The Doctoral Theses collection contains digital copies of AUT doctoral theses deposited with the Library since 2004 and made available open access. All theses for doctorates awarded from 2007 onwards are required to be deposited in Tuwhera Open Theses unless subject to an embargo.
For theses submitted prior to 2007, open access was not mandatory, so only those theses for which the author has given consent are available in Tuwhera Open Theses. Where consent for open access has not been provided, the thesis is usually recorded in the AUT Library catalogue where the full text, if available, may be accessed with an AUT password. Other people should request an Interlibrary Loan through their library.
Browsing Doctoral Theses by Author "Abdul Rahman, Muhammad Azizi"
(Auckland University of Technology, 2012) Abdul Rahman, Muhammad Azizi
Advancement of High Temperature Superconductor (HTS) materials has increased interest in research and development of superconducting transformers. One of the challenges in the design and development of HTS transformer is the modelling of load flow, system losses and over current phenomena to be experienced by the transformer. Even though HTS technology is claimed to offer better power system and other packaging benefits, the performance and reliability need to be proven and verified through the proper modelling of power system network.
The study set up the electrical characteristics of a HTS transformer. The parameters need to be validated before any effort is to carry out to model the behaviour of a distribution network under a range of conditions. The characteristics is then adapted into existing network models and analysed under various system loading and over current conditions. The modelling is carried out on a three phase distribution feeder which is inherently unbalanced. The technical information on the HTS transformer is based on a preliminary baseline design. The exploration is intended to establish the performance of HTS transformers in comparison with that of conventional transformers.
The short circuit design of a transformer is one of the most significant and challenging criteria. Additional generating capacity and interconnections due to the growth of electrical power demand have contributed to an increase in short circuit capacity of power networks. One of the consequences is that the short circuit duty to be undertaken by transformers becomes more severe. The short circuit strength of a transformer is designed to withstand through fault currents due to external short circuit. Any weakness in the strength may result in a mechanical collapse of windings and deformation to clamping structures. The internal faults initiated by the external short circuits may lead to bushing blowouts, tank bursting, fire hazard, etc.
Meanwhile, the inrush current of a transformer can be as high as seven to ten times the rated current. Inrush current events are more frequent compared to short circuits. They also last for much longer time (few seconds) compared to short circuits (tens of milliseconds). The users are very anxious about the repeated switching of a transformer. Even though the inrush currents are usually not seriously looked into in the mechanical design considerations, the forces generated due to an extensive number of switching in a day may weaken the winding over a period of time. The continuous occurrences may lead to winding looseness and subsequent failure.
The active development of HTS materials has led to extensive research and development studies of superconducting transformers worldwide. Considerable benefits have been accomplished with the introduction of HTS transformer such as reduced power loss, size and transformer weight. This study focuses on modelling the effects of various distribution network conditions towards a distribution HTS transformer and looks comprehensively into the thermal effects of short circuit current on various architectural designs and fault current limiting properties of HTS transformer winding conductor. Recent advancement in the design technology has looked into the HTS transformer’s ability to also perform as Fault Current Limiter (FCL). This study presents the computation of the thermal effects of short circuit currents on a non-FCL HTS transformer and demonstrates how it will behave with a HTS-FCL winding conductor.