Topological structure and method of single-bridge-arm power electronic transformer

Abstract

The invention discloses an improved single-bridge arm power electronic transformer topological structure, which belongs to the field of power electronic converters and comprises a three-phase alternating current filter inductor Lg, a phase main bridge arm circuit, a three-phase common half-bridge circuit, a high-frequency series inductor capacitor L1 / C1, a high-frequency isolation transformer, a low-voltage side rectifier and a common isolation DC-DC converter. The topological structure belongs to a single-stage structure, each phase only needs a string of bridge arms, alternating-current and direct-current side power transmission is realized by directly adopting hybrid frequency conversion, the number of devices of a power electronic transformer can be reduced, and the cost is reduced; a three-phase common half-bridge circuit is introduced, so that the multi-frequency coupling degree of a bridge arm is reduced, and control is simplified; and the reasonable circuit parameter design is combined, the number of the main bridge arm sub-modules is optimized, and the efficiency can be improved. The topology is suitable for all application occasions needing conversion from medium-voltage alternating current to low-voltage direct current.

Description

Technical Field

[0001] The invention belongs to the field of power electronic converters, and in particular relates to a single-bridge-arm power electronic transformer topology structure and method. Background Art

[0002] "Intelligence", "cleanliness" and "power electronics" will be the inevitable development trends of future power grid systems. However, traditional power frequency transformers are no longer sufficient to meet the needs and challenges of future power grids due to their inherent defects such as fixed voltage-frequency conversion, inflexible adjustment methods, large size and weight, poor anti-disturbance and harmonic performance. As a new type of power electronic device, power electronic transformers have rich functions such as adjustable voltage conversion, reactive power control, harmonic elimination, unbalanced operation, etc., and can greatly reduce volume and weight. They are a powerful choice to replace traditional power frequency transformers. Power electronic transformers can be divided into AC (alternating current)-AC (alternating current) conversion type, AC (alternating current)-DC (direct current) conversion type and DC-DC conversion type according to the type of input and output ports. The present invention focuses on the AC-DC type. The input AC port is connected to the medium and high voltage AC power grid, and the output DC port is connected to the low voltage DC bus. It is suitable for occasions such as data center power supply, new energy grid connection, super charging station, AC / DC hybrid distribution network, etc. that require DC connection.

[0003] The existing AC-DC power electronic transformer topology is mainly divided into two-stage and single-stage structures. The two-stage structure usually adopts a series H-bridge or modular multi-level structure. This type of topology often has a large number of transformation stages, a large number of modules (number of devices), and a large number of passive devices such as capacitors, inductors, and high-frequency transformers, which greatly increases the cost and volume of the device. The single-stage topology can effectively reduce the number of devices by reducing the power conversion link, but the device efficiency is often lost, and the control complexity and implementation complexity are increased. Summary of the invention

[0004] In view of the above problems, the present invention proposes a single-arm power electronic transformer topology structure and method, the purpose of which is to reduce the number of components of the power electronic transformer, reduce costs, and simplify control by improving the existing power electronic transformer topology structure. Combined with appropriate circuit parameter design, efficiency is improved. This topology structure is suitable for all applications that require medium voltage AC to low voltage DC conversion.

[0005] In order to achieve the above purpose, the present invention adopts the following technical means: In a first aspect, the present invention provides a single-arm power electronic transformer topology structure, comprising: a three-phase AC filter inductor L g , phase main bridge arm circuit, three-phase common half-bridge circuit, high-frequency series inductor and capacitor L1 / C1, high-frequency isolation transformer, low-voltage side rectifier and common isolation DC-DC converter; The three-phase AC filter inductor Lg is used to filter out high-frequency harmonics in the three-phase AC current; The phase main bridge arm circuit adopts hybrid frequency conversion to convert three-phase AC power into high-frequency AC power; The three-phase common half-bridge circuit is used to assist the main bridge arm circuit of the phase to convert voltage and current; The high-frequency series inductor and capacitor L1 / C1 are used to filter out the power frequency fundamental wave and high-frequency PWM harmonics generated by the phase main bridge arm circuit and the three-phase common half-bridge circuit; High-frequency isolation transformer, used to achieve electrical isolation and voltage conversion, and transmit high-frequency AC power from the primary side to the secondary side; The low-voltage side rectifier is located on the secondary side of the high-frequency isolation transformer and is used to convert high-frequency AC power into low-voltage DC power, transmit most of the power of the circuit, and maintain the bus voltage stable; The public isolated DC-DC converter is used to supplement the transmission of the remaining power and maintain the voltage stability at both ends.

[0006] As a further improvement of the present invention, the phase main bridge arm circuit is composed of N sub-modules connected in series, N is a positive integer greater than or equal to 1; the sub-module is a full-bridge circuit composed of four switching tubes and a DC side capacitor, the upper and lower switching tubes are complementary and turned on, and the midpoints of the two half bridges are respectively connected to external ports Q1 / Q2; wherein the Q2 port of one sub-module is connected to the Q1 port of another sub-module, and they are connected in series in sequence to form the phase main bridge arm.

[0007] As a further improvement of the present invention, the main bridge arms of the three phases a, b and c have the same structure, the main bridge arm of the phase a leads to upper and lower ports Ma1 / Ma2, the main bridge arm of the phase b leads to upper and lower ports Mb1 / Mb2, and the main bridge arm of the phase c leads to upper and lower ports Mc1 / Mc2.

[0008] As a further improvement of the present invention, the upper port of the phase main bridge arm is connected to the three-phase medium voltage AC power grid through a filter inductor: for phase a, port Ma1 is connected to phase a power grid va through a filter inductor Lg; for phase b, port Mb1 is connected to phase b power grid vb through a filter inductor Lg; for phase c, port Mc1 is connected to phase c power grid vc through a filter inductor Lg; The upper port of the phase main bridge arm is also connected to the high-frequency isolation transformer through a high-frequency series inductor and capacitor: For phase a, port M a1 Connect the high-frequency isolation transformer T through the high-frequency series inductor and capacitor L1 / C1 a port; for phase b, port M b1 Connect the high-frequency isolation transformer T through the high-frequency series inductor and capacitor L1 / C1 b port; for phase c, port M c1Connect the high-frequency isolation transformer T through the high-frequency series inductor and capacitor L1 / C1 c port.

[0009] As a further improvement of the present invention, the three-phase common half-bridge circuit includes three half-bridges composed of six switch tubes, and the three half-bridges have a common DC side port; the midpoints of the three half-bridges are respectively connected to the M of the three-phase main bridge arms a, b, and c. a2 / M b2 / M c2 The common DC side port is connected to the input port P of the common isolated DC-DC converter. com / N com connected.

[0010] As a further improvement of the present invention, the common isolated DC-DC converter is a DC-DC converter with isolation and bidirectional power flow functions; The common isolated DC-DC converter is selected from: a dual active bridge converter, a CLLC resonant converter, and all other DC-DC converters that meet the isolation and bidirectional power flow functions.

[0011] As a further improvement of the present invention, the primary side of the high-frequency isolation transformer includes ports Ta, Tb, Tc and Tcom, and the secondary side includes ports S1~S i , i is a positive integer greater than 1; The primary side Tcom port is the common port of the three primary windings, which is connected to the DC side Ncom port of the three-phase common half-bridge circuit; the secondary side S1~S i The port is connected to the low voltage side rectifier.

[0012] As a further improvement of the present invention, the high-frequency isolation transformer is selected from three independent two-winding transformers, a three-input three-output six-winding transformer or a three-input single-output four-winding transformer.

[0013] As a further improvement of the present invention, the low-voltage side rectifier structure may have a three-full-bridge DC side parallel structure and a single full-bridge structure according to the number of secondary side ports of the high-frequency isolation transformer.

[0014] In a second aspect, the present invention provides a working method of a single-arm power electronic transformer topology structure, characterized by comprising: The three-phase AC power source filters out high-frequency harmonics through the three-phase AC filter inductor Lg and enters the phase main bridge arm circuit. The phase main bridge arm circuit and the three-phase common half-bridge circuit convert the three-phase AC power into high-frequency AC power through mixed frequency conversion control, and transmits energy through the resonant circuit composed of high-frequency series inductor and capacitor L1 / C1; The high-frequency AC power is electrically isolated and transmitted through a high-frequency isolation transformer, and the secondary side generates corresponding high-frequency AC power; The low-voltage side rectifier converts high-frequency AC power into low-voltage DC power, undertakes the main power transmission and controls the stable bus voltage; the public isolated DC-DC converter connects the three-phase public half-bridge DC side with the low-voltage DC bus, transmits the remaining small part of the power, and controls and maintains the voltage stability at both ends.

[0015] The present invention has the following technical effects: In the power electronic transformer of the present invention, the three-phase AC power source enters the phase main bridge arm circuit through the three-phase AC filter inductor Lg. The phase main bridge arm circuit and the three-phase common half-bridge circuit convert the three-phase AC into high-frequency AC through mixed frequency conversion, and transmit energy through the resonant circuit composed of high-frequency series inductor capacitor L1 / C1. The high-frequency AC is electrically isolated and transmitted through a high-frequency isolation transformer, and the secondary side generates corresponding high-frequency AC. The low-voltage side rectifier converts the high-frequency AC into low-voltage DC, transmits most of the power, maintains the stability of the low-voltage DC bus, and supplements the remaining power in combination with the public isolation DC-DC converter to further ensure the stability of the system. The transmission efficiency of the power electronic transformer is improved through the high-frequency resonant circuit and the optimized control strategy. The number of conversion stages is reduced, the power density is improved, and the entire system is more compact and lightweight. The high-frequency isolation transformer realizes electrical isolation between input and output, and improves the safety of the system. Different high-frequency isolation transformers and low-voltage side rectifier structures can be selected according to specific application requirements to meet different conversion requirements. The present invention belongs to a single-stage structure, only one bridge arm is required for each phase, the number of devices is small, and it has cost advantages. At the same time, by introducing the three-phase common half-bridge module, the multi-frequency coupling degree of the bridge arm is reduced, simplifying the difficulty of control implementation. Potentially, if combined with optimized circuit parameter design, the efficiency of this topology can be further improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0017] Figure 1 An improved single-arm power electronic transformer topology diagram; Figure 2 Submodule SM structure diagram; Figure 3 Public isolated DC-DC converter: dual active bridge converter; Figure 4 Public isolated DC-DC converter: CLLC resonant converter; Figure 5 High frequency isolation transformer structure 1; Figure 6 High frequency isolation transformer structure 2; Figure 7 High frequency isolation transformer structure 3; Figure 8 Low voltage side rectifier structure 1: three full bridges in parallel on the DC side; Fig. 9 Low voltage side rectifier structure 2: single full bridge; Fig.10 Specific embodiments of the present invention; Fig.11 Steady-state operating waveform of a specific embodiment of the present invention. DETAILED DESCRIPTION

[0018] In order to make the purpose and technical solution of the present invention clearer and easier to understand, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. The specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0019] The technical solution of the present invention will be described clearly and completely below in conjunction with the accompanying drawings and specific embodiments. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of the present invention.

[0020] In order to promote the better application of power electronic transformers, it is urgent to provide a topological structure with low cost, easy control operation and high efficiency. The present invention proposes an improved single-arm power electronic transformer topology, which is suitable for all applications requiring medium voltage AC to low voltage DC conversion. Figure 1 As shown, including three-phase AC filter inductor L g , phase main bridge arm circuit, three-phase common half-bridge circuit, high-frequency series inductor and capacitor L1 / C1, high-frequency isolation transformer, low-voltage side rectifier and common isolation DC-DC converter.

[0021] Three-phase AC filter inductor Lg: used to filter out high-frequency harmonics in the three-phase AC current and ensure the quality of the grid current at the input end of the power electronic transformer.

[0022] Phase main bridge arm circuit: Each phase is equipped with a main bridge arm circuit, which, together with the three-phase common half-bridge circuit, is used to convert the three-phase AC into high-frequency AC. These main bridge arm circuits are usually composed of multiple switching devices (such as IGBTs or MOSFETs), and the switching devices are controlled by a mixed frequency conversion strategy to achieve voltage and current conversion.

[0023] Three-phase common half-bridge circuit: This is a key improvement point. The traditional single-stage power electronic transformer bridge arm contains a variety of high and low frequency components, which makes the module balance control complicated. The present invention introduces a three-phase common half-bridge circuit to share the frequency coupling of the main bridge arm, simplify the control, and improve the reliability of the system. The common half-bridge circuit is also composed of switching devices, and works in conjunction with the phase main bridge arm circuit to achieve voltage and current conversion.

[0024] High-frequency series inductor and capacitor L1 / C1: These components form a bandpass filter to filter out the industrial frequency fundamental wave and high-frequency PWM harmonics generated by the phase main bridge arm circuit and the three-phase common half-bridge circuit, ensuring that the current flowing into the high-frequency transformer is only the set high-frequency AC power.

[0025] High-frequency isolation transformer: This is one of the core components of power electronic transformers. It is used to achieve electrical isolation and voltage conversion, and transmit high-frequency AC from the primary side to the secondary side. High-frequency isolation transformers have the advantages of small size and light weight, and are the key to achieving higher power density for power electronic transformers than industrial frequency transformers.

[0026] Low-voltage side rectifier: Located on the secondary side of the high-frequency isolation transformer, it is used to convert high-frequency AC power into low-voltage DC power. The rectifier consists of active switching tubes (such as MOSFET) and converts AC power into DC power through rectification.

[0027] Public isolation DC-DC converter: used to connect the three-phase public half-bridge DC side with the low-voltage DC bus to stabilize the voltage at both ends. There are multiple optional topologies. The public isolation DC-DC converter also has electrical isolation function to ensure the safety and stability of the system.

[0028] Furthermore, the phase main bridge arm circuit is composed of N sub-modules (SM) connected in series, where N is a positive integer greater than or equal to 1. The sub-module structure is as follows Figure 2 As shown in the figure, it is a full-bridge circuit composed of four switch tubes and a DC side capacitor. The upper and lower switch tubes are complementary and turned on, and the midpoints of the two half-bridges lead to external ports Q1 / Q2 respectively. The Q2 port of one submodule is connected to the Q1 port of another submodule, and in this way, they are connected in series to form the main bridge arm of each phase. The main bridge arms of phases a, b, and c have the same structure. Finally, the main bridge arm of phase a leads to the upper and lower ports M a1 / M a2 , the main bridge arm of phase b leads to the upper and lower ports M b1 / M b2 , c-phase main bridge arm leads to upper and lower ports M c1 / M c2 .

[0029] In the above scheme, the phase main bridge arm circuit proposed in the present invention is designed to be composed of N sub-modules (SM) connected in series, which provides higher flexibility and scalability for the power electronic transformer. Each sub-module is composed of four switching tubes (such as IGBT or MOSFET) and a DC side capacitor to form a full-bridge circuit. The upper and lower switching tubes are complementary and turned on to ensure the stability and efficiency of the circuit. The midpoints of the two half bridges lead to external ports Q1 and Q2 respectively, which are used for series connection between sub-modules. The Q2 port of one sub-module is connected to the Q1 port of another sub-module to form a series chain. In this way, the phase main bridge arm can be composed of any number of sub-modules as needed, which improves the flexibility of the design.

[0030] Furthermore, the structures of the three-phase main bridge arms a, b, and c are the same, and are all composed of N submodules in series. Each phase main bridge arm leads to upper and lower ports (such as M a1 / M a2 、M b1 / M b2 、M c1 / M c2 ), which is used for connection with subsequent circuits. The modular design makes the manufacture and maintenance of power electronic transformers easier. The capacity and performance of the transformer can be adjusted by increasing or decreasing the number of submodules to meet different application requirements. Since the submodules are connected in series, the capacity of the phase main bridge arm can be easily expanded. This enables the power electronic transformer to adapt to power systems of different sizes and improves its scope of application. The modular design also provides redundancy. When a submodule fails, it can be bypassed or replaced without affecting the operation of the entire system. This improves the reliability and stability of the system and reduces the risk of power outages caused by failures.

[0031] Specifically, the upper port of the phase main bridge arm is connected to the three-phase medium voltage AC power grid through a filter inductor: for phase a, port M a1 Connect the a-phase grid va through the filter inductor Lg; for the b-phase, port M b1 The b-phase grid vb is connected via the filtering inductor Lg; for the c-phase, the port Mc1 is connected to the c-phase grid vc via the filtering inductor Lg.

[0032] At the same time, the upper port of the phase main bridge arm is also connected to the high-frequency isolation transformer through a high-frequency series inductor and capacitor: For phase a, port M a1 Connect the high-frequency isolation transformer T through the high-frequency series inductor and capacitor L1 / C1 a port; for phase b, port M b1 Connect the high-frequency isolation transformer T through the high-frequency series inductor and capacitor L1 / C1 b port; for phase c, port M c1 Connect the high-frequency isolation transformer T through the high-frequency series inductor and capacitor L1 / C1c port.

[0033] Specifically, the three-phase common half-bridge circuit includes three half-bridges composed of six switch tubes, and the three half-bridges have a common DC side port. The midpoints of the three half-bridges are respectively connected to the M of the abc three-phase main bridge arm. a2 / M b2 / M c2 The common DC side port is connected to the input port P of the common isolated DC-DC converter. com / N com connected.

[0034] The circuit consists of six switching tubes, divided into three half-bridges, each half-bridge contains two switching tubes. The three half-bridges share a common DC side port, namely P com and N com The midpoint of each half bridge is respectively connected to the M of the abc three-phase main bridge arm. a2 、M b2 、M c2 The common isolation DC-DC converter is a power electronic device with isolation and bidirectional power flow functions. Its input port P com and N com It is connected to the common DC side port of the three-phase common half-bridge circuit. The output ports DC+ and DC- are used to provide a stable DC voltage or current to meet the needs of subsequent circuits or loads.

[0035] Specifically, the common isolated DC-DC converter can be any DC-DC converter with isolation and bidirectional power flow functions, such as a dual active bridge converter, a CLLC resonant converter, and the structures are as follows: Figure 3 , Figure 4 The input port of the common isolated DC-DC converter is P com and N com , the output ports are DC+ and DC-. Among them, Figure 3 Public isolated DC-DC converter: dual active bridge converter; Figure 4 Common isolated DC-DC converter: CLLC resonant converter.

[0036] In the above scheme, the three-phase common half-bridge circuit and the common isolated DC-DC converter only need to process a small part of the power of the whole system, and the required device capacity is small, the loss is small, and the cost is low. . . At the same time, the high efficiency of the common isolated DC-DC converter also ensures the high efficiency of the whole system. The bidirectional power flow function of the common isolated DC-DC converter enables the system to flexibly adapt to different load requirements and working modes.

[0037] Specifically, the primary side of the high-frequency isolation transformer includes ports Ta, Tb, Tc and Tcom, and the secondary side includes ports S1~S i , i is a positive integer greater than 1, and the value of i depends on the specific form of the high-frequency isolation transformer. The high-frequency isolation transformer can be three independent two-winding transformers, a three-input three-output six-winding transformer, and a three-input single-output four-winding transformer, respectively. Figure 5 , Figure 6 , Figure 7 As shown. Original side T com The port is the common port of the three primary windings, which is connected to the DC side N of the three-phase common half-bridge circuit. com The secondary side S1~S i The port is connected to the low voltage side rectifier. Figure 5 High frequency isolation transformer structure 1; Figure 6 High frequency isolation transformer structure 2; Figure 7 High frequency isolation transformer structure3.

[0038] As a specific solution, the low-voltage side rectifier structure can have a three-full-bridge DC side parallel structure and a single full-bridge structure according to the number of secondary side ports of the high-frequency isolation transformer. The former can be adapted to high-frequency transformer structure 1 and structure 2, such as Figure 8 As shown; the latter can be adapted to the high-frequency transformer structure 3, such as Fig. 9 As shown in the figure, the output ports of the low voltage side rectifier are DC+ and DC-.

[0039] As an example, the structure of the low-voltage side rectifier will also vary depending on the number of secondary ports of the high-frequency isolation transformer. Three full-bridge DC side parallel structure: Applicable to high-frequency isolation transformers with multiple secondary ports, which can ensure that each port can effectively output power. Single full-bridge structure: Applicable to high-frequency isolation transformers with only one secondary port, with a simpler structure.

[0040] It is particularly noted that the switch tube in the embodiment of the present invention includes all fully-controlled active switch devices, such as metal-oxide semiconductor field effect transistors (MOSFETs), insulated gate bipolar transistors (IGBTs), etc.

[0041] Among them, the high-frequency isolation transformer is a device that transmits high-frequency voltage and current. It is suitable for high-frequency electronic equipment and switching power supplies. Due to its high operating frequency, its volume can be greatly reduced. According to different application requirements, the number of windings and the connection method will also be different. Three independent two-winding transformers: Each winding has independent input and output ports, and the transformer is easy to design and manufacture. Three-input three-output six-winding transformer: Multiple windings share a common magnetic core, the structure is more compact, but the processing is more complicated. Three-input single-output four-winding transformer: Multiple input windings jointly drive one output winding. In these structures, the primary side of the T com The N terminal is the common terminal of the three windings and is usually connected to the DC side of the three-phase common half-bridge circuit. com The secondary side S1~S i The port is connected to the low voltage side rectifier.

[0042] As an example, the high efficiency of high-frequency isolation transformers makes them suitable for various high-power electronic devices and switching power supplies. Electrical isolation between input and output is achieved through electromagnetic induction, which improves the safety of the circuit. Different winding numbers and connection methods can meet various complex circuit requirements.

[0043] Finally, all DC+ ports including the common isolated DC-DC converter and the low-voltage side rectifier are connected to form a positive low-voltage DC bus, and all DC- ports are connected to form a negative low-voltage DC bus, and the output DC voltage v Ldc . Figure 8 Low voltage side rectifier structure 1: three full bridges in parallel on the DC side; Fig. 9 Low-voltage side rectifier structure 2: single full-bridge.

[0044] The single-arm power electronic transformer topology structure of the present invention comprises the following steps: Input stage: The three-phase AC power supply is filtered by the three-phase AC filter inductor Lg and then enters the phase main bridge arm circuit.

[0045] High-frequency conversion stage: The phase main bridge arm circuit and the three-phase common half-bridge circuit convert the three-phase AC into high-frequency AC through mixed frequency conversion control, and transmit energy through the resonant circuit composed of high-frequency series inductor and capacitor L1 / C1.

[0046] Isolation and transmission stage: High-frequency AC power is electrically isolated and transmitted through a high-frequency isolation transformer, and the secondary side generates corresponding high-frequency AC power.

[0047] Rectification and output stage: The low-voltage side rectifier converts high-frequency AC power into low-voltage DC power, controls and stabilizes the DC bus voltage, and combines with a public isolated DC-DC converter to ensure the stability of the DC side of the three-phase public half-bridge circuit.

[0048] In order to verify the feasibility of the present invention, the proposed topological structure is further described here in conjunction with a specific embodiment. The topological structure of the specific embodiment is as follows: Fig.10 As shown, the three-phase main bridge arm is composed of N full-bridge sub-modules connected in series. The upper port of the main bridge arm is connected to the three-phase AC grid through the inductor Lg, and the lower port is connected to the three-phase common half-bridge. At the same time, the upper port is also connected to the high-frequency transformer through the inductor capacitor L1C1. The high-frequency isolation transformer adopts three independent two-winding transformers. The secondary side of the transformer is connected to the low-voltage side rectifier. The low-voltage side rectifier adopts a three-full-bridge DC side parallel structure. The DC side of the three-phase common half-bridge circuit is connected to the common isolation DC-DC converter. The common isolation DC-DC converter adopts a CLLC resonant converter, and the resonant converter works at the resonance point. All low-voltage side rectifier outputs and the CLLC resonant converter output are connected in parallel to form a low-voltage DC bus.

[0049] Table 1 Circuit parameters of the embodiment

[0050] Table 1 shows the specific circuit parameters of this embodiment: the three-phase AC side is connected to a 10 kV grid, and the rated power is set to 1 MVA; each phase main bridge arm is composed of 11 submodules, and the rated voltage of each submodule is set to 1.1 kV, and the equivalent switching frequency of the AC side is 20 kHz; the AC filter inductance is 10 mH; the low-voltage DC side outputs a 1 kV voltage, and the low-voltage side switching frequency is 10 kHz; the high-frequency transformer ratio is 3.3:1. Based on the above parameters, the present invention uses Matlab / Simulink to build a simulation of the power electronic transformer, and obtains the steady-state operation waveform of this embodiment, as shown in FIG. Fig.11 As shown, Fig.11 Steady-state operating waveform of a specific embodiment of the present invention.

[0051] Fig.11 From top to bottom, they are three-phase AC grid voltage, three-phase AC grid current, three-phase main bridge arm submodule voltage, low-voltage DC bus voltage, and low-voltage DC bus current. As can be seen from the figure, the topology can stably realize power transmission on the AC and DC sides, ensure the good sinusoidal characteristics of the three-phase AC current, and have low harmonic content. At the same time, the submodule voltage of each phase main bridge arm can be stabilized at the rated voltage of 1.1kV, the low-voltage DC bus voltage is stabilized at 1kV, and the low-voltage DC bus current is 1kA.

[0052] Based on the above introduction, the improved single-arm power electronic transformer topology proposed in the present invention has multiple advantages such as high efficiency, high power density, electrical isolation and flexibility, and can be widely used in various occasions requiring medium-voltage AC to low-voltage DC conversion. This topology is suitable for all applications requiring medium-voltage AC to low-voltage DC conversion, such as smart grids, electric vehicle charging stations, data centers, etc.

[0053] Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work should fall within the scope of protection of the present invention.

[0054] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, ordinary technicians in the relevant field should understand that the specific implementation methods of the present invention can still be modified or replaced by equivalents. Any modification or equivalent replacement that does not depart from the spirit and scope of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A single-arm power electronic transformer topology structure, characterized in that: include: Three-phase AC filter inductor L g , phase main bridge arm circuit, three-phase common half-bridge circuit, high-frequency series inductor and capacitor L1 / C1, high-frequency isolation transformer, low-voltage side rectifier and common isolation DC-DC converter; The three-phase AC filter inductor Lg is used to filter out high-frequency harmonics in the three-phase AC current; The phase main bridge arm circuit is used to convert the three-phase alternating current into high-frequency alternating current by adopting hybrid frequency conversion; The three-phase common half-bridge circuit is used to assist the main bridge arm circuit of the phase to convert voltage and current; The high-frequency series inductor and capacitor L1 / C1 are used to filter out the power frequency fundamental wave and high-frequency PWM harmonics generated by the phase main bridge arm circuit and the three-phase common half-bridge circuit; High-frequency isolation transformer, used to achieve electrical isolation and voltage conversion, and transmit high-frequency AC power from the primary side to the secondary side; The low-voltage side rectifier is located on the secondary side of the high-frequency isolation transformer and is used to convert high-frequency AC power into low-voltage DC power, transmitting most of the power of the circuit; The common isolated DC-DC converter is used to supplement and transmit the remaining power.

2. A single-arm power electronic transformer topology structure according to claim 1, characterized in that: The phase main bridge arm circuit is composed of N sub-modules connected in series, where N is a positive integer greater than or equal to 1; the sub-module is a full-bridge circuit composed of four switching tubes and a DC side capacitor, the upper and lower switching tubes are complementary and turned on, and the midpoints of the two half-bridges are respectively connected to external ports Q1 / Q2; wherein the Q2 port of one sub-module is connected to the Q1 port of another sub-module, and they are connected in series in sequence to form the phase main bridge arm.

3. The single-arm power electronic transformer topology structure according to claim 1, characterized in that: The main bridge arms of the three phases a, b and c have the same structure. The main bridge arm of phase a leads to upper and lower ports Ma1 / Ma2, the main bridge arm of phase b leads to upper and lower ports Mb1 / Mb2, and the main bridge arm of phase c leads to upper and lower ports Mc1 / Mc2.

4. The single-arm power electronic transformer topology structure according to claim 3 is characterized in that: The ports on the phase main bridge arm are connected to the three-phase medium voltage AC power grid via a filter inductor: for phase a, port Ma1 is connected to phase a power grid va via a filter inductor Lg; for phase b, port Mb1 is connected to phase b power grid vb via a filter inductor Lg; for phase c, port Mc1 is connected to phase c power grid vc via a filter inductor Lg; The upper port of the phase main bridge arm is also connected to the high-frequency isolation transformer through a high-frequency series inductor and capacitor: For phase a, port M a1 Connect the high-frequency isolation transformer T through the high-frequency series inductor and capacitor L1 / C1 a port; for phase b, port M b1 Connect the high-frequency isolation transformer T through the high-frequency series inductor and capacitor L1 / C1 b port; for phase c, port M c1 Connect the high-frequency isolation transformer T through the high-frequency series inductor and capacitor L1 / C1 c port.

5. The single-arm power electronic transformer topology structure according to claim 3 is characterized in that: The three-phase common half-bridge circuit includes three half-bridges composed of six switch tubes, and the three half-bridges have a common DC side port; the midpoints of the three half-bridges are respectively connected to the M of the three-phase main bridge arms a, b, and c. a2 / M b2 / M c2 The common DC side port is connected to the input port P of the common isolated DC-DC converter. com / N com connected.

6. The single-arm power electronic transformer topology structure according to claim 1, characterized in that: The common isolated DC-DC converter is a DC-DC converter with isolation and bidirectional power flow functions; The common isolated DC-DC converter is selected from: a dual active bridge converter and a CLLC resonant converter.

7. The single-arm power electronic transformer topology structure according to claim 1, characterized in that: The primary side of the high-frequency isolation transformer includes ports Ta, Tb, Tc and Tcom, and the secondary side includes ports S1~S i , i is a positive integer greater than 1; The primary side Tcom port is the common port of the three primary windings, which is connected to the DC side Ncom port of the three-phase common half-bridge circuit; the secondary side S1~S i The port is connected to the low voltage side rectifier.

8. The single-arm power electronic transformer topology structure according to claim 1, characterized in that: The high frequency isolation transformer is selected from three mutually independent two-winding transformers, a three-input three-output six-winding transformer or a three-input single-output four-winding transformer.

9. The single-arm power electronic transformer topology structure according to claim 1, characterized in that: The low-voltage side rectifier structure may have a three-full-bridge DC side parallel structure or a single full-bridge structure according to the number of secondary side ports of the high-frequency isolation transformer.

10. A working method of a single-arm power electronic transformer topology structure, characterized in that: include: The three-phase AC power source filters out high-frequency harmonic current through the three-phase AC filter inductor Lg and enters the phase main bridge arm circuit. The phase main bridge arm circuit and the three-phase common half-bridge circuit convert the three-phase AC power into high-frequency AC power through mixed frequency conversion control, and transmits energy through the resonant circuit composed of high-frequency series inductor and capacitor L1 / C1; The high-frequency AC is electrically isolated and transmitted through a high-frequency isolation transformer, and the secondary side generates corresponding high-frequency AC; The low-voltage side rectifier converts high-frequency AC power into low-voltage DC power, performs power transmission and controls the voltage stability of the low-voltage DC bus; the public isolated DC-DC converter connects the three-phase public half-bridge DC side with the low-voltage DC bus, transmits the remaining small part of the power, and maintains the voltage stability at both ends.

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