The cyclical loads generally vary with the ambient temperature due to air conditioning and heating. However, summer and winter profiles are very different. Summer peak loads occur when peak temperatures occur.
The reverse occurs in winter. In fall and spring, not only is the ambient temperature mild, but there is little or no air conditioning or heating load. The thermal stress on a transformer with a cyclical normal load during fall and spring is negligible. It is also usually negligible during winter peak loading.
The thermal rating of a power transformer differs from the thermal rating of other current-carrying elements in a substation. Examples of other elements are conductor, buswork, connectors, disconnectors, circuit breakers, etc. The insulation system for these elements is air and solid support insulators. The cooling system is passive (ambient air). The thermal limits depend on the properties of the conductor itself. These elements are maximum-rated devices. In a power transformer, the cooling system is active.
The thermal limits depend on the dielectric and mechanical properties of the cellulose and oil insulation system. As a maximum-rated device, the transformer capability may be 200% of the maximum nameplate rating (IEEE, 1995).
If ambient conditions differ from the power transformer nameplate criteria, then the user must adjust the transformer capability accordingly. IEEE C57.91-1995 provides tables and equations for making these adjustments. A good approximation is an adjustment of 1% of the maximum nameplate rating for every degree C above or below the nameplate rating (IEEE, 1995). If the transformer operates in 40˚C average ambient, then the user must de-rate the nameplate kVA by 10% in order to meet the nameplate thermal-rating criteria. Conversely, operating in a 0˚C average ambient environment allows the user to up-rate the transformer by approximately 30%.
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