Medium-voltage power electronic transformer

Abstract

The invention discloses a medium-voltage power electronic transformer, and relates to the technical field of power transformers, the transformer is based on a CHB topological structure and is composed of a plurality of input-series output-parallel power subunits, and each subunit comprises an AC / DC rectifier module and a dual-active full-bridge isolation conversion module and outputs low-voltage AC and DC ports at the same time. A fault current control circuit is arranged, so that a direct-current short-circuit fault can be quickly limited and broken, and line voltage can be adjusted; a modular design is adopted, and a central controller and a local controller are configured for cooperative control. The rated capacity of the transformer is 1MVA, the rated working condition efficiency exceeds 96%, and the fault current-limiting response time is less than 0.5 ms. Compared with a traditional transformer, renewable energy access conversion links can be reduced, flexible networking and interconnection and complementation of different types of energy sources are achieved, the energy utilization efficiency is effectively improved, the system control capacity is enhanced, and the transformer has wide application prospects in an alternating current and direct current hybrid system containing large-scale renewable energy sources.

Description

Technical Field

[0001] The present invention relates to the technical field of power transformers, and particularly to a medium-voltage power electronic transformer. Background Art

[0002] In recent years, distributed renewable energy in China has grown rapidly. The large-scale access of distributed renewable energy to the power grid poses new challenges and higher requirements for the flexible access and effective management and control of the system. Conventional iron-core transformers have many AC-DC conversion links in the current renewable energy access technology, which reduces the efficiency and affects the convenience of access. In addition, the lack of interconnection and mutual assistance and flexible regulation capabilities in the distribution network also restricts the full consumption and efficient utilization of distributed renewable energy, as specifically shown in the appendix Figure 1 as follows. Summary of the Invention

[0003] In view of the deficiencies of the prior art, the present invention provides a medium-voltage power electronic transformer, which solves the problems raised in the above background art.

[0004] To achieve the above objectives, the present invention is realized through the following technical solutions: A medium-voltage power electronic transformer, comprising: A cascaded structure based on the CHB topology, the cascaded structure comprising a plurality of power sub-units connected in series at the input and in parallel at the output; Each of the power sub-units includes an AC / DC rectification module and a dual-active full-bridge isolation conversion module connected in sequence; A low-voltage AC port and a DC port connected to the output end of the cascaded structure.

[0005] Further, the dual-active full-bridge isolation conversion module includes a high-frequency transformer, and the primary side and the secondary side of the high-frequency transformer are respectively connected with an H-bridge inverter.

[0006] Further, the medium-voltage power electronic transformer further includes: A fault current control circuit integrated in the DC port, the fault current control circuit including a solid-state switch, a current-limiting reactor and a dissipative resistor connected in series; A mechanical circuit breaker connected in parallel with the solid-state switch.

[0007] Further, the fault current control circuit further includes: A current sensor for detecting a DC short-circuit fault; A controller connected to the current sensor, the controller being configured to trigger the solid-state switch to turn off when a short-circuit fault is detected and trigger the mechanical circuit breaker to trip after a preset time.

[0008] Furthermore, the multiple power sub-units adopt a modular design, and each power sub-unit is encapsulated in an independent cabinet structure, which is configured with standard electrical interfaces and mechanical interfaces.

[0009] Furthermore, the low-voltage AC port includes a three-phase bridge inverter circuit, and the DC side of the three-phase bridge inverter circuit is connected to the DC output end of the cascade structure.

[0010] Furthermore, the AC / DC rectification module adopts a three-phase PWM rectification circuit, and the three-phase PWM rectification circuit is configured with a neutral-point clamping structure.

[0011] Furthermore, the medium-voltage power electronic transformer further includes: A central controller for coordinately controlling multiple power sub-units; A local controller configured for each power sub-unit, and the local controller is communicatively connected to the central controller.

[0012] Furthermore, the magnetic core of the high-frequency transformer is made of nanocrystalline alloy or ferrite material.

[0013] Furthermore, the low-voltage AC port and the DC port are respectively configured with filter circuits, and the filter circuits include LC filter structures or LCL filter structures.

[0014] The present invention provides a medium-voltage power electronic transformer. Compared with the prior art, it has the following beneficial effects: Through the cascade design based on the CHB topology structure, the integrated fault current control circuit, the modular construction and the cooperative control architecture, the medium-voltage power electronic transformer constructs a bidirectional multi-port medium-voltage power electronic transformer, which can flexibly integrate distributed renewable energy at multiple AC and DC voltage levels, reduce the power conversion links, improve the energy utilization efficiency, enhance the system control ability, realize the interconnection, complementarity and full consumption of different types of renewable energy, effectively solve the disadvantages of traditional transformers in renewable energy access, and show significant advantages of small volume, flexible regulation, convenient networking, high-efficiency and stable operation in the AC-DC hybrid system.

[0015] Combined with a power electronic converter and a high-frequency transformer, compared with traditional transformers, the PET has great advantages in reducing its own volume and weight, power regulation, etc., and has great application value in AC-DC hybrid systems containing large-scale renewable energy. Using a bidirectional multi-port power electronic transformer to construct an AC-DC hybrid system can achieve flexible networking, integrate distributed renewable energy at multiple AC-DC voltage levels, and achieve flexible and safe access; and reduce the conversion link, improve energy utilization efficiency, enhance system control capabilities, realize the interconnection and complementarity of different types of renewable energy in a larger range, fully absorb renewable energy, which is an important future development direction and has broad application prospects. Brief Description of the Drawings

[0016] Figure 1 It is a comparison diagram of the access system mode for distributed renewable energy; Figure 2 It is a topological structure diagram of the PET based on CHB; Figure 3 It is an internal structure diagram of the medium-voltage power electronic transformer; Figure 4 It is a framework diagram of the medium-voltage power electronic transformer; Figure 5 It is an inter-domain mutual assistance block diagram of the medium-voltage power electronic transformer. Detailed Embodiment

[0017] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0018] Please refer to Figure 2-5 , the present invention provides a technical solution: a medium-voltage power electronic transformer, including a topological structure. The medium-voltage power electronic transformer is based on the CHB topological structure, and the medium-voltage input stage and the intermediate isolation stage are composed of multiple power sub-units in a way of input series and output parallel. Each power sub-unit includes an AC / DC rectification module and a dual-active full-bridge isolation conversion module connected in sequence. Among them, the AC / DC rectification module adopts a three-phase PWM rectification circuit and is configured with a midpoint clamping structure for converting the input three-phase alternating current into direct current. The dual-active full-bridge isolation conversion module includes a high-frequency transformer. The primary side and the secondary side of the high-frequency transformer are respectively connected with an H-bridge inverter circuit. By controlling the switching state of the H-bridge inverter circuit, bidirectional power transmission and voltage conversion between the primary and secondary sides are realized. The high-frequency transformer uses a magnetic core made of nanocrystalline alloy or ferrite material to meet the magnetic performance requirements under high-frequency working conditions; Figure 2 As shown, for the medium-voltage input stage and the intermediate isolation stage of the CHB-based PET topology, multiple sub-units are connected in series at the input and in parallel at the output. Each sub-unit consists of an AC / DC converter and a dual-active full-bridge converter. The PET can output a low-voltage AC port and a DC port simultaneously. Since CHB has the advantages of being easy to modularize, highly scalable, and easy to implement redundant design, it is one of the most commonly used topologies for PET. Studying its control method is the key path to realizing the medium-voltage power electronic transformer.

[0019] Medium-voltage power electronic transformer fault current controller; In the AC-DC hybrid distributed renewable energy system, the DC network has low damping, large DC short-circuit fault current amplitude, and high rising rate. It is necessary to quickly limit and interrupt the fault current. At the same time, due to the volatility and intermittency of distributed renewable energy, the power flow of long feeders and the voltage at the renewable energy access point change greatly. It is necessary to regulate the line voltage to ensure the stability of the voltage at the renewable energy access point and reduce network losses. Therefore, power flow control, voltage compensation, and fault current limitation in the AC-DC hybrid distributed renewable energy system and DC distribution are important technical issues that must be faced and solved in its future development.

[0020] Optimal operation of medium-voltage power electronic transformers; The various distributed power sources, loads, and new power equipment connected to the AC-DC hybrid renewable energy system containing power electronic transformers have obvious differences in time scales, which increases the overall operation and control difficulty of the system. At the same time, the complementary optimal operation and coordinated scheduling between "source-load-storage" are technical problems that need to be solved urgently.

[0021] Connected to the output end of the cascade structure are a low-voltage AC port and a DC port. The low-voltage AC port includes a three-phase bridge inverter circuit. The DC side of the three-phase bridge inverter circuit is connected to the DC output end of the cascade structure and is used to convert DC electrical energy into three-phase AC electrical energy for output to supply power to AC loads. The DC port directly outputs DC electrical energy to supply power to equipment such as DC sources and loads. Moreover, filter circuits are respectively configured for the low-voltage AC port and the DC port. The filter circuits adopt LC filter structures or LCL filter structures to filter out harmonic components in the output electrical energy and improve the power quality; The transformer is integrated with a fault current control circuit, which includes a series of a solid-state switch, a current-limiting reactor, and a dissipative resistor. At the same time, a mechanical circuit breaker is connected in parallel with the solid-state switch. In addition, a current sensor for detecting DC short-circuit faults is provided, as well as a controller connected to the current sensor. The current sensor monitors the magnitude of the current at the DC port in real time and transmits the current signal to the controller; It should be noted that the invention proposes a medium-voltage power electronic transformer, which integrates a hybrid topology and intelligent cooperative control. Its core innovative structure is as follows 1. Enhanced CHB Hybrid Topology Structure Three-stage transformation architecture: High-voltage stage: A 12-pulse CHB rectifier (6 sub-units in series per phase) is adopted, and a star-delta hybrid-connected transformer is used to reduce the total harmonic distortion (THD) of the grid-side current to less than 1.2%; Isolation stage: Each power sub-unit integrates a dual-active full-bridge (DAB) converter and a high-frequency transformer. The magnetic core uses a nanocrystalline-ferrite composite material, and the core loss is reduced by 35% at a switching frequency of 100 kHz; Low-voltage stage: An innovative AC / DC parallel output structure is designed. The AC port adopts a three-level LCL filter inverter, and the DC port integrates a superconducting current-limiting unit (critical current 1.5 kA).

[0022] 2. Dual-stage Fault Current Suppression System Fast current-limiting unit: The solid-state switch adopts a parallel structure of SiC MOSFET and IGBT, and the turn-off time < 100 ns; A series superconducting current-limiting reactor (L = 50 μH) suppresses the current rising rate to 1 kA / ms within 50 μs after a fault occurs; Energy dissipation unit: A parallel composite arc extinguishing device, including a silicon carbide varistor (with a current-carrying capacity of 10 kA) and a permanent magnet mechanism circuit breaker, and the opening time < 0.3 ms; Configure a fault recording module, which can record the current and voltage waveforms within 20 ms for fault analysis.

[0023] 3. Three-layer Intelligent Cooperative Control Architecture

[0024] In the 10 kV / 1 MVA application scenario, the specific structure of the transformer is as follows: High-voltage stage: 3 phases × 6 sub-units in series, each sub-unit adopts a three-level NPC rectifier circuit, and the DC bus capacitor is configured as 4500 μF / 1.2 kV; Isolation stage: The high-frequency transformer uses a nanocrystalline magnetic core (size EC95), with a turns ratio of 10:1, and realizes 200 kW power transmission at a switching frequency of 100 kHz; Low-voltage stage: The AC port outputs 380 V / 50 Hz, configures an LCL filter (L1 = 1.5 mH, L2 = 0.5 mH, C = 10 μF), the DC port outputs 400 V, and integrates a 1 kA / 400 V superconducting current limiter.

[0025] Fault handling process When a DC side short - circuit fault is detected: The current sensor (sampling rate 1MHz) identifies the fault within 10μs; The FPGA triggers the SiC MOSFET to turn off, and the superconducting reactor immediately enters the current - limiting state; At 50μs, the PLC issues an instruction to make the permanent - magnet circuit breaker start to operate; At 0.3ms, the circuit breaker trips, and the silicon carbide varistor absorbs the remaining energy; The central controller starts the fault - location algorithm and determines the faulty sub - unit within 10ms.

[0026] Energy management strategy In an AC - DC hybrid microgrid with PV / energy storage: When the PV output exceeds the load demand by 20%, the central controller instructs the DC port to charge the energy - storage battery, and the charging efficiency > 95%; When the grid voltage drops by 10%, the power - compensation mode of the DAB module is started, and 10% of reactive power support is provided within 5ms; During the night low - load period, the sub - unit sleep strategy is executed, and the no - load loss is reduced to less than 1.5kW.

[0027] Working process: When the controller detects a DC short - circuit fault through the current sensor, it quickly triggers the solid - state switch to turn off, uses the current - limiting reactor to limit the rising speed of the fault current, and at the same time the energy - consuming resistor consumes the fault energy. At a preset time after the solid - state switch turns off, the mechanical circuit breaker is triggered to trip, completely cutting off the fault - current path, so as to achieve rapid current - limiting and interruption of the DC short - circuit fault current. At the same time, according to the line power flow and the voltage of the renewable - energy access point, the controller can also adjust the line voltage to ensure the stability of the renewable - energy access - point voltage and reduce the network loss; Multiple power sub - units adopt a modular design. Each power sub - unit is encapsulated in an independent cabinet structure, and the cabinet structure is configured with standard electrical interfaces and mechanical interfaces, which is convenient for installation, maintenance, and expansion; The transformer is provided with a central controller for coordinating and controlling multiple power sub-units, and local controllers configured for each power sub-unit. The local controllers are communicatively connected to the central controller. The central controller sends control instructions to each local controller according to the system operation state and control objectives. After receiving the instructions, the local controllers perform specific control operations on the AC / DC rectification module, dual-active full-bridge isolation conversion module, etc. of the power sub-unit where they are located, so as to realize the coordinated control of the overall operation of the transformer. At the same time, through this control architecture, it is possible to achieve the complementary and optimized operation and coordinated scheduling between the "source - load - storage", so as to cope with the time-scale differences of distributed power sources, loads and new power equipment in the AC / DC hybrid renewable energy system containing the transformer.

[0028] Through the collaborative innovation of hybrid topology optimization, two-stage fault suppression and three-layer intelligent control, the technical bottleneck of traditional PET in the access of high-penetration renewable energy is solved. It is especially suitable for scenarios such as urban distribution networks and integrated energy systems in industrial parks, providing key equipment support for building a new power system.

Claims

1. A medium-voltage power electronic transformer, characterized in that, Comprising: A cascaded structure based on the CHB topology, the cascaded structure comprising a plurality of power sub-units with series-connected inputs and parallel-connected outputs; Each of the power sub-units comprises an AC / DC rectification module and a dual-active full-bridge isolation conversion module connected in sequence; A low-voltage AC port and a DC port connected to the output end of the cascaded structure.

2. The medium-voltage power electronic transformer according to claim 1, characterized in that, The dual-active full-bridge isolation conversion module comprises a high-frequency transformer, and H-bridge inverters are respectively connected to the primary side and the secondary side of the high-frequency transformer.

3. The medium voltage power electronic transformer according to claim 1, characterized in that, It further comprises: A fault current control circuit integrated in the DC port, the fault current control circuit comprising a solid-state switch, a current-limiting reactor and a dissipative resistor connected in series; A mechanical circuit breaker connected in parallel with the solid-state switch.

4. The medium-voltage power electronic transformer according to claim 3, characterized in that, The fault current control circuit further comprises: A current sensor for detecting a DC short-circuit fault; A controller connected to the current sensor, the controller being configured to trigger the solid-state switch to turn off when a short-circuit fault is detected and trigger the mechanical circuit breaker to trip after a preset time.

5. The medium-voltage power electronic transformer according to claim 1, characterized in that, The plurality of power sub-units adopt a modular design, each power sub-unit is encapsulated in an independent cabinet structure, and the cabinet structure is configured with standard electrical interfaces and mechanical interfaces.

6. The medium-voltage power electronic transformer according to claim 1, wherein, The low-voltage AC port comprises a three-phase bridge inverter circuit, and the DC side of the three-phase bridge inverter circuit is connected to the DC output end of the cascaded structure.

7. The medium-voltage power electronic transformer according to claim 1, wherein The AC / DC rectification module adopts a three-phase PWM rectification circuit, and the three-phase PWM rectification circuit is configured with a neutral-point clamping structure.

8. A medium-voltage power electronic transformer according to claim 1, wherein, It further comprises: A central controller for coordinately controlling a plurality of power sub-units; A local controller configured in each power sub-unit, the local controller being communicatively connected to the central controller.

9. The medium-voltage power electronic transformer according to claim 2, characterized in that, The magnetic core of the high-frequency transformer is made of nanocrystalline alloy or ferrite material.

10. The medium-voltage power electronic transformer according to claim 1, characterized in that, The low-voltage AC port and the DC port are respectively configured with a filter circuit, and the filter circuit comprises an LC filter structure or an LCL filter structure.

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