Distribution Transformer Harmonic Derating Guidelines

The objective of this project is to develop harmonic derating guidelines for distribution transformers that account for the extra heating caused by non-linear load currents. The excess heating developed within transformers is a function of both the spectral content of the current waveform and the internal construction of the transformer. The current spectrum is determined by the type of load served while the transformer losses are influenced by construction materials and design.


The approach used was to perform frequency scans of both single and three phase transformers to determine the effective resistance as a function of frequency. The measured effective resistance is in effect a resistance representing both winding and stray loss effects. The measured resistances are used to iteratively calculate the losses due to composite 60 Hz and harmonic loads such that the total losses yield the same Watts loss that occur when the transformer operates at rated 60 Hz current. To ensure that transformers with different constructions are included in the sample, transformers were measured at utilities with different A and B factors.

The project provides much needed information on the harmonic capability of distribution transformers. The industry is currently guided by ANSI C.57.110 for power transformers which is based on a combination of conservative assumptions, calculation estimates, and manufacturer’s experience. This project represents an effort to measure a wide range of individual distribution transformers and summarize their response to harmonics. There are a number of major conclusions reached in this project based on field test data and their analysis. They are summarized in the conclusions section of the 141-page final report.

Each DSTAR utility needs to review the guidelines included in this report and make an individual assessment relative to their application. Depending on the utility’s past experiences with derating transformers for harmonic loads, and the degree of conservatism desired in such applications, the utility may continue to implement existing practices based on experiences, the IEEE approach, or the guidelines developed in this project.


This project was conducted in P4-2