A new principle based on Pearson correlation coefficient to avoid mal-operation of the restricted earth fault protection

This paper presents a new principle based on the Pearson correlation coefficient principle to avoid maloperation of the restricted earth fault protection (REF), by calculating the correlation coefficient of 3I0 (residual current) and IN (star-point current) to determine internal or external fault. When the transformer earth fault occurs, the direction and intensity of 3I0 (residual current) between IN (starpoint current) almost change in the same way, that means perfect positive correlation. When the external earth fault occurs, 3I0 and IN are close to be perfect negative correlation. When CT saturation or inrush condition happens, the correlation coefficient is also significantly negative. Therefore, it can make use of the correlation coefficient calculation, in case 3I0 and IN are strong negative correlation then restraint or delay the REF protection trip. In this way can improve the reliability of the REF protection, while avoiding the harmonic restraint principle leads to the REF protection's sensitivity reduction. The Matlab software was used to analyse fault data which recorded from RTDS test environment obtained typical power transformer fault simulation. The test item includes: internal and external metal fault, internal and external fault occurs with CT saturation, transformer winding to ground fault and the high resistance ground fault, through theoretical analysis and dynamic simulation experiment verifies the feasibility and validity of the new principle. This new method also can easily to apply on modern digital protection device. (4 pages)