Flexible DC power transmission system DC line fault ride-through control method and system

Abstract

The invention discloses a DC line fault ride-through control method and system for a flexible DC power transmission system, and the method comprises the steps: comparing the number of times of DC line protection actions with a set upper limit value when a certain DC line breaks down, and if the number of times of DC line protection actions is greater than or equal to the set upper limit value, executing the step 1; all converter units connected to the DC line are locked, connection between all converter units and an AC power grid is disconnected, and the DC line fault ride-through control process is ended; otherwise, all the converter units connected to the direct current line are locked, and thyristors of all the converter units are conducted; and after the fault current is isolated, restarting the flexible direct-current power transmission system, and ending the direct-current line fault ride-through control flow. According to the technical scheme, when various types of direct current line faults occur in the flexible direct current power transmission system provided with the thyristor, the faults can be rapidly judged and isolated, the direct current system continues to operate after the direct current line faults occur instantaneously, and equipment safety and stable operation of the system are guaranteed.

Description

Technical Field

[0001] The present invention belongs to the technical field of relay protection and safety control of power systems, and particularly relates to a control method and system for DC line fault ride-through in a flexible DC transmission system. Background Art

[0002] With the development and maturity of modular multilevel converters (MMC) technology, the voltage levels, rated capacities, and transmission distances of flexible DC transmission or hybrid DC transmission projects are also increasing. In long-distance DC transmission projects, overhead lines are mainly used for DC lines. However, overhead DC transmission lines not only have a wide distribution range, but also are in a harsh geographical environment and complex meteorological environment, with a high probability of being struck by lightning and experiencing faults. DC fault clearing is one of the core key issues faced by long-distance large-capacity flexible DC transmission projects.

[0003] One technical route for DC fault clearing is to use high-voltage DC circuit breakers, but high-voltage DC circuit breakers still face significant challenges in terms of technology maturity, cost, and volume. Another technical route is to improve the traditional half-bridge MMC topology to enable it to have the ability to self-clear DC faults. Typical representatives are full-bridge sub-module structures, clamped double sub-modules, cross-clamped sub-modules, etc., and then cut off the DC current. Currently, the hybrid full-half-bridge sub-module MMC has a relatively high acceptance in practical applications. However, in order to achieve rapid DC fault clearing, a relatively high proportion of full-bridge sub-modules is required, which increases equipment investment and results in significant energy losses during operation.

[0004] Chinese Patent Application No. 202310707687.1, with the invention title "An MMC with Self-breaking Current Ability Based on Thyristor Ring and Its Control Method", discloses an MMC converter configured with thyristors and its control method, which has a low proportion of full-bridge sub-modules, high efficiency, high economy, and the ability to quickly cut off DC faults. However, it fails to fully utilize the advantages of this topology to enable the DC system to continue operating after an instantaneous DC line fault, and the proposed DC line fault criterion is too simple compared with the DC protection principle in engineering applications.

[0005] Therefore, the applicant conducts research on the flexible DC transmission system based on the above topology, proposes a DC line fault ride-through control strategy, and this case is thus generated. Summary of the Invention

[0006] The object of the present invention is to provide a control method and system for DC line fault ride-through in a flexible DC transmission system, which can quickly identify and isolate faults when various types of DC line faults occur in a flexible DC transmission system configured with thyristors, enable the DC system to continue operating after an instantaneous DC line fault, and ensure the safe operation of equipment and the stable operation of the system.

[0007] To achieve the above object, the solution of the present invention is:

[0008] A control method for DC line fault ride-through in a flexible DC transmission system, the flexible DC transmission system includes several groups of converter modules connected in parallel, each converter module includes a first converter unit and a second converter unit, the AC terminal of the first converter unit is connected to the AC power grid, and the DC terminal is connected to the DC terminal of the second converter unit through a DC line, and the AC terminal of the second converter unit is connected to the AC power grid on the other side; the control method includes that when a certain DC line fails,

[0009] Compare the number of DC line protection actions with the set upper limit value. If the number of DC line protection actions is greater than or equal to the set upper limit value, execute the first strategy; if the number of DC line protection actions is less than the set upper limit value, execute the second strategy;

[0010] The first strategy includes locking all converter units connected to the DC line and disconnecting the connections between all converter units and the AC power grid, and ending the DC line fault ride-through control process;

[0011] The second strategy includes,

[0012] Lock all converter units connected to the DC line and turn on the thyristors of all converter units; after isolating the fault current, restart the flexible DC transmission system and end the DC line fault ride-through control process.

[0013] Among them, after isolating the fault current and restarting the flexible DC transmission system, it includes,

[0014] Judge that the DC current disappears and disconnect the connections between all converter units and the DC line;

[0015] Turn off the thyristors of all converter units and connect all converter units to the DC line;

[0016] Unlock the constant DC voltage station, unlock the non-constant DC voltage station, and restart the flexible DC transmission system.

[0017] Among them, judging that the DC current disappears includes that when the DC circuit is lower than the set threshold value, it is considered that the DC current disappears.

[0018] Among them, the unlocking of the fixed DC voltage station, the unlocking of the non-fixed DC voltage station, and the restart of the flexible DC power transmission system include

[0019] Unlocking the fixed DC voltage station, and then determining whether the DC line fault is cleared:

[0020] If it is determined that the DC line fault has not been cleared, continue to compare the number of DC line protection actions with the set upper limit value, and execute the first strategy or the second strategy; if it is determined that the DC line fault has been cleared, unlock the non-fixed DC voltage station, restart the flexible DC power transmission system, and end the DC line fault ride-through control process.

[0021] The above flexible DC power transmission system is a bipolar system. Specifically, the flexible DC power transmission system is a series system of high-voltage and low-voltage converters.

[0022] A DC line fault ride-through control system for a flexible DC power transmission system. The flexible DC power transmission system includes several groups of converter modules connected in parallel. Each converter module includes a first converter unit and a second converter unit. The AC terminal of the first converter unit is connected to the AC power grid, and the DC terminal is connected to the DC terminal of the second converter unit through a DC line. The AC terminal of the second converter unit is connected to the AC power grid on the other side; the control system includes

[0023] A comparison module configured to compare the number of DC line protection actions with a set upper limit value when a certain DC line fails. If the number of DC line protection actions is greater than or equal to the set upper limit value, start the first strategy execution module; if the number of DC line protection actions is less than the set upper limit value, start the second strategy execution module;

[0024] The first strategy execution module is configured to perform the following actions: block all converter units connected to the DC line and disconnect the connection between all converter units and the AC power grid; and

[0025] The first strategy execution module is configured to perform the following actions: block all converter units connected to the DC line and turn on the thyristors of all converter units; after isolating the fault current, restart the flexible DC power transmission system.

[0026] The above first strategy execution module performs the following actions: after isolating the fault current, restart the flexible DC power transmission system, including

[0027] Determine that the DC current disappears, and disconnect the connection between all converter units and the DC line;

[0028] Turn off the thyristors of all converter units and connect all converter units to the DC line;

[0029] Unlock the constant DC voltage station, unlock the non-constant DC voltage station, and restart the flexible DC transmission system.

[0030] The above-mentioned first strategy execution module determines the disappearance of DC current, including that when the DC circuit is lower than the set threshold, it is considered that the DC current has disappeared.

[0031] The above-mentioned first strategy execution module performs the following actions: unlock the constant DC voltage station, unlock the non-constant DC voltage station, and restart the flexible DC transmission system, including

[0032] Unlock the constant DC voltage station, and then determine whether the DC line fault has been cleared:

[0033] If it is determined that the DC line fault has not been cleared, continue to compare the relationship between the number of DC line protection actions and the set upper limit value, and execute the first strategy or the second strategy; if it is determined that the DC line fault has been cleared, unlock the non-constant DC voltage station and restart the flexible DC transmission system.

[0034] The above-mentioned flexible DC transmission system is a bipolar system. Specifically, the flexible DC transmission system is a series system of high-voltage and low-voltage converters.

[0035] After adopting the above solution, the present invention can restart the DC system after quickly isolating the fault current by blocking the converter and conducting the thyristor, enabling the DC system to continue operating after an instantaneous DC line fault and improving the transmission reliability. Description of the Drawings

[0036] Figure 1 It is a schematic diagram of a bipolar flexible DC transmission system configured with thyristors;

[0037] Figure 2 It is a schematic diagram of the converter structure;

[0038] Figure 3 It is a schematic diagram of the DC line fault clearing and restart process of the DC transmission system;

[0039] Figure 4 It is a schematic diagram of the series structure of high-voltage and low-voltage converters adopted by the flexible DC transmission system configured with thyristors. Detailed Embodiment

[0040] The present invention provides a method for controlling the DC line fault ride-through of a flexible DC transmission system. The flexible DC transmission system includes several groups of converter modules connected in parallel. Each converter module includes a first converter unit and a second converter unit. The AC terminal of the first converter unit is connected to the AC power grid, and the DC terminal is connected to the DC terminal of the second converter unit through a DC line. The AC terminal of the second converter unit is connected to the AC power grid on the other side; the method includes that when a certain DC line fails:

[0041] Compare the number of DC line protection operations with the set upper limit value. If the number of DC line protection operations is greater than or equal to the set upper limit value, execute the first strategy; if the number of DC line protection operations is less than the set upper limit value, execute the second strategy;

[0042] The first strategy includes blocking all converter units connected to the DC line and disconnecting all the converter units from the AC power grid, thus ending the DC line fault ride-through control process;

[0043] The second strategy includes,

[0044] Blocking all converter units connected to the DC line and turning on the thyristors of all the converter units; after isolating the fault current, restart the flexible DC power transmission system, thus ending the DC line fault ride-through control process.

[0045] Among them, after isolating the fault current and restarting the flexible DC power transmission system, it includes,

[0046] Judge that the DC current disappears, and disconnect all the converter units from the DC line;

[0047] Turn off the thyristors of all the converter units and connect all the converter units to the DC line;

[0048] Unlock the constant DC voltage station, unlock the non-constant DC voltage station, and restart the flexible DC power transmission system.

[0049] Among them, judging that the DC current disappears includes that when the DC circuit is lower than the set threshold value, it is considered that the DC current disappears.

[0050] Among them, unlocking the constant DC voltage station, unlocking the non-constant DC voltage station, and restarting the flexible DC power transmission system includes,

[0051] Unlock the constant DC voltage station, and then judge whether the DC line fault is cleared:

[0052] Judge that the DC line fault has not been cleared, continue to compare the relationship between the number of DC line protection operations and the set upper limit value, and execute the first strategy or the second strategy; judge that the DC line fault has been cleared, unlock the non-constant DC voltage station, restart the flexible DC power transmission system, and end the DC line fault ride-through control process.

[0053] Among them, the flexible DC power transmission system is a bipolar system, or the flexible DC power transmission system is a series system of high-voltage and low-voltage converters.

[0054] The present invention also provides a DC line fault ride-through control system for a flexible DC power transmission system. The flexible DC power transmission system includes a plurality of groups of converter modules connected in parallel. Each converter module includes a first converter unit and a second converter unit. The AC terminal of the first converter unit is connected to the AC power grid, and the DC terminal is connected to the DC terminal of the second converter unit through a DC line. The AC terminal of the second converter unit is connected to the AC power grid on the other side. It includes,

[0055] A comparison module, configured to compare the number of DC line protection actions with a set upper limit value when a certain DC line fails. If the number of DC line protection actions is greater than or equal to the set upper limit value, start the first strategy execution module; if the number of DC line protection actions is less than the set upper limit value, start the second strategy execution module;

[0056] The first strategy execution module, configured to perform the following actions: block all converter units connected to the DC line, and disconnect the connection between all converter units and the AC power grid; and,

[0057] The first strategy execution module, configured to perform the following actions: block all converter units connected to the DC line, and turn on the thyristors of all converter units; after isolating the fault current, restart the flexible DC power transmission system.

[0058] Wherein, the first strategy execution module performs the following actions: after isolating the fault current, restart the flexible DC power transmission system, including,

[0059] Judge that the DC current disappears, and disconnect the connection between all converter units and the DC line;

[0060] Turn off the thyristors of all converter units, and connect all converter units to the DC line;

[0061] Unlock the constant DC voltage station, unlock the non-constant DC voltage station, and restart the flexible DC power transmission system.

[0062] Wherein, the first strategy execution module judges that the DC current disappears, including that when the DC circuit is lower than the set threshold, it is considered that the DC current disappears.

[0063] Wherein, the first strategy execution module performs the following actions: unlock the constant DC voltage station, unlock the non-constant DC voltage station, and restart the flexible DC power transmission system, including,

[0064] Unlock the constant DC voltage station, and then judge whether the DC line fault is cleared:

[0065] If it is determined that the DC line fault has not been cleared, continue to compare the number of DC line protection actions with the set upper limit value, and execute the first strategy or the second strategy; if it is determined that the DC line fault has been cleared, unlock the non-fixed DC voltage station and restart the flexible DC transmission system.

[0066] Among them, the flexible DC transmission system is a bipolar system, or the flexible DC transmission system is a series system of high- and low-voltage converters.

[0067] The following will use Figure 1 The bipolar flexible DC transmission system configured with thyristors shown as an example to illustrate the technical solution of the present invention. Among them, the flexible DC transmission system configured with thyristors is a bipolar system using a modular multilevel converter. Among them, both the first converter unit and the second converter unit use a single converter and can cooperate with Figure 2 As shown, the two converters are symmetrically connected. The upper bridge arm thereof includes a full-bridge module and a half-bridge module. A thyristor T is configured on the AC side of the converter, and a starting resistor R and its bypass device S are configured, and are connected to the AC grid through an AC switch.

[0068] As Figure 3 shown, a DC line fault ride-through control method for a flexible DC transmission system adopted in this embodiment. When a fault occurs on a certain DC line, start the fault ride-through control method for this DC line, which specifically includes the following steps:

[0069] S1. Determine whether the number of DC line protection actions T1 reaches the set upper limit value T2. If it is confirmed that the number of DC line protection actions T1 is greater than or equal to the set upper limit value T2, go to S2; if it is confirmed that the number of DC line protection actions T1 is less than the set upper limit value T2, go to S3;

[0070] According to the exemplary embodiment, after a DC line fails, DC protection acts, and the control device receives the action signal of the protection device, and the control device records the number of DC line protection action signals T1.

[0071] According to the exemplary embodiment, the set upper limit value T2 of the number of protection action signals is determined by the superior dispatching.

[0072] S2. Block all the converters connected to the DC line of the flexible DC system, disconnect the AC switch, the flexible DC system exits the operation, and the DC line fault ride-through control process ends;

[0073] According to the exemplary embodiment, the flexible DC system exits the operation mode as follows: the control device sends a blocking signal to the converter of the flexible DC transmission system and sends a disconnection signal to the AC switch, so that the converter is blocked and the AC switch is disconnected.

[0074] S3. Block the converter and conduct the thyristor;

[0075] According to the exemplary embodiment, the control device sends a blocking signal to the converter and does not send a disconnection signal to the AC switch, blocking the converter and keeping the AC switch closed. At the same time, the control device sends a conduction signal to the thyristor to turn on the thyristor;

[0076] S4. After the converter is blocked and the thyristor is turned on, wait for the DC current Idp to disappear;

[0077] According to the exemplary embodiment, when the DC current Idp is lower than the set threshold, it can be considered that the DC current has disappeared. The set threshold of the DC current should be less than the disconnection capacity of the DC switch.

[0078] S5. After the DC current Idp disappears, disconnect the DC switch;

[0079] S6. Turn off the thyristor and close the DC switch;

[0080] S7. Unlock the fixed DC voltage station of the flexible DC transmission system and restore the DC voltage;

[0081] According to the exemplary embodiment, one converter station in the flexible DC system is responsible for controlling the DC voltage and is the fixed DC voltage station. The remaining converter stations are responsible for controlling the frequency of the AC system or the active power transmitted by the flexible DC system. The fixed DC voltage station of the flexible DC system is unlocked first to restore the DC voltage and provide a stable DC voltage for the flexible DC system. The remaining non-fixed DC voltage stations are unlocked later.

[0082] S8. Determine whether the DC line fault has been cleared. If the DC line fault has not been cleared, go to S1; if the DC line fault has been cleared, go to S9.

[0083] According to the exemplary embodiment, for a persistent fault that has not been actually cleared on the DC line, as the DC voltage increases, the converter will inject current into the fault point again, and the line protection will act again. At this time, it is determined that the DC line fault has not been cleared; for an instantaneous fault that has been actually cleared on the DC line, as the DC voltage increases, the line protection will not act again. At this time, it is determined that the DC line fault has been cleared.

[0084] S9. Unlock and put into operation the remaining non-fixed DC voltage stations, and the flexible DC system restarts successfully, ending the DC line fault ride-through control process.

[0085] Figure 4 Schematic diagram of the series structure of high-voltage and low-voltage converters for a flexible DC transmission system with thyristors configured according to an exemplary embodiment; to meet the requirements of long-distance and large-capacity power transmission, the DC transmission system can adopt the technology of connecting two or more converters in series to increase the DC voltage level and transmission capacity of the system. According to some embodiments, the flexible DC transmission system adopts a series structure of high-voltage and low-voltage converters.

[0086] In several embodiments provided by the present invention, it should be understood that the disclosed devices and methods can be implemented in other ways. The device embodiments described above are merely illustrative. For example, the division of the units is only a logical function division. In actual implementation, there may be other division methods. For example, multiple units or components can be combined, or integrated into another system, or some features can be ignored, or not executed. In addition, the couplings between the various components shown or discussed, or direct couplings, or communication connections can be through some interfaces. The indirect couplings or communication connections of devices or units can be electrical, mechanical, or other forms.

[0087] Those skilled in the art should understand that the embodiments of the present invention can be provided as a method, a system, or a computer program product. Therefore, the present invention can take the form of a complete hardware embodiment, a complete software embodiment, or an embodiment combining software and hardware aspects. Moreover, the present invention can take the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code. The solutions in the embodiments of the present invention can be implemented in various computer languages. For example, object-oriented programming languages such as Java and interpreted scripting languages such as JavaScript.

[0088] The present invention is described with reference to the flowcharts and / or block diagrams of methods, devices (systems), and computer program products according to the embodiments of the present invention. It should be understood that each flow and / or block in the flowchart and / or block diagram, and the combination of flows and / or blocks in the flowchart and / or block diagram, can be implemented by computer program instructions. These computer program instructions can be provided to the processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing devices to generate a machine, such that the instructions executed by the processor of the computer or other programmable data processing devices generate means for implementing the functions specified in Figure 1 one flow or multiple flows and / or blocks Figure 1 one block or multiple blocks.

[0089] These computer program instructions can also be stored in a computer-readable memory that can direct a computer or other programmable data processing device to work in a specific manner, such that the instructions stored in the computer-readable memory generate a manufacture including an instruction device, and the instruction device implements the functions specified in Figure 1 one flow or multiple flows and / or blocks Figure 1 one block or multiple blocks.

[0090] These computer program instructions can also be loaded onto a computer or other programmable data processing apparatus, so that a series of operation steps are executed on the computer or other programmable apparatus to produce a computer-implemented process, thereby providing instructions for implementing the functions specified in one process or a plurality of processes and / or blocks Figure 1 one process or a plurality of processes and / or blocks Figure 1 steps for implementing the functions specified in one block or a plurality of blocks.

[0091] Although the preferred embodiments of the present invention have been described, additional changes and modifications can be made to these embodiments by those skilled in the art once they learn of the basic inventive concept. Therefore, the appended claims are intended to be construed to cover the preferred embodiments as well as all changes and modifications that fall within the scope of the present invention.

[0092] Obviously, those skilled in the art can make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if these modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalent technologies, the present invention is also intended to include these modifications and variations.

Claims

1. A control method for a flexible DC transmission system to ride through DC line faults. The flexible DC transmission system includes several groups of converter modules connected in parallel. Each converter module includes a first converter unit and a second converter unit. The AC terminal of the first converter unit is connected to the AC grid, and the DC terminal is connected to the DC terminal of the second converter unit through a DC line. The AC terminal of the second converter unit is connected to the AC grid on the other side. It is characterized in that: including, when a DC line fails, comparing the number of operations of the DC line protection with a set upper limit value. If the number of operations of the DC line protection is greater than or equal to the set upper limit value, execute the first strategy; if the number of operations of the DC line protection is less than the set upper limit value, execute the second strategy; the first strategy includes blocking all converter units connected to the DC line and disconnecting all converter units from the AC grid, and ending the DC line fault ride-through control process; the second strategy includes, blocking all converter units connected to the DC line and turning on the thyristors of all converter units; after isolating the fault current, restart the flexible DC transmission system and end the DC line fault ride-through control process.

2. The method according to claim 1, characterized in that: After isolating the fault current, restarting the flexible DC transmission system includes, judging that the DC current disappears, and disconnecting all converter units from the DC line; turning off the thyristors of all converter units and connecting all converter units to the DC line; unlocking the constant DC voltage station, unlocking the non-constant DC voltage station, and restarting the flexible DC transmission system.

3. The method according to claim 2, wherein: judging that the DC current disappears, including that when the DC circuit is lower than a set threshold value, it is considered that the DC current disappears.

4. The method according to claim 2, wherein: Unlocking the constant DC voltage station, unlocking the non-constant DC voltage station, and restarting the flexible DC transmission system includes, unlocking the constant DC voltage station, and then judging whether the DC line fault is cleared: judging that the DC line fault is not cleared, continuing to compare the relationship between the number of operations of the DC line protection and the set upper limit value, and executing the first strategy or the second strategy; judging that the DC line fault has been cleared, unlocking the non-constant DC voltage station, restarting the flexible DC transmission system, and ending the DC line fault ride-through control process.

5. The method according to claim 1, wherein: The flexible DC transmission system is a bipolar system.

6. A DC line fault ride-through control system for a flexible DC power transmission system, the flexible DC power transmission system comprising a plurality of groups of converter modules connected in parallel, each converter module including a first converter unit and a second converter unit, the AC side of the first converter unit being connected to an AC power grid, and the DC side being connected to the DC side of the second converter unit through a DC line, and the AC side of the second converter unit being connected to the AC power grid on the other side; characterized in that: including, a comparison module configured to compare the number of operations of the DC line protection with a set upper limit value when a certain DC line fails. If the number of operations of the DC line protection is greater than or equal to the set upper limit value, start the first strategy execution module; if the number of operations of the DC line protection is less than the set upper limit value, start the second strategy execution module; a first strategy execution module configured to perform the following actions: blocking all converter units connected to the DC line and disconnecting all converter units from the AC grid; and, a first strategy execution module configured to perform the following actions: blocking all converter units connected to the DC line and turning on the thyristors of all converter units; after isolating the fault current, restart the flexible DC transmission system.

7. The system according to claim 6, wherein: The first strategy execution module performs the following actions: after isolating the fault current, restart the flexible DC transmission system, including, judging that the DC current disappears, and disconnecting all converter units from the DC line; turning off the thyristors of all converter units and connecting all converter units to the DC line; unlocking the constant DC voltage station, unlocking the non-constant DC voltage station, and restarting the flexible DC transmission system.

8. The system according to claim 7, characterized in that: The first strategy execution module judges that the DC current disappears, including that when the DC circuit is lower than a set threshold value, it is considered that the DC current disappears.

9. The system according to claim 7, characterized in that: The first strategy execution module performs the following actions: unlocking the constant DC voltage station, unlocking the non-constant DC voltage station, restarting the flexible DC power transmission system, including, unlocking the constant DC voltage station, and then determining whether the DC line fault has been cleared: If it is determined that the DC line fault has not been cleared, continue to compare the number of DC line protection actions with the set upper limit value, and execute the first strategy or the second strategy; if it is determined that the DC line fault has been cleared, unlock the non-constant DC voltage station and restart the flexible DC power transmission system.

10. The system according to claim 6, wherein: The flexible DC power transmission system is a bipolar system.

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