Emergency-stability controls through HVDC links

The paper deals with some power modulation controls achievable by using the high voltage direct current (HVDC) interconnections between different grids or subnetworks. HVDC links can be in fact effectively used for damping electromechanical oscillations, stabilizing power swings, reducing the life-fatigue effects of sub-synchronous resonance (SSR), etc. The development of voltage source converter (VSC) technology (used for instance in HVDC Light interconnections) can improve also the network voltage compensation and the transmission grid load-ability, reducing the risks of voltage instability and collapse. The paper mainly focuses on the power modulation strategies designed for two Italian HVDC links, the Sardinia-Corsica-Italy link (SACOI), with more than ten years of commercial operation after refurbishment by ENEL and EDF, and the Italy-Greece link (GRITA), more recently put in service by ENEL and PPC. These mono-polar links with sea return (rated voltage 300 kV and 400 kV respectively, rated current 1500 A and 1250 A respectively, rated power 300 MW and 500 MW respectively) are based on conventional grid commuted double thyristor bridges. The permanent or transient critical conditions of network islanding are faced by high speed and low droop frequency regulators, characterized by flexible and effective control operations.