HVDC tie links in smart power grids: a versatile control strategy of VSC converters

The forced switching converters are the kernel of the modern paradigm of VSC-HVDC system. It is widely recognized that appropriate control laws make these converters more advantageous than the traditional line commutated ones. They can play a versatile role by properly controlling active and reactive powers between electrical networks, improving the dynamic performances of the whole system. Besides this is more and more motivated by the wide spreading penetration of renewable resources. In this paper after a few references to basic concepts, a possible control strategy of VSC-HVDC transmission system is discussed. With respect to a two-area system with AC-DC link, a combined control law is investigated for improving inertial and primary frequency response. The control law is based upon the equivalent HVDC tie link synchronization coefficient which allows determining the reference value of the AC voltage reference angles of the converters. A damping action is further added for improving the power swings damping. The simulations performed with a simple but quite general case study permit to confirm the validity and the goodness of the proposed control strategy.