To meet the precise protection needs for hybrid AC/DC residual currents. First

based on the voltage-type magnetic modulation principle

a mathematical model of the dynamic variation process of the excitation current was constructed

and a calibration method for accurate residual current protection was proposed. Second

a firing angle model capable of separating pulsating DC components and high-frequency components was established based on the firing angle

and a skewness-based firing angle calculation method was employed to precisely characterize the intrinsic relationship between the firing angle and the residual current protection threshold. Third

a protection algorithm combining both RMS (root mean square) and instantaneous value protection methods was proposed. Finally

experimental verification was conducted under conditions of varying amplitudes and DC offsets. By applying the proposed method

the measurement accuracy of open-loop fluxgate residual current sensors can be improved to within 1.5%

and the measurement range can be extended by over 30%. The firing angle calculation error can be controlled within 1.3%. The skewness-based firing angle calculation method enables real-time adjustment of the residual current protection threshold based on the detected firing angle

thereby achieving the goal of precise protection for hybrid AC/DC residual currents.