The Distribution Transformers main concern is for heating caused by loading. Radiators are added to the transformer to help the insulating fluid control the steady state temperature rise, but these do not help during fault conditions. Heat generated during a fault happens in such a short period of time (usually seconds) that the calculation assumes "all heat is stored" in the conductor because heat dissipation does not occur fast enough to combat the rapidly heating conductors. The GT takes this into account and is designed such that the conductor can handle the fault heating without relying on insulating oil for heat transfer during the fault.
Many GT specifications recognize this and allow the steady state cooling to be calculated using the magnetizing current and HV I2R loss resulting from energizing the core only. This leads to some misconception that the DT is better cooled, but the opposite is during faults.
Another subtle difference is the way the two devices "see" faults. The DT typically sees a line to ground fault or maybe, a line to line fault, but since the GT is providing a return path to the network, it typically sees a zero sequence fault which impresses the fault current equally on all three legs simultaneously. To combat the forces generated, GT conductors are always copper for maximum strength to cross section ratio, and because copper has a higher thermal withstand capability. GT coils are always circular on cruciform cores to gain the maximum form stability. Distribution transformers often utilize rectangular coil construction which does not have the same form stability offered by the circular coil technology.
Transformers under load generate heat due to winding (copper) and core losses occurring during operation. There is an 'acceptable' temperature rise for transformers used in power applications, and this can even limit their size. This acceptable temperature rise is directly related to the limitations of the transformer materials; safety regulations; or component parts in close proximity that may have high-temperature reliability problems.
Many GT specifications recognize this and allow the steady state cooling to be calculated using the magnetizing current and HV I2R loss resulting from energizing the core only. This leads to some misconception that the DT is better cooled, but the opposite is during faults.
Another subtle difference is the way the two devices "see" faults. The DT typically sees a line to ground fault or maybe, a line to line fault, but since the GT is providing a return path to the network, it typically sees a zero sequence fault which impresses the fault current equally on all three legs simultaneously. To combat the forces generated, GT conductors are always copper for maximum strength to cross section ratio, and because copper has a higher thermal withstand capability. GT coils are always circular on cruciform cores to gain the maximum form stability. Distribution transformers often utilize rectangular coil construction which does not have the same form stability offered by the circular coil technology.
Transformers under load generate heat due to winding (copper) and core losses occurring during operation. There is an 'acceptable' temperature rise for transformers used in power applications, and this can even limit their size. This acceptable temperature rise is directly related to the limitations of the transformer materials; safety regulations; or component parts in close proximity that may have high-temperature reliability problems.
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