Title:
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A measurement system for the monitoring and measurement of earthing grid performance at large high voltage substations
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Earthing grids are a major part of high voltage plant at electrical installations ensuring
good operation of equipment and the safety of personnel and the general public. Therefore,
for improved safety, it is essential to keep the earthing grid impedance as low as
possible at all times.
A continuous on-line monitoring of the earthing grid is, therefore, desirable. This
thesis presents the investigations and improvements to existing practice which were
carried out to help the development of such a monitoring system. It also describes the
specification, analysis, set up and performance tests of a prototype monitoring system.
Extensive measurements in a laboratory environment, at the Cardiff University Outdoor
Test Site, and at operating electrical substations and tower lines were used to validate
the developed system.
The literature review carried out in this work concentrated on measurement techniques
used in earthing systems and safety issues related to electrical substations and
tower lines.
The accuracy of the existing impedance measurement system, IMS, developed at
Cardiff, was examined, and the errors involved were quantified for the first time. The
investigation was then concentrated on developing a fully computerised earth impedance
measurement system, CEIMS, which not only allows more accurate evaluation of the
test-electrode parameters but also permits savings on expensive lock-in amplifiers. The
developed new algorithms allow frequency discrimination using advanced digital signal
processing techniques.
The extensive experimental work carried out within this programme, in the laboratory,
at the Cardiff University Outdoor Test Site, and at operating substations and tower
lines, has led to improvements in field test set-ups and better isolation techniques. More
importantly, two new methods were developed; one is for the measurement of earth electrode
resistance using the power consumption principle, while the other is for monitoring
the earthing impedance of large substations. The latter technique does not require any
auxiliary current injection but takes advantage of earth-wire currents flowing into/out
of the substation earth grid. It is shown that if a large proportion of these currents is
measured, changes to the earth grid impedance can be assessedc ontinuously.
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