A Method for Dual Dynamic Blocking Detection of CT Open Circuits in Relay Protection Systems Under Low-Current Conditions

Under low-current conditions, the residual current caused by CT open circuits and the zero-sequence current resulting from single-phase ground faults exhibit amplitude aliasing and spectral indistinguishability between classes in the high-order harmonic attenuation spectrum within the low signal-to-noise ratio range. This causes traditional zero-sequence threshold discrimination methods to frequently result in false tripping or missed alarms due to linear separability issues in the feature space. To address this, this paper proposes a dual dynamic blocking discrimination method based on the temporal entropy of the zero-sequence current waveform and the fundamental-to-harmonic energy ratio. A sliding window is used to calculate the waveform temporal entropy of the zero-sequence current in real time, quantifying the difference in time-domain disorder between open-circuit and ground-fault conditions; an energy ratio feature between the fundamental and the 3rd and 5th harmonics is constructed, and adaptive thresholds are employed to distinguish between CT open-circuit residual current and ground-fault current; A collaborative blocking logic is designed using the rms value of the zero-sequence current and the aforementioned dual features, which blocks the open-circuit alarm only when the rms value exceeds the threshold and both the temporal entropy and energy ratio indicate ground fault characteristics; otherwise, normal discrimination is permitted. Experimental results show that, under 1%–5% of rated current, this method achieves a discrimination success rate exceeding 90% with a response time of less than 100 ms, significantly reducing the rates of missed detections and false alarms while enhancing the reliability of CT open-circuit detection in low-current scenarios.