Investigation and analysis of the causes of 11/0.4 kV distribution transformers' high failure rate : case study - Swaziland Electricity Company
The distribution network of the Swaziland Electricity Company consists of a number of components like distribution lines, wooden poles, insulators, conductors, pole mounted transformers and metering, to mention but a few. The most expensive of these components per unit cost is the pole mounted transformer and it takes longer to install. The distribution network is the less reliable compared to the transmission network, and its components fail regularly, and the pole mounted transformers are among the components which have a significant contribution to the failures. Some interventions in the past have been tried, but the failure rate of transformers has not reduced to reasonable limits, thereby compromising the reliability of the distribution network. This research therefore tries to identify the causes of the high failure rate of transformers, the pattern of failure, areas where the failures are prevalent and what can be done to counter the root causes. Statistical data on the transformer failures was gathered over a period of four years, where on a daily basis, the number of failed transformers, weather conditions and time of day when the failures occurred were recorded. The weather conditions and time of day when the failures occurred made it easier to identify the possible causes of the failures. For instance, if there was a thunderstorm in a certain area and there were some failed units which were discovered immediately after the storm, then the failures of those units was attributed to lightning. Similarly, when there were no thunderstorms, but a transformer has failed windings, the likely cause of failure is overloading. After replacing that failed unit, its load was monitored to confirm overload and then take a corrective action. It is a standard practice that a failed transformer is replaced and the failed one taken for repairs and then later kept as a spare at the Swaziland Electricity Company. Basic tests on the failed transformers and visual inspections were carried out to determine the nature of damage on the failed unit, for instance, winding damage and bushing damage. Footing resistances were also recorded at the transformer structures and if there was a need, they were reduced to 10 Ω or less using the crowfoot earthing method or a combination of the crowfoot earthing method and conductive cement.
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