Prediction of core saturation instability at an HVDC converter

Core saturation instability has occurred on several HVDC schemes resulting from interactions between second harmonic and DC quantities (voltages and currents) on the AC side of the converter and fundamental frequency quantities on the DC side of the converter. The instability can be reinforced by unbalanced saturation of the converter transformers. The paper presents an analytical method which can be used to quickly screen AC and DC system operating conditions to predict where core saturation instability is likely to occur. Analytical results have been confirmed using the digital transients simulation program PSCAD/EMTDC.