Research On Dielectric Loss Factor Of Insulation On-line Monitoring For Cross-linked Polyethylene Power Cable

The on-line insulation monitoring technique of power apparatus, which is based on the state repair, is one of the rising essential techniques in modem electric power industry. By sensitively detection of insulations, faults can be found in time to avoid the coming accidents, so that the safety, stability, and economy of the power system can ensured.The power cables have always been the key apparatus in the power transform and transmission network and the XLPE power cable has been taking up the leading place in power cable field because of its relatively light weight, simple processing, easy setting up and the favorable electric performance and heat endurance. However, after years' service, faults of power cables will occur if suffer from situations such as mechanical injuring, moisture, insulation aging, erosion of cable shields, over-voltage and heat. The interruption of power supply will surely cause great losses because of the XLPE cables' high capacity. So, to avoid the power cut accidents, it's necessary to build the efficient precise on-line insulation monitoring for XLPE power cables.At present, method of on-line monitoring for XLPE cables is limited to direct current parameter method yet. The present on-line monitoring systems for capacitive apparatus still have shortages in precise and stability. In this paper, based on the analysis of the electrical characteristics of XLPE power cables, an on-line insulation dielectric loss factor (tg 8 ) monitoring system for XLPE cables has been accomplished. And the system focuses on measuring the value of tg 8 accurately and stably. A series of rules for choosing the proper current sensor for tg 8 's measurement are given with a detailed analysis of sensors' error mechanism. In order to remove interference on-site, which influences the result, a method that replaces the hardware functions with software when possible is put forward. Furthermore, an integrative anti- interference solutions are provided, which are consisted of both hardware ones and software ones. Solutions in hardware include a multi-layer shielding structure and proper groundings of the whole system. Solutions in software include digital data pretreatment based on wavelet, FIR digital filter and algorithm of improved high order sine fit method. They do resolve the corresponding problems in on-site monitoring caused by strong electromagnetic field, noise in signals and harmonic and the fluctuation of power system frequency. The real time data acquisition software with a database function is also offered intuitively and stably. Finally, the field experiments show that the measurement of tg 8 of XLPE power cables is fairly precise and stable. And the monitoring system could achieve the on-site monitoring task of power system well.