Thermal Modeling of Power Transformers: Comparative Analysis of IEC 60076-7 and IEEE C57.91 for Hotspot Temperature Estimation

Abstract:
The global increase in electricity demand due to innovative loads and the growing share of volatile renewable energy generation pose significant challenges. Ensuring the reliable operation of power grids depends on the functionality of power transformers, which are essential for energy transmission. However, increasing system stress accelerates transformer aging. Accurately assessing the condition of power transformers and estimating their remaining lifetime is crucial for optimizing maintenance expenses and extending asset life. This urgency is further intensified by rising delivery times and costs of new transformers, necessitating their operation beyond the typical service life. Within the principle grid optimization before reinforcement before expansion (NOVA) and the associated increased utilization of electrical grids, the objective is to operate the transformers beyond their rated load. This is only possible with precise monitoring of the hotspot temperature. This study analyzes thermal models for determining the hotspot temperature of power transformers, with a focus on the International Electrotechnical Commission (IEC) 60076-7 and Institute of Electrical and Electronics Engineers (IEEE) C57.91 standards. The advantages and limitations of these models are examined through sensitivity analyses, which assess the impact of various input parameters. The results indicate that model complexity can be significantly reduced by approximating certain parameters more coarsely. A comparative evaluation using two real cases highlights that the top-oil temperature is the critical factor in hotspot temperature determination. By directly measuring and incorporating this parameter, estimation errors can be significantly reduced, thereby improving the reliability of transformer thermal models.