Series-parallel direct-current transmission line and single-end protection method

Abstract

The invention discloses a series-parallel direct-current power transmission line and a single-end protection method. The defects of a traditional single-end protection scheme are effectively overcome. According to the constructed series-parallel direct-current transmission line, a single-end protection device is arranged at the head end, and a circuit breaker is arranged at a direct-current bus, so that a foundation is laid for fault monitoring and processing. According to the single-end protection method, the voltage change rate and the double-protection criterion are used for fault judgment, and accurate protection is achieved. Compared with a traditional scheme, the method has obvious advantages. On one hand, in a traditional scheme, a current-limiting inductor at the tail end of the line is taken as a boundary, the transition resistance tolerance is weak, and in the scheme, a protection boundary is expanded to a current-limiting inductor at the head end of an adjacent line, so that the high-resistance fault identification capability is effectively improved; and on the other hand, traditional single-end protection is difficult to adapt to the hybrid direct current line, while the scheme is specially designed for the hybrid situation and is also suitable for pure cables and pure overhead lines, the application range is greatly widened, and safe and stable operation of the hybrid direct current transmission system can be more reliably and efficiently guaranteed.

Description

Technical Field

[0001] This application relates to the technical field of transmission line protection, and more specifically, to a hybrid series - parallel HVDC transmission line and a single - end protection method. Background Art

[0002] In the scenario where the multi - terminal flexible HVDC transmission technology based on modular multilevel converters is applied to large - scale offshore wind power transmission, the hybrid DC line is composed of a series connection of a DC submarine cable and an overhead line on land. When a fault occurs, the DC voltage drops suddenly and the fault current surges, seriously threatening the safe operation of the system. Therefore, the accurate and rapid response of DC line protection has become the key to ensuring system stability.

[0003] There are obvious shortcomings in the current single - end protection technology for DC lines. First, existing single - end protections generally use the current - limiting inductor at the end of the line as the protection boundary. When a high - resistance fault occurs at the end of the line, due to the limitation of this boundary setting, the electrical characteristics generated by the fault are weak and difficult to be effectively captured, resulting in the protection device being unable to identify the fault in a timely and accurate manner, seriously affecting the fault handling efficiency and system reliability. Second, existing single - end protection schemes are mostly designed for pure overhead lines or pure cable lines, while hybrid DC lines have the electrical characteristics of both cables and overhead lines, and the operating conditions are more complex. Traditional schemes are difficult to adapt to the influence brought by the alternating changes of the characteristics of cables and overhead lines in hybrid lines. When identifying faults in hybrid DC lines, misjudgment or missed judgment is very likely to occur, resulting in protection failure.

[0004] Due to the deficiencies of existing single - end protections for DC lines in high - resistance fault identification and adaptability to hybrid lines, they cannot meet the protection requirements of hybrid DC lines in offshore wind power transmission systems. There is an urgent need to develop a new protection scheme to effectively deal with hybrid DC line faults and ensure the safe and stable operation of the system. Summary of the Invention

[0005] This application provides a hybrid series - parallel HVDC transmission line and a single - end protection method. By extending the protection boundary to the current - limiting inductor at the head of the adjacent line, it overcomes the defect of the traditional scheme's weak tolerance to transition resistance. At the same time, it is designed for hybrid lines and applicable to various line forms, significantly improving the ability to identify high - resistance faults and the application scope, and more reliably and efficiently ensuring the safe and stable operation of the hybrid HVDC transmission system.

[0006] A hybrid series - parallel HVDC transmission line is composed of a hybrid connection of a DC cable and a DC overhead line, with the head connected to the converter station at the new energy output end and the tail connected to the DC bus through a current - limiting inductor;

[0007] Multiple two - port DC circuit breakers or a multi - port DC circuit breaker are configured at the DC bus;

[0008] The head end is far away from the DC bus side and is configured with a single-ended protection device for real-time detection of line faults and triggering protection actions.

[0009] Optionally, the DC bus is connected to at least one adjacent line of the hybrid DC transmission line, wherein each of the adjacent lines is connected via the current-limiting inductor.

[0010] A single-ended protection method for a hybrid DC transmission line, applied to any of the above-mentioned hybrid DC transmission lines, comprising:

[0011] Real-time measurement of the voltage at the protection installation location of the single-ended protection device in the hybrid DC transmission line and calculation of the voltage change rate;

[0012] Setting a first protection criterion and a second protection criterion, wherein the first protection criterion is that when the voltage change rate is less than a first predetermined threshold, it is determined that a fault has occurred in the hybrid DC line; and the second protection criterion is that when the voltage change rate is less than a second predetermined threshold, the current at the protection installation is continuously monitored within a predetermined time window, and if the current is always greater than zero within the predetermined time window, it is determined that a fault has occurred in the hybrid DC line;

[0013] The establishment results of the first protection criterion and the second protection criterion are determined according to the voltage change rate, and corresponding protection actions are triggered.

[0014] Optionally, the first predetermined threshold is determined based on a minimum voltage change rate of a metallic bipolar short circuit fault at the DC bus, and the second predetermined threshold is determined based on a minimum voltage change rate of a metallic bipolar short circuit fault at the head end of the adjacent line.

[0015] Optionally, triggering a corresponding protection action according to the satisfaction of the first protection criterion and the second protection criterion includes:

[0016] When the first protection criterion is met, it is determined that a fault occurs inside the hybrid DC line and a first protection action is triggered;

[0017] When the second protection criterion is met and the current is continuously greater than zero within the predetermined time window, it is determined that a fault has occurred inside the hybrid DC line and a second-stage protection action is triggered;

[0018] The triggering priority of the second-stage protection action is lower than that of the first-stage protection action.

[0019] Optionally, also include:

[0020] If the current at the protection installation is detected to be zero-crossing within the predetermined time window, it is determined to be a DC bus fault and the protection action is locked.

[0021] Optionally, the duration of the predetermined time window is greater than the maximum duration required for the current at the protection installation location to pass through zero during a DC bus fault.

[0022] Optionally, the first predetermined threshold is the product of the minimum voltage change rate of a solid double-pole short-circuit fault at the DC bus and the first reliability coefficient;

[0023] The second predetermined threshold is the product of the minimum voltage change rate of a solid double-pole short-circuit fault at the head end of the adjacent line and the second reliability coefficient;

[0024] The first reliability coefficient and the second reliability coefficient are adjusted and determined based on the real-time monitored fault transition resistance value and a preset mapping relationship.

[0025] Optionally, it further includes:

[0026] After the protection action is triggered, an in-zone fault signal is sent to the head end and the converter station at the new energy sending end connected to the head end.

[0027] As can be seen from the above technical solutions, an HVDC transmission line provided in an embodiment of the present application is composed of a hybrid connection of a DC cable and a DC overhead line. The head end is connected to the converter station at the new energy sending end, and the tail end is connected to the DC bus through a current-limiting inductor. Multiple two-port DC circuit breakers or a multi-port DC circuit breaker are configured at the DC bus, and a single-end protection device is configured on the side of the head end far from the DC bus. The present application also provides a single-end protection method applied to the HVDC transmission line. By measuring the voltage at the protection installation location in real time and calculating the voltage change rate, a first protection criterion and a second protection criterion are set, and the result of the criterion establishment is determined according to the voltage change rate and the protection action is triggered.

[0028] Compared with the traditional single-end protection scheme for DC lines, the protection scheme proposed in the present application has significant advantages. First, the traditional scheme takes the current-limiting inductor at the end of this line as the boundary and has limited ability to withstand the transition resistance. However, this scheme makes full use of the boundary effect of the current-limiting inductor at the head end of the adjacent DC line and extends the protection boundary to the current-limiting inductor at the head end of the adjacent line, so that when facing a high-resistance fault, the fault characteristics can be captured more clearly, effectively improving the protection's ability to withstand the transition resistance. Second, the traditional single-end protection has poor adaptability to the hybrid DC line of cable-overhead line. However, this scheme is designed by fully considering the hybrid situation of cable-overhead line. It is not only applicable to the hybrid DC line, but also applicable to the pure cable line and the pure overhead line because they are special cases of the hybrid line. This scheme greatly broadens the application scope of the protection scheme. Compared with the existing protection schemes that are only applicable to pure overhead or pure cable lines, it has better adaptability and can more reliably and accurately identify the faults of the hybrid DC line, ensuring the safe and stable operation of the system. Description of the Drawings

[0029] To more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the drawings described below are only the embodiments of the present application. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on the provided drawings.

[0030] Figure 1 It is a flowchart of a single - end protection method for a hybrid - series DC transmission line disclosed in the embodiments of the present application;

[0031] Figure 2 It is a schematic diagram of a five - terminal DC power transmission system in a verification example disclosed in the embodiments of the present application;

[0032] Figure 3 It is a schematic diagram of the simulation results of a metallic fault in a verification example disclosed in the embodiments of the present application, where Figure 3 (a)is the curve graph of the voltage between poles at R1 in the metallic fault simulation, Figure 3 (b)is the curve graph of the rate of change of the voltage between poles at R1 in the metallic fault simulation;

[0033] Figure 4 It is a schematic diagram of the simulation results of a high - resistance fault in a verification example disclosed in the embodiments of the present application, where Figure 4 (a)is the curve graph of the voltage between poles at R1 in the high - resistance fault simulation, Figure 4 (b)is the curve graph of the rate of change of the voltage between poles at R1 in the high - resistance fault simulation, Figure 4 (c)is the curve graph of the current at R1 in the high - resistance fault simulation;

[0034] Figure 5 It is a schematic diagram of the simulation results of a DC bus fault in a verification example disclosed in the embodiments of the present application, where Figure 5 (a)is the curve graph of the voltage between poles at R1 in the DC bus fault, Figure 5 (b)is the curve graph of the rate of change of the voltage between poles at R1 in the DC bus fault, Figure 5 (c)is the curve graph of the current at R1 in the DC bus fault. Detailed implementation manners

[0035] The following will clearly and completely describe the technical solutions in the embodiments of the present application in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments of the present application, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present application.

[0036] Next, the solution of this application will be introduced. The following technical solution is proposed in this application. Please refer to the following text for details.

[0037] The hybrid DC transmission line provided by this application is composed of a hybrid connection of DC cables and DC overhead lines. The head end is connected to the converter station at the new energy output end, and the tail end is connected to the DC bus through a current-limiting inductor.

[0038] A plurality of two-port DC circuit breakers or a multi-port DC circuit breaker are configured at the DC bus.

[0039] A single-ended protection device is configured on the side of the head end far from the DC bus, which is used to detect line faults in real time and trigger protection actions.

[0040] Furthermore, at least one adjacent line of the hybrid DC transmission line is connected to the DC bus, and each adjacent line is connected through the current-limiting inductor.

[0041] Specifically, the hybrid DC transmission line provided by this application is composed of a hybrid connection of DC cables and DC overhead lines. Among them, the DC cable has the characteristics of strong anti-electromagnetic interference ability and small occupied space. In the marine environment, it can effectively resist the influence of complex electromagnetic environments and does not occupy additional marine space, making it suitable for marine laying; the DC overhead line has the advantages of low construction cost and small transmission loss. In the land environment, it can achieve efficient power transmission at a low cost. The hybrid connection of the two realizes complementary advantages and perfectly adapts to the large-scale offshore wind power output scenario, and can efficiently achieve long-distance transmission of clean energy.

[0042] The head end of this line is connected to the converter station at the new energy output end. The converter station converts the alternating current generated by offshore wind power into direct current, providing a basis for the DC transmission of electric energy. The tail end of the line is connected to the DC bus through a current-limiting inductor. At the same time, at least one other DC line is also connected to this DC bus through a current-limiting inductor. The setting of the current-limiting inductor can effectively limit the magnitude of the fault current when a fault occurs, prevent the fault current from being too large and damaging the system equipment, ensure stable interconnection between multiple lines, and provide a basic guarantee for the safe operation of the system.

[0043] At the DC bus, a plurality of two-port DC circuit breakers (the number of two-port DC circuit breakers is equal to the number of DC lines connected to the DC bus) or a multi-port DC circuit breaker are configured. When a fault occurs in a certain line, these circuit breakers can quickly act to cut off the connection between the faulty line and the DC bus, accurately isolate the faulty area, greatly shorten the fault handling time, effectively prevent the fault from spreading to other lines, and significantly improve the reliability and stability of the overall system operation.

[0044] The single-ended protection device proposed in this application is located at the head end of the hybrid DC line (away from the DC bus side). This single-ended protection device monitors local electrical quantities in real time without relying on two-end communication, completely getting rid of the constraints of two-end communication delay, and can quickly sense and respond at the moment of fault occurrence. In addition, this line uses the current-limiting inductor at the head end of the adjacent line to expand the protection boundary, which effectively enhances the ability to withstand transition resistance compared to the traditional protection scheme that uses the current-limiting inductor at the end of this line as the boundary. This enables this line to accurately identify both high-resistance faults and complex hybrid line faults. At the same time, since pure cable lines and pure overhead lines can be regarded as special cases of hybrid lines, the configuration of this line is also applicable to all types of lines, greatly expanding the scope of application.

[0045] The hybrid DC transmission line provided in this application, through a unique system configuration design, structurally integrates the advantages of cables and overhead lines, functionally achieves rapid isolation and accurate identification of faults, and in application achieves wide adaptability to various line types.

[0046] Figure 1 This is a flow chart of a single-ended protection method for a hybrid DC transmission line disclosed in an embodiment of the present application.

[0047] This method is applied to the above-mentioned hybrid DC transmission lines, such as Figure 1 As shown, the method may include:

[0048] Step S1: measuring the voltage at the protection installation location of the single-ended protection device in the hybrid DC transmission line in real time, and calculating the voltage change rate.

[0049] Specifically, the system measures the voltage at the protection installation location of single-ended protection devices in hybrid DC transmission lines in real time and calculates the voltage rate of change. As a key electrical quantity reflecting the operating status of a line, the voltage rate of change can sensitively reflect abnormalities in the line. Through high-precision measurement equipment and fast calculation algorithms, the voltage and its rate of change can be accurately acquired in real time, providing a reliable data foundation for subsequent fault diagnosis.

[0050] Step S2: Setting a first protection criterion and a second protection criterion.

[0051] Specifically, the first protection criterion is that when the rate of voltage change is less than the first predetermined threshold, it is determined that a fault has occurred inside the hybrid DC line. When the rate of voltage change is less than the first predetermined threshold, it is determined that a fault has occurred inside the hybrid DC line. Based on the principle that the voltage will drop rapidly when a fault occurs, resulting in a significant change in the rate of voltage change, this criterion can quickly capture the occurrence of an internal fault in the line and achieve a rapid initial response to the fault. In practical applications, the first predetermined threshold needs to be accurately set according to the specific parameters of the line, operating conditions, etc. to balance the sensitivity and reliability of the protection.

[0052] The second protection criterion is that when the rate of voltage change is less than the second predetermined threshold, the current at the protection installation location is continuously monitored within a predetermined time window. If the current is always greater than zero within the predetermined time window, it is determined that a fault has occurred inside the hybrid DC line. When the rate of voltage change is less than the second predetermined threshold, the current at the protection installation location is continuously monitored within a predetermined time window. If the current is always greater than zero within the predetermined time window, it is determined that a fault has occurred inside the hybrid DC line. This criterion further combines the current characteristics to make up for the possible limitations of relying solely on the rate of voltage change for judgment. For some high-resistance faults or complex fault scenarios, the voltage may not change significantly, but by the continuous presence of the current within a certain period of time, the fault can be more accurately identified, effectively avoiding missed judgments and false judgments. Similarly, the second predetermined threshold and the predetermined time window need to be optimized according to a large amount of simulation analysis and actual test data.

[0053] In the single-ended protection method for hybrid DC transmission lines, the reasonable setting of the protection criterion threshold and the predetermined time window is the key to achieving accurate fault identification and reliable protection. The specific content is as follows:

[0054] (I) Setting of the protection criterion threshold

[0055] The first predetermined threshold is determined based on the minimum rate of voltage change of a metallic bipolar short-circuit fault at the DC bus. The second predetermined threshold is determined based on the minimum rate of voltage change of a metallic bipolar short-circuit fault at the head end of the adjacent line.

[0056] Specifically, the first predetermined threshold is the product of the minimum rate of voltage change of a metallic bipolar short-circuit fault at the DC bus and the first reliability coefficient;

[0057] The second predetermined threshold is the product of the minimum rate of voltage change of a metallic bipolar short-circuit fault at the head end of the adjacent line and the second reliability coefficient;

[0058] The first reliability coefficient and the second reliability coefficient are adjusted and determined based on the real-time monitored fault transition resistance value and a preset mapping relationship.

[0059] The first predetermined threshold: The first predetermined threshold is used for the first protection criterion. When the voltage change rate at the installation location of the hybrid HVDC transmission line protection is less than this threshold, it is determined that a fault has occurred inside the line. This threshold is determined based on the minimum voltage change rate of a solid bipolar short-circuit fault at the DC bus, specifically, the product of the minimum voltage change rate of a solid bipolar short-circuit fault at the DC bus and the first reliability coefficient. Among them, the minimum voltage change rate of a solid bipolar short-circuit fault at the DC bus is obtained through a large number of simulation calculations and actual tests on the system under this fault condition, which reflects the extreme situation of voltage change when the most serious short-circuit fault occurs at the DC bus. The first reliability coefficient is adjusted and determined based on the real-time monitored fault transition resistance value and the preset mapping relationship. The fault transition resistance will affect the electrical quantity characteristics during the fault. By establishing the mapping relationship between the fault transition resistance value and the reliability coefficient, the first reliability coefficient can be dynamically adjusted according to the actual fault transition resistance situation, enabling the first predetermined threshold to effectively balance the sensitivity and reliability of the protection in different fault scenarios and avoiding misjudgment or missed judgment in cases such as high-resistance faults due to a fixed threshold.

[0060] The second predetermined threshold: The second predetermined threshold is applied to the second protection criterion. When the voltage change rate is less than this threshold, the current needs to be further monitored within a predetermined time window to determine whether a fault has occurred in the line. This threshold is determined based on the minimum voltage change rate of a solid bipolar short-circuit fault at the head end of the adjacent line, which is the product of the minimum voltage change rate of a solid bipolar short-circuit fault at the head end of the adjacent line and the second reliability coefficient. Similarly, the minimum voltage change rate of a solid bipolar short-circuit fault at the head end of the adjacent line is obtained through simulation and actual measurement of this fault scenario, which reflects the voltage change characteristics when a serious short-circuit fault occurs at the head end of the adjacent line. The second reliability coefficient is also adjusted according to the real-time monitored fault transition resistance value and the preset mapping relationship. Considering the influence of the fault transition resistance on the fault characteristics, the second predetermined threshold is dynamically optimized, enabling the second protection criterion to more accurately identify internal faults in the line in the face of complex fault scenarios, especially high-resistance faults, making up for the deficiency of relying solely on the voltage change rate for judgment and reducing the risk of missed judgment and misjudgment.

[0061] (2) Setting of the predetermined time window

[0062] The duration of the predetermined time window is greater than the maximum duration required for the current at the protection installation location to pass through zero during a DC bus fault.

[0063] The predetermined time window plays a key role in the second protection criterion. Its duration is set to be greater than the maximum time required for the current to cross zero at the protection installation in the event of a DC bus fault. In the event of a DC bus fault, the DC bus protection will activate rapidly, and the DC circuit breaker at the DC bus will disconnect the faulty line after a delay. This process will cause the current at the protection installation to oscillate and cross zero. However, when a fault occurs within the line, the fault current will typically persist before this protection recognizes the fault. By setting the duration of the predetermined time window to be greater than the maximum time required for the current to cross zero in the event of a DC bus fault, it is ensured that during a DC bus fault, the second protection criterion will not be falsely triggered due to brief oscillations of the current crossing zero, effectively distinguishing between DC bus faults and internal line faults. This further improves the reliability and accuracy of the protection scheme in complex fault conditions and ensures that the single-ended protection method for hybrid DC transmission lines can operate stably and reliably.

[0064] Step S3: determining whether the first protection criterion and the second protection criterion are satisfied according to the voltage change rate, and triggering corresponding protection actions.

[0065] Specifically, the voltage change rate determines whether the first and second protection criteria are met, triggering the corresponding protection actions. If the first protection criterion is met, the protection device quickly initiates the rapid protection process, enabling emergency response to the line fault. If the first protection criterion is not met, but the second protection criterion is met, the protection device promptly triggers the corresponding protection measures, ensuring reliable protection against hybrid DC transmission line faults.

[0066] At the same time, the first-stage protection and second-stage protection criteria in this protection method cooperate with the above-mentioned first protection criterion and second protection criterion. The first-stage protection uses the current-limiting inductor at the end of the line as the protection boundary, and quickly responds to obvious faults on the line through the first protection criterion; the second-stage protection extends the protection boundary to the current-limiting inductor of the adjacent line, and effectively responds to complex fault scenarios based on the second protection criterion.

[0067] Triggering corresponding protection actions according to the results of the first protection criterion and the second protection criterion being met, specifically including:

[0068] When the first protection criterion is met, it is determined that a fault occurs inside the hybrid DC line and a first protection action is triggered;

[0069] When the second protection criterion is met and the current is continuously greater than zero within the predetermined time window, it is determined that a fault has occurred inside the hybrid DC line and a second-stage protection action is triggered;

[0070] The triggering priority of the second-stage protection action is lower than that of the first-stage protection action.

[0071] Specifically, when the first protection criterion is met—that is, the voltage change rate at the protection installation is less than a first predetermined threshold—a fault is quickly determined to have occurred within the hybrid DC line, and the first-stage protection action is immediately triggered. The first-stage protection action is intended to provide an emergency response to internal line faults, isolating the fault as quickly as possible and minimizing its impact on the system.

[0072] If the second protection criterion holds and the current at the protection installation remains above zero for a predetermined time window, a fault is detected within the hybrid DC line, triggering the second-stage protection. This second-stage protection is designed to address complex fault scenarios, such as high-resistance faults, that are not covered by the first-stage protection. Furthermore, the second-stage protection has a lower triggering priority than the first-stage protection, ensuring that the protection system can optimally allocate response strategies to address critical faults, regardless of fault severity.

[0073] In addition, if the current at the protection installation is detected to be zero-crossing within the predetermined time window, it is determined to be a DC bus fault and the protection action is locked.

[0074] Specifically, if the current at the protection device crosses zero within a predetermined time window, it is determined to be a DC bus fault. In this case, to prevent the protection device from malfunctioning, the system will lock out the protection action, preventing any unnecessary impact to the system caused by erroneous protection operation.

[0075] Furthermore, the present application can also start a fault signal sending mechanism within the zone after the protection action is triggered, specifically:

[0076] When the protection action is triggered, an intra-area fault signal is sent to the head end and the converter station of the new energy sending end connected to the head end.

[0077] Specifically, when the protection action (first-stage or second-stage protection) is triggered, the fault signal transmission process is immediately initiated. Key information, such as the fault occurrence time, fault line number, and protection action type (first-stage or second-stage protection), is encoded and integrated into a specific format to generate a fault signal. This signal is then rapidly transmitted to the line headend and the connected sending-end converter station. Upon receiving the fault signal, the sending-end converter station implements different action strategies based on the equipment configuration at the line headend. If a DC circuit breaker is not configured at the line headend, the half-bridge converter station must quickly shut down, cutting off energy transmission to the faulty line to prevent further fault expansion. Full-bridge and hybrid-bridge converter stations immediately implement an active current limiting control strategy, adjusting the operating state of their internal power electronic components to limit the fault current and maintain system stability. If a DC circuit breaker is configured at the line headend, upon receiving the fault signal, the converter station coordinates with the protection device to send a trip command to the DC circuit breaker. The DC circuit breaker then rapidly executes, rapidly isolating the faulty line and achieving precise fault clearance.

[0078] The protection scheme and intra-area fault signal transmission mechanism proposed in this application fully take into account the above-mentioned various configuration situations. Regardless of whether a DC circuit breaker is configured at the head end of the line and what topology the converter station adopts, effective fault identification and signal transmission can be achieved. It can also be well adapted to the action strategies of the converter station and DC circuit breaker, ensuring that intra-area faults can be handled quickly and reliably in different configuration scenarios, thereby ensuring the safe and stable operation of the hybrid DC transmission system.

[0079] As can be seen from the above technical solution, the embodiment of the present application provides a hybrid DC transmission line consisting of a hybrid DC cable and a DC overhead line. The first end is connected to the converter station at the new energy transmission end, and the terminal is connected to the DC bus through a current-limiting inductor. Multiple two-port DC circuit breakers or a multi-port DC circuit breaker are configured at the DC bus, and a single-ended protection device is configured at the first end away from the DC bus. The present application also provides a single-ended protection method for the hybrid DC transmission line. By measuring the voltage at the protection installation in real time and calculating the voltage change rate, a first protection criterion and a second protection criterion are set. The result of the criterion is determined based on the voltage change rate and the protection action is triggered.

[0080] Compared with the traditional single-ended protection scheme for DC lines, the protection scheme proposed in this application has significant advantages. First, the traditional scheme uses the current-limiting inductor at the end of the line as the boundary, and its ability to withstand transition resistance is limited. However, this scheme makes full use of the boundary effect of the current-limiting inductor at the beginning of the adjacent DC line, and extends the protection boundary to the current-limiting inductor at the beginning of the adjacent line, so that when facing high-resistance faults, it can also capture the fault characteristics more clearly, effectively improving the protection ability to withstand transition resistance. Second, the traditional single-ended protection has poor adaptability to cable-overhead line hybrid DC lines, while this scheme fully considers the hybrid situation of cable-overhead lines and is designed. It is not only applicable to hybrid DC lines, but also to pure cable lines and pure overhead lines, which are special cases of hybrid lines. This scheme is also applicable, which greatly broadens the application scope of the protection scheme. Compared with the existing protection scheme that is only applicable to pure overhead or pure cable lines, it has better adaptability and can more reliably and accurately identify hybrid DC line faults to ensure safe and stable operation of the system.

[0081] The following is an example to verify the technical solution of this application.

[0082] Based on PSCAD / EMTDC platform, Figure 2 The five-terminal DC transmission system shown uses a symmetrical single-pole connection. A multi-port DC circuit breaker is configured at the DC busbar, with an operating time of 2 ms and a DC busbar protection time of 0.1 ms. All transmission lines use a frequency-dependent model with a protection sampling frequency of 20 kHz. Other key system parameters are shown in Table 1.

[0083]

[0084] Using the single - end protection device R1 in Figure 2 as the protection device of this protection, and using the hybrid DC line composed of DC cables and DC overhead lines as this line. Metallic bipolar short - circuit faults are respectively set at the DC bus and the head of the adjacent line. The voltage change rate setting values corresponding to the first protection criterion and the second protection criterion obtained by simulation are the first predetermined threshold K1α1 = - 991 and the second predetermined threshold K2α2 = - 396 respectively. The lower the transition resistance of the DC bus fault, the greater the time length t2 required for the current at the single - end protection device R1 to pass through zero; in the case of a metallic fault on the DC bus, t2 = 3.97ms; reserving a certain safety margin, the predetermined time window length t1 = 4.5ms.

[0085] Verification of the feasibility of the first - stage protection criterion:

[0086] To verify the feasibility of the first protection criterion mentioned above, a bipolar metallic fault (fault transition resistance Rf = 0Ω) is set at 150km away from the single - end protection device R1. The inter - pole voltage and its change rate at the single - end protection device R1 are as Figure 3 shown. In the figure, the fault occurrence time is set as time zero. As Figure 3 (a) shows, when a metallic fault occurs at 150km of the line, after the fault traveling wave reaches the single - end protection device R1, the voltage drops rapidly; its voltage change rate is as Figure 3 (b) shows. At 0.8ms after the fault occurs, the voltage change rate is less than the first predetermined threshold K1α1, meeting the first protection criterion. The protection device identifies that a fault has occurred inside the hybrid DC line and triggers the first - stage protection action.

[0087] Verification of the high - resistance fault tolerance ability of the second - stage protection criterion:

[0088] To verify the tolerance ability of the second protection criterion proposed in this paper for high - resistance faults within the zone, a high - resistance fault (fault transition resistance Rf = 200Ω) is set at 150km away from the single - end protection device R1. The voltage and its change rate at the single - end protection device R1 are as Figure 4 (a) and (b) show. As Figure 4 (b) shows, the voltage change rate is always greater than the first predetermined threshold K1α1 within 1.5ms. Therefore, the first - stage protection does not act. At 0.76ms, the voltage change rate is less than the second predetermined threshold K2α2, and the second - stage protection starts timing; the current waveform is as Figure 4 (c) shows. Within 4.5ms after the voltage change rate is less than the second predetermined threshold K2α2, the current at the protection installation location is always greater than 0. Therefore, the second protection criterion is established and the second - stage protection action is triggered. It can be seen that the proposed protection strategy can identify high - resistance faults within the zone.

[0089] Verification of DC bus faults:

[0090] When a fault occurs in the DC bus, the voltage change rate and current at the single - end protection device R1 are as Figure 5 shown in (a) and (b). As Figure 5 shown in (a), after the fault traveling wave arrives at the single - end protection device R1, the voltage has different degrees of drop under different fault transition resistances Rf. As Figure 5 shown in (b), when a high - resistance fault (Rf = 200Ω) occurs, the voltage change rate is always greater than the second predetermined threshold K2α2, and neither the first protection criterion nor the second protection criterion is activated. When a bipolar metallic fault (Rf = 0Ω) occurs in the DC bus, its voltage change rate is less than the second predetermined threshold K2α2 at 1.02 ms. At this time, the second - stage protection is activated and starts timing; the current waveform is as Figure 5 shown in (c). After the voltage change rate condition of the second protection criterion at the single - end protection device R1 is satisfied, the current at the single - end protection device R1 passes through zero within 4.5 ms. Therefore, the second - stage protection will not malfunction, and the system locks out the protection action.

[0091] Finally, it should also be noted that in this article, relational terms such as first and second are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the term "comprising", "including" or any other variant thereof is intended to cover non - exclusive inclusion, so that a process, method, article or device comprising a series of elements not only includes those elements, but also includes other elements not expressly listed, or elements inherent to such process, method, article or device. Without further limitation, an element defined by the statement "comprising an..." does not exclude the presence of additional identical elements in the process, method, article or device comprising the said element.

[0092] The various embodiments in this specification are described in a progressive manner. Each embodiment focuses on the differences from other embodiments, and the same or similar parts among the various embodiments can be referred to each other.

[0093] The above description of the disclosed embodiments enables those skilled in the art to implement or use the present application. Various modifications to these embodiments will be obvious to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to these embodiments shown herein, but will be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A hybrid series-parallel HVDC transmission line, characterized in that, It is composed of a series-parallel connection of a DC cable and a DC overhead line, with the head end connected to a converter station at the new energy output end and the tail end connected to the DC bus through a current-limiting inductor; A plurality of two-port DC circuit breakers or a multi-port DC circuit breaker are configured at the DC bus; A single-end protection device is configured on the side of the head end far from the DC bus for real-time detection of line faults and triggering of protection actions.

2. The hybrid series-parallel HVDC transmission line according to claim 1, wherein At least one adjacent line of the series-parallel DC transmission line is connected to the DC bus, and each adjacent line is connected through the current-limiting inductor.

3. A single - end protection method for a hybrid - series DC transmission line, characterized in that, Applied to the series-parallel DC transmission line according to any one of claims 1 or 2, the method includes: Real-time measure the voltage at the protection installation location of the single-end protection device in the series-parallel DC transmission line and calculate the voltage change rate; Set a first protection criterion and a second protection criterion, where the first protection criterion is that when the voltage change rate is less than a first predetermined threshold, it is determined that a fault occurs inside the series-parallel DC line, and the second protection criterion is that when the voltage change rate is less than a second predetermined threshold, continuously monitor the current at the protection installation location within a predetermined time window. If the current is always greater than zero within the predetermined time window, it is determined that a fault occurs inside the series-parallel DC line; Determine the establishment results of the first protection criterion and the second protection criterion according to the voltage change rate and trigger corresponding protection actions.

4. The method according to claim 3, characterized in that The first predetermined threshold is determined based on the minimum voltage change rate of a metallic bipolar short-circuit fault at the DC bus, and the second predetermined threshold is determined based on the minimum voltage change rate of a metallic bipolar short-circuit fault at the head end of the adjacent line.

5. The method according to claim 3, characterized in that Triggering corresponding protection actions according to the establishment results of the first protection criterion and the second protection criterion includes: When the first protection criterion is established, it is determined that a fault occurs inside the series-parallel DC line and a first-stage protection action is triggered; When the second protection criterion is established and the current continues to be greater than zero within the predetermined time window, it is determined that a fault occurs inside the series-parallel DC line and a second-stage protection action is triggered; The trigger priority of the second-stage protection action is lower than that of the first-stage protection action.

6. The method according to claim 3, wherein It also includes: If the current at the protection installation location is detected to cross zero within the predetermined time window, it is determined that a DC bus fault occurs and the protection action is blocked.

7. The method according to claim 3, characterized in that, The duration of the predetermined time window is greater than the maximum duration required for the current at the protection installation location to cross zero during a DC bus fault.

8. The method according to claim 4, wherein The first predetermined threshold is the product of the minimum voltage change rate of a metallic bipolar short-circuit fault at the DC bus and a first reliability coefficient; The second predetermined threshold is the product of the minimum voltage change rate of a metallic bipolar short-circuit fault at the head end of the adjacent line and a second reliability coefficient; The first reliability coefficient and the second reliability coefficient are adjusted and determined based on the real-time monitored fault transition resistance value and a preset mapping relationship.

9. The method according to claim 3, wherein It also includes: After the protection action is triggered, send an in-zone fault signal to the head end and the converter station at the new energy output end connected to the head end.

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