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Fundamentals of short-circuit protection for transformers

This paper reviews principles of protection against internal short circuits in transformers of various constructions. Transformer fundamentals are reviewed as pertaining to protection. In particular, the electromagnetic circuit of a transformer is reviewed that links the terminal currents, winding currents, fluxes, and ampere-turns (ATs) in a set of balance equations for a given transformer. These balance equations are used to explain the sensitivity of protection to various types of transformer faults. The paper shows that the classical transformer differential compensation rules have roots in the first principles — they reflect the AT balance of the protected transformer. The rule of building transformer differential protection equations following the AT balance is used in this paper to derive differential equations for autotransformers; power zig-zag, Scott-T, and Le-Blanc transformers; and phase shifters. The restricted earth fault (REF) and negative-sequence transformer differential (87TQ) functions are explained as a means to detect ground faults near the neutral and turn-to-turn faults, respectively.