| The function of the semicon layer of the high-voltage cable is grading electric field distribution,so the properties of semicon are related with the longterm reliability of the cable.At present,the DC resistivity of semicon material is used as the evaluation index in national standard,but the power cable is actually working under the AC voltage.During the power transmission line switching after the maintenance,the power cable accessories often break down.The power cable transmission may be subjected to the switching over-voltage in the process of the power transmission line switching,and the transient electric field distribution analysis of the failure for the cable accessories can be conducted,only after obtaining the dielectric properties before and after aging of the semicon material used for the stress cone of the cable accessories.Therefore,it is very necessary to test the AC dielectric properties(resistivity,permitivity,loss factor)of the semicon material before and after aging.The corresponding test equipment must be based on the foundation of the hardware for the test of AC dielectric properties of semicon material.However,the existing test equipment of broadband AC dielectric property is mainly the impedance analyzer,which cannot be widely used due to its high expense.So it is very necessary that developing a set of low cost device for testing the broadband AC dielectric properties of the semicon material.According to the relevant test requirements stipulated by the national standard for semicon material resistivity,a four-terminal electrode system has been designed and made in this paper.According to the requirement of broadband dielectric AC properties test about semicon material,a design scheme of testing system based on the virtualinstrument for the dielectric properties of semicon material has been proposed.According to the proposed test system,the hardware system includes a voltagecontrolled current source,a differential voltage pre-amplifier,a four-terminal electrode system,a temperature acquisition module,a data acquisition card and a host computer;the software includes the data acquisition,voltage and current signal acquisition analysis and dielectric properties calculation.According to the resistance of standard sample in the four-terminal electrode system ranging from 1k? to 50k?,and its broadband dielectric properties needs to be measured under 0~1MHz,so a voltage-controlled current source with output current 0~50m A RMS,effective value of 0~5V voltage and 0~1MHz bandwidth has been designed by using the chip Ths4011 CD and Howland current pump circuit.Meanwhile,a differential voltage pre-amplifier has been designed with 2 times the gain and 0~1MHz bandwidth by using the AD8421 chip.According to the test bandwidth and temperature measurement required,a sampling rate of 10 MHz PCI-1714 UL data acquisition card and the sampling rate of 8Hz IPAM-5502 temperature acquisition module has been configured within the hardware system.The temperature acquisition and the test programs of the semicon material dielectric properties within the software system have been developed by combining with Labview software platform.The test system needs to be calibrated after the entire test system has been completed.Taking the parameters obtained by the impedance analyzer of the resistance-capacitance parallel model of the sample as the standard to calibrate the test system designed in this paper is the main method of system calibration.After the completion of the system calibration,the AC dielectric properties of a standard sample made of the semicon material are measured,and the test results confirmed that the test system can accurately measure AC dielectric properties of the semicon material of the cable and can accurately measure and record the excitation current and response of voltage signal. |
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