Differential protection generally is considered to be the "best" protection for any given zone-of-protection on a power system. It is sensitive, secure and faster than other options. It is particularly appealing for protection of power transformers, due to the critical nature in the power system configuration. Fast clearing is desired to minimize damage for internal faults. Security is important, since a transformer will need to be tested after an operation (no reclosing). Transformer differential protection generally has been recommended for transformers 10MVA and above (4,5), but the economics of multifunction, numeric relay platforms and the overall decrease in cost per function has led to expansion of differential protection to circuits where it previously was not justifiable.
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