CN-121995161-A - 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. The voltage common mode signal is injected at the neutral point and is equally superimposed on each phase of cable to form a common mode voltage signal, the electric potential of each phase of cable is the same under the action of the common mode voltage signal, no current is induced in the connected load, non-invasive on-line monitoring is realized, meanwhile, the generated leakage current only flows through cable insulation, the change of the load does not influence the leakage current, and the system is suitable for various operation conditions of the system. Compared with the traditional direct current or low frequency voltage superposition method, the power frequency voltage superposition method can amplify the insulation capacity characteristics of the cable, and improves the monitoring precision and sensitivity. The required monitoring voltage amplitude is thus reduced, making the proposed monitoring scheme easier to implement. Meanwhile, the magnitude of the measured leakage current can be changed by adjusting the amplitude of the injection monitoring voltage, so that the monitoring flexibility is improved.

Inventors

• LI HAOFENG
• XIAO YAN
• YANG XIN
• DING XIAOCHENG
• ZUO WEINING
• Liu Longnan
• MA PENGTAO

Assignees

• 国网甘肃省电力公司庆阳供电公司

Dates

Publication Date

20260508

Application Date

20241105

Claims (3)

1. 1. A frequency common mode signal injection method is characterized in that voltage common mode signals are injected at neutral points and are equally superimposed on each phase of cable to form common mode voltage signals, under the action of the common mode voltage signals, the electric potentials of each phase of cable are the same, current cannot be induced in connected loads, non-invasive on-line monitoring is achieved, meanwhile, generated leakage current only flows through cable insulation, load changes cannot affect the leakage current, and the method is suitable for various operation conditions of a system.
2. 2. The method for injecting frequency common mode signals according to claim 1, wherein the common mode voltage superimposed at the neutral point is a common mode voltage, and the induced leakage current also has the characteristic of a common mode under the action of the common mode monitoring voltage, and the common mode leakage current sensor extracts the leakage current from a larger load current to realize on-line and quantitative cable insulation state monitoring.
3. 3. The method for injecting frequency common mode signals according to claim 1, wherein the power frequency voltage superposition is adopted to amplify the cable insulation capacity characteristics, thereby improving the monitoring precision and sensitivity, reducing the required monitoring voltage amplitude, and simultaneously, changing the measured leakage current by adjusting the amplitude of the injected monitoring voltage, thereby increasing the monitoring flexibility.

Description

Frequency common mode signal injection method Technical Field The application belongs to the field of cable fault monitoring, and particularly relates to a frequency common mode signal injection method. Background The 10kV crosslinked polyethylene cable has a severe running environment, insulation is often aged due to damp and overload thermal effects, insulation breakdown and grounding short circuit faults are finally caused, and safe and reliable running of the urban power grid is seriously endangered. Under complex working conditions such as multi-source load connection, the insulation aging problem is more remarkable, the load current of the cable can be rapidly fluctuated, the cable insulation bears larger thermal stress, the switching frequency of the converter and the harmonic waves of times of the switching frequency are coupled to the cable through various ways, and the insulation aging is accelerated, but no definite research result exists for an insulation aging model in the scene at present. The electrical indicators that characterize the degree of insulation aging that are widely used today mainly include the resistance and capacitance of the cable insulation, and the dielectric loss angle that can be represented by both. Underground cable insulation is mainly composed of crosslinked polyethylene (XLPE) and like polymers. In the cable manufacturing process, a faster cooling and curing process exists, so that the produced cable contains a large number of voids, and along with long-time high-current operation of the cable, the number of voids is further increased due to the movement of the insulation formed by the polymer to a thermal balance point. Moisture and impurities invade and diffuse into the cavities, resulting in uneven distribution of the electric field inside the insulation, reduced insulation strength, and even possible generation of internal water and electric trees, resulting in rapid breakdown and damage of the insulation. Since moisture and impurities often have a large dielectric constant and conductivity, phenomena of reduced resistance, increased capacitance, and increased dielectric loss angle are often detected in aged cables. In addition, the high temperature operation of the cable caused by overload can cause breakage and excessive interconnection of molecular chains in the polymer, so that the internal trap level and the relaxation coefficient change, and the phenomena of dielectric constant increase, capacitance increase and resistance decrease can also be caused. However, the measurement of the insulation resistance and capacitance of the cable is difficult, and most of the measurement can be realized only through an off-line test mode at present. And the severe working environment of the underground cable further increases the difficulty and cost for measuring the resistance and capacitance of the cable in operation. Therefore, the invention provides a frequency common mode signal injection method which can reflect the insulation capacitance characteristic of the cable by measuring the injection signal characteristic. Disclosure of Invention The application aims to solve the problem of on-line monitoring of the leakage current of a distribution cable. In order to achieve the above object, the present application proposes a frequency common mode signal injection method. Aiming at the problem that insulation aging of a distribution cable cannot be monitored on line, the application provides a common mode signal injection method in a frequency mode, and insulation characteristics of the cable are judged by monitoring injection signal information. Experiments show that the insulation leakage current of the distribution cable gradually increases along with the increase of the aging degree of the cable, but the leakage current with smaller amplitude can be submerged in the load current with larger amplitude, and is difficult to sense and extract. In order to solve the problem, the application provides a cable insulation on-line monitoring technology based on common mode signal injection. Firstly, a microampere ultra-low frequency (0.01 Hz-1 Hz) broadband modulation signal is superimposed between a neutral point of a cable system to be tested and the ground, and a high-precision common-mode leakage current sensor is used for measuring common-mode leakage current generated under the action of an injected monitoring signal and converting weak current into an output voltage signal. The multichannel acquisition and data processing device is used for acquiring square wave excitation source signals and output voltage signals, the signals are subjected to wavelet feature extraction after being acquired, the fundamental wave frequency and the phase of the square wave excitation source and the amplitude and the phase of even harmonic waves of the output voltage are further obtained, and microampere-level current can be obtained according to the information, so that the insulation capa