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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.

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