IEC 60076-7 loading guide thermal model constants estimation

Abstract

SUMMARY When fiber optic probes were taken into use to record local hot-spots in windings and oil ducts, it was noticed that the hot-spot temperature rise over top-oil temperature due to load changes is a function depending on time as well as the transformer loading (overshoot time dependent function). This results in winding hottest spot temperatures higher than those predicted by the old loading guide during transient states after the load current increases, before the corresponding steady states have been reached. The new loading guide, (IEC 60076–7), takes into account these findings by introducing two thermal models. The first one, which is given as an exponential equation solution, is suitable for a load variation according to a step function. This method is particularly suited to determination of the heat transfer parameters by test, especially by manufacturers. The second one, which is given as a differential equations solution, is suitable for arbitrarily time-varying load factor K and time-varying ambient temperature θa. This method is particularly applicable for on-line monitoring. This method is a mathematical variation of the exponential model. A significant advantage of the suggested thermal models is that they are tied to measured parameters that are readily available (i.e., data obtained from a non-truncated heat run test performed by the transformer manufacturer). However, it has been reported recently by many users that this new approach causes a lot of technical hitches. It is unclear when and why the exponential method should be used instead of or the differential method and vice versa. Also, is there any difference between the recommended models, especially in their output? How the new thermal constants, (k11, k21, and k22), should be applied and how they could be derived? Answers to these questions and more are given in the text below. Copyright © 2012 John Wiley & Sons, Ltd.