Secure differential protection using unique algorithms

Modern microprocessor-based low-impedance differential relays use a percentage differential principle that utilizes traditional dual slope characteristics. Many differential mis-operations occur with these traditional methods as a result of CT saturation, DC offset, or current subsidence. One reason is because the traditional operating and restraining principles rely heavily on the current magnitude, and neglect directionality. This paper describes the application of four independent principles to distinguish external and internal faults, and to enhance the security of the differential protection: traditional operate/restraint slope characteristic, computationally efficient dot product phase current phase angle comparison, extension of the dot product comparison to zero sequence and residual earth current, and the comparison of the rate of change of the restraint and operate currents.