Stability analysis of a digitally based HVDC firing-pulse synchronization control

A step-by-step analysis of the performance and stability characteristics of a digitally based DC firing-pulse synchronizing control is presented. The transient response of the synchronizing loop is shown to be strongly affected by AC system resonances. The effect of the AC system becomes more pronounced as the system becomes weaker. In general, inverter synchronizing loop performance is worse than rectifier synchronizing loop performance. Without any remedial action, the inverter´s synchronizing loop tends to destabilize low-frequency modes of oscillation of the overall system, this is particularly true for weak system applications. High DC power operation at the highest possible angle is the worst case for assessing the stability of the system with respect to the inverter synchronizing loop. A first-order low-pass filter in the predictive path of the inverter´s synchronizing loop is an effective means to stabilize the low-frequency mode of oscillation