Integrated digital power transformer protection

In the paper the hardware and software prototype realisation of the integrated digital three-phase power transformer protection for selective operation with all kinds of short circuits, ground faults and turn-to-turn faults is presented. The proposed integrated protection concept was developed on the basis of a nonlinear mathematical transformer model which includes nonlinear effects of saturation, hysteresis and eddy currents. The protection algorithm includes a percentage differential relay with harmonic restraint at inrush and overexcitation conditions, an overcurrent relay with instantaneous and time-lag operation, a sensitive relay for selective operation at high-impedance ground faults and a fault recorder. The protection algorithm was implemented on powerful signal processor. Presented are the results of real-time laboratory and field testing which include the comparison of the standard static protection performance with the proposed digital one in relation to speed and accuracy.