Method and device for obtaining secondary current and differential protection method

Abstract

The present application relates to a method for obtaining secondary current, a device, a computer device, a storage medium, and a differential protection method. Stray current data is obtained, and the stray current data includes the amplitudes of each low-frequency component and the fundamental wave frequencies of each low-frequency component; the primary current of the current transformer is obtained according to the amplitude of the low-frequency component, the fundamental wave frequency, the preset amplitude of the primary-side power frequency current, and the power frequency; the bias magnetic quantity is obtained according to the amplitude of the primary current, and the transformation rule corresponding to the bias magnetic quantity is determined; the secondary current of the current transformer is obtained according to the primary current and the transformation rule. This solution obtains the bias magnetic quantity by obtaining the amplitude of the primary current of the current transformer, determines the transformation rule corresponding to the bias magnetic quantity, obtains the secondary current of the current transformer according to the primary current and the transformation rule, and further obtains the influence of stray current on differential protection, which can answer whether stray current will affect differential protection for actual power grid operation and propose warning values and corresponding improvement measures.

Description

Technical Field

[0001] The present application relates to the technical field of power grids, and particularly to a method for obtaining secondary current, a device, a computer device, a storage medium, and a differential protection method. Background Art

[0002] With the large-scale construction of urban rail transit, the stray current leaking to the ground has become increasingly serious. On the one hand, the stray current will exacerbate the electrochemical corrosion of the grounding grid and reduce the structural durability; on the other hand, the stray current flows into the power grid through the grounding neutral point of the substation transformer, which may cause the phenomenon of DC bias in ferromagnetic equipment of the power system, resulting in hazards such as increased temperature, vibration, and noise.

[0003] The intrusion of stray current into the power grid and its propagation in the power grid are affected by various factors. The operating conditions of the subway, soil resistivity, substation location, leakage reactance of the transformer, and the resistivity of the earth near it will all affect the intrusion of stray current into the neutral point of a certain substation transformer, and the propagation path of stray current in the power grid is mainly the transmission line; regarding the characteristics of the invading stray current, relevant literature has recorded that the amplitude of the stray current invading the transformer is high, the duration is long, the coverage range is wide, and the current fluctuation is random. The strategy of switching on and off the DC-blocking device according to the amplitude of the neutral point current obviously cannot meet the requirements, and clarifying the characteristics of rail transit stray current can effectively guide the suppression of stray current intrusion and provide more accurate input data support for analyzing the DC bias effect of stray current on power grid equipment.

[0004] Currently, the research on the DC bias of electromagnetic current transformers (CTs) mainly focuses on the DC bias phenomena caused by high-voltage direct current transmission (HVDC) with a constant DC and monopole-ground loop operation mode, and ground induced current (GIC). Under the action of power frequency superimposed with a constant bias DC, the DC bias will accelerate the local transient saturation of the CT, and will interact with the residual magnetism to affect the initial saturation time of the CT; relevant literature has derived a quantitative calculation formula for the transfer error of the metering CT under constant DC using the harmonic balance principle. The bias current caused by HVDC has a fixed value and direction under the condition of unchanged system operation mode. The bias current invaded by GIC has irregular time-varying magnitude and direction, and its influence on the DC bias of the CT is more complex. However, the amplitude difference between the bias current and the power frequency current in the existing research is not large, and the magnetic flux growth margin that the CT can withstand is ignored, so only the saturation state of the CT is selectively analyzed while ignoring the process of the iron core reaching saturation.

[0005] The correct transformation of the CT for protection is crucial for the correct operation of relay protection. The distortion of the secondary current caused by CT saturation will seriously affect the judgment of relay protection. At present, most of the identifications of CT saturation are aimed at the CT saturation caused by external faults, whose characteristics are obvious and easy to identify. However, there is little research on the CT saturation caused by DC bias magnetization. Its saturation characteristics are not as obvious as the former, but the threat to relay protection is not small. There are relevant literatures that simulate and analyze the impact of local transient saturation caused by GIC on differential protection, but there is no modeling analysis of CT saturation characteristics and differential protection, lacking the protection of transmission lines, which further threatens the normal operation of relay protection and the safe operation of the power system. Summary of the Invention

[0006] As described in the background art, the existing research on the electromagnetic current transformer (CT) affected by DC bias mainly focuses on the DC bias phenomena caused by constant DC, monopole-ground loop operation mode of high voltage direct current (HVDC) and ground induced current (GIC). After research by the inventor, it is found that there is little research on the CT saturation caused by DC bias magnetization caused by stray current. Its saturation characteristics are not as obvious as the former, but the threat to relay protection is not small. For the above reasons, the present invention provides a method for obtaining secondary current, a device, a computer device, a storage medium and a differential protection method.

[0007] In a first aspect, the present application provides a method for obtaining secondary current, including:

[0008] Obtain stray current data, where the stray current data includes the amplitude of each low-frequency component and the fundamental frequency of each low-frequency component;

[0009] Obtain the primary current of the current transformer according to the amplitude of the low-frequency component, the fundamental frequency, the preset amplitude of the primary side power frequency current and the power frequency;

[0010] Obtain the bias magnetization amount according to the amplitude of the primary current, and determine the transformation rule corresponding to the bias magnetization amount;

[0011] Obtain the secondary current of the current transformer according to the primary current and the transformation rule.

[0012] In one embodiment, before determining the transformation rule corresponding to the bias magnetization amount, it further includes:

[0013] Obtain the exciting branch current of the current transformer;

[0014] Determine a plurality of bias ranges according to the bias magnet consumption calculation model and the exciting branch current, and obtain the corresponding transfer rules for each of the bias ranges.

[0015] In one embodiment, obtaining the exciting branch current of the current transformer includes:

[0016] Obtain the exciting branch current according to the primary current, secondary current converted to the secondary side, and the equivalent circuit of the current transformer;

[0017] Wherein, the equivalent circuit is established according to the primary winding impedance, equivalent exciting resistance, exciting reactance, secondary winding impedance, and secondary load impedance of the current transformer.

[0018] In one embodiment, determining the transfer rule corresponding to the bias amount includes:

[0019] When the bias amount is less than the first bias threshold, determine the corresponding first transfer rule;

[0020] Wherein, when the bias amount is less than the first bias threshold, the iron core operates in the linear transfer region, and the first transfer rule is to obtain the secondary current according to the low-frequency component of the primary current and the turns ratio of the winding.

[0021] In one embodiment, determining the transfer rule corresponding to the bias amount includes:

[0022] When the bias amount is greater than or equal to the first bias threshold and less than the second bias threshold, determine the corresponding second transfer rule, where the second bias threshold is greater than the first bias threshold;

[0023] Wherein, when the bias amount is greater than or equal to the first bias threshold and less than the second bias threshold, the iron core operates in the transient saturation region, and the second transfer rule is to obtain the secondary current according to the low-frequency component of the primary current, the transfer coefficient, and the turns ratio of the winding, and the transfer coefficient is determined according to the bias amount.

[0024] In one embodiment, determining the transfer rule corresponding to the bias amount includes:

[0025] When the bias amount is greater than or equal to the second bias threshold, determine the third transfer rule;

[0026] Wherein, the third transfer rule is to obtain the secondary current according to the amplitude and phase angle of the harmonic component of the primary current, and the amplitude and phase angle of the harmonic component are determined according to the stray current data.

[0027] In a second aspect, the present application further provides a differential protection method, including:

[0028] Obtain the differential protection type;

[0029] Determine the side of the transformer affected by stray current according to the differential protection type, where the differential protection type is one of transformer differential protection and transmission line pilot current differential protection, and the side affected by stray current is at least one of the primary side and the secondary side;

[0030] Obtain the secondary current of the side affected by stray current by using the method for obtaining the secondary current described in any of the above embodiments, and obtain the preset secondary current of the side not affected by stray current;

[0031] Obtain the differential current of the transformer according to the secondary current of the primary side and the secondary current of the secondary side of the transformer;

[0032] Perform differential protection on the transformer according to the differential current.

[0033] In a third aspect, the present application further provides an apparatus for obtaining a secondary current, including:

[0034] An acquisition module, configured to acquire stray current data, where the stray current data includes the amplitudes of each low-frequency component and the fundamental frequency of each low-frequency component, obtain the primary current of the current transformer according to the amplitude of the low-frequency component, the fundamental frequency, the preset amplitude of the primary side power frequency current, and the power frequency, obtain the bias magnetic quantity according to the amplitude of the primary current, determine the transfer rule corresponding to the bias magnetic quantity, and obtain the secondary current of the current transformer according to the primary current and the transfer rule.

[0035] In a fourth aspect, the present application further provides a computer device, including a memory and a processor, where the memory stores a computer program, and is characterized in that when the processor executes the computer program, the steps of the method for obtaining the secondary current described in any of the above embodiments are implemented.

[0036] In a fifth aspect, the present application further provides a computer-readable storage medium, on which a computer program is stored, and is characterized in that when the computer program is executed by a processor, the steps of the method for obtaining the secondary current described in any of the above embodiments are implemented.

[0037] The above method for obtaining the secondary current, apparatus, computer device, storage medium, and differential protection method obtain the primary current of the current transformer by acquiring stray current data, the preset amplitude of the primary side power frequency current, and the power frequency, then obtain the bias magnetic quantity according to the amplitude of the primary current, determine the transfer rule corresponding to the bias magnetic quantity, and obtain the secondary current of the current transformer according to the primary current and the transfer rule, and further obtain the influence of stray current on differential protection, which can answer whether stray current will affect differential protection and propose warning values and corresponding improvement measures for the actual operation of the power grid. Description of the Drawings

[0038] To more clearly illustrate the technical solutions in the embodiments of the present application or in the prior art, the following will briefly introduce the accompanying drawings required for the description of the embodiments or the prior art. Obviously, the accompanying drawings in the following description are only some embodiments of the present application. For those of ordinary skill in the art, without creative efforts, other accompanying drawings can also be obtained based on these drawings.

[0039] Figure 1 is a flowchart of the method for obtaining the secondary current in the embodiments of the present disclosure;

[0040] Figure 2 is a flowchart of the differential protection method in the embodiments of the present disclosure;

[0041] Figure 3 is a schematic diagram of the flow path of stray current in the power system in the embodiments of the present disclosure;

[0042] Figure 4 is a schematic diagram of the intrusion current at the neutral point of the #1 main transformer under the action of stray current in the embodiments of the present disclosure;

[0043] Figure 5 is a schematic diagram of the power spectrum of the intrusion current at the neutral point in the embodiments of the present disclosure;

[0044] Figure 6 is a schematic diagram of the equivalent analysis model of the current transformer in the embodiments of the present disclosure;

[0045] Figure 7 is a schematic diagram of the core loss of the current transformer under different bias magnetic fields in the embodiments of the present disclosure;

[0046] Figure 8 is a schematic diagram of the small hysteresis loop under the bias magnetic field in the embodiments of the present disclosure;

[0047] Figure 9 is a schematic diagram of the magnetic flux change of the current transformer under the influence of stray current in the embodiments of the present disclosure;

[0048] Figure 10 is a schematic diagram of the comparison of the influence of stray current on the differential current of transformer differential protection in the embodiments of the present disclosure;

[0049] Figure 11 is a schematic diagram of the comparison of the influence of stray current on the low-frequency component of the secondary current in the embodiments of the present disclosure;

[0050] Figure 12 is a schematic diagram of the comparison of the influence of stray current on the amplitude of the second harmonic in the embodiments of the present disclosure;

[0051] Figure 13Schematic diagram of differential current and second harmonic restraint ratio under pure DC in the embodiments of the present disclosure;

[0052] Figure 14 Schematic diagram for comparing the influence of stray current on differential current of longitudinal differential protection of transmission line in the embodiments of the present disclosure;

[0053] Figure 15 Schematic diagram for comparing the influence of stray current on low-frequency components of secondary current in the embodiments of the present disclosure;

[0054] Figure 16 Schematic diagram of differential current and restraint current under pure DC in the embodiments of the present disclosure;

[0055] Figure 17 Logic circuit block diagram of transformer differential protection modeling in the embodiments of the present disclosure;

[0056] Figure 18 Schematic diagram of ratio restraint characteristic of line longitudinal differential protection in the embodiments of the present disclosure;

[0057] Figure 19 Schematic diagram of acquisition and processing of stray current data based on PSCAD in the embodiments of the present disclosure. Detailed implementation manners

[0058] In order to make the objectives, technical solutions and advantages of the present application clearer, the present application will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application, but not to limit the present application.

[0059] As Figure 1 shown, in one of the embodiments, a method for obtaining secondary current is provided, including the following steps 101 to 104. The stray current data of each embodiment of the present application can be read through the fileread module in PSCAD.

[0060] Step 101, obtain stray current data, where the stray current data includes the amplitudes of each low-frequency component and the fundamental frequencies of each low-frequency component;

[0061] Specifically, according to the subway stray current monitoring system, when the stray current interference is serious, the leaked stray current causes the change of the local potential nearby, thereby affecting the AC system in the area. The existence of the potential difference between transformers causes the stray current to invade the internal AC system, triggering various magnetic bias phenomena. The circulation path of the stray current in the power system is as Figure 3 shown by the dotted line in. To clarify the characteristics of the stray current, 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 at a certain station. FromFigure 4 It can be seen that the neutral point stray current has a period of positive or negative current, and there are more drastic current changes during the morning and evening rush hours, with a larger sudden change amplitude. The sudden change difference can reach 40 A, and there is no obvious change pattern. In order to clarify the composition of stray current, the spectrum analysis of the neutral point intrusion current is performed based on FFT. The results are as follows: Figure 4 As shown. Figure 5 It can be seen that the stray DC current at the neutral point of the main transformer contains rich frequency components and is highly random. The frequency range of stray DC current is between 0.001 Hz and 0.1 Hz, mainly 0.01 Hz~0.025 Hz components, and the maximum amplitude of each low-frequency component is only 3.293 A. Compared with the biased DC caused by HVDC and GIC, the overall amplitude of the rail transit stray current is smaller but the fluctuation is more violent, and the frequency components are richer. In addition, the stray current of rail transit is generally transmitted through the transmission lines in the high-level power grid, which affects the electromagnetic equipment in the entire network. Under the influence of the bias caused by stray current, the internal magnetic flux of the electromagnetic CT will show more complex and continuous changes, which will more easily cause changes in the CT transmission characteristics, thereby affecting the performance of the power grid relay protection. Use the fileread module in PSCAD to read the stray current data.txt, and use the following Figure 19 The controlled voltage source shown realizes the control of the transformer's ground potential, wherein the leftmost module is the fileread module in PSCAD, which is used to read txt data information and output a digital sequence to control the voltage amplitude of the controlled voltage source; a resistor is superimposed on the basis of the controlled power supply, and the current emitted by the controlled source is made equal to the actual matched neutral point intrusion current by processing the sequence data output by the fileread module.

[0062] Step 102, obtaining a primary current of a current transformer according to the low-frequency component amplitude, the fundamental frequency, a preset primary-side power frequency current amplitude, and the power frequency;

[0063] Specifically, under the influence of stray current, the primary current of CT can be described as:

[0064] (4)

[0065] Where, I1 is the amplitude of the primary side power frequency current; ω1 is the power frequency; I m is the amplitude of each low-frequency component of the stray current; ω2 is the fundamental frequency of the low-frequency component of the stray current.

[0066] Step 103, obtaining a bias magnetic quantity according to the amplitude of the primary current, and determining a transmission rule corresponding to the bias magnetic quantity;

[0067] According to the law of electromagnetic induction and the CT primary circuit equation, the CT excitation flux density can be obtained from the primary current:

[0068] (5)

[0069] where a and b are constants, ; ; S is the cross-sectional area of the iron core. B ac represents the power-frequency related change of the magnetic flux; B dc represents the change related to the stray current. It can be seen from Equation (5) that there is a change related to the stray current on the basis of the power-frequency change of the magnetic flux. When the value of the stray current is positive, the bias magnetization increases and the hysteresis loop linearly rises along the magnetization curve. When the value of the stray current is negative, the bias magnetization decreases and the hysteresis loop linearly descends along the magnetization curve, as shown in segment ① of the simplified magnetization curve in Figure 8 (a), and the hysteresis loop of this process is as shown in Figure 8 (b).

[0070] Step 104, obtain the secondary current of the current transformer according to the primary current and the transformation rule.

[0071] Among them, the secondary current of the current transformer under different bias magnetization conditions can be reflected by the transformation characteristic model of the current transformer considering the stray current. The transformation characteristic model of the current transformer is established by combining the stray current characteristics, the equivalent model of the current transformer, the bias magnet loss calculation model, and the iron core hysteresis model with Faraday's electromagnetic induction law equation, where the iron core hysteresis model is the hysteresis loop when the current transformer works.

[0072] In this embodiment, by obtaining the stray current data, combining the primary current of the current transformer to obtain the bias magnetization amount of the current transformer, further determining the transformation rule corresponding to the bias magnetization amount, and finally determining the secondary current of the current transformer under different bias magnetization conditions. Since the stray current will affect the bias magnetization amount of the current transformer to varying degrees, the secondary current determined considering the stray data will be more accurate. And the distortion of the secondary current will directly affect the calculation of the differential current and the correct operation of the differential protection. Therefore, obtaining accurate secondary current can make the calculation of the differential current and the operation of the differential protection more accurate.

[0073] In one embodiment, before determining the transformation rule corresponding to the bias magnetization amount, it includes:

[0074] Obtain the exciting branch current of the current transformer;

[0075] Determine multiple bias magnetization ranges according to the bias magnet loss calculation model and the exciting branch current, and obtain the transformation rules corresponding to each of the bias magnetization ranges one by one.

[0076] Specifically, the Bertotti classical model divides the iron loss under sinusoidal current power supply into three items: hysteresis loss, eddy current loss, and additional loss. The direct current bias mainly affects the hysteresis loss, and the eddy current loss can be ignored. At this time, the calculation model of the biased iron loss is as follows:

[0077] (2)

[0078] In the formula, P Fe is the biased iron loss affected by the direct current bias, B m is the amplitude of the alternating magnetic flux density under rated alternating current excitation without direct current bias; B dc is the bias amount brought by the direct current; P h0 is the alternating current loss without bias under the same magnetic flux density amplitude. When B m = 0.05T, the calculation model is as shown in Figure 7 shown.

[0079] It can be seen that the additional iron loss brought by low bias can be ignored. As the bias increases above 1.4T, the iron loss gradually increases and shows a sharp growth trend above 1.7T. The relationship between the iron loss and the excitation is:

[0080] (3)

[0081] In the formula, P Fe is the iron loss; I e is the value of the excitation current. The magnitude 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 iron core is extremely small and basically remains constant. The excitation resistance is defaulted to a constant. Therefore, the iron loss increased due to the increase of the bias amount will be balanced by the increase of the excitation current. The comparison between the limit hysteresis loop of the CT iron core for protection and the small hysteresis loop under the influence of direct current bias is as shown in Figure 8 shown, Figure 8 The dotted line in (a) is the limit hysteresis loop of the iron core when the CT accurately limits the current; Figure 8 (b), Figure 8 (c), and Figure 8 (d) and their corresponding positions in Figure 8 (a) are the hysteresis loops under the influence of direct current bias in the rated state. The magnetization process of the CT under the influence of direct current can be simplified into three sections, that is, corresponding to three transmission rules. As shown by the solid broken line in Figure 8 (a) is the simplified magnetization curve of the silicon steel sheet.

[0082] In one embodiment, obtaining the excitation branch current of the current transformer includes:

[0083] Obtaining the excitation branch current according to the primary current, secondary current converted to the secondary side, and the equivalent circuit of the current transformer;

[0084] Among them, the equivalent circuit is established according to the primary winding impedance, equivalent exciting resistance, exciting reactance, secondary winding impedance, and secondary load impedance of the current transformer.

[0085] Specifically, as Figure 6 shown in the equivalent circuit of the current transformer, where Z1, R m , L m are respectively the primary winding impedance, equivalent exciting resistance, and exciting reactance of the CT, and R s , R L are respectively the secondary winding impedance and secondary load impedance of the CT, and their sum is the secondary impedance R2; i1, i2, i e are respectively the primary current, secondary current, and exciting current converted to the secondary side; E is the induced electromotive force on the exciting impedance used to characterize excitation, and the exciting branch current can be obtained by Figure 6 calculation as:

[0086] (1)

[0087] In the formula, N1 and N2 are the number of turns of the primary and secondary windings.

[0088] In one embodiment, determining the transformation rule corresponding to the bias magnetic quantity includes:

[0089] When the bias magnetic quantity is less than the first bias magnetic threshold, determining the corresponding first transformation rule;

[0090] Among them, when the bias magnetic quantity is less than the first bias magnetic threshold, the iron core operates in the linear transformation region, and the first transformation rule is to obtain the secondary current according to the low-frequency component of the primary current and the winding turn ratio.

[0091] Specifically, since the iron loss does not change when the bias magnetic is small, the exciting current will not change either. Therefore, when the stray current has a small impact on the growth of the bias magnetic, the secondary current can be expressed as:

[0092] (6)

[0093] That is, when the bias magnetic is small, the secondary current can accurately transform the low-frequency component in the primary current.

[0094] In one embodiment, determining the transformation rule corresponding to the bias magnetic quantity includes:

[0095] When the bias magnetic quantity is greater than or equal to the first bias magnetic threshold and less than the second bias magnetic threshold, determining the corresponding second transformation rule, where the second bias magnetic threshold is greater than the first bias magnetic threshold;

[0096] Wherein, when the bias magnetic quantity is greater than or equal to the first bias magnetic threshold and less than the second bias magnetic threshold, the iron core operates in the transient saturation region, and the second transformation rule is to obtain the secondary current according to the low-frequency component of the primary current, the transformation coefficient, and the winding turn ratio, and the transformation coefficient is determined according to the bias magnetic quantity.

[0097] Specifically, when the absolute value of the bias magnetic increases to a certain value, the continuous increase of the absolute value of the bias magnetic will lead to an increase in the bias magnetic iron loss. At this time, the magnetic flux bias change amount and the excitation current bias amount are:

[0098] (7)

[0099] (8)

[0100] Combining Equation (2), Equation (7), and Equation (8), it can be seen that the increase of the bias magnetic will increase the iron loss, resulting in a larger DC bias in the excitation current. Therefore, the growth rate of the absolute value of the bias magnetic slows down, and the hysteresis loop will move along Figure 8 section ② of the simplified magnetization curve in (a). According to the law of total current and combined with Figure 8 analysis of (c), in the iron core saturation region, compared with the linear transformation interval, to maintain the same magnetic flux change, a larger AC excitation is required, which makes the AC component of the excitation current increase and distort. Therefore, the bias magnetic and the secondary current in this interval can be expressed as:

[0101] (9)

[0102] In the formula, ρ is the transformation coefficient of the secondary current to the low-frequency component of the primary side current, ρ ∈ (0, 1) and will decrease with the increase of B dc and decrease.

[0103] In one embodiment, determining the transformation rule corresponding to the bias magnetic quantity includes:

[0104] When the bias magnetic quantity is greater than or equal to the second bias magnetic threshold, determine the third transformation rule;

[0105] Wherein, the third transformation rule is to obtain the secondary current according to the amplitude and phase angle of the harmonic component of the primary current, and the amplitude and phase angle of the harmonic component are determined according to the stray current data.

[0106] Specifically, if there is an alternation of stray current during this process, according to Equation (7), it can be known that the absolute value of the bias magnetic will decrease, so that the CT returns to the linear transformation region. As the absolute value of the bias magnetic, the iron loss, and the excitation current bias continue to increase, the low-frequency current will be completely used for excitation to balance the iron loss. At this time, it can be known from Equation (7) that the magnetic flux no longer increases under the action of the low-frequency current in the same direction as the bias magnetic, and the low-frequency component of the secondary current is 0, and the hysteresis loop remains constant. The hysteresis loop of this process is asFigure 8 as shown in (d). Therefore, the secondary current can be expressed as:

[0107] (10)

[0108] In the formula, I 2n and β n are respectively the amplitude and phase angle of the nth harmonic component, and their magnitudes are related to the primary side stray current. Under the action of the reverse current, a demagnetization phenomenon occurs, causing the CT to return to the above two stages in the transfer of stray current.

[0109] In one embodiment, when the CT has residual magnetism under the influence of stray current, the growth margin of the core to withstand the residual magnetism in the same direction of the bias magnetic field will be further reduced, and the linear transfer region will be reduced; while for the reverse bias magnetic field, the bias magnetic field tolerance margin and the size of the linear transfer region will be increased.

[0110] The transfer characteristic model of the protection CT under the influence of stray current is shown in formulas (6), (9) and (10). The stray current with a lower amplitude will affect the core to generate bias magnetism, and different bias magnetisms will affect the change of iron loss, thus causing different degrees of distortion in the transfer of the CT to the primary current. The distortion of the secondary current will directly affect the calculation of the differential current and the correct operation of the differential protection.

[0111] In one embodiment, according to Figure 17 the logic diagram to determine the operation of the differential protection, where I φ.r > I φ.set.r (φ can be A, B, C) represents a differential relay with braking characteristics for a certain phase;

[0112] I φ.2 >K2I φ1 (φ can be A, B, C) represents the second harmonic braking element for a certain phase, which together with H2, "NOT gate" and Y1 constitutes the second harmonic braking scheme for three-phase OR gate braking;

[0113] I φ.r > I set.r (φ can be A, B, C) represents a differential current instantaneous trip relay for a certain phase. Since the operating current I set.r is very large, a differential relay without braking characteristics is used. I φ.r > I set.r together with H1 constitutes the differential current instantaneous trip protection.

[0114] In one embodiment, for the transformer differential protection, affected by the path of the stray current invading the transformer, only one side of the CT is affected by the stray current.

[0115] When stray current invades the power system from the neutral point of the transformer and propagates in the transmission line, the secondary currents of the CTs on both sides of the transformer can be described as follows:

[0116] (11)

[0117] (12)

[0118] (13)

[0119] In the formula, I t1 , I t2 are the secondary currents of the CTs on the primary and secondary sides of the transformer; I t is the amplitude of the primary current of the CT on the primary side of the transformer; K1 and K2 are the transformation ratios of the CTs on the primary and secondary sides of the transformer; K T is the transformation ratio of the transformer; the differential current I r can be described as follows:

[0120] (14)

[0121] (15)

[0122] It can be seen from formula (14) that the changes in the low-frequency components and harmonics of the differential current are consistent with the changes in the secondary current of the CT under the influence of stray current, and there is a corresponding relationship between the power-frequency component of the differential current and the power-frequency fundamental wave after the local transient saturation of the CT. The attenuation of the low-frequency components and harmonics after the local transient saturation of the CT on the side affected by the stray current will directly affect the changes in the differential current and the second-harmonic braking ratio.

[0123] In one embodiment, for the pilot current differential protection of the transmission line, the ratio restraint characteristic is as Figure 18 shown. It can be known that the operating characteristic is as follows:

[0124] I res =I res.0 When, I r > I r.min ;

[0125] I res >I res.0 When, I r > KI res

[0126] The CTs on both sides of the line will be affected by the stray current, and since the positive directions of the currents of the CTs on both sides are opposite, the stray current has opposite bias magnetic effects on the two CTs. The calculation of the secondary currents and differential current of the CTs on both sides can be described as follows:

[0127] (16)

[0128] (17)

[0129] Wherein, I l1 , I l2 are the secondary currents of CTs on both sides of the line; I l1n , I l2n are the fundamental wave and the amplitudes of each harmonic of the secondary currents of CTs on both sides of the line; β 1n , β 2n are the phase offsets of the fundamental wave and each harmonic of the secondary currents of CTs on both sides of the line; ρ1 and ρ2 are the transfer coefficients of CTs on both sides of the line for stray current. Without considering the structural differences between the two CTs, under ideal conditions, the differential current can be calculated as:

[0130] (18)

[0131] (19)

[0132] (20)

[0133] In one embodiment, when the residual magnetisms of the two CTs are opposite numbers, the magnetic fluxes and saturation degrees of the two CTs change in the same way under the same changing stray current, so ρ1 = ρ2, β 1n = β 2n , I l1n = I l2n , and the differential current is 0; in other cases, the magnetic fluxes and saturation degrees of the two CTs are inconsistent, resulting in ρ1≠ρ2, β 1n ≠β 2n , I l1n ≠ I l2n . The differential current not only has a change in power frequency amplitude but also has a change accompanied by the change in stray current amplitude. And when the CT on the side with high residual magnetism has undergone local transient saturation and the CT on the side with low residual magnetism is still working in the linear transfer region, the low-frequency change of the differential current is the largest. However, as the CT on the side with low residual magnetism also approaches saturation under the influence of stray current, |ρ1 - ρ2| gradually approaches 0, and the low-frequency component of the differential current will gradually decrease. The closer the residual magnetisms of the two CTs are, the closer ρ1 and ρ2 are, and the smaller the change in the differential current is.

[0134] In one embodiment, a differential protection method is provided, including:

[0135] Obtain the differential protection type;

[0136] Determine the side of the transformer affected by stray current according to the differential protection type, where the differential protection type is one of transformer differential protection and transmission line pilot current differential protection, and the side affected by stray current is at least one of the primary side and the secondary side;

[0137] Obtain the secondary current on the side affected by stray current by using the method for obtaining secondary current described in the above embodiments, and obtain the preset secondary current on the side not affected by stray current;

[0138] Obtain the differential current of the transformer according to the secondary current on the primary side and the secondary current on the secondary side of the transformer;

[0139] Perform differential protection on the transformer according to the differential current.

[0140] In addition, in order to verify the accuracy of the above differential protection, the inventor also verified the reliability of the foregoing method through simulation experiments. Specifically, according to Figure 2 Establish a simulation model to perform simulation analysis on the influence of DC bias caused by stray current on the pilot current differential protection of ultra-high voltage transmission lines and the transformer differential protection. In the model, the 500kV transformer adopts the Y, Y connection method, the rated current of the 220kV transmission line is 2400A, the CT transformation ratio is 2400A / 1A, and the Jiles-Atherton model is adopted. Temporarily do not consider the influence of stray current on the transformer, assume that the three phases of the transformer are symmetrical, and the stray current is evenly distributed in the three-phase line. Inject Figure 4 The indicated stray current, the magnetic flux change is as Figure 9 Shown. Since the existing stray current has a small amplitude and a short duration of the same-direction amplitude, the alternating of positive and negative amplitudes causes the magnetic flux of the CT iron core to rise and fall to varying degrees, but generally has a small influence on the bias of the protection CT, and the CT iron core is still in the linear transfer region, that is, the secondary current can accurately transfer the stray current on the primary side.

[0141] In practice, with the development of rail transit and the power system, the amplitude of stray current has increased. To realize the simulation analysis of the influence of stray current on differential protection, a low-frequency (0.006Hz) sinusoidal quantity with an increased amplitude is used to simulate the stray current to analyze the influence of stray current on differential protection.

[0142] Considering the double influence of residual magnetism and stray current, perform simulation analysis on the influence of stray current on transformer differential protection under the condition of no residual magnetism and 0.8T residual magnetism of the CT. Figure 10 It is a comparison diagram of differential currents in two cases. Among them, I r1 , I r2 Are the changes in differential current under the influence of no residual magnetism and residual magnetism respectively, Figure 11 Is the change in the low-frequency quantity of the CT secondary side current correspondingly. Among them, I td1 Is the low-frequency quantity of the CT primary side current (converted to the secondary side) for comparison; I td2_1 , I td2_2 Are the changes in the low-frequency quantity of the CT secondary side current under the influence of no residual magnetism and residual magnetism respectively.

[0143] It can be seen that the stray current will cause DC bias magnetization in the CT, resulting in saturation. The differential current will have DC offset and changes in AC amplitude. The presence of residual magnetism will affect the saturation time of the CT under the influence of stray current. When there is no influence of residual magnetism, the CT starts to enter the second-stage transformation at 30 s. The DC component on the secondary side starts to decay and decays to 0 at 50 s. At the same time, the phase shifts at 30 s, and the corresponding AC amplitude of Ir1 starts to increase from 30 s. However, under the influence of residual magnetism, the above process is advanced by approximately 10 s. When the low-frequency component alternates after 80 s, since the CTs in both cases have maintained the local transient saturation state, the magnetic flux changes are basically the same when demagnetizing with negative current. The low-frequency component can be well transformed in both cases without being affected by the previously applied residual magnetism, and the differential current changes in the same way as the low-frequency component.

[0144] At the same time, the gradual saturation of the CT will cause the fundamental wave of the CT secondary-side current to decrease and the harmonic to increase, resulting in an increase in the second-harmonic restraint ratio. The changes in the second-harmonic restraint ratio with and without the influence of residual magnetism are as Figure 12 shown. I 22_1 、I 22_2 are the changes in the second-harmonic amplitude without and with the influence of residual magnetism respectively. The time when I 22_2 reaches the peak value and the starting time of the change are advanced by I 22_ 110 s. The demagnetization effect caused by the alternation of the low-frequency component that appears after 80 s gradually reduces the magnetic bias and saturation degree of the CT. The CT gradually enters the linear transformation region, the harmonic amplitude decreases, and the same magnetic flux change condition after saturation makes the second-harmonic changes in both cases always the same.

[0145] From Figure 10 、 Figure 11 and Figure 12 it can be seen that the magnetic flux of the CT iron core is the key to affecting the differential current and the second-harmonic restraint ratio, and it is affected by residual magnetism and stray-current magnetic bias. To achieve the purpose of engineering warning, in this embodiment, a simulation test of the tolerable margin of differential protection with a constant direct current is carried out.

[0146] When the DC current at the neutral point of the transformer reaches 385 A and the CT is saturated, the second-harmonic restraint ratio reaches 15%, which is sufficient to trigger the blocking of differential protection and threaten the safe operation of the power system. Under the influence of this DC, the absolute value of the differential current with DC bias in the linear transformation region of the CT is at most 0.0688 A, and the maximum value of the differential current when local transient saturation is triggered is 0.0655 A, both of which cannot trigger the action of differential protection. The changes in the differential current and the second-harmonic restraint ratio under this DC are as Figure 13 shown.

[0147] Regarding the maloperation characteristics of differential protection, two reasons are considered: the sudden appearance of DC bias causes the secondary current to have a DC bias and thus be higher than the restraint current; or a large phase shift occurs in the saturated CT, resulting in an increase in the AC amplitude of the differential current that is higher than the restraint current. For the ratio differential starting current setting value of 0.5I e in the first case, at least 3600 A of DC injection is required, and in the second case, an equation is established on the premise of ignoring the current distortion caused by harmonics:

[0148] (21)

[0149] It is solved that the phase angle difference β needs to reach 17.375°, and at least 6297.5 A of DC injection is required at the transformer neutral point.

[0150] For the two cases of DC current affecting the maloperation of differential protection, which are much larger than the rated current, they can be ignored. Therefore, for the correct operation of the 500 kV transformer differential protection, at least a warning value of 385 A is required to prevent the differential protection from refusing to operate due to blocking during a fault.

[0151] Considering the influence of residual magnetism, the influence of stray current on differential current under different residual magnetism differences is analyzed, as Figure 14 shown. I r3 、I r4 are the changes in differential current when one side CT has 1 T of residual magnetism, the other side CT has no residual magnetism and -0.8 T of residual magnetism respectively, Figure 15 is the change in the low-frequency component of the secondary current of the corresponding two-side CT, I ld1 、I ld1_1 are the low-frequency component of the primary current (converted to the secondary side) and the change in the low-frequency component of the secondary current of the 1 T residual magnetism CT; I ld2 、I td2_1 、I td2_2 are the low-frequency component of the primary current (converted to the secondary side) of the other side CT, the low-frequency component changes when there is no residual magnetism and -0.8 T of residual magnetism respectively.

[0152] From Figure 14 and Figure 15 it can be seen that the CT with larger residual magnetism saturates first, and the decay of the DC component occurs in advance, resulting in a DC offset of the differential current. The offset amplitude is consistent with the sum of the low-frequency components of the two-side CT, and the gradual increase in the phase shift when the CT saturates causes an increase in the AC amplitude of the differential current at the same time. When the iron loss of the CT with smaller residual magnetism starts to increase significantly, the phase shift and DC decay of the two-side CT gradually approach, and the AC amplitude and DC offset of the differential current decrease.

[0153] Meanwhile, after the CT reaches local transient saturation, the alternation of the low-frequency component has the same effect on both CTs, and the effects cancel each other out when calculating the differential current. Therefore, the alternation of the low-frequency component will have no effect on the differential current. As shown in Figure 14 the I from 80s to 160s in r3 、I r4 .

[0154] By comparing the changes in differential current under different remanence differences, it can be found that due to the reduction of the remanence difference, the saturation times of the two CTs are closer, the start times of DC decay and phase shift are closer, and the offsets of the two CTs can be canceled out in a shorter time, resulting in a reduction in the change of the differential current.

[0155] By comparing the results of whether the remanence is consistent and the influence of stray current, it can be found that the reason for the increase in the longitudinal differential current of the transmission line is the inconsistency of the saturation times of the CTs on both sides of the line under the influence of stray current and remanence, resulting in differences in the decay of the DC components and the angular difference changes of the secondary currents of the two CTs. The difference in remanence is the main cause of the change in the differential current, and the stray current provides the growth of the bias magnetic field. Therefore, for engineering prevention, the remanence of the CT iron cores on both sides of the line should be made opposite as much as possible.

[0156] To achieve the purpose of engineering early warning, in this embodiment, the withstand margin simulation test of differential protection is also carried out with a constant direct current. To obtain the maximum distortion of the differential current, a remanence of 1T is given to one CT to ensure that the difference in DC and phase of the secondary currents of the two CTs can reach the maximum. When the neutral point current of the transformer reaches 654.5A, the comparison of the differential current and the braking current is as Figure 16 shown.

[0157] The CT with remanence reaches saturation in advance, causing the DC component of its secondary current to decay to 0 and a phase shift to occur. As a result, the differential current has a DC offset of 0.09A and an increase in the AC amplitude due to the phase angle difference of 2.7367° between the two CTs, causing the differential current to be greater than the braking current at 1.309s and resulting in a misoperation of the differential protection.

[0158] In the transmission line, due to the existence of stray current, there is a DC bias phenomenon in the current transformer, which threatens the normal operation of relay protection and the safe operation of the power system. The randomness and volatility of stray current are greater, and the characteristics such as sudden change determine that its influence on the CT is more complex. Therefore, in this embodiment, through the stray current characteristics, the equivalent model of the current transformer, the bias magnet loss calculation model and the core hysteresis model, and combined with the Faraday electromagnetic induction law equation, the transfer characteristic model of the protection CT under the influence of stray current is obtained, and its influence on transformer differential protection and line longitudinal differential protection is analyzed through theory and simulation; due to the changes in bias and iron loss, the transfer characteristics of the CT under the influence of stray current are divided into three-stage changes, and the remanence combined with the bias caused by stray current has an impact on different differential protections, and relevant preventive measures and stray current warning value analysis are given. For example, for transformer differential protection, try to make the remanence of the CT core on the side where stray current invades as small as possible; for line differential protection, try to make the remanence of the CT cores on both sides of the line as small as possible or opposite to each other.

[0159] In one embodiment, a device for obtaining secondary current is provided, including:

[0160] An acquisition module for acquiring stray current data, where the stray current data includes the amplitude of each low-frequency component and the fundamental frequency of each low-frequency component, obtaining the primary current of the current transformer according to the amplitude of the low-frequency component, the fundamental frequency, the preset amplitude of the primary-side power frequency current and the power frequency, obtaining the bias magnetic quantity according to the amplitude of the primary current, determining the transfer rule corresponding to the bias magnetic quantity, and obtaining the secondary current of the current transformer according to the primary current and the transfer rule.

[0161] In one embodiment, a computer device is included, including a memory and a processor, and when the processor executes the computer program, the steps of the method for obtaining secondary current in any of the above embodiments are implemented.

[0162] In one embodiment, a computer-readable storage medium is provided, on which a computer program is stored, and the computer program is executed by the processor to implement the steps of the method for obtaining secondary current in any of the above embodiments.

[0163] Those of ordinary skill in the art can understand that all or part of the processes in the methods of the above embodiments can be completed by instructing relevant hardware through a computer program. 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 methods. Among them, any reference to a memory, storage, database, or other medium used in the embodiments provided in the present application can include at least one of non-volatile and volatile memories. Non-volatile memory can include read-only memory (ROM), magnetic tape, floppy disk, flash memory, or optical memory, etc. Volatile memory can include random access memory (RAM) or external cache memory. 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), etc.

[0164] In the description of this specification, the description referring to terms such as "some embodiments", "other embodiments", "ideal embodiments", etc. means that the specific features, structures, materials, or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic description of the above terms does not necessarily refer to the same embodiment or example.

[0165] The technical features of the above embodiments can be combined arbitrarily. For the sake of concise description, 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, it should be considered as the scope described in this specification.

[0166] The above-described embodiments merely represent several implementation manners of the present application. The description is relatively specific and detailed, but it should not be construed as a limitation on the scope of the invention patent. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present application, several modifications and improvements can still be made, and these all belong to the protection scope of the present application. Therefore, the protection scope of the patent of the present application should be subject to the appended claims.

Claims

1. A method for obtaining secondary current, characterized in that, Comprising: Obtain stray current data, where the stray current data includes the amplitudes of each low-frequency component and the fundamental frequencies of each low-frequency component; Obtain the primary current of the current transformer according to the amplitude of the low-frequency component, the fundamental frequency, a preset amplitude of the primary-side power frequency current, and the power frequency; Obtain the bias magnetic quantity according to the amplitude of the primary current, and determine the transformation rule corresponding to the bias magnetic quantity; Obtain the secondary current of the current transformer according to the primary current and the transformation rule; Before determining the transformation rule corresponding to the bias magnetic quantity, it further includes: Obtain the exciting branch current of the current transformer; Determine multiple bias magnetic ranges according to the bias magnet core loss calculation model and the exciting branch current, and obtain the transformation rules corresponding to each of the bias magnetic ranges one by one.

2. The acquisition method according to claim 1, wherein The obtaining of the exciting branch current of the current transformer includes: Obtain the exciting branch current according to the primary current, secondary current converted to the secondary side, and the equivalent circuit of the current transformer; Wherein, the equivalent circuit is established according to the primary winding impedance, equivalent exciting resistance, exciting reactance, secondary winding impedance, and secondary load impedance of the current transformer.

3. The acquisition method according to claim 1, characterized in that, The determining of the transformation rule corresponding to the bias magnetic quantity includes: When the bias magnetic quantity is less than the first bias magnetic threshold, determine the corresponding first transformation rule; Wherein, when the bias magnetic quantity is less than the first bias magnetic threshold, the iron core operates in the linear transformation region, and the first transformation rule is to obtain the secondary current according to the low-frequency component of the primary current and the turns ratio of the winding.

4. The acquisition method according to claim 1, wherein The determining of the transformation rule corresponding to the bias magnetic quantity includes: When the bias magnetic quantity is greater than or equal to the first bias magnetic threshold and less than the second bias magnetic threshold, determine the corresponding second transformation rule, where the second bias magnetic threshold is greater than the first bias magnetic threshold; Wherein, when the bias magnetic quantity is greater than or equal to the first bias magnetic threshold and less than the second bias magnetic threshold, the iron core operates in the transient saturation region, and the second transformation rule is to obtain the secondary current according to the low-frequency component of the primary current, the transformation coefficient, and the turns ratio of the winding, and the transformation coefficient is determined according to the bias magnetic quantity.

5. The obtaining method according to claim 1, wherein The determining of the transformation rule corresponding to the bias magnetic quantity includes: When the bias magnetic quantity is greater than or equal to the second bias magnetic threshold, determine the third transformation rule; Wherein, the third transformation rule is to obtain the secondary current according to the amplitude and phase angle of the harmonic component of the primary current, and the amplitude and phase angle of the harmonic component are determined according to the stray current data.

6. A differential protection method, characterized in that, Comprising: Obtain the differential protection type; Determine the side of the transformer affected by the stray current according to the differential protection type, where the differential protection type is one of transformer differential protection and transmission line pilot current differential protection, and the side affected by the stray current is at least one of the primary side and the secondary side; Obtain the secondary current of the side affected by the stray current by using the method for obtaining the secondary current according to any one of claims 1 to 5, and obtain the preset secondary current of the side not affected by the stray current; Obtain the differential current of the transformer according to the secondary current on the primary side and the secondary current on the secondary side of the transformer; Perform differential protection on the transformer according to the differential current.

7. An acquisition device for secondary current, characterized in that, Comprising: An acquisition module, configured to acquire stray current data, where the stray current data includes the amplitudes of each low-frequency component and the fundamental frequencies of each low-frequency component, acquire the primary current of a current transformer according to the amplitudes of the low-frequency components, the fundamental frequencies, a preset primary-side power-frequency current amplitude, and a power-frequency frequency, acquire a bias magnetic quantity according to the amplitude of the primary current, determine a transformation rule corresponding to the bias magnetic quantity, and acquire the secondary current of the current transformer according to the primary current and the transformation rule; Before determining the transformation rule corresponding to the bias magnetic quantity, it further includes: Acquire the exciting branch current of the current transformer; Determine a plurality of bias magnetic ranges according to a bias magnet core loss calculation model and the exciting branch current, and acquire each transformation rule corresponding to each of the bias magnetic ranges one by one.

8. A computer device, comprising a memory and a processor, the memory storing a computer program, characterized in that, When the processor executes the computer program, it implements the steps of the method according to any one of claims 1 to 6.

9. A computer-readable storage medium having a computer program stored thereon, characterized in that, When the computer program is executed by the processor, it implements the steps of the method according to any one of claims 1 to 6.

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