Design Of 0.1Hz VLF Dielectric Loss Measurement Device

As an important transmission method for urban power grids,the safe operation of power cables is a prerequisite for ensuring the stability of the power system.During the polymerization process,residual moisture,insulation aging,or poorly sealed joints in the cable insulation layer may form water branches under the action of operating voltage and gradually develop into electrical branches,ultimately leading to cable breakdown accidents after water ingress at the end and intermediate joints.Therefore,it is necessary to strengthen the detection of insulation water tree aging in power cables.The insulation medium loss characteristics of power cables under 0.1Hz ultra-low frequency voltage are obvious,which can reflect the polarization loss caused by water tree degradation,and has technical advantages for on-site detection of power cable defects.At present,ultra-low frequency dielectric loss measurement devices are increasingly widely used in China,and relevant theories have a certain foundation.However,the technology for ultra-low frequency dielectric loss measurement devices is not mature,and only imported ultra-low frequency dielectric loss measurement devices are market-oriented.Therefore,it is necessary to conduct research on 0.1Hz ultra-low frequency dielectric loss measurement devices.In this paper,the dielectric loss measurement method is theoretically analyzed,the signal extraction circuit based on sampling capacitance is designed,and the value conditions of circuit components are given.According to the quasi synchronous sampling harmonic analysis method,the dielectric loss calculation formula is derived,and the anti-interference ability of dielectric loss measurement under this method is simulated and analyzed.A 0.1Hz ultra-low frequency sine wave generator circuit based on voltage demodulation by the voltage stabilizing tube is proposed,and the calculation methods of various parameters in the circuit are provided.The correctness of the circuit design is verified through simulation using Multisim software.Based on the principle of dielectric loss measurement methods,the hardware circuit of the ultra-low frequency dielectric loss measurement device was designed,software programs were written,and a dielectric loss measurement device test platform was built.The dielectric loss values under different signals were measured.The research results indicate that the 0.1Hz ultra-low frequency sine wave generator circuit can generate an ultra-low frequency sine wave voltage with waveform distortion less than 3%,which meets the DL/T 849.4-2004 standard.The signal extraction method designed under this voltage can effectively solve the problem of signal pulsation and extract stable voltage and current signals.The measurement error of the dielectric loss of the ideal signal and the added harmonic signal is less than 0.01%,which meets the DL/T 1694.6-2020 standard.At the same time,it is verified that the dielectric loss measurement under this method has good anti-interference ability.