Phasor Estimation in the Presence of DC Offset and CT Saturation

A hybrid algorithm for phasor estimation is proposed that is immune to DC offset and current transformer (CT) saturation problems. The algorithm utilizes partial sum (PS)-based and multistage least-squares (MLS)-based methods before and after CT saturation is detected, respectively. The MLS-based method is initiated when the third difference of the secondary current detects the start point of the first saturation period. The determination of each saturation period is based on the sum of the secondary current from the start point of the first saturation period. A least-squares (LS) technique estimates the DC offset parameters from the single-cycle difference of the secondary current in the unsaturated periods. Removal of DC offset from the secondary current yields the sinusoidal waveform portion. Finally, the LS technique is used once again to estimate the phasor from the sinusoidal waveform portion. The performance of the algorithm was evaluated for a-g faults on a 345-kV 100-km overhead transmission line. The electromagnetic transient program was used to generate fault current signals for different fault angles and remanent fluxes. The performance evaluation shows that the proposed algorithm accurately estimates the phasor of a current signal regardless of DC offset and CT saturation. The paper concludes by describing the hardware implementation of the algorithm on a prototype unit based on a digital signal processor.

I. Introduction

Modern protective devices depend on knowing the phasors of the voltage and current signals. Any fault-induced dc offset must be removed from the current signal to estimate the current phasor accurately. Since a dc offset is a nonperiodic signal whose spectrum covers all frequencies, the presence of such a dc offset may result in a phasor estimation error of almost 20%, depending on the algorithm used. It is well known that the saturation of a current transformer (CT) also has an adverse influence on the estimation of the current phasor. Since dc offset itself is one of main causes of CT saturation, dc offset, and CT saturation should be considered together when estimating the phasor of a current signal.