A comparison in THD values of conventional 3-phase VSI with different conduction angles of thyristors

Multi-level (ML) converters, now a day's seems to be one of the finest solutions for the DC-to-AC power conversion in the high power high-voltage applications like HVDC transmission system as well as in medium power medium-voltage applications such as renewable energy system. ML converters uses at least three or more voltage levels (steps) in order to modulate the output ac voltage, which leads to the reduction in the voltage distortion in the power system. For the medium power applications, we have the topologies like cascaded H-Bridge, diode-clamped, flying-capacitor type etc., but in the high power applications, all of these suffers from a common problem of high switching losses, requires devices with high switching capability like MOSFETs, IGBTs, GTOs. In this paper two simulink models of the conventional VSI with their firing circuits are presented and a comparison of the conventional VSI (with a single DC energy source) for the different possible conduction angles of the thyristors is done with the help of FFT tool in MATLAB. Finally the optimization of the thyristors conduction angle for the least possible THD value is proposed by the tabulation method and it is verified that for a conventional VSI, the THD value can also be reduced by varying conduction angle of thyristors to a particular value.