Two-level VSC type photovoltaic integrated dual-terminal flexible DC distribution network fault location method, system, and application

Abstract

The present invention belongs to the technical field of fault location for flexible DC distribution networks with distributed power sources, and discloses a fault location method, system and application for a two-level VSC-type flexible DC distribution network with photovoltaic at both ends. By using the current direction of the positive pole of each DC feeder during a fault, the fault section is identified; according to the equivalent circuits in each transient stage, an inter-pole short-circuit fault ranging model covering the interactive responses of each system is established; the fault distance is solved by using the parameter information of each fault electrical quantity to achieve fault location. The present invention can correctly identify the fault section and achieve accurate fault location by using the parameter information of each fault electrical quantity, providing a key technology for the rapid location of inter-pole short-circuit faults in flexible DC distribution networks, further improving the power supply reliability of the system, effectively avoiding the errors caused by the differential terms of traditional ranging methods, having a more accurate positioning result and better stability, being conducive to promoting the further development of flexible DC distribution networks, and providing assistance for the construction of a new power system under the background of energy transformation.

Description

Technical Field

[0001] The invention belongs to the technical field of fault location for flexible DC distribution networks with distributed power sources, and particularly relates to a fault location method, system and application for a two-level VSC type flexible DC distribution network with photovoltaic at both ends. Background Art

[0002] The flexible DC distribution network (referred to as the flexible DC distribution network for short) has the characteristics of high operating efficiency, large power supply radius, excellent power quality, high reliability, strong controllability, and is convenient for high-proportion access of distributed power sources. It has become an important part of the new power system. The two-level VSC (Voltage Sources Convertor) is a key device for constructing the flexible DC distribution network. Its structure is simple and the cost is low. With the continuous development of SiC devices, its application prospect in the field of medium- and low-voltage flexible DC distribution networks is becoming increasingly broad. At the same time, with the continuous increase in the penetration rate of photovoltaic power sources, the flexible DC distribution network with photovoltaic based on the two-level VSC has attracted extensive attention from the academic circles at home and abroad. The DC side pole-to-pole short-circuit fault of the flexible DC distribution network is the most harmful to the system. Accurate fault location is the premise for ensuring the safe and reliable operation of the system; while the pole-to-pole fault of the VSC-based flexible DC distribution network has the characteristics of fast current rising speed, large amplitude and many influencing factors, which brings great challenges to the fault location technology. The existing flexible DC distribution network location methods can be mainly divided into the traveling wave method, the active injection method and the fault analysis method according to the principle. The traveling wave method usually calculates the fault distance by identifying the traveling wave head and measuring the time difference for a round trip between the detection end and the fault point, and has been widely used in DC transmission lines. However, it has high requirements for the sampling frequency, and at the same time, the ranging accuracy is interfered by factors such as wave head identification and wave speed calculation. The active injection method usually adds additional auxiliary equipment after the protection action, and forms a discharge loop with the DC network to calculate the fault distance, but the installation of the auxiliary equipment makes this method not economically advantageous. At the same time, the above two methods have certain limitations on the line length. Due to the short DC lines in the medium- and low-voltage flexible DC distribution network, their application in the fault location of the flexible DC distribution network based on the two-level VSC is greatly restricted. The fault analysis method is to carry out fault location by writing the relationship expression between the fault distance and the electrical quantity in the flexible DC distribution network. It can directly utilize the fault transient information without the need to additionally set signal injection equipment. At the same time, its location accuracy is independent of the line length, and it has high applicability for realizing fault location in the flexible DC distribution network based on the two-level VSC. However, the existing transient full-process analysis model for the pole-to-pole short-circuit fault of the two-level VSC-based flexible DC distribution network is not accurate enough. It usually only covers the transient output characteristics of the two-level VSC, ignoring the coupling relationship between the AC system and the distributed power sources (Distributed Energy Resources, DERs) system at the fault point, and directly used for fault location will bring certain errors. In addition, the traditional flexible DC distribution network location method based on the fault analysis method usually only uses the information at a certain time section and the corresponding model for fault location, and the information utilization is not sufficient enough to ensure the robustness of the location method. And the traditional location method usually uses the difference of two sampling values to replace the differential at the midpoint moment, and the average value to replace the instantaneous value at the midpoint moment. The substitution error brought by this processing method will affect the location accuracy. Therefore, it is urgent to design a new fault location method for the flexible DC distribution network with distributed power sources.

[0003] Through the above analysis, the problems and defects existing in the prior art are as follows: The existing transient full-process analysis model for the pole-to-pole short-circuit fault of the two-level VSC type flexible DC distribution network is not accurate enough. It usually only covers the transient output characteristics of the two-level VSC and ignores the coupling relationship between the AC system and the Distributed Energy Resources (DERs) system at the fault point. Directly using it for fault location will bring certain errors. In addition, the traditional fault location method for flexible DC distribution networks based on the fault analysis method usually only uses the information at a certain time section and the corresponding model for fault location. The utilization of information is not sufficient enough to ensure the robustness of the location method. And the traditional location method usually uses the difference of two sampling values to replace the differential at the midpoint moment and the average value to replace the instantaneous value at the midpoint moment. The substitution error brought by this processing method will affect the location accuracy.

[0004] The difficulty in solving the above problems and defects is as follows: During the fault, the internal structure of the converter is complex. It is necessary to analyze the conduction situation of the internal diodes of the converters in different subsystems, divide different transient stages of the pole-to-pole short-circuit fault, and the phased division principle and the construction of the fault analysis model for different transient stages are still blank. At the same time, the structure of the two-level VSC type flexible DC distribution network with photovoltaic is complex, and the order of the ranging state equation for each transient stage of the fault is relatively high, and the difficulty in solving the location result is relatively large.

[0005] The significance of solving the above problems and defects is as follows: Based on the typical topological structure of the two-level VSC type photovoltaic double-terminal flexible DC distribution network, considering the coupling relationship between the photovoltaic and the AC subsystem during the pole-to-pole short-circuit fault, a precise ranging model for the transient stage of the DC side pole-to-pole short-circuit fault is constructed, and the fault location of the two-level VSC type flexible DC distribution network is carried out by using the information of each fault electrical quantity parameter, which provides a key technology for realizing the rapid location of the pole-to-pole short-circuit fault of the flexible DC distribution network and can further improve the power supply reliability of the system. It is beneficial to promote the further development of the flexible DC distribution network and provide assistance for the construction of a new power system under the background of energy transformation. Summary of the Invention

[0006] To overcome the problems existing in the related technologies, the disclosed embodiments of the present invention provide a fault location method and system for a two-level VSC type photovoltaic double-terminal flexible DC distribution network, and in particular, a method, system, medium, device, and terminal for pole-to-pole short-circuit fault location of a two-level VSC type photovoltaic double-terminal flexible DC distribution network based on parameter identification. The technical solution is as follows:

[0007] The present invention is implemented as follows. A fault location method for a two-level VSC type photovoltaic double-terminal flexible DC distribution network, the fault location method for the two-level VSC type photovoltaic double-terminal flexible DC distribution network includes:

[0008] Utilize the positive current directions of each DC feeder during a fault to identify the fault section; based on the equivalent circuits of each transient stage of the fault, establish an inter-pole short-circuit fault ranging model covering the interactive responses of each system; solve the fault distance using the information of each fault electrical quantity parameter to achieve fault location.

[0009] Furthermore, the fault location method for the two-level VSC-type photovoltaic integrated dual-terminal flexible DC distribution network includes the following steps:

[0010] Step 1, specify the T-junction as the intersection of the DC feeder on the DC side of the photovoltaic subsystem and the DC feeder on the DC side of the AC subsystem, and determine the fault section according to the positive current directions of each DC feeder at the T-junction during the fault.

[0011] Step 2, establish an inter-pole short-circuit fault ranging model covering the interactive responses of each system through the equivalent circuits of each transient stage of the inter-pole short circuit on the DC side of the two-level VSC-type flexible DC distribution network.

[0012] Step 3, determine the sampling data window length using the boundary conditions of the fault transient stage, calculate the fault distance at each sampling moment based on the information of each fault electrical quantity parameter; take the average value of the fault distances obtained at several sampling points to achieve accurate positioning of the inter-pole short-circuit fault.

[0013] In one embodiment, in Step 1, the direction from the converter of the AC subsystem or the photovoltaic subsystem to the DC line is specified as the positive direction, and i VSC1 , i VSC2 , i DC / DC are the positive currents at the T-junction between the T-junction and VSC1, VSC2, and the DC / DC outlet respectively. If i VSC1 is negative during the fault, the fault occurs between VSC1 and the T-junction; if i VSC2 is negative during the fault, the fault occurs between VSC2 and the T-junction; if i DC / DC is negative during the fault, the fault occurs between DC / DC and the T-junction.

[0014] In one embodiment, in Step 2, immediately after the fault occurs, the IGBTs of the converters of each subsystem are blocked. According to the conduction and current feeding conditions of the internal diodes of the two-level VSC and the photovoltaic DC / DC, analyze the coupling relationship of each subsystem at the fault point during the fault, and determine the boundary conditions and equivalent circuits of each transient stage of the inter-pole short-circuit fault; construct the corresponding fault state equation according to Kirchhoff's voltage and current laws; represent the elements inside the state equation that change with the fault distance and the transition resistance, and then establish an inter-pole short-circuit fault ranging state equation containing the fault distance and the transition resistance.

[0015] In one embodiment, in the third step, based on the fault location state equation, electrical values of each state variable are collected at the same sampling moment, and an objective optimization problem under multiple constraints is formed according to the least squares method. The fault distance at the corresponding sampling moment is obtained by solving this problem.

[0016] The start and end moments of each stage are determined by using the boundary conditions of the fault transient stage, and the difference between the two is the length of the sampling data window. Based on the parameter information of each fault electrical quantity, a multi-constraint objective optimization problem at the same sampling moment is constructed according to the least squares method:

[0017] min{[χ1(t i ,x,R g ) - χ1(t i )] 2 + [χ2(t i ,x,R g ) - χ2(t i )] 2 + ··· + [χ k (t i ,x,R g ) - χ k (t i )] 2 + ··· + [χ n (t i ,x,R g ) - χ n (t i )] 2}

[0018]

[0019] The fault distance at each sampling moment will be calculated by solving this problem; the average value of the fault distances obtained at several sampling points is obtained to achieve accurate positioning of the pole-to-pole short-circuit fault;

[0020]

[0021] Another object of the present invention is to provide a two-level VSC type photovoltaic double-ended flexible DC distribution network fault location system applying the two-level VSC type photovoltaic double-ended flexible DC distribution network fault location method described above. The two-level VSC type photovoltaic double-ended flexible DC distribution network fault location system includes:

[0022] A fault occurrence section judgment module, which is used to define the T connection point as the intersection of the DC feeder of the photovoltaic subsystem and the DC feeder of the AC subsystem, and judge the fault occurrence section according to the positive current direction of each DC feeder at the T connection point during the fault;

[0023] A fault location model construction module, which is used to establish an inter-pole short-circuit fault location model covering the interactive responses of each system through the equivalent circuits of each transient stage of the fault;

[0024] A fault distance calculation module, which is used to determine the sampling data window length by using the boundary conditions of the fault transient stage, and calculate the fault distance at each sampling moment based on the parameter information of each fault electrical quantity;

[0025] A short-circuit fault location module, which is used to calculate the average value of the fault distances obtained at several sampling points to achieve accurate positioning of the inter-pole short-circuit fault.

[0026] Another object of the present invention is to provide a program storage medium for receiving user input, and the stored computer program enables an electronic device to execute any one of the claims, including the following steps:

[0027] Utilize the current directions of the positive poles of each DC feeder during the fault to identify the fault section; establish an inter-pole short-circuit fault location model covering the interactive responses of each system according to the equivalent circuits of each transient stage; utilize the parameter information of each fault electrical quantity to solve the fault distance and achieve fault location.

[0028] Another object of the present invention is to provide a computer device, the computer device includes a memory and a processor, the memory stores a computer program, and when the computer program is executed by the processor, the processor executes the following steps:

[0029] Utilize the current directions of the positive poles of each DC feeder during the fault to identify the fault section; establish an inter-pole short-circuit fault location model covering the interactive responses of each system according to the equivalent circuits of each transient stage; utilize the parameter information of each fault electrical quantity to solve the fault distance and achieve fault location.

[0030] Another object of the present invention is to provide a computer-readable storage medium, storing a computer program, and when the computer program is executed by a processor, the processor executes the following steps:

[0031] Utilize the current directions of the positive poles of each DC feeder during the fault to identify the fault section; establish an inter-pole short-circuit fault location model covering the interactive responses of each system according to the equivalent circuits of each transient stage; utilize the parameter information of each fault electrical quantity to solve the fault distance and achieve fault location.

[0032] Another object of the present invention is to provide an information data processing terminal, and the information data processing terminal is used to provide a user input interface to implement the two-level VSC type photovoltaic double-ended flexible DC distribution network fault location system when executed on an electronic device.

[0033] Combining all the above technical solutions, the advantages and positive effects of the present invention are:

[0034] First, in view of the technical problems existing in the above-mentioned prior art and the difficulty of solving these problems, closely combined with the technical solution to be protected by the present invention, as well as the results and data during the R & D process, etc., analyze in detail and profoundly how the technical solution of the present invention solves the technical problems and the creative technical effects brought about after solving the problems. The specific description is as follows:

[0035] The two-level VSC type photovoltaic double-ended flexible DC distribution network pole-to-pole short-circuit fault location method based on parameter identification provided by the present invention has significant differences compared with traditional location methods:

[0036] 1. Traditional location methods do not consider the coupling relationship between the AC system and the photovoltaic system feed current at the fault point during the fault, and the location accuracy is poor when the PV access capacity is large. The two-level VSC type photovoltaic double-ended flexible DC distribution network pole-to-pole short-circuit fault location method based on parameter identification provided by the present invention constructs a pole-to-pole short-circuit fault ranging model covering the interaction responses of each subsystem according to the conduction and feed current conditions of the internal diodes of the two-level VSC and the photovoltaic DC / DC, considering the coupling relationship between each subsystem at the fault point, and has higher location accuracy.

[0037] 2. Traditional location methods usually use the difference of two-point sampling values to replace the differential at the midpoint moment, and the average value to replace the instantaneous value at the midpoint moment, and their accuracy is greatly affected by the data window size and the sampling frequency. The two-level VSC type photovoltaic double-ended flexible DC distribution network pole-to-pole short-circuit fault location method based on parameter identification provided by the present invention does not need to consider the substitution error caused by the above-mentioned differential value processing, and can still ensure accuracy within a small data window, with higher accuracy and better stability.

[0038] Second, the present invention proposes a two-level VSC type photovoltaic double-ended flexible DC distribution network pole-to-pole short-circuit fault location method based on parameter identification, which can correctly identify the fault occurrence section, realize accurate fault location by using the parameter information of each fault electrical quantity, effectively avoid the error caused by the differential item of the traditional ranging method, and has a more accurate location result and better stability.

[0039] Third, as a creative auxiliary evidence of the claims of the present invention, it is also reflected that the present invention provides a key technology for realizing the rapid location of pole-to-pole short-circuit faults in flexible DC distribution networks, which can further improve the power supply reliability of the system, is conducive to promoting the further development of flexible DC distribution networks, and provides assistance for the construction of a new power system under the background of energy transformation. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] The drawings here are incorporated into the specification and form a part of this specification, showing embodiments consistent with the present disclosure, and are used together with the specification to explain the principles of the present disclosure.

[0041] Figure 1 It is the flowchart of the fault location method for a two-level VSC type photovoltaic-integrated dual-terminal flexible DC distribution network provided by an embodiment of the present invention;

[0042] Figure 2 It is the schematic diagram of the fault location method for a two-level VSC type photovoltaic-integrated dual-terminal flexible DC distribution network provided by an embodiment of the present invention;

[0043] Figure 3 It is the system structure block diagram of the fault location for a two-level VSC type photovoltaic-integrated dual-terminal flexible DC distribution network provided by an embodiment of the present invention;

[0044] Figure 4 It is the topology diagram of the pole-to-pole short circuit fault for a photovoltaic-integrated dual-terminal flexible DC distribution network based on a two-level VSC provided by an embodiment of the present invention;

[0045] Figure 5 It is the equivalent circuit diagram of the capacitor discharge - capacitor discharge - photovoltaic feeder conduction stage provided by an embodiment of the present invention;

[0046] Figure 6 It is the typical structure diagram of a photovoltaic-integrated dual-terminal flexible DC distribution network based on a two-level VSC provided by an embodiment of the present invention;

[0047] Figure 7 It is the comparison diagram of the relative errors of different ranging methods provided by an embodiment of the present invention;

[0048] In the figure: 1. Fault occurrence section judgment module; 2. Fault ranging model construction module; 3. Fault distance calculation module; 4. Short circuit fault location module. Specific implementation manners

[0049] To make the above objects, features, and advantages of the present invention more obvious and understandable, the specific implementation manners of the present invention will be described in detail below with reference to the accompanying drawings. Many specific details are set forth in the following description in order to fully understand the present invention. However, the present invention can be implemented in many other ways different from those described herein, and those skilled in the art can make similar improvements without departing from the connotation of the present invention. Therefore, the present invention is not limited by the specific implementations disclosed below.

[0050] I. Explanation of the embodiment:

[0051] Embodiment 1

[0052] As Figure 1 shown, the fault location method for a two-level VSC type photovoltaic-integrated dual-terminal flexible DC distribution network provided by an embodiment of the present invention includes the following steps:

[0053] S101, stipulate that the T connection point is the intersection of the DC feeder of the photovoltaic subsystem and the DC feeder of the AC subsystem, and judge the fault occurrence section according to the positive current directions of each DC feeder at the T connection point during the fault;

[0054] S102. Establish an inter - pole short - circuit fault location model covering the interaction responses of each system through the equivalent circuits of each transient stage.

[0055] S103. Determine the sampling data window length using the boundary conditions of the fault transient stage, calculate the fault distance at each sampling moment based on the parameter information of each fault electrical quantity; take the average value of the fault distances obtained at several sampling points to achieve accurate positioning of the inter - pole short - circuit fault.

[0056] As Figure 2 shown, the two - level VSC - type flexible DC distribution network fault location method with photovoltaic power generation provided by the embodiment of the present invention utilizes the positive - current directions of each DC feeder during the fault to identify the fault section; according to the equivalent circuits of each transient stage of the fault, establish an inter - pole short - circuit fault location model covering the interaction responses of each system, and solve the fault distance using the parameter information of each fault electrical quantity to achieve accurate fault location. The specific content of the technical solution adopted in the embodiment of the present invention is as follows:

[0057] 1. Define the T - junction as the intersection of the DC feeder on the photovoltaic subsystem side and the DC feeder on the AC subsystem side, and the direction from the converter of the AC subsystem or the photovoltaic subsystem to the DC line is the positive direction. i VSC1 、i VSC2 、i DC / DC are the positive - pole currents at the T - junction and at the T - junctions between the T - junction and VSC1, VSC2, and the DC / DC outlet respectively. According to the positive - current directions of each DC feeder at the T - junction during the fault, judge the fault - occurring section. If i VSC1 is negative during the fault, the fault occurs between VSC1 and the T - junction; if i VSC2 is negative during the fault, the fault occurs between VSC2 and the T - junction; if i DC / DC is negative during the fault, the fault occurs between DC / DC and the T - junction.

[0058] 2. After the fault occurs, the IGBTs of the converters in each subsystem are immediately blocked. According to the conduction and current - feeding conditions of the internal diodes of the two - level VSC and the photovoltaic DC / DC, considering the coupling relationship of each subsystem at the fault point during the fault, determine the boundary conditions and equivalent circuits of each transient stage of the inter - pole short - circuit fault, establish an inter - pole short - circuit fault state equation including the fault distance and the transition resistance, and construct an inter - pole short - circuit fault location model covering the interaction responses of each system.

[0059] 3. Determine the sampling data window length using the boundary conditions in the fault transient stage. Based on the fault location state equation, collect the electrical values of each state variable at the same sampling moment. Form an objective optimization problem under multiple constraints according to the least squares method. Calculate the fault distance at each sampling moment based on the parameter information of each fault electrical quantity, and obtain the average value of the fault distances obtained at several sampling points to achieve accurate positioning of the pole-to-pole short circuit fault.

[0060] As Figure 3 shown, the two-level VSC type photovoltaic-integrated dual-terminal flexible DC distribution network fault location system provided by the embodiment of the present invention includes:

[0061] A fault occurrence section judgment module 1, which is used to specify the T connection point as the intersection of the DC feeder of the photovoltaic subsystem and the DC feeder of the AC subsystem, and judge the fault occurrence section according to the positive current direction of each DC feeder at the T connection point during the fault;

[0062] A fault location model construction module 2, which is used to establish a pole-to-pole short circuit fault location model covering the interactive responses of each system through the equivalent circuits in each transient stage;

[0063] A fault distance calculation module 3, which is used to determine the sampling data window length using the boundary conditions in the fault transient stage, and calculate the fault distance at each sampling moment based on the parameter information of each fault electrical quantity;

[0064] A short circuit fault location module 4, which is used to obtain the average value of the fault distances obtained at several sampling points to achieve accurate positioning of the pole-to-pole short circuit fault.

[0065] Embodiment 2

[0066] The present invention proposes a method for locating the pole-to-pole short circuit fault of a two-level VSC type photovoltaic-integrated dual-terminal flexible DC distribution network. The overall flowchart of its location is as Figure 2 shown. The present invention will be described in detail below in combination with the example of a two-level VSC type photovoltaic-integrated dual-terminal flexible DC distribution network and the accompanying drawings.

[0067] As a preferred embodiment, as Figure 2 shown, the method for locating the fault of a two-level VSC type photovoltaic-integrated dual-terminal flexible DC distribution network provided by the embodiment of the present invention specifically includes the following steps:

[0068] 1. Location start criterion

[0069] After a pole-to-pole short circuit fault occurs in a photovoltaic-integrated dual-terminal flexible DC distribution network based on a two-level VSC, the fault current rises rapidly. When the DC side current rises to 3 times the rated current, the IGBT inside the system converter quickly locks, and the fault location link starts. At the same time, the numerical values of each electrical quantity at the moment of locking are collected.

[0070] 2. Fault section identification

[0071] It is stipulated that the T - junction is the intersection point of the DC feeder of the PV subsystem and the DC feeder of the AC subsystem. The direction from the converter of the AC subsystem or the PV subsystem to the DC line is the positive direction. i VSC1 、i VSC2 、i DC / DC are respectively the positive - pole currents at the T - junction between the T - junction and the outlets of VSC1, VSC2, and DC / DC. According to the positive - pole current directions of each DC feeder at the T - junction during the fault, the fault - occurring section is judged. If i VSC1 is negative during the fault, the fault occurs between VSC1 and the T - junction; if i VSC2 is negative during the fault, the fault occurs between VSC2 and the T - junction; if i DC / DC is negative during the fault, the fault occurs between DC / DC and the T - junction.

[0072] 3. Transient stage and data - window identification

[0073] Taking the capacitor - discharging - capacitor - discharging - PV - current - conducting stage of the pole - to - pole short - circuit fault as an example, when the DC - side capacitor voltage of the AC system satisfies u dc (t)=0, the process of the capacitor - discharging transient stage ends. In addition, when the DC - side capacitor voltage of the PV system drops to zero, the process of the PV - current - conducting transient stage ends. According to the boundary conditions of their respective transient stages, the duration of the capacitor - discharging - capacitor - discharging - PV - current - conducting stage of the pole - to - pole short - circuit fault in the two - level VSC - type PV - integrated dual - terminal flexible DC distribution network can be determined. Assume that t VSC1 、t VSC2 are respectively the end times of the capacitor - discharging stage after the faults of VSC1 and VSC2, and t DC / DC is the end time of the PV - current - conducting stage of the PV system. Then the end time of the capacitor - discharging - capacitor - discharging - PV - current - conducting stage is t = min{t VSC1 ,t VSC2 ,t DC / DC}. The data required for the fault - location method provided by the present invention are the electrical - quantity data from the IGBT locking of the converter to the end time of the capacitor - discharging - capacitor - discharging - PV - current - conducting stage. According to the end time of the capacitor - discharging - capacitor - discharging - PV - current - conducting stage, the duration of this transient process can be determined, and then the width of the corresponding data window can be obtained.

[0074] 4. Ranging - model construction

[0075] The topology of the pole - to - pole short - circuit fault in the PV - integrated dual - terminal flexible DC distribution network based on the two - level VSC is as Figure 4As shown in the figure, taking the pole-to-pole short-circuit fault occurring at f1 as an example, assuming that the unit resistance and inductance of the DC line are r0 and l0, the line length between VSC1 and the T connection point is l, the fault distance is x, and taking the capacitor discharge - capacitor discharge - photovoltaic feed current conduction stage of the pole-to-pole short-circuit fault as an example, its equivalent circuit is as follows Figure 5 As shown. From Figure 5 it can be obtained that the state equation for this stage is: where,

[0076]

[0077]

[0078]

[0079]

[0080]

[0081] M = L 22 L 23 + L 22 L′ 21 + L 23 L′ 21 ,

[0082] For a certain fixed dual-terminal flexible DC distribution network with photovoltaic, its system unit impedance and other parameters are certain. When R g and x are determined, the numerical values of each electrical quantity at any moment can be solved according to the 4th-order classical Runge-Kutta formula using the state equation.

[0083] 5. Calculation of positioning results

[0084] Collect the electrical values u i (t dc1 ), u i (t dc2 ), u i (t dc3 ), i b (t i ), i 21 (t i ), i 22 (t i ) and i 23 (t i ) at time t i in sequence. Then, according to the least squares method, an objective optimization problem with multiple constraint conditions can be formed.

[0085]

[0086]

[0087] By solving this equation, t can be obtained. i The fault distance x at the moment i Similarly, the corresponding fault distances at several sampling points within the data window can be obtained, and the average value of several fault distances is used as the final ranging result.

[0088]

[0089] II. Application Example:

[0090] In the above embodiment, it can be implemented in whole or in part by software, hardware, firmware, or any combination thereof. When implemented in whole or in part in the form of a computer program product, the computer program product includes one or more computer instructions. When the computer program instructions are loaded or executed on a computer, the processes or functions described in the embodiments of the present invention are generated in whole or in part. The computer may be a general-purpose computer, a special-purpose computer, a computer network, or other programmable devices. The computer instructions may be stored in a computer-readable storage medium, or transmitted from one computer-readable storage medium to another computer-readable storage medium. For example, the computer instructions may be transmitted from one website, computer, server, or data center to another website, computer, server, or data center by wire (such as coaxial cable, optical fiber, digital subscriber line (DSL)) or wireless (such as infrared, wireless, microwave, etc.). The computer-readable storage medium may be any available medium that can be accessed by a computer or a data storage device such as a server or data center that includes one or more integrated available media. The available medium may be a magnetic medium (for example, a floppy disk, a hard disk, a magnetic tape), an optical medium (for example, a DVD), or a semiconductor medium (for example, a solid-state disk (SSD)).

[0091] III. Evidence of the Related Effects of the Embodiment:

[0092] Build a simulation model of a photovoltaic-based two-terminal flexible DC distribution network based on a two-level VSC as shown in Figure 6 in Matlab / Simulink. The simulation parameters are shown in Table 1. The lengths of the DC distribution lines are set as follows: T-VSC1: 3 km, T-VSC2: 2 km, and T-DC / DC: 0.5 km.

[0093] Table 1 Simulation Model Parameters

[0094]

[0095] The lines of T-VSC1 section, T-VSC2 section, and T-DC / DC section are respectively selected as the research objects, with the outlets of each converter as the reference points. According to the existing literature, the transition resistances of the pole-to-pole short-circuit faults are respectively set as: 0.001 Ω, 0.01 Ω, 0.1 Ω, 0.5 Ω. The fault distances of the T-VSC1 section line are set as: 0.2 km, 1 km, 2 km, 3 km, the fault distances of the T-VSC2 section line are set as: 0.2 km, 1 km, 1.5 km, 2 km, and the fault distances of the T-DC / DC section line are set as: 0.2 km, 0.35 km, 0.5 km. At the same time, the influence of temperature change on the transient output of the photovoltaic is relatively small, so the temperature is set to be constant during the fault, which is 25 °C. The light intensity is set as: -1000 W / m² for the fault of the T-VSC1 section line 2 , -800 W / m² for the fault of the T-VSC2 section line 2 , -600 W / m² for the fault of the T-DC / DC section line 2 . The above-mentioned several settings combine to produce 44 kinds of fault situations, and simulation verification is carried out for each situation. When the DC circuit current reaches three times the rated current, the IGBT is immediately blocked. The sampling frequency is set to 20 KHz, and the positioning error is calculated as shown in the following formula. The ranging results under the pole-to-pole fault are shown in Table 2

[0096]

[0097] Table 2 Simulation Results of Fault Location

[0098]

[0099]

[0100]

[0101] It can be seen from Table 2 that the ranging method proposed by the present invention has high ranging accuracy. And with the changes of the transition resistance and the fault distance, there are slight fluctuations in the ranging accuracy, but the maximum relative error does not exceed 0.2%, which proves the effectiveness and accuracy of the positioning method proposed by the present invention

[0102] In addition, taking the fault occurring between T-VSC1 as an example, the light intensity is 1000 W / m² 2, the transition resistance of the pole-to-pole short-circuit fault is set to: 0.001 Ω, 0.01 Ω, 0.1 Ω, 0.5 Ω respectively, and the fault distance is set to: 0.2 km, 1 km, 2 km, 3 km. The positioning accuracies of the traditional ranging method and the ranging method proposed in the present invention are compared. Among them, the traditional ranging method (1) is the flexible DC distribution network positioning method that ignores the feed current of the photovoltaic system. The traditional ranging method (2) is the flexible DC distribution network positioning method based on the RLC differential equation. The relative errors of different ranging methods are compared as shown in Table 3, and its relative error is as Figure 7 shown.

[0103] Table 3 Comparison of positioning accuracies of different ranging methods

[0104]

[0105]

[0106] Compared with the two traditional ranging methods, the ranging method proposed in the present invention has more accurate positioning results and better stability under different transition resistances and fault distances. At the same time, it can still ensure high accuracy within a short data window and has a lower requirement for the sampling frequency.

[0107] The above is only the preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention, any modification, equivalent replacement, and improvement made within the spirit and principle of the present invention shall be covered by the protection scope of the present invention.

Claims

1. A fault location method for a two-level VSC type photovoltaic integrated dual-terminal flexible DC distribution network, characterized in that, The two-level VSC type photovoltaic-integrated dual-terminal flexible DC distribution network fault location method includes: using the positive current direction of each DC feeder during a fault to identify the fault section; establishing an inter-pole short-circuit fault ranging model covering the interactive responses of each system according to the equivalent circuits of each transient stage of the inter-pole short circuit on the DC side of the two-level VSC type flexible DC distribution network; using the parameter information of each fault electrical quantity to solve the fault distance and achieve fault location; The two-level VSC type photovoltaic-integrated dual-terminal flexible DC distribution network fault location method includes the following steps: Step 1: Define the T-junction as the intersection of the DC feeder of the photovoltaic subsystem and the DC feeder of the AC subsystem. Determine the fault section according to the positive current direction of each DC feeder at the T-junction during the fault; Step 2: Establish an inter-pole short-circuit fault ranging model covering the interactive responses of each system through the equivalent circuits of each transient stage of the inter-pole short circuit on the DC side of the two-level VSC type flexible DC distribution network; Step 3: Use the boundary conditions of the fault transient stage to determine the sampling data window length, and calculate the fault distance at each sampling moment based on the parameter information of each fault electrical quantity; take the average value of the fault distances obtained at several sampling points to achieve accurate positioning of the inter-pole short-circuit fault; In the first step, the direction from the converter of the AC subsystem or the PV subsystem to the DC line is defined as the positive direction. , , are the positive-pole currents at the T-joint and the T-joint between the VSC1, VSC2, and the DC / DC outlet respectively. If is negative during the fault, the fault occurs between the VSC1 and the T-joint; if is negative during the fault, the fault occurs between the VSC2 and the T-joint; if is negative during the fault, the fault occurs between the DC / DC and the T-joint. In Step 2, after a fault occurs, the IGBTs of the converters in each subsystem are immediately blocked. According to the conduction and current feeding conditions of the internal diodes of the two-level VSC and the photovoltaic DC / DC, analyze the coupling relationship of each subsystem at the fault point during the fault, and determine the boundary conditions and equivalent circuits of each transient stage of the inter-pole short-circuit fault; according to Kirchhoff's voltage and current laws, construct the corresponding fault state equation; represent the elements inside the state equation that change with the fault distance and the transition resistance, and then establish an inter-pole short-circuit fault ranging state equation including the fault distance and the transition resistance.

2. The fault location method for a two-level VSC type photovoltaic integrated dual-terminal flexible DC distribution network according to claim 1, characterized in that, In Step 3, based on the fault ranging state equation, collect the electrical values of each state variable at the same sampling moment, form an objective optimization problem under multiple constraints according to the least squares method, and obtain the fault distance at the corresponding sampling moment by solving this problem; Use the boundary conditions of the fault transient stage to determine the start and end times of each stage, and the difference between the two is the sampling data window length; Based on the parameter information of each fault electrical quantity, construct an objective optimization problem with multiple constraints at the same sampling moment according to the least squares method: ； By solving this problem, the fault distance at each sampling moment will be calculated; take the average value of the fault distances obtained at several sampling points to achieve accurate positioning of the inter-pole short-circuit fault; ； Wherein, represents the moment, represents the fault distance at the moment, represents the fault distance, , , , , , , and respectively represent the electrical values of each electrical quantity at the moment.

3. A fault location system for a two-level VSC type photovoltaic integrated dual-terminal flexible DC distribution network applying the fault location method for a two-level VSC type photovoltaic integrated dual-terminal flexible DC distribution network according to any one of claims 1 to 2, characterized in that, The two-level VSC type photovoltaic-integrated dual-terminal flexible DC distribution network fault location system includes: A fault section judgment module (1) for defining the T-junction as the intersection of the DC feeder of the photovoltaic subsystem and the DC feeder of the AC subsystem, and determining the fault section according to the positive current direction of each DC feeder at the T-junction during the fault; A fault ranging model construction module (2) for establishing an inter-pole short-circuit fault ranging model covering the interactive responses of each system through the equivalent circuits of each transient stage; A fault distance calculation module (3) for using the boundary conditions of the fault transient stage to determine the sampling data window length, and calculating the fault distance at each sampling moment based on the parameter information of each fault electrical quantity; The short-circuit fault location module (4) is used to calculate the average value of the fault distances obtained at several sampling points, so as to achieve accurate positioning of the pole-to-pole short-circuit fault.

4. A program storage medium for receiving user input, characterized in that, The stored computer program causes the electronic device to execute the steps of the two-level VSC type photovoltaic double-ended flexible DC distribution network fault location method described in any one of claims 1 to 2.

5. A computer device, characterized in that, The computer device includes a memory and a processor. The memory stores a computer program. When the computer program is executed by the processor, the processor is caused to execute the steps of the two-level VSC type photovoltaic double-ended flexible DC distribution network fault location method described in any one of claims 1 to 2.

6. A computer-readable storage medium, characterized in that, There is a stored computer program. When the computer program is executed by the processor, the processor is caused to execute the steps of the two-level VSC type photovoltaic double-ended flexible DC distribution network fault location method described in any one of claims 1 to 2.

7. An information data processing terminal, characterized in that, The information data processing terminal is used to provide a user input interface when implemented on an electronic device to implement the two-level VSC type photovoltaic double-ended flexible DC distribution network fault location system described in claim 3.

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