Comparative study of the effect of different gate firing schemes on the operation of a prototype HVDC transmission system

Two high voltage direct current transmission line circuits have been simulated in the PSIM software, one employing the Individual Phase Control scheme and the other, Equidistant Pulse Control scheme. This paper presents the circuitry employed in the two simulations and the results obtained from both. It is shown that the circuit employing the former scheme suffers from an excess of non-characteristic harmonics when operating with a weak AC system whereas the latter can work on weak AC systems without problems of harmonic instability because it generates firing pulses independent of the AC source. The implementation of the two schemes clearly demonstrates how one is dependent on the feedback from the source whilst the other utilizes a Phase Locked Loop to circumvent this problem.