This guide focuses primarily on application of protective relays for the protection of power transformers, with an emphasis on the most prevalent protection schemes and transformers. Principles are emphasized. Setting procedures are only discu. This guide focuses primarily on application of protective relays for the protection of power transformers, with an emphasis on the most prevalent protection schemes and transformers. Principles are emphasized. Setting procedures are only discussed in a general nature in the material to follow. Refer to specific instruction manuals for your relay. T. The reader interested in additional information, advanced or unusual application advice, and detailed settings guidance should refer to Ref. 1. This document includes extensive references and bibliographies. Also, Ref. 2 and 3, textbooks on protective relaying, contain chapters on trans-former protection, and Ref. 4, another IEEE standard, includes. Fuses are economical, require little mainte-nance, and do not need an external power source to clear a fault, which is of great cost and maintenance benefit. As discussed above, MVA of a transformer is an imperfect guide to the appropriate level of transformer protection, but it may be noted that fuses are probably the predominant choice for transf. Typically, there is some small difference in the summation of the sensed currents so that, even after appropriate tap settings, currents sensed by the relay do not sum to an effective 0A. The error (or difference) current becomes the operate current which, if it rises too high, will cause relay operation. The operate current arises due to dissimila. A recovery inrush occurs at the clearing of an external fault as a result of the sudden increase in voltage from the depressed and unbalanced level that exists during the fault. This voltage transient causes a flux transient, with accompa-nying abnormally high exciting current. The current level is less than that seen during trans-former energizati.
