Prediction and control of lighting generated overvoltage in medium overhead distribution lines in Dar es Salaam: the case of Kinondoni Area

The study focused on the prediction and control of lightning generated overvoltages conducted specifically on the medium voltage overhead electrical distribution lines in Kinondoni Dar es Salaam. The study evaluated and established the nature and types of transformer failures and power distribution outages in Kinondoni. According to data collected from TANESCO (Tanzania Electric Supply Company- Power utility company) six failure modes have been identified in the study, these are lightning strikes initiated problems, transformer core problems, tap changer initiated problems (OLTC), ageing of winding/insulation, short circuits and others. Others stand for bushing, unknown and operational errors. The various failure modes in transformers of 20-100 kVA rating and in transformers with rating above 100 kVA respectively have been analyzed accordingly. Relying on the analysis above most failures in small transformers 20-100 kVA occur due to Lightning and tap changers. In medium size transformers, 100-400 kVA, the ageing, lightning and others failures stands for the most failures. The study has also evaluated and established the expected lightning performance of the existing MV power line (in Tanzania) based on various researcher models such as Erickson, Love, Whitehead-Armstrong. By using Electrogeometric Model the study has given the estimates on the lightning minimum attractive radius or striking distance to the distribution line. Distribution transformers can fail or be damaged due to lightning generated overvoltage’s despite the traditional precautions for lightning protection. The transformer failure due to lightning generated overvoltage is attributed to the lack of protection in the low voltage-side of distribution Transformers.