Flexible DC line adaptive reclosing method and device based on transient voltage ratio

Abstract

The invention is suitable for the technical field of power systems, and provides a flexible direct current line adaptive reclosing method and device based on a transient voltage ratio, and the method comprises the steps: controlling hybrid direct current circuit breakers at two ends of a flexible direct current line to be disconnected when the fault of the flexible direct current line is detected; measuring the transient voltage at the two ends of the first hybrid DC circuit breaker and the transient voltage at the line side; calculating the ratio of the two to serve as a transient voltage ratio; based on the transient voltage ratio, the fault type of the flexible DC line is judged; if the fault type is a permanent fault, controlling reclosure of the hybrid DC circuit breaker not to act; and if the fault is an instantaneous fault, controlling the first hybrid direct current circuit breaker and the second hybrid direct current circuit breaker to perform reclosing action. According to the method, the fault state identification basis is constructed through the transient voltage ratio, so that the line fault can be accurately judged, the influence of the transient process is small, and the secondary impact on the system caused by the reclosing action during the permanent fault can be avoided.

Description

Technical Field

[0001] The present invention belongs to the technical field of power systems, and in particular relates to a flexible DC line adaptive reclosing method and device based on transient voltage ratio. Background Art

[0002] At present, the reclosing methods of flexible DC systems are mainly divided into reclosing methods based on converter control and reclosing methods based on DC circuit breakers. The reclosing method based on converter control injects characteristic signals into the fault line through the converter station control system, and uses the difference characteristics of the characteristic signal in the two states of line fault presence and fault disappearance to judge the nature of the fault. This type of method has fast judgment speed and high reliability, but this type of method needs to lock the converter immediately after the fault, and needs to add additional control devices. It has poor applicability to different types of converter stations, and it is also necessary to consider whether the injected signal will affect the normal operation of the system.

[0003] The reclosing method based on DC circuit breaker switches the DC circuit breaker branch and uses the difference in voltage or current between transient fault and permanent fault to identify the nature of the fault. The principle of this method is simple, but most existing methods rely on the topological structure of the DC circuit breaker, which is less practical and greatly affected by transient processes. The reliability is low and it is easy to cause secondary impact to the system. Summary of the invention

[0004] The embodiments of the present invention provide a flexible DC line adaptive reclosing method and device based on transient voltage ratio to solve the problem that the existing reclosing method is prone to secondary impact on the system due to inaccurate judgment of transient faults and permanent faults.

[0005] The present invention is achieved through the following technical solutions:

[0006] In a first aspect, an embodiment of the present invention provides a method for adaptive reclosing of a flexible DC line based on a transient voltage ratio, comprising:

[0007] When a fault in the flexible DC line is detected, controlling a first hybrid DC circuit breaker and a second hybrid DC circuit breaker at both ends of the flexible DC line to be disconnected;

[0008] measuring a transient voltage across the first hybrid DC circuit breaker and a transient voltage on the line side;

[0009] calculating a ratio of a transient voltage across the first hybrid DC circuit breaker and a transient voltage on the line side as a transient voltage ratio;

[0010] Based on the transient voltage ratio, determining a fault type of the flexible DC line;

[0011] If the fault type is a permanent fault, the reclosing of the first hybrid DC circuit breaker and the second hybrid DC circuit breaker is controlled not to operate; if the fault type is a transient fault, the first hybrid DC circuit breaker and the second hybrid DC circuit breaker are controlled to perform reclosing.

[0012] In combination with the first aspect, in some embodiments, the method further includes:

[0013] The operating state of the flexible DC line is detected to determine whether a fault occurs in the flexible DC line.

[0014] In combination with the first aspect, in some embodiments, the method further includes:

[0015] Obtaining an equivalent circuit diagram of the instantaneous fault of the first hybrid DC circuit breaker, and analyzing the equivalent circuit diagram of the instantaneous fault to obtain a first expression of a transient voltage across the first hybrid DC circuit breaker and a transient voltage on the line side when the first hybrid DC circuit breaker has a transient fault;

[0016] Acquiring an equivalent circuit diagram of a permanent fault of the first hybrid DC circuit breaker, and performing circuit analysis on the equivalent circuit diagram of the permanent fault to obtain a second expression of a transient voltage across the first hybrid DC circuit breaker and a transient voltage on the line side when a permanent fault occurs in the first hybrid DC circuit breaker;

[0017] A target ratio is determined based on the first expression and the second expression, and a judgment threshold is determined based on the target ratio; wherein the target ratio is the ratio of three times the equivalent capacitance of the flexible DC line to the equivalent capacitance of the transfer branch of the first hybrid DC circuit breaker; and the judgment threshold is greater than the target ratio.

[0018] In combination with the first aspect, in some embodiments, before calculating the ratio of the transient voltage across the hybrid DC circuit breaker and the transient voltage on the line side, the method further includes:

[0019] The transient voltage across the first hybrid DC circuit breaker and the transient voltage on the line side are filtered.

[0020] In combination with the first aspect, in some embodiments, the transient voltage ratio is calculated based on the following formula:

[0021]

[0022] Where, K is the transient voltage ratio of the flexible DC line, is the DC circuit breaker transient voltage, is the transient voltage on the line side, ω is the angular frequency of the flexible DC line, ω 0 is the preset filter frequency.

[0023] In combination with the first aspect, in some embodiments, judging the fault type of the flexible DC line based on the transient voltage ratio includes:

[0024] Establish the basis for fault status identification:

[0025]

[0026] Among them, K set is the judgment threshold, K is the transient voltage ratio of the flexible DC line, G is the fault state of the flexible DC line, G=0 is a permanent fault, G=1 is a transient fault;

[0027] The fault type of the flexible DC line is determined according to the transient voltage ratio and the fault state identification basis.

[0028] In combination with the first aspect, in some embodiments, the method further includes:

[0029] When the transient voltage on the line side is 0, the flexible DC line is in a permanent fault.

[0030] In combination with the first aspect, in some embodiments, if the fault of the flexible DC line is a transient fault, controlling the first hybrid DC circuit breaker and the second hybrid DC circuit breaker to perform a reclosing action includes:

[0031] If the fault of the flexible DC line is a transient fault, the transient voltage ratio is recalculated after a delay of a preset time length;

[0032] The fault type of the flexible DC line is re-determined based on the recalculated transient voltage ratio; if the re-determined fault type of the DC line is a transient fault, the first hybrid DC circuit breaker and the second hybrid DC circuit breaker are controlled to perform a reclosing action.

[0033] In combination with the first aspect, in some embodiments, if the re-determined fault type of the DC line is a transient fault, controlling the first hybrid DC circuit breaker and the second hybrid DC circuit breaker to perform a reclosing action includes:

[0034] If the re-determined fault type of the flexible DC line is a transient fault, controlling the first hybrid DC circuit breaker to perform a reclosing action;

[0035] The voltage of the flexible DC line is detected, and if the voltage of the flexible DC line is within a normal range, the second hybrid DC circuit breaker is controlled to perform a reclosing action.

[0036] In a second aspect, an embodiment of the present invention provides a transient voltage ratio flexible DC line adaptive reclosing device, comprising:

[0037] A detection module, configured to control the first hybrid DC circuit breaker and the second hybrid DC circuit breaker at both ends of the flexible DC circuit to be disconnected when a fault is detected in the flexible DC circuit;

[0038] A measuring module, used for measuring the transient voltage at both ends of the first hybrid DC circuit breaker and the transient voltage on the line side;

[0039] a calculation module, configured to calculate a ratio of a transient voltage across the first hybrid DC circuit breaker and a transient voltage on the line side as a transient voltage ratio;

[0040] A judging module, configured to judge a fault type of the flexible DC line based on the transient voltage ratio;

[0041] A control module is used to control the reclosing of the first hybrid DC circuit breaker and the second hybrid DC circuit breaker not to operate if the fault type is a permanent fault; and to control the first hybrid DC circuit breaker and the second hybrid DC circuit breaker to perform reclosing if the fault type is a transient fault.

[0042] The embodiment of the present invention provides a flexible DC line adaptive reclosing method and device based on transient voltage ratio, which detects the fault state of the flexible DC line and controls the hybrid DC circuit breakers at both ends to disconnect according to the fault state, and then obtains the transient voltage ratio by measuring the transient voltage at both ends of the circuit breaker and the transient voltage on the line side, and finally determines the fault type, and determines whether reclosing action is required according to the fault type. By determining the fault type through the transient voltage ratio, the adverse effect of the transient process on the fault judgment is reduced, and it will not be affected by the fault location and fault type, and has a strong ability to withstand transient resistance. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings required for use in the embodiments or related technical descriptions are briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0044] Figure 1It is a flow chart of a method for adaptive reclosing of a flexible DC line based on transient voltage ratio provided by an embodiment of the present invention;

[0045] Figure 2 is a topological structure diagram of a DC power grid provided by an embodiment of the present invention;

[0046] Figure 3 is a topological structure diagram of a hybrid DC circuit breaker provided in one embodiment of the present invention;

[0047] Figure 4 It is a schematic diagram of the electronic power switch provided by one embodiment of the present invention;

[0048] Figure 5 is an instantaneous fault equivalent circuit diagram of a hybrid DC circuit breaker provided by an embodiment of the present invention;

[0049] Figure 6 is a permanent fault equivalent circuit diagram of a hybrid DC circuit breaker provided by an embodiment of the present invention;

[0050] Figure 7 is a relationship diagram between transient voltage ratio and time of a flexible DC line provided by an embodiment of the present invention when a transient single-stage grounding fault occurs;

[0051] Figure 8 is a relationship diagram between transient voltage ratio and time of a flexible DC line provided by an embodiment of the present invention when a permanent single-pole grounding fault occurs;

[0052] Fig. 9 is a relationship diagram between transient voltage ratio and time when a transient single-pole grounding fault occurs at different positions of a flexible DC line provided by an embodiment of the present invention;

[0053] Fig.10 is a relationship diagram between transient voltage ratio and time of a flexible DC line provided by an embodiment of the present invention when a permanent single-pole grounding fault occurs at different positions of the flexible DC line;

[0054] Fig.11 It is a structural schematic diagram of a flexible DC line adaptive reclosing device based on transient voltage ratio provided in an embodiment of the present invention. DETAILED DESCRIPTION

[0055] In the following description, specific details such as specific system structures, technologies, etc. are provided for the purpose of illustration rather than limitation, so as to provide a thorough understanding of the embodiments of the present invention. However, it should be clear to those skilled in the art that the present invention may be implemented in other embodiments without these specific details. In other cases, detailed descriptions of well-known systems, devices, circuits, and methods are omitted to prevent unnecessary details from obstructing the description of the present invention.

[0056] It should be understood that when used in the present specification and the appended claims, the term "comprising" indicates the presence of described features, integers, steps, operations, elements and / or components, but does not exclude the presence or addition of one or more other features, integers, steps, operations, elements, components and / or combinations thereof.

[0057] It should also be understood that the term "and / or" used in the present description and the appended claims refers to and includes any and all possible combinations of one or more of the associated listed items.

[0058] As used in the present specification and the appended claims, the term "if" may be interpreted as "when" or "upon" or "in response to determining" or "in response to detecting", depending on the context. Similarly, the phrase "if it is determined" or "if [described condition or event] is detected" may be interpreted as meaning "upon determination" or "in response to determining" or "upon detection of [described condition or event]" or "in response to detecting [described condition or event]", depending on the context.

[0059] In addition, in the description of the present specification and the appended claims, the terms "first", "second", "third", etc. are only used to distinguish the descriptions and cannot be understood as indicating or implying relative importance.

[0060] References to "one embodiment" or "some embodiments" etc. described in the present specification mean that one or more embodiments of the present invention include specific features, structures or characteristics described in conjunction with the embodiment. Therefore, the statements "in one embodiment", "in some embodiments", "in some other embodiments", "in some other embodiments", etc. that appear in different places in this specification do not necessarily refer to the same embodiment, but mean "one or more but not all embodiments", unless otherwise specifically emphasized in other ways. The terms "including", "comprising", "having" and their variations all mean "including but not limited to", unless otherwise specifically emphasized in other ways.

[0061] Figure 1 is a flow chart of a flexible DC line adaptive reclosing method based on transient voltage ratio provided by an embodiment of the present invention, with reference to Figure 1 The flexible DC line adaptive reclosing method based on transient voltage ratio is described as follows:

[0062] S110: When a fault is detected in the flexible DC line, a first hybrid DC circuit breaker and a second hybrid DC circuit breaker at both ends of the flexible DC line are controlled to be disconnected.

[0063] Figure 2is the topological structure diagram of the DC grid, where Line 12 is the flexible DC line, MMC is the flexible DC converter station (Modular Multilevel Converter, MMC), R 12 and R 21 For hybrid DC circuit breaker, M side and N side represent the two ends of the circuit, L 12 and L 21 Indicates a current limiting reactor.

[0064] like Figure 2 As shown, the first hybrid DC circuit breaker R 12 and the second hybrid DC circuit breaker R 21 Located at both ends of the flexible DC line, the line length in Table 1 refers to the length of the flexible DC line, and some parameters in the table are not shown in the figure.

[0065] Combine the following Figure 3 Introduction of hybrid DC circuit breaker:

[0066] A hybrid DC circuit breaker is usually composed of a main branch, a transfer branch and an energy consumption branch in parallel. The main branch is composed of a fast mechanical switch (Ultra-Fastdisconnector Diode, UFD) and a power electronic switch PSM, the transfer branch is composed of a cascade of pure electronic power switches PST, and the energy consumption branch is composed of metal oxide nonlinear resistors. Figure 4 As shown, the electronic power switch can have various configurations.

[0067] When the system is in steady-state operation, the main branch is turned on and the load current flows through the main branch. When a fault occurs in the system, the DC circuit breaker is activated, the main branch submodule is locked, and the electronic power switch PSM is turned off first, and the current is transferred to the transfer branch. After the main branch current is 0, the fast mechanical switch UFD is turned off. After that, the transfer branch is locked, and the pure electronic power switch PST on the transfer branch is turned off. At this time, the current is transferred to the energy-consuming branch, and a reverse voltage higher than the rated voltage of the system is established during the transfer, and the fault current is cleared through the energy-consuming branch.

[0068] If the fault is a transient fault, after the hybrid DC circuit breaker is actuated, after a period of time, the arc at the fault point is extinguished and the fault channel is fully freed, and the hybrid DC circuit breakers on both sides of the fault line are reclosed.

[0069] If the fault is a permanent fault, the reclosing operation will not be performed. Performing the reclosing operation in the case of a permanent fault will generate a large fault current, causing serious secondary impact on the fault line and related equipment.

[0070] In a possible implementation, before step S110, the method further includes:

[0071] The operating status of the flexible DC line is detected to determine whether a fault occurs in the flexible DC line.

[0072] Optionally, when a fault occurs in the flexible DC line, both the first hybrid DC circuit breaker and the second hybrid DC circuit breaker will be disconnected.

[0073] S120: Measure the transient voltage across the first hybrid DC circuit breaker and the transient voltage on the line side.

[0074] In this embodiment, Figure 2 As shown, the transient voltage across the first hybrid DC circuit breaker is 12 The voltage at both ends, the line-side transient voltage is the voltage of the flexible DC line adjacent to the first hybrid DC circuit breaker.

[0075] In this embodiment, during the disconnection process of the DC circuit breaker, the voltage will change briefly, and the voltage measured during the brief change process is the transient voltage.

[0076] When analyzing the faults of flexible DC lines, it is found that when permanent faults and transient faults occur in the circuit, the transient voltage ratio has obvious differences in value. Therefore, the fault state of the flexible DC line can be determined by the transient voltage ratio. The specific derivation process is as follows:

[0077] In a possible implementation, before step S120, the following steps are included:

[0078] Obtaining an equivalent circuit diagram of the transient fault of the first hybrid DC circuit breaker, and analyzing the equivalent circuit diagram of the transient fault to obtain a first expression of a transient voltage across the first hybrid DC circuit breaker and a transient voltage on the line side when the first hybrid DC circuit breaker has a transient fault;

[0079] Obtaining an equivalent circuit diagram of a permanent fault of the first hybrid DC circuit breaker, and performing circuit analysis on the equivalent circuit diagram of the permanent fault to obtain a second expression of a transient voltage across the first hybrid DC circuit breaker and a transient voltage on the line side when a permanent fault occurs in the first hybrid DC circuit breaker;

[0080] A target ratio is determined based on the first expression and the second expression, and a judgment threshold is determined based on the target ratio; wherein the target ratio is three times the ratio of the equivalent capacitance of the flexible DC line to the equivalent capacitance of the transfer branch of the first hybrid DC circuit breaker; and the judgment threshold is greater than the target ratio.

[0081] In this embodiment, the instantaneous fault equivalent circuit diagram of the first hybrid DC circuit breaker is as follows: Figure 5 As shown, L m is the current limiting reactor, C 1 is the DC circuit breaker transfer branch capacitance, L 2 is the equivalent inductance of the DC line, R 2 is the equivalent resistance of the DC line, C 2 is the equivalent capacitance of the DC line. By analyzing the equivalent circuit diagram of the transient fault, it can be obtained that, when a transient fault occurs, the first expression of the transient voltage across the first hybrid DC circuit breaker is:

[0082]

[0083] Among them, u 1 (s) is the transient voltage across the first hybrid DC circuit breaker during a transient fault, s is the complex frequency domain, and E(s) is the voltage between the current limiting reactor and the equivalent capacitance of the DC circuit breaker transfer branch.

[0084] In case of instantaneous fault, the first expression of transient voltage on the line side is:

[0085]

[0086] Among them, u 2 (s) is the transient voltage on the line side during an instantaneous fault, s is the complex frequency domain, and E(s) is the voltage between the current limiting reactor and the equivalent capacitance of the transfer branch of the DC circuit breaker.

[0087] In this embodiment, the permanent fault equivalent circuit diagram of the first hybrid DC circuit breaker is as follows: Figure 6 As shown, L m is the current limiting reactor, C 1 is the DC circuit breaker transfer branch capacitance, L 2 is the equivalent inductance of the DC line, R 2 is the equivalent resistance of the DC line, C 2 is the DC line equivalent capacitance, R f is the fault resistance. By analyzing the permanent fault equivalent circuit diagram, it can be obtained that, in the case of a permanent fault, the second expression of the transient voltage across the first hybrid DC circuit breaker is:

[0088]

[0089] Among them, u′ 1 (s) is the transient voltage across the first hybrid DC circuit breaker during a permanent fault, s is the complex frequency domain, and E(s) is the voltage between the current limiting reactor and the equivalent capacitance of the transfer branch of the DC circuit breaker.

[0090] In case of permanent fault, the second expression of transient voltage on the line side is:

[0091]

[0092] Among them, u′ 2 (s) is the transient voltage on the line side during a permanent fault, s is the complex frequency domain, and E(s) is the voltage between the current limiting reactor and the equivalent capacitance of the transfer branch of the DC circuit breaker.

[0093] In case of a transient fault, the transient voltage ratio of the transient fault can be obtained according to the first expression of the transient voltage across the first hybrid DC circuit breaker and the transient voltage on the line side:

[0094]

[0095] The amplitude-frequency characteristic of the transient voltage ratio of a transient fault can be expressed as:

[0096]

[0097] Where j is a complex unit and ω is the voltage frequency.

[0098] In the case of transient faults, as the frequency increases, the amplitude of the transient voltage ratio increases first and then decreases.

[0099] In case of permanent fault, the transient voltage ratio of permanent fault can be obtained according to the second expression of the transient voltage across the first hybrid DC circuit breaker and the transient voltage on the line side:

[0100]

[0101] The amplitude-frequency characteristic of the transient voltage ratio of a permanent fault is:

[0102]

[0103] Where j is a complex unit and ω is the voltage frequency.

[0104] In the case of permanent fault, as the frequency increases, the transient voltage ratio amplitude tends to decrease overall.

[0105] In case of permanent fault, the transient voltage ratio is calculated as The frequency value at time can be obtained:

[0106]

[0107] In case of transient fault, when the frequency is less than ω 0 When the transient voltage ratio is less than In the case of permanent fault, when the frequency is less than ω 0 When the transient voltage ratio is greater than

[0108] S130: Calculate a ratio of a transient voltage across the first hybrid DC circuit breaker and a transient voltage on the line side as a transient voltage ratio.

[0109] Through the above derivation, the transient voltage ratio can be selected as the basis for judgment.

[0110] In an optional embodiment, before calculating the ratio of the transient voltage across the hybrid DC circuit breaker and the transient voltage on the line side, the method further includes:

[0111] The transient voltage across the first hybrid DC circuit breaker and the transient voltage on the line side are filtered.

[0112] Among them, through the above calculation, the filtering frequency is determined to be ω 0 .

[0113] In an optional embodiment, the transient voltage ratio is calculated based on the following formula:

[0114]

[0115] Where, K is the transient voltage ratio of the flexible DC line, is the transient voltage of the DC circuit breaker, is the transient voltage on the line side, ω is the angular frequency of the flexible DC line, ω 0 is the preset filter frequency.

[0116] S140: Determine the fault type of the flexible DC line based on the transient voltage ratio.

[0117] In an optional embodiment, judging the fault type of the flexible DC line based on the transient voltage ratio includes:

[0118] Establish the basis for fault status identification:

[0119]

[0120] Among them, K set is the judgment threshold, K is the transient voltage ratio of the flexible DC line, G is the fault state of the flexible DC line, G=0 is a permanent fault, G=1 is a transient fault;

[0121] The fault type of the flexible DC line is determined based on the transient voltage ratio and the fault state identification basis.

[0122] The basis for determining the judgment threshold is: However, it is necessary to take into account the errors that may occur during actual operation and add a certain margin on this basis, which is generally taken as 10.

[0123] In this embodiment, the fault state identification basis is obtained through the above derivation process. By constructing the fault identification basis and combining the fault identification basis with the transient voltage ratio, the operating state of the flexible DC circuit is judged. If the calculated transient voltage ratio is greater than or equal to the judgment threshold, the fault occurring in the flexible DC line is a permanent fault. If the calculated transient voltage ratio is less than the judgment threshold, the fault occurring in the flexible DC line is a transient fault.

[0124] In an optional embodiment, it also includes:

[0125] When the transient voltage on the line side is 0, the flexible DC line is in a permanent fault.

[0126] When a short circuit occurs between the outlet of the flexible DC line and the ground, the transient voltage on the line side is 0. At this time, the denominator of the transient voltage ratio is 0, and the transient voltage ratio cannot be obtained. In the case of this fault, the fault can be directly determined as a permanent fault based on the transient voltage on the line side being 0.

[0127] By calculating the transient voltage ratio and combining it with the judgment threshold, the fault state of the flexible DC line is determined, making the judgment process simpler and not affected by the circuit transient state, which can improve the efficiency and accuracy of determining the fault type of the flexible DC line system.

[0128] S150: If the fault type is a permanent fault, control the reclosing of the first hybrid DC circuit breaker and the second hybrid DC circuit breaker not to operate; if the fault type is a transient fault, control the first hybrid DC circuit breaker and the second hybrid DC circuit breaker to perform reclosing operation.

[0129] Reference Figure 3 After detecting that the fault of the system has been cleared (for example, the voltage or current returns to normal), the reclosing action is performed, and the pure electronic power switch PST on the transfer branch is closed first, so that the transfer branch is turned on. If there is no fault on the flexible DC line after the transfer branch is turned on, the fast mechanical switch UFD on the main branch is closed, and then the electronic power switch PSM on the main branch is closed. At this time, the reclosing action is completed, and a stable current flows on the main branch.

[0130] When a fault occurs in the flexible DC line, the two hybrid DC breakers at both ends of the line are disconnected. The transient voltage at both ends of the first hybrid DC breaker on one side of the line and the transient voltage on the line side are measured, and the transient voltage ratio is calculated. The type of fault of the line is determined by the transient voltage ratio, and whether the first hybrid DC breaker can perform a reclosing action is determined by the fault type. The system is less affected by the transient process of the fault, avoiding secondary shock to the system, and can correctly judge the nature of the fault in different fault locations and different fault types, and realize rapid restart of the system in the case of transient faults, and has strong transient resistance resistance.

[0131] In an optional embodiment, if the fault of the flexible DC line is a transient fault, controlling the first hybrid DC circuit breaker and the second hybrid DC circuit breaker to perform a reclosing action includes:

[0132] If the fault of the flexible DC line is a transient fault, the transient voltage ratio is recalculated after a delay of a preset time;

[0133] The fault type of the flexible DC line is re-determined based on the recalculated transient voltage ratio. If the re-determined fault type of the DC line is a transient fault, the first hybrid DC circuit breaker and the second hybrid DC circuit breaker are controlled to perform a reclosing action.

[0134] In an optional embodiment, if the re-determined fault type of the DC line is a transient fault, controlling the first hybrid DC circuit breaker and the second hybrid DC circuit breaker to perform a reclosing action includes:

[0135] If the re-determined fault type of the flexible DC line is a transient fault, the first hybrid DC circuit breaker is controlled to perform a reclosing action;

[0136] The voltage of the flexible DC line is detected, and if the voltage of the flexible DC line is within a normal range, the second hybrid DC circuit breaker is controlled to perform a reclosing action.

[0137] If the fault of the flexible DC line determined by the transient voltage ratio is a transient fault, then after Δt 2 After a delay, the transient voltage at both ends of the first hybrid DC circuit breaker and the line side voltage are collected again, and the transient voltage ratio is recalculated. If the recalculated transient voltage ratio still meets the condition of the transient fault, the first hybrid DC circuit breaker is controlled to perform a reclosing action. After the first hybrid DC circuit breaker completes the reclosing action, the voltage state of the flexible DC line is judged again. If the voltage of the flexible DC line is stable within a normal range, the second hybrid DC circuit breaker is controlled to perform a reclosing action. Otherwise, the second hybrid DC circuit breaker does not perform a reclosing action.

[0138] Therefore, in the case of a permanent fault, the hybrid DC circuit breakers on both sides of the line will not reclose, and reclosing will only be performed in the case of a transient fault, avoiding reclosing on a permanent fault and causing a secondary impact on the system. In addition, there is no need for communication between the two sides of the line, so it will not be interfered by the communication system, avoiding the influence of the communication system on the reclosing method and is easy to design and implement.

[0139] The following are some examples from practical applications to illustrate how the transient voltage ratio can be adapted to different situations:

[0140] In this embodiment, Figure 2 is the topological structure diagram of the DC power grid, such as Figure 2 As shown, f 1 It indicates that an intra-area fault has occurred in the flexible DC line. The main parameters for simulating the flexible DC line in the DC grid are shown in Table 1:

[0141] Table 1 Main operating parameters of MMC-based HVDC system

[0142]

[0143] Example 1:

[0144] Reference Figure 7 When a transient single-pole grounding fault occurs in the flexible DC line and the transition resistance varies from 0 to 300Ω, the circuit breaker clears the fault at 0 seconds and the secondary arc is extinguished at 0.05 seconds. The transient voltage under this fault condition is as follows: Figure 7 shown.

[0145] Depend on Figure 7 It can be seen that when the time is less than 0.05 seconds, the DC line is in the secondary arc stage, the arc will experience repeated reignition and extinguishing processes, and the transient voltage ratio will fluctuate accordingly, but it is greater than the threshold value, and the reclosing switch will not operate. When t>0.05s, the transient voltage ratio decreases rapidly and is less than the threshold value. It can be judged that a transient fault has occurred in the DC line and the secondary arc has been extinguished, and the reclosing switch is started. Figure 7 It can be seen that when the fault has not disappeared, the greater the fault resistance, the greater the transient voltage ratio. This is because when the fault resistance increases, the attenuation rate of the high-frequency component of the voltage will also increase, and the line side voltage is closer to 0, so the calculated transient voltage ratio is larger.

[0146] According to the above analysis, this method can quickly and accurately identify the nature of the fault, is less affected by the transient process of the fault, and has a strong ability to withstand transition resistance.

[0147] Example 2:

[0148] Reference Figure 8 When a permanent single-pole grounding fault occurs in the flexible DC line and the transition resistance varies from 0 to 300Ω, the circuit breaker clears the fault at the 0th second. The transient voltage under this fault condition is as follows: Figure 8 shown.

[0149] Depend on Figure 8 It can be seen that when a permanent fault occurs in the flexible DC line, the transient voltage ratio is always greater than the threshold value during the entire fault process. Therefore, it can be identified as a permanent fault, and no restart operation is performed, and the reclosing switch is locked. Figure 8 In the figure, as time increases, the transient voltage ratio fluctuates to a certain extent. At 0.0237 seconds and the transition resistance is 0Ω, K has a minimum value of 49.91, which is much larger than the threshold value.

[0150] Example 3:

[0151] Reference Fig. 9 When a transient single-pole grounding fault occurs at different locations of the flexible DC line and the transition resistance is 300Ω, the circuit breaker clears the fault at 0 seconds and the secondary arc is extinguished at 0.05 seconds. The transient voltage under this fault condition is as follows: Fig. 9 shown.

[0152] Depend on Fig. 9 It can be seen that under the same transition resistance conditions, when the time is less than 0.05 seconds, as the fault distance increases, the transient voltage ratios at different time sections are all greater than the threshold value, and this is identified as a transient fault secondary arc stage or a permanent fault. When the time is greater than 0.05 seconds, the transient voltage ratio decreases rapidly and is less than the threshold value, and it can be judged that a transient fault has occurred in the DC line and the secondary arc has been extinguished, and the reclosing is started. After the secondary arc is extinguished, the transient voltage ratio is relatively stable and is not affected by the fault location, which is consistent with the theoretical analysis.

[0153] According to the above analysis, the fault nature identification criterion proposed in this paper is less affected by the fault location and can realize the identification of fault nature of the entire line.

[0154] Example 4:

[0155] Reference Fig.10 , when a permanent single-pole grounding fault occurs at different locations of the flexible DC line and the transition resistance is 300Ω, the circuit breaker clears the fault at 0 seconds. The transient voltage under this fault condition is as follows Fig.10 shown.

[0156] Depend on Fig.10 It can be seen that after the fault is removed, the transient voltage ratio is always greater than the threshold value, which is determined to be a permanent fault and no restart operation is performed. Fig.10In the same time section, as the fault distance increases, the transient voltage ratio gradually decreases. When a fault occurs at 100% of the distance from the head end, the transient voltage ratio at the 0.05 second value is 441.5, which is much larger than the threshold value and can be accurately identified as a permanent fault.

[0157] From the above analysis, it can be seen that when permanent faults occur at different locations of the DC line, the method proposed in this paper can correctly identify the nature of the fault and prevent the problem of multiple overcurrent shocks caused by overlapping with the permanent fault.

[0158] When a fault occurs in the flexible DC line, the hybrid DC circuit breakers at both ends are controlled to disconnect, and the transient voltage at both ends of the first hybrid DC circuit breaker and the transient voltage on the line side are measured to obtain the transient voltage ratio, and then the relationship between the transient voltage ratio and the judgment threshold is used to determine whether it is a transient fault or a permanent fault, and the first hybrid DC circuit breaker is controlled to perform a reclosing action in the case of a transient fault, and the second hybrid DC circuit breaker is controlled to perform a reclosing action after the reclosing action is completed and the circuit voltage is restored. No additional control strategy is required, and no communication is required between the two sides of the line, and there will be no interference from the communication system. The first hybrid DC circuit breaker is controlled not to perform a reclosing action in the case of a permanent fault, which can avoid interference from the transient process and avoid the hybrid DC circuit breaker performing a reclosing action in the case of a permanent fault. The secondary impact on the system caused by the reclosing action. In addition, this solution can accurately judge the nature of the fault when different fault locations and different fault types occur, can achieve rapid restart of the system in the case of a transient fault, and has a strong ability to withstand transition resistance.

[0159] It should be understood that the order of execution of the steps in the above embodiment does not necessarily mean the order of execution. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiment of the present invention.

[0160] Corresponding to the flexible DC line adaptive reclosing method based on transient voltage ratio in the above embodiment, Fig.11 A schematic structural diagram of a flexible DC line adaptive reclosing device based on transient voltage ratio provided in an embodiment of the present invention is shown. For ease of description, only the parts related to the embodiment of the present invention are shown.

[0161] See also Fig.11 The flexible DC line adaptive reclosing device based on transient voltage ratio in the embodiment of the present invention may include:

[0162] The detection module 111 is used to control the first hybrid DC circuit breaker and the second hybrid DC circuit breaker at both ends of the flexible DC circuit to be disconnected when a fault is detected in the flexible DC circuit;

[0163] The measuring module 112 is used to measure the transient voltage across the first hybrid DC circuit breaker and the transient voltage on the line side;

[0164] A calculation module 113, used to calculate the ratio of the transient voltage across the first hybrid DC circuit breaker and the transient voltage on the line side as a transient voltage ratio;

[0165] A judgment module 114, configured to judge the fault type of the flexible DC line based on the transient voltage ratio;

[0166] The control module 115 is used to control the reclosing of the first hybrid DC circuit breaker and the second hybrid DC circuit breaker to not operate if the fault type is a permanent fault; and to control the reclosing of the first hybrid DC circuit breaker and the second hybrid DC circuit breaker to operate if the fault type is a transient fault.

[0167] In some embodiments, the detection module 111 is specifically configured to:

[0168] The operating status of the flexible DC line is detected to determine whether a fault occurs in the flexible DC line.

[0169] In some embodiments, the computing module 112 is specifically configured to:

[0170] Obtaining an equivalent circuit diagram of the transient fault of the first hybrid DC circuit breaker, and analyzing the equivalent circuit diagram of the transient fault to obtain a first expression of the transient voltage across the first hybrid DC circuit breaker and the transient voltage on the line side when the first hybrid DC circuit breaker has a transient fault;

[0171] Obtaining an equivalent circuit diagram of a permanent fault of the first hybrid DC circuit breaker, and performing circuit analysis on the equivalent circuit diagram of the permanent fault to obtain a second expression of a transient voltage across the first hybrid DC circuit breaker and a transient voltage on the line side when a permanent fault occurs in the first hybrid DC circuit breaker;

[0172] A target ratio is determined based on the first expression and the second expression, and a judgment threshold is determined based on the target ratio; wherein the target ratio is three times the ratio of the equivalent capacitance of the flexible DC line to the equivalent capacitance of the transfer branch of the first hybrid DC circuit breaker; and the judgment threshold is greater than the target ratio.

[0173] In some embodiments, the measurement module 112 is further configured to:

[0174] The transient voltage across the first hybrid DC circuit breaker and the transient voltage on the line side are filtered.

[0175] In some embodiments, the computing module 113 is specifically configured to:

[0176] The transient voltage ratio is calculated based on the following formula:

[0177]

[0178] Where, K is the transient voltage ratio of the flexible DC line, is the transient voltage of the DC circuit breaker, is the transient voltage on the line side, ω is the angular frequency of the flexible DC line, ω 0 is the preset filter frequency.

[0179] In some embodiments, the determination module 114 is specifically configured to:

[0180] Establish the basis for fault status identification:

[0181]

[0182] Among them, K set is the judgment threshold, K is the transient voltage ratio of the flexible DC line, G is the fault state of the flexible DC line, G=0 is a permanent fault, G=1 is a transient fault;

[0183] The fault type of the flexible DC line is determined based on the transient voltage ratio and the fault state identification basis.

[0184] In some embodiments, the determination module 114 is further configured to:

[0185] When the transient voltage on the line side is 0, the flexible DC line is in a permanent fault.

[0186] In some embodiments, the control module 115 is specifically configured to:

[0187] If the fault of the flexible DC line is a transient fault, the transient voltage ratio is recalculated after a delay of a preset time;

[0188] The fault type of the flexible DC line is re-determined based on the recalculated transient voltage ratio. If the re-determined fault type of the DC line is a transient fault, the first hybrid DC circuit breaker and the second hybrid DC circuit breaker are controlled to perform a reclosing action.

[0189] In some embodiments, the control module 115 is specifically configured to:

[0190] If the re-determined fault type of the flexible DC line is a transient fault, the first hybrid DC circuit breaker is controlled to perform a reclosing action;

[0191] The voltage of the flexible DC line is detected, and if the voltage of the flexible DC line is within a normal range, the second hybrid DC circuit breaker is controlled to perform a reclosing action.

[0192] In the above embodiments, the description of each embodiment has its own emphasis. For parts that are not described or recorded in detail in a certain embodiment, reference can be made to the relevant descriptions of other embodiments.

[0193] Those of ordinary skill in the art will appreciate that the templates, units, and algorithm steps of each example described in conjunction with the embodiments disclosed herein can be implemented in electronic hardware, or a combination of computer software and electronic hardware. Whether these functions are performed in hardware or software depends on the specific application and design constraints of the technical solution. Professional and technical personnel can use different methods to implement the described functions for each specific application, but such implementation should not be considered to be beyond the scope of the present invention.

[0194] If the module / unit is implemented in the form of a software functional unit and sold or used as an independent product, it can be stored in a computer-readable storage medium. Based on this understanding, the present invention implements all or part of the processes in the above-mentioned embodiment method, and can also be completed by instructing the relevant hardware through a computer program. The computer program can be stored in a computer-readable storage medium. When the computer program is executed by the processor, it can implement the steps of the above-mentioned flexible DC line adaptive reclosing method embodiment based on transient voltage ratio. Among them, the computer program includes computer program code, and the computer program code can be in source code form, object code form, executable file or some intermediate form. The computer-readable medium may include: any entity or device that can carry the computer program code, recording medium, U disk, mobile hard disk, disk, optical disk, computer memory, read-only memory, random access memory, electric carrier signal, telecommunication signal and software distribution medium.

[0195] The embodiments described above are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that the technical solutions described in the aforementioned embodiments may still be modified, or some of the technical features may be replaced by equivalents. Such modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention, and should all be included in the protection scope of the present invention.

Claims

1. A flexible DC line adaptive reclosing method based on transient voltage ratio, characterized in that: include: When a fault in the flexible DC line is detected, controlling a first hybrid DC circuit breaker and a second hybrid DC circuit breaker at both ends of the flexible DC line to be disconnected; measuring a transient voltage across the first hybrid DC circuit breaker and a transient voltage on the line side; calculating a ratio of a transient voltage across the first hybrid DC circuit breaker and a transient voltage on the line side as a transient voltage ratio; Based on the transient voltage ratio, determining a fault type of the flexible DC line; If the fault type is a permanent fault, the reclosing of the first hybrid DC circuit breaker and the second hybrid DC circuit breaker is controlled not to operate; if the fault type is a transient fault, the first hybrid DC circuit breaker and the second hybrid DC circuit breaker are controlled to perform reclosing.

2. The method for adaptive reclosing of flexible DC lines based on transient voltage ratio according to claim 1, characterized in that: The method further comprises: The operating state of the flexible DC line is detected to determine whether a fault occurs in the flexible DC line.

3. The method for adaptive reclosing of flexible DC lines based on transient voltage ratio according to claim 1, characterized in that: Also includes: Obtaining an equivalent circuit diagram of the instantaneous fault of the first hybrid DC circuit breaker, and analyzing the equivalent circuit diagram of the instantaneous fault to obtain a first expression of a transient voltage across the first hybrid DC circuit breaker and a transient voltage on the line side when the first hybrid DC circuit breaker has a transient fault; Acquiring an equivalent circuit diagram of a permanent fault of the first hybrid DC circuit breaker, and performing circuit analysis on the equivalent circuit diagram of the permanent fault to obtain a second expression of a transient voltage across the first hybrid DC circuit breaker and a transient voltage on the line side when a permanent fault occurs in the first hybrid DC circuit breaker; Determine a target ratio based on the first expression and the second expression, and determine a judgment threshold based on the target ratio; wherein the target ratio is the ratio of the three times equivalent capacitance of the flexible DC line to the equivalent capacitance of the transfer branch of the first hybrid DC circuit breaker; The judgment threshold is greater than the target ratio.

4. The method for adaptive reclosing of flexible DC lines based on transient voltage ratio according to claim 1, characterized in that: Before calculating the ratio of the transient voltage across the hybrid DC circuit breaker and the transient voltage on the line side, the method further includes: The transient voltage across the first hybrid DC circuit breaker and the transient voltage on the line side are filtered.

5. The method for adaptive reclosing of flexible DC lines based on transient voltage ratio according to claim 1, characterized in that: The transient voltage ratio is calculated based on the following formula: Where, K is the transient voltage ratio of the flexible DC line, is the transient voltage of the DC circuit breaker, is the transient voltage on the line side, ω is the angular frequency of the flexible DC line, and ω0 is the preset filtering frequency.

6. The method for adaptive reclosing of flexible DC lines based on transient voltage ratio according to claim 1, characterized in that: The determining, based on the transient voltage ratio, the fault type of the flexible DC line comprises: Establish the basis for fault status identification: Among them, K set is the judgment threshold, K is the transient voltage ratio of the flexible DC line, G is the fault state of the flexible DC line, G=0 is a permanent fault, G=1 is a transient fault; The fault type of the flexible DC line is determined according to the transient voltage ratio and the fault state identification basis.

7. The method for adaptive reclosing of flexible DC lines based on transient voltage ratio according to claim 1, characterized in that: The method further comprises: When the transient voltage on the line side is 0, the flexible DC line is in a permanent fault.

8. The method for adaptive reclosing of flexible DC lines based on transient voltage ratio according to claim 1, characterized in that: If the fault of the flexible DC line is a transient fault, controlling the first hybrid DC circuit breaker and the second hybrid DC circuit breaker to perform a reclosing action includes: If the fault of the flexible DC line is a transient fault, the transient voltage ratio is recalculated after a delay of a preset time length; The fault type of the flexible DC line is re-determined based on the recalculated transient voltage ratio; if the re-determined fault type of the DC line is a transient fault, the first hybrid DC circuit breaker and the second hybrid DC circuit breaker are controlled to perform a reclosing action.

9. The method for adaptive reclosing of flexible DC lines based on transient voltage ratio according to claim 8, characterized in that: If the re-determined fault type of the DC line is a transient fault, controlling the first hybrid DC circuit breaker and the second hybrid DC circuit breaker to perform a reclosing action, comprising: If the re-determined fault type of the flexible DC line is a transient fault, controlling the first hybrid DC circuit breaker to perform a reclosing action; The voltage of the flexible DC line is detected, and if the voltage of the flexible DC line is within a normal range, the second hybrid DC circuit breaker is controlled to perform a reclosing action.

10. A flexible DC line adaptive reclosing device based on transient voltage ratio, characterized in that: include: A detection module, configured to control the first hybrid DC circuit breaker and the second hybrid DC circuit breaker at both ends of the flexible DC circuit to be disconnected when a fault is detected in the flexible DC circuit; A measuring module, used for measuring the transient voltage at both ends of the first hybrid DC circuit breaker and the transient voltage on the line side; a calculation module, configured to calculate a ratio of a transient voltage across the first hybrid DC circuit breaker and a transient voltage on the line side as a transient voltage ratio; A judging module, configured to judge a fault type of the flexible DC line based on the transient voltage ratio; A control module is used to control the reclosing of the first hybrid DC circuit breaker and the second hybrid DC circuit breaker not to operate if the fault type is a permanent fault; and to control the first hybrid DC circuit breaker and the second hybrid DC circuit breaker to perform reclosing if the fault type is a transient fault.

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