Power electronic transformer

Abstract

A power electronic transformer includes three modules, each module including a first sub-module; a second sub-module; the third sub-module comprises a first power conversion module which comprises N first power conversion units, and the input ends of the N first power conversion units are connected with the N output ends of the first sub-module in a one-to-one correspondence mode; the second power conversion module comprises M first power conversion units, and the input ends of the M first power conversion units are connected with the M output ends of the second submodule in a one-to-one correspondence mode. One terminal of a first input end of a first sub-module of the three modules is connected with three phases of a first three-phase alternating current in a one-to-one correspondence mode, and the other terminal of the first input end of the first sub-module of the three modules is connected with three phases of a second three-phase alternating current in a one-to-one correspondence mode. One terminal of the first input end of the second sub-module of the three modules is connected with three phases of a first three-phase alternating current in a one-to-one correspondence mode, the other terminal of the first input end of the second sub-module of the three modules is connected with a neutral point, and the output ends of the N first power conversion units and the output ends of the M first power conversion units are connected in parallel to form a direct-current bus used for supplying power to a load.

Description

Technical Field

[0001] This case involves the fields of flexible interconnection of AC and DC power grids in power systems and power electronics technology, specifically a power electronic transformer. Background Art

[0002] In recent years, the increasingly serious environmental crisis and shortage of fossil energy have promoted the pace of change in the energy consumption structure. New clean and renewable energy represented by wind energy and solar energy will gradually replace fossil energy as the dominant energy. Under the background of energy structure transformation and new infrastructure layout, not only distributed power sources such as photovoltaics, wind power, and energy storage will be connected to the power grid on a large scale, but also high-power DC loads such as electric vehicle supercharging stations and data centers will be widely connected to the distribution network, which will have a profound impact on the development trend of the future distribution network: 1) Traditional AC distribution networks are mainly one-way power supply, and the control ability of primary control equipment (on-load voltage regulators, capacitors, tie switches, etc.) is insufficient. With the widespread access of new sources and loads, the distribution network has problems such as poor operation flexibility and unbalanced feeder loads; 2) With the large-scale application of new source-load storage and supercharging station technologies, the distribution network faces a large-capacity development trend, and the single distribution network feeder capacity is difficult to support the access of ultra-large power DC sources and loads; 3) With the increasing development and construction of cities, facing the limitations of urban development and power supply corridors, it is difficult for distribution networks to achieve rapid capacity expansion and large-scale energy storage configuration, and it is difficult to face the high-speed electrification process.

[0003] To solve the above problems, multiple AC feeders are used to achieve interconnection and mutual assistance, which can make full use of the remaining capacity of the access feeders to realize the joint supply to low-voltage DC users. The low voltage is less than 1500V, and the real-time and rapid control of power electronic transformers can be used to realize power interaction between different feeders, solving the problem of reduced equipment and feeder utilization caused by dynamic imbalance in load distribution in space. At present, some scholars have proposed a multi-medium voltage AC port solid-state transformer and its control method. This scheme uses a small power single-phase converter to form a common DC bus, and then realizes the joint supply of the low-voltage DC bus by connecting a cascaded H-Bridge Power Electronics Transformer (CHB-PET). However, the common DC bus voltage of the series circuit formed in the 10KV AC distribution network reaches 2500V, and the rated voltage of the switching device is generally required to be higher than twice the DC bus voltage, while the existing conventional IGBT withstand voltage level is only about 600V-1700V. Therefore, this scheme has great limitations in device selection, and it is difficult to meet the low-cost and large-scale promotion and application of the distribution network, and it is difficult to apply to higher voltage distribution networks.

[0004] Therefore, it is necessary to propose a new solution that can not only solve the problem of the voltage resistance level of the switching device, but also maintain the solution that can combine multiple AC feeders to realize the joint supply of the low-voltage DC bus, where the low voltage is less than 1500V. Summary of the invention

[0005] The purpose of this section is to summarize some aspects of the embodiments of the case and briefly introduce some preferred embodiments. Some simplifications or omissions may be made in this section and the abstract and title of the application to avoid blurring the purpose of this section, the abstract and the title of the application, and such simplifications or omissions shall not be used to limit the scope of the case.

[0006] In view of the above existing problems, this case proposes a power electronic transformer, which solves the problem that the common DC bus voltage of the original multi-medium voltage AC port solid-state transformer is relatively large, resulting in insufficient device withstand voltage level, and can be applied to high-voltage distribution networks by increasing the cascade number of small-power second sub-modules in the multi-medium voltage AC port flexible interconnection module.

[0007] In order to solve the above technical problems, the present case provides the following technical solutions: a power electronic transformer, characterized in that it includes three modules, each of which includes: a first submodule, including a first input terminal and N output terminals, N is an integer greater than 0; a second submodule, including a first input terminal and M output terminals, M is an integer greater than 0; and a third submodule, including a first power conversion module and a second power conversion module, the first power conversion module includes N first power conversion units, the input terminals of the N first power conversion units are connected to the N output terminals of the first submodule in a one-to-one correspondence, and the second power conversion module includes M first A power conversion unit, the input ends of the M first power conversion units are connected to the M output ends of the second submodules in one-to-one correspondence, one of the terminals of the first input ends of the first submodules of the three modules are connected to the three phases of the first three-phase alternating current in one-to-one correspondence, and the other terminal is connected to the three phases of the second three-phase alternating current in one-to-one correspondence, one of the terminals of the first input ends of the second submodules of the three modules are connected to the three phases of the first three-phase alternating current in one-to-one correspondence, and the other terminal is connected to the neutral point, and the output ends of the N first power conversion units of the three modules and the output ends of the M first power conversion units are connected in parallel to form a DC bus for powering the load.

[0008] As a preferred solution of a power electronic transformer in this case, the first submodule includes N second power conversion units, the input ends of the N second power conversion units are connected in series to form the first input end of the first submodule, and the output ends of the N second power conversion units form N output ends of the first submodule.

[0009] As a preferred solution of a power electronic transformer in this case, the second submodule includes M second power conversion units, the input ends of the M second power conversion units are connected in series to form the first input end of the second submodule, and the output ends of the M second power conversion units form M output ends of the second submodule.

[0010] As a preferred solution of a power electronic transformer in this case, the first submodule also includes a second input end, P output ends and P second power conversion units, P is an integer greater than 0, the input ends of the P second power conversion units are connected in series to form the second input end of the first submodule, the output ends of the P second power conversion units form the P output ends of the first submodule, one of the terminals of the second input end of the first submodule of the three modules is respectively connected to the three phases of the third three-phase alternating current in a one-to-one correspondence, and the other terminal is respectively connected to the three phases of the first three-phase alternating current in a one-to-one correspondence, the first power conversion module also includes P first power conversion units, the input ends of the P first power conversion units are connected to the P output ends of the first submodule in a one-to-one correspondence, and the output ends of the P first power conversion units of the three modules are connected in parallel with the DC bus.

[0011] As a preferred solution of a power electronic transformer in this case, the first power conversion unit is an isolated DC-DC converter.

[0012] As a preferred solution of a power electronic transformer in this case, the first power conversion unit is an isolated bidirectional DC-DC converter.

[0013] As a preferred solution of a power electronic transformer in this case, the first power conversion unit is a dual active full-bridge DC-DC converter.

[0014] As a preferred solution of a power electronic transformer in this case, the first power conversion unit is a half-bridge DC-DC converter.

[0015] As a preferred solution of a power electronic transformer in this case, the second power conversion unit is a full-bridge converter.

[0016] As a preferred solution of a power electronic transformer in this case, the second submodule is a voltage source converter.

[0017] The beneficial effects of the present invention are as follows: the first submodule in the present invention provides a plurality of AC ports, realizes the interconnection between the various AC ports, and can perform decoupling control of the power between the feeders by adjusting the size of the series voltage source, realizes the interconnection and mutual assistance of the various AC ports, and is conducive to the high efficiency, multi-port flexible expansion, flexible control, and control decoupling operation of the distribution network; the DC side of each submodule of the second submodule and the first submodule in the present invention is connected to one end of the third submodule, and the other end of the third submodule is connected in parallel to form a low-voltage DC bus, realizes the joint supply to the low-voltage DC side, and improves the operation reliability of the low-voltage DC distribution network, and the low voltage is less than 1500V; the first submodule in the present invention adopts a modular design, and the expansion of the AC port can be realized by increasing the number of SCHBs, and the rapidity and economy of the expansion of the AC port are realized; the SCHB in the present invention adopts a low-power cascade full-bridge design, which reduces the DC side voltage level, so that the device can choose a lower withstand voltage level, which is conducive to more economical and safe operation of the distribution network; the SCHB in the present invention can be applied to a distribution network with a higher voltage level by adding a cascade full bridge. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] In order to more clearly illustrate the technical solutions of the embodiments of this case, the following briefly introduces the drawings required for describing the embodiments. Obviously, the drawings described below are only some embodiments of this case. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative labor. Among them:

[0019] Figure 1 A schematic diagram of the structure of a power electronic transformer provided in the first embodiment of the present invention;

[0020] Figure 2 A schematic diagram showing a typical topology example of a first power conversion unit and a second power conversion unit of a power electronic transformer provided in the first embodiment of the present invention;

[0021] Figure 3 A schematic diagram of the structure of a power electronic transformer provided in the second embodiment of the present invention;

[0022] Figure 4 for Figure 3 The control strategy block diagram of the power electronic transformer shown;

[0023] Figure 5 A schematic diagram of the structure of a power electronic transformer provided in the third embodiment of the present invention;

[0024] Figure 6 The power waveform diagram of the power electronic transformer provided for this case for power flow control of each feeder and joint power supply to the low-voltage DC bus under three working conditions;

[0025] Figure 7 The current waveform diagram of feeder 1 of the power electronic transformer provided for this case under three working conditions;

[0026] Figure 8 The current waveform diagram of feeder 2 of the power electronic transformer provided for this case under three working conditions;

[0027] Fig. 9 Current waveform diagram of feeder 3 of the power electronic transformer provided for this case under three working conditions;

[0028] Fig.10 The voltage waveform of the SCHB submodule capacitor of feeder 2 under three working conditions of the power electronic transformer provided for this case;

[0029] Fig.11 The capacitor voltage waveform of the SCHB submodule of feeder 3 under three working conditions of the power electronic transformer provided for this case;

[0030] Fig.12 The capacitor voltage waveform diagram of the CHB submodule under three working conditions of the power electronic transformer provided for this case;

[0031] Fig.13 The low voltage DC bus capacitor voltage waveform diagram of the power electronic transformer provided for this case under three working conditions;

[0032] Fig.14 Schematic diagram of the AC / DC hybrid distribution network system with multiple medium voltage AC ports interconnected with the power electronic transformer provided for this case. DETAILED DESCRIPTION

[0033] In order to make the above-mentioned purposes, features and advantages of this case more obvious and easy to understand, the specific implementation methods of this case are described in detail below in conjunction with the drawings of the specification. Obviously, the described embodiments are part of the embodiments of this case, not all of them. Based on the embodiments in this case, all other embodiments obtained by ordinary persons in this field without creative work should fall within the scope of protection of this case.

[0034] In the following description, many specific details are set forth to facilitate a full understanding of the present case, but the present case may also be implemented in other ways different from those described herein, and those skilled in the art may make similar generalizations without violating the connotation of the present case. Therefore, the present case is not limited to the specific embodiments disclosed below.

[0035] Secondly, the term "one embodiment" or "embodiment" as used herein refers to a specific feature, structure or characteristic that may be included in at least one implementation of the present invention. The term "in one embodiment" that appears in different places in this specification does not necessarily refer to the same embodiment, nor does it refer to a separate or selective embodiment that is mutually exclusive with other embodiments.

[0036] This case is described in detail with reference to the schematic diagram. When describing the embodiments of this case in detail, for the sake of convenience, the cross-sectional diagrams showing the device structure will not be partially enlarged according to the general scale, and the schematic diagrams are only examples, which should not limit the scope of protection of this case. In addition, in actual production, the three-dimensional dimensions of length, width and depth should be included.

[0037] At the same time, in the description of this case, it should be noted that the directions or positional relationships indicated by the terms "upper, lower, inner and outer" are based on the directions or positional relationships shown in the drawings, and are only for the convenience of describing this case and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific direction, be constructed and operated in a specific direction, and therefore cannot be understood as a limitation of this case. In addition, the terms "first, second or third" are only used for descriptive purposes and cannot be understood as indicating or implying relative importance.

[0038] In this case, unless otherwise clearly specified and limited, the terms "install, connect, connect" should be understood in a broad sense, for example: it can be a fixed connection, a detachable connection or an integral connection; it can also be a mechanical connection, an electrical connection or a direct connection, or it can be indirectly connected through an intermediate medium, or it can be the internal connection of two components. For ordinary technicians in this field, the specific meanings of the above terms in this case can be understood according to the specific circumstances.

[0039] The technical solution of this case is further explained below in conjunction with the accompanying drawings, but it is not limited to this. Any modification or equivalent replacement of the technical solution of this case that does not depart from the spirit and scope of the technical solution of this case should be within the protection scope of this case.

[0040] Figure 1 This is a schematic diagram of the structure of the power electronic transformer 10 provided in the first embodiment of the present invention. Figure 1 As shown, the power electronic transformer 10 includes three modules, corresponding to phase A, phase B and phase C respectively, and each of the three modules includes three submodules, namely a first submodule 11, a second submodule 12 and a third submodule. The first submodule 11 includes a first input terminal and N output terminals, where N is an integer greater than 0; the second submodule 12 includes a first input terminal and M output terminals, where M is an integer greater than 0; and the third submodule includes a first power conversion module 131 and a second power conversion module 132.

[0041] like Figure 1As shown, the first power conversion module 131 includes N first power conversion units, and the input ends of the N first power conversion units are connected to the N output ends of the first submodule 11 in a one-to-one correspondence. The second power conversion module 132 includes M first power conversion units, and the input ends of the M first power conversion units are connected to the M output ends of the second submodule 12 in a one-to-one correspondence. One of the terminals P1 of the first input end of the first submodule 11 is connected to the three phases of the first three-phase alternating current in a one-to-one correspondence, and the other terminal P2 is connected to the three phases of the second three-phase alternating current in a one-to-one correspondence. Figure 1 , the first three-phase alternating current corresponds to feeder 1, and the second alternating current corresponds to feeder 2. One of the terminals P1' of the first input end of the second submodule 12 is connected to the three phases of the first three-phase alternating current one by one, and the other terminal P2' is connected to the neutral point. The output ends of the N first power conversion units and the output ends of the M first power conversion units are connected in parallel to form a low-voltage DC bus for powering the load.

[0042] Continue to refer to Figure 1 The first submodule 11 includes N second power conversion units, the input ends of the N second power conversion units are connected in series to form a first input end of the first submodule 11, and the output ends of the N second power conversion units form N output ends of the first submodule 11. The second submodule 12 includes M second power conversion units, the input ends of the M second power conversion units are connected in series to form a first input end of the second submodule 12, and the output ends of the M second power conversion units form M output ends of the second submodule 12.

[0043] Figure 2 The first embodiment of the present invention provides a schematic diagram of a typical topology of the first power conversion unit and the second power conversion unit of the power electronic transformer 10. Figure 2 As shown, the first power conversion unit is an isolated DC-DC converter with a medium-high frequency isolation transformer in the middle. Furthermore, the first power conversion unit is an isolated bidirectional DC-DC converter, and its topology is a dual-active full-bridge DC-DC converter or a half-bridge DC-DC converter. The second power conversion unit is a full-bridge converter, wherein the second submodule 12 formed by M cascaded full-bridge converters is a voltage source converter. Figure 1 and Figure 2 Each second power conversion unit of the first submodule 11 is connected to the low-voltage DC bus through a first power conversion unit, and each second power conversion unit of the second submodule 12 is connected to the low-voltage DC bus through a first power conversion unit.

[0044] Figure 3 This is a schematic diagram of the structure of the power electronic transformer 20 provided in the second embodiment of the present invention. Figure 3As shown, the power electronic transformer 20 includes three modules, corresponding to phase A, phase B and phase C respectively, and each of the three modules includes three submodules, namely a first submodule 21, a second submodule 22 and a third submodule. The first submodule 21 includes a first input terminal and N output terminals, where N is an integer greater than 0; the second submodule 22 includes a first input terminal and M output terminals, where M is an integer greater than 0; and the third submodule includes a first power conversion module 231 and a second power conversion module 232.

[0045] The first power conversion module 231 includes N first power conversion units, and the input ends of the N first power conversion units are connected to the N output ends of the first submodule 21 in a one-to-one correspondence. The second power conversion module 232 includes M first power conversion units, and the input ends of the M first power conversion units are connected to the M output ends of the second submodule 22 in a one-to-one correspondence. One of the terminals P1 of the first input end of the first submodule 21 is connected to the feeder 1 in a one-to-one correspondence, and the other terminal P2 is connected to the feeder 2 in a one-to-one correspondence. One of the terminals P1' of the first input end of the second submodule 22 is connected to the feeder 1 in a one-to-one correspondence, and the other terminal P2' is connected to the neutral point. The output ends of the N first power conversion units and the output ends of the M first power conversion units are connected in parallel to form a low-voltage DC bus for powering the load. The power electronic transformer 20 can realize the device topology and system connection of two feeder power flow control and low-voltage DC joint power supply.

[0046] The first submodule 21 includes N second power conversion units, the input ends of the N second power conversion units are connected in series to form a first input end of the first submodule 21, and the output ends of the N second power conversion units form N output ends of the first submodule 21. The second submodule 22 includes M second power conversion units, the input ends of the M second power conversion units are connected in series to form a first input end of the second submodule 22, and the output ends of the M second power conversion units form M output ends of the second submodule 22. In this embodiment, the first power conversion unit is an isolated bidirectional DC-DC converter (Isolated Bidirectional DC-DC Converter, hereinafter referred to as IBDC), the second power conversion unit is a full-bridge converter (Full Bridge Sub-Module, hereinafter referred to as FBSM), the first sub-module 21 is formed by cascading N full-bridge converters FBSM, and the cascaded N full-bridge converters FBSM included in the first sub-module 21 are referred to as SCHB (Series Cascaded H-Bridge) hereinafter, and the second sub-module 22 is formed by cascading M full-bridge converters FBSM, and the second sub-module 12 is referred to as CHB (Cascaded H-Bridge) hereinafter.

[0047] To meet the energy balance at the system level, the power electronic transformer satisfies the following power equation:

[0048]

[0049] Among them, P k is the active power expression of the feeder k of the power electronic transformer, 1≤k≤l, l≥2, P L is the low voltage DC port power of the power electronic transformer.

[0050] Furthermore, combined with Figure 1 The power electronic transformer 10 shown has a power equation that satisfies:

[0051]

[0052] Among them, P 1 is the active power expression of the feeder 1 of the power electronic transformer, P k is the active power expression of the feeder k of the power electronic transformer, 2≤k≤l, l≥2, P L is the low voltage DC port power of the power electronic transformer, P loss is the active power loss of the system.

[0053] The power electronic transformer 20 provided in this case also includes a controller (not shown) for executing a control method to realize active control of active power and reactive power on the feeder by controlling the equivalent voltage AC component connected in series in the feeder. The control method includes a power flow control loop, a cascade full-bridge control loop, a SCHB voltage balance control loop and a low-voltage DC bus voltage balance control loop.

[0054] Figure 4 The second embodiment of the present invention provides a dual-port power electronic transformer topology structure in which the second power conversion unit of the power electronic transformer adopts a full-bridge topology and the first power conversion unit adopts a dual-active full-bridge DC converter topology, and a block diagram of a control strategy for interconnecting two AC feeders. Figure 4 As shown, the control strategy of the power electronic transformer includes a power flow control loop, a cascade full-bridge control loop, a SCHB voltage balance control loop, and a low-voltage DC bus voltage balance control loop.

[0055] The power flow control loop calculates the d-axis component reference value of the power flow control feeder current based on the active power reference value and reactive power reference value of the power flow control feeder. and q-axis component reference value The calculation method is as follows:

[0056]

[0057] Among them, P l* Indicates the active power reference value, Indicates the reactive power reference value.

[0058] Furthermore, the feeder power flow control loop is performed in the dq coordinate system. When the feeder power is controlled, the feeder d-axis current and the q-axis current are coupled with each other, and are also affected by the disturbances of the feeder node voltage and the feeder 1 node voltage. Efforts should be made to eliminate these adverse effects on the control effect.

[0059] Furthermore, the proportional-integral controller is used for control, and the calculation method is as follows:

[0060]

[0061] in, and Multiply it by the Park inverse transformation matrix to obtain the feeder power flow control output reference voltage in the abc coordinate system Right now v 1 is the AC voltage on feeder 1, i 1 is the AC current on feeder 1, v l is the AC voltage on feeder line l, i l is the AC voltage on the feeder l, ω represents the AC frequency of the feeder, L represents the equivalent inductance of the feeder, subscript d represents the d-axis component, subscript q represents the q-axis component, superscript * represents the reference value, k p is the proportional link gain coefficient of the proportional-integral controller, k i is the integral link gain coefficient of the proportional-integral controller.

[0062] Preferably, the cascade full-bridge control loop includes a voltage outer loop, a reactive outer loop and a current inner loop. The voltage outer loop controls the sum of the capacitor voltages of the second submodule, inputs the deviation between its actual value and the reference value into the PI regulator, calculates the d-axis current of CHB, and then subtracts it from the d-axis current of feeder 2 to obtain the reference value of the current inner loop. The reactive outer loop is used to adjust the reactive power of feeder 1, and its controller output is the reference value of the current inner loop. The calculation method is as follows:

[0063]

[0064] in Indicates the reactive power reference value of feeder 1, v 1d Represents the d-axis component of the AC voltage on feeder 1.

[0065] Furthermore, the d-axis and q-axis current sampling values ​​of feeder 1 and the d-axis and q-axis current reference values ​​of feeder 1 are input into the PI regulator, and the current coupling compensation term ωL is added.1 L 1q ,ωL 1 L 1d To achieve current decoupling control, thus obtaining the CHB output voltage dq axis reference value Using the Parker inverse transformation matrix, the dq-abc coordinate system transformation is performed, and the reference value of the CHB output voltage in the abc coordinate system is obtained as follows: and The calculation method is as follows:

[0066]

[0067] Furthermore, after obtaining the CHB output voltage reference value, the CHB each phase submodule output voltage reference value can be calculated by the following formula:

[0068]

[0069] When the IBDC is a dual active full-bridge converter, the SCHB voltage balance control loop and the low voltage DC bus voltage balance control loop include:

[0070] The low-voltage DC bus capacitor voltage control adopts single-phase shift (SPS) modulation. The IBDC control target is to adjust the active power flowing into the low-voltage DC bus capacitor to maintain its capacitor voltage V LVDC stability.

[0071] Furthermore, the SCHB capacitor voltage control adopts single phase shift (SPS) modulation, and the control target of the third submodule is to adjust the active power flowing out of the capacitor of the first submodule in series, so as to maintain its capacitor voltage V SCHB stability.

[0072] Figure 5 The structure of the power electronic transformer 30 provided in the third embodiment of this case. Figure 5 As shown, the device topology and system connection of three feeder power flow control and low-voltage DC combined power supply are realized using a power electronic transformer 30. In this embodiment, the power electronic transformer 30 includes three modules, corresponding to phase A, phase B and phase C respectively, and the three modules each include three submodules, namely a first submodule 31, a second submodule 32 and a third submodule. The first submodule 31 includes N second power conversion units, and the second submodule 32 includes M second power conversion units. The third submodule includes a first power conversion module 331 and a second power conversion module 332. The first power conversion module 331 includes N first power conversion units, and the second power conversion module 331 includes M first power conversion units. The structure and connection relationship of the second submodule 32 and the third submodule are basically the same as those of the second embodiment.

[0073] In this embodiment, the first submodule 31 includes two SCHBs, namely SCHB-1 and SCHB-2, wherein SCHB-1 has the same structure as the SCHB in the second embodiment. SCHB-2 includes a second input terminal, P output terminals and P second power conversion units, P is an integer greater than 0, the second power conversion unit is a full-bridge converter (FBSM), the input terminals of the P second power conversion units are connected in series to form the second input terminal of the first submodule 31, the output terminals of the P second power conversion units form the P output terminals of the first submodule 31, one of the terminals P4 of the second input terminal of the first submodule 31 is respectively connected to the three phases of the third three-phase alternating current, the third three-phase alternating current corresponds to feeder 3, and the other terminal P3 is respectively connected to feeder 1. In other implementations, the first submodule of the power electronic transformer may also include more SCHBs to realize the device topology and system connection of multiple feeder power flow control and low-voltage DC combined power supply, which is not limited to this case.

[0074] In the third embodiment, the first power conversion module 331 also includes P first power conversion units, the input ends of the P first power conversion units are connected one-to-one to the P output ends of the first sub-module 31, and the output ends of the P first power conversion units are connected in parallel with the low-voltage DC bus.

[0075] The control strategy of the power electronic transformer 30 of this embodiment is the same as that of the second embodiment. Figure 4 .

[0076] The application of the structure and method in the above embodiment is further explained below in conjunction with specific simulation examples.

[0077] In combination with the above embodiment, MATLAB / Simulink software is used to perform simulation verification on the system, and the simulation parameters are shown in Table 1.

[0078] Table 1: Power electronic transformer simulation parameters

[0079] parameter value parameter value Feeder capacity 3MVA Low voltage DC bus voltage 750V Medium voltage AC feeder voltage 10kV SCHB submodule capacitor 1.6mF Feeder equivalent inductance 10.61mH CHB submodule capacitor 1.6mF CHB submodule voltage 800V Low voltage DC bus capacitor 4mF CHB cascade quantity 12 SCHB module switching frequency 3kHz SCHB submodule voltage 800V CHB module switching frequency 3kHz SCHB cascade quantity 3 IBDC switching frequency 3kHz

[0080] The power electronic transformer 30 is connected to three feeders, and its connection diagram is shown in FIG. Figure 5 , the control method of the simulation example is referenced Figure 4. The power electronic transformer 30 includes two SCHBs to control the active power and reactive power on the feeders (feeder 2, feeder 3). Feeder 1 is a balanced feeder that can balance the active power of other feeders and the low-voltage DC bus load, and the corresponding control loop is a feeder flow control loop. The reactive power of feeder 1 is controlled by the second submodule 32, and the capacitor voltage balance of the second submodule 32 is controlled by the amplitude and phase of the second submodule 32, and the corresponding control loop is a cascade full-bridge control loop. The first power conversion unit connected to each second power conversion unit of the SCHB controls the capacitor voltage balance of the SCHB, and the corresponding control loop is the SCHB voltage balance control loop. The first power conversion unit connected to each second power conversion unit of the second submodule 32 controls the capacitor voltage balance of the low-voltage DC bus, and the corresponding control loop is the low-voltage DC bus voltage balance control loop.

[0081] In order to verify the active power flow control capability of the hybrid transformer, three operating conditions were set in the simulation.

[0082] Working condition 1: Feeder 1 neither emits nor absorbs reactive power, feeder 2 emits 0.3pu active power and neither emits nor absorbs reactive power, feeder 3 emits 0.3pu active power and neither emits nor absorbs reactive power, and the low-voltage DC bus absorbs 0.9pu.

[0083] Working condition 2: Feeder 1 neither emits nor absorbs reactive power, feeder 2 emits 0.3pu active power and absorbs 0.1pu reactive power, feeder 3 emits 0.4pu active power, neither emits nor absorbs reactive power, and the low-voltage DC bus absorbs 0.9pu.

[0084] Working condition 3: Feeder 1 neither emits nor absorbs reactive power, feeder 2 emits 0.3pu active power and absorbs 0.3pu reactive power, feeder 3 emits 0.4pu active power and 0.1pu reactive power, and the low-voltage DC bus absorbs 0.9pu.

[0085] Figure 6 The simulation results of the power flow control of each feeder under three working conditions of the power electronic transformer and the joint power supply of the low-voltage DC bus include the active power P of feeder 1. 1 Waveform, reactive power Q of feeder 1 1 Waveform, active power P of feeder 2 2 Waveform, reactive power Q of feeder 2 2 Waveform, active power P of feeder 3 3 Waveform, reactive power Q of feeder 3 3 Waveform diagram and low voltage DC bus P LVDC Active power waveform. Figure 7 Figure 2 is the current waveform of feeder 1 of the power electronic transformer under three working conditions. Figure 8 The current waveforms of feeder 2 of the power electronic transformer under three working conditions. Fig. 9 Graphs showing the current waveforms of feeder 3 of the power electronic transformer under three operating conditions. Fig.10 The waveforms of the capacitor voltage in the SCHB submodule of feeder 2 of the power electronic transformer under three working conditions are shown. Fig.11 The waveform diagram of the capacitor voltage in the SCHB submodule of feeder 3 of the power electronic transformer under three working conditions. Fig.12 The waveforms of capacitor voltage in the CHB submodule of the power electronic transformer under three working conditions. Fig.13 The waveform diagram of the low-voltage DC bus capacitor voltage of the power electronic transformer under three working conditions.

[0086] The simulation waveform results show that when the power electronic transformer is connected to three transmission lines, it can not only realize the active power flow control of decoupling active power and reactive power on each feeder, as well as the ability to jointly supply power to low-voltage DC, but also maintain the internal energy balance of the device, that is, the capacitor voltage is stable, and has the ability to expand ports.

[0087] Fig.14 The schematic diagram of the AC / DC hybrid distribution network system provided for this case with multiple medium voltage AC ports interconnected with power electronic transformers is as follows: Fig.14 As shown, the power electronic transformer provides multiple AC ports, which correspond to AC line 1-AC feeder 1 respectively, realizing the interconnection between various AC ports. One end of the isolated DC-DC converter is connected in parallel to form a low-voltage DC feeder, realizing the joint supply to the low-voltage DC side. High-power DC loads such as charging stations, photovoltaics, and base stations can be connected to the distribution network, improving the operating reliability of the low-voltage DC distribution network.

[0088] The first submodule of the power electronic transformer in this case provides multiple AC ports, realizes the interconnection between each AC port, and can decouple the power between feeders by adjusting the size of the series voltage source, realizes the interconnection and mutual assistance of each AC port, which is conducive to the high efficiency, flexible expansion of multiple ports, flexible control, and control decoupling of the distribution network; the DC side of each second power conversion unit of the second submodule and the first submodule in this case is connected to one end of the isolated DC-DC converter, and the other ends of the isolated DC-DC converters are connected in parallel to form a low-voltage DC bus, realizing the joint supply of the low-voltage DC side and improving the operation reliability of the low-voltage DC distribution network; the first submodule in this case adopts a modular design, and the expansion of the AC port can be realized by increasing the number of SCHBs, realizing the rapidity and economy of the expansion of the AC port; the SCHB in this case adopts a small-power cascade full-bridge design, which reduces the DC side voltage level, so that the device can choose a lower withstand voltage level, which is conducive to the more economical and safe operation of the distribution network; the SCHB in this case can be applied to distribution networks with higher voltage levels by adding cascade full bridges.

[0089] Those skilled in the art will appreciate that the embodiments of the present application can be provided as methods, systems, or computer program products. Therefore, the present application can adopt the form of complete hardware embodiments, complete software embodiments, or embodiments in combination with software and hardware. Moreover, the present application can adopt the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) that contain computer-usable program code. The scheme in the embodiments of the present application can be implemented in various computer languages, for example, object-oriented programming language Java and literal scripting language JavaScript, etc.

[0090] The present application is described with reference to the flowcharts and / or block diagrams of the methods, devices (systems), and computer program products according to the embodiments of the present application. It should be understood that each process and / or box in the flowchart and / or block diagram, as well as the combination of the processes and / or boxes in the flowchart and / or block diagram, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing device to generate a machine, so that the instructions executed by the processor of the computer or other programmable data processing device generate instructions for implementing the processes in the flowchart and / or block diagram. Figure 1 A process or multiple processes and / or boxes Figure 1 A device that provides the functions specified in a block or multiple blocks.

[0091] These computer program instructions may also be stored in a computer-readable memory capable of directing a computer or other programmable data processing device to operate in a specific manner, so that the instructions stored in the computer-readable memory produce an article of manufacture comprising an instruction device, which implements the process Figure 1 A process or multiple processes and / or boxes Figure 1 A function specified in one or more boxes.

[0092] These computer program instructions can also be loaded onto a computer or other programmable data processing device so that a series of operating steps are executed on the computer or other programmable device to produce a computer-implemented process, thereby providing instructions for implementing the process. Figure 1 A process or multiple processes and / or boxes Figure 1 The steps for the functions specified in one or more boxes.

[0093] Although the preferred embodiments of the present application have been described, those skilled in the art may make other changes and modifications to these embodiments once they have learned the basic creative concept. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments and all changes and modifications falling within the scope of the present application.

[0094] Obviously, those skilled in the art can make various changes and modifications to the present application without departing from the spirit and scope of the present application. Thus, if these modifications and variations of the present application fall within the scope of the claims of the present application and their equivalents, the present application is also intended to include these modifications and variations.

Claims

1. A power electronic transformer, characterized in that, it includes three modules, and the three modules all include: A first sub-module, including a first input terminal and N output terminals, where N is an integer greater than 0; A second sub-module, including a first input terminal and M output terminals, where M is an integer greater than 0; and A third sub-module, including a first power conversion module and a second power conversion module, The first power conversion module includes N first power conversion units, and the input terminals of the N first power conversion units are connected to the N output terminals of the first sub-module in one-to-one correspondence, The second power conversion module includes M first power conversion units, and the input terminals of the M first power conversion units are connected to the M output terminals of the second sub-module in one-to-one correspondence, One of the terminals of the first input terminal of the first sub-module of the three modules is respectively connected to the three phases of the first three-phase alternating current in one-to-one correspondence, and the other terminal is respectively connected to the three phases of the second three-phase alternating current in one-to-one correspondence, One of the terminals of the first input terminal of the second sub-module of the three modules is respectively connected to the three phases of the first three-phase alternating current in one-to-one correspondence, and the other terminal is connected to the neutral point, The output terminals of the N first power conversion units and the output terminals of the M first power conversion units of the three modules are connected in parallel to form a DC bus for supplying power to the load.

2. The power electronic transformer according to claim 1, characterized in that, The first sub-module includes N second power conversion units, the input terminals of the N second power conversion units are connected in series to form the first input terminal of the first sub-module, and the output terminals of the N second power conversion units form the N output terminals of the first sub-module.

3. The power electronic transformer according to claim 1, characterized in that, The second sub-module includes M second power conversion units, the input terminals of the M second power conversion units are connected in series to form the first input terminal of the second sub-module, and the output terminals of the M second power conversion units form the M output terminals of the second sub-module.

4. The power electronic transformer according to claim 1, characterized in that, The first sub-module further includes a second input terminal, P output terminals and P second power conversion units, where P is an integer greater than 0, the input terminals of the P second power conversion units are connected in series to form the second input terminal of the first sub-module, and the output terminals of the P second power conversion units form the P output terminals of the first sub-module, One of the terminals of the second input terminal of the first sub-module of the three modules is respectively connected to the three phases of the third three-phase alternating current in one-to-one correspondence, and the other terminal is respectively connected to the three phases of the first three-phase alternating current in one-to-one correspondence, The first power conversion module further includes P first power conversion units, and the input terminals of the P first power conversion units are connected to the P output terminals of the first sub-module in one-to-one correspondence, The output terminals of the P first power conversion units of the three modules are connected in parallel with the DC bus.

5. The power electronic transformer according to claim 1, characterized in that, The first power conversion unit is an isolated DC-DC converter.

6. The power electronic transformer according to claim 5, It is characterized in that The first power conversion unit is an isolated bidirectional DC-DC converter.

7. The power electronic transformer according to claim 1, It is characterized in that The first power conversion unit is a dual active full-bridge DC-DC converter.

8. The power electronic transformer according to claim 1, It is characterized in that The first power conversion unit is a half-bridge DC-DC converter.

9. The power electronic transformer according to any one of claims 2 to 4, It is characterized in that The second power conversion unit is a full-bridge converter.

10. The power electronic transformer according to claim 1, It is characterized in that The second submodule is a voltage source converter.

Images