Fundamental principles of transformer thermal loading and protection

Power transformers are traditionally protected by differential protection schemes that use voltages and currents to detect abnormalities in the differential zone of protection. For this type of scheme, a short circuit or high magnitude current must be present to initiate a trip. However, this scheme might not be ideal when transformers need to be overloaded to mitigate contingency conditions. Using the IEEE Guide for Loading of Oil-Immersed Power Transformers C57.91–1995, one can thermally rate transformers beyond their nameplate conditions to a level that is safe for operation. Using the guide, engineers can establish protection practices along with continuous, emergency and short term emergency transformer ratings.