A new hot-spot temperature model and methodology to determine the loss of insulation life in distribution transformers

2015-01-22In electric utility service areas with extreme ambient temperatures, strong solar irradiation, and high electric loads, hot-spot temperatures in transformer windings often exceed the recommended 110° Celsius limit. Exceeding this limit can considerably reduce the insulation life of the transformer. Specifically, every 8°-Celsius increase in temperature beyond the limit yields a doubling of the rate at which the insulation deteriorates, or a halving of the transformer life expectancy. Therefore, it is critical to include all of the heat sources when calculating the transformer hot-spot temperature and loss-of-life. ❧ In this thesis, IEEE C57.91 Clause-7 hot-spot temperature and loss-of-life equations are analyzed, and their gross errors and omissions are identified. A new hot-spot temperature model and methodology is presented that includes the additional effects of microclimate ambient temperatures, strong solar irradiation, and high electric loads (based on Net-Power-for-Load curves). ❧ Additionally, the new hot-spot temperature model and methodology is based on 8760 hours of data for ambient temperatures and electric loads as opposed to the IEEE C57.91 model and methodology that uses 24-hour averages. The 8760 hours of data captures the electric loading and temperatures spikes that occur during heat storms or high temperature days that are masked when using the 24-hour average methodology. ❧ Three sample distribution transformer loss-of-life scenarios were compared using the IEEE C57.91 Clause-7 model and methodology and the new model and methodology of this thesis. The latter is shown to be superior for transformers located in areas with extreme ambient temperatures, strong solar irradiation, and high electric loads