High CIGs penetration sensibly affects the active and reactive power flows, hence influencing the electromechanical vibration characteristics. To assess whether this influence is positive or negative, factors such as penetration level, type, location, force, operation, and the control strategy/parameters (PTLSOC) ^[1][4] should be analyzed. As far as transient stability is concerned, both positive and negative effects have been reported in the literature, depending mainly on the size and location of the disturbance and the type of the implemented control ^[2][3][17,18]. In general, there are many case studies reporting positive effects of CIGs on transient stability due to greater flexibility in active and reactive power control ^[4][19].

Additionally, the presence of HVDC converters strongly affects power system stability. In this context, ^[5][20] proposes the installation of an additional POD (Power Oscillations Damping) controller since the introduction of HVDC lines into the system induced inter-area oscillation modes around 1 Hz, which are different from the typical modes of traditional transmission lines around 0.2 Hz. Therefore, to mitigate these higher frequency oscillations, it is necessary to improve the design of POD controllers so as to avoid unwanted interactions with modes of a different nature that are close to those of the inter-area target modes. Reference ^[6][21] evaluates the impact of control structure parameters on the system’s intrinsic mode damping and frequency. Indeed, the control system of HVDC converters can be likened to a first-order dynamic system and can be described through two parameters: the control system gain (kDC) and the system time constant (TDC). The study sought to identify parameter pairs that constitute a compromise between network stability and reactivity. Reference ^[7][22] proposes a control strategy that is based on a combined approach, which identifies the desired value of the synchronization power by a model-based approach. This control strategy allows improving both the inertial and the frequency response, as demonstrated by detailed simulation studies developed by considering realistic operation scenarios.

Since unstable oscillations can even lead to a system collapse, real-time power system monitoring represents an effective tool for reliably assessing the system stability. To this aim, the spread of phasor measurement units (PMUs) in power transmission systems enables the development of advanced monitoring functions for early detection of dynamic perturbations. In this context, an interesting application is based on the deployment of modal analysis for damping estimation, which is currently used by a Finnish power system operator ^[8][23]. Additionally, ref. ^[9][24] analyzes real-time monitoring using PMUs, but proposes a decentralized architecture to estimate modal properties highlighting the advantages of such an approach, especially in the detection of local oscillations. PMUs can also be used to detect dangerous oscillations in advance and take appropriate measures. In this regard, in ^[10][25], through an optimization of the Hilbert solution method, the measurements provided by PMUs are processed in order to estimate and classify the oscillation components, and the characteristic parameters such as frequency and damping. The proposed method estimated the frequency and damping values with a very good accuracy and also had the advantage of being able to classify oscillations characterized by very close frequency values, so that distinct peaks in the signal spectrum could not be displayed.

The recent wide area measurement system (WAMS) technology can also be used to monitor inter-area oscillations. Reference ^[11][26] highlights the advantages of using WAMS technology over conventional power system stabilizers (PSSs). In particular, it analyzes a novel wide-area control technique aimed at modulating the active power injections in the task of damping the critical frequency oscillations, which include the inter-area oscillations and the transient frequency swing. The advantage of using WAMS technology is that multiple remote signals can be used for designing effective control strategies, whereas the PSSs process only local signals. This results in greater robustness of the control actions, since even if the centralized controller link fails, the local controllers continue to operate. Reference ^[12][27] evaluates the benefits deriving by the installation of a wide-area power oscillation damper for simultaneously damping both forced oscillations and inter-area modes. The proposed technique is based on an event-triggered strategy, which activates the adaptive control scheme, modulating the voltage set-points of the power oscillation damper. Time synchronized measurements can also enable the development of wide-area damping controllers (WADC), which can be used to damp inter-area oscillations, enhancing the grid observability and controllability ^[13][28].

An effective method for analyzing the grid resilience and assessing the correct operation of the main grid components is based on restoration tests, which are based on measurement campaigns aimed at analyzing low-frequency oscillations (LFO) in active power, reactive power and voltage magnitude profiles. Among the possible causes of this phenomenon, Reference ^[14][29] focuses on the mutual coupling between the restoration path and the transmission grid in normal operation, identifying, through electromechanical simulations, the main parameters influencing the amplitude and phase of LFOs.

Reference ^[15][30] proposes a tool to detect synchronization torque deficiency as a method of prevention against aperiodic rotor angle instability due to small disturbances (ASD). In a first step, the potential buses that can influence the stability margin of the ASD generator are identified by using a Thevenin impedance analysis with respect to a change in nodal admittance; subsequently, the buses are reduced by implementing a sensitivity analysis based on self-propagating graphs, and, finally, the optimal mitigation measures are identified.

The replacement of large synchronous generators with CIGs leads to a dramatic reduction of power system inertia, which could sensibly threaten the frequency stability, especially when the penetration of wind and photovoltaic systems exceed some critical levels ^[16][9]. Low system inertia causes a higher rate of change of frequency (ROCOF), and lower/higher nadir/zenith frequency, hence requesting more complex and faster protection systems. In this context, many technical solutions are currently under investigation, such as ultrafast control, virtual inertia, and grid-forming converter control ^[17][14]. These techniques aim at allowing generators to support the grid in the presence of transient perturbations by improving the frequency stability ^[18][31]. Other approaches propose intentional islanding as a preventive measure aimed at improving the system resilience ^[19][32], which has been shown to be an effective method for preventing cascading phenomena induced by HILP (High Impact Low Probability) events, facilitating the identification of pre-contingency mitigation actions.

Dynamic Security Assessment (DSA) is another relevant tool that has been attracted a great amount of attention from both researchers and industry in recent years, mainly due to its effectiveness in accurately describing the grid dynamics in the presence of both external and internal perturbations. However, its application requires the analysis of a large number of operational and failure events, making its online deployment extremely challenging. Hence, DSA is traditionally performed offline, by deploying Monte Carlo simulations for analyzing the dynamic grid evolution in the presence of “credible” contingencies, and estimating the corresponding stability limits. The study ^[20][33] draws attention to the possibility of using DSA as an “online” decision support tool by deploying high performance computing (HPC) paradigms in the task of promptly solving multiple contingency analyses under different operation scenarios, according to the strictly time-constraints requested by real-time power system operation functions ^[21][22][34,35]. The adoption of HPC-based DSA has been reported in many case studies: (i) the PJM control centre analyses a 13,500-bus system and can process up to 3000 contingencies every 15 min, (ii) EPRI simulates 1000 contingencies of a 20,000-bus system in about 27 min, (iii) iTesla, which combines offline and online simulations for reducing the total amount of scenarios to be analyzed ^[23][36].

To improve power system transient stability, it is possible to adjust in real-time the control strategies of CIGs, by dynamically modulating the generated active and reactive power profiles. In particular, the study ^[24][37] analyses the factors mainly influencing power system transient stability, and suggests performing a sensitivity analysis based on a multivariate optimization procedure in order to identify a proper mapping between the active and reactive power profiles that should be generated during a grid contingency.

The frequency stability is strongly related to the inertia of the power system. Reference ^[25][38] presents a method for “online” inertia estimation, using the obtained estimation for assessing the grid vulnerability to dynamic perturbations. The proposed estimation method, which is based on the integration of the Wold’s decomposition method and then Goertzel’s algorithm, uses a second-order infinite impulse response filter to describe the grid dynamics, assuming logistic distributions, and deploying first-order autoregressive models. The proposed method exhibited good performances in a realistic operation scenario, by reliably identifying the actual system inertia, and also detecting critical operation states in the presence of sensible penetration of renewable power generators. Another interesting approach aimed at managing low inertia power systems is proposed in ^[26][39], which analyzes the role of synchronous compensators (SC) in the task of increasing the short-circuit power, enhancing the voltage magnitude dynamic after severe grid contingencies. Detailed experimental results obtained on real case studies are also presented in this paper, demonstrating the effectiveness of SCs in enhancing the dynamic performance of low-inertia transmission systems.

Active Power Gradient (APG) control represents another promising technique for enhancing the frequency stability, as demonstrated in ^[27][40], which proposed two interesting mathematical methods for adjusting the parameters of APG controllers in order to reduce the frequency deviations after severe power imbalances. Both methods are formulated by a constrained single-objective optimization problem, but the first method aimed at minimizing the instantaneous variation of the kinetic energy of the synchronous areas, while the second one aimed at minimizing the spatial shifts between the dynamic trajectories of the temporal responses in different synchronous areas.

The massive penetration of CIGs into the transmission and distribution grid makes the voltage regulation process much more complex. In this context, short-term voltage stability could be compromised due to the fast dynamic response of the converters, which could induce severe transient voltage disturbances (e.g., over/undervoltages) ^[28][41].

Among the possible mitigation techniques that could be deployed for enhancing the voltage stability, those based on synchrophasors-based data processing are the most promising. These techniques allow detecting voltage instability and implementing load reduction techniques by processing time-synchronized measurements acquired by a network of PMUs. However, the application of these techniques is still embryonic, and several open problems should be fixed in order to obtain a complete and reliable system observability. In this context, the uncertainties induced by measurement errors, the accuracy and integrity of the time-synchronization sources, and the vulnerability of PMUs to cyberattacks are some of the relevant issue to address ^[29][30][42,43].

A useful analysis of the main effects induced by DFIG-based wind generators on voltage stability is presented in ^[31][44], which identifies the effects of these generators on maximum loadability limits and the maximum scalable active power demand, clearly emphasizing that an increasing penetration of wind power generators can overload critical grid equipment, hence limiting their hosting capacity. Similarly, in ^[32][45], the impact on the long-term voltage stability (LTVS) of photovoltaic generation is assessed. In particular, this study showed that solar generation can also have both negative and beneficial effects on voltage stability. Furthermore, the parameters that most affect LTVS were identified, including solar irradiation and ambient temperature, inverter power, reactive power gain and current-limiting strategies.

A further cause of voltage instability is the decommissioning of conventional synchronous generators, which lowers the available reactive power reserve. In this context, Reference ^[33][46] performs a comprehensive voltage stability analysis computing the VQ stability margins, emphasizing the fundamental role played by the reactive power control strategy and reactive power capacity of converter-interfaced technologies in ensuring voltage stability. According to this important result, several studies suggest the deployment of additional devices aimed at enhancing voltage stability by improving reactive power reserves, such as synchronous capacitors, synchronous static compensators (STATCOM), switchable and non-switchable shunt capacitors and reactors, and on-load tap changing transformers (OLTCs) ^[34][47].

Many experimental studies developed worldwide demonstrated converter-driven stability issues that could induce unstable power system operations due to a number of complex dynamic phenomena, which include sub-synchronous oscillations between wind generators and series compensated lines ^[35][48], and harmonic instability induced by solar generators ^[36][49]. These phenomena can be generated by a combination of multiple effects, such as interactions between the CIGs control units, power system weakness, converter-interfaced load dynamics, and congested power lines ^[37][50]. In particular, the fast dynamic of the CIGs power/voltage control unit could induce rapid variations of the grid frequency and transient distortions of the voltage waveform, which could trigger the grid protections ^[38][51]. In this context, it is extremely important to accurately analyze and classify all the potential sources of converter-driven instabilities, in order to design and implement robust corrective actions aimed at mitigating the dynamic perturbation effects of these phenomena on correct power system operation.

Resonance stability issues are mainly related to the torsional impacts of flexible alternating current transmission systems and/or high-voltage direct current transmission systems, and the electrical resonances originating from the grid interactions of the CIGs controllers. The analysis of these oscillatory phenomena can be performed according to different computing techniques, which include time-domain electromagnetic power system simulation, state-space analysis, and frequency-domain impedance-based methods. In particular, electromagnetic power system simulation requires the numerical solution of the differential equations describing the dynamics of the power electronic components of the CIGs. Although these techniques allow accurately modeling the grid oscillations, they could not allow analyzing the intrinsic phenomena ruling the oscillations ^[39][52]. State-space analysis-based methods allow overcoming this limitation, by analyzing the eigenvalues of the state-matrix in the task of detecting unstable oscillation modes, determining the main parameters influencing their evolution ^[40][53]. Despite these benefits, developing a state-space model for power systems in the presence of a large number of CIGs is a challenging issue. In this context, the complexities in describing the reactive effects of the power transmission system, and the dynamic of the power electronic converters are some of the most relevant issues to address. The deployment of frequency-domain impedance-based methods represent a promising alternative approach for resonance stability analysis. The mathematical backbone supporting these approaches is based on the Nyquist stability criterion of the single port-impedance, which can be easily developed by using different modeling approaches, such as small-disturbance models, harmonic linearization, and the measurement methods ^[40][53]. Recently, more effective modeling approaches based on the s-domain nodal admittance matrix analysis have been proposed in the task of accurately analyzing the resonance stability of power systems in the presence of a large-scale penetration of CIGs ^[41][54]. The application of these modeling approaches allows identifying the main phenomena affecting resonance stability, and analyzing the potential technical solutions aimed at enhancing the grid robustness against these phenomena. In this context, the most promising mitigation techniques include the deployment of static var compensators for damping the torsional resonance, and the smart coordination of the CIGs controllers in the task of damping the electrical resonance ^[37][50].