A new method for decaying dc offset removal for digital protective relays

Voltage and current signals used in protection devices may contain harmonics and decaying dc offset in transient states. Because of this, protection devices must contain filters to remove the part of the signal which is not of interest and estimate the value of the component or components which are to be used for protection functions.Traditionally, the discrete Fourier transform (DFT) filter has been used in digital protection devices. This filter has the advantage of being easily applied, but the disadvantage that when an exponentially decaying component is present the results obtained are incorrect. Because of this, different algorithms based on the DFT have been developed in order to remove the decaying dc offset and reduce digital processing errors.This paper describes a new method for removing the exponential component associated with electrical signals in transient states. Its ease of application results in a low computational load as compared with other methods. This characteristic, together with the accuracy of results, makes this new method suitable for application in real time on protection devices.