Pilot protection method and system for flexible DC line

Abstract

The invention discloses a pilot protection method and system for a flexible DC line, and relates to the field of power system relay protection, and the method comprises the steps: collecting voltage and current data in a DC power transmission system, and constructing a coupling model between the DC power transmission system and the ground; selecting a smoothing reactor and an adjacent line thereof as boundary conditions, and constructing a transient energy signal by using fault voltage and current information covering a full frequency band to judge a fault position; each measuring point collects voltage and current electrical quantities in real time, and whether protection is started or not is judged by comparing voltage values at the two ends of the smoothing reactor with a starting threshold value; after protection is started, transient power is calculated through voltage and current transient electrical quantities collected by all measuring points of the converter station, and then transient energy and a transient energy ratio of the transient power are calculated; fault area identification is carried out by comparing the transient energy ratio of each measuring point between the two converter stations with a protection setting value; and obtaining a numerical value of a pole selection function through positive and negative pole voltage fault components, and comparing the numerical value with a protection setting value to identify a fault pole.

Description

Technical Field

[0001] The present invention relates to the field of relay protection in power systems, and specifically to a method and system for longitudinal protection of flexible DC lines. Background Art

[0002] With the introduction of VSC and MMC, multi-terminal flexible DC grids can achieve independent control of active and reactive power, thereby realizing intelligent regulation of the power grid. This not only improves the response speed and stability of the power grid, but also reduces the risk of power grid failures. In addition, multi-terminal flexible DC grids also have better compatibility and can accommodate more types of clean energy, such as wind power, solar energy, etc., thus promoting the large-scale development and utilization of clean energy. Compared with traditional DC transmission methods, multi-terminal flexible DC grids have higher power supply reliability. Due to the adoption of advanced commutation technologies and control strategies, the failure rate of the power grid is greatly reduced, and the power supply quality is significantly improved. At the same time, since multi-terminal flexible DC grids can achieve flexible distribution and scheduling of electric energy, they also have stronger capabilities in dealing with emergencies and ensuring power supply to important users. In terms of investment costs and operating expenses, multi-terminal flexible DC grids also show obvious advantages. Although the initial investment may be relatively high, due to its high operating efficiency, low maintenance cost, long service life and other characteristics, it can achieve lower costs in long-term operation. In addition, with the continuous progress and maturity of technology, the investment cost of multi-terminal flexible DC grids is also expected to be further reduced. With the continuous progress of technology and the continuous popularization of applications, multi-terminal flexible DC grids will play a more important role in future power systems. It can not only meet the growing power demand, but also promote the large-scale application and development of clean energy, and make important contributions to building a clean, low-carbon and efficient energy system.

[0003] However, as an emerging power transmission technology, flexible DC transmission systems face a series of challenges in market promotion and practical applications. Since the selective fast isolation technology for DC line faults has not been perfected, to a certain extent, this restricts the further promotion and application of flexible DC transmission systems. In a flexible DC grid, the rapid handling of DC line faults is crucial. Once a DC line fails, in order to ensure that non-faulty lines can continue to operate normally, the rapid response ability of DC circuit breakers becomes particularly important. This rapid action ability helps to reduce the impact of faults on the system and ensure the stable and reliable operation of the DC system. However, the fault characteristics of DC grids are significantly different from those of traditional AC grids. When a fault occurs, the line voltage will also drop significantly within a very short time, which poses a great challenge to the stability of the entire system. Therefore, the research and development of more mature and reliable DC line fault rapid handling technology has become an important direction for the development of flexible DC transmission systems.

[0004] Based on the above analysis, it is particularly urgent and important to study a reliable, rapid, sensitive and selective multi-terminal flexible DC transmission line protection scheme. The present invention will deeply analyze the fault characteristics of the DC power grid, explore the line protection principle, and strive to propose a DC line protection scheme that is both practical and meets various requirements, so as to ensure the stability and reliability of the power system. Summary of the Invention

[0005] The present invention provides the following technical solution: A longitudinal protection method for a flexible DC line, which includes the following steps.

[0006] Collect voltage and current data in the DC transmission system to construct a coupling model between the DC transmission system and the ground.

[0007] Select the smoothing reactor and its adjacent line as the boundary conditions, and use the fault voltage and current information covering the full frequency band to construct a transient energy signal to determine the fault location.

[0008] Each measuring point real-time collects voltage and current electrical quantities, and determines whether the protection starts by comparing the voltage value at both ends of the smoothing reactor with the starting threshold.

[0009] After the protection starts, calculate the transient power from the transient electrical quantities of voltage and current collected by each measuring point of the converter station, and then calculate its transient energy and transient energy ratio.

[0010] Identify the fault area by comparing the transient energy ratio between each measuring point between two converter stations with the protection setting value.

[0011] Compare the value of the pole selection function obtained from the positive and negative pole voltage fault components with the protection setting value to identify the fault pole.

[0012] As a preferred scheme of the longitudinal protection method for a flexible DC line described in the present invention, wherein: the construction of the coupling model between the DC transmission system and the ground is that in the DC transmission system, electrical quantities such as voltage and current flow between the positive and negative lines, and also interact with the ground to form a coupling relationship. The Karrenbauer transform is used for phase-mode transformation, and the expression is:

[0013]

[0014] where i p , i n , u p , u n are respectively the current and voltage components of the positive and negative poles in the phase domain, and i0, i1, u0, u1 are respectively the line mode and zero mode components of current and voltage in the mode domain.

[0015] As a preferred embodiment of the flexible DC line pilot protection method according to the present invention, wherein: the smoothing reactor and its adjacent line are selected as boundary conditions, and the transient energy signal is constructed by using the fault voltage and current information covering the full frequency band to determine the fault location. Based on the boundary defined by the original smoothing reactor, the shunt effect of the adjacent line is integrated to expand the boundary range, and the protection criterion is constructed by combining the current fault components and voltage fault components at points a and m;

[0016] When a single-pole fault occurs in the Line1 of the DC transmission line, the positive voltage fault component and current fault component are |Δu m | > |Δu a |, |Δi m | > |Δi a |. For the measuring points n and b on the MMC2 side of the line, the voltage fault component and current fault component are |Δu n | > |Δu b |, |Δi n | > |Δi b |;

[0017] Wherein, Δu m 、Δu n 、Δu a 、Δu b are the voltage fault components of the measuring points m, n, a, and b, and Δi m 、Δi n 、Δi a 、Δi b are the current fault components of the measuring points m, n, a, and b;

[0018] When a bipolar fault occurs in the Line1 of the DC transmission line, the voltage and current fault shunts at the measuring points a and m are |Δu m | > |Δu a |, |Δi m | > |Δi a |. For the voltage and current fault shunts of the measuring points n and b on the opposite side, they are |Δu n | > |Δu b |, |Δi n | > |Δi b |;

[0019] When a forward out-of-zone fault occurs in the Line1 of the DC transmission line, the current fault components at points m and a satisfy |Δi m | > |Δi a |, the voltage fault components at points m and a satisfy |Δu m | > |Δu a |, and the measuring points n and b satisfy |Δi n|<|Δi b |, the voltage drop effect of the smoothing reactor satisfies |Δu n |<|Δu b |;

[0020] When an external reverse fault occurs, the voltage and current fault components of the external reverse fault are |Δu m |<|Δu a |, |Δi m |<|Δi a |, |Δu n |>|Δu b |, |Δi n |>|Δi b |.

[0021] As a preferred scheme of a flexible DC line pilot protection method described in the present invention, wherein: each measuring point collects voltage and current electrical quantities in real time, and determines whether the protection is started by comparing the voltage value at both ends of the smoothing reactor with the starting threshold. The protection starting margin u L0 value is set. When the DC power grid is operating normally, the voltages at both ends of each smoothing reactor are equal, and the voltage at both ends of the smoothing reactor can be approximated as 0. When a fault occurs in the DC transmission line, the voltage difference at both ends of its smoothing reactor is large at the initial stage of the fault. A protection starting criterion is constructed, and the expression is:

[0022] u L (t) > u L0

[0023] Wherein, u L (t) is the voltage difference at both ends of the boundary measuring points, that is, the voltage of the smoothing reactor; u L0 is the protection margin of the DC power grid under normal operation.

[0024] As a preferred scheme of a flexible DC line pilot protection method described in the present invention, wherein: after the protection is started, the transient power is calculated from the voltage and current transient electrical quantities collected by each measuring point of the converter station, and then the transient energy and transient energy ratio are calculated according to the current and voltage fault components measured at each measuring point. The transient power of the four measuring points a, b, m, and n is expressed as:

[0025]

[0026] Wherein, P a 、P b 、P m 、P n are the absolute values of the transient power of each measuring point a, b, m, and n respectively;

[0027] When a fault occurs in the DC transmission line, among the four measuring points a, m, n, and b, P m / P a >1, P n / P b >1. When a fault occurs outside the MMC1 side of the DC transmission line, P m / P a <1. For the non-faulty end of the MMC2 terminal, P n / P b >1.

[0028] When a fault occurs outside the MMC2 side of the DC transmission line, P n / P b <1. For the non-faulty end of the MMC1 terminal, P m / P a >1;

[0029] The transient energy of each measuring point m, n, a, b in the full frequency band is expressed as:

[0030]

[0031] Among them, P m 、P n 、P a 、P b are the sampling values of the full frequency band transient power at the measuring points m, n, a, b during the sampling time window, N = 1, 2…, N is the number of sampling points within the sampling time window length;

[0032] Construct an identification criterion for the flexible DC transmission line during internal and external faults, and the expression is:

[0033]

[0034] Among them, K set is the protection setting value, E a 、E b 、E m 、E n are the transient energies of the measuring points a, b, m, n.

[0035] As a preferred scheme of a longitudinal protection method for a flexible DC line described in the present invention, among them: the identification of the fault area by comparing the transient energy ratio between each measuring point between the two converter stations with the protection setting value K set includes,

[0036] When the transient energy ratio E m / E a ≤K set or E n / E b ≤K setIf so, it is determined that a fault occurs outside the zone, and the protection is reset;

[0037] When E m / E a > K set and E n / E b > K set , it is determined that a fault occurs within the zone.

[0038] 8. As a preferred solution of a flexible DC line pilot protection method according to the present invention, wherein: the numerical value W of the pole selection function obtained from the positive and negative voltage fault components and the protection setting value W set for comparison and identification of the fault pole include,

[0039] When W ≥ W set1 When W, it is determined that a positive pole fault within the zone occurs;

[0040] When W ≤ W set2 , it is determined that a negative pole fault within the zone occurs;

[0041] When W set2 ≤ W ≤ W set1 , it is determined that a bipolar fault within the zone occurs;

[0042] Construct the fault pole selection function as follows:

[0043]

[0044] wherein, Δu m1 , Δu m2 are the positive and negative voltage fault components at the measuring point m where the protection is installed respectively;

[0045] When a negative pole fault occurs in the DC transmission line, the fault pole selection function W < 1;

[0046] When a positive pole grounding fault occurs, the fault pole selection function W > 1;

[0047] When a bipolar fault occurs, the fault pole selection function W ≈ 1;

[0048] Construct the pole selection criterion according to the different values of the pole selection function of the fault pole as follows:

[0049]

[0050] wherein, W set1 , W set2 are the setting values of the fault pole selection criterion.

[0051] Another object of the present invention is to provide a flexible DC line pilot protection system, which can collect electrical quantities such as voltage and current at each measurement point in the DC transmission system in real time, construct a coupling model between the system and the ground, detect the occurrence of a fault by using the voltage change at both ends of the smoothing reactor, and accurately identify the fault area and fault polarity based on the transient energy ratio and the fault pole selection function, solving the problems of slow response speed, high misjudgment rate, inability to accurately distinguish internal and external faults, and positive and negative pole faults in the existing flexible DC transmission line during the fault identification process.

[0052] To solve the above technical problems, the present invention provides the following technical solutions: A flexible DC line pilot protection system includes: a fault startup module, a fault identification module, and a fault pole selection module;

[0053] The fault startup module is to collect and store the voltage and current at each measurement point in real time. If the protection startup criterion is met, the protection starts and enters the fault identification module; otherwise, it does not start.

[0054] The fault startup module includes:

[0055] A data acquisition unit that collects the analog signals output from the secondary sides of the voltage and current transformers at each measurement point in real time;

[0056] An analog-to-digital conversion unit converts the voltage and current analog signals collected at the measurement point into digital signals;

[0057] The protection startup unit determines whether the voltage digital signal is greater than the startup threshold set in this protection scheme. If so, it reads the startup time and stores the data, and the protection starts;

[0058] The fault identification module is to identify internal and external faults when the protection starts. If it is judged as an external fault, the protection does not act; if it is judged as an internal fault, it enters the fault pole selection module;

[0059] The fault identification module includes:

[0060] A line mode conversion unit that converts the line mode components of the measured voltage and current;

[0061] A parameter setting unit sets the protection threshold value, protection threshold W set1 、W set2 ;

[0062] A numerical calculation unit calculates the transient energy and transient energy ratio at each measurement point, and compares the transient energy ratio with the set threshold value;

[0063] The fault pole selection module is to judge whether the system has a positive pole, negative pole or bipolar fault through the fault pole selection function, and the relevant protection acts;

[0064] The fault pole selection module includes:

[0065] The data measurement unit measures the positive and negative voltage fault components and obtains the value W of the pole selection function;

[0066] The polarity judgment unit judges the positive and negative voltage fault components, obtains the value W of the pole selection function, and compares it with the protection setting value W set for comparison;

[0067] Identify the faulty pole. When W≥W set1 it is determined that a positive pole fault occurs in the zone;

[0068] When W≤W set2 it is determined that a negative pole fault occurs in the zone;

[0069] When W set2 ≤W≤W set1 it is determined that a bipolar fault occurs in the zone.

[0070] A computer device includes a memory and a processor. The memory stores a computer program, and when the processor executes the computer program, the steps of the above-mentioned flexible DC line pilot protection method are implemented.

[0071] A computer-readable storage medium stores a computer program, and when the computer program is executed by a processor, the steps of the above-mentioned flexible DC line pilot protection method are implemented.

[0072] Advantages of the present invention: Compared with traditional pilot protection, the implementation principle of the present invention is clear and simple, without the need for cumbersome processing of voltage and current characteristic quantities, the calculation amount is relatively small, the response is rapid, and the requirement for the sampling rate is not high. The present invention has no strict requirements for two-end communication and does not require strict synchronization of two-end data; when a relatively serious out-of-zone fault occurs, the protection scheme can still reliably brake. The protection scheme of the present invention shows excellent ability to withstand transition resistance, and can accurately identify and distinguish different types of faults. In the case of in-zone faults, it can reliably determine whether a single-pole fault or a bipolar fault occurs, can reliably and quickly protect the entire length of the line, has good selectivity, and ensures the safe and stable operation of the DC line. BRIEF DESCRIPTION OF THE DRAWINGS

[0073] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following will briefly introduce the drawings required for the description of the embodiments. Obviously, the following drawings are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0074] Figure 1 It is the overall flowchart of a flexible DC line pilot protection method provided by the first embodiment of the present invention;

[0075] Figure 2 For a flexible DC line pilot protection method provided by the first embodiment of the present invention, the simulation diagrams of the transient energy, energy ratio, and pole selection function at each negative side point when a negative pole metal grounding fault occurs at 10 km near the line start within the Line1 area of the DC line;

[0076] Figure 3 The simulation topology diagram for a flexible DC line pilot protection method provided by the second embodiment of the present invention;

[0077] Figure 4 For a flexible DC line pilot protection method provided by the second embodiment of the present invention, the simulation diagrams of the transient energy and energy ratio at measurement points m and a, and the transient energy and energy ratio at measurement points n and b when an out-of-area fault occurs on the transmission line on the Line1 side;

[0078] Figure 5 The framework diagram of a flexible DC line pilot protection system provided by the embodiment of the present invention. Detailed implementation manners

[0079] To make the above objects, features, and advantages of the present invention more obvious and understandable, the following detailed description of the specific implementation manners of the present invention will be given in conjunction with the accompanying drawings of the specification. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of them. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0080] Embodiment 1, referring to Figures 1 to 3 For an embodiment of the present invention, a flexible DC line pilot protection method is provided, including:

[0081] The four-terminal double-mesh flexible DC power grid model is shown in the appendix Figure 3 As shown, the DC voltage is 380 kV. Taking the line Line1 as the research object, the length of the transmission line is 100 km, and the sampling frequency of each electrical quantity is set to 10 kHz. The data sampling time window length is set to 5 ms. In the simulation model, a negative pole metal grounding fault (f1) occurs at 10 km near the line start within the Line1 area of the DC line.

[0082] Referring to Figure 1 , Step1: In the DC power transmission system, electrical quantities such as voltage and current not only flow between the positive and negative pole lines but also interact with the ground to form a coupling relationship. The Karrenbauer transformation is used for phase-mode transformation as follows:

[0083]

[0084] Step 2: On the basis of the boundary defined by the original smoothing reactor, further incorporate the shunt effect of adjacent lines to expand the scope of the boundary. Select the smoothing reactor and its adjacent lines as boundary conditions, and use the fault voltage and current information covering the entire frequency band to construct a transient energy signal to identify the fault location.

[0085] Step 2.1: Given the unique voltage reduction characteristics of the smoothing reactors on both sides of the HVDC transmission line, on the basis of the boundary defined by the original smoothing reactor, further incorporate the shunt effect of adjacent lines to expand the scope of the boundary. In this way, the characteristics of the boundary are more significantly highlighted, providing a clearer and more accurate basis for subsequent fault identification.

[0086] Step 2.2: When a monopole fault occurs at the location of DC transmission line Line1, for the protection installation points a and m on the MMC1 side, the positive voltage fault component and current fault component satisfy |Δu m | > |Δu a |, |Δi m | > |Δi a |. For the measurement points n and b on the MMC2 side of the line, according to the same analysis above, it can be known that their voltage fault components and current fault components satisfy |Δu n | > |Δu b |, |Δi n | > |Δi b |;

[0087] Step 2.3: When a bipolar fault occurs in the zone of DC transmission line Line1, the voltage and current fault shunts at the measurement points a and m are: |Δu m | > |Δu a |, |Δi m | > |Δi a |. The analysis principle for the measurement points n and b on the opposite side is the same as that for the measurement points a and m, and it can be obtained that: |Δu n | > |Δu b |, |Δi n | > |Δi b |;

[0088] Step 2.4: When a forward out-of-zone fault occurs in DC transmission line Line1, due to the shunt effect of line4 and Line5, the current fault components at points m and a satisfy |Δi m | > |Δi a |. Due to the voltage drop effect of the smoothing reactor, the voltage fault components at points m and a satisfy |Δu m | > |Δu a |; Similarly, due to the shunt effect of line Line2, points n and b satisfy |Δi n|<|Δi b |, due to the voltage drop of the smoothing reactor satisfying |Δu n |<|Δu b |. When a reverse external fault occurs, the theoretical analysis is the same as that for a forward external fault: |Δu m |<|Δu a |, |Δi m |<|Δi a |, |Δu n |>|Δu b |, |Δi n |>|Δi b |;

[0089] Step3: Each measuring point real - time collects voltage and current electrical quantities, and determines whether the protection starts by comparing the voltage value at both ends of the smoothing reactor with the starting threshold.

[0090] Step3.1: Set the protection starting margin u L0 value under the normal operation of the DC power grid;

[0091] Step3.2: When the DC power grid is operating normally, the voltages at both ends of each smoothing reactor are basically equal, and the voltage across the smoothing reactor can be approximated as 0; when a fault occurs in the DC transmission line, in the initial stage of the fault, the voltage difference across the smoothing reactor is large. The protection starting criterion is constructed as follows:

[0092] u L (t)>u L0 (2)

[0093] where u L (t) is the voltage difference at both ends of the boundary measuring points, that is, the voltage of the smoothing reactor; u L0 is the protection margin under the normal operation of the DC power grid.

[0094] Step4: After the protection starts, calculate the transient power from the voltage and current transient electrical quantities collected at each measuring point of the converter station, and then calculate its transient energy and transient energy ratio.

[0095] Step4.1: According to the current and voltage fault components measured at each measuring point, the transient powers of the four measuring points a, b, m, and n are as follows:

[0096]

[0097] Step4.2: When a fault occurs within the DC transmission line, among the four measuring points a, m, n, and b, P m / P a >1, P n / P b> 1;

[0098] Step4.3: When an external fault occurs on the MMC1 side of the DC transmission line, due to the voltage drop characteristic of the smoothing reactor and the shunt effect of the DC transmission line, its P m / P a < 1. For the MMC2 terminal (non-fault terminal), P n / P b > 1;

[0099] Step4.4: When an external fault occurs on the MMC2 side of the DC transmission line, due to the voltage drop characteristic of the smoothing reactor and the shunt effect of the DC transmission line, its P n / P b < 1. For the MMC1 terminal (non-fault terminal), P m / P a > 1;

[0100] Step4.5: According to the differences in the characteristics of internal and external faults above, the transient energy values of all frequency bands at measuring points m, n, a, and b are as follows:

[0101]

[0102] Among them, P m is the sampling value of the transient power of all frequency bands at measuring point m within the sampling time window. P n , P a , P b are the same; N = 1, 2..., N is the number of sampling points within the sampling time window length. According to the above analysis, the identification criteria for the four-terminal double-mesh flexible DC grid during internal and external faults are constructed as follows:

[0103]

[0104] Based on a large number of simulation experiments and considering a certain margin, set the setting threshold value k set = 1.5.

[0105] Step5: By comparing the transient energy ratios between the measuring points of the two converter stations with the protection setting value K set to identify the fault area.

[0106] Step5.1: When the transient energy ratio E m / E a ≤ K set or E n / E b ≤ K set then it is determined that an external fault has occurred and the protection is reset;

[0107] Step5.2: When E m / Ea > K set and E n / E b > K set , continuously judge the calculated value of the transient energy ratio three times, and determine that a fault occurs within the zone;

[0108] Step6: Obtain the value W of the pole selection function and the protection setting value W through the positive and negative pole voltage fault components set for comparison to further identify the faulty pole.

[0109] Step6.1: Construct the fault pole selection function as follows:

[0110]

[0111] where, Δu m1 , Δu m2 are the voltage fault components of the positive and negative poles at the measuring point m where the protection is installed respectively. When a negative pole grounding fault occurs on the DC transmission line, its fault pole selection function W < 1; when a positive pole grounding fault occurs, its fault pole selection function W > 1; when a bipolar fault occurs, its fault pole selection function W ≈ 1. Construct the pole selection criterion according to the different values of the fault pole selection function, as follows:

[0112]

[0113] where, W set1 , W set2 are the setting values of the fault pole selection criterion. After a large number of simulation experiments and considering a certain margin, select W set1 = 2, W set2 = 0.7.

[0114] Step6.2: When a positive pole grounding fault occurs on the DC transmission line, the change amount of the positive pole voltage fault component will far exceed that of the negative pole. When W ≥ W set1 , it is determined that a positive pole fault within the zone occurs;

[0115] Step6.3: When a negative pole grounding fault occurs on the DC transmission line, the change amount of the negative pole voltage fault component will be significantly greater than that of the positive pole. When W ≤ W set2 , it is determined that a negative pole fault within the zone occurs;

[0116] Step6.4: When a bipolar grounding fault occurs on the DC transmission line, the change amounts of the voltage fault components of the positive and negative poles are basically equivalent. When W set2 ≤ W ≤ W set1 , it is determined that a bipolar fault within the zone occurs.

[0117] The fault simulation diagram it makes is referred to Figure 2, the transient energy and transient energy ratio are calculated through the algorithm, and it can be seen that the transient energy ratio is greater than the setting threshold value k set = 1.5, it is judged as an internal fault; the fault pole selection algorithm is started and its value is less than W set2 = 0.7, it is judged as a negative pole fault.

[0118] Example 2, refer to Figures 3 to 4 Another embodiment of the present invention provides a longitudinal protection method for a flexible DC line, including a four-terminal double-mesh flexible DC power grid model as shown in the appendix Figure 3 . The DC voltage is 380 kV. Taking line Line1 as the research object, the transmission line is 100 km long. It is assumed that a fault occurs on the DC transmission line at 4.5 s, and the duration of the fault is 0.5 s. The sampling frequency of each electrical quantity is set to 10 kHz. At the same time, in order to analyze the change of electrical quantities before and after the fault, the data sampling time window length is set to 5 ms, and a forward external fault (f2) occurs on the transmission line on the line1 side of the DC line area.

[0119] The fault simulation diagram made is referred to Figure 4 . Through the algorithm, the transient energy and transient energy ratio are calculated. It can be seen that the transient energy ratios on the a and m sides are less than the setting threshold value k set = 1.5, while the transient energy ratios on the n and b sides are greater than the setting threshold value k set = 1.5, it is judged as a forward external fault, and the fault pole selection module is not started.

[0120] Example 3, refer to Figure 5 Another embodiment of the present invention provides a system for a longitudinal protection method for a flexible DC line, including: a fault start module for real-time collecting and storing the voltage and current of each measuring point. If the protection start criterion is met, the protection starts and enters the fault identification module; otherwise, it does not start.

[0121] A fault identification module, when the protection starts, is used to identify internal and external faults. If it is judged as an external fault, the protection does not act; if it is judged as an internal fault, it enters the fault pole selection module.

[0122] A fault pole selection module, which judges whether the system has a positive pole, negative pole or bipolar fault through a fault pole selection function, and the relevant protection acts.

[0123] The described longitudinal protection system for a flexible DC line is characterized in that the fault start module specifically includes:

[0124] A data acquisition unit for real-time collecting the analog signals output by the secondary sides of the voltage and current transformers at each measuring point;

[0125] An analog-to-digital conversion unit for converting the voltage and current analog signals collected at the measuring point into digital signals;

[0126] The protection start-up unit is used to determine whether the voltage digital signal is greater than the start-up threshold set by this protection scheme. If so, the start-up time is read and the data is stored.

[0127] The flexible DC line longitudinal protection system is characterized in that the fault identification module specifically includes:

[0128] Parameter setting unit, used to set the protection threshold value K set , protection threshold W set1 , W set2 ; After a large number of simulation experiments and considering a certain margin, the threshold value k is set set =1.5, select W set1 =2,W set2 =0.7;

[0129] The numerical calculation unit is used to calculate the transient energy and transient energy ratio of each measuring point, and compare the transient energy ratio with the set threshold value. The simulation results in Example 1 are as follows: Figure 3 As shown, E m / E a >K set And E n / E b >K set , it is determined that an in-area fault has occurred; the simulation results in Example 2 are as follows Figure 4 As shown, E m / E a ≤K set And E n / E b >K set , it is determined that a fault outside the forward zone has occurred.

[0130] The flexible DC line longitudinal protection system is characterized in that the fault pole selection module specifically includes:

[0131] Data measurement unit: used to measure the positive and negative voltage fault components and obtain the value of the pole selection function;

[0132] Polarity judgment unit: compares the value of the pole selection function obtained by judging the positive and negative pole voltage fault components with the protection setting value to further identify the fault pole. In embodiment 1, it is determined that a negative pole fault occurs in the area; Figure 3 In Example 2, it is determined that a fault outside the forward zone has occurred; fault selection is no longer performed, such as Figure 4 shown.

[0133] If the above-described functions are implemented in the form of software function units and sold or used as independent products, they can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of the present invention, in essence, or the part that contributes to the prior art, or a part of this technical solution, can be embodied in the form of a software product. This computer software product is stored in a storage medium and includes several instructions for causing a computer device (which may be a personal computer, a server, or a network device, etc.) to execute all or part of the steps of the methods described in various embodiments of the present invention. The aforementioned storage medium includes: USB flash drives, mobile hard disks, read-only memories (ROMs), random access memories (RAMs), magnetic disks, or optical discs, etc., which can store program codes of various kinds.

[0134] The logic and / or steps represented in the flowchart or described in other ways herein, for example, can be considered as a definite sequence list of executable instructions for implementing logical functions, and can be specifically implemented in any computer-readable medium for use by an instruction execution system, apparatus, or device (such as a computer-based system, a system including a processor, or other systems that can fetch instructions from the instruction execution system, apparatus, or device and execute the instructions), or in combination with these instruction execution systems, apparatuses, or devices. For the purposes of this specification, a "computer-readable medium" can be any device that can contain, store, communicate, propagate, or transport a program for use by or in combination with an instruction execution system, apparatus, or device.

[0135] More specific examples (non-exhaustive list) of computer-readable media include the following: electrical connection parts with one or more wirings (electronic devices), portable computer disk cartridges (magnetic devices), random access memories (RAMs), read-only memories (ROMs), erasable programmable read-only memories (EPROMs or flash memories), fiber optic devices, and portable compact disc read-only memories (CDROMs). Additionally, the computer-readable medium can even be paper or other suitable media on which the program can be printed, because the program can be obtained electronically, for example, by optically scanning the paper or other media, then editing, interpreting, or processing it in other suitable ways when necessary, and then storing it in a computer memory.

[0136] It should be understood that each part of the present invention can be implemented by hardware, software, firmware or a combination thereof. In the above embodiments, multiple steps or methods can be implemented by software or firmware stored in a memory and executed by a suitable instruction execution system. For example, if implemented by hardware, as in another embodiment, any one of the following techniques known in the art or a combination thereof can be used: discrete logic circuits having logic gate circuits for implementing logical functions on data signals, application specific integrated circuits having appropriate combinational logic gate circuits, programmable gate arrays (PGAs), field programmable gate arrays (FPGAs), etc.

[0137] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present invention can be modified or equivalently replaced without departing from the spirit and scope of the technical solutions of the present invention, and they should all be covered within the scope of the claims of the present invention.

Claims

1. A longitudinal protection method for a flexible DC line, characterized in that, Including: Collect voltage and current data in the HVDC transmission system, and construct a coupling model between the HVDC transmission system and the ground; Select the smoothing reactor and its adjacent line as the boundary conditions, and use the fault voltage and current information covering the full frequency band to construct a transient energy signal to determine the fault location; Each measuring point collects voltage and current electrical quantities in real time, and determines whether the protection starts by comparing the voltage value at both ends of the smoothing reactor with the starting threshold; When the protection starts, calculate the transient power from the voltage and current transient electrical quantities collected by each measuring point of the converter station, and then calculate its transient energy and transient energy ratio; Identify the fault area by comparing the transient energy ratios of each measuring point between the two converter stations with the protection setting value; Compare the value of the pole selection function obtained from the positive and negative pole voltage fault components with the protection setting value to identify the fault pole.

2. The flexible DC line pilot protection method according to claim 1, wherein: The construction of the coupling model between the HVDC transmission system and the ground is that in the HVDC transmission system, electrical quantities such as voltage and current flow between the positive and negative lines, and also interact with the ground to form a coupling relationship. The Karrenbauer transform is used for phase-mode transformation, and the expression is: Among them, i p , i n , u p , u n are respectively the current and voltage components of the positive and negative electrodes in the phase domain. i0, i1, u0, and u1 are respectively the line mode and zero mode components of the current and voltage in the modal domain.

3. The method for longitudinal protection of a flexible DC line according to claim 2, wherein: The selection of the smoothing reactor and its adjacent line as the boundary conditions, and the use of the fault voltage and current information covering the full frequency band to construct a transient energy signal to determine the fault location is based on the boundary defined by the original smoothing reactor, integrating the shunt effect of the adjacent line to expand the boundary range, and combining the current fault component and voltage fault component at points a and m to construct a protection criterion; When a single-pole fault occurs within the DC transmission line Line1, the fault component of the positive-pole voltage and the fault component of the current are |Δu m | > |Δu a |, |Δi m | > |Δi a |. For the measurement points n and b on the MMC2 side of the line, the fault component of the voltage and the fault component of the current are |Δu n | > |Δu b |, |Δi n | > |Δi b |; where, Δu m , Δu n , Δu a , Δu b are the voltage fault components of each measuring point m, n, a, b, and Δi m , Δi n , Δi a , Δi b are the current fault components of each measuring point m, n, a, b; When a bipolar fault occurs within the DC transmission line Line1, the voltage and current fault shunts at measurement points a and m are such that |Δu m | > |Δu a |, |Δi m | > |Δi a |. For the voltage and current fault shunts at the opposite measurement points n and b, |Δu n | > |Δu b |, |Δi n | > |Δi b |; When a forward external fault occurs on the DC transmission line Line1, the current fault components at m and a satisfy |Δi m | > |Δi a |, and the voltage fault components at m and a satisfy |Δu m | > |Δu a |. Due to the shunt effect of the line Line2, at n and b, |Δi n | < |Δi b |, and due to the voltage drop effect of the smoothing reactor, |Δu n | < |Δu b |; When a reverse out-of-zone fault occurs, the voltage and current fault components of the reverse out-of-zone fault are |Δu m | < |Δu a |, |Δi m | < |Δi a |, |Δu n | > |Δu b |, |Δi n | > |Δi b |.

4. The method for pilot protection of a flexible DC line according to claim 3, characterized in that: Each measurement point collects voltage and current electrical quantities in real time. By comparing the voltage value at both ends of the smoothing reactor with the starting threshold, it is determined whether the protection is started, and the protection starting margin u of the DC power grid under normal operation is set. L0 Value. When the DC power grid is operating normally, the measured voltages at both ends of each smoothing reactor are equal, and the voltage at both ends of the smoothing reactor can be approximated as 0. When a fault occurs in the DC transmission line, the voltage difference at both ends of its smoothing reactor is large at the initial stage of the fault. A protection starting criterion is constructed, and the expression is: u L (t) > u L0 Among them, u L (t) is the voltage difference between the measuring points at both ends of the boundary, that is, the voltage of the smoothing reactor; u L0 is the protection margin of the DC power grid under normal operation.

5. The longitudinal differential protection method for a flexible DC line according to claim 4, wherein: When the protection starts, calculate the transient power from the voltage and current transient electrical quantities collected by each measuring point of the converter station, and then calculate its transient energy and transient energy ratio. According to the current and voltage fault components measured at each measuring point, the transient power of the four measuring points a, b, m, and n, the expression is: Among them, P a , P b , P m , P n are the absolute values of the transient power at measuring points a, b, m, and n respectively; When a fault occurs within the DC transmission line, among the four measuring points a, m, n, and b, P m / P a >1, P n / P b >1. When a fault occurs outside the MMC1 side of the DC transmission line, P m / P a <1. For the non-faulty end of the MMC2 terminal, P n / P b >1. When an external fault occurs on the MMC2 side of the DC transmission line, P n / P b <1. For the MMC1 terminal, the non-faulty terminal, P m / P a >1; The transient energy of each measuring point m, n, a, and b in the full frequency band, the expression is: Among them, P m , P n , P a , P b are respectively the sampling values of the full-band transient power at measurement points m, n, a, and b within the sampling time window, N = 1, 2…, N is the number of sampling points within the sampling time window length; Construct an identification criterion for internal and external faults occurring in the flexible HVDC transmission line, the expression is: Among them, K set is the protection setting value, and E a , E b , E m , E n are the transient energies of each measuring point a, b, m, and n.

6. The method for pilot protection of a flexible DC line according to claim 5, characterized in that: By comparing the transient energy ratios of each measurement point between the two converter stations with the protection setting value K set to identify the fault area, including When the transient energy ratio E m / E a ≤K set or E n / E b ≤K set , it is determined that an external fault has occurred, and the protection is reset; When E is satisfied m / E a > K set and E n / E b > K set , it is determined as a fault within the zone.

7. A pilot protection method for a flexible DC line according to claim 6, characterized in that: The value W of the pole selection function obtained from the positive and negative pole voltage fault components is compared with the protection setting value W set Comparing and identifying the faulty pole includes When W ≥ W set1 When W, it is determined that a positive pole fault has occurred in the occurrence area; When W ≤ W set2 it is determined that a negative pole fault has occurred in the occurrence area; When W set2 ≤ W ≤ W set1 , it is determined that a bipolar fault has occurred within the area; Construct a fault pole selection function as follows: where, Δu m1 and Δu m2 are the voltage fault components of the positive and negative poles at the measuring point m at the protection installation location, respectively; When a negative pole fault occurs in the HVDC transmission line, the fault pole selection function W < 1; When a positive pole ground fault occurs, the fault pole selection function W > 1; When a bipolar fault occurs, the fault pole selection function W ≈ 1; Construct a pole selection criterion according to the different values of the pole selection function of the fault pole as follows: Among them, W set1 and W set2 are the setting values of the fault pole selection criterion.

8. A system adopting a flexible DC line pilot protection method as described in any one of claims 1 to 7, characterized in that: Including a fault start module, a fault identification module, and a fault pole selection module; The fault start module is to collect and store the voltage and current of each measuring point in real time. If the protection start criterion is met, the protection starts and enters the fault identification module; Otherwise, it does not start; The fault start module includes: A data acquisition unit that collects the analog signals output by the secondary sides of the voltage and current transformers at each measuring point in real time; The analog-to-digital conversion unit converts the voltage and current analog signals collected by the measuring point into digital signals; The protection start unit determines whether the voltage digital signal is greater than the start threshold set by this protection scheme. If so, it reads the start time and stores the data, and the protection starts; The fault identification module is to identify internal and external faults when the protection starts. If it is judged as an external fault, the protection does not operate; if it is judged as an internal fault, it enters the fault pole selection module; The fault identification module includes: The line-mode conversion unit converts the line-mode components of the measured voltage and current; The parameter setting unit sets the protection threshold value and the protection threshold W set1 、W set2 ; The numerical calculation unit calculates the transient energy and transient energy ratio of each measuring point, and compares the transient energy ratio with the set threshold value; The fault pole selection module judges whether a positive-pole, negative-pole or bipolar fault occurs in the system through the fault pole selection function, and relevant protections act; The fault pole selection module includes: The data measurement unit measures the voltage fault components of the positive and negative poles and obtains the value W of the pole selection function; The polarity judgment unit judges the positive and negative voltage fault components to obtain the value W of the pole selection function and compares it with the protection setting value W set for comparison; Identify the faulty pole. When W ≥ W set1 , it is determined that a positive pole fault within the zone has occurred; When W ≤ W set2 it is determined that a negative pole fault has occurred in the occurrence area; When W set2 ≤ W ≤ W set1 , it is determined that a bipolar fault has occurred within the area.

9. A computer device, comprising a memory and a processor, the memory storing a computer program, characterized in that, When the processor executes the computer program, the steps of a flexible DC line pilot protection method described in any one of claims 1 to 7 are implemented.

10. A computer-readable storage medium having a computer program stored thereon, characterized in that, When the computer program is executed by the processor, the steps of a flexible DC line pilot protection method described in any one of claims 1 to 7 are implemented.

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