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Power Transformer Loading

Providing guidelines for effective utilization and extension of Distribution transformer asset life.

Project Objectives

The objective of this project for a large North American distribution utility was to provide guidelines for the loading of Distribution Station power transformers, along with detailed background rationale for these loading recommendations. The work covered Distribution Station power transformers up to 20 MVA with secondary voltages of 27.6 kV and below.

Project Conclusions

After analysis of the utility’s loading policies, transformer types, and local environment, our study provided the following conclusions:

  • In the climate area of the utility in question, station transformer peak loading limits of 160% ONAN rating in the winter and 135% ONAN in the summer, were acceptable in terms of loss of life. Power transformer loading limits of 130% ONAF in the winter and 105% ONAF in the summer were acceptable. Higher power transformer loading in emergencies of up to 180% ONAN was considered acceptable in winter without extensive loss-of-life. These levels were proposed as planning target levels at which action was to be initiated to reduce loading. The increased cost of losses while operating at values above these levels was noted as an area of concern.
  • Approximately 10% of the utility’s Distribution Station transformers had peak loads beyond acceptable levels and were subject to accelerated loss-of-life.
  • Benchmarked against others, this utility loaded many Distribution Station power transformers beyond the levels applied by similar installations. This philosophy was based on the use of Mobile Unit Substations in the event of failure, as opposed to a dual transformer scheme.
  • Many factors were considered when choosing the appropriate loading, including loss-of-life, hot spot temperatures, total ownership cost, current economic parameters, the treatment of loss costs, and the effects on thermal loading limitations of ambient temperature and use of auxiliary cooling.
  • The study also included a review of loading impacts on the life and health of Under Load Tap Changers (ULTCs). Some conclusions regarding ULTCs were as follows:
  • ULTCs were the major cause of power transformer failures at this utility. ULTCs can experience 60,000 to 80,000 operations in 5 years. The ANSI C37.131 standard requires a service duty test of 50,000 operations.
  • Little information was available on the effect of power transformer loading on ULTC life other than qualitative statements that high loading can accelerate contact erosion and contact wear may be proportional to the square of the current. Coking of ULTC contacts increases the contact resistance and increases heating, possibly leading to thermal runaway.
  • The ANSI Standard C57.131 indicates that ULTCs should be designed to match the performance specified in applicable power transformer loading guides and should not exceed a 20°C rise at 120% rated load. There was insufficient evidence to indicate that ULTCs impose limits beyond those for power transformer loadability, particularly considering that ULTCs are maintainable and replaceable. Therefore their effect on transformer loss-of-life is reversible to some degree.

Effective Recommendations

  • It was recommended that the utility take action to reduce loading or increase the capacity of the outlier Distribution Station power transformers that are operating beyond the acceptable limits.
  • Remote monitoring of the power transformer temperature was highly recommended for power transformers that had planned loading above rated levels. A low-cost monitoring system at critical Distribution Stations would be an appropriate method to extend the life of power transformer assets and defer capital expense. Acoustic and temperature differential monitoring is available for under-load tap changers. Knowledge of the power transformer loading history is the first step in the application of Reliability-Centered Maintenance (RCM).
  • Loading guidelines must be applied to the appropriate air-cooled or forced-air ratings of the transformers.
  • Testing and model development to evaluate the effects of loading and temperature on the life expectation of ULTCs would advance the state of the art.
  • Development is required to obtain typical loading profiles based on monitoring information from DSs.
  • This utility was well suited for Demand Management initiatives that would improve load factors on power transformers, thereby extending equipment life and deferring capital expenditure.