Fault blocking control method and system for flexible loop closing device

Abstract

The invention discloses a fault blocking control method and system for a flexible loop closing device. The method comprises the following steps: detecting whether a short-circuit fault occurs in an alternating-current power grid to cause voltage drop in real time; judging whether the on-off states of the third switch and the fourth switch need to be switched or not, and executing switching operation; a power semiconductor device in the straight string sub-module is controlled, so that the output voltage of the closed-loop commutation chain is increased, and the increased output voltage value is equal to the voltage drop value of the faulted alternating current power grid; if the current exceeds the safety range of the current, locking a power semiconductor device in the direct-string sub-module, and increasing the direct-current capacitor voltage of the sub-module; and if the DC capacitor voltage value exceeds the safety range, the first switch and the second switch are separated. Through the arrangement of three levels, the safety of the device is ensured, the sensitivity of the device to power grid faults can be reduced, additional cost does not need to be increased, the device is simple and easy to implement, and the reliability of the device is greatly improved.

Description

Technical Field

[0001] The present application belongs to the technical field of alternating current power transmission, and specifically relates to a fault blocking control method and system for a flexible loop closing device. Background Art

[0002] In urban load centers, with the development of distribution networks, grid equipment has become overloaded due to unexpected load growth in some local areas. As the proportion of new energy increases, problems such as “power reverse transmission” have become increasingly serious, requiring the provision of reasonable nodes and paths for the absorption of new energy.

[0003] At present, the traditional methods of adding new grid points are limited by line corridor resources and require high investment. There is a problem of electromagnetic ring network when connecting lines through connecting switches. Therefore, it is necessary to use flexible loop technology to achieve flexible power regulation and dynamic capacity expansion.

[0004] In the prior art, there are schemes for realizing flexible loop closing by using power electronic converters, such as connecting the AC lines of two power sources by using AC-DC-AC converters, with DC isolation between the two back-to-back converters, and adjusting the AC voltage amplitude and phase on the AC side, so as to realize flexible loop closing. However, the cost of this scheme is relatively high, and two sets of back-to-back full-power converters are required for rectification and inversion, and the parallel flexible loop closing scheme is not cost-effective. In addition, there is currently a flexible AC controllable interconnection that uses a sub-module cascade method, directly connects the AC lines of the two power sources in series through a reactor, and uses the voltage difference of the two-side system to adjust the transmission power. The converter valve of this scheme has low loss, high efficiency, and low cost and space. However, since the converter valve of the series scheme is directly connected between the two power sources, it is more sensitive to faults on the grid side than the parallel scheme. There is an urgent need for a fault blocking control method suitable for the series flexible loop closing, which can not only avoid the frequent tripping of the flexible loop closing device due to faults, but also ensure the safety of the device. Summary of the invention

[0005] In order to solve the above problems existing in the prior art, the present invention provides a fault blocking control method and control system for a flexible loop closing device.

[0006] In order to achieve the above-mentioned object of the invention, the present invention specifically adopts the following technical solutions.

[0007] On the one hand, the present invention discloses a fault blocking control method for a flexible loop closing device, wherein a three-phase commutation chain of the flexible loop closing device is connected in series between a first AC power grid and a second AC power grid through a first switch and a second switch respectively, and an energy supply unit of the flexible loop closing device is connected in parallel to the first AC power grid through a third switch and / or connected in parallel to the second AC power grid through a fourth switch; each phase commutation chain includes M direct string sub-modules cascaded, M is an integer greater than or equal to 1, and the direct string sub-module includes an H-type bridge circuit composed of four groups of power semiconductor devices, the DC side of the H-type bridge circuit is connected to a DC capacitor, and the AC side is led out of the cascade; the fault blocking method includes:

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

[0009] S02: judging whether it is necessary to switch the on / off state of the third switch and the fourth switch according to the AC power grid where the voltage drop occurs and the states of the third switch and the fourth switch, and performing the switching operation if switching is necessary;

[0010] S03: Control the power semiconductor devices in the direct string submodule to increase the output voltage of the closed-loop commutation chain, the increased output voltage value is equal to the voltage drop value of the AC power grid where 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 safety range, the power semiconductor devices in the direct series submodule are locked, and the DC capacitor voltage of the submodule increases;

[0012] S05: If the DC capacitor voltage value exceeds the safety range, separate the first switch and the second switch.

[0013] More preferably,

[0014] The direct string submodule further comprises a bypass switch and a submodule control unit. The bypass switch is connected in parallel with the AC side of the direct string submodule. The submodule control unit controls the on and off of the power semiconductor device.

[0015] More preferably,

[0016] The energy supply unit includes a multi-winding transformer and 3M rectifier units. The multi-winding transformer includes 3M secondary sides. 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 direct-current string module. The primary side of the multi-winding transformer is connected to the third switch and the fourth switch.

[0017] More preferably,

[0018] The third switch and the fourth switch are not allowed to be closed at the same time.

[0019] More preferably,

[0020] In S02, according to the AC power grid with voltage drop and the states of the third switch and the fourth switch, it is determined whether the on / off states of the third switch and the fourth switch need to be switched:

[0021] When the voltage of the first AC power grid drops, and the third switch is closed and the fourth switch is open, the switch state is switched to the third switch being open and the fourth switch being closed;

[0022] When the voltage of the second AC power grid drops, and the fourth switch is closed and the third switch is opened, the switch state is switched to the third switch being closed and the fourth switch being opened;

[0023] No need to switch in other cases.

[0024] More preferably,

[0025] In S03, the current safety range of 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 locking command needs to be sent to the power semiconductor devices in all the direct string sub-modules at the same time, so that the DC capacitor voltages of the direct string sub-modules rise evenly.

[0028] More preferably,

[0029] In S05, the safety range of the DC capacitor voltage value is: no more than 1.3 to 1.5 times the rated current of the DC string module.

[0030] More preferably,

[0031] During the execution of S01 to S05, if the sub-module control unit finds that there is an abnormality in the sub-module, it controls the bypass switch of the sub-module to close, and the sub-module status is marked as exited; when the number of exited sub-modules in a certain phase is greater than the preset redundant number N, all sub-module control units marked as exited control the power semiconductor devices to lock and trip the first switch, the second switch, the third switch and the fourth switch.

[0032] More preferably,

[0033] The redundant number N is set to not exceed 20% of the total number of submodules.

[0034] On the other hand, the present invention also discloses a fault blocking control system of a flexible loop closing device based on the aforementioned fault blocking control method, including a voltage drop detection module, an energy 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 switch switching operation module of the energy supply unit determines whether the on / off state of the third switch and the fourth switch needs to be switched:

[0037] When the voltage of the first AC power grid drops, and the third switch is closed and the fourth switch is opened, the third switch is controlled to be opened and the fourth switch is closed; when the voltage of the second AC power grid drops, and the fourth switch is closed and the third switch is opened, the third switch is controlled to be closed and the fourth switch is opened;

[0038] The power semiconductor device control module controls the power semiconductor devices in the direct string submodule to increase the output voltage of the closed-loop commutation chain, and the increased output voltage value is equal to the voltage drop value of the AC power grid where the 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 safety range, the power semiconductor device control module locks the power semiconductor devices in the direct string submodule;

[0039] The DC capacitor voltage detection module detects the DC capacitor voltage value of the power semiconductor device in the locked direct string submodule. If the DC capacitor voltage value exceeds the safety range, the flexible closing device series switch control module separates 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 the present invention solves the problem that the flexible loop closing device is sensitive to faults on the grid side. The method includes several levels: first, an AC voltage is output through the commutation chain to compensate for the voltage difference caused by the grid voltage drop, so as to control the current flowing through the device within a safe range. If the grid drops severely and the current cannot be limited, the power semiconductor device locking method can be used to reduce the fault current. During the fault blocking process, if it is detected that the DC voltage of the sub-module exceeds the safe range, the switches at both ends of the loop closing device are directly separated to completely isolate the device from the system. The three-level setting not only ensures the safety of the device, but also reduces the sensitivity of the device to grid faults. No additional cost is added, it is simple and easy to implement, and the reliability of the device is greatly improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 It is a structural schematic diagram of a flexible loop closing device with a fault blocking function of the present invention;

[0043] Figure 2 It is a schematic diagram of the structure of the straight string module;

[0044] Figure 3 It is a schematic diagram of the connection relationship between the rectifier unit and the direct string submodule;

[0045] Figure 4 It is a schematic flow chart of the fault blocking control method of the present invention. DETAILED DESCRIPTION

[0046] The following will be combined with the drawings in the embodiments of the present application to clearly and completely describe the technical solutions in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work are within the scope of protection of the present 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 component. Therefore, the first component discussed below can be referred to as the second component without departing from the teachings of the concepts of the present application. As used herein, the term "and / or" includes any one of the associated listed items and all combinations of one or more.

[0048] Those skilled in the art will appreciate that the drawings are only schematic diagrams of example embodiments and may not be to scale. The modules or processes in the drawings are not necessarily required to implement the present application and therefore cannot be used to limit the scope of protection of the present application.

[0049] like Figure 1 As shown, this embodiment provides a fault blocking control method for a flexible closed-loop device, wherein the flexible closed-loop device includes a three-phase closed-loop commutation chain 1, a reactor 2, a first switch K1, a second switch K2, a third switch K3, a fourth switch K4 and an energy supply unit 3, wherein each phase commutation chain includes M direct-series submodule cascades 4, where M is an integer greater than or equal to 1, wherein:

[0050] The direct string submodule comprises four groups of power semiconductor devices and DC capacitors. The four groups of power semiconductor devices form an H-type bridge circuit. The DC side of the H-type bridge circuit is connected to the DC capacitor, and the AC side is cascaded.

[0051] Each phase closed-loop commutation 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 grid S1, and the other end is connected to the second AC grid S2; one end of each phase closed-loop commutation chain is connected in series with the energy supply unit and the third switch to form a first parallel branch; the other end of each phase closed-loop commutation chain is connected in series with the energy supply unit and the fourth switch to form a second parallel branch;

[0052] The fault blocking method comprises:

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

[0054] S02: determining whether the on / off states of the third switch and the fourth switch need to be switched, and if so, performing the switching operation;

[0055] S03: Control the power semiconductor devices in the direct string submodule to increase the output voltage of the closed-loop commutation chain, the increased output voltage value is equal to the voltage drop value of the AC power grid where 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 safety range, the power semiconductor devices in the direct series submodule are locked, and the DC capacitor voltage of the submodule increases;

[0057] S05: If the DC capacitor voltage value exceeds the safety range, separate the first switch and the second switch.

[0058] The direct string 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 direct string submodule to protect the submodule. The submodule control unit controls the on and off of the power semiconductor device. The direct string submodule is implemented as follows. Figure 2 shown.

[0059] The energy supply unit includes a multi-winding transformer T1 and 3M rectifier units 5, the multi-winding transformer includes 3M secondary sides, 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 direct string module, the primary side of the multi-winding transformer is connected to the third switch and the fourth switch, the rectifier unit is composed of 4 or 6 groups of power semiconductor devices to form a bridge circuit, and the connection implementation of the rectifier unit and the direct string module is as follows: Figure 3 shown.

[0060] Figure 4 This is a diagram showing an embodiment of the flexible closed-loop fault blocking control method of this embodiment.

[0061] The third switch and the fourth switch are not allowed to be closed at the same time.

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

[0063] When the voltage of the first AC power grid drops, and the third switch is closed and the fourth switch is open, the switch state is switched to the third switch being open and the fourth switch being closed;

[0064] When the voltage of the second AC power grid drops, and the fourth switch is closed and the third switch is opened, the switch state is switched to the third switch being closed and the fourth switch being opened;

[0065] No need to switch in other cases;

[0066] Wherein, the current safety range of the flexible loop closing device in step S03 is: not more than 1.5 to 2 times of the rated current of the device.

[0067] Wherein, in the step S04, it is necessary to send a locking command to the power semiconductor devices in the direct string sub-modules at the same time, so that the DC capacitor voltages of the direct string sub-modules rise evenly.

[0068] The safety range of the DC capacitor voltage value in step S05 is: no more than 1.3 to 1.5 times the rated current of the DC string module.

[0069] Wherein, during the execution of steps S01 to S05, if the submodule control unit finds that the submodule is abnormal, it controls the bypass switch of the submodule to close, and the submodule state is marked as exited; when the number of exited submodules of a certain phase is greater than the preset redundant number N, the submodule control unit controls the power semiconductor device to lock and trip the first switch, the second switch, the third switch and the fourth switch. In the embodiment of the present invention, the redundant number N is set to not more than 20% of the total number of submodules.

[0070] It should be clearly understood that the present application describes how to form and use specific examples, but the present application is not limited to any details of these examples. On the contrary, based on the teaching of the content disclosed in the present application, these principles can be applied to many other embodiments.

[0071] In addition, it should be noted that the above figures are only schematic illustrations of the processes included in the method according to the exemplary embodiments of the present application, and are not intended to be limiting. It is easy to understand that the processes shown in the above figures do not indicate or limit the time sequence of these processes. In addition, it is also easy to understand that these processes can be performed synchronously or asynchronously, for example, in multiple modules.

[0072] The exemplary embodiments of the present application are specifically shown and described above. It should be understood that the present application is not limited to the detailed structures, configurations or implementations described herein; on the contrary, the present application is intended to cover various modifications and equivalent configurations included in 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 commutation chain of the flexible loop closing device is connected in series between the first AC power grid and the second AC power grid through a first switch and a second switch respectively, and the energy supply unit of the flexible loop closing device is connected in parallel to the first AC power grid through a third switch and / or connected in parallel to the second AC power grid through a fourth switch; each phase commutation chain comprises M direct string submodules cascaded, M is an integer greater than or equal to 1, and the direct string submodule comprises an H-type bridge circuit composed of four groups of power semiconductor devices, the DC side of the H-type bridge circuit is connected to a DC capacitor, and the AC side is led out of the cascade; characterized in that The fault blocking method comprises: S01: The flexible loop closing device detects that a short circuit fault occurs in the first AC power grid or the second AC power grid, resulting in a voltage drop; S02: judging whether it is necessary to switch the on / off state of the third switch and the fourth switch according to the AC power grid where the voltage drop occurs and the states of the third switch and the fourth switch, and performing the switching operation if switching is necessary; S03: Control the power semiconductor devices in the direct string submodule to increase the output voltage of the closed-loop commutation chain, the increased output voltage value is equal to the voltage drop value of the AC power grid where 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 safety range, the power semiconductor devices in the direct series submodule are locked, and the DC capacitor voltage of the submodule increases; S05: If the DC capacitor voltage value exceeds the safety range, separate the first switch and the second switch.

2. The fault blocking control method of the flexible loop closing device according to claim 1, characterized in that: The direct string submodule further comprises a bypass switch and a submodule control unit. The bypass switch is connected in parallel with the AC side of the direct string submodule. The submodule control unit controls the on and off of the power semiconductor device.

3. The fault blocking control method of the flexible loop closing device according to claim 1, characterized in that: The energy supply unit includes a multi-winding transformer and 3M rectifier units. The multi-winding transformer includes 3M secondary sides. 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 direct-current string module. The primary side of the multi-winding transformer is connected to the third switch and the fourth switch.

4. The fault blocking control method of the flexible loop closing device according to claim 1, characterized in that: The third switch and the fourth switch are not allowed to be closed at the same time.

5. The fault blocking control method of the flexible loop closing device according to claim 1, characterized in that: In S02, according to the AC power grid with voltage drop and the states of the third switch and the fourth switch, it is determined whether the on / off states of the third switch and the fourth switch need to be switched: When the voltage of the first AC power grid drops, and the third switch is closed and the fourth switch is open, the switch state is switched to the third switch being open and the fourth switch being closed; When the voltage of the second AC power grid drops, and the fourth switch is closed and the third switch is opened, the switch state is switched to the third switch being closed and the fourth switch being opened; No need to switch in other cases.

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

7. The fault blocking control method of the flexible loop closing device according to claim 1, characterized in that: In S04, a locking command needs to be sent to the power semiconductor devices in all the direct string sub-modules at the same time, so that the DC capacitor voltages of the direct string sub-modules rise evenly.

8. The fault blocking control method of the flexible loop closing device according to claim 1, characterized in that: In S05, the safety range of the DC capacitor voltage value is: no more than 1.3 to 1.5 times the rated current of the DC string module.

9. The fault blocking control method of the flexible loop closing device according to claim 2, characterized in that: During the execution of S01 to S05, if the submodule control unit finds that the submodule is abnormal, it controls the bypass switch of the submodule to be closed, and the state of the submodule is marked as exit; When the number of exited submodules of a certain phase is greater than the preset redundant number N, all submodule control units marked as exited control the power semiconductor devices to lock and trip the first switch, the second switch, the third switch and the fourth switch.

10. The fault blocking control method according to claim 9, characterized in that: The redundant number N is set to not exceed 20% of the total number of submodules.

11. A fault blocking control system for a flexible loop closing device based on the fault blocking control method according to any one of claims 1 to 10, comprising a voltage drop detection module, an energy 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 switch switching operation module of the energy supply unit determines whether the on / off state of the third switch and the fourth switch needs to be switched: When the voltage of the first AC power grid drops, and the third switch is closed and the fourth switch is opened, the third switch is controlled to be opened and the fourth switch is closed; when the voltage of the second AC power grid drops, and the fourth switch is closed and the third switch is opened, the third switch is controlled to be closed and the fourth switch is opened; The power semiconductor device control module controls the power semiconductor devices in the direct string submodule to increase the output voltage of the closed-loop commutation chain, and the increased output voltage value is equal to the voltage drop value of the AC power grid where the 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 safety range, the power semiconductor device control module locks the power semiconductor devices in the direct string submodule; The DC capacitor voltage detection module detects the DC capacitor voltage value of the power semiconductor device in the locked direct string submodule. If the DC capacitor voltage value exceeds the safety range, the flexible closing device series switch control module separates the first switch and the second switch.

Images