Light-weight medium-voltage distribution network flexible loop closing device

Abstract

The invention relates to a light-weight medium-voltage distribution network flexible loop closing device, and belongs to the field of power electronic power conversion. The loop closing device comprises a parallel current converter and a series current converter, the series current converter comprises a plurality of series modules, and each series module comprises an isolation transformer and some peripheral converters, so that active and reactive power decoupling control and flexible mutual assistance of power flow among different medium-voltage distribution lines are realized; wherein the isolation transformer is a medium-frequency transformer or a high-frequency transformer, comprises a primary winding and three secondary windings, and has the advantages of small size and compact structure compared with an industrial frequency transformer. Therefore, the problem that partial power type flexible interconnection equipment in the prior art is large in size due to the fact that the partial power type flexible interconnection equipment is connected with the power grid through the power frequency series transformer is solved.

Description

Technical Field

[0001] The invention relates to a lightweight medium-voltage distribution network flexible ring closing device, belonging to the field of power electronic power conversion. Background Art

[0002] The flexible loop closure technology of the distribution network aims to replace the traditional feeder interconnector based on circuit breakers with controllable power electronic converters, thereby realizing normalized flexible "soft connection" between feeders. It can provide flexible, fast and precise power exchange control and power flow optimization capabilities, realize power failure support and power quality management, tap the power supply potential of the distribution network, and improve power supply reliability.

[0003] According to the different ways of accessing the distribution network, flexible loop closing equipment is divided into full-power flexible interconnection equipment and partial-power flexible interconnection equipment.

[0004] Full-power flexible interconnection equipment is connected in parallel between feeders, which can realize various functions such as rapid fault isolation, power-off feeder support, asynchronous feeder interconnection, and interconnection of different voltage levels. However, the equipment capacity is within the flow control range. When used in medium-voltage distribution networks, it will bring problems of high equipment cost and large size, and it is difficult to adapt to urban distribution networks with tight land resources; partial-power flexible interconnection equipment connects some equipment in series to the interconnection line, and uses the small voltage output by the series equipment to adjust the line flow on a large scale. The flow control range is higher than the equipment capacity, which has cost advantages compared to full-power flexible interconnection devices. However, since the series equipment usually uses an industrial frequency series transformer to connect to the power grid, it still has the problem of large size. Summary of the Invention

[0005] The purpose of the present invention is to provide a lightweight medium-voltage distribution network flexible loop closing device to solve the problem of large size of some power-type flexible interconnection devices in the prior art due to the use of industrial frequency series transformers to connect to the power grid.

[0006] To achieve the above object, the solution of the present invention includes:

[0007] A lightweight medium-voltage distribution network flexible closing device of the present invention includes a parallel converter and a series converter. The series converter includes at least one series module, each series module includes three output converters, three independent converters, an isolation transformer and a common converter; the isolation transformer includes a primary winding and three secondary windings, the DC end of the common converter serves as the DC end of the series module, the AC end of the common converter is connected to the primary winding of the isolation transformer, the AC ends of the three independent converters are respectively connected to the three secondary windings of the isolation transformer, the DC ends of the three independent converters are respectively connected to the DC ends of the three output converters, and the three phases formed by the AC ends of the three output converters are the three-phase AC ends of the series module; the DC ends of each series module are connected in parallel and then connected in parallel with the DC end of the parallel converter; the three-phase AC ends of each series module are connected in series and then connected to two three-phase AC lines that need to be closed; the AC end of the parallel converter is connected to a three-phase AC line that needs to be closed; the isolation transformer is a medium-frequency transformer or a high-frequency transformer.

[0008] Furthermore, the primary winding of the isolation transformer is connected to the AC end of the common converter via a primary inductor.

[0009] Furthermore, the three secondary windings of the isolation transformer are connected to the AC terminals of the three independent converters through corresponding secondary inductors.

[0010] Furthermore, the output converter, the independent converter and the common converter are all full-bridge converters.

[0011] Furthermore, the parallel converter is a three-phase low-voltage converter, the DC end of the three-phase low-voltage converter is connected in parallel with the DC end of each series module, and the AC end of the three-phase low-voltage converter is connected to a three-phase AC line that needs to be closed through its internal step-down transformer.

[0012] Furthermore, the capacitor between the common converter and the DC end of the parallel converter is a shared DC capacitor, or is a DC capacitor independently configured at the DC end of the common converter and the DC end of the parallel converter.

[0013] Furthermore, the DC capacitors between the three independent converters and the three output converters are shared DC capacitors, or are DC capacitors independently configured at the DC ends of the independent converters and the DC ends of the output converters.

[0014] Furthermore, when there are two or more parallel converters, the DC sides of the parallel converters are connected in parallel, and the AC sides are connected to the three-phase AC line that needs to be closed on the same side or the AC lines that need to be closed on different sides.

[0015] The beneficial effects of the present invention are as follows: as an improved invention, the ring closing device includes a parallel converter and a series converter, the series converter includes multiple series modules, each series module uses a medium-frequency transformer or a high-frequency transformer as an isolation transformer, in order to realize the active and reactive power decoupling control and flexible mutual assistance of power flow between different medium-voltage distribution lines, the isolation transformer includes a primary winding and three secondary windings, the primary winding is connected to the AC end of the common converter, and the three secondary windings are respectively connected to the AC ends of the three independent converters; the DC ends of the three independent converters are respectively connected to the DC ends of the three output converters, and the AC ends of the three output converters are connected to the three-phase AC line that needs to be closed as the three-phase AC ends of the series module, the DC end of the common converter is connected to the DC end of the parallel converter as the DC end of the series module, and the AC end of the parallel converter is connected to a three-phase AC line that needs to be closed. Since the isolation transformer used in the present invention has the advantages of small size and compact structure compared to the power frequency transformer, it solves the problem in the prior art that some power-type flexible interconnection devices are connected to the power grid using a power frequency series transformer, resulting in a large size of the interconnection devices. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 This is a structural diagram of a lightweight medium-voltage distribution network flexible ring closing device of the present invention;

[0017] Figure 2 It is a three-phase integrated series module structure diagram based on full-bridge converter. DETAILED DESCRIPTION

[0018] In order to make the objectives, technical solutions and advantages of the present invention more clear, the present invention is described in detail below in detail with reference to the accompanying drawings and embodiments.

[0019] The concept of the present invention is that the ring closing device includes a parallel converter and a series converter. The series converter includes at least one series module. The series module includes an isolation transformer and some peripheral converters to achieve active and reactive power decoupling control and flexible mutual assistance of power flow between different medium-voltage distribution lines. The isolation transformer is a medium-frequency transformer or a high-frequency transformer, which has the advantages of small size and compact structure compared to the industrial frequency transformer.

[0020] Example of lightweight medium voltage distribution network flexible ring closing device:

[0021] like Figure 1 The structure diagram of a lightweight medium-voltage distribution network flexible ring closing device shown in FIG. includes a parallel converter and a three-phase integrated series converter (hereinafter referred to as the series converter) composed of multiple three-phase integrated series modules (hereinafter referred to as the series modules). Each series module, such as Figure 2As shown, it includes three output converters and a four-active bridge circuit, wherein the four-active bridge circuit includes three independent converters, a four-winding isolation transformer and a common converter; the DC end of the common converter serves as the DC port of the series module, the AC end of the common converter is connected in series with the primary side of the four-winding isolation transformer, the AC ends of the three independent converters are connected in series with the three secondary sides of the four-winding isolation transformer respectively, the DC ends of the three independent converters are connected back-to-back with the DC ends of the three output converters respectively, and the three phases formed by the three-phase AC ends of the three output converters serve as the three-phase AC ports of a series module respectively. After the DC end of each series module is connected in parallel, it serves as the DC end of the series converter and is connected to the DC end of the parallel converter. The three-phase AC end of each series module forms the three-phase AC port of the series converter, which is respectively connected to the single-phase AC line that needs to be closed; the AC end of the parallel converter is connected to a three-phase AC line that needs to be closed ( Figure 1 ( denoted by A, B, and C, three-phase AC lines of line 1). It is worth noting that the four-winding isolation transformer of this embodiment is a medium-frequency transformer or a high-frequency transformer, with a frequency generally between 1kHz and 20kHz. Compared to power-frequency transformers, the four-winding isolation transformer of this embodiment has the advantages of small size and compact structure. In summary, through the loop closing device of this embodiment, the series converter is used to output an AC voltage with controllable amplitude and phase to control the active and reactive power flows between the lines. The parallel converter is used to provide reactive power support for one-side lines, thereby achieving decoupling control of active and reactive power and flexible mutual assistance of power flows between different medium-voltage distribution lines.

[0022] Specifically, the parallel converter is a three-phase low-voltage converter containing a step-down transformer. It converts the high voltage of the series converter to a low voltage before connecting it to the AC line. The DC side of the three-phase low-voltage converter is connected in parallel with the DC side of each series module via a DC capacitor. The AC side of the three-phase low-voltage converter is connected to a three-phase AC line that needs to be closed via its internal step-down transformer. The closing device can have one or more parallel converters. When multiple parallel converters are used, the DC sides of the parallel converters are connected in parallel, and the AC sides can be connected to the same or different AC lines. Furthermore, the three-phase low-voltage converter can adopt a variety of topologies, such as two-level or three-level.

[0023] also, Figure 2 In the four-way active bridge circuit, inductors are connected in series to the primary and secondary sides of the four-winding isolation transformer, respectively. Energy transfer is achieved by phase-shifting the output voltage of the primary and secondary full-bridge converters. Alternatively, in actual applications, the primary and secondary inductors do not need to be configured separately and can be replaced by the leakage inductance of the four-winding isolation transformer. Alternatively, capacitors can be connected in series to the primary and secondary sides of the isolation transformer to form a resonant four-way active bridge circuit, which can also achieve energy transfer between the primary and secondary sides.

[0024] In this embodiment, the common converter, independent converters, and output converter of the series modules are all full-bridge converters. In other implementations, the common converter and independent converters can be replaced with other types of converters with alternating positive and negative voltage outputs, such as three-level neutral-point clamped half-bridge converters. Furthermore, in this embodiment, the DC capacitors between the common converter and the parallel converters can be shared or configured independently; the DC capacitors between the independent converters and the output converter can be shared or configured independently.

[0025] It should be noted that this closing device can also be used to close multiple AC lines. It is only necessary to expand the number of series converters and the capacity of parallel converters as needed. If three AC lines are to be connected, two series converters are used. Series converter one connects the ABC phases of line one with the ABC phases of line two; series converter two connects the ABC phases of line one with the ABC phases of line two. The capacity of the parallel converter is updated, and only one converter is needed. The line flow regulation amount is related to the voltage amplitude and phase of the series-connected line, as well as the line impedance. Therefore, according to the flow regulation target and the line impedance, the output voltage range of the series converter can be obtained, and then according to the line current, the capacity of the series converter can be obtained. The active capacity of the series converter is consistent with that of the parallel converter. Considering the reactive support capacity of the parallel converter, the capacity of the parallel converter can also be obtained.

Claims

1. A lightweight medium voltage distribution network flexible ring closing device, characterized in that: It includes a parallel converter and a series converter, the series converter includes at least one series module, each series module includes three output converters, three independent converters, an isolation transformer and a common converter; the isolation transformer includes a primary winding and three secondary windings, the DC end of the common converter serves as the DC end of the series module, the AC end of the common converter is connected to the primary winding of the isolation transformer, the AC ends of the three independent converters are respectively connected to the three secondary windings of the isolation transformer, the DC ends of the three independent converters are respectively connected to the DC ends of the three output converters, and the three phases formed by the AC ends of the three output converters are the three-phase AC ends of the series module; the DC ends of each series module are connected in parallel and then connected in parallel with the DC end of the parallel converter; the three-phase AC ends of each series module are connected in series and then connected to two three-phase AC lines that need to be closed; the AC end of the parallel converter is connected to a three-phase AC line that needs to be closed; the isolation transformer is a medium frequency transformer or a high frequency transformer.

2. The lightweight medium voltage distribution network flexible loop closing device according to claim 1 is characterized in that: The primary winding of the isolation transformer is connected to the AC terminal of the common converter through a primary inductor.

3. The lightweight medium voltage distribution network flexible loop closing device according to claim 1 is characterized in that: The three secondary windings of the isolation transformer are connected to the AC terminals of the three independent converters through corresponding secondary inductors.

4. The lightweight medium voltage distribution network flexible loop closing device according to claim 1 is characterized in that: The output converter, the independent converter and the common converter are all full-bridge converters.

5. The lightweight medium voltage distribution network flexible loop closing device according to claim 1 is characterized in that: The parallel converter is a three-phase low-voltage converter, the DC end of the three-phase low-voltage converter is connected in parallel with the DC end of each series module, and the AC end of the three-phase low-voltage converter is connected to a three-phase AC line that needs to be closed through its internal step-down transformer.

6. The lightweight medium voltage distribution network flexible loop closing device according to claim 1, characterized in that: The capacitor between the common converter and the DC end of the parallel converter is a shared DC capacitor, or is a DC capacitor independently configured at the DC end of the common converter and the DC end of the parallel converter.

7. The lightweight medium voltage distribution network flexible loop closing device according to claim 1, characterized in that: The DC capacitors between the three independent converters and the three output converters are shared DC capacitors, or are DC capacitors independently configured at the DC ends of the independent converters and the DC ends of the output converters.

8. The lightweight medium voltage distribution network flexible loop closing device according to claim 1, characterized in that: When there are two or more parallel converters, the DC sides of the parallel converters are connected in parallel, and the AC sides are connected to the three-phase AC line that needs to be closed on the same side or the AC lines that need to be closed on different sides.

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