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Multiterminal dc networks are drawing a lot of interest lately in applications related to distributed generation, particularly in those that also integrate energy storage (ES). A few 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 custom droop characteristics obtained by combining concepts of droop and dc-bus signaling control. This approach is designed to be generic and takes into account the various operating states of the network. Five operating bands, similar to the operating states of the ac grids, as well as various droop characteristics for different elements connected to the dc network, are defined. For the ES, the state of charge is taken into account at the primary control level and included in the droop characteristic, creating a two-variable droop surface. The proposed control strategy is validated through simulation and experimental results obtained from a case study that involves amicro dc network composed of a photovoltaic generator, a lead-acid battery, and a connection point to the ac grid., Peer Reviewed, Postprint (published version)

The connection of Distributed Generation (DG) units to existing networks may give rise to harmful effects on the network and the DG units itself. Therefore, due to increasing penetration of DG units, it is important to consider these effects in order to avoid some of harmful events. Ferroresonance is one of these destructive phenomena. Over voltages due to this phenomena can damage devices which are connected to the network. In DG systems, due to the use of capacitor banks which are used for VAR compensation, grid connection filters, etc. ferroresonance may appear. Also, special operations like islanding, may lead these systems to ferroresonance. In traditional systems ferroresonance normally appear after there is an unbalance in the system. Nevertheless, for networks with significant penetration of DG units, the situation that gives rise to ferroresonance may be different from traditional systems and they can happen without any unbalancing occurrence. In this paper a distribution system integrating wind (WT) and photovoltaic (PV) power systems has been evaluated in order to analyze the performance of ferroresonance in such systems. Finally, the most appropriate techniques to prevent its appearance, and the associated over voltages, have been studied and implemented using PSCAD/EMTDC., Peer Reviewed, Postprint (published version)

Battery Energy Storage Systems (BESSs) could contribute to the generation/consumption balance of the grid and could provide advanced functionalities at different grid levels (generation, T&D, end-user and RES integration). In this paper an analysis and comparison of Battery Energy Storage (BES) technologies for grid applications is carried out. The comparison is focused on the most installed technologies in the recent experimental BESS installations. Furthermore, the paper presents a new methodology aimed at selecting the most suitable BES technology for a specific grid application. This methodology defines a priority level for each technical and economical characteristic of the BES technologies. Finally, the proposed methodology is applied for a specific grid application confirming its contribution in the selection of the best-suited technology., Peer Reviewed, Postprint (published version)

This paper presents a method for improving the design procedure of AC-DC power converters for power generation applications. The proposed methodology is based on selection of the most convenient configuration, in terms of switching frequency and design of the line filter, in order to minimize the overall losses in the system. In this work two types of semiconductor devices are considered: silicon IGBT and silicon carbide MOSFET for the design of a 100 kVA system, and they performance is compared. The AC-DC converter parameters, the design methodology and preliminary simulation results for representatives of Si IGBT and SiC MOSFET modules are presented., Peer Reviewed, Postprint (published version)

The high penetration of renewable energy resources, such as wind and photovoltaic, as well as the development of power electronics devices play an important role in the development of DC grids. However, the evolution of DC-systems from point-to-point connections to ‘Multi-terminal’ DC (MTDC) networks is not so straight forward. At the current moment there is a large gap in the literature concerning the dynamics, control and operation of MTDC networks. On the other hand, conventional AC networks have been studied exhaustively in the past decades providing for a rich inspiration source. In this paper we are going to establish a comprehensive analogy between the characteristics of conventional AC grids and the emerging DC grids. The key similarities and differences between the two types of grids are identified in an attempt to better understand the characteristics and behavior of DC grids. In the end, the extent to which concepts like planning for operation and control of AC grids can be conveniently extrapolated to DC grids is evaluated., 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)

Multi-terminal DC (MTDC) systems are drawing a lot of interest lately in applications related to distributed generation, especially in those that integrate wind or PV systems together with energy storage. This paper proposes an improved strategy for the primary control of MTDC integrating energy storage (ES) units. The concept is based on the combination of a droop control method and dc-bus signaling in order to provide a generic approach to be used as basis for a hierarchical control structure in a multi-point DC-bus system. The proposed method considers the State of Charge (SoC) of the energy storage system at the primary control level, yielding a surface characteristic for a droop control that depends both on the SoC and the dc bus voltage. Finally, a method for designing and tuning the system is proposed and analyzed theoretically and through simulations and experiments on a 10kVA system with energy storage., 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)

Peer Reviewed, Postprint (published version)

This paper presents a methodology for designing the link passive filter of the power converters installed in a multi-megawatt wave power plant. Such filter is based on the superposition of both LCL and trap filter configurations with the purpose of achieving enhanced grid interaction of a twolevel voltage source converter. The LCL+Trap filter design is highly dependent on the electrical network characteristics; hence the distribution system of the plant is introduced in the paper to take into account its effects on the tuning parameters. The proposed LCL+trap filter topology arises as a successful alternative solution for the conventional LCL filter topology, thanks to its improved filtering capability while ensuring a reduced filter size. The validation of the proposed filter performance is evaluated in simulation by performing a comparative analysis between the LCL+trap and the LCL filters when operate in a wave farm application., Peer Reviewed, Postprint (published version)

The high penetration of distributed generation, as PV or wind power, has forced the Transmission System Operators (TSOs) to set restrictive requirements for the operation of such systems. As it can be extracted from the forthcoming grid codes drafts, the future distributed generation systems will be requested to have the equivalent performance of a synchronous generator, which is seen from the TSOs as the only solution if a massive integration of renewable in the electrical network should be achieved. In this paper a method for controlling PV grid connected power converters as a synchronous generator, namely Synchronous Power Controller (SPC), is presented. As a difference with previous works this method permits to take advantage of emulating the synchronous behavior meanwhile it is able to get rid of its drawbacks. The main concept of the SPC, as well as some simulation and experimental results will be shown in this paper considering a PV power plant as a study case., Peer Reviewed, Postprint (published version)

This paper presents the analysis of shunt active filter (SAF) in harmonic damping. SAF with voltage-based detection control act as an active damping and able to attenuate harmonic propagation throughout the power distribution network. The active filter is implemented on two different power line networks to assess its practicality in different network configuration. The first network is 220-V distribution power line and the second network is the reduced equivalent WPP network of 150-kV, 444.4-MVA. Assessment on the best place for active filter implementation is supported by using harmonic resonance mode analysis method., Peer Reviewed, Postprint (published version)

The connection of electronic power converters to the electrical network is increasing mainly due to massive integration of renewable energy systems. However, the electrical dynamic performance of these converters does not match the behavior of the network, which is mainly formed by generation facilities based on big synchronous generation systems. Depending on the desired electrical operation mode different control structures can be implemented in the converters in order to get adapted with the grid conditions. However, changing between different control structures and operation is not an optimal solution, as the resulting system results complex and is not highly robust. As an alternative, this paper presents a new control technique for grid connected power converters based on the concept of virtual admittance. The proposed control permits to emulate the electrical performance of generation facilities based on classical synchronous generators with a power converter, with no need of implementing different control structures, giving rise to a system that provides a friendly and robust operation with the network., Peer Reviewed, Postprint (published version)

This paper proposes a novel control strategy in multi-terminal DC (MTDC) grids with offshore wind generation based on the idea of generalized voltage droop (GVD). In the proposed strategy, the GVD characteristics are assigned to the voltage-regulating converter station, enhancing their control maneuverability and enabling them to perform necessary DC voltage control and power sharing tasks. By adjusting the coefficients of the corresponding GVD characteristic, a converter station can be operated in three operating modes, namely (1) fixed active power control mode, (2) conventional voltage droop control mode or (3) fixed DC voltage control mode. Results of simulations on a test four-terminal DC grid including two offshore wind farm and two AC grids show the capabilities of the proposed control strategy. Moreover, simulation results confirmed stable operation of the GVD control in transition from one operating mode to another., Postprint (published version)

This paper presents a filter design methodology and control design strategy for LCL+Trap filters deployed in large scale wave power plants. This filter topology appears as an alternative for the widely used LCL filters, since effective harmonic mitigation can be achieved while reducing the overall passive filter components size. As a result, the use of LCL+Trap filters for two-level power converters arise as a suitable power processing solution to match the requirements of large scale wave energy applications, since such well-known power converter technology is very robust, reliable and very cost effective. The designed LCL+Trap filter is validated through simulation by performing a comparative analysis between LCL and LCL+Trap filters, and finally, the control of the converter is introduced with the purpose of validating the grid connection compliance of the entire power processing system., Peer Reviewed, Postprint (published version)

In this paper a five-level Multi-Clamped Multilevel Converter (5L-MLC2) topology is analyzed based on a typical grid integration application for renewable energy system. The loss and thermal distribution of the power devices in different switching legs are investigated and illustrated under rated condition. Afterwards, a loss and efficiency evaluation method is proposed and applied which take into account various injected reactive power ranges regulated by the grid codes. It is concluded that the loss and thermal distribution of 5L-MLC2 multilevel topology is unequal between the clamping switching legs and the main switching legs, it also shows a good and consistent efficiency characteristic under different output power ranges when complying with the grid codes., Preprint

This paper addresses the important issue of Multi-Terminal DC (MTDC) grid control strategy based on the particle swarm optimization (PSO) technique. The MTDC grid is controlled by the concept of vector control, in which the AC currents and voltages are transformed into the rotating directquadrature (d-q) reference quantities which are subsequently used for decoupled control of the active and reactive powers as well as the DC and AC voltages. The paper employs an efficient PSO algorithm for optimal tuning of the controllers’ parameters in an MTDC grid. In addition, voltage droop control scheme is utilized to ensure the balance of active power within the MTDC grid. Simulation results that has obtained through detailed modeling of a four-terminal DC grid, demonstrate the efficiency of the proposed approach. Comparison to controllers optimized by genetic algorithm (GA) also confirmed the favorable performance of the proposed PSO-tuned controllers., Postprint (published version)

In this paper a new application of PSO technique for optimizing the controller parameters of multi modular converters is presented. PSO based LQR controller is designed for averaged model of modular converter and to obtain better control parameters though the minimizing a quadraticperformance index. In this method in order to have more accuracy and better design for controller parameters, PSO algorithm is added to find the gain matrix of the controller. Simulation results show that with using this method better dynamic responses are accessible., Peer Reviewed, Postprint (published version)

This paper presents a hierarchical control structure for Multi terminal HVDC grids. The hierarchical control structure is inspired by the three-layer control structure of largescale AC power systems and it is divided into primary, secondary and tertiary control. This structure permits to create an easy interaction between power dispatch and the lower controllers. Moreover, a generalized voltage droop (GVD) characteristic is also proposed which facilitates control issues inside the Multi terminal HVDC grids. In the proposed control structure, the coefficients of the GVD characteristics are set based on the solution of a DC power flow algorithm, executed in the secondary control level., Postprint (published version)

There is a growing interest in electrical grid connection of single unit Wave Energy Converters (WECs), as well as the multiple units based Wave Power Plant (WPP) applications. Due to the increasing integration of high power converters in WPP, additional concerns on the harmonic current mitigation of WEC device arise which requires the grid filter installation. Furthermore, the identification and appropriate mitigation of additional harmonics at the WPP level is needed, as any inductances and capacitors of the WPP can interact with the resonant frequency of the entire system and lead to detrimental effects on equipment. The present paper focuses on the potential harmonic studies of a realistic proposed WPP layout, in which the system resonances are identified by performing the frequency scan and modal analysis (HRMA) methods. The later technique has the ability of determining the participation factor of each bus, finding in this way the nodes in the system that have more influence on the obtained results. In this way, the most suitable nodes in which additional filtering solutions should be implemented can be identified., Peer Reviewed, Postprint (published version)

In the wave energy sector, an increasing concern regarding the grid connection of wave energy converters is recently being experienced, as well as in the main challenges resulting from the grid integration of large scale wave power plants. This increasing interest arise due to the near-commercial developments of several wave energy converter devices, which have already proved its hydrodynamic and power take-off performance. Therefore, this paper proposes a suitable power processing system configuration and its suitable control design used for interfacing the grid connection of such high power wave energy converters while ensuring acceptable grid connection standards compliance. An advanced filter topology will be introduced based on the superposition of LCL+Trap filters as an alternative solution to the conventional LCL filters, as it allows for smaller filter size, while ensuring a reliable and cost-effective power processing solution. Finally, the evaluation of the filter design as well as the power converter control will be introduced in simulation with the purpose of validating the grid connection of wave energy converters., Peer Reviewed, Postprint (published version)

A new application of LQR controller with the ability of reference tracking for a multi modular converter with several cells is proposed. First, the mathematical model for the grid connected MMC is presented and then, the small signal state space model is obtained. MMC is a very complex MIMO system with several state variables and global control for this system is very important issue. Optimal advanced controller based on linear quadratic regulator with integral action is a very good candidate for controlling the AC signals and energy balancing in multi cells MMC system. The performance and validity of obtained model and designed linear controller are evaluated with non-linear model of MMC in SimPower System toolbox/Matlab. Simulation results are verified the excellent performance of the proposed approach in the transient and steady state operational conditions., Peer Reviewed, Postprint (published version)

Electrical Energy generation is an issue that is continuously cause of concern around the world. Many efforts have been done in this sense to cover the requirements of the constantly growing in the electrical energy demand. But not only the electrical energy demand is growing but also clean electrical energy demand. In this sense, many countries are taking advantage from the renewable energy generation systems, considering mainly wind and solar energy. Solar energy systems provide a high percentage of the total energy production, according with the latest report of the International Energy Agency (IEA) regarding Photovoltaic Power System Program (PVPS), the cumulative installed PV power at the end of 2009 it was around 20.3 GW out of which 6.188 GW were installed in 2009. From the total PV power installed in 2009, 6.113 are grid connected systems. The growing of the PV systems is due to the new technologies and developments that have permitted to reduce costs in the total design and installation of a PV source. As the major percentage of the total PV energy installed is from grid connected systems, this thesis work deals with the analysis and proposals in the transformerless grid-connected PV systems which can provide higher efficiencies regarding PV system with transformer. In this sense, when there is not transformer between the electrical grid and the power converter, a problem regarding leakage ground currents appears, this is the main issue in this thesis work. The main research task in this thesis work is to analyze and evaluate the operation of the different transformerless topologies presented in the bibliography and then to provide some solutions to minimize the leakage ground current phenomenon in order to comply with the standard requirements., Postprint (published version)

In this paper a five-level Multi-Clamped Multilevel Converter (5L-MLC2) topology is analyzed based on a typical grid integration application for renewable energy generation system. The loss and thermal distribution of the power devices in different switching legs are investigated and illustrated under rated operation. Afterwards, a loss and efficiency evaluation method is proposed and applied which takes into account various reactive power operation boundaries regulated by the grid codes. It is concluded that the 5L-MLC2 multilevel topology shows a good and consistent efficiency characteristic under different output active powers when complying with the active/reactive power operation range regulated by e.g. the German grid codes, Postprint (published version)

High penetration of wind energy into the grid may introduce stability and power quality problems due to the fluctuating nature of the wind and the increasing complexity of the power system. By implementing advanced functionalities in power converters, it is possible to improve the performance of the wind farm and also to provide grid support, as it is required by the grid codes. One of the main compliance difficulties that can be found in such power plants are related to reactive power compensation and to keep the harmonics content between the allowed limits, even if the power of the WPP converters is increasing. This paper deals with an advanced control strategy design of a three-level converter performing STATCOM and Active Filter functionalities. The proposed system is called Smart-STATCOM since it has the capability of self-controlling reactive power and harmonic voltages at the same time. Therefore, deciding the amount of non-active currents (fundamental, 5th and 7th harmonics) to be injected depending on the PCC voltage quality. Experimental results of the proposed control strategy are analyzed in order to validate the performance of the entire system. Keywords- STATCOM, Active Filter, Power Quality, Postprint (published version)

In the last few years, restrictive grid codes have arisen to ensure the performance and stability of electrical networks, which experience amassive integration of renewable energy sources and distributed generation systems that are normally connected to the grid through electronic power converters. In these codes, the injection of positive- and negative-sequence current components becomes necessary for fulfilling, among others, the low-voltage ride-through requirements during balanced and unbalanced grid faults. However, the performance of classical dq current controllers, applied to power converters, under unbalanced grid-voltage conditions is highly deficient, due to the unavoidable appearance of current oscillations. This paper analyzes the performance of the double synchronous reference frame controller and improves its structure by adding a decoupling network for estimating and compensating the undesirable current oscillations. Experimental results will demonstrate the validity of the proposed decoupled DSRF controller, Postprint (published version)

A new combined method based on optimal control is proposed to enhance load frequency control dynamics. This method include of PSO and optimal output feedback controller. In the output feedback method only the measurable state variables within each control area is required to use for feedback. The optimal control law is determined by minimizing a performance index under the output feedback conditions leading to a coupled matrix equation. But for more accuracy and better design for this controller, PSO algorithm is applied to find the gain matrix of the controller. The results of the simulation are shown that with using this method, the load frequency control requirements in a practical environment are satisfied and also better dynamic responses are accessible, Postprint (published version)

Harmonic is one of main concern in Wind Power Plant (WPP) as growing usage of power electronic converter is observed. Besides the injection of harmonic by individual wind turbine (WT) as source of harmonic, its propagation in network and problem of resonance is also an issue. By implementing passive filter for harmonic mitigation, additional resonance peak is observed due to its interaction with system side impedance. This paper explores the solution for harmonic mitigation with capability of damping resonance in WPP. An aggregated model of 400-MW WPP network is analyzed in correlating resonance and amplification. A passive filter and hybrid filter is designed to mitigate harmonic accordingly. It is shown that hybrid filter able to provide damping capability and improve the performance, Postprint (published version)

A novel optimization algorithm is presented to improve the design of optimal controllers for load frequency control problem. This algorithm is applied for two area LFC system with using an output feedback controller. In the practical power system, access to some of the state variables in LFC system is limited and measuring is also impossible. So an optimal output feedback controller with a practical viewpoint is proposed. The optimal control law is determined by minimizing a performance index under the output feedback conditions leading to a coupled matrix equation. In order to solve these equations traditional methods may be used. But for more accuracy and better design for this controller, ICA algorithm is applied to find the global optimal gain matrix of the controller. Simulation results of ICA are compared with the conventional design. Comparison shows the success of ICA for design of optimal controller, Postprint (published version)

This paper addresses a detailed design of a wind power plant and turbine slope voltage control in the presence of communication delays for a wide short-circuit ratio range operation. The implemented voltage control scheme is based upon the secondary voltage control concept, which offers fast response to grid disturbances, despite the communication delays, i.e., this concept is based on a primary voltage control, located in the wind turbine, which follows an external voltage reference sent by a central controller, called secondary voltage control, which is controlling the voltage at the point of connection with the grid. The performance has been tested using PSCAD/EMTDC program. The plant layout used in the simulations is based on an installed wind power plant, composed of 23 doubly fed generator wind turbines. The resulting performance is evaluated using a compilation of grid code voltage control requirements. The results show that fast response to grid disturbances can be achieved using the secondary voltage control scheme, and the fulfillment of the design requirements can be extended for a wide range of short-circuit ratios., Peer Reviewed, Postprint (published version)

This paper proposes a simple method for inherently frequency-adaptive virtual flux (VF) estimation intended for voltage-sensor-less grid synchronization and control of voltage source converters. The frequency-adaptive characteristics are obtained by using a second-order generalized integrator configured as a quadrature signal generator (SOGI-QSG) with the grid frequency as an explicit input variable. It is also shown how the properties of VF estimation based on SOGI-QSGs can be utilized to achieve a simple and effective synchronization technique that integrates frequency-adaptive bandpass filtering, VF estimation, and symmetrical component sequence separation into one operation. This new method avoids cascaded delays of VF estimation and sequence separation and is labeled as dual SOGI-based VF (DSOGI-VF) estimation since it is based on two parallel SOGI-QSGs. The properties and performance of the DSOGI-VF estimation are analyzed in comparison to conventional configurations of VF estimation and sequence separation, demonstrating that it is a simpler structure with improved dynamic response. In fact, similar response time as for grid synchronization based on voltage measurements is achieved. The operation and performance of the proposed DSOGI-VF estimation method have been verified by laboratory experiments in a small-scale converter setup., Peer Reviewed, Postprint (published version)

Wind turbine power capability is an essential set of data for both wind turbine manufacturers/operators and transmission system operators since the power capability determines whether a wind turbine is able to fulfill transmission system reactive power requirements and how much it is able to provide reactive power support as an ancillary service. For multimegawatt full-scale wind turbines, power capability depends on converter topology and semiconductor switch technology. As power capability limiting factors, switch current, semiconductor junction temperature, and converter output voltage are addressed in this study for the three-level neutral-point-clamped voltage source converter (3L-NPC-VSC) and 3L Active NPC VSC (3L-ANPC-VSC) with press-pack insulated gate bipolar transistors employed as a gridside converter. In order to investigate these VSCs’ power capabilities under various operating conditions with respect to these limiting factors, a power capability generation algorithm based on the converter electrothermal model is developed. Built considering the VSCs’ operation principles and physical structure, the model is validated by a 2MV·A single-phase 3L-ANPC-VSC test setup. The power capability investigations regarding a sample grid code’s reactive power requirement showthat 3L-ANPC-VSC results in 32% better power capability than 3L-NPC-VSC at the switching frequency of 1050 Hz. Furthermore, 3L-ANPC-VSC with 57% higher switching frequency (1650 Hz) and 33% smaller switching ripple filter can yield close power capability compared to 3L-NPC-VSC with 1050 Hz., Peer Reviewed, Postprint (published version)

In this paper the losses of the DCM-232 topology for a particular modulation strategy has been analyzed. This modulation has as particular that no common mode currents are presented. Some simulation and experimental results are given in order to prove the good behavior of the topology regarding common mode voltage (CMV), the converter efficiency and the THD, Postprint (published version)

Conditioning of network impedance by insertion of a virtual inductance as an inherent feature of the converter control system has the potential to extend the range of stable operation of the overall system. This paper presents a scenario in which such potential is explored and its range of application is identified. It shows first that implementation of a virtual inductance is naturally suited for combination with Virtual Flux-based control systems, and that it can be extended to the more general case of a virtual reactance. The paper then presents the structure of a control system for operation under unbalanced conditions, where the virtual inductance directly influences the current reference calculation. Compared with the standard approach of unity power factor control of grid integrated converters, the suggested technique allows for improving the stability range of converters operating in high-impedance grids, and for remote point control of reactive power or power factor., Peer Reviewed, Postprint (published version)

Harmonic resonance is among imperative aspect need to be analyzed in a wind power plant (WPP). It becomes complex since it involves a large network with high operating power. This paper presents a study on harmonic resonances in a WPP, particularly on parallel resonance. Important factors including length of cable, number of operating wind turbine and usage of output grid-side filter are analyzed. The damping of resonance by using common passive filter and an alternative hybrid filter is also explored. An aggregated WPP model of 400 MW with 40 wind turbines of 10 MW each are used as a study case, Postprint (published version)

This paper proposes a novel optimized fuzzy logic controller (OFLC) for current-mode control (CMC)-based voltage regulation in DC-DC converters. The implementation of the proposed fuzzy logic controller (FLC) does not depend on the mathematical model of the converter and only requires knowledge about its operation modes. The fast and efficient algorithm of particle swarm optimization (PSO) has been employed to enhance the performance of the FLC by fine tuning its membership functions. The reference current for the current control loop has been obtained in the outer voltage loop by measuring load current and output voltage and calculating the load resistive impedance. The proposed OFLC has been used in four different case studies for evaluating its performance under different conditions of input voltage and load variations as well as variations in the system parameters, e.g. inductance and capacitance of the converter circuit. The results of simulations and comparison to a proportional-integral (PI) controller demonstrates the fast and robust performance of the proposed controller, able to effectively regulate the output voltage in presence of high perturbations in the input voltage and high variation of the converter components nominal values, Postprint (published version)

This paper analyzes the minimum energy capacity ratings that an energy-storage (ES) system should accomplish in order to achieve a defined constant power production in a photovoltaic (PV) power plant. ES is a key issue for the further integration of intermittent and stochastic renewable energy sources, which are not currently dispatchable due to their dependence on real-time weather conditions, as is the case of PV technology. This paper proposes and describes an energy management strategy (EMS) for operating PV power plants with ES in the future. The goal of this EMS is to endow these power plants with a constant production that can be controlled and hence traded on electricity markets. This paper presents simulated results on the percentage of time throughout the year during which a large-scale PV+ES plant operates properly with different ES ratings and according to different configurations of this EMS. Finally, a test bench was developed, and experimental results validating this EMS were obtained, Postprint (published version)

With the rapid proliferation of the DC distribution systems, special attentions are paid to the photovoltaic (PV) generations. This paper addresses the problem of maximum power point tracking (MPPT) for PV systems using a local neuro fuzzy (LNF) network and steepest descent (SD) optimization algorithm. The proposed approach, termed LNF + SD, first identifies a valid an accurate model for the PV system using the LNF network and through measurement data. Then the identified PV model is used for MPPT by SD optimization algorithm. The salient modeling abilities of the proposed LNF network results in a reliable and dependable PV model which takes voltage, temperature and insolation variations into account. The proposed approach is evaluated using several identification and MPPT simulations. The simulation results showed the accuracy of the LNF network in modeling of PV systems. Furthermore, simulations carried out for assessment of the MPPT performance during insolation transients demonstrated the high efficiency of the proposed LNF + SD approach for MPPT applications. Performance of the proposed method MPPT, while the PV array was supplying loads through DC-DC converters was also analyzed. Comparisons to the perturb-and-observe (P&O) and fuzzy logic methods revealed the superiority of the proposed approach, Postprint (published version)

The Transmission System Operators are specially concerned about the Low Voltage Ride Through requirements of distributed generation power plants. Solutions based on the installation of STATCOMs and DVRs, as well as on advanced control functionalities for the existing power converters have contributed to enhance their response under faulty and distorted scenarios, and hence to fulfill these requirements. In order to achieve satisfactory results it is necessary to count on accurate and fast grid voltage synchronization algorithms, which are able to work under unbalanced and distorted conditions. This paper analyzes and compares the synchronization capability of three advanced synchronization systems: the Decoupled Double Synchronous Reference Frame-Phase-Locked Loop, the Dual Second Order Generalized Integgrator- Phase-Locked Loop and the Three-Phase Enhanced Phase-Locked Loop, designed to work under such conditions. Its response will be analyzed with respect the synchronization needs that can be extracted from the standards, Postprint (published version)

The High Voltage Direct Current (HVDC) system gains much more flexibility on a basis of multi terminal operation. Having extra converters brings also new ideas in sharing the active power and one of the solutions is the use of virtual impedance correlated with a droop controller. This paper analysis the Multi Terminal Direct Current (MTDC) operation and focuses on the sharing of active power produced by an offshore Wind Power Plant (WPP). The first objective was to model the system in PSCAD/EMTDC simulation software and then control structure tested under different situations. The second objective was to validate the simulation on a laboratory platform using 15 kW Voltage Source Converters (VSC) and a Real Time Interface (RTI). As a result, the power sharing is validated using such methodology and the influence in the parameters can be evaluated, Postprint (published version)

This study presents a solution for reducing the frequency oscillations in multi-area interconnected power system by means of using an VSC-HVDC interconnection transmission line. The upcoming power-electronics based HVDC transmission system offers new features that can be advantageous for improving the frequency control and thus for enhancing the stability in transmission networks. This paper is structured in two main parts; the first one is be devoted to introduce the basic elements of HVDC links and the second part will be focused on the effects of the HVDC links in the frequency control of interconnected power systems. In order to show the positive effects of HVDC links under random load disturbance, a model with parallel AC-DC interconnected power system, including a new power modulation controller of bi-directional VSC-HVDC link, is proposed and simulated. Comparisons between HVAC and HVDC transmissions are presented and its performance is evaluated using Matlab/Simulink, Postprint (published version)

Harmonic distortion is one of important factor to consider in power system network. In a big network such as wind power plant (WPP), careful analysis and mitigation steps is essential. Many aspects including resonances created from interactions between inductive and capacitive elements must be considered. Passive filter or tuned filters are common solution for mitigating harmonics by creating path for specific harmonic but many times it created a resonance issue. By implementing hybrid filter, an active filter connected in series with shunt passive filter, it enhances the ability of damping the resonant. The concept is adapted from previous research works and in this paper it is implemented on a WPP network. This paper presents a study on harmonic resonances in a WPP, particularly on parallel resonance. The damping of resonance by using common passive filter and an alternative hybrid filter is proved through simulation in PSCAD-EMTDC software. An aggregated WPP model of 400 MW with 40 wind turbines of 10 MW each are used as a study case, Postprint (published version)

Due to the growing changes in the electrical network related to the new distributed generation scheme and the integration of renewable energy sources, new requirements for grid-connected power converters are being defined in the new grid codes. The injection of positive- and negativesequence current components is becoming necessary for achieving new capabilities like the reactive power injection during a grid fault. This paper deals with a fundamental issue in this topic, i.e., the performance of the current controller. Classical dq controllers, which are extensively used in industrial applications, degrade its performance when both sequences are involved, as unavoidable oscillations in the dq axes appear. In this paper, a new scheme for controlling positive- and negative-sequence currents using dq controllers is proposed, based on an enhanced Double Synchronous Reference Frame (DSRF) controller. In this paper the DSRF controller is improved by adding a decoupling network which counteracts the oscillations caused by the presence of both sequences. Experimental results will demonstrate the validity of the proposed Decoupled DSRF (DDSRF) controller., Postprint (published version)

This paper analyzes the integration of lithium ion (Li-ion) batteries into large scale grid-connected PV plants. It performs a systematic analysis on both the operation improvement obtained by a PV+ES power plant and the ageing experienced by the Li-ion batteries used as Energy Storage (ES) system when operating under different energy management strategies (EMS). In this paper, the PV+ES power plant structure is presented and the selection of Li-on batteries as ES system (ESS) is justified. Moreover, the simulation model used for studying the Li-ion battery ageing is explained and tested for the different EMS. All the analyses conducted in this paper are based on actual data recorded from an operative PV power plant and using verified real models., Postprint (published version)

Prize Letter Award, atorgat per la IEEE Power Electronics Society, This letter introduces a new series of multilevel (ML) converters based on the ML clamping concept. By applying this technique, a ML clamping unit (MCU) conveys additional levels for synthesizing the output waveforms of a diode-clampedML dc– ac power converter. The basic building block of the ML clamping scheme is the ML clamping cell, which is composed of a pair of dc sources associated with one single-pole/triple-throw type of switch arrangement. The number of series-connected ML-clamping cells will set the number of levels of the synthesized waveform. By depending on the MCU arrangements, different converter configurations can be derived, namely, the common clamping and the modular types. Both approaches can be accomplished by employing classical three-level neutral-point-clamped (NPC) technology. Thus, the overall structure of the resultant converter is kept simple, wherein the number of parts count can be reduced, when compared with conventional NPC counterparts. In this letter, emphasis is given on the common clamping converter configuration. The attractiveness of the proposed ML concept is evidenced by conducting a comparative cost analysis. Experimental results are presented as a practicality proof of the proposed ML power converter., Peer Reviewed, Award-winning, Postprint (published version)

Larger percentages of wind power penetration translate to more demanding requirements from grid codes. Recently, voltage support at the point of connection has been introduced by several grid codes from around the world, thus, making it important to analyze this control when applied to wind power plants. This paper addresses the analysis of two different voltage control strategies for a wind power plant, i.e. decentralized and centralized voltage control schemes. The analysis has been performed using the equivalent and simplified transfer functions of the system. Using this representation, it is possible to investigate the influence of the plant control gain, short circuit ratio, and time delays on the system stability, as well as the fulfillment of the design requirements., Peer Reviewed, Postprint (published version)

The operation of distributed power generation systems under grid fault conditions is a key issue for the massive integration of renewable energy systems. Several studies have been conducted to improve the response of such distributed generation systems under voltage dips. In spite of being less severe, unbalanced fault cause a great number of disconnections, as it may produce the injection of unbalanced currents that can easily conduct to the tripping of the converter. In this paper a current control strategy for power converters able to deal with these grid conditions, without exceeding the current ratings of the converter is introduced. Moreover, a novel flexible algorithm has been proposed in order to regulate easily the injection of positive and negative currents for general purpose applications., Peer Reviewed, Postprint (published version)

A possible methodology for the power sharing is based on DC voltage droop control at onshore station converters. The aim is to present an evaluation tool for the droop control parameter description in support of DC power sharing. Main limitation for this method regarding sharing factors definition and overvoltages are going to be revised. A study case using two different onshore stations are going to be used. Both of them are sized with different power ratings and they have different distances from the offshore converters. This benchmark is suitable as starting point for future prospects of DC supergrids. Using the concept of virtual impedances, the droop parameters are limited by the boundaries of the DC voltage limits and line characteristics. For the power sharing point of view, mainly the cable resistances are considered. Simulation results using time domain simulation (EMTDC/PSCAD) will reinforce the methodology. The approach can be considered as a general case from the previous control schemes based on voltage droop. The work extends the purpose to power sharing characteristic and it can be useful when the converters have different sizes or/and there are restrict agreements among all the operators at onshore AC systems, Postprint (published version)

Voltage source converter (VSC) based high voltage dc (HVDC) transmission is an attractive technique for large offshore wind power plants, especially when long cable transmission is required for connection to the onshore grid. New multi-MW wind turbines are likely to be equipped with full scale converters to meet the stringent grid code requirements. In such a scenario, the offshore grid is terminated to the power electronic converters on all the ends. This paper presents a control scheme for the synchronization and control of the grid side converters (GSC) of the wind turbine generators (WTG). Current limit control enables the GSC to sustain the fault currents during short circuits in the offshore wind collector system grid. However, power transmission is affected, and the fault has to be isolated. It can be resynchronized after the fault has been cleared and the breaker reclosed. Healthy WTG converters can remain connected. The scheme is demonstrated through PSCAD/EMTDC simulation., Peer Reviewed, Postprint (published version)

This paper focuses on the estimation of series resistance changes for flat silicone PV panels or arrays during operation, without moving the operating point far away from the Maximum Power Point1. The method is based on the measurement of the slope of the IV curve at a current level that differs from the short-circuit current with a fixed value, regardless of irradiance conditions. The method causes less power loss and power ripple due to measurements and it is easily integrated in a hill-climbing MPPT algorithm., Peer Reviewed, Postprint (published version)