Evaluating harmonic-induced transformer heating

The proliferation of nonlinear loads on power systems has increased the awareness of the potential reduction of a transformer's life due to increased losses. Over the past few years, several manufacturers and users have been applying a harmonic rating system to specify transformers based on relationships developed in ANSI/IEEE C57.110. The specification or evaluation of transformers in a harmonic environment requires a knowledge of the load mix as well as details of the load current harmonic content. The additional heating experienced by a transformer depends on the harmonic content of the load current, and the design of the unit. A heat run circuit was devised to produce harmonic rich load current on a 25 kVA oil-immersed distribution transformer. The test was conducted at 100% of nameplate current using a purely resistive load, and a 2.4 ms current pulse. The temperature rise experienced under harmonic excitation was observed and documented.