Thesis (PhD)--University of Stellenbosch, 2002. / ENGLISH ABSTRACT: This dissertation considers the wideband modelling of capacitive voltage sensors for open-air transmission
line applications. Two novel topologies were introduced, namely a differential parallel
plate sensor with a floating faraday cage for the interface instrumentation and a coaxial sensor
mounted around the earth conductor with a faraday cage connected to the earth conductor. The
modelling and analysis procedures included the derivation of equivalent circuit models in order to
simulate the calibration factor, the loading effect of the interface instrumentation and the effect of
leakage 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 very
high frequencies (several MHz) it is important that the interface instrumentation have a high input
impedance and galvanic isolation be maintained. This was achieved by developing interface
instrumentation with a fibre-optic link operated from battery power. The instrumentation represents
a fairly unique approach in that the data is digitised before transmission across the serial
fibre-optical link, where-as conventional interfaces use analogue optical technology. Despite the
added complexity and high power requirements introduced by the digitising process, the improved
versatility is expected to yield a superior interface solution. The instrumentation has a bandwidth
of 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 ground
will introduce variations in the frequency response towards the low-frequency end. Leakage of a
100 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 into
the differential sensor topology and the associated electric fields. The improvement of a differential
plate sensor over the traditional single element plate sensor was demonstrated using twodimensional
simulations. Further simulations with a three-dimensional package showed that the
two-dimensional simulations are insufficient, because the boundary conditions and end effects have
a great influence on the calibration factor of the sensor.
Extensive laboratory tests were also undertaken to evaluate the sensor topology as well as the effects
of the interface instrumentation and leakage to ground. Excellent correlation were found
between the measured and simulated waveforms, both in the time- and frequency domains regarding
the calibration factor as well as the added poles or zeros at low frequencies. It can therefore
be deduced that a valid circuit model was suggested for these sensor topologies in the frequency
range from 10 Hz to 1 MHz.
Keywords: Capacitive sensors, Open-air voltage sensors / AFRIKAANSE OPSOMMING: Hierdie verhandeling beskou die wyeband modellering van kapasitiewe spanningsensors vir opelug
transmissie lyn toepassings. Twee oorspronklike topologieë is voorgestel, naamlik 'n differensiële
parallel plaat sensor met 'n aparte faraday hok vir die koppelvlak instrumentasie en
'n koaksiale sensor wat rondom die aardgeleier monteer word met die faraday hok ook aan die
aardgeleier gekoppel. Die modellerings en analise prosedures het ingesluit die afleiding van ekwivalente
stroombaanmodelle vir simulasie van die kalibrasiefaktor asook die belasingseffek van
die koppelvlak instrumentasie en lekweerstand na grond in beide die tyd- en frekwensie gebiede.
Om 'n plat frekwensieweergawe te verkry vanaf baie lae frekwensies (laer as 5 Hz) tot by baie
hoë frekwensies Cn paar MHz), is dit belangrik dat die koppelvlak instrumentasie 'n hoë intreeimpedansie
het en galvaniese isolasie verseker word. Dit was bereik deur koppelvlak instrumentasie
te ontwikkel met 'n optiese vesel koppeling wat met battery krag aangedryf word. Die instrumentasie
verskaf 'n redelik unieke aanslag in die opsig dat die data gemonster word voordat
dit oor die seriële optiese vesel skakel gestuur word, terwyl konvensionele koppelvlakke analoog
optiese tegnologie gebruik. Ten spyte van die toegevoegde kompleksiteit en hoë drywingsvereistes
van die versyferingsproses, het die instrumentasie se veelsydigheid toegeneem tot die mate dat dit
as 'n beter koppelvlak oplossing beskou word. Die instrumentasie het 'n bandwydte van ongeveer
6 MHz, met 'n opsionele teen-vou filter by 1 MHz.
Die ondersteuningstruktuur is van besondere belang aangesien enige ongebalanseerde lekweerstand
na grond afwykings in die frekwensieweergawe sal veroorsaak aan die lae frekwensie kant.
Lekweerstand van 100 MQ sal die frekwensieweergawe beïvloed tot by ongeveer 1 kHz.
Uitgebreide simulasies is gedoen om kwalitatiewe en kwantitatiewe insig in die differensiële sensor
topologie en die geassosieerde elektriese velde te verkry. Die verbetering van 'n differensiële
parallel plaat sensor in vergelyking met die tradisionele enkel element plaat sensor is demonstreer
met twee-dimensionele simulasies. Verdere simulasies met 'n drie-dimensionele pakket het gewys
dat die twee- dimensionele simulasies onvoldoende is aangesien grensvoorwaardes en randeffekte
'n groot invloed het op die kalibrasiefaktor van die sensor.
Uitgebreide laboratorium toetse is ook gedoen om die sensor topologie sowel as die effekte van die
koppelvlak instrumentasie en lekweerstand na grond te evalueer. Uitstekende korrelasie is gevind
tussen gemete en voorspelde golfvorms, in beide die tyd- en frekwensie gebied met betrekking tot
die kalibrasie faktor sowel as die toegevoegde pole en zeros by lae frekwensies. Die gevolgtrekking
is dus dat 'n geldige stroombaanmodel voorgestel is vir die sensor topologieë vir die frekwensie
bereik van 10Hz to 1 MHz.
Sleutelwoorde: Kapasitiewe sensors, Ope-lug spanningsensors
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/53222 |
| Date | 12 1900 |
| Creators | Schilder, Melanie |
| Contributors | Vermeulen, H. J., Stellenbosch University. Faculty of Engineering. Dept. of Electrical and Electronic Engineering. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | Unknown |
| Type | Thesis |
| Format | 198 p. : ill. |
| Rights | Stellenbosch University |
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