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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%.

Dramatic inertia reduction suffered in worldwide grids caused by the transition towards renewable and distributed generation, raises the risk of disturbances and blackouts due to frequency related issues. Subsequently, ENTSO-E has recently published several documents stating rules and recommendations regarding frequency support; such documents highlight the importance of developing new techniques in a grid dominated by generators with a power electronic-based interface. In fact, they are considered the first guidelines of an up-coming frequency control standard, which contemplates the possibility of having renewable and hybrid plants providing frequency stability as an ancillary service; but also proposes a new terminology and establishes the path to be followed in the following years. Therefore, the main goal of this paper is to assess the suitability of requirements provided in current standards. The paper critically reviews the last releases of ENTSO-E regarding frequency regulation and ancillary services provided by renewable and HyPP, identifies issues and points out gaps that have not been considered so far. Thereafter, a new approach aiming to avoid second and subsequent frequency dips after a large excursion is proposed.

Diode rectifiers have been gaining traction as a viable alternative for connecting offshore wind farms (OWFs) to HVdc networks. However, before technical connection requirements compatible with such solutions can be determined, more studies are needed to assess their capabilities to contribute to the secure operation of the networks linked to them. This study assesses the capability of such an OWF to provide support to an onshore ac network by means of primary frequency response (PFR). A semi-aggregated OWF representation is considered in order to examine the dynamics of each grid-forming wind turbine (WT) within a string when providing PFR. Simulation results corroborate that such an OWF can indeed provide PFR, while its grid-forming WTs share the reactive power and keep the offshore frequency and voltages within their normal operating ranges.

Contribution to the power systems’ frequency support is expected to be one of the essential ancillary services that wind power plants (WPPs) shall provide. The high-voltage DC (HVDC) connected offshore WPPs may provide this service with and without using fast communication links between onshore and offshore. In the case of offshore HVDC grid, implementing the communication-less frequency support is challenging. Although it increases the reliability of the frequency control, among other challenges, it is not straightforward to comply with relevant grid code requirements. In this paper, this issue is mathematically described and a static model is developed to calculate the deviation of various electrical parameters of an HVDC grid in case of frequency drop on the land ac systems. A solution for the aforementioned problem is presented and its associated concerns are addressed. The study is verified by simulations of a four terminals dc grid with two offshore WPPs and two inland ac systems.

Lower costs and higher reliability make diode rectifiers (DRs) a promising alternative for connecting offshore wind power plants to HVdc networks. However, such offshore substations lack the grid-forming capability provided by voltage source converters. Hence, the grid-side converters of type-4 (fullconverter) wind turbines (WTs) have been proposed as viable candidates to take over such responsibility, apart from controlling the WT output power. In this work, a common synchronization reference (CSR) is proposed for the synchronization of such converters. Such reference is generated in the offshore HVdc substation and transmitted to the converters through the optical fibers embedded in the power cables. The proposed CSR enables the use of conventional current control for grid-forming WTs. Additionally, a scheme which combines communication-less and communication-based onshore frequency support is proposed and implemented. The proposed controls are validated by simulations under different operational scenarios.

The work presented in this paper aims to show how modern wind turbines can help to control the frequency in a small grid which suffers from large power imbalances. It is shown for an exemplary situation, which occurred in Flensburg’s distribution grid in 2019: a major blackout, which occurred after almost two hours in islanding operation, affecting almost the entire distribution grid, which supplies approximately 55,000 households and businesses. For the analysis, a wind turbine model and a grid support controller developed at the Wind Energy Technology Institute are combined with real measurements from the day of the blackout to generate a fictional yet realistic case study for such an islanding situation. For this case study, it is assumed that wind turbines with grid support functionalities are connected to the medium voltage distribution grid of the city. It is shown to what extent wind turbines can help to operate the grid by providing grid frequency support in two ways: By supplying synthetic inertia only, where the wind turbines can help to limit the rate of change of frequency in the islanded grid directly after losing the connection to the central European grid. In combination with the primary frequency control capabilities of the wind turbines (WTs), the disconnection of one gen set in the local power station might have been avoided. Furthermore, wind turbines with primary frequency control capabilities could have restored the grid frequency to 50 Hz shortly after the islanding situation even if the aforementioned gen-set was lost. This would have allowed connecting a backup medium voltage line to the central European grid and thereby avoiding the blackout.

Plėtojant elektros tinklus HVDC technologija yra patraukli dėl savo plačių valdymo galimybių ir darbo režimų elektros tinklo stabilumo gerinimui. HVDC sistemos pasižymi specifinėmis galimybėmis, priklausomai nuo to, kokia technologija yra naudojama. Šiame darbe bus tiriami „VSC“ technologijos HVDC keitikliai. Keitiklio darbo režimai, aktualūs šiame darbe, taip pat ir užtikrinantys lanksčias el. tinklo darbo galimybes, yra šie : galios srautų krypties pakeitimas, dažnio palaikymas, išjungtos sistemos paleidimas (angl. „black-start“). Keitikliai, veikiantys su elektros sistemomis, turi tam tikrus apribojimus, priklausomai nuo elektros sistemos parametrų. Šiame darbe bus tiriama elektros sistemos parametrų įtaka minėtiems keitiklio darbo režimams. Taip pat bus ištirta dažnio palaikymo funkcija keitikliu bei skirtingi išjungtos elektros sistemos paleidimo atvejai., In the process of power grids development, HVDC technology is attractive because of its wide control options and operating modes to improve electrical power grid stability. HVDC system has specific capabilities, depending on the technology being used. \"VSC\" based HVDC converter technology will be investigated in this work. Converter operating modes, relevant to this work, as well as ensuring flexible electrical grid operating modes are as follows: power flow direction reversal, frequency support, “dead” electrical system start up (also called \"black-start). Converters, working with electrical systems have certain limitations, depending on the electrical system parameters. Electrical system parameters will be investigated on previously mentioned converter operating modes. In this work There will also be investigated converters frequency support function and different off the electrical system start-up situations.

Contemporary data is often supported by an irregular structure, which can be conveniently captured by a graph. Accounting for this graph support is crucial to analyze the data, leading to an area known as graph signal processing (GSP). The two most important tools in GSP are the graph shift operator (GSO), which is a sparse matrix accounting for the topology of the graph, and the graph Fourier transform (GFT), which maps graph signals into a frequency domain spanned by a number of graph-related Fourier-like basis vectors. This alternative representation of a graph signal is denominated the graph frequency signal. Several attempts have been undertaken in order to interpret the support of this graph frequency signal, but they all resulted in a one-dimensional interpretation. However, if the support of the original signal is captured by a graph, why would the graph frequency signal have a simple one-dimensional support? That is why, for the first time, we propose an irregular support for the graph frequency signal, which we coin the dual graph. The dual GSO leads to a better interpretation of the graph frequency signal and its domain, helps to understand how the different graph frequencies are related and clustered, enables the development of better graph filters and filter banks, and facilitates the generalization of classical SP results to the graph domain., 5 pages, 2 figures

With the aim of distributing the total power required at the wind farm control level for each wind turbine, we proposed a multi-objective model predictive control scheme able to (i) ensure the tracking of the power demanded by the electrical grid; (ii) provide primary frequency control by de-loading the wind turbines; and (iii) maximize the total power reserve when the available power is higher than the power demand.

New high voltage direct current (HVDC) installations are expected to be able to provide stability support to the main synchronous networks to which they are connected. In multi-terminal HVDC (MTDC) schemes incorporating offshore wind farm this issue could be addressed by setting aside a reserve of wind energy by the curtailment of wind turbine generators (WTGs). This paper proposes a communication-less dc voltage cooperative control strategy for MTDC transmission systems. A grid side converter, the master, and a wind farm are designed to work cooperatively to maintain a stable dc link voltage, facilitating normal power dispatch orders and the provision of frequency support. The proposed control maintains a wind energy reserve and uses a flexible dc link voltage at the master converter. Allowing the local master dc voltage to vary within certain limits encourages the wind farm to participate in dc link voltage control and hence no communication system is required. The master converter automatically assumes control of the dc link voltage in the absence of wind or when the wind reserve is used up. A four terminal MTDC system comprising one master converter, two active/reactive power converters and one wind farm is studied. The effectiveness of the proposed control strategy is validated through simulation using MATLAB ® -SIMULINK ® .

For some applications, High voltage direct current (HVDC) transmission systems are more suitable than conventional ac transmission. With recent advances in this technology and the controllability that it offers, Voltage SourceConverter basedHigh voltage direct current (VSC-HVDC) systems have proven to be a more advantageous choice for integration of renewable energy sources, mostly for offshore wind power plants which are generally located far away from the main grid. Their interaction with the onshore system influences the power grid stability and calls for different dynamic analysis. As well as conventional generation, VSC-HVDC connected generation units have to fulfil strict grid codes determined by the Transmission System Operator (TSO). Consequently, the impact on the grid stability that regulations such as frequency support, Fault Ride Through (FRT) or post fault active power recovery rate needs to be studied. This work aims to analyze the impact on of VSC-HVDC systems on the grid stability in response to large voltage and frequency deviations. The software tool PowerFactory is used as a platform for the studies, which original templates have been modified in order to include new ancillary services and control strategies. Through several case simulations, it was found that the implemented functionalities improved the VSC-HVDC dynamic performance.

With regard to a successive penetration of renewable energies and the implied need for system flexibility, stationary battery storage systems (BSS) are seen as hybrids, which can manoeuvre either on the demand or on the supply side, due to the bidirectional transformation process. In the scientific debate, the revenue side for BSS are often just titled and partially mapped for individual markets, which frequently leads to the conclusion that the investment in BSS does not pay off. Therefore, the core concept of the paper is the strategy to combine applications and their values, in order to extend the financial attractiveness. To specify and exemplify monetary value propositions (applications) and value networks (benefits) as well as the combination theory, a single and combined revenue model is examined: energy trading via day-ahead market (arbitrage) and energy trading in combination with frequency support via secondary control. The results show, that depending on the technology, the combined revenue model reduces the load factor and thus theoretically expands the BSS lifespan. Moreover, with modified market rules and individual bidder strategies there is a potential to generate higher proceeds with the combined revenue model.

Electric vehicles are growing in popularity as a zero emission and efficient mode of transport against traditional internal combustion engine-based vehicles. Considerable as flexible distributed energy storage systems, by adjusting the battery charging process they can potentially provide different ancillary services for supporting the power grid. This paper presents modeling and analysis of the benefits of primary frequency regulation by electric vehicles in a microgrid. An innovative control logic algorithm is introduced, with the purpose of curtailing the number of current set-point variations that the battery needs to perform during the regulation process. It is shown that, compared to traditional droop-control approaches, the proposed solution assures same effects in terms of frequency containment, by employing a considerably lower number of variations of battery current set-point. The modeled low voltage microgrid is built to reproduce a real configuration of the experimental facility SYSLAB-PowerLabDK. Root-mean-square simulation studies have been carried out in DIgSILENT PowerFactory environment for the validation of the controller.

For an islanded micro-grid with a high penetration of photovoltaic (PV) power generators, the low inertia reserve and the maximum peak power tracking control may increase the difficulty of maintaining the system&rsquo, s supply&ndash, demand balance, and cause frequency instability, especially when the available generation is excessive. This will require changes in the way the PV inverter is controlled. In this paper, a virtual inertia frequency control (VIFC) strategy is proposed to let the two-stage PV inverters emulate inertia and support the system frequency with a timely response (e.g., inertia response), and the required power for inertia emulation is obtained from both the DC-link capacitor and the PV reserved energy. As the rate of the system frequency change can be reduced with the inertia increase, the proposed method can mitigate the frequency contingency event before the superior-level coordination control is enabled for the frequency restoration. The simulation results demonstrate the effectiveness of the proposed method.

Modern large wind farms are required to provide frequency regulation service like conventional synchronous generation units. The frequency support capability of modern wind farms has been widely investigated and implemented. Remotely located large offshore wind farms are probably connected to the onshore system grid through voltage-source converter-based–high voltage direct current (VSC-HVdc) transmission systems. Due to the decoupling of VSC-HVdc and signal transmission delay, offshore wind farms may not be able to respond to the onshore grid frequency excursion in time and, consequently, the stability and security of the power system will be put at risk, especially for those with high wind penetration. This paper proposes a coordinated control scheme to allow VSC-HVdc link to contribute to the system frequency regulation by adjusting its dc-link voltage. By means of this approach, the dc capacitors of VSC-HVdc are controlled to absorb or release energy so as to provide frequency support. To further enhance the system frequency response, the frequency support from VSC-HVdc is also finely coordinated with that from offshore wind farm according to the latency of offshore wind farm responding to onshore grid frequency excursion. The control scheme is evaluated for both underfrequency and overfrequency events, and results are presented to demonstrate its effectiveness.

This work aims to study different ancillary services such as frequency support, rotor angle stability and voltage stability that can be provided to independent AC areas through Multi-Terminal HVDC grid. An agent-based control scheme has been suggested to support the frequency of the imbalanced AC area using other intact areas during a disturbance. The frequency support is carried out through some agents/controller, where the objective of the agents is to reach a common value for a given state, i.e. frequency, using the local and neighbouring information. In this scheme, the concept of Adaptive Graph (AG) has been introduced to the consensus problem to deal with the limitation of original scheme in considering the rated capacity of HVDC converters. In this case, the frequency consensus can be reached to a new stable value by removing one or several converters from the communication graph when reaching rated active power capacity. In order to test the proposed scheme, a model of 7-terminal dc-grid and corresponding AC areas is developed in real time simulator OPAL-RT. The results show that the adaptive graph scheme improves the reliability of the system during distributed frequency support.

The modulated wideband converter (MWC) is a kind of sub-Nyquist sampling system which is developed from compressed sensing theory. It accomplishes highly accurate broadband sparse signal recovery by multichannel sub-Nyquist sampling sequences. However, when the number of sparse sub-bands becomes large, the amount of sampling channels increases proportionally. Besides, it is very hard to adjust the number of sampling channels when the sparsity changes, because its undersampling board is designed by a given sparsity. Such hardware cost and inconvenience are unacceptable in practical applications. This paper proposes a distributed modulated wideband converter (DMWC) scheme innovatively, which regards one sensor node as one sampling channel and combines MWC technology with a broadband cooperative spectrum sensing network perfectly. Being different from the MWC scheme, DMWC takes phase shift and transmission loss into account in the input terminal, which are unavoidable in practical application. Our scheme is not only able to recover the support of broadband sparse signals quickly and accurately, but also reduces the hardware cost of the single node drastically. Theoretical analysis and numerical simulations show that phase shift has no influence on the recovery of frequency support, but transmission loss degrades the recovery performance to a different extent. Nevertheless, we can increase the amount of cooperative nodes and select satisfactory nodes by a different transmission distance to improve the recovery performance. Furthermore, we can adjust the amount of cooperative nodes flexibly when the sparsity changes. It indicates DMWC is extremely effective in the broadband cooperative spectrum sensing network.

Summarization: Photovoltaic (PV) systems constitute a significant percentage of the worldwide installed electrical power plants. They exploit a renewable energy source and they do not require frequent maintenance, since they do not contain mechanical parts. These, combined with the fact that their components get cheaper over time, explain the increasing penetration of PV power plants in modern electricity grids. To avoid frequency disturbance and to improve the grid’s power quality, a way of controlling the output power of PV systems is the Flexible Power Point Tracking (FPPT), where a reference value is configured for the PV array’s output power. Since PV modules are usually shaded, conventional FPPT algorithms cannot operate efficiently, thus the concept of the Global FPPT (GFPPT) algorithm was introduced. In this thesis, two novel GFPPT algorithms based on machine learning are proposed. To evaluate their effectiveness, an experimental system was developed. One GFPPT algorithm from the literature and the two new ones were tested experimentally on varying operating conditions, as it could happen in real world applications. The experimental results demonstrated that the two new algorithms developed within the framework of this thesis converged in significantly less time, while they achieved an almost same steady-state tracking error. Περίληψη: Τα φωτοβολταϊκά (Φ/Β) συστήματα αποτελούν ένα σημαντικό ποσοστό των παγκοσμίως εγκατεστημένων μονάδων παραγωγής ηλεκτρικής ενέργειας. Εκμεταλλεύονται μια ανανεώσιμη πηγή ενέργειας και δεν απαιτούν συχνή συντήρηση, αφού δεν περιέχουν μηχανικά μέρη. Αυτά, σε συνδυασμό με το γεγονός ότι τα εξαρτήματά τους γίνονται φθηνότερα με την πάροδο του χρόνου, εξηγούν την αυξανόμενη διείσδυση των Φ/Β σταθμών παραγωγής ισχύος στα σύγχρονα δίκτυα παραγωγής και διανομής ηλεκτρικής ενέργειας. Για να αποφευχθεί η διαταραχή της συχνότητας και να βελτιωθεί η ποιότητα ισχύος του δικτύου, ένας τρόπος ελέγχου είναι η εφαρμογή της διαδικασίας Flexible Power Point Tracking (FPPT), όπου η ισχύς του Φ/Β συστήματος ρυθμίζεται σε μια τιμή αναφοράς. Εφόσον τα Φ/Β πάνελ συνήθως σκιάζονται, οι συμβατικοί αλγόριθμοι FPPT δεν λειτουργούν αποδοτικά, επομένως αναπτύχθηκε η κατηγορία των Global FPPT (GFPPT) αλγορίθμων. Στην εργασία αυτή προτείνονται δυο νέοι GFPPT αλγόριθμοι, βασισμένοι στη μηχανική μάθηση. Για να αξιολογηθεί η αποτελεσματικότητά τους, αναπτύχθηκε ένα πειραματικό σύστημα. Ένας GFPPT αλγόριθμος από τη βιβλιογραφία και οι δυο νέοι δοκιμάστηκαν πειραματικά υπό μεταβαλλόμενες συνθήκες λειτουργίας, όπως θα μπορούσε να συμβεί και σε πραγματικές εφαρμογές. Τα πειραματικά αποτελέσματα έδειξαν ότι οι δυο νέοι αλγόριθμοι που αναπτύχθηκαν στο πλαίσιο αυτής της εργασίας συνέκλιναν σε σημαντικά μικρότερο χρονικό διάστημα, ενώ πέτυχαν ένα σχεδόν ίδιο σφάλμα παρακολούθησης στη μόνιμη κατάσταση.

Διπλωματική Διατριβή που υποβλήθηκε στη σχολή ΗΜΜΥ του πολυτεχνείου Κρήτης για την απόκτηση προπτυχιακού διπλώματος Περίληψη: Τα τελευταία χρόνια, τα ηλεκτρικά οχήματα (PEVs) έχουν γίνει εξαιρετικά δημοφιλή και βρίσκονται σε μια τροχιά συνεχούς ανάπτυξης. Κατά συνέπεια, τα συστήματα ηλεκτρικής ενέργειας αντιμετωπίζουν νέα ζητήματα και προκλήσεις που απειλούν την ασφάλεια και την αξιοπιστία του δικτύου. Τέτοιου είδους οχήματα, επί του παρόντος, συμπεριφέρονται ως απλά φορτία λόγω της χαμηλής διείσδυσής τους στην αγορά. Ωστόσο, καθώς το πλήθος τους αυξάνεται, τα PEVs μπορούν να αξιοποιηθούν ως κατανεμημένες μονάδες αποθήκευσης ενέργειας, παρέχοντας βοηθητικές υπηρεσίες στο σύστημα. Οι μπαταρίες που χρησιμοποιούνται σε PEVs έχουν κατασκευαστεί για να μπορούν να αυξομειώνουν ταχύτατα την ενεργό ισχύ που παρέχουν, καθιστώντας τις μια εξαιρετική λύση για στήριξη συχνότητας. Σε αυτή την εργασία προτείνεται ένα λεπτομερές δυναμικό μοντέλο που μπορεί να προβλέψει την στήριξη συχνότητας από μεγάλο αριθμό ηλεκτρικών οχημάτων, χρησιμοποιώντας μία ισοδύναμη μπαταρία, μειώνοντας έτσι τον υπολογιστικό χρόνο και την πολυπλοκότητα. Η προτεινόμενη μέθοδος εφαρμόστηκε στο σύστημα της Κρήτης χρησιμοποιώντας πραγματικά δεδομένα μπαταριών και δρομολογίων των οχημάτων. Από τις προσομοιώσεις αποδεικνύεται ότι τα PEVs μπορούν αποτελεσματικά να περιορίσουν τις διακυμάνσεις της συχνότητας και να βελτιώσουν τη συνολική αντοχή του συστήματος, προς όφελος τόσο του ιδιοκτήτη όσο και του ηλεκτρικού δικτύου. Summarization: In recent years, plug-in electric vehicles (PEVs) have gained massive popularity and are on a trajectory for continued growth. As a consequence, power systems are facing new issues and challenges threatening the safety and reliability of the grid. PEVs are currently treated as simple loads due to their low penetration in the market. However, as their numbers are growing, PEVs could be potentially exploited as distributed energy storages, providing ancillary services to the power system. Batteries used in PEVs are developed to deliver instantaneous active power, making them an excellent solution for load frequency support. This diploma proposes a fully detailed dynamic model that is able to predict frequency support from a large number of PEVs, using an aggregated equivalent of the battery, with respect to the constraints, reducing the computational time and complexity. The proposed method is applied to the power system of Crete using real battery data and transportation distributions. It is proved that PEVs can effectively suppress frequency fluctuations and improve the overall robustness of the grid, benefiting both the owner and the electric system.

Summarization: The increasing popularity of Plug-in Electric Vehicles (PEVs) will impose a significant strain on the electricity networks. However, Vehicle-to-grid (V2G) operation can provide ancillary services, improving the overall stability and reliability of the grid. In this paper, an optimal power management system with frequency support capability is proposed. Particle Swarm Optimization (PSO) algorithm is used for the calculation of the optimal power exchange between PEV's battery and the grid throughout the day. Additionally, the ability of the PEV to provide frequency support to the grid is examined. Using a frequency-PEV load droop control approach, the deviation of PEV's power from its optimal set-point is determined. Once the need for frequency support ends, the aforementioned algorithm reschedules the new optimal operation of the PEV, satisfying at the same time all predetermined goals and constraints. Παρουσιάστηκε στο: 2021 10th International Conference on Modern Circuits and Systems Technologies

Summarization: In recent years, plug-in electric vehicles (PEVs) have gained immense popularity and are on a trajectory of constant growth. As a result, power systems are confronted with new issues and challenges, threatening their safety and reliability. PEVs are currently treated as simple loads due to their low penetration. However, as their numbers are growing, PEVs could potentially be exploited as distributed energy storage devices providing ancillary services to the network. Batteries used in PEVs are developed to deliver instantaneously active power, making them an excellent solution for system frequency support. This paper proposes a detailed dynamic model that is able to simulate frequency support capability from a large number of PEVs, using an innovative aggregate battery model that takes into account the most significant constraints at PEV and aggregate battery levels. The cost optimization algorithm, which is the most time-consuming process of the problem, is executed only at the aggregate battery level, thereby reducing the computational requirements of the model without compromising the obtained accuracy. The proposed method is applied to the power system of Crete exploiting detailed statistical data of EV mobility. It is proven that PEVs can effectively support power system frequency fluctuations without any significant deviation from their optimal operation. Presented on: Inventions