Alternating-current fault ride-through control method and system for multi-terminal low-frequency power transmission system

Abstract

The invention belongs to the technical field of low-frequency alternating-current power transmission, and provides an alternating-current fault ride-through control method and system of a multi-terminal low-frequency power transmission system, and the technical scheme is based on an alternating-current fault ride-through control strategy of the multi-terminal low-frequency power transmission system. The fault frequency conversion station controls and reduces the amplitude of the alternating voltage of the low-frequency side, so that other frequency conversion stations receive a fault generation signal, and then one or more power control frequency conversion stations with the power flow direction opposite to that of the fault frequency conversion station control and reduce the active power flowing into / out of the low-frequency power transmission line. Therefore, the serious overvoltage / undervoltage phenomenon in the fault frequency conversion station is avoided. And reliable fault ride-through of the multi-terminal low-frequency power transmission system is ensured.

Description

Technical Field

[0001] The present invention belongs to the technical field of low-frequency alternating current power transmission, and in particular relates to an alternating current fault ride-through control method and system for a multi-terminal low-frequency power transmission system. Background Art

[0002] The statements in this section merely provide background information related to the present invention and do not necessarily constitute prior art.

[0003] The existing power grid mainly uses industrial frequency AC transmission. In order to improve the power supply capacity of the transmission system, compared with the DC transmission method that requires the re-laying of DC cables, the low-frequency AC transmission method can use the original AC lines in the power grid, and the lower transmission frequency can increase the transmission capacity of the power grid and increase the transmission distance. It is more economical in application scenarios such as new energy transmission, urban power supply and long-distance power supply in remote areas. At the same time, the low-frequency power grid can use the circuit breakers and other control and protection devices in the industrial frequency power grid, which is easier to form a ring network, has strong fault tolerance and high reliability.

[0004] For the application scenarios of urban power supply and long-distance power supply in remote areas, multiple low-frequency converter stations will be connected through low-frequency transmission lines to form a multi-terminal low-frequency transmission system. In order to improve the power supply stability of the multi-terminal low-frequency system, when an AC fault occurs in the system, a certain fault ride-through control strategy needs to be adopted to ensure the safety and stability of the system.

[0005] Usually in a flexible DC transmission system, when a fault occurs on the AC grid side of a converter station at one end, the fault signal will be transmitted to another converter station via the communication optical fiber. The converter station that receives the fault signal will reduce the power command value according to the severity of the fault. This fault ride-through strategy relies heavily on communication signals. Once the communication optical fiber fails, the fault ride-through will fail. This problem also exists in the fault ride-through condition of a multi-terminal low-frequency transmission system, which leads to unstable operation safety of the multi-terminal low-frequency transmission system. Summary of the invention

[0006] In order to solve at least one technical problem existing in the above-mentioned background technology, the present invention provides an AC fault ride-through control method and system for a multi-terminal low-frequency power transmission system, which can achieve AC fault ride-through without relying on communication signals between frequency conversion stations.

[0007] In order to achieve the above object, the present invention adopts the following technical solution:

[0008] A first aspect of the present invention provides an AC fault ride-through control method for a multi-terminal low-frequency power transmission system, comprising the following steps:

[0009] Obtain the AC voltage amplitude on the power frequency network side of the AC voltage control frequency conversion station;

[0010] Determine whether an AC fault occurs based on the AC voltage amplitude on the power frequency side of the AC voltage control frequency conversion station;

[0011] When an AC fault occurs, an additional deviation controller in the AC voltage fault control frequency conversion station is enabled to control and reduce the AC voltage amplitude on the low-frequency side of the AC voltage fault control frequency conversion station;

[0012] The AC voltage amplitude on the low frequency side of the AC voltage control frequency conversion station is consistent with the AC voltage amplitude on the low frequency side of the AC power control frequency conversion station;

[0013] The controlled AC power frequency conversion station detects whether an AC fault occurs according to the AC voltage amplitude on its low-frequency side. When an AC fault is detected, the controlled AC power frequency conversion station with a power flow direction opposite to that of the controlled AC voltage fault frequency conversion station is selected as the power reduction station, and an additional deviation controller is enabled in the power reduction station to control the reduction of the active power flowing into or out of the low-frequency AC transmission line.

[0014] Further, the selecting of an AC power control frequency conversion station having a power flow direction opposite to that of the AC voltage fault control frequency conversion station as a power reduction station includes:

[0015] When the AC voltage fault frequency conversion station is the power receiving end, the AC power frequency conversion station as the power sending end is selected as the power reduction station;

[0016] When the controlled AC voltage fault frequency conversion station is the power sending end, the controlled AC power frequency conversion station as the power receiving end is selected as the power reduction station.

[0017] Furthermore, when an AC fault occurs, an additional deviation controller in the AC voltage fault control frequency conversion station is enabled to control and reduce the AC voltage amplitude on the low-frequency side of the AC voltage fault control frequency conversion station, including:

[0018] After enabling the additional deviation controller in the controlled AC voltage fault frequency conversion station, the average value of the overall voltage of the submodule in the additional deviation controller is higher than the command value of the submodule voltage, and the deviation command value of the AC voltage amplitude output by the additional deviation controller is a negative value, so that the d-axis component of the actual command value of the AC voltage amplitude is reduced, that is, under the action of the additional deviation controller of the controlled AC voltage fault frequency conversion station, the AC voltage amplitude on the low-frequency side is reduced.

[0019] Furthermore, an additional deviation controller is enabled in the power reduction station to control and reduce the active power flowing into the low-frequency AC transmission line, including:

[0020] After enabling the additional deviation controller in the power reduction station, the feedback value of the voltage amplitude on the low-frequency side decreases due to the reduction of the low-frequency AC voltage amplitude and is less than the command value of the voltage amplitude on the low-frequency side. Then the deviation command value of the AC active power output by the additional deviation controller is a positive value, which reduces the actual command value of the AC active power in the power reduction station. That is, under the action of the additional deviation controller of the power reduction station, the active power input to the low-frequency side line by the power reduction station will be reduced, thereby reducing the surplus power in the system.

[0021] Furthermore, when the fault is cleared, the industrial frequency side of the controlled AC voltage frequency conversion station returns to normal, the fault signal disappears, the additional deviation controller in the controlled AC voltage fault frequency conversion station is disabled, the low voltage side of the controlled AC voltage frequency conversion station returns to the rated value, and the corresponding controlled AC power frequency conversion station resumes steady-state control.

[0022] A second aspect of the present invention provides an AC fault ride-through control system for a multi-terminal low-frequency power transmission system, comprising: at least one AC voltage control frequency conversion station and one AC power control frequency conversion station, the low-frequency side of each frequency conversion station is connected together via a low-frequency AC transmission line, and the industrial frequency side is connected to the respective industrial frequency AC power grid;

[0023] The AC fault ride-through control system of the multi-terminal low-frequency power transmission system performs the following steps when in operation:

[0024] Obtain the AC voltage amplitude on the power frequency network side of the AC voltage control frequency conversion station;

[0025] Determine whether an AC fault occurs based on the AC voltage amplitude on the power frequency side of the AC voltage control frequency conversion station;

[0026] When an AC fault occurs, an additional deviation controller in the AC voltage fault control frequency conversion station is enabled to control and reduce the AC voltage amplitude on the low-frequency side of the AC voltage fault control frequency conversion station;

[0027] The AC voltage amplitude on the low frequency side of the AC voltage control frequency conversion station is consistent with the AC voltage amplitude on the low frequency side of the AC power control frequency conversion station;

[0028] The controlled AC power frequency conversion station detects whether an AC fault occurs according to the AC voltage amplitude on its low-frequency side. When an AC fault is detected, the controlled AC power frequency conversion station with a power flow direction opposite to that of the controlled AC voltage fault frequency conversion station is selected as the power reduction station, and the additional deviation controller in the power reduction station is enabled to control the reduction of the active power flowing into or out of the low-frequency AC transmission line.

[0029] Furthermore, each frequency conversion station is provided with a modular multi-level matrix converter, each modular multi-level matrix converter includes three sub-converters, each sub-converter includes three bridge arms, and each bridge arm is composed of a plurality of full-bridge sub-modules connected in series.

[0030] Furthermore, the control structure of the modular multi-level matrix converter includes three parts, a power frequency side control part, a low frequency side control part and a circulating current control part, wherein the power frequency side control part is responsible for controlling the overall stability of the sub-module voltage inside the converter, the low frequency side control part is responsible for controlling the AC voltage on the low frequency side of the converter or the active power and reactive power on the low frequency side, and the circulating current control part is responsible for controlling the internal circulating current of the converter to ensure the balance of the sub-module voltage between each bridge arm.

[0031] Further, the selecting of an AC power control frequency conversion station having a power flow direction opposite to that of the AC voltage fault control frequency conversion station as a power reduction station includes:

[0032] When the AC voltage fault frequency conversion station is the power receiving end, the AC power frequency conversion station as the power sending end is selected as the power reduction station;

[0033] When the controlled AC voltage fault frequency conversion station is the power sending end, the controlled AC power frequency conversion station as the power receiving end is selected as the power reduction station.

[0034] Furthermore, when the fault is cleared, the industrial frequency side of the controlled AC voltage frequency conversion station returns to normal, the fault signal disappears, the additional deviation controller in the controlled AC voltage fault frequency conversion station is disabled, the low voltage side of the controlled AC voltage frequency conversion station returns to the rated value, and the corresponding controlled AC power frequency conversion station resumes steady-state control.

[0035] Compared with the prior art, the present invention has the following beneficial effects:

[0036] The AC fault ride-through control strategy of the multi-terminal low-frequency power transmission system proposed in the present invention does not rely on communication signals. The fault frequency conversion station reduces the AC voltage amplitude on the low-frequency side by controlling, so that other frequency conversion stations receive the signal of the fault occurrence, and then one or more power control frequency conversion stations with the opposite power flow direction to the fault frequency conversion station control to reduce the active power flowing into / out of the low-frequency transmission line, thereby avoiding serious overvoltage / undervoltage phenomenon in the fault frequency conversion station. Ensure that the multi-terminal low-frequency power transmission system can achieve reliable fault ride-through.

[0037] Advantages of additional aspects of the present invention will be given in part in the following description, and in part will become obvious from the following description, or will be learned through practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] The accompanying drawings in the specification, which constitute a part of the present invention, are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.

[0039] Figure 1 A flow chart of an AC fault ride-through control strategy for a multi-terminal low-frequency power transmission system provided by an embodiment of the present invention;

[0040] Figure 2 A schematic diagram of the structure of a three-terminal low-frequency power transmission system provided by an embodiment of the present invention;

[0041] Figure 3 A schematic diagram of the M3C topology structure provided by an embodiment of the present invention;

[0042] Figure 4 A block diagram of the basic M3C control strategy provided by an embodiment of the present invention;

[0043] Figure 5 A block diagram of a low-frequency side fault ride-through control strategy for controlling a low-frequency AC voltage frequency conversion station in a multi-terminal low-frequency power transmission system provided by an embodiment of the present invention;

[0044] Figure 6 A block diagram of a low-frequency side fault ride-through control strategy for controlling an AC power frequency conversion station in a multi-terminal low-frequency power transmission system provided in an embodiment of the present invention. DETAILED DESCRIPTION

[0045] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0046] It should be noted that the following detailed descriptions are all illustrative and intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meanings as those commonly understood by those skilled in the art to which the present invention belongs.

[0047] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit exemplary embodiments according to the present invention. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprising" and / or "including" are used in this specification, it indicates the presence of features, steps, operations, devices, components and / or combinations thereof.

[0048] Embodiment 1

[0049] This embodiment provides an AC fault ride-through control method for a multi-terminal low-frequency power transmission system, which can achieve AC fault ride-through without relying on communication signals between frequency conversion stations. The method takes the control of the active power flow of the AC power frequency conversion station to the control of the AC voltage frequency conversion station as an example. Figure 1 As shown, the control method specifically includes the following steps:

[0050] Obtain the AC voltage amplitude on the power frequency network side of the AC voltage control frequency conversion station;

[0051] Determine whether an AC fault occurs based on the AC voltage amplitude on the power frequency side of the AC voltage control frequency conversion station;

[0052] When an AC fault occurs, an additional deviation controller in the AC voltage fault control frequency conversion station is enabled to control and reduce the AC voltage amplitude on the low-frequency side of the AC voltage fault control frequency conversion station;

[0053] The AC voltage amplitude on the low frequency side of the AC voltage control frequency conversion station is consistent with the AC voltage amplitude on the low frequency side of the AC power control frequency conversion station;

[0054] The controlled AC power frequency conversion station detects whether an AC fault occurs according to the AC voltage amplitude on its low-frequency side. When an AC fault is detected, the controlled AC power frequency conversion station with a power flow direction opposite to that of the controlled AC voltage fault frequency conversion station is selected as the power reduction station, and an additional deviation controller is enabled in the power reduction station to control and reduce the active power flowing into the low-frequency AC transmission line.

[0055] Among them, the criteria for judging the occurrence of a fault are:

[0056] When a fault occurs, when the positive sequence voltage amplitude of the three-phase AC voltage decreases to less than the set first threshold, it is determined that a fault occurs and a fault signal is enabled;

[0057] During the fault recovery process, when the positive sequence voltage amplitude of the three-phase AC voltage decreases to be greater than the second threshold value, it is determined that the fault is cleared and the fault signal is disabled;

[0058] In this embodiment, the first threshold is 0.85pu and the second threshold is 0.88pu;

[0059] In this embodiment, the step of selecting an AC power control frequency conversion station having a power flow direction opposite to that of an AC voltage fault control frequency conversion station as a power reduction station includes:

[0060] When the AC voltage fault frequency conversion station is the power receiving end, the AC power frequency conversion station as the power sending end is selected as the power reduction station;

[0061] When the controlled AC voltage fault frequency conversion station is the power sending end, the controlled AC power frequency conversion station as the power receiving end is selected as the power reduction station.

[0062] like Figure 5 As shown, in this embodiment, when an AC fault occurs, an additional deviation is enabled in the AC voltage fault control frequency conversion station, and the AC voltage amplitude on the low-frequency side of the AC voltage fault control frequency conversion station is controlled to be reduced, including:

[0063] When an AC fault occurs on the power grid side of the controlled AC voltage frequency conversion station in the system, the output power of the frequency conversion station is blocked, and its input power remains basically unchanged in a short period of time after the fault, then there will be a power surplus inside the controlled AC voltage frequency conversion station, resulting in an increase in the voltage of the submodules in the frequency conversion station. At the same time, the frequency conversion station will detect a decrease in the power grid voltage on the power grid side and determine that a fault has occurred, thereby enabling the "additional submodule voltage deviation controller", in which the average value of the overall voltage of the submodule U sm_ave Higher than the command value U of the submodule voltage sm_set , then the deviation command value of the AC voltage amplitude output by the controller is △U d is a negative value, making the d-axis component U of the actual command value of the AC voltage amplitude d_ref Reduced, that is, under the action of the frequency conversion station controller, the AC voltage amplitude on the low-frequency side will be reduced.

[0064] like Figure 6 As shown, in this embodiment, the additional deviation controller in the power reduction station is enabled to control the reduction of active power flowing into the low-frequency AC transmission line, including:

[0065] For the sending-end controlled AC power frequency conversion station, its low-frequency side is connected to the low-frequency side of the fault station. Therefore, when the AC voltage of the fault station is reduced, the sending-end controlled AC power frequency conversion station detects the reduction of the AC voltage on the low-frequency side and determines that a fault has occurred, and enables the "additional voltage amplitude deviation controller" in the station controller. At this time, the feedback value Ud_ of the voltage amplitude on the low-frequency side in the additional controller is fdb2 Due to the decrease of the low-frequency AC voltage amplitude, it decreases and is less than the command value Ud_ of the low-frequency side voltage amplitude set , then the deviation command value of the AC active power output by the additional controller is △P ref is a positive value, so that the actual AC active power command value P in the frequency conversion station control system ref Reduced, that is, under the action of the additional deviation controller of the frequency conversion station, the active power input to the low-frequency side line will be reduced, thereby reducing the surplus power in the system. In this embodiment, when the fault is cleared, the power frequency side of the AC voltage control frequency conversion station returns to normal, the fault signal disappears, the additional deviation controller in the AC voltage fault control frequency conversion station is disabled, the low-voltage side of the AC voltage control frequency conversion station returns to the rated value, and the corresponding AC power control frequency conversion station resumes steady-state control.

[0066] It should be noted that when the active power flow direction changes from the AC voltage-controlled frequency conversion station to the AC power-controlled frequency conversion station, the AC fault ride-through control strategy of the multi-terminal low-frequency transmission system has the same principle and does not rely on communication signals. The fault frequency conversion station controls to reduce the AC voltage amplitude on the low-frequency side, so that other frequency conversion stations receive the signal of the fault occurrence, and then one or more power-controlled frequency conversion stations with the opposite power flow direction to the fault frequency conversion station control to reduce the active power of the low-frequency transmission line, thereby ensuring that the multi-terminal low-frequency transmission system can achieve reliable fault ride-through.

[0067] Embodiment 2

[0068] This embodiment provides an AC fault ride-through control system for a multi-terminal low-frequency power transmission system, comprising at least one AC voltage control frequency conversion station and one AC power control frequency conversion station, wherein the low-frequency side of each frequency conversion station is connected together via a low-frequency AC transmission line, and the industrial frequency side is connected to the respective industrial frequency AC power grid;

[0069] The AC fault ride-through control system of the multi-terminal low-frequency power transmission system performs the following steps when in operation:

[0070] Obtain the AC voltage amplitude on the power frequency network side of the AC voltage control frequency conversion station;

[0071] Determine whether an AC fault occurs based on the AC voltage amplitude on the power frequency side of the AC voltage control frequency conversion station;

[0072] When an AC fault occurs, an additional deviation controller in the AC voltage fault control frequency conversion station is enabled to control and reduce the AC voltage amplitude on the low-frequency side of the AC voltage fault control frequency conversion station;

[0073] The AC voltage amplitude on the low frequency side of the AC voltage control frequency conversion station is consistent with the AC voltage amplitude on the low frequency side of the AC power control frequency conversion station;

[0074] The controlled AC power frequency conversion station detects whether an AC fault occurs according to the AC voltage amplitude on its low-frequency side. When an AC fault is detected, the controlled AC power frequency conversion station with a power flow direction opposite to that of the controlled AC voltage fault frequency conversion station is selected as the power reduction station, and an additional deviation controller is enabled in the power reduction station to control the reduction of the active power flowing into or out of the low-frequency AC transmission line.

[0075] The step of selecting an AC power control frequency conversion station having a power flow direction opposite to that of an AC voltage fault control frequency conversion station as a power reduction station includes:

[0076] When the AC voltage fault frequency conversion station is the power receiving end, the AC power frequency conversion station as the power sending end is selected as the power reduction station;

[0077] When the controlled AC voltage fault frequency conversion station is the power sending end, the controlled AC power frequency conversion station as the power receiving end is selected as the power reduction station.

[0078] This embodiment is described by taking a three-terminal low-frequency power transmission system as an example;

[0079] Figure 2 It is a schematic diagram of the structure of a three-terminal low-frequency power transmission system. Figure 2 As shown, the three-terminal low-frequency power transmission system includes a receiving end and a sending end;

[0080] The receiving end is a frequency conversion station for controlling AC voltage, and the sending end includes two frequency conversion stations for controlling AC power, namely frequency conversion station 1 and frequency conversion station 2. The low-frequency sides of the frequency conversion station for controlling AC voltage and the two frequency conversion stations for controlling AC power are connected together through a low-frequency AC transmission line, and the industrial frequency sides are connected to their respective industrial frequency AC power grids.

[0081] Figure 3 It is a schematic diagram of M3C topology structure, such as Figure 3 As shown, each frequency conversion station is mainly composed of a modular multilevel matrix converter (M3C), each M3C contains three sub-converters, each sub-converter contains three bridge arms, each bridge arm is composed of multiple full-bridge sub-modules connected in series, that is, there are a total of 9 bridge arms in the M3C.

[0082] Figure 4 This is the basic control strategy block diagram of M3C, such as Figure 4 As shown in the figure, the control of M3C mainly includes three parts: power frequency side control part, low frequency side control part and circulating current control part. The power frequency side control part is responsible for controlling the overall stability of the sub-module voltage inside the converter, the low frequency side control part is responsible for controlling the AC voltage on the low frequency side of the converter or the active power and reactive power on the low frequency side, and the circulating current control part is responsible for controlling the internal circulating current of the converter to ensure the balance of the sub-module voltage between each bridge arm.

[0083] like Figure 5 and Figure 6 As shown, in order to cope with AC fault conditions, the present invention adds some control strategies on the basis of the M3C basic control strategy. Since the submodule voltage stability is the basis for the implementation of each converter control strategy, this embodiment only adds a fault ride-through strategy to the M3C low-frequency side control part.

[0084] When an AC fault occurs on the power grid side of the AC voltage control frequency conversion station in the three-terminal low-frequency power transmission system, the output AC power is blocked while the input power remains basically unchanged, and a power surplus will occur in the frequency conversion station, further causing the submodule voltage in the frequency conversion station to increase. The continuous increase in submodule voltage endangers equipment safety. In order to avoid serious overvoltage in the frequency conversion station, an AC fault ride-through strategy is configured in the AC voltage control frequency conversion station and the AC power control frequency conversion station;

[0085] For the controlled AC voltage frequency conversion station at the receiving end, when a fault occurs on the power frequency network side of the station, the sub-module voltage of the frequency conversion station will increase. At the same time, the frequency conversion station detects that the amplitude on the power frequency network side has dropped to a certain level and determines that a fault has occurred. Then, the "additional sub-module voltage deviation controller" of the low-frequency control part of the station M3C is enabled. The amplitude command value of the low-frequency side voltage will decrease as the sub-module voltage increases, thereby controlling the voltage reduction on the low-frequency AC transmission line in the system.

[0086] Figure 5 The block diagram of the low-frequency side fault ride-through control strategy for controlling the low-frequency AC voltage frequency conversion station in a multi-terminal low-frequency transmission system is shown in FIG. Figure 5 As shown, U sm_set is the command value of the submodule voltage in the AC voltage frequency conversion station, U sm__ave is the average value of the overall voltage of the submodules in the frequency conversion station; ΔU d is the deviation command value of the AC voltage amplitude in the frequency conversion station, U dref_ori is the d-axis component of the initial command value of the AC voltage amplitude; U d_ref is the d-axis component of the actual command value of the AC voltage amplitude, U d_fdb is the d-axis component of the actual feedback value of the AC voltage amplitude; U q_fdb It is the q-axis component of the actual feedback value of the AC voltage amplitude.

[0087] The specific control process is:

[0088] When an AC fault occurs on the power grid side of the controlled AC voltage frequency conversion station in the system, the output power of the frequency conversion station is blocked, and its input power remains basically unchanged in a short period of time after the fault, then there will be a power surplus inside the controlled AC voltage frequency conversion station, resulting in an increase in the voltage of the submodules in the frequency conversion station. At the same time, the frequency conversion station will detect a decrease in the power grid voltage on the power grid side and determine that a fault has occurred, thereby enabling the "additional submodule voltage deviation controller", in which the average value of the overall voltage of the submodule U sm_ave Higher than the command value U of the submodule voltage sm_set , then the deviation command value of the AC voltage amplitude output by the controller is △U d is a negative value, making the d-axis component U of the actual command value of the AC voltage amplitude d_ref Reduced, that is, under the action of the frequency conversion station controller, the AC voltage amplitude on the low-frequency side will be reduced.

[0089] For the AC power frequency conversion station at the sending end, when the frequency conversion station detects a fault according to the voltage reduction degree on the low-frequency side, a power control frequency conversion station is selected as the power reduction frequency conversion station. Here, frequency conversion station 1 is selected as the priority power reduction station. After frequency conversion station 1 detects the fault, the "additional voltage amplitude deviation controller" of the low-frequency side control part is enabled, and the AC active power command value of the frequency conversion station will decrease as the voltage on the low-frequency side decreases, thereby controlling the active power flowing from the sending-end frequency conversion station 1 to the low-frequency line to decrease, thereby achieving the purpose of reducing the surplus power in the receiving-end frequency conversion station.

[0090] Figure 6 The block diagram of the low-frequency side fault ride-through control strategy for controlling the AC power frequency conversion station in a multi-terminal low-frequency transmission system is shown in Figure 6 As shown, U d_set is the command value of the voltage amplitude on the low-frequency side of the power frequency conversion station, U d__fdb2 is the feedback value of the voltage amplitude on the low-frequency side of the frequency conversion station; ΔP ref is the deviation command value of the AC active power in the frequency conversion station, P ref_ori is the initial command value of AC active power; P ref is the actual command value of AC active power, P fdb is the actual feedback value of AC active power; Q ref is the actual command value of AC reactive power, Q fdb It is the actual feedback value of AC reactive power.

[0091] The specific control process is:

[0092] For the sending-end controlled AC power frequency conversion station, its low-frequency side is connected to the low-frequency side of the fault station. Therefore, when the AC voltage of the fault station is reduced, the sending-end controlled AC power frequency conversion station detects the reduction of the AC voltage on the low-frequency side and determines that a fault has occurred, and enables the "additional voltage amplitude deviation controller" in the station controller. At this time, the feedback value Ud_ of the voltage amplitude on the low-frequency side in the additional controller is fdb2 Due to the decrease of the low-frequency AC voltage amplitude, it decreases and is less than the command value Ud_ of the low-frequency side voltage amplitude set , then the deviation command value of the AC active power output by the additional controller is △P ref is a positive value, so that the actual AC active power command value P in the frequency conversion station control system refReduced, that is, under the action of the frequency conversion station controller, the active power input to the low-frequency side line will be reduced, thereby reducing the surplus power in the system. In this way, under the action of the above-mentioned fault ride-through control strategy, the multi-terminal low-frequency transmission system can complete AC fault ride-through without relying on communication signals. In the designed control strategy, the low-frequency AC voltage control frequency conversion station at the receiving end and the AC power control frequency conversion station at the sending end need to coordinate and cooperate, using the low-frequency AC voltage amplitude as the electrical quantity for transmitting the fault signal, and dynamically adjusting the output power of the sending end station through closed-loop control, thereby avoiding serious overvoltage in the receiving end frequency conversion station and achieving system fault ride-through.

[0093] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. An AC fault ride-through control method for a multi-terminal low-frequency power transmission system, characterized in that: The steps include: Obtain the AC voltage amplitude on the power frequency network side of the AC voltage control frequency conversion station; Determine whether an AC fault occurs based on the AC voltage amplitude on the power frequency side of the AC voltage control frequency conversion station; When an AC fault occurs, an additional deviation controller in the AC voltage fault control frequency conversion station is enabled to control and reduce the AC voltage amplitude on the low-frequency side of the AC voltage fault control frequency conversion station; The AC voltage amplitude on the low frequency side of the AC voltage control frequency conversion station is consistent with the AC voltage amplitude on the low frequency side of the AC power control frequency conversion station; The controlled AC power frequency conversion station detects whether an AC fault occurs according to the AC voltage amplitude on its low-frequency side. When an AC fault is detected, the controlled AC power frequency conversion station with a power flow direction opposite to that of the controlled AC voltage fault frequency conversion station is selected as the power reduction station, and the additional deviation controller in the power reduction station is enabled to control the reduction of the active power flowing into the low-frequency AC transmission line.

2. The AC fault ride-through control method of a multi-terminal low-frequency power transmission system according to claim 1, characterized in that: The step of selecting an AC power control frequency conversion station having a power flow direction opposite to that of an AC voltage fault control frequency conversion station as a power reduction station comprises: When the AC voltage fault frequency conversion station is the power receiving end, the AC power frequency conversion station as the power sending end is selected as the power reduction station; When the controlled AC voltage fault frequency conversion station is the power sending end, the controlled AC power frequency conversion station as the power receiving end is selected as the power reduction station.

3. The AC fault ride-through control method of a multi-terminal low-frequency power transmission system according to claim 1, characterized in that: When an AC fault occurs, the additional deviation controller in the AC voltage fault control frequency conversion station is enabled to control and reduce the AC voltage amplitude on the low-frequency side of the AC voltage fault control frequency conversion station, including: After enabling the additional deviation controller in the controlled AC voltage fault frequency conversion station, the average value of the overall voltage of the submodule in the additional deviation controller is higher than the command value of the submodule voltage, and the deviation command value of the AC voltage amplitude output by the additional deviation controller is a negative value, so that the d-axis component of the actual command value of the AC voltage amplitude is reduced, that is, under the action of the additional deviation controller of the controlled AC voltage fault frequency conversion station, the AC voltage amplitude on the low-frequency side is reduced.

4. The AC fault ride-through control method of a multi-terminal low-frequency power transmission system according to claim 1, characterized in that: Enable additional deviation controller in power reduction station to control and reduce active power flowing into low-frequency AC transmission line, including: After enabling the additional deviation controller in the power reduction station, the feedback value of the voltage amplitude on the low-frequency side decreases due to the reduction of the low-frequency AC voltage amplitude and is less than the command value of the voltage amplitude on the low-frequency side. Then the deviation command value of the AC active power output by the additional deviation controller is a positive value, which reduces the actual command value of the AC active power in the power reduction station. That is, under the action of the additional deviation controller of the power reduction station, the active power input to the low-frequency side line by the power reduction station will be reduced, thereby reducing the surplus power in the system.

5. The AC fault ride-through control method of a multi-terminal low-frequency power transmission system according to claim 1, characterized in that: When the fault is cleared, the power frequency side of the AC voltage control frequency conversion station returns to normal, the fault signal disappears, the additional deviation controller in the AC voltage fault control frequency conversion station is disabled, the low voltage side of the AC voltage control frequency conversion station returns to the rated value, and the corresponding AC power control frequency conversion station resumes steady-state control.

6. An AC fault ride-through control system for a multi-terminal low-frequency power transmission system, characterized in that: At least one AC voltage control frequency conversion station and one AC power control frequency conversion station are included. The low-frequency side of each frequency conversion station is connected together through a low-frequency AC transmission line, and the industrial frequency side is connected to the respective industrial frequency AC power grid; The AC fault ride-through control system of the multi-terminal low-frequency power transmission system performs the following steps when in operation: Obtain the AC voltage amplitude on the power frequency network side of the AC voltage control frequency conversion station; Determine whether an AC fault occurs based on the AC voltage amplitude on the power frequency side of the AC voltage control frequency conversion station; When an AC fault occurs, an additional deviation controller in the AC voltage fault control frequency conversion station is enabled to control and reduce the AC voltage amplitude on the low-frequency side of the AC voltage fault control frequency conversion station; The AC voltage amplitude on the low frequency side of the AC voltage control frequency conversion station is consistent with the AC voltage amplitude on the low frequency side of the AC power control frequency conversion station; The controlled AC power frequency conversion station detects whether an AC fault occurs according to the AC voltage amplitude on its low-frequency side. When an AC fault is detected, the controlled AC power frequency conversion station with a power flow direction opposite to that of the controlled AC voltage fault frequency conversion station is selected as the power reduction station, and an additional deviation controller is enabled in the power reduction station to control the reduction of the active power flowing into or out of the low-frequency AC transmission line.

7. The AC fault ride-through control system of a multi-terminal low-frequency power transmission system according to claim 6, characterized in that: Each frequency conversion station is provided with a modular multi-level matrix converter, each modular multi-level matrix converter includes three sub-converters, each sub-converter includes three bridge arms, and each bridge arm is composed of a plurality of full-bridge sub-modules connected in series.

8. The AC fault ride-through control system of a multi-terminal low-frequency power transmission system according to claim 7, characterized in that: The control structure of the modular multi-level matrix converter consists of three parts: the power frequency side control part, the low frequency side control part and the circulating current control part. The power frequency side control part is responsible for controlling the overall stability of the sub-module voltage inside the converter, the low frequency side control part is responsible for controlling the AC voltage on the low frequency side of the converter or the active power and reactive power on the low frequency side, and the circulating current control part is responsible for controlling the internal circulating current of the converter to ensure the balance of the sub-module voltage between each bridge arm.

9. The AC fault ride-through control system of a multi-terminal low-frequency power transmission system according to claim 6, characterized in that: The step of selecting an AC power control frequency conversion station having a power flow direction opposite to that of an AC voltage fault control frequency conversion station as a power reduction station comprises: When the AC voltage fault frequency conversion station is the power receiving end, the AC power frequency conversion station as the power sending end is selected as the power reduction station; When the controlled AC voltage fault frequency conversion station is the power sending end, the controlled AC power frequency conversion station as the power receiving end is selected as the power reduction station.

10. The AC fault ride-through control system of a multi-terminal low-frequency power transmission system according to claim 7, characterized in that: When the fault is cleared, the power frequency side of the AC voltage control frequency conversion station returns to normal, the fault signal disappears, the additional deviation controller in the AC voltage fault control frequency conversion station is disabled, the low voltage side of the AC voltage control frequency conversion station returns to the rated value, and the corresponding AC power control frequency conversion station resumes steady-state control.

Images