An Analytical Fast Decaying DC Mitigation Method for Digital Relaying Applications

This paper proposes a novel fast and accurate method to mitigate the decaying DC (DDC) component from the fault current signal. At the first stage, it starts to identify the DDC parameters by integrating six sub-cycle samples of current signal considering the symmetry feature of odd harmonics. By receiving the next samples, the estimation of DDC parameters is updated and improved. This procedure continues until the integral of a full cycle current signal is calculated. To improve the accuracy of phasor estimation procedure, in each stage, by subtracting the DDC component from original current signal, the discrete Fourier transform method is applied to estimate the fundamental phasor. Performance of the proposed method for different DDC parameters and fault conditions is verified through various test studies, and its robustness against harmonics, noise, off-nominal frequency, and multiple DDCs conditions is also investigated. Besides, to demonstrate the reliability of proposed method in terms of dependability and security, it is implemented on the phasor estimation block of a distance relay. Three types of data including mathematical, simulated, and real power system signals are used to evaluate the performance of proposed method. Obtained results confirm the desirable performance of proposed method in various operating conditions.

I. Introduction

Short circuit fault currents in power systems contain a decaying DC (DDC) component along with the fundamental frequency component and harmonics, due to the resistive and inductive nature of grid components. The DDC is a non-periodic component that its time constant and initial magnitude depends on the X/R ratio of system and the fault inception angle, respectively. This component disturbs the phasor estimation performance and results in inaccuracy of the results. However, for timely fault location and isolation, fast and accurate phasor estimation of voltage and current signals are necessary.