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The energy transition towards renewables must be accelerated to achieve climate targets. To do so, renewable power plants, such as wind power plants (WPPs) must replace conventional power plants (CPPs). Transmission System Operators require this replacement to be made without weakening the frequency response of power systems, so it must be ensured that WPPs match the response of CPPs to grid frequency variations. CPPs consist of grid-tied synchronous generators that inherently react to frequency variations by modifying their stored kinetic energy and their output power, thereby contributing to grid stability. Such response is known as inertial response. By contrast, wind turbines (WTs) are mostly based on either doubly-fed induction generators (DFIG) or permanent magnet synchronous generators (PMSG). Their power electronics interface decouples the electromechanical behavior of the generator from the power grid, leading to a negligible inertial response. Therefore, in order to replace CPPs with WPPs, WTs must be able to react to frequency variations by changing their output power, i.e., emulating an inertial response. Currently implemented inertia emulation strategies in WTs rely on pitch control and stored kinetic energy variation. This paper proposes an alternative strategy, using the energy stored in a supercapacitor directly connected to the back-to-back converter DC link to emulate inertia. Its performance is validated by means of simulation for both DFIG and PMSG. Compared to state-of-the-art techniques, it allows a more accurate emulation of grid-tied synchronous generators, favoring the replacement of these generators by WTs. This work was supported in part by Siemens Gamesa Renewable Energy, in part by the Spanish State Research Agency, AEI/10.13039/501100011033 under Grants PID2019- 111262RB-I00 and PID2019-110956RB-I00, and in part by the Government of Navarre under Grant 0011-1411-2022-000049 through EnVeNA Project.

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.

Bulk integration of Distributed Energy Resources (DERs) into the power grids reduces the system’s inherent inertial response.The reduced inertial response causes a high Rate-of-Change-of-Frequency (RoCoF) and poses formidable operational challenges for the grid frequency stability. Interconnections around the world comprehend the role and value of the Synthetic Inertia Control (SIC), which is considered as a subset of the Fast Frequency Response (FFR) and as one of the potential solutions to arrest high RoCoF in low- inertia power systems. This paper proposes an intelligent SIC model with an adaptive Fuzzy Logic Controller for a low-inertia microgrid. The proposed Fuzzy-SIC (FSIC) design optimizes the DER output to fulfill the FFR requirements of the system for various operating conditions. The particle swarm optimization algorithm is applied to tune the SIC unit parameters along with the secondary Proportional-Integral-Derivative control. The proposed approach is examined in a control area with distinct degrees of DERs and load. Case studies and numerical results demonstrate about 87% improvement in RoCoF and frequency nadir in comparison to the system without a synthetic inertia emulation. Furthermore, the robustness of the proposed approach is evaluated using various case studies and a time-domain analysis is conducted to demonstrate the impact of incremental SIC parameters on the system parameters.

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.

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.

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.

The increasing interest in renewable energy has significantly increased in the last decades. The increasing amount of variable renewable energy resources in the grid, which are connected via power electronics, reduces the total mechanical system inertia. Frequency-regulating resources such as hydropower will become more important in balancing variable renewable energy resources, setting higher requirements on stability and performance to maintain a stable electrical grid. This thesis concerns the decreased mechanical inertia from non-directly electrically coupled generation units. The thesis starts with a description of the grid system inertia situation today and presents two methods for estimating the grid frequency derivative used to provide synthetic inertia and one method used to enhance the mechanical inertia response of a synchronous generator. The synthetic inertia and enhanced inertia methods are tested in a small-scale experimental setup and compared with results from tests in the Nordic grid. A full-scale hybrid energy storage system was designed and built using a split frequency method as a power controller. The results show that a power-frequency derivative controller-based synthetic inertia method achieved an improved grid frequency quality during regular operation in the nano-grid experimental setup. The results are evaluated both via simulations and experimental tests. The results from the hybrid energy storage solution showed the possibility of increasing frequency quality by using a slow run of the river hydroelectric power plants and a battery energy storage system for frequency containment reserve.

In den vergangenen Jahren wurden die Auswirkungen des Klimawandels, in Form von extremen Wetterereignissen sowie Veränderungen unseres Planeten, zunehmend wahrnehmbar. Um dieser Entwicklung entgegenzuwirken, werden vor allem im Energiesektor immer mehr Anstrengungen unternommen, um die Erzeugung, den Transport sowie den Verbrauch elektrischer Energie möglichst umweltschonend zu gestalten. Diese Bemühungen beziehen sich neben etwaigen Effizienzsteigerungen vor allem auf den Ausbau von erneuerbaren Energieerzeugern, wie Wind und Photovoltaik. Da solche erneuerbaren Energieerzeuger einen hohen Grad an Volatilität ins System bringen und meist keinen wesentlichen Beitrag zur Systemstabilität leisten, stellt ihre erfolgreiche Integration eine große Herausforderung für die Aufrechterhaltung der Frequenzstabilität des Systems dar.Diese Dissertation behandelt mit der Einführung von schnellen Regelleistungen eine Möglichkeit, um die Frequenzstabilität im kontinentaleuropäischen Verbundnetz trotz der voranschreitenden Transformation des Systems auch zukünftig gewährleisten zu können. Zu diesem Zweck werden vier verschiedene schnelle Regelleistungen vorgestellt und in Bezug auf deren Auslegung und Parametrierung im Detail untersucht. Darüber hinaus wird auch eine Betrachtung des kombinierten Einsatzes dieser schnellen Regelleistungen sowie der sich aus Sicht der erbringenden Anlagen ergebenden Anforderungen vorgenommen. Schließlich werden in dieser Arbeit, aufgrund ihres großen Einflusses auf die Systemstabilität, auch sogenannte Deterministic Frequency Deviations sowie mögliche Maßnahmen gegen diese betrachtet.Die enge Verknüpfung der in dieser Arbeit betrachteten Fragestellungen mit verschiedenen Forschungsprojekten sowie die Implementierung von ähnlichen hochdynamischen Regelleistungen in anderen Synchrongebieten zeigen die praktische Relevanz dieser Untersuchungen. Entsprechend sollen die in dieser Dissertation präsentierten Ergebnisse als Ausgangspunkt für die Auslegung, Parametrierung und Integration von neuen schnellen Regelleistungsprodukten im kontinentaleuropäischen Verbundsystem dienen., The extreme weather conditions as well as the changing landscape of our planet in recent years have made the effects of the ongoing climate change increasingly evident. In order to counteract this development sizeable efforts are being made, especially in the energy sector. The main goal of these actions is to make the generation, transport, and consumption of electricity as environmentally friendly as possible. The means to achieve these goals include efficiency improvements as well as a substantial extension of renewable energy sources, such as wind and solar energy. However, since renewable energy sources introduce a high degree of volatility into the system and they usually do not contribute to system inertia, their successful integration poses a major challenge for maintaining frequency stability.This thesis proposes the introduction of fast frequency control services as a way to cope with the ongoing transformation of the system and to ensure future frequency stability in the synchronous area Continental Europe. For this purpose, four different fast frequency control services are presented and examined in detail with regard to their design and parametrization. Furthermore, this thesis investigates the combined use of these fast frequency control services as well as the resulting requirements from the point of view of the providing units. Finally, due to their impact on system stability, this thesis also considers so-called Deterministic Frequency Deviations and possible measures against them.The practical relevance of the conducted investigations in this thesis is underlined by the research projects connected to it, as well as the implementation of similar highly dynamic frequency control services in other synchronous areas. Therefore, the results of this thesis shall serve as a starting point for the design, parametrization, and integration of new fast frequency control services in the synchronous area Continental Europe.

Reconstructable dynamic simulation models of modern variable-speed wind turbines (WTs), which are integrable into any simulation software, are crucial to the scientists investigating the contribution of WTs to counteracting the current power system stability issues. The structural similarity between a doubly fed induction-generator-based (DFIG-based) WT model and a full-scale-convertor-based (FSC-based) WT model using induction generator offers the possibility of integrating them into a combined modular model with little effort and the same used parameter set. This article presents a combined root mean square (RMS) WT model, which contains a DFIG-based WT and a FSC-based WT using induction generator. The model is designed based on fundamental machine and converter equations and can be applied for classical network stability analyses. Furthermore, analogous well-performing initialization procedures for both DFIG-based and FSC-based WT models are also introduced. As an example, to demonstrate the performance of the WT model in frequency stability studies, the model is extended with a droop-based fast frequency response (FFR) controller and is implemented in a MATLAB-based RMS simulation tool. The results of the case studies confirmed a solid functionality of initialization procedures. Furthermore, they illustrate feasible and comparable general behavior of both WT models as well as their plausible responses in the event of a frequency drop in a 220 kV test system.

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.

This work examines the operation of the autonomous power system of a geographical island assuming the integration of significant generation shares from renewable energy sources and the installation of the required storage systems. The frequency stability of the system is investigated considering different operating conditions, in terms of load demand and renewable power generation. The main focus of the work is an original control strategy specifically designed for power converters interfacing storage units to the grid. The proposed strategy is based on an extended frequency droop control, which selects specific droop settings depending on the operating mode—charge or discharge—of the storage unit. A simulation model of the whole electrical system is developed for dynamic analysis. The model also implements the possibility of including specific auxiliary frequency controls for synthetic inertia and primary reserve. The results of the simulation and analysis indicate that the proposed control strategy has a significant positive effect, making the storage units able to provide a fundamental and more effective support to the frequency stability of the system. The application of the proposed control strategy to storage units also reduces the need for a contribution to the frequency control from intermittent and variable sources, making the whole system more robust, stable and reliable.

The paper presents the results of a theoretical study on the behaviour of a battery storage system operated as a Fast Reserve Unit and equipped with additional synthetic inertia control. The Fast Reserve Unit is assumed connected to the transmission system of Sicily, operated as an isolated grid in order to show more clearly the effect of the Fast Reserve Unit intervention during a power imbalance. The unit is controlled also to provide synthetic inertia with a conventional control scheme and with a new scheme proposed by the authors and named “Modular Synthetic Inertia”. The latter has been conceived for offering a modular response as a function of the Rate of Change of Frequency, to avoid short repetitive charging/discharging of the battery.

Inertia reduction due to high-level penetration of converter interfaced components may result in frequency stability issues. The paper proposes and analyzes different strategies using synchronous condenser (SC), synthetic inertia (SI) of wind power plant, and their combination to enhance the frequency stability of low inertia systems under various scenarios and wind conditions. Furthermore, one of the SC models includes hardware of automatic voltage regulator (AVR) for better representation of the reality is implemented. The simplified Western Danish power system simulated in real time digital simulator (RTDS) is used as a test system of low inertia to demonstrate the effectiveness of the strategies. The comparative results show that the combination of SC with AVR hardware-in-the-loop test and SI offers a betterimprovement not only on frequency stability (rate of change of frequency and frequency deviation) but also on the system synchronism under various operating conditions.

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.

In this paper, the synthetic inertia need of the small island of Pantelleria in the Mediterranean Sea is assessed. Firstly, the optimal renewable energy mix able to minimize the Levelized Cost of Energy for the generation system of the island is evaluated, considering the yearly load demand and the characteristics of the local natural resources. The optimal energy mix introduces in the island a quota of power generation with static power converters interfaced to the grid that could jeopardize the security of the system if not suitably controlled. Therefore, the inherent inertial response of the diesel generators of the system is calculated hour by hour for each month of one year, considering the new schedule of the generators for various cases characterized by different percentages of production from the optimum renewable energy mix. Finally, the rated power and capacity of a battery energy storage system is calculated in order to compensate the reduction of the inertial response in the power system thanks to a suitable synthetic inertia control. The calculation is done by considering the maximum allowed frequency nadir in case of a disturbance. For each scenario, the maximum disturbance is also assessed with and without synthetic inertia. The results are compared with those obtained in a previous study on the same island where batteries were used for different purposes.

Baigiamojo magistro darbo tikslas - atlikti sintetinės inercijos, kuriamos baterijų energijos kaupimo sistemos, įtakos analizę elektros sistemos dažninėms charakteristikoms. Šis baigiamasis magistro tyrimas sudarytas iš trijų pagrindinių dalių. Visų pirma, pirmajame darbo skyriuje pateikiama literatūros apžvalga. Joje aptariami kitų autorių moksliniai darbai apie natūraliai susidarančios elektros sistemos inercijos mažėjimo priežastis, pasekmes bei problemos sprendimo būdus naudojant sintetinę inerciją, išgaunamą iš baterijų energijos kaupimo sistemų. Antroje dalyje aprašomas tiriamosios elektros sistemos modelis. Taip pat pateiktos modelyje naudojamos elementų bei jų valdymo sistemos, jas aprašančios lygtys bei schemos. Galutinėje, trečioje tyrimo dalyje, pateikiami bei analizuojami gauti darbo rezultatai. Juose atsispindi sinchroninių generatorių, sintetinės inercijos funkcijos bei jos valdiklio kintamųjų įtaka elektros sistemos dažnio charakteristikoms. Pabaigoje pasitelkus gautų rezultatų duomenis suformuotos tyrimo išvados., The aim of Master‘s final degree thesis – to perform impact analysis of the synthetic inertia developed by the battery energy storage system on frequency characteristics of the electric power system. This research of Master‘s final degree consists of three main parts. First of all, the first chapter provides a review of the literature. It discusses the scientific work of other authors on the causes and consequences of the reduction of inertia by naturally occurring from electric power system and the ways of solving the problem using synthetic inertia extracted from battery energy storage systems. The second part describes the model of the investigated electric power system. The elements used in the model and their control systems, equations and schemes describing them are also presented. In the final, third part of the research, the obtained work results are presented and analyzed. They reflect the influence of synchronous generators, function of synthetic inertia and its controller variables on frequency characteristics of the electric power system. At the end, the research conclusions were formed using the data of the obtained results.

Power system inertia is an essential part of grid frequency control. The number of synchronously connected machines, which inherently provide inertia, is decreasing due to the transition to renewable energies. Conventional generation units are being replaced by renewable generation units which are connected to the grid via frequency converters. Some power systems already suffer from too little power system inertia. Hence, inertia is a valuable yet non-traded commodity. A day-ahead dispatch methodology to secure power system inertia was developed and is applied and assessed in this work. Day-ahead market data of the combined market of the Republic of Ireland and Northern Ireland is used. If the superimposition of sell and buy bids results in insufficient inertia, the dispatch algorithm is applied. In decreasing price order, non-inertia-providing sell bids get replaced by the following sell bids in the merit order. The iterative process is repeated until sufficient inertia is in the system. The provision of synthetic inertia by wind turbines is considered in the process. The costs for additional stored kinetic energy for the assessed time periods and scenarios result in costs ranging from 1.02 to 4.49 EUR/kgm2.

Battery Energy Storage Systems (BESS) based on Li-Ion technology are considered to be one of the providers of services in the future power system. Although prices for Li-Ion batteries are falling continuously, it is still difficult to achieve profitability from a single service today. Multi-use operation of BESS in order to reach a so-called “value-stacking” of services therefore is a hotly debated topic in literature, since such an operation holds the potential to increase profitability dramatically. The multi-use operation of a BESS can be divided into two parts: the operational planning phase and the real-time operation. While the operational planning phase has been examined in many studies, there seems to be a lack of discussion for the real-time operation. This paper therefore tries to address the topic of the real-time operation in more detail. For this reason, this paper discusses concepts for implementing a real-time multi-use operation and introduces the novel concept of dynamic prioritization, which allows resolving conflicts of services. Besides the ability to cope with abnormal grid conditions, this concept also holds potential for a better utilization of resources during normal grid conditions. A mathematical framework is used to describe several services and their interaction, taking into account the concept of dynamic prioritization. Several applications are presented in order to demonstrate the behavior of the concept during normal and abnormal grid conditions. These applications are simulated in Matlab/Simulink for specific events and in the form of long-time simulations.

Battery Energy Storage Systems (BESS) based on Li-Ion technology are considered to be one of the providers of services in the future power system. Although prices for Li-Ion batteries are falling continuously, it is still difficult to achieve profitability from a single service today. Multi-use operation of BESS in order to reach a so-called “value-stacking” of services therefore is a hotly debated topic in literature, since such an operation holds the potential to increase profitability dramatically. The multi-use operation of a BESS can be divided into two parts: the operational planning phase and the real-time operation. While the operational planning phase has been examined in many studies, there seems to be a lack of discussion for the real-time operation. This paper therefore tries to address the topic of the real-time operation in more detail. For this reason, this paper discusses concepts for implementing a real-time multi-use operation and introduces the novel concept of dynamic prioritization, which allows resolving conflicts of services. Besides the ability to cope with abnormal grid conditions, this concept also holds potential for a better utilization of resources during normal grid conditions. A mathematical framework is used to describe several services and their interaction, taking into account the concept of dynamic prioritization. Several applications are presented in order to demonstrate the behavior of the concept during normal and abnormal grid conditions. These applications are simulated in Matlab/Simulink for specific events and in the form of long-time simulations.

Along with the increasing share of non-synchronous power sources, the inertia of power systems is being reduced, which can give rise to frequency containment problems should an outage of a generator or a power infeed happen. Low system inertia is eventually unavoidable, thus power system operators need to be prepared for this condition. This paper addresses the problem of low inertia in the power system from two different perspectives. At a system level, it proposes an operation planning methodology, which utilises a combination of power flow and dynamic simulation for calculation of existing inertia and, if need be, synthetic inertia (SI) to fulfil the security criterion of adequate rate of change of frequency (RoCoF). On a device level, it introduces a new concept for active power controller, which can be applied virtually to any power source with sufficient response time to create synthetic inertia. The methodology is demonstrated for a 24 h planning period, for which it proves to be effective. The performance of SI controller activated in a battery energy storage system (BESS) is positively validated using a real-time digital simulator (RTDS). Both proposals can effectively contribute to facilitating the operation of low inertia power systems.

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.

The increasing wind power penetration in electrical power systems results in a reduction of operative conventional power plants. These plants include synchronous generators directly connected to the grid. Facing a change in grid frequency, these generators inherently respond by varying their stored kinetic energy and their output power, which contributes to grid stability. Such a response is known as inertial response. Wind turbines (WTs) are mostly based on Doubly-Fed Induction Generator (DFIG) or Permanent Magnet Synchronous Generator (PMSG) machines. Their power electronics interface decouples the electromechanical behaviour of the generator from the power grid, making their inertial response null or insignificant. Therefore, in order not to weaken the frequency response of the power system, WTs must be able to react to frequency variations by changing their output power, i.e., emulating an inertial response. Common techniques for inertia emulation in WTs rely on pitch control and stored kinetic energy variation. This contribution proposes a strategy (applicable for both DFIG and PMSG) which uses the energy stored in a supercapacitor connected to the back-to-back converter DC link to emulate the inertial response. Its performance is compared by simulation with aforementioned common techniques, showing ability to remove certain limitations. This work has been supported by Siemens Gamesa Renewable Energy, by the Spanish State Research Agency (AEI/ 10.13039/501100011033) under grants PID2019-111262RB-I00 and PID2019-110956RB-I00, and by the Public University of Navarra under project ReBMS PJUPNA1904

Orientadora: Profa. Dra. Ahda Pionkoski Grilo Pavani Dissertação (mestrado) - Universidade Federal do ABC, Programa de Pós-Graduação em Energia, Santo André, 2021. Diferentemente, dos geradores síncronos convencionais, os geradores eólicos, devido ao acoplamento à rede elétrica por conversores de eletrônica de potência, não apresentam resposta inércial. O crescente aumento de geração eólica conectada à rede elétrica provoca a redução da inércia do sistema, resultando em excursões de frequência maiores em casos de distúrbios no sistema. Com isso, um requisito que vem sendo adotado pelos operadores do sistema de vários países, inclusive já está presente no Brasil, denominado de inércia sintética, requer suporte de frequência pelos geradores eólicos. Este requisito tem o objetivo de aumentar o valor da frequência mínima alcançada pelo sistema em caso de distúrbios. Para provimento deste suporte, os geradores injetam parte da energia cinética de seu rotor na rede elétrica até que os reguladores de velocidade dos geradores síncronos restabeleçam a frequência. Após este restabelecimento ou quando a contribuição de potência do gerador eólico é retirada, os geradores eólicos atingen um valor abaixo do ponto de operação predisturbio e devem recuperar o ponto de operação perdido devido à energia liberada durante o período de suporte de frequência. Neste trabalho, é realizada uma análise do suporte de frequência realizado por geradores eólicos que empregam geradores de indução duplamente alimentados, considerando o método de Controle Inércial progressivo (SIC) empregado em geradores eólicos para provimento de inércia sintética, dando ênfase no requisito atualmente em vigência pelo Operador Nacional do Sistema. Para isso, são realizadas simulações no ANATEM, software do CEPEL. Unlike conventional synchronous generators, wind generators, due to the connection to the electrical network by power electronics converters, do not present an inertial response. In this sense, reducing the system's inertia results in higher frequency excursions in cases of system disturbances. As a result, a requirement that has been adopted by system operators in several countries, which is already present in Brazil, called synthetic inertia, requires frequency support from wind generators. This requirement is intended to increase the value of the minimum frequency reached by the system in the event of disturbances. To provide this support, the generators inject part of the kinetic energy of their rotor into the electrical network until the speed regulators of the synchronous generators restore the frequency. After this restoration or when the power contribution of the wind generator is withdrawn, the wind generators reach a value below the pre-disturbance operating point and must recover the operating point lost due to the energy released during the frequency support period. In this work, an analysis of the frequency support performed by wind generators that employ double fed induction generators is performed. Considering the stepwise inertial control (SIC) method used in wind generators for synthetic inertia, with emphasis on the current National System Operator requirements. For that, simulations are carried out in ANATEM, CEPEL software.

This paper explores the impact of ultracapacitor (UC) modelling on fast frequency response performance. The dynamic model of the UC bank is based on the nonlinear dynamic model of the UC cell which takes into account the dependence of the UC parameters on the applied DC voltage. The complete system is modelled and tested on IEEE 39-bus system in DIgSILENT PowerFactory for overfrequency and underfrequency events. The performance of the realistic UC model is compared against an ideal ultracapacitor model to show that the ideal representation is not always appropriate, although for most cases the ideal model will suffice.

Providing fast frequency regulation by means of energy storage systems is currently considered as a viable solution to low-inertia issues, caused by power electronics-interfaced generators. In particular, hybrid energy storage systems, composed by more energy storage technologies having different power and energy ratings, can optimally support the frequency regulation. A supercapacitor/battery storage system, for example, can exploit the supercapacitor dynamic active power response for synthetic inertia control, while the battery can provide primary and secondary frequency regulation. However, the optimal energy management of hybrid energy storage systems during transients needs to be addressed further in literature. In this paper, a State of Charge (SOC) feedback control scheme is proposed, that adjusts the active power output reference depending on the state of charge, avoiding excessive stress on the components and limiting the state of charge excursions. Control system parameters are optimally tuned minimizing a weighted multi-objective function in the solution of an optimal control problem. Test results adopting different weights are presented and discussed.

There is a trend in Sweden towards increasing the electricity production from renewable energy sources in the electric grid. The increased share of renewables could be seen as essential for Sweden to be able to meet the obligated climate goals. Integration of renewables will enable Sweden to be a progressive part in reducing greenhouse gases and decreasing the global warming. However, one issue with renewable energy sources is the inverter governed production. This, together with the decommission of larger synchronous generators, results in decreasing the inertia and increasing the instability inthe grid. This thesis is dedicated to elaborating on frequency stability issues and investigating how Variable Speed Wind Turbines (VSWT) could contribute towards stabilized operation when included in frequency support services. The study is generated through an extensive research process where focus areas are identified. Questions are purposed and then discussed through interviews with experienced people in the field. Estimated power production series from a wind turbine park (WTP) are applied in a constructed model to study the possibilities appearing when including VSWTs in frequency support services. The income generated from including VSWTs in different regulation power market services is with the model compared against solely procuring the production capacity on the day ahead market. The studied frequency support services are then compared altogether to generate favorable solutions. The study examines both economic as well as technical features of the inclusion of VSWTs in frequency support services. Results found in the study were that inclusion of wind turbines for power regulation purposes could be seen as increasingly manageable and needed in the electric grid. The maturity of technical solutions alongwith a transition in the regulation power market could be observed as leading factors. The diversification of regulation abilities and the increasing economic incentives in the regulation power market was also found to be important reasons for including wind turbines in the regulation power market. In the study, it was also found thataggregating the power production from several VSWT could increase the ability to deliver the studied services. It was concluded that inclusion of VSWT in the frequency containment reserve during disturbed operation for down-regulation purposes as well as the fast frequency reserve was the most promising frequency support products for the inclusion of wind turbines. When including battery energy storages and to a larger part managing the durability demands for the services then the frequency contain reserve for normal operation and the frequency containment reserve for disturbed operation for upregulation purposes could be observed as preferable alternatives. Regulation abilities were concluded as achievable with the use of pitch and torque regulation techniques available in the variable speed wind turbine.

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.

The power grid faces stability problems due to loss of inertia. The ancillary services balancing the system must be improved to maintain stability. In contrast to earlier studies, this thesis estimates how the future grid frequency quality in both the short and long term is affected by different control methods, using an ensemble of quality measures. The thesis uses conventional one-area models for the power grid and FCR-N (frequency containment reserve – normal operation) but develops new models for FCR-D (– disturbed operation), FFR (fast frequency reserve), and synthetic inertia (SI). To acquire proper input data the thesis uses an inverse model of the power grid to compute the momentary load disturbance, i.e., the difference between load and generation, from the grid frequency. The thesis makes a difference between proportional and derivative SI, where the latter is the one commonly associated with the term SI. The results show that derivative SI can improve some quality measures but requires very high power capacity. In contrast, proportional SI improves almost every measure the most, for a fraction of the required capacity. Derivative SI is therefore worth less from a system perspective. However, it is shown that the quality measures improved by derivative SI are related to hydropower wear and tear, making it more interesting for hydro power owners. Moreover, FFR gives no benefit to long time quality measures. Considering the short-term analysis where a large disturbance occurs, FFR gives almost no benefit and risks worsening the stability. Also, for the large disturbance, proportional SI performs the best. The results are limited by the small number of simulations that were performed and can only provide indications of trends. A more solid conclusion, however, is that one cannot expect transmission system operators (TSO) to introduce an ancillary service that builds on derivative SI.

The overall kinetic energy reduction due to increasing share of converter-interfaced generation (CIG) has been investigated so far by several works for systems of different types and sizes. The so called synthetic inertia is one of the most common and reviewed solutions to the issue of system inertia reduction. The paper investigates the effects of synthetic inertia as frequency service provided by the CIG sources for the Continental Europe (CE) synchronous area using a detailed large scale dynamic model. The model - provided in the basic form by the System Protection & Dynamics (SP&D) group of the ENTSOE - is properly developed substituting a determined amount of synchronous machines with CIG sources. The CIG sources implemented the typical current vector control and the inclusion of synthetic inertia schemes. The system is then analyzed through combined time-domain simulations and modal analysis computations, exploring the effects of CIG sources participation to the frequency response of the CE system when delivering synthetic inertia against power unbalances. The specific impact on the inter-area oscillations of the system is recognized and discussed.

In small–scale power systems, distributed virtual inertia (DVI)–based converters can effectively participate in primary frequency regulation. In this method, the synthetic inertia provision is fulfilled by discharging the preserved energy of dc–side capacitors employed in grid–interactive converters. Nevertheless, the eigenvalue analyses provided by small–signal state–space model of the converter reveal that the DVI function induces instability to the converter operating in weak grid connection. To address this issue aimed at improving grid frequency stability during frequency events, a new compensator is presented in this work. The compensator is designed so as eliminates the negative effect of DVI regulator on system stability. The effectiveness of proposed approach is illustrated by the time–domain simulations in MATLAB. The results show that the frequency rate of change following a frequency perturbation is enhanced by 40.38% compared with the scenario in which the synthetic inertia functionality is nullified.

The paper presents a study of primary frequency control within the power system of Continental Europe (CE) including the dynamics of the high voltage direct current (HVDC) link between France and Great Britain (GB). The large-scale dynamic model of the CE system originally provided by ENTSO-E is extended with a more detailed representation of the whole HVDC interconnection. The dynamics of the GB synchronous area is also considered in the analysis, implementing a system equivalent suitable for primary frequency control studies. The opportunity of frequency containment reserve exchange between the two areas through the link is also investigated, considering supplementary frequency controls within the HVDC control system, namely synthetic inertia and fast frequency response. The overall system is then simulated in two different scenarios: a base case of all synchronous generation, and a reduced inertia scenario with the introduction of a given amount of non-synchronous generation share. The reference incident for the CE system of 3 GW loss is considered for the analysis. The specific effects on the frequency dynamics of the system in the different operating conditions are illustrated and discussed.

The increasing amount of renewable energy sources in the electrical energy system leads to an increasing number of converter-based generators connected to the electrical power grid. Other than conventional power plants that are often connected to the grid via synchronous generators, converter-based generators do not provide mechanical inertia intrinsically. Therefore, ensuring frequency stability in the electrical power grid might become even more difficult in the future. With the concept of synthetic inertia, the converter-based generators partially imitate the behavior of conventional generators. By implementing such a concept in converters, they are capable of contributing to frequency stability as well. This paper compares two strategies to realize synthetic inertia by modeling converter-based generators in MATLABmathsizesmall/SIMULINKmathsizesmall and simulating their behavior in a small Microgrid. The results prove that any kind of realization of synthetic inertia helps to improve frequency stability. Each of the two investigated strategies may have their scope of application in a future electrical energy system.

The paper proposes a method to assess the equivalent inertial response provided by converter-interfaced generation equipped with fast frequency response capability and its impact on the minimum amount of rotational inertia required to guarantee power system's stability. The method is based on the solution of an optimization problem aimed at finding the minimum inertia required by the system to contain frequency and rate of change of frequency deviations within the limits required by system operators. The effectiveness of the proposed method is illustrated through the WSCC 9-bus test system.

In this paper, the implementation of an innovative synthetic inertia control mechanism for a wind generator is presented. Basically, it emulates the inertial response provided by traditional generators during frequency events, exploiting the energy content of the DC link, suitably sized. Such mechanism can be applied to whatever type of plant connected to the grid through a fully rated converter, provided that a sufficient energy buffer is available. This strategy can enhance the grid stability after under-frequency events, especially in low-inertia networks. The proposed control strategy has been first studied in a simplified network and then, to corroborate its effectiveness, tested on a portion of the Italian Transmission Network (Sicily).

A significant amount of conventional power plants in the European power system is anticipated to be replaced by solar and wind power in the future. This may require alternative sources for inertia support. The purpose of the paper is to learn about the consequences on the frequency deviation after a fault in the European power system when more wind and solar are introduced and when wind is considered as a possible provider of inertia. This study quantifies the expected maximum requirement for additional inertia in the future European power system up to 2050. Furthermore, we investigated the possibility of wind power to meet this additional need by providing emulated inertia. The European power system of the EU-28 countries has been clustered to the five synchronous grids, UCTE, Nordic, UK, Baltic and Irish. The future European energy mix is simulated considering twelve different scenarios. Production units are dispatched according to their expected environmental impacts, which closely follow the minimum natural contribution of inertia, in descending order. The available capacity for all the types of production is considered the same as the installed. For all the simulated scenarios the worst case is examined, which means that a sudden disconnection of the largest production unit of the dispatched types is considered. Case study results reveal that, in most cases, additional inertia will be required but wind power may fully cover this need for up to 84% of all simulated horizons among all the scenarios on the UCTE grid, and for up to 98%, 86%, 99% and 86% on the Nordic, UK, Baltic and Irish grids, respectively.

In this paper, a novel concept to integrate High Voltage Direct Current (HVDC)-connected offshore wind power plants with the onshore grid is presented. The concept makes use of a holistic wind farm controller along with a fully instrumented conventional synchronous generator at the point of common coupling. In our approach, the wind farm is able to replicate the natural response of the generator to a system, even enabling the wind farm to reproduce, in a scaled up manner, a range of ancillary services without having to rely on indirect frequency measurements which are prone to noise and delays. Simulation results are presented to validate the proposed solution.

Conceitos como a descentralização, a descarbonização e a democratização do uso da energia elétrica têm impulsionado e ocasionado um considerável aumento da geração distribuída. Em razão desse aumento e principalmente no que se refere às fontes intermitentes de energia, novos paradigmas e desafios aos sistemas de distribuição são propostos, com destaque para as microrredes. Essas estruturas reduzidas facilitam a integração das fontes distribuídas, dos sistemas de armazenamento e cargas locais, uma vez que são fundamentais para o avanço das chamadas redes inteligentes. Além disso, esses sistemas possuem a versatilidade de poderem operar tanto conectados à rede principal, ou de maneira ilhada. No entanto, apesar dos benefícios que essas arquiteturas apresentam, particularidades dessas redes devem ser consideradas, principalmente no que tange problemas de estabilidade, como a baixa inércia inerente destes sistemas e os eventos de transição entre os modos de operação. Os sistemas armazenadores de energia, principalmente as baterias, também têm se destacado, vivenciando um maior quadro quanto ao mercado e conjunto de aplicações. Apresentando-se dentre outras possibilidades, como ferramentas promissoras no que concerne aos desafios citados e a outros no que concerne à adequada operação das microrredes. Neste contexto, esta dissertação analisa por meio de simulações em ambiente Simulink/Matlab a contribuição que um sistema armazenador de energia, utilizando da tecnologia de lítio-íon, pode prover a estabilidade de frequência de uma microrrede frente a um distúrbio e sucessiva operação em modo ilhado. Para isso, utilizou-se um sistema teste baseado em uma microrrede real da Ilha de Ulleungdo, Coreia do Sul. Os resultados obtidos demonstram os benefícios que o sistema armazenador por baterias pode prover por meio dos controles por droop e inércia sintética, em relação ao desvio e taxa de variação de frequência da microrrede frente a eventos que ocasionem no desbalanço de potência ativa. Além disso, propõe-se também neste trabalho uma melhoria ao controle convencional por inércia sintética, em relação ao tempo de recuperação, um sistema de gerenciamento para controle da operação e manutenção do estado de carga do sistema armazenador. Concepts such as decentralization, decarbonization, and democratization of electricity use have an impact to increase distributed generation. Due to this growth and mainly concerning about intermittent sources of energy, new paradigms and challenges for distribution systems are proposed, with a highlight towards the microgrids. These reduced structures facilitate the integration of distributed sources, storage systems and local loads, being pointed as fundamental architectures for the advance of the so-called smart grids. In addition, these systems have the versatility to be able to operate either connected to the main grid or on an islanded mode. However, despite the benefits that these architectures present, one must also take into account the particularities of these systems, especially regarding their stability, such as the inherent low inertia of these systems and the transition events between modes of operation. Energy storage systems, especially batteries, have also attracted attention, experiencing a larger picture concerning the market and set of applications. Presenting itself among other possibilities, as promising tools with regard to the challenges mentioned and others concerning the proper operation of the microgrids. In this context, this work analyzes through simulations in Simulink / Matlab environment the contribution that an energy storage system, using lithium-ion technology, can provide the frequency stability of a microgrid in front of a disturbance and successive islanded mode operation. For this, it is used as a test system based on a real microgrid from Ulleungdo Island, South Korea. The obtained results demonstrate the benefits that the battery energy storage system employed can provide by means of droop and synthetic inertia controls, in relation to the deviation and rate of change of frequency of the microgrid before events that cause active power unbalance. Moreover, it is also proposed an improvement about synthetic inertia control over recovery time, a management system for controlling the operation and maintenance of the state of charge of the storage system. Dissertação (Mestrado)

The increasing penetration of renewable energy sources has been leading to the progressive phase-out of synchronous generators, which constitute the main source of frequency stability for electric power systems. In the light of these changes, over the past years, some power systems started to exhibit an odd frequency distribution characterised by a bimodal behavior. This results in increased wear and tear of turbine governors and, in general, in degraded frequency performances. This is a cause of concern for grid operators, which have become increasingly interested in understanding the factors shaping frequency distribution. This paper explores the root causes of unwanted frequency distributions. The influence of some main aggregate system parameters on frequency distribution is detailed. The paper also shows that the implementation of the so-called synthetic inertia can lead to a robust unimodal frequency distribution.

Variable renewable energy sources are today increasingly integrated in the power system as a step towards the renewable society. The large-scale introduction of converter-based energy sources brings challenges in terms of reduced damping to the power system due to the reduced number of synchronous generators. This can be manifested as high rate-of-change-of-frequency and decreased grid stability. To forestall this reduced performance, it is suggested that the grid-following control of today’s converters are restructured to a grid-forming control, enabling the converter to behave closer to a synchronous machine. This thesis compares grid-following and grid-forming control and seeks to further describe this grid-forming behavior by applying a grid-forming control method on an energy storage enhanced STATCOM-system. A continuous time model and a linearized model based on state space representations are constructed in order to investigate the grid-forming behavior but also how the converter stability is affected by a restructure from grid-following to grid-forming control. The results indicate that the investigated grid-forming control method displays a behavior similar to synchronous machines and incorporates the ability to provide frequency response services and so called “synthetic inertia” to the grid. The results also show that the stability of the converter (the ability to provide a bounded output when the system is perturbed) is ensured when the control method is restructured from grid-following to grid-forming and that the investigated grid-forming method is stable also in weak grid situations.

Diese Arbeit befasst sich im Wesentlichen mit der Analyse der Kleinsignalstabilität von miteinander verbundenen Stromversorgungssystemen, die durch die Abnahme der Gesamtträgheit des Netzes beeinflusst werden.Die Frequenzstabilität wird als Hauptbestandteil dieser Analyse betrachtet, die an einem nichtlinearen zwei verbundenen Stromversorgungssystemen mit mehreren Maschinen durchgeführt wird.In konventionellen Stromversorgungssystemen werden große Synchronmaschinen auf die Hauptteile des Systems bezogen, deren Trägheitsmoment eine entscheidende Rolle spielt, um die Stabilität des Stromversorgungssystems gegen kleine Störungen zu gewährleisten.Der wachsende Anteil erneuerbarer dezentraler Stromerzeugung wie Wind- und Solarkraftwerke auf Stromversorgungssystemen führt jedoch zu einer geringeren Nutzung der konventionellen synchronen Stromerzeugung, was dazu führt, dass die Wirksamkeit der Aufgabe des Trägheitsmoments im Netz nachlässt als Figur bei der Senkung der Systemdynamik. Mit anderen Worten, wie in den folgenden Kapiteln gezeigt wird, wird das Fehlen eines Trägheitsmoments durch ein Stromversorgungssystem mit geringerer Robustheit und Empfindlichkeit gegenüber kleinen Störungen beendet.Andererseits führt die inhärente Unsicherheit, die bei Generationen auf erneuerbarer Basis aufgrund ihrer Abhängigkeit von den Wetterbedingungen besteht, dazu, dass das Auftreten von Störungen bemerkenswert und häufig zunimmt. Dieses unbestimmte Verhalten gefährdet wiederum die Stabilität des Systems weiter und macht das System unzuverlässiger.Da das Trägheitsmoment die Reaktion des Systems auf Störungen von Natur aus verlangsamt, spart dies mehr Zeit, damit die Primärfrequenzsteuerungen Maßnahmen ergreifen können. Je geringer das Trägheitsmoment im System ist, desto schneller sind Frequenzaktionen erforderlich. Wie noch erläutert werden wird, scheint es in integrierten Stromversorgungssystemen ineffizient zu sein, die Frequenz des Systems gegenüber Störungen wie Schwankungen der Lasten durch langsame herkömmliche Steueraktionen wie Statiksteuerung und automatische Spannungssteuerung stabil zu halten.Daher wird das Konzept der synthetischen Trägheit als schnelle Frequenzgang steuerungstechnik untersucht, wie effektiv diese Lösung die oben erwähnte steigende Herausforderung bewältigen kann., This thesis fundamentally deals with Small signal stability analysis of interconnected power systems influenced by decline in the total inertia of the grid.Frequency stability is considered as a main part of this analysis which is done on non-linear multi-machine two area power system model.In conventional power systems, big scale synchronous machines are referred to the major parts of the system which their moment of inertia play a vital role to keep power system stable against small disturbances.While the growing share of renewable-based distributed generation, such as wind and solar power plants, on power systems lead to decrease in utilizing the conventional synchronous-based generation but it causes fading effectiveness of the task of moment of inertia on the grid as a figure in lowering system dynamic [1]. In other words, as it will be shown in the following chapters, lowering of moment of inertia is terminated a power system with lower robustness and higher sensitivity to small disturbances [1].On the other hand, the inherent uncertainty that exists in renewable-based generations, due to their dependence on the weather conditions, results to increase of occurrence of disturbances remarkably and frequently. This undetermined behavior in turn threatens the stability of the system further and makes the system more unreliable [2].Since the moment of inertia inherently makes slowing down the response of the system to disturbances then it causes to save more time for primary frequency controls to take action [3]. The lower moment of inertia in the system the faster frequency actions are essential [3]. As will be discussed, in integrated power systems keeping the frequency of the system still stable against the disturbances, like variation of the loads, by the slow conventional control actions like droop control and automatic voltage control seems to be inefficient.Hence, the concept of synthetic inertia as a fast frequency response control technique is investigated how effective this solution can overcome the above mentioned rising challenges

Power converter technology partially or fully electrically decouples the wind energy source from the grid which results in the decrease of system inertia. However, when those units participate in virtual inertial response their electromechanical dynamics become coupled to the grid electromechanical modes. To date, there were no comprehensive studies on how do different elements and parameters of a wind energy conversion system (WECS) impact its virtual inertial response provision. This is important from the standpoint of understanding the expected wind farm response during frequency containment process as well as from the standpoint of developing better inertial response controllers. In this paper we have investigated how do operating point, line-side and machine-side converter, phase-locked loop and pitch angle control impact the inertial response of the total power controlled type III WECS (DFIG) which is one of the most common wind turbine topologies used today. We show that the operating point, pitch angle control and outer loop of the machine-side converter have a visible impact on strength of the inertial response, while other elements do not and some can even be neglected in inertial response studies.

A number of jurisdictions across the world are committed to reducing their carbon emissions. A key route to decarbonisation is by raising the share of renewable generation. However, this will result in reduction of the system inertia and consequently deterioration of frequency stability. Fast acting reserve (FAR) is seen as a possible solution. Various approaches concerning the control of FAR (e.g. frequency gradient and frequency deviation based triggering) have previously been proposed. There is however a lack of clarity regarding the volume of FAR that can potentially compensates for reduction in system inertia. This paper carries out a quantitative assessment of FAR to limit the rate of change of frequency. All island power system of Ireland is adopted as a test case for analysis. The study concludes that FAR can mitigate the RoCoF and it presents also the quantitative relationship between FAR and conventional inertia which depends on system conditions.

The future global energy transition favour renewables such as wind power, which is predicted to be one of the predominant sources harvesting abundant amounts of energy onwards. Consequently, causing several conventional synchronous generators to be decommissioned in a near future to achieve an overall reduction in greenhouse gases related to electricity generation. However, this evolution comes with new challenges regarding power system stability that could jeopardize the reliability of the grid as we today know it. Therefore, this thesis will examine how high penetrations of wind power are impacting each fundamental criterion regarding power system stability. For this purpose, are two different scenarios being carried out in Siemens PSS/E, representing a futuristic case as well as a present one. The simulation results themselves are being compared with analogies drawn from previous studies conducted within the field to determine if it can be improved.

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%.

The displacement of conventional generation by converter connected resources reduces the available rotational inertia in the power system, which leads to faster frequency dynamics and consequently a less stable frequency behaviour. This study aims at presenting the current requirements and challenges that transmissionsystem operators are facing due to the high integration of inertia-less resources. The manuscript presents a review of the various solutions and technologies that could potentially compensate for reduction in system inertia. The solutions are categorized into two groups, namely synchronous inertia and emulated inertia employing fast acting reserves (FARs). Meanwhile, FAR are divided into three groups based on the applied control approach, namely virtual synchronous machines, synthetic inertia control and fast frequency control. The analytical interdependency between the applied control approaches and the frequency gradient is also presented. It highlights the key parameters that can influence the units’ response and limit their ability in participating in such services. The manuscript presents also a trade-off analysis among the most prominent control approaches and technologies guiding the reader through benefits and drawbacks of each solution.

Synthetic inertia provision through the control of doubly-fed induction generator (DFIG) wind turbines is an effective means of providing frequency support to the wider electrical network. There are numerous control topologies to achieve this, many of which work by making modifications to the DFIG power controller and introducing additional loops to relate active power to electrical frequency. How these many controller designs compare to one-another in terms of their contribution to frequency response is a much studied topic, but perhaps less studied is their effect on the small-signal stability of the system. The concept of small-signal stability in the context of a power system is the ability to maintain synchronism when subjected to small disturbances, such as those associated with a change in load or a loss of generation. Amendments made to the control system of a large-scale wind farm will inevitably have an effect on the system as a whole, and by making a DFIG wind turbine behave more like a synchronous generator, which synthetic inertia provision does, may incur consequences relating to electromechanical oscillations between generating units. This work compares the implications of two prominent synthetic inertia controllers of varying complexity and their effect on small-signal stability. Eigenvalue analysis is conducted to highlight the key information relating to electromechanical modes between generators for the two control strategies, with a focus on how these affect the damping ratios. It is shown that as the synthetic inertia controller becomes both more complex and more effective, the damping ratio of the electromechanical modes is reduced, signifying a decreased system stability.

The increasing penetration of Renewable Energy Sources (RES) and, more in general, of converter connected power generation is progressively reducing the overall inertia of the electricity system introducing new challenges in the network Frequency Support (FS). Therefore, FS strategies for RES are becoming an important aspect to be developed in order to ensure the secure operation of the electricity power grid. The aim of the present article is to propose an innovative approach for the inertial emulation of Wind Turbine Generators (WTGs) in order to ensure a positive effect on the system frequency avoiding unstable operations of the WTG and reducing the negative impact of the rotor speed recovery on the secondary frequency drop. Moreover, the paper provides a detailed definition of the triggering logics adopted to activate and deactivate the FS making it suitable for implementation in available industrial controllers. The proposed controller is firstly tested on a simplified configuration to show its enhanced performances with respect to previously developed approaches. Then, simulations are carried out in a more realistic network to assess the positive impact of the controller on the system frequency. Results highlighted a 40% reduction of the system Rate of Change of Frequency while the frequency deviation at the transient nadir is improved by 35%.