Integration of low carbon technologies poses a technical challenge on distribution transformers due to the dynamic loading and potentially frequent overloading scenarios. Transformer dynamic thermal rating is hence required, which is the most economical approach to tackle this challenge and ensure a safe operation. To reach the aim, it is important to enhance the accuracy of the dynamic thermal model, where the top-oil temperature is a key thermal parameter. In this study, a wide range of constant load temperature-rise tests were carried out on an 11/0.433 kV distribution transformer to study the dynamic thermal behaviour of the top-oil temperature. A model based on the IEC 60076-7 thermal model but with an improved oil time constant calibration was deduced for top-oil temperature modelling. The oil time constant calibration was inspired by IEEE C57.91 and verified by eight temperature-rise tests with load factors ranging from 0.7 to 1.4 pu. In addition, the improved top-oil temperature modelling was further verified in experiments under multiple load profiles.