Distance protection method, computer device and computer readable storage medium

Abstract

The present application relates to a distance protection method, computer equipment, and computer-readable storage medium. The method includes: obtaining power frequency current data, stray current data, current transformer parameters, and power system model data; obtaining the primary current of the current transformer based on the power frequency current data and the stray current data; obtaining the secondary current of the current transformer based on the primary current and the current transformer parameters; obtaining the bias magnetic quantity at the current moment based on the secondary current; when a transient fault occurs, obtaining the transient secondary current of the current transformer based on the bias magnetic quantity, the power system model data, and the current transformer parameters; using the transient secondary current as the input current of the impedance relay, the input current including a fundamental component and a harmonic component; and performing distance protection based on the fundamental component. This method can perform distance protection more comprehensively and accurately.

Description

Technical Field

[0001] The present application relates to the field of power grid technology, and in particular to a distance protection method, a computer device, and a computer-readable storage medium. Background Art

[0002] With the large-scale construction of urban rail transit, stray current leakage to the ground is becoming increasingly serious. Advances in power grid technology have led to the emergence of distance protection for transmission lines. Existing technologies primarily focus on the operating characteristics of distance protection under the influence of constant DC current, but their analysis of the factors and forms of influence affecting distance protection is not yet precise. Summary of the Invention

[0003] Based on this, it is necessary to provide a distance protection method, computer equipment and computer-readable storage medium that can more comprehensively and accurately analyze influencing factors and influencing forms in response to the above technical problems.

[0004] In a first aspect, the present application provides a distance protection method, the method comprising:

[0005] Obtain power frequency current data, stray current data, current transformer parameters and power system model data;

[0006] acquiring a primary current of a current transformer according to the power frequency current data and the stray current data;

[0007] Obtaining a secondary current of the current transformer according to the primary current and the current transformer parameter;

[0008] Obtaining the bias magnetic amount at the current moment according to the secondary current;

[0009] When a transient fault occurs, obtaining a transient secondary current of the current transformer according to the bias magnetic amount, the power system model data and the current transformer parameters;

[0010] Using the transient secondary current as the input current of the impedance relay, wherein the input current includes a fundamental component and a harmonic component;

[0011] Distance protection is performed according to the fundamental wave component.

[0012] In one embodiment, before obtaining the transient secondary current of the current transformer according to the bias magnetic amount, the power system model data and the current transformer parameters, the method further includes:

[0013] When no transient fault occurs, the secondary current at the next moment is obtained according to the bias magnetic amount, the primary current and the current transformer parameters.

[0014] In one embodiment, obtaining the secondary current at the next moment according to the bias magnetic amount, the primary current, and the current transformer parameter includes:

[0015] Obtaining bias magnet loss according to the bias magnet amount;

[0016] Obtaining the excitation branch current according to the bias magnetic loss;

[0017] The secondary current at the next moment is obtained according to the excitation branch current, the primary current and the current transformer parameters.

[0018] In one embodiment, the current transformer parameters include the number of turns of the primary winding and the number of turns of the secondary winding of the current transformer, and the secondary current is obtained according to the following formula:

[0019]

[0020] Where, I2 is the secondary current, I1 is the primary current, I e is the excitation branch current, N1 is the number of turns of the primary winding, and N2 is the number of turns of the secondary winding.

[0021] In one embodiment, the obtaining of the transient secondary current of the current transformer according to the bias magnetic amount, the power system model data, and the current transformer parameters includes:

[0022] acquiring, according to the power system model data, a transient primary current of the current transformer when a transient fault occurs;

[0023] The transient secondary current is obtained according to the bias magnetic amount, the current transformer parameter and the transient primary current.

[0024] In one embodiment, the power system model data includes the steady-state peak value of the fault current when there is no bias, the voltage phase angle at the initial stage of the short circuit, and the impact time. The transient primary current of the current transformer when a transient fault occurs is obtained according to the following formula:

[0025]

[0026] Where i1(t) is the transient primary current, t is the impact time, I f is the steady-state peak value of the fault current, θ is the voltage phase angle, T p is the primary time constant of the current transformer.

[0027] In one embodiment, the distance protection according to the fundamental wave component includes:

[0028] Obtaining a measured impedance based on the fundamental wave component;

[0029] When the modulus of the measured impedance is not greater than the modulus of the preset setting impedance, the distance protection is activated;

[0030] When the modulus of the measured impedance is greater than the modulus of the preset setting impedance, the distance protection does not operate.

[0031] In one embodiment, the measured impedance is obtained according to the following formula:

[0032]

[0033] Where Z m is the measured impedance, Z1 is the line positive sequence unit impedance, L is the distance from the fault point to the protection relay, is the fundamental component, is the measured current flowing through the protection relay, is the fault current at the other power supply end, R g is the transition resistor.

[0034] In a second aspect, the present application further provides a computer device comprising a memory and a processor, wherein the memory stores a computer program, and the processor implements the steps of the above method when executing the computer program.

[0035] In a third aspect, the present application further provides a computer-readable storage medium having a computer program stored thereon, which implements the steps of the above method when executed by a processor.

[0036] The above-mentioned distance protection method, computer device and computer-readable storage medium analyze the influence of stray current on the saturation characteristics of the current transformer, obtain the secondary current of the current transformer when no transient fault occurs and the transient secondary current of the current transformer when a transient fault occurs based on the stray current data analysis, and further comprehensively analyze the influence of stray current and current transformer saturation on distance protection, that is, analyze the operating characteristics of the distance protection based on the fundamental component of the secondary current, so as to perform distance protection more comprehensively and accurately. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 This is one of the flow charts of a distance protection method in one embodiment;

[0038] Figure 2 is a schematic diagram of a flow path of stray current in a power system according to an embodiment;

[0039] Figure 3 Schematic diagram of the intrusion current at the neutral point of the #1 main transformer under the action of stray current in one embodiment;

[0040] Figure 4 FIG1 is a schematic diagram of obtaining and processing stray current data based on PSCAD in one embodiment;

[0041] Figure 5 is a schematic diagram of an extreme hysteresis loop and a small hysteresis loop in one embodiment;

[0042] Figure 6 Schematic diagram of iron loss under different bias magnetic conditions in one embodiment;

[0043] Figure 7 is a schematic diagram of an equivalent analysis model of a current transformer in one embodiment;

[0044] Figure 8 This is a structural block diagram of a distance protection module in one embodiment;

[0045] Figure 9 A schematic diagram of the operating characteristics of two impedance elements under the influence of stray current in a dual-side power supply embodiment;

[0046] Figure 10 This is a second flow chart of a distance protection method in one embodiment;

[0047] Figure 11 Schematic diagram of the structure of extracting the fundamental wave of fault voltage and fault current and calculating the positive sequence, negative sequence and zero sequence current in one embodiment;

[0048] Figure 12 A schematic diagram of calculation of measured impedance and comparison with impedance relay characteristics in one embodiment;

[0049] Figure 13 Schematic diagram of the change of the magnetic flux of the iron core under the influence of stray current in one embodiment;

[0050] Figure 14 Schematic diagram of changes in magnetic flux measurement in the first cycle after a fault at different saturation levels in one embodiment;

[0051] Figure 15 A schematic diagram showing a comparison of measured impedances in the first cycle after a fault at different saturation levels in one embodiment;

[0052] Figure 16 is a schematic diagram of current comparison at different saturation levels in one embodiment;

[0053] Figure 17 Schematic diagram of fundamental wave amplitude variation at different saturation levels in one embodiment;

[0054] Figure 18 Schematic diagram of fundamental wave phase change at different saturation levels in one embodiment;

[0055] Figure 19 A schematic diagram showing a comparison of measured impedances in the first cycle after a fault at different saturation levels in one embodiment;

[0056] Figure 20 Schematic diagram showing comparison of measured impedance in the second period after a fault at different saturation levels in one embodiment;

[0057] Figure 21 Schematic diagram showing comparison of measured impedance in the third period after a fault at different saturation levels in one embodiment;

[0058] Figure 22 FIG. 1 is a diagram showing the internal structure of a computer device in one embodiment. DETAILED DESCRIPTION

[0059] In order to make the purpose, technical solutions and advantages of this application more clear, the following further describes this application in detail with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain this application and are not intended to limit this application.

[0060] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art in the art of the present application. The terms used herein in the description of the embodiments of the present application are only for the purpose of describing specific embodiments and are not intended to limit the embodiments of the present application.

[0061] In one embodiment, Figure 1 As shown, a distance protection method is provided. This embodiment uses the method applied to a terminal as an example for illustration. It is understandable that the method can also be applied to a server, and can also be applied to a system including a terminal and a server, and implemented through the interaction between the terminal and the server. This embodiment provides a distance protection method for a transmission line. In this embodiment, the method includes the following steps:

[0062] Step 100: Acquire power frequency current data, current transformer parameters, and power system model data.

[0063] Step 200: Acquire the primary current of the current transformer according to the power frequency current data and the stray current data.

[0064] Step 300: Obtain the secondary current of the current transformer according to the primary current and the current transformer parameters.

[0065] Step 400: Obtain the bias magnetic amount at time t according to the secondary current.

[0066] Step 500: When a transient fault occurs, obtain the transient secondary current of the current transformer according to the bias magnetic quantity, the power system model data and the current transformer parameters.

[0067] Step 600: Using the transient secondary current as the input current of the impedance relay, the input current includes a fundamental component and a harmonic component.

[0068] Step 700: Perform distance protection according to the fundamental wave component.

[0069] According to the subway stray current monitoring system, when the stray current interference is serious, the leaked stray current causes the nearby ground potential to change, thereby affecting the AC system in the area and causing various magnetic bias phenomena. The flow path of the stray current in the power system is as follows: Figure 2 In this embodiment, when processing the acquired stray current data of rail transit, the processed controlled voltage source is used to simulate the potential change caused by the leaked stray current, thereby allowing the stray current to invade the power system.

[0070] In order to clarify the characteristics of the stray current, in this embodiment, a DC Hall sensor and a portable data acquisition device are used to test the intrusion current of the stray current into the neutral point of the transformer. The test object is the intrusion current of the neutral point of the #1 main transformer under the action of the stray current of a station in the Southern Power Grid. Figure 3 As shown in the figure, since the sampling interval is 2s, the measured data can be considered as a DC change trend. The obtained stray current is processed by PSCAD, as shown in the figure. Figure 4 As shown, the processed data is equivalent to the stray current injected into the transformer.

[0071] Depend on Figure 3 It can be seen that the stray current at the neutral point of the main transformer has the following characteristics: First, the neutral point current is distributed periodically, with a daily cycle, and is close to 0A during the period of 00:00 to 06:00 every day, and fluctuates continuously between -20 and 30A during the rest of the time; second, the stray current at the neutral point has a period of positive or negative current, and there are more drastic current changes and larger sudden changes during the morning and evening rush hours. The sudden change difference can reach 40A, and there is no obvious change pattern.

[0072] When the stray current of rail transit invades the urban power grid, the components of the stray current will exist in the transmission lines, lightning conductors and the earth of the urban power grid, and the DC current flowing in the transmission lines will be the main component. As a result, the primary side of the current transformer will be a superposition of power frequency current and low-amplitude low-frequency DC current. When the stray current flows through the transmission line, the primary current of the current transformer is: Where, I1 is the power frequency current data, Id is the stray current data. d The current transformer parameter includes the transformation ratio of the current transformer, and the secondary current can be calculated based on the transformation ratio and the primary current.

[0073] The accuracy limit coefficient of the current transformer used for protection is very large and the secondary load is far lower than the rated value. Compared with the hysteresis loop when the core is at the limit of transmission, the working hysteresis loop of the current transformer core under the rated state is very small, such as Figure 5 As shown. Among them, Figure 5 The dotted line in (a) is the limit hysteresis loop of the current transformer under the extreme transmission condition. Figure 5 (b) shows the hysteresis loop under the primary current rating.

[0074] Since the rated AC magnetic flux density is extremely small compared to the limiting hysteresis loop, and the ferromagnetism has basically no growth when the bias is small, that is, the current of the excitation branch will not change, which means that the DC component in the primary current will be completely transferred to the secondary side, and bias magnetization will appear in the core. Figure 5 The broken line in (a) represents the current transformer core magnetization curve, which moves up and down. When the DC current is positive, the bias increases, causing the hysteresis loop to rise linearly along the magnetization curve. When the DC current is negative, the bias decreases, causing the hysteresis loop to fall linearly along the magnetization curve. The first segment, nearly perpendicular to the X-axis, represents the current transformer's linear transition range. This also indicates that the protective current transformer can withstand significant bias when the primary AC current is at its rated current.

[0075] The magnetic flux of the current transformer can be calculated according to Faraday's law:

[0076]

[0077] Where, t is the impact time, B dc is the bias magnetization amount.

[0078] Under the influence of stray current, the magnetic flux of the current transformer grows slowly. This is because the stray current amplitude is too small relative to the power frequency current amplitude. The magnitude of the stray current amplitude affects the slope of the magnetic flux growth. The larger the stray current amplitude, the faster the magnetic flux of the current transformer grows. The longer the stray current stays near a constant value, the greater the magnetic flux growth of the current transformer. The reverse current portion has a demagnetizing effect, and the relationship between the magnetic flux change and the stray current amplitude is the same as that of the forward current.

[0079] In this embodiment, the analysis of distance protection is based on the circuit logic of traditional distance protection. This embodiment analyzes the effect of stray current on the saturation characteristics of the current transformer, further comprehensively analyzing the impact of stray current and current transformer saturation on distance protection. This provides a more comprehensive and accurate analysis of the factors and forms of influence on distance protection, which has practical significance for the precise operation of distance protection.

[0080] In one embodiment, before obtaining the transient secondary current of the current transformer based on the bias magnetization, the power system model data, and the current transformer parameters, the method further includes: when no transient fault occurs, obtaining the secondary current at the next moment based on the bias magnetization, the primary current, and the current transformer parameters. If the current moment is t, then let t = t + Δt represent the next moment, thereby sequentially obtaining the bias magnetization at each moment through the secondary current at each moment. In this embodiment, an equivalent analysis model of the current transformer of the transmission line can be constructed using the stray current data to obtain the secondary current, and further obtain the bias magnetization at each moment when no transient fault occurs.

[0081] In one embodiment, obtaining the secondary current at the next moment according to the bias magnetic amount, the primary current, and the current transformer parameter includes:

[0082] The bias magnet loss is obtained according to the bias magnet amount.

[0083] In this embodiment, the iron loss when powered by sinusoidal current is divided into three items: hysteresis loss, eddy current loss, and additional loss. The DC bias magnetic loss is mainly affected by the hysteresis loss, and the eddy current loss can be ignored. The bias magnetic loss calculation model is as follows:

[0084]

[0085] Where, P Fe is the bias magnet loss under the influence of DC bias magnetization, B m B is the AC magnetic flux amplitude at rated AC excitation without DC bias, dc P is the bias magnetic amount caused by DC. h0 B is the AC loss without bias magnetization under the same magnetic flux density amplitude. m = 0.05T when the calculation model is as follows Figure 6 shown.

[0086] It can be seen that the additional iron loss caused by low bias is negligible. When the absolute value of the bias increases to a certain value, the bias iron loss begins to increase. As the bias increases above 1.4T, the iron loss gradually increases and increases sharply above 1.7T.

[0087] The excitation branch current is obtained according to the bias magnetic loss.

[0088] Combined with the relationship between the bias magnetic loss and the excitation branch current: have:

[0089]

[0090] Where, P Fe is the bias magnet loss; e is the current value of the excitation branch current.

[0091] And due to the bias magnetization:

[0092]

[0093] The size of the excitation resistance is related to the main magnetic flux. Under the action of the rated primary current, the main magnetic flux of the core is extremely small and basically remains constant. The excitation resistance is constant by default, so the increase in the bias magnetic loss due to the increase in the bias magnetic amount will be balanced by the increase in the excitation branch current.

[0094] Combining the above formula, it can be seen that the increase of the bias magnetic amount will increase the bias magnetic loss, causing the excitation branch current to have a larger DC bias, and the absolute value growth rate of the bias magnetic amount will also slow down. According to the full current law and combined with the following Figure 5 (a) The second segment of the magnetization curve and Figure 5 (b) Analysis shows that the core saturation region requires greater AC excitation participation to maintain the same flux change compared to the linear transmission region, which increases the AC component of the excitation branch current and causes distortion.

[0095] As the absolute value of the bias magnetization increases, the bias magnetic loss and the excitation branch current offset gradually increase until the low-frequency current is completely used for the excitation branch current to balance the bias magnetic loss. Under the action of the low-frequency current in the same direction as the bias magnetization, the magnetic flux no longer increases, the low-frequency component of the secondary current is zero, and the hysteresis loop remains constant. Under the action of the reverse current, demagnetization occurs, causing the current transformer's transmission of the stray current to return to the two stages described above.

[0096] When the current transformer contains residual magnetism when affected by the stray current, the growth margin of the iron core that can withstand the residual magnetism in the same direction of bias magnetization will be further reduced, and the linear transfer area will be reduced; while for reverse bias magnetization, the bias magnetization tolerance margin and the size of the linear transfer area will be increased.

[0097] The secondary current at the next moment is obtained according to the excitation branch current, the primary current and the current transformer parameters.

[0098] In this embodiment, since the intrusion current caused by the stray current in the transmission line has a very small amplitude relative to the power frequency current, the hysteresis eddy current loss and the core magnetic flux of the current transformer are affected by the continuous direct current and show a continuous and slow change. It takes a certain amount of time for the core to reach the saturation state. Therefore, for the excitation characteristics under large power frequency current and small direct current, the influence of the excitation resistance cannot be ignored.

[0099] The equivalent circuit of the electromagnetic current transformer considering the core loss is as follows Figure 7 As shown, R1, L1, R m , L m are the primary winding resistance, reactance, equivalent excitation resistance and excitation reactance of the current transformer respectively; R2, R L are the secondary winding resistance and secondary load resistance of the current transformer (ignoring reactance), which together form the secondary impedance R2; I1, I2, I e are the primary current, the secondary current, and the excitation branch current converted to the secondary side, respectively; E is the induced electromotive force on the excitation impedance used to characterize the excitation. The secondary current can be obtained based on the above parameters.

[0100] In one embodiment, reference Figure 7 The current transformer parameters include the number of turns of the primary winding and the number of turns of the secondary winding of the current transformer, and the secondary current is obtained according to the following formula:

[0101]

[0102] Where, I2 is the secondary current, I1 is the primary current, I e is the excitation branch current, N1 is the number of turns of the primary winding, and N2 is the number of turns of the secondary winding.

[0103] In this embodiment, an equivalent analysis model of the current transformer of the transmission line is constructed using the stray current data, so that the secondary current of the current transformer when no transient fault occurs is obtained using the stray current.

[0104] In one embodiment, obtaining the transient secondary current of the current transformer based on the bias magnetic amount, the power system model data, and the current transformer parameters includes: obtaining the transient primary current of the current transformer when a transient fault occurs based on the power system model data. Furthermore, obtaining the transient secondary current based on the transient primary current, the bias magnetic amount when no transient fault occurs, and the current transformer parameters.

[0105] In one embodiment, the power system model data includes the steady-state peak value of the fault current when there is no bias magnetization and the voltage phase angle and impact time at the initial stage of the short circuit. The transient primary current of the current transformer when a transient fault occurs is obtained according to the following formula:

[0106]

[0107] Where i1(t) is the transient primary current, t is the impact time, I f is the steady-state peak value of the fault current, θ is the voltage phase angle, T p is the primary time constant of the current transformer.

[0108] In this embodiment, a transient fault occurs when the stray current influences the time Δts. At the moment of the transient fault, the core magnetic flux of the current transformer and the transient primary current are:

[0109]

[0110]

[0111] Where, Because I m < f , and considering the short-circuit current 100% offset, that is, the case when θ = 0, the transient primary current is:

[0112]

[0113] Combining the electromagnetic induction relationship and the secondary side circuit equation of the current transformer, the excitation flux of the current transformer during normal transmission can be solved as follows:

[0114]

[0115] Where, T s is the secondary time constant of the current transformer.

[0116] In one embodiment, the distance protection based on the fundamental wave component includes: obtaining a measured impedance based on the fundamental wave component. When the modulus of the measured impedance is not greater than the modulus of a preset set impedance, the distance protection is activated; when the modulus of the measured impedance is greater than the modulus of the preset set impedance, the distance protection is not activated.

[0117] ​In this embodiment, a distance protection model can be constructed based on the fundamental component of the transient secondary current. The fundamental component includes a modulus and a phase. The preset set impedance refers to the maximum impedance at which the impedance relay operates. The modulus of the preset set impedance is equivalent to the diameter of the characteristic circle of the directional characteristic impedance relay. When a fault occurs within the influence time of the stray current, the effect of the stray current on the iron core will affect the initial saturation time and transient saturation of the current transformer in a manner similar to residual magnetism, which may affect the relevant relay protection operation.

[0118] For distance protection of double-ended power supply network, the protection module structure diagram is as follows Figure 8 As shown in Figure 1, it is assumed that the line positive sequence unit impedance is Z1 and the distance from the fault point to the protection relay is L. Figure 2 In the system shown, the measured current flowing through the protection relay is I m +I d , and the fault point current becomes I m +I d +I n , then the voltage at the protected measurement point is:

[0119]

[0120] In contrast, I m >>I d , then the measured impedance of the impedance relay under the double-side power supply condition is the ratio of the voltage at the protection measurement point to the fundamental component of the transient secondary current, and I d Can be ignored.

[0121] From the above derivation and analysis, it can be seen that when a fault occurs in the transmission line, the short-circuit current is provided by the power supply on both sides. Due to the presence of the stray current, the transmission characteristics of the current transformer will be distorted, that is, the fundamental component of the transient secondary current will change, thereby causing the measurement impedance of the impedance relay to change, which may cause incorrect operation of the distance protection device.

[0122] In this embodiment, the directional circular characteristic impedance relay is used as an example for analysis, and the results are as follows: Figure 9 Without considering the operating characteristics of the transient secondary current distortion caused by the saturation of the current transformer, the measurement impedance changes as shown in Figure 9 Considering the action characteristics of the impedance relay when the current transformer is saturated and the transient secondary current is distorted, the change of the measured impedance is as shown in FIG. Figure 9 As shown by the solid line in . Figure 9Analysis shows that the modulus of the measured impedance shown by the dotted line is smaller than the diameter of the characteristic circle of the directional circle characteristic impedance relay, that is, it is not greater than the modulus of the preset setting impedance, so the distance protection is activated; the modulus of the measured impedance shown by the solid line is greater than the modulus of the preset setting impedance, so the distance protection does not activate.

[0123] Because stray currents cause magnetic flux offset in the current transformer, the transient primary current suddenly increases and the magnetic flux surges when a transient fault occurs. The superposition of transient magnetic flux and bias magnetization causes the current transformer to transiently saturate, resulting in distortion of the transient secondary current. At this point, the low-frequency component of the transient secondary current is extremely small. Abnormal changes in the amplitude and phase of the transient secondary current can cause changes in the measured impedance, leading to incorrect operation of the distance protection. This is more serious in directional impedance relays. Improper operation of the distance protection is more serious when the magnetic flux increase caused by the stray current is large or the fault is located at the end of the distance protection.

[0124] In one embodiment, the measured impedance is obtained according to the following formula:

[0125]

[0126] Where Z m is the measured impedance, Z1 is the line positive sequence unit impedance, L is the distance from the fault point to the protection relay, is the fundamental component, is the measured current flowing through the protection relay, is the fault current at the other power supply end, R g is the transition resistor.

[0127] In one embodiment, Figure 10 As shown, step 100 obtains power frequency current data, stray current data, current transformer parameters, and power system model data. Step 200 obtains the primary current of the current transformer based on the power frequency current data and the stray current data. Step 300 obtains the secondary current of the current transformer based on the primary current and the current transformer parameters. Step 400 obtains the current bias magnetic value based on the secondary current.

[0128] In step 410, when no transient fault occurs, the bias magnetic loss is obtained based on the bias magnetic amount. In step 420, the excitation branch current is obtained based on the bias magnetic loss. In step 430, the secondary current at the next moment is obtained based on the excitation branch current, the primary current, and the current transformer parameters. Specifically, the current transformer parameters include the number of turns of the primary winding and the number of turns of the secondary winding of the current transformer. The secondary current is obtained according to the following formula: Where, I2 is the secondary current, I1 is the primary current, I e is the excitation branch current, N1 is the number of turns of the primary winding, and N2 is the number of turns of the secondary winding.

[0129] In step 510, when a transient fault occurs, a transient primary current of the current transformer is obtained according to the power system model data. Specifically, the power system model data includes a steady-state peak value of the fault current when there is no bias, a voltage phase angle at the initial stage of a short circuit, and an impact time. The transient primary current of the current transformer when a transient fault occurs is obtained according to the following formula: Where i1(t) is the transient primary current, t is the impact time, I f is the steady-state peak value of the fault current, θ is the voltage phase angle, T p is the primary time constant of the current transformer. Step 520 obtains the transient secondary current based on the bias magnetic quantity, the current transformer parameters, and the transient primary current. Step 600 uses the transient secondary current as the input current of the impedance relay, where the input current includes a fundamental component and a harmonic component. Step 710 obtains the measured impedance based on the fundamental component. Specifically, the measured impedance is obtained according to the following formula: Where Z m is the measured impedance, Z1 is the line positive sequence unit impedance, L is the distance from the fault point to the protection relay, is the fundamental component, is the measured current flowing through the protection relay, is the fault current at the other power supply end, R g In step 720, when the modulus of the measured impedance is not greater than the modulus of the preset set impedance, the distance protection is activated; when the modulus of the measured impedance is greater than the modulus of the preset set impedance, the distance protection is not activated.

[0130] In this embodiment, the mechanism of local transient saturation of the current transformer under the influence of stray current and DC bias magnetic field is analyzed to investigate the saturation of the current transformer caused by stray current and its impact on transmission line distance protection. This allows for a more comprehensive and precise analysis of the factors and forms of influence on distance protection. The low amplitude and long duration of stray current can increase the magnetic flux of the current transformer, affecting both steady-state and transient transmission. This can cause amplitude and phase shifts in the measured impedance, leading to under-range failure of the distance protection.

[0131] Compared to the biased DC current caused by high-voltage direct current (HVDC) and grounding circuit breakers (GIC), the stray current in rail transit has a smaller overall amplitude but more dramatic fluctuations and richer frequency components. Furthermore, the stray current in rail transit generally propagates through transmission lines in high-level power grids, affecting electromagnetic devices throughout the entire grid. Under the influence of the DC bias caused by the stray current, the magnetic flux inside the electromagnetic current transformer will exhibit more complex and continuous changes, making it more likely to cause changes in the current transformer's transmission characteristics, thereby affecting the performance of the grid's relay protection.

[0132] It should be understood that although the flowcharts involved in the above embodiments Figure 1 and Figure 10 The steps in the flowchart are shown in the order indicated by the arrows, but these steps are not necessarily executed in the order indicated by the arrows. Unless otherwise specified in this document, there is no strict order restriction for the execution of these steps, and these steps can be executed in other orders. Figure 1 and Figure 10 At least part of the steps may include multiple steps or multiple stages. These steps or stages are not necessarily performed at the same time, but can be performed at different times. The order of execution of these steps or stages is not necessarily one by one, but can be performed in turn or alternately with other steps or at least part of the steps or stages in other steps.

[0133] Based on the above analysis, Figure 2 A 220 kV transmission line simulation model was established. The power angle difference between the two ends was 20°, the line rated current was 2400 A, the line positive-sequence unit impedance Z1 = 0.0258 + 0.2941 Ω / km, and the zero-sequence unit impedance was 0.181 + 0.8579 Ω / km. The impedance relay used a directional circular characteristic, and the distance protection range was 85% of the total line length. The current transformer model was based on 220 kV, and the line protection was constructed using the current transformer data. The basic data is shown in Table 1.

[0134] Table 1 Current transformer parameters

[0135]

[0136] The construction of the transmission line distance protection model is divided into two steps:

[0137] 1. Extraction of fault voltage and fault current fundamentals and calculation of positive sequence, negative sequence and zero sequence currents, such as Figure 11 shown.

[0138] 2. Calculation of the measured impedance and comparison with the impedance relay characteristics, such as Figure 12shown.

[0139] by Figure 3 Taking the stray current superimposed on the power frequency current as an example, the change of the core magnetic flux under the influence of the power frequency current superimposed on the stray current is discussed. Figure 13 As shown in the figure, since the existing stray current has a small amplitude and a short duration of the same-direction amplitude, the bias magnetization effect on the current transformer used for protection is small, and the current transformer is completely in the linear transmission range, which does not affect the transmission in steady state and transient state.

[0140] In practice, with the development of rail transportation and power systems, the amplitude of the stray current has increased. In order to analyze the impact of the stray current on line distance protection, a simulation analysis is performed by superimposing the stray current on the DC current.

[0141] like Figure 3 The stray current shown here, superimposed with a 20A DC component, is injected into the neutral point of one transformer to simulate the impact of severe stray current on the current transformer saturation and relay protection. The measured impedance changes during a single-phase-to-ground non-metallic fault at the 40% line point are simulated and analyzed. The 40% line point is chosen to clearly demonstrate the effects of stray current and to avoid potential failure to operate due to proximity to the relay terminal. Currently, most actual project faults are non-metallic.

[0142] When the stray current has little effect on the bias magnetization, the magnetic flux at the time of the fault does not reach the saturation flux value of the current transformer in the first cycle. The schematic diagrams of the magnetic flux change and the measured impedance comparison in the first cycle after the fault at different saturation levels are shown as follows: Figure 14 and Figure 15 The transient secondary current and the fundamental component will not be distorted in the first cycle. The calculation of the measured impedance in the first cycle after the fault is the same as when there is no bias magnetic influence. The distance protection can be correctly judged and acted. Figure 15 As shown, 0.02s is a cycle, 50s is the fault start time, Figure 15 The straight line from outside the circle to inside the circle on the right side can be understood as the first cycle. circle , Z m , Z m1 They are respectively the characteristic circle of the impedance relay, and the measured impedance changes without and with the influence of the stray current.

[0143] When the bias magnetic field under the influence of the stray current is large and causes the transient secondary current of the current transformer to be distorted within the first cycle, the change of the measurement impedance becomes more complicated. Figure 16The current waveforms of the single-phase short-circuit grounding fault under the influence of the stray current at different times are compared with the transient secondary current I of the current transformer when the fault occurs without the influence of the stray current. a , I b , I c The following are the fault current waveforms on the secondary side of the current transformer under the influence of the stray current at 33.88s, 36.66s, and 43.24s, respectively. It can be seen that after a period of severe stray current influence, the current transformer's transmission of the fault current begins to show errors in the first cycle. The greater the magnetic bias caused by the stray current, the greater the saturation of the current transformer and the greater the current distortion.

[0144] The impedance relay calculates the measured impedance based on the fundamental component of the transient secondary current, so the change of the fundamental component at different saturation levels is analyzed. Figure 17 and Figure 18 represent the amplitude and phase changes of the fundamental component of the transient secondary current of the current transformer at different saturation levels, I a1 , I b1 , I c1 They respectively represent the fundamental wave components at 33.88s, 36.66s and 43.24s under the influence of the stray current.

[0145] Depend on Figure 17 and Figure 18 It can be seen that the saturation of the current transformer will reduce the amplitude of the fundamental component and the phase shift. When the relay protection does not remove the fault, the amplitude and phase changes of the fundamental component of the transient secondary current will gradually approach the normal situation. Figure 15 Analysis shows that the reduction in the amplitude and phase shift of the fundamental component causes the measured impedance to shift toward the "downward right" compared to normal conditions. The differential impact of different saturation levels is reflected in the first two cycles of the fault current. The longer the impact lasts, the greater the saturation of the current transformer, the greater the transient secondary current distortion, the greater the amplitude distortion of the fundamental component, and the greater the phase shift distortion. Consequently, the measured impedance shifts increasingly toward the "downward right" as the saturation level increases. Figure 19 、 Figure 20 and Figure 21 The following are comparison diagrams of the changes in the measured impedance in the first three cycles when the fault occurs at different saturation levels of the current transformer. Among them, 0.02s is one cycle, and the first 0.02s after the fault is taken as the first cycle, the second 0.02s as the second cycle, and the third 0.02s as the third cycle. m , Z m1 , Z m2, Z m3 These are the measured impedance changes when there is no influence of the stray current and when the stray current influences the conditions of 33.88s, 36.66s, and 43.24s, respectively.

[0146] Based on this, it can be clearly seen that, without the influence of the stray current, the measured impedance is within the distance protection range in the first cycle, and the distance protection device can operate correctly and clear the fault in a timely manner within the first cycle. However, when the stray current is present, the measured impedance is outside the distance protection range in the first cycle due to the offset in the first quadrant. It is not until the second cycle that the measured impedance enters the distance protection range. At a high saturation level (when the stray current influence lasts for 30 seconds), the measured impedance in the second cycle is at the edge of the distance protection range. At different saturation levels, the fundamental component after the fault changes more consistently in the third cycle, but the amplitude and phase of the fundamental component are still lower than normal, and the measured impedance still has a certain offset. It can be inferred that the farther the fault point is from the impedance relay, the more likely the distance protection will fail to operate due to under-range in the first few cycles. As the transient secondary current of the current transformer gradually approaches the normal current, the distance protection will only operate. Whether the measured impedance is within the distance protection range is determined by whether the modulus of the measured impedance is not greater than the modulus of the preset setting impedance.

[0147] Therefore, the stray currents present in the transmission line can affect the transient transmission characteristics of the current transformer by affecting the magnetic flux of the current transformer, thereby affecting the correct operation of the distance protection. The larger the positive amplitude of the stray current and the longer the impact lasts, the more likely it is that the current transformer will saturate under transient conditions, the greater the amplitude and phase amplitude of the fundamental component, the greater the offset of the measured impedance, and the more serious the under-range rejection of the distance protection in the first few cycles.

[0148] This embodiment analyzes the changes in the transmission characteristics of the current transformer under the influence of the stray current and its impact on the distance protection. The stray current existing in the transmission line will affect the transmission characteristics of the current transformer in the form of affecting the magnetic flux of the current transformer. When the bias magnetization caused by the stray current is small, the stray current will not affect the correct operation of the distance protection; when the bias magnetization is large, the stray current causes the transient secondary current of the current transformer to have a transmission error in the first cycle during a fault, and the measured impedance has an increase in amplitude and a phase shift, which may cause the distance protection to have an under-range refusal phenomenon in the first few cycles. With the further development of rail transit and power grid systems, the impact of the stray current on the power system will be more serious. It is of greater practical significance to study the stray current in the power grid and its impact on the transmission characteristics of the current transformer.

[0149] An embodiment of the present application further provides a computer device, which includes a memory and a processor, wherein the memory stores a computer program, and the processor implements the steps of the above-mentioned route planning method when executing the computer program.

[0150] Those skilled in the art will understand that Figure 22 The structure shown in the figure is only a block diagram of a part of the structure related to the present application scheme, and does not constitute a limitation on the computer device to which the present application scheme is applied. The specific computer device may include Figure 22 More or fewer components may be shown, or some components may be combined, or the components may be arranged differently.

[0151] The embodiment of the present application further provides a computer-readable storage medium having a computer program stored thereon, which implements the steps of the above-mentioned distance protection method when executed by a processor.

[0152] Those skilled in the art will appreciate that all or part of the processes in the above-mentioned embodiment methods can be implemented by instructing the relevant hardware through a computer program, and the computer program can be stored in a non-volatile computer-readable storage medium. When the computer program is executed, it can include the processes of the embodiments of the above-mentioned methods. Among them, any reference to memory, database or other media used in the embodiments provided in this application may include at least one of non-volatile and volatile memory. Non-volatile memory may include read-only memory (ROM), magnetic tape, floppy disk, flash memory, optical memory, high-density embedded non-volatile memory, resistive random access memory (ReRAM), magnetic random access memory (MRAM), ferroelectric random access memory (FRAM), phase change memory (PCM), graphene memory, etc. Volatile memory may include random access memory (RAM) or external cache memory, etc. By way of illustration and not limitation, RAM can be in various forms, such as static random access memory (SRAM) or dynamic random access memory (DRAM). The database involved in the various embodiments provided herein may include at least one of a relational database and a non-relational database. Non-relational databases may include, but are not limited to, distributed databases based on blockchains. The processor involved in the various embodiments provided herein may be, but are not limited to, a general-purpose processor, a central processing unit, a graphics processing unit, a digital signal processor, a programmable logic unit, a data processing logic unit based on quantum computing, and the like.

[0153] The technical features of the above embodiments can be combined arbitrarily. To make the description concise, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0154] The above-described embodiments merely represent several implementation methods of the present application. While the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present application. It should be noted that a person of ordinary skill in the art may make various modifications and improvements without departing from the spirit of the present application, and these modifications and improvements fall within the scope of protection of the present application. Therefore, the scope of protection of the present application shall be determined by the appended claims.

Claims

1. A distance protection method, characterized in that: The method comprises: Obtain power frequency current data, stray current data, current transformer parameters and power system model data; acquiring a primary current of a current transformer according to the power frequency current data and the stray current data; Obtaining a secondary current of the current transformer according to the primary current and the current transformer parameter; Obtaining the bias magnetic amount at the current moment according to the secondary current; When a transient fault occurs, the transient secondary current of the current transformer is obtained according to the bias magnetization, the power system model data and the current transformer parameters; the obtaining of the transient secondary current of the current transformer according to the bias magnetization, the power system model data and the current transformer parameters includes: obtaining the transient primary current of the current transformer when a transient fault occurs according to the power system model data; obtaining the transient secondary current according to the bias magnetization, the current transformer parameters and the transient primary current; the power system model data includes the steady-state peak value of the fault current when there is no bias magnetization, the voltage phase angle at the initial stage of the short circuit and the impact time, and the transient primary current of the current transformer when a transient fault occurs is obtained according to the following formula: , where is the transient primary current, t is the impact time, is the steady-state peak value of the fault current, is the voltage phase angle, T p is the primary time constant of the current transformer; Using the transient secondary current as the input current of the impedance relay, wherein the input current includes a fundamental component and a harmonic component; Distance protection is performed according to the fundamental wave component; the distance protection according to the fundamental wave component includes: obtaining a measured impedance according to the fundamental wave component; when the modulus value of the measured impedance is not greater than the modulus value of the preset setting impedance, the distance protection is activated; when the modulus value of the measured impedance is greater than the modulus value of the preset setting impedance, the distance protection is not activated.

2. The method according to claim 1, characterized in that Before obtaining the transient secondary current of the current transformer according to the bias magnetic amount, the power system model data and the current transformer parameters, the method further includes: When no transient fault occurs, the secondary current at the next moment is obtained according to the bias magnetic amount, the primary current and the current transformer parameters.

3. The method according to claim 2, characterized in that The acquiring the secondary current at a next moment according to the bias magnetic amount, the primary current, and the current transformer parameter includes: Obtaining bias magnet loss according to the bias magnet amount; Obtaining the excitation branch current according to the bias magnetic loss; The secondary current at the next moment is obtained according to the excitation branch current, the primary current and the current transformer parameters.

4. The method according to claim 3, characterized in that The current transformer parameters include the number of turns of the primary winding and the number of turns of the secondary winding of the current transformer, and the secondary current is obtained according to the following formula: I2= I1-I e Where, I2 is the secondary current, I1 is the primary current, I e is the excitation branch current, N1 is the number of turns of the primary winding, and N2 is the number of turns of the secondary winding.

5. The method according to claim 1, wherein The measured impedance is obtained according to the following formula: ； Where, For the measured impedance, is the line positive sequence unit impedance, is the distance from the fault point to the protection relay, is the fundamental component, is the measured current flowing through the protection relay, is the fault current at the other power supply end, is the transition resistor.

6. A computer device comprising a memory and a processor, wherein the memory stores a computer program, wherein: When the processor executes the computer program, the steps of the method according to any one of claims 1 to 5 are implemented.

7. A computer-readable storage medium having a computer program stored thereon, characterized in that: When the computer program is executed by a processor, the steps of the method according to any one of claims 1 to 5 are implemented.

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