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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.

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.

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.

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.

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

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.

Droop control is a common method to parallel operation of inverters. This method uses the average values of power components which are usually obtained through low-pass filters. However, in single-phase systems the power components are oscillating at twice fundamental frequency and the filters cut-off frequency must be very low. Thus, there is a tradeoff between dynamic response and oscillation attenuation. In this paper, a method that uses a fictitious quadrature reference frame to calculate the average power components is presented. This method improves the dynamic response of the power sharing control and eliminates the double-frequency oscillation over the voltage and frequency synthesized by the inverter. Moreover, it is able to deal with nonlinear loads since the harmonic components that appear in the instantaneous power are filtered. This paper also presents a comparison between this method and the classical one through simulation carried out in Matlab/Simulink and experimental results.

Master-Slave configuration is a suitable alternative to droop control method used in microgrids. In this configuration, only one inverter is the master, while the others are slaves. The slave inverters are always current controlled whereas the master inverter should have two selectable operation modes: current controlled, when the microgrid is connected to the grid; and voltage controlled, when it is operating in island mode. In gridconnected mode, the master needs a synchronization system to perform the accurate control of its delivered power, and, in island mode, it needs a voltage reference oscillator that serves as a reference to the slave inverters. Based on the master-slave concept, this paper proposes a single system that perform both functions, i.e., it can act as a synchronization system or as a voltage reference oscillator depending on an input selector. Moreover, the system ensures a smoothly transition between the two operation modes, guaranteeing the safety operation of the microgrid. Experimental results are provided to confirm the effectiveness of the proposed system.

The high penetration of wind power systems in the electrical network has introduced new issues in the stability and transient operation of the grid. By means of providing advanced functionalities to the existing power converters of such power plants it is possible to enhance their performance and also to support the grid operation, as the new grid codes demand. The connection of FACTS based on power converters, such as STATCOMs, are also contributing to the integration of renewable energies improving their behavior under contingencies. However, in both cases it is needed to have a grid voltage synchronization system, able to work under unbalanced and distorted conditions. This paper presents the discrete representation and performance of three PLL’s structures, designed to work in that kind of situations. Their synchronization capability will be tested in different scenarios and their performance constraints will be established according to the Grid Code Requirements (GCRs)

The increasing penetration of generation power plants based on renewable energies in the electrical networks has boosted the number of systems connected to the grid. In this scenario the microgrid, providing advanced functionalities to improve the stability and operation of the network, have become very popular. In this paper a control strategy for the microgrid management is presented. The proposed system improves the performance of microgrids and its interaction with the main network, or another microgrid, under grid voltage transients. This technique gives rise to a simple and robust control scheme to ensure the appropriate microgrid disconnection, and further resynchronization and reconnection, from the main grid. In the following, the control algorithm for the intelligent connection agent will be shown, and the grid synchronization system, as well as the voltage and current control loops will be detailed. Finally, results obtained using the MATLAB/Simulink & PSIM platform will be presented and discussed.

This paper presents a solution for filtering current harmonics in three-phase four-wire networks based on the usage of a four-branch star (FBS) filter topology. The specific layout of single-phase inductors and capacitors in this filter topology allows achieving a power filter with two independent and simultaneous resonance frequencies, i.e., one for positive- /negative-sequence and another one for zero-sequence components. The FBS filter topology can work either as a passive filter, or as a hybrid filter, when its performance is improved by integrating a power converter into its structure. The paper analyzes the FBS topology and derives fundamental concepts about the control of a generic FBS hybrid power filter. From this analysis, a neutral current hybrid power filter and var compensator is presented as an illustrative example applying the FBS filtering topology to a three-phase four-wire system. An evaluation using simulation and experimental results is conducted as well.

This paper presents a three-level three-phase neutral-point-clamped back-to-back converter applied to a wind emulator, where a wind power generator is connected to the grid through the back-to-back converter. The converter regulates the power extracted from the generator and controls how this power is injected to the grid with the desired power factor. Two phase locked loops are in charge of synchronizing the generator-side and the grid-side converters, regardless of the rotor speed of the generator. DC-link neutral point voltage balance is performed by the modulation strategy of the back-to- back converter. Simulation and experimental results are provided to verify the correct performance of the system.

International audience; Recent contributions in pulse width modulations (PWM) for multilevel diode-clamped converters enable the use of these converters with passive front-ends, any number of levels, and small dc-link capacitors. Highly compact converters designs based on these topologies can be envisioned. However, the design of the gate-driver power-supply for the multiple controlled semiconductor devices remains an important issue to be addressed. This paper focuses on the design of such circuits and the analysis of the resulting multilevel converter performance. A simple circuit connected across each power switch and monolithically integrable within it is selected. These circuits lead to simple, compact, and efficient converter designs. The main issues arising from the operation of the multilevel converter with such circuits are analyzed, and both hardware and software solutions are proposed. In particular, a new PWM strategy is presented. Experimental results are provided verifying the good performance of all proposed solutions.

In the previous literature, the introduction of the virtual space vector concept for the three-level three-leg neutral-point-clamped converter has led to the definition of pulsewidth modulation (PWM) strategies guaranteeing the dc-link capacitor voltage balance under any type of load, with the only requirement being that the addition of the three phase currents equals zero. This paper presents the definition of the virtual space vectors for the general case of an n-level converter, suggests guidelines for designing virtual-space-vector PWM strategies, and provides the expressions of the phase duty-ratio waveforms corresponding to this family of PWMs. Modulations defined upon these vectors enable the use of diode-clamped topologies with passive front-ends. The performance of these converters operated with the proposed PWMs is compared to the performance of alternative designs through analysis, simulation and experiments.

In photovoltaic (PV) power systems where a set of series-connected PV arrays (PVAs) is connected to a conventional two-level inverter, the occurrence of partial shades and/or the mismatching of PVAs leads to a reduction of the power generated from its potential maximum. To overcome these problems, the connection of the PVAs to a multilevel diode-clamped converter is considered in this paper. A control and pulsewidth-modulation scheme is proposed, capable of independently controlling the operating voltage of each PVA. Compared to a conventional two-level inverter system, the proposed system configuration allows one to extract maximum power, to reduce the devices voltage rating (with the subsequent benefits in device-performance characteristics), to reduce the output-voltage distortion, and to increase the system efficiency. Simulation and experimental tests have been conducted with three PVAs connected to a four-level three-phase diode-clamped converter to verify the good performance of the proposed system configuration and control strategy.

In this paper, a variable and smart energy storage system, consisting on a two-level voltage source converter with LCL filter and Microbial Electrosynthesis (MES) stack, is presented. This energy storage system is able to contribute to damp the variability of renewables and provide grid services, by controlling the active and reactive power exchanged with the grid and can be considered as another alternative in the market of storage normally centered in batteries. The proposed system converts electrical energy into methane, taking waste water as another input. The converted energy inside the MES presents a new degree of freedom, which can be exploited to provide ancillary services such as energy management, frequency regulation, voltage profile improvement and oscillation damping. The big challenge for controlling this system lays on the fact that the dc bus voltage of the converter has to be changed in order to regulate the exchanged active power with the grid. This paper presents a cascaded approach to control such system by means of combining extern control loops with fast inner loops. The outer power loop, with a PI controller with special limitation values and anti-windup capability, is used to generate dc bus voltage reference. An intermediate loop is used for dc bus voltage regulation and current reference generation. A new proportional resonant controller is used to track the current reference. The proposed scheme is validated through real-time simulation in OPAL OP4510 hardware in the loop system.