No description available.

Thesis (MScEng)--University of Stellenbosch, 2005. / ENGLISH ABSTRACT: The calibration and field evaluation of an Insulator Pollution Monitoring Relay (IPMR) were the main aims of this research programme. A repeatable artificial wetting test method was developed after several modifications were made to the steam system, test chamber and the test routine. The IPMR was successfully calibrated with insulators that were artificially polluted according to the solid layer method. Linear and polynomial relationships were determined after curve-fitting techniques were performed on the results. The calibration showed that the IPMR is capable as a device relating the maximum conductivity during artificial wetting to the ESDD, a severity classification parameter. The IPMR was successfully used in a salt fog chamber to determine if the device is capable to evaluate the severity of an instantaneous pollution event. The IPMR was successfully installed at a natural pollution test site along the Cape west coast. The conductivity measurements with natural wetting showed good correlation to flashovers experienced. A rule of thumb, developed to indicate a possible risk of flashover, was based on observations made on the relationship between humidity and surface conductivity. The measured IPMR data was successfully applied to quantify the site severity according to the conductivity measurement with natural wetting. This calculated severity value could be used in the assessment of flashover probability of high voltage insulators. / AFRIKAANSE OPSOMMING: Die hoofdoel van die tesis was die kalibrasie en veldtoetse van 'n isolatorbesoedelingsmonitor (IPMR). 'n Herhaalbare nagebootste benatting-toetsmetode is ontwikkel na veranderings aan die stoomstelsel, toetsruimte en die toetsproses. Die IPMR is suksesvol gekalibreer met isolators wat besoedel was met 'n nagebootste besoedeling volgens die "solid layer method". Liniêre sowel as kwadratiese verwantskappe is ontwikkel na krommepassings op die resultate uitgevoer was. Die kalibrasie het gewys dat die IPMR in staat is om die maksimum geleidingsvermoë wat d.m.v. nagebootste benatting verkry is, met die ESDD, 'n besoedelingsklassifikasie, kan vergelyk. Die apparaat is ook suksesvol gebruik tydens soutmistoetse om te bepaal of dit in staat is om 'n skielike besoedelingsgebeurtenis te kan meet. Na die afhandeling van laboratorium werk is die apparaat by 'n natuurlike isolator besoedeling-toetsstasie langs die Kaapse weskus geïnstalleer. Die geleidingsvermoë metings met natuurlike benatting het goeie korrelasie getoon met isolator oorvonkings. 'n Skattingsmetode wat ontwikkel is om moontlike oorvonkings te voorspel, is gebaseer op waarnemings wat gemaak is van die humiditeit sowel as die oppervlakte geleidingsvermoë. Die IPMR se geleidingsvermoë metings met natuurlike benatting is aangewend om die besoedelingsgraad van die gebied te bepaal. Die bepaalde besoedelingsgraad kan verder gebruik word om die waarskynlikheid van die oorvonking van isolators vas te stel.

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.

Thesis (M.Ing.)--University of Stellenbosch, 2000. / ENGLISH ABSTRACT: The application of cycloaliphatic epoxide (CE) insulation in the Republic of South Africa is investigated within this thesis. There was a request from industry to evaluate the product and indicate its suitability for application within the various geographic and climatic zones typical of the RSA. An extensive survey was conducted into the historic origins of the material, and the experiences of international utilities applying the insulation medium in respect of polluted conditions. The properties of the Automatic Pressure Gelation (APG) CE insulator manufacturing process are discussed with reference to the manufacture of glass and porcelain insulation, and typical areas for concern are discussed with reference to insulators found to deviate from specification. Typical insulators were obtained from the manufacturers and utilities associated with the product, and were subjected to electrical, material and visual examination tests. The electrical tests included AC wet and dry, lightning impulse, clean fog, salt fog, mould release resiliency and the IEC 1109 voltage test. The material tests included the identification of the epoxide systems in use in the RSA, ultraviolet radiation aging simulation, water hydrolysis and salt deposit density tests. The main findings are: • CE insulation is sensitive to marine pollution and continuous-wetting pollution types. • The use of CE insulation is promoted in respect of inland and medium industrial pollution types. • Class B pin-type CE insulation is prone to partial discharge related failures. • Surface roughening on the insulator surfaces leads to reduced AC wet flashover voltages and increased pollution catch. Additionally: • A comprehensive aging hypothesis was developed detailing three independent aging stages/modes applying to CE insulation. • A hypothesis was developed for a new evaluation method for documenting changes on insulator surfaces due to the action of aging mechanisms: The Surface Area Index. / AFRIKAANSE OPSOMMING: Die aanwending van sikloalifatiese epoksied (SE) isolasie in die Republiek Van Suid Afrika word in hierdie skripsie ondersoek. Hierdie ondersoek is geloods op versoek van die elektrisiteitsbedryf om die toepaslike aanwending van die produk onder die verskeie heersende streeksweersomstandighede binne die RSA te evalueer. Breedvoerige navorsing is gedoen om die geskiedkundige oorsprong van die materiaal vas te stel, sowel as om die prestasie daarvan as insulasiemiddel te boekstaaf. Daar is veral gelet op die prestasie van die materiaal onder besoedelende omstandighede en ondervinding wat opgedoen is deur intemasionale ondememings. Die eienskappe van die automatiese drukjelvormings-vervaardigingsproses (APG) word behandel met verwysing na die maak van glas en porselein insulators. Tipiese probleemareas word bespreek van insulators wat afwyk van spesifikasie af. Verteenwoordigende isolators is vanaf vervaardigers en ondememings verkry wat met die produk bemoei is. Hulle is blootgestel aan elektriese, materiaal en visuele toetse. Die elektriese toetse het die volgende behels: nat en droog wisselstroom, skoon waterdamp, sout waterdamp en die IEe 1109 spanningstoets. Die effek van die verlies van die anti-kleefmiddel (gebruik tydens die gietproses) as gevolg van veroudering is ook in detail behandel. Die materiaaltoetse het ingesluit: die uitkenning van die epoksied stelsels in gebruik in die RSA, gesimuleerde veroudering deur ultravioletligbestraling, water hidroliese en soutlaag digtheids toetse. Die hoofbevindings is: • SE isolasie word bemvloed deur seelug besoedeling sowel as volgehoue benatting. Die gebruik van SE isolasie word voorgestel vir binnelandse gebruik sowel as in gebiede met mediumvlak nywerheidsbesoedeling. • Klas B pen-tipe SE isolasie is geneig tot deelontladings wat dan tot faling lei. • Oppervlakvergroffing lei tot In verminderde wisselstroom orvonksspanning onder nat toestande sowel as verhoogde vangs van besoedeling. Bykomend hiertoe is: • 'n breedvoerige SE verouderingshipotese ontwikkel wat drie onafhanklike stadia en modusse van die verouderingsproses uitgewys het. • 'n hipotese ontwikkel vir In nuwe evaluasiemetode om die verandering in die isolatoroppervlak as gevolg van veroudering te dokumenteer: Oppervlakte- Area Indeks (SAl).

Variable speed drives allow for more precise speed control of induction motors, are of high power factor, and offer fast response characteristics, compared to older technologies, such as motor-generator sets and eddy current clutches. However, due to the high switching frequencies as well as the high dV/dt in the output increased dielectric stresses are produced in the insulation system of the motor they supply. Due to the use of these solid state drives there have been concerns of premature failure in large, medium and high voltage, motors. To fully understand and deal with these concerns requires studying the degradation mechanisms, in the insulation system, caused by these drives; which, on an actual motor is both extremely costly as well as impractical. Therefore, coil samples which accurately represent the construction of the actual insulation system, must be aged and studied instead. In addition, to ideally replicate the aging process, the same waveform that the motor is subjected to must be applied to these samples. As a result of this requirement, a low power, two-level, high voltage PWM inverter has been built to replicate the most important characteristics of the output waveform of a variable speed drive. This power modulator allows for testing the insulation systems considering a real PWM waveform in which both the fast pulses and the fundamental low frequency are included. The results of these tests show that the effects of PWM waveforms cannot be entirely replicated by a unipolar pulse generator.

High-power voltage sourced converters(VSC’s) are becoming increasingly prevalent in modern transmission systems. These systems primarily use switching schemes generating kHz range harmonics, which may be magnified by one or more system resonances. Despite the high frequency harmonics, VSC systems widely use insulated equipment designed for operation at power frequencies; this includes critical substation components such as medium voltage polymeric cables and terminations. The stress grading systems of non-geometric (compact) cable terminations are susceptible to insulation degradation and eventual flashover failure, under high frequency harmonic stresses. As such, the present work studies high frequency voltage effects in cross-linked polyethylene cable terminations, and their relationship to stress grading (SG) design and material properties. Finite element modeling (FEM) has been used to analyze electric field and resistive heating in termination designs, in response to parametric variations in SG material properties. Experimental studies investigate thermal behaviour in a variety of commercial termination designs, using a high voltage, high frequency test setup developed to replicate conditions of high frequency harmonic resonance in a VSC system. The study results show that high frequency voltage application increases the electric field, resistive heating, and surface temperature rise, in non-geometric (compact) termination designs using field-dependant stress grading materials. Geometric (stress cone) designs are insensitive to high frequency harmonics; however, they have disadvantages compared to compact designs, making them a less practical long-term solution for high frequency applications. Among non-geometric designs, the field-dependent electrical conductivity σ (E), the permittivity ε, and the temperature dependencies of σ (E) and ε strongly influence the termination electrical and thermal behaviour under high frequency stress. Since thermal hotspots in cable termination SG areas may lead to material degradation and eventual failure, recommendations are made for an optimal non-geometric stress grading design, for terminations operating in environments where high frequency harmonics may be present.

Variable speed drives allow for more precise speed control of induction motors, are of high power factor, and offer fast response characteristics, compared to older technologies, such as motor-generator sets and eddy current clutches. However, due to the high switching frequencies as well as the high dV/dt in the output increased dielectric stresses are produced in the insulation system of the motor they supply. Due to the use of these solid state drives there have been concerns of premature failure in large, medium and high voltage, motors. To fully understand and deal with these concerns requires studying the degradation mechanisms, in the insulation system, caused by these drives; which, on an actual motor is both extremely costly as well as impractical. Therefore, coil samples which accurately represent the construction of the actual insulation system, must be aged and studied instead. In addition, to ideally replicate the aging process, the same waveform that the motor is subjected to must be applied to these samples. As a result of this requirement, a low power, two-level, high voltage PWM inverter has been built to replicate the most important characteristics of the output waveform of a variable speed drive. This power modulator allows for testing the insulation systems considering a real PWM waveform in which both the fast pulses and the fundamental low frequency are included. The results of these tests show that the effects of PWM waveforms cannot be entirely replicated by a unipolar pulse generator.

High-power voltage sourced converters(VSC’s) are becoming increasingly prevalent in modern transmission systems. These systems primarily use switching schemes generating kHz range harmonics, which may be magnified by one or more system resonances. Despite the high frequency harmonics, VSC systems widely use insulated equipment designed for operation at power frequencies; this includes critical substation components such as medium voltage polymeric cables and terminations. The stress grading systems of non-geometric (compact) cable terminations are susceptible to insulation degradation and eventual flashover failure, under high frequency harmonic stresses. As such, the present work studies high frequency voltage effects in cross-linked polyethylene cable terminations, and their relationship to stress grading (SG) design and material properties. Finite element modeling (FEM) has been used to analyze electric field and resistive heating in termination designs, in response to parametric variations in SG material properties. Experimental studies investigate thermal behaviour in a variety of commercial termination designs, using a high voltage, high frequency test setup developed to replicate conditions of high frequency harmonic resonance in a VSC system. The study results show that high frequency voltage application increases the electric field, resistive heating, and surface temperature rise, in non-geometric (compact) termination designs using field-dependant stress grading materials. Geometric (stress cone) designs are insensitive to high frequency harmonics; however, they have disadvantages compared to compact designs, making them a less practical long-term solution for high frequency applications. Among non-geometric designs, the field-dependent electrical conductivity σ (E), the permittivity ε, and the temperature dependencies of σ (E) and ε strongly influence the termination electrical and thermal behaviour under high frequency stress. Since thermal hotspots in cable termination SG areas may lead to material degradation and eventual failure, recommendations are made for an optimal non-geometric stress grading design, for terminations operating in environments where high frequency harmonics may be present.

With the continuous growth of non-linear power electronic components and the increasing penetration of the distributed generation (DG), the potential for degradation in the power quality of the existing grid exists. There are concerns that the total harmonic distortion (THD) could reach unacceptable levels of 5% or higher. Moreover, there is additional potential of the presence of amplified harmonic components in the power network grid when the harmonic frequencies align with the resonant frequencies that are being injected by power electronic components of the DG. The above conditions could increase the electrical stresses on the insulation system of the power system components, and in particular, cable terminations are a concern. Standard cable terminations are designed to operate under power frequency in the power system network and their service life is considered accordingly. The research work aims to provide an understanding of the performance of the stress grading (SG) system of a commercial cable termination when the voltage waveform is distorted due to the presence of harmonics and when the high frequency and high dV/dt voltage waveforms are present from a typical power electronic drive. An aging experiment was performed for over a 600 hour time period using the pulse width modulated (PWM) high-voltage generator to quantify the impact of high frequency stress on SG system of cable termination. Furthermore, the cable termination was tested under power frequency, distorted voltage waveforms composed of fundamental and low order harmonics using an experiment setup that generate distorted voltage waveforms. Diagnostic techniques such as surface potential distribution measurements and surface temperature monitoring are used to analyze the termination performance. The surface tangential field is calculated based on the gradient of the termination surface potential as measured with an electrostatic voltmeter. The study shows that distorted voltage waveforms with high frequency and high dV/dt components, increase the electric field, resistive heating, and surface temperature rise in the terminations that use the field-dependent SG materials. The rise of electric field by as high as 27.1% and surface temperature rise of as high as 17C demonstrates the severity on the cable terminations. Such electric field enhancements for a period of time have a potential to initiate partial discharge that could lead to degradation of the termination. Moreover, surface temperature rise of 17 deg C could reduce the allowable ampacity of the cable conductor, reduce the short circuit levels, and reduce the feeder loading limits. The field-dependent electrical conductivity (σ(E,T)), permittivity (ε), and the temperature dependencies of (σ(E,T) and ε) have strong impact to degrade the electrical and thermal properties of the termination due to stress from the non-sinusoidal distorted voltage waveform. In order to minimize the surface temperature rise from the hotspot and electrical stress enhancement that eventually lead to insulation degradation and failure, the following recommendations are made for a suitable SG design for a termination to handle the severe voltage stress: Apply the capacitively graded termination in the grid where the distortion levels are low. Under the increased total harmonic distortion levels and HF components, resistively grading with higher degree of nonlinearity (achieved through the use of ZnO filler) is beneficial. The utilities could take preventive maintenance on medium voltage power cable accessories to prevent the termination failure before it actually occurs. Researchers could focus to resolve and minimize the rising power quality issues when the distribution generations are operated, improve the power electronic converters, and provide cost-effective harmonic filter solutions for harmonic mitigation.

With the continuous growth of non-linear power electronic components and the increasing penetration of the distributed generation (DG), the potential for degradation in the power quality of the existing grid exists. There are concerns that the total harmonic distortion (THD) could reach unacceptable levels of 5% or higher. Moreover, there is additional potential of the presence of amplified harmonic components in the power network grid when the harmonic frequencies align with the resonant frequencies that are being injected by power electronic components of the DG. The above conditions could increase the electrical stresses on the insulation system of the power system components, and in particular, cable terminations are a concern. Standard cable terminations are designed to operate under power frequency in the power system network and their service life is considered accordingly. The research work aims to provide an understanding of the performance of the stress grading (SG) system of a commercial cable termination when the voltage waveform is distorted due to the presence of harmonics and when the high frequency and high dV/dt voltage waveforms are present from a typical power electronic drive. An aging experiment was performed for over a 600 hour time period using the pulse width modulated (PWM) high-voltage generator to quantify the impact of high frequency stress on SG system of cable termination. Furthermore, the cable termination was tested under power frequency, distorted voltage waveforms composed of fundamental and low order harmonics using an experiment setup that generate distorted voltage waveforms. Diagnostic techniques such as surface potential distribution measurements and surface temperature monitoring are used to analyze the termination performance. The surface tangential field is calculated based on the gradient of the termination surface potential as measured with an electrostatic voltmeter. The study shows that distorted voltage waveforms with high frequency and high dV/dt components, increase the electric field, resistive heating, and surface temperature rise in the terminations that use the field-dependent SG materials. The rise of electric field by as high as 27.1% and surface temperature rise of as high as 17C demonstrates the severity on the cable terminations. Such electric field enhancements for a period of time have a potential to initiate partial discharge that could lead to degradation of the termination. Moreover, surface temperature rise of 17 deg C could reduce the allowable ampacity of the cable conductor, reduce the short circuit levels, and reduce the feeder loading limits. The field-dependent electrical conductivity (σ(E,T)), permittivity (ε), and the temperature dependencies of (σ(E,T) and ε) have strong impact to degrade the electrical and thermal properties of the termination due to stress from the non-sinusoidal distorted voltage waveform. In order to minimize the surface temperature rise from the hotspot and electrical stress enhancement that eventually lead to insulation degradation and failure, the following recommendations are made for a suitable SG design for a termination to handle the severe voltage stress: Apply the capacitively graded termination in the grid where the distortion levels are low. Under the increased total harmonic distortion levels and HF components, resistively grading with higher degree of nonlinearity (achieved through the use of ZnO filler) is beneficial. The utilities could take preventive maintenance on medium voltage power cable accessories to prevent the termination failure before it actually occurs. Researchers could focus to resolve and minimize the rising power quality issues when the distribution generations are operated, improve the power electronic converters, and provide cost-effective harmonic filter solutions for harmonic mitigation.