A voltage integral-based method for determining saturation of a transformer core. For each phase voltage on one side of a transformer, when a voltage variation exceeds a set threshold value, a sudden change of the state of the transformer is determined, the calculation of a half-cycle integral S1(t) of an absolute value of a voltage begins after said sudden voltage state change; an absolute value S2(t) of an integral of a half-cycle voltage variation which begins at the moment of said sudden voltage state change is calculated; assuming that, if any phase satisfies S(t)>S0, then determining that the transformer core is saturated, wherein: Se represents a per-unit baseline value, and S0 is a transformer core saturation threshold value. The method focuses on the root causes that generate a magnetizing inrush current, and the magnetizing inrush currents caused by voltage changes, such as a transformer non-load closing magnetizing inrush current, a magnetizing inrush current caused by voltage recovery following fault removal, a sympathetic inrush current, and an inrush current caused by a fault or certain operations during normal operations of a transformer, may all be determined in about half a cycle.