A new Decaying DC Component Estimation method for Digital Relaying

In protection relaying schemes, the digital filter unit plays the essential roles to calculate the accurate phasor. However, while the fault current contains plentiful decaying dc component, the over-reach of distance protection will cause several problems. This work develops an algorithm which can estimate and eliminate the decaying dc component from fault current signals after one cycle from the fault instant. Also, it can be applied to a conventional discrete Fourier transform to calculate phasor quantities of fault currents in a digital protection relay. In this algorithm the magnitude and time constant of the decaying dc component are estimated exactly by integrating fault current during one cycle. Decaying dc offset removal is carried out before applying the current signal to the digital filter used for phasor estimation. A relay is modeled in ATP using this algorithm to calculate correct phasor. Simulations results illustrate the effectiveness of this new algorithm for relaying applications.

Keywords:

decaying DC , discrete Fourier transform (DFT) , dc time constant , dc magnitude , fault current