Minimisation of uncharacteristic harmonics in HVDC convertors through firing angle modulation

A modulating signal (MS) applied to the control voltage has a profound effect on the DC and AC-side uncharacteristic harmonic currents. The presence of harmonic distortion in the AC-busbar voltage causes distortion on the DC-line current ripple. A method of harmonic minimisation based on approximate linear relationships between uncharacteristic harmonics and the MS was developed and implemented on a convertor model. A circuit to select, adjust and inject the MS into the control voltage connected to the output of the injection circuit inhibits the MS injection during transients. This control feature is important because it allows fast transient response of the control and does not contribute to oscillatory behaviour. A full set of analytical predictions and model tests using the injection circuit described for an MS of 50 Hz, 100 Hz and 150 Hz is presented