Design, Fabrication and Testing of Transformer Static Differential Relay Circuit with Harmonic Blocking

H.B. Ramachandraiah, J. Balakrishnan, and S.H. Jangamshetti (India)


Differential Protection, False Tripping, Harmonic, Inrush Current, Power Transformer.


An integrated approach that provides a compact, reliable, cost effective solution to the problem of false tripping of differential relays during inrush is presented. The circuitry for detection of inrush current and second harmonic is implemented using low cost integrated devices. The implementation is for an inrush activated blocking scheme based on an instantaneous level of differential current and its rate of change. The relay allows for second harmonic ratio detection using active band pass filters and to avoid tripping during inrush condition. A prototype which prevents false tripping of the differential relay during inrush and second harmonic detection is designed and tested at Karnataka Power Transmission Corporation Limited, Bangalore, Karnataka, India. The behavior of inrush current with and without load and second harmonic detection circuits to avoid false tripping is designed and experimented. The corresponding waveforms under test conditions were captured and analyzed. The measurements of primary and secondary currents of transformer using the prototype are tabulated. The present system can be extended to a three phase system with minor modifications. The system is applicable in detection of inrush current, detection of second harmonic and differential current calculation.

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