Cable insulation aging monitoring method for alternating current and direct current mixed signals

Abstract

The invention provides a cable insulation aging monitoring method based on alternating-current and direct-current mixed signals aiming at an omnibearing monitoring means of a transient state and a steady state change process of cable insulation aging, and the cable insulation aging monitoring method based on the alternating-current and direct-current mixed signals is based on Hall alternating-current and TMR (Tunnel Magnet Resistance) direct-current combined online monitoring. The comprehensive detection function of the grounding short circuit fault and the chronic insulation aging defect caused by long-term operation is achieved, and the effectiveness and the practical feasibility of the method are verified through experiments and simulation under a complex topological structure. The method has a great practical value, and can solve the problem that the operation and maintenance of the 10kV cable are not linear.

Description

Technical Field

[0001] This application belongs to the field of cable fault monitoring, specifically relating to a method for monitoring cable insulation aging using AC / DC mixed signals. Background Technology

[0002] Underground cables operate in harsh environments, and under multiple stresses from electricity, heat, and machinery, they are highly susceptible to insulation aging, ultimately leading to line faults and jeopardizing the reliable operation of the distribution network. Cable-related faults account for as much as 59% of those in medium-voltage distribution networks, with about half of these faults caused by the cable itself. Currently, most research focuses on timely disconnection after a cable fault occurs to prevent cascading failures; however, this still struggles to prevent regional faults, making this protection method rather "passive." If high-precision sensing of cable conditions could be achieved during operation, cable inspection and replacement could be performed when severe aging occurs, shifting system maintenance from "passive" to "proactive," fundamentally preventing faults and improving system reliability.

[0003] The status sensing of power distribution network cables mainly includes two aspects: 1) determining whether the cable has aged or suffered a serious fault through online monitoring methods; 2) locating localized aging to enable targeted maintenance. Serious faults, such as short circuits, damage, overvoltage, and overload, will cause a sudden, large pulse in the AC current, making them easily detectable. However, the changes in leakage current electrical parameters during the long-term aging of cable insulation are often very subtle, making online and accurate sensing difficult. Therefore, this invention can monitor both large AC fault currents and the subtle leakage currents caused by insulation aging. Summary of the Invention

[0004] The purpose of this application is to overcome the problem of online monitoring of insulation aging defects in AC / DC hybrid power distribution cables.

[0005] To achieve the above objectives, this application proposes a method for monitoring cable insulation aging using AC / DC mixed signals.

[0006] To monitor the insulation aging of AC and DC cables, reasonable online monitoring methods are proposed. Methods based on online measurement of electrical parameters mainly measure the dielectric loss tangent or leakage current of the cable insulation. Common online methods for measuring the dielectric loss tangent include the zero-crossing comparison method, the grounding box lead current measurement method, the two-terminal electrical quantity measurement method, and the system transient response analysis method. Essentially, these methods establish the relationship between measurable electrical quantities of the cable system and the dielectric loss tangent, and infer the magnitude of the dielectric loss tangent by measuring the relevant electrical quantities. However, the change in the dielectric loss tangent during insulation aging is often small, and the resulting changes in related electrical quantities are even weaker, leading to low monitoring accuracy. Even a small measurement error can cause a large error in the estimated dielectric loss tangent, making these methods difficult to use in practical systems. In contrast, measuring the leakage current of the cable insulation can directly reflect the insulation state without complex calculations, thus improving monitoring accuracy.

[0007] Compared with the prior art, the advantages of the present invention are:

[0008] This invention, based on a comprehensive analysis of 10kV cable aging, identifies transient and steady-state changes in cable insulation aging. Transient aging is generally caused by sudden changes in load current (e.g., ground fault short circuits), resulting in sudden changes in current amplitude in the cable's AC signal, indicating a transient ground fault. Secondly, during long-term operation, cables gradually age due to factors such as the cable structure itself, tunnel environment, and load variations. This aging process is an accumulated process, exhibiting gradual changes. By injecting a common-mode DC current signal source and utilizing TMR electromagnetic current detection, weak DC signals can be detected, allowing for the assessment of steady-state aging parameters. Using both AC and DC monitoring parameters as criteria for cable insulation aging allows for a comprehensive understanding of the cable insulation's operating status, providing strong on-site data support for cable operation management by cable work teams. Attached Figure Description

[0009] Figure 1 This is a graph showing the trend of leakage current changes corresponding to the aging degree of a 10kV cable.

[0010] Figure 2 This is a schematic diagram of the TMR leakage current detection principle.

[0011] Figure 3 A block diagram for the overall hardware structure design. Detailed Implementation

[0012] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments.

[0013] Example

[0014] This invention relates to a method for monitoring cable insulation aging using a mixed AC / DC signal.

[0015] like Figure 1 The figure shows the leakage current variation trend corresponding to the aging degree of a 10kV cable, where (a) is the capacitive leakage current and (b) is the resistive leakage current.

[0016] (1) Alternating Current Hall Monitoring Design

[0017] The Hall effect is a type of magnetoelectric effect. Components, circuits, and systems based on the Hall effect are called Hall sensors. These are non-contact measurement sensors whose output voltage is proportional to the magnetic field strength and independent of the rate of change of the magnetic field. Structurally, Hall current sensors are divided into closed-loop and open-loop types. Considering the current operating status of 10kV cable monitoring, most of the cables to which current sensors are installed are already laid; therefore, subsequent installations can only use an open-loop design. The inner diameter of the magnetic coil can be customized later. An open-loop Hall current sensor has a single closed magnetic circuit core with an inner diameter of d. The Hall chip is placed in the open air gap of the core, with a gap length of L. The element can directly detect the magnetic field signal, and the output is a voltage signal Vo. ut The proportional relationship between the output voltage and the magnetic field B is as follows:

[0018]

[0019] I represents the driving current of the Hall element, B represents the magnetic field strength of the air gap in the magnetic ring, and R represents the Hall coefficient, which is often determined by the material of the Hall element. The Hall current sensor converts the magnetic field generated around a current-carrying conductor through a magnetic ring into a voltage signal on the order of mV. Open-loop Hall sensors have simple external circuitry and relatively low cost, but due to their poor accuracy and linearity, slow response time, and large temperature drift, they are more suitable for detecting transient AC fault currents. To avoid these shortcomings of Hall sensors, this invention makes precise judgments in the selection of Hall element materials and magnetic materials, with the main objective of improving magnetic focusing and measurement linearity performance.

[0020] (2) TMR DC leakage current monitoring

[0021] Cable insulation aging is often accompanied by changes in relative permittivity. During cable operation, the number of voids in the insulation continues to increase, and moisture and impurities in the environment can easily enter these voids, further increasing the relative permittivity of the cable insulation. This paper mainly studies the types of insulation aging in cables that involve changes in the relative permittivity. Extensive experimental results show that measuring cable insulation leakage current can monitor the insulation capacitance and resistance, thereby estimating the degree of insulation aging. The primary focus is on monitoring insulation capacitance. As the degree of aging increases, the upward trend of insulation capacitance remains consistent across a wide frequency band, providing significant flexibility for monitoring. Furthermore, at power frequency or higher frequencies, the capacitive portion of the insulation leakage current is much larger than its resistive portion, resulting in higher accuracy and sensitivity in monitoring insulation capacitance. The amplitudes of capacitive and resistive leakage currents of cables at different insulation aging degrees at power frequency are shown below. Figure 2 As shown, both capacitive and resistive leakage currents gradually increase with the degree of cable aging. Throughout the entire aging process, the capacitive leakage current is consistently much larger than the resistive leakage current. In the early stages of cable aging, the capacitive leakage current increases significantly, while the resistive leakage current changes relatively little. When the aging is severe, although the resistive leakage current increases rapidly, it still differs from the capacitive leakage current by an order of magnitude. Therefore, at power frequency, capacitive leakage current accounts for the vast majority of the insulation leakage current, and measuring the insulation leakage current allows for effective monitoring of insulation capacitance.

[0022] Experiments revealed a direct proportionality between cable insulation leakage current and the magnetic field generated by the injected current. Therefore, TMR (tunneling magnetoresistance) technology can be used to measure the magnitude of the magnetic field generated by the injected current. Since the magnetic field from leakage current is within a very small range, TMR technology was chosen for measurement. TMR is a novel magnetoresistance measurement technique that utilizes the tunneling magnetoresistance effect of magnetic multilayer film materials to induce a magnetic field. Figure 2 Measurement principle for TMR open loop:

[0023] Assume the current flowing through the conductor is I, and the distance between the sensor and the conductor is d. When the current changes, the magnetic field changes accordingly, and the resistance of the TMR also changes. A bridge circuit is used to output the change in resistance as a voltage signal. Because the TMR resistance and the magnetic field exhibit a linear relationship, the output voltage is proportional to the measured current, thus achieving the function of measuring the current signal.

[0024] (3) Design of integrated monitoring and sensing devices

[0025] This design incorporates AC / DC integrated cable current sensor principles and objectives, focusing on compactness, low power consumption, and miniaturization. The sensor features a single sampling input and two independent signal outputs, enabling simultaneous monitoring of transient and steady-state fault currents for a more comprehensive real-time understanding of cable operating conditions. The integrated current sensor hardware structure primarily consists of three main components: coil, chip, and conditioning circuit. The design methodology for each of these core components is described in detail below. The overall hardware structure block diagram is shown below. Figure 3 As shown:

[0026] 1) Coil: The selection of the coil is crucial, as the current value it collects also serves as the signal source for AC / DC signal analysis. As mentioned earlier, the coil material should possess characteristics such as high permeability, low temperature coefficient, and high hysteresis coefficient. Magnetic materials with high initial permeability and good linearity can improve the sensitivity and accuracy of the current sensor. Materials with good magnetic focusing effects generally include silicon steel sheets, iron-nickel soft magnetic alloys, amorphous alloys, iron-based microcrystalline alloys, and permalloy. Microcrystalline and permalloy are two magnetic materials with significant application value. Microcrystalline is an amorphous magnetic material with a grain size on the nanometer scale; permalloy is a strongly magnetic crystalline alloy material. They differ in magnetic properties, physical characteristics, and applications. The magnetic properties of microcrystalline and permalloy are one of their most fundamental differences. First, microcrystalline has a higher saturation magnetic induction intensity than permalloy. Second, microcrystalline has lower hysteresis loss than permalloy, exhibiting better high-frequency characteristics. On the other hand, permalloy has lower iron loss and higher permeability than microcrystalline. Taking all factors into consideration, iron-based microcrystalline alloys were selected as the magnetic coil material.

[0027] The coil design of this device adopts a double-layer structure. The first layer collects the AC current value generated by the cable joint, and the second layer collects the leakage current generated by the signal injected into the cable. The ingenious design of the two layers enables the acquisition of current values ​​with different performance parameters. On the other hand, the outer shielding layer can shield and cancel large AC currents through the inner coil, thereby improving the measurement accuracy of the small current in the outer layer.

[0028] 2) Chip Selection and Design: GaAs material was used for the Hall current device, as GaAs offers the best output linearity and can withstand temperatures up to 200℃. A double-ended lead-out package Hall element was designed and deployed at the top of a coil layer, where the top provides the best magnetic focusing effect. The THS119 chip was selected, and the thickness of the Hall element generally does not exceed 2mm. The MAGIC2001 electromagnetic chip was chosen for TMR, which can still ensure high measurement accuracy even in the presence of DC and harmonic components.

[0029] 3) Modulation Circuit Design: The core of the modulation circuit includes temperature compensation, signal conditioning, amplification, and zero-adjustment circuits. The temperature compensation circuit adjusts the operational amplifier parameters according to the ambient temperature to compensate for the sensitivity of the internal TMR component. A temperature-sensitive voltage source is designed to power the TMR component to compensate for temperature drift errors. Given that the TMR component's output voltage is on the order of mV, a signal amplification circuit is designed to amplify the signal to the order of V. A pre-amplifier circuit and a secondary signal amplification circuit are designed. Since the chip component uses a bridge structure, the initial values ​​of the four internal resistors cannot be guaranteed to be completely consistent due to process limitations. Furthermore, influenced by surrounding stray magnetic fields, the TMR component has a fixed output bias voltage; therefore, a bias zero-adjustment circuit is designed.

[0030] The AC / DC integrated cable current sensor can simultaneously monitor both AC and DC current in real time. Based on this principle, a prototype device has been developed that can achieve the following performance indicators:

[0031] (1) AC signal measurement of 6-10kV cable busbar: The principle of Hall sensor current detection AC signal is adopted, and the data detection indicators are as follows:

[0032] A. Measurement range: 0-400A

[0033] B. Measurement accuracy: 0.5%;

[0034] C. Frequency response: 50-1kHz;

[0035] D. Output characteristics: 4-20mA;

[0036] (2) The DC leakage current signal of 6-10kV cable busbar is measured using the TMR chip detection principle. The data detection indicators are as follows:

[0037] A. Measurement range: 0-1mA;

[0038] B. Measurement accuracy: 0.5%;

[0039] C. Output characteristics: 4-20mA;

[0040] D. Output response: 3s.

[0041] 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 monitoring cable insulation aging using AC / DC mixed signals, characterized in that, The AC cable signal adopts Hall effect monitoring, a non-contact measurement method that solves the problem of difficult installation of power distribution cables. Furthermore, the selection of high magnetic materials as coils greatly improves magnetic concentration and measurement linearity performance.

2. The method for monitoring cable insulation aging using AC / DC mixed signals according to claim 1, characterized in that, The DC selection uses the TMR leakage current detection mode, which effectively monitors insulation capacitance by measuring insulation leakage current. It offers high output linearity and convenient open-loop installation.

3. The method for monitoring cable insulation aging using AC / DC mixed signals according to claim 1, characterized in that, Adopting an AC / DC integrated sensing device monitoring design mode, and with design concepts such as compactness, low power consumption, and miniaturization, it forms an integrated sensor with one sampling input and two independent signal outputs, which can simultaneously monitor transient and steady-state fault currents, and provide a more comprehensive real-time understanding of the cable's operating status.

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