Method for calculating AC resistance value of cable core by using oscillogram obtained by parallel acquisition method

Abstract

The invention relates to the technical field of cable testing, in particular to a cable core alternating current resistance calculation method based on a parallel acquisition method. According to the scheme, through the differential input and image comparison method, the measurement precision can be effectively improved, and errors are avoided. The method is simple to operate and easy to implement, and does not need a complex calculation process. The method is high in measurement speed and high in efficiency, and can meet the requirements of practical engineering application.

Description

Technical Field

[0001] The present invention relates to the technical field of cable testing, and particularly to a method for calculating the AC resistance value of a cable core by using a waveform diagram obtained by a parallel acquisition method. Background Art

[0002] Cables are an important part of the power system, and their performance directly affects the safe and stable operation of the power system. The AC resistance of the cable core is one of the important parameters of the cable performance, and its magnitude directly affects the transmission efficiency and loss of the cable.

[0003] Traditional methods for measuring the AC resistance of the cable core mainly include the bridge method, the volt-ampere method, and the micro-ohmmeter method, etc. These methods have the following disadvantages:

[0004] Low measurement accuracy: Traditional methods are greatly affected by environmental factors and the accuracy of the instrument itself, and the measurement results are prone to errors.

[0005] Complicated operation: Traditional methods require cumbersome wiring operations, the operation process is complex, and it is easy to make mistakes.

[0006] Low efficiency: Traditional methods have a slow measurement speed and low efficiency, and it is difficult to meet the requirements of actual engineering applications.

[0007] In order to overcome the disadvantages of traditional methods, in recent years, some methods for measuring the AC resistance of the cable core based on modern signal processing technology have emerged. Summary of the Invention

[0008] To solve the above problems, the present invention discloses a method for calculating the AC resistance value of a cable core by using a waveform diagram obtained by a parallel acquisition method.

[0009] The present application discloses a method for calculating the AC resistance value of a cable core by using a waveform diagram obtained by a parallel acquisition method, including the following steps:

[0010] Collect the sine waveform data of voltage and current through an acquisition card with differential analog signal input;

[0011] Generate a waveform diagram according to the collected data and obtain its initial phase angle value;

[0012] Extract the data of three key nodes, the Y-axis and X-axis values corresponding to the images of t1, tMax, and tMin, according to the collected waveform diagram;

[0013] Extract three key nodes from the collected data, calculate the theoretical initial phase value after the data corresponding to the time points of t1, tMax, and tMin;

[0014] Determine the compliance of the calculated initial phase value by the image comparison method;

[0015] Calculate the AC resistance value of the cable core according to the initial phase value and the collected voltage and current data.

[0016] Among them, the AC resistance calculation process includes the following steps:

[0017] According to the vector calculation formula: V a ∠θ a =V a (cosθ a +jsinθ a ), calculate and determine the voltage drop along each phase, between the measurement points, to obtain Va∠θa, Vb∠θb, Vc∠θc measured; where va is the modulus of the complex number, θa is the argument of the complex number, and j is the imaginary unit;

[0018] According to the calculation formula: Calculate the average conductor voltage drop over the entire cable length;

[0019] According to the condition that the power supply rotates with a positive sequence phase at power frequency: a = 1∠120 = cos 120 + jsin 120, through the calculation formula: Calculate the average conductor current;

[0020] Through the calculation formula: Calculate the three-phase AC impedance, and the real part of it is the AC resistance value.

[0021] The process of determining the compliance of the calculated initial phase value by the image comparison method includes: when the image has ωt as the horizontal axis, the inspection range is the magnitude of the phase difference between the two sinusoidal quantities; when the image has time t as the horizontal axis, the inspection range should be the time period △Ф / ω seconds. If it exceeds this range, the judgment result is the opposite.

[0022] The image comparison method includes determining the compliance of the initial phase by the requirements of waveform matching and the function formula must be consistent.

[0023] The AC resistance calculation process includes: for phase differences greater than π, they should be expressed as negative phase differences greater than -π; for phase differences less than -π, they should be expressed as positive phase differences less than π.

[0024] The solution of this application can effectively improve the measurement accuracy and avoid errors through differential input and the image comparison method. This method is simple to operate, easy to implement, and does not require complex calculation processes. This method has a fast measurement speed and high efficiency, and can meet the requirements of actual engineering applications. Brief Description of the Drawings

[0025] Figure 1 It is the data source provided by the table acquisition card and the initial phase verification diagram in the embodiment of this application;

[0026] Figure 2Data source and initial phase verification diagram provided for the acquisition card in the embodiments of this application;

[0027] Figure 3 Schematic structural diagram of the acquisition card in the embodiments of this application. Detailed implementation manners

[0028] To make the objectives, features, and advantages of the present invention more obvious and understandable, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the embodiments described below are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present invention. The principles and features of the present invention will be described below in conjunction with the accompanying drawings. The examples given are only for explaining the present invention and are not used to limit the scope of the present invention.

[0029] The term "including" in the specification or claims of the present invention and other similar expressions mean covering non-exclusive inclusion. For example, a process, method, system, or device including a series of steps or units is not limited to the listed steps or units.

[0030] Embodiment: A method for calculating the AC resistance value of a cable core using a waveform diagram obtained by the parallel acquisition method, including the following steps:

[0031] Collect the sine waveform data of voltage and current through an acquisition card with differential analog signal input;

[0032] Generate a waveform diagram based on the collected data and obtain its initial phase angle value;

[0033] Extract the data of three key nodes, the Y-axis and X-axis values corresponding to the images of t1, tMax, and tMin, from the collected waveform diagram;

[0034] Draw the collected data into a waveform diagram to intuitively show the change trends of voltage and current;

[0035] Extract three key nodes from the collected data, calculate the theoretical initial phase value after the data corresponding to the time points of t1, tMax, and tMin;

[0036] Extract the Y-axis and X-axis values corresponding to the images of the three key nodes t1, tMax, and tMin in the waveform diagram for calculating the theoretical initial phase value;

[0037] Determine the compliance of the calculated initial phase value through the image comparison method;

[0038] Calculate the AC resistance value of the cable core according to the initial phase value and the collected voltage and current data.

[0039] Among them, the AC resistance calculation process includes the following steps:

[0040] According to the vector calculation formula: V a ∠θ a = V a (cosθ a + jsinθ a ), calculate and determine the voltage drop along each phase, between the measurement points, so as to measure Va∠θa, Vb∠θb, Vc∠θc; where va is the modulus of the complex number, θa is the argument of the complex number, and j is the imaginary unit;

[0041] According to the calculation formula: Calculate the average conductor voltage drop over the entire cable length;

[0042] According to the condition that the power supply rotates in the positive sequence phase of the power frequency: a = 1∠120 = cos 120 + jsin 120, through the calculation formula: Calculate the average conductor current;

[0043] Through the calculation formula: Calculate the three-phase AC impedance, and the real part value thereof is the AC resistance value.

[0044] When the image has ωt as the horizontal axis, the inspection range is the magnitude of the phase difference between the two sinusoidal quantities; when the image has time t as the horizontal axis, the inspection range should be the time period △Ф / ω seconds. If it exceeds this range, the determination result is opposite.

[0045] The image comparison method includes determining the compliance of the initial phase by the requirements that the waveforms match and the functional expressions must be the same.

[0046] The AC resistance calculation process includes: for phase differences greater than π, they should be expressed as negative phase differences greater than -π; for phase differences less than -π, they should be expressed as positive phase differences less than π.

[0047] In a possible implementation manner, the structure of the acquisition card is as shown below Figure 3 as shown in:

[0048] The acquisition card includes:

[0049] Signal connector: an interface connected to an external device or sensor, responsible for receiving external signals;

[0050] Conditioning circuit: including multiple analog-to-digital conversion channels (such as CH10, CH15, etc.), responsible for conditioning the input signal and preparing for analog-to-digital conversion (ADC); for different signal channels, different conditioning requirements are carried out;

[0051] ADC (Analog-to-Digital Converter): Converts the conditioned analog signal into a digital signal for subsequent processing;

[0052] Core circuit: FPGA (Field-Programmable Gate Array), which is responsible for processing the digital signal obtained from the ADC and performing real-time calculations or data processing;

[0053] DAC (Digital-to-Analog Converter): Converts the processed digital signal back into an analog signal for practical applications;

[0054] EEPROM: A non-volatile memory used to store configuration or status information, enabling the system to retain important data even after power-off;

[0055] FX3: A controller for USB interfaces or other communication interfaces, responsible for data input and output;

[0056] Port0, Port1, Port2: Used for different signal inputs and outputs.

[0057] The acquisition card processes the external signal through conditioning, conversion, and processing, ultimately generating a usable output signal, while also possessing the capabilities of storage and communication.

[0058] Exemplarily, taking Figure 1 the data collected in as an example, the above method includes:

[0059] Generating a waveform diagram from the data collected by the system and obtaining the requirement for its initial phase angle value;

[0060] Obtaining the initial phases of voltage and current of the sine waveform diagram and specifying its inspection range. Among them, when the image has ωt as the horizontal axis, the inspection range is the magnitude of the phase difference between the two sine quantities; when the image has time t as the horizontal axis, the inspection range should be the time period △Ф / ω seconds. If it exceeds this range, the judgment result is opposite;

[0061] From Figure 2 the waveform diagram collected in / Table 3, extract the data of three key nodes, the Y-axis and X-axis values corresponding to the images of t1, t Max , t Min ;

[0062] Extract three key nodes from the collected data, the data corresponding to the time points of t1, t Max , t Min and then calculate the theoretical initial phase value;

[0063] Determine the compliance of the calculated initial phase value through the image comparison method;

[0064] Figure 1 And Figure 2 the description of the coincidence determination of the data source and the waveform diagram in;

[0065] (1) The data in column A is the Y-axis data of a complete sine wave graph. The acquisition card completes the data acquisition of 1000 points within one period (0.02 seconds).

[0066] (2) Column B is the time value corresponding to each acquisition point within one period of 0.02 seconds.

[0067] (3) The value in cell D2 is obtained through function 1: MAX(A2:A1002).

[0068] (4) Cell C2 is obtained through function 2: INDEX(B:B,MATCH(MAX(A:A),A:A,0)).

[0069] (5) The value in cell D4 is obtained through function 3: Min(A2:A1002).

[0070] (6) Cell C4 is obtained through function 3: INDEX(B:B,MATCH(Min(A:A),A:A,0)).

[0071] (7) The upper waveform graph in Figure 4 is generated by extracting values. There are 3 initial phase values, and only one graph coincides with the graph generated by the acquisition card, that is, the graph with the initial phase of θ3.

[0072] (8) The judgment rule for the initial phase calculated by the graphical method: regardless of the waveform fluctuation, the phase law is the first, positive in the middle and turning, negative in the middle and flat; positive positive positive turning, negative negative negative flat; negative positive negative turning, positive negative positive flat.

[0073] (9) The graphs with the initial phases of θ1 and θ2 in the upper waveform graph in Figure 4 do not coincide with the graph generated by the acquisition card. The graph with the initial phase of θ1 appears symmetric with the collected waveform about the X-axis. The graph with the initial phase of θ2 appears symmetric with the collected waveform about the straight line passing through the image intersection point and parallel to the Y-axis. In this case, the calculated value is judged with time t as the horizontal axis, and its range is not within the time period of △Ф / ω seconds. If it exceeds this range, the judgment result may be opposite. Therefore, the conformity of the initial phase is judged by the requirement that the waveform coincidence and the functional formula must be consistent.

[0074] (10) The lower waveform graph in Figure 4 is obtained through the phase lead or lag of the waveform graph and the requirements of the initial phase judgment rule, resulting in the lower waveform Figure 1 Consistent. To verify the accuracy of its measurement and judgment, the error rate in column O of the data table proves that the method of inferring the initial phase value from the collected waveform graph is feasible.

[0075] (11) The graphs with the initial phases of θ1, θ2 and θ3 appear in the waveform graph in Figure 5 and coincide with the graph generated by the acquisition card

[0076] The coincidence phenomenon occurs when the acquisition cut-in point is within the time period of △Ф / ω seconds. When judged with time t as the horizontal axis, the actually acquired waveform is very close to a complete sine or cosine waveform.

[0077] (12) To prevent the graphical error at the first time cut-in acquisition point and the deviation that occurs when calculating the initial phase, t1 is taken as 0 and 0.00002 for calculation. Therefore, the values in cell H2 and cell F32 appear. Finally, the average value of the sum of the 4 determined initial phase values is taken as the initial phase value of the current or voltage. The Amax amplitude is also obtained in sequence.

[0078] 6. Mathematical expression and calculation process requirements for calculating the AC resistance.

[0079] (1) When determining the phase relationship between two sinusoidal quantities of voltage and current according to the mathematical expression, the phase difference should be calculated first, and then the leading and lagging relationships should be determined using the positive and negative of the phase difference. If △Ф = Ф1 - Ф2 > 0, the first sinusoidal quantity leads the second sinusoidal quantity; if △Ф = Ф1 - Ф2 < 0, the first sinusoidal quantity lags the second sinusoidal quantity. From this, it can be inferred whether the system belongs to capacitive impedance or inductive impedance.

[0080] (2) To make the phase difference of the sinusoidal quantity have a single value and be able to explain the leading and lagging relationships between the two sinusoidal quantities in terms of phase or time, the phase difference must be expressed in an angle with an absolute value less than π. For a positive phase difference greater than π, it should be converted into a negative phase difference greater than -π; for a negative phase difference less than -π, it should be converted into a positive phase difference less than π. For example, △Ф = 240° = 4 / 3π above should be converted into △Ф = -120° = -2 / 3π.

[0081] Therefore, the phase difference must be expressed in an angle with an absolute value less than 180° (π radians).

[0082] Exemplarily, taking a group of high-voltage cables as an example:

[0083] When a high-voltage cable operates in the power grid, 3 single-core cables or 1 three-core cable are required to form a loop. Therefore, during actual use, the AC resistance is affected by both the skin effect and the proximity effect. Measuring the AC resistance of a single-phase conductor alone does not conform to the actual situation and has little reference significance. Only by selecting a three-core cable for AC resistance measurement can it be consistent with the actual application scenario, and the result is more meaningful for reference.

[0084] Therefore, in this test, a 3×2000mm copper conductor high-voltage AC submarine cable with a length of 10m 2 is selected as a sample for AC resistance test research.

[0085] 2 Test process and theoretical calculation

[0086] Connect the electrical circuit, apply a current of 1000 A to the sample through a large current AC generator. After the temperature change value is less than 0.5 °C within half an hour, collect the waveforms and phases of the voltage and current of the sample through the data acquisition system. The sampling rate is 50,000 samples per second. To improve the accuracy of the collected data, the collection time is set to 2 seconds, and a total of 100 groups of parallel samples are collected. Through software fitting, 1 group of final data is obtained for calculation. During the test, a thermocouple is used to collect the temperature of the sample conductor synchronously.

[0087] The test data processing process is shown in Table 1.

[0088] The data processing formula has been programmed into the AC resistance test software program. The following is a schematic calculation process:

[0089] (1) Sinusoidal electrical quantity (time function), current and voltage values.

[0090] i = Im * sin(100πt + φ1);

[0091] (2) The change of sinusoidal electrical quantity (time function) to vector (complex number), current and voltage formula

[0092] Im∠φ1 = Im(cosφ1 + jsinφ1);

[0093] Um∠φ2 = um(cosφ2 + jsinφ2);

[0094] (3) 3-core cable current vector operation formula

[0095] I total∠φ total = (Iamcosφa1 + Ibmcosφb1 + Icmcosφc1) + j

[0096] (Iamsinφa1 + Ibmsinφb1 + Icmsinφc1)

[0097] Let: X = I am cosφ a 1 + Ib m cosφb1 + I cm cosφ c 1;

[0098] Y = Iamsinφa1 + Ibmsinφb1 + Icmsinφc1;

[0099] That is: I total = sqrt(pow(X, 2) + pow(Y, 2));

[0100] ∠φI total = ATAN(Y / X);

[0101] (4) 3 - core cable voltage vector operation formula

[0102] ∠φU total = (Uamcosφa2 + Ubmcosφb2 + Ucmcosφc2) + j

[0103] (Uamsinφa2 + Ubmsinφb2 + Ucmsinφc2)

[0104] Let: D = Uamcosφa2 + Ubmcosφb2 + Ucmcosφc2;

[0105] E = Uamsinφa2 + Ubmsinφb2 + Ucmsinφc2;

[0106] That is: U total = 3sqrt(pow(D, 2) + pow(E, 2)); (Note: Y - type connection, sum of 3 - core voltages)

[0107] ∠φU total = ATAN(E / D);

[0108] (5) 3 - phase AC impedance

[0109] Z = (U total / I total)∠(φU total - φI total), the real - part value of which is the 3 - core AC resistance value,

[0110] That is: R test value = (U total / I total) * cos(φU total - φI total);

[0111] (6) Average value of 3 - core AC resistance value

[0112] R = R test value / L / 3 * k, where L: length of the measured cable core; k: temperature coefficient. After the sample returns to room temperature, apply a current of 1500A to the sample through a large - current AC generator, and repeat the above steps to obtain the second set of data;

[0113] After the sample returns to room temperature, apply a current of 1900A to the sample through a large - current AC generator, and repeat the above steps to obtain the third set of data;

[0114] The temperatures of the conductor at different test currents collected by the thermocouple are shown in Table 1.

[0115] The DC resistance of the conductor of the sample obtained by the DC resistance test equipment at 20°C is 0.0088Ω / km. Calculate the DC resistance of the sample at different test temperatures according to the calculation method specified in GB / T3956 - 2008, as shown in Table 1;

[0116] After processing the test data, the AC resistance of the sample at different currents and temperatures is obtained, as shown in Table 1.

[0117] Table 1: Relationship between AC resistance / DC resistance at different test temperatures

[0118]

[0119] Note: The calculation of DC resistance is carried out according to the calculation method specified in GB / T3956 - 2008.

[0120] Table 2: Test Results

[0121] Preface Test Electricity Sample Conductor Alternating Current Alternating Current Corresponding Temperature Alternating Current Measurement of Alternating Current Resistance 1 1000 39.3 0.011 0.011 0.0095 1.253 1.252 2 1500 58.7 0.012 0.013 0.0101 1.238 1.297 3 1900 75.7 0.013 0.013 0.0107 1.215 1.299

[0122] Note 1: According to the calculation method recommended by IEC60287 - 1 - 1.

[0123] Note 2: According to the calculation method specified in GB / T3956 - 2008.

[0124] 3 Result Analysis

[0125] Analyzing the above data, when the temperature of the sample conductor is 39.3 °C, the calculated value and the measured value of its AC resistance are basically the same. As the temperature continues to rise, the ratio of the calculated AC resistance to the DC resistance shows a downward trend, but the ratio of the measured AC resistance to the DC resistance remains stable and the two are still relatively close.

[0126] There are two reasons for the difference between the theoretical calculation and the measured value of the AC resistance. First, since there is a water-blocking gel between the single wires of the submarine cable conductor, which plays a certain insulating role, but the insulation between the single wires is not complete, and the actual situation does not exactly match the third situation given in the test results of Table 2. Therefore, the selection of the ks and kp values is not completely accurate, resulting in a certain deviation between the theoretical calculated value and the actual situation, so the theoretical calculated value and the measured value are not exactly the same, and the theoretical calculation of the AC resistance needs to be further considered. Second, due to certain errors in the current, voltage, and temperature acquisition of the test device, the comprehensive analysis shows that the systematic error of the test device is about 1%, which leads to a certain deviation between the measured value and the actual value. In the future, the error can be reduced by changing the length of the lead wire for equipment measurement, the wiring method, and the anti-interference performance.

[0127] In addition, the processing method adopted for the loss of the steel wire armor in the calculation process is different from the actual situation, which is also one of the reasons for the difference between the calculated value and the measured value of the AC resistance.

[0128] In summary, the AC resistance test device and test method introduced in this paper can accurately measure the AC resistance of large-section 3-core high-voltage AC cables, which has good practical significance.

[0129] The technical means disclosed by the solution of the present invention are not limited to the technical means disclosed in the above embodiments, but also include technical solutions composed of any combination of the above technical features. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and refinements can be made, and these improvements and refinements are also regarded as the protection scope of the present invention.

Claims

1. A method for calculating the AC resistance of a cable core using a waveform diagram obtained by a parallel acquisition method, characterized in that: The following steps are involved: The acquisition card with differential analog signal input collects the sinusoidal waveform data of voltage and current; Generate a waveform diagram based on the collected data and obtain its initial phase angle value; According to the collected waveform, extract the Y-axis and X-axis values ​​corresponding to the images of three key node data, t1, tMax, and tMin; Extract the data corresponding to three key nodes, t1, tMax, and tMin, from the collected data and calculate the theoretical initial phase value; By using the image comparison method, the consistency of the calculated initial phase value is determined; The AC resistance of the cable core is calculated based on the initial phase value and the collected voltage and current data.

2. The method according to claim 1, characterized in that The AC resistance calculation process comprises the following steps: According to the vector calculation formula: , calculate and determine the voltage drop along each phase, between the measurement points, so that the measurement is Va∠θa, Vb∠θb, Vc∠θc; where va is the modulus of the complex number, θa is the argument of the complex number, and j is the imaginary unit; According to the following calculation formula , calculate the average conductor voltage drop over the entire cable length; According to the condition of power supply being industrial frequency positive sequence phase rotation: ; By calculation formula: , calculate the average conductor current; By calculation: ; Calculate the three-phase AC impedance, and its actual value is the AC resistance value.

3. The method according to claim 1, characterized in that: When the image is based on ωt as the horizontal axis, the investigation range is the phase difference between the two sinusoidal quantities; when the image is based on time t as the horizontal axis, the investigation range should be the time period △Ф / ω seconds. If it exceeds this range, the judgment result will be the opposite.

4. The method according to claim 1, characterized in that: The image comparison method includes determining the conformity of the initial phase by requiring waveform matching and functional consistency.

5. The method according to claim 1, characterized in that The AC resistance calculation process includes: a phase difference greater than π should be converted into a negative phase difference greater than -π; a phase difference less than -π should be converted into a positive phase difference less than π.

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