Rapid pilot protection method for flexible DC line

Abstract

The invention discloses a rapid pilot protection method for a flexible DC line, and the method comprises the steps: firstly collecting the positive and negative current data of the DC line in real time, calculating the current change rate, and starting protection if the current change rate exceeds a preset threshold value; locking a fault moment data window, and calculating an H distance of a positive and negative electrode current sequence based on a Hausdorff distance so as to distinguish an inter-electrode fault or a single-electrode fault; thirdly, calculating a fault current break variable sequence by using the current data in the data window to obtain a current included angle of the side; then, current included angle information of the two sides is interacted with an opposite-side protection device; calculating a product average value, namely a fault current included angle entropy, according to the current included angles on the two sides; and finally, by judging whether the included angle entropy is greater than an internal and external fault threshold value or not, accurately identifying an internal fault and triggering tripping, or judging the internal fault as an external fault. According to the method, millisecond-level rapid protection is realized through the current included angle characteristic quantity and the included angle entropy criterion, and the method has high transition resistance resistance and low communication synchronization requirements.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of power system protection, in particular to a flexible DC line fast pilot protection method. BACKGROUND

[0002] The flexible DC transmission (MMC-HVDC) technology is based on the modular multilevel converter, has the advantages of independent control of active and reactive power, no need of reactive power compensation, easy to form a multi-terminal DC network, and has become an ideal solution for long-distance power transmission, renewable energy grid connection and consumption. However, the rapid development of the flexible DC system puts unprecedented high requirements on the line protection performance. Because of the use of fully controlled power devices (such as IGBT), its overcurrent capacity is very poor, and it may be damaged by fault current impact within a few milliseconds, thereby causing the entire converter station to be locked out or even the system to be shut down. Therefore, the ideal flexible DC line protection must have ultra-high speed action ability to realize the rapid identification and isolation of faults and ensure the safety of the system main equipment.

[0003] At present, the research on the line protection of the flexible DC system mainly focuses on the two-terminal quantity protection principle. The two-terminal quantity protection uses the electrical quantity information on both sides of the communication line, has good selectivity, but the time-frequency algorithm has a large amount of calculation, and the differential protection has strict requirements on the data synchronization of communication, which to some extent increases the exit time of the two-terminal quantity protection and reduces the speed.

[0004] Therefore, how to improve the anti-transition resistance performance under the premise of ensuring ultra-high speed action has become a technical problem to be solved. SUMMARY

[0005] In view of the above technical problems, the present application provides a flexible DC line fast pilot protection method, which extracts the fault current angle characteristics and optimizes the information interaction logic to quickly determine the in-zone and out-zone faults and improve the speed and anti-transition resistance of the flexible DC line protection.

[0006] In order to solve the above technical problems, the present application adopts the following technical scheme:

[0007] A flexible DC line fast pilot protection method, comprising the following steps:

[0008] S1, collecting the current data of the positive and negative poles of the flexible DC line, and calculating the current change rate of the current data;

[0009] S2, judging whether the current change rate is greater than a preset threshold value; if yes, it is determined that a fault occurs, and step S3 is executed; otherwise, return to step S1;

[0010] S3, lock the data window based on the fault time, obtain the positive and negative electrode current sequence in the data window; calculate the H distance of the positive and negative electrode current sequence based on the Hausdorff distance, and determine the fault type through the H distance;

[0011] S4, based on the local side current data in the locked data window, calculate the fault current mutation sequence to obtain the local side current included angle;

[0012] S5, send the local side current included angle to the protection device of the opposite side of the line, and receive the opposite side current included angle sent by the opposite side protection device;

[0013] S6, the local side and the opposite side respectively calculate the fault current included angle entropy based on the local side current included angle and the opposite side current included angle;

[0014] S7, judge whether the fault current included angle entropy of the local side and the opposite side is greater than the preset intra-zone and extra-zone fault criterion threshold; if yes, it is judged as intra-zone fault, and the corresponding circuit breaker is combined with the fault type action to isolate the fault line; otherwise, it is judged as extra-zone fault.

[0015] As a preferred scheme, in step S2, the current rate of change is used as the fault judgment basis, which is specifically represented as:

[0016] |di fault / dt|>k1|di load / dt| max ;

[0017] In the formula, |di load / dt| max is the peak value of the current rate of change under normal operation of the flexible DC line; k1 is a reliability coefficient; |di fault / dt| is the current rate of change when the flexible DC line fails.

[0018] As a preferred scheme, in step S3, the H distance of the positive and negative electrode current sequence based on the Hausdorff distance is represented as:

[0019]

[0020] In the formula, A and B are the positive and negative electrode current sequences of the local side in the data window; H(·) is the Hausdorff distance; a i and b j are the i th and j th current sampling values in the positive and negative electrode current sequences, respectively;

[0021] As a preferred scheme, in step S3, the judgment of the fault type is specifically represented as:

[0022]

[0023] H SET is a preset setting value;

[0024] In step S7, if the fault is determined to be an inter-pole fault and an intra-zone fault, the circuit breakers on both sides of the fault line are operated to isolate the fault line; if the fault is determined to be a single-pole grounding fault, it is further determined whether the fault is a positive pole fault or a negative pole fault: when the fault is a positive pole fault and an intra-zone fault, the circuit breaker of the positive pole line is operated to isolate the fault line; when the fault is a negative pole fault and an intra-zone fault, the circuit breaker of the negative pole line is operated to isolate the fault line.

[0025] As a preferred solution, in step S4, the local-side current angle is obtained by the following steps:

[0026] Based on the sequence of fault current mutation variables, the sequence of instantaneous current angles is obtained according to the relative position of the data window and the fault time; the sequence of instantaneous current angles is smoothed to obtain the local-side current angle.

[0027] As a preferred solution, the sequence of instantaneous current angles is represented by the following formula:

[0028] When the data window starts at the fault time, the sequence of instantaneous current angles is calculated by the following formula:

[0029] θ1' = arctan(H / N);

[0030] In the formula, N is the number of data window sampling points; H is the change amplitude of the current amplitude;

[0031] When the data window starts before or after the fault, the sequence of instantaneous current angles is calculated by the following formula:

[0032] θ1" = (θ 11 + θ 12 ) / 2;

[0033] In the formula, θ 11 and θ 12 are two angles obtained based on the start point of the data window and the fault point;

[0034] In summary, the sequence of instantaneous current angles θ1 = {θ1', θ1"}.

[0035] As a preferred solution, in step S5, before sending the local-side current angle, an auxiliary criterion step is further included: the short-term average value of the local-side current angle is calculated, and if the short-term average value is greater than a preset high-resistance fault threshold value for multiple times in succession, the sending operation is performed.

[0036] As a preferred solution, the auxiliary criterion is represented by:

[0037]

[0038] wherein,

[0039] wherein, θ1 is the current angle of the local side; is the short-term average of the current angle of the local side; is the high-resistance fault threshold value; θ2 is the current angle of the opposite side; is the short-term average of the current angle of the opposite side; N1 and N2 are the number of angles for calculating the average of the current angle of the local side and the opposite side, respectively.

[0040] As a preferred solution, in step S6, the fault current angle entropy is expressed as:

[0041]

[0042] wherein, ∫θ is the fault current angle entropy; M is the number of data points in the current data window; θ 1i and θ 2i are the i-th current angle of the local side and the opposite side of the line, respectively.

[0043] As a preferred solution, in step S6, the fault current angle entropy is taken as the final fault criterion and is specifically expressed as:

[0044]

[0045] wherein, ∫θ SET is the preset threshold value of the fault criterion inside and outside the region.

[0046] Compared with the prior art, the present application has the following technical effects:

[0047] 1. The present application uses the current rate of change as the fault starting criterion, which has a much faster response speed than the criterion based on the current amplitude, thereby greatly improving the speed of the protection; at the same time, the present application uses the Hausdorff distance as the fault pole selection criterion, which only needs to analyze the similarity of the positive and negative current waveforms of the local side, and can distinguish between inter-pole faults and pole-to-ground faults in a short time after the fault, thereby realizing the localized and rapid selection of the fault type.

[0048] 2. The present application converts the fault current mutation sequence into a current angle with better characterization, fuses the amplitude and rate of change information of the fault current mutation, thereby effectively filtering out the interference of the steady-state load current, showing extremely high sensitivity to high transition resistance faults, and the positive / negative sign of its value strictly corresponds to the fault direction, thereby providing a basis for subsequent longitudinal discrimination.

[0049] 3. The information interaction link of the longitudinal protection of the present invention only needs to transmit the current angle. Compared with the traditional longitudinal differential protection that requires high-speed synchronous sampling data, it reduces the amount of communication data and the transmission delay, thereby directly improving the overall protection action speed and optimizing communication efficiency and reliability.

[0050] 4. This invention uses the fault current angle entropy as the criterion to fuse the current angles on both sides, which can clearly distinguish the fault area. When the fault is within the line area, the current on both sides flows to the fault point, the angles are both positive, and their product is positive, resulting in a significantly positive angle entropy. When the fault is outside the line area, the current on one side must be in the opposite direction, the angle is negative, and the product is negative, resulting in a negative or very small angle entropy. This effectively prevents false tripping and failure to trip, and improves the accuracy of fault judgment. Attached Figure Description

[0051] To make the objectives, technical solutions, and advantages of the invention clearer, the invention will now be described in further detail with reference to the accompanying drawings, wherein:

[0052] Figure 1 This is a topology diagram of a true bipolar MMC-HVDC system according to an embodiment of the present invention;

[0053] Figure 2 This is a flowchart of the longitudinal protection scheme according to an embodiment of the present invention;

[0054] Figure 3 This is a fault current angle characteristic diagram of an embodiment of the present invention;

[0055] Figure 4 This is a fault current path diagram for a DC line according to an embodiment of the present invention;

[0056] Figure 5 Additional network circuit diagrams are provided for faults within the area specified in this embodiment of the invention;

[0057] Figure 6 This is a diagram showing the current angle characteristics under fault conditions in an embodiment of the present invention;

[0058] Figure 7 This is a diagram showing the current angle characteristics under an external fault according to an embodiment of the present invention.

[0059] Figure 8 This is a schematic diagram of the angle between the sudden changes in fault current according to an embodiment of the present invention;

[0060] Figure 9 This is a logic diagram of information transmission according to an embodiment of the present invention;

[0061] Figure 10 This is a diagram showing the angle of fault current under fault conditions within the region according to an embodiment of the present invention.

[0062] Figure 11This is a diagram showing the angle of fault current under an external fault in an embodiment of the present invention. Detailed Implementation

[0063] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but only to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0064] The present invention will now be described in further detail with reference to the accompanying drawings.

[0065] Existing technologies, especially longitudinal current differential protection as the main protection, face severe difficulties in practical applications: on the one hand, to ensure selectivity, they require acquiring a long data window and performing complex calculations, resulting in a long inherent delay, making it difficult to meet the extreme requirements of "fast operation" in flexible DC systems; on the other hand, their criteria are greatly affected by the distributed capacitance current of the line, resulting in insufficient sensitivity in high-resistance grounding faults, and stringent requirements for the data synchronization performance of the communication channel. All of these factors restrict their protection performance and engineering applicability. Therefore, a longitudinal protection method that can balance ultra-high operating speed and strong resistance to transition resistance is needed. To address the above-mentioned problems, this invention proposes a fast longitudinal protection method for flexible DC lines. This method quickly initiates protection based on the rate of change of current, uses the H distance to distinguish fault types, and its core lies in extracting the current angle, which can sensitively and reliably characterize fault characteristics, as a feature quantity. The main criterion for faults inside and outside the protection zone is constructed by calculating the average of the product of the angles on both sides of the line (i.e., the angle entropy). At the same time, auxiliary criteria are introduced to optimize the communication logic, exchanging angle information with the opposite side only when it is confirmed that the fault on this side is positive.

[0066] This invention provides a fast longitudinal protection method for flexible DC lines, and the research objects involved are as follows: Figure 1 As shown, the method includes the following steps, such as Figure 2 As shown:

[0067] S1. Collect current data of the positive and negative poles of the flexible DC line and calculate the rate of change of the current data;

[0068] In specific implementation, such as Figure 3As shown, due to the rapid increase in current during the fault, an angle is formed between the current growth curve and the time axis. This angle essentially represents the degree of abrupt change in the fault current. The sampling frequency of the electrical quantities of this line is mostly 10kHz, and the time interval Δt is very small. Therefore, under a short time scale, the magnitude of the current abrupt change is large, and the calculated current angle characteristic is obvious. Thus, compared with the magnitude increment of the current abrupt change, the current angle has a larger range of values, [-90, 90], which can more sensitively reflect the fault current abrupt change within a short window.

[0069] S2. Determine whether the rate of change of current is greater than a preset threshold; if it is, determine that a fault has occurred and proceed to step S3; otherwise, return to step S1.

[0070] In practical implementation, under normal operation of the MMC-HVDC, the DC line current value is constant, and the rate of change is low, approximately zero. When a short-circuit fault occurs on the DC side, the fault current surges, and the rate of change of the fault current also surges. Therefore, using the rate of change of current as the basis for fault judgment can sensitively capture the instantaneous current change during a fault, greatly improving the speed of protection.

[0071] Specifically, it is expressed as follows:

[0072] |di fault / dt|>k1|di load / dt| max ;

[0073] In the formula, |di load / dt| max The peak current rate of change is the current under normal operation of the flexible DC line; k1 is the reliability coefficient; |di fault / dt| represents the rate of change of current when a fault occurs in the flexible DC line.

[0074] S3. Lock the data window based on the fault time, and obtain the positive and negative current sequences of the local side within the data window; calculate the H distance of the positive and negative current sequences based on the Hausdorff distance, and compare the H distance with the preset setting value to determine the fault type;

[0075] The H-distance calculated based on the Hausdorff distance for the positive and negative electrode current sequences is expressed as:

[0076]

[0077] In the formula, A and B are the positive and negative current sequences on this side within the data window, respectively; H(·) is the Hausdorff distance; a i and b j These are the i-th and j-th current sample values ​​in the positive and negative current sequences, respectively;

[0078] The determination of the fault type is specifically expressed as follows:

[0079]

[0080] In the formula, H SET This is the preset setting value.

[0081] In this embodiment, based on the comparison between the preset setting value and the H distance, if it is determined to be an inter-pole fault and an intra-zone fault, the circuit breakers on both sides of the faulty line are activated to isolate the faulty line; if it is determined to be a single-pole grounding fault, it is further determined whether it is a positive or negative fault (the absolute value of the fault current can be used to determine whether it is a positive or negative fault): when it is a positive fault and an intra-zone fault, the circuit breaker of the positive line is activated to isolate the faulty line; when it is a negative fault and an intra-zone fault, the circuit breaker of the negative line is activated to isolate the faulty line.

[0082] In practice, Figure 4 The diagram illustrates the fault current path in a DC line. Under inter-pole faults, the two current sequences approximately overlap, and the calculated distance H is approximately zero. Considering measurement errors, calculation errors, and the effects of unbalanced operation between the positive and negative poles, the distance H will be slightly greater than zero; therefore, the value of HSET should not be too close to zero. Under unipolar faults, the calculated distance H is greater than zero, but as the transition resistance increases, the current amplitude decreases, and the calculated distance H decreases simultaneously. To ensure strong resistance to transition resistance, the value of HSET should not be too high. This embodiment selects a setting value H... SET The value is set to 0.1. Thus, by using the Hausdorff distance as the fault polarity selection criterion, it is only necessary to analyze the similarity of the positive and negative current waveforms on this side, which can distinguish between inter-pole faults and pole-to-ground faults in a short time after a fault, realizing localized rapid polarity selection based on fault type.

[0083] S4. Based on the local current data within the locked data window, calculate the fault current mutation sequence to obtain the local current angle.

[0084] In practice, power is normally transferred from MMC1 to MMC2. For example... Figure 5 As shown, under intra-area fault conditions, both MMC converter stations simultaneously inject fault current I into the fault point. AB and I BA MMC1 side current I AB The current I injected into the MMC2 side is in the same direction as the power flow. BA Opposite to the power flow direction, the fault current direction changes. At this time, the fault current abrupt changes on both sides of MMC1 and MMC2 are positive, and the corresponding current abrupt change angles θ1 and θ2 are also always greater than 0, such as... Figure 6As shown. Under external fault conditions, the fault point may appear at f1 or f5, with the current angle as shown. Figure 7 As shown. After a fault occurs at point f1, the power transmission path of MMC1 is interrupted, and the current at both measuring points on both sides of the line originates from the power receiving end MMC2. The fault current I AB and I BA The direction changes; the fault current mutation on the MMC1 side is negative, and the current angle θ1 is always less than 0, while the fault current mutation on the MMC2 side is positive, and the current angle θ2 is always greater than 0. After the fault occurs at point f5, the current at the measuring points on both sides of the line comes from the power sending end MMC1, and the fault current I... AB and I BA With the direction unchanged, the fault current mutation on the MMC1 side is positive, and the current angle θ1 is always greater than 0, while the fault current mutation on the MMC2 side is negative, and the current angle θ2 is always less than 0.

[0085] In this embodiment, based on the fault current mutation sequence, the instantaneous current angle sequence is obtained according to the relative position of the data window and the fault time; the instantaneous current angle sequence is smoothed to obtain the local current angle.

[0086] This embodiment calculates the instantaneous current angle corresponding to the fault current over a period of time by continuously collecting data from N points. However, due to the different collection times, the fault current can exhibit three different angle scenarios, such as... Figure 8 As shown, the first case involves collecting N data points backward from the time of the fault, with a unique included angle. The included angle θ1 of the local current is calculated using trigonometric functions: θ1' = arctan(H / N); where N is the number of data points collected in the data window, and H is the magnitude of the change in current amplitude. The second case involves collecting N data points backward from before the fault, in which case there are two included angles, θ1' and θ2'. 11 and θ 12 The third scenario involves collecting N data points backward from the point of failure, which also involves two included angles, θ. 11 and θ 12 The average value is θ1”. The angle of the local current is calculated using the following formula for the two cases mentioned above: θ1”=(θ 11 +θ 12 ) / 2.

[0087] Considering the angle obtained from the fault current on the MMC1 side of the line under the above various conditions, the logic and process for calculating the angle on the MMC2 side are similar:

[0088] θ1 = {θ1', θ1”};

[0089] θ2 = {θ2', θ2”};

[0090] In the formula, θ2 is the angle obtained from the fault current on the MMC2 side of the line, and θ2' and θ2" are the angles obtained from the start of the data window at the fault time, before the fault, or after the fault.

[0091] The current angle between the two sides of the flexible DC line after the sliding window smoothing process is expressed as:

[0092]

[0093] In the formula, These represent the current angles on both sides of the flexible DC line after the sliding window smoothing process; x1, x2, ..., x m It calculates the m-th angle and then averages it.

[0094] S5. Send the local current angle to the protection device on the opposite side of the line, and receive the opposite current angle sent by the protection device on the opposite side.

[0095] In specific implementation, before transmitting the local current angle, an auxiliary criterion step is included: calculating the short-term average value of the local current angle; if the short-term average value is greater than a preset high-resistance fault threshold value multiple times consecutively, then the transmission operation is performed. In the specific application of this embodiment, when the auxiliary criterion is met five times consecutively, a positive fault is identified, and the instantaneous current angle sequence calculated in the five times is transmitted to the other side.

[0096] The auxiliary criterion is expressed as follows:

[0097]

[0098] in,

[0099] In the formula, θ1 is the included angle of the local current; This represents the short-term average value of the current angle on this side; θ1 is the included angle corresponding to a high-resistance fault (1000Ω); θ2 is the included angle of the current on the opposite side; N1 represents the short-term average value of the current angle on the opposite side; N2 and N1 represent the number of angles used to calculate the average current angle on this side and the opposite side, respectively.

[0100] The specific information transmission logic is as follows: Figure 9As shown, this means that the information is considered reliable only when the calculated current angle on this side meets the auxiliary criterion. The current angle information on this side is then sent to the other end via the communication channel. Simultaneously, the protection device on this side also receives the current angle information sent by the protection device at the other end after it meets its auxiliary criterion. During a forward fault, the calculated current angle on this side is positive, and after continuous calculation and averaging, its value is stable and greater than the high-resistance fault threshold. At this time, the auxiliary criterion is met, the system considers the information reliable, and thus triggers communication to send the angle information on this side to the other side. During a reverse fault, the calculated current angle on this side is negative and cannot meet the auxiliary criterion; therefore, this side will not send any information to the other side. This ensures that the data transmitted to the other side is reliable information from the actual fault path, which has undergone preliminary verification. This greatly reduces the possibility of protection maloperation due to measurement interference or external faults. Furthermore, communication is only performed when necessary, reducing unnecessary data transmission, lowering the load on the communication channel, and reducing the extreme requirements for real-time performance.

[0101] S6. The fault current angle entropy is calculated on the local side and the opposite side based on the current angle on the local side and the current angle on the opposite side, respectively.

[0102] In practical implementation, to reduce the influence of discrete points, the included angle entropy feature is defined to determine the fault section. The formula for calculating the included angle entropy is as follows:

[0103]

[0104] In the formula, ∫θ is the entropy of the fault current angle; M is the number of data points in the current data window; θ 1i and θ 2i These are the i-th current angles on the local and opposite sides of the line, respectively.

[0105] S7. Determine whether the entropy of the angle between the fault current on this side and the opposite side is greater than the preset threshold for faults inside and outside the zone; if it is greater, it is determined to be a fault inside the zone (a fault inside the zone means that the fault point is in this section of the line), and a trip command is issued in combination with the fault type to activate the corresponding circuit breaker to isolate the faulty line. Isolating the faulty line means that after determining that it is a fault inside the zone, the line is disconnected by using a DC circuit breaker to facilitate rapid fault recovery and also to facilitate the overall controller; otherwise, it is determined to be a fault outside the zone.

[0106] In practice, after a line fault, the fault current continues to increase, and the corresponding fault current angle remains near a large value. Extending the time window can reduce the impact of anomalies in the short time window and increase the reliability of the judgment. The fault current angle entropy is used as the final fault criterion, specifically expressed as follows:

[0107]

[0108] In the formula, ∫θSET The threshold values ​​for fault determination within and outside the preset area are used.

[0109] Compared with existing technologies, traditional current differential protection requires comparing the instantaneous current values ​​on both sides of the line at the same moment. If the clocks of the protection devices on both sides are not synchronized, the current being compared is not at the "same moment," and the calculation results will be severely distorted, leading to false tripping or failure to trip of the protection. Therefore, in this embodiment, the current sampling value within the short data window is first calculated as the instantaneous current angle characterizing the current change trend. Then, a stable local current angle is obtained by multiple averaging and auxiliary criteria. This process transforms the criterion basis from time-sensitive instantaneous values ​​to time-insensitive characteristic trend quantities. Finally, the protection criterion distinguishes the fault area by comparing the sign and intensity trend (i.e., angle entropy) of the current angle on both sides of the line, rather than relying on strictly synchronized instantaneous value comparison, thereby achieving rapid and accurate fault judgment without the need for precise clock synchronization.

[0110] To better illustrate the effectiveness of this method, a specific example will be used to illustrate the method of this invention below.

[0111] This embodiment takes a true bipolar half-bridge MMC-HVDC system as the research object, and its topology is as follows: Figure 1 As shown in the figure. The MMC submodule adopts an equivalent model. The MMC2 converter station uses constant active power control, and the MMC1 converter station uses constant DC voltage control. It is assumed that the power is sent from the MMC1 side to the MMC2 side of the converter station, and the current flowing from the bus into the line is defined as positive. Both MMCs adopt a symmetrical bipolar structure, and the parameters are the same. The parameters of the true bipolar MMC-HVDC system are shown in Table 1.

[0112] Table 1 Parameters of MMC-HVDC Converter Station

[0113]

[0114] Assume that a short circuit fault occurs on the DC side line at time t = 0 ms.

[0115] Taking the inter-pole fault at point f3 as an example, Figure 10 The waveforms of the currents on both sides of the line are given, and the corresponding current angles are all positive. At t = 1.9 ms, the angle entropy calculated from the MMC1 and MMC2 sides is greater than the threshold, and a trip command is issued to the corresponding DC circuit breaker. Tests were conducted under different fault distances, different fault types, and different transition resistances. The test results show that the proposed longitudinal protection can determine the fault within the zone as quickly as 1.4 ms and as late as no more than 3.1 ms at the beginning and end of the zone, with a maximum withstand transition resistance of 1000Ω.

[0116] Taking an inter-pole fault at point f1 as an example, Figure 11The current waveforms on both sides of the line are given. For MMC1, it is a reverse out-of-zone fault, and the current angle is negative. For MMC2, it is a forward out-of-zone fault, and the calculated current angle is positive. Tests were conducted under different fault types and transition resistances. The results show that under out-of-zone fault conditions, the proposed fast longitudinal protection system determines an out-of-zone fault at one MMC and a normal condition at the other MMC. Regardless of the condition, the line protection device reliably does not operate.

[0117] In summary, the proposed fast longitudinal protection method for flexible straight lines in this embodiment can cope with different line fault conditions and quickly determine faults inside and outside the fault zone. Compared with traditional differential protection and similarity protection, the proposed protection has better resistance to transition resistance and faster operation. Specifically, this embodiment first uses the rate of change of current as a fast start-up criterion to achieve instantaneous fault perception. Then, it constructs a fault selection mechanism by analyzing the positive and negative current matching degree through Hausdorff distance analysis. At the same time, this embodiment optimizes the information interaction logic of the proposed fast longitudinal protection by converting the time-domain current signal into a more representative angle feature quantity and combining it with information interaction under the control of auxiliary criteria. Finally, reliable fault discrimination between inside and outside the fault zone is achieved through the angle entropy criterion. This makes the protection system no longer dependent on strictly synchronized instantaneous value comparison, thereby significantly reducing the requirements for communication synchronization.

[0118] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described with reference to preferred embodiments, those skilled in the art should understand that various changes in form and detail can be made without departing from the spirit and scope of the invention as defined in the appended claims.

Claims

1. A method of flexible DC line fast pilot protection, characterized in that, The method comprises the following steps: S1, collecting current data of positive and negative poles of the flexible DC line, and calculating a current change rate of the current data; S2, judging whether the current change rate is greater than a preset threshold value; if yes, it is determined that a fault occurs, and step S3 is executed; otherwise, returning to step S1; S3, locking a data window with a fault time as a reference, obtaining a positive and negative pole current sequence in the data window, calculating an H distance of the positive and negative pole current sequence based on a Hausdorff distance, and determining a fault type through the H distance; S4, calculating a fault current mutation variable sequence based on the current data in the locked data window, to obtain a self-side current included angle; S5, sending the self-side current included angle to a protection device on the opposite side of the line, and receiving an opposite-side current included angle sent by the opposite-side protection device; S6, calculating a fault current included angle entropy based on the self-side current included angle and the opposite-side current included angle respectively; S7, judging whether the fault current included angle entropy of the self-side and the opposite-side is greater than a preset intra-zone and extra-zone fault criterion threshold value; if yes, it is determined that it is an intra-zone fault, and a circuit breaker corresponding to a fault line is isolated in combination with a fault type action; otherwise, it is determined that it is an extra-zone fault. In step S2, the current change rate is used as a fault judgment basis, which is specifically represented as:

2. The method of flexible DC line fast pilot protection according to claim 1, characterized in that, In step S3, the H distance of the positive and negative pole current sequence calculated based on the Hausdorff distance is represented as: | di fault | di load | di max ; In the formula, |di load / dt| max is the peak value of the current rate of change under normal operation of the flexible DC line; k1 is a reliability coefficient; |di fault / dt| is the current rate of change when the flexible DC line fails.

3. The method of flexible DC line fast pilot protection according to claim 1, characterized in that, In step S3, the judgment of the fault type is specifically represented as: where A and B are the positive and negative current sequences of the local side in the data window, respectively; H(·) is the Hausdorff distance; a i and b j are the i th and j th current sampling values in the positive and negative current sequences, respectively.

4. The method of flexible DC line fast pilot protection according to claim 3, characterized in that, In step S7, if it is determined that it is an inter-pole fault and an intra-zone fault, a circuit breaker on both sides of the fault line is operated to isolate the fault line; if it is determined that it is a single-pole grounding fault, it is further judged whether it is a positive pole fault or a negative pole fault: when it is a positive pole fault and an intra-zone fault, a circuit breaker of a positive pole line is operated to isolate the fault line; when it is a negative pole fault and an intra-zone fault, a circuit breaker of a negative pole line is operated to isolate the fault line. In the formula, H SET is a preset setting value; In step S4, the self-side current included angle is obtained through the following steps:

5. The method of flexible DC line fast pilot protection according to claim 1, characterized in that, Based on the fault current mutation variable sequence, an instantaneous current included angle sequence is obtained according to the relative position of the data window and the fault time; the instantaneous current included angle sequence is smoothed to obtain the self-side current included angle. The instantaneous current included angle sequence is represented by the following formula:

6. The method of flexible DC line fast pilot protection according to claim 5, characterized in that, When the data window starts at the fault time, the instantaneous current included angle sequence is calculated by the following formula: θ1'=arctan(H / N); In the formula, N is the number of data window sampling points; H is the change amplitude of the current amplitude; When the data window starts before or after the fault, the instantaneous current included angle sequence is calculated by the following formula: In summary, the instantaneous current included angle sequence θ1={θ1', θ1''}. θ1" = (θ 11 + θ 12 ) / 2; where θ 11 and θ 12 are two angles based on the start of the data window and the fault point; In step S5, before sending the self-side current included angle, an auxiliary criterion step is further included: calculating a short-term average value of the self-side current included angle, and if the short-term average value is greater than a preset high-resistance fault threshold value for a plurality of times in succession, the sending operation is executed.

7. The method of flexible DC line fast pilot protection according to claim 1, characterized in that, The auxiliary criterion is represented as:

8. The method of flexible DC line fast pilot protection according to claim 7, characterized in that, In step S6, the fault current included angle entropy is represented as: wherein In the formula, θ1 is the current angle on the side; is the short-term average of the current angle on the side; is the high-impedance fault threshold value; θ2 is the current angle on the opposite side; is the short-term average of the current angle on the opposite side; N1 and N2 are the number of angles for calculating the average of the current angles on the side and the opposite side, respectively.

9. The method of flexible DC line fast pilot protection according to claim 1, characterized in that, In the formula, ∫θ is the fault current included angle entropy; M is the number of data points in the current data window. ​ θ 1i and θ 2i are the i-th current angles on the local side and on the opposite side of the line, respectively.

10. The method of flexible DC line fast pilot protection according to claim 1, characterized in that, In step S6, the fault current included angle entropy is taken as the final fault criterion and is specifically expressed as: In the formula, ∫θ SET is a preset fault criterion threshold value inside and outside the region.

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