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The transition towards an electricity system that is dominated by asynchronous and non-dispatchable generators, such as wind and solar power, entails challenges related to balancing the load and, thereby, keeping the grid frequency stable. Many technologies can contribute to load balancing and frequency control. This study investigates the interactions between electricity generation and frequency control in terms of investments and operation, using cost-minimizing, linear optimization modeling. The model is applied in three different geographic cases and for four future time-points, starting off with the already existing transmission and generation capacities, so as to yield insights into different systems and different stages along the energy transition. The results show that frequency control constraints in the optimization model have a weak impact on the system composition and cost, and that batteries are important for minimizing the impact. Furthermore, inertia requirements without a reserve demand show no impact on the cost or system composition. When allowing for vehicle-to-grid from battery electric vehicles, a large proportion of stationary grid battery investments is displaced, and the impact on system cost from adding frequency control constraints is removed. [ABSTRACT FROM AUTHOR]

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This work presents a technique to estimate on-line the global inertia of an electric power system by exploiting the footprint of the principal frequency system dynamics. This method can estimate the inertia provided as a whole by synchronous machines, as well as by converter-interfaced generators controlled to emulate the behavior of the former through virtual inertia. Probing tones are injected by a grid-forming converter-interfaced generator and its virtual rotor speed is used to extrapolate its footprint. As a result, the method requires neither measuring the active power exchange of each synchronous generator nor extrapolating their rotor speeds. Since the proposed technique is entirely data driven, it does not require any model of the power system generators/prime-movers/controllers and of the interconnecting grid. The method is comprehensively tested on a modified version of the IEEE 39-BUS system and a dynamic version of the IEEE 118-BUS system, both containing grid-forming converter-interfaced generators.

In the transition from non-renewable to renewable energy sources, wind and solar power plants have emerged as primary sources for reducing atmospheric emissions. However, ensuring grid stability remains a challenge in the absence of traditional power plants. Renewable energy sources are often referred to as "inertia-less," highlighting their vulnerability in transient stability scenarios in which system inertia is crucial. This paper introduces the concept of "synthetic inertia estimation" using an Extended Kalman Filter as a novel solution. Using the synthetic inertia estimation from a wind power plant, one can use the residual kinetic energy, and through the DC/AC converter, inject it into the system during times of stability disturbances. This paper offers practical insights into synthetic inertia estimation and its implementation for enhancing grid stability in a sustainable energy landscape. [ABSTRACT FROM AUTHOR]

Increasing the replacement of conventional synchronous machines by non-synchronous renewable machines reduces the conventional synchronous generator (SG) inertia in the modern network. Synthetic inertia (SI) control topologies to provide frequency support are becoming a new frequency control tactic in new networks. However, the participation of SI in the market of RES-rich networks to provide instant frequency support when required proposes an increase in the overall marginal operation cost of contemporary networks. Consequently, depreciation of operation costs by optimizing the required SI in the network is inevitable. Therefore, this paper proposes a flexible SI optimization method. The algorithm developed in the proposed method minimizes the operation cost of the network by giving flexible SI at a given SG inertia and different sizes of contingency events. The proposed method uses Box's evolutionary optimizer with a self-tuning capability of the SI control parameters. The proposed method is validated using the modified New England 39-bus network. The results show that provided SIs support the available SG inertia to reduce the RoCoF values and maintain them within acceptable limits to increase the network's resilience. [ABSTRACT FROM AUTHOR]

The increasing use of renewable energy sources in place of conventional generation units is leading to a reduction in onshore inertia and to the development of offshore wind park grids connected by multiple high-voltage direct current (HVDC) connections to the onshore alternating current (AC) grid. For AC-side meshed offshore grids with voltage-source converter (VSC) and diode rectifier unit (DRU) HVDC connections towards onshore grids, this study focuses on the energetic feasibility of synthetic inertia (SI) and primary control reserve (PCR) contributions triggered locally at the onshore converters of both connection types. To this end, the obstacles preventing contributions for VSC HVDC connections and the mechanisms allowing contributions for DRU HVDC connections are identified first. Based on these findings, the article proposes an enhancement of the offshore HVDC converter controls that is continuously active and allows locally triggered onshore contributions at all onshore HVDC converters of both connection types without using communication and requiring only minimal system knowledge. Additional simulations confirm that, although the enhancement is continuously active, the operational performance of the offshore HVDC converter controls for normal offshore grid operation and its robustness against offshore AC-side faults are not affected. [ABSTRACT FROM AUTHOR]

The rapid growth of renewable energy capacity, in particular photovoltaic systems, is creating challenges associated with changing the rate of transient processes in the power system. This is due to the approach PV systems are connected to the grid using power converters and the absence of a rotating mass in the PV power plant. One of the most pressing challenge is the participation of PV stations in the process of frequency control in power systems, including in emergency modes. Simultaneously with PV power plants, it is efficient to use energy storage systems, including hydrogen ones. This is due to the fact that it is possible to obtain hydrogen for such energy storage systems using excess energy from PV power plants. The article proposes to solve the problem of frequency regulation in the power system by using an algorithm that allows to control the frequency in the power system using a synthetic inertia block of PV station, including at different levels of insolation and temperature of PV panels. The robustness of the proposed algorithm allows it to be used at different levels of power generated by the PV station, as well as in emergency modes. [ABSTRACT FROM AUTHOR]

Large-scale integration of inverter-based renewables is displacing synchronous machine generation, causing a reduction in the inertia of electrical power systems. This reduction is reflected in an increase in the rate of change of frequency (RoCoF). Additionally, the variation of the RoCoF will depend on the uncertainty associated with the generation of non-conventional renewable energy sources. For the planning of the operation of the system, it is essential to know the range of variation of the RoCoF when there are disturbances in the system and uncertainties in the generation of non-conventional sources of renewable energy. This paper proposes to establish the calculation of a confidence interval of the RoCoF variation that considers these uncertainties. So, this paper proposes a method to consider these uncertainties based on the probabilistic point estimate method (PEM); considering multiple renewable non-conventional sources with correlated or uncorrelated behavior in their powers injected into the system. On the other hand, as there are different proposals to calculate the RoCoF, this paper presents the application of the uncertainty model with three different RoCoF proposed calculation methods. [ABSTRACT FROM AUTHOR]

This paper focuses on a comprehensive stability study of a two-area power system with wind power integration and synthetic inertia control in each area, considering the effects of varying the interconnection link. Normally, synthetic inertia proposals are analyzed in one-area systems, in which stability is tested without considering transmission system phenomena, such as coherency. As modern power systems are progressively becoming interconnected, the possibility of forming two or more non-coherent areas is likely, which poses a challenge to synthetic inertia control techniques that use system frequency as a main feedback signal. In this context, this work addresses a crucial gap in the existing literature and provides a valuable starting point for studying more complex interconnected power systems with wind power integration. Simulations were performed in Matlab-Simulink considering a data-driven frequency dynamics model of the Chilean Electric System, and a wind power model with synthetic inertia control H 2 norm minimization in each area. The results showed that it is possible to find local optimal feedback gains, preserving the stability of the global system under significant variations in the interconnection link. RoCoF and Nadir indicators are provided, highlighting the benefits of synthetic inertia control, particularly in low-inertia situations. [ABSTRACT FROM AUTHOR]

The development of electric power systems determines the growing probability of low-frequency oscillations, which can be reason of system faults. Traditionally, the task of damping low-frequency oscillations is assigned to synchronous generators, by appropriate setting of the automatic voltage regulator parameters. However, as the installed capacity of renewable generation units increases, research aimed to analyse the impact of these generation units on the power oscillations in the electric power system is becoming more relevant. Moreover, an evaluation of the potential of renewables to solve the problems of power oscillation damping seems necessary to expand the operation functions of the these generation units and to increase the stability of the electric power systems. This paper presents an analysis of renewable energy plants, in particular photovoltaic stations, on damping of these power oscillations. Achieving such damping function is possible via upgrading the automatic control system of photovoltaic stations by using a synthetic inertia block and the possibilities of the “underload” mode. Different scenarios, including small disturbances and severe fault condition for strong and weak grids depending on the location and capacity of photovoltaic station were considered. To evaluate the impact of renewable generation unit to the damping properties of electric power system, the transient damping ratio was calculated. The calculation was carried out in accordance with the guidelines in case of three-phase short-circuit for a different penetration level of renewable generation unit, as well as with and without the synthetic inertia block.

In the transition from non-renewable to renewable energy sources, wind and solar power plants have emerged as primary sources for reducing atmospheric emissions. However, ensuring grid stability remains a challenge in the absence of traditional power plants. Renewable energy sources are often referred to as “inertia-less,” highlighting their vulnerability in transient stability scenarios in which system inertia is crucial. This paper introduces the concept of “synthetic inertia estimation” using an Extended Kalman Filter as a novel solution. Using the synthetic inertia estimation from a wind power plant, one can use the residual kinetic energy, and through the DC/AC converter, inject it into the system during times of stability disturbances. This paper offers practical insights into synthetic inertia estimation and its implementation for enhancing grid stability in a sustainable energy landscape.

Increasing the replacement of conventional synchronous machines by non-synchronous renewable machines reduces the conventional synchronous generator (SG) inertia in the modern network. Synthetic inertia (SI) control topologies to provide frequency support are becoming a new frequency control tactic in new networks. However, the participation of SI in the market of RES-rich networks to provide instant frequency support when required proposes an increase in the overall marginal operation cost of contemporary networks. Consequently, depreciation of operation costs by optimizing the required SI in the network is inevitable. Therefore, this paper proposes a flexible SI optimization method. The algorithm developed in the proposed method minimizes the operation cost of the network by giving flexible SI at a given SG inertia and different sizes of contingency events. The proposed method uses Box’s evolutionary optimizer with a self-tuning capability of the SI control parameters. The proposed method is validated using the modified New England 39-bus network. The results show that provided SIs support the available SG inertia to reduce the RoCoF values and maintain them within acceptable limits to increase the network’s resilience.

Integration of wind generation units around the world is a promising solution to the energy crisis. In power systems with a predominant share of wind power plants connected through power converters, their influence on the dynamic properties of the power system becomes significant. One of the important problems is reducing the total inertia of the power system. In view of this, transmission system operators place demands on plug-in wind farms to manage active power and participate in frequency support. The proposed solution is the use of synthetic inertia in the control system of wind turbines. Research in this field focuses on improving the frequency stability and coordination of the synthetic inertia controller with other wind turbine control systems. However, the influence of synthetic inertia and its tuning on the stability of the power system are poorly studied. The paper analyzes the effect of synthetic inertia on the transient stability of the power system. It is shown that by adjusting the synthetic inertia parameters, it is possible to provide not only the required inertial response, but also to increase the transient stability of power systems. The influence of the measurement time window and calculation of the rate of change of frequency, which is important under certain modes, is also investigated. The results are obtained for a test scheme and a real-scale power system scheme. [ABSTRACT FROM AUTHOR]

The reduction in system inertia under the penetration of power electronic-interfaced generation has fostered various proposals on wind synthetic inertia. Several works in the state of the art propose control strategies to optimize synthetic inertia performance; however, most of the performance metrics indirectly represent frequency control adequacy indicators such as rate of change of frequency or frequency nadir. Also, control definitions assume that there is one variable speed wind turbine and one controller, which limits the applicability of the proposals as power systems normally count with various wind turbines/farms over extended geographical areas with different wind regimes. This work presents a $H_{2}$ optimal control approach for a variable speed wind turbine synthetic inertia controller. First, the objective of the formulation is to explicitly minimize the rate of change of power system frequency. Second, a stability proof for the multi-wind turbine case is proposed, allowing optimal controllers to be independently implemented for an arbitrary number of wind turbines and wind regimes. The effectiveness of the proposed control scheme is demonstrated with a numerical example, considering an empirically-validated, reduced-order model of the Electric Reliability Council of Texas frequency dynamics.

This paper proposes a dynamic synthetic inertia control method, considering the fatigue loads of the wind turbine. The control objectives include reducing the rate of change of frequency and frequency nadir of the power system and the fatigue load of the shaft and tower of the wind turbine. A frequency regulation model of the power system containing the primary operating dynamics of the wind turbine is established. The dynamic synthetic inertia control method is proposed according to the relationship between fatigue load, wind velocity, and frequency. Case studies are conducted with a wind turbine fatigue load under a synthetic inertia control with different weights for different wind velocities and system loads. Therefore, the dynamic weights are obtained. Comparing the rate of change of frequency and frequency nadir and equivalent damage load, the efficacy of the proposed method is verified.

One of the trends in the development of the electric power industry in the world is the use of renewable resources. In recent years, the development of hydrogen energy has been widely discussed, which, when combined with renewable energy sources, makes it possible to obtain "green" hydrogen. One of the promising solutions for obtaining green hydrogen is the development of wind energy-hydrogen systems. However, one of the limiting factors, in addition to economic aspects, are technical limitations. Sustainable hydrogen production is complicated by the variability of wind energy. In addition, energy systems with a predominant share of wind power units have a low total inertia value. Consequently, such systems become more sensitive to changes in load modes in the grid and become less stable in emergency modes. Together, these facts reduce the reliability of wind energy-hydrogen system. The paper proposes to increase the stability of wind energy-hydrogen system and power system as a whole to apply synthetic inertia for wind power plants. It is shown that by adjusting the parameters of synthetic inertia can provide not only the required inertial response, but also increase the dynamic stability of the power system and as a consequence wind energy-hydrogen system. The influence of the measurement window time and calculation of the frequency change rate, which is important in certain modes, is also investigated. [ABSTRACT FROM AUTHOR]

The increasing use of renewable energy sources in place of conventional generation units is leading to a reduction in onshore inertia and to the development of offshore wind park grids connected by multiple high-voltage direct current (HVDC) connections to the onshore alternating current (AC) grid. For AC-side meshed offshore grids with voltage-source converter (VSC) and diode rectifier unit (DRU) HVDC connections towards onshore grids, this study focuses on the energetic feasibility of synthetic inertia (SI) and primary control reserve (PCR) contributions triggered locally at the onshore converters of both connection types. To this end, the obstacles preventing contributions for VSC HVDC connections and the mechanisms allowing contributions for DRU HVDC connections are identified first. Based on these findings, the article proposes an enhancement of the offshore HVDC converter controls that is continuously active and allows locally triggered onshore contributions at all onshore HVDC converters of both connection types without using communication and requiring only minimal system knowledge. Additional simulations confirm that, although the enhancement is continuously active, the operational performance of the offshore HVDC converter controls for normal offshore grid operation and its robustness against offshore AC-side faults are not affected.

Increasing the share of grid frequency converter-connected renewables reduces power system inertia, which is crucial for grid frequency stability. However, this development is insufficiently covered by energy system modelling and analysis as well as related scientific literature. Additionally, only synchronous inertia from fossil fuel-emitting power plants is represented, although renewable generators are a source of synthetic inertia, thus resulting in increased must-run capacities, CO2 emissions and system costs. The work at hands adds an analysis of the future German power system considering sufficient inertia to the literature. Therefore, results of an novel open-source energy system model are analysed. The model depicts minimum system inertia constraints as well as wind turbines and battery storage systems as a carbon-dioxide-free source for a synthetic inertial response. Results indicate that integrating system inertia constraints in energy system models has a high impact on indicators such as system costs. Especially when investments in additional storage units providing an inertial response are necessary. With respect to researched scenarios, system cost increases range from 1% up to 23%. The incremental costs for providing additional inertia varies between 0.002 EURO/kg·m2 and 0.61 EURO/kg·m2. [ABSTRACT FROM AUTHOR]

This work presents a technique to estimate on-line the global inertia of an electric power system by exploiting the footprint of the principal frequency system dynamics. This method can estimate the inertia provided as a whole by synchronous machines, as well as by converter-interfaced generators controlled to emulate the behavior of the former through virtual inertia. Probing tones are injected by a grid-forming converter-interfaced generator and its virtual rotor speed is used to extrapolate its footprint. As a result, the method requires neither measuring the active power exchange of each synchronous generator nor extrapolating their rotor speeds. Since the proposed technique is entirely data driven, it does not require any model of the power system generators/prime-movers/controllers and of the interconnecting grid. The method is comprehensively tested on a modified version of the IEEE 39-BUS system and a dynamic version of the IEEE 118-BUS system, both containing grid-forming converter-interfaced generators. • An important and timely issue: how to estimate the inertia of an entire power system? • We present a technique to estimate on-line the inertia of an electric power system. • The proposed technique is entirely data driven. • This approach is adequate to accurately estimate also the inertia by converter-interfaced generators. [ABSTRACT FROM AUTHOR]

This paper summarizes the results of a power system stability analysis realized for the EU project OSMOSE. The case study is the electrical network of Sicily, one of the two main islands of Italy, in a scenario forecasted for 2050, with a large penetration of renewable generation. The objective is to establish if angle and voltage stabilities can be guaranteed despite the loss of the inertia and the regulation services provided today by traditional thermal power plants. To replace these resources, new flexibility services, potentially provided by renewable energy power plants, battery energy storage systems, and flexible loads, are taken into account. A highly detailed dynamical model of the electrical grid, provided by the same transmission system operator who manages the system, is modified to fit with the 2050 scenario and integrated with the models of the mentioned flexibility services. Thanks to this dynamic model, an extensive simulation analysis on large and small perturbation angle stability and voltage stability is carried out. Results show that stability can be guaranteed, but the use of a suitable combination of the new flexibility services is mandatory. [ABSTRACT FROM AUTHOR]

In this article, two different strategies to enhance power system stability exploiting synthetic inertia are presented and compared to assess the possibility of increasing the penetration of converter-based generation in power systems, keeping a suitable stability margin. Synthetic inertia is the emulation, carried out by converted-based generation, of the inertial response provided by rotating generators during frequency events. Two approaches are hence described, focusing on wind power. In the first approach, the primary power source is connected to the grid by a grid-following converter, and it exploits the energy content of the direct current link, suitably sized (or, with the same concept, of a suitable energy buffer available). The second approach uses a grid-forming converter model, whose features resemble more a traditional synchronous machine. The article aims at assessing the contribution to frequency stability that can be provided by converter-based generation. The simulation results demonstrate that converter-based generation can efficiently guarantee frequency stability, thus proving that future power systems will be able to manage ever-increasing renewable energy source converter-based generation. [ABSTRACT FROM AUTHOR]

The ability to use the algorithm synthetic inertia (SI) is one of the most important properties of renewable energy sources (RES) generating units connected to the network via a power converter (GUPC). Through the use of SI algorithm there is an opportunity to increase the inertia and damping properties of such plants. The effectiveness of the SI algorithm depends on the mains frequency input value, which is formed by the phase locked loop (PLL), which is an integral part of the power converter control system. However, the operation of the PLL can lead to oscillations with different frequencies when the GUPC is installed in weak electrical networks and, accordingly, adversely affect the performance of the SI algorithm. The studies have shown that the PLL in the photovoltaic plant (PV) control system allows to influence the performance of the SI algorithm, but the nature of this influence depends on the electric network density and can be positive or negative. The influence obtained on the test EPS is also confirmed for the power system of large-scale. [ABSTRACT FROM AUTHOR]

Frequency stability in today's power systems has become more critical than ever due to the growing contribution of renewable energy sources. This situation has arisen because of the electro-mechanical decoupling between renewable generation sources and the main grid caused by their connection through power electronic converters. This paper designs two synthetic inertia controllers that adhere to the grid code requirements from two different countries and then to utilize them to aid in the integration of high levels of wind power penetration in a test power system. The controllers are designed for a full converter wind turbine generator and are validated in an EMT real-time simulation with isolated testing at different wind speeds and different wind power penetration. The role of synchronous inertia in maintaining frequency stability is also explored through the use of synchronous condensers. Finally, the economic aspect of inertia is discussed, using the real-world example of the Bornholm island power system.

Power system inertia is an essential part for grid frequency stability and decreases due to the replacement of fossil fuel fired power plants with variable renewable energy sources. This development is not represented sufficiently in unit commitment and economic dispatch models. If considered at all, only synchronous inertia from fossil fuel driven power plants is modelled. This results in increased CO2 emissions, curtailed renewable energy and high system costs. While wind turbines are a source for synthetic inertia and an important renewable energy source, their capability to provide inertia is not incorporated into energy system models. The work at hand closes this research gap and applies a methodology to depict synthetic inertia provided by wind turbines as part of the optimisation dispatch model. A unit commitment and economic inertia dispatch model of the the all-Island Irish power system is created. The potential of wind inertia is analysed and quantified by assessing CO2 emissions, curtailed renewable energy and system costs. Results show that synthetic inertia provided by wind turbines can save up 30.99% of the CO2 emissions, reduce curtailment by up to 39.90% and reduce system costs by 32.72%. [ABSTRACT FROM AUTHOR]

The penetration of renewable energy sources on a large scale in conventional electric networks increases exponentially for environmental preservation. However, the increase in RESs leads to some technical problems that include (a) fluctuations in power production, (b) less or no generation units for power balancing, (c) reduced inertia due to RESs decoupling from the conventional grid using converters. Micro-hydro power plants (MHPPs) are emerging as a mature balancing technology and a great alternative to large hydropower plants as they encounter population displacement and many environmental problems. Modern pump storage MHPPs uses a power converter between the grid and machine to control the consumption of power during pumping mode. This power electronic interface loses the ability of the rotating mass of the machine to contribute to the grid inertia by making it independent of the grid frequency. This problem is solved through the control of power converters in such a way that the inertia effect is synthesized termed as synthetic inertia. MHPPs can also be operated in a standalone mode where they are not connected to the grid. This paper reviews control schemes applied in literature for the frequency, voltage, and inertia control, in both the grid-connected and standalone modes. This study starts with the standalone MHPPs, covering the literature review and control structure for voltage and frequency control of standalone MHPPs. Then it presents a detailed operation, control principle, synthetic inertia concepts, and architecture of grid-connected MHPPs. The mathematical formulation of the grid, permanent magnet synchronous generator, synthetic inertia inclusion, and control structures, including direct torque control, virtual synchronous machine, and model predictive control, is also presented. Finally, the paper presents the concluding remarks with the comparative analysis of various control structures to include synthetic inertia, its suitability, and future scope for MHPPs.

In order to prevent the potential frequency instability due to the high Power Electronics (PE) penetration under an unintentional islanding event, this paper presents a novel microgrid scheduling approach which includes the system frequency dynamics as well as the uncertainty associated with renewable energy resources and load. Synthetic Inertia (SI) control is applied to regulating the active power output of the Inverter-Based Generators (IBGs) to support the post-islanding frequency evaluation. The uncertainty associated with the noncritical load shedding is explicitly modeled based on the distributionally robust formulation to ensure resilient operation during islanding events. The resulted frequency constraints are derived analytically and reformulated into Second-Order Cone (SOC) form, which are further incorporated into the microgrid scheduling model, enabling optimal frequency services provision from the micorgrid perspective. With the SOC relaxation of the AC power flow constraints, the overall problem is constructed as a mixed-integer SOC Programming (MISOCP). The effectiveness of the proposed model is demonstrated based on modified IEEE 14-bus system. [ABSTRACT FROM AUTHOR]

In this paper, a virtual inertia and damping control (VIDC) strategy is proposed to enable bidirectional DC (bi-DC) converter to dampen the voltage oscillation, using the energy stored in energy storage system (ESS) to emulate the inertia of DC microgrid (DC-MG) without modifications of system hardware. Both inertia part and damping part are modelled in the VIDC regulator, by analogizing with DC machine. Droop control is introduced to achieve multi-parallel operation of ESS. Further, its small signal model is established and the dynamic characteristic of the DC bus voltage during power fluctuation is analyzed. A 2nd-order equivalent model is introduced to simplify the parameters design. By analyzing the control performance indices, the VIDC parameters are optimized. At last, based on the principle of equivalent machine, an inertia matching method of the multi-parallel bi-DC converters is proposed. The simulation results verify that ESS, by acting as a synthetic inertia, can improve the voltage dynamic characteristics and stability in islanded DC-MG. Compared with other control methods, it has better control effect. [ABSTRACT FROM AUTHOR]

This study investigates how the inclusion of frequency control constraints in electricity system modeling impacts the levels of investment and dispatch in electricity generation and storage technologies for futures that include high-level penetration of variable renewable energy. This is achieved using a linear cost-minimizing investment and dispatch model using historic load, wind and solar conditions from Spain, Ireland, Sweden and Hungary for Year 2050. With an hourly time-resolution, constraints are added so as to ensure that, within each hour, sufficient inertial power and reserves are available to control the frequency of the power grid. Comparing the results obtained with and without these constraints reveals that the main impacts on the results are from battery investments and operation. Furthermore, it is found that the reserve requirements exert a greater impact on system composition and operation than do the inertial power requirements. • Constraints on operating reserve and inertia have been tested in wind- and solar PV-dominated systems. • Inertia and operating reserve constraints influence investments more than they influence dispatch. • Batteries are the main source of reserves, while both wind power and batteries provide synthetic inertia. • In the absence of synthetic inertia, investments in synchronous condensers cover the inertia demand. • The overall impact is largely predictable and isolated from energy generation. [ABSTRACT FROM AUTHOR]

• Pioneer Spain: first country to include in grid code the synthetic inertia capability. • Synthetic inertial response of a PV inverter studied based on the Spanish grid code. • Analyzed the role of power plant controller in the dynamic response of the facility. • PV system able to contribute to frequency control providing an inertial response. As the large-scale integration of renewable energy grows, inertia in power systems is reduced, which causes the rate of change of frequency to increase and worsens the frequency nadir during disturbances. There is, therefore, an urgent need to accelerate the development of international standards and regulations that require solar PV systems and other renewable installations to also contribute to stability in the network by emulating inertia. Spain is a pioneer in this field: it is currently the only country that includes the synthetic inertia technical requirement in its grid code. In this context, the present paper analyzes the synthetic inertial response of a PV system model that forms part of an actual PV power plant in Spain under two scenarios, according to its grid code: i) when measurements are conducted at the power conversion PV system level and hence only its control algorithms are executed; and ii) when measurements are conducted at the power plant level and the power plant controller also influences the response of the entire installation. Among its aims, this paper seeks to thoroughly assess the response of an actual PV power conversion system that has the capability of actively contributing to frequency regulation in a network and to critically analyze the suitability of having a power plant controller when the PV facility contributes to power-frequency control by means of an inertial response. The results reveal, at the power conversion PV system level, that the inertia emulation module significantly reduces the response times of the installation to changes in frequency -by more than 68% in the worst case under frequency increases, and by 73% under frequency decreases, thus contributing to more rapid power-frequency regulation in the network. However, at the power plant level, the power plant controller disturbs the PV power conversion system's behavior and causes the opposite effect: response times increase -up to more than 7% in the most unfavorable case when the inertia module is enabled. [ABSTRACT FROM AUTHOR]

Direct current (DC) building microgrids allow the integrations of DC distributed energy resources (DERs) and loads with a simpler topology and the elimination of alternating current (AC) system issues, such as frequency transients and harmonics. Because of the domination of power-electronics-interfaced generators, islanded DC microgrids in general present very low inertia, which reveals subtle stability threats to the systems. In addition, the conventional droop-based DER power-sharing mechanisms may suffer from poor power-sharing accuracy and DC bus voltage deviations that require secondary restorations. In this paper, a novel control scheme for building-scale islanded DC microgrids is proposed based on finite control set model predictive control (FCS-MPC). A new DER power-sharing mechanism is devised by introducing a current-sharing vector in the controller formulation, which eliminates bus voltage deviation during load/DER fluctuation and offers plug-and-play capabilities. Moreover, for the first time, virtual capacitance control for DC microgrids is implemented in an FCS-MPC manner to enhance the adaptive synthetic inertia of DC bus. Both the simulation and experimental case studies are carried out to provide verification for the promising performance of the proposed method.

Increasing the share of grid frequency converter-connected renewables reduces power system inertia, which is crucial for grid frequency stability. However, this development is insufficiently covered by energy system modelling and analysis as well as related scientific literature. Additionally, only synchronous inertia from fossil fuel-emitting power plants is represented, although renewable generators are a source of synthetic inertia, thus resulting in increased must-run capacities, CO2 emissions and system costs. The work at hands adds an analysis of the future German power system considering sufficient inertia to the literature. Therefore, results of an novel open-source energy system model are analysed. The model depicts minimum system inertia constraints as well as wind turbines and battery storage systems as a carbon-dioxide-free source for a synthetic inertial response. Results indicate that integrating system inertia constraints in energy system models has a high impact on indicators such as system costs. Especially when investments in additional storage units providing an inertial response are necessary. With respect to researched scenarios, system cost increases range from 1% up to 23%. The incremental costs for providing additional inertia varies between 0.002 EURO/kg·m2 and 0.61 EURO/kg·m2.

Emulation mechanisms of the physical synchronous generators obtained through convert-interfaced units are becoming extremely popular in literature. Still the impact of these strategies on networks frequency transients often does not take into account the effect of the converter DC-bus regulation, that plays a significant role in the dynamics definition. Thus two of the main synthetic inertia architectures are analysed both mathematically and experimentally in this article: differently from the available literature, the goal is not just to compare their macroscopic effects on network behaviour, but rather to identify the impact of the multiple internal control feedbacks on the inertia provision itself. [ABSTRACT FROM AUTHOR]

This paper summarizes the results of a power system stability analysis realized for the EU project OSMOSE. The case study is the electrical network of Sicily, one of the two main islands of Italy, in a scenario forecasted for 2050, with a large penetration of renewable generation. The objective is to establish if angle and voltage stabilities can be guaranteed despite the loss of the inertia and the regulation services provided today by traditional thermal power plants. To replace these resources, new flexibility services, potentially provided by renewable energy power plants, battery energy storage systems, and flexible loads, are taken into account. A highly detailed dynamical model of the electrical grid, provided by the same transmission system operator who manages the system, is modified to fit with the 2050 scenario and integrated with the models of the mentioned flexibility services. Thanks to this dynamic model, an extensive simulation analysis on large and small perturbation angle stability and voltage stability is carried out. Results show that stability can be guaranteed, but the use of a suitable combination of the new flexibility services is mandatory.

Power system inertia is an essential part for grid frequency stability and decreases due to the replacement of fossil fuel fired power plants with variable renewable energy sources. This development is not represented sufficiently in unit commitment and economic dispatch models. If considered at all, only synchronous inertia from fossil fuel driven power plants is modelled. This results in increased CO2 emissions, curtailed renewable energy and high system costs. While wind turbines are a source for synthetic inertia and an important renewable energy source, their capability to provide inertia is not incorporated into energy system models. The work at hand closes this research gap and applies a methodology to depict synthetic inertia provided by wind turbines as part of the optimisation dispatch model. A unit commitment and economic inertia dispatch model of the the all-Island Irish power system is created. The potential of wind inertia is analysed and quantified by assessing CO2 emissions, curtailed renewable energy and system costs. Results show that synthetic inertia provided by wind turbines can save up 30.99% of the CO2 emissions, reduce curtailment by up to 39.90% and reduce system costs by 32.72%.

Traditionally, power system's inertia has been estimated according to the rotating masses directly connected to the grid. Due to the significant penetration of renewable generation units, the conventional grid inertia is decreasing, subsequently affecting both reliability analysis and grid stability. As a result, concepts such as 'synthetic inertia', 'hidden inertia' or 'virtual inertia', together with alternative spinning reserves, are currently under discussion. Under this new framework, an algorithm to estimate the minimum inertia needed to fulfil the European network of transmission system operators for electricity requirements for rate of change of frequency values is proposed and assessed. Both inertia and additional active power can come from different sources, such as storage solutions, renewable sources decoupled from the grid, interconnections, or a combination of them. The power system under consideration includes thermal, hydro‐power plants, and renewable generation units, in line with the most current and future European power systems. More than 700 generation mix scenarios are simulated, varying the renewable integration, the power imbalance, and the inertia constant of conventional power plants. The solutions studied here provide important information to ease the massive integration of renewable resources, without reducing the grid capacity in terms of stability and response to contingencies. [ABSTRACT FROM AUTHOR]

The undergoing transition from conventional to converter-interfaced renewable generation leads to significant challenges in maintaining frequency stability due to declining system inertia. In this paper, a novel control framework for Synthetic Inertia (SI) provision from Wind Turbines (WTs) is proposed, which eliminates the secondary frequency dip and allows the dynamics of SI from WTs to be analytically integrated into the system frequency dynamics. Furthermore, analytical system frequency constraints with SI provision from WTs are developed and incorporated into a stochastic system scheduling model, which enables the provision of SI from WTs to be dynamically optimized on a system level. Several case studies are carried out on a Great Britain 2030 power system with different penetration levels of wind generation and inclusion of frequency response requirements in order to assess the performance of the proposed model and analyze the influence of the improved SI control scheme on the potential secondary frequency dip. The results demonstrate that the inclusion of SI provision from WTs into Unit Commitment (UC) can drastically impact the overall system costs. [ABSTRACT FROM AUTHOR]

Summary: This paper proposes an optimization approach for the participation of wind turbines in large scale power systems. The wind turbine participation optimization aims to regulate the droop slopes and the synthetic inertia of the aggregated wind turbines to command the power contribution of wind turbines proportional to their power ratings in the primary frequency response schemes. A reduced second order model of power system is firstly presented to simplify the analysis of large scale power system. Meanwhile, the internal dynamics of general wind turbines are analyzed and the concept of stability margin is proposed to describe the wind turbines' capability of enduring frequency dips at the bus node. A convex optimization based approach is then presented to regulate the droop slopes and synthetic inertia of the wind turbines incorporating their stability margin. Five case studies are conducted in New England 39‐bus system, IEEE 118‐bus power system and the Polish 3375‐bus system. The simulation results indicate that the proposed approach is capable of transiting the bus frequency to the new steady state in a smoother and more stable manner. [ABSTRACT FROM AUTHOR]

It is well-known that wind power plants can provide short-term frequency control responses; however, when wind turbines are operated to extract maximum power from the wind, any additional energy supplied to the grid in response to the frequency event must be returned to the wind turbine to accelerate it back to optimum speed. The wind turbine must temporarily lower its power output to facilitate acceleration back to optimum speed. In power systems that have many wind turbines providing short-term frequency control services, the reduction in power generation associated with optimum speed recovery could be significant enough to cause a secondary frequency dip shortly after the initial dip, the nadir of which could be lower than the initial dip. This paper proposes 1) an improved speed-recovery scheme for wind turbines that provide synthetic inertia, and 2) a coordinated control scheme for wind turbines and BESSs to mitigate the secondary frequency dip that can occur during the synthetic inertia speed-recovery phase. The control schemes presented can be easily integrated into existing wind turbine and BESS active power controllers as implementation can be achieved using only the active power and rotational speed measurements that are available in the wind turbine's active power controller. [ABSTRACT FROM AUTHOR]

In many electrical grids, the share of renewable energy generation increases. As these generators are typically connected to the grid via inverters, the level of grid inertia decreases. Such grids may therefore suffer from high rates of change of frequency during power imbalances. Modern wind turbines can help in controlling the frequency in such grids by providing synthetic inertia. A controller to provide synthetic inertia with wind turbines was developed at the Wind Energy Technology Institute in collaboration with Suzlon Energy. For this controller the influence of providing synthetic inertia on the mechanical loads of the wind turbine is assessed for different grid frequency scenarios. Such a scenario-based load analysis has not been published before, especially as the scenarios are derived from real measurements. The effect of the loads strongly depends on the analyzed grid frequency behavior. Ten months of high quality grid frequency measurements of the Indian grid are analyzed in order to derive inputs for the load calculation. Different types of grid frequency abnormities are identified and categorized with respect to their severity. Based on the observed occurrences of the grid frequency abnormities, realistic scenarios for the load calculations are chosen. The load calculations are performed for a state-of-the-art Suzlon wind turbine generator. The load increases caused by the supply of synthetic inertia are calculated for individual components assuming an otherwise undisturbed power production of the wind turbine in turbulent wind. Furthermore, a hardware-in-the-loop test bench is used to show how the measured grid frequencies are actually perceived by the control system of a typical wind turbine. The original frequency data were recorded with high quality measurement equipment, which is faster and more accurate than a multi-function relay, often used in wind turbines. For exemplary time traces, the effect of the reduced measurement accuracy on the reaction of the wind turbine is shown. This aspect has not been investigated in the literature yet. The results show that wind turbines can provide synthetic inertia without a considerable effect on the lifetime of the wind turbine. However, there are still problems with providing synthetic inertia reliably at high power operating points, which have to be solved.

High penetration of renewable energies through fast-response power converters results in a considerable displacement of conventional synchronous generators and losing of system inertia for frequency control. Modular multilevel converters (MMCs) can be employed serving as an interface between the large-scale renewable generation and power grids. In a microgrid with high shares of renewables integrating through MMCs, submodule (SM) capacitors can be used as energy storage to provide a degree of synthetic inertia for system frequency support. This paper quantitatively exploits the MMC synthetic inertia in a microgrid with flexible renewable penetration levels. An MMC inertia coefficient concept is proposed. It is mainly affected by the penetration ratio, the SM capacitance, and the modulation index of an MMC, with the system operation constraints. According to the analysis, a substantial portion of system inertia can be provided by properly designed MMCs. Detailed MMC frequency control loops are presented. The capacitor average voltage is proportionally linked to the frequency deviation, in order to flexibly adjust the capacitor energy during frequency events. The MMC output active power is deliberately and simultaneously regulated according to the capacitor energy change rate. By controlling the MMC capacitor voltage, dc side power, and output active power, an amount of energy can be released or absorbed by SM capacitors to improve the system frequency response during the frequency event transients, while the renewable generation is scarcely influenced. The proposed concept is experimentally verified and the results show that, with proper system parameters and control loops, the line frequency deviation and the rate of change of frequency can be significantly reduced. Good agreements of the system frequency characteristics have been achieved between the theoretical calculation and experimental results. [ABSTRACT FROM AUTHOR]