Oscillographic records of the mercury arc valve disturbances experienced at the Sylmar Converter Station (the southern terminus of the Pacific Northwest-Southwest HVDC Intertie) are presented and discussed. The disturbances experienced include arc-throughs, arc-backs, consequential arc-backs and a new disturbance combination called a simultaneous arc-back. This presentation will provide a reference for mercury arc valve disturbance characteristics, and their effects on both the valve bridge outputs and the entire HVDC system.