Dynamic assessment of Power System Strength in systems with a large share of generation from renewable sources

The energy transition involves the gradual phasing out of conventional power generation and the massive integration of Renewable Energy Sources (RES) connected to the power system through inverters. In fact, the European climate-energy objectives define the goal of climate neutrality ("net zero") by 2050, along with a reduction in greenhouse gas (GHG) emissions of at least 55% by 2030 compared to 1990 levels. The decarbonisation targets defined in the Energy-Climate Plan, at the time of writing of this paper, sets new challenges for the electricity sector. By 2030, around 70 GW of new renewable capacity will be installed in Italy to cover at least 65% of the demand with renewable energy power plants. In this context, the number of initiatives proposed by private investors to develop renewable energy power plants is very encouraging. In the electricity system of the near future, characterised by a generation mix strongly based on Inverter-Based Resources (IBR), the system operators will have to face the challenge of providing the grid with those services mainly provided today by synchronous generators, such as frequency and voltage control, in order to maintain the security of the electricity system and the quality of service. In a network area characterized by high density of IBR, their dynamic behaviour that could lead the system to become unstable has to be considered. In this emerging scenario, the concept of Power System Strength needs to be properly investigated, considering the effect of several IBR in the system. Typically, system strength is derived from the existing Short Circuit Ratio (SCR)-based methods that may not effectively reflect the effects of multiple RES power plants. Several scholars have shown that indexes referring to different ways of calculating short circuit ratio are not able to give a complete evaluation of the actual stability of a system with a large share of inverter-based resources connected. The reason why of the inadequacy of these methods resides in that the concept of system strength is basically related to the short circuit power (Ssc) which is mainly provided by synchronous generators. Furthermore, High Voltage Direct Current (HVDC) systems adopting Voltage Source Technology (VSC) offer full control of energy flows in the DC grid and represent the most suitable assets to deal with these new power system challenges while providing necessary regulation services to enhance the system strength levels of the AC grid. Infact, when needed, the decoupling of active and reactive power control enable the VSC converter station to operate as a Static Synchronous Compensator (STATCOM) providing voltage regulation services. In this paper, results of system strength evaluation using MISCR static index are shown in the first section. Furthermore, in section 3 stability assessments based on input-admittance methods are carried out exploring the dynamic parameters of the components including control and synchronization loops and the results are compared using dynamic simulations. Both HVDC VSC and IBRs with different control logics are considered. 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, by mitigating power fluctuations and enabling better control of the voltage profiles possibly using control schemes based on the so-called Grid Forming (GFM) operating mode which is briefly introduced and tested in the paper.