Boundary-free flexible direct current system direction protection based on energy attenuation characteristics

Abstract

The invention discloses a boundary-free flexible direct current system direction protection based on energy attenuation characteristics, which comprises the following steps of: judging whether a fault occurs in a flexible direct current system by utilizing an improved voltage gradient algorithm, and determining a protection action time window; calculating high-frequency and low-frequency transient energy of 1-mode fault voltage traveling waves on two sides of the bus by using wavelet packet transformation; the fault occurrence position of the flexible direct current system is judged by calculating a k value reflecting the traveling wave energy attenuation characteristic; fault pole selection is realized through positive and negative pole fault voltage variation measured by protection measurement points at two ends of the line. A boundary-free flexible direct-current power transmission system has remarkable advantages in the aspects of system dynamics, stability and economical efficiency, a direction criterion is constructed according to the difference between high-frequency energy attenuation characteristics and low-frequency energy attenuation characteristics of refracted waves at a bus, and a protection setting strategy which is not affected by fault control is put forward; and fault starting, fault identification and fault pole selection criteria are combined to construct a complete pilot direction protection scheme, so that the sensitivity and reliability of protection are improved.

Description

Technical Field

[0001] The present invention relates to the field of relay protection and control of power systems, and in particular to a directional protection for a borderless flexible direct current transmission system taking into account the influence of source-side current limiting, and in particular to a directional protection for a borderless flexible direct current system based on energy attenuation characteristics. Background Art

[0002] Flexible direct current transmission system has become the most suitable power transmission mode for large-scale renewable energy access to the power grid due to its significant advantages of flexibility, controllability and ability to smooth the volatility and randomness of renewable energy generation. Existing flexible direct current systems mostly install boundary elements such as current limiting reactors at both ends of the DC line, which can create conditions for protection schemes while limiting the rapid development of fault currents; while the boundaryless flexible direct current system transfers the current limiting reactors from both ends of the line to the converter station outlet, which has significant advantages in system dynamics, stability and economy, but it is necessary to find a protection scheme that does not rely on boundary elements at both ends of the line.

[0003] The protection schemes of flexible DC systems can be divided into single-ended protection, longitudinal differential protection and longitudinal directional protection. Since there are no obvious boundaries between the lines in the borderless flexible DC system, single-ended protection cannot effectively distinguish between the end faults in the area and the head faults outside the area; the longitudinal differential protection has a strong ability to withstand transition resistance, but the data transmission volume is large, and it has high requirements for data synchronization and communication capabilities; while the longitudinal directional protection has high reliability, excellent performance, and is easy to implement because it only needs to transmit logic signals. Therefore, it is of great significance to study the longitudinal directional protection applicable to the borderless flexible DC system.

[0004] At present, the protection technology research for the borderless flexible DC system considers the impact of different transition resistances, noise and other factors on the protection when conducting adaptability analysis, but does not consider the impact of fault current limiting on the protection scheme. The investment of fault current limiting will change the original change of the fault characteristic quantity, thereby reducing the protection sensitivity and even the failure of the original protection principle. Fault current limiting can be divided into source-side fault current limiting and grid-side fault current limiting according to the different implementation locations. Source-side fault current limiting refers to the realization of fault current limiting by changing the topology or control strategy of the modular multilevel converter. Compared with the grid-side fault current limiting method of installing various current limiting devices at both ends of the DC line, source-side fault current limiting does not require additional devices, has a lower cost, and can shorten the system fault recovery time by hundreds of milliseconds, but the introduction of source-side fault current limiting will bring challenges to the original protection scheme.

[0005] At present, the longitudinal direction protection designed for the borderless flexible DC system uses the low-frequency energy, amplitude integral and polarity of the traveling wave. However, the protection scheme using the low-frequency energy and amplitude integral of the traveling wave will have the problem of reduced protection sensitivity and reduced transition resistance performance under the influence of source-side fault current limiting. The longitudinal direction protection scheme using the polarity of the second voltage traveling wave will have the problem of changing the polarity of the traveling wave after introducing the source-side fault current limiting, which will lead to the failure of the protection principle. Therefore, when conducting adaptability analysis on the protection scheme designed for the borderless flexible DC system, the impact of source-side fault current limiting needs to be considered. Summary of the invention

[0006] In order to solve the shortcomings and deficiencies in the prior art, the present application proposes a directional protection for a borderless flexible direct current system based on energy attenuation characteristics.

[0007] The purpose is to utilize the significant advantages of the borderless flexible direct current transmission system in terms of system dynamics, stability and economy, but the line protection principle designed based on boundary elements is difficult to apply to this topology. At the same time, taking into account the impact of fault current limiting will bring greater challenges to the protection of the borderless flexible direct current system. Based on this, the present invention designs a directional protection principle that does not rely on line boundary elements and takes into account the impact of source-side fault current limiting. The difference in the high- and low-frequency energy attenuation characteristics of the refracted waves at the busbar is used to construct a directional criterion, and a protection setting strategy that is not affected by fault control is proposed; and the fault initiation, fault identification and fault pole selection criteria are combined to construct a complete longitudinal directional protection scheme to improve the sensitivity and reliability of the protection.

[0008] Specifically, the present application proposes a directional protection for a borderless flexible DC system based on energy attenuation characteristics, which is applicable to a borderless flexible DC system and maintains high sensitivity and reliability while tolerating the influence of source-side current limiting. The method comprises the following steps:

[0009] Step (1) using an improved voltage gradient algorithm to determine whether a fault occurs in the flexible DC system and determine a protection action time window;

[0010] Step (2) using wavelet packet transform to calculate the high-frequency and low-frequency transient energy of the traveling wave of the 1-mode fault voltage on both sides of the bus;

[0011] Step (3) determining the location of the flexible DC system fault by calculating the k value of the energy attenuation characteristic of the traveling wave;

[0012] Step (4) implements fault pole selection by measuring the positive and negative fault voltage changes at the protection measurement points at both ends of the line.

[0013] Furthermore, the improved voltage gradient algorithm specifically uses the positive and negative voltages measured at the protection measurement points at both ends of the DC line as u p 、u n, the specific calculation formula is as follows:

[0014]

[0015] Where ▽u p (k), ▽u n (k) is the positive and negative voltage gradient operator, and k represents the voltage value of the kth sampling point.

[0016] Furthermore, the protection action time window Δt is constrained by the protection forward direction line length l1 and the reverse direction line length l2, and the constraint conditions are as follows:

[0017]

[0018] Where, wave speed v≈300km / ms;

[0019] Furthermore, the specific steps of calculating the high-frequency and low-frequency transient energy of the traveling wave of the 1-mode fault voltage on both sides of the bus in step (2) are as follows:

[0020] Get the positive and negative voltages measured at the protection measurement points at both ends of the DC line as u p 、u n , calculate the positive and negative voltage fault components;

[0021] The positive and negative voltage fault components are converted into 0-mode and 1-mode fault voltage and current components through pole mode transformation. The pole mode transformation formula used is as follows:

[0022]

[0023] Where: Δu, Δi represent the voltage and current fault components respectively; subscripts 0 and 1 represent the 0-mode and 1-mode components respectively; subscripts p and n represent the positive and negative pole quantities respectively.

[0024] The 1-mode fault voltage forward wave u reaching the protection installation can be obtained by using the obtained 0-mode and 1-mode fault voltage and current components. q With the anti-travelling wave u f , for u q 、u f Perform m-layer wavelet packet decomposition and get 2 m The frequency range of each frequency band is:

[0025]

[0026] Where: f s is the signal sampling frequency; j is the frequency band number (arranged in order from low frequency to high frequency).

[0027] The square of the coefficient decomposed by wavelet packet transform represents the transient energy of the 1-mode fault voltage traveling wave at the sampling point. Then the wavelet packet energy E of the k-th sampling point in the j-th frequency band signal is j (k) is:

[0028] E j (k)=|d j (k) 2 |

[0029] Where: d j (k) represents the wavelet packet decomposition coefficient of the kth sampling point in the jth frequency band signal.

[0030] The energy of the wavelet packet E in the jth frequency band can be obtained by summing the energy of all sampling points in the jth frequency band. j :

[0031]

[0032] Where: n represents the total number of sampling points in the jth frequency band.

[0033] Calculate the high and low frequency energy (E) of the reverse traveling wave at the protection installation location f-H 、E f-L ) and the high and low frequency energy of the forward traveling wave at the protection installation on the other side of the adjacent busbar (E q-H 、E q-L ). The high-frequency energy is the highest frequency energy that can be obtained in the wavelet packet decomposition, that is, j = 2 m ; Low-frequency energy is the lowest frequency band energy that can be obtained in wavelet packet decomposition, that is, j=1.

[0034] Furthermore, the specific steps of determining the location of the flexible DC system fault by calculating the k value of the reaction traveling wave energy attenuation characteristic in step (3) are:

[0035] make k L The degree of refraction and attenuation of the low-frequency energy of the traveling wave, k H It reflects the degree of refraction and attenuation of the high-frequency energy of the traveling wave, and k reflects the difference in the high- and low-frequency energy attenuation characteristics of the traveling wave.

[0036] When the protection forward line fails, k L <k H , k<1, when a reverse line fault occurs, the protection detects that the high-frequency and low-frequency energies in the reverse traveling wave are both small, that is, the following formula is satisfied:

[0037] (E f-L <E f-Lset )∩(E f-H <E f-Hset )=1

[0038] This time

[0039]

[0040] The fault direction identification criteria are as follows:

[0041]

[0042] When the k value of the reaction wave energy attenuation characteristic is less than or equal to the setting value k set When the protection at both ends of the line identifies a forward fault, it is judged as an in-zone fault, otherwise it is judged as an out-of-zone fault.

[0043] Furthermore, the specific steps of fault pole selection in step (4) are:

[0044] Get the positive and negative voltages measured at the protection measurement points at both ends of the DC line as u p 、u n , calculate the positive and negative voltage fault components;

[0045] The fault pole selection criterion is constructed by summing the amplitudes of the positive and negative line fault voltage components at k sampling points and making comparisons. The fault pole selection expression is as follows:

[0046]

[0047] In the formula, Δu p (k), Δu n (k) is the value of the positive and negative pole voltages of the fault at the kth sampling point, and P is the pole selection factor.

[0048] The fault selection criteria are as follows:

[0049]

[0050] Where P set Select the setting value for protection.

[0051] An electronic device includes one or more processors, and the processor executes any of the above-mentioned directional protection of the borderless flexible direct current system based on energy attenuation characteristics.

[0052] A computer storage medium, wherein the computer readable storage medium stores computer instructions, wherein the computer instructions are used to enable a computer to execute any of the above-mentioned directional protection of a borderless flexible direct current system based on energy attenuation characteristics.

[0053] Compared with the prior art, the present invention has the following beneficial effects:

[0054] 1. The protection scheme proposed in the present invention collects information from adjacent outgoing lines at both ends of the busbar, and identifies faults by judging the k value that reflects the high and low frequency energy attenuation characteristics of the traveling wave. The protection scheme does not rely on the boundary elements at both ends of the line, which solves the problem that the existing protection principle based on boundary element design cannot be applied to the borderless flexible direct current system.

[0055] 2. The parameter setting strategy in the proposed protection scheme fully takes into account the impact of fault current limiting. After the source-side fault current limiting is put into use, it can not only correctly realize fault identification and fault pole selection, but also has high sensitivity and reliability, effectively solving the problem of reduced protection performance or even failure under the influence of fault current limiting.

[0056] 3. The proposed protection scheme is not affected by the fault location, can identify various types of faults, has strong tolerance to transition resistance and noise, and has low requirements on communication capabilities because it only needs to transmit logic signals, and has strong engineering applicability. BRIEF DESCRIPTION OF THE DRAWINGS

[0057] In order to more clearly illustrate the technical solution of the present invention, the accompanying drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other accompanying drawings can be obtained based on these accompanying drawings without paying creative work.

[0058] Figure 1 This is the topological structure diagram of the four-terminal borderless flexible direct current system;

[0059] Figure 2 This is the simulation waveform of the traveling wave of the 1-mode fault voltage;

[0060] Figure 3 This is the simulation waveform of the traveling wave of the 1-mode fault voltage;

[0061] (a) Wavelet packet coefficients of the backward wave at each frequency band; (b) Wavelet packet coefficients of the forward wave at each frequency band;

[0062] Figure 4 This is the MMC equivalent RLC circuit diagram taking into account the source side current limiting;

[0063] Figure 5 This is the Peterson equivalent circuit diagram taking into account the source side current limiting;

[0064] Figure 6 is the amplitude-frequency characteristic curve of the refractive index;

[0065] Figure 7 Schematic diagram of the traveling wave propagation process taking into account the source-side current limiting during reverse fault.

[0066] Figure 8 It is a flowchart of the protection scheme; DETAILED DESCRIPTION

[0067] In order to make the structure and advantages of the present invention more clear, the structure of the present invention will be further described below in conjunction with the accompanying drawings.

[0068] A directional protection for a borderless flexible DC system based on energy attenuation characteristics, the steps are as follows:

[0069] Step (1) uses an improved voltage gradient algorithm to detect the DC voltage change to determine whether the flexible DC system has a fault and determine the protection action time window. The present invention defines the positive and negative voltages measured at the protection measurement points at both ends of the DC line as u p 、u n , the improved voltage gradient algorithm expression is as follows:

[0070]

[0071] Where ▽u p (k), ▽u n (k) is the positive and negative voltage gradient operator, and k represents the voltage value of the kth sampling point.

[0072] The fault start criteria are as follows:

[0073]

[0074] Where ▽ set When the positive and negative voltage gradient operators are both greater than the fault start setting value, it is judged that a fault has occurred and the protection is started, otherwise the protection is locked.

[0075] The protection action time window Δt is constrained by the protection forward direction line length l1 and the reverse direction line length l2. The constraints are as follows:

[0076]

[0077] Where, wave speed v≈300km / ms;

[0078] Step (2) uses wavelet packet transform to calculate the high-frequency and low-frequency transient energy of the 1-mode fault voltage traveling wave on both sides of the busbar. The positive and negative fault voltage and current components obtained by protection measurement are converted into 0-mode and 1-mode fault voltage and current components through pole mode transformation. The pole mode transformation formula used is as follows:

[0079]

[0080] Where: Δu, Δi represent the voltage and current fault components respectively; subscripts 0 and 1 represent the 0-mode and 1-mode components respectively; subscripts p and n represent the positive and negative pole quantities respectively.

[0081] The 0-mode and 1-mode fault voltage and current components obtained above can be used to calculate the 1-mode forward and reverse voltage traveling waves reaching the protection installation. The calculation formula is as follows:

[0082]

[0083] Where: u q 、u f Indicates the forward and reverse waves of the 1-mode fault voltage, Z c1 Represents the line 1 mode wave impedance, where the positive direction of current is defined as flowing from the busbar to the line.

[0084] The forward and reverse traveling waves u of the 1-mode fault voltage detected by the protection installation within the Δt time window q 、u f Perform m-layer wavelet packet decomposition and get 2 m The frequency range of each frequency band is:

[0085]

[0086] Where: f s is the signal sampling frequency; j is the frequency band number (arranged in order from low frequency to high frequency).

[0087] The square of the coefficient decomposed by wavelet packet transform represents the transient energy of the 1-mode fault voltage traveling wave at the sampling point. Then the wavelet packet energy E of the k-th sampling point in the j-th frequency band signal is j (k) is:

[0088] E j (k)=|d j (k) 2 |

[0089] Where: d j (k) represents the wavelet packet decomposition coefficient of the kth sampling point in the jth frequency band signal.

[0090] The energy of the wavelet packet E in the jth frequency band can be obtained by summing the energy of all sampling points in the jth frequency band. j :

[0091]

[0092] Where: n represents the total number of sampling points in the jth frequency band.

[0093] Calculate the high and low frequency energy (E) of the reverse traveling wave at the protection installation location f-H 、E f-L ) and the high and low frequency energy of the forward traveling wave at the protection installation on the other side of the adjacent busbar (E q-H 、E q-L). The high-frequency energy is the highest frequency energy that can be obtained in the wavelet packet decomposition, that is, j = 2 m ; Low-frequency energy is the lowest frequency energy that can be obtained in wavelet packet decomposition, that is, j = 1. Where: E represents the fault voltage traveling wave energy; subscripts q and f represent forward traveling wave and reverse traveling wave respectively; subscripts L and H represent low-frequency component and high-frequency component respectively.

[0094] Step (3) compares the obtained wavelet packet energy meter to determine the fault location of the flexible DC system.

[0095] make k L Reflects the refraction attenuation degree of the low-frequency energy of the traveling wave, k H It reflects the degree of refraction and attenuation of the high-frequency energy of the traveling wave, and k reflects the difference in the high- and low-frequency energy attenuation characteristics of the traveling wave.

[0096] After the source side fault current limiting is activated, when the protection forward line fails, k L <k H , k<1, when a reverse line fault occurs, the protection detects that the high-frequency and low-frequency energies in the reverse traveling wave are both small, that is, the following formula is satisfied:

[0097] (E f-L <E f-Lset )∩(E f-H <E f-Hset )=1

[0098] This time

[0099] k>>1, and the k value is approximately tens of thousands to hundreds of thousands.

[0100] The fault direction identification criteria are as follows:

[0101]

[0102] When the k value of the reaction wave energy attenuation characteristic is less than or equal to the setting value k set When , it is identified as a forward fault, otherwise it is identified as a reverse fault. There is an obvious difference in the k value between forward and reverse faults. set The value range is large, and even if source-side fault current limiting is introduced, the protection still maintains high sensitivity and reliability.

[0103] When the protection at both ends of the line identifies a forward fault, it is judged to be an in-zone fault, otherwise it is judged to be an out-of-zone fault.

[0104] Step (4) implements fault pole selection by measuring the positive and negative fault voltage changes at the protection measurement points at both ends of the DC line. The fault pole selection expression is as follows:

[0105]

[0106] In the formula, Δu p (k), Δu n (k) is the value of the positive and negative pole voltages of the fault at the kth sampling point, and P is the pole selection factor.

[0107] The fault selection criteria are as follows:

[0108]

[0109] Where P set Select the setting value for protection.

[0110] The following embodiments are described in conjunction with the accompanying drawings:

[0111] (1) The protection is started by detecting the change of DC voltage using the improved voltage gradient algorithm.

[0112] When the flexible DC system is operating normally, the positive and negative DC voltages are approximately the rated voltage ±500kV. However, after a fault occurs, the DC voltage changes suddenly. Therefore, the improved voltage gradient algorithm can be used to detect the change in DC voltage to determine whether the flexible DC system has a fault. If the system is detected to have a fault, the protection action time window is determined based on the length of the forward and reverse protection lines.

[0113] (2) The high-frequency and low-frequency transient energy of the traveling wave of the 1-mode fault voltage on both sides of the bus are calculated using wavelet packet transform.

[0114] The topology of multi-terminal boundaryless flexible DC system is as follows: Figure 1 As shown. The network after the flexible DC system fault occurs can be equivalent to the superposition of the normal network and the fault component network. Because the fault characteristics of the system are mainly determined by the traveling wave fault component, the present invention only analyzes the fault component network. The forward and reverse traveling waves of the 1-mode fault voltage are obtained by calculating the voltage and current fault components collected and detected at the protection installation, as shown in Figure 2 As shown, the wavelet packet coefficients in each frequency band can be obtained by performing wavelet packet transform, such as Figure 3 As shown, by selecting the highest frequency band and the lowest frequency band respectively for calculation, the high-frequency and low-frequency transient energy of the 1-mode fault voltage traveling wave can be obtained.

[0115] (3) Constructing fault identification criteria using differences in energy attenuation characteristics

[0116] Since the essence of source-side fault current limiting technology is to suppress the development of fault current by reducing the outlet voltage on the DC side of the converter station, it can be equivalent to inserting additional virtual inductance and resistance in series in the converter station, such as Figure 4 As shown in the figure, the MMC equivalent RLC circuit diagram taking into account the source side fault current limiting when a forward fault occurs is as follows Figure 5shown.

[0117] According to the equivalent circuit diagram, the traveling wave refraction coefficient α of the 1-mode fault voltage at the busbar of the unbounded flexible direct current system can be calculated. * The frequency domain expression of is:

[0118]

[0119] By taking values ​​of virtual inductance and resistance, the amplitude-frequency characteristic curve of the refractive index can be drawn, such as Figure 6 As shown in the figure, during a forward fault, the refractive index of the fault voltage traveling wave in the low frequency band is smaller than that in the high frequency band.

[0120] The traveling wave propagation process of reverse fault is shown in the figure Figure 7 As shown, before the reflected wave of the adjacent line reaches the protection installation, the reverse voltage traveling wave cannot be detected. At this time, the refractive index of the low-frequency band and the refractive index of the high-frequency band are both very large.

[0121] The high- and low-frequency transient energy of the fault voltage traveling wave is used to react to the above characteristics. When a forward fault occurs, the ratio of the low-frequency energy of the forward and reverse traveling waves is smaller than the ratio of the high-frequency energy. At this time, the k value reflecting the difference in attenuation characteristics of different frequencies is smaller than the direction identification setting value. When a reverse fault occurs, an auxiliary discrimination formula is introduced:

[0122] (E f-L <E f-Lset )∩(E f-H <E f-Hset )=1

[0123] At this time, the k value that reflects the difference in attenuation characteristics of different frequencies is greater than the direction identification setting value, and the k value is significantly different during forward and reverse faults. Based on this, the direction of the fault can be identified with high reliability and sensitivity. By communicating the direction information at both ends, it can be determined whether the fault location is inside or outside the area.

[0124] (4) Fault polarity selection is achieved by detecting the change in the positive and negative fault voltages.

[0125] When an inter-pole short circuit fault occurs, the transient information of the positive and negative poles to ground voltage is basically the same; when a single-pole ground short circuit fault occurs, the transient information of the two poles to ground voltage is different, and the amplitude of the fault component of the fault pole voltage is greater than that of the sound pole. Based on the above differences, the ratio of the accumulated values ​​of the positive and negative pole fault voltages is used as the fault pole selection criterion.

[0126] The flowchart of the new protection principle proposed by the present invention is as follows Figure 8 The specific implementation process is as follows:

[0127] First, the improved voltage gradient is calculated to determine whether to start the protection;

[0128] After the protection is started, the traveling wave of the 1-mode fault voltage on both sides of the busbar is transformed by wavelet packet transformation and its high-frequency transient energy and low-frequency transient energy are calculated. Then, the direction of the fault is determined by calculating the k value.

[0129] If both ends of the protection identify a forward fault, it is determined that an internal fault has occurred. Then the fault pole is identified by the fault pole selection criterion, and finally the protection is activated to clear the fault.

[0130] If one of the two protection ends identifies a reverse fault, it is determined that an out-of-zone fault has occurred and the protection is restored.

[0131] The present application also provides an electronic device, which may include: at least one processor and at least one memory.

[0132] The processor may include one or more processing cores. The processor uses various interfaces and lines to connect various parts of the entire server, and executes various functions of the server and processes data by running or executing instructions, programs, code sets or instruction sets stored in the memory, and calling data stored in the memory.

[0133] Optionally, the processor can be implemented in at least one hardware form of digital signal processing (DSP), field programmable gate array (FPGA), and programmable logic array (PLA). The processor can integrate one or a combination of a central processing unit (CPU), a graphics processing unit (GPU), and a modem. Among them, the CPU mainly processes the operating system, user interface, and application programs; the GPU is responsible for rendering and drawing the content to be displayed on the display; and the modem is used to process wireless communications. It is understandable that the above-mentioned modem may not be integrated into the processor, but may be implemented separately through a chip.

[0134] Among them, the memory may include a random access memory (Random Access Memory, RAM) or a read-only memory (Read-Only Memory). Optionally, the memory includes a non-transitory computer-readable storage medium. The memory can be used to store instructions, programs, codes, code sets or instruction sets. The memory may include a program storage area and a data storage area, wherein the program storage area may store instructions for implementing an operating system, instructions for at least one function (such as a touch function, a sound playback function, an image playback function, etc.), instructions for implementing the above-mentioned various method embodiments, etc.; the data storage area may store data involved in the above-mentioned various method embodiments, etc. The memory may also be optionally at least one storage device located away from the aforementioned processor. The memory as a computer storage medium may include an operating system, a network communication module, a user interface module, and an application program for a small incision thyroid surgery image acquisition system.

[0135] The present application also provides a computer-readable storage medium, which stores instructions. When executed by one or more processors, the electronic device executes one or more of the methods described in the above embodiments.

[0136] In addition, each functional unit in the present application may be integrated into one processing unit, or each unit may exist physically separately, or two or more units may be integrated into one unit. The above integrated unit may be implemented in the form of hardware or in the form of software functional units.

[0137] If the integrated unit is implemented in the form of a software functional unit and sold or used as an independent product, it can be stored in a computer-readable memory. Based on this understanding, the technical solution of the present application, or the part that contributes to the prior art, or all or part of the technical solution can be embodied in the form of a software product, which is stored in a memory and includes several instructions for a computer device (which can be a personal computer, server or network device, etc.) to perform all or part of the steps of the various embodiments of the present application. The aforementioned memory includes: various media that can store program codes, such as USB flash drives, mobile hard drives, magnetic disks or optical disks.

[0138] The above descriptions are merely embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A directional protection for a borderless flexible direct current system based on energy attenuation characteristics, characterized in that: The following steps are involved: Step (1) using an improved voltage gradient algorithm to determine whether a fault occurs in the flexible DC system and determine a protection action time window; Step (2) using wavelet packet transform to calculate the high-frequency and low-frequency transient energy of the traveling wave of the 1-mode fault voltage on both sides of the bus; Step (3) determining the location of the flexible DC system fault by calculating the k value of the energy attenuation characteristic of the traveling wave; Step (4) implements fault pole selection by measuring the positive and negative fault voltage changes at the protection measurement points at both ends of the line.

2. The directional protection of the borderless flexible direct current system based on energy attenuation characteristics according to claim 1 is characterized in that: The improved voltage gradient algorithm in step (1) specifically uses the positive and negative voltages measured at the protection measurement points at both ends of the DC line as u p 、u n , the specific calculation formula is as follows: Where ▽u p (k), ▽u n (k) is the positive and negative voltage gradient operator, and k represents the voltage value of the kth sampling point.

3. The directional protection of the borderless flexible direct current system based on energy attenuation characteristics according to claim 1 is characterized in that: The protection action time window Δt in step (1) is constrained by the protection forward direction line length l1 and the reverse direction line length l2. The constraint conditions are as follows: In the formula, the wave speed v≈300km / ms.

4. The directional protection of the borderless flexible direct current system based on energy attenuation characteristics according to claim 1 is characterized in that: The specific steps of calculating the high-frequency and low-frequency transient energy of the traveling wave of the 1-mode fault voltage on both sides of the bus in step (2) are as follows: Get the positive and negative voltages measured at the protection measurement points at both ends of the DC line as u p 、u n , calculate the positive and negative voltage fault components; The positive and negative voltage fault components are converted into 0-mode and 1-mode fault voltage and current components through pole mode transformation. The pole mode transformation formula used is as follows: Where: Δu, Δi represent the voltage and current fault components respectively; subscripts 0 and 1 represent the 0-mode and 1-mode components respectively; subscripts p and n represent the positive and negative pole quantities respectively; The 1-mode fault voltage forward wave u reaching the protection installation can be obtained by using the obtained 0-mode and 1-mode fault voltage and current components. q With the anti-travelling wave u f , for u q 、u f Perform m-layer wavelet packet decomposition and get 2 m The frequency range of each frequency band is: Where: f s is the signal sampling frequency; j is the frequency band number; The square of the coefficient decomposed by wavelet packet transform represents the transient energy of the 1-mode fault voltage traveling wave at the sampling point. Then the wavelet packet energy E of the k-th sampling point in the j-th frequency band signal is j (k) is: E j (k)=|d j (k) 2 Where: d j (k) represents the wavelet packet decomposition coefficient of the kth sampling point in the jth frequency band signal; The energy of the wavelet packet E in the jth frequency band can be obtained by summing the energy of all sampling points in the jth frequency band. j : Where: n represents the total number of sampling points in the jth frequency band; Calculate the high and low frequency energy (E) of the reverse traveling wave at the protection installation location f-H 、E f-L ) and the high and low frequency energy of the forward traveling wave at the protection installation on the other side of the adjacent busbar (E q-H 、E q-L ), where high-frequency energy is the highest frequency energy that can be obtained in wavelet packet decomposition, i.e. j = 2 m ; Low-frequency energy is the lowest frequency band energy that can be obtained in wavelet packet decomposition, that is, j=1.

5. The directional protection of the borderless flexible direct current system based on energy attenuation characteristics according to claim 1 is characterized in that: The specific steps of determining the location of the flexible DC system fault by calculating the k value of the reaction traveling wave energy attenuation characteristic in step (3) are as follows: make k L The degree of refraction and attenuation of the low-frequency energy of the traveling wave, k H The degree of refraction and attenuation of high-frequency energy of the traveling wave is reflected, and k reflects the difference in the attenuation characteristics of high- and low-frequency energy of the traveling wave; When the protection forward line fails, k L <k H , k<1, when a reverse line fault occurs, the protection detects that the high-frequency and low-frequency energies in the reverse traveling wave are both small, that is, the following formula is satisfied: (AND f-L <And f-Lset )∩(And f-H <And f-Hset )=1 This time The fault direction identification criteria are as follows: When the k value of the reaction wave energy attenuation characteristic is less than or equal to the setting value k set When the protection at both ends of the line identifies a forward fault, it is judged as an in-zone fault, otherwise it is judged as an out-of-zone fault.

6. The directional protection of the borderless flexible direct current system based on energy attenuation characteristics according to claim 1 is characterized in that: The specific steps of fault pole selection in step (4) are: Get the positive and negative voltages measured at the protection measurement points at both ends of the DC line as u p 、u n , calculate the positive and negative voltage fault components; The fault pole selection criterion is constructed by summing the amplitudes of the positive and negative line fault voltage components at k sampling points and making comparisons. The fault pole selection expression is as follows: In the formula, Δu p (k), Δu n (k) is the value of the fault positive and negative pole voltages at the kth sampling point, and P is the pole selection factor; The fault selection criteria are as follows: Where P set Select the setting value for protection.

7. An electronic device, characterized in that: It comprises one or more processors, and the processor executes the directional protection of the borderless flexible direct current system based on energy attenuation characteristics as described in any one of claims 1 to 6.

8. A computer storage medium, characterized in that: The computer-readable storage medium stores computer instructions, and the computer instructions are used to enable a computer to execute the directional protection of a borderless flexible direct current system based on energy attenuation characteristics as described in any one of claims 1 to 6.

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