Derating method for dry type power transformers based on current distortion parameters

Abstract

Harmonic loads on a transformer cause losses and unwanted heating. This leads to reduced lifetime and unexpected downtime caused by insulation failure. Many publications deal with the life time estimation of transformers. Life time estimation of transformers based on ambient, surface, winding and hotspot temperature is constrained by lack of data such as measurements from inside the transformer. In order to protect the transformer against overload when harmonics are present, derating of the transformer is applied. Derating is usually performed based on the K-factor in the EU and K-rating is used in the US. This information is not always available in current measurement equipment, especially panel meters. Panel meters typically measure only basic power quality parameters and in many cases they calculate only the THD(I). The K-factor is only calculated in dedicated power quality analyzers. This makes a quick assessment of the required derating difficult. Consequently this paper offers an interesting point of view. This paper presents the use of THD(I) as a parameter for derating of dry type transformers next to and in relation with the K-Factor. But it also gives the opportunity to determine the US K-factor based on THD(I). The relation of these parameters is determined based on field data from the industry on a large number of transformers and load conditions.