Analysis of energy flows in the power supply system with a thyristor voltage converte

Abstract

A numerical analysis of energy flows in a three-phase power supply system with a sine wave EMF source, active load and a thyristor converter that distorts the sine waveform and generates higher harmonics is performed. The influence of the thyristor non-sinusoidal voltage on the energy flows and the power factor of the source is analyzed. Thyristor converter is considered as a nonlinear electric power unit consuming electricity from the primary power source at a fundamental frequency, part of which is converted and returned back to the network as a secondary energy stream of zero-sequence systems and higher harmonics, thereby reducing the overall power factor and the quality of electricity becomes worse. It is analyzed the influence of the considered harmonics quantitative composition on the accuracy of the currents and voltages calculations. The dependence of the higher harmonics amplitudes on the thyristor firing angle is established. The constant component and the 2nd order harmonic voltage are the most denoted in the thyristor harmonics spectrum. In general case it is shown that in the nonlinear circuit with a sinusoidal power source, even in the absence of inductive and capacitive electric load, the fundamental current harmonic of the power source does not coincide in phase with the sinusoidal EMF of the source. Thus, the thyristor can be considered as a reactive inductive element that consumes reactive power of the fundamental frequency of current, regardless of the its firing angle and the presence of other reactive elements in the system. Therefore, in order to increase the power factor, it is necessary to compensate the reactive power of displacement only at the current fundamental frequency, which should be recorded by electronic meters. It is proposed to use the relative current value of the first harmonic of the power source to characterize the effect of the secondary energy flow on the total power factor. In a four-wire network, the presence of a thyristor converter causes a significant current increasing in the neutral wire, even under symmetrical phase loading, due to the zero sequence currents flowing, which are multiple of three, and they must be taken into account during wire cross-section selection and power losses calculation. It is proposed the practical measures to limit the level of higher harmonics and improve the electricity quality.