Three-phase non-isolated cascaded power electronic transformer with zero load

Abstract

The application discloses a three-phase non-isolated cascade type power electronic transformer with no-load, and belongs to the technical field of transformers. Each phase input is connected to a neutral point through a cascade converter structure. The cascade converter structure comprises a plurality of rectification-inversion modules, a front stage of which is a cascade rectification structure and a rear stage of which is a cascade inversion structure. The cascade converter structure comprises at least one no-load module, which is arranged at the head end of the cascade converter structure. When the number of the rectification-inversion modules is three, the second rectification-inversion module is a three-bridge-arm structure, and the third rectification-inversion module is a two-bridge-arm structure. When the number of the rectification-inversion modules is greater than three, the rectification-inversion modules after the second one are all two-bridge-arm structures. The application solves the short-circuit path problem of the power electronic transformer with N points, and also makes the input and output realize common ground.

Description

Technical Field

[0001] This invention relates to the field of transformer technology, and more particularly to a three-phase common N-point non-isolated cascaded power electronic transformer with no-load operation. Background Technology

[0002] Power electronic transformers possess powerful frequency conversion and voltage regulation capabilities, making them particularly suitable for the control and speed regulation of high-voltage motors. Traditionally, power electronic transformers drive motors using three independent, isolated power electronic transformers. However, these transformers require high-frequency transformers to act as isolation structures, resulting in complex circuitry, numerous switching devices, and consequently, larger size, heavier weight, and higher cost.

[0003] The main solutions currently available in the industry to address this issue are:

[0004] 1. For example Figure 1 The three-phase non-isolated power electronic transformer shown uses a non-isolated cascaded module to replace the original isolated cascaded module with a high-frequency transformer. This significantly reduces the size and weight of the converter, reduces the number of energy transmission stages, and lowers the power loss on the converter.

[0005] 2. For example Figure 2 The three-phase non-isolated power electronic transformer shown has an added no-load module compared to a pure non-isolated power electronic transformer. The added no-load module can increase the upper limit of the converter's speed regulation mode to a certain extent, enabling higher voltage motor speed control.

[0006] However, the existing solution has the following problems:

[0007] 1. In actual operation, three-phase non-isolated power electronic transformers have many short-circuit paths of switching transistors that need to be avoided, resulting in many switching states being unavailable and limiting the operating range of the converter. In addition, new short-circuit paths will appear after N points of input and output across the three phases.

[0008] 2. A three-phase non-isolated power electronic transformer with an unloaded module increases the degree of freedom in the input and output port levels, which can greatly expand the operating range of the original non-isolated power electronic transformer. However, after the three-phase output reaches N points, new short-circuit paths will appear, and there are switching transistors that can be simplified compared to the original short-circuit paths.

[0009] Furthermore, for the converter to have N input and output points, the potential difference between the N input points and N output points of the three phases must be the same, meaning the potential difference between the N input points and N output points of the three phases is zero. However, due to the existence of short-circuit paths, a potential difference exists between the N input points and the negative terminals of the three output phases. The negative terminals of the three output phases with different potentials cannot be directly connected, thus preventing the achievement of a common N-point connection. In this case, a common-mode voltage will be generated when driving a three-phase motor, causing bearing current and damaging the motor. In addition, the existence of redundant switching paths leads to increased power loss. Summary of the Invention

[0010] The purpose of this invention is to overcome the problems existing in the prior art and to provide a three-phase common N-point non-isolated cascaded power electronic transformer with no-load operation.

[0011] The objective of this invention is achieved through the following technical solution:

[0012] A three-phase, N-point, non-isolated cascaded power electronic transformer with no-load capability is provided. Each phase input is connected to the neutral point through a cascaded converter structure. The cascaded converter structure includes multiple rectifier-inverter modules, with a cascaded rectifier structure at the front end and a cascaded inverter structure at the rear end. The positive terminal of the cascaded rectifier structure in the first-end rectifier-inverter module is connected to each phase input, and the positive terminal of the cascaded inverter structure in the first-end rectifier-inverter module is connected to a certain phase of the three-phase motor AC power.

[0013] The cascaded converter structure includes at least one unloaded module, which is located at the first end of the cascaded converter structure.

[0014] When the number of rectifier inverter modules is 3, the second rectifier inverter module has a three-bridge arm structure and the third rectifier inverter module has a two-bridge arm structure; when the number of rectifier inverter modules is greater than 3, the second and subsequent rectifier inverter modules all have a two-bridge arm structure.

[0015] In some embodiments, when the number of unloaded modules is greater than 1, all unloaded modules are located on the same side of the first end of the cascaded converter structure.

[0016] In some embodiments, the negative terminal of the cascaded inverter structure of each rectifier-inverter module is directly connected to the neutral point.

[0017] It should be further noted that the technical features corresponding to the above embodiments can be combined or substituted with each other to form new technical solutions without conflict.

[0018] Compared with the prior art, the beneficial effects of the present invention are:

[0019] 1. This invention provides a simplified three-phase, N-point, non-isolated cascaded power electronic transformer with no-load operation. Each phase input is connected to the neutral point via a cascaded converter structure. In this cascaded converter structure, the second rectifier-inverter module has a three-arm structure, while subsequent rectifier-inverter modules have two-arm structures, eliminating the bridge connection between subsequent modules. The negative terminal of the cascaded inverter structure of the second rectifier-inverter module is directly connected to the neutral point. This locks the output potential to the neutral point, discards redundant switches in some arms that are unusable, avoids potential differences between input and output caused by redundant switching states, ensures that the three-phase input and three-phase output share N points, and improves system reliability. It also solves the short-circuit path problem of the power electronic transformer after sharing N points and enables the input and output to share a common ground, reducing the possibility of short-circuit faults.

[0020] 2. This invention eliminates the switching transistors that could cause short circuits while maintaining the original operating range, further reducing the size and cost of the original power electronic transformer. It also standardizes the current flow path of the topology, thereby reducing conduction and switching losses on the switching transistors.

[0021] 3. This invention simplifies the modulation logic, enabling clearer selection of the voltage balancing vector for each module, clarifying the judgment conditions for the effect of the voltage balancing vector, and better balancing of unloaded modules. It also improves the topology's operational stability and its ability to handle unbalanced power loads. Attached Figure Description

[0022] Figure 1 It is a traditional three-phase non-isolated power electronic transformer;

[0023] Figure 2 It is a traditional three-phase non-isolated power electronic transformer with an unloaded module;

[0024] Figure 3 This is a schematic diagram of the topology of a three-module cascaded transformer with one unloaded module at the beginning of the present invention;

[0025] Figure 4 This is a schematic diagram of a four-module cascaded transformer topology with one unloaded module at the beginning of the present invention;

[0026] Figure 5 This is a schematic diagram of the N-module cascaded transformer topology with one unloaded module at the beginning of the present invention;

[0027] Figure 6 This is a schematic diagram of the N-point topology of the three-module cascaded transformer of the present invention. Detailed Implementation

[0028] The technical solution of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. The components of the embodiments of this application described and shown in the accompanying drawings can generally be arranged and designed in various different configurations. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0029] It should be noted that the defects in the solutions in the prior art are all the results of the inventors' practice and careful research. Therefore, the discovery process of the above problems and the solutions proposed by the embodiments of this application in the following text should be the inventors' contributions to this application in the process of invention and creation, and should not be understood as technical content known to those skilled in the art.

[0030] In one exemplary embodiment, a three-phase N-point non-isolated cascaded power electronic transformer with no-load is provided, wherein each phase input is connected to the neutral point N through a cascaded converter structure; the cascaded converter structure includes multiple rectifier-inverter modules, the front stage of which is a cascaded rectifier structure and the rear stage is a cascaded inverter structure; wherein, the positive terminal of the cascaded rectifier structure in the first-end rectifier-inverter module is connected to each phase input, and the positive terminal of the cascaded inverter structure in the first-end rectifier-inverter module is connected to a certain phase of the three-phase motor AC power;

[0031] The cascaded converter structure includes at least one unloaded module, which is located at the first end of the cascaded converter structure.

[0032] When the number of rectifier inverter modules is 3, the second rectifier inverter module is a three-bridge-arm structure and the third rectifier inverter module is a two-bridge-arm structure; when the number of rectifier inverter modules is greater than 3, the second and subsequent rectifier inverter modules are all two-bridge-arm structures, and the two-bridge-arm structures are all non-isolated back-to-back H-bridge structures.

[0033] Each phase input is connected to the cascaded converter structure via a reactor. When the number of unloaded modules is greater than one, all unloaded modules are located on the same side of the first end of the cascaded converter structure.

[0034] Furthermore, the negative terminal of the cascaded inverter structure of each rectifier-inverter module is directly connected to the neutral point. This locks the output potential to the neutral point, discards redundant switches in some bridge arms that are unusable in the switching state, avoids potential differences between the input and output caused by redundant switching states, ensures that the three-phase input and three-phase output share N points, and improves the reliability of the system.

[0035] In one example, such as Figure 3As shown, the cascaded converter structure includes three cascaded rectifier-inverter modules. The first rectifier-inverter module is an unloaded module, where the positive terminal of the rectifier structure is connected to phase A of the three-phase AC power, and the positive terminal of the inverter structure is connected to phase a of the three-phase AC power of the motor. The second rectifier-inverter module has a three-arm structure, and the third rectifier-inverter module has a two-arm structure, eliminating some of the arm structures. Figure 2 The structure reduces the number of three bridge arms and corresponding switching transistors in the cascaded converter.

[0036] In one example, such as Figure 4 As shown, the cascaded converter structure includes four cascaded rectifier-inverter modules. The first rectifier-inverter module is an unloaded module, the second rectifier-inverter module has a three-arm structure, and the third and fourth rectifier-inverter modules have a two-arm structure. Figure 2 The structure reduces the number of five bridge arms and corresponding switching transistors in the cascaded converter.

[0037] In one example, when an unloaded module needs to be added, simply add an unloaded module at the front end and connect them in series on the same side. When there are more than three modules, simply add two bridge arm structures below. Figure 5 As shown, the cascaded converter structure includes N cascaded rectifier-inverter modules. The first rectifier-inverter module is an unloaded module, the second rectifier-inverter module has a three-arm structure, and the third and subsequent rectifier-inverter modules all have a two-arm structure. Figure 2 The structure reduces the number of (2N-3) bridge arms and corresponding switching transistors in the cascaded converter.

[0038] In one example, such as Figure 6 As shown in the figure, the three-phase AC motor M includes , , The input sides of the three cascaded converters are connected to the three phases of the three-phase AC power supply (A, B, and C in the diagram). Taking a three-module example, the input ports of the cascaded converter structure are connected to one phase of the three-phase AC power supply and the neutral point N, respectively. The output ports are connected to one phase of the output three-phase AC motor and the upper N points, achieving a total of N input and output points. When there is 1 unloaded module and 3 cascaded rectifier-inverter modules, taking phase A as an example, the positive terminal of the single-phase input power supply is connected to the positive terminal A1P of the first module (unloaded module) of the cascaded converter structure via reactor L1. The negative terminal A1N of the rectifier structure of the first module is connected to the positive terminal A2P of the rectifier structure of the second module. The negative terminal A2N of the rectifier structure of the second module is connected to the positive terminal A3P of the rectifier structure of the third module. The negative terminal A3N of the rectifier structure of the third module is connected to the neutral point N.

[0039] The positive terminal B1P of the inverter structure in module 1 is connected to a phase of the three-phase motor. The negative terminal B1N of the inverter structure in module 1 is connected to the positive terminal B2P of the inverter structure in module 2, and finally outputs one of the three phases. The negative terminal of the input phase is connected to point N of the three input phases, causing the output potential to lock to the neutral point. The connection of the other two phases is similar to that of phase A, and will not be described in detail here. This invention reduces some of the switching transistors in the non-isolated topology module, reduces switching losses, and avoids the impact of redundant switching states on the three-phase N-point connection.

[0040] The above detailed embodiments are a description of the present invention. It should not be considered that the specific embodiments of the present invention are limited to these descriptions. For those skilled in the art, several simple deductions and substitutions can be made without departing from the concept of the present invention, and all of these should be considered to fall within the protection scope of the present invention.

Claims

1. A three-phase common N-point non-isolated cascaded power electronic transformer with no-load, characterized in that, Each phase input is connected to a neutral point through a cascaded converter structure; the cascaded converter structure comprises a plurality of rectifier-inverter modules, the front stage of which is a cascaded rectifier structure and the rear stage of which is a cascaded inverter structure; wherein the positive end of the cascaded rectifier structure in the first rectifier-inverter module is connected to the AC power of a certain phase, and the positive end of the cascaded inverter structure in the first rectifier-inverter module is connected to the AC power of a certain phase of the three-phase motor; the negative end of the cascaded inverter structure of each rectifier-inverter module is directly connected to the neutral point; The cascaded converter structure comprises at least one idle module, and the idle module is arranged at the front end of the cascaded converter structure. When the number of rectifier-inverter modules is 3, the second rectifier-inverter module is a three-bridge arm structure, and the third rectifier-inverter module is a two-bridge arm structure; when the number of rectifier-inverter modules is greater than 3, the rectifier-inverter modules after the second are all two-bridge arm structures, and the cross-bridge connection between the subsequent rectifier-inverter modules is cancelled, so that the potential of the output side is locked to the neutral point.

2. The three-phase common N-point non-isolated cascaded power electronic transformer with no-load according to claim 1, characterized in that, When the number of idle modules is greater than 1, the idle modules are all arranged on the same side of the front end of the cascaded converter structure.

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