A practical look at harmonics in power transformer differential protection

Differential protection of transformers traditionally uses second and/or fifth harmonics to block its operation during inrush conditions. The second and/or fifth harmonics produced during fault condition, when a protection system is required to operate, have in some cases blocked or delayed operation of transformer differential protection. The problem is to distinguish fault and inrush situations and allow the protection system to operate when there is fault, and to block it during inrush conditions. In order to achieve this, we propose to measure the rate of change of the third harmonic as an additional indicator of the fault condition. If the rate of change of the third harmonic exceeds some predefined threshold and the second and/or fifth harmonic are detected, the protection system will be allowed to operate, otherwise it is blocked.