A fault blocking control method and system for a flexible loop closing device

Abstract

A fault-blocking control method and system for a flexible loop-closing device includes: real-time detection of whether a short-circuit fault in the AC power grid causes a voltage drop; determining whether it is necessary to switch the on / off states of a third and fourth switch, and performing the switching operation; controlling the power semiconductor devices in the direct-connect submodule to increase the output voltage of the loop-closing converter chain, the increased output voltage value being equal to the voltage drop value of the AC power grid during the fault; if the current exceeds the safe range, then blocking the power semiconductor devices in the direct-connect submodule, and increasing the DC capacitor voltage of the submodule; if the DC capacitor voltage value exceeds the safe range, then opening the first and second switches. This invention, through a three-level setup, ensures the safety of the device while reducing its sensitivity to power grid faults, without incurring additional costs, is simple to implement, and greatly improves the reliability of the device.

Description

Technical Field

[0001] This application belongs to the field of AC power transmission technology, specifically relating to a fault blocking control method and system for a flexible loop closing device. Background Technology

[0002] In urban load centers, with the development of the distribution network, some areas are experiencing heavy loads due to unexpected load growth. Furthermore, with the increase in the proportion of renewable energy, problems such as "power backflow" are becoming increasingly serious, necessitating the provision of reasonable nodes and pathways for renewable energy consumption.

[0003] Currently, traditional methods for adding new power grid points are limited by line corridor resources and involve high investment. Connecting lines through tie switches can lead to electromagnetic loop problems. Therefore, flexible loop technology is needed to achieve flexible power regulation and dynamic capacity expansion.

[0004] Existing technologies include flexible loop-closing schemes using power electronic converters. For example, an AC-DC-AC converter connects the AC lines of two power sources, with DC isolation between the two back-to-back converters. The AC voltage amplitude and phase are adjusted separately on the AC side to achieve flexible loop closure. However, this scheme is costly, requiring two sets of back-to-back full-power converters for rectification and inversion, making the parallel flexible loop-closing scheme inefficient. Another approach is flexible AC controllable interconnection using cascaded submodules. Reactors are directly connected in series to the AC lines of two power sources, and the power transmission is regulated by the voltage difference between the two systems. This scheme has low converter valve losses, high efficiency, and lower cost and footprint. However, because the converter valve in the series scheme is directly connected between the two power sources, it is more sensitive to grid-side faults compared to the parallel scheme. Therefore, a fault-blocking control method suitable for series flexible loop closure is urgently needed to prevent frequent tripping of the flexible loop device due to faults while ensuring device safety. Summary of the Invention

[0005] To address the aforementioned problems in existing technologies, this invention provides a fault blocking control method and control system for a flexible loop closing device.

[0006] To achieve the above-mentioned objectives, the present invention specifically adopts the following technical solution.

[0007] On one hand, this invention discloses a fault-blocking control method for a flexible loop-closing device. The three-phase converter chain of the flexible loop-closing device is connected in series between a first AC power grid and a second AC power grid via a first switch and a second switch, respectively. The power supply unit of the flexible loop-closing device is connected in parallel to the first AC power grid via a third switch and in parallel to the second AC power grid via a fourth switch. Each phase converter chain includes M cascaded DC-to-serial submodules, where M is an integer greater than or equal to 1. Each DC-to-serial submodule includes an H-type bridge circuit composed of four sets of power semiconductor devices. The DC side of the H-type bridge circuit is connected to a DC capacitor, and the AC side is cascaded. The fault-blocking control method includes:

[0008] S01: The flexible loop closing device detects a short circuit fault in the first AC power grid or the second AC power grid, causing a voltage drop.

[0009] S02: Based on the AC power grid where the voltage drop occurred and the status of the third and fourth switches, determine whether it is necessary to switch the open / closed status of the third and fourth switches. If switching is necessary, execute the switching operation.

[0010] S03: Control the power semiconductor devices in the direct-connect submodule to increase the output voltage of the closed-loop converter chain. The increased output voltage value is equal to the value of the AC grid voltage drop when the fault occurs, so that the current flowing through the flexible closed-loop device is limited to a safe range.

[0011] S04: If the current exceeds the safe range, the power semiconductor devices in the DC-DC submodule will be blocked, and the DC capacitor voltage of the submodule will increase.

[0012] S05: If the DC capacitor voltage value exceeds the safe range, then disconnect the first switch and the second switch.

[0013] More preferably,

[0014] The DC-DC submodule also includes a bypass switch and a submodule control unit. The bypass switch is connected in parallel with the AC side of the DC-DC submodule, and the submodule control unit controls the switching on and off of the power semiconductor devices.

[0015] More preferably,

[0016] The power supply unit includes a multi-winding transformer and 3M rectifier units. The multi-winding transformer includes 3M secondary windings. Each secondary winding is connected to the input side of the rectifier unit. The output side of the rectifier unit is connected to the DC capacitor of the DC-DC submodule. The primary winding of the multi-winding transformer is connected to the third switch and the fourth switch.

[0017] More preferably,

[0018] The third and fourth switches are not allowed to be closed simultaneously.

[0019] More preferably,

[0020] In S02, based on the AC power grid where the voltage drop occurred and the states of the third and fourth switches, it is determined whether it is necessary to switch the open / closed states of the third and fourth switches:

[0021] When the voltage of the first AC mains drops, and the third switch is closed and the fourth switch is open, the switch state switches to the third switch open and the fourth switch closed.

[0022] When the voltage of the second AC mains drops, and the fourth switch is closed and the third switch is open, the switch state changes to the third switch being closed and the fourth switch being open.

[0023] No switching is required in other cases.

[0024] More preferably,

[0025] In S03, the safe current range for the flexible loop closing device is: no more than 1.5 to 2 times the rated current of the device.

[0026] More preferably,

[0027] In S04, a latching command needs to be sent to the power semiconductor devices in all direct-to-serial submodules simultaneously to ensure that the DC capacitor voltages of each direct-to-serial submodule rise evenly.

[0028] More preferably,

[0029] In S05, the safe range for the DC capacitor voltage value is: no more than 1.3 to 1.5 times the rated voltage of the DC-DC submodule.

[0030] More preferably,

[0031] During the execution of S01 to S05, if the submodule control unit detects an abnormality in the submodule, it controls the bypass switch of the submodule to close, and the submodule status is marked as exited. When the number of exited submodules in a certain phase is greater than the preset number of redundancies N, the control units of all exited submodules control the power semiconductor devices to lock out and trip the first switch, the second switch, the third switch and the fourth switch.

[0032] More preferably,

[0033] The number of redundancies N is set to no more than 20% of the total number of submodules.

[0034] On the other hand, the present invention also discloses a fault blocking control system for a flexible loop closing device based on the aforementioned fault blocking control method, including a voltage drop detection module, a power supply unit switch switching operation module, a power semiconductor device control module, a flexible loop closing device current detection module, a DC capacitor voltage detection module, and a flexible loop closing device series switch control module.

[0035] The voltage drop detection module detects in real time whether a voltage drop occurs in the first AC power grid or the second AC power grid.

[0036] The power supply unit switch switching operation module determines whether it is necessary to switch the open / closed state of the third and fourth switches:

[0037] When the voltage of the first AC mains drops, and the third switch is closed and the fourth switch is open, the control will open the third switch and close the fourth switch; when the voltage of the second AC mains drops, and the fourth switch is closed and the third switch is open, the control will close the third switch and open the fourth switch.

[0038] The power semiconductor device control module controls the power semiconductor devices in the direct-series submodule to increase the output voltage of the closed-loop converter chain. The increased output voltage value is equal to the value of the AC grid voltage drop when a fault occurs. The flexible closed-loop device current detection module simultaneously detects the current value flowing through the flexible closed-loop device. If the current exceeds the safe current range, the power semiconductor device control module blocks the power semiconductor devices in the direct-series submodule.

[0039] The DC capacitor voltage detection module detects the DC capacitor voltage value of the power semiconductor device in the locked DC-series submodule. If the DC capacitor voltage value exceeds the safe range, the flexible loop-closing device series switch control module will separate the first switch and the second switch.

[0040] Compared with the prior art, the present invention has the following beneficial technical effects:

[0041] The fault blocking control method provided by this invention solves the problem of the sensitivity of flexible loop closing devices to grid-side faults. The method includes several levels: First, AC voltage is output through the converter chain to compensate for the voltage difference caused by grid voltage drops, so as to control the current flowing through the device within a safe range. If a severe grid voltage drop occurs and the current cannot be limited, a power semiconductor device blocking method can be used to reduce the fault current. During the fault blocking process, if the DC voltage of the submodule is detected to exceed the safe range, the switches at both ends of the loop closing device are directly opened to completely isolate the device from the system. The three-level setup not only ensures the safety of the device, but also reduces the device's sensitivity to grid faults. It does not require additional costs, is simple to implement, and greatly improves the reliability of the device. Attached Figure Description

[0042] Figure 1 This is a schematic diagram of the flexible loop-closing device with fault-blocking function of the present invention.

[0043] Figure 2 This is a schematic diagram of the straight-line submodule structure;

[0044] Figure 3 This is a schematic diagram showing the connection relationship between the rectifier unit and the direct-to-serial submodule;

[0045] Figure 4 This is a schematic diagram of the fault blocking control method of the present invention. Detailed Implementation

[0046] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this application. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0047] It should be understood that although the terms first, second, etc., may be used herein to describe various components, these components should not be limited by these terms. These terms are used to distinguish one component from another. Therefore, the first component discussed below may be referred to as the second component without departing from the teachings of this application. As used herein, the term "and / or" includes all combinations of any and more of the associated listed items.

[0048] Those skilled in the art will understand that the accompanying drawings are merely schematic diagrams of exemplary embodiments and may not be to scale. The modules or processes shown in the drawings are not necessarily essential for implementing this application and therefore should not be used to limit the scope of protection of this application.

[0049] like Figure 1 As shown, this embodiment provides a fault blocking control method for a flexible loop-closing device. The flexible loop-closing device includes a three-phase loop-closing converter chain 1, a reactor 2, a first switch K1, a second switch K2, a third switch K3, a fourth switch K4, and a power supply unit 3. Each phase converter chain includes M cascaded direct-to-serial submodules 4, where M is an integer greater than or equal to 1, wherein:

[0050] The direct-connect submodule consists of four sets of power semiconductor devices and a DC capacitor. The four sets of power semiconductor devices form an H-bridge circuit. The DC side of the H-bridge circuit is connected to the DC capacitor, and the AC side is cascaded.

[0051] Each phase-to-phase converter chain is connected in series with the reactor, the first switch, and the second switch to form a series branch. One end of the series branch is connected to the first AC power grid S1, and the other end is connected to the second AC power grid S2. One end of each phase-to-phase converter chain is connected in series with the power supply unit and the third switch to form a first parallel branch. The other end of each phase-to-phase converter chain is connected in series with the power supply unit and the fourth switch to form a second parallel branch.

[0052] The fault-blocking method includes:

[0053] S01: The flexible loop closing device detects a short circuit fault in the first AC power grid or the second AC power grid, causing a voltage drop.

[0054] S02: Determine whether it is necessary to switch the open / closed state of the third and fourth switches. If it is necessary to switch, then perform the switching operation.

[0055] S03: Control the power semiconductor devices in the direct-connect submodule to increase the output voltage of the closed-loop converter chain. The increased output voltage value is equal to the value of the AC grid voltage drop when the fault occurs, so that the current flowing through the flexible closed-loop device is limited to a safe range.

[0056] S04: If the current exceeds the safe range, the power semiconductor devices in the DC-DC submodule will be blocked, and the DC capacitor voltage of the submodule will increase.

[0057] S05: If the DC capacitor voltage value exceeds the safe range, then disconnect the first switch and the second switch.

[0058] The DC-DC submodule also includes a bypass switch 6 and a submodule control unit 7. The bypass switch is connected in parallel with the AC side of the DC-DC submodule to protect the submodule. The submodule control unit controls the switching on and off of the power semiconductor devices. The DC-DC submodule implements, for example... Figure 2 As shown.

[0059] The power supply unit includes a multi-winding transformer T1 and 3M rectifier units 5. The multi-winding transformer has 3M secondary windings, each secondary winding being connected to the input side of the rectifier unit. The output side of the rectifier unit is connected to the DC capacitor of the DC-DC submodule. The primary winding of the multi-winding transformer is connected to the third and fourth switches. The rectifier unit consists of a bridge circuit composed of 4 or 6 groups of power semiconductor devices. The connection between the rectifier unit and the DC-DC submodule is implemented as follows: Figure 3 As shown.

[0060] Figure 4 This is an embodiment diagram of the flexible loop-closing fault blocking control method in this example.

[0061] The third and fourth switches are not allowed to be closed simultaneously.

[0062] The specific operation method in step S02 is as follows:

[0063] When the voltage of the first AC mains drops, and the third switch is closed and the fourth switch is open, the switch state switches to the third switch open and the fourth switch closed.

[0064] When the voltage of the second AC mains drops, and the fourth switch is closed and the third switch is open, the switch state changes to the third switch being closed and the fourth switch being open.

[0065] No switching is required in other cases;

[0066] In step S03, the safe current range of the flexible loop closing device is no more than 1.5 to 2 times the rated current of the device.

[0067] In step S04, a latching command needs to be sent to the power semiconductor devices in the DC-DC submodules simultaneously to ensure that the DC capacitor voltages of each DC-DC submodule rise uniformly.

[0068] In step S05, the safe range of the DC capacitor voltage value is no more than 1.3 to 1.5 times the rated voltage of the DC-DC submodule.

[0069] During steps S01 to S05, if the submodule control unit detects an anomaly in a submodule, it controls the bypass switch of that submodule to close, at which point the submodule's status is marked as exited. When the number of exited submodules in a certain phase exceeds a preset redundancy number N, the submodule control unit controls the power semiconductor devices to lock out and trips the first, second, third, and fourth switches. In this embodiment of the invention, the redundancy number N is set to not exceed 20% of the total number of submodules.

[0070] It should be clearly understood that this application describes how specific examples are formed and used, but this application is not limited to any details of these examples. Rather, based on the teachings of the disclosure of this application, these principles can be applied to many other embodiments.

[0071] Furthermore, it should be noted that the above figures are merely illustrative representations of the processes included in the method according to exemplary embodiments of this application, and are not intended to be limiting. It is readily understood that the processes shown in the above figures do not indicate or limit the temporal order of these processes. Additionally, it is readily understood that these processes may be executed synchronously or asynchronously, for example, in multiple modules.

[0072] While the exemplary embodiments of the present application have been specifically illustrated and described above, it should be understood that the present application is not limited to the detailed structures, configurations, or implementations described herein; rather, the present application is intended to encompass various modifications and equivalent configurations within the spirit and scope of the appended claims.

Claims

1. A fault-blocking control method for a flexible loop-closing device, wherein the three-phase converter chain of the flexible loop-closing device is connected in series between a first AC power grid and a second AC power grid via a first switch and a second switch, respectively; the power supply unit of the flexible loop-closing device is connected in parallel to the first AC power grid via a third switch and in parallel to the second AC power grid via a fourth switch; each phase converter chain includes M cascaded DC-to-serial submodules, where M is an integer greater than or equal to 1; each DC-to-serial submodule includes an H-type bridge circuit composed of four sets of power semiconductor devices; the DC side of the H-type bridge circuit is connected to a DC capacitor, and the AC side is cascaded; characterized in that... The fault-blocking control method includes: S01: The flexible loop closing device detects a short circuit fault in the first AC power grid or the second AC power grid, causing a voltage drop. S02: Based on the AC mains voltage drop and the states of the third and fourth switches, determine whether it is necessary to switch the open / closed states of the third and fourth switches. If switching is required, execute the switching operation: When the voltage of the first AC mains drops and the third switch is closed while the fourth switch is open, the switch state is switched to the third switch open and the fourth switch closed; when the voltage of the second AC mains drops and the fourth switch is closed while the third switch is open, the switch state is switched to the third switch closed and the fourth switch open; no switching is required in other cases. S03: Control the power semiconductor devices in the direct-connect submodule to increase the output voltage of the closed-loop converter chain. The increased output voltage value is equal to the value of the AC grid voltage drop when the fault occurs, so that the current flowing through the flexible closed-loop device is limited to a safe range. S04: If the current exceeds the safe range, the power semiconductor devices in the DC-DC submodule will be blocked, and the DC capacitor voltage of the submodule will increase. S05: If the DC capacitor voltage value exceeds the safe range, then disconnect the first switch and the second switch.

2. The fault blocking control method for the flexible loop closing device as described in claim 1, characterized in that: The DC-DC submodule also includes a bypass switch and a submodule control unit. The bypass switch is connected in parallel with the AC side of the DC-DC submodule, and the submodule control unit controls the switching on and off of the power semiconductor devices.

3. The fault blocking control method for the flexible loop closing device as described in claim 1, characterized in that: The power supply unit includes a multi-winding transformer and 3M rectifier units. The multi-winding transformer includes 3M secondary windings. Each secondary winding is connected to the input side of the rectifier unit. The output side of the rectifier unit is connected to the DC capacitor of the DC-DC submodule. The primary winding of the multi-winding transformer is connected to the third switch and the fourth switch.

4. The fault blocking control method for the flexible loop closing device as described in claim 1, characterized in that: The third and fourth switches are not allowed to be closed simultaneously.

5. The fault blocking control method for the flexible loop closing device as described in claim 1, characterized in that: In S03, the safe current range for the flexible loop closing device is: no more than 1.5 to 2 times the rated current of the device.

6. The fault blocking control method for the flexible loop closing device as described in claim 1, characterized in that: In S04, a latching command needs to be sent to the power semiconductor devices in all direct-to-serial submodules simultaneously to ensure that the DC capacitor voltages of each direct-to-serial submodule rise evenly.

7. The fault blocking control method for the flexible loop closing device as described in claim 1, characterized in that: In S05, the safe range for the DC capacitor voltage value is: no more than 1.3 to 1.5 times the rated voltage of the DC-DC submodule.

8. The fault blocking control method for the flexible loop closing device according to claim 2, characterized in that: During the execution of S01 to S05, if the submodule control unit detects an abnormality in the submodule, it controls the bypass switch of the submodule to close, and at this time the status of the submodule is marked as exited. When the number of exiting submodules in a certain phase exceeds the preset redundancy number N, the control units of all exiting submodules control the power semiconductor devices to lock out and trip the first switch, the second switch, the third switch, and the fourth switch.

9. The fault blocking control method as described in claim 8, characterized in that: The number of redundancies N is set to no more than 20% of the total number of submodules.

10. A fault-blocking control system for a flexible loop-closing device based on the fault-blocking control method of any one of claims 1-9, comprising a voltage drop detection module, a power supply unit switch switching operation module, a power semiconductor device control module, a flexible loop-closing device current detection module, a DC capacitor voltage detection module, and a flexible loop-closing device series switch control module, characterized in that: The voltage drop detection module detects in real time whether a voltage drop occurs in the first AC power grid or the second AC power grid. The power supply unit switch switching operation module determines whether it is necessary to switch the open / closed state of the third and fourth switches: When the voltage of the first AC mains drops, and the third switch is closed and the fourth switch is open, the control will open the third switch and close the fourth switch; when the voltage of the second AC mains drops, and the fourth switch is closed and the third switch is open, the control will close the third switch and open the fourth switch. The power semiconductor device control module controls the power semiconductor devices in the direct-series submodule to increase the output voltage of the closed-loop converter chain. The increased output voltage value is equal to the value of the AC grid voltage drop when a fault occurs. The flexible closed-loop device current detection module simultaneously detects the current value flowing through the flexible closed-loop device. If the current exceeds the safe current range, the power semiconductor device control module blocks the power semiconductor devices in the direct-series submodule. The DC capacitor voltage detection module detects the DC capacitor voltage value of the power semiconductor device in the locked DC-series submodule. If the DC capacitor voltage value exceeds the safe range, the flexible loop-closing device series switch control module will separate the first switch and the second switch.

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