Compact medium / low-voltage AC / DC solid-state transformer based on MMC midpoint embedded submodule and frequency mixing modulation method of compact medium / low-voltage AC / DC solid-state transformer

Abstract

The invention provides a compact medium / low-voltage AC / DC solid-state transformer based on an MMC midpoint embedded sub-module. The compact medium / low-voltage AC / DC solid-state transformer comprises a low-voltage DC port, a three-phase DC / AC converter, a three-phase circuit topology and a medium-voltage AC port, the three-phase circuit topology comprises three same single-phase lines, and each single-phase line comprises an upper bridge arm, an upper bridge arm inductor, an embedded module, a lower bridge arm and a lower bridge arm inductor. The embedded module comprises a first switching tube, a second switching tube, a third switching tube, a fourth switching tube, a fifth switching tube, a sixth switching tube, a direct-current capacitor and a high-frequency transformer. According to the solid-state transformer, by introducing one embedded module, the number of high-frequency transformers of a traditional solid-state transformer is reduced to only one high-frequency transformer for each phase, and the size of the solid-state transformer is greatly reduced; as the upper and lower bridge arms can generate high-frequency voltage pulses, high-frequency pulse voltage generated by the embedded module can be eliminated. Therefore, the high-frequency pulse voltage output by the embedded module does not deteriorate the circulating current of the bridge arm.

Description

Technical Field

[0001] The present application relates to the technical field of power systems, and in particular to a compact medium / low voltage AC / DC solid-state transformer and a mixing frequency modulation method based on an MMC midpoint embedded submodule. Background Art

[0002] Solid state transformers (SSTs) are usually composed of power electronic circuits and high-frequency transformers. They are a new type of power electronic equipment that has the voltage level conversion and electrical isolation functions of traditional power frequency transformers. By increasing the operating frequency of the isolation transformer, SSTs significantly reduce the size and weight of the transformer. Compared with traditional power frequency transformers, SSTs are not only more flexible, but can also easily realize fault tolerance, power quality regulation, DC access to renewable energy and energy storage equipment, and multi-port operation.

[0003] As an electric energy conversion device, the topology and control method of SST are closely related to the specific application scenario. In early studies, the input side of SST is usually connected to a medium voltage or medium voltage AC distribution network. Cascaded H-Bridge (CHB) and Modular Multilevel Converter (MMC) are the two most common input stage topologies. In an SST with a CHB structure (CHB-SST), the DC side of each CHB submodule is connected to a dual active bridge (DAB) based on a high-frequency isolation transformer, and the other side of these DAB units is connected in parallel to the low-voltage DC bus to achieve high current output. In an SST with an MMC structure (MMC-SST), the DC side of each submodule is also connected to a DAB unit, and the output ends are connected in parallel to form a low-voltage DC port.

[0004] Compared with CHB-SST, MMC-SST can directly realize the output of medium voltage AC (MVAC) and medium voltage DC (MVDC) ports, so the number of conversion stages is less. Since each MMC submodule in MMC-SST is equipped with a dual active bridge (DAB) conversion unit, this topology requires the number of DAB units to be consistent with the number of MMC submodules, resulting in too many transformers and high insulation level requirements, which leads to problems of large size and high cost. Summary of the invention

[0005] In order to overcome the above technical defects, the present application provides a compact medium / low voltage AC / DC solid-state transformer and a mixing modulation method based on an MMC midpoint embedded submodule. To achieve the above purpose, the present application is implemented according to the following technical solutions: In a first aspect, the present application provides a compact medium / low voltage AC / DC solid-state transformer based on an MMC midpoint embedded submodule, characterized in that it includes: a low voltage DC port, a three-phase DC / AC converter, a three-phase circuit topology, and a medium voltage AC port; The three-phase circuit topology includes three identical single-phase lines, and the single-phase lines include an upper bridge arm, an upper bridge arm inductor, an embedded module, a lower bridge arm, and a lower bridge arm inductor; The embedded module includes a first switch tube, a second switch tube, a third switch tube, a fourth switch tube, a fifth switch tube and a sixth switch tube, as well as a DC capacitor and a high-frequency transformer. The emitter node of the first switch tube, the collector node of the second switch tube, one end of the primary winding side of the high-frequency transformer and one end of the upper bridge arm inductor are connected to form a node m, the emitter node of the fourth switch tube, the collector node of the third switch tube, the other end of the primary winding side of the high-frequency transformer and one end of the lower bridge arm inductor are connected to form a node n, the emitter nodes of the first switch tube, the third switch tube and the sixth switch tube are connected to the negative end of the DC capacitor, the collector nodes of the second switch tube, the fourth switch tube and the fifth switch tube are connected to the positive end of the DC capacitor, one end of the secondary winding of the high-frequency transformer is connected to the three-phase DC / AC converter, and one end of the secondary winding of the high-frequency transformer is connected to the low-voltage intermediate-stream node; The upper bridge arm and the lower bridge arm each include a plurality of half-bridge sub-modules, and the plurality of half-bridge sub-modules are connected in series; One end of the last half-bridge submodule in the upper bridge arm is connected to the other end of the upper bridge arm inductor, and one end of the first half-bridge submodule in the lower bridge arm is connected to the other end of the lower bridge arm inductor; The positive terminal and the negative terminal of the low-voltage DC port are respectively connected to the three-phase DC / AC converter; The medium voltage AC port is formed by three ports respectively led out from the connection line between the fifth switch tube and the sixth switch tube included in the embedded module on the three single-phase lines.

[0006] Optionally include: One end of the first half-bridge submodule in the upper bridge arm is connected to one end of the first half-bridge submodule in the upper bridge arms of the other two phases, and one end of the last half-bridge submodule in the lower bridge arm is connected to the last half-bridge submodule in the lower bridge arms of the other two phases.

[0007] In a second aspect, the present application provides a mixing frequency modulation method, for a compact medium / low voltage AC / DC solid-state transformer based on an MMC midpoint embedded submodule according to the first aspect, comprising: An embedded module is arranged between the upper bridge arm and the lower bridge arm of the single-phase line, and the upper and lower ports of the embedded module respectively generate a first high-frequency voltage pulse as an input voltage of the primary side of the high-frequency transformer; The upper bridge arm and the lower bridge arm generate a second high-frequency voltage pulse respectively, wherein the first high-frequency voltage pulse and the second high-frequency voltage pulse are equal in magnitude and opposite in direction.

[0008] Optionally, when the upper and lower ports of the embedded module respectively generate the first high-frequency voltage, they include a first working state, a second working state, a third working state, and a fourth working state; The first working state is: the first switch tube, the fourth switch tube, and the fifth switch tube are turned on, the second switch tube, the third switch tube, and the sixth switch tube are turned off, and the DC capacitor is reversely input. m , n Port output voltage u mn for- U c , m , x Port output voltage u mx for- U c , x , n Port output voltage u mx is 0.

[0009] Optionally, the second working state is: the first switch tube, the fourth switch tube, and the sixth switch tube are turned on, the second switch tube, the third switch tube, and the fifth switch tube are turned off, and the DC capacitor is reversely input. m , n Port output voltage u mn for- U c , m , x Port output voltage u mx is 0, x , n Port output voltage u mx for- U c .

[0010] Optionally, the third working state is: the second switch tube, the third switch tube, and the fifth switch tube are turned on, the first switch tube, the fourth switch tube, and the sixth switch tube are turned off, and the DC capacitor is positively input. m , n Port output voltage umn for U c , m , x Port output voltage u mx is 0, x , n Port output voltage u mx for U c .

[0011] Optionally, the fourth working state is: the second switch tube, the third switch tube, and the sixth switch tube are turned on, the first switch tube, the fourth switch tube, and the fifth switch tube are turned off, and the DC capacitor is positively input. m , n Port output voltage u mn for U c , m , x Port output voltage u mx for U c , x , n Port output voltage u mx is 0.

[0012] This application has the following beneficial effects: The CMMC-SST topology provided in the present application reduces the number of high-frequency transformers of the traditional solid-state transformer to only one per phase by introducing an embedded module, thereby greatly reducing the volume of the solid-state transformer; in addition, since the upper and lower bridge arms will generate high-frequency voltage pulses, the high-frequency pulse voltage generated by the embedded module will be eliminated. Therefore, the high-frequency pulse voltage output by the embedded module will not deteriorate the circulating current of the bridge arm.

[0013] In addition to the above-described purposes, features and advantages, the present application has other purposes, features and advantages. The present application will be further described in detail below with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The drawings constituting a part of the present application are used to provide a further understanding of the present application. The illustrative embodiments and descriptions of the present application are used to explain the present application and do not constitute an improper limitation on the present application. In the drawings: Figure 1 It is a schematic diagram of a topological structure of a compact medium / low voltage AC / DC solid-state transformer based on an MMC midpoint embedded submodule provided in an embodiment of the present application; Figure 2It is an embedded module provided in the embodiment of the present application m , n Schematic diagram of high-frequency pulse voltage at the port; FIG3 (a) is an equivalent circuit diagram of an embedded module provided in an embodiment of the present application when it is in a first working state; FIG3 (b) is an equivalent circuit diagram of the embedded module provided in an embodiment of the present application when it is in a second working state; FIG3 (c) is an equivalent circuit diagram of the embedded module provided in an embodiment of the present application when it is in a third working state; FIG3 (d) is an equivalent circuit diagram of the embedded module provided in an embodiment of the present application when it is in a fourth working state; Figure 4 It is a schematic diagram of the modulation principle of the upper and lower bridge arms provided in the embodiment of the present application; FIG. 5 (a) is a diagram of an embedded module provided in an embodiment of the present application in a second working state and a fourth working state. u mx Schematic diagram of FIG5 (b) is a diagram of an embedded module provided in an embodiment of the present application in a first working state and a third working state. u xn Schematic diagram of FIG. 6 (a) is a diagram of an embedded module provided in an embodiment of the present application in a second working state and a fourth working state. u mx Schematic diagram of FIG6 (b) is a diagram of an embedded module provided in an embodiment of the present application in a second working state and a fourth working state. u mx Schematic diagram of . DETAILED DESCRIPTION

[0015] The embodiments of the present application are described in detail below with reference to the accompanying drawings; however, the present application can be implemented in many different ways as defined and covered by the claims.

[0016] In order to solve the problems raised in the background technology, such as Figure 1 As shown, the present application provides a compact medium / low voltage AC / DC solid-state transformer (compact MMC-SST, CMMC-SST) based on an MMC midpoint embedded submodule, comprising: a low voltage DC port, a three-phase DC / AC converter, a three-phase circuit topology, and a medium voltage AC port; The three-phase circuit topology includes three identical single-phase lines, namely phase A, phase B, and phase C. The single-phase line includes an upper bridge arm, an upper bridge arm inductor, and a L 1 , embedded module, lower bridge arm, lower bridge arm inductor L 2; That is to say, the structure of each single-phase line is the same and contains the above devices; The embedded module includes a first switch tube S 1. Second switch tube S 2. The third switch tube S 3. The fourth switch tube S 4. The fifth switch tube S 5 and the sixth switch tube S 6, and DC capacitors C and high frequency transformer T 1 , six upstream switch tubes, mainly six switch devices based on silicon-based insulated gate bipolar transistors (IGBT), and the embedded module realizes the construction of the DC port of CMMC-SST. The specific connection method is as follows: The first switch tube S 1 emitter node, the second switch tube S 2 collector node, high frequency transformer T 1 The primary winding side and the upper bridge arm inductor L 1 The node formed by connecting one end of S 4 emitter node, the third switch tube S 3 collector node, high frequency transformer T 1 The other end of the primary winding and the lower bridge arm inductor L 2 is connected to form a node n, and the first switch tube S 1. The third switch tube S 3 and the sixth switch tube S The emitter node of 6 is connected to the negative terminal of the DC capacitor, and the second switch tube S 2. The fourth switch tube S 4 and the fifth switch tube S The collector node of 5 is connected to a DC capacitor C The positive terminal of the high frequency transformer T 1 One end of the secondary winding is connected to the three-phase DC / AC converter, high-frequency transformer T 1 One end of the secondary winding is connected to the low voltage midstream node; The upper bridge arm and the lower bridge arm each include a plurality of half-bridge submodules HSM, the plurality of half-bridge submodules are connected in series, one end of the last half-bridge submodule in the upper bridge arm is connected to the other end of the upper bridge arm inductor, and one end of the first half-bridge submodule in the lower bridge arm is connected to the other end of the lower bridge arm inductor; The positive terminal and the negative terminal of the low voltage DC port are respectively connected to the three-phase DC / AC converter; The medium voltage AC port is formed by three ports respectively led out from the connection lines between the fifth switch tube and the sixth switch tube included in the embedded modules on the three single-phase lines.

[0017] The CMMC-SST contains two ports, low voltage DC and medium voltage AC, and each phase contains only one high-frequency transformer ( T 1) Realize energy exchange and electrical isolation.

[0018] The embedded module of the CMMC-SST topology mentioned above can lead out the DC port through only one high-frequency transformer. Compared with the capacitor series voltage divider structure used in the MVDC port, it can greatly reduce the number of high-frequency transformers and reduce the size of the device.

[0019] This application proposes a frequency mixing modulation strategy for the above CMMC-SST topology. An embedded module is set between the upper bridge arm and the lower bridge arm of the single-phase line. By controlling the closing of the switch tube of the embedded module, the embedded module m , n The port generates a first high-frequency pulse voltage as the input voltage of the primary side of the high-frequency transformer. In addition, the upper and lower bridge arms generate an opposite second high-frequency pulse voltage to offset m , n The influence of high-frequency pulse voltage at the port on the circulating current is analyzed as follows: (1) Modulation principle of the intermediate embedded module Built-in modules m , n Ports can generate U c 、- U c There are two levels and the specific switching modes are shown in Table 1.

[0020] Table 1 Embedded module switch mode , Note: "1" means the device is closed; "0" means the device is open By controlling the switch devices of the embedded module, m , n The modulation voltage of the port is a high-frequency pulse voltage, and its modulation principle is as follows Figure 2 shown.

[0021] As can be seen from Table 1, according to the different conduction states of the switch tube, the four working states of the embedded module are shown in Figure 3 (a) to Figure 3 (d): First working state (a): The first switch tube S 1. The fourth switch tube S 4. The fifth switch tube S 5 is turned on, the second switch tube S2. The third switch tube S 3. The sixth switch tube S 6 shutdown, the DC capacitor is reversed, m , n Port output voltage u mn for- U c , m , x Port output voltage u mx for- U c , x , n Port output voltage u mx is 0.

[0022] Second working state (b): The first switch tube S 1. The fourth switch tube S 4. The sixth switch tube S 6 is turned on, the second switch tube S 2. The third switch tube S 3. The fifth switch tube S 5 shutdown, the DC capacitor is reversed, m , n Port output voltage u mn for- U c , m , x Port output voltage u mx is 0, x , n Port output voltage u mx for- U c .

[0023] The third working state (c): the second switch tube S 2. The third switch tube S 3. The fifth switch tube S 5 is turned on, the first switch tube S 1. The fourth switch tube S 4. The sixth switch tube S 6 is turned off, the DC capacitor is positively input, m , n Port output voltage u mn for U c , m , x Port output voltage umx is 0, x , n Port output voltage u mx for U c .

[0024] Fourth working state (d): the second switch tube S 2. S 3. The sixth switch tube S 6 is turned on, the first switch tube S 1. The fourth switch tube S 4. The fifth switch tube S 5 is turned off, the DC capacitor is positively input, m , n Port output voltage u mn for U c , m , x Port output voltage u mx for U c , x , n Port output voltage u mx is 0.

[0025] 2) Modulation principle of upper and lower bridge arms From the working status analysis of the embedded module, we can see that m , n The high-frequency pulse voltage at the port will change the sum of the MMC DC voltage. If the upper and lower bridge arms are not controlled, the pulse voltage will be superimposed on the bridge arm inductance, increasing the system circulating current. m , n The influence of the high-frequency pulse voltage of the port on the total voltage of the MMC bridge arm. The upper and lower bridge arms need to output an additional reverse high-frequency voltage respectively. u mx and- u xn ,like Figure 4 shown.

[0026] Figure 2 It can be seen that m , n The high-frequency pulse voltage of the port can be decomposed into m , x Ports and x , n Two high frequency voltages at the ports u mx and u xn, as shown in Figure 5 (a)-Figure 5 (b) and Figure 6 (a)-Figure 6 (b). According to the working status of the middle embedded module, the working status of the upper and lower bridge arms can also be divided into the following two situations: When the embedded module operates in the second working state (b) or the fourth working state (d), m , x Port equivalent output a + U c Level pulse voltage, x , n Port equivalent output one - U c The pulse voltage. m , x The port output voltage is + U c When , the upper bridge arm needs to invest one less submodule; when x , n The port output voltage is - U c When the lower bridge arm is used, one more submodule is required.

[0027] When the embedded module operates in the first working state (a) or the third working state (c), m , x Port equivalent output one - U c Level pulse voltage, x , n Port equivalent output a + U c The pulse voltage. m , x The port output voltage is - U c When , the upper bridge arm needs to invest one more submodule; when x , n The port output voltage is + U c When the lower bridge arm is used, one less submodule is required.

[0028] In summary, the proposed MMC-SST operation principle can eliminate the middle embedded module by cooperating with the upper and lower bridge arm submodules. m , n The influence of the high-frequency pulse voltage of the port on the bridge arm voltage.

[0029] In summary, the CMMC-SST topology provided in this application reduces the number of high-frequency transformers in the traditional solid-state transformer to only one per phase by introducing an embedded module, thereby greatly reducing the size of the solid-state transformer; in addition, the high-frequency pulse voltage output by the embedded module will not deteriorate the circulating current of the bridge arm.

[0030] The above description is only the preferred embodiment of the present application and is not intended to limit the present application. For those skilled in the art, the present application may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A compact medium / low voltage AC / DC solid-state transformer based on MMC midpoint embedded submodule, characterized in that: include: Low voltage DC port, three-phase DC / AC converter, three-phase circuit topology, medium voltage AC port; The three-phase circuit topology includes three identical single-phase lines, and the single-phase lines include an upper bridge arm, an upper bridge arm inductor, an embedded module, a lower bridge arm, and a lower bridge arm inductor; The embedded module includes a first switch tube, a second switch tube, a third switch tube, a fourth switch tube, a fifth switch tube and a sixth switch tube, as well as a DC capacitor and a high-frequency transformer. The emitter node of the first switch tube, the collector node of the second switch tube, one end of the primary winding side of the high-frequency transformer and one end of the upper bridge arm inductor are connected to form a node m, the emitter node of the fourth switch tube, the collector node of the third switch tube, the other end of the primary winding side of the high-frequency transformer and one end of the lower bridge arm inductor are connected to form a node n, the emitter nodes of the first switch tube, the third switch tube and the sixth switch tube are connected to the negative end of the DC capacitor, the collector nodes of the second switch tube, the fourth switch tube and the fifth switch tube are connected to the positive end of the DC capacitor, one end of the secondary winding of the high-frequency transformer is connected to the three-phase DC / AC converter, and one end of the secondary winding of the high-frequency transformer is connected to the low-voltage intermediate-stream node; The upper bridge arm and the lower bridge arm each include a plurality of half-bridge sub-modules, and the plurality of half-bridge sub-modules are connected in series; One end of the last half-bridge submodule in the upper bridge arm is connected to the other end of the upper bridge arm inductor, and one end of the first half-bridge submodule in the lower bridge arm is connected to the other end of the lower bridge arm inductor; The positive terminal and the negative terminal of the low-voltage DC port are respectively connected to the three-phase DC / AC converter; The medium voltage AC port is formed by three ports respectively led out from the connection line between the fifth switch tube and the sixth switch tube included in the embedded module on the three single-phase lines.

2. The solid-state transformer according to claim 1, characterized in that: include: One end of the first half-bridge submodule in the upper bridge arm is connected to one end of the first half-bridge submodule in the upper bridge arms of the other two phases, and one end of the last half-bridge submodule in the lower bridge arm is connected to the last half-bridge submodule in the lower bridge arms of the other two phases.

3. A mixing frequency modulation method for a compact medium / low voltage AC / DC solid-state transformer based on an MMC midpoint embedded submodule, characterized in that: The compact medium / low voltage AC / DC solid-state transformer based on the MMC midpoint embedded submodule according to any one of claims 1 to 2 comprises: An embedded module is arranged between the upper bridge arm and the lower bridge arm of the single-phase line, and the upper and lower ports of the embedded module respectively generate a first high-frequency voltage pulse as an input voltage of the primary side of the high-frequency transformer; The upper bridge arm and the lower bridge arm generate a second high-frequency voltage pulse respectively, wherein the first high-frequency voltage pulse and the second high-frequency voltage pulse are equal in magnitude and opposite in direction.

4. The method according to claim 3, characterized in that When the upper and lower ports of the embedded module generate the first high-frequency voltage respectively, the first working state, the second working state, the third working state and the fourth working state are included; The first working state is: the first switch tube, the fourth switch tube, and the fifth switch tube are turned on, the second switch tube, the third switch tube, and the sixth switch tube are turned off, and the DC capacitor is reversely input. m , n Port output voltage u mn for- U c , m , x Port output voltage u mx for- U c , x , n Port output voltage u mx is 0.

5. The method according to claim 4, characterized in that The second working state is: the first switch tube, the fourth switch tube, and the sixth switch tube are turned on, the second switch tube, the third switch tube, and the fifth switch tube are turned off, and the DC capacitor is reversely input. m , n Port output voltage u mn for- U c , m , x Port output voltage u mx is 0, x , n Port output voltage u mx for- U c .

6. The method according to claim 4, characterized in that The third working state is: the second switch tube, the third switch tube, and the fifth switch tube are turned on, the first switch tube, the fourth switch tube, and the sixth switch tube are turned off, and the DC capacitor is positively input. m , n Port output voltage u mn for U c , m , x Port output voltage u mx is 0, x , n Port output voltage u mx for U c .

7. The method according to claim 4, characterized in that The fourth working state is: the second switch tube, the third switch tube, and the sixth switch tube are turned on, the first switch tube, the fourth switch tube, and the fifth switch tube are turned off, and the DC capacitor is positively input. m , n Port output voltage u mn for U c , m , x Port output voltage u mx for U c , x , n Port output voltage u mx is 0.

Images