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The refurbishment of the three-terminal HVDC (High Voltage Direct Current) link SA.CO.I.2 is an interesting study case in terms of possible techno-economic benefits and improved Sardinian system flexibility for operational security that can be provided by the interconnector. The present paper firstly investigates the key techno-economic benefits, evaluated by the application of a OPF (Optimal Power Flow)-based market simulator for zonal analyses. Starting from the optimized power schedules and generation unit commitment, main focus is on the Sardinian system flexibility adequacy that is verified by means of the comparison between the power reserve margins availability and the operational security requirements. The results provide indexes of the effectiveness that the new infrastructure may have on the Sardinian system in a 2030 time horizon in line with the scenario assumptions of the Italian National Energy Strategy.

This work focuses on two innovative ancillary services for power system frequency control, namely fast primary regulation and synthetic inertia supply. A method for automatic tuning of the controller parameters is proposed. These are here computed by solving a non-linear optimization problem with constraints both on the transient response of grid frequency and on the amount of power that can be exchanged by the control actions. Some penalty coefficients are also taken into account to favor the former or the latter regulation, in particular by considering possible different prices for the two services. Thus, the performance index to be minimized is related both to the employed energy and to its cost. It is obtained starting from a simple model of the regulators both in the continuous and in the discrete time domain. In the simulations, this latter formulation has allowed a considerable reduction of the computation time, so as to ease the analysis of a complete 2030 annual scenario for the Sardinian power system, in case of loss of a DC link pole that connects the island to the Italian mainland.

High Voltage Direct Current (HVDC) links help to integrate large amounts of power from Renewable Energy Sources (RES), however they may interact in complex ways with the control, defense, and protection systems of the power grid. The paper analyses some of the stability phenomena and interactions that may affect small systems weakly connected with the bulk power system via HVDC links, and characterized by low inertia, low short circuit power, and high production from RES. To this purpose, detailed time-domain simulations are performed, based on phasor models, to study severe disturbances such as short circuits close to the AC terminals of HVDC link. The stability outcome is significantly dependent on the control and defense system parameters. A sensitivity analysis is performed and the resulting changes in system performance indicators are discussed.

The EU FP7 BEST PATHS research project is investigating in one of its Demos the possible refurbishment of the three-terminal HVDC link “SA.CO.I.2” (300 MW) between Sardinia, Corsica and the continental grid. The present paper proposes a dedicated control solution, based on the “voltage margin method”, for the future SA.CO.I.3 assuming the adoption of Voltage Source Converters “VSC” with a bipolar configuration and a 600 MW transmission capacity. The three-terminal VSC-HVDC SA.CO.I.3 has been modelled together with a “horizon year 2030” representation of the interconnected AC systems in the DigSILENT PowerFactory® software. The simulation of electromechanical transients in Sardinia demonstrated that the proposed control solution is able to exploit the VSC technical potentialities with advantage, in particular, for the insular Sardinian/Corsican frequency stability.

To prepare the reader for the following parts of the book on monitoring, control and protection of interconnected power systems, this chapter provides a general overview of the power system dynamic phenomena: the purpose is to provide general concepts of the power system stability focusing on interconnection between bulk power systems and in particular for large interconnected systems such as the perspective one consequent to the possible interconnection between the Continental Europe “CE” synchronous system, including Turkey, and the IPS/UPS system of the Russian Confederation.