When the ground fault by the contact between high and low voltage lines occurs, the line ground current flows into the earth resistance in the secondary side of the pole transformer. So the ground voltage rises from 300 to 600V, it is necessary to open the electric circuit within 1 second for safety. The methods to detect the ground fault are classified into “direct detection” and “indirect detection”. Previously, the direct detection is to detect the zero phase voltage by the over voltage ground relay from high voltage distribution line side. In this paper, we propose a new method to detect the ground fault directly from low voltage distribution line side for three phase power conditioners. During the ground fault, line currents are three phase unbalanced currents, and make the unbalance of the line voltage changes through the line impedance. Proposed method detects the unbalance of the line voltage changes from the view point of both the scalar change and the vector change. Firstly, we describe the detection principle by using the simple high voltage distribution line model. Next, we have developed the analysis model for three phase unbalanced power flow in considering the influence of various power system conditions, and clarified the detection setting values for the proposed method.
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