Removal of Decaying DC Component in Current SignalUsing a ovel Estimation Algorithm

In protection relaying schemes, thedigital filter unit plays the essential roles tocalculate the accurate phasor. However, whilethe fault current contains plentiful decaying dccomponent, the over-reach of distanceprotection will cause several problems. Thiswork develops an algorithm which can estimateand eliminate the decaying dc component fromfault current signals after one cycle from thefault instant. Also, it can be applied to aconventional discrete Fourier transform tocalculate phasor quantities of fault currents in adigital protection relay. In this algorithm themagnitude and time constant of the decaying dccomponent are estimated exactly by integratingfault current during one cycle. Decaying dcoffset removal is carried out before applying thecurrent signal to the digital filter used forphasor estimation. A relay is modeled in ATPusing this algorithm to calculate correctphasor. Simulations results illustrate theeffectiveness of this new algorithm for relayingapplications.

Keywords:

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