Frequency common-mode signal injection method

Abstract

The invention belongs to the field of cable fault monitoring, and relates to a frequency common-mode signal injection method. A voltage common-mode signal is injected at the neutral point, the voltage signal is equally superposed on each phase cable to form a common-mode voltage signal, under the action of the common-mode voltage signal, the potentials of each phase cable are the same, current cannot be induced in a connected load, and non-intrusive online monitoring is realized; meanwhile, the generated leakage current only flows through the cable insulator, the change of the load does not affect the leakage current, and the system is suitable for various operation conditions of the system. Compared with a traditional direct current or low-frequency voltage superposition method, power frequency voltage superposition can amplify the insulation capacitive characteristics of the cable, and the monitoring precision and sensitivity are improved. And the required monitoring voltage amplitude is also reduced, so that the proposed monitoring scheme is easier to implement. Meanwhile, the magnitude of the measured leakage current can be changed by adjusting the amplitude of the injected monitoring voltage, and the monitoring flexibility is improved.

Description

Technical Field

[0001] This application belongs to the field of cable fault monitoring, specifically relating to a frequency common-mode signal injection method. Background Technology

[0002] 10kV cross-linked polyethylene (XLPE) cables operate in harsh environments, and their insulation often ages due to moisture and overload thermal effects, ultimately leading to insulation breakdown and grounding short-circuit faults, seriously jeopardizing the safe and reliable operation of urban power grids. Under complex conditions such as multi-source load access, insulation aging becomes even more pronounced. The cable's load current may fluctuate rapidly, causing significant thermal stress on the cable insulation. The switching frequency of the converter and its multiples of harmonics couple to the cable through various pathways, accelerating insulation aging. However, there are currently no definitive research results on insulation aging models for this scenario. Currently, widely used electrical indicators characterizing the degree of insulation aging mainly include the resistance and capacitance of the cable insulation layer, and the dielectric loss angle, which can be represented by these two parameters. Underground cable insulation is mainly composed of polymers such as cross-linked polyethylene (XLPE). During cable manufacturing, a rapid cooling and curing process occurs, resulting in cables containing numerous voids. As the cable operates under high current for extended periods, the movement of the polymer insulation towards thermal equilibrium further increases the number of voids. Moisture and impurities infiltrate and diffuse into cavities, leading to uneven electric field distribution within the insulation, reducing insulation strength, and potentially causing internal water trees and electrical trees, resulting in rapid insulation breakdown and damage. Since moisture and impurities often have high dielectric constants and conductivity, decreased resistance, increased capacitance, and increased dielectric loss angle are commonly detected in aging cables. Furthermore, overload-induced high-temperature operation of cables causes molecular chain breakage and excessive interconnection within the polymer, leading to changes in internal trap energy levels and relaxation coefficients, which also result in increased dielectric constant, increased capacitance, and decreased resistance. However, measuring cable insulation resistance and capacitance is difficult and currently mostly achieved through offline testing. The harsh operating environment of underground cables further increases the difficulty and cost of measuring the resistance and capacitance of cables in operation. Therefore, this invention proposes a frequency common-mode signal injection method that can reflect the characteristics of cable insulation capacitance by measuring the characteristics of the injected signal. Summary of the Invention

[0003] The purpose of this application is to solve the problem of online monitoring of leakage current in power distribution cables.

[0004] To achieve the above objectives, this application proposes a frequency common-mode signal injection method. Addressing the problem of the inability to monitor the aging of distribution cable insulation online, this invention proposes a frequency-mode common-mode signal injection method to determine the cable insulation characteristics by monitoring the injected signal information. Experiments show that the leakage current of distribution cable insulation gradually increases with the degree of cable aging, but the smaller leakage current is submerged in the larger load current, making it difficult to detect and extract. To solve this problem, this invention proposes an online cable insulation monitoring technology based on common-mode signal injection. First, a microampere-level ultra-low frequency (0.01Hz-1Hz) broadband modulation signal is superimposed between the neutral point and ground of the cable system under test; then, a high-precision common-mode leakage current sensor is used to measure the common-mode leakage current generated under the action of the injected monitoring signal, and the weak current is converted into an output voltage signal. A multi-channel acquisition and data processing device is used to acquire square wave excitation source signals and output voltage signals. After the signals are acquired, wavelet features are extracted to obtain the fundamental frequency and phase of the square wave excitation source, as well as the amplitude and phase of the even harmonics of the output voltage. Based on this information, the magnitude of the microampere current can be obtained, and thus the insulation capacitance of the cable can be obtained.

[0005] High-voltage transmission lines supply power to medium-voltage distribution networks via step-down transformers, and the energy is then transmitted to loads via cables. A power frequency monitoring signal is superimposed on the neutral point of the secondary side of the step-down transformer. This monitoring signal can be obtained by stepping down a phase of the transmission line or generated by power electronic devices such as inverters. The common-mode leakage current generated on the cable under the action of the monitoring signal is measured, thereby enabling online monitoring of the insulation aging condition.

[0006] Compared with the prior art, the advantages of the present invention are:

[0007] (1) The voltage signal at the neutral point is equally superimposed on each phase cable, therefore this signal belongs to the common-mode voltage signal. Under the action of the common-mode voltage signal, the potential of each phase cable is the same, and no current will be induced in the connected load, realizing non-intrusive online monitoring. At the same time, the leakage current generated only flows through the cable insulation, and the change of load will not affect the leakage current, making the proposed scheme applicable to various operating conditions of the system;

[0008] (2) On the other hand, the common-mode voltage superimposed at the neutral point makes online leakage current measurement possible. Under the action of the common-mode monitoring voltage, the induced leakage current also has the characteristics of common-mode. The application of a common-mode leakage current sensor can extract it from a large load current, making online, quantitative monitoring of cable insulation status possible;

[0009] (3) Compared with traditional DC or low-frequency voltage superposition methods, using power frequency voltage superposition can amplify the capacitive characteristics of cable insulation, improving monitoring accuracy and sensitivity. The required monitoring voltage amplitude is also reduced, making the proposed monitoring scheme easier to implement. At the same time, the measured leakage current can be changed by adjusting the amplitude of the injected monitoring voltage, increasing the flexibility of monitoring.

[0010] Studies have shown that cable insulation can withstand overvoltages several times its rated voltage for a short period. In the proposed scheme, the superimposed monitoring voltage amplitude is 5%-10% of the rated voltage, therefore it will not adversely affect the cable insulation. Furthermore, since insulation aging is a slow and long-term process, online monitoring of the cable does not need to be performed frequently; monitoring using the proposed scheme only needs to be conducted periodically. Attached Figure Description

[0011] Figure 1 Schematic diagram of online insulation aging measurement principle;

[0012] Figure 2 Here is a flowchart of the overall scheme for the micro-injection method;

[0013] Figure 3 This is a system schematic diagram of the present invention. Detailed Implementation

[0014] The technical solution of this application will be described in detail below with reference to the accompanying drawings.

[0015] Example

[0016] This invention presents a common-mode signal injection method. In power distribution systems, two types of equipment are needed for online monitoring of cable insulation aging: a common-mode (CM) monitoring signal injection device and a CM leakage current measurement device. The measurement principle is as follows: Figure 1 As shown.

[0017] The CM monitoring signal injection device injects CM monitoring signals into power cables at a controlled frequency by modifying the secondary side of existing current transformers (CTs) in the distribution network system, without requiring an additional injection voltage source. The CM leakage current induced by the injected monitoring signal is then measured online in real time by a CM leakage current measuring device to monitor the severity of cable insulation aging. In common-mode leakage current measurement technology, the monitoring signal is injected as a weak signal from the system's grounded neutral point via CM. Signal injection can be achieved by modifying the secondary side of the CTs in the distribution network, ensuring equal energization of all phases. This prevents current from being induced in any connected loads, and the frequency of the injected monitoring signal can be easily and remotely adaptively adjusted via software. Using a higher frequency monitoring signal amplifies the characteristics of insulation capacitance, improving current monitoring accuracy. Common-mode leakage current characteristics are used to describe the insulation state of cables. Due to its small amplitude and the influence of varying distribution network structures and conductor positions, a magnetic ring made of high permeability material is used to wrap the cable under test. A magnetic field sensor is then placed outside the magnetic ring to collect magnetic induction intensity, thereby obtaining the common-mode leakage current.

[0018] The flowchart of the micro-injection method is as follows: Figure 2 As shown: First, a microampere-level ultra-low frequency (0.01Hz-1Hz) broadband modulation signal is superimposed between the neutral point and ground of the cable system under test. A novel weak current sensor, namely a magnetic modulator, is used to measure the injected weak signal and convert the weak current into an output voltage signal. A multi-channel acquisition and data processing device is used to acquire the square wave excitation source signal and the output voltage signal. After the signal is acquired, it is subjected to Fourier decomposition to obtain the fundamental frequency and phase of the square wave excitation source, as well as the amplitude and phase of the even harmonics of the output voltage. Based on this information, the magnitude of the microampere-level current can be determined, and thus the insulation resistance of the cable can be obtained.

[0019] Online monitoring of power distribution cable insulation status using micro-injection method reveals the following: When no ultra-low frequency (ULF) signal is injected into the cable, wavelet transform of the magnetic modulator's output voltage reveals only odd-order harmonic voltages, with no even-order harmonic voltages. However, when an ULF signal is injected, wavelet transform of the magnetic modulator's output voltage reveals both odd-order and even-order harmonic voltages. The signal magnetic field strength is proportional to the ULF injected current signal, i.e.:

[0020] H0=K0I0 (1)

[0021] Where H0 is the signal magnetic field generated after the injected signal current, K is a proportionality constant, I0 is the ultra-low frequency injected current signal, and I0 can reflect the insulation resistance and thus determine the insulation status of the power distribution cable.

[0022] Based on the above principles, a system scheme for the injection and acquisition device is designed, such as... Figure 3 As shown.

[0023] This invention proposes a common-mode leakage current monitoring signal injection and control system based on the secondary side modification of a distribution network current transformer (CT). The system consists of three parts: a monitoring signal generation module, a low-pass filter, and a data acquisition and control switch module. The monitoring signal generation module and the data acquisition and control switch are the core components of the system, generating and injecting monitoring signals at a controlled frequency.

[0024] (1) Monitoring signal generation module: a resistor R s Connected to the secondary side of the CT, the CT is connected in parallel with two anti-parallel MOSFETs and two uncontrolled diodes. The two MOSFETs are controlled by the same PWM signal. When not under online monitoring, both MOSFETs remain in the "on" state, and the CT operates normally with the secondary side short-circuited. When under online monitoring, the MOSFETs are controlled using the PWM signal to switch between "on" and "off" states at a selected frequency.

[0025] (2) The low-pass filter consists of resistors and capacitors, used to filter out unwanted high-frequency components in the generated signal. The online monitoring switch module consists of two anti-parallel MOSFETs and two uncontrolled diodes, used as a bidirectional switch. The two anti-parallel MOSFETs are controlled by the monitoring mode signal. When online monitoring is not performed, both anti-parallel MOSFETs receive a "turn-off" command, and the neutral point of the system remains ungrounded. When online monitoring is performed, the two anti-parallel MOSFETs will receive a "turn-on" command, and the generated signal will be applied to the neutral point of the system as the CM monitoring signal;

[0026] (3) The real-time acquisition and control module of the embedded system uses a 24-bit analog-to-digital converter chip + FPGA + DSP architecture to realize the data acquisition and processing architecture. In the software, a method of sacrificing space for continuous time is used. Combining these two points, the function of continuous data acquisition and reading is realized. It is difficult to achieve high-precision calculation by using only FPGA to complete the control and calculation, which will seriously affect the accuracy of weak signal measurement; therefore, the DSP with floating-point operation is used to enhance the calculation of the acquired data to obtain accurate switching control calculation results and accurately control the frequency and amplitude of PWM signal generation.

[0027] Finally, it should be noted that 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 embodiments, those skilled in the art should understand that modifications or equivalent substitutions to the technical solutions of the present invention do not depart from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for injecting a frequency common-mode signal, characterized in that, A common-mode voltage signal is injected at the neutral point, and the voltage signal is equally superimposed on each phase cable to form a common-mode voltage signal. Under the action of the common-mode voltage signal, the potential of each phase cable is the same, and no current is induced in the connected load, thus realizing non-intrusive online monitoring. At the same time, the leakage current generated only flows through the cable insulation, and the change of load will not affect the leakage current, making it suitable for various operating conditions of the system.

2. The frequency common-mode signal injection method according to claim 1, characterized in that, The common-mode voltage superimposed at the neutral point; under the action of the common-mode monitoring voltage, the induced leakage current also has the characteristics of common-mode. The common-mode leakage current sensor extracts the leakage current from the larger load current, realizing online and quantitative monitoring of cable insulation status.

3. The frequency common-mode signal injection method according to claim 1, characterized in that, Using superimposed power frequency voltage can amplify the capacitive characteristics of cable insulation, improve monitoring accuracy and sensitivity, and thus reduce the required monitoring voltage amplitude. At the same time, by adjusting the amplitude of the injected monitoring voltage, the magnitude of the measured leakage current can be changed, increasing the flexibility of monitoring.

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