A new method for the calculation of subtransmission and distribution system transients based on the FFT

This paper presents an interesting method for the calculation of bus voltage transients in an electric power system. The method is specifically intended for application to distribution and subtransmission networks. The transient bus voltages are calculated during the application of large industrial loads. In this sense, the calculation is a power quality assessment. In particular, the case of load currents which are electronically switched may be analyzed. The essence of the method is the Fourier transform of Ohm's law. The fast Fourier transform is used in order to give computational efficiency. An interesting method for obtaining the Fourier transform of the bus transfer impedance, Z_ij(ω), is developed from the theory of parallel n-port networks. Two approximations are found for the calculation of this transfer impedance and one of these is found to be applicable to the cited problem. Examples are used to illustrate the calculation of bus voltage transients and harmonic content. Although the latter is not a transient, harmonic voltages may be calculated by the proposed method under certain circumstances. The main application illustrated is that of a transient which has dynamics which occur in a time interval of similar order as the period of the power frequency (i.e., 1/60 sec for a 60 Hz system). With the advent of power electronic switches, this type of transient is a serious reality at distribution power levels.