Browsing by Author "Solomon, Anthea"
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- ItemWideband modelling and parameter estimation of capacitor voltage transformers using a pseudo-random impulse sequence(Stellenbosch : Stellenbosch University, 2021-12) Solomon, Anthea; Vermeulen, Johan; Mwaniki, Fred; Stellenbosch University. Faculty of Engineering. Dept. of Electrical and Electronic Engineering.ENGLISH ABSTRACT: Capacitor voltage transformers are widely used on power transmission networks for voltages of 275 kV and above for voltage measurement, installed on high voltage busbars and transmission lines. The main purpose of line capacitor voltage transformer arrangements is to step down high voltages to measurable values for protection and metering devices, as well as for quality of supply and control applications. In addition, they provide a low impedance path for the high frequency power line carrier signal used for communication and accelerated tripping between protection devices. For accurate capacitor voltage transformer modelling and frequency response simulations a suitable broadband circuit model must be used. This project examines the time-domain and frequency-domain behaviour of a broadband model topology for a practical capacitive voltage transformer. The mathematical equations for the circuit topology are derived and simulation results of the secondary input impedances, voltage transformation ratios and transconductance ratio are presented and critically appraised. Transfer functions for various subcircuits are derived, and evaluated against simulated and calculated responses. A new method of using waveform pattern recognition for the detection of internal capacitor voltage transformer failures experienced during network transients, energization, de-energization and steady state conditions was examined and discussed. This can be a useful tool that would minimize time spent on fault finding, and limit the impact on surrounding plant if imminent failures are identified and attended to early enough. In addition, it can prompt timeous replacement of the equipment before severe failures occur, considering long lead times regarding procurement processes and unavailability of strategic spares. The approach is based on evidence collection, where analog voltage recordings, are critically appraised and used as a tool for failure detection and condition monitoring. Results from simulated internal failures correlated well with voltage waveforms obtained from fault recordings retrieved during network transients of faulted capacitor voltage transformers. Use of a novel perturbation signal for the determination of frequency responses of capacitor voltage transformers, namely the pseudo-random impulse sequence [1], was explored. This broadband signal can be easily generated for high voltage applications. Measured frequency responses were presented for the transformation ratio, secondary input impedance and secondary transconductance ratio of a 400 kV/110 V capacitor voltage transformer. The measured and simulated frequency responses observed correlated well. Measurement results revealed that resonance at much higher frequencies will occur for transfer functions of capacitor voltage transformers that have reached their useful life. It is concluded that the measurement arrangement for testing of secondary input transfer functions using the pseudo-random impulse sequence perturbation current source should be done with the lower stack capacitor C2 open circuited, to reduce the impedance seen on the secondary side. Most of the circuit parameters of capacitive voltage transformers are propriety and not available to the end user. The development of a parameter estimation technique for capacitive voltage transformers is therefore crucial as a means of obtaining these circuit parameters. A novel approach based on an objective function derived from a comparison between simulated and measured time domain signals is introduced and analysed. Previous work on parameter estimation for capacitive voltage transformers focused on optimization of an objective function defined in terms of impedances. The proposed parameter estimation methodology, furthermore, makes use of a novel perturbation signal for this type of application, namely the pseudo-random impulse sequence. Results are presented for a range of case studies. The results show excellent correlation between the actual and estimated parameters for most of the cases studies. The frequency responses obtained with the estimated parameters, furthermore, represent the responses obtained with the actual parameters closely. Overall, it is concluded that the proposed methodology, with an objective function derived from comparison of time domain voltage time signals, performs well for estimating parameters of practical capacitive voltage transformers.