Traditional methods for measuring the capacitive current by injecting signals are improved. By detecting voltage amplitudes of the injecting signals at the neutral point， the network resonance frequency used in calculating the system capacitance is determined so that the zero sequence admittance is obtained in monitoring the network insulation. The frequency range of the injecting signals is defined without overlapping the power network frequency. A practical signal-processing algorithm is proposed to suppress the noise interference. It is proved that the system capacitive current can be detected accurately and the high-resistance grounding can be distinguished effectively even with serious power-frequency disturbance.