Online cable fault detection device and calibration method based on broadband impedance spectroscopy

Abstract

The invention discloses an online cable fault detection device and calibration method based on broadband impedance spectroscopy, belongs to the technical field of cable fault detection, is used for cable fault detection, and comprises a direct digital frequency synthesizer DDS, a micro control unit MCU, an analog-to-digital converter ADC, four low-pass filters, two multipliers and an electromagnetic coupling device. The DDS is respectively connected with the MCU, the first low-pass filter and the second low-pass filter, and the analog-to-digital converter ADC is respectively connected with the MCU, the third low-pass filter and the fourth low-pass filter; the calibration process of the method includes obtaining a measured value of the cable reflection coefficient and a true value of the cable reflection coefficient. According to the invention, cable defects are monitored and positioning is realized during on-line operation of the cable, and the method is suitable for cables of various specifications and three-phase cables, has universality, and reduces the detection difficulty; the broadband impedance spectroscopy measurement precision is improved, the measurement error is reduced, and the cable fault defect positioning precision is improved.

Description

Technical Field

[0001] This invention discloses an online cable fault detection device and calibration method based on broadband impedance spectrum, belonging to the field of cable fault detection technology. Background Technology

[0002] With urban development, power grids have become increasingly complex and closely linked to production and daily life. Various external factors lead to frequent cable faults, making the rapid elimination of faults and restoration of power supply to affected areas crucial. Therefore, cable fault detection is widely used in engineering. Currently, most cable fault detection technologies are based on high-voltage discharge technology, requiring the connection of high-voltage discharge equipment to the cable under test. Different cable specifications necessitate the use of discharge equipment with different discharge levels, increasing both complexity and the risk factor in the testing process. Broadband impedance spectroscopy-based methods offer advantages such as non-destructive testing, simple connection, and fast measurement speed. However, compared to high-voltage discharge measurements, broadband impedance spectroscopy signals have high-frequency characteristics, which can affect the signal injected into the cable. Accurate measurement requires signal calibration, posing a challenge to achieving accurate measurement of the cable under test. Summary of the Invention

[0003] The purpose of this invention is to provide an online cable fault detection device and calibration method based on broadband impedance spectrum, so as to solve the problem of inaccurate cable fault detection in the prior art.

[0004] An online cable fault detection device based on broadband impedance spectrum includes a direct digital frequency synthesizer (DDS), a microcontroller unit (MCU), an analog-to-digital converter (ADC), four low-pass filters, two multipliers, and an electromagnetic coupling device. The DDS is connected to the MCU, the first low-pass filter, and the second low-pass filter, respectively. The ADC is connected to the MCU, the third low-pass filter, and the fourth low-pass filter, respectively.

[0005] The CH1 terminal of the DDS is sequentially connected to the first low-pass filter, the first capacitor, the first 2x operational amplifier, and the first resistor. Then, it has a branch. The first branch has a second resistor. After the second resistor, there is a branch. The first branch is connected to the second 2x operational amplifier, the first multiplier, the third low-pass filter, the first 20x operational amplifier, and the CH0 terminal of the ADC. The second branch is connected to the third 2x operational amplifier, the second multiplier, the fourth low-pass filter, the second 20x operational amplifier, and the CH1 terminal of the ADC. The second branch has a third resistor. After the third resistor, there is a fork. The first fork has an electromagnetic coupling device. The second fork is connected to the third 2x operational amplifier.

[0006] The CH0 terminal of the DDS is connected in sequence to the second low-pass filter, the second capacitor, and the first multiplier.

[0007] The DDS and MCU are connected via a Sequence Peripheral Interface (SPI), and the ADC and MCU are connected via a Sequence Peripheral Interface (SPI).

[0008] The first and second low-pass filters have a frequency of 200MHz, and the third and fourth low-pass filters have a frequency of 30kHz.

[0009] A power supply branch is provided between the first low-pass filter and the CH1 terminal of the DDS, and between the second low-pass filter and the CH0 terminal of the DDS. The power supply branch is connected in sequence to the power supply resistor and the 1.8V filter power supply.

[0010] A first grounding branch is provided between the first capacitor and the first 2x operational amplifier. The first grounding branch is connected in sequence to a grounding resistor and a grounding terminal. The second resistor is also connected to the second grounding branch.

[0011] A branch line connecting the second multiplier is provided between the second capacitor and the first multiplier.

[0012] The online cable fault detection method based on broadband impedance spectrum uses the aforementioned online cable fault detection device based on broadband impedance spectrum, and includes a measurement process and a calibration process. The calibration process includes obtaining the measured value and the true value of the cable reflection coefficient.

[0013] The calibration process includes measuring the cable reflection coefficient. and the true value of cable reflection coefficient for:

[0014] ;

[0015] ;

[0016] In the formula, It is a directional error. It is the reverse tracking error. It is the source matching error.

[0017] The calibration process includes:

[0018] ;

[0019] ;

[0020] ;

[0021] In the formula, It is a cable Calibration under load , It is a cable Calibration under negative load , It is a cable Calibration under load .

[0022] Compared with the prior art, the present invention has the following advantages: the present invention monitors and locates cable defects when the cable is running online, and is applicable to various specifications of cables and three-phase cables, which has universality and reduces the difficulty of detection; it improves the accuracy of broadband impedance spectrum measurement, reduces measurement error, and improves the accuracy of cable fault defect location. Attached Figure Description

[0023] Figure 1 This is a block diagram of the device in an example of the present invention;

[0024] Figure 2 This is a signal flow graph in an example of the present invention;

[0025] Figure 3 This is a calibration diagram from an example of the present invention;

[0026] Figure 4 The reflection coefficient spectrum;

[0027] Figure 5 This is the result of the integral transform of the reflection coefficient spectrum. Detailed Implementation

[0028] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention are described clearly and completely below. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0029] Online cable fault detection device based on broadband impedance spectrum, such as Figure 1 It includes a direct digital frequency synthesizer (DDS), a microcontroller unit (MCU), an analog-to-digital converter (ADC), four low-pass filters, two multipliers, and an electromagnetic coupling device. The DDS is connected to the MCU, the first low-pass filter, and the second low-pass filter, respectively. The analog-to-digital converter (ADC) is connected to the MCU, the third low-pass filter, and the fourth low-pass filter, respectively.

[0030] The CH1 terminal of the DDS is sequentially connected to the first low-pass filter, the first capacitor, the first 2x operational amplifier, and the first resistor. Then, it has a branch. The first branch has a second resistor. After the second resistor, there is a branch. The first branch is connected to the second 2x operational amplifier, the first multiplier, the third low-pass filter, the first 20x operational amplifier, and the CH0 terminal of the ADC. The second branch is connected to the third 2x operational amplifier, the second multiplier, the fourth low-pass filter, the second 20x operational amplifier, and the CH1 terminal of the ADC. The second branch has a third resistor. After the third resistor, there is a fork. The first fork has an electromagnetic coupling device. The second fork is connected to the third 2x operational amplifier.

[0031] The CH0 terminal of the DDS is connected in sequence to the second low-pass filter, the second capacitor, and the first multiplier.

[0032] The DDS and MCU are connected via a Sequence Peripheral Interface (SPI), and the ADC and MCU are connected via a Sequence Peripheral Interface (SPI).

[0033] The first and second low-pass filters have a frequency of 200MHz, and the third and fourth low-pass filters have a frequency of 30kHz.

[0034] A power supply branch is provided between the first low-pass filter and the CH1 terminal of the DDS, and between the second low-pass filter and the CH0 terminal of the DDS. The power supply branch is connected in sequence to the power supply resistor and the 1.8V filter power supply.

[0035] A first grounding branch is provided between the first capacitor and the first 2x operational amplifier. The first grounding branch is connected in sequence to a grounding resistor and a grounding terminal. The second resistor is also connected to the second grounding branch.

[0036] A branch line connecting the second multiplier is provided between the second capacitor and the first multiplier.

[0037] The online cable fault detection method based on broadband impedance spectrum uses the aforementioned online cable fault detection device based on broadband impedance spectrum, and includes a measurement process and a calibration process. The calibration process includes obtaining the measured value and the true value of the cable reflection coefficient.

[0038] The calibration process includes measuring the cable reflection coefficient. and the true value of cable reflection coefficient for:

[0039] ;

[0040] ;

[0041] In the formula, It is a directional error. It is the reverse tracking error. It is the source matching error.

[0042] The calibration process includes:

[0043] ;

[0044] ;

[0045] ;

[0046] In the formula, It is a cable Calibration under load , It is a cable Calibration under negative load , It is a cable Calibration under load .

[0047] The measurement process includes connecting the online cable fault detection device based on broadband impedance spectrum to the cable core and shielding layer at the beginning of the cable under test via a coaxial cable with clamps. After inputting the parameters, the measurement begins. For a length of... The cable is [length] from the beginning. Input impedance at for:

[0048] ;

[0049] In the formula, and These are the propagation coefficient and characteristic impedance of the cable, respectively. The reflection coefficient at the terminal is... It is a natural constant;

[0050] When a cable fault occurs, the location of the fault is at least 1 meter from the beginning of the cable. impedance at With the head impedance at They are respectively:

[0051] ;

[0052] ;

[0053] ;

[0054] ;

[0055] In the formula, , They are , The reflection coefficient, Distance from the head The fault resistor at the location, This indicates that two devices are connected in parallel. This is the load impedance.

[0056] To obtain a broadband impedance spectrum containing cable fault information, this invention requires the generation of a high-frequency swept sinusoidal signal. A direct digital frequency synthesizer (DDS) is used to generate this swept signal. However, due to the weak driving capability of the DDS, a signal conditioning circuit is subsequently designed to amplify the swept signal. After amplification, the signal is fed into a directional bridge to separate the reflected aliasing signal into the reflected signal and the source signal. These are then fed into a mixer along with the same local oscillator signal for down-conversion to an intermediate frequency (IF) signal. An ADC samples the reflected signal and transmits it to an MCU. After processing by the MCU, the broadband impedance spectrum of the cable input is obtained. Following the hardware design, a signal transmission control and acquisition program and a human-machine interface (HMI) are required. The signal transmission control and acquisition program is compiled and written to the MCU using KEIL5 as the IDE. The HMI is implemented using QT, allowing users to set the starting frequency, number of steps, and overall control of the device via a communication interface. It also processes the measurement data acquired and processed by the device using a positioning algorithm and displays the results on the host computer interface, thus achieving cable fault location.

[0057] Because high-frequency signals are highly susceptible to external environmental influences, and under high-frequency conditions, signals transmit within the device as electromagnetic waves, generating electromagnetic radiation interference that affects the quality of both the reflected and source signals, it is necessary to eliminate errors caused by internal electromagnetic interference to improve the system's measurement accuracy. To achieve this, a calibration method is proposed. This method first proposes an error model based on the device, and then uses the Mason formula to obtain the relationship between the error value and the true value. The process of obtaining the error value consists of two steps: the first step is error correction acquisition, which determines the device's system error by characterizing known calibration components; the second step is error correction application, which measures the broadband impedance spectrum of the cable and uses the error correction algorithm to obtain the correct result.

[0058] After the measurement is completed, this invention can obtain the broadband impedance spectrum of the cable termination containing the fault location. The broadband impedance of a cable is a function of its characteristic impedance, propagation coefficient, and terminal reflection coefficient. When a fault exists in the cable, the cable parameters at that location change, affecting the terminal reflection coefficient and ultimately reflected in the broadband impedance spectrum. Therefore, using broadband impedance spectrum measurement technology, a swept-frequency signal is injected into the device while simultaneously measuring the cable reflection coefficient. The data is acquired by an ADC and fed into an MCU, where FFT is used to obtain the incident signal and its amplitude and phase characteristics. The input impedance at various frequencies of the cable is then calculated. Finally, a host computer-based fault location algorithm is used to locate the cable fault.

[0059] The excitation circuit of this invention uses a DDS to generate a sweep frequency signal. The host computer controls the MCU via a serial port, and the MCU controls the DDS to generate the required sweep frequency signal via SPI communication. A directional bridge is used for signal separation. The midpoint of the left arm of the directional bridge contains only the incident signal, while the reflected signal at the midpoint of the right arm is superimposed on the incident signal. Differential operation is performed on the two points to obtain the reflected signal; the incident signal is the potential at the midpoint of the left arm of the directional bridge. The acquisition circuit and the MCU core circuit use an external ADC for synchronous sampling at a sampling frequency of 128kHz. The MCU is mainly responsible for controlling the excitation source to generate the required signal, communicating with the external ADC, transmitting the reflection coefficient to the host computer via serial communication, and interacting with the host computer. The measurement process involves first connecting the calibrated device to the cable core and shielding layer at the cable head end via a coaxial cable with clamps. After inputting parameters into the host computer, clicking "Start Measurement" and waiting for the measurement to complete, clicking "Generate Location Result" yields the cable fault location.

[0060] The signal flow graph of this invention is as follows: Figure 2 ,in Represents the reflection coefficient. and Indicates both ends of the cable. When calibrating the reflection parameters of the device, such as... Figure 3 As shown, the model mainly includes three error terms: directional error, backtracking error, and source matching error. , yes The values ​​at both ends of the cable under load, , yes The values ​​at both ends of the cable under load, , yes The values ​​at both ends of the cable under load. The transfer function of the error network can be represented using Mason's formula and signal flow graph, thus showing the relationship between the measured value and the true value of the device under test. The specific calibration process is as follows: the device is connected via a coaxial cable with clamps. Load, the device records the results of this calibration. The value is denoted as ,for and Repeat this process until you get , , Then, tests were conducted in a laboratory environment to verify the feasibility of cable fault location. The experimental object was a 300m long communication coaxial cable with a ground fault at 200m (including the insulation layer) via a resistor. The reflection coefficient spectrum of the cable before and after calibration was obtained and transmitted to a host computer for algorithm processing to obtain the fault location. Figure 4 and Figure 5 As shown, calibration can effectively remove the head-end oscillation, making the positioning results more obvious. Moreover, the fault location is the same as the experimental setting location, proving the correctness of the method proposed in this embodiment.

[0061] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. An online cable fault detection device based on broadband impedance spectrum, characterized in that, It includes a direct digital frequency synthesizer (DDS), a microcontroller unit (MCU), an analog-to-digital converter (ADC), four low-pass filters, two multipliers, and an electromagnetic coupling device. The DDS is connected to the MCU, the first low-pass filter, and the second low-pass filter, respectively. The ADC is connected to the MCU, the third low-pass filter, and the fourth low-pass filter, respectively. The CH1 terminal of the DDS is sequentially connected to the first low-pass filter, the first capacitor, the first 2x operational amplifier, and the first resistor. Then, it has a branch. The first branch has a second resistor. After the second resistor, there is a branch. The first branch is connected to the second 2x operational amplifier, the first multiplier, the third low-pass filter, the first 20x operational amplifier, and the CH0 terminal of the ADC. The second branch is connected to the third 2x operational amplifier, the second multiplier, the fourth low-pass filter, the second 20x operational amplifier, and the CH1 terminal of the ADC. The second branch has a third resistor. After the third resistor, there is a fork. The first fork has an electromagnetic coupling device. The second fork is connected to the third 2x operational amplifier. The CH0 terminal of the DDS is connected in sequence to the second low-pass filter, the second capacitor, and the first multiplier.

2. The online cable fault detection device based on broadband impedance spectrum according to claim 1, characterized in that, The DDS and MCU are connected via a Sequence Peripheral Interface (SPI), and the ADC and MCU are connected via a Sequence Peripheral Interface (SPI).

3. The online cable fault detection device based on broadband impedance spectrum according to claim 2, characterized in that, The first and second low-pass filters have a frequency of 200MHz, and the third and fourth low-pass filters have a frequency of 30kHz.

4. The online cable fault detection device based on broadband impedance spectrum according to claim 3, characterized in that, A power supply branch is provided between the first low-pass filter and the CH1 terminal of the DDS, and between the second low-pass filter and the CH0 terminal of the DDS. The power supply branch is connected in sequence to the power supply resistor and the 1.8V filter power supply.

5. The online cable fault detection device based on broadband impedance spectrum according to claim 4, characterized in that, A first grounding branch is provided between the first capacitor and the first 2x operational amplifier. The first grounding branch is connected in sequence to a grounding resistor and a grounding terminal. The second resistor is also connected to the second grounding branch.

6. The online cable fault detection device based on broadband impedance spectrum according to claim 5, characterized in that, A branch line connecting the second multiplier is provided between the second capacitor and the first multiplier.

7. A calibration method for an online cable fault detection device based on broadband impedance spectrum, wherein the online cable fault detection device based on broadband impedance spectrum uses the online cable fault detection device based on broadband impedance spectrum as described in claim 6, characterized in that... It includes a measurement process and a calibration process. The calibration process includes obtaining the measured value and the true value of the cable reflection coefficient.

8. The calibration method for the online cable fault detection device based on broadband impedance spectrum according to claim 7, characterized in that, The calibration process includes measuring the cable reflection coefficient. and the true value of cable reflection coefficient for: ； ； In the formula, It is a directional error. It is the reverse tracking error. It is the source matching error.

9. The calibration method for the online cable fault detection device based on broadband impedance spectrum according to claim 8, characterized in that, The calibration process includes: ； ； ； In the formula, It is a cable Calibration under load , It is a cable Calibration under negative load , It is a cable Calibration under load .

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