A new scalar product restraint transformer differential protection algorithm

Scalar product restraint principle is widely used in transformer differential protection for high sensitivity. Based on the magnitude and phase angle relationships of the two side currents, this paper gives an ideal annular sector restraint region in the complex plane for transformer differential protection. And a novel differential criterion with the ideal region is presented by using scalar product restraint principle for reference. The new criterion defines restraint values in magnitude and phase angle respectively, to take account of both the magnitude and phase angle errors due to CT saturation. The phase angle constraint takes a form of similar scalar product restraint, and by combining it with magnitude constraint, the sensitivity of the new criterion is highly enhanced. The restraint region is adjusted by two coefficients to ensure protection stable in CT saturation during serious external faults. The new criterion provides a new idea for enhancing transformer differential protection performance theoretically. The analysis and simulation results show that compared with traditional percentage restraint differential protection, the new criterion is more sensitive during internal faults and has better anti-CT saturation performance during external faults.