Single-phase Grounding Fault Distance Protection Method, Device, Equipment and Medium for Double-ended Weak-feed System

Abstract

The present invention relates to the technical field of relay protection in power systems, and particularly to a single-phase grounding fault distance protection method, device, equipment and medium for a double-ended weakly-fed system. The method includes collecting real-time electrical quantities of three-phase voltages and currents, calculating the current change rate; monitoring whether the change rate continuously exceeds the starting threshold value; calculating the measured impedance in combination with the control characteristics of the double-ended weakly-fed system; solving the adaptive compensation coefficient, performing phase shift compensation on the measured impedance to obtain the compensated measured impedance and line impedance; extracting the imaginary part to calculate the fault distance and eliminating the influence of the transition resistance error; logically judging the fault section and triggering the protection action. This method solves the additional impedance angle and the adaptive compensation coefficient through the three-phase electrical quantities of the flexible converter, compensates the measured impedance by using reverse phase shift, and combines the mathematical analysis method to eliminate the influence of the transition resistance on the protection performance, realizing fast and accurate protection, reducing the hardware requirements and calculation amount, and ensuring the safe and reliable operation of the system.

Description

Technical Field

[0001] The present invention relates to the technical field of relay protection in power systems, and in particular to a single-phase grounding fault distance protection method, device, equipment and medium for a double-ended weakly-fed system line. Background Technique

[0002] In recent years, with the development of new energy grid-connected power generation technology, the power electronic characteristics of the power grid have become increasingly obvious. This new development trend has significantly changed the fault characteristics of the power system. Especially with the emergence of ultra-long-distance and large-capacity power transmission scenarios such as deep-sea and far-sea offshore wind power and "desert, gobi, and wasteland" new energy bases, the flexible DC transmission technology has attracted extensive attention from domestic and foreign scholars. For example, a demonstration project of offshore wind power transmitted through flexible DC has been built in Rudong, Jiangsu.

[0003] The transmission lines of the offshore wind power transmitted through flexible DC system are all typical double-ended weakly-fed system lines. The converters at both ends of the line are composed of a large number of power electronic devices. The fault characteristics are affected by the control strategies of the converters on both sides and have the characteristics of limited amplitude and controlled phase. Due to different converter control strategies in different scenarios, in the case of a single-phase grounding fault, the positive-sequence, negative-sequence and zero-sequence equivalent networks in the composite sequence network are connected in series, and the current in the zero-sequence network also shows a controlled characteristic, which will cause the traditional distance protection to be unable to correctly identify the single-phase grounding fault of the double-ended weakly-fed system outgoing line.

[0004] The information disclosed in this background technical section is only intended to deepen the understanding of the overall background technology of the present invention, and should not be regarded as an admission or any form of implication that this information constitutes the prior art known to those skilled in the art. Summary of the Invention

[0005] The present invention provides a single-phase grounding fault distance protection method, device, equipment and medium for a double-ended weakly-fed system line, thereby effectively solving the problems in the background technology.

[0006] In order to achieve the above object, the technical solution adopted by the present invention is: a single-phase grounding fault distance protection method for a double-ended weakly-fed system line, including the following steps:

[0007] S10: Collect the real-time electrical quantities of three-phase voltage and three-phase current through the voltage transformer and current transformer installed on the MMC side, and calculate the change rates of d-axis and q-axis currents;

[0008] S20: Monitor whether the change rates of the d-axis and q-axis currents continuously exceed the starting threshold value within a set time window. If so, the protection device starts; if not, return to step S10 to continue monitoring;

[0009] S30: After the protection device is activated, the collected three-phase current is converted into positive-sequence, negative-sequence, and zero-sequence components using the symmetrical component method, and combined with the control characteristics in the double-ended weakly-fed system, the measured impedance during single-phase grounding faults is calculated.

[0010] S40: According to the negative-sequence voltage suppression control strategy on the MMC side and the negative-sequence current suppression control strategy of the wind turbine generator set, the adaptive compensation coefficient is solved to eliminate the influence of the transition resistance on the measured impedance.

[0011] S50: The adaptive compensation coefficient is respectively used for phase shift compensation with the measured impedance phase and the line impedance phase to obtain the compensated measured impedance and the compensated line impedance.

[0012] S60: Based on the compensated measured impedance and the compensated line impedance, the imaginary part is extracted and the fault distance α is calculated, and the calculation error caused by the transition resistance is eliminated through mathematical analysis methods to ensure the accuracy of the fault distance.

[0013] S70: A logical judgment is made on the calculation result of the fault distance α to determine the fault section, and corresponding protection actions are triggered according to the fault section.

[0014] Further, in step S20, within the set time window, it is monitored whether the change rates of the d-axis and q-axis currents continuously exceed the startup threshold value. If so, the protection device is activated; if not, return to step S10 to continue monitoring. The startup criterion of the protection device includes:

[0015]

[0016] ΔI d =I d (t)-I d (t - Δt);

[0017] ΔI q =I q (t)-I q (t - Δt);

[0018] Δt = 1 / f s ;

[0019] Wherein, I d (t) and I q (t) are the per-unit values of the d-axis and q-axis currents of the current inner loop of the sending-end MMC controller, f s is the protection sampling frequency, and ε1 and ε2 are the startup threshold values.

[0020] Further, in step S30, after the protection device is started, the collected three-phase current is converted into positive sequence, negative sequence, and zero sequence components by using the symmetrical component method, and combined with the control characteristics in the double-ended weakly-fed system, the measured impedance during single-phase ground fault is calculated. The model of the measured impedance includes:

[0021]

[0022] In the formula, Z MMC-AG is the measured impedance during single-phase ground fault, is the phasor of phase A voltage, is the phasor of phase A current, is the zero sequence component of phase A current, K = (z0 - z1) / (3z1), z1 is the positive sequence impedance per unit length of the AC transmission line, and z0 is the zero sequence impedance per unit length of the AC transmission line.

[0023] Further, in step S40, according to the negative sequence voltage suppression control strategy on the MMC side and the negative sequence current suppression control strategy of the wind turbine generator set, the adaptive compensation coefficient is solved to eliminate the influence of the transition resistance on the measured impedance. The model of the adaptive compensation coefficient includes:

[0024]

[0025] In the formula, is the adaptive compensation coefficient, j is the imaginary unit, is the negative sequence component of phase A current, is the zero sequence component of phase A current, is the phasor of phase A current, K = (z0 - z1) / (3z1), z1 is the positive sequence impedance per unit length of the AC transmission line, and z0 is the zero sequence impedance per unit length of the AC transmission line.

[0026] Further, in step S50, the adaptive compensation coefficient is respectively used for offset compensation with the phase of the measured impedance and the phase of the line impedance to obtain the compensated measured impedance and the compensated line impedance. The models of the compensated measured impedance and the line impedance respectively include:

[0027]

[0028] In the formula, Z′ MMC-AG is the compensated measured impedance, Z MMC-AG is the measured impedance, Z L ′ is the compensated line impedance, Z L is the line impedance, α is the ratio of the distance from the fault point to the head of the line on the MMC side, Z add is the additional impedance, j is the imaginary unit, is the adaptive compensation coefficient.

[0029] Further, in step S60, based on the compensated measured impedance and the line impedance, take the imaginary parts of both parts and perform a mathematical derivation model:

[0030]

[0031] In the formula, Z′ MMC-AG is the compensated measured impedance, α is the fault distance, Z L is the line impedance, Z L ′ is the compensated line impedance, is the adaptive compensation coefficient, Z add is the additional impedance.

[0032] Further, in step S60, the model of the fault distance α includes:

[0033]

[0034] In the formula, α is the fault distance, Z′ MMC-AG is the compensated measured impedance, Z′ L is the compensated line impedance.

[0035] Further, in step S70, perform a logical judgment on the calculation result of the fault distance α to determine the fault section, and trigger corresponding protection actions according to the fault section, including:

[0036] If the calculation result of the fault distance α satisfies the in-zone fault determination condition 0 < α < α set.1 for x1 consecutive sampling points, then the protection determines that it is an in-zone fault of the AC transmission line, meeting the action criterion of the first section of the distance protection on the MMC side;

[0037] If the calculation result of the fault distance α satisfies the lower-level line fault condition α set.1 < α < α set.2 for x2 consecutive sampling points, then the protection determines that it is a lower-level line fault of the AC transmission line, meeting the time-delay action criterion of the second section of the distance protection on the MMC side, and set the time delay Δt1 to 0.2 s as the backup protection criterion of this protection method;

[0038] If the above determination conditions are not satisfied within the set time Δt2, then the protection should determine that it is an out-of-zone fault of the AC transmission line or the system is in normal operation, and the protection does not operate.

[0039] The present invention also includes a single-phase grounding fault distance protection device for a double-ended weakly-fed system line, using the method as described above, including:

[0040] The data acquisition unit is used to collect the real-time electrical quantities of three-phase voltage and three-phase current through the voltage transformer and current transformer installed on the MMC side, and calculate the change rates of d-axis and q-axis currents;

[0041] The protection startup unit is used to monitor whether the change rates of the d-axis and q-axis currents continuously exceed the startup threshold value within a set time window. If so, the protection device is started; if not, it returns to step S10 to continue monitoring;

[0042] The measured impedance calculation unit is used, after the protection device is started, to convert the collected three-phase currents into positive-sequence, negative-sequence, and zero-sequence components by using the symmetrical component method, and combine the control characteristics in the double-ended weakly-fed system to calculate the measured impedance during single-phase ground fault;

[0043] The adaptive compensation unit is used to solve the adaptive compensation coefficient according to the negative-sequence voltage suppression control strategy on the MMC side and the negative-sequence current suppression control strategy of the wind turbine generator set, and is used to eliminate the influence of the transition resistance on the measured impedance;

[0044] The measured impedance compensation unit is used to perform offset compensation on the phase of the measured impedance and the phase of the line impedance respectively with the adaptive compensation coefficient to obtain the compensated measured impedance and the compensated line impedance;

[0045] The fault distance calculation unit is used to extract the imaginary part based on the compensated measured impedance and the compensated line impedance and calculate the fault distance α, and eliminate the calculation error caused by the transition resistance through a mathematical analysis method to ensure the accuracy of the fault distance;

[0046] The fault judgment unit makes a logical judgment on the calculation result of the fault distance α to determine the fault section, and triggers corresponding protection actions according to the fault section.

[0047] The present invention further includes a computer device, including a memory, a processor, and a computer program stored on the memory and executable on the processor. When the processor executes the computer program, the method as described above is implemented.

[0048] The present invention further includes a storage medium, on which a computer program is stored. When the computer program is executed by a processor, the method as described above is implemented.

[0049] The beneficial effects of the present invention are:

[0050] 1. This method takes the offshore wind power system with negative sequence voltage control on the MMC converter side and negative sequence current control on the wind turbine side as a typical application scenario of a double-ended weak system, constructs a composite sequence network diagram of the system under single-phase grounding fault, calculates the fault distance using the electrical quantities on the MMC converter side, and based on this, a new protection strategy is established to solve the problem of incorrect operation faced by existing distance protection in double-ended weak feed systems.

[0051] 2. At the same time, this method combines a single-phase grounding impedance relay and the electrical quantity equation of the composite sequence network, calculates the adaptive coefficient based on unilateral information, adaptively compensates the phase of the measured impedance and the line impedance, eliminates the influence of the transition resistance on the protection, and can adapt to various complex fault conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the drawings described below are only some embodiments recorded in the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0053] Figure 1 It is a schematic diagram of a double-ended weak feed system represented by offshore wind power integrated into the grid through a flexible DC link;

[0054] Figure 2 It is a schematic flow diagram of the single-phase grounding fault distance protection method for the double-ended weak feed system line;

[0055] Figure 3 It is a system topology diagram during single-phase grounding fault;

[0056] Figure 4 It is a system topology diagram of the simulation model;

[0057] Figure 5 It is a comparison diagram of calculation results when a fault occurs at different positions on the line with a 0.1Ω transition resistance;

[0058] Figure 6 It is a comparison diagram of calculation results when a fault occurs at different positions on the line with a 20Ω transition resistance;

[0059] Figure 7 It is a comparison diagram of calculation results when a fault occurs at different positions on the line with a 60Ω transition resistance;

[0060] Figure 8 It is a schematic structural diagram of the single-phase grounding fault distance protection device for the double-ended weak feed system line;

[0061] Figure 9 It is a schematic structural diagram of a computer device. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0062] Next, in combination with the accompanying drawings in the embodiments of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments.

[0063] Embodiment 1:

[0064] In order to overcome the problems of refusal to operate and misoperation that may be faced when a single-phase ground fault occurs in the AC line of a double-ended weakly-fed system, the present invention proposes a single-phase ground fault distance protection method for the line of a double-ended weakly-fed system based on the phase adaptive compensation of the measured impedance, and takes the line of a double-ended weakly-fed system in which an offshore wind turbine is connected to an MMC converter station through a collection line and an AC transmission line as an example for analysis. The schematic diagram of the offshore wind power connected to the power grid is as Figure 1 shown. The offshore wind farm is connected to a 220 kV step-up substation, and then sent to the flexible DC transmission system through the AC transmission line and incorporated into the land system.

[0065] As Figure 2 shown: A single-phase ground fault distance protection method for the line of a double-ended weakly-fed system includes the following steps:

[0066] S10: Through the voltage transformer and current transformer installed on the MMC (flexible DC converter) side, collect the real-time electrical quantities of the three-phase voltage and three-phase current, and calculate the change rates of the d-axis and q-axis currents;

[0067] S20: Within the set time window, monitor whether the change rates of the d-axis and q-axis currents continuously exceed the starting threshold value. If so, the protection device starts; if not, return to step S10 to continue monitoring;

[0068] S30: After the protection device starts, use the symmetrical component method to convert the collected three-phase current into positive-sequence, negative-sequence, and zero-sequence components, and combine the control characteristics in the double-ended weakly-fed system to calculate the measured impedance during a single-phase ground fault;

[0069] S40: According to the negative-sequence voltage suppression control strategy on the MMC (flexible DC converter) side and the negative-sequence current suppression control strategy of the wind turbine, solve the adaptive compensation coefficient to eliminate the influence of the transition resistance on the measured impedance;

[0070] S50: Perform offset compensation on the measured impedance phase and the line impedance phase with the adaptive compensation coefficient respectively to obtain the compensated measured impedance and the compensated line impedance;

[0071] S60: Based on the compensated measured impedance and the compensated line impedance, extract the imaginary part and calculate the fault distance α, and eliminate the calculation error caused by the transition resistance through a mathematical analysis method to ensure the accuracy of the fault distance;

[0072] S70: Logically judge the calculation result of the fault distance α to determine the fault section, and trigger corresponding protection actions according to the fault section.

[0073] This method focuses on solving the problem that distance protection may malfunction incorrectly in the case of single-phase grounding faults in the AC transmission lines of a double-ended weakly-fed system. The research object is a typical double-ended weakly-fed system, where one end of the transmission line is a marine wind turbine and the other end is a flexible DC converter. A single-phase grounding fault distance protection method for the transmission lines of a double-ended weakly-fed system based on the phase adaptive compensation of the measured impedance is proposed. During the fault, the flexible DC converter adopts a negative-sequence voltage suppression control strategy, and the wind turbine adopts a negative-sequence current suppression control strategy. The adaptive phase shift compensation coefficient is calculated using the electrical quantities on the MMC side, the fault distance is obtained using a deformed form of the existing ground distance impedance relay expression, and the solution result is logically judged, enabling the protection to quickly and reliably distinguish between internal and external faults, which is of great significance for ensuring the safe and reliable operation of the transmission lines of the double-ended weakly-fed system.

[0074] The additional impedance angle and the adaptive compensation coefficient are solved through the three-phase electrical quantities of the flexible DC converter. The measured impedance is adaptively compensated by reverse phase shift. The influence of the transition resistance on the protection performance is theoretically eliminated using mathematical analysis methods, and the fault location is accurately calculated, enabling the protection to correctly and quickly act under different operating modes, reactive power support requirements, transition resistances, and fault locations, ensuring the safe and reliable operation of the transmission lines of the double-ended weakly-fed system, reducing the hardware requirements and computational load of the protection device, and having good engineering application prospects.

[0075] In a double-ended weakly-fed system, the fault characteristics are affected by the control strategies of the converters at both ends. Traditional distance protection methods are prone to refusal or misoperation. The present invention combines the negative-sequence voltage suppression control strategy of the flexible DC converter and the negative-sequence current suppression control strategy of the wind turbine, and uses the adaptive compensation technology to adapt to the special requirements of the weakly-fed system and ensure the effectiveness and reliability of the protection system in such an environment.

[0076] When calculating the fault distance, the error caused by the transition resistance is eliminated using mathematical analysis methods, improving the system's ability to resist transition resistance. Even in the presence of a large transition resistance, the system can maintain a high fault location accuracy and protection action reliability.

[0077] Taking the A-phase grounding fault of the AC outgoing line as an example, the fault system topology is as Figure 3 shown, where R g is the grounding transition resistance, α is the ratio of the distance from the fault point to the line head end of the MMC side, and Z Lis the line impedance. The HVDC side distance protection device respectively collects the three-phase voltages at the protection installation location on this side through a voltage transformer and the three-phase currents flowing through the protection installation location through a current transformer, and converts the electrical real-time quantities u MMC-A (t), u MMC-B (t), u MMC-C (t) and i MMC-A (t), i MMC-B (t), i MMC-C (t) into the three-phase voltage phasors and the three-phase current phasors for analysis.

[0078] In this embodiment, the protection device collects the d-axis and q-axis values of the current in the current inner loop of the sending-end MMC converter controller, and calculates the d-axis and q-axis current change rates. The time window for protection startup judgment is adjusted according to the reliability requirements in actual projects. In this method, the judgment time window δt of the protection startup criterion is set to 1 ms, and the sampling frequency is 1200 Hz. When the difference of the d-axis and q-axis currents with respect to time continuously exceeds the startup threshold value within the time window δt, the protection starts.

[0079] As an optimization of the above embodiment, in step S20, within the set time window, it is monitored whether the d-axis and q-axis current change rates continuously exceed the startup threshold value. If so, the protection device starts; if not, it returns to step S10 to continue monitoring. The startup criterion of the protection device includes:

[0080]

[0081] ΔI d =I d (t)-I d (t - Δt);

[0082] ΔI q =I q (t)-I q (t - Δt);

[0083] Δt=1 / f s ;

[0084] In the formula, I d (t) and I q (t) are the per-unit values of the d-axis and q-axis currents of the current inner loop of the sending-end MMC controller, f s is the protection sampling frequency, ε1 and ε2 are the startup threshold values, and 10 times the maximum change rate of I d and I q with time within the first 5 ms of the sampling points is used as the protection startup threshold.

[0085] Using the rate of change of d-axis and q-axis currents as the starting criterion can quickly capture the current change characteristics during a fault. By setting the time window and starting threshold, the protection device can be triggered to start within a short time, reducing the impact of the fault on the system operation and improving the response speed of the protection action; adopting the maximum change value of the rate of change of d-axis and q-axis currents within the first 5 ms before the sampling point and taking 10 times of it as the starting threshold can effectively avoid false triggering caused by normal system fluctuations or small disturbances, and can maintain the stability and anti-interference ability of the protection in a complex power grid operation environment.

[0086] In this embodiment, in step S30, after the protection device starts, the collected three-phase currents are converted into positive-sequence, negative-sequence, and zero-sequence components by using the symmetrical component method, and combined with the control characteristics in the double-ended weak-feed system, the measured impedance during a single-phase ground fault is calculated. The model of the measured impedance includes:

[0087]

[0088] In the formula, Z MMC-AG is the measured impedance during a single-phase ground fault, is the phasor of phase A voltage, is the phasor of phase A current, is the zero-sequence component of phase A current, K = (z0 - z1) / (3z1), z1 is the positive-sequence impedance per unit length of the AC transmission line, and z0 is the zero-sequence impedance per unit length of the AC transmission line.

[0089] The measured impedance calculated by the phase A ground distance relay on the MMC side can be decomposed into:

[0090]

[0091] The additional impedance Z add and its phase φ add The expressions are:

[0092]

[0093] Among them, in step S40, according to the negative-sequence voltage suppression control strategy on the MMC side and the negative-sequence current suppression control strategy of the wind turbine generator set, the adaptive compensation coefficient is solved to eliminate the influence of the transition resistance on the measured impedance. The model of the adaptive compensation coefficient includes:

[0094]

[0095] In the formula, is the adaptive compensation coefficient, j is the imaginary unit, is the negative-sequence component of phase A current, is the zero-sequence component of phase A current, is the phasor of phase A current, K = (z0 - z1) / (3z1), where z1 is the positive sequence impedance per unit length of the AC transmission line, and z0 is the zero sequence impedance per unit length of the AC transmission line.

[0096] By combining the proportional relationship between negative sequence current and zero sequence current, and the difference between positive sequence and zero sequence impedances of the line, an adaptive compensation coefficient is constructed, effectively eliminating the influence of transition resistance on the measured impedance, improving the accuracy of fault measurement, especially the adaptability in high transition resistance scenarios.

[0097] As an optimization of the above embodiment, in step S50, the adaptive compensation coefficient is respectively used for offset compensation of the phases of the measured impedance and the line impedance to obtain the compensated measured impedance and the compensated line impedance. The models of the compensated measured impedance and the line impedance respectively include:

[0098]

[0099] In the formula, Z′ MMC-AG is the compensated measured impedance, Z MMC-AG is the measured impedance, Z′ L is the compensated line impedance, Z L is the line impedance, α is the proportion of the distance from the fault point to the head of the line on the MMC side, Z add is the additional impedance, j is the imaginary unit, is the adaptive compensation coefficient.

[0100] Introducing the adaptive compensation coefficient to perform offset compensation on the phases of the measured impedance and the line impedance effectively eliminates the interference of system characteristics and operating state changes on the protection action, making the protection system more stable under complex operating conditions.

[0101] Among them, in step S60, based on the compensated measured impedance and the line impedance, the imaginary parts of the two parts are taken simultaneously for mathematical derivation of the model:

[0102]

[0103] In the formula, Z′ MMC-AG is the compensated measured impedance, α is the fault distance, Z L is the line impedance, Z′ L is the compensated line impedance, is the adaptive compensation coefficient, Z add is the additional impedance.

[0104] Through the above imaginary part operation model, the impedance measurement error caused by transition resistance and weak feed characteristics is eliminated, ensuring the positioning accuracy in complex fault scenarios.

[0105] In this embodiment, in step S60, the model of the fault distance α includes:

[0106]

[0107] In the formula, α is the fault distance, and Z′ MMC-AG is the compensated measured impedance, and Z′ L is the compensated line impedance.

[0108] By calculating through the imaginary part of the compensated impedance, the error caused by the change of impedance amplitude or other interference factors is avoided, ensuring the stability and robustness of the calculation result. The model calculation is based on simple imaginary part operations, with a lightweight algorithm, capable of quickly completing fault location, meeting the high requirements for response speed in real-time protection scenarios.

[0109] As an optimization of the above embodiment, in step S70, a logical judgment is made on the calculation result of the fault distance α to determine the fault section, and corresponding protection actions are triggered according to the fault section, including:

[0110] If the calculation result of the fault distance α satisfies the in-zone fault determination condition 0 < α < α for x1 consecutive sampling points set.1 then the protection determines that it is an in-zone fault of the AC transmission line, meeting the action criterion of the first section of the distance protection on the MMC side;

[0111] If the calculation result of the fault distance α satisfies the lower-level line fault condition α set.1 < α < α set.2 for x2 consecutive sampling points, then the protection determines that it is a fault of the lower-level line of the AC transmission line, meeting the action criterion of the second section of the distance protection on the MMC side with a time delay of Δt1 set to 0.2 s as the backup protection criterion of this protection method;

[0112] If the above determination conditions are not satisfied within the set time Δt2, then the protection should determine that it is an out-of-zone fault of the AC transmission line or the system is in normal operation, and the protection does not operate.

[0113] Among them, the above α set.1 and α set.2 are the protection ranges of the distance protection. The range of the first section is 80%, and the range of the second section is 120%.

[0114] Based on the monitoring of consecutive sampling points of the fault distance and the logical judgment of the interval conditions, it can accurately distinguish in-zone faults, lower-level line faults, and out-of-zone faults; through the dynamic sampling point determination conditions 0 < α < α set.1 and α set.1 < α < α set.2 the influence of transient interference or measurement error on the determination result is effectively avoided, ensuring the reliability of the protection action.

[0115] For internal faults, the first section of the distance protection on the MMC side can act quickly to isolate the fault point in time and prevent the expansion of the fault.

[0116] For faults on the downstream line, the second section of the protection acts with a time delay to provide reliable backup protection and avoid incorrect tripping or affecting system operation due to overly fast operation.

[0117] For external faults or normal operating conditions, the protection does not act, reducing the possibility of incorrect operation and ensuring system stability.

[0118] This method takes the offshore wind power system with negative sequence voltage control on the MMC converter side and negative sequence current control on the wind turbine side as a typical application scenario of a double-ended weak system, constructs the system composite sequence network diagram under single-phase grounding faults, calculates the fault distance using the electrical quantities on the MMC converter side, and establishes a new protection strategy on this basis to solve the problem of incorrect operation faced by existing distance protection in double-ended weak feed systems.

[0119] At the same time, this method combines the single-phase grounding impedance relay and the composite sequence network electrical quantity equation, calculates the adaptive coefficient based on unilateral information, adaptively compensates the phase of the measured impedance and the phase of the line impedance, eliminates the influence of the transition resistance on the protection, and can adapt to various complex fault conditions.

[0120] Example 2:

[0121] A simulation model is established based on the PSCAD / EMTDC simulation platform. The system topology is as Figure 4 shown. The offshore wind farm is connected to the 220 kV main transformer through a 35 kV collector system. The capacity of the wind farm is 600 MW. The positive sequence impedance of the protected line is Z1 = 0.018 + j0.3 Ω / km, the zero sequence impedance is Z0 = 0.295 + j1.040 Ω / km, and the line length is 100 km. F1, F2, and F3 are located at 50%, 70%, and 90% of the line starting end from the MMC side respectively. F4 is located on the collector line. The signal acquisition frequency is 1200 Hz. Take x1 = 30, x2 = 40, α set.1 = 0.8, α set.2 = 1.2. The calculation results are as Figures 5 to 7 shown.

[0122] As the transition resistance changes, the protection can act within 40 ms after the fault occurs. After 70 ms, α stabilizes near the fault location, and the fault location can be accurately judged. The protection action time is shown in Table 1. t Ⅰ is the action time of the first section of the protection, and t Ⅱ is the action time of the second section of the protection.

[0123] Table 1 Simulation protection action conditions

[0124]

[0125] As can be seen from the above table, the proposed protection algorithm can correctly calculate the fault distance under the ground fault, and the protection can operate within 40 ms, and has strong ability to resist the transition resistance.

[0126] The characteristics of the weak-feed system are closely related to the system capacity. The capacity of the offshore wind farm is set at 800 MW, and the protection operation is shown in Table 2.

[0127] Table 2 Protection operation after the change of the wind farm capacity

[0128]

[0129] As can be seen from the above table, the proposed protection strategy can still operate normally under different capacities of the offshore wind farm, can correctly calculate the fault distance, and the proposed protection scheme is little affected by the weak-feed characteristics of the offshore wind farm.

[0130] The present invention further includes a single-phase ground fault distance protection device for a double-ended weak-feed system line, using the method as described above, as Figure 8 shown, including:

[0131] A data acquisition unit, configured to collect real-time electrical quantities of three-phase voltages and three-phase currents through a voltage transformer and a current transformer installed on the MMC side, and calculate the change rates of d-axis and q-axis currents;

[0132] A protection startup unit, configured to monitor whether the change rates of d-axis and q-axis currents continuously exceed the startup threshold value within a set time window. If so, the protection device starts; if not, return to step S10 to continue monitoring;

[0133] A measured impedance calculation unit, configured to, after the protection device starts, convert the collected three-phase currents into positive-sequence, negative-sequence and zero-sequence components by using the symmetrical component method, and calculate the measured impedance during the single-phase ground fault in combination with the control characteristics in the double-ended weak-feed system;

[0134] An adaptive compensation unit, configured to solve the adaptive compensation coefficient according to the negative-sequence voltage suppression control strategy on the MMC side and the negative-sequence current suppression control strategy of the wind turbine generator set, and used to eliminate the influence of the transition resistance on the measured impedance;

[0135] A measured impedance compensation unit, configured to perform offset compensation on the measured impedance phase and the line impedance phase respectively with the adaptive compensation coefficient to obtain the compensated measured impedance and the compensated line impedance;

[0136] A fault distance calculation unit, configured to extract the imaginary part and calculate the fault distance α based on the compensated measured impedance and the compensated line impedance, and eliminate the calculation error caused by the transition resistance through a mathematical analysis method to ensure the accuracy of the fault distance;

[0137] The fault judgment unit makes a logical judgment on the calculation result of the fault distance α to determine the fault section, and triggers corresponding protection actions according to the fault section.

[0138] Please refer to Figure 9 the structural schematic diagram of the computer device provided by the embodiment of the present application shown. A computer device 400 provided by an embodiment of the present application includes: a processor 410 and a memory 420. The memory 420 stores a computer program executable by the processor 410. When the computer program is executed by the processor 410, the above method is executed.

[0139] An embodiment of the present application also provides a storage medium 430. A computer program is stored on the storage medium 430. When the computer program is run by the processor 410, the above method is executed.

[0140] Among them, the storage medium 430 can be implemented by any type of volatile or non-volatile storage device or a combination thereof, such as static random access memory (abbreviated as SRAM), electrically erasable programmable read-only memory (abbreviated as EEPROM), erasable programmable read-only memory (abbreviated as EPROM), programmable read-only memory (abbreviated as PROM), read-only memory (abbreviated as ROM), magnetic memory, flash memory, magnetic disk or optical disc.

[0141] In the description of the present invention, the terms "first" and "second" are only used for descriptive purposes, and cannot be understood as indicating or implying relative importance or implicitly indicating the quantity of the indicated technical features. Thus, the features defined with "first" and "second" may explicitly or implicitly include one or more of such features. The meaning of "plurality" is two or more, unless otherwise specifically defined.

[0142] In the present invention, unless otherwise clearly defined or limited, terms such as "installed", "connected", "coupled", "fixed", etc. shall be understood in a broad sense. For example, it may be a fixed connection, a detachable connection, or integrated; it may be a mechanical connection or an electrical connection; it may be directly connected or indirectly connected through an intermediate medium, and it may be the internal communication of two components or the interaction relationship between two components. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0143] In the description of this specification, the description with reference to terms such as "one embodiment", "some embodiments", "example", "specific example", or "some examples" means that the specific features, structures, materials, or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described may be combined in any one or more embodiments or examples in a suitable manner. In addition, without contradiction, those skilled in the art may combine and combine the different embodiments or examples described in this specification and the features of different embodiments or examples.

[0144] Any process or method description shown in a flowchart or otherwise described herein can be understood to represent a module, segment, or portion of code including one or more executable instructions for implementing a specific logical function or process. The scope of the preferred embodiments of the present invention includes additional implementations, where the functions may be executed in a substantially simultaneous manner or in a reverse order according to the functions involved, rather than in the order shown or discussed, which should be understood by those skilled in the art to which the embodiments of the present invention pertain.

[0145] The logic and / or steps represented in the flowchart or otherwise described herein, for example, can be considered as a definitional sequence list of executable instructions for implementing logical functions, and can be specifically implemented in any computer-readable medium for use by an instruction execution system, apparatus, or device (such as a computer-based system, a system including a processor, or other systems that can fetch and execute instructions from the instruction execution system, apparatus, or device), or in conjunction with these instruction execution systems, apparatus, or devices. For the purposes of this specification, a "computer-readable medium" can be any device that can contain, store, communicate, propagate, or transport a program for use by or in conjunction with an instruction execution system, apparatus, or device. More specific examples (a non-exhaustive list) of the computer-readable medium include the following: an electrical connection portion having one or more wirings (electronic device), a portable computer diskette (magnetic device), a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or flash memory), an optical fiber device, and a portable compact disc read-only memory (CDROM). Additionally, the computer-readable medium can even be paper or other suitable media on which the program can be printed, because the program can be obtained electronically, for example, by optically scanning the paper or other media, followed by editing, interpretation, or otherwise processing as appropriate, and then storing it in a computer memory.

[0146] It should be understood that various parts of the present invention can be implemented by hardware, software, firmware, or a combination thereof. In the above-described embodiments, multiple steps or methods can be implemented by software or firmware stored in a memory and executed by a suitable instruction execution system. For example, if implemented by hardware, as in another embodiment, any one or a combination of the following techniques well known in the art can be used: discrete logic circuits having logic gate circuits for implementing logical functions on data signals, application-specific integrated circuits having suitable combinational logic gate circuits, programmable gate arrays (PGAs), field-programmable gate arrays (FPGAs), etc.

[0147] Those of ordinary skill in the art of the present technology can understand that all or part of the steps carried by the method of implementing the above embodiments can be completed by a program instructing relevant hardware, and the program can be stored in a computer-readable storage medium. When the program is executed, it includes one or a combination of the steps of the method embodiments.

[0148] The above-mentioned storage medium can be a read-only memory, a magnetic disk, an optical disc, etc. Although the embodiments of the present invention have been shown and described above, it can be understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those of ordinary skill in the art can make changes, modifications, substitutions, and variations to the above embodiments within the scope of the present invention.

Claims

1. A single-phase ground fault distance protection method for a double-terminal weak-feedback system line, characterized in that: The steps include: S10: The real-time electrical quantities of the three-phase voltage and the three-phase current are collected through the voltage transformer and the current transformer installed on the MMC side, and the change rates of the d-axis and q-axis currents are calculated; S20: within the set time window, monitoring whether the d-axis and q-axis current change rates continuously exceed the start threshold value, if so, the protection device is started, if not, returning to step S10 to continue monitoring; S30: After the protection device is started, the collected three-phase current is converted into positive sequence, negative sequence and zero sequence components by using a symmetrical component method, and the measured impedance during a single-phase grounding fault is calculated in combination with the control characteristics in a double-terminal weak-feedback system; S40: according to the negative sequence voltage suppression control strategy on the MMC side and the negative sequence current suppression control strategy on the wind turbine generator set, solving the adaptive compensation coefficient to eliminate the influence of the transition resistance on the measurement impedance; The model of the adaptive compensation coefficient includes: In the formula, is the adaptive compensation coefficient, j is the imaginary unit, is the negative sequence component of phase A current, is the zero-sequence component of phase A current, is the A-phase current phasor, K=(z0-z1) / (3z1), z1 is the positive-sequence impedance per unit length of the AC transmission line, z0 is the zero-sequence impedance per unit length of the AC transmission line; S50: performing offset compensation on the adaptive compensation coefficient and the measured impedance phase and the line impedance phase respectively to obtain a compensated measured impedance and a compensated line impedance; S60: extracting the imaginary part and calculating the fault distance α based on the compensated measured impedance and the compensated line impedance, eliminating the calculation error caused by the transition resistance by a mathematical analytical method, and ensuring the accuracy of the fault distance; S70: Performing logical judgment on the calculation result of the fault distance α to determine the fault section, and triggering corresponding protection actions according to the fault section.

2. The single-phase ground fault distance protection method for a double-terminal weak-feed system line according to claim 1 is characterized in that: In step S20, within the set time window, monitor whether the d-axis and q-axis current change rates continuously exceed the start threshold value. If so, the protection device is started. If not, return to step S10 to continue monitoring. The start criteria of the protection device include: △I d =I d (t)-I d (t-△t); △I q =I q (t)-I q (t-△t); △t=1 / f s ; In the formula, I d (t) and I q (t) is the per unit value of the d-axis and q-axis currents in the inner loop of the MMC controller at the sending end, f s To protect the sampling frequency, ε1 and ε2 are the start threshold values.

3. The single-phase grounding fault distance protection method for a double-terminal weak-feed system line according to claim 1 is characterized in that: In step S30, after the protection device is started, the collected three-phase current is converted into positive sequence, negative sequence and zero sequence components by using the symmetrical component method, and the measurement impedance during the single-phase grounding fault is calculated in combination with the control characteristics of the double-terminal weak feed system. The model of the measurement impedance includes: In the formula, Z MMC-AG is the measured impedance during a single-phase ground fault, is the voltage phasor of phase A, is the current phasor of phase A, is the zero-sequence component of phase A current, K = (z0-z1) / (3z1), z1 is the positive-sequence impedance per unit length of the AC transmission line, and z0 is the zero-sequence impedance per unit length of the AC transmission line.

4. The single-phase ground fault distance protection method for a double-terminal weak-feed system line according to claim 1 is characterized in that: In step S50, the adaptive compensation coefficient is offset-compensated with the measured impedance phase and the line impedance phase to obtain a compensated measured impedance and a compensated line impedance. The models of the compensated measured impedance and the line impedance respectively include: In the formula, Z′ MMC-AG is the measured impedance after compensation, Z MMC-AG To measure impedance, Z′ L is the line impedance after compensation, Z L is the line impedance, α is the distance ratio between the fault point and the line head end on the MMC side, Z add is the additional impedance, j is the imaginary unit, is the adaptive compensation coefficient.

5. The single-phase ground fault distance protection method for a double-terminal weak-feed system line according to claim 1 is characterized in that: In step S60, based on the compensated measured impedance and line impedance, the imaginary parts of both parts are taken to perform a mathematical derivation model: In the formula, Z′ MMC-AG is the measured impedance after compensation, α is the fault distance, Z L is the line impedance, Z L ′ is the line impedance after compensation, is the adaptive compensation coefficient, Z add is the additional impedance.

6. The single-phase ground fault distance protection method for a double-terminal weak-feed system line according to claim 1 is characterized in that: In step S60, the model of the fault distance α includes: Where α is the fault distance, Z′ MMC-AG is the measured impedance after compensation, Z′ L is the line impedance after compensation.

7. The single-phase ground fault distance protection method for a double-terminal weak-feed system line according to claim 1 is characterized in that: In step S70, a logical judgment is performed on the calculation result of the fault distance α to determine the fault section, and a corresponding protection action is triggered according to the fault section, including: If the calculation result of the fault distance α satisfies the fault judgment condition within the zone 0<α<α for x1 consecutive sampling points set.1 When the fault occurs, the protection is judged as a fault in the AC transmission line area, which meets the action criteria of the MMC side distance protection stage I; If the calculation result of the fault distance α satisfies the lower line fault condition α for x2 consecutive sampling points set.1 <α<α set.2 When the protection is judged as a fault in the lower line of the AC transmission line, the delay action criterion of the distance protection stage II on the MMC side is met, and the delay △t1 is set to 0.2s as the backup protection criterion of this protection method; If the above judgment conditions are not met within the set time △t2, the protection should be judged as a fault outside the AC transmission line area or the system is in normal operation, and the protection will not operate.

8. A double-terminal weak-feedback system line single-phase ground fault distance protection device, characterized in that: Use of the method according to any one of claims 1 to 7, comprising: The data acquisition unit is used to collect the real-time electrical quantities of three-phase voltage and three-phase current through the voltage transformer and current transformer installed on the MMC side, and calculate the change rate of the d-axis and q-axis currents; A protection start-up unit is used to monitor whether the change rate of the d-axis and q-axis current continuously exceeds the start-up threshold value within a set time window. If so, the protection device is started; if not, the process returns to step S10 to continue monitoring; A measurement impedance calculation unit, which is used to convert the collected three-phase current into positive sequence, negative sequence and zero sequence components by using a symmetrical component method after the protection device is started, and calculate the measurement impedance when a single-phase grounding fault occurs in combination with the control characteristics in a double-terminal weak feed system; An adaptive compensation unit is used to solve an adaptive compensation coefficient according to the negative sequence voltage suppression control strategy on the MMC side and the negative sequence current suppression control strategy on the wind turbine generator set, so as to eliminate the influence of the transition resistance on the measurement impedance; A measurement impedance compensation unit, used to perform offset compensation on the adaptive compensation coefficient and the measurement impedance phase and the line impedance phase, respectively, to obtain a compensated measurement impedance and a compensated line impedance; A fault distance calculation unit, used to extract the imaginary part and calculate the fault distance α based on the compensated measured impedance and the compensated line impedance, and eliminate the calculation error caused by the transition resistance through a mathematical analytical method to ensure the accuracy of the fault distance; The fault judgment unit performs a logical judgment on the calculation result of the fault distance α to determine the fault section, and triggers a corresponding protection action according to the fault section.

9. A computer device comprising a memory, a processor and a computer program stored in the memory and executable on the processor, characterized in that: When the processor executes the computer program, the method according to any one of claims 1 to 7 is implemented.

10. A storage medium having a computer program stored thereon, characterized in that: When the computer program is executed by a processor, the method according to any one of claims 1 to 7 is implemented.

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