Modelling and performance evaluation of a three-phase capacitive voltage sensor topology
Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2006.
This research project investigates the design, modelling and application of an open-air capacitive voltage sensor assembly for the measurement of wideband High Voltage signals on three-phase transmission lines. The advantages and disadvantages of conventional methods used to measure these voltages are reviewed and the advantages of the open-air capacitive sensor are established. The main research objective of this project involves extending the application of previously developed single-phase capacitive sensor topologies to three-phase applications. A three-phase set of mobile, compact and relatively inexpensive capacitive voltage sensors for open-air application under overhead transmission lines are designed and constructed, including a data acquisition triggering system for the measurement of transient waveforms. Equivalent circuit models, using a Thévenin equivalent approach, are developed for the three-phase sensor topology and the associated three-phase transmission line configuration. A number of different methods for simplifying the associated Thévenin equivalent impedance and voltage equations are evaluated. The decoupling of the voltage waveforms measured by the individual sensors for a three-phase transmission line configuration is subsequently examined with the view to derive mathematical relationships for determining the phase conductor voltages from the measured sensor voltages. The performance of the sensor assembly is experimentally evaluated under laboratory conditions as well as field conditions. An outdoor HV test facility, representing a scaled three-phase flat transmission line structure, is developed for evaluation of the three-phase sensor topology in a controlled environment. The methodology for decoupling the phase voltages and reconstructing the phase conductor voltages from the voltages induced on the sensors is evaluated using measured data obtained with this HV test facility. It is shown that the three-phase capacitive voltage sensor topology as applied in the test facility delivers good results for the measurement of three-phase transient voltage waveforms.