Thermodynamic modelling of buried transformer substations for dynamic loading capability assessment considering underground heat accumulative effect

The lifetime cycle and secured service of buried transformers are constrained by their thermal insulation and loading conditions. This paper proposes an extended thermal circuit model for direct-buried transformer substations to dynamically evaluate the transformer loading capability. In the proposed model, the underground thermal interactions and energy balances among heat generation, transfer and storage in the transformer substation are represented with nonlinear thermal resistances and capacitances based on thermal-electrical analogies, and then hot-spot temperature (HST) dynamics can be captured from the nodal analysis on this R-C thermal equivalent circuit. Furthermore, the underground thermal accumulative effect is investigated for dynamic loading capability assessment considering the combined impact of heat accumulation in the surrounding soil caused by fluctuating transformer loads during prior operating periods. Finally, the finite element analysis with measured data is implemented for parameter tuning and model verification of the proposed thermodynamic model, and numerical simulations confirm the improvements of the proposed model for the transformer life extension and load management.