| Summary: | Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2009. === ENGLISH ABSTRACT: From a systems operation and design perspective, it is important to understand the behaviour
of HVDC system insulation when presented with high voltage transients, such as induced by
lightning and switching operations. Therefore, this thesis investigates the design, operation
and performance of a circuit arrangement that can be used in high voltage laboratories to
generate impulse voltage waveforms superimposed on a dc bias voltage. The circuit
arrangement consists of an impulse generator and a dc source that supplies continuous dc
voltage to stress the test object, which can be any type of insulator, i.e. composite, porcelain,
glass, gap arrangements, etc. The composite waveform obtained from the test arrangement is
used experimentally to investigate the impulse flashover of insulators.
For modelling and analysis purposes, the test circuit was transformed to a Laplace equivalent
in order to derive the applicable nodal voltage equations. After substitution of circuit
parameter values, the voltage equations are then transformed to time domain equations that
predict the time-domain behaviour of the circuit. To validate this mathematical approach, the
voltage waveforms obtained with this mathematical model is compared with the waveforms
measured under laboratory conditions and also with waveforms simulated with HSPICE
software. These comparisons are performed using graphical representations. Good correlation
was obtained and the results are presented in this thesis.
The final stage of this thesis discusses the application of the designed test arrangement for
flashover and withstands tests on a silicon rubber insulator. The determination of the flashover
values is done by using the existing statistical methods. The insulator was tested under dry
conditions and also under polluted wet conditions for both positive and negative impulses
compared to the DC bias voltage polarity. The results show that the dc bias voltage does not
affect the total voltage flashover of the insulator significantly. It was also observed that
wetting affects the flashover for negative impulse more severely, while the influence of
wetting is minimal with positive impulse voltages === AFRIKAANSE OPSOMMING: Vanuit ‘n stelselbedryf en ontwerp perspektief, is dit is belangrik om die gedrag van HSGS
stelsels te verstaan wanneer dit onderwerp word aan hoogspanning oorgangsverskynsels soos
veroorsaak deur weerlig en skakeloperasies. Daarom ondersoek hierdie tesis die ontwerp,
werking en werkverrigting van ‘n stroombaanopstelling wat gebruik kan word in
hoogspanningslaboratoriums om impulsspannings gesuperponeer op gelykspanning
voorspannings op te wek. Die stroombaan bestaan uit ’n impulsgenerator en ‘n gs-bron wat
die langdurige gs-spanning voorsien aan die toetsvoorwerp, wat enige tipe isolator kan wees.
bv. porselein, glas, gapings, ens. Die saamgestelde golfvorm wat met die toetsopstelling
verkry word, is eksperimenteel gebruik om die impulsoorvonking van isolators te ondersoek.
Vir die doel van modellering and analise, is die stroombaan na ‘n Laplace ekwivalent
getransformeer om die toepaslike knooppunt spanningsvergelykings af te lei. Na substitusie
van die stroombaan parameter waardes, word die spanningsvergelykings getransformeer na
die tydgebied vergelykings wat die tydgebied gedrag van die stroombaan voorspel. Om die
wiskundige benadering te toets, word die spanningsgolfvorms wat met die wiskundige model
voorspel word, vergelyk met golfvorms wat onder laboratorium toestande gemeet is en ook
met golfvorms wat met HSPICE programmatuur gesimuleer is. Hierdie vergelykings word
gedoen met behulp van grafiese voorstellings. Goeie korrelasie is verkry en die resultate word
in die tesis gegee.
Die finale stadium van hierdie tesis bespreek die toepassing van die ontwerpte toetsopstelling
vir oorvonk- en weerstaantoetse op ‘n silikonrubber isolator. Die bepaling van die
oorvonkwaardes word gedoen deur bestaande statistiese metodes te gebruik. Die isolator is
onder droë en nat besoedelde toestande gedoen, vir beide positiewe sowel as negatiewe
impulse met verwysing na die GS voorspan spanning. Die resultate toon dat die gsvoorspanning
nie die oorvonkspanning van die isolator beïnvloed nie. Dit is ook waargeneem
dat die benatting die oorvonking neer beïnvloed met ‘n negatiewe impuls terwyl die invloed
minimaal is met positiewe impulsspannings.
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