Conditions for Failure of Numerical Estimation of Decaying DC Component in Fault Current

The estimation of the Decaying DC component (DDC) in fault currents plays a key role in the electrical protection of the power network. Several methods have been proposed in the literature for the estimation of DDC parameters, viz., the peak magnitude and decay rate E. These methods can be broadly categorized as: methods that filter DDC (category-I) and methods that numerically estimate DDC (category-II). However, even today, the industry prefers mimic and cosine filters (category-I methods) for network protection. This paper presents the first in-depth mathematical analysis and associated conditions for which category-II methods may fail to provide valid numeric estimates of E. This analysis is also supported by several numeric results for fault currents in the presence of Gaussian noise.