Hybrid High Voltage AC and DC System Strength evaluation with large penetration of renewable energy sources

The Italian electricity system is undergoing significant evolutions as part of the energy transition in the country. The European targets of the Fit-for-55 package include a 55% reduction of CO2 emissions by 2030 (compared to 1990 levels). This means that energy produced from Renewable Energy Sources (RES) in Italy must cover at least 65% of final consumption in the electricity sector by 2030 (compared to 55% previously envisaged by the National Integrated Energy and Climate Plan (PNIEC)). This will require an additional 70 GW (mainly new photovoltaic and wind power) by 2030, compared to the additional 40 GW envisaged in the PNIEC. Direct current (DC) technology, with its many advantages over alternating current (AC) transmission, is the most suitable means of transporting this large amount of renewable generation to the grid in a safe and stable manner. High Voltage Direct Current (HVDC) systems will allow full control of energy flows in the DC grid, which is not possible in conventional AC grids. In this way, it will be possible to better manage the energy flows generated by the increasing number of RES power plants, which by their nature are not predictable, over long distances thereby reducing losses for the same transmitted power. The increasing penetration of RES can challenge Transmission System Operators (TSOs) to maintain the security of supply and system reliability. Potential power system stability issues may arise when a large number of RES are connected to a weak power system. In this scenario the concept of System Strength should be properly investigated taking into account the interaction of multiple Inverter-Based Resources (IBRs) in the system. When several IBRs are connected to the same portion of the network, they can interact each other even dynamically through control loops actions, causing the system to become unstable. From a static point of view the existing Short Circuit Ratio (SCR)-based methods may be not effective in reflecting the interaction effects among multiple RES power plants. Indeed, the static indicators found in the literature tend to underestimate or overestimate the system strength. They show critical issues related to their formulation based on short-circuit power, which in case of large penetration of RES power plants does not longer represent an appropriate indicator of system stability. In this paper, in order to assess system strength, dynamic simulations are carried out in MATLAB/Simulink environment. High Voltage Direct Current (HVDC) Voltage Source Converters (VSC) and IBR with different control logics are implemented. The aim is to investigate their dynamic response analyzing various control logics configurations applied on power sources. HVDC links using VSC technology will be able to improve the AC system stability, containing power fluctuations and ensuring a better control of the voltage profiles. In addition, appropriate control strategies for VSC converters of the IBRs generation are investigated