A method for detecting three-phase unbalanced current in cables based on phase difference

Abstract

The present invention proposes a method for detecting three-phase unbalanced current of a cable based on phase difference, which analyzes the phase difference of the magnetic field signals generated by the radial and tangential directions of the three-core three-phase cable in different directions, thereby obtaining the phase difference images of the radial and tangential magnetic field signals of the three-core three-phase cable in different directions, and uses the magnetic field phase difference image generated by the three-phase current balance of the cable as a reference to monitor whether the three-phase current of the three-core three-phase cable meets the balance requirement, so as to achieve the purpose of real-time monitoring. At the same time, using the phase difference as a monitoring means for the three-core three-phase cable can ensure the long-term operation of the equipment, so the time period of regular calibration is greatly extended, reducing the workload of sensor calibration; even if the signal amplitude exceeds the amplitude measurement range of the magnetic sensor to produce saturation distortion as the phase difference as a monitoring means, it can be judged by the phase difference of the two signals.

Description

Technical Field

[0001] The present invention belongs to the field of current sensing and relates to a method for detecting three-phase unbalanced current in a cable based on phase difference, which is used to detect the phase difference of magnetic field signals generated radially and tangentially in different directions of a three-core three-phase cable, and based on this, judge whether the three-phase current passed by the three-core three-phase cable meets the three-phase balance requirement. Background Art

[0002] The current value is an important monitoring quantity used in the power system to evaluate the operating status of the power grid. Excessive three-phase unbalanced current will have an adverse effect on the healthy operation of the power grid. Only when the three-phase balance is guaranteed can the loss in the power transmission be minimized, the lowest energy consumption can be achieved, and effective energy saving can be achieved. The commonly used method at present is to use a clamp meter to perform fixed-point measurement at the neutral point grounding of the transformer, but this method requires on-site detection and cannot display the cable-related conditions in real time. When there is a problem with the cable, it is impossible to provide timely feedback. For the measurement method based on the amplitude of the magnetic field signal, although it can solve the problem of real-time performance, for the measurement of amplitude, the long-term operation of the sensor will cause drift, resulting in inaccurate amplitude measurement, and regular calibration is required; for the magnetic sensor based on the amplitude, when the magnetic field exceeds the amplitude measurement range, saturation distortion will occur. Therefore, a new three-phase current balance state detection technology is needed that can ensure real-time performance, long-term stability, and a large measurement range.

[0003] The present invention proposes to analyze the phase difference of the magnetic field signals generated by the radial and tangential directions of the three-core three-phase cable in different directions, thereby obtaining the phase difference images of the radial and tangential magnetic field signals of the three-core three-phase cable in different directions, and using the magnetic field phase difference image generated by the three-phase current balance of the cable as a reference to monitor whether the three-phase current of the three-core three-phase cable meets the balance requirement, so as to achieve the purpose of real-time monitoring. At the same time, using the phase difference as a monitoring means for the three-core three-phase cable is not affected by the amplitude drift of the sensor output, and can ensure the long-term operation of the equipment, so the time period of regular calibration is greatly extended, reducing the workload of sensor calibration; as a monitoring means, even if the signal strength exceeds the amplitude measurement range of the magnetic sensor, thereby generating saturation distortion, it can also be judged by the phase difference of the two signals. Summary of the invention

[0004] One purpose of the present invention is to propose a method for detecting three-phase unbalanced current of a cable based on phase difference, without increasing the complexity of the equipment, taking the phase difference of the magnetic field signals generated by the radial and tangential directions of the three-core three-phase cable in different directions as the judgment basis. The present invention uses the analysis of the phase difference of the magnetic field signals generated by the radial and tangential directions of the three-core three-phase cable in different directions to draw phase difference images of the three-core three-phase cable in different directions, and based on this, makes a judgment on the balance of the three phases of the three-core three-phase cable.

[0005] The working mechanism of the present invention is as follows: after three-phase electricity is passed through a three-core three-phase cable (7), a magnetic field is generated around it, and the magnitude and direction of the magnetic field are different, so that the phase difference of the radial and tangential magnetic field signals generated by the three-core three-phase cable (7) in different directions is different. A magnetic sensor array (9) composed of a plurality of vector magnetic sensors at equal angles composed of a vector magnetic sensor (8) detects the radial and tangential phase differences in each direction of the three-core three-phase cable (7) axially around one circle, and draws phase difference images of the three-core three-phase cable (7) in different directions. When an unbalanced current and a balanced current are passed through the three-core three-phase cable (7), the generated phase images have obvious differences. Therefore, using the phase difference image drawn by the magnetic field generated by the balanced three-phase current as a basis for judgment, it is easy to judge whether the three-phase current of the three-core three-phase cable (7) is balanced.

[0006] A method for detecting three-phase unbalanced current in a cable based on phase difference. The method is based on a three-core three-phase cable current detection device. The three-core three-phase cable current detection device comprises a magnetic sensor system (1) and a phase detection system (2). The magnetic sensor system (1) and the phase detection system (2) are connected in sequence via a wire.

[0007] The magnetic sensing system (1) comprises a magnetic sensor array (8) composed of a plurality of vector magnetic sensors (7) for acquiring a magnetic field generated by a three-core three-phase cable (6); the three-core three-phase cable (6) comprises an A core (3), a B core (4), and a C core (5); when a three-phase current exists in the A core (3), the B core (4), and the C core (5) of the three-core three-phase cable (6), the magnetic sensor array (8) acquires the magnetic field generated by the A core (3), the B core (4), and the C core (5) in radial and tangential directions of the three-core three-phase cable (6) at different orientations.

[0008] Preferably, the magnetic sensing system (1) comprises n vector magnetic sensors (7); the n vector magnetic sensors (7) are equally spaced and distributed on a circle with a radius R and a center point of the three cores of the three-core three-phase cable (6), that is, each vector magnetic sensor (7) is at the same distance from the axis of the three-core three-phase cable (6); wherein R≥d, d represents 1 / 2 of the outer diameter of the three-core three-phase cable (6); n=360 / θ and is an integer, θ represents the center angle between two adjacent sampling points;

[0009] Preferably, the magnetic sensing system (1) comprises a vector magnetic sensor (7), a circular track, and a motor; the circular track is located on the periphery of the three-core three-phase cable (6), and its center coincides with the center points of the three cores of the three-core three-phase cable (6); the vector magnetic sensor (7) is slidably connected to the inner side of the circular track; the motor drives the vector magnetic sensor (7) to slide along the inner side of the circular track; the radius of the circular track R ≥ d, d represents 1 / 2 of the outer diameter of the three-core three-phase cable (6);

[0010] The phase detection system (2) comprises a vector magnetic sensor signal receiving device (9) and a vector magnetic sensor signal processing device (10); the signal acquired by the magnetic sensor array (8) is read by the vector magnetic sensor signal receiving device (9), and the vector magnetic sensor signal processing device (10) draws images of the radial and tangential magnetic field phase differences at different orientations acquired by the vector magnetic sensor signal receiving device (9).

[0011] The method for detecting three-phase unbalanced current of a cable based on phase difference specifically comprises the following steps:

[0012] Step (1): Build a test platform:

[0013] 1-1 Select a suitable position on a three-core three-phase cable (6) to install a magnetic sensor array (8) capable of detecting radial and tangential magnetic fields in different directions;

[0014] 1-2 Turn on the vector magnetic sensor signal receiving device (10), receive the radial and tangential magnetic field signals of the three-core three-phase cable (6) at different orientations collected by the magnetic sensor array (8), and calculate the phase difference of the radial and tangential magnetic fields at different orientations.

[0015] Step (2), determining the three-core calibration position of the three-core three-phase cable (6):

[0016] 2-1 Only the current of the A core (3) of the three-core three-phase cable (6) is passed, and the current of the B core (4) and the C core (5) is not passed, and the magnetic field magnetic induction intensity received by the magnetic sensor array (8) is used to find the position with the maximum magnetic induction intensity as the calibration position of the A core (3) of the three-core three-phase cable (6);

[0017] 2-2 Only the current of the B core (4) of the three-core three-phase cable (6) is passed, and the current of the A core (3) and the C core (5) is not passed, and the magnetic field magnetic induction intensity received by the magnetic sensor array (8) is used to find the position with the maximum magnetic induction intensity as the calibration position of the B core (4) of the three-core three-phase cable (6);

[0018] 2-3 Only the current of the C core (5) of the three-core three-phase cable (6) is passed, and the current of the A core (3) and the B core (4) is not passed, and the magnetic field magnetic induction intensity received by the magnetic sensor array (8) is used to find the position with the maximum magnetic induction intensity as the calibration position of the C core (5) of the three-core three-phase cable (6);

[0019] Step (3), constructing a standard image with the magnetic field sampling point position as the horizontal coordinate and the phase difference as the vertical coordinate;

[0020] 3-1 regulating the currents of the A core (3), the B core (4) and the C core (5) in the three-core three-phase cable (6) so that the three-phase currents are balanced;

[0021] 3-2 The radial and tangential magnetic fields in different directions are acquired by the magnetic sensor array (8) and transmitted to the vector magnetic sensor signal receiving device (9);

[0022] The 3-3 vector magnetic sensor signal receiving device (9) obtains the phase difference of the radial and tangential magnetic fields of the three-core three-phase cable (6) at different directions;

[0023] 3-4 The vector magnetic sensor signal receiving device (10) uses the magnetic field sampling point position as the horizontal coordinate, wherein one of the three calibration positions of the three-core three-phase cable (6) determined in step (2) is selected as the initial position, and the phase difference is used as the vertical coordinate to draw the standard image S 0 ;

[0024] Step (4), real-time detection of the three-phase current balance of the three-core three-phase cable (6):

[0025] 4-1: Without changing the platform built in the above step (1) and the calibration position of the three-core three-phase cable (6) in the above step (2), the magnetic sensor array (8) acquires the radial and tangential magnetic fields of the three-core three-phase cable (6) in different directions in real time, the vector magnetic sensor signal receiving device (9) calculates the phase difference of the radial and tangential magnetic fields of the three-core three-phase cable (6) in different directions according to the magnetic field signal acquired by the magnetic sensor array (8), and then the vector magnetic sensor signal receiving device (10) draws a real-time image S i ;

[0026] 4-2: Real-time image S i With the standard image S 0 If they are the same, the three-phase current is considered balanced. Otherwise, it is necessary to determine whether the phase difference corresponding to the magnetic field sampling point position is consistent with the standard image S 0 Whether the absolute value of the difference corresponding to the phase difference is less than or equal to the threshold, if so, it is considered that the three-phase current is balanced, otherwise it is considered that the three-phase current is unbalanced.

[0027] Preferably, the radial and tangential magnetic fields of the three-core three-phase cable (7) in each direction are B x, B y , specifically:

[0028]

[0029]

[0030] Among them A a ,A b ,A c are the magnetic field amplitudes generated by the A core (3), the B core (4), and the C core (5) of the three-core three-phase cable (7); α, β, and γ are the angles between the A core (3), the B core (4), and the C core (5) of the three-core three-phase cable (7) and the vector magnetic sensor (7) with respect to the horizontal direction; and w is the angular frequency of the current;

[0031] The magnetic fields generated by the currents in the A core (3), B core (4) and C core (5) of the three-core three-phase cable (7) are combined to obtain B x , B y Initial phase of magnetic field Specifically:

[0032]

[0033]

[0034] Among them A xbc ,A ybc , for

[0035]

[0036]

[0037]

[0038]

[0039] So the phase difference Expressed as

[0040]

[0041] The beneficial effects of this method are:

[0042] 1. Compared with using magnetic field strength as the standard, using phase difference as the standard can reduce the workload of calibration and ensure that the sensor works stably for a long time.

[0043] 2. It can meet the real-time requirements of cable detection and detect the cable status in real time.

[0044] 3. Compared with the magnetic field strength as the standard, the present invention proposes to use the phase difference as the standard, which can greatly improve the detection range of the magnetic sensor for the three-phase unbalanced current. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Figure 1 is a schematic diagram of the process of this method;

[0046] Figure 2 Detailed flowchart of this method;

[0047] Figure 3 This method is a result diagram calculated by the finite source analysis method; a is a comparison diagram of the result calculated by the finite source analysis method when only one phase current decreases and the result calculated by the finite source analysis method when the three phases are balanced, and b is a comparison diagram of the result calculated by the finite source analysis method when only one phase current increases and the result calculated by the finite source analysis method when the three phases are balanced;

[0048] Figure 4 Graphs showing the results of the actual experiment of this method; a is a comparison graph of the actual experimental results when only one phase current decreases and the actual experimental results when the three phases are balanced, and b is a comparison graph of the actual experimental results when only one phase current increases and the actual experimental results when the three phases are balanced;

[0049] Figure 5 These are the radial and tangential magnetic field diagrams of this method when different three-phase balanced currents are passed; among them, a is the radial and tangential magnetic field diagram when 2A current is passed through all three phases of the three-phase cable, b is the radial and tangential magnetic field diagram when 13A current is passed through all three phases of the three-phase cable, and c is the radial and tangential magnetic field diagram when 30A current is passed through all three phases of the three-phase cable. DETAILED DESCRIPTION

[0050] The method is further analyzed below in conjunction with the accompanying drawings.

[0051] like Figure 1 As shown, the three-core three-phase cable current detection device includes a magnetic sensor system 1 and a phase detection system 2; the magnetic sensor system 1 and the phase detection system 2 are connected in sequence through wires.

[0052] like Figure 2As shown, the magnetic sensing system 1 includes a three-phase A core 3 of the cable, a three-phase B core 4 of the cable, a three-phase C core 5 of the cable, a three-core three-phase cable 6, a vector magnetic sensor 7, and a magnetic sensor array 8 composed of multiple equiangularly spaced vector magnetic sensors; when there is a three-phase current in the three-phase A core 3 of the cable, the three-phase B core 4 of the cable, and the three-phase C core 5 of the cable, the magnetic sensor array 8 composed of multiple equiangularly spaced vector magnetic sensors composed of the vector magnetic sensor 7 obtains the radial and tangential magnetic fields generated by the three-phase A core 3 of the cable, the three-phase B core 4 of the cable, and the three-phase C core 5 of the cable in different directions of the three-core three-phase cable 6.

[0053] The phase detection system 2 includes a vector magnetic sensor signal receiving device 9 and a vector magnetic sensor signal processing device 10; the magnetic sensor array 8 composed of a plurality of vector magnetic sensors 7 at equal angles obtains signals which are read by the vector magnetic sensor signal receiving device 9 to obtain the phase difference between the radial and tangential magnetic fields of the three-core three-phase cable 6 at different orientations, and the vector magnetic sensor signal processing device 10 processes the phase difference.

[0054] The vector magnetic sensor 7 in the actual example uses a fluxgate sensor, and the magnetic sensor array 8 composed of a plurality of vector magnetic sensors spaced at equal angles is distributed on a circle with a radius R and a center point of the three cores of the three-core three-phase cable (6), that is, each vector magnetic sensor (7) is at the same distance from the axis of the three-core three-phase cable (6); wherein R is 1 / 2 of the outer diameter of the three-core three-phase cable (6); and the center angle between two adjacent sampling points is 5 degrees.

[0055] The magnetic sensing system (1) described in this embodiment includes a vector magnetic sensor (7), a circular track, and a motor; the circular track is located on the outer circumference of the three-core three-phase cable (6), and its center coincides with the center points of the three cores of the three-core three-phase cable (6); the vector magnetic sensor (7) is slidably connected to the inner side of the circular track; the motor drives the vector magnetic sensor (7) to slide along the inner side of the circular track; the radius R of the circular track is 1 / 2 of the outer diameter of the three-core three-phase cable (6). Figure 2 At the position shown, a fluxgate sensor collects data from the three-core three-phase cable 6 at every 5 degrees as a sampling point. This is equivalent to a magnetic sensor array 8 composed of multiple vector magnetic sensors at equal angles. The vector magnetic sensor signal receiving device 9 uses an oscilloscope, which directly reads the phase difference of the two signals through the radial and tangential magnetic field signals of the three-core three-phase cable 6 in different directions of the fluxgate sensor of the oscilloscope; the vector magnetic sensor signal processing device 10 manually collects the phase difference of each direction of the three-core three-phase cable 6 and draws an image of the total phase difference.

[0056] During the experiment, a circular track with a scale of 5 degrees was added to the three-core three-phase cable 6, and the vector magnetic sensor 7 was fixed on the circular track, and the sampling point position was tangent to the three-core three-phase cable 6. The detected signal was passed to the vector magnetic sensor signal receiving device 9. Only the A core 3 of the three-phase cable was energized, and the vector magnetic sensor signal receiving device 9 displayed different sampling point amplitudes. The maximum amplitude position was found to be the position directly above the A core 3 of the three-phase cable. The positions directly above the B core 4 and the C core 5 of the three-phase cable were found in the same way as the three calibration positions. Record the calibration position of the A core 3 of the three-phase cable as the initial point, and pass a current of 20A into the A core 3 of the three-phase cable, the B core 4 of the three-phase cable, and the C core 5 of the three-phase cable. Starting from the initial point, the vector magnetic sensor signal receiving device 10 samples at every 5 degrees as a sampling point, and records the phase difference of each sampling point. Draw an image with the position of each point starting from the initial point as the horizontal coordinate and the phase difference as the vertical coordinate. In the same way, collect images of the current with only the current value of the B core 5 of the three-phase cable changed to 22A, 21A, 19A, and 18A for comparative experiments.

[0057] like Figure 3 As shown, this figure is the result calculated using the finite source analysis method.

[0058] like Figure 4 As shown in the figure, this figure is the result of actual measurement.

[0059] It can be seen from the two figures that there is a clear difference between the phase diagram when the three-phase is unbalanced and the phase diagram when the three-phase is balanced. Therefore, the method of using phase difference to detect the three-phase balance of the cable is feasible.

[0060] like Figure 5 As shown, in the radial and tangential magnetic field diagrams of the present invention when different three-phase balanced currents are passed, it can be seen that when the current reaches 30A and exceeds the range of the magnetic sensor amplitude measurement, when saturation distortion occurs, the measurement result of the phase difference is still consistent with that when it does not exceed the range of the magnetic sensor amplitude measurement, which fully proves that using the phase difference as a detection standard can greatly improve the detection range of the magnetic sensor for the three-phase unbalanced current.

Claims

1. A method for detecting three-phase unbalanced current in a cable based on phase difference, the method being based on a three-core three-phase cable current detection device, the three-core three-phase cable current detection device comprising a magnetic sensor system (1) and a phase detection system (2); the magnetic sensor system (1) and the phase detection system (2) are sequentially connected via a wire; The magnetic sensing system (1) comprises a magnetic sensor array (8) composed of a plurality of vector magnetic sensors (7) for acquiring a magnetic field generated by a three-core three-phase cable (6); the three-core three-phase cable (6) comprises an A core (3), a B core (4), and a C core (5); when a three-phase current exists in the A core (3), the B core (4), and the C core (5) of the three-core three-phase cable (6), the magnetic sensor array (8) acquires the magnetic field generated by the A core (3), the B core (4), and the C core (5) in radial and tangential directions of the three-core three-phase cable (6) at different orientations; The phase detection system (2) comprises a vector magnetic sensor signal receiving device (9) and a vector magnetic sensor signal processing device (10); the signal acquired by the magnetic sensor array (8) is read by the vector magnetic sensor signal receiving device (9), and the vector magnetic sensor signal processing device (10) draws images of the radial and tangential magnetic field phase differences at different orientations acquired by the vector magnetic sensor signal receiving device (9); Features The method comprises the following steps: Step (1): Build a test platform: 1-1 A magnetic sensor array (8) capable of detecting radial and tangential magnetic fields in different directions is installed on a three-core three-phase cable (6); 1-2 Turning on the vector magnetic sensor signal receiving device (10), receiving radial and tangential magnetic field signals of the three-core three-phase cable (6) at different orientations collected by the magnetic sensor array (8), and calculating the phase difference of the radial and tangential magnetic fields at different orientations; Step (2), determining the three-core calibration position of the three-core three-phase cable (6): 2-1 Only the current of the A core (3) of the three-core three-phase cable (6) is passed, and the current of the B core (4) and the C core (5) is not passed, and the magnetic field magnetic induction intensity received by the magnetic sensor array (8) is used to find the position with the maximum magnetic induction intensity as the calibration position of the A core (3) of the three-core three-phase cable (6); 2-2 Only the current of the B core (4) of the three-core three-phase cable (6) is passed, and the current of the A core (3) and the C core (5) is not passed, and the magnetic field magnetic induction intensity received by the magnetic sensor array (8) is used to find the position with the maximum magnetic induction intensity as the calibration position of the B core (4) of the three-core three-phase cable (6); 2-3 Only the current of the C core (5) of the three-core three-phase cable (6) is passed, and the current of the A core (3) and the B core (4) is not passed, and the magnetic field magnetic induction intensity received by the magnetic sensor array (8) is used to find the position with the maximum magnetic induction intensity as the calibration position of the C core (5) of the three-core three-phase cable (6); Step (3), constructing a standard image with the magnetic field sampling point position as the horizontal coordinate and the phase difference as the vertical coordinate; 3-1: Regulating the currents of the A core (3), the B core (4), and the C core (5) in the three-core three-phase cable (6) so that the three-phase currents are balanced; 3-2: The radial and tangential magnetic fields in different directions are acquired by the magnetic sensor array (8) and transmitted to the vector magnetic sensor signal receiving device (9); The 3-3 vector magnetic sensor signal receiving device (9) obtains the phase difference of the radial and tangential magnetic fields of the three-core three-phase cable (6) at different directions; 3-4 The vector magnetic sensor signal receiving device (10) uses the magnetic field sampling point position as the horizontal coordinate, wherein one of the three calibration positions of the three-core three-phase cable (6) determined in step (2) is selected as the initial position, and the phase difference is used as the vertical coordinate to draw the standard image S 0 ; Step (4), real-time detection of the three-phase current balance of the three-core three-phase cable (6): 4-1: Without changing the platform built in the above step (1) and the calibration position of the three-core three-phase cable (6) in the above step (2), the magnetic sensor array (8) acquires the radial and tangential magnetic fields of the three-core three-phase cable (6) in different directions in real time, the vector magnetic sensor signal receiving device (9) calculates the phase difference of the radial and tangential magnetic fields of the three-core three-phase cable (6) in different directions according to the magnetic field signal acquired by the magnetic sensor array (8), and then the vector magnetic sensor signal receiving device (10) draws a real-time image S i ; 4-2: Real-time image S i With the standard image S 0 If the three-phase currents are the same, the three-phase currents are considered balanced. Otherwise, the phase difference corresponding to the magnetic field sampling point position and the standard image S are determined again. 0 Whether the absolute value of the difference corresponding to the phase difference is less than or equal to the threshold, if so, it is considered that the three-phase current is balanced, otherwise it is considered that the three-phase current is unbalanced.

2. The method according to claim 1, Features The radial and tangential magnetic fields of the three-core three-phase cable (7) in each direction are respectively x , B y , specifically: Among them A a ,A b ,A c are the magnetic field amplitudes generated by the A core (3), the B core (4), and the C core (5) of the three-core three-phase cable (7); α, β, and γ are the angles between the A core (3), the B core (4), and the C core (5) of the three-core three-phase cable (7) and the vector magnetic sensor (7) with respect to the horizontal direction; and w is the angular frequency of the current; The magnetic fields generated by the currents in the A core (3), B core (4) and C core (5) of the three-core three-phase cable (7) are combined to obtain B x , B y Initial phase of magnetic field Specifically: Among them A xbc ,A ybc , for So the phase difference Expressed as 3. The method according to claim 1, Features The magnetic sensing system (1) comprises n vector magnetic sensors (7); the n vector magnetic sensors (7) are evenly spaced and distributed on a circle with a radius R and a center point of the three cores of the three-core three-phase cable (6) as the center, that is, each vector magnetic sensor (7) is at the same distance from the axis of the three-core three-phase cable (6); wherein R≥d, d represents 1 / 2 of the outer diameter of the three-core three-phase cable (6); n=360 / θ and is an integer, and θ represents the center angle between two adjacent sampling points.

4. The method according to claim 1, Features The magnetic sensing system (1) comprises a vector magnetic sensor (7), a circular track, and a motor; the circular track is located on the outer circumference of the three-core three-phase cable (6), and its center coincides with the center points of the three cores of the three-core three-phase cable (6); the vector magnetic sensor (7) is slidably connected to the inner side of the circular track; the motor drives the vector magnetic sensor (7) to slide along the inner side of the circular track; the radius R of the circular track is ≥ d, and d represents 1 / 2 of the outer diameter of the three-core three-phase cable (6).

Images