An AC-DC solid-state transformer based on an MMC embedded module and a mixing modulation method

Abstract

This application provides an AC-DC solid-state transformer based on an MMC embedded module, including: 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 each single-phase line includes an upper arm, an upper-arm inductor, an embedded module, a lower arm, and a lower-arm inductor; the embedded module includes a first switch, a second switch, a third switch, a fourth switch, a fifth switch, and a sixth switch, as well as a DC capacitor and a high-frequency transformer. By introducing an embedded module, this application reduces the number of high-frequency transformers in a traditional solid-state transformer to only one per phase, greatly reducing the volume of the solid-state transformer; in addition, since high-frequency voltage pulses will be generated in the upper and lower arms, 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 arm circulating current.

Description

Technical Field

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

[0002] A solid state transformer (SST) is usually composed of a power electronic circuit and a high-frequency transformer, and is a new type of power electronic device with the functions of traditional power frequency transformer voltage level conversion and electrical isolation. By increasing the operating frequency of the isolation transformer, the SST significantly reduces the volume and weight of the transformer. Compared with traditional power frequency transformers, the SST not only has higher flexibility, but also can conveniently achieve various functions such as fault tolerance, power quality regulation, DC access of renewable energy and energy storage devices, and multi-port operation.

[0003] As a power conversion device, the topology and control method of the SST are closely related to the specific application scenarios. In the early research, the input side of the SST was usually connected to a medium voltage or medium voltage AC distribution network. The cascaded H-bridge (CHB) and the modular multilevel converter (MMC) are the two most common input stage topologies. In the SST adopting the CHB structure (CHB-SST), the DC side of each CHB sub-module is connected to a dual active bridge (DAB) based on a high-frequency isolation transformer, and the other sides of these DAB units are connected in parallel on the low-voltage DC bus to achieve high-current output. In the SST adopting the MMC structure (MMC-SST), the DC side of each sub-module 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 the CHB-SST, the MMC-SST can directly achieve the output of medium voltage AC (MVAC) and medium voltage DC (MVDC) ports, so the number of conversion stages is less. Since each MMC sub-module in the MMC-SST is equipped with a dual active bridge (DAB) conversion unit, this topology requires the number of DAB units to be the same as the number of MMC sub-modules, resulting in too many transformers and high insulation level requirements, thus causing problems of large volume and high cost. Summary of the Invention

[0005] In order to overcome the above technical defects, this application provides an AC-DC solid state transformer based on an MMC embedded module and a mixing modulation method. To achieve the above object, this application is implemented according to the following technical solutions:

[0006] In a first aspect, the present application provides an AC-DC solid-state transformer based on an MMC embedded module, which is characterized by comprising: a low-voltage DC port, a three-phase DC / AC converter, a three-phase circuit topology, and a medium-voltage AC port;

[0007] The three-phase circuit topology includes three identical single-phase lines, and each single-phase line includes an upper arm, an upper-arm inductor, an embedded module, a lower arm, and a lower-arm inductor;

[0008] The embedded module includes a first switch, a second switch, a third switch, a fourth switch, a fifth switch, and a sixth switch, as well as a DC capacitor and a high-frequency transformer. A node formed by connecting the collector node of the first switch, the emitter node of the second switch, one end of the primary winding side of the high-frequency transformer, and one end of the upper-arm inductor is m. A node formed by connecting the emitter node of the fourth switch, the collector node of the third switch, the other end of the primary-side winding of the high-frequency transformer, and one end of the lower-arm inductor is n. The emitter nodes of the first switch, the third switch, and the sixth switch are connected to the negative terminal of the DC capacitor. The collector nodes of the second switch, the fourth switch, and the fifth switch are connected to the positive terminal of the DC capacitor. One end of the secondary-side winding of the high-frequency transformer is connected to the three-phase DC / AC converter, and one end of the secondary-side winding of the high-frequency transformer is connected to a low-voltage mid-stream node;

[0009] Both the upper arm and the lower arm include a plurality of half-bridge sub-modules, and the plurality of half-bridge sub-modules are connected in series;

[0010] One end of the last half-bridge sub-module in the upper arm is connected to the other end of the upper-arm inductor, and one end of the first half-bridge sub-module in the lower arm is connected to the other end of the lower-arm inductor;

[0011] The positive and negative terminals of the low-voltage DC port are respectively connected to the three-phase DC / AC converter;

[0012] The medium-voltage AC port is formed by three ports respectively led out from the connection lines between the fifth switch and the sixth switch included in the embedded modules on the three single-phase lines.

[0013] Optionally, it includes:

[0014] One end of the first half-bridge sub-module in the upper arm is connected to one end of the first half-bridge sub-module in the upper arms of the other two phases, and one end of the last half-bridge sub-module in the lower arm is connected to one end of the last half-bridge sub-module in the lower arms of the other two phases.

[0015] In a second aspect, the present application provides a mixing modulation method for a compact medium / low voltage AC-DC solid-state transformer based on the MMC midpoint embedding sub-module according to the first aspect, including:

[0016] An embedded module is provided between the upper bridge arm and the lower bridge arm of a single-phase line, and first high-frequency voltage pulses are generated at the upper and lower ports of the embedded module respectively, serving as the input voltage of the primary side of the high-frequency transformer;

[0017] The upper bridge arm and the lower bridge arm respectively generate second high-frequency voltage pulses, where the first high-frequency voltage pulse and the second high-frequency voltage pulse are equal in magnitude and opposite in direction.

[0018] Optionally, when the first high-frequency voltage is generated at the upper and lower ports of the embedded module respectively, it includes a first working state, a second working state, a third working state, and a fourth working state;

[0019] Among them, 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 connected, m 、 n The output voltage of the port u mn is - U c , m 、 x The output voltage of the port u mx is - U c , x 、 n The output voltage of the port u mx is 0.

[0020] 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 connected, m 、 n The output voltage of the port u mn is - U c , m 、 x The output voltage of the port u mx is 0, x 、 n The output voltage of the port u mx is - U c 。

[0021] Optionally, the third operating state is that 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 connected. m 、 n The output voltage of the u mn is U c , m 、 x The output voltage of the u mx is 0, x 、 n The output voltage of the u mx is U c 。

[0022] Optionally, the fourth operating state is that 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 connected. m 、 n The output voltage of the u mn is U c , m 、 x The output voltage of the u mx is U c , x 、 n The output voltage of the u mx is 0.

[0023] The present application has the following beneficial effects:

[0024] The CMMC-SST topology provided by the present application reduces the number of high-frequency transformers of the traditional solid-state transformer to only 1 per phase by introducing an embedded module, greatly reducing the volume of the solid-state transformer; in addition, since high-frequency voltage pulses will be generated in the upper and lower bridge arms, 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.

[0025] In addition to the purposes, features, and advantages described above, the present application has other purposes, features, and advantages. The following will refer to the accompanying drawings to further elaborate on the present application in detail. Description of the Drawings

[0026] The accompanying drawings, which form a part of this application, are used to provide a further understanding of this application. The schematic embodiments of this application and their descriptions are used to explain this application and do not constitute an improper limitation of this application. In the drawings:

[0027] Figure 1 is a schematic diagram of a AC-DC solid-state transformer topology structure based on an MMC embedded module provided by an embodiment of this application;

[0028] Figure 2 is the embedded module provided by an embodiment of this application m 、 n schematic diagram of the high-frequency pulse voltage of the port;

[0029] Figure 3(a) is an equivalent circuit diagram of the embedded module in the first working state provided by an embodiment of this application;

[0030] Figure 3(b) is an equivalent circuit diagram of the embedded module in the second working state provided by an embodiment of this application;

[0031] Figure 3(c) is an equivalent circuit diagram of the embedded module in the third working state provided by an embodiment of this application;

[0032] Figure 3(d) is an equivalent circuit diagram of the embedded module in the fourth working state provided by an embodiment of this application;

[0033] Figure 4 is a schematic diagram of the modulation principle of the upper and lower bridge arms provided by an embodiment of this application;

[0034] Figure 5(a) is the schematic diagram of u mx when the embedded module is in the second working state and the fourth working state provided by an embodiment of this application;

[0035] Figure 5(b) is the schematic diagram of u xn when the embedded module is in the first working state and the third working state provided by an embodiment of this application;

[0036] Figure 6(a) is the schematic diagram of u mx when the embedded module is in the second working state and the fourth working state provided by an embodiment of this application;

[0037] Figure 6(b) is the schematic diagram of u mx when the embedded module is in the second working state and the fourth working state provided by an embodiment of this application. Detailed implementation manners

[0038] The embodiments of the present application will be described in detail below with reference to the accompanying drawings. However, the present application can be implemented in many different ways defined and covered by the claims.

[0039] To solve the problems raised in the background art, as Figure 1 shown, the present application provides a compact modular multilevel converter-based AC-DC solid-state transformer (CMMC-SST), including: a low-voltage DC port, a three-phase DC / AC converter, a three-phase circuit topology, and a medium-voltage AC port;

[0040] 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 arm, an upper arm inductor L 1 , an embedded module, a lower arm, and a lower arm inductor L 2; that is to say, the structure of each single-phase line is the same and all include the above components;

[0041] The embedded module includes a first switch S 1, a second switch S 2, a third switch S 3, a fourth switch S 4, a fifth switch S 5, and a sixth switch S 6, as well as a DC capacitor C and a high-frequency transformer T 1 . The six upper-stream switches are mainly 6 switching devices based on silicon-based insulated gate bipolar transistors (IGBTs). The embedded module realizes the construction of the DC port of the CMMC-SST. The specific connection method is as follows:

[0042] The collector node of the first switch S 1, the emitter node of the second switch S 2, one end of the primary winding side of the high-frequency transformer T 1 and one end of the upper arm inductor L 1 are connected to form a node m. The emitter node of the fourth switch S 4, the collector node of the third switch S 3, the other end of the primary side winding of the high-frequency transformer T 1 and one end of the lower arm inductor L 2 are connected to form a node n. The first switch S 1, the third switch S 3, and the sixth switch SThe emitter node of 6 is connected to the negative terminal of the DC capacitor, and the second switching transistor S 2. The fourth switching transistor S 4 and the fifth switching transistor S 5 have their collector nodes connected to the positive terminal of the DC capacitor. One end of the secondary side winding of the high-frequency transformer C is connected to the three-phase DC / AC converter, and one end of the secondary side winding of the high-frequency transformer T 1 is connected to the low-voltage medium-current node; T 1 One end of the secondary side winding of the high-frequency transformer is connected to the low-voltage medium-current node;

[0043] Both the upper bridge arm and the lower bridge arm contain multiple half-bridge sub-modules HSM. The multiple half-bridge sub-modules are connected in series. One end of the last half-bridge sub-module in the upper bridge arm is connected to the other end of the inductor in the upper bridge arm, and one end of the first half-bridge sub-module in the lower bridge arm is connected to the other end of the inductor in the lower bridge arm;

[0044] The positive and negative terminals of the low-voltage DC port are respectively connected to the three-phase DC / AC converter;

[0045] The medium-voltage AC port is formed by three ports respectively led out from the connection lines between the fifth and sixth switching transistors included in the embedded modules on three single-phase lines.

[0046] The CMMC-SST includes a total of two ports, namely a low-voltage DC port and a medium-voltage AC port. Each phase contains only one high-frequency transformer ( T 1) to achieve energy exchange and electrical isolation.

[0047] For the embedded module of the CMMC-SST topology mentioned above, a DC port can be led out through only one high-frequency transformer. Compared with the capacitor series voltage division structure adopted at the MVDC port, the number of high-frequency transformers can be greatly reduced, and the volume of the device can be reduced.

[0048] This application proposes a mixing modulation strategy for the above CMMC-SST topology. An embedded module is set between the upper and lower bridge arms of a single-phase line. By controlling the closing of the switching transistors of the embedded module, a first high-frequency pulse voltage is generated at the m 、 n ports of the embedded module, which serves as the input voltage of the primary side of the high-frequency transformer. In addition, opposite second high-frequency pulse voltages are generated on the upper and lower bridge arms respectively to cancel the influence of the high-frequency pulse voltage at the m 、 n ports on the circulating current. The specific analysis is as follows:

[0049] (1) Modulation principle of the intermediate embedded module

[0050] For the m 、 nThe port can generate U c 、 - U c two levels, and the specific switching modes are shown in Table 1.

[0051] Table 1 Switching Modes of Embedded Modules

[0052] ,

[0053] Note: "1" represents the device is closed; "0" represents the device is open

[0054] By controlling the switching 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 Figure 2 shown.

[0055] As can be seen from Table 1, according to the different conduction states of the switching tubes, the four working states of the embedded module are shown in Figure 3(a) - Figure 3(d):

[0056] The first working state (a): The first switching tube S 1, the fourth switching tube S 4, and the fifth switching tube S 5 are conducting, the second switching tube S 2, the third switching tube S 3, and the sixth switching tube S 6 are off, the DC capacitor is reversely connected, m 、 n the output voltage of the port u mn is - U c , m 、 x the output voltage of the port u mx is - U c , x 、 n the output voltage of the port u mx is 0.

[0057] The second working state (b): The first switching tube S 1, the fourth switching tube S 4, and the sixth switching tube S 6 are conducting, the second switching tube S 2, the third switching tube S 3, and the fifth switching tube S 5 are off, the DC capacitor is reversely connected, m 、 n the output voltage of the port umn is - U c , m and x the output voltage of the port u mx is 0. x and n the output voltage of the port u mx is - U c .

[0058] The third working state (c): The second switching tube S 2, the third switching tube S 3, the fifth switching tube S 5 is turned on, the first switching tube S 1, the fourth switching tube S 4, the sixth switching tube S 6 is turned off, the DC capacitor is positively connected, m and n the output voltage of the port u mn is U c , m and x the output voltage of the port u mx is 0, x and n the output voltage of the port u mx is U c .

[0059] The fourth working state (d): The second switching tube S 2, S 3, the sixth switching tube S 6 is turned on, the first switching tube S 1, the fourth switching tube S 4, the fifth switching tube S 5 is turned off, the DC capacitor is positively connected, m and n the output voltage of the port u mn is U c , m and x the output voltage of the port u mx is U c , x and n the output voltage of the port u mx is 0.

[0060] 2) Modulation principle of the upper and lower bridge arms

[0061] It can be seen from the analysis of the working state of the embedded module that m 、 n The high-frequency pulse voltage at the port will change the sum of the MMC DC voltages. If the upper and lower bridge arms are not controlled, this pulse voltage will be superimposed on the bridge arm inductance, increasing the system circulating current. In order to eliminate m 、 n The influence of the high-frequency pulse voltage at the port on the total voltage of the MMC bridge arm. The upper and lower bridge arms respectively need to output an additional reverse high-frequency voltage - u mx and - u xn , as shown in Figure 4 .

[0062] Figure 2 It can be known that m 、 n The high-frequency pulse voltage at the port can be decomposed into m 、 x ports and x 、 n ports two high-frequency voltages u mx and u xn , as shown in Figures 5(a)-Figure 5(b), Figures 6(a)-Figure 6(b). According to the working state of the intermediate embedded module, the working states of the upper and lower bridge arms can also be divided into the following two cases:

[0063] When the embedded module operates in the second working state (b) and the fourth working state (d), m 、 x The port equivalently outputs a pulse voltage of + U c level, x 、 n The port equivalently outputs a pulse voltage of - U c . When m 、 x The output voltage of the port is + U c , the upper bridge arm needs to invest 1 less sub-module; when x 、 n The output voltage of the port is - U c , the lower bridge arm needs to invest 1 more sub-module.

[0064] When the embedded module operates in the first working state (a) and the third working state (c), m 、 xThe port equivalently outputs a - U c pulse voltage of a level, x 、 n and the port equivalently outputs a + U c pulse voltage. When m 、 x the port output voltage is - U c , one more sub-module needs to be invested in the upper bridge arm; when x 、 n the port output voltage is + U c , one less sub-module needs to be invested in the lower bridge arm.

[0065] In summary, in the operating principle of the proposed MMC-SST, the intermediate embedded module and the upper and lower bridge arm sub-modules cooperate with each other, which can eliminate the influence of the high-frequency pulse voltage at the m 、 n ports of the intermediate embedded module on the arm voltage.

[0066] In summary, the CMMC-SST topology provided by this application reduces the number of high-frequency transformers of the traditional solid-state transformer to only 1 per phase by introducing an embedded module, greatly reducing the volume of the solid-state transformer; in addition, since the high-frequency pulse voltage output by the embedded module will not deteriorate the arm circulating current.

[0067] The above are only the preferred embodiments of this application and are not used to limit this application. For those skilled in the art, this application can have various changes and modifications. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of this application shall be included within the protection scope of this application.

Claims

1. A hybrid AC / DC solid-state transformer based on MMC embedded modules, characterized in that, Comprising: 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 each single-phase line includes 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, a second switch, a third switch, a fourth switch, a fifth switch, and a sixth switch, as well as a DC capacitor and a high-frequency transformer. A node formed by connecting the collector node of the first switch, the emitter node of the second switch, one end of the primary winding side of the high-frequency transformer, and one end of the upper bridge arm inductor is m. A node formed by connecting the emitter node of the fourth switch, the collector node of the third switch, the other end of the primary side winding of the high-frequency transformer, and one end of the lower bridge arm inductor is n. The emitter nodes of the first switch, the third switch, and the sixth switch are connected to the negative terminal of the DC capacitor. The collector nodes of the second switch, the fourth switch, and the fifth switch are connected to the positive terminal of the DC capacitor. One end of the secondary side winding of the high-frequency transformer is connected to the three-phase DC / AC converter, and one end of the secondary side winding of the high-frequency transformer is connected to the low-voltage medium-current node; Both the upper bridge arm and the lower bridge arm 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 sub-module 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 sub-module in the lower bridge arm is connected to the other end of the lower bridge arm inductor; The positive and negative terminals 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 and the sixth switch included in the embedded modules on the three single-phase lines.

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

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

4. The method according to claim 3, characterized in that, When the first high-frequency voltage pulses are respectively generated at the upper and lower ports of the embedded module, it includes a first working state, a second working state, a third working state, and a fourth working state; Among them, the first working state is that 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 connected, m and n the output voltage of the u mn port is - U c , m and x the output voltage of the u mx port is - U c , x and n the output voltage of the u mx port is 0.

5. The method according to claim 4, wherein The second working state is that 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 connected, m and n the output voltage of port u mn is - U c , m and x the output voltage of port u mx is 0, x and n the output voltage of port u mx is - U c .

6. The method according to claim 4, wherein The third working state is that the second switch transistor, the third switch transistor, and the fifth switch transistor are turned on, while the first switch transistor, the fourth switch transistor, and the sixth switch transistor are turned off, and the DC capacitor is positively connected. m and n the output voltage of the u mn is U c , m and x the output voltage of the u mx is 0, x and n the output voltage of the u mx is U c .

7. The method according to claim 4, characterized in that, The fourth working state is that 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 connected. m , n The output voltage of the u mn is U c , m , x The output voltage of the u mx is U c , x , n The output voltage of the u mx is 0.

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