HARMONIC VOLTAGE PROFILE CALCULATIONS OF ELECTRIC POWER TRANSMISSION SYSTEMS

Abstract

Rapid developments in the semiconductor industry have made possible a revolutionary improvement in the power handling capabilities of the rectifying devices. This has led to increased number and size of solid state power conditioning equipment installed on power system networks. The direct consequence of this is the increase in the stationary harmonic frequency voltages and currents on power system networks causing distortion of the sinusoidal waveforms. The renewed interest in the harmonics and a need to study the impact of such harmonics is the basis for the work reported in this thesis. Studies reported in this thesis are concerned with the harmonic voltage profile calculation of an electric power transmission network. The transmission lines due to their long lengths, tend to exhibit standing wave patterns comprising nodes and antinodes at harmonic frequencies. The studies are directed specifically towards the determination of the harmonic voltage profile of transmission lines. The effect of various parameters of the transmission system on the harmonic character of the transmission line voltage and current is studied. An iterative algorithm based on non-linear frequency domain analysis is presented in this thesis. An important factor in this effort is the accurate modelling of various equipment of the power system including sources of harmonics. A dynamic model of a six pulse ac/dc converter is presented and the effect of the impedance of the converter transformer on the commutation delay is included to accurately determine the harmonic spectrum of the line current of the converter. The phase shifting behaviour of the transformer is preserved in the equivalent admittance matrix model of a transformer for the harmonic frequency analysis. The cascaded equivalent pi model of the transmission line is used in order to minimise the errors. Also, a cascaded pi model lends itself to obtaining the harmonic voltage profile. The skin effect, due to increased harmonic frequency, is also taken care of in modelling the transmission system. An iterative approach is proposed for the overall algorithm based upon the criterion of mismatch of active and non-active power. Five example applications of the algorithm are presented ranging from a power system comprised long transmission lines to an industrial power system. The algorithm is general and can be used for any kind of balanced power system to study the harmonic bus voltages, harmonic line currents, harmonic voltage profiles of transmission lines etc. due to the harmonic loads, and resonance and filtering effects of VAR compensation. Other information available from the program is the distortion factor and bus impedance versus frequency table. The approach taken is general and well suited for the analysis of balanced power system networks with ac/dc converters of various sizes.