A Thermally-Based Dynamic Approach to the Load Management of Distribution Transformers

The future demand for electric vehicle (EV) charging power is expected to increase the peak loading of existing distribution transformers. In the absence of appropriate solutions, this may lead to the replacement of transformers with higher-rated types, incurring significant expense. Therefore, a novel approach is proposed to improve the management of transformer EV charging loads, avoiding or postponing the need for a transformer upgrade. It allows for the determination of whether loads that exceed transformer nameplate ratings are acceptable in terms of power level, time and duration. This is achieved with knowledge of the maximum charging duration, transformer thermal time constants, current loading levels, voltage regulation limits, oil pressure and temperature limits, and desired transformer lifetime requirements. The results of a case study are presented based on a future EV car park scenario for which an EV charging load is added to the existing base load on a 300 kVA transformer. The results show the proposed approach has advantages over the traditional approach of limiting loading to nameplate levels. These include reduced waiting time and increased levels of charging power, which are achieved without compromising the desired transformer lifetime target.