Flexible direct current transmission line protection method and system based on modulus high-frequency component

Abstract

The invention discloses a flexible direct-current transmission line protection method and system based on modulus high-frequency components. The method comprises the following steps: acquiring a fault characteristic frequency range in a transmission line area in advance by using a full-element analysis method; thirdly, collecting positive voltage and negative voltage of a multi-terminal flexible direct current power transmission line protection installation position at multiple different moments, and realizing protection starting detection by using positive and negative break variables; the positive electrode grounding fault, the negative electrode grounding fault and the bipolar fault are detected by using the accumulated zero-mode voltage; and finally, according to a fault line mode voltage characteristic frequency range in a preset area and a multi-dimensional variational mode decomposition algorithm, carrying out time-frequency characteristic component extraction on the line mode voltage of the line, and constructing an adaptive peak voltage quantitative index to realize quantitative detection on the fault area of the power transmission line. According to the invention, a full-element analysis method, a multi-dimensional variational mode decomposition algorithm and a self-adaptive voltage peak value are utilized, so that the single-ended protection method can tolerate 1500 ohm transition resistance, and the sensitivity of the protection method is effectively improved.

Description

Technical Field

[0001] This invention relates to a highly sensitive multi-terminal flexible DC transmission line protection method and system based on high-frequency modulus components, belonging to the field of relay protection technology, especially the field of transmission line protection in relay protection. Background Technology

[0002] With the continuous expansion of renewable energy power generation and the increasing demand for inter-regional power grid interconnection, multi-terminal flexible DC transmission technology has become a key technology for building a new generation of power grids and achieving optimal energy allocation due to its outstanding advantages such as flexible control, ease of grid connection, and ability to supply power to passive networks. However, when a fault occurs in a multi-terminal flexible DC power grid transmission line, the fault current rise rate (d) i / d t The fault threshold is extremely high, and can usually reach the safety threshold that endangers converter station devices within a few milliseconds. Therefore, line protection must be able to accurately identify faults in a very short time (usually around 3ms).

[0003] Currently, protection schemes applied to DC lines mainly draw on the experience of AC systems or DC systems at both ends, and can be divided into traditional traveling wave protection methods, differential undervoltage protection methods, longitudinal differential protection, and high-frequency component protection methods based on wavelet transform. Traditional protection methods typically use the amplitude and rate of change of the fault modulus traveling wave to identify faults, but they face problems with weak tolerance to fault transition resistance and interference resistance. Regarding differential undervoltage protection methods, because undervoltage differential protection utilizes the output DC voltage, the measured information includes not only reverse traveling waves but also forward traveling waves, making the protection criterion's action performance significantly affected by traveling wave reflections. Although traveling wave protection using polar waves is unaffected by line reflections, it still faces the problem of unreliable fault identification when the protection time window is short. As for longitudinal differential protection, although it has absolute selectivity in principle, the long transmission line distances and inherent communication channel delays do not meet the speed requirements of multi-terminal flexible DC grids. While wavelet transform-based high-frequency component protection methods are more sensitive to the detection of singularities and abrupt changes, thus improving the sensitivity of the protection method to some extent, they still face challenges such as wavelet basis selection and susceptibility to noise, making it difficult to achieve a good balance between sensitivity and speed.

[0004] Existing technology CN115015687A discloses a method and system for fault location in a four-terminal ring-shaped flexible DC power grid. However, the line mode wave impedance selected in this scheme is a constant value, while the electrical characteristics of high-voltage transmission lines are affected by frequency, and this scheme does not consider this influencing factor. Furthermore, this patent is mainly applied to fault location and does not explain how to distinguish between faults within and outside the transmission line area. Existing technology CN115097253A discloses a method and system for fault location in an MMC-HVDC DC transmission line, but this scheme only achieves fault location and does not explain how to distinguish between faults within and outside the transmission line area.

[0005] Therefore, this invention proposes a highly sensitive multi-terminal flexible DC transmission line protection method based on high-frequency modulus components to improve the sensitivity of protection methods more effectively. Summary of the Invention

[0006] This invention provides a highly sensitive protection method for multi-terminal flexible DC transmission lines based on high-frequency modulus components, which effectively addresses the problem of insufficient sensitivity in existing protection methods.

[0007] To address the aforementioned technical problems, the technical solution adopted in this invention is as follows: Based on the system topology and transmission line parameters, the characteristic frequency range of fault line-mode voltage within the multi-terminal flexible DC power grid area is pre-obtained using a full-component analysis method; positive and negative voltages at multiple different times are collected at the protection installation points of the multi-terminal flexible DC transmission line; wherein, the multi-terminal flexible DC transmission line is a DC transmission line with single-terminal quantity protection; based on the obtained positive and negative voltages, the sudden change in positive and negative voltages is used to detect whether protection needs to be activated; the zero-mode voltage accumulation value is used to detect the fault pole; the multi-dimensional variational mode decomposition (MVMD) algorithm is used to extract the time-frequency components of the line-mode voltage of the multi-terminal flexible DC transmission line; characteristic time-frequency components are selected through the characteristic frequency range of fault line-mode voltage within the area, and an adaptive voltage peak value is constructed to achieve fault area detection.

[0008] This invention discloses a protection method for flexible DC transmission lines based on high-frequency modulus components, the protection method comprising: S1: Based on the system topology and transmission line parameters, the characteristic frequency range of fault line mode voltage in the multi-terminal flexible DC power grid area is obtained in advance by using full component analysis; S2: Collect the positive and negative voltages at multiple different times at the installation location of the multi-terminal flexible DC transmission line protection; where the multi-terminal flexible DC transmission line is a DC transmission line with single-terminal quantity protection. S3: Detect grid disturbances based on the sudden changes in positive and negative voltages. If a disturbance occurs, proceed to step 4 and record the disturbance time; otherwise, return to S2. S4: Modulus voltage is calculated based on the recorded disturbance time. The modulus voltage calculation includes the calculation of accumulated zero-mode voltage and line-mode voltage. The calculated accumulated zero-mode voltage, line-mode voltage, pre-acquired characteristic frequency range of fault line-mode voltage in the region, and multidimensional variational mode decomposition are used to detect fault poles and fault regions. The modulus voltage includes zero-mode voltage and line-mode voltage.

[0009] More preferably, In S1, based on the topology of the multi-terminal flexible DC grid, for faults in the high-resistivity zone at the end of the transmission line and faults outside the near-field metallic zone of the transmission line, the line-mode node voltage equivalent circuits of the fault frequency domain supplementary network, including all modular multilevel converter stations and all components at both ends of the line, are drawn respectively. Write the loop voltage equations for the equivalent circuits of each line-mode node voltage and solve for the node voltages to obtain the frequency domain expressions of the line-mode voltages for faults in the high-resistivity region at the end of the transmission line and faults outside the near-metallic region. Plot the line-mode voltage amplitude curves for faults in the high-resistivity region at the end of the transmission line and faults outside the near-metallic region using the transmission line parameters to obtain the characteristic frequency range of the line-mode voltage for faults in the high-resistivity region at the end of the transmission line.

[0010] More preferably, In S3, when the positive voltage changes abruptly... U pm ( t or negative electrode voltage sudden change U nm ( t When the value exceeds the disturbance threshold, a disturbance is determined to have occurred in the DC power grid, and this moment is recorded and defined as the disturbance occurrence moment. t fs The disturbance threshold is taken as the rated value of the positive electrode of the system. sx The ratio is 1.5 times, among which the selection is based on the principle that even the largest possible single-pole grounding fault transition resistance in a multi-terminal flexible DC power grid can ensure normal startup. sx The value of .

[0011] More preferably, Positive voltage change U pm ( t ), negative electrode voltage change U nm ( t Calculated as follows: ; ; in, t Time, in seconds. f s Sampling frequency, t d This refers to the current moment.

[0012] More preferably, In S4, the accumulated zero-mode voltage is calculated as follows: Acquire the positive electrode voltage 0.5ms before and 0.5ms after the disturbance. and analysis of negative electrode voltage ; Calculate zero-mode voltage and accumulated zero-mode voltage ; ; ; in, This represents the moment of DC grid disturbance.

[0013] More preferably, In S4, the following method is used for fault detection: If the zero-mode voltage is accumulated U ZO Less than the voltage accumulation threshold - U jx If so, it is determined to be a positive ground fault; if U ZO Greater than the voltage accumulation threshold U jx If the fault is between - U jx and U jx If the value is between these two values, then a bipolar fault is determined.

[0014] More preferably, Voltage accumulation threshold U jx The value is 200kV. U jx The value is -200kV.

[0015] More preferably, The following methods are used to detect fault areas: Calculate the line-mode voltage after DC grid disturbance U M ( t ), and obtain the normalized line-mode voltage. U GM ( t ); Using MVMD U GM ( t )conduct cn Layer decomposition, to obtain cn Each time-frequency component SP cn ( t Based on the fault characteristic frequency range within the multi-terminal flexible DC power grid area obtained in advance through full component analysis, the first... Kn Each time-frequency component is a characteristic component. F c ( t ), No. Kn The frequency range of each time-frequency component must be greater than or equal to the minimum value of the fault characteristic frequency range within the pre-obtained multi-terminal flexible DC power grid area; Linear mode voltage U M ( t First-order difference calculations are performed to obtain the difference line modulus. U DM ( t ); Calculate adaptive voltage peak AP ;

[0016] In the formula, max( F c ( t ))express F c ( t The maximum value of ), max( U DM ( t ))express U DM ( t The maximum value of ). when AP Greater than the fault area detection threshold ε If the fault is within the zone, it is determined to be a fault within the zone; otherwise, it is determined to be a fault outside the zone.

[0017] More preferably, The fault area detection threshold ε The value is 1100kV.

[0018] The second aspect of the present invention discloses a flexible DC transmission line protection system based on the aforementioned protection method, including a fault line mode voltage characteristic frequency range acquisition module, a multi-terminal flexible DC transmission line electrical quantity acquisition module, a disturbance start judgment module, an accumulated zero-mode voltage calculation module, and a fault pole and fault area detection module; The fault line mode voltage characteristic frequency range acquisition module obtains the fault line mode voltage characteristic frequency range in advance within the multi-terminal flexible DC power grid area based on the system topology and transmission line parameters and using full component analysis. The electrical quantity acquisition module for multi-terminal flexible DC transmission lines collects the positive and negative voltages at multiple different times at the protection installation points of multi-terminal flexible DC transmission lines. The disturbance initiation judgment module detects power grid disturbances based on the sudden changes in positive and negative voltage. The fault pole and fault area detection module calculates the accumulated zero-mode voltage after a power grid disturbance occurs, and determines the fault pole based on the accumulated zero-mode voltage; it calculates the adaptive voltage peak after the disturbance based on the pre-acquired characteristic frequency range of the fault line-mode voltage in the area and the line-mode voltage, and determines the fault inside or outside the area based on the adaptive voltage peak.

[0019] A third aspect of the present invention discloses a computer-readable storage medium storing one or more programs, characterized in that the one or more programs include instructions that, when executed by a computing device, cause the computing device to perform a flexible DC transmission line protection method based on modulus high-frequency components proposed in the present invention.

[0020] The fourth aspect of the present invention discloses one or more processors, one or more memories, and one or more programs, wherein the one or more programs are stored in the one or more memories and configured to be executed by the one or more processors, the one or more programs including methods for performing a flexible DC transmission line protection method based on modulus high-frequency components.

[0021] Compared with the prior art, the beneficial effects achieved by the present invention are as follows: 1. This invention obtains a more accurate characteristic frequency range of fault line mode voltage within the transmission line area of ​​a multi-terminal flexible DC power grid by using a full-component analysis method; 2. Using a multidimensional variational mode decomposition algorithm to extract high-frequency components of line-mode voltage can improve the calculation time for high-frequency component extraction and avoid the selection of wavelet basis. Moreover, the calculation speed is faster than existing methods including variational mode decomposition, empirical mode decomposition and continuous variational mode decomposition, which can better meet the protection requirements of flexible DC transmission lines. 3. This invention utilizes the time-frequency component of the line-mode voltage characteristic and the line-mode voltage change rate to construct an adaptive voltage peak, which can further improve the sensitivity of the protection method while ensuring rapid protection startup, and can withstand higher transition resistances up to 1500Ω. Attached Figure Description

[0022] Figure 1 A flowchart illustrating an embodiment of a flexible DC transmission line protection method based on high-frequency modulus components; Figure 2This is a topology diagram of a multi-terminal flexible DC power grid; Figure 3 Additional frequency domain analysis network diagram for faults at the end of the zone via transition resistors; Figure 4 An additional frequency domain equivalent circuit diagram for faults at the end of the zone via a transition resistor; Figure 5 A fault additional frequency domain analysis network diagram for near-end metallic faults outside the zone; Figure 6 The additional frequency domain equivalent circuit diagram for near-end metallic faults outside the zone; Figure 7 Amplitude-frequency curves of the proportional coefficient of fault line mode voltage in different regions; Figure 8 Positive voltage diagrams at various protection points under operating conditions A and B; Figure 9 Negative voltage diagrams at various protection points under operating conditions A and B; Figure 10 Characteristic component diagrams of each protection point under operating condition A; Figure 11 Amplitude-frequency diagrams of characteristic components of protection N under operating conditions A and B; Figure 12 Characteristic component diagrams of each protection point under operating condition B. Detailed Implementation

[0023] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of this invention. The embodiments described in this application are merely some embodiments of this invention, and not all embodiments. Based on the spirit of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the protection scope of this invention.

[0024] This application discloses a protection method for flexible DC transmission lines based on high-frequency modulus components, characterized by comprising: S1: Based on the system topology and transmission line parameters, the characteristic frequency range of fault line mode voltage in the multi-terminal flexible DC power grid area is obtained in advance by using full component analysis; Based on the topology of the multi-terminal flexible DC grid, for faults in the high-resistivity zone at the end of the transmission line and faults outside the near-field metallic zone of the transmission line, the line-mode node voltage equivalent circuits of the fault frequency domain supplementary network, including all modular multilevel converter stations and all components at both ends of the line, are drawn respectively. Write the loop voltage equations for the equivalent circuits of each line-mode node voltage and solve for the node voltages to obtain the frequency domain expressions of the line-mode voltages for faults in the high-resistivity region at the end of the transmission line and faults outside the near-metallic region. Plot the line-mode voltage amplitude curves for faults in the high-resistivity region at the end of the transmission line and faults outside the near-metallic region using the transmission line parameters to obtain the characteristic frequency range of the line-mode voltage for faults in the high-resistivity region at the end of the transmission line.

[0025] S2: Collect the positive and negative voltages at multiple different times at the installation location of the multi-terminal flexible DC transmission line protection; where the multi-terminal flexible DC transmission line is a DC transmission line with single-terminal quantity protection. S3: Detect grid disturbances based on the sudden changes in positive and negative voltages. If a disturbance is detected, proceed to step 4; otherwise, return to S2. Positive voltage change U pm ( t ), negative electrode voltage change U nm ( t Calculated as follows: ; ; in, t Time, in seconds. f s Sampling frequency, t d This refers to the current moment.

[0026] When the positive voltage changes abruptly U pm ( t or negative electrode voltage sudden change U nm ( t If the value exceeds the disturbance threshold, a disturbance is determined to have occurred in the DC power grid. The disturbance threshold is taken as the rated value of the system's positive pole. sx times.

[0027] Based on the maximum possible single-pole grounding fault transition resistance in a multi-terminal flexible DC power grid R gmax Based on the principle that it can also start normally, proceed as follows sx Selection of values. In this invention, for the case of a single-pole grounding fault with a transition resistance of 1500Ω in a ±500kV multi-terminal flexible DC power grid, the following values ​​are selected: sx It is 0.06 times the system's single-pole rated voltage.

[0028] S4: Calculate the accumulated zero-mode voltage based on the current moment. Use the accumulated zero-mode voltage, the pre-acquired characteristic frequency range of the fault line-mode voltage in the region, and multidimensional variational mode decomposition to detect the fault pole and the fault region. The modulus voltage includes the zero-mode voltage and the line-mode voltage.

[0029] Determine if a disturbance has occurred in the multi-terminal flexible DC power grid, and record the value at that moment. t fs And obtain and analyze the positive electrode voltage. U pf ( t ) and analysis of negative electrode voltage U nf ( t This will be used for subsequent fault polarity detection and fault area detection. U pf ( t )and U nf ( t )for:

[0030]

[0031]

[0032]

[0033] in, for The positive voltage of a multi-terminal flexible DC transmission line at any given time. for The negative voltage of a multi-terminal flexible DC transmission line at any given time. , These are multi-terminal flexible DC transmission lines in The average rated voltage of the positive electrode and the average rated voltage of the negative electrode within the interval, where S is the number of seconds.

[0034] Utilizing the zero-mode voltage of multi-terminal flexible DC transmission lines U Z ( t Accumulate zero-mode voltage U ZO Calculate, where the zero-mode voltage U Z ( t and accumulated zero-mode voltage U ZO The calculation method is as follows:

[0035]

[0036] If the zero-mode voltage is accumulated U ZO Less than the voltage accumulation threshold - U jx If so, it is determined to be a positive ground fault; if U ZO Greater than the voltage accumulation threshold U jx If the fault is between - U jx and U jx If the value is between these two values, then a bipolar fault is determined.

[0037] Based on the maximum possible single-pole grounding fault transition resistance in a multi-terminal flexible DC power grid R gmax It can also accurately select the pole for faults. U jx Selection of the value. In this invention, for the condition of a single-pole grounding fault with a transition resistance of 1500Ω in a ±500kV multi-terminal flexible DC power grid, the preferred value is... U jx The value is 200kV. U jx The value is -200kV.

[0038] The following methods are used to detect fault areas: Calculate the line-mode voltage after DC grid disturbance U M ( t ), and obtain the normalized line-mode voltage. U GM ( t ); Using MVMD U GM ( t )conduct cn Layer decomposition, to obtain cn Each time-frequency component SP cn ( t Based on the fault characteristic frequency range within the multi-terminal flexible DC power grid area obtained in advance through full component analysis, the first... Kn Each time-frequency component is a characteristic component. F c ( t ), this first Kn The frequency range of each time-frequency component must be greater than or equal to the minimum value of the fault characteristic frequency range within the pre-acquired multi-terminal flexible DC power grid area; in this invention, the signal sampling frequency is 20kHz, and the pre-acquired fault characteristic frequency range within the multi-terminal flexible DC power grid area is set to 2000Hz~10kHz. cnSelect 8. Kn If we select 4, the frequency range of the fourth time-frequency component will be greater than 2000Hz. Linear mode voltage U M ( t First-order difference calculations are performed to obtain the difference line modulus. U DM ( t ); Calculate adaptive voltage peak AP ;

[0039] In the formula, max( F c ( t ))express F c ( t The maximum value of ), max( U DM ( t ))express U DM ( t The maximum value of ). when AP Greater than the fault area detection threshold ε If the fault is within the zone, it is determined to be a fault within the zone; otherwise, it is determined to be a fault outside the zone.

[0040] The fault area detection threshold ε The value is set to 1100kV. When encountering systems with different voltage levels and topologies, a threshold can be selected after simulation experiments based on the method proposed in this invention. ε .

[0041] This application also discloses a protection system for a multi-terminal flexible DC transmission line based on the aforementioned protection method, including a fault line mode voltage characteristic frequency range acquisition module, a multi-terminal flexible DC transmission line electrical quantity acquisition module, a disturbance start judgment module, an accumulated zero-mode voltage calculation module, and a fault pole and fault area detection module.

[0042] The fault line mode voltage characteristic frequency range acquisition module obtains the fault line mode voltage characteristic frequency range in advance within the multi-terminal flexible DC power grid area based on the system topology and transmission line parameters and using full component analysis. The electrical quantity acquisition module for multi-terminal flexible DC transmission lines collects the positive and negative voltages at multiple different times at the protection installation points of multi-terminal flexible DC transmission lines. The disturbance initiation judgment module detects power grid disturbances based on the sudden changes in positive and negative voltage. The fault pole and fault area detection module calculates the accumulated zero-mode voltage after a power grid disturbance occurs, and determines the fault pole based on the accumulated zero-mode voltage; it calculates the adaptive voltage peak after the disturbance based on the pre-acquired characteristic frequency range of the fault line-mode voltage in the area and the line-mode voltage, and determines the fault inside or outside the area based on the adaptive voltage peak.

[0043] Example 1: like Figure 1 The image shows a preferred embodiment of the high-sensitivity multi-terminal flexible DC transmission line protection method based on high-frequency modulus components of the present invention, which includes the following steps: Step 1: Based on the system topology and transmission line parameters, use full component analysis to pre-obtain the characteristic frequency range of fault line mode voltage in the multi-terminal flexible DC power grid area; Based on the topology of the multi-terminal flexible DC grid, for faults in the high-resistance zone (high resistance in this embodiment refers to greater than 500Ω) at the end of the transmission line and faults outside the near-field metallic zone of the transmission line, the line-mode node voltage equivalent circuits of the fault frequency domain supplementary network are drawn, including all components such as the fault equivalent resistance of all modular multilevel converter stations, series components of capacitors and inductors, fault supplementary equivalent power supply, fault transition resistance, fault equivalent wave impedance components of all transmission lines, and equivalent current-limiting inductor components at both ends of the line. Write the loop voltage equations for the equivalent circuits of the line-mode node voltages and solve for the node voltages to obtain the frequency domain expressions of the line-mode voltages for faults in the high-resistivity region at the end of the transmission line and faults outside the near-metallic region. Plot the line-mode voltage amplitude curves for faults in the high-resistivity region at the end of the transmission line and faults outside the near-metallic region using the transmission line parameters to obtain the characteristic frequency range of the line-mode voltage for faults in the high-resistivity region at the end of the transmission line.

[0044] With attachment Figure 2 Taking a multi-terminal flexible DC grid as an example, this paper introduces a method for full-component fault analysis of transmission lines in a multi-terminal flexible DC grid to pre-obtain the characteristic frequency range of fault line mode voltage within the multi-terminal flexible DC grid area.

[0045] Figure 2 A topology diagram of a multi-terminal flexible DC grid is given. Figure 2 In the diagram, S1, S2, S3, and S4 are the AC power supply numbers; B 12 B 21 B 13 B 31 B 24 B 42 B 34 B 43 MMC1, MMC2, MMC3, and MMC4 are the converter station numbers; M, N, O, P, Q, R, S, and T are the protection device numbers. l 1. l2. l 3. l 4 represents the line number; taking protection N as an example, f1 represents a fault at the end of the zone via a transition resistor; f2 represents a near-end metallic fault outside the zone. (The last part, "for line N," appears to be a typo and can be left as is.) l For example, when a fault occurs in a multi-terminal flexible DC power grid transmission line, the voltage at the fault point will drop rapidly to zero. At this time, the fault point is equivalent to having a voltage superimposed on the normal load voltage. U dc Additional fault voltage sources with equal amplitude but opposite direction U f That is, satisfying U f =- U dc , U dc This is the system's rated voltage. Therefore, for a fault-dependent network, the fault-dependent voltage source will generate a fault traveling wave and propagate it to both ends of the line. Figure 2 Taking protection N in the example, when in the line l After a fault f1 occurs on 1, taking the linear mode component as an example, the traveling wave theory can be used to obtain... Figure 3 The faulty f1 frequency domain additional network shown is in Figure 2 middle, Z M1 ( ω ), Z M2 ( ω ), Z M3 ( ω )and Z M4 ( ω ) are the frequency domain impedances of converter stations MMC1, MMC2, MMC3 and MMC4, respectively; Z 1( ω ), Z 2( ω ), Z 3( ω )and Z 4( ω These are transmission lines. l 1. l 2. l 3. l 4. Frequency domain impedance; ω Angular frequency; L d1 , L d2 , L d3 and L d4 Transmission lines l 1. l2. l 3. l 4 current-limiting inductors; U MM To protect the voltage at point M; U ff The voltage at the fault point; R g This is the fault transition resistor.

[0046] Depend on Figure 3 Can be drawn Figure 4 The diagram shows the equivalent circuit of the node voltage for fault f1. In the figure, 1, 2, 3, 4, and 5 represent node numbers and corresponding node voltages. U 1. U 2. U 3. U 4 and U 5, U 1 corresponds to the voltage at MM1 on the MMC1 bus of the converter station. U 2. Voltage at the corresponding fault point U 3 corresponds to the voltage at MM2 on the MMC2 bus of the converter station. U 4 corresponds to the voltage at MM4 on the MMC4 bus of the converter station. U 5 corresponds to the voltage at MM3 on the MMC3 bus of the converter station, and the currents of each branch are numbered sequentially as follows: I 1. I 2. I 3. I 4. I 5. I 6. I 7. I 8. I 9 and I 10 , I 1 represents the current flowing from bus MM1 of converter station MMC1 to protection point M. I 2 represents the current flowing from bus MM1 to bus MM3 of the converter station. I 3 represents the current flowing to the converter station MMC1. I 4 represents the current flowing to the fault point. I 5 represents the current flowing from the fault point to the MMC2 busbar of the converter station. I 6 represents the current flowing to converter station MMC2. I 7 represents the current flowing to the MMC4 busbar of the converter station. I 8 represents the current flowing to converter station MMC4. I 9 represents the current flowing to the MMC3 busbar of the converter station. I 10 This represents the current flowing to the converter station MMC3.

[0047] Furthermore, according to Kirchhoff's laws, the frequency domain equations for the voltages at each node can be obtained, as shown in equation (1). Equation (1) can be solved to obtain... U ff The voltage expression, that is, the node voltage. U 2. Simultaneously, considering that the reverse traveling wave of the fault can be ignored when calculating the initial fault voltage at the protection measurement point, and only the propagation characteristics of the forward traveling wave along the transmission line need to be considered, then the line-mode transient voltage at protection point N during a fault within the zone is... U NN As shown in equation (2).

[0048] (1) (2) Similarly, with Figure 2 Taking protection N in the example, when in the line l After a fault f2 occurs on 1, taking the linear mode component as an example, the traveling wave theory can be used to obtain... Figure 5 The faulty f2 frequency domain additional network shown is composed of Figure 5 Can be drawn Figure 6 The node voltage equivalent circuit of fault f2 shown is in Figure 6 In the diagram, 1, 2, 3, and 4 represent node numbers and corresponding node voltages. U 1. U 2. U 3 and U 4, of which, U 1 indicates the voltage at bus MM1 of station MMC1. U 2 indicates the voltage at bus MM2 of MMC2 station. U 3 indicates the voltage at bus MM4 of station MMC4. U 4 represents the voltage at bus MM3 of MMC3 station, and the currents of each branch are numbered sequentially as follows: I 1. I 2. I 3. I 4. I 5. I 6 and I 7, of which, I 1 indicates the current flowing from the MMC1 busbar to protection point M. I 2 represents the current flowing to the MMC2 station. I 3 indicates the current flowing from the MMC2 bus to the MMC3 converter station. I 4 indicates the current flowing to converter station MMC4. I 5 represents the current flowing from converter station MMC3 to converter station MMC1. I 6 indicates the current flowing to converter station MMC3. I7 represents the current flowing from converter station MMC3 to converter station MMC4. According to Kirchhoff's laws, the frequency domain equations for the voltages at each node can be obtained, as shown in equation (3). From equation (3), the voltage at protection point N can be further obtained. U NW The frequency domain equation is shown in equation (4).

[0049] (3) (4) Based on equations (2) and (4), by using typical transmission line parameters and setting different transition resistances, the results are obtained. U NN Amplitude greater than U NW The frequency range of the amplitude, such as Figure 7 As shown, this frequency band range is the characteristic frequency range of the fault within the area. Under the condition of a 20kHz signal sampling frequency, considering a certain margin, the characteristic frequency range of the line-mode voltage fault of high-impedance fault (maximum transition resistance is 1500Ω) within the area can be selected as 2000Hz~10kHz; Step 2: Collect positive voltage data at multiple different times at the installation location of the multi-terminal flexible DC transmission line protection system. U p ( t ) and negative voltage U n ( t ); Step 3: Utilize the positive fault sudden voltage U pm ( t ) and negative electrode fault sudden voltage U nm ( t Determine whether the protection is activated and obtain and analyze the positive voltage. U pf ( t ) and analysis of negative electrode voltage U nf ( t ); Step 3.1 Using positive voltage U p ( t ) and negative voltage U n ( t Obtain the positive fault sudden change voltage U pm ( t ) and negative electrode fault sudden voltage U nm ( t ), Upm ( t )and U nm ( t The calculation methods are as follows:

[0050]

[0051]

[0052]

[0053] in, t For the sampling time point, U rp The positive rated voltage, U rn The rated voltage is the negative terminal. Step 3.2 If detected U pm ( t )or U nm ( t () Greater than the rated voltage of the system's positive terminal U rp of sx Times, in this invention sx If the value is set to 0.06, a disturbance is determined to have occurred in the multi-terminal flexible DC power grid. Record the value at that moment. t fs Proceed to step 4; otherwise, return to step 2. Step 4: Analyze the positive voltage U pf ( t ) and analysis of negative electrode voltage U nf ( t To determine the faulty pole; Step 4.1 If detected U pm ( t )or U nm ( t (Greater than the system's positive electrode rated) sx times, sx If the value is set to 0.06, a disturbance is determined to have occurred in the multi-terminal flexible DC power grid. Record the value at that moment. t fs And obtain and analyze the positive electrode voltage. U pf ( t ) and analysis of negative electrode voltage U nf ( tThis will be used for subsequent fault polarity detection and fault area detection. U pf ( t )and U nf ( t )for:

[0054]

[0055]

[0056]

[0057] in, for The positive voltage of a multi-terminal flexible DC transmission line at any given time. for The negative voltage of a multi-terminal flexible DC transmission line at any given time. , These are multi-terminal flexible DC transmission lines in The average rated voltage of the positive electrode and the average rated voltage of the negative electrode within the interval, where S is the number of seconds.

[0058] Step 4.2 Utilize the analysis of positive electrode voltage U pf ( t ) and analysis of negative electrode voltage U nf ( t Calculate the zero-mode voltage U Z ( t ), U Z ( t The calculation method for ) is as follows:

[0059] Step 4.3 Utilize U Z ( t Calculate the accumulated zero-mode voltage U ZO , U ZO The calculation method is as follows:

[0060] Step 4.4 If U ZO Less than the voltage accumulation threshold - U jx If so, it is determined to be a positive ground fault; if U ZOGreater than the voltage accumulation threshold U jx If the fault is between - U jx and U jx If the polarity is between these values, a bipolar fault is determined. In this invention, U jx The value is 200kV. U jx The value is -200kV.

[0061] Step 5: Analyze the positive voltage U pf ( t ), Analysis of negative electrode voltage U nf ( t The system uses multivariate variational mode decomposition (MVMD) and other algorithms to determine the fault region. Step 5.1 Analyze the positive voltage U pf ( t ) and analysis of negative electrode voltage U nf ( t To calculate the line-mode voltage of a multi-terminal flexible DC transmission line. U M ( t The calculation method is as follows:

[0062] Step 5.2 Check the line-mode voltage U M ( t Normalize the voltage to obtain the normalized line-mode voltage. U GM ( t The calculation method is as follows:

[0063] in, M ma The maximum value after normalization. M mi The minimum value after normalization, max( U M ( t ))express U M ( t The maximum value of ), min( U M ( t))express U M ( t The minimum value of ), in this invention, M ma =1, M mi It is -1.

[0064] Step 5.3 Using MVMD U GM ( t )conduct cn Layer decomposition can obtain cn Each time-frequency component SP cn ( t Based on the fault characteristic frequency range within the multi-terminal flexible DC power grid area pre-obtained using the full component analysis method, the first... Kn Each time-frequency component is a characteristic component. F c ( t ), cn It is 8. Kn It is 4; the first Kn The frequency range of each time-frequency component must be greater than or equal to the minimum value of the fault characteristic frequency range within the pre-acquired multi-terminal flexible DC power grid area; in this invention, the signal sampling frequency is 20kHz, and the pre-acquired fault characteristic frequency range within the multi-terminal flexible DC power grid area is set to 2000Hz~10kHz. cn Select 8. Kn Choosing 4, the frequency range of the fourth time-frequency component is greater than 2000Hz. 5.4 pairs U M ( t First-order difference calculations are performed to obtain the difference line modulus. U DM ( t ), U DM ( t The calculation method for ) is as follows:

[0065] In the formula, f s The sampling frequency of the polarity voltage is, in this invention, f s Set the frequency to 20kHz.

[0066] 5.5 Utilization F c ( t )and U DM ( t Calculate adaptive voltage peak AP, AP The calculation method is as follows:

[0067] In the formula, max( F c ( t ))express F c ( t The maximum value of ), max( U DM ( t ))express U DM ( t The maximum value of ).

[0068] 5.6 If AP Greater than the fault area detection threshold ε If the fault is detected, it is determined to be an intra-zone fault; otherwise, it is determined to be an extra-zone fault. In this invention, ε Set it to 1100.

[0069] Select Figure 1 The multi-terminal flexible DC grid topology shown is illustrated in Table 1, with specific parameters listed below. l 1. l 2. l 3 and l The lengths of the four lines are 227km, 126km, 219km and 66km respectively, and the transmission line parameters are shown in Table 2.

[0070] Table 1 Parameters of Multi-Terminal Flexible DC Grid

[0071] Based on the parameters shown in Tables 1 and 2, taking protection N as an example, the voltage at the end of the zone after passing through different transition resistors can be plotted. U NN The amplitude-frequency curve of the proportional coefficient between the fault and the auxiliary power supply can also be used to plot near-end metallic faults outside the fault zone. U NW The amplitude-frequency curve of the proportional coefficient between the faulty accessory power supply and the faulty accessory power supply, such as Figure 5 As shown. In Figure 5 As can be seen from this, when the frequency is greater than 2000Hz, even a 1500Ω high-resistance grounding fault at the end... U NN The proportionality coefficient between the auxiliary power supply and the faulty power supply will be greater than U NW The proportionality coefficient between the fault-addition power supply and the fault-specific power supply. Therefore, the fault characteristic frequency range within the preset zone is set to be greater than 2000Hz.

[0072] Table 2 Transmission Line Parameters

[0073] Next, we will take typical operating conditions A and B as examples to further illustrate the method of the present invention.

[0074] Operating condition A: Output power is rated power, fault location is f1, positive terminal is grounded, and transition resistance is 1500Ω; Operating condition B: Output power is rated power, fault location is f2, bipolar short circuit, metallic fault.

[0075] When operating under conditions A and B Figure 8 This refers to the positive voltage at each protection point of the multi-terminal flexible DC power grid. Figure 9 This refers to the negative voltage at each protection point of the multi-terminal flexible DC power grid. Figure 10 These are the characteristic components of a multi-terminal flexible DC power grid operating under condition A. Figure 11 The amplitude-frequency diagrams of the characteristic components at point N of the protection device are shown under operating conditions A and B. Figure 12 These are the characteristic components of a multi-terminal flexible DC power grid operating under condition B.

[0076] Based on the positive and negative voltages of operating conditions A and B, it can be seen that the voltage amplitude of operating condition B is greater than that of operating condition A. While voltage amplitude alone is insufficient for effectively detecting fault areas, both conditions can be effectively started, allowing for accurate analysis of the positive and negative voltages.

[0077] For operating condition A, utilizing Figure 8 and Figure 9 The accumulated zero-mode voltages at protection points M, N, O, P, Q, R, S, and T are -1046.07kV, -1123.96kV, -1011.73kV, -930.40kV, -793.67kV, -749.21kV, -1228.23kV, and -912.20kV, respectively. These values ​​are all much lower than -200kV, indicating a positive ground fault, consistent with the preset fault. Furthermore, the line-mode voltage at each protection point is decomposed into eight layers using MVMD, and the fourth-layer time-frequency component is obtained as the characteristic component, such as... Figure 10 As shown. Figure 11 The Fourier amplitude-frequency diagrams of the characteristic components at point N of the protection device are given under operating condition A. Figure 11 This indicates that the time-frequency component effectively includes frequency components greater than 2000Hz, thus enabling the acquisition of adaptive voltage peak values ​​at each protection point. AP The values ​​are 4382.07, 8750.78, 7.64, 20.81, 33.03, 1.13, 0.28, and 0.20, respectively. Only the values ​​at protection M and protection N are among these. APThe value is greater than the threshold of 1100, which indicates that the proposed single-ended quantity protection method can effectively determine the fault area.

[0078] For condition B, utilizing Figure 8 and Figure 9 The accumulated zero-mode voltages at protection points M, N, O, P, Q, R, S, and T are -7.6419kV, -3.8024kV, -8.8343kV, -8.4493kV, -4.0884kV, -4.4436kV, -4.9976kV, and -8.1113kV, respectively. These values ​​are all between -200kV and 200kV, indicating a bipolar fault, consistent with the preset fault. Furthermore, the MVMD method is used to perform an 8-level decomposition of the line-mode voltage at each protection point in condition B, selecting the fourth-level component as the characteristic component, such as... Figure 12 As shown. This allows for the acquisition of adaptive voltage peak values ​​at each protection point. AP The values ​​are 780.81, 189.3, 9.20, 15.50, 189.43, 608.00, 6.56, and 1.621, respectively. All these values ​​are less than the threshold of 1100 and are therefore classified as out-of-area faults, consistent with the actual settings. Additionally, Figure 11 The Fourier amplitude-frequency diagram of the characteristic components at point N of the protection under operating condition B is also given. The amplitude of operating condition B is much smaller than that of operating condition A, which also shows the correctness of the theoretical analysis results.

[0079] This disclosure can be a system, method, and / or computer program product. A computer program product may include a computer-readable storage medium having computer-readable program instructions loaded thereon for causing a processor to implement various aspects of this disclosure.

[0080] Computer-readable storage media can be tangible devices capable of holding and storing instructions for use by an instruction execution device. Computer-readable storage media can be, for example—but not limited to—electrical storage devices, magnetic storage devices, optical storage devices, electromagnetic storage devices, semiconductor storage devices, or any suitable combination of the foregoing. More specific examples (a non-exhaustive list) of computer-readable storage media include: portable computer disks, hard disks, random access memory (RAM), read-only memory (ROM), erasable programmable read-only memory (EPROM or flash memory), static random access memory (SRAM), portable compact disc read-only memory (CD-ROM), digital multifunction disc (DVD), memory sticks, floppy disks, mechanical encoding devices, such as punch cards or recessed protrusions storing instructions thereon, and any suitable combination of the foregoing. The computer-readable storage media used herein are not to be construed as transient signals themselves, such as radio waves or other freely propagating electromagnetic waves, electromagnetic waves propagating through waveguides or other transmission media (e.g., light pulses through fiber optic cables), or electrical signals transmitted through wires.

[0081] The computer-readable program instructions described herein can be downloaded from computer-readable storage media to various computing / processing devices, or downloaded via a network, such as the Internet, local area network, wide area network, and / or wireless network, to an external computer or external storage device. The network may include copper transmission cables, fiber optic transmission, wireless transmission, routers, firewalls, switches, gateway computers, and / or edge servers. A network adapter card or network interface in each computing / processing device receives the computer-readable program instructions from the network and forwards them to the computer-readable storage media in the respective computing / processing device.

[0082] Computer program instructions used to perform the operations of this disclosure may be assembly instructions, instruction set architecture (ISA) instructions, machine instructions, machine-dependent instructions, microcode, firmware instructions, status setting data, or source code or object code written in any combination of one or more programming languages, including object-oriented programming languages ​​such as Smalltalk, C++, etc., and conventional procedural programming languages ​​such as the "C" language or similar programming languages. The computer-readable program instructions may execute entirely on the user's computer, partially on the user's computer, as a standalone software package, partially on the user's computer and partially on a remote computer, or entirely on a remote computer or server. In cases involving a remote computer, the remote computer may be connected to the user's computer via any type of network—including a local area network (LAN) or a wide area network (WAN)—or may be connected to an external computer (e.g., via the Internet using an Internet service provider). In some embodiments, electronic circuitry, such as programmable logic circuitry, field-programmable gate arrays (FPGAs), or programmable logic arrays (PLAs), is personalized by utilizing the status information of the computer-readable program instructions to implement various aspects of this disclosure.

[0083] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention should be covered within the protection scope of the claims of the present invention.

Claims

1. A protection method for flexible DC transmission lines based on high-frequency modulus components, characterized in that, include: S1: Based on the system topology and transmission line parameters, the characteristic frequency range of fault line mode voltage in the multi-terminal flexible DC power grid area is obtained in advance by using full component analysis; S2: Collect the positive and negative voltages at multiple different times at the installation location of the multi-terminal flexible DC transmission line protection; where the multi-terminal flexible DC transmission line is a DC transmission line with single-terminal quantity protection. S3: Detect grid disturbances based on the sudden changes in positive and negative voltages. If a disturbance occurs, proceed to step 4 and record the disturbance time; otherwise, return to S2. S4: Modulus voltage is calculated based on the recorded disturbance time. The modulus voltage calculation includes the calculation of accumulated zero-mode voltage and line-mode voltage. The calculated accumulated zero-mode voltage, line-mode voltage, pre-acquired characteristic frequency range of fault line-mode voltage in the region, and multidimensional variational mode decomposition are used to detect fault poles and fault regions. The modulus voltage includes zero-mode voltage and line-mode voltage.

2. The flexible DC transmission line protection method based on high-frequency modulus components according to claim 1, characterized in that: In S1, based on the topology of the multi-terminal flexible DC grid, for faults in the high-resistivity zone at the end of the transmission line and faults outside the near-field metallic zone of the transmission line, the line-mode node voltage equivalent circuits of the fault frequency domain supplementary network, including all modular multilevel converter stations and all components at both ends of the line, are drawn respectively. Write the loop voltage equations for the equivalent circuits of each line-mode node voltage and solve for the node voltages to obtain the frequency domain expressions of the line-mode voltages for faults in the high-resistivity region at the end of the transmission line and faults outside the near-metallic region. Plot the line-mode voltage amplitude curves for faults in the high-resistivity region at the end of the transmission line and faults outside the near-metallic region using the transmission line parameters to obtain the characteristic frequency range of the line-mode voltage for faults in the high-resistivity region at the end of the transmission line.

3. The flexible DC transmission line protection method based on high-frequency modulus components according to claim 1, characterized in that: In S3, when the positive voltage changes abruptly... U pm ( t or negative electrode voltage sudden change U nm ( t When the value exceeds the disturbance threshold, a disturbance is determined to have occurred in the DC power grid, and this moment is recorded and defined as the disturbance occurrence moment. t fs The disturbance threshold is taken as the rated value of the positive electrode of the system. sx The ratio is 1.5 times, among which the selection is based on the principle that even the largest possible single-pole grounding fault transition resistance in a multi-terminal flexible DC power grid can ensure normal startup. sx The value of .

4. The flexible DC transmission line protection method based on high-frequency modulus components according to claim 3, characterized in that: Positive voltage change U pm ( t ), negative electrode voltage change U nm ( t Calculated as follows: ； ； in, t Time, in seconds. f s Sampling frequency, t d This refers to the current moment.

5. The protection method for multi-terminal flexible DC transmission lines based on high-frequency modulus components according to claim 1 or 4, characterized in that: In S4, the accumulated zero-mode voltage is calculated as follows: Acquire the positive electrode voltage 0.5ms before and 0.5ms after the disturbance. and analysis of negative electrode voltage ; Calculate zero-mode voltage and accumulated zero-mode voltage ; ； ； in, This represents the moment of DC grid disturbance.

6. The flexible DC transmission line protection method based on high-frequency modulus components according to claim 1, characterized in that: In S4, fault detection is performed using the following method: If the zero-mode voltage is accumulated U ZO Less than the voltage accumulation threshold - U jx If so, it is determined to be a positive ground fault; if U ZO Greater than the voltage accumulation threshold U jx If the fault is between - U jx and U jx If the value is between these two values, then a bipolar fault is determined.

7. The protection method for multi-terminal flexible DC transmission lines based on high-frequency modulus components according to claim 6, characterized in that: Voltage accumulation threshold U jx The value is 200kV. U jx The value is -200kV.

8. The flexible DC transmission line protection method based on high-frequency modulus components according to claim 6, characterized in that: The following methods are used for fault area detection: Calculate the line-mode voltage after DC grid disturbance U M ( t ), and obtain the normalized line-mode voltage. U GM ( t ); Using MVMD U GM ( t )conduct cn Layer decomposition, to obtain cn Each time-frequency component SP cn ( t Based on the fault characteristic frequency range within the multi-terminal flexible DC power grid area obtained in advance through full component analysis, the first... Kn Each time-frequency component is a characteristic component. F c ( t ), No. Kn The frequency range of each time-frequency component must be greater than or equal to the minimum value of the fault characteristic frequency range within the pre-obtained multi-terminal flexible DC power grid area; Linear mode voltage U M ( t Perform first-order difference calculations to obtain the difference line modulus. U DM ( t ); Calculate adaptive voltage peak AP ; In the formula, max( F c ( t ))express F c ( t The maximum value of ), max( U DM ( t ))express U DM ( t The maximum value of ). when AP Greater than the fault area detection threshold ε If the fault is within the zone, it is determined to be a fault within the zone; otherwise, it is determined to be a fault outside the zone.

9. The flexible DC transmission line protection method based on high-frequency modulus components according to claim 8, characterized in that: The fault area detection threshold ε The value is 1100kV.

10. A flexible DC transmission line protection system based on the protection method of any one of claims 1-9, comprising a fault line mode voltage characteristic frequency range acquisition module, a multi-terminal flexible DC transmission line electrical quantity acquisition module, a disturbance start judgment module, an accumulated zero-mode voltage calculation module, and a fault pole and fault area detection module; characterized in that: The fault line mode voltage characteristic frequency range acquisition module obtains the fault line mode voltage characteristic frequency range in advance within the multi-terminal flexible DC power grid area based on the system topology and transmission line parameters and using full component analysis. The electrical quantity acquisition module for multi-terminal flexible DC transmission lines collects the positive and negative voltages at multiple different times at the protection installation points of multi-terminal flexible DC transmission lines. The disturbance initiation judgment module detects power grid disturbances based on the sudden changes in positive and negative voltage. The fault pole and fault area detection module calculates the accumulated zero-mode voltage after a power grid disturbance occurs, and determines the fault pole based on the accumulated zero-mode voltage; it calculates the adaptive voltage peak after the disturbance based on the pre-acquired characteristic frequency range of the fault line-mode voltage in the area and the line-mode voltage, and determines the fault inside or outside the area based on the adaptive voltage peak.

11. A computer-readable storage medium for storing one or more programs, characterized in that: The one or more programs include instructions that, when executed by a computing device, cause the computing device to perform any of the methods according to claims 1 to 9.

12. A computing device, characterized in that, include: One or more processors, one or more memories, and one or more programs, wherein the one or more programs are stored in the one or more memories and configured to be executed by the one or more processors, the one or more programs including instructions for performing any of the methods according to claims 1 to 9.

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