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©2023 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works., The high penetration of distributed energy resources, based on power electronics, is giving rise to stability issues in the electrical network, as initially those plants were not required to provide either voltage or frequency support. Due to this, transmission system operators (TSOs) and distribution system operators (DSOs) have set new requirements for them to provide grid support functionalities. This article presents a grid forming control strategy for power converters which is based on a virtual admittance loop. By means of this strategy it is possible to use a conventional current control based inverter, just adding this outer loop. By doing so, the converter is not only able to perform current control, but also frequency and voltage support functionalities. In this article, the proposed control is described and analyzed, checking as well its stability boundaries. After this analysis, the results obtained in a hardware-in-the-loop platform, where faulty scenarios, islanding or black start conditions, will be shown. Finally, the results obtained in an experimental true scale workbench will show its effectiveness for providing such services., This work was supported in part by LC-SC3-ES5-2018-2020-INTERRFACE project under Grant GA 824330 and in part by FlexiGRID project under Grant RTI2018-100921-B-C21., Peer Reviewed, Postprint (author's final draft)

The integration of renewable energies based on photovoltaic power plants (PVPPs) is increasing exponentially fast. In consequence, stability problems associated with the lack of inertia are becoming a serious concern. Transmission and distribution system operators are demanding more requirements in PV plants to provide grid functionalities in terms of frequency and voltage support. Therefore, power converters based on virtual synchronous controllers have been proposed to emulate the dynamic behavior of synchronous generators. Control strategies such as the synchronous power controller have been proposed for PV applications to fulfill this requirement. However, most of these control strategies are integrated locally as an add-on firmware embedded in each converter, providing the demanded support in their connection point. For PVPPs, the inherent synchronous characteristic is expected at the connection point with the electrical grid, which is not always the connection point of each power converter; thus, unexpected power support levels can be obtained. The aggregation of several power converters operating in a coordinated synchronous action would solve this issue, due to the fact that they can provide voltage and frequency support in a specific point. In this article, a centralized control strategy based on a synchronous central architecture is proposed. The control strategy emulates an aggregated synchronous control to regulate a PVPP as a coordinated system. The simulation and experimental results validate the effectiveness of the control proposed., Peer Reviewed, Postprint (author's final draft)

With the advent of renewable energy, stability of power system is becoming more and more crucial. Recently, grid-forming converter concept has attracted much attention from the industry and academia as a potential solution to improve power system stability. This paper proposes a frequency-selective power oscillation damper for grid-forming power converter to improve its flexibility and performance in damping electromechanical modes. The proposed controller employs the multi-rotor concept where a virtual synchronous machine with multiple rotors is emulated in the control part of a grid-forming power converter. In addition to the enhancement in suppressing power oscillation, the main advantages of the proposed controller include the ability to provide the damping selectively for each electromechanical mode. That is, the proposed controller allows setting the desired damping individually for each oscillation mode. The performance of the proposed controller is validated by using the three-machine infinite bus system., This work was supported by the European Commission under the project FLEXITRANSTORE - H2020-LCE-2016-2017-SGS-774407, Peer Reviewed, Postprint (published version)

This paper proposes a management strategy based on a transactive mechanism to improve the efficiency of energy storage systems. The proposed control scheme relies on a non-cooperative strategy where each system is at the liberty to set its price in order to maximize owner profit while maintaining power system stability. The novelty of this approach comes from the simple yet effective design of the management strategy. In fact, different from existing works, the proposed management scheme is treated in the framework of classical control which does not only result in reliable implementation but also is easy to be incorporated in stability studies. Simulation and experimental results of a utility-scale battery energy storage system are provided to validate the effectiveness of the proposed management strategy., This work was supported by the European Commission under the project FLEXITRANSTORE H2020-LCE-2016-2017-SGS-774407 and the project INTERRFACE H2020-LC-SC3-2018-ES-SCC - GA 824330., Peer Reviewed, Postprint (published version)

During the years, grid connected operation in power systems has been the key factor for renewable energy resource to be able to exchange energy to the electrical grid. However, with the increase of small power systems and micro-grids, the standalone operation and the blackstart of the power system has been increasingly considered a demand in most of power converters connected to the micro-grid system. The gridforming power converter strategy has been widely used in micro-grid scenarios, taking the virtual synchronous machine concept behind it. This emulation of a synchronous machine benefits the power converter with additional features, such as island operation or blackstart. In this regard, this paper presents a resynchronization system for a grid-forming power converters, based on the synchronous power control, the match the phase angle of the utility grid prior to its connection., This project has been partially funded by Spanish Ministry of Science Innovation and Universities under the project RTI2018-100921-B-21 and the H2020-LC-SC3-2018-ES-SCC project Interrface GA824330., Peer Reviewed, Postprint (published version)

Grid-forming power converters (GFMC) have been widely adopted in power systems as an attractive solution against the challenges imposed by the ever-increasing penetration of renewables. Despite its versatility, GFMC is employed only to provide islanded operation, grid regulations, and synthetic inertia. To further extend the use of GFMC in enhancing power system stability, this paper proposes a multi-rotor virtual machine (MRVM) controller to attenuate sub-synchronous oscillations. Driven by the formulation of a virtual synchronous machine (VSM), the proposed MRVM implements a VSM-based GFMC with several virtual rotors whose electromechanical characteristics can be individually adjusted to target specific oscillatory modes in the system. In this work, the MRVM’s working principle is described in detail and tuning guidelines are proposed to simplify the selection of control parameters by using frequency-domain techniques and the eigenvalue locus analyses. To validate the performance of the MRVM, an IEEE benchmark grid model is adopted namely, the three-machine-infinite-bus system. It is evident from the results that the MRVM (i) provides higher degrees of freedom when dealing with sub-synchronous oscillations, and (ii) outperforms conventional GFMC, especially in damping intra-area power oscillations., This work was supported by the European Commission under Project FLEXITRANSTORE-H2020-LCE-2016-2017-SGS-774407., Peer Reviewed, Postprint (published version)

The integration of non-synchronous generation units and energy storage through power electronics is introducing new challenges in power system dynamics. Specifically, the rotor angle stability has been identified as one of the major obstacle with regards to power electronics dominated power systems. To date, conventional power system stabilizer (PSS) devices are used for damping electromechanical oscillations, which are only tuned sporadically leading to significant deterioration in performance against the ever-changing operating conditions. In this paper, an intelligent power oscillation damper (iPOD) is proposed for grid-forming converters to attenuate electromechanical inter-area power oscillation. In particular, the iPOD is applied to a synchronous power controller (SPC) based grid-forming power converter to increases gain of the active power control loop at the oscillatory frequency. Predictions regarding the mode frequency, corresponding to the current operating points, are given by an artificial intelligence ensemble model called Random Forests. The performance of the proposed controller is verified using the two area system considering symmetrical fault for random operating points. In addition, a comparison with PSS installed in each generator reveals the individual contribution with respect to the inter-area mode damping., This work was supported in part by the European Commission under Project FLEXITRANSTORE—-H2020-LCE-2016-2017-SGS774407, and in part by the Spanish Ministry of Science under Project ENE2017-88889-C2-1-R., Peer Reviewed, Postprint (published version)

The series-connection of modules in multilevel converters are prone to energy imbalances in the dc capacitor due to the differences between the power absorbed and consumed. In renewable energy applications where the primary source is directly connected to each module, energy imbalances can be even worse if the primary sources are affected by unpredictable weather conditions. Therefore, control strategies are required to compensate such energy imbalances, while maintaining the correct converter operation. Focusing our attention on a cascaded transformer multilevel inverter called Isolated MultiModular Converter, this paper introduces the combination of two control strategies aimed at providing a wide range of operation under imbalanced energy states. A general analytical model, including the regulation capability and differences with an existing strategy are presented to demonstrate the performance of the control proposed. The effectiveness of the proposal is validated through experimental results based on a three-phase multilevel prototype., This work was supported in part by the European Commission under Project FLEXITRANSTORE—H2020-LCE-2016-2017-SGS-774407 and Project INTERRFACE—H2020-LC-SC3-2018-ES-SCC-824330, and in part by the Spanish Ministry of Science under Project ENE2017-88889-C2-1-R., Peer Reviewed, Postprint (published version)

©2019 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works., Grid operators require grid connected RGUs to provide specific dynamic features. RGUs must support dynamic performance of grid and operate in such a way to ward off new issues in grid. Recently, unfavorable oscillatory modes have appeared through grid connection of RGUs due to their dynamic interaction with other classical components of grid. Therefore, it is essential to develop a novel technique for dynamic design of RGUs to mitigate such risky oscillations. In this paper, a novel analytical method is proposed for dynamic tuning of a RSSG-SPC. Proposed method is based on mathematical analysis of derivative function of general damping ratio formula. The analytical results establishes generalized solutions that cover all operation mode of RSSG-SPC. Further on, solutions are implemented into dynamic model of RSSG-SPC to obtain clear, accurate and trustable criteria for tuning of virtual damping and virtual inertia. The dynamic tuning aims towards avoiding new oscillations in system and to mitigate natural oscillations of grid. Proposed approach was used for stability enhancement in high penetrated generation area and to support synchronization between interconnected areas in Kundur system. Proposed method was validated through modal analysis, time domain simulations and real-time evaluations which ensured that proposed approach is a reliable technique for dynamic design of RSSG-SPCs., Peer Reviewed, Postprint (author's final draft)

Grid-connected power converters have been increasingly utilized to inject renewable energy into the grid. To reduce installation cost and the same time to increase system stability, this paper presents control design for grid-connected power converters with only current sensors. The proposed control scheme mainly consists of a state-feedback controller and a state observer. To guarantee the stability of the closed-loop system, linear matrix inequality approach is employed to obtain control gains. As opposed to the existing approaches, the proposed scheme offers superior dynamic performance under voltage sag. Such improvement is achieved by incorporating the internal model of proportional controller into the state-feedback controller. The effectiveness of the proposed controller is validated by simulations under various operating conditions., This work was supported by the European Union’s Horizon 2020 research and innovation programme under grant agreement No 824330, project INTERRFACE H2020-LCSC3-2018-ES-SCC and by the Spanish Ministry of Science under project ENE2017-88889-C2-1-R., Peer Reviewed, Postprint (published version)

©2020 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works., This article presents a robust voltage sensorless control scheme to eliminate the need of line-voltage sensors in an LCL-filtered grid-connected inverter system. The proposed control scheme is based on an extended-state estimator structure which consists of a state feedback regulator and an estimator. The main contribution of this article relies on the utilization of the linear matrix inequality approach and the internal model principle to synthesis a robust control scheme that equips a grid-connected inverter with the ability to withstand grid disturbances. As opposed to the existing works, the proposed control scheme can simultaneously deal with all types of grid disturbances, such as grid voltage distortion, grid voltage imbalance, and grid impedance variations. The effectiveness of the proposed controller is highlighted through comparative simulation results and in-depth analyses. Experimental results are also given to validate the practicality of the proposed control scheme., This research was funded by the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie, grant number 712949 and the Spanish Ministry of Economy and Competitiveness under the project RTI2018-100921-B-C21., Peer Reviewed, Postprint (author's final draft)

© 2019 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes,creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works., Renewable energies has shown to be a promising solution to improve the energy injected to the electrical network. However, the lack of inertia in such systems have proven to affect the grid stability, as those technologies operate as current controlled sources they not provide grid support functionalities. This issue has fostered the implementation of new control strategies to enable power plants to provide a coordinated action to support some inertia and damping into the system. Synchronous controllers have proven to be an excellent option for distributed renewable generation, which allows to emulate the inertia of a synchronous generators, having a better interaction between the power converter and the electrical grid. However, the implementation of such control strategies require power converter to be smarter and include advanced control algorithms. In this paper, a new control strategy to control different distributed generation units to have a coordinated action in front of the electrical grid is presented, reducing the cost of the power converter by extracting the synchronous control from the internal controller and replacing in by a central synchronous controller., Peer Reviewed, Postprint (author's final draft)

©2019 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works., This paper presents a network clustering (NC) method for active distribution networks (ADNs). Following the outage of a section of an ADN, the method identifies and forms an optimum cluster of microgrids within the section. The optimum cluster is determined from a set of candidate microgrid clusters by estimating the following metrics: total power loss, voltage deviations, and minimum load shedding. To compute these metrics, equivalent circuits of the clusters are estimated using measured data provided by phasor measurement units (PMUs). Hence, the proposed NC method determines the optimum microgrid cluster without requiring information about the network’s topology and its components. The proposed method is tested by simulating a study network in a real-time simulator coupled to physical PMUs and a prototype algorithm implementation, also executing in real time., Peer Reviewed, Postprint (author's final draft)

©2019 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works., Grid stability is one of the main concerns in renewable energies. The lack of inertia and their low capability to provide frequency support has created the need for implementing new control strategies to solve this problem. In current networks, frequency and voltage support are performed through synchronous generators, which provide an inherent grid support due to the inertia presented in their mechanical rotors. Based on the same concept, renewable energies based on power converters have introduced synchronous controllers to emulate the dynamic behavior of synchronous generators and provide voltage and frequency support. However, most synchronous control strategies integrate their controllers as an add-on firmware embedded in each power converter, without presenting a coordinated synchronous performance when several converters operate in a PV power plant. The aggregation of several power converters operating with a coordinated synchronous response would be advantageous in these cases, since they can provide a harmonic response with an automatic power distribution when grid support is required. This paper presents a synchronous control strategy for photovoltaic power plants, which manages several power converters as an aggregated synchronous system., Peer Reviewed, Postprint (author's final draft)

This paper presents a Unified Power Flow Controller (UPFC) application of the Custom Power Active Transformer (CPAT); a power electronics integrated transformer which provides services to the grid through its auxiliary windings. The CPAT structure integrates three single-phase transformers into one shunt-series combining transformer. This integration empowers a sub-station with the capability of dynamically regulating the terminal voltage and current of a transformer through isolated power electronics converters. This paper investigatesthe CPAT’s capability to provide UPFC services which includes power flow control, reactive power compensation, voltage regulation and harmonics elimination. Simulations of the CPAT-UPFC with a stiff grid and a 5-bus power system demonstrates its functionality as an inter-bus couplingtransformer that provides the required grid services. Moreover, the impact of the CPAT-UPFC during load perturbations on the power system is investigated to further validate its transient and steady-state response. Furthermore, an experimental prototype reveals the operation of the three-phase CPAT-UPFC confirming its stable operation according to the theoretical expectations., Peer Reviewed, Postprint (author's final draft)

Multilevel converters are widely used in medium and high voltage applications. Their high performance, power quality, efficiency and smaller filters make them interesting for renewable energy distribution systems. In utility- scale photovoltaic plants, these topologies could provide multiple benefits since they are able to connect string of photovoltaic panels to independent modules. However, high floating voltages caused by high number of modules limit multilevel converters in medium and high voltage applications, since all of them are not suitable to provide isolation to each module. To offer a solution, this paper presents a novel multi-modular converter that provides multiple isolated modules connected in series through low frequency transformers to operate at medium voltage levels. This topology is able to achieve the power balancing between the connected modules and independently adjust the dc voltage of each module by means of controlling a circulating current which flows through the arms. Furthermore, the topology implemented in photovoltaic renewable energy systems and the control strategy required to regulate the circulating and the output current are presented. The main principle behind this concept and the performance of the converter are evaluated and validated through simulation and experimental results., Peer Reviewed, Postprint (author's final draft)

© 2019 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes,creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works., The control scheme of grid-tied converters is often implemented in the dq-frame due to simplicity of design. However, with this transformation, there exists an inherent cross-coupling term between the d-and q-channels which is often compensated for by using a feed-forward term in the current-control loop. It is shown, by applying the generalized Nyquist criterion (GNC) to the dq-frame ac impedance of the converter, that the inclusion of this decoupling term, in fact, degrades the stability of the controller when increasing the bandwidth of the synchronous reference frame phase-locked loop (SRF-PLL). Harware-in-the-loop (HIL) experiments are also conducted and verify these results., Peer Reviewed, Postprint (author's final draft)

© 2019 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes,creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works., Synchronous power controller (SPC) has emerged as a suitable technique to equip grid-connected inverters with grid supporting functionalities such as inertial emulation and frequency/voltage support by mimicking the behavior of synchronous machines. Although the feasibility of the SPC has been experimentally verified under various operating conditions, parameter tuning for the SPC to ensure a stable inverter system has not been adequately addressed in the literature. To fill this gap, this paper presents a robust control design for the SPC to ensure its stable operation under the grid impedance variation. The proposed design procedure consists of system modelling and robust optimal parameter selection by using linear matrix inequality approach. The effectiveness of the proposed control design is proven by means of simulations and experiments., Peer Reviewed, Postprint (author's final draft)

The increasing penetration of grid-connected RES systems, advanced control algorithms have been developed to operate under grid faults and fulfill strict requirements of the grid codes. In order to overcome this, the current controller performance is critical considering it as the inner control loop of any grid-connected RES system. Based on the resonant control concept, this paper presents a modified structure for this controller which results advantageous when implemented on RES systems, as it permits better performance during the dynamic state of the controller. This paper also deals with the analysis of the decoupling terms in the aß reference frame, as well as the capability to generate a decoupled control of the positive and the negative sequence. The proposed controller will be analyzed, discussed and finally validated by means of simulation analysis., Peer Reviewed, Postprint (author's final draft)

Peer Reviewed, Postprint (author's final draft)

The increasing penetration of renewable energy systems force the grid-connected power converters to use advanced active power controls, which needs to add more functionalities to their control strategies. A power reserve control is being required for grid support capabilities on Renewable Energy Sources (RES). Classical algorithms as the Maximum Power Point Tracking (MPPT) cannot withstand power reserve on its control, due to this, different control strategies have to be formulated to gain the power reserve capability. On this paper a cost-effective solution to perform the power reserve is presented, which provide a good solution for frequency regulation as well as for general grid fault conditions. Simulation test have been performed to verify the control capacity of this cost-effective solution., Peer Reviewed, Postprint (author's final draft)

This study deals with impacts of the shunt conductances associated with HVDC cables and VSC-HVDC stations on optimal operation of Multi-Terminal HVDC (MT-HVDC). In this study, for the first time, shunt conductances are integrated to HVDC Optimal Power-Flow (OPF) program that is executed at the Power Dispatch Center (PDC) of the MT-HVDC grid. With the objective of losses minimization, optimal reference operation points of the VSC-HVDC stations are derived. The operating points of the power converter stations are adjusted based on the calculations performed in the dispatch center. CIGRE DCS3 MT-HVDC grid, structured by CIGRE B4 working group, is taken as the test platform. Test results have revealed the optimum voltages and loss pattern change. Moreover, the findings are compared with the case of neglecting the shunt conductances., Peer Reviewed, Postprint (author's final draft)

Multi-modular converters have proven to be one of the most suitable topology to be used in high and power applications. The modularity of these configurations offer several advantages, such as: high quality voltage, redundancy and high efficiency. In these converters, the series connection of modules increase the number of voltage levels, but also the floating voltage of modules with respect the neutral point connection which may give rise to isolation issues at the DC side. Because of this, modules with two or more conversion stages are used to provide isolation and to prevent high floating voltages. However, increasing the number of conversion stages, increase also the losses and the implementation costs. In order to overcome these drawbacks, a new configuration of multi-modular converter is proposed in this paper. The proposed converter provides isolation with one conversion stage by means of using low frequency transformers in each module. The main principle behind this concept, the control structure and simulation results are presented to validate the proposed configuration., Peer Reviewed, Postprint (author's final draft)

Future Flexible HVDC Transmission Systems will consist of several active and passive infrastructures including DC Power Flow Controller (DC-PFC) device. The addition of the DC-PFC to the future Multi-Terminal HVDC (MT-HVDC) grids arises some key concerns such as stability, system interoperability, and possible adverse interactions. Hence, a suitable model is necessary to conduct deep frequency domain analysis. In this context, this paper proposes a linearized model for small-signal stability analysis of flexible MT-HVDC grids in state space framework that can be straightforwardly utilized in the process of control design. In this paper, a spick-and-span, systematic and step-by-step process to derive the small signal model of all flexible MT-HVDC grid components is presented such that each sub-system is modeled individually and then all are integrated together. The derived model is cross-verified by time domain simulations of a nonlinear system model in a MATLAB/SIMULINK platform for CIGRE DCS3 MT-HVDC test grid., Peer Reviewed, Postprint (author's final draft)

Distributed renewable energy sources progressively gain importance in power systems, with which new challenges, but also new opportunities, arise. Therefore, it is necessary to analyze the interaction between this type of generators and the grid, and to study adequate control methods that allow small generating units to operate harmoniously within power systems and contribute to their control, stability, and reliability. In this paper, the stability of a distribution system with several renewable energy generators is studied, focusing on its ability to maintain power supply during a transmission blackout. The response of the distribution system after a sudden disconnection from the main network is simulated and analyzed, comparing the results obtained using a conventional control for the renewable generators with those of controllers that enable a synchronous interaction between these generators and the power system., Peer Reviewed, Postprint (author's final draft)

This paper proposes an efficient control framework that utilizes dc-dc converters to achieve flexible power flow control in multiterminal dc (MTDC) grids. The dc-dc converter employed in this paper is connected in cascade with the dc transmission line, and is therefore named cascaded power flow controller (CPFC). In this paper, a two-layer control strategy is developed for the operation and control of voltage source converter stations and CPFC station in MTDC grids. At the primary control layer, a novel differential voltage droop control is developed, while at the secondary control layer, a modified dc power flow algorithm-employing the new CPFC framework-is implemented. The overall control strategy enables the CPFC to regulate the power flow in the dc transmission line. The primary control guarantees the transient stability of the CPFC, and the secondary control system ensures the desired steady-state operation. The proposed voltage droop control framework helps the MTDC grid to remain stable in the event of a communication failure between the primary and secondary control layers. Static analysis and dynamic simulations are performed on the CIGRE B4 dc grid test system, in order to confirm the effectiveness of the proposed control framework for power flow regulation in MTDC grids., Peer Reviewed, Postprint (author's final draft)

Photovoltaic power plants (PV) are equipped with anti-islanding algorithms, embedded in the converters controller, to avoid the island operation. However, the current trends in the development of the future electrical networks evidence that it is technically feasible and economically advantageous to keep feeding islanded systems under these situations, without cutting the power supply to the loads connected to the network. Nevertheless, commercial PV power converters are programmed as grid-feeding converters, and they are unable to work in island mode if there is not an agent forming the grid. In order to overcome this problem the Synchronous Power Controller (SPC) is presented in this paper as a suitable alternative for controlling PV inverters. As it will be further discussed this controller permits PV plants to operate seamless in grid connected and island mode, with no need of changing the control structure in any case. Moreover, the participation of SPC based power converters integrating energy storage enables other grid-feeding systems to contribute to the grid operation in island conditions. The results achieved with the SPC will be shown in simulations and also with experiments considering a real PV power plant combining SPC and comercial converters., Peer Reviewed, Postprint (author's final draft)

Development of Medium Voltage DC (MVDC) distribution systems require using proper protection schemes whereas due to the behavior of dc fault current, the conventional fault detection methods for ac distribution systems cannot provide a selective protection for these networks. In this paper, a differential based protection method using Ethernet communication has been implemented for a radial MVDC distribution system with integrated PV and WT. This method must be fast enough to detect the faults before involving the main converters and separate only the faulty feeder in order to increase the reliability of the power system. The experimental validation is performed by using Hardware-in-the-Loop (HIL) approach and communication based on the IEC61850 protocol. A real time simulator (OPAL-RT) and a development board (DK60) are used to evaluate the method and to calculate the time delay of this fault detection algorithm., Peer Reviewed, Postprint (author's final draft)

Connecting renewable power plants to the grid must comply with certain codes and requirements. One requirement is the ramp rate constraint, which must be fulfilled in order to avoid penalties. As this service becomes compulsory with an increased grid penetration of renewable, all possible solutions must be explored especially that large battery energy storage systems are still expensive solutions. Thus, in order to make battery investment economic viable, the use of second life batteries is investigated in the present work. This paper proposes a method for determining firstly, the optimal rating of a second life battery energy storage system (SLBESS) and secondly, to obtain the power exchange and battery state of charge profiles during the operation. These will constitute the cycling patterns for testing batteries and studying the ageing effect of this specific application. Real data from the Spanish electricity market for a whole year are used for validating the results., Peer Reviewed, Postprint (author's final draft)

Global warming is already having significant and costly effects on our communities, our health, and our climate. Nowadays, In developing countries (G20) around the world, decisions are being considered and a shifting of our dependence from fossil fuels to renewable energy sources (RES). Among RES solar energy, specially photovoltaic system (PV) play a key role in this challenge, thus, this paper provides a comparative study on the performances of three transformer-less inverters for photovoltaic applications (H-Bridge, H5, HERIC and NPC inverters) in terms of power losses, considering the use of different kinds of switches. Silicon (Si) and Silicon Carbide (SiC) diodes have been combined with two topologies of IGBTs in the performed tests. Simulation results have been obtained for each of the systems in different operating conditions by using PLECS software. Power losses and European Efficiency (EE) have been calculated for all the topologies, taking into account the real characteristics of the switches by using the thermal models provided by the software library., Peer Reviewed, Postprint (published version)

The impact caused by the large scale penetration of renewable energy sources into electrical grid has been given an increasing concern in the past decade. Multiple challenges will occur in the future when the share of the traditional synchronous generators is reduced. One of the main issues is the lack of rotational inertia in the grid, which may cause stability issues. Therefore, the renewable energy generation plants have been asked for new control objectives and services. In this paper, three types of active power synchronizing controllers are analyzed and compared with each other. All the three types of controllers are able to provide inertia, damping and droop characteristics, which are the favored features for the future renewable energy generation plants. The comparisons are hence conducted in these three aspects. Theoretical and experimental comparisons are both given, with the comments and conclusions., Peer Reviewed, Postprint (published version)

The increase of renewable energy sources and distributed generation may have a negative impact on power systems, specially on weak ones. Therefore, their control systems are acquiring growing importance, with new proposals that aim to contribute to the control and stability of the power system as conventional generating units. This paper studies the effect that a PV system using a synchronous power controller may have on a small power system constituted by a diesel generator, two loads and the PV generating unit. The response of this power system to realistic disturbances is compared with the case where the PV unit is controlled to provide its maximum power production regardless of the state of the system., Peer Reviewed, Postprint (author's final draft)

This paper proposes a novel wave energy converter (WEC) control technique based on an adaptive vector controller for maximum power absorption of the wave energy resource. The proposed controller arises as a suitable solution for a realistic wave energy converter deployment, as it serves from its own wave energy converter velocity to instantaneously determine the required power-take-off force needed for maximizing the power extraction from the waves. The adaptive characteristic of the wave energy controller is achieved thanks to the implementation of advanced signal monitoring and synchronization processes along with vector control algorithms. As a result, maximum power absorption can be instantaneously achieved regardless of the dominant characteristics of the irregular waves, Peer Reviewed, Postprint (author's final draft)

The continuous integration of renewable energy sources into power systems has led to the construction of large power plants based on power electronics, with power ratings up to hundreds of megawatts, in order to accomodate technologies like solar PV and wind energy. Due to the size of these plants, they must provide support services for the grid and it is necessary to analyze the impact they have on power systems. Opposing to conventional power plants formed by a reduced number of synchronous generators in the range of several hundreds of megawatts, large power plants based on PV and wind energy are usually composed of a high number of individual generating units with power ratings of a few megawatts. Therefore, it is of great importance to develop aggregated models of power plants formed by multiple power converters to be used in the dynamic analysis of large power systems. This paper presents the control system of a 20 MW PV plant, derives an equivalent aggregated model and studies the performance of this model through simulation., Peer Reviewed, Postprint (author's final draft)

In this paper, application of Posicast control method to generator excitation system is presented. The method is one of the simplest possible control design methods that can be applied to damp the oscillations caused by changing the excitation reference signal. Stability and robustness of the designed controller are shown using extensive time domain simulations. All the detailed simulations are carried out in MATLAB/Simulink environment., Peer Reviewed, Postprint (published version)

This paper proposes a distributed algorithm for Fault Location, Isolation, and Service Restoration (FLISR) to be implemented as part of the intelligent software for controlling the operation of each circuit breaker. The paper proposes an event driven finite-state machine design that assures the self-reconfiguration of the distribution power grid when a fault occurs on a certain section of a feeder. The benefits of such an approach are first of all the modularity, as each breaker is controlled by the same algorithm and requires data exchange only with the neighboring protection devices. Secondly, by using the event driven technique, the FLISR algorithm is perfectly suited for taking advantage of the features and benefits defined by the IEC 61850 protocol. Using GOOSE messages for transferring status information between the neighboring circuit breakers, the proposed distributed algorithms act in a collective manner for reconfiguring the distribution grid., Peer Reviewed, Postprint (published version)

In this paper, DC-link voltage control in DC microgrids with photovoltaic (PV) generation and battery, is addressed based on an intelligent approach. The proposed strategy is based on the modeling of the power interface, i.e. power electronic converter, located between the PV array, battery and DC bus, by use of measurement data. For this purpose, a local model network (LMN) is developed to model the converter and then a local linear control (LLC) strategy is designed based on the LMN. Simulation results on a DC microgrid demonstrates efficacy of the proposed approach., Peer Reviewed, Postprint (published version)

The increasing penetration of renewable energy sources in power systems requires new control systems that provide services similar to those of conventional generators. This paper proposes an active power controller that is able to automatically achieve synchronization with the grid and to control the DC link voltage. Through this controller, the converter can be identified with a virtual synchronous generator whose kinetic energy is stored in the DC link. Moreover, the active power loop becomes a second order system whose damping can be programmed freely., Peer Reviewed, Postprint (published version)

This paper develops a flexible DC transmission system (FDCTS) to power control and load flow control within the multi-terminal (MTDC) grids. The FDCTS include static power electronics-based elements for electric energy transmission inside a DC grid. Considering the current advancements in the field of power electronics, this paper proposes a cascaded power flow controller (CPFC) as the first element of the FDTCS for providing power control in MTDC grids. It is shown through dynamic simulations that the CPFC is able to control power flowing through a particular DC transmission line, and thus enhancing controllability in the MTDC grids., Peer Reviewed, Postprint (published version)

This paper develops a hybrid power flow controller (HPFC) to take one step forward for building future flexible DC transmission system (FDCTS). The idea for proposing HPFC is inspired by the successful operation of flexible AC transmission systems (FACTS) devices, the FDCTS includes static power electronics-based elements to provide voltage regulation, power control and load flow control in the multi-terminal DC (MTDC) grids. The HPFC, as one of the devices of the FDCTS, has a hybrid connection scheme (i.e. series-parallel) to the grid and allows power flow control through DC transmission lines by imposing a series voltage to the controlled DC line. In this work, in order to demonstrate the eligibilities of the HPFC, the performance of this device is included in the DC power flow formulation for the power flow control purposes. Finally, it will be shown through dynamic simulations that the proposed HPFC is able to control the power flow through a particular DC transmission line., Peer Reviewed, Postprint (published version)

Power generation facilities based on renewable energy sources, such as PV or wind, are frequently connected to the grid through three-phase power converters. As the power of the plant increases, the design of the grid connection filters becomes critical, as a fast dynamic performance is needed while the injection of current harmonics to the network should be prevented. LCL filters, together with trap filters tuned at the switching frequency, provide a good trade-off between both needs. However, the tuning of the controllers becomes more complex as the LCL+trap filter lead to a considerably higher order system. In this paper a tuning method for determining the parameters of proportional resonant current controllers for three-phase high power converters linked to the grid through a LCL+trap filter is proposed and analyzed. In order to validate the proposed solution, different results taken in different scenarios are shown., Peer Reviewed, Postprint (published version)

Because renewable energy sources are environment-friendly and inexhaustible, more and more said renewable energy power plants have been integrated into power grids worldwide. To compensate for their inherent variability, STATCOMs are typically installed at the point of common coupling (PCC) to help their operation by regulating the PCC voltage. However under different contingencies, PCC voltage fluctuations in magnitude and frequency may impede the STATCOM from tracking the grid frequency correctly, hence worsening its overall compensation performance, and putting at risk the operation of the power plant. Further, the virtual synchronous machine (VSM) concept has recently been introduced to control grid-connected inverters emulating the behavior of rotating synchronous machines, in an effort to eliminate the shortcomings of conventional d-q frame phase-locked loops (PLL). In this paper, the VSM concept is extended by developing a STATCOM controller with it, which then behaves like a fully-adjustable synchronous condenser, including the adjustment of its “virtual” inertia and impedance. The proposed controller is compared against existent d-q frame STATCOM control strategies, evincing how the VSM-based approach guarantees not just better synchronization, but an improved voltage regulation performance at the PCC, attained through its virtual impedance, as well as a lower sensitivity to system disturbances, attained through its virtual inertia. The paper will include the complete design procedure for the VSM-STATCOM, and the verification of key results through detailed simulation studies., Peer Reviewed, Postprint (published version)

Renewable energy sources are increasing their penetration in power systems, making necessary new control systems that offer services usually provided only by conventional generators. In this paper, an active power controller able to achieve synchronization with the grid and to control the DC link voltage is proposed. This controller allows identifying the converter with a virtual synchronous generator whose inertia can be modified online, considering that its virtual kinetic energy is stored in the DC link. Additionally, the resulting active power loop is a second order system whose damping factor can be defined freely., Peer Reviewed, Postprint (published version)

Large penetration of renewable energy is currently attenuated by concerns regarding their impact on the controllability and reliability of the electrical system. As the inclusion of energy storage is to a great extent the solution to these issues, this paper proposes a methodology for approaching the calculation of the size of the energy storage to be connected to a PV power plant for providing inertia emulation, primary control, and the reduction of power fluctuation. A complete control strategy, developed for the inclusion of these services in the operation of a dc-ac grid connected converter has been implemented and validated through simulation results. The obtained results are validated by experimental tests performed on a scaled 10 kW prototype., Peer Reviewed, Postprint (published version)

A fault location method and a fault clearance strategy are presented in this paper for medium voltage dc (MVDC) distribution system. MVDC systems are applicable for connection between microgrids (MGs) and integration of renewable energy systems (RESs) to distribution systems. Due to the specifications of fault current in dc systems, it is difficult to coordinate the over current (O/C) relays based on the time inverse grading. Hence, in this paper, a communication link between O/C relays is used to diagnose the fault location. On the other hand, the fault clearance is done by the operation of dc circuit breakers (DCCB) and isolator switches. In this protection strategy, O/C relays detect the faulty part using communication links and after the fault extinguishing by DCCBs, the dc switches isolate the faulty part. Finally, the sound parts of the system re-energize when DCCB are re-closed. Moreover, data transmission by communication links is based on the standard messages of IEC61850 protocol., Peer Reviewed, Postprint (published version)

The current route of achieving the ultimate plan for flawless operation and control of the multi-terminal DC (MTDC) grids can be significantly accelerated by learning from the vast and valuable experiences gained from the operation of the AC power grids for more than a century. This paper introduces concept of flexible DC transmission system (FDCTS), inspired by the successful operation of flexible AC transmission systems (FACTS), to provide voltage regulation, power control and load flow control within MTDC grids. Considering the current advancements in the field of power electronics, this paper recognizes DC-DC converters as the first element of the FDTCS for providing voltage and power control in MTDC grids. By use of DC-DC converters, this paper developes two elements of the FDCTS, namely the cascaded power flow controller (CPFC) and hybrid power flow controller (HPFC). In this paper, to demonstrate the eligibility of the CPFC and HPFC to play the role of an FDCTS, they are included in the DC power flow formulation for DC voltage regulation and power flow control purposes., Peer Reviewed, Postprint (published version)

This paper proposes an effective dc voltage and power-sharing control structure for multiterminal dc (MTDC) grids based on an optimal power flow (OPF) procedure and voltage-droop control implemented in the different hierarchical layers. In the proposed approach, an OPF algorithm is executed at the secondary control level of the MTDC grid to find the optimal reference values for the dc voltages and active power of the voltage-regulating converters. Then, at the primary control level, the voltage-droop characteristics of the voltage-regulating converters are tuned based upon the OPF results. In this control structure, the optimally-tuned voltage-droop controllers lead to the optimal operation of the MTDC grid. In case of variation in load or generation of the grid, a new stable operating point is achieved based on the voltage-droop characteristics. Then by executing a new OPF, the voltage-droop characteristics are returned for optimal operation of the MTDC grid after the load or generation variations. This paper also considers the integration of frequency support loop in the proposed control framework in case of connection of weak ac grids. The simulations performed on a study case inspired by the CIGRE B4 dc grid test system demonstrate the efficient grid performance under the proposed control strategy., Peer Reviewed, Postprint (published version)

This paper proposes a hierarchical control architecture designed for an arbitrary high voltage multiterminal dc (MTDC) network. In the proposed architecture, the primary control of the MTDC system is decentralized and implemented using a generalized droop strategy. Design criteria for dimensioning the primary control parameters, including voltage limits, are offered by analyzing the transients appearing in the system. The proposed secondary control is centralized and regulates the operating point (OP) of the network so that optimal power flow (OPF) is achieved. Compared to previous works, this paper further elaborates, both analytically and through simulations, on the coordination between the primary and secondary control layers. This includes how local primary controllers have to be driven by the centralized controller in order to ensure a smooth transition to the optimal OP., Peer Reviewed, Postprint (published version)

Multi-terminal dc (MTDC) networks are drawing a lot of interest lately in applications related to distributed generation, especially in those that integrate energy storage. Several approaches for controlling the operation of such systems have been proposed in the literature; however the existing structures can be significantly enhanced. This paper proposes an improved primary control layer, based on droop and dc-bus signaling to serve as a base framework for implementing a hierarchical control structure in a MTDC system. As it will be further discussed in this work, five operating bands as well as various droop characteristics for different elements connected to the dc-bus were defined. For the energy storage the state of charge (SoC) was taken into account at the primary control level and it was included in the droop characteristic, creating a two variables droop surface. The proposed control strategy was validated through simulation and experimental results obtained from a case study that involves a micro dc network composed of a PV generator, a lead-acid battery and one connection point to the ac grid. © 2013 IEEE., Postprint (published version)

This paper proposes a generalized voltage droop (GVD) control strategy for control and power sharing in voltage source converter (VSC)-based multi-terminal DC (MTDC) grids. In the proposed approach, the conventional voltage droop characteristics of voltage-regulating VSC stations are replaced by the GVD characteristics. The proposed GVD control strategy can be operated in three different control modes including conventional voltage droop control, fixed active power control and fixed DC voltage control by proper adjustment of the GVD characteristics of the voltage-regulating converters. The proposed strategy improves the control and power-sharing capabilities of the conventional voltage droop, and enhances its maneuverability. Simulation results, obtained by detailed modeling in a four-terminal high voltage DC grid demonstrated the efficacy of the proposed approach and its flexibility in active power sharing. Moreover, based on the obtained results, switching between operating mode of the GVD control approach does not results in oscillations of the active powers flowing inside the MTDC grids., Postprint (published version)