Abstract: This paper describes a current differential relay for transformer protection that operates in conjunction with a core saturation detection-based blocking algorithm. The differential current for the magnetic inrush or over-excitation has a point of inflection at the start and end of each saturation period of the transformer core. At these instants, discontinuities arise in the first-difference function of the differential current. The second- and third-difference functions convert the points of inflection into pulses, the magnitudes of which are large enough to detect core saturation. The blocking signal is activated if the third-difference of the differential current is larger than the threshold and is maintained for one cycle. In addition, a method to discriminate between transformer saturation and current transformer (CT) saturation is included. The performance of the proposed blocking scheme was compared with that of a conventional harmonic blocking method. The test results indicate that the proposed scheme successfully discriminates internal faults even with CT saturation from the magnetic inrush, over-excitation, and external faults with CT saturation, and can significantly reduce the operating time delay of the relay.
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Lee, B.E.; Lee, J.; Won, S.H.; Lee, B.; Crossley, P.A.; Kang, Y.C. Saturation Detection-Based Blocking Scheme for Transformer Differential Protection. Energies 2014, 7, 4571-4587.
Lee BE, Lee J, Won SH, Lee B, Crossley PA, Kang YC. Saturation Detection-Based Blocking Scheme for Transformer Differential Protection. Energies. 2014; 7(7):4571-4587.
Lee, Byung E.; Lee, Jinsik; Won, Sung H.; Lee, Byongjun; Crossley, Peter A.; Kang, Yong C. 2014. "Saturation Detection-Based Blocking Scheme for Transformer Differential Protection." Energies 7, no. 7: 4571-4587.