Research On On-line Insulation Monitoring Of Three-Phase Power Cable Under The Shielding Interconnection

The cross-linked power cable has the merit of less circulation circuit current of the shielding layer and is more economic and safe than the ordinary layout.But it is difficult to monitor the insulation situation of each segment of the electrical cable.The reason is that the current which flows cross the main insulation of each segment of the cable is hard to be got by direct measurement.In order to solve the problem,analyzing and introducing the shielding interconnection mode of three-phase power cable in detail.Through analyzing the interconnection mode,found that the conductor wire core is not cross-linked.Hereby,twin CT method as a new method of on-line monitoring has been put forward,and the working principle of the twin CT method is described in detail.According to Kirchhoff's Current Law,the expression of twin CT method is derived.The result shows that the new method is suitable for all kinds insulation of on-line monitoring of power cable,whether metallic sheath cross linked or not.For the problem of the voltage drop of the long distance cable caused by load current,analyzing the influence on voltage drop caused by the load current.The results show that measuring tan? to judge the cable insulation condition should be used the average of both ends voltage of the cable as the reference vector.According to the theoretical analysis,a solution combined with twin CT method is presented,which constitute the on-line insulation tan? monitoring system of long distance three-phase cable.The current signal measurement by using the Rogowski of PCB coils as sensor and the voltage signal measurement by using the sensor on the low arm of capacitive voltage divider.The whole hardware of the monitoring system is designed,including the signal processing circuit,the measuring and control part of circuit and the power circuit.The monitoring system is developed with the TMS320F2812 as the core of data processing and logic controlling.The ADS8364 is used for the data sampling as the A/D converter;GPS's time function is used to provide synchronous sampling signal for system;GPRS is used for the data wireless transmssion.For the software,the following function is given such as the data synchronous sampling and the main controlling,and so on.The terminal of on-line monitoring of the three-phase cable system is designed based on LABVIEW software,the realization of the measurement of dielectric loss factor of cable insulation by using the harmonic analysis method.The experiment result is shown that the on-line monitoring systen of the dielectric loss factor has high accuracy after comparing the data of measurement with the result of theoretical calculations,verifying the validity and correctness of the on-line monitoring system.