With the increasing age of the primary equipment of the electrical grids there exists also an increasing need to know its internal condition. For this purpose, off- and online diagnostic methods and systems for power transformers have been developed in recent years. Online monitoring is used continuously during operation and offers possibilities to record the relevant stresses which can affect the lifetime. The evaluation of these data offers the possibility of detecting oncoming faults early. In comparison to this, offline methods require disconnecting the transformer from the electrical grid and are used during planned inspections or when the transformer is already failure suspicious. This contribution presents the status and current trends of different diagnostic techniques of power transformers. It provides significant tutorial elements, backed up by case studies, results and some analysis. The broadness and improvements of the presented diagnostic techniques show that the power transformer is not anymore a black box that does not allow a view into its internal condition. Reliable and accurate condition assessment is possible leading to more efficient maintenance strategies.

Loading capability of transformers depends strongly on their thermal conditions which can be investigated by means of a dynamic thermal model. It is typically a set of simple differential equations representing very complex physical phenomena; however, there are several factors affecting the accuracies of the models which should be kept in mind while using these models. This study provides a review on the dynamic thermal models of power transformers from very simple to rather complex thermal models proposed in the last hundred years, the affecting factors on their accuracies, and their fields of applications. It is worthwhile to note that this study focuses on the thermal models which are simple and accurate and able to be integrated into monitoring systems.

With the integration of the renewable energy-based distributed small power plants, whose generated energies are uncertain, in addition to the increasing usage of the charging stations for electric vehicles, the determination of the dynamic loading of distribution transformers appears to be crucial. Therefore, the issues of the model-based online monitoring of distribution transformers may achieve acceptance, if a cost-benefit compromise is reached. In this paper, a new thermal model is proposed for calculation of the top-oil temperature of indoor distribution transformers using calculated tank temperature which accounts for convection, radiation, conduction, and ventilation heat transfer phenomena. The model is validated using measurements during the in-service operation of two indoor distribution transformers. Moreover, a method for calculating loading capability of indoor distribution transformers without sacrificing the transformer's life is presented which can be easily adjusted for other distribution transformers. Furthermore, the results of the hardware implementing of the proposed model and method for a model-based online assessment of the loading capability during the in-service operation of an indoor distribution transformer are presented.