When a failure occurs in the low voltage side differential protection zone of a 220 kV step-down transformer

two sets of longitudinal differential protection act differently on different breaking principles. According to offline analyses and calculations with MATLAB software upon protection device recorded wave data

it has been found why free oscillation of the harmonic current of the low voltage side parallel compensation capacitor impacts the transformer longitudinal differential protection when inter-phase fault occurs. Also

an analysis is made of the cause of generation of harmonic current and its amplitude frequency characteristics of the parallel compensation capacitor during the fault. Suggestions for improvement of the differential protection principles are also made.