Multi-terminal flexible direct current transmission line protection method based on injection signal

Abstract

The invention discloses a multi-terminal flexible direct current transmission line protection method based on injection signals, and relates to the technical field of power distribution network line protection. After the bipolar short-circuit fault is detected, the modularized multi-level converter is controlled to enter an active current limiting mode, and the fault current is limited to be 1.1 times of rated current, so that the impact on equipment is reduced; a main control station injects current sinusoidal signals with different angular frequencies twice, current and voltage signals of a port of the main control station are extracted, the impedance amplitude difference under the two angular frequencies is obtained, and the fault section protection method based on the impedance amplitude difference is established. Through a cut-and-trial method, the number of current-limiting reactors through which fault current flows is calculated, a criterion for matching and detecting a fault line section is established, and the fault section is isolated by a protection action. According to the method, synchronous communication is not needed, fault line protection can be realized under high resistance and noise, and the protection reliability and the recovery efficiency of the complex topology multi-terminal flexible direct current power grid are remarkably improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of distribution network line protection, and in particular relates to a multi-terminal flexible direct current transmission line protection method based on injection signals. Background Art

[0002] DC faults in flexible DC distribution networks can easily cause system overcurrent, seriously threatening the safe operation of the power grid. Flexible DC distribution networks based on full-bridge modular multilevel converters (FBSM-MMCs) mostly use MMC blocking to cut off fault currents, but this blocking can cause short-term power outages in the entire DC distribution network, reducing power supply reliability. To date, there is no effective solution. Mei Jun, Zhang Bingtian, Zhu Pengfei, et al. (Fault location method for flexible DC distribution networks based on active fault current control [J]. Automation of Electric Power Systems, 2021, 45(24): 133-141) proposed a fault location method for flexible DC distribution networks based on active fault current control. This method utilizes the high controllability of hybrid modular multilevel converters (MMCs) to divide the fault current into phase I and phase II. Phase I mainly achieves rapid suppression of the fault current and provides conditions for a smooth transition to phase II; phase II achieves accurate fault location by injecting high-frequency reverse current of a specific frequency. Simulation results show that this method can determine the fault location within 10ms and has high positioning accuracy, solving the problem of fault location difficulty caused by multiple refraction and reflection of traveling waves in short lines in traditional methods.

[0003] Zheng Tao, Wu Qiong, Lv Wenxuan, et al. (DC distribution network protection and fault isolation scheme based on active current limiting control [J]. Automation of Electric Power Systems, 2020, 44(5): 114-121) proposed a flexible DC distribution network protection and fault isolation scheme based on active current limiting control of full-bridge modular multilevel converter (FBSM-MMC). The scheme is divided into three stages: first, the MMC active current limiting control is used to limit the DC current output of the converter to about 1.2 times the rated current (stage 1), then the fault line is identified based on whether the DC current at both ends of each line has a synchronous zero-crossing feature (stage 2), and finally, the DC circuit breaker and fast mechanical switch are coordinated to complete the fault isolation (stage 3). Simulation results show that the scheme can effectively solve the problem of short-term power outages in the entire network caused by MMC locking, while reducing the breaking capacity requirements of the DC circuit breaker and significantly saving engineering costs.

[0004] Song Guobing, Yang Jiayi, Chang Zhongxue, et al. (Principle of single-ended full-line fast-acting protection using converters to construct active boundaries [J]. Automation of Electric Power Systems, 2024, 48(13): 160-167) proposed a single-ended full-line fast-acting protection method based on converters to actively construct active boundaries. This method uses the high controllability of power electronic equipment to actively generate characteristic electrical quantities reflecting the fault location when a fault occurs, thereby constructing active boundary characteristics. Specifically, a modular multilevel converter (MMC) is used to superimpose a specific response signal during fault ride-through, so that the protection end can achieve fault identification within the single-ended full-line range based solely on this characteristic signal. Simulation results show that this method still has high reliability and sensitivity in the case of high-resistance faults, solving the performance degradation problem of traditional passive boundary protection caused by weakening or eliminating filtering equipment.

[0005] Chinese invention patent CN119253547A discloses a "Three-terminal Hybrid DC Transmission Line Protection Method Based on Line-Mode Power Characteristics." Through multi-constraint joint optimization, this method addresses the adaptability of directional current protection in distribution networks with distributed photovoltaic integration. Its core value lies in directly linking protection performance with renewable energy capacity, providing a quantitative tool for the safe operation of high-penetration distribution networks. Future research is needed to further explore algorithm lightweighting and multi-timescale collaborative optimization to enhance engineering practicality.

[0006] The existing technologies listed above primarily target four-terminal DC distribution networks or those with specific topologies, and each has its own shortcomings. Their protection methods may fail for larger, multi-terminal DC grids with more complex topologies. To address these issues, the present invention proposes a multi-terminal flexible DC transmission line protection method based on signal injection. Summary of the Invention

[0007] The object of the present invention is to propose a multi-terminal flexible direct current transmission line protection method based on injection signals to solve the problem that the existing line protection method proposed in the background art may fail for a multi-terminal direct current grid.

[0008] To achieve the above objectives, the present invention adopts the following technical solutions:

[0009] A multi-terminal flexible direct current transmission line protection method based on injection signals comprises the following steps:

[0010] S1: Real-time monitoring of the inter-pole voltage at each converter station port in the flexible DC ring network. When the inter-pole voltage change rate exceeds the set start threshold, it is determined that a bipolar short circuit fault has occurred in the system, and the DC circuit breaker at the DC / DC input port is triggered to quickly operate.

[0011] S2. The MMC control system of each converter station quickly switches to the active current limiting mode, setting the fault current reference value to 1.1 times the rated current. Through current regulation, the port current of each converter station is stabilized near the fault current reference value to ensure that the power electronic equipment in the converter station avoids the risk of overcurrent damage;

[0012] S3. Preselect a converter station as the master control station. When the fault current is detected to be stable near the fault current reference value, the master control station changes the control strategy of the MMC control system and injects a first sinusoidal AC current signal with an angular frequency of ω1 into the DC transmission line for a duration of Δt. The other converter stations remain in the current limiting mode. After the injection of the first sinusoidal AC current signal is completed, the master control station again injects a second sinusoidal AC current signal with an angular frequency of ω2 into the DC transmission line for a duration of Δt. The other converter stations remain in the current limiting mode.

[0013] S4. Use the master station to collect the port current and voltage during the two sinusoidal AC current injection processes and calculate the two impedance values. Then obtain The absolute value of the amplitude difference is called the impedance amplitude difference Δz under the two current injections;

[0014] S5. Use the operation described in S4 to construct a calculation formula for the impedance amplitude difference Δz, and then derive a calculation formula for the fault distance;

[0015] S6. Using the trial-and-error method, substitute the number of current-limiting reactors into the fault distance calculation formula in ascending order. Match the calculated fault distance with the previously obtained physical distance from each reactor to the measurement point to identify the fault section. The master control station protection system sends trip commands to the DC circuit breakers on both sides of the fault section to effectively isolate the fault section.

[0016] Preferably, the S1 specifically includes the following contents:

[0017] After a bipolar short circuit fault occurs, the inter-pole voltage drops rapidly and the current at the receiving converter station port flows in the opposite direction. The multi-point inter-pole voltage gradient detection is used as the starting criterion. The multi-point inter-pole voltage gradient value is:

[0018]

[0019] Where u(q+p) and u(qp) are the line voltage sampling values ​​of p sampling periods before and after time q respectively;

[0020] The protection start criteria are established as follows:

[0021]

[0022] Where, D is the protection start setting value;

[0023] When the startup criteria are met, the DC circuit breaker at the DC / DC input port quickly trips due to overcurrent.

[0024] Preferably, the S2 specifically includes the following contents:

[0025] When a bipolar short circuit fault is detected, all converter stations are switched to active current limiting mode, and the fault current is adjusted by the converter station to follow the fault current reference value I dcref ;

[0026] When each converter station MMC enters the active current limiting mode, the outer loop control system remains unchanged, and the current loop compares the real-time current I dc and the fault current reference value I dcref After PI adjustment, it is superimposed with the output instruction of the circulating current suppressor to generate the final modulation signal U dc The modulation signal U dc After inverse transformation, it is decomposed into a three-phase modulation wave, input into the submodule, and enters the negative input mode to limit the fault current to the fault current reference value I dcref to ensure that the fault current is within the safe operating range of the converter and DC circuit breaker, and to avoid damage to the power electronic equipment in the converter station due to overcurrent.

[0027] Preferably, the S3 specifically includes the following contents:

[0028] (1) Selection of angular frequency of the two injected current signals:

[0029] The frequency of the injected current signal is 100-500 Hz, and the corresponding angular frequency ω is 628.32-3141.59 rad / s;

[0030] (2) Selection of the amplitude of the two injected current signals:

[0031] The injected current signal amplitude satisfies:

[0032] I inj <2I n (3)

[0033] Where, I inj Indicates the current signal amplitude; I n Indicates rated current;

[0034] (3) Selection of injection time and injection duration of the two injection current signals:

[0035] The moment when the current at the master station port stabilizes near the fault current reference value is defined as T1, and the injection duration Δt=50ms; the moment after the first sinusoidal AC current signal is injected is defined as T2, and the injection duration Δt=50ms.

[0036] Preferably, the S4 specifically includes the following contents:

[0037] S4.1. Based on the collected voltage and current signals at the master station port measurement point, the voltage amplitude U is obtained through Fourier transform. ω and the current amplitude I ω ;

[0038] S4.2, according to the voltage amplitude U ω and the current amplitude I ω Calculate the impedance corresponding to angular frequencies ω1 and ω2 Amplitude:

[0039]

[0040] S4.3, for two impedance values calculate The absolute value of the amplitude difference between:

[0041]

[0042] Where Δz represents the impedance amplitude difference under two current injections, referred to as the impedance amplitude difference.

[0043] Preferably, the S5 specifically includes the following contents:

[0044] When a bipolar fault occurs in a section, the fault current starts from the converter station port and flows through an even number of reactors on the positive line. The impedance at the injected current angular frequency of ω1 and ω2 is calculated. and

[0045]

[0046] Subtract the two equations in equation (6) and then take the modulus, and combine equation (5) to obtain the impedance amplitude difference ΔZ as follows:

[0047] ΔZ=l0n|ω1-ω2|+2x|l ω1 -l ω2 | (7)

[0048] From formula (7), the calculation formula of the fault distance x is derived as follows:

[0049]

[0050] Where x represents the distance between the fault point and the measurement point; n represents the number of current-limiting reactors through which the fault current flows, which is an even number; l0 represents the inductive reactance value of a single current-limiting reactor; They represent the inductive reactance per unit length of the line at current angular frequencies of ω1 and ω2 respectively; R f is the transition resistance value at the fault point.

[0051] Preferably, the S6 specifically includes the following contents:

[0052] Obtain n through trial and error and determine the fault section;

[0053] Using the impedance amplitude difference ΔZ, different numbers of current-limiting reactors are used as trial variables k, which are substituted into equation (8) from small to large. The value of x corresponding to k is calculated by replacing n in equation (8). The value is then matched with the previously obtained physical distances from each reactor to the measurement point to correctly identify the fault section. The specific steps are as follows:

[0054] 1) Try different trial variables k

[0055] From small to large, take different trial variables k, k = 0, 2, 4, ..., M, where M is the total number of current-limiting reactors in the DC transmission system;

[0056] 2) Calculate the candidate fault distance x k

[0057] For each trial variable k, substitute into formula (8) to solve the corresponding candidate fault distance x k :

[0058]

[0059] 3) Match the fault section

[0060] Starting from the voltage and current signal measurement point of the master station port, with the instantaneous clock as the positive direction, obtain the current limiting reactors L2, L4, ..., L M The physical distances to the measurement points are d2, d4, ..., d M ; Establish a mapping relationship between the physical distance between each section and each current limiting reactor, that is, the i-th section S i The physical distance interval of the corresponding current limiting reactor is [d 2i ,d 2i+4 ], i≥1; the special section S0 is the section from the measurement point to the current-limiting reactor L2, and the physical distance interval corresponding to the current-limiting reactor is [d0, d2], d0=0;

[0061] For x calculated by trial variable k k , check x k Does it satisfy the following formula:

[0062] d k <x k <d k+2 (10)

[0063] If formula (10) is satisfied, the fault section is judged to be S k / 2Otherwise, substitute k+2 into formula (9) and calculate the corresponding x k+2 , then check x k 、x k+2 Does it satisfy the following formula:

[0064] d k+2 <x k && x k+2 <d k+2 (11)

[0065] In the formula, && represents the logical and relationship;

[0066] If equation (11) is satisfied, it means the fault occurs in section S k / 2 At the end of the fault section, it is judged that the fault section is S k / 2 ; If neither equation (10) nor equation (11) is satisfied, then check x k 、x k+2 Does it satisfy the following formula:

[0067] d k+2 <x k && x k+2 <d k (12)

[0068] If equation (12) is satisfied, it means the fault occurs in section S k / 2 At this time, the fault section is judged to be S k / 2 ; The fault section judgment is completed;

[0069] If equations (10), (11), and (12) are not satisfied, then add 2 to k and return to equation (9) for the next round of trial and error.

[0070] When the master control station identifies the faulty section, it sends instructions to the DC circuit breakers on both sides of the faulty section, which then operate to isolate the faulty line.

[0071] Compared with the prior art, the present invention has the following beneficial effects:

[0072] (1) Control and insurance coordination strategy

[0073] After a fault occurs, without any additional equipment, only the control strategy of the converter station is changed to limit the fault current to 1.1 times the rated current, avoiding overcurrent damage to the equipment and creating conditions for subsequent protection actions.

[0074] (2) Key technologies for active detection protection

[0075] After current limiting, the system actively injects two detection signals of different frequencies into the DC transmission system according to a predetermined rule. This system uses differential impedance to construct a location criterion, thereby determining the fault section. This solves the problem of traditional protection systems relying on passive detection, which results in insufficient fault information.

[0076] (3) Single-ended method does not require information synchronization

[0077] Since the single-ended method is adopted, there is no need to consider the issue of information synchronization. It can distinguish short-circuit faults inside and outside the line area, prevent the generation of dead zones, and better adapt to fault detection and protection of multi-terminal flexible DC distribution network lines.

[0078] (4) Adapt to complex topological structures

[0079] The present invention proposes a multi-terminal flexible DC transmission line protection method based on injection signals. It is aimed at large-scale, complex topology multi-terminal flexible DC grids, solves the problem that traditional methods may fail in ring networks, and is suitable for fault detection and protection of multi-terminal flexible DC distribution network lines. BRIEF DESCRIPTION OF THE DRAWINGS

[0080] To more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, a brief introduction to the drawings involved in the embodiments is now provided. It is obvious that the drawings described below are only schematic illustrations of some embodiments of the present invention. Those skilled in the art can construct other forms of drawings based on these drawings without inventive effort.

[0081] Figure 1 This is a flow chart of the multi-terminal flexible DC transmission line protection method based on injection signals proposed by the present invention;

[0082] Figure 2 This is a working mode diagram of the MMC mentioned in Example 1 of the present invention in normal operation of the power transmission network;

[0083] Figure 3 This is a working mode diagram of the MMC mentioned in Example 1 of the present invention when operating in a power transmission network fault;

[0084] Figure 4 The voltage and current detected at the measurement point when the angular frequency of the current signal mentioned in Example 1 of the present invention is 2513.27 rad / s;

[0085] Figure 5 The voltage and current detected at the measurement point when the angular frequency of the current signal mentioned in Example 1 of the present invention is 3141.59 rad / s;

[0086] Figure 6 This is a schematic diagram of the fault current flow path mentioned in Example 1 of the present invention;

[0087] Figure 7 This is a simplified schematic diagram of the fault equivalent mentioned in Example 1 of the present invention;

[0088] Figure 8 This is a network topology diagram of the 10kV flexible DC transmission system of the present invention;

[0089] Figure 9 This is a control block diagram of the MMC control system with control and security coordination functions mentioned in Example 2 of the present invention;

[0090] Figure 10 This is the current simulation result of the system mentioned in Example 2 of the present invention running for a period of 2.95s to 3.20s. DETAILED DESCRIPTION

[0091] The following will provide a clear and complete description of the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0092] The present invention addresses the problem that existing protection schemes are susceptible to transition resistance and noise interference and lack coordination with the control link. The present invention proposes a single-ended quantity protection method that combines active current limiting and boundary effects. After detecting a bipolar short-circuit fault, the method controls the modular multilevel converter (MMC) to enter the active current limiting mode, limiting the fault current to 1.1 times the rated current to reduce the impact on the equipment; then, the master control station injects current signals of different frequencies twice, extracts the current and voltage signals of the single-ended port, and establishes a fault distance model based on differential impedance. By analyzing the number of reactance boundaries through which the current signal flows, combined with the trial-and-error method to match the system topology, the fault section is accurately located. The present invention does not require synchronous communication, solves the problems of low positioning accuracy and protection dead zones in high-resistance and noisy environments in traditional methods, and significantly improves the protection reliability and recovery efficiency of complex topology multi-terminal flexible DC power grids. It has the advantages of anti-noise interference, good transition resistance tolerance, and no need for synchronous communication. The following is an explanation of the multi-terminal flexible DC transmission line protection method based on injection signals proposed by the present invention in conjunction with relevant drawings and specific examples. The specific content is as follows.

[0093] Example 1:

[0094] See also Figure 1 The present invention provides a method for determining a target area for geological disaster monitoring in a small watershed based on multi-source data fusion, comprising the following steps:

[0095] Step 1:

[0096] The inter-pole voltage at each converter station port in the flexible DC ring network is monitored in real time. When the inter-pole voltage change rate exceeds the set start threshold, the system is judged to have a bipolar short circuit fault. The DC circuit breaker at the DC / DC input port will quickly operate due to overcurrent. The specific procedures include the following:

[0097] When a bipolar fault occurs, the inter-pole voltage will drop rapidly and the current at the receiving converter station port will flow in the opposite direction. The multi-point inter-pole voltage gradient detection is used as the starting criterion. The multi-point inter-pole voltage gradient values ​​are as follows:

[0098]

[0099] Where u(q+p) and u(qp) are the line voltage sampling values ​​of p sampling periods before and after time q respectively;

[0100] The protection start criteria are established as follows:

[0101]

[0102] Where D is the protection start setting value; considering a certain margin, 5kV is taken;

[0103] When the startup criteria are met, the DC circuit breaker at the DC / DC input port will quickly trip due to overcurrent;

[0104] Step 2:

[0105] See also Figure 2 , the working mode of MMC during normal operation of the transmission network is as follows Figure 2 As shown; the working mode when the transmission network fails is as follows Figure 3 As shown, at this time, the MMC control system of each converter station quickly switches to the active current limiting mode, setting the fault current reference value to 1.1 times the rated current. After a short period of current regulation, the port current of each converter station stabilizes to near the fault current reference value, ensuring that the power electronic equipment in the converter station will not be damaged by overcurrent. The specific contents include the following:

[0106] All converter stations are switched to active current limiting mode. The fault current is adjusted by the converter station for a certain period of time and follows the fault current reference value I dcref ;

[0107] When each converter station MMC enters the active current limiting mode, the outer loop control system remains unchanged, and the current loop compares the real-time current I dc and the fault current reference value I dcref After PI adjustment, it is superimposed with the output instruction of the circulating current suppressor to generate the final modulation signal U dc The signal is decomposed into a three-phase modulated wave through inverse transformation and input into the submodule, which enters the negative input mode to achieve current limiting.

[0108] When a DC fault occurs, the short-circuit current can rapidly rise to dozens of times the rated current, far exceeding the tolerance limit of power electronic devices. Active current limiting control limits the fault current to 1.1 times the rated current, ensuring that the fault current is within the safe operating range of the converter and DC circuit breaker, preventing device damage due to overcurrent.

[0109] Step 3:

[0110] A converter station is selected in advance as the master control station. When the detected fault current stabilizes near the fault current reference value, at time T1, the master control station changes the control strategy of the MMC control system and injects a sinusoidal AC current signal with an angular frequency of ω1 into the DC transmission line for a duration of Δt. The other converter stations remain in current limiting mode. Subsequently, at time T2, the master control station again injects a sinusoidal AC current signal with an angular frequency of ω2 into the DC transmission line for a duration of Δt. The specific steps are as follows:

[0111] (1) Selection of angular frequency of the two injected current signals:

[0112] The basic principle of selection is: under the premise of meeting the performance constraints of the injection equipment, the response characteristics are most obvious, so that the detection is more sensitive. Therefore, the higher the frequency of the injected current signal, the better. In order to locate the fault in 10ms, the frequency of the injected current signal should be greater than 100Hz, and the corresponding angular frequency should be greater than 628.32rad / s. However, in actual engineering, the frequency of the injected current signal is limited by the switching frequency of the converter submodule. Therefore, the frequency of the injected current signal should be much smaller than the switching frequency of the converter submodule. If the frequency is too high, it will be greater than the switching frequency of the converter submodule. Cutting the frequency will cause the injected current signal to differ significantly from the set injected current signal. The simulation results of changing the frequency of injecting different current signals show that when the frequency is below 500Hz, the injected current signal can be more accurately consistent with the set injected current signal; while when the frequency is 500Hz and above, the injected current signal differs significantly from the set injected current signal, which has a greater impact on the fault distance calculation. Therefore, the frequencies of the two injected current signals are 400Hz and 500Hz respectively, and the corresponding angular frequencies ω1 and ω2 are 2513.27rad / s and 3141.59rad / s respectively.

[0113] (2) Selection of the amplitude of the two injected current signals:

[0114] The basic principle of selection is: when selecting the amplitude of the injected current signal, it is necessary to consider the tolerance of the power electronic devices of the converter station and whether the fault current and voltage signals are within the measurement range of the measuring components; in order to meet the above requirements, it is necessary to adjust the amplitude of the injected current signal I injThe withstand current capability of the key equipment of the converter station is only the rated current I n The injected current signal amplitude cannot exceed 3 times the rated current. Considering a certain margin, the following conditions must be met:

[0115] I inj <2I n (3)

[0116] Where, I n is the rated current.

[0117] Refer to the voltage and current specified for the 10kV DC distribution network in GB / T 35727-2017 "Medium and Low Voltage DC Distribution System", and their deviation range is -10% to +5%. At the same time, considering that the fault current reference value in step 2 is set to 1.1 times the rated current, the actual injection current signal amplitude is 1.2 times the rated current.

[0118] (3) Selection of injection time and injection duration of the two injection current signals:

[0119] The moment when the current at the master control station port stabilizes near the fault current reference value is T1. At this time, a sinusoidal AC current signal of ω1 is injected into the DC transmission line. The basic principle for selecting the injection duration is to inject the signal for as short a time as possible while ensuring that one cycle of the injected signal can be effectively extracted to avoid affecting the normal operation of the healthy line. Therefore, the injection duration Δt is 50ms. Subsequently, at time T2, a sinusoidal AC current signal of ω2 is injected into the DC transmission line. The injection duration is also Δt = 50ms. To avoid mutual interference between the injected current signals, the voltage and current data 20ms after the injection time are extracted for subsequent processing.

[0120] Step 4:

[0121] During the two sinusoidal AC current injection processes, the master station measured the port current and voltage respectively and calculated the two impedance values. The absolute value of their amplitude difference is obtained, which is called the impedance amplitude difference ΔZ under the two current injections, hereinafter referred to as the impedance amplitude difference; specifically, it includes the following contents:

[0122] First, according to the collected voltage and current signals of the main control station port measurement point, the voltage and current amplitude U is obtained through Fourier transform. ω and I ω ;

[0123] Calculate the impedance corresponding to angular frequencies ω1 and ω2 Amplitude:

[0124]

[0125] For two impedance values The absolute value of their amplitude difference is obtained, which is called the impedance amplitude difference ΔZ under the two current injections:

[0126]

[0127] Step 5:

[0128] A formula for impedance amplitude difference is established in advance, from which the formula for fault distance is derived. The specific contents include the following:

[0129] like Figure 6 As shown in the figure, if a fault occurs at f1, the fault current will flow through two reactors L1 and L2; if a fault occurs at f2, the fault current will flow through four reactors L1 to L4; if a fault occurs at f3, the fault current will flow through six reactors L1 to L6;

[0130] according to Figure 7 Equivalent circuit diagram of the fault line. When a bipolar fault occurs in a section, the fault current starts from the converter station port, flows through multiple reactors, lines, transition resistors on the positive line, and returns to the master control station from the negative line and reactor, that is, it passes through an even number of reactors.

[0131] By fault circuit Figure 7 , the impedances at injected current angular frequencies of ω1 and ω2 are obtained respectively and The expression is:

[0132]

[0133] Where n is the number of current-limiting reactors through which the fault current flows, which is an even number; l0 is the inductive reactance value of a single current-limiting reactor; They correspond to the inductive reactance per unit length of the line at current angular frequencies of ω1 and ω2 respectively; R f is the transition resistance value of the fault point; x is the distance between the fault point and the measurement point;

[0134] Subtract the two equations in equation (6) and then take the modulus to obtain the impedance amplitude difference ΔZ as follows:

[0135]

[0136] From formula (7), the calculation formula of the fault distance x is derived as follows:

[0137]

[0138] Step 6:

[0139] Using the fault distance expression obtained in step 5, a trial-and-error method is used. Using the impedance amplitude difference ΔZ, different numbers of current-limiting reactors are substituted into the fault distance formula from smallest to largest. The calculated fault distance is matched with the previously obtained physical distances from each reactor to the measurement point to correctly identify the faulty section. The master control station sends trip commands to the DC circuit breakers on both sides of the faulty section, tripping them and effectively isolating the faulty section. This includes the following:

[0140] Since there are two unknown quantities n and x in formula (8), it is necessary to obtain n through trial and error to determine the fault section;

[0141] Using the impedance amplitude difference ΔZ, we try to use different numbers of current-limiting reactors as trial variables k, and substitute them into the fault distance formula (8) from small to large. We replace n in formula (8), calculate x under the corresponding k, and then match it with the physical distance from each reactor to the measurement point obtained in advance, so as to correctly identify the fault section. The specific steps are as follows:

[0142] 1) Try different trial variables k

[0143] From small to large, take different trial variables k, k = 0, 2, 4, ..., M, where M is the total number of current-limiting reactors in the DC transmission system;

[0144] 2) Calculate the candidate fault distance x k

[0145] For each trial variable k, substitute into formula (8) to solve the corresponding candidate fault distance x k ,as follows:

[0146]

[0147] 3) Match the fault section

[0148] Starting from the voltage and current signal measurement point of the master station port, with the instantaneous clock as the positive direction, obtain the current limiting reactors L2, L4, ..., L M The physical distances to the measurement points are d2, d4, ..., d M ; Establish a mapping relationship between the physical distance between each section and each current limiting reactor, that is, the i-th section S i The physical distance interval of the corresponding current limiting reactor is [d 2i ,d 2i+4 ], i≥1; the special section S0 is the section from the measurement point to the current-limiting reactor L2, and the physical distance interval corresponding to the current-limiting reactor is [d0, d2], d0=0;

[0149] For x calculated by trial variable k k , check xk Does it satisfy the following formula:

[0150] d k <x k <d k+2 (10)

[0151] If formula (10) is satisfied, the fault section is judged to be S k / 2 Otherwise, substitute k+2 into formula (9) to calculate the corresponding x k+2 , then check x k 、x k+2 Does it satisfy the following formula:

[0152] d k+2 <x k &&x k+2 <d k+2 (11)

[0153] In the formula, && represents the logical and relationship;

[0154] If equation (11) is satisfied, it means the fault occurs in section S k / 2 At the end of the fault section, it is judged that the fault section is S k / 2 ; If neither equation (10) nor equation (11) is satisfied, then check x k 、x k+2 Does it satisfy the following formula:

[0155] d k+2 <x k &&x k+2 <d k (12)

[0156] If equation (12) is satisfied, it means the fault occurs in section S k / 2 At this time, the fault section is judged to be S k / 2 ; The fault section judgment is completed;

[0157] If equations (10), (11), and (12) are not satisfied, then add 2 to k and return to equation (9) for the next round of trial and error.

[0158] When the master control station identifies the faulty section, it sends instructions to the DC circuit breakers on both sides of the faulty section, which then operate to isolate the faulty line.

[0159] Example 2:

[0160] Based on Example 1, but different in that this example is based on the PSCAD / EMTDC simulation platform, Figure 8The 10kV multi-terminal ring flexible DC grid system model is shown in Table 1. The simulation parameters are shown in Table 1. The sampling frequency is 20kHz and the data window length is 5ms. MMC1 is the master control station and adopts constant DC voltage and constant reactive power control. It serves as the balancing node of the entire DC system. The measurement point is placed at its outlet. MMC2, 3, and 5 adopt constant active power and constant reactive power control. MMC4 adopts island droop control. The system is set to have a bipolar short circuit fault at 3s. Its control block diagram is shown in the figure below. Figure 9 As shown, the current simulation results of the system during the period of 2.95s to 3.20s are as follows Figure 10 shown.

[0161] Table 1 Parameters of the simulation system

[0162]

[0163]

[0164] To define the mapping relationship between each section in the flexible DC ring network, the corresponding circuit breaker, and the physical distance, we precisely divide the sections and associate the physical locations of current-limiting reactors, providing a clear topology matching basis for the subsequent fault location algorithm. This mapping table fully considers the typical characteristics of line segmentation and reactor distribution in actual systems, ensuring that the algorithm can efficiently identify the number of reactors through which the fault current flows and accurately locate the fault section. The corresponding topological segment mapping table is shown in Table 2.

[0165] Table 2 Segment mapping table

[0166]

[0167] (1) Verification of the impact of different fault distances and transition resistances

[0168] exist Figure 7 In the multi-terminal flexible DC grid model, bipolar short-circuit faults were set at 2.5, 4, 7.5, 9, 12.5, and 14 km from the measurement point. The fault section determination results are shown in Table 3.

[0169] Table 3 Verification results of different fault distances and transition resistances

[0170]

[0171]

[0172] It can be seen from Table 3 that the method of the present invention is not affected by the fault distance and transition resistance, and can identify the fault section more accurately.

[0173] (2) Dead zone verification

[0174] In order to verify whether this method can correctly identify the faulty section when the fault occurs at the beginning or end of the section. Figure 7 Bipolar faults are set at the beginning and end of different sections in the central line, and simulation experiments are carried out to verify whether there is a dead zone when the method is used to identify the fault section. The results are shown in Table 4.

[0175] Table 4 Dead zone verification results

[0176]

[0177] It can be seen from the experimental results in Table 4 that when the fault occurs at the beginning or end of the section, this method can still correctly identify the faulty section, indicating that this method does not have a dead zone problem.

[0178] (3) Impact of noise

[0179] To verify the noise immunity of this method, a simulation was conducted by superimposing white noise with a signal-to-noise ratio of 25dB on the current and voltage signals. Even with the noise added, the fault location could still be determined through trial and error. This demonstrates that the proposed algorithm has a certain degree of noise immunity. The results of the noise test are shown in Table 5.

[0180] Table 5 Verification results under 25dB noise

[0181]

[0182] In summary, the present invention proposes a multi-terminal flexible direct current transmission line protection method based on injection signals, which uses MMC active current limiting and injects current signals of different frequencies to realize fault section identification. By injecting the number of boundaries of the current flowing through the reactor twice, a fault section identification formula is established, and the trial-and-error method is used to solve the distance through which the fault current flows, thereby determining the fault section and completing the protection. Simulation results show that this scheme does not require synchronous communication, solves the problems of low positioning accuracy and protection dead zones in traditional methods in high-resistance and noisy environments, and significantly improves the protection reliability and recovery efficiency of complex topology multi-terminal flexible direct current power grids.

[0183] The above description is only used to help understand the method and core essence of the present invention, but the scope of protection of the present invention is not limited thereto. For those skilled in the art, equivalent replacements or modifications based on the technical solutions and inventive concepts of the present invention within the technical scope disclosed by the present invention should be included in the scope of protection of the present invention. In summary, the contents of this specification should not be understood as limiting the present invention.

Claims

1. A multi-terminal flexible direct current transmission line protection method based on injection signal, characterized in that: The following steps are involved: S1: Real-time monitoring of the inter-pole voltage at each converter station port in the flexible DC ring network. When the inter-pole voltage change rate exceeds the set start threshold, it is determined that a bipolar short circuit fault has occurred in the system, and the DC circuit breaker at the DC / DC input port is triggered to quickly operate. S2. The MMC control system of each converter station quickly switches to the active current limiting mode, setting the fault current reference value to 1.1 times the rated current. Through current regulation, the port current of each converter station is stabilized near the fault current reference value to ensure that the power electronic equipment in the converter station avoids the risk of overcurrent damage; S3. Preselect a converter station as the master control station. When the fault current is detected to be stable near the fault current reference value, the master control station changes the control strategy of the MMC control system and injects a first sinusoidal AC current signal with an angular frequency of ω1 into the DC transmission line for a duration of Δt. The other converter stations remain in the current limiting mode. After the injection of the first sinusoidal AC current signal is completed, the master control station again injects a second sinusoidal AC current signal with an angular frequency of ω2 into the DC transmission line for a duration of Δt. The other converter stations remain in the current limiting mode. S4. Use the master station to collect the port current and voltage during the two sinusoidal AC current injection processes and calculate the two impedance values. Then obtain The absolute value of the amplitude difference is called the impedance amplitude difference Δz under the two current injections; S5. Use the operation described in S4 to construct a calculation formula for the impedance amplitude difference Δz, and then derive a calculation formula for the fault distance; S6. Using the trial-and-error method, substitute the number of current-limiting reactors into the fault distance calculation formula in ascending order. Match the calculated fault distance with the previously obtained physical distance from each reactor to the measurement point to identify the fault section. The master control station protection system sends trip commands to the DC circuit breakers on both sides of the fault section to effectively isolate the fault section.

2. The method according to claim 1, characterized in that The S1 specifically includes the following contents: After a bipolar short-circuit fault occurs, the inter-pole voltage drops rapidly and the current at the receiving converter station port flows in the opposite direction. Taking the multi-point inter-pole voltage gradient detection as the starting criterion, the multi-point inter-pole voltage gradient value at time q is: Where u(q+p) and u(qp) are the line voltage sampling values ​​of p sampling periods before and after time q respectively; The protection start criteria are established as follows: Where, D is the protection start setting value; When the startup criteria are met, the DC circuit breaker at the DC / DC input port quickly trips due to overcurrent.

3. The method according to claim 2, characterized in that The S2 specifically includes the following contents: When a bipolar short circuit fault is detected, all converter stations are switched to active current limiting mode, and the fault current is adjusted by the converter station to follow the fault current reference value I dcref ; When each converter station MMC enters the active current limiting mode, the outer loop control system remains unchanged, and the current loop compares the real-time current I dc and the fault current reference value I dcref After PI adjustment, it is superimposed with the output instruction of the circulating current suppressor to generate the final modulation signal U dc The modulation signal U dc After inverse transformation, it is decomposed into a three-phase modulation wave, input into the submodule, and enters the negative input mode to limit the fault current to the fault current reference value I dcref to ensure that the fault current is within the safe operating range of the converter and DC circuit breaker, and to avoid damage to the power electronic equipment in the converter station due to overcurrent.

4. The method according to claim 3, characterized in that The S3 specifically includes the following contents: (1) Selection of angular frequency of the two injected current signals: The frequency of the injected current signal is 100-500 Hz, and the corresponding angular frequency ω is 628.32-3141.59 rad / s; (2) Selection of the amplitude of the two injected current signals: The injected current signal amplitude satisfies: AND inj <2I n (3) Where, I inj Indicates the current signal amplitude; I n Indicates rated current; (3) Selection of injection time and injection duration of the two injection current signals: The moment when the current at the master station port stabilizes near the fault current reference value is defined as T1, and the injection duration Δt=50ms; the moment after the first sinusoidal AC current signal is injected is defined as T2, and the injection duration Δt=50ms.

5. The method according to claim 4, characterized in that The S4 specifically includes the following contents: S4.1 obtains the voltage amplitude U through Fourier transform based on the voltage and current signals collected at the master station port measurement point ω and the current amplitude I ω ; S4.2 According to the voltage amplitude U ω and the current amplitude I ω Calculate the impedance corresponding to the angular frequencies ω1 and ω2 Amplitude: S4.3 for two impedances calculate The absolute value of the amplitude difference ΔZ: Where ΔZ is the impedance amplitude difference.

6. The method according to claim 5, characterized in that The S5 specifically includes the following contents: When a bipolar fault occurs in a section, the fault current starts from the converter station port and flows through an even number of reactors on the positive line. The impedance at the injected current angular frequencies ω1 and ω2 is calculated. and Subtract the two equations in equation (6) and then take the modulus, and combine them with equation (5) to obtain the expression of the impedance amplitude difference ΔZ as follows: From formula (7), the calculation formula of the fault distance x is derived as follows: Where x is the distance between the fault point and the measurement point; n is the number of current-limiting reactors through which the fault current flows, which is an even number; l0 represents the inductive reactance value of a single current-limiting reactor; are the inductive reactance per unit length of the line at the current angular frequencies ω1 and ω2 respectively; R f is the transition resistance value at the fault point.

7. The method according to claim 6, characterized in that The S6 specifically includes the following contents: Obtain n through trial and error and detect the faulty section; Using the impedance amplitude difference ΔZ, the number of different current-limiting reactors is used as the trial variable k, which is substituted into formula (8) from small to large. The n in formula (8) is replaced to calculate the x value corresponding to k. Then, it is matched with the physical distance from each current-limiting reactor to the measurement point obtained in advance, and the fault section is correctly identified. The specific steps are as follows: 1) Try different trial variables k From small to large, take different trial variables k, k = 0, 2, 4, ..., M, where M is the total number of current-limiting reactors in the DC transmission system; 2) Calculate the candidate fault distance x k For each trial variable k, substitute it into formula (8) and solve it to get the corresponding candidate fault distance x k : 3) Match and detect the fault section Starting from the voltage and current signal measurement point of the master station port, with the instantaneous clock as the positive direction, obtain the current limiting reactors L2, L4, ..., L M The physical distances to the measurement points are d2, d4, ..., d M ; Establish a mapping relationship between each line section and the physical distance of each current limiting reactor, that is, the i-th section S i The physical distance interval corresponding to the current limiting reactor is [d 2i ,d 2i+4 ], i≥1; the special section S0 is the section between the measurement points of the current-limiting reactor L2, and the corresponding physical distance interval of the current-limiting reactor is [d0, d2], d0=0; For x calculated by trial variable k k , check x k Does it satisfy the following formula: d k <x k <d k+2 (10) If formula (10) is satisfied, the fault section is judged to be S k / 2 , end the detection of the fault section; Otherwise, substitute k+2 into formula (9) and calculate the corresponding x k+2 , then check x k 、x k+2 Does it satisfy the following formula: d k+2 <x k && x k+2 <d k+2 (11) In the formula, && represents the logical and relationship; If equation (11) is satisfied, the fault occurs in section S k / 2 At the end of the fault section, it is judged that the fault section is S k / 2 , end the detection of the fault section; If neither equation (10) nor equation (11) is satisfied, check x k 、x k+2 Does it satisfy the following formula: d k+2 <x k && x k+2 <d k (12) If equation (12) is satisfied, the fault occurs in section S k / 2 At this time, the fault section is judged to be S k / 2 , end the detection of the fault section; If equations (10), (11), and (12) are not satisfied, then add 2 to k and return to equation (9) for the next round of trial and error. When the master control station determines the fault section, it sends instructions to the DC circuit breakers on both sides of the fault section, which then operate to isolate the faulty line.

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