Comparison and extension of IEC thermal models for dynamic rating of distribution transformers

Unison Networks Ltd (UNL) in Hastings, New Zealand is in the process of implementing the smart grid initiative, as part of advancing Life Cycle Asset Management (LCAM) practices. As a part of that initiative, ETEL Transformers Ltd, a subsidy of UNL has been assigned the task of developing a smart transformer kiosk. As condition monitoring and diagnosis are now an essential part of a transformer management system, ETEL is developing an intelligent framework for dynamically measuring and assessing the condition of the transformer. ETEL has developed a unique low cost fibre optics based transformer oil and winding temperature monitoring system. ETEL in collaboration with Robinson Research Institute is also developing the Distribution Transformer Dynamic Rating (DT-DR) algorithm. The extension of the two IEC thermal models for the DT-DR algorithm is proposed in this paper under unbalanced loading with different harmonic distortions in each phase.