Stability and control of HVDC and AC transmissions in parallel

A generic conccpl combining transienl anglc and vollagc stabilitics of parallel acldc transmission is introduced. It proves that HVDC schcmcs in parallel opcration with ac transmission arc prone lo both transienl swing anglc and voltagc inslabilities. This tendency is inlensilied as the parallel ac interconncctioii becomcs more strongcr. Thc concept also explains why an HVDC schcme fitted with classical dc powcr conlrols docs no1 contrihutc to system synchroni7,ing torque and may increasc thc risks for instability. The conccpt oCCcrs n solid incans Lo dcvclop robust stralcgies to control thc dynamic opcration of dc and ac lransniissions in parallel. With such slrategies, the HVDC sclicmc can posilively participate in the iiistnntancous rcschcduling of power according to arising network conditions, particularly during disturbancc conditions