ADAPTIVE DIGITAL DIFFERENTIAL RELAY OF PARABOLIC CHARACTERISTIC FOR TRANSFORMER PROTECTION

ABSTRACT

In the present paper a variable-bias digital differential relay for power transformers protection is proposed. The amount of bias is varied linearly with the value of through current (mean of the currents entering and leaving the transformer). To take care of mismatch between CT ratios on primary and secondary sides of the power transformer, secondary currents of CT are corrected using a precalculated correction factor for the power transformer tap in operation. In the event of an abnormal operation, a fault discriminant based on fundamental, second and fifth harmonic components of differential current is used. It provides discrimination between faults and magnetizing inrush /overexcitation copditions. Trip and indication signals arc issued in the former case, while only an indication is given in the latter case.

The relay scheme is simulated on a personal computer. The advantage of the proposed scheme is its ability to detect even low-level internal faults, which may not be detected by a fixed-bias relay. The relay is restrained from operations on magnetizing inrush and overexcitation. The proposed relay is stable on those heavy through faults on which a fixed-bias relay may operate.