Dynamic Thermal Modeling of MV/LV Prefabricated Substations

With the expansion and infilling of urban areas, the demand for electric power is driving the design and capacity of distribution substations to their thermal limits. Distribution transformer substations are increasingly required to be compact, reliable, safe, and intelligent. To efficiently utilize city space and to support the intermittent load flows imposed by smart-grid features, such as distributed generation, the transformers are expected to operate close to or occasionally over their ratings, with stalled or little air circulation inside the safety enclosure. Dynamic thermal models with physically validated convection and radiation heat-transfer components are essential for the real-time thermal rating of substations. Natural convection via the air inside the cabin to the outside ambient air plays the major role in cooling down a transformer. In this study a scale model of a prefabricated substation is examined to draft a numerical solution which is based on stack ventilation principles. A clear and expandable first principle approach is used to quantify heat transfer through ventilation openings. Measurements from actual cabins and 3-D finite element method simulations are used to validate the numerical model.