Removal of System and CT Decaying DC Component in Fault Signal Using Selective Interval Integration

The fundamental current component at the secondary side of current transformer (CT) is often accompanied with decaying direct current (DC) component produced by the system and the CT. The discrete Fourier transform (DFT) is known as a powerful tool in digital protection for estimation of the fundamental component in the presence of harmonics. The decaying DC component, unlike harmonics, is not limited to a distinct frequency but include a wide range of low frequencies affecting the DFT output by undesirable oscillations. This paper proposes a method based on two selective interval integration for accurate estimation and elimination of the decaying DC component from the fundamental component of fault current. The two data intervals are separated by half a cycle minus one interval. The proposed algorithm is first tested using data derived by mathematical formulation under different conditions such as sampling frequency, noise, and characteristics of the DC component. The algorithm is also tested by simulation study using PSCAD/EMTDC for different fault conditions in a typical transmission system. In order to test the algorithm performance for practical applications, a Mho distance relay is simulated in the Matlab/Simulink environment. All the test cases verify efficiency, accuracy and speed of the proposed algorithm.