A dissertation submitted to the Faculty of Engineering and the Built Environment,
University of the Witwatersrand, in fulfilment of the requirements of the degree of
Master of Science in Electrical Engineering
June 2016 / Through the Renewable Energy Independent Power Producer Procurement Program
(REIPPPP) the South African government has awarded opportunities for growth
of renewable energy through bidding rounds. Round 1 saw a total capacity of
397 MW being awarded to independent power producers (IPP). Subsequently
Rounds 2, 3 and 4 each had a total capacity of 333 MW auctioned. The advent
of renewables on the market has brought upon its own associated problems with
regards to power quality issues and failure of HV equipment. This thesis will
address transformer failures that occurred at an onshore wind farm. The nature
of the transformer failures suggest transient overvoltages are mainly to blame.
A comparison between transformer failures in South African and Brazil suggest
a common failure mechanism. The failure starts with an inter-turn insulation
failure which propagates to an inter-layer insulation. In worst cases the failure
mode results in a puncture through the LV-HV barrier and punctures through the
LV winding. An extensive literature review was performed to find appropriate
methods to predict and explain the failure mode in wind turbine LV-MV step-up
transformers. Of the different models which were reviewed the most notable
was the Multi-conductor Transmission Line (MTL) model which was chosen as
the preferred model due to its ability to predict the inter-turn/inter-layer voltage
stresses. Verification of the developed MTL model by the author was then compared
to published results of an MTL model of a disc winding transformer. The results
of the comparison revealed a relatively good agreement between the developed
model and the published model. The application of the MTL model to represent
the voltage stresses in transformer windings was then extended to two specially
constructed wind turbine step-up transformer prototypes. The prototypes differed
in the winding arrangement of the MV coil. The other used two separate MV coils
separated by an oil gap whereas the other had a single MV coil. To validate the
model accuracy, a comparison of measured results versus those obtained analytically
was done for the two prototypes. The analytical and measured results also had a
relatively good agreement for the two prototypes considered. Measurement of
switching surges was done on-site at the wind farm to understand the nature of the
transients. Using analysis tools such as FFT and frequency domain severity factor
it was possible to understand the impact the nature of these transients would pose
on the transformer insulation. Different mitigation techniques which can be used
to alleviate the transient overvoltages to within safe levels were investigated. The
most notable protection device considered was the RL choke device which offered
a significant reduction of the pre-strikes and is virtually transparent under power
frequency operation. / MT2017
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/22657 |
| Date | January 2016 |
| Creators | Banda, Cedric Amittai |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | Online resource (xiv, 147 leaves), application/pdf |
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