Wideband modelling of capacitive voltage sensors for open-air transmission line applications

[摘要] ENGLISH ABSTRACT:This dissertation considers the wideband modelling of capacitive voltage sensors for open-air transmissionline applications. Two novel topologies were introduced, namely a differential parallelplate sensor with a floating faraday cage for the interface instrumentation and a coaxial sensormounted around the earth conductor with a faraday cage connected to the earth conductor. Themodelling and analysis procedures included the derivation of equivalent circuit models in order tosimulate the calibration factor, the loading effect of the interface instrumentation and the effect ofleakage to ground, both in the time- and frequency domain.In order to obtain a flat frequency response from very low frequencies (less than 5 Hz) to veryhigh frequencies (several MHz) it is important that the interface instrumentation have a high inputimpedance and galvanic isolation be maintained. This was achieved by developing interfaceinstrumentation with a fibre-optic link operated from battery power. The instrumentation representsa fairly unique approach in that the data is digitised before transmission across the serialfibre-optical link, where-as conventional interfaces use analogue optical technology. Despite theadded complexity and high power requirements introduced by the digitising process, the improvedversatility is expected to yield a superior interface solution. The instrumentation has a bandwidthof approximately 6 MHz, with an optional anti-aliasing filter at 1 MHz.Special consideration should be given to the support structure as any unbalanced leakage to groundwill introduce variations in the frequency response towards the low-frequency end. Leakage of a100 MQ was found to influence the frequency response of the circuit up to frequencies of 1 kHz.Extensive simulation studies were conducted to obtain qualitative and quantitative insight intothe differential sensor topology and the associated electric fields. The improvement of a differentialplate sensor over the traditional single element plate sensor was demonstrated using twodimensionalsimulations. Further simulations with a three-dimensional package showed that thetwo-dimensional simulations are insufficient, because the boundary conditions and end effects havea great influence on the calibration factor of the sensor.Extensive laboratory tests were also undertaken to evaluate the sensor topology as well as the effectsof the interface instrumentation and leakage to ground. Excellent correlation were foundbetween the measured and simulated waveforms, both in the time- and frequency domains regardingthe calibration factor as well as the added poles or zeros at low frequencies. It can thereforebe deduced that a valid circuit model was suggested for these sensor topologies in the frequencyrange from 10 Hz to 1 MHz.Keywords: Capacitive sensors, Open-air voltage sensors

[发布机构] Stellenbosch University