ABSTRACT
This chapter describes an improved approach for transformer differential protection using current-only inputs. The approach ensures security for external faults, inrush, and overexcitation conditions, and it provides dependability for internal faults. Transformer overexcitation causes transformer heating and increases exciting current, noise, and vibration. Because it is difficult, with differential protection, to control the amount of overexcitation that a transformer can tolerate, transformer differential protection tripping for an overexcitation condition is not desirable. Overexcitation of a power transformer is a typical case of ac saturation of the core that produces odd harmonics in the exciting current. In the case of transformer-differential protection, the effect of CT saturation is double-edged. Combining restraint and blocking into an independent restraint/blocking method provides an improved approach to transformer differential protection. In contrast to the odd harmonics ac CT saturation generates, even harmonics are a clear indicator of magnetizing inrush.
