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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.

Lung function (breath duration, respiratory flow [], and tidal volume [VT]), and end-expiratory O2 were measured in 19 adult bottlenose dolphins (Tursiops spp.) while at rest in water or beached for up to 10 min. The results show that inspiratory VT, expiratory VT, or inspiratory did not differ on land or in water. The average expiratory for all dolphins on land decreased by 16%, and the expiratory and total breath durations increased by 5% and 4%, respectively, compared with in water. There were temporal changes observed during beaching, where expired and inspired VT and inspired decreased by 13%, 16%, and 9%, respectively, after 10 min on land. These data suggest that dolphins compensate for the effect of gravity by adjusting respiration to maintain alveolar ventilation and gas exchange, but during extended durations, the increased work of breathing may impede ventilation and gas exchange. Continuous monitoring of lung function and gas exchange may help prevent long-term damage during out-of-water medical procedures, optimize animal transport conditions, and improve survival during stranding events.

Lung function (breath duration, respiratory flow [

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.

The increasing amount of renewable power generation systems is a challenging issue for the control and operation of the electrical networks. One of the main issues is their lack of inertia, which is becoming a greater problem as much as the share of the power plants based on traditional synchronous generators gets reduced. In this regard, the new grid codes ask these plants to provide new functionalities such as the frequency support and inertia emulation. In this paper, a synchronous power controller for grid-connected converters is proposed as a good solution for the renewable generation systems with energy storage. It provides inertia, damping, and flexible droop characteristics. Different from the faithful replication of the swing equation of synchronous machines, an alternative control structure is proposed, by which the damping and inherent droop slope can be configured independently to meet the requirements in both dynamics and frequency regulations. Analysis and experimental results are both shown to validate the proposed controller.

The implementation of ancillary services in renewable energy based generation systems requires controlling bidirectional power flow. For such applications, integrated energy storage systems (ESSs) in such generation platforms have emerged as a promising solution. However, a large variety of ESS solutions are available in the market, and even hybrid systems are proposed to optimize the overall performance. In this regard, the integration of supercapacitors (SCs) and electrochemical batteries is an attractive and feasible solution, as it takes the most of the combination of the large storage capacity of batteries with the high dynamic performance of SCs. This combination enables the implementation of energy and power services, such as inertia emulation, black start, or power oscillation damping among others. This solution avoids oversizing ESS and enlarges the battery lifetime, since the SCs absorve the high dynamic response meanwhile the battery feeds an attenuated power profile. This paper presents a strategy to manage mixed energy storage technologies, composed by a direct connection of a battery and an SC bank interfaced through a dc-dc converter. A power control loop distributes the power flow through each element in order to achieve an optimized performance, meanwhile, it permits to provide grid-frequency support and minimizes the number of cycles applied to the battery. The analysis made and the initial sections are endorsed by simulation and experimental results collected in a 50-kW test-bench. This work was supported in part by Spanish Ministry of Science under Projects ENE2016-79493-R and ENE2017-88889-C2-1-R and in part by the European Commission under Project FLEXITRANSTORE —H2020-LCE-2016-2017-SGS-774407. Peer Reviewed Objectius de Desenvolupament Sostenible::7 - Energia Assequible i No Contaminant

We measured respiratory flow ( V ), breathing frequency ( f R ), tidal volume ( V T ), breath duration and end-expired O 2 content in bottlenose dolphins ( Tursiops truncatus ) before and after static surface breath-holds ranging from 34 to 292 s. There was considerable variation in the end-expired O 2 , V T and f R following a breath-hold. The analysis suggests that the dolphins attempt to minimize recovery following a dive by altering V T and f R to rapidly replenish the O 2 stores. For the first breath following a surface breath-hold, the end-expired O 2 decreased with dive duration, while V T and f R increased. Throughout the recovery period, end-expired O 2 increased while the respiratory effort ( V T , f R ) decreased. We propose that the dolphins alter respiratory effort following a breath-hold according to the reduction in end-expired O 2 levels, allowing almost complete recovery after 1.2 min.

We measured respiratory flow (

Multiterminal dc (MTDC) systems are drawing a lot of interest lately in applications related to distributed generation, especially in those that integrate wind or photovoltaic (PV) generation with energy storage (ES). Several approaches for controlling the operation of such systems have been proposed in the literature; however, the existing structures are mainly application specific and, thus, can be still improved in order to provide a more generic approach. This paper proposes an improved primary control layer for an MTDC system. The concept is based on the combination of a droop control method and dc bus signaling in order to provide a more generic and flexible solution. In this paper, different droop characteristics are proposed for the various elements connected to the dc bus. All of them are specifically tailored around five operation bands, which depend on the dc bus voltage level. Special attention is paid to the integration of ES: the state of charge (SoC) is considered at the primary control level, yielding a surface characteristic that depends on the SoC and the dc bus voltage. The scaling of the system has been analyzed together with the proposed control strategy and the overall operation has been validated through simulations by considering a 100 kW PV system with energy storage. Experimental results were obtained on a scaled laboratory prototype rated at 10 kW.

The control of active and reactive power for the Renewable Energy Sources (RES) based power plants are very important. The injection of active and reactive power to the grid is normally controlled at the Point of Common Connection (PCC) where this point is typically far away from the power converter station. This paper proposed a controlling principle which is based on virtual flux approach that permits to control remotely the power injected at the PCC. The results will show that the Virtual Flux (VF) estimation is capable to estimate the grid voltage in any point of the network as well as the capability of the control principle to inject the specific amount of active and reactive power at a point that can be some kilometers away. In this paper, the basic principle for the remote power control is presented and the effectiveness of the proposed system has been validated by experimental studies.

In power conversion applications grid synchronization systems should face very different network operating conditions such as: generic grid connection, faulty or distorted scenarios or islanded conditions among others. However, all the existing synchronization systems have issues operating in all these scenarios if the control tuning parameters cannot be adapted to the grid conditions. This paper proposes a Fuzzy Gain Scheduling PLL (FGS-PLL) to be used for grid synchronization, which has a robust performance in case of severe voltage sag, where there is even a null value of voltage, and phase jump conditions. Moreover, the proposed method employs a fuzzy gain scheduling technique to adjust the proportional and integral gains of the proposed PLL during amplitude, phase and frequency variations in the grid voltage waveform to build a flexible PLL in case of severe grid conditions. This PLL structure permits to adjust the performance in different conditions as well as to provide a voltage reference, even in case of 100% voltage sag.

©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. Virtual Synchronous Generators (VSGs) are an interesting control approach for the connection of renewable energies to the electrical grid through power converters. These power devices have shown their ability to enable grid supportive services in terms of both voltage and frequency. The implementation form shown in the paper allows islanding as well as grid forming operation of the device. A challenging problem that arises is to make the grid supportive services compatible with the instantaneous power limits that renewable energies can provide. In this paper, an Active Power Limiter (APL) is proposed, designed and tested to manage output active power of power converters connected to a renewable source. The implementation of APL is tested on the Synchronous Power Controller (SPC) that emulates inertia and output admittance. The simulation and experimental results shown in the paper confirm that the proposed controller can efficiently control and limit the active power while still providing grid supportive services. This work was supported by the European Commission under projects INTERRFACE H2020-LC-SC3-2018-ES-SCC824330 and by the Spanish Ministry of Science under project RTI2018-100921-B-C21.

In this paper, a new type of single-stage interlinking converters for Microbial Electrosynthesis (MES) energy storage applications connected to the three-phase grid is presented, in which impedance source network (ZSN) is employed to reduce cost and volume, and meanwhile improve the efficiency. Impedance source converter (ISC) with buck-boost characteristic is able to feed MES with a wide range of voltages in DC side, also, converter grid distortion in AC side is reduced because there is no need to dead time for switching. Different ZSNs are studied and discussed for MES energy application. The applicable ones are selected and a comprehensive comparison is presented. Also, the proposed efficient control for ISC consists of two parts, first ZSN capacitor control plus reactive power control are done by controlling modulation index, and second the objective of tracking active power reference can be achieved by means of controlling shoot-through duty cycle. Simulation results are presented to certify the comparison between ZSNs and to verify the merit of the proposed control scheme.

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.

Management of Renewable Static Synchronous Generation units based on Synchronous Power Controller (RSSG-SPC), not only provides grid synchronization without using PLL and gives proper dynamic to output active power, moreover yields simple solution to overcome technical constraints on these units. Control of active power level injected to the grid is one of these key constraints which can overcome by an Active Power Limiter (APL). In this work, an APL designed based on the classic power equation is proposed for this kind of modern RSSG-SPCs. Primary Simulink analysis and real time test results confirm that suggested APL can control and limit both maximum and minimum level of output active power for several grid connection cases.

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.

In this paper the design and the implementation of a method oriented to carry out a safe and a stable intentional disconnection, and re-connection, of local micro-grids from the main distribution network under generic grid conditions, avoiding the appearance of undesired transients, will be presented. In this work a seamless transfer between grid-connected and isolated mode is achieved by means of using an adapted frequency based synchronization control loop, which works in the ab stationary reference frame, whose response can be dynamically adapted in function of the micro-grid operating conditions. In addition to this feature, the inner loops of the control algorithm proposed in this application have been implemented in order to provide a proper operation of the system under generic conditions, even in the presence of unbalanced loads. In order to prove the reliability and good performance of the proposed method simulation and experimental results will be provided and discussed in the last sections of the paper.

Supporting frequency inside of each generation area as well as keeping proper synchronism condition between interconnected areas are two main challenges for grid operators. In this paper, after analysis of structure of Renewable Static Synchronous Generation unit based on Synchronous Power Controller (RSSG-SPC) for holding and control of frequency, it is shown that how a renewable unit based on SPC can contribute to frequency support inside of generation areas and in this way it will support synchronism of neighbor areas. Results of mathematical analysis and state space modelling, as well as time domain investigation then real time test of two area system in presence of RSSG-SPC confirm that by reducing Rate of Change of Frequency (ROCOF), increasing level of Frequency Nadir (FN) and providing stiffer Center Of Inertia (COI) for whole of the grid, the RSSG-SPC not only supports frequency on the generation areas, furthermore it will cover synchronism of these interconnected areas.

The appearance of harmonics in the grid voltage is an issue for the generation systems and grid connected consumers. This paper presents the basic control strategy to be implemented in Synchronous Power Control (SPC) based power converters, which contributes to reduce the harmonic content in the voltage by means of injecting different harmonic current to the grid using a separated virtual admittance algorithm. This control strategy is capable of generating such a current reference that attenuates the harmonic values at the point of connection of the converter.

High penetration of Renewable Generation Units (RGU), has led to that the interaction of this newfound participators in power network with conventional units be more critical challenges for network operators. Effect on the phase stability in emplacement as well as on neighbor generation units is one of this essential challenges. In this paper, a RGU controlled by Synchronous Power Controller (SPC) which behaves as a big Static Synchronous Generation (SSG-SPC) units, are presented as solution to cope with this challenge. Small signal modelling of a power network in presence of SSG-SPC unit is used to analysis this solution. After illustrating SPC dynamic capabilities, study on IEEE-14B test system based on modal analysis, time domain investigation and real time test confirms that a SSG-SPC not only has not damaging impact on phase stability and dynamic of network, but on the contrary can improve it. Moreover, by having at least two dynamic freedom degrees in SPC, the SSG-SPC can adapt itself for keeping this improvement effect.

VSCs connected to the grid through LCL filters are common in the integration of renewable energies into electrical systems. Therefore, in this paper small signal stability of the current loop of this configuration is performed. The analysis includes the getting of a state-space representation of the system, the calculation of their eigenvalues and the sensitivity analysis to the variation of some relevant parameters. Real-time simulations have also been carried out in order to validate the theoretical analysis.

Grid synchronization based on virtual flux (VF) estimation allows for control of grid-connected power converter without depending on ac-voltage measurements. This is useful in voltage-sensor-less applications for reducing cost and complexity of the control hardware, and can be utilized in case of limited reliability or availability of voltage measurements at the intended point of synchronization to the grid. However, for voltage source converters (VSC) with LCL filters, the influence of the capacitor current must be taken into account to ensure accurate VF estimation at the point of common coupling (PCC) with the grid. This paper presents a comparative evaluation of three VF-based methods for grid synchronization of VSCs with LCL filters, with three different ways of obtaining the capacitor current. The VF estimation in the first method is based only on the measured converter currents. The second method includes capacitor voltage measurements used for estimating the capacitor currents, while the capacitor currents are measured in the third approach. Comparative results from time-domain simulations are presented, demonstrating good performance of the estimation and accurate control of the active and reactive power at the PCC with all three methods, as long as sufficiently accurate filter parameters and current measurements are available. However, the approach based on capacitor current measurements is sensitive to noise due to the high ripple current compared with the fundamental frequency current in the capacitors. The operation of a converter with VF-based grid synchronization including estimation of the capacitor current is demonstrated by experimental results, verifying the voltage sensor-less operation with LCL filter. © 1972-2012 IEEE.

Daily increasing in renewable energy sources installed in dispersed location, has led to greater need for long transmission lines to use these green sources. Voltage and phase stability are major challenges for transmission system operators in the operation of these lines. In this paper, a Static Synchronous Generators based in Synchronous Power Controller (SSG-SPC) is presented and implemented in a long AC transmission system in order to reducing stability problems. Power control strategies, compatibility with the grid codes and improvement in voltage and phase stability by SSG-SPC are investigated. Simulations and Experimental test results confirms SSG-SPC capability for participation in reactive compensation, damping active power oscillations and modify stability margins.

Growing universal demand for greater use of renewable energy resources (RES) installed in dispersed location, has caused to more urgent need for long transmission systems to use these green sources. Small signal stability is one of vital challenges for transmission system operators in presence of big RES. In this paper, a Static Synchronous Generators based in Synchronous Power Controller (SSG-SPC) is presented and its capabilities for dynamic stability enhancement in a long AC transmission system is investigated. Dynamic modelling of a long line and then study in Uncompensated and Compensated cases by using time domain simulation and modal analysis, and then experimental test confirms that SSG-SPC can be effective in active damping of power oscillations and reducing small signal stability challenges.

The enabling of ac microgrids in distribution networks allows delivering distributed power and providing grid support services during regular operation of the grid, as well as powering isolated islands in case of faults and contingencies, thus increasing the performance and reliability of the electrical system. The high penetration of distributed generators, linked to the grid through highly controllable power processors based on power electronics, together with the incorporation of electrical energy storage systems, communication technologies, and controllable loads, opens new horizons to the effective expansion of microgrid applications integrated into electrical power systems. This paper carries out an overview about microgrid structures and control techniques at different hierarchical levels. At the power converter level, a detailed analysis of the main operation modes and control structures for power converters belonging to microgrids is carried out, focusing mainly on grid-forming, grid-feeding, and grid-supporting configurations. This analysis is extended as well toward the hierarchical control scheme of microgrids, which, based on the primary, secondary, and tertiary control layer division, is devoted to minimize the operation cost, coordinating support services, meanwhile maximizing the reliability and the controllability of microgrids. Finally, the main grid services that microgrids can offer to the main network, as well as the future trends in the development of their operation and control for the next future, are presented and discussed.

The high penetration of distributed generation power plants, based on renewable energy sources (RESs), is boosting the connection of power converters to the electrical network. This generation concept would permit to form local networks, microgrids, when the main grid falls due to any kind of contingency in the network. However, the connection and disconnection of these local networks may give rise to undesired transient overcurrents that should be avoided. In order to solve this drawback, this paper presents a method oriented to carry out a stable intentional disconnection/reconnection of local grids from the main electrical network under grid-fault conditions. This control method has been implemented in a grid-connected power converter that acts as an intelligent connection agent (ICA) and adapts its operation mode according to its connection state. The proposed control also manages the operation of a controlled switch, which is responsible of disconnecting/reconnecting the microgrid from the mains. In this paper, the behavior of the ICA under transient conditions will be discussed, and finally, its simulated and experimental performance will be shown.

Several pulsewidth modulation (PWM) strategies have been proposed for the three-level three-phase diode-clamped DC-AC converter. Among them, the nearest-three virtual-space-vector PWM guarantees the DC-link capacitor voltage balance under any operating condition, provided that the addition of the three phase currents equals zero. This paper extends this modulation concept to the four level converter. The new virtual vectors are presented and a simple modulation solution is defined. Conventional nearest-three space vector PWM cannot comprehensively achieve balanced and stable DC-link voltages. The proposed modulation solution enables the practical use of the four-level converter since it guarantees the dc-link capacitor voltage balance for any operating condition and load, provided that the addition of the three phase currents equals zero. Simulation and experimental results prove the goodness of the presented approach.

This paper presents a closed-loop control scheme for the three-level three-phase neutral-point-clamped dc-ac converter using the optimized nearest three virtual-space-vector pulsewidth modulation, which is a modulation that produces low output-voltage distortion with a significant reduction of the dc-link capacitance. A new specific loop modifying the modulating waveforms is proposed to rapidly control possible perturbations in the neutral-point voltage balance. An online estimation of the load displacement angle and load linear/nonlinear nature is introduced at no extra cost. The remaining part of the control is analogous to the control for a two-level converter with an appropriate interfacing to the selected modulation. The closed-loop control is designed for the case of a renewable-energy source connected to the ac mains, and its performance is analyzed through simulation and experiments.

The actual grid code requirements for the grid connection of distributed generation systems, mainly wind and photovoltaic (PV) systems, are becoming very demanding. The transmission system operators (TSOs) are especially concerned about the low-voltage-ride-through requirements. Solutions based on the installation of STATCOMs and dynamic voltage regulators (DVRs), as well as on advanced control functionalities for the existing power converters of distributed generation plants, have contributed to enhance their response under faulty and distorted scenarios and, hence, to fulfill these requirements. In order to achieve satisfactory results with such systems, 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 the synchronization capability of three advanced synchronization systems: the decoupled double synchronous reference frame phase-locked loop (PLL), the dual second order generalized integrator PLL, and the three-phase enhanced PLL, designed to work under such conditions. Although other systems based on frequency-locked loops have also been developed, PLLs have been chosen due to their link with dq0 controllers. In the following, the different algorithms will be presented and discretized, and their performance will be tested in an experimental setup controlled in order to evaluate their accuracy and implementation features.

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.

Improving the dynamics of the widely applied gridconnected power converters has been drawing a lot of interests in recent years. Since the currently installed grid-connected converters doesn’t shown inertia effect in power processing, the lack of inertia in the electrical networks will become increasingly significant and less admissible when more and more renewable energy sources penetrate into the grid through grid-connected converters. An active power synchronizing controller for grid-connected power converters with configurable natural droop characteristics is proposed in this paper. The proposed controller is able to incorporate inertia, damping and configurable natural droop effect in the dynamics of the controlled converter. And in addition to the inertia characteristics, the droop and damping behaviors are discussed in detail. By a modified form of the swing equation of the synchronous generators, the droop behavior can be properly configured without undermining the damping characteristics. Experimental results are shown to validate the proposed controller

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.

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.

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.

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.

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.

In this paper a new multilevel power conversion concept is proposed, whereby an additional auxiliary circuit composed of multilevel clamping cells is applied to a main DC-AC converter. The resultant structure, the multilevel-clamped multilevel converter, or simply multilevel-clamped converter (MLC2), has the number of levels increased, while keeping circuitry simple, wherein the number of parts count can be reduced, when compared with classical NPC counterparts. Hence, the well established three-level neutral-point-clamped (NPC) technology can be applied, which has been the preferred choice in medium-voltage (MV) high power industry for several industrial products. A description of the proposed concept is provided, on which different topologies are derived. Moreover, a cost comparison analysis is shown to illustrate their attractiveness. Simulation and experimental results based on the space vector pulse width modulation (SVPWM) are shown, in order to illustrate the operation and effectiveness of the new multilevel method.