Tamara Mandić

The increasing integration of renewable energy sources into distribution power networks, particularly solar power plants, leads to significant changes in power flows within the system. Increasing the installed capacity of solar power plants in the distribution power system may lead to the occurrence of reverse power flow through the transformer, which can result in higher loading than in the case when only consumer demand was supplied. Considering that solar power plants produce the highest amount of energy during the summer period, when ambient temperatures are also at their highest, such an operating regime can significantly affect the thermal loading of the transformer and consequently accelerate its aging. The objective of this paper is to estimate the maximum capacity of solar power plants that can be safely connected within the supply area of a distribution transformer while preserving its lifetime and respecting technical operating limits. The thermal model of the transformer was developed in accordance with relevant IEC standards that define thermal limits, winding temperatures, and insulation aging criteria. The application of Dynamic Thermal Rating (DTR) enables a realistic and time-dependent estimation of the maximum transformer loading depending on variable operating and ambient conditions, including time series of transformer loading, ambient temperature, and solar irradiation intensity. In contrast to the conventional static approach for determining permissible loading, which is based on conservative assumptions and constant environmental conditions, the dynamic approach enables a more accurate representation of the real operating conditions of the transformer over time. In addition to the basic thermal loading, the analysis also includes additional factors that contribute to an increase in the overall heating of the transformer. Particular attention is given to the impact of voltage variations caused by the connection of solar power plants, which lead to changes in Joule losses in the windings and magnetic losses in the transformer core. Furthermore, the influence of solar irradiation on the heating of the external surface of the transformer tank is considered, which additionally increases the temperatures of the oil and windings. These effects significantly contribute to the overall thermal loading and must be included in the calculations in order to ensure a realistic assessment of the permissible loading of the transformer. Based on the developed thermal model, an algorithm was formulated to enable an integrated assessment of the maximum permissible capacity of solar power plants within the supply area of the transformer, taking into account actual operating conditions, additional thermal loading, and voltage-related effects, as well as the application of standardized aging criteria defined by IEC standards. The proposed approach enables a more detailed analysis of the impact of solar power plants on the operation of distribution transformers and provides a reliable basis for determining the connection limits of new generation capacities. The application of the proposed algorithm provides a reliable and comprehensive framework for planning the integration of solar capacities into the distribution network while preserving the lifetime of the transformer and respecting the technical operating constraints of the power system.

Keywords: Transformer thermal model, DTR, Solar power plants, Voltage variations