Lightweight medium voltage distribution network flexible closing device

Abstract

The utility model relates to a kind of lightweight medium-voltage distribution network flexible ring closing device, belong to the field of power electronic power conversion.The ring closing device includes parallel current converter and series current converter, series current converter includes multiple series modules, each series module includes an isolation transformer and some peripheral converters, to realize the active, reactive power decoupling control and the flexible mutual aid of tidal flow between different medium-voltage distribution lines, wherein isolation transformer is medium-frequency transformer or high-frequency transformer, including one primary winding and three secondary windings, compared to power frequency transformer has the advantages of small size and compact structure.Therefore, solve the problem that the volume of interconnection equipment is large in the prior art due to the use of power frequency series transformer and power grid connection.

Description

Technical Field

[0001] This utility model relates to a lightweight medium-voltage distribution network flexible loop-closing device, belonging to the field of power electronic power conversion. Background Technology

[0002] The flexible loop-connection technology for distribution networks aims to replace traditional circuit breaker-based feeder interconnection switches with controllable power electronic converters, thereby achieving a normalized flexible "soft connection" between feeders. It can provide flexible, fast, and accurate power exchange control and power flow optimization capabilities, realize power outage support and power quality management, tap the power supply potential of the distribution network, and improve power supply reliability.

[0003] Depending on the method of connection to the distribution network, flexible loop-connected devices are divided into full-power flexible interconnection devices and partial-power flexible interconnection devices.

[0004] Full-power flexible interconnection equipment is connected in parallel between feeders, enabling various functions such as rapid fault isolation, power outage feeder support, asynchronous feeder interconnection, and interconnection of different voltage levels. However, the equipment capacity is limited to the power flow control range, which leads to high equipment cost and large size when applied to medium-voltage distribution networks, making it difficult to adapt to urban distribution networks with limited land resources. Partial-power flexible interconnection equipment connects some equipment in series to the interconnection line, using the small voltage output of the series equipment to regulate the line power flow over a wide range. The power flow control range is higher than the equipment capacity, and it has a cost advantage compared to full-power flexible interconnection devices. However, since the series equipment usually uses a power frequency series transformer to connect to the grid, it still has the problem of large size. Utility Model Content

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

[0006] To achieve the above objectives, the solution of this utility model includes:

[0007] This utility model discloses a lightweight medium-voltage distribution network flexible loop-closing device, comprising a parallel converter and a series converter. The series converter includes at least one series module, and each series module includes three output converters, three independent converters, an isolation transformer, and a common converter. The isolation transformer includes one primary winding and three secondary windings. The DC terminal of the common converter serves as the DC terminal of its respective series module, and the AC terminal of the common converter is connected to the primary winding of the isolation transformer. The AC terminals of the three independent converters are respectively connected to the three secondary windings of the isolation transformer, and the DC terminals of the three independent converters are respectively connected to the DC terminals of the three output converters. The three phases formed by the AC terminals of the three output converters constitute the three-phase AC terminals of the series module. The DC terminals of each series module are connected in parallel and then connected in parallel to the DC terminals of the parallel converter. The three-phase AC terminals of each series module are connected in series and then respectively connected to the two three-phase AC lines that need to be loop-closing. The AC terminal of the parallel converter is connected to one three-phase AC line that needs to be loop-closing. 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 terminal 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 three independent converters through corresponding secondary inductors.

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

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

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

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

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

[0015] The beneficial effects of this utility model are as follows: As an improved invention, this loop-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 achieve decoupling control of active and reactive power 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 terminal of the common converter, and the three secondary windings are respectively connected to the AC terminals of three independent converters. The DC terminals of the three independent converters are then respectively connected to the DC terminals of three output converters. The AC terminals of the three output converters are connected to the three-phase AC line that needs to be looped, serving as the three-phase AC terminals of the series module. The DC terminal of the common converter is connected to the DC terminal of the parallel converter, serving as the DC terminal of the series module. The AC terminal of the parallel converter is connected to a three-phase AC line that needs to be looped. Since the isolation transformer used in this utility model has the advantages of small size and compact structure compared with the power frequency transformer, it solves the problem that the size of the interconnection equipment is large due to the use of power frequency series transformers to connect to the power grid in some existing power-type flexible interconnection equipment. Attached Figure Description

[0016] Figure 1 This is a structural diagram of the lightweight medium-voltage power distribution network flexible loop-closing device of this utility model;

[0017] Figure 2 This is a structural diagram of a three-phase integrated series module based on a full-bridge converter. Detailed Implementation

[0018] To make the objectives, technical solutions, and advantages of this utility model clearer, the present utility model will be described in a clear and complete manner below with reference to the accompanying drawings and embodiments.

[0019] The present invention is based on the following: the loop-connected 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 decoupling control of active and reactive power 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 with power frequency transformers.

[0020] Example of a lightweight medium-voltage power distribution network flexible loop closing device:

[0021] like Figure 1 The diagram shows a lightweight medium-voltage distribution network flexible loop-connection device, including 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 series modules). Each series module, such as... Figure 2As shown, the circuit includes three output converters and a four-winding active bridge circuit. The four-winding active bridge circuit comprises three independent converters, a four-winding isolation transformer, and a common converter. The DC terminal of the common converter serves as the DC port of the series module. The AC terminal of the common converter is connected in series with the primary winding of the four-winding isolation transformer. The AC terminals of the three independent converters are connected in series with the three secondary windings of the four-winding isolation transformer, respectively. The DC terminals of the three independent converters are connected back-to-back with the DC terminals of the three output converters. The three-phase AC terminals of the three output converters form three-phase AC ports for each series module. The DC terminals of each series module are connected in parallel to form the DC terminals of the series converter and connected to the DC terminals of the parallel converters. The three-phase AC terminals of each series module form the three-phase AC ports of the series converter, which are connected to the single-phase AC lines requiring loop closure. The AC terminals of the parallel converters are connected to a three-phase AC line requiring loop closure. Figure 1 (The image shows the three-phase AC lines A, B, and C of line 1). It is worth noting that the four-winding isolation transformer in this embodiment is a medium-frequency or high-frequency transformer, typically with a frequency between 1kHz and 20kHz. Compared to power-frequency transformers, the four-winding isolation transformer in this embodiment has the advantages of smaller size and more compact structure. In summary, through the loop-closing device of this embodiment, a series converter outputs an AC voltage with controllable amplitude and phase to control the active and reactive power flow between lines. By using parallel converters to provide reactive power support for a single-sided line, decoupling control of active and reactive power and flexible mutual assistance of power flow between different medium-voltage distribution lines can be achieved.

[0022] Specifically, the parallel converter is a three-phase low-voltage converter containing a step-down transformer, used to convert the high voltage of the series converter to low voltage and connect it to the AC line. The DC terminal of the three-phase low-voltage converter is connected in parallel with the DC terminals of each series module through a DC capacitor. The AC terminal of the three-phase low-voltage converter is connected to a three-phase AC line that needs to be looped through its internal step-down transformer. There can be one or more parallel converters in this loop-closing device. When multiple parallel converters are used, the DC sides of each parallel converter are connected in parallel, and the AC sides can be connected to the same AC line or different AC lines. Furthermore, the three-phase low-voltage converter can adopt various topologies such as two-level or three-level.

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

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

[0025] It should be noted that this loop-closing device can also be used for loop-closing multiple AC lines, simply by expanding the number of series converters and the capacity of the parallel converters as needed. To connect three AC lines, two series converters are used: one connects phases ABC of line one to phases ABC of line two; the other connects phases ABC of line one to phases ABC of line two. The capacity of the parallel converter is updated, requiring only one unit. The power flow regulation is related to the voltage amplitude and phase of the input lines, as well as the line impedance. Therefore, based on the power flow regulation target and line impedance, the output voltage range of the series converters can be obtained. Then, based on the line current, the capacity of the series converters can be determined. The active power capacity of the series converters is the same as that of the parallel converters. Considering the reactive power support capacity of the parallel converters, the capacity of the parallel converters can also be obtained.

Claims

1. A lightweight medium-voltage distribution network flexible loop-closing device, characterized in that, The system includes parallel converters and series converters. 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 one primary winding and three secondary windings. The DC terminal of the common converter serves as the DC terminal of its series module. The AC terminal of the common converter is connected to the primary winding of the isolation transformer. The AC terminals of the three independent converters are respectively connected to the three secondary windings of the isolation transformer. The DC terminals of the three independent converters are respectively connected to the DC terminals of the three output converters. The three phases formed by the AC terminals of the three output converters constitute the three-phase AC terminals of the series module. The DC terminals of each series module are connected in parallel and then connected in parallel to the DC terminals of the parallel converter. The corresponding three-phase AC terminals of each series module are connected in series and then connected to the two three-phase AC lines that need to be closed in a loop. The AC terminal of the parallel converter is connected to one three-phase AC line that needs to be closed in a loop. 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, 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, characterized in that, The three secondary windings of the isolation transformer are connected to the AC terminals of three independent converters through their respective secondary inductors.

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

5. The lightweight medium-voltage distribution network flexible loop-closing device according to claim 1, characterized in that, The parallel converter is a three-phase low-voltage converter. The DC terminal of the three-phase low-voltage converter is connected in parallel with the DC terminals of each series module. The AC terminal of the three-phase low-voltage converter is connected to a three-phase AC line that needs to be closed in a loop 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 DC terminals of the common converter and the parallel converter is a shared DC capacitor, or a DC capacitor independently configured at the DC terminals of the common converter and 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 either shared DC capacitors or DC capacitors that are independently configured at the DC terminals of the independent converters and the output converters, respectively.

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 each parallel converter are connected in parallel, and the AC sides are connected to the same three-phase AC line that needs to be closed in a loop or to AC lines that need to be closed in a loop on different sides.

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