Risk assessment using transformer loss of life data

The IEEE guide has evolved in sophistication for modeling transformer insulation life to models that include bottom and duct oil rises, fluid viscosities, and specific heats of materials as well as the parameters needed for the analysis shown (i.e., hot-spot gradient, oil rise over ambient, hot-spot time constant, top oil time constant, total harmonic losses, no load losses, exponential power of loss versus temperature, loading, and ambient temperature). Because data for these various inputs are missing and the loading history is difficult to retrieve, this article assesses risk without data for bottom and duct oil rises, fluid viscosities, or specific heat and does so without requiring any history of the load on the transformer. Previous IEEE guides have used equations similar to the ones used here but with differing parameters for aging rates, life end point criteria, and base lives, making management of the HVDC converter transformers in question difficult to assess. In evaluating risks, a program developed by Manitoba Hydro (TLD™ Ver 1.0) for transformer loading is used, which incorporates a simple GUI (graphical user interface) and allows input data to be easily adjusted. The approach in the risk assessment presented here is to begin with an older IEEE C57.92-1981 guide and equate it to the per unit quantities of the most recent IEEE C57.91-1995 guide and then compare the present rate of loss of life to nominal. Only the thermal life of the insulation is considered in this analysis. Other forms of deterioration caused by aging, such as reduced dielectric strength or reduced mechanical strength, are factors in the overall transformer life, and the approach used here is limited to the transformer thermal insulation life.