General Smart Meter Guidelines to Accurately Assess the Aging of Distribution Transformers

Smart grid technology has enhanced the ability to monitor numerous aspects of the distribution system for improving system performance and reducing system losses. One benefit is the continuous assessment of insulation degradation within distribution transformers, which is commonly referred to as a loss of life assessment. Calculating the expended life of a transformer consists of determining its winding hotspot temperature, which fluctuates with customer demand, ambient temperature, and cooling characteristics. Most residential transformers in service today do not possess the ability to connect to a smart grid communication system for continuous monitoring; therefore, they rely upon fusing for protection against extreme loading conditions. Smart meters can provide utility companies with the information required to identify distribution transformers that are experiencing higher than rated losses that can ultimately reduce their expected life. The research presented in this paper defines the smart meter functions required to accurately assess the aging of distribution transformers according to IEEE Std C57.91 and C57.110. To establish general accuracy guidelines, transformer loading indices were developed to evaluate acute excessive loading, long-term excessive loading, and excessive loading due to harmonics. Metering functions evaluated included power factor, harmonic demand, and ambient temperature.