Distantly-located offshore energy hubs need to be connected to the shore via High Voltage Direct Current (HVDC) links to allow for an efficient bulk power exchange. A bipolar configuration of the HVDC link is suitable for a point-to-point connection, as it provides redundancy, and, therefore, a larger reliability, e.g. half of the rated transfer capacity can still be transferred via one of the poles in case of a fault occurring on the other pole. Nevertheless, a control strategy of the converters that can effectively enable a situation-dependent power routing between the two poles constitutes a research challenge. In this paper, two control strategies are proposed for the offshore Modular Multi-level Converters (MMCs) of a bipolar HVDC link connecting a 2 GW offshore hub to the shore. The strategies, based on DC current and DC voltage measurements, respectively, enable to track and adjust the amount of power flowing through each pole of the link. Real-time digital simulations show that both strategies can effectively route the power exchanges through the bipolar HVDC link, e.g. operation under balanced or unbalanced conditions. The strategy based on DC current seems more suitable to manage the dynamic performance of the HVDC link.@en

Modernization of power systems leads to more power electronic interfaced units in the generation, demand, and transmission. Examples are remotely installed renewable energy sources, loads with constant power, or high voltage direct current (HVDC) corridors. These changes significantly affect the frequency stability margins of the system and thus special control techniques should be applied in the converters of the new installed units so as to shoulder the frequency regulation in case of commonly occurred active power imbalances. The response of such units has to be cooperative in order to avoid problems such as insufficient reactions or overshoots. In this chapter, a coordinative tuning approach of the active power gradient control scheme applied to the controllers of modular multilevel converter (MMC)-based HVDC links and proton exchange membrane electrolyzers with the provision of fast frequency support in a multiarea hybrid HVDC-HVAC power system with responsive demand units is proposed. This tuning uses an optimization approach based on mean variance mapping optimization and is able to minimize the frequency excursions in all interconnected areas participating in the frequency regulation even without communication between the system nodes. This technique has shown great results in terms of quality and convergence rate within a short number of fitness evaluations achieving a set of frequency responses within acceptable limits set by operators even in case of the loss of the largest generating unit in the weakest system area. It has also revealed the applicability of such a method in more complex systems and the necessity for sophisticated tuning methods according to the application needs and the system characteristics.

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Power electronic dominated power systems formed nowadays are characterized by fast and frequent dynamics, limited short circuit support, low inertia conditions and lack of inertial support. Under these conditions, coping with active power imbalances in a power system may becomes a significant challenge for transmission system operators (TSOs) that may experience extensive frequency deviations and steep rates of change of frequency (RoCofs). To deal with the frequency stability issues encountered, power electronic interfaced (PEI) units can rapidly respond to provide fast frequency support (FFS) taking advantage of their controllability levels and their rapid response to setpoint changes. FFS may depend on the active power gradient (APG) control strategy that determines the required amount of active power, and the rate the power injection takes place. However, when multiple elements try to regulate simultaneously the frequency adverse control actions such as insufficient or over frequency regulation may be encountered. To solve this issue, this paper proposes a formulation for the optimal and coordinative tuning of the APG controllers of PEI elements installed in a multi-area, multi-energy hybrid HVDC/HVAC power system with modular multilevel converter (MMC) HVDC links and proton exchange membrane (PEM) electrolyzers. This formulation focuses on creating an artificially coupled frequency response for an electromagnetically decoupled multi-area system taking advantage of the available active power reserves and the inertia levels of each area. In that way, an active power imbalance can be optimally shared among the interconnected areas leading to effectively improved frequency response for the affected and supporting areas. The proposed formulation is solved using the mean variance mapping optimization (MVMO) algorithm after a series of RMS simulations is performed in DIgSILENT PowerFactory 2021.

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This paper proposes a Electro-Magnetic Transient (EMT) model of a 2 GW offshore network with the parallel operation of two Modular Multi-level Converter (MMC)—High Voltage Direct Current (HVDC) transmission links connecting four Offshore Wind Farms (OWFs) to two onshore systems, which represent a large scale power system. Additionally, to mitigate the challenges corresponding to voltage and frequency stability issues in large scale offshore networks, a Direct Voltage Control (DVC) strategy is implemented for the Type-4 Wind Generators (WGs), which represent the OWFs in this work. The electrical power system is developed in the power system simulation software RSCAD™, that is suitable for performing EMT based simulations. The EMT model of 2 GW offshore network with DVC in Type-4 WGs is successfully designed and it is well-coordinated between the control structures in MMCs and WGs.

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The increase in Power Electronic (PE) converters due to the increase in offshore wind energy deployment have given rise to technical challenges (e.g., due to unprecedented fast dynamic phenomena) related to voltage and frequency stability in the power system. In the Offshore Wind Farms (OWFs), the currently available current injection-based voltage control for PE converters are not suitable for voltage control in PE dominated systems due to the absence of continuous voltage control and ineffectiveness during islanding. Moreover, in such power systems, the conventional controllers are not suitable for frequency control due to the absence of dynamic frequency control. The paper presents the Direct Voltage Control (DVC) strategy in a real-time environment to mitigate challenges related to voltage and frequency stability during islanding of OWFs. The control strategy is implemented in the average Electro-magnetic Transient (EMT) model of Type-4 Wind Generator (WG) in RSCAD® Version 5.011.1. It is compared with the benchmark model of the control strategy in DIgSILENT PowerFactory™ 2019 SP2 (×64) in EMT platform. The comparison based on shortterm voltage stability and reactive current injection reveals that both the models provide similar results, confirming the validation of the RSCAD model. Moreover, the detailed representation of the converters in the RSCAD model provides a better depiction of the real-world operation.

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This chapter presents a general overview of the experience learned with the use of DIgSILENT PowerFactory in the design of theoretical lectures and practical sessions of a power system dynamics course at postgraduate level. This chapter focuses on the experiences acquired in the course that is part of the MSc program in Electrical Engineering of TU Delft, Department of Electrical Sustainable Energy. The discussion provided in this chapter focuses on power systems application with special focus on (i) Steady-state, Dynamic, (ii) Voltage Stability and (iii) rotor angle stability. The main objective of using PowerFactory at MSc level is to expose the postgraduate students to real-life application, however, without lack of generalisation this chapter is dedicated to the is to expose to the application above by using a very well-known two area-four machine test power system (2A4G), it gives students insights and experience with cases closer to actual power systems. Results of this pedagogical experience demonstrate the importance of incorporating appropriate power system simulations tools in the postgraduate level.

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In this chapter, a generic model of fuel cells and electrolysers suitable for power system stability studies has been developed in PowerFactory. Both theoretical modelling background and software implementation of fuel cells and electrolysers are detailed. Furthermore, a case study based on a three area test system has been performed, which provides valuable insight into the benefits that the synergy between the electricity and hydrogen sectors can bring to power system stability.

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Traditionally, electrical power systems have been based on fossil-fuel fired generation plants to satisfy the load demand. However, due to environmental targets for significant CO2 reduction, a gradual decommission of the aforementioned plants is observed whereas renewable energy sources are gaining gradually increasing momentum, which entails radical changes in the dynamic behavior of electrical power systems. Among the existing renewable energy technologies, variable speed wind generators which utilize full–scale power electronics units, are a preferred technological solution to tackle the variability of renewable energy. Increasing renewable power generation caused a reduction of system inertia and short circuit capacity. This reduction challenges the rotor angle stability of remaining synchronous generators when large disturbance occur. This paper presents a study on modifications of the outer control loops of the grid side converter of wind generators type IV to limit the magnitude of the first rotor angle swing while increasing the overall damping performance of a power system. The study includes a comparison between three different wind generation controllers. Namely, a basic Low Voltage Right Through (LVRT) with a post-fault ramp in the active power injection strategy, a voltage dependent active power injection scheme and a Supplementary Damping control (SDC) method are examined and tested through a power hardware-in-the-loop (PHIL) based test bench. It has been found that SDC supports quick damping of oscillations and high reduction of magnitude of the first swing with respect to the other two control schemes.@en

This study presents an investigation of the impact of the quasi-stationary voltage support provided by a voltage source converter (VSC) connected to a single point of a power system. Based on the directional derivative concept, an analytical method is developed to quantify the sensitivities of the AC bus voltage with respect to the VSC reactive power control modes. Based on a real case study, it is shown that the method applies to VSC units that are part of VSC-HVDC systems, which can operate in a point-to-point or multi-terminal configuration. Time-domain simulations are performed to verify the findings from the application of the analytical method on a reduced size power system.

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The decommissioning of synchronous generators, and their replacement by decoupled renewable power plants, has a significant impact on the transient stability performance of a power system. This paper concerns with an investigation of the degree of transient stability enhancement that can be achieved in power systems with high shares (e.g., around 75%) of wind generation. It is considered that the wind generators can work either under the principle of current control or under the principle of fast local voltage control. In both cases, a power–angle modulation (PAM) controller is superimposed on the current control loops of the grid side converters of the wind generators. The investigation of the degree of enhancement takes into account different approaches of the tuning of PAM. It considers a simple approach in the form of parametric sensitivity, and also a sophisticated approach in the form of a formal optimization problem. Besides, the paper gives insight on what is a suitable objective function of the optimization problem, which entails the best performance of PAM. The whole investigation is conducted based on a synthetic model of the Great Britain (GB) system

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During the last few years, electric power systems have undergone a widespread shift from conventional fossil-based generation toward renewable energy-based generation. Variable speed wind generators utilizing full-scale power electronics converters are becoming the preferred technology among other types of renewable-based generation, due to the high flexibility to implement different control functions that can support the stabilization of electrical power systems. This paper presents a fundamental study on the enhancement of transient stability in electrical power systems with increasing high share (i.e., above 50%) of power electronic interfaced generation. The wind generator type IV is taken as a representative form of power electronic interfaced generation, and the goal is to investigate how to mitigate the magnitude of the first swing while enhancing the damping of rotor angle oscillations triggered by major electrical disturbances. To perform such mitigation, this paper proposes a power-angle modulation (PAM) controller to adjust the post-fault active power response of the wind generator type IV, after a large disturbance occurs in the system. Based on a small size system, the PAM concept is introduced. The study is performed upon time-domain simulations and analytical formulations of the power transfer equations. Additionally, the IEEE 9 BUS system and the test model of Great Britain's system are used to further investigate the performance of the PAM controller in a multi-machine context, as well as to perform a comparative assessment of the effect of different fault locations, and the necessary wind generators that should be equipped with PAM controllers.

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In this study, a novel methodology is proposed for sensitivity-based tuning and analysis of derivative-based fast active power injection (FAPI) controllers in type-4 wind turbine units integrated into a low-inertia power system. The FAPI controller is attached to a power electronic interfaced generation (PEIG) represented by a generic model of wind turbines type 4. It consists of a combination of droop and derivative controllers, which is dependent on the measurement of the frequency. The tuning methodology performs parametric sensitivity to search for the most suitable set of parameters of the attached FAPI that minimises the maximum frequency deviation in the containment period. The FAPI is adjusted to safeguard system stability when increasing the share of PEIG. Since the input signal of the FAPI is the measured frequency, the impact of different values and parameter settings of the phase-locked loop used for the FAPI controller is also investigated. Detailed validation with a full-scaled wind power converter is also provided with a real-time digital simulator testbed. Obtained simulation results using a three-area test system, identify the maximum achievable degree of increase in the share of wind power when a proper combination of wind park locations considering their suggested settings for inertia emulation.@en

Power System Stability is a major domain of renewed interest for electrical power system researchers worldwide. Among the different stability classification domains, large disturbance rotor angle (transient) stability studies are of high concern due to the decommissioning of conventional power plants which leads to a dramatic decrease of inertia and shortcircuit capacity. In this paper, the superiority of a proposed Supplementary Damping Control (SDC) scheme, concerning with transient stability enhancement, is demonstrated against other existing controls, namely, a common form of low-voltage ride through (LVRT) controller with a post-fault ramp, and Voltage Dependent Active Power Injection (VDAPI) control strategy. Based on the analysis done with the modified IEEE 9 bus system with 52% and 75 % share of wind generation, it has been found that proposed SDC has quick damping of oscillations, and also causes a higher reduction of the magnitude of the first rotor angle swing, and has lesser impact on the overall system frequency performance. The controllers’ performance against rotor angle stability threats is tested via EMT modelling and simulation with RSCAD software, which is a real-time digital simulation (RTDS) platform.@en

This paper presents different forms of Fast Active Power Injection (FAPI) control schemes for the analysis and development of different mitigation measures to address the frequency stability problem due to the growth of the penetration level of the Power Electronic Interfaced Generation (PEIG) in sustainable interconnected energy systems. Among the studied FAPI control schemes, two different approaches in the form of a derivative-based control and a virtual synchronous power (VSP) based control for wind turbine applications are also proposed. All schemes are attached to the PEIG represented by a generic model of wind turbines type 4. The derivative-based FAPI control is applied as an extension of the droop based control scheme, which is dependent on the measurement of the network frequency. By contrast, the proposed VSP-based FAPI is fed by the measurement of the active power deviation. Additionally, unlike existing approaches for virtual synchronous machines, which are characterized by high-order transfer functions, the proposed VSP-based FAPI is defined by a second-order transfer function, which can contribute to fast mitigation of the system primary frequency deviations during containment period. The Great Britain (GB) test system, for the Gone-Green planning scenario for the year 2030 (GG2030), is used to evaluate the effects of the proposed FAPI controllers on the dynamics of the system frequency within the frequency containment period. Thanks to proposed FAPI controllers, it is possible to reach up to 70% for the share of wind power generation without violating the threshold limits for frequency stability. For verification purposes, a full-scale wind turbine facilitated with each FAPI controller is tested in EMT real-time simulation environment.@en

Coping with severe active power imbalances constitutes a challenging task in low inertia multi-energy systems. The phase out of the majority of conventional power plants with large size synchronous generators entails that power electronic interfaced devices should take over the primary task of active power balance-frequency control. In view of this, the active power gradient (APG) control of these devices should be carefully designed to ensure effectiveness and to prevent collateral effects on other stability phenomena. This paper presents two novel formulations for the optimal tuning of the parameters of the APG controllers. Both formulations are defined as a constrained single objective optimization problem. The goal is to optimally manage the APG controllers to quickly bound the instantaneous frequency deviations excited by a large active power imbalance. The first formulation concerns with the minimization of the instantaneous variation of the kinetic energy of the synchronous areas of an interconnected system, whereas the second formulation concerns with the minimization of the spatial displacements between the dynamic trajectories of the time frequency responses in different synchronous areas. The formulations are solved by using a new proposed variant of the mean-variance mapping optimization algorithm (MVMO), and their conceptual implications are illustrated based on numerical simulations performed on a small-size multi-energy system.@en

Hydrogen as an energy carrier holds promising potential for future power systems. An excess of electrical power from renewables can be stored as hydrogen, which can be used at a later moment by industries, households or the transportation system. The stability of the power system could also benefit from electrolysers as these have the potential to participate in frequency and voltage support. Although some electrical models of small electrolysers exist, practical models of large electrolysers have not been described in literature yet. In this publication, a generic electrolyser model is developed in RSCAD, to be used in real-time simulations on the real-time digital simulator. This model has been validated against field measurements of a 1 MW pilot electrolyser installed in the northern part of The Netherlands. To study the impact of electrolysers on power system stability, various simulations have been performed. These simulations show that electrolysers have a positive effect on frequency stability, as electrolysers are able to respond faster to frequency deviations than conventional generators.

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The connection of offshore wind turbines to the European grid has been growing in the recent years. Many European countries are adopting this renewable energy and are increasing the number of wind power plant additions into their electrical transmission networks. In this paper, the impacts of harmonic frequencies introduced by the wind parks in a low-inertia grid are studied. Despite of classical methods which are mainly based on single-input single-output (SISO) systems, a novel approach, based on Singular Value Decomposition (SVD) techniques, considering a multiple-input multiple-output (MIMO) system is presented and discussed. The proposed SVD is a powerful mathematical tool to discover the harmonic frequencies. It can be used to analyse the system at a certain harmonic frequency and show which input(s) of the system will have more influence in the system dynamics and which output(s) will be the most affected by that input(s). According to the presented study, an SVD based methodology is provided to model any electrical network via its passive electrical elements, and to perform a harmonic analysis.@en

The active power gradient (APG) control of MMC-HVDC links can contribute to frequency support in AC networks affected by severe active power imbalances. This functionality is particularly convenient if the coupled AC systems (through MMC-HVDC) have similar levels of available inertia. This paper tackles the problem of determining the optimal APG parameters that entail bounding frequency excursions within acceptable limits while helping to quickly damp out electromechanical oscillations. The tuning task is tackled as a single objective computationally expensive optimization problem. Since the optimization search procedure involves repetitive time-domain (RMS) simulations, the major challenge resides in solving the problem within reduced amount of fitness evaluations. In view of this, an emerging metaheuristic algorithm, namely, the mean-variance mapping optimization (MVMO) is selected. Numerical results prove the effectiveness of MVMO in finding the optimal solution within a few fitness evaluations.

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When a PtP link is expanded into a multi-terminal HVDC (MTDC) system by interconnecting an additional offshore wind farm (OWF) converter, the existing operation strategy changes. The OWF power production should be considered as a determining factor to operate the system. In this paper, a new power capability curve for the existing converters is proposed. This new power capability curve is formulated as a function of OWF power production. Furthermore, different converter control strategies are also described and compared. It has been found that a multi-slope droop control strategy is the most suitable strategy for the 3-terminal HVDC system with an OWF converter.

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