Design and implementation of a digital differential relay for a 3-phase power transformer based on Kalman filtering theory

The design and implementation of a digital differential relay for a three-phase power transformer based on Kalman filtering theory are presented. The differential current is modeled as a sum of a fundamental, second harmonic, and exponentially decaying DC component, plus a random noise of known variance. The fundamental and second harmonic components are optimally estimated using a five-state Kalman filter. Inrush and internal fault conditions are distinguished by the relative magnitudes of their differential currents. The algorithm is implemented on a TMS 320 digital signal processor and tested using a three-phase transformer. Results show that the relay provides good restraint for inrush conditions and the relay operation time for internal faults is less than half a cycle