Double-end weak feed system line single-phase earth fault distance protection method, device, equipment and medium

Abstract

The invention relates to the technical field of power system relay protection, in particular to a double-end weak feed system line single-phase earth fault distance protection method, device and equipment and a medium, and the method comprises the steps: collecting the real-time electrical quantity of three-phase voltage and current, and calculating the current change rate; monitoring whether the change rate continuously exceeds a starting threshold value or not; measuring impedance is calculated by combining control characteristics of the double-end weak feed system; solving an adaptive compensation coefficient, and performing phase offset compensation on the measurement impedance to obtain compensated measurement impedance and line impedance; extracting an imaginary part to calculate a fault distance, and eliminating a transition resistance error; according to the method, the additional impedance angle and the self-adaptive compensation coefficient are solved through the three-phase electrical quantity of the flexible DC converter, the measurement impedance is compensated by using the reverse phase shift, and the influence of the transition resistance on the protection performance is eliminated by combining a mathematical analysis method, so that the rapid and accurate protection is realized, the hardware demand and the calculated amount are reduced, and the fault protection efficiency is improved. Safe and reliable operation of the system is ensured.

Description

Technical Field

[0001] The present invention relates to the technical field of relay protection for electric power systems, and in particular to a method, device, equipment and medium for distance protection of single-phase grounding faults in a double-terminal weak-feedback system line. Background Art

[0002] In recent years, with the development of new energy grid-connected power generation technology, the electronic characteristics of power grids have become increasingly obvious. This new development trend has led to significant changes in the fault characteristics of the power system, especially with the emergence of ultra-long-distance and large-capacity transmission scenarios such as deep-sea offshore wind power and the "Shagohuang" new energy base. Flexible direct current transmission technology has attracted widespread attention from scholars at home and abroad. For example, Rudong, Jiangsu has built a demonstration project for offshore wind power transmission via flexible direct current.

[0003] The transmission lines of offshore wind power through the flexible DC transmission system are all typical double-end weak-feed 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 the 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 are connected in series in the composite sequence network diagram, and the current in the zero sequence network also shows controlled characteristics, which will cause the traditional distance protection to be unable to correctly identify the single-phase grounding fault of the double-end weak-feed system transmission line.

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

[0005] The present invention provides a method, device, equipment and medium for distance protection of single-phase grounding fault of a double-terminal weak-feedback 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 distance protection method for single-phase ground fault of a double-terminal weak-feed system line, comprising the following steps:

[0007] 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;

[0008] 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;

[0009] 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 of a double-terminal weak-feedback system;

[0010] 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;

[0011] 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;

[0012] 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;

[0013] 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.

[0014] Further, in step S20, within the set time window, it is monitored 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, it returns to step S10 to continue monitoring. The start criteria of the protection device include:

[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] 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.

[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 a symmetrical component method, and the measurement impedance during a single-phase grounding fault is calculated in combination with the control characteristics of a double-terminal weak feed system. The model of the measurement impedance includes:

[0021]

[0022] 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.

[0023] Further, in step S40, according to the MMC side negative sequence voltage suppression control strategy and the wind turbine negative sequence current suppression control strategy, an adaptive compensation coefficient is solved to eliminate the influence of the transition resistance on the measurement 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 A-phase current phasor, 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 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:

[0027]

[0028] 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.

[0029] Further, 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:

[0030]

[0031] 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.

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

[0033]

[0034] Where α is the fault distance, Z′ MMC-AG is the measured impedance after compensation, Z′ L is the line impedance after compensation.

[0035] Furthermore, 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:

[0036] 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;

[0037] 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 Δt1 is set to 0.2s as the backup protection criterion of this protection method;

[0038] If the above judgment conditions are not met within the set time Δt2, 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.

[0039] The present invention also includes a double-terminal weak-feedback system line single-phase grounding fault distance protection device, using the above method, 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 rate of the d-axis and q-axis currents;

[0041] 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;

[0042] 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;

[0043] 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;

[0044] 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;

[0045] 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;

[0046] 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.

[0047] The present invention also includes a computer device, including a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein when the processor executes the computer program, the method described above is implemented.

[0048] The present invention also includes a storage medium on which a computer program is stored. When the computer program is executed by a processor, the method 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 the two-terminal weak system, constructs a system composite sequence network diagram under a single-phase grounding fault, and uses the electrical quantities on the MMC converter side to obtain the fault distance. On this basis, a new protection strategy is established to solve the problem of incorrect operation of the existing distance protection in the two-terminal weak feedback system.

[0051] 2. 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 the single-side information, adaptively compensates the measured impedance phase and line impedance, eliminates the influence of transition resistance on protection, and can adapt to a variety of complex fault conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0053] Figure 1 It is a schematic diagram of a double-terminal weak-feedback system represented by offshore wind power connected to the grid via flexible direct current.

[0054] Figure 2 It is a flow chart of a distance protection method for single-phase ground fault of a double-terminal weak-feed system line;

[0055] Figure 3 This is the system topology diagram for single-phase grounding fault;

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

[0057] Figure 5 This is a comparison chart of calculation results when a fault occurs at different positions of the line through a 0.1Ω transition resistor;

[0058] Figure 6 This is a comparison chart of calculation results when a fault occurs at different positions of the line through a 20Ω transition resistor;

[0059] Figure 7 This is a comparison chart of calculation results when a fault occurs at different positions of the line through a 60Ω transition resistor;

[0060] Figure 8 It is a structural schematic diagram of a single-phase ground fault distance protection device for a double-terminal weak-feedback system line;

[0061] Fig. 9 A schematic diagram of the structure of a computer device. DETAILED DESCRIPTION

[0062] The technical solutions in the embodiments of the present invention will be described clearly and completely below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only 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 false operation that may be faced when a single-phase grounding fault occurs in the AC line of a double-terminal weak-feed system, the present invention proposes a distance protection method for single-phase grounding faults in a double-terminal weak-feed system line based on measuring impedance phase adaptive compensation, and takes the double-terminal weak-feed system line in which an offshore wind turbine is connected to an MMC converter station via a collection line and an AC transmission line as an example for analysis. The schematic diagram of offshore wind power access to the grid is shown in Figure 1. Figure 1 As shown, the offshore wind farm is connected to a 220kV booster station, and then transmitted to the flexible DC transmission system through an AC transmission line and integrated into the land system.

[0065] like Figure 2 As shown: A distance protection method for single-phase ground fault of a double-terminal weak-feed system line comprises the following steps:

[0066] 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 (flexible DC converter) side, and the change rates of the d-axis and q-axis currents are calculated;

[0067] 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 activated. If not, return to step S10 to continue monitoring.

[0068] 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 measured impedance during the single-phase grounding fault is calculated by combining the control characteristics in the double-terminal weak feed system;

[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 on the wind turbine generator set, solving the adaptive compensation coefficient to eliminate the influence of the transition resistance on the measurement impedance;

[0070] 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;

[0071] S60: Based on the compensated measured impedance and the compensated line impedance, the imaginary part is extracted and the fault distance α is calculated. The calculation error caused by the transition resistance is eliminated by a mathematical analytical method to ensure the accuracy of the fault distance.

[0072] S70: Perform 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.

[0073] This method focuses on solving the problem that distance protection may not operate correctly in the case of single-phase grounding fault in double-terminal weak-feedback system AC transmission line. The research object is a typical double-terminal weak-feedback system, in which one end of the transmission line is an offshore wind turbine and the other end is a flexible DC converter. A single-phase grounding fault distance protection method for double-terminal weak-feedback system line based on measured impedance phase adaptive compensation 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 quantity on the MMC side, and the fault distance is calculated using the deformed form of the existing ground distance impedance relay expression. The solution result is logically judged, so that the protection can quickly and reliably distinguish between in-zone and out-of-zone faults, which is of great significance for ensuring the safe and reliable operation of the double-terminal weak-feedback system line.

[0074] The additional impedance angle and adaptive compensation coefficient are solved by the three-phase electrical quantities of the flexible DC converter, the measured impedance is adaptively compensated with the help of reverse phase shift, and the influence of transition resistance on protection performance is theoretically eliminated by mathematical analytical methods. The fault location is accurately calculated, so that the protection can act correctly and quickly under different operating modes, reactive support requirements, transition resistance and fault locations, ensuring the safe and reliable operation of the double-terminal weak feed system line, reducing the hardware requirements and calculation amount required for the protection device, and has good engineering application prospects.

[0075] In a double-terminal weak-feedback system, the fault characteristics are affected by the control strategies of the converters at both ends. Traditional distance protection methods are prone to refusal to operate or false operation. 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 utilizes adaptive compensation technology to adapt to the special needs of the weak-feedback 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 through mathematical analytical methods, which improves the system's ability to resist transition resistance. Even in the presence of large transition resistance, the system can maintain high fault location accuracy and protection action reliability.

[0077] Take the case of a phase A ground fault on the AC transmission line as an example. The fault system topology is as follows: Figure 3 As shown, R g is the grounding transition resistance, α is the distance between the fault point and the head end of the line on the MMC side, Z LThe flexible DC side distance protection device collects the three-phase voltage at the protection installation on this side and the three-phase current flowing through the protection installation through the voltage transformer and the current transformer, and converts the electrical real-time quantity 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) Convert to three-phase voltage phasor and three-phase current phasors Conduct analysis.

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

[0079] As a preferred embodiment of the above, in step S20, within the set time window, it is monitored 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, it returns to step S10 to continue monitoring. The start criteria of the protection device include:

[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) 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, and the I d and I q 10 times of the maximum rate of change over time is used as the protection start threshold.

[0085] Using the change rate of the d-axis and q-axis currents as the starting criterion, the current change characteristics when a fault occurs can be quickly captured. By setting the time window and the starting threshold value, the protection device can be triggered to start in a short time, reducing the impact of the fault on the system operation and improving the response speed of the protection action. The maximum change value of the d-axis and q-axis current change rate within 5ms before the sampling point is adopted, and 10 times of it is taken as the starting threshold, which effectively avoids 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 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:

[0087]

[0088] 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.

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

[0090]

[0091] Additional impedance Z add and its phase φ add The expression is:

[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, the adaptive compensation coefficient is solved to eliminate the influence of the transition resistance on the measurement 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 A-phase current phasor, 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.

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

[0097] As a preferred embodiment of the above, in step S50, the adaptive compensation coefficient is offset-compensated with the measured impedance phase and the line impedance phase respectively 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 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.

[0100] An adaptive compensation coefficient is introduced to compensate for the phase offset of the measured impedance and the line impedance, effectively eliminating the interference of system characteristics and operating status changes on the protection action, making the protection system more stable under complex operating conditions.

[0101] 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:

[0102]

[0103] 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.

[0104] Through the above-mentioned 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] Where α is the fault distance, Z′ MMC-AG is the measured impedance after compensation, Z′ L is the line impedance after compensation.

[0108] By calculating the compensated imaginary part of the impedance, errors caused by changes in impedance amplitude or other interference factors are avoided, ensuring the stability and robustness of the calculation results. The model calculation is based on simple imaginary part operations, the algorithm is lightweight, and fault location can be completed quickly, meeting the high requirements for response speed in real-time protection scenarios.

[0109] As a preference of the above embodiment, 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:

[0110] 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;

[0111] 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 Δt1 is set to 0.2s as the backup protection criterion of this protection method;

[0112] If the above judgment conditions are not met within the set time Δt2, 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.

[0113] Among them, the above α set.1 , α set.2 The protection range of distance protection is 80% for Section I and 120% for Section II.

[0114] Based on the monitoring of continuous sampling points of fault distance and logical judgment of interval conditions, it can accurately distinguish between faults within the zone, faults of lower-level lines and faults outside the zone; through dynamic sampling point judgment conditions 0<α<α set.1 and α set.1 <α<α set.2 , effectively avoiding the influence of transient interference or measurement error on the judgment result and ensuring the reliability of protection action.

[0115] For faults within the zone, the distance protection stage I on the MMC side can act quickly to isolate the fault point in time and avoid the expansion of the fault.

[0116] For lower-level line faults, the stage II protection will delay operation to provide reliable backup protection and avoid mis-tripping or impact on system operation due to overly fast operation.

[0117] For out-of-zone faults or normal operating conditions, the protection will not operate, reducing the possibility of false operation and ensuring the stability of the system.

[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 the two-terminal weak feed system, constructs a composite sequence network diagram of the system under a single-phase grounding fault, and uses the electrical quantities on the MMC converter side to calculate the fault distance. On this basis, a new protection strategy is established to solve the problem of incorrect operation faced by the existing distance protection in the two-terminal weak feed system.

[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 the single-side information, adaptively compensates the measured impedance phase and the line impedance phase, eliminates the influence of transition resistance on protection, and can adapt to a variety of complex fault conditions.

[0120] Embodiment 2:

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

[0122] As the transition resistance changes, the protection can be activated within 40ms after the fault occurs. After 70ms, α stabilizes near the fault location, and the fault location can be accurately determined. The protection action time is shown in Table 1, t Ⅰ To protect the I stage action time, t Ⅱ To protect the II stage action time.

[0123] Table 1 Simulation protection action

[0124]

[0125] It can be seen from the above table that the proposed protection algorithm can normally calculate the fault distance under ground fault, the protection can be activated within 40ms, and it has a strong ability to resist 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 to 800MW, and the protection action is shown in Table 2.

[0127] Table 2 Protection action after wind farm capacity changes

[0128]

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

[0130] The present invention also includes a double-terminal weak-feedback system line single-phase ground fault distance protection device, using the above method, such as Figure 8 As shown, including:

[0131] 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;

[0132] 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;

[0133] The measuring impedance calculation unit is used to convert the collected three-phase current into positive sequence, negative sequence and zero sequence components by using the symmetrical component method after the protection device is started, and calculate the measuring impedance when a single-phase grounding fault occurs in combination with the control characteristics in the double-terminal weak-feedback system;

[0134] 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;

[0135] 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;

[0136] A fault distance calculation unit is 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;

[0137] The fault judgment unit performs 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] See also Fig. 9 A computer device 400 provided in an embodiment of the present application includes: a processor 410 and a memory 420, wherein the memory 420 stores a computer program executable by the processor 410, and when the computer program is executed by the processor 410, the above method is executed.

[0139] The embodiment of the present application further provides a storage medium 430 on which a computer program is stored. 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 (SRAM), electrically erasable programmable read-only memory (EEPROM), erasable programmable read-only memory (EPROM), programmable red-only memory (PROM), read-only memory (ROM), magnetic storage, flash memory, disk or optical disk.

[0141] In the description of the present invention, the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" and "second" may explicitly or implicitly include one or more of the features. "Multiple" means two or more, unless otherwise clearly and specifically defined.

[0142] In the present invention, unless otherwise clearly specified and limited, the terms "installed", "connected", "connected", "fixed" and the like should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium, it can be the internal connection of two elements or the interaction relationship between two elements. For ordinary technicians in this field, 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 the terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" etc. means that the specific features, structures, materials or characteristics described in conjunction 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, those skilled in the art may combine and combine the different embodiments or examples described in this specification and the features of the different embodiments or examples, unless they are contradictory.

[0144] Any process or method description in a flowchart or otherwise described herein may be understood to represent a module, segment or portion of code that includes one or more executable instructions for implementing the steps of a specific logical function or process, and the scope of the preferred embodiments of the present invention includes alternative implementations in which functions may not be performed in the order shown or discussed, including performing functions in a substantially simultaneous manner or in the reverse order depending on the functions involved, which should be understood by those skilled in the art to which the embodiments of the present invention belong.

[0145] The logic and / or steps represented in the flowchart or otherwise described herein, for example, can be considered as an ordered list of executable instructions for implementing logical functions, and can be embodied in any computer-readable medium for use by an instruction execution system, device or apparatus (such as a computer-based system, a system including a processor, or other system that can fetch instructions from an instruction execution system, device or apparatus and execute instructions), or in combination with these instruction execution systems, devices or apparatuses. For the purposes of this specification, a "computer-readable medium" can be any device that can contain, store, communicate, propagate or transmit a program for use by an instruction execution system, device or apparatus, or in combination with these instruction execution systems, devices or apparatuses. More specific examples of computer-readable media (a non-exhaustive list) include the following: an electrical connection with one or more wires (electronic devices), a portable computer disk box (magnetic device), a random access memory (RAM), a read-only memory (ROM), an erasable and programmable read-only memory (EPROM or flash memory), a fiber optic device, and a portable compact disk read-only memory (CDROM). In addition, the computer-readable medium may even be paper or other suitable medium on which the program is printed, since the program may be obtained electronically, for example, by optically scanning the paper or other medium and then editing, interpreting or processing in other suitable ways if necessary, and then stored in a computer memory.

[0146] It should be understood that the various parts of the present invention can be implemented by hardware, software, firmware or a combination thereof. In the above-mentioned embodiments, a plurality of 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, it can be implemented by any one of the following technologies known in the art or their combination: a discrete logic circuit having a logic gate circuit for implementing a logic function for a data signal, a dedicated integrated circuit having a suitable combination of logic gate circuits, a programmable gate array (PGA), a field programmable gate array (FPGA), etc.

[0147] A person skilled in the art may understand that all or part of the steps in the method for implementing the above-mentioned embodiment may be completed by instructing related hardware through a program, and the program may be stored in a computer-readable storage medium, which, when executed, includes one or a combination of the steps of the method embodiment.

[0148] The storage medium mentioned above may be a read-only memory, a magnetic disk or an optical disk, 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 cannot be understood as limiting the present invention. A person of ordinary skill in the art may change, modify, replace and modify 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 of 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; 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: ΔI d =I d (t)-I d (t-Δt); ΔI q =I q (t)-I q (t-Δt); Δt=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, 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 S40, according to the MMC side negative sequence voltage suppression control strategy and the wind turbine negative sequence current suppression control strategy, an adaptive compensation coefficient is solved 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, and z0 is the zero-sequence impedance per unit length of the AC transmission line.

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 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.

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, 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.

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 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.

8. 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 Δt1 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 Δt2, 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.

9. A double-terminal weak-feed system line single-phase ground fault distance protection device, characterized in that: Use of the method according to any one of claims 1 to 8, 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.

10. 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 8 is implemented.

11. 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 8 is implemented.

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