No description available.

This thesis presents new configurations for three-phase AC-to-DC single-stage, softswitched, high frequency (HF) transformer isolated converters with power factor correction (PFC) and low harmonic distortion. Four different configurations are presented. Topology of all these four configurations is based on integration of a front-end DCM boost with a soft switching HF transformer isolated DC-to-DC PWM converter with fixed frequency. DCM operation of the front-end boost provides natural PFC with low total harmonic distortion (THD) and the DC-to-DC HF transformer isolated soft switching PWM converter with an appropriate gating scheme provides output voltage regulation. A double switch AC-to-DC converter is presented in Chapter 2. Due to unsymmetrical gating scheme, DC blocking capacitors are required to avoid transformer saturation. To reduce this problem, a new gating scheme is proposed in Chapter 3, which can be used in full bridge converters providing ZVS. This gating scheme is first used in a DC-to-DC bridge converter. In the next three chapters, this new gating scheme is applied to three different types of single-stage AC-to-DC boost integrated fixed-frequency bridge converters. These configurations are: (i) boost integrated single inductor linear current DC-to-DC PWM bridge converter, (ii) boost integrated series resonant DC-to-DC bridge converter and (iii) boost integrated parallel resonant DC-to-DC bridge converter. The steady state operation of each converter and modes of operation are explained with equivalent circuits for each interval of HF cycle. The general solutions for all the intervals are derived and design curves are obtained based on steady state relations. The design procedure is illustrated with a design example. Detailed PSPICE simulation results and experimental results obtained from a laboratory prototype model are given for all the converters to verify the theory and analysis. THD of the line current without any complex control circuit remains in a reasonable range of 8% to 13% for the total range of operation. Input line current waveforms for all suggested converters shows a low harmonic distortion similar to a single 3-Φ DCM boost. The difference would be in increase or decrease of DC bus voltage in each case, which can affect THD of the boost converter. Three switches in the full bridge converter operate with zero-voltage switching (ZVS) while the main switch operates with ZVS at full load, minimum line voltage and with ZVT at lower loads conditions. Soft switching of all the switches helps in lower loss. Chapter 6 presents the contributions of this thesis, summarizes the advantages and disadvantages of each configuration and gives some suggestions for future work. / Graduate

A dissertation submitted in fulfilment of the requirements for the degree of Master of Science in Engineering to the Faculty of Engineering and the Built Environment, School of Electrical and Information Engineering, University of the Witwatersrand, Johannesburg, 2018 / Power cable terminations and joints experience high electrical stress due to the abrupt change in geometry of the cable; hence the insulation at these points is more prone to partial discharges and has been reported as the main root cause of power cable system failures. Electrical failure of the insulation is known to occur due to a phenomenon of Partial Discharges (PD). Modern practice (especially in higher voltage installations) entails installation of PD sensors at strategic locations during installation of electrical equipment such as cable joints and terminations. This enables continuous monitoring of PD events in the plant, and this is termed on-line PD diagnosis. However, with limited knowledge to interpret the meaning of certain PD changes during the service period, this practice remains limited. It is therefore the interest of the study to understand the time evolution behaviour of PD characteristics in order to discern the insulation condition or deteriorating stages. The present study is on XLPE power cables, focusing on PD in artificial defects in the cable termination insulation that in most cases arise from poor workmanship. The power cables were subjected to accelerated ageing to emulate their ageing under service conditions. PD measurements were then conducted at periodic time intervals and characterized PD in terms of PD Inception Voltage (PDIV), maximum apparent PD magnitude (Qmax), Pulse Repetition Rate (PRR) and Phase-Resolved-PartialDischarge-Pattern (PRPDP). The findings are that, Qmax, PRR and PDIV did not show any time-evolution trends unique to a defect, the general trends observed were that of a fairly constant PDIV with several fluctuations of a 5 kV band. Qmax showed a decreasing trend over ageing time. The PRR decreased overall, with a pick up increase near the end of the tests. Qmax and PRR were noted significantly fluctuative between 23% and 57% of the total ageing period, distinct characteristics were that, the tram line had the largest PRR which is expected since it is a flat cavity, and iii the PRPDP appeared more skewed than other defects. The semicon feather had a PRPDP that seemed like a combination of a void discharge and corona discharge. The ring cut PRPDP was similar to that of the tram line except that it was not skewed. Furthermore, a capacitance PD model was constructed in Matlab R Simulink R to emulate experimental observed PD behaviour and therefore confirm the theory explaining the observed time-dependency of PD phenomena. Simulated void discharge PRPDP which corresponded with experimentally measured PRPDP were obtained for the unaged, moderately aged and severely aged cavity defect. The corona-surface discharge effect observed in the semicon PRPDP was also successfully emulated. The study outcomes suggest that PD characteristics evolve over time, and that the behaviour of the observed trend is unique at different stages during ageing. The time evolution characteristics of PD are The PRPDP signatures did not change with time of ageing despite the variations in Qmax and PRR. This means that, defect signatures obtained prior ageing or in-service operation of the cables can still serve as a good reference of identifying the nature of the defect at different ageing stages except in the event of PD evanescence. From the simulations, it was derived that the PD region surface conductivity as well as the geometry of the defect are the main contributing factors to the unique signatures observed at different stages and per defect. / XL2019

No description available.

Thesis (Ph.D.) - Carleton University, 2007. / Includes bibliographical references (p. 180-182). Also available in electronic format on the Internet.

published_or_final_version / Electrical Engineering / Master / Master of Science in Engineering

published_or_final_version / Electrical Engineering / Master / Master of Science in Engineering

published_or_final_version / Electrical and Electronic Engineering / Master / Master of Philosophy

The relationship between impulse breakdown stress and stressed oil volume has been extensively investigated for uniform field electrodes. In large high voltage transformers it is essential to extend this relationship to paper insulated non-uniform field situations which are more frequently encountered in practice. This paper develops the stated relationship by experimental tests, using insulated cable to plane configurations, and statistical methods. These tests show that the impulse breakdown strength of an insulated cable to plane or nonuniform field system has a two parameter Weibull distribution, when the oil is considered to be the weak-link of the system. A non-linear regression analysis is then used to find that the Weibull parameters for insulated non-uniform field electrodes are approximately the same as the parameters for the uninsulated uniform field electrodes provided their stressed oil volumes are equivalent. Statistical calculations are used to derive the fundamental relationship between the Weibull parameters for any stressed oil volume and the Weibull parameters for the unit oil volume. With this relationship it is possible for design engineers to approximately determine the probability of electrical breakdown of any insulated cable to plane configuration in large power transformers.

A dissertation submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Master of Science in Engineering in the High Voltage Research Group School of Electrical and Information Engineering Johannesburg, June 2017 South Africa / Servitude availability in space-constrained built-up areas within the Johannesburg or Central Load Network (CLN) poses every-day challenges for power system engineers. Strengthening the backbone 88/275 kV transmission system within the CLN becomes even more difficult when multi-circuit transmission lines are required for increased power transfer capabilities. When uprating is considered to increase the power transfer capability, the withstand levels of existing external insulation demands an optimisation to find a new stress versus strength balance that allows reliable operation of substations at higher voltages. The research includes primarily an investigative simulation study to evaluate the current Eskom available design clearances in terms of their withstand capability when subjected to over-voltage transients. Two voltage range classes were evaluated and the results are discussed. For voltage range 1, it was found that the over-voltage stress was low enough to allow for a higher nominal operating voltage while maintaining the existing clearances. For voltage range 2, existing clearances are also found to be conservative and smaller safety margins will most likely be acceptable. From a transient analysis evaluation, voltage uprating is considered as a very attractive option to increase the power transfer capability of existing substations. Current Eskom clearances for 88 kV and 275 kV are expected to perform well during transients generated in uprated systems. Electrode grading to improve the field gradients in the substation will require attention to increase gap factors. Additional surge arresters are considered to be a cost effective solution to control over-voltages throughout the whole uprated substation. The physical modification of substations to replace strung conductors with tubular conductors, ensuring sufficient outage time to refurbish and rebuild with new equipment will be the most challenging part of uprating existing substations. / MT 2017