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© 2022 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. With the aim to increase standardization, reduce the cost, and obtain advanced performance features, the design of voltage-source power converter legs can be undertaken by combining several instances of a standard switching cell, properly connected in active neutral-point-clamped structures to reach the desired voltage and current ratings. These switching cells can be organized into switching-cell arrays. This design approach introduces several degrees of freedom into the design. Namely, the different options to interconnect the cells and the distribution of switching losses among these cells. This article aims to define an optimization problem to explore this design space. The design problem is formulated in different scenarios, involving different conversion configurations (dc-dc and dc-ac), different leg number of levels (two and three), and different types of available cells (standard and conduction-optimized in combination with switching-optimized). A weighted objective function is then defined in terms of leg simplicity, efficiency, and reliability. The value of the design variables that minimize the objective function with different sets of weighting factors are obtained under selected scenarios and operating conditions, to illustrate the flexibility of the converter design approach under study. The solution of the optimization problem is obtained using a surrogate optimization algorithm in MATLAB, well suited to quickly solve optimization problems involving a combination of integer design variables (the number of parallel switching cells in each converter leg position) and continuous design variables (the proportion of switching losses taken by each cell), together with linear and nonlinear constraints.

This article explores the thermal and reliability benefits of configuring the power semiconductor block of power converters with several small standard power semiconductor devices instead of a few larger ones at a fixed total chip area. The effectiveness of a finer power semiconductor device granularity is analyzed in the context of a synchronous buck converter design under different operating conditions and employing a recently proposed component, the switching-cell array, to implement the semiconductor-component block of the converter. Comparative evalua tions are undertaken under different degrees of granularity and in terms of semiconductor loss distribution, semiconductor temperature distribution, and converter mean time to failure. The article also discusses the thermo-electrical and reliability models employed, and the assumptions made for a fair and straightforward comparison in a generic case. The comparison results reveal that a finer granularity can potentially provide remarkable benefits, although the incremental improvements decrease as the granularity becomes finer. A tradeoff solution must be sought in each specific case balancing the benefits achieved with the added complexity

© 2021 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 paper addresses an optimization-based algorithm to calculate the references of the Modular Multilevel Converter (MMC) during normal and constrained scenarios (when the prioritization of quantities is required). The optimization problem prioritizes to satisfy the positive- and negative-sequence active and reactive current set-points demanded by the Transmission System Operator (TSO) through the corresponding grid code. If the TSO’s requirements are achieved, the algorithm minimizes the arm impedances losses. Otherwise, it attempts to reduce the error between the current components and the TSO’s current set-points. The optimization-based current reference calculation is derived based on the steady-state equations of the MMC, considering the maximum currents that can flow through the MMC’s arms, the maximum and minimum arm applied voltages and the maximum sub-module capacitor’s voltage. Simulation in the time-domain have been conducted and the results indicate that this method can be potentially employed to calculate the converter’s references during both normal and faulted conditions.

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

A grid-forming power converter based on the synchronous power controller (SPC) is a solution that through the flexible and adaptive emulation of the main functional blocks of a synchronous machine, allows to take advantage of its benefits while avoiding or minimizing its weaknesses. In this paper a small signal stability analysis of a grid-forming power converter based on SPC is performed. The analysis includes obtaining a representation of the system’s state space, calculating its eigenvalues and analysing the sensitivity to variation of some relevant parameters. Some simulations and experimental tests have also been carried out to validate the theoretical analysis. 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.

During the years, synchronization systems in power converter control has been mainly used for estimating the phase angle and the magnitude of the grid voltage, this estimated information is usually used for droop control strategies to ensure power sharing among power converters. Most of existing synchronization systems rely on the voltage measurements at the point of common coupling of the power converter, which may be distorted or unbalanced. This paper presents a voltage sensorless grid-forming structure for power converter in microgrids. By means of reading the voltage reference of the power converter, the system is able to synchronize with the grid. In addition, the virtual admittance compensation provides grid-forming capabilities to the power converter, which can emulate the inertia and damping of the synchronous machine in its synchronization structure, based on the power balance synchronization. This work was supported by the European Commission under projects INTERRFACE H2020-LC-SC3-2018-ESSCC-824330 and by the Spanish Ministry of Science under project RTI2018-100921-B-C21.

Synchronization plays a fundamental role in connecting power electronic converters with the grid. Conventionally, the synchronous reference frame phase-locked loop (SRF-PLL) has been widely used to estimate the grid phase angle. Though SRF-PLL is effective under normal grid voltage conditions, its performance degrades drastically under weak grid conditions. This paper presents a control strategy for current controller of grid-connected power converters that contributes in the reduction of the negative effects of SRF-PLL during grid disturbances. The main control concept is based on virtual admittance. This approach is capable of operating effectively power converters when connected to weak grids and permit grid-forming operation. Simulation and experimental results are provided to validate the proposed controller. This project has been partially funded by Spanish Ministry of Science Innovation and Universities under the project RTI2018-100921-B-21 and the H2020-LC-SC3-2018-ES-SCC project Interrface GA824330.

A novel closed-loop capacitance-to-pulse width converter (CPC) suitable for single element capacitive sensors that use sinusoidal excitation is presented in this paper. Its operation is realized using a new configuration based on a simple, yet effective, auto-balancing scheme. The hardware prototype of the proposed CPC is relatively less complex to implement than those presented so far in the literature. It provides a quasi-digital output at a high update rate. Additionally, the output is insensitive to parasitic capacitances of the sensor. The output possesses high linearity, with respect to change in the sensor capacitance, ranging +/-5 pF, with a nominal capacitance as high as 200 pF. It exhibits a maximum non-linearity error of 0.061%FS. The output of the prototype has a resolution of 13.31 bits. Also, its response time for a step-change in the sensor capacitance is about 13 ms. This sophisticated and inexpensive closed-loop CPC is a perfect fit as an interfacing circuit for single element capacitive sensors.

© 2019 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes,creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works. Thispaperpresents the design and structure of aversatileworkbench simulator forevaluating the performance of a five-phase inverter andPermanent Magnet assisted Synchronous Reluctance Motor(PMa-SynRM). The simulatorallows for adding variations tothe modulationtechniques, changingthe inverter structure’s semiconductordevice, and calculatingtheinverter’spower losses. Itcanalso facilitate observingthe current, voltage,andthe jointtemperature ofthe semiconductors devices. Furthermore,wecanobtain a perform that is close to anactualPMa-SynRM, dependingon the desired conditionsof speed and torque. The workbench simulator wasdevelopedby combining three software: Matlab/Simulink, PLECSand Altair Flux.

This paper proposes a cascaded single phase multilevel inverter using an off-the-shelf three-phase inverter and transformer. The concept is based on a cascaded connection of two inverter legs using a typical three phase inverter in such a way that the third leg is shared between the other two phases. The cascaded connection is achieved through an integrated series transformer with a typical three-phase transformer core. Utilization of a special transformer design has been previously proposed in the Custom Power Active Transformer. However, cascaded connection of inverter legs has not been previously investigated with such a concept. In this way, a three-leg inverter and a three-phase transformer are converted into an isolated multilevel single-phase inverter based on an unique configuration and modulation technique.

In this paper, the switching dynamics of a dc-ac resonant self-oscillating inverter is considered. Using bifurcation analysis coexisting steady-state solutions are predicted, which are increasingly relevant for low values of the quality factor of the resonant circuit. A repelling sliding region is found to be connected with the two unstable limit cycles that split the phase plane in three basins of attraction. Simulation results obtained from the switched model confirm the theoretical derivations.

This paper proposes the outline design of a platform that can be used to implement multiple emulators to replicate the behaviour of different energetic resources, storage systems, loads or electric grids. This platform can be used to test control algorithms of microgrids with different elements connected into it.

This article presents the analysis of two topologies of power converters. Voltage Source Inverter (VSI) and Current Source Inverter (CSI) proposals for traction system applications, these topologies are implemented with silicon carbide devices. The use of SiC semiconductors allow working at high switching frequency (100KHz), increase the working temperature range and decreasing power losses during conduction and activation of the semiconductors. The objective is analyze these topologies and select the one that provides the best performance and behavior at high frequency to improve it on a electric traction system.

This work proposes a technique to inject a discontinuous zero sequence signal (D-ZSS) into voltage modulation of the modular multilevel converter arms. The core of the technique is the clamping of the arm voltages to a particular level and interval, in such a way that reduces the number of switching events on the instants that the load stress on the semiconductors is high, and consequently the dissipated energy. The clamping voltage and interval has some degrees of freedom which are also discussed. The description of the methodology is complemented with its simulation. Either in steady-state or in dynamic converter operation this technique is able to reduce the switching events and the correspondent total losses up to 6.6%. Hence contributing to a more efficient VSC-HVdc solution.

The safe operation of grid connected power converters during abnormal condition is a key issue in order to guarantee its operation and to avoid undesired trips. In this paper different control methods for the operation of a D-STATCOM are evaluated, where the reference currents are determined in such a way that none of the phase currents goes over the limits, as well as the DC voltage fluctuations remain in safe operation limit. Therefore, the contribution of this paper lays on the combination of the DC voltage oscillations and the current limit control. As it is shown in the following, three different control strategies are evaluated. The amplitude of the oscillations which are superimposed on the DC voltage as well as peak amplitude of the phase currents are calculated for each, considering a generic imbalance in the network. The effectiveness of the presented control strategies are verified by simulating a D-STATCOM tied to an industrial distribution network. Moreover a scaled scenario has been reproduced experimentally which shows that the results cope well with the analytical equations and the simulation results.

Control of the circulating current in the modular multilevel converter (MMC) is one of the major tasks for the proper operation of the converter topology. This paper develops a controller for the circulating current of the MMC that utilises the redundant voltage levels in 2N+1 modulated MMCs in order to regulate the current to its reference. The main advantages of the approach are the elimination of control loops that generate the voltages for the control of the circulating current, simple implementation and very fast dynamic performance. The application of the control is also independent of the circulating current reference. The simplicity and effectiveness of the proposed controller is illustrated through simulations and experimental results.

This paper discusses the implementation of proportional resonant (PR) current controllers for a Voltage Source Converter (VSC) with LCL filter which is synchronized to the grid by virtual flux (VF) estimation with inherent sequence separation. Even though there is an extensive amount of literature and studies on the PR current controller for tracking the current reference of a VSC in the stationary reference frame, there is no discussion taking into account voltage sensor-less operation based on virtual flux estimation with an LCL-filter. Separate estimation of the positive and negative sequence virtual flux components at the grid-side of the LCL-filter, as well as current sequence separation, using the Second Order Generalized Integrator-Frequency Locked Loop (SOGI-FLL) is presented as part of a proposed method. The LCL filter is characterized in order to reduce the parameter deviation that might affect the virtual flux estimation. The stability of the proposed method is analyzed in the frequency domain while the operation and performance of the proposed system is verified by simulation studies.

This paper presents an analytical development for the Nyquist trajectory of the closed loop system of a Voltage Source Inverter with resonant control strategy. Both inner current loop and outer voltage loop are taken into account, and the interaction between them is studied. The stability boundaries depending on different parameters are determined. Firstly, the system is modelled and analysed in continuous time. Secondly, the analysis is conducted in the discrete time domain. A numeric study supported with graphical representations is provided for complex analytic situations.

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

This article shows the proposal of a linear–assisted converter or linear–&–switching hybrid converter with a constant switching frequency. The control loop of the system is based on the current–mode technique. The main disadvantage of a converter with current–mode control is the inherent instability of the loop when switch duty ratios are greater than 0.5. In order to make stable the proposed linear–assisted converter, the article shows the technique based on a slope compensation.

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.

Modular Multilevel Converters (MMC) technology is today one of the preferred technology in HVDC and large STATCOM applications due to modularity, fault tolerance and high efficiency, but the voltage balancing becomes very challenging, especially during negative sequence current injection as required for load unbalance compensation. This paper proposes an improved balancing strategy for the delta configuration to overcome this situation based on the injection of a third harmonic zero sequence current. Simulation results have proved the effectiveness of the proposal even where different switching losses and capacitor tolerance are taken in consideration.

In this paper a 10 kVA photovoltaic emulator is presented. The system implemented generates photovoltaic V-I characteristic curves to analyse and test maximum power point tracking algorithms of real photovoltaic inverters according to the standard EN50530. The power converter used includes a threephase ac-dc active rectifier. The power stage selected allows to minimize the harmonic current content and permits to be closed to the unity power factor with a fast response. The emulator is controlled by a digital signal processor that communicates with a human-machine-interface. It allows to emulate static and dynamic responses. Furthermore, the interface is also capable of simulating weather evolution and situations under shaded conditions.

This work presents a study of the influence of different gate driver circuits on the switching behavior of SiC MOSFET devices used in a buck converter. The paper is based on several tests performed to determine the switching times and switching losses, using different reverse bias V GS voltage levels and different passive RCD (Resistance Capacitor Diode) circuits to interface the driver to the SiC MOSFET gate. The study reveals that the gate bias and the different coupling circuits have little influence on response time, nevertheless choosing the right combination of reverse bias and coupling capacitor may reduce the switching losses.

This paper reports a comparison study of different carrier-based PWM techniques applied to the modular multilevel converter. Phase-disposition PWM (PD-PWM) and phase-shifted pulse-width modulation (PS-PWM) with non-interleaving and interleaving are considered in this study. In PS-PWM, two cases are evaluated. In the first case, the particular SMs that have to be activated/deactivated are defined by a voltage balancing algorithm, which is the same one implemented in PD-PWM. In addition, an algorithm to restrict the number of switching SMs is also implemented to reduce the switching frequency of the power devices. In the other case of PS-PWM, each sub-module (SM) has a carrier signal associated to it and capacitor voltage balance is achieved by individual control of its capacitor voltage. The circulating current is controlled to be a dc component in all the cases. Simulation and experimental results are presented to evaluate the quality of the line-to-line output voltages and SM capacitor voltage ripples for the different cases under study. © 2014 IEEE.

In this work, a typical structure of cascaded converters present in DC microgrids is analyzed. From the control point of view, converters acting as loads add a nonlinear effect to the supply bus, caused by its constant power load behavior. A realistic analysis of the equivalent load is presented. The open loop dynamics of the system is discussed, taking into account the equivalent load profile. A nonlinear sliding mode controller based on a proportional-integral controller is proposed to regulate the bus voltage under unknown load variations. The proposed methodology is validated trough simulation and experimental results.

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.

This paper presents two control techniques to minimize the circulating currents of the modular multilevel converter. The control techniques reduce the amplitude of each capacitor voltage ripple. Multicarrier level-shifted pulse-width-modulation is applied and the performance of interleaving and non-interleaving the carrier waveforms between the upper and the lower arms is reported. The total harmonic distortion of the output common voltage and rms value of the arms currents are benchmarked against the case when such techniques are not employed. The techniques have been verified by simulation using MATLAB/Simulink and PLECS Blockset software. Experimental results from a low power phase-leg prototype built with five sub-modules per arm are also provided.

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.

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.

n flying capacitor (FC) converters, phase-shifted pulse-width modulation (PS-PWM) provides natural voltage balancing. However, for a practical application, a more robust balancing mechanism of maintaining the FC voltages at the desired values is required. This paper proposes a new closed-loop voltage balancing method for multilevel FC converters using PS-PWM. The proposed method balances the voltages of the FCs by modifying the duty cycle of each switch of the FC converter using a proportional (P) controller. The crossed effect between FC currents and duty cycles is considered and is used for FC voltage balancing. The Simulation results verify that the proposed voltage balancing method is very robust to different operating conditions, such as load transients and non-linear loads.

This work deals with the modeling, control and experimental validation of a flywheel test bench which is part of IREC’s lab-scale microgrid. The storage device has been designed as a proof of concept. It is based on a low-speed rotating disk mechanically coupled to a Permanent Magnet Synchronous Machine. The electrical power is exchanged with the external grid by means of a set of back-to-back power converters. These power electronics control the speed of the machine, and thus the active power absorbed or injected by the device, and also regulate the reactive power at the point of common coupling with the external grid. Vector control techniques are used for designing the converter controllers: a field oriented vector control algorithm is implemented for governing the servomotor while the instantaneous power theory-based algorithm is used to manage the active and reactive currents flowing from the grid side converter. The control implementation in the experimental setup has been carried out by means of programming Digital Signal Processors (DSP’s). The modeling and control system design has been validated after executing several experiments.

The flying capacitor (FC) multilevel converter has attracted a great deal of interest in the recent years because of its easier extension to a higher number of levels (n>;3), as compared to its counterpart, the diode-clamped converter (DCC). The main focus of this paper is to develop a voltage balancing scheme of FCs for a five-level FC converter based on phase disposition pulse-width modulation (PD-PWM). Since there are multiple states that produce the same output voltage at the leg of the converter, such a redundancy is used to regulate the FC voltages at their desired levels. The selection of the optimal states is performed by minimizing a cost function. A drawback observed when using standard symmetrical triangular carriers for the PD-PWM, is the additional switching events that are produced due to transitions within the same voltage level. Nevertheless, this fact can be avoided by using sawtooth carrier waveforms instead. Simulation results verify the robustness of the proposed voltage balancing scheme against static and dynamic load conditions. Moreover, using sawtooth carriers a significant reduction of the switching frequency is achieved as compared to the use of standard triangle carriers while maintaining the FC voltage balanced.

A back-to-back HVDC converter system is typically used to connect two separate electrical grids or a remote large-scale renewable energy system with the electrical grid through a DC bus. The DC bus typically contains a large electrolytic capacitor giving a robust performance to the converter system. However, it is well known that the electrolytic DC bus capacitor is bulky and has a low reliability. Therefore, a possible solution to these drawbacks is to use a reduced DC bus capacitance, which will allow film capacitors to be used instead of electrolytic capacitors. This paper presents the performance evaluation of a five-level flying capacitor (FC) back-to-back converter system with reduced DC bus capacitance under different modulation schemes. Two carrier-based modulation strategies are studied, namely the phase shifted pulse-width modulation (PS-PWM) and the phase disposition pulse-width modulation (PD-PWM), along with their appropriate flying capacitor voltage balancing techniques. The classical synchronous reference frame control strategy has been implemented in order to provide DC bus voltage regulation and active/reactive power control. The performance of the two different modulation schemes is tested against the DC bus and FCs voltage ripples using the MATLAB/Simulink-PLECS software platform. The analyzed case studies demonstrate that the PS-PWM strategy is more suitable to operate the proposed FC converter system using reduced values for the DC bus and flying capacitors.

Digital control of power converters has been an area of considerable research interest in recent times. One of the problems which arises in these systems is that of the limit cycle oscillations that occur due to quantization in the feedback loop. This paper investigates the limit cycle oscillations that occur in the digitally controlled version of the buck converter with a proportional-integral controller. The amplitude and frequency of the oscillations that may occur on two duty cycle levels are investigated and related to the controller gain parameters. The analysis shows that it is not possible to guarantee that limit cycle oscillations on two levels will not occur simply by adjusting the gain parameters, and yields a condition which will prevent oscillations on two levels from occurring.

The droop control uses the local average values of the active and reactive power components for sharing the load power demand among inverters in parallel. In this paper, a method that uses a virtual quadrature reference frame to calculate the average power components injected by singlephase inverters is presented. This method improves the dynamic response of the power sharing control, as well as, completely eliminates the double frequency oscillation over the voltage and frequency synthesized by the inverter in steady state. Moreover, it is able to deal with nonlinear loads since the harmonic components that appears in the instantaneous power are filtered. The method is compared with the classical method, which is based on low pass filters, through simulation carried out in Matlab/Simulink and experimental results.

This work presents a switching converter topology which can boost and step down the supply voltage and can deliver both positive and negative voltages to a grounded load (bipolar converter topology with non-floating output). This topology, which uses five switches to perform the power conversion, is mostly intended for switching amplifiers, especially for applications which require both boosting and inverting the supply voltage (to supply bipolar voltage signals), and/or EMI-sensitive applications. Using a single-stage switching amplifier based on this 5-switch topology, it is feasible to supply a grounded load with a symmetrical bipolar voltage signal whose magnitude extends up to several times the magnitude of the supply voltage.

In this paper a modulation strategy for the DCM-232 power converter is proposed. This modulation has as particular a symmetrical ripple and 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).

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 DCAC 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.

With three different DC-side and AC-side connections, the three-level H-bridge voltage source converters (3L-HB-VSCs) are alternatives to 3L neutral-point-clamped VSCs (3L-NPC-VSCs) for interfacing large wind turbines with electricity grids. In order to assess their feasibility for large wind turbines, they should be investigated in terms of power density, which is one of the most important design criteria for wind turbine converters due to turbine nacelle space limitation. In this study, by means of the converter electro-thermal models based on the converter characteristics, the power capabilities, DC capacitor sizes, converter cabinet volumes of the three 3LHB- VSCs utilizing press-pack IGBTs are investigated in order to quantify and compare the power densities of the 3L-HB-VSCs employed as grid-side converters. Also, the suitable transformer types for the 3L-HB-VSCs are determined and comparatively studied in terms of volume and weight in order to estimate the size effects of the 3L-HB-VSC topology on the whole wind turbine connection system. Finally, based on the power density and transformer-size investigations, the feasibility of each 3LHB- VSC is discussed.

The continuous and accurate information of generator speed in wind energy application is an essential requirement in order to get the maximum power at given tip speed ratio, since the maximum power is almost an cubic function of generator speed. This paper studies a Frequency Locked Loop (FLL) - based controller for estimating the rotor speed of the Permanent Magnet Synchronous Generator (PMSG) in a Wind Energy Conversion Systems (WECS). The proposed system consist on get the direct sequence component of generator voltage by means a Dual Second Order Generalized Integrator (DSOGI) and applying a FLL for estimating the generator frequency and, hence, its rotor speed. Using a Proportional Resonant (PR) control, the rotor position is not required and thus a lot of mathematical operations are avoided. Simulation using MATLAB/Simulink and PowerSim shows that using a FLL leads to a simpler and reliable sensorless technique.

This work presents the first design and modelling of bonding-wire-based triangular spiral inductors (Fig. 1), targeting their application to on-chip switching power converters. It is demonstrated that the equilateral triangular shape compared to other polygonal shapes best balances the inductive density as well as the total Equivalent Series Resistance (ESR). Afterwards, a design procedure is presented in order to optimize the inductor design, in terms of ESR and occupied area reduction. Finally, finite-elements simulation results of an optimized design (27nH, 1 Ω) are presented to validate the proposed expressions

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.

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.

Nowadays transformerless PV converters are widely used in the energy conversion stage of a PV plant, because their good characteristics regarding efficiency, size, cost and weight. The main problem in transformerless PV systems is the common mode voltage variations that can generate leakage ground currents (LGC) through the parasitic capacitance formed between the PV panels and ground. These LGC are a risk for the users in a PV system therefore they should be avoided. The LGC has a clear origin on the pulse width modulation (PWM) strategy and the converter structure. In this paper a three-phase inverter with constant common mode voltage (CMV) is proposed. Simulation results are also provided.

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.

This paper presents a Sliding Mode Control for a wound rotor synchronous machine acting as an isolated generator connected to an unbalanced load. In order to simplify the control methodology, the standard dq-model of the machine is connected to a balanced resistive load. A switching function is defined in order to fulfill the control objective. From the desired surface, the standard sliding methodology is applied to obtain a robust and very simple controller. Then, the actual measured voltage of the machines is acquired and treated trough a frequency locked loop algorithm in order to extract the positive sequence and control it to guarantee a good definition of the park transformation. A phase locked loop algorithm is also used to avoid speed sensors. Numerical simulations a validate the control law and show good performance and a fast response to load and reference changes.

The increasing influence of wind power in the power system operation has caused the power system operators to include wind power plants in the control of the overall power system, both in steady-state and transient operation. Therefore, the grid connection requirements of the power system operators involve wind power systems. Low voltage ride-through requirement is probably the most demanding grid connection requirement to meet, at least from the point of view of the wind energy conversion system. In presence of grid voltage dips, the low voltage ride-through compliance produces a mismatch between the generated active power and the active power delivered to the grid. The management of this mismatch supposes a challenge for the wind energy conversion system. In this work, a control scheme for the back-to-back neutral-pointclamped converter is proposed. Under grid voltage dip, the controllers for generator-side and grid-side converters work concurrently to meet the low voltage ride-through requirement by storing the active power surplus in the inertia of the generator and keeping constant the dc-link voltage. Simulation results verify the proposed control scheme.

This paper proposes one novel sliding mode controller for the Half-bridge converter to reduce the line and load disturbance. Based on the bilinear model of the Half-bridge DC/DC converter, the small signal model and the design consideration of the sliding mode controller for this converter are deduced and presented. The controller is simply implemented in the analog controller board. The validation of the controller is verified both by theory analysis and experimental results. By applying the novel sliding mode controller, the Half-bridge converter can achieve good transient response.

This work extends the Asynchronous ΣΔ Modulator and its Adaptive version to multi-level modulators, targeting wideband high-efficiency switching power amplifiers. The performance of these modulators is compared with the multi-level Pulse Width Modulator, the most common modulator in multilevel switching power amplifiers. Behavioural model simulation results show that, unlike the multi-level PWM (which can only be used at high switching frequencies), the Asynchronous ΣΔ Modulators can also be used at average switching frequencies close to the signal bandwidth. At high switching frequencies, the performance of all the modulators is similar. The Asynchronous ΣΔ Modulators have further and more important advantages, such as high PSRR (even at the input signal frequency), the ability of handling non-regular distributed voltage levels, or a low ratio between the average switching frequency and the maximum instantaneous switching frequency.