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 continuous, emergency and short term emergency transformer ratings. Operators can use these ratings until the contingency conditions are mitigated. However, once the transformer has surpassed the short term emergency ratings, the transformer might reach critical temperatures and could possibly sustain damage. Protection engineers can avoid further transformer damage by using the thermal protection principles of the IEEE standard. This paper discusses the fundamental thermal principles of power transformers, philosophies of operations and the implementations of thermal protection.