Microprocessor-implemented digital filters for the calculation of symmetrical components

Symmetrical components of voltages and currents of three-phase power systems are used for protection purposes and power-system automatic control. Until now, passive and active analogue filters have been used for determining the symmetrical components from the phase quantities. A disadvantage of analogue filters can be the fact that it is difficult to suppress harmonic components that may be present in the phase quantities. It will be demonstrated that very suitable digital filters can be implemented with a microprocessor which are able to suppress the second- and third-harmonic components as well as the DC components that are present in the measured signals. Tue input of these digital filters consists of three phase quantities and the output consists of three numbers representing the amplitude of the zero-, positive- and negative-sequence quantities.