The negative electrode of the second DC bus capacitor C d2 The positive electrode;

[0081] The DC input ports X3 of the variable level bridge arms of all phases are connected in parallel and connected to the second DC bus capacitor C d2 The negative electrode and the third DC bus capacitor C d3 The positive electrode;

[0082] The DC input ports X4 of the variable level bridge arms of all phases are connected in parallel and connected to the third DC bus capacitor C d3 The negative electrode;

[0083] The first DC bus capacitor C d1 The positive electrode and the third DC bus capacitor C d3 A DC power source such as a photovoltaic string or a battery cluster is connected between the negative poles, and the AC output ports of the three variable-level bridge arms jointly realize three-phase AC voltage output.

[0084] It is understandable that according to Figure 3 As shown in the structural diagram, the switch devices S1 and S1', S2 and S2', S3 and S3' are complementary, that is, when one of the switch devices is turned on, the corresponding complementary switch device is turned off. According to the switching state of the bypass switch K, the variable level converter has two working modes: four-level mode and three-level mode.

[0085] In one embodiment of the present application, when the bypass switch K is disconnected, the variable level converter operates in a four-level mode, having four switching states capable of outputting four different levels, as shown in Table 1. It can be seen from Table 1 that the switch device S3 must be turned on before S2, and the switch device S2 must be turned on before the switch device S1.

[0086] Table 1 Relationship between switch state and output voltage in four-level mode

[0087] S1 S2 S3 <![CDATA[U o ]]> 1 1 1 <![CDATA[U d1 +U d2 +U d3 ]]> 0 1 1 <![CDATA[U d2 +U d3 ]]> 0 0 1 <![CDATA[U d3 ]]> 0 0 0 0

[0088] In the four-level mode, the four switching states correspond to four different levels as follows:

[0089] (1) When the switch devices S1, S2, and S3 are turned on and the other switch devices are turned off, the output voltage of the variable level converter is:

[0090] Uo =U dc =U d1 +U d2 +U d3

[0091] Among them, U o It represents the output voltage of the variable level converter with the negative pole of the DC bus as reference ground potential, U dc Indicates the total DC bus voltage, U d1 Represents the first DC bus capacitor C d1 Voltage, U d2 Represents the second DC bus capacitor C d2 Voltage, U d3 Represents the third DC bus capacitor C d3 voltage.

[0092] (2) When the switch devices S1', S2, and S3 are turned on and the other switch devices are turned off, the output voltage of the variable level converter is:

[0093] U o =U d2 +U d3

[0094] (3) When the switch devices S1', S2', and S3 are turned on and the other switch devices are turned off, the output voltage of the variable level converter is:

[0095] U o =U d3

[0096] (4) When the switch devices S1', S2', and S3' are turned on and the other switch devices are turned off, the output voltage of the variable level converter is:

[0097] U o =0

[0098] In another embodiment of the present application, when the bypass switch K is closed, the variable level converter operates in a three-level mode, has four switch states and can output three different levels, as shown in Table 2.

[0099] Table 2 Relationship between three-level mode switch state and output voltage

[0100] S1 S2 S3 <![CDATA[U o ]]> 1 1 1 <![CDATA[U d1 +U d3 ]]> 0 1 0 <![CDATA[U d3 ]]> 1 0 1 <![CDATA[U d3 ]]> 0 0 0 0

[0101] In the three-level mode, the three different levels corresponding to the four switching states are as follows:

[0102] (1) When the switch devices S1, S2, and S3 are turned on and the other switch devices are turned off, the output voltage of the variable level converter is:

[0103] Uo =U dc =U d1 +U d3

[0104] (2) When the switch devices S1', S2, and S3' are turned on and the other switch devices are turned off, the output voltage of the variable level converter is:

[0105] U o =U d3

[0106] (3) When the switch devices S1, S2', and S3 are turned on and the other switch devices are turned off, the output voltage of the variable level converter is still:

[0107] U o =U d3

[0108] (4) When the switch devices S1', S2', and S3' are turned on and the other switch devices are turned off, the output voltage of the variable level converter is:

[0109] U o =0

[0110] Figure 4 1 is a flow chart of a control method of a variable level converter according to an embodiment of the present application, comprising:

[0111] Step 101, determining the state of the bypass switch according to a comparison result between the total DC bus voltage and the rated voltage of the first DC bus capacitor.

[0112] It is understandable that according to Figure 3 As shown in the structural diagram, the DC bus capacitor C d1 With C d3 The voltage remains equal, and the rated working voltage is set to E, with a value range of:

[0113] U dc / 3 <E<U dc / 2

[0114] For photovoltaic and energy storage applications, the DC output voltage of photovoltaic strings and battery clusters will vary over a wide range depending on the lighting conditions, temperature, and battery SOC. In these applications, the variable level converter is based on the total DC bus voltage U dc Different work in different modes.

[0115] In one embodiment of the present application, if U dc >2E, the bypass switch K is disconnected and the variable level converter operates in the four-level mode.

[0116] In another embodiment of the present application, if U dc ≤2E, the bypass switch K is closed and the variable level converter operates in the three-level mode.

[0117] Step 102, in response to the bypass switch being disconnected, determining that the variable level converter operates in a four-level mode, adopting a carrier overlapping PWM modulation strategy to control the voltages of the three DC bus capacitors, reducing only the voltage of the second DC bus capacitor when the total DC bus voltage decreases, and controlling the voltages of the first and third DC bus capacitors to maintain the rated values ​​unchanged, and closing the bypass switch when the voltage of the second DC bus capacitor approaches 0.

[0118] When the variable level converter operates in the four-level mode, the first DC bus capacitor C is controlled d1 , the third DC bus capacitor C d3 The voltage of the second DC bus capacitor C is kept constant at the rated value E. d2 The voltage is U d2 =U dc -2E, at this time, the voltage borne by the switch devices S1, S1', S3, S3' is the rated voltage E, and lower voltage switch devices can be used, while the voltage borne by the switch devices S2, S2' is E+U d2 , a higher voltage switch device is required; when the DC bus voltage U dc When the maximum value of is constant, the voltages borne by the low-voltage switch devices S1, S3, S1', S3' and the high-voltage switch devices S2, S2' can be optimized by reasonably setting the rated voltage E.

[0119] In one embodiment of the present application, for the highest DC voltage U dc =2000V photovoltaic and energy storage system, when E=750V is selected, U d2 =500V, the voltage of S1, S3, S1', S3' is 750V, and 1200V low-voltage switch devices can be used. At this time, the voltage of S2 and S2' is 1250V, and 1700V high-voltage switch devices can be used. If the traditional method is used, the DC bus capacitor C d1 , C d2 and C d3 When the voltages are all equal and take 667V, S1, S3, S1', S3' still need to use 1200V switching devices, while the voltage that S2 and S2' withstand is 1333V. Using 1700V switching devices will cause the problem of insufficient voltage margin.

[0120] Moreover, when the DC bus voltage U dc When the light conditions or SOC changes decrease, due to the C d1 and Cd3 The voltage of E remains unchanged, C d2 The voltage U d2 The DC bus voltage U dc The voltage E+U on S2 and S2' decreases rapidly with the decrease of d2 The switching loss of the high-voltage switching devices S2 and S2' will also decrease accordingly, thereby reducing the switching loss of the high-voltage switching devices S2 and S2'.

[0121] In addition, in one embodiment of the present application, in order to achieve effective control of the voltages of the three DC bus capacitors, a carrier overlap PWM modulation strategy is adopted, such as Figure 5 As shown. Three carriers C r1 , C r2 , C r3 is a triangular wave with the same period and phase, where the first carrier C r1 , and the third carrier C r3 The amplitude of the second carrier C r2 The amplitude is 2; the first carrier C r1 The lowest point is 0, the highest point is 1, and the second carrier C r2 The lowest point is -1, the highest point is 1, and the third carrier C r3 The lowest point is -1 and the highest point is 0. The switch devices S1 and S1' correspond to the upper carrier C r1 , the switch devices S2 and S2' correspond to the intermediate carrier C r2 , the switch devices S3 and S3' correspond to the lower carrier C r3 Assume that the output reference voltage is u ref , the range is -1≤u ref ≤1, change u ref With carrier C r1 , C e2 , C r3 By comparing respectively, the control signals of the switch devices S1, S2, and S3 can be obtained. When the reference voltage is greater than the carrier, the corresponding switch device control signal is at a high level, otherwise it is at a low level.

[0122] Step 103, in response to the bypass switch being closed, determining that the variable level converter operates in a three-level mode, and adopting a hybrid modulation strategy to control the high voltage switch device at the fundamental frequency.

[0123] When the variable level converter operates in three-level mode, C d2 The voltage U d2 After it drops to 0, it is bypassed, and the DC bus capacitor C is controlled at this time. d1 and C d3 The voltage is U d1 =U d3 =U dc / 2, at this time, the voltage borne by all switching devices is Udc / 2, the switching losses of S2 and S2' can be further reduced.

[0124] In one embodiment of the present application, in order to reduce the switching loss of the high voltage device, a hybrid modulation strategy is adopted, such as Figure 6 As shown. At this time, high-voltage devices S2 and S2' operate at the fundamental frequency, and low-voltage devices S1, S3, S1', and S3' operate at the carrier frequency. ref ≥0, S2 is turned on and S2' is turned off. ref By comparing with the triangular carrier, the control signals of the switch devices S1 and S3 can be obtained; when u ref <0, S2' is turned on and S2 is turned off. At this time, 1+u ref With carrier C r By comparison, control signals of the switch devices S1 and S3 can be obtained.

[0125] It should also be noted that in order to implement the above embodiments, according to the different voltage levels of the DC bus, different types of switching devices with corresponding voltage levels can be selected, such as MOSFET (Metal-Oxide-SemiconductorField-Effect Transistor), IGBT (Insulated Gate Bipolar Transistor) or IGCT (Integrated Gate Commutated Thyristor) made of silicon materials, as well as wide bandgap semiconductor switching devices made of materials such as silicon carbide and gallium nitride.

[0126] In addition, the bypass switch can select different types of switches such as mechanical switches, solid-state switches, relays, contactors, etc. according to different voltage and current levels.

[0127] The present application proposes a novel variable level converter, which consists of three DC bus capacitors and a three-phase variable level bridge arm. The DC bus capacitors are connected in series in sequence, and each phase variable level bridge arm includes six switching devices and a bypass switch. On the one hand, by mixing and using switching devices of different voltage levels, the number of devices can be reduced and the cost can be reduced; on the other hand, the output level voltage can be changed as the DC bus voltage changes, and the switching loss can be reduced by reducing the working voltage of the high-voltage device. When the DC bus voltage drops to a certain level, it can directly switch from a four-level working mode to a three-level working mode. At this time, the high-voltage device directly works at the fundamental frequency, which greatly improves the system efficiency. The present application has the advantages of fewer switching devices and high system efficiency, and is suitable for occasions such as photovoltaic and energy storage grid-connected inverters.

[0128] Figure 71 is a block diagram of a control device 10 for a variable level converter according to an embodiment of the present application, comprising:

[0129] The bypass switch state determination module 100 is used to determine the state of the bypass switch according to the comparison result between the total DC bus voltage and the rated voltage of the first DC bus capacitor;

[0130] A four-level mode operation module 200, in response to the bypass switch being disconnected, is used to determine that the variable level converter operates in the four-level mode, and adopts a carrier overlap PWM modulation strategy to control the voltages of the three DC bus capacitors, and when the total DC bus voltage decreases, only the voltage of the second DC bus capacitor is reduced, and the voltages of the first and third DC bus capacitors are controlled to remain unchanged at the rated values, and when the voltage of the second DC bus capacitor is close to 0, the bypass switch is closed;

[0131] The three-level mode operation module 300 is used to determine that the variable level converter operates in the three-level mode in response to the bypass switch being closed, and to control the high-voltage switch device at the fundamental frequency using a hybrid modulation strategy.

[0132] Regarding the device in the above embodiment, the specific manner in which each module performs operations has been described in detail in the embodiment of the method, and will not be elaborated here.

[0133] It should be understood that in the above-mentioned embodiments, the structure, location, and connection of each component can be changed. On the basis of the technical solution of the present invention, improvements and equivalent transformations of individual components should not be excluded from the scope of protection of the present invention.

[0134] It should be understood that the various forms of processes shown above can be used to reorder, add or delete steps. For example, the steps recorded in this application can be executed in parallel, sequentially or in different orders, as long as the expected results of the technical solution of this application can be achieved, and this document is not limited here.

[0135] The above specific implementations do not constitute a limitation on the protection scope of this application. It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and substitutions can be made according to design requirements and other factors. Any modifications, equivalent substitutions and improvements made within the spirit and principles of this application should be included in the protection scope of this application.

Claims

1. A control method for a variable level converter, characterized in that: The method is applied to a variable level converter, wherein the variable level converter is composed of three DC bus capacitors and three-phase variable level bridge arms, wherein the DC bus capacitors are sequentially connected in series, and each phase variable level bridge arm includes six switch devices and a bypass switch, and the three DC bus capacitors are connected in series. , and Sequentially connected in series, each phase variable level bridge arm includes switch device S1, switch device S2, switch device S3, switch device S1', switch device S2', switch device S3', and a total of four DC input ports X1, X2, X3, X4 and one AC output port X5 are derived, wherein, The emitter of the switching device S1 is connected to the collector of the switching device S1'; The emitter of the switch device S3 is connected to the collector of the switch device S3'; The collector of the switching device S2 is connected to the emitter of the switching device S1; The emitter of the switch device S2' is connected to the collector of the switch device S3'; The emitter of the switch device S2 is connected to the collector of the switch device S2' and serves as the AC output terminal X5 of the variable level bridge arm; The collector of the switch device S1 is connected to the DC input port X1; The emitter of the switch device S1' is connected to the DC input port X2; The collector of the switch device S3 is connected to the DC input port X3; The emitter of the switch device S3' is connected to the DC input port X4; The bypass switch K is connected in parallel between the DC input ports X2 and X3; The DC input ports X1 of the variable level bridge arms of all phases are connected in parallel and connected to the first DC bus capacitor The positive electrode; The DC input ports X2 of the variable level bridge arms of all phases are connected in parallel and connected to the first DC bus capacitor The negative electrode of the second DC bus capacitor The positive electrode; The DC input ports X3 of the variable level bridge arms of all phases are connected in parallel and connected to the second DC bus capacitor The negative electrode and the third DC bus capacitor The positive electrode; The DC input ports X4 of the variable level bridge arms of all phases are connected in parallel and connected to the third DC bus capacitor The negative electrode; The method comprises: Determining the state of the bypass switch according to a comparison result of the total DC bus voltage and the rated voltage of the DC bus capacitor; In response to the bypass switch being disconnected, determining that the variable level converter operates in a four-level mode, using a carrier overlap PWM modulation strategy to control the voltages of the three DC bus capacitors, reducing only the voltage of the second DC bus capacitor when the total DC bus voltage decreases, and controlling the voltages of the first and third DC bus capacitors to maintain a rated value unchanged, and closing the bypass switch when the voltage of the second DC bus capacitor is 0; In response to the bypass switch being closed, it is determined that the variable level converter operates in a three-level mode, and a hybrid modulation strategy is adopted to control the high-voltage switch device at the fundamental frequency; when the variable level converter operates in a four-level mode, it has four switch states and can output four different levels, including: When the switch devices S1, S2, and S3 are turned on and the other switch devices are turned off, the output voltage of the variable level converter is: in, represents the output voltage of the variable level converter with the negative pole of the DC bus as the reference ground potential, Indicates the total DC bus voltage, Indicates the first DC bus capacitance The voltage, Represents the second DC bus capacitor The voltage, Indicates the third DC bus capacitor Voltage; When the switch devices S1', S2, and S3 are turned on and the other switch devices are turned off, the output voltage of the variable level converter is: When the switch devices S1', S2', and S3 are turned on and the other switch devices are turned off, the output voltage of the variable level converter is: When the switch devices S1', S2', and S3' are turned on and the other switch devices are turned off, the output voltage of the variable level converter is: 。 2. The method according to claim 1, characterized in that When the variable level converter operates in the three-level mode, it has four switching states and can output three different levels, including: When the switch devices S1, S2, and S3 are turned on and the other switch devices are turned off, the output voltage of the variable level converter is: When the switch devices S1', S2, and S3' are turned on and the other switch devices are turned off, the output voltage of the variable level converter is: When the switch devices S1, S2', and S3 are turned on and the other switch devices are turned off, the output voltage of the variable level converter is: When the switch devices S1', S2', and S3' are turned on and the other switch devices are turned off, the output voltage of the variable level converter is: 。 3. The method according to claim 2, characterized in that Determining the state of the bypass switch according to a comparison result between the total DC bus voltage and the rated voltage of the first DC bus capacitor includes: like , the bypass switch K is disconnected, and the variable level converter operates in a four-level mode, Indicates the first DC bus capacitance Or the third DC bus capacitor Rated voltage; like , the bypass switch K is closed, and the variable level converter operates in a three-level mode.

4. The method according to claim 3, characterized in that Also includes: According to the different voltage levels of the DC bus, select different types of switching devices with corresponding voltage levels; Depending on the voltage and current levels, choose different types of bypass switches with corresponding voltage and current levels.

5. A control device for a variable level converter, characterized in that: The device is applied to a variable level converter, which is composed of three DC bus capacitors and three-phase variable level bridge arms. The DC bus capacitors are connected in series in sequence. Each phase variable level bridge arm includes six switch devices and a bypass switch. The three DC bus capacitors , and Sequentially connected in series, each phase variable level bridge arm includes switch device S1, switch device S2, switch device S3, switch device S1', switch device S2', switch device S3', and a total of four DC input ports X1, X2, X3, X4 and one AC output port X5 are derived, wherein, The emitter of the switching device S1 is connected to the collector of the switching device S1'; The emitter of the switch device S3 is connected to the collector of the switch device S3'; The collector of the switching device S2 is connected to the emitter of the switching device S1; The emitter of the switch device S2' is connected to the collector of the switch device S3'; The emitter of the switch device S2 is connected to the collector of the switch device S2' and serves as the AC output terminal X5 of the variable level bridge arm; The collector of the switch device S1 is connected to the DC input port X1; The emitter of the switch device S1' is connected to the DC input port X2; The collector of the switch device S3 is connected to the DC input port X3; The emitter of the switch device S3' is connected to the DC input port X4; The bypass switch K is connected in parallel between the DC input ports X2 and X3; The DC input ports X1 of the variable level bridge arms of all phases are connected in parallel and connected to the first DC bus capacitor The positive electrode; The DC input ports X2 of the variable level bridge arms of all phases are connected in parallel and connected to the first DC bus capacitor The negative electrode of the second DC bus capacitor The positive electrode; The DC input ports X3 of the variable level bridge arms of all phases are connected in parallel and connected to the second DC bus capacitor The negative electrode and the third DC bus capacitor The positive electrode; The DC input ports X4 of the variable level bridge arms of all phases are connected in parallel and connected to the third DC bus capacitor The negative electrode; The device comprises: A bypass switch state determination module, used to determine the state of the bypass switch according to a comparison result between the total DC bus voltage and the rated voltage of the DC bus capacitor; a four-level mode operation module, in response to the bypass switch being disconnected, for determining that the variable level converter operates in the four-level mode, adopting a carrier overlap PWM modulation strategy to control the voltages of the three DC bus capacitors, reducing only the voltage of the second DC bus capacitor when the total DC bus voltage decreases, and controlling the voltages of the first and third DC bus capacitors to maintain the rated values ​​unchanged, and closing the bypass switch when the voltage of the second DC bus capacitor is 0; A three-level mode operation module, in response to the bypass switch being closed, for determining that the variable level converter operates in a three-level mode, and adopting a hybrid modulation strategy to control the high-voltage switch device at a fundamental frequency; When the variable level converter operates in the four-level mode, it has four switch states and can output four different levels, including: When the switch devices S1, S2, and S3 are turned on and the other switch devices are turned off, the output voltage of the variable level converter is: in, represents the output voltage of the variable level converter with the negative pole of the DC bus as the reference ground potential, Indicates the total DC bus voltage, Indicates the first DC bus capacitance The voltage, Represents the second DC bus capacitor The voltage, Indicates the third DC bus capacitor Voltage; When the switch devices S1', S2, and S3 are turned on and the other switch devices are turned off, the output voltage of the variable level converter is: When the switch devices S1', S2', and S3 are turned on and the other switch devices are turned off, the output voltage of the variable level converter is: When the switch devices S1', S2', and S3' are turned on and the other switch devices are turned off, the output voltage of the variable level converter is: 。

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