Extra-high voltage three-terminal hybrid direct current transmission line protection method and system based on MVMD

Abstract

The invention discloses an extra-high voltage three-terminal hybrid direct current transmission line protection method and system based on MVMD. The method comprises the following steps: calculating a protection starting criterion, a fault direction criterion, an intra-region and extra-region criterion and a fault pole selection criterion of a direct current transmission line; collecting line mode current signals of lines on two sides of a T area of the direct current transmission line, inputting the current signals into a protection starting criterion, and judging whether a fault occurs or not; when it is judged that the fault occurs, the line mode current signal is input into a fault direction criterion, and the fault direction is judged; after the fault direction is judged, the zero-crossing rate of the line mode current break variable IMF5 of the fault side is calculated, the zero-crossing rate is input into the internal and external criteria, and a fault occurrence area is judged; and acquiring an anode current instantaneous variable quantity and a cathode current instantaneous variable quantity in a time window when the fault occurs, calculating a ratio of the anode current instantaneous variable quantity to the cathode current instantaneous variable quantity, inputting the ratio into a fault pole selection criterion, and determining a fault result.

Description

Technical Field

[0001] The present invention belongs to the technical field of high-voltage direct current transmission line protection in an electric power system, and in particular relates to a protection method and system for an ultra-high voltage three-terminal hybrid direct current transmission line based on MVMD. Background Art

[0002] Ultra-high voltage direct current (HVDC) technology plays a key role in large-scale, long-distance power transmission. Modular multilevel converter (MMC) and hybrid direct current transmission technology (LCC+MMC) combine the advantages of phase-commutated converter (LCC) and modular multilevel converter (MMC) and are increasingly widely used. However, DC line faults have become a common problem in direct current systems. Hybrid direct current transmission systems have flexible operation modes and complex topological structures, which pose severe challenges to line protection reliability and rapid and accurate fault identification.

[0003] At present, there have been many studies on the protection of UHV three-terminal hybrid DC transmission lines at home and abroad. Most of the existing protection schemes use the differences between the inside and outside of the area and the different characteristics of the fault direction to construct the judgment criteria, and some combine intelligent algorithms to realize fault identification, but they all have defects such as poor resistance resistance, inability to identify T-zone faults, and poor resistance resistance. Therefore, it is necessary to propose a new protection scheme to solve the above problems and improve the reliability, accuracy and adaptability of the protection of UHV three-terminal hybrid DC transmission lines. Summary of the invention

[0004] The present invention aims to solve the deficiencies of the prior art and provides the following solutions:

[0005] The MVMD-based ultra-high voltage three-terminal hybrid direct current transmission line protection method comprises the following steps:

[0006] Calculate the protection start-up criteria, fault direction criteria, in-zone and out-of-zone criteria, and fault pole selection criteria of DC transmission lines;

[0007] Collecting line mode current signals of the lines on both sides of the T zone of the DC transmission line, inputting the current signals into the protection start criterion, and judging whether a fault occurs;

[0008] When it is determined that a fault occurs, the line mode current signal is input into the fault direction judgment criterion to determine the fault direction;

[0009] After the fault direction is determined, the zero-crossing rate of the line mode current mutation amount IMF5 on the fault side is calculated, and the zero-crossing rate is input into the inside and outside zone judgment criteria to determine the fault occurrence zone;

[0010] The instantaneous change of the positive current and the instantaneous change of the negative current within the time window of the fault occurrence are obtained, the ratio of the instantaneous change of the positive current to the instantaneous change of the negative current is calculated, and the ratio is input into the fault pole selection criterion to clarify the fault result.

[0011] Preferably, the protection start criterion includes:

[0012] Compare the mutation amounts of the line mode current signals of the two lines, select the side with the larger mutation amount as the fault judgment basis, and then judge the size of the mutation amount and the starting integral value to decide whether to start the protection:

[0013] max(Δi L , Δi R )>Δi set

[0014] Among them, max means taking the maximum value, Δi L Indicates the line mode current mutation on the left side of the T zone, Δi R Indicates the line mode current mutation on the right side of the T zone, Δi set Indicates the start integer value;

[0015] When the sudden change of the line mode current signal is greater than its preset ratio, it is determined that a line fault occurs.

[0016] Preferably, the fault direction criterion includes:

[0017] Calculate the signal energy of the mutation amount of the line mode current signal:

[0018]

[0019] Where E represents the signal energy, x[n] represents the sample value of the discrete signal in the signal, N represents the total number of samples of the signal, and n represents the sample of the signal;

[0020] The difference rate of signal energy is calculated based on the signal energy:

[0021]

[0022] Among them, P represents the difference rate, E L Represents the energy of the line mode current mutation on the left, E R Indicates the energy of the line mode current mutation on the left, and max indicates the maximum value;

[0023] Determine the fault direction based on the difference rate:

[0024]

[0025] Among them, Δ setT1 Indicates the setting value of the fault direction criterion.

[0026] Preferably, the in-zone and out-zone criteria include:

[0027] Performing MVMD decomposition on the line mode current signal to obtain a decomposed signal;

[0028] Calculate the IMF5 zero-crossing rate of the decomposed signal:

[0029]

[0030] Where C represents the IMF5 zero-crossing rate, n' represents the sample of the decomposed signal, x[n'] represents the sample value of the discrete signal in the signal, N represents the total number of samples of the signal, and sgn() represents the sign function;

[0031] Based on the IMF5 zero-crossing rate, the faults inside and outside the area are judged:

[0032]

[0033] Among them, Δ setT2 Indicates the fault protection setting value.

[0034] Preferably, the fault pole selection criterion includes:

[0035]

[0036] Wherein, K represents the ratio of the instantaneous change of the positive electrode current to the instantaneous change of the negative electrode current.

[0037] The present invention also provides a UHV three-terminal hybrid DC transmission line protection system based on MVMD, the system applies any of the above methods, including: a criterion calculation module, a fault occurrence judgment module, a fault direction judgment module, a fault occurrence area judgment module and a fault pole selection judgment module;

[0038] The criterion calculation module is used to calculate the protection start-up criterion, fault direction criterion, inside-outside-area criterion and fault pole selection criterion of the DC transmission line;

[0039] The fault occurrence judgment module is used to collect line mode current signals of the lines on both sides of the T zone of the DC transmission line, input the current signals into the protection start judgment criterion, and judge whether a fault occurs;

[0040] When it is determined that a fault occurs, the fault direction determination module inputs the line mode current signal into the fault direction determination criterion to determine the fault direction;

[0041] After the fault direction is determined, the fault occurrence area determination module calculates the zero-crossing rate of the line mode current mutation amount IMF5 on the fault side, and inputs the zero-crossing rate into the zone inside and outside judgment criteria to determine the fault occurrence area;

[0042] The fault pole selection judgment module obtains the instantaneous change of the positive current and the instantaneous change of the negative current within the time window of the fault occurrence, calculates the ratio of the instantaneous change of the positive current to the instantaneous change of the negative current, and inputs the ratio into the fault pole selection judgment criterion to clarify the fault result.

[0043] Compared with the prior art, the present invention has the following beneficial effects:

[0044] (1) High accuracy and reliability: The present invention can accurately identify the fault direction, fault inside and outside the zone, and fault polarity under different fault locations, transition resistances, and fault types. The high-frequency intrinsic mode components are extracted through multivariate variational mode decomposition (MVMD), and the fault is identified by combining the zero-crossing rate difference, ensuring the rapidity, accuracy, and reliability of fault judgment;

[0045] (2) Only current is needed, and the equipment is simple: The present invention only needs current to identify the fault, and does not require complex voltage or other signal measurement equipment, thus simplifying the system structure. The fault direction is determined by using the energy difference of the current line mode components on both sides of the T zone, and the fault area is distinguished by combining the difference in the zero-crossing rate of the intrinsic mode components of the fault inside and outside the zone, further simplifying the fault identification process;

[0046] (3) Strong anti-interference ability and strong adaptability: The present invention utilizes the attenuation characteristics of high-frequency transient signals of the boundary elements of the DC transmission line, which can effectively resist noise interference and has strong robustness. At the same time, the method has low requirements on sampling frequency, high resistance resistance, simple calculation, strong adaptability, and can adapt to complex actual working conditions;

[0047] (4) Fast full-line protection: The present invention can realize fast full-line protection of UHV three-terminal hybrid DC transmission lines, significantly improving the reliability and stability of the transmission lines. By real-time monitoring of the line mode current mutation on both sides of the T zone, combined with multivariate variational mode decomposition (MVMD) and zero-crossing rate difference, it can quickly and accurately identify faults and trigger protection actions;

[0048] (6) Broad engineering application prospects: The present invention is applicable to ultra-high voltage multi-terminal hybrid DC transmission systems, is easy to implement in actual projects, and has broad application prospects. Its simple equipment, low sampling frequency requirements, and strong anti-interference capabilities make it have high practical value in complex power systems.

[0049] In summary, the present invention can effectively solve the problems of the difficulty in accurately identifying faults in the T zone of the UHV three-terminal hybrid DC transmission line and the poor protection resistance tolerance capability. It also has the advantages of simple equipment, strong anti-interference ability, strong adaptability, etc., and has high engineering application value. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] In order to more clearly illustrate the technical solution of the present invention, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0051] Figure 1 A schematic diagram of a method flow of an embodiment of the present invention;

[0052] Figure 2 A topological structure diagram of a three-terminal hybrid direct current transmission system according to an embodiment of the present invention;

[0053] Figure 3 This is a fault equivalent circuit diagram of the left side of the T zone according to an embodiment of the present invention;

[0054] Figure 4 This is a fault equivalent circuit diagram of the right side of the T zone according to an embodiment of the present invention;

[0055] Figure 5 is a T-zone fault equivalent circuit diagram of an embodiment of the present invention;

[0056] Figure 6 It is the inner and outer boundary map of the line area on the north side of Kunming according to an embodiment of the present invention;

[0057] Figure 7 This is a graph showing the amplitude-frequency characteristics of the transfer function of the boundary element on the Kunbei rectifier side according to an embodiment of the present invention;

[0058] Figure 8 It is an inner and outer boundary diagram of the gantry side line area according to an embodiment of the present invention;

[0059] Fig. 9 The amplitude-frequency characteristic diagram of the transmission function of the boundary element on the gantry inverter side according to an embodiment of the present invention;

[0060] Fig.10 Schematic diagram of the Kunliulong ±800kV UHV three-terminal hybrid DC system model. DETAILED DESCRIPTION

[0061] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0062] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0063] First, let’s introduce the fault characteristics of the three-terminal hybrid DC transmission line:

[0064] The research object is the Kunliulong ±800kV UHV three-terminal hybrid DC transmission system. The sending end of the system is located on the Kunbei side, using LCC type converters, and the receiving end is on the Liubei side and Longmen side, using MMC type converters. The line and line are DC transmission lines. The protection device is installed on both sides of the T area, that is, M 1 、M 2 , used to measure the positive and negative currents on both sides of the T zone. Its topological structure is as follows Figure 2 shown.

[0065] When the fault occurs on the left side of the T zone, the equivalent circuit diagram of its fault component can be obtained, such as Figure 3 As shown. Among them, Z L1 and Z L2 Line L 1 and L 2 The equivalent impedance, U f is the additional line-mode voltage source at the fault point, R f is the transition resistance at the fault, ΔI L and ΔI R They are the line mode current fault components on the left and right sides of the T zone, Z LCC , Z MMC1 and Z MMC2 are the equivalent impedances of the converters at Kunbei, Liubei and Longmen sides, respectively. 1 is the composite equivalent impedance of the smoothing reactor and DC filter at Kunbei side, Z 2 and Z 3 Equivalent impedance of smoothing reactors on the Liubei side and the Longmen side.

[0066] We can get ΔI L and ΔI R The relationship is:

[0067]

[0068] According to the above formula, ΔI L Greater than ΔI R Substitute the system equivalent parameters into the formula to calculate ΔI L / ΔI R The value of is approximately 2.

[0069] When the fault occurs on the right side of the T zone, the equivalent circuit diagram of its fault component can be obtained, such as Figure 4 As shown. L and ΔI R The relationship is:

[0070]

[0071] ΔI L <ΔI R

[0072] In this case, ΔI R Greater than ΔI L , substitute the system equivalent parameters into the formula to calculate ΔI L / ΔI R The value of is about 0.38.

[0073] When the fault occurs in the T zone, the equivalent circuit diagram of its fault component is as follows: Figure 5 As shown. L and ΔI R The relationship is:

[0074]

[0075] Since line L 1 and line L 2 The rated current values ​​of the two types of rectifiers are different, and their wire diameters are also different. 1 , its wire diameter is thicker, which makes its corresponding impedance relatively small. 1 and L 2 There are some differences, but their impedance values ​​are still close. Therefore, when a fault occurs inside the T zone, line L 1 and L 2 The current change ΔI L and ΔI R It will be closer.

[0076] It can be seen that when the fault occurs in the non-T zone, the line mode current mutation on the fault side is much greater than that on the non-fault side; when the fault occurs in the T zone, the line mode current mutation on both sides is close, and this characteristic can be used to distinguish the fault direction.

[0077] The converter station at the sending end (Kunbei side) of the Kunliulong UHV three-terminal hybrid DC transmission system uses a traditional grid-commutated converter (LCC), which makes the DC system have large harmonics. For this reason, smoothing reactors and DC filters are installed on the line, which together constitute the physical boundary of the rectifier side. This boundary is as follows Figure 6 As shown in the figure, U 1 is the transient voltage outside the area, U 2 is the voltage after attenuation by the boundary element. The impedance of the smoothing reactor is:

[0078] Z 1 (jω)=jωL p1

[0079] Among them, Z 1represents the impedance of the smoothing reactor, j represents the imaginary unit, ω represents the angular frequency, L p1 Indicates the inductance of the smoothing reactor, which is 300mH;

[0080] The DC filter impedance is:

[0081]

[0082] Among them, Z 2 Indicates the DC filter impedance, parameter value: C 1 =1.2μF, L 1 =9.345mH, C 2 =2.84μF, L 2 =15.919mH, C 3 =2.647μF, L 3 =4.656mH, || indicates parallel connection;

[0083] In order to study the transmission characteristics of the boundary element, the line side is assumed to be open circuit and the inverter side boundary transfer function G is defined as 1 (jω) is:

[0084]

[0085] Substituting specific parameters, the amplitude-frequency characteristic curve of the transfer function of the Kunbei rectifier boundary element can be obtained, such as Figure 7 As shown, when the frequency f is less than 100 Hz, the value of the transfer function is approximately 1, and when 100 Hz is less than f is less than 2000 Hz, it reaches several extreme values; when the frequency f is greater than 2000 Hz, the value of the transfer function is close to 0, which indicates that the boundary element composed of the DC filter and the smoothing reactor has a significant attenuation effect on high-frequency transient signals.

[0086] The receiving end (Longmen side) converter station of the Kunliulong UHV multi-terminal hybrid DC transmission system adopts modular multilevel converter (MMC). The smoothing reactor installed at the end of L2 is used to form the inverter side boundary at the end of the line. Figure 8 shown.

[0087] Assume the line side is in an open circuit state and define the inverter side boundary transfer function G 2 (jω) is:

[0088]

[0089] Among them, U 3 Indicates the transient voltage outside the area, U 4 Represents the voltage after attenuation of the boundary element, L p2 Indicates the inductance value of the current limiting reactor, which is 150mH, G gIt represents a capacitance to ground, which is 0.006208μF.

[0090] Substituting the specific parameters into the input, we can obtain the amplitude-frequency characteristic curve of the transfer function of the boundary element on the gantry inverter side, such as Fig. 9 As shown in the figure, when the frequency f is less than 1300Hz, the value of the transfer function is about 1; when 1300<f<11000Hz, the amplitude-frequency characteristic curve changes suddenly; when f>11000Hz, the amplitude of the transfer function is close to 0. This shows that the boundary element composed of the current limiting reactor and the DC transmission line capacitance to ground also has a significant attenuation effect on high-frequency transient signals.

[0091] Embodiment 1

[0092] In this embodiment, if Figure 1 As shown, the UHV three-terminal hybrid DC transmission line protection method based on MVMD includes the following steps:

[0093] S1. Calculate the protection start-up criteria, fault direction criteria, in-zone and out-of-zone criteria, and fault pole selection criteria for DC transmission lines.

[0094] S2. Collect the line mode current signals of the lines on both sides of the T zone of the DC transmission line, input the current signals into the protection start judgment criteria, and determine whether a fault occurs.

[0095] When a fault occurs in the DC system, the line currents on both sides of the T zone will change significantly. The protection start-up criteria include: comparing the mutation amount of the line mode current signal of the lines on both sides, selecting the side with the larger mutation amount as the fault judgment basis, and then judging the size of the mutation amount and the start integral value to decide whether to start the protection:

[0096] max(Δi L , Δi R )>Δi set

[0097] Among them, max means taking the maximum value, Δi L Indicates the line mode current mutation on the left side of the T zone, Δi R Indicates the line mode current mutation on the right side of the T zone, Δi set Indicates the start integer value; when the sudden change of the line mode current signal is greater than its preset ratio, it is determined that the line has a fault. In this embodiment, the preset ratio can be set to 10%.

[0098] S3. When it is determined that a fault has occurred, the line mode current signal is input into the fault direction judgment criterion to determine the fault direction.

[0099] When the fault current passes through the T zone, the line mode current on the left and right sides will differ. Therefore, it is proposed to use the line mode current energy difference on both sides of the line in the T zone as the criterion for fault direction determination. The line mode current mutation on both sides of the T zone is extracted within the set data window and its signal energy is calculated. The signal energy is usually defined as the total energy of the signal over time.

[0100] Fault direction criteria include:

[0101] Calculate the signal energy of the sudden change of the line mode current signal:

[0102]

[0103] Where E represents the signal energy, x[n] represents the sample value of the discrete signal in the signal, N represents the total number of samples of the signal, and n represents the sample of the signal;

[0104] Calculate the difference rate of signal energy based on signal energy:

[0105]

[0106] Among them, P represents the difference rate, E L Represents the energy of the line mode current mutation on the left, E R Indicates the energy of the line mode current mutation on the left, and max indicates the maximum value;

[0107] Determine the fault direction based on the difference rate:

[0108]

[0109] Among them, Δ setT1 Indicates the setting value of the fault direction criterion. The selection of this value should ensure that it is greater than the maximum possible value of the standard energy difference when a fault occurs in the T area, and less than the minimum value of the standard energy difference when a fault occurs on the lines on both sides of the T area. This setting principle is intended to ensure the accuracy and selectivity of the fault direction criterion, that is, in the face of various fault situations, it can accurately determine the location of the fault and avoid false operations. setT1 The most reasonable calculation is 30%.

[0110] S4. After the fault direction is determined, the zero-crossing rate of the line mode current mutation amount IMF5 on the fault side is calculated, and the zero-crossing rate is input into the zone inside and outside judgment criteria to determine the fault occurrence zone.

[0111] When the fault line mode current signal crosses a specific boundary, its IMF5 zero-crossing rate after MVMD decomposition shows obvious differences. In view of this phenomenon, it is proposed to use the difference in IMF5 zero-crossing rate detected by the signals on both sides of the boundary as a criterion for determining the specific location of the fault.

[0112] Zero Crossing Rate (ZCR) refers to the number of times a signal crosses the zero line in a given time window. It is an important feature in signal processing, used to describe the frequency of signal changes, especially in audio signals and time series analysis. It represents the number of times a signal crosses the zero value per unit time, and is often used to analyze the characteristics and dynamics of a signal. It refers to the number of times a signal crosses the zero line in a given time window.

[0113] The criteria for in-zone and out-of-zone include:

[0114] Perform MVMD decomposition on the line mode current signal to obtain a decomposed signal;

[0115] Calculate the IMF5 zero-crossing rate of the decomposed signal:

[0116]

[0117] Where C represents the IMF5 zero-crossing rate, n' represents the sample of the decomposed signal, x[n'] represents the sample value of the discrete signal in the signal, N represents the total number of samples of the signal, and sgn() represents the sign function;

[0118] Determine the fault inside and outside the zone based on the zero-crossing rate of IMF5:

[0119]

[0120] Δ setT2 =K rel *C f

[0121] Among them, Δ setT2 Indicates the fault protection setting value, K rel is the reliability coefficient, the value is 3, C f is the C value calculated by the protection when an out-of-zone fault occurs. When a line fault occurs outside the zone, the line protection device should be reliable and not act. Therefore, it is most reasonable to set the protection value according to the fault. In this embodiment, Δ setT2 Take 0.21.

[0122] S5. Obtain the instantaneous change of the positive current and the instantaneous change of the negative current within the time window of the fault occurrence, calculate the ratio of the instantaneous change of the positive current and the instantaneous change of the negative current, and input the ratio into the fault pole selection criterion to clarify the fault result.

[0123] When a unipolar fault occurs in the system, the fault has a relatively small impact on the non-faulty pole, and the change amplitude of its control characteristic quantity is smaller than that of the faulty pole; when the system encounters a bipolar fault, a symmetrical loop will be formed between the faulty lines. In this case, the currents of the positive and negative poles remain basically consistent, and the degree of change of the control characteristic quantities of the two poles is also roughly the same.

[0124] Based on the change characteristics of the bipolar control characteristic quantities under different fault conditions mentioned above, this characteristic can be used to carry out related work on fault polarity selection. The selection of fault polarity is discussed using the instantaneous change of the positive and negative currents within the time window. The ratio K is defined as the integral ratio of the absolute value of the instantaneous change of the positive current and the negative current within the time window:

[0125]

[0126] Among them, S p Indicates the instantaneous change of positive current, S n Indicates the instantaneous change of negative current.

[0127] When a bipolar fault occurs, the defined K value will approach 1. When a positive ground fault occurs, the K value will be greater than 1; and when a negative ground fault occurs, the q value will be less than 1. In order to ensure the accuracy and reliability of the judgment, a certain margin is reserved to construct the fault pole selection criterion, which includes:

[0128]

[0129] Wherein, K represents the ratio of the instantaneous change of the positive electrode current to the instantaneous change of the negative electrode current.

[0130] Embodiment 2

[0131] In this embodiment, the UHV three-terminal hybrid DC transmission line protection system based on MVMD includes: a criterion calculation module, a fault occurrence judgment module, a fault direction judgment module, a fault occurrence area judgment module and a fault pole selection judgment module.

[0132] The criterion calculation module is used to calculate the protection start-up criterion, fault direction criterion, inside-outside-zone criterion and fault pole selection criterion of the DC transmission line.

[0133] The fault occurrence judgment module is used to collect the line mode current signals of the lines on both sides of the T zone of the DC transmission line, input the current signals into the protection start judgment criteria, and judge whether a fault occurs.

[0134] When it is determined that a fault occurs, the fault direction determination module inputs the line mode current signal into the fault direction determination criterion to determine the fault direction.

[0135] After the fault direction is determined, the fault occurrence area determination module calculates the zero-crossing rate of the line mode current mutation amount IMF5 on the fault side, and inputs the zero-crossing rate into the zone inside and outside determination criteria to determine the fault occurrence area.

[0136] The fault pole selection judgment module obtains the instantaneous change of the positive current and the instantaneous change of the negative current within the time window of the fault occurrence, calculates the ratio of the instantaneous change of the positive current to the instantaneous change of the negative current, and inputs the ratio into the fault pole selection judgment criterion to clarify the fault result.

[0137] Embodiment 3

[0138] In order to better understand the purpose, structure and function of the present invention, Fig.10 , the present invention is further described in detail.

[0139] Simulation model: According to the parameters of the Kunliulong ±800kV UHV three-terminal hybrid DC transmission system, a UHV three-terminal hybrid DC transmission system model was built on the PSCAD / EMTDC platform. The system starts from Kunbei Converter Station in Yunnan, and the intermediate landing point and terminal are Liubei Converter Station in Guangxi and Longmen Converter Station in Guangdong respectively. The sending end is two single-pole dual 12-pulse LCC converters connected in series, and a constant DC current control method is adopted. Both receiving ends use full half-bridge submodule hybrid MMC converters, and the control methods are constant DC power control and constant DC voltage control respectively. The total length of the line is 912.5km, and the total length of the line is 552.5km.

[0140] The Kunliulong ±800kV UHV three-terminal hybrid DC system model built using PSCAD is shown in the figure. Fig.10 As shown, where f 1 Corresponding to the left side of the T zone, f 2 、f 3 and f 4 are the first, middle and end faults in the left area of ​​T area, f 5 Corresponding to the fault outside the right side of T zone, f 6 、f 7 and f 8 Corresponding to the fault in the right area of ​​​​the T area, f 9 Corresponds to T zone fault. P, N, D represent positive, negative and bipolar faults.

[0141] The model sampling frequency is 50kHz, and the fault is triggered at 3 seconds and lasts for 0.1 seconds. A 4ms data window is used for data analysis, and a total of 200 sampling points are taken to verify the effect of the scheme.

[0142] The simulation results of different fault locations and transition resistances are shown in Tables 1 to 3.

[0143] Table 1

[0144]

[0145] Table 2

[0146]

[0147] Table 3

[0148]

[0149] Case 1: Protection start-up criteria, fault direction criteria, zone in-zone criteria, and fault pole selection criteria

[0150] Outside the right side of the T zone (f 5-N ) fault as an example, the protection device M 1 With M 2 The measured line mode current mutation I L with I R Rapidly increase to meet the protection start criterion. Extract the 4ms data window for MVMD decomposition and calculate E L =36.801, E R =173.172, P=-78.75%, the fault direction is judged to be the right of T zone, the zero-crossing rate of IMF5 is calculated to be 0.060, it is judged to be an out-of-zone fault, K=0.226 is calculated, it is judged to be a negative pole fault, and the fault result is a negative pole fault outside the right side of T zone.

[0151] Case 2:

[0152] In the left area of ​​​​T zone (f 3-P ) fault as an example, the protection device M 1 With M 2 The measured line mode current mutation I L with I R Rapidly increase to meet the protection start criterion. Extract the 4ms data window for MVMD decomposition and calculate E L =366.232, E R =63.335, P = 82.71%, the fault direction is judged to be the left of T zone, the zero crossing rate of IMF5 is calculated to be 0.930, it is judged to be an out-of-zone fault, K is calculated to be 15.231, it is judged to be a positive pole fault, and the fault result is a positive pole fault in the left zone of T zone

[0153] The simulation data in Tables 1 to 3 are analyzed: in terms of fault direction judgment, the line mode current energy difference rate criterion on both sides of the T zone can correctly judge under different fault directions; in terms of internal and external fault judgment, the criterion of IMF5 zero-crossing rate difference is used, and the data in the table shows that the internal and external fault judgment is accurate; when selecting the fault pole, the criterion based on the K value is reliable, and the K value performance under different fault types is consistent with the theory; the proposed protection method can accurately judge the fault direction, distinguish internal and external faults, and correctly select the pole under various working conditions, verifying its reliability and effectiveness.

[0154] Table 4

[0155]

[0156] It can be seen from Table 4 that the P and C calculated at a 40 dB SNR are different from those without noise shown in Tables 1 to 3, but still meet the fault direction criterion and the in-zone and out-of-zone criterion. Therefore, the proposed protection has strong robustness under noise interference.

[0157] The embodiments described above are only descriptions of the preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Without departing from the design spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by ordinary technicians in this field should all fall within the protection scope determined by the claims of the present invention.

Claims

1. A UHV three-terminal hybrid DC transmission line protection method based on MVMD, characterized in that: The following steps are involved: Calculate the protection start-up criteria, fault direction criteria, in-zone and out-of-zone criteria, and fault pole selection criteria of DC transmission lines; Collecting line mode current signals of the lines on both sides of the T zone of the DC transmission line, inputting the current signals into the protection start criterion, and judging whether a fault occurs; When it is determined that a fault occurs, the line mode current signal is input into the fault direction judgment criterion to determine the fault direction; After the fault direction is determined, the zero-crossing rate of the line mode current mutation amount IMF5 on the fault side is calculated, and the zero-crossing rate is input into the inside and outside zone judgment criteria to determine the fault occurrence zone; The instantaneous change of the positive current and the instantaneous change of the negative current within the time window of the fault occurrence are obtained, the ratio of the instantaneous change of the positive current to the instantaneous change of the negative current is calculated, and the ratio is input into the fault pole selection criterion to clarify the fault result.

2. The UHV three-terminal hybrid DC transmission line protection method based on MVMD according to claim 1 is characterized in that: The protection start criteria include: Compare the mutation amounts of the line mode current signals of the two lines, select the side with the larger mutation amount as the fault judgment basis, and then judge the size of the mutation amount and the starting integral value to decide whether to start the protection: max(Δi L ,Δi R )>Δi set Among them, max means taking the maximum value, Δi L Indicates the line mode current mutation on the left side of the T zone, Δi R Indicates the line mode current mutation on the right side of the T zone, Δi set Indicates the start integer value; When the sudden change of the line mode current signal is greater than its preset ratio, it is determined that a line fault occurs.

3. The UHV three-terminal hybrid DC transmission line protection method based on MVMD according to claim 1 is characterized in that: The fault direction criterion includes: Calculate the signal energy of the mutation amount of the line mode current signal: Where E represents the signal energy, x[n] represents the sample value of the discrete signal in the signal, N represents the total number of samples of the signal, and n represents the sample of the signal; The difference rate of signal energy is calculated based on the signal energy: Among them, P represents the difference rate, E L Represents the energy of the line mode current mutation on the left, E R Indicates the energy of the line mode current mutation on the left, and max indicates the maximum value; Determine the fault direction based on the difference rate: Among them, Δ setT1 Indicates the setting value of the fault direction criterion.

4. The UHV three-terminal hybrid DC transmission line protection method based on MVMD according to claim 1 is characterized in that: The in-zone and out-of-zone criteria include: Performing MVMD decomposition on the line mode current signal to obtain a decomposed signal; Calculate the IMF5 zero-crossing rate of the decomposed signal: Where C represents the IMF5 zero-crossing rate, n' represents the sample of the decomposed signal, x[n'] represents the sample value of the discrete signal in the signal, N represents the total number of samples of the signal, and sgn() represents the sign function; Based on the IMF5 zero-crossing rate, the faults inside and outside the area are judged: Among them, Δ setT2 Indicates the fault protection setting value.

5. The UHV three-terminal hybrid DC transmission line protection method based on MVMD according to claim 1 is characterized in that: The fault selection criteria include: Wherein, K represents the ratio of the instantaneous change of the positive electrode current to the instantaneous change of the negative electrode current.

6. A UHV three-terminal hybrid DC transmission line protection system based on MVMD, wherein the system applies the method described in any one of claims 1 to 5, characterized in that: include: Judgment calculation module, fault occurrence judgment module, fault direction judgment module, fault occurrence area judgment module and fault pole selection judgment module; The criterion calculation module is used to calculate the protection start-up criterion, fault direction criterion, inside-outside-area criterion and fault pole selection criterion of the DC transmission line; The fault occurrence judgment module is used to collect line mode current signals of the lines on both sides of the T zone of the DC transmission line, input the current signals into the protection start judgment criterion, and judge whether a fault occurs; When it is determined that a fault occurs, the fault direction determination module inputs the line mode current signal into the fault direction determination criterion to determine the fault direction; After the fault direction is determined, the fault occurrence area determination module calculates the zero-crossing rate of the line mode current mutation amount IMF5 on the fault side, and inputs the zero-crossing rate into the zone inside and outside judgment criteria to determine the fault occurrence area; The fault pole selection judgment module obtains the instantaneous change of the positive current and the instantaneous change of the negative current within the time window of the fault occurrence, calculates the ratio of the instantaneous change of the positive current to the instantaneous change of the negative current, and inputs the ratio into the fault pole selection judgment criterion to clarify the fault result.

Images