Mokberdoran, A.; Gomis-Bellmunt, O.; Silva, N.; Carvalho, A. IEEE transactions on power electronics Vol. 33, num. 2, p. 1323-1334 DOI: 10.1109/TPEL.2017.2688412 Data de publicació: 2018-02 Article en revista
This paper proposes a new device by combining features of an interline dual H-bridge current flow controller with the core idea of a hybrid HVdc circuit breaker for meshed HVdc grid application. The proposed device can substitute two dc circuit breakers at a dc bus with at least two adjacent transmission lines. In addition to the current interruption action, the current in one of the adjacent lines can be controlled by the embedded current flow controller. The system-level behavior of the proposed current flow controlling hybrid dc circuit breaker is similar to that of the typical hybrid dc circuit breaker and the interline dual H-bridge current flow controller. The operation principles of the proposed device are introduced and analyzed in this work. The component ratings are compared to the existing solution, and the functionality of the proposed device is verified by simulation.
Inverter-based distributed generation plays a vital role in the stability and reliability of new power systems. Under voltage sags, these systems must remain connected to the electrical network according to the stringent requirements of grid codes. Low-voltage ride-through control strategies are becoming a common trend in power electronics research. However, previous studies of these control strategies have not dealt with the different possible scenarios presented by new grid codes, and many of them focus on a very limited number of control objectives. In this study, an algorithm to maximize the converter capabilities was developed and subjected to experimental tests during different voltage sags. In this research, based on unbalanced voltage drops of several severity levels, six different cases of current injection are identified while taking into consideration the restrictions imposed by grid codes. The research results represent a further step towards the development of flexible controllers adaptable to the environments of intelligent electricity grids with high integration of distributed generation.
This paper describes the design of an interleaved sliding mode control for a multiphase synchronous buck converter, which inherits the properties of sliding mode control, operates with fixed switching frequency in the steady-state, and ensures current equalization among phases. Moreover, a power management algorithm is added in order to decide the number of active phases as function of the power load demand, thus optimizing the converter efficiency. The system uses a Master-Slave structure where each phase can actuate as the Master one in such a way that the overall system reliability is improved. Experimental results in a 1.5 kW eight-phase synchronous buck converter show that interleaving operation, robust output voltage regulation, phase current equalization, switching frequency regulation, and power management are achieved.
Busquets-Monge, S.; Àlber Filbà Martínez; Alepuz, S.; Calle, A. IEEE transactions on power electronics Vol. 32, num. 10, p. 7521-7533 DOI: 10.1109/TPEL.2016.2636839 Data de publicació: 2017-10-01 Article en revista
This paper proposes a modulation strategy for multilevel multiphase diode-clamped dc–ac converters (also applicable to other functionally equivalent topologies) able to keep the dc-link capacitor voltages balanced under passive front ends, low frequency modulation indices (i.e., low number of switching transitions per fundamental cycle), any value of the amplitude modulation index, and any ac-side displacement power factor. A suitable phase voltage pattern with minimum number of switching transitions is presented for the n-level three-phase case. Subsequently, it is extended to higher number of switching transitions per fundamental cycle and to higher number of phases. Simulation results with three, four, and five levels; three and five phases; and several frequency-modulation-index values are presented to validate the proposed modulation strategy. Experimental results obtained with a four-level three-phase dc–ac converter prototype are also provided. The proposed modulation strategy enables the use of this type of converters in applications where the ac fundamental frequency may be close to the switching frequency, such as in high-power systems and variable-speed motor drives.
Matas, J.; Martin, M.; De La Hoz, J.; Abusorrah, A.; Al-Turki, Y.; Al-Hindawi, M. IEEE transactions on power electronics Vol. PP, num. 99, p. 1-13 DOI: 10.1109/TPEL.2017.2748920 Data de publicació: 2017-09-04 Article en revista
IEEE This paper applies the adaptive gradient descent method to the second-order generalized integrator (SOGI) filter in order to find an online estimation algorithm for the grid frequency, which leads to the proposal of three possible estimators. One of them is identical to the FLL algorithm reported in literature, which proves that it should be understood as a gradient descent estimation algorithm and not as a "frequency locked loop". The proposed gradient descent estimators are simple and suitable to be implemented into a digital processor with small computational burden. However, due to the SOGI characteristics, the estimators show to be especially sensitive to subharmonic and dc-offset voltage distortion. These problems are removed adopting a cascaded double SOGI approach, which strongly increases the rejection capability to harmonics and enhances the response to voltage sags. Simulation and experimental results are provided to validate the proposed contribution.
This paper applies the adaptive gradient descent method to the second-order generalized integrator (SOGI) filter in order to find an online estimation algorithm for the grid frequency, which leads to the proposal of three possible estimators. One of them is identical to the FLL algorithm reported in literature, which proves that it should be understood as a gradient descent estimation algorithm and not as a "frequency locked loop". The proposed gradient descent estimators are simple and suitable to be implemented into a digital processor with small computational burden. However, due to the SOGI characteristics, the estimators show to be especially sensitive to subharmonic and dc-offset voltage distortion. These problems are removed adopting a cascaded double SOGI approach, which strongly increases the rejection capability to harmonics and enhances the response to voltage sags. Simulation and experimental results are provided to validate the proposed contribution.
IEEE This paper presents a new transformer, i.e., the Custom Power Active Transformer (CPAT) which integrates both series and shunt power conditioning through power electronics in a single transformer. This is achieved through a distinct design of the magnetic circuit and auxiliary windings of the transformer. In this paper, a single-phase CPAT is proposed as well as a preface into its extension to multi-phase systems. Through its magnetic equivalent circuit model, several design considerations and control limitations are revealed in the paper. Analysis of the resulting CPAT structure shows some prospects in material saving as well as size and cost reduction when compared to the traditional multi-transformer based configuration. In this paper, the proposed single-phase CPAT is utilized in a distribution system application, where the control architecture is designed to attenuate voltage and current distortions at both the load and the grid side, respectively. Performance and effectiveness of the proposed CPAT are evaluated through simulation and experiments.
Guzman, R.; Garcia de Vicuña, J.; Castilla, M.; Miret, J.; Martin, M. IEEE transactions on power electronics Vol. 33, num. 5, p. 4512-4522 DOI: 10.1109/TPEL.2017.2723638 Data de publicació: 2017-07-05 Article en revista
This paper presents a variable structure control in natural frame for a three-phase voltage source inverter. The proposed control method is based on modifying the converter model in natural reference frame, preserving the low frequency state space variables dynamics. Using this model in a Kalman filter, the system state-space variables are estimated allowing to design three robust current sliding-mode controllers in natural frame. The main closed-loop features of the proposed method are: 1) robustness against grid inductance variations because the proposed model is independent of the grid inductance, 2) the power losses are reduced since physical damping resistors are avoided, and 3) the control bandwidth can be increased due to the combination of a variable hysteresis controller with a Kalman filter. To complete the control scheme, a theoretical stability analysis is developed. Finally, selected experimental results validate the proposed control strategy and permit illustrating all its appealing features.
López, I.; Ceballos, S.; Pou, J.; Zaragoza, J.; Andreu, J.; Ibarra, E.; Konstantinou, G. IEEE transactions on power electronics Vol. 32, num. 6, p. 4878-4890 DOI: 10.1109/TPEL.2016.2599872 Data de publicació: 2017-06-12 Article en revista
This paper presents a generalized pulse width modulation (PWM)-based control algorithm for multiphase neutral-point-clamped (NPC) converters. The proposed algorithm provides a zero sequence to be added to the reference voltages that contributes to improve the performance of the converter by: 1) Regulating the neutral-point (NP) current to eliminate/attenuate the low-frequency NP voltage ripples; 2) reducing the switching losses of the power semiconductors; and 3) maximizing the range of modulation indices for linear operation mode. The control method is formulated following a carrier-based PWM approach. Hence, dealing with complex space-vector diagrams to solve the modulation problem for multiphase converters is avoided. The recursive approach means that it can be easily extended to n-phase converters without increasing the complexity and computational burden, making it especially attractive for digital implementation. The proposed method allows regulating the NP voltage without the need for external controllers; therefore, no parameter tuning is required. The algorithm has been tested in a four-leg NPC converter prototype performing as a three- and four-phase system and operating with balanced and unbalanced loads.
Luna, A.; Meng, L.; Díaz Aldana, N.; Graells, M.; Vasquez, J.; Guerrero, J. IEEE transactions on power electronics num. 99, p. 1-15 DOI: 10.1109/TPEL.2017.2700083 Data de publicació: 2017-05-02 Article en revista
Microgrids are energy systems that can work independently from the main grid in a stable and self-sustainable way. They rely on energy management systems to schedule optimally the distributed energy resources. Conventionally, the main research in this field is focused on scheduling problems applicable for specific case studies rather than in generic architectures that can deal with the uncertainties of the renewable energy sources. This paper contributes a design and experimental validation of an adaptable energy management system implemented in an online scheme, as well as an evaluation framework for quantitatively assess the enhancement attained by different online energy management strategies. The proposed architecture allows the interaction of measurement, forecasting and optimization modules, in which a generic generation-side mathematical problem is modeled, aiming to minimize operating costs and load disconnections. The whole energy management system has been tested experimentally in a test bench under both grid-connected and islanded mode. Also, its performance has been proved considering severe mismatches in forecast generation and load. Several experimental results have demonstrated the effectiveness of the proposed EMS, assessed by the corresponding average gap with respect to a selected benchmark strategy and ideal boundaries of the best and worst known solutions.
Pérez, J.; Cóbreces, S.; Griño, R.; Rodriguez, F.J. IEEE transactions on power electronics Vol. 32, num. 4, p. 3180-3191 DOI: 10.1109/TPEL.2016.2574560 Data de publicació: 2017-04-01 Article en revista
This paper presents a current controller that shapes, in the frequency domain, the input admittance of voltage-source converters connected to the grid. The controller is obtained by means of a H8 synthesis procedure, which minimizes the difference between the application closed-loop input admittance and a model-reference defined by the designer. This formulation achieves good accuracy in both modulus and phase. The proposed methodology allows the fulfilment of other current control objectives, such as current tracking, by defining frequency regions where each objective is desired. Experimental results show the good response of the proposed controller, both in frequency and time domain
Luna, A.; Díaz, N.; Graells, M.; Vasquez, J.; Guerrero, J. IEEE transactions on power electronics Vol. 32, num. 4, p. 2769-2783 DOI: 10.1109/TPEL.2016.2581021 Data de publicació: 2017-04-01 Article en revista
Microgrids are energy systems that aggregate distributed energy resources, loads, and power electronics devices in a stable and balanced way. They rely on energy management systems to schedule optimally the distributed energy resources. Conventionally, many scheduling problems have been solved by using complex algorithms that, even so, do not consider the operation of the distributed energy resources. This paper presents the modeling and design of a modular energy management system and its integration to a grid-connected battery-based microgrid. The scheduling model is a power generation-side strategy, defined as a general mixed-integer linear programming by taking into account two stages for proper charging of the storage units. This model is considered as a deterministic problem that aims to minimize operating costs and promote self-consumption based on 24-hour ahead forecast data. The operation of the microgrid is complemented with a supervisory control stage that compensates any mismatch between the offline scheduling process and the real time microgrid operation. The proposal has been tested experimentally in a hybrid microgrid at the Microgrid Research Laboratory, Aalborg University.
Fixing the switching frequency is a key issue in sliding mode control implementations. This paper presents a hysteresis band controller capable of setting a constant value for the steady-state switching frequency of a sliding mode controller in regulation and tracking tasks. The proposed architecture relies on a piecewise linear modeling of the switching function behavior within the hysteresis band, and consists of a discrete-time integral-type controller that modifies the amplitude of the hysteresis band of the comparator in accordance with the error between the desired and the actually measured switching period. For tracking purposes, an additional feedforward action is introduced to compensate the time variation of the switching function derivatives at either sides of the switching hyperplane in the steady state. Stability proofs are provided, and a design criterion for the control parameters to guarantee closed-loop stability is subsequently derived. Numerical simulations and experimental results validate the proposal.
Konstantinou, G.; Pou, J.; Ceballos, S.; Picas, R.; Zaragoza, J.; Agelidis, V. IEEE transactions on power electronics Vol. 31, num. 11, p. 7761-7769 DOI: 10.1109/TPEL.2015.2512842 Data de publicació: 2016-11-01 Article en revista
Among the main control targets in a modular multilevel converter (MMC) is the control of the circulating currents within the phase legs of the topology. This paper presents a controller for the circulating current of the MMC that utilizes the available redundancies of the multilevel waveform in 2N + 1 modulated MMCs in order to regulate the circulating current to its reference. The main advantages of the approach are the elimination of control loops that generate the reference voltages for the control of the circulating current, simple implementation and very fast dynamic performance. The controller is implemented at the modulation stage and its operation is independent of the circulating current reference. An extension of the controller to track large deviations in the circulating current is also demonstrated. The simplicity and effectiveness of the proposed controller is illustrated through detailed simulations and experimental results from a single-phase laboratory prototype.
Guzman, R.; Garcia de Vicuña, J.; Morales, J.; Castilla, M.; Miret, J. IEEE transactions on power electronics Vol. 32, num. 7, p. 5637-5650 DOI: 10.1109/TPEL.2016.2605858 Data de publicació: 2016-09-02 Article en revista
This paper presents a robust model-based active
damping control in natural frame for a three-phase voltage
source inverter with LCL filter. The presence of the LCL
filter complicates the design of the control scheme, particularly
when system parameters deviations are considered. The proposed
control method is addressed to overcome such difficulties and
uses a modified converter model in an state observer. In this
proposal, the converter model is modified by introducing a virtual
damping resistor. Then, a Kalman filter makes use of this model
to estimate the system state-space variables. Although the state
estimates do not obviously match the real world system variables,
they permit designing three current sliding-mode controllers that
provide the following features to the closed loop system: a) robust
ande active damping capability like in the case of using a physical
damping resistor, b) robustness because the control specifications
are met independently of variation in the system parameters, c)
noise immunity due to the application of the Kalman filter, and
d) power loss minimization because the system losses caused
by the physical damping resistor are avoided. An interesting
side effect of the proposed control scheme is that the sliding
surfaces for each controller are independent. This decoupling
property for the three controllers allows using a fixed switching
frequency algorithm that ensures perfect current control. To
complete the control scheme, a theoretical stability analysis is
developed. Finally, selected experimental results validate the
proposed control strategy and permit illustrating all its appealing
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Ghias, A.; Pou, J.; Capella, G.; Acuna, P.; Agelidis, V. IEEE transactions on power electronics Vol. 31, num. 8, p. 5384-5388 DOI: 10.1109/TPEL.2016.2521803 Data de publicació: 2016-08-01 Article en revista
This letter proposes a phase-shifted pulse-width modulation (PS-PWM) technique for the flying capacitor (FC) multilevel converter that improves the quality of the line-to-line voltages. In traditional PS-PWM, the line-to-line voltages include intervals that switch between nonadjacent voltage levels, which deteriorates the harmonic performance. The proposed PS-PWM constantly achieves line-to-line voltages with switching transitions between two adjacent levels. The proposed modulation technique is implemented using two sets of n evenly phase-shifted carriers that are alternatively applied depending on the instantaneous value of the reference signal of each phase-leg. Furthermore, it can be implemented in FC multilevel converters with any number of levels while maintaining natural capacitor voltage balance. Experimental results from a four-level FC converter prototype are presented.
Sosa, J.; Castilla, M.; Miret, J.; Matas, J.; Al-Turki, Y. IEEE transactions on power electronics Vol. 31, num. 4, p. 3314-3323 DOI: 10.1109/TPEL.2015.2451674 Data de publicació: 2016-04-01 Article en revista
Under voltage sags, grid-tied photovoltaic inverters should remain connected to the grid according to low-voltage ride-through requirements. During such perturbations, it is interesting to exploit completely the distributed power provisions to contribute to the stability and reliability of the grid. In this sense, this paper proposes a low-voltage ride-through control strategy that maximizes the inverter power capability by injecting the maximum-rated current during the sag. To achieve this objective, two possible active power situations have been considered, i.e., high- and low-power production scenarios. In the first case, if the source is unable to deliver the whole generated power to the grid, the controller applies active power curtailment to guarantee that the maximum rated current is not surpassed. In the second case, the maximum allowed current is not reached, thus, the control strategy determined the amount of reactive power that can be injected up to reach it. The control objective can be fulfilled by means of a flexible current injection strategy that combines a proper balance between positive- and negative-current sequences, which limits the inverter output current to the maximum rated value and avoid active power oscillations. Selected experimental and simulation results are reported in order to validate the effectiveness of the proposed control strategy.
This paper presents a fault-tolerant configuration for the modular multilevel converter (MMC). The procedure is able to detect faults in voltage sensors and semiconductor switching devices, and it can reconfigure the system so that it can keep on operating. Both switch and sensor faults can be detected by comparing the output voltage of a set of submodules (SMs), which is measured by a so-called supervisory sensor, with two calculated reference voltages. Faults in the supervisory sensors are also considered. Sensor faults are overcome by using a measuring technique based on estimates that are periodically updated with the voltage measurements of the supervisory sensors. Additional SMs are included in the arms so that the MMC can bypass a faulty SM and continue operating without affecting the output voltage of the phase-leg. Experimental results obtained from a low-power MMC prototype are presented in order to demonstrate the effectiveness of the proposed techniques.
Picas, R.; Zaragoza, J.; Pou, J.; Ceballos, S.; Balcells, J. IEEE transactions on power electronics Vol. 31, num. 1, p. 177-187 DOI: 10.1109/TPEL.2015.2412658 Data de publicació: 2016-01-01 Article en revista
This paper presents a new technique for measuring the capacitor voltages in a modular multilevel converter using a reduced number of voltage sensors. With this technique, the minimum number of voltage sensors per arm is two. Each sensor measures the output voltage of a set of submodules (SMs) connected in series and acquires a new measurement when there is only one SM activated within the set. The acquired value corresponds to the capacitor voltage of the activated SM minus the voltage drops produced in the switches. A simple mathematical model is used to estimate all the SM capacitor voltages, and it is then updated whenever there is a new measurement available. An algorithm that enforces the periodic update of the voltage measurements is also presented. The proposed measuring technique highly reduces the number of voltage sensors; hence reducing the complexity and costs of the signal conditioning and data acquisition stages. Simulation and experimental results are presented to demonstrate the efficiency of the proposed technique.
This letter analyzes how the efficiency of boost dc/dc converters operating in burst mode under light-load conditions can be improved by an appropriate selection of the inductor current that transfers energy from the input to the output. A theoretical analysis evaluates the main power losses (fixed, conduction, and switching losses) involved in such converters, and how do they depend on the inductor current. This analysis shows that there is an optimal value of this current that causes minimum losses and, hence, maximum efficiency. These theoretical predictions are then compared with experimental data resulting from a commercial boost dc/dc converter (TPS61252), whose average inductor current is adjustable. Experimental results show that the use of the optimal inductor current, which was around 340 mA for an output voltage of 5 V, provides an efficiency increase of 7%.
Guzman, R.; Garcia de Vicuña, J.; Morales, J.; Castilla, M.; Matas, J. IEEE transactions on power electronics Vol. 31, num. 1, p. 758-769 DOI: 10.1109/TPEL.2015.2403075 Data de publicació: 2016-01-01 Article en revista
This paper presents an improved variable hysteresis-band current-control in natural frame for a three-phase unity power rectifier. The proposed control algorithm is based on three decoupled sliding-mode controllers combined with three independent Kalman filters. The use of Kalman filters instead of a nonadaptive state observer improves the quality of the estimated signals in presence of noise, increasing the immunity of the control loop in noisy environments. To reduce drastically the computational load in the Kalman algorithm, a reduced bilinear model is derived which allows to use a Kalman filter algorithm instead of an extended Kalman filter. A fast output-voltage control is also presented which avoids output-voltage variations when a sudden change in the load or a voltage sag appears. Moreover, a fixed switching frequency algorithm is proposed which uses a variable hysteresis-band in combination with a switching decision algorithm, ensuring a switching spectrum concentrated around the desired switching frequency. The overall control proposal has been fully integrated into a digital signal processor. Selected experimental results are introduced to validate the theoretical contributions of this paper.
This paper presents an improved variable hysteresisband current-control in natural frame for a three-phase unity power rectifier. The proposed control algorithm is based on three decoupled sliding-mode controllers combined with three independent Kalman filters. The use of Kalman filters instead of a non-adaptive state observer improves the quality of the estimated signals in presence of noise, increasing the immunity of the control loop in noisy environments. To reduce drastically the computational load in the Kalman algorithm, a reduced bilinear model is derived which allows to use a Kalman filter algorithm instead of an extended Kalman filter. A fast output-voltage control is also presented which avoids output-voltage variations when a sudden change in the load or a voltage sag appears. Moreover, a fixed switching frequency algorithm is proposed which uses a variable hysteresis-band in combination with a switching decision algorithm, ensuring a switching spectrum concentrated around the desired switching frequency. The overall control proposal has been fully integrated into a digital signal processor. Selected experimental results are introduced to validate the theoretical contributions of this paper.
Ghias, A.; Pou, J.; Capella, G.; Agelidis, V.; Aguilera, R.; Meynard, T. IEEE transactions on power electronics Vol. 30, num. 10, p. 5376-5380 DOI: 10.1109/TPEL.2015.2427201 Data de publicació: 2015-10-01 Article en revista
This letter proposes a new implementation of phase-disposition pulse-width modulation (PD-PWM) for multilevel flying capacitor (FC) converters using a single triangular carrier. The proposed implementation is much simpler than conventional PD-PWM techniques based on multiple trapezoidal-shaped carriers, generates the same results as far as natural capacitor voltage balance is concerned and offers better quality line-to-line voltages when compared to phase-shifted PWM. The proposed algorithm is based on reshaping the reference signal to fit within the range of a single carrier and assigning each crossing of the reference signal with the carrier to a particular pair of switches at any time. The proposed algorithm is suitable for digital implementation taking maximum benefit from the PWM units available in the processor. Simulation and experimental results are presented from the five-level FC converter to verify the proposed PD-PWM implementation.
Darus, R.; Pou, J.; Konstantinou, G.; Ceballos, S.; Picas, R.; Agelidis, V. IEEE transactions on power electronics Vol. 30, num. 8, p. 4119-4127 DOI: 10.1109/TPEL.2014.2359005 Data de publicació: 2015-08-01 Article en revista
This paper introduces a low complexity implementation of the voltage balancing algorithm aiming to reduce the switching frequency of the power devices in modular multilevel converters (MMCs). The proposed algorithm features a relatively simple implementation without any conditional execution requirements and is easily expandable regardless of the number of submodules (SMs). Two modulation techniques are evaluated, namely the staircase modulation and the phase-disposition pulse width modulation (PD-PWM) under the conventional and the proposed algorithm. Using a circulating current controller in an MMC with 12 SMs per arm, PD-PWM yields better results compared to the staircase modulation technique. The test condition for this comparison is such that the power devices operate at a similar switching frequency and produce similar amplitudes to the capacitor voltage ripples in both modulation techniques. The results are verified through extensive simulations and experiments on a low power phase-leg MMC laboratory prototype.
Picas, R.; Ceballos, S.; Pou, J.; Zaragoza, J.; Konstantinou, G.; Agelidis, V. IEEE transactions on power electronics Vol. 30, num. 7, p. 4714-4725 DOI: 10.1109/TPEL.2014.2368055 Data de publicació: 2015-04-15 Article en revista
López, I.; Ceballos, S.; Pou, J.; Zaragoza, J.; Andreu, J.; Kortabarria, I.; Agelidis, V. IEEE transactions on power electronics Vol. 31, num. 2, p. 928-941 DOI: 10.1109/TPEL.2015.2416911 Data de publicació: 2015-02-01 Article en revista
This paper presents a novel modulation strategy for n-phase neutral-point-clamped (NPC) converters. The proposed modulation strategy is able to control and completely remove the low-frequency neutral-point (NP) voltage oscillations for any operation point and load types. Even when unbalanced and/or nonlinear loads are considered, the NP voltage remains under total control. Consequently, the strategy is very attractive for n-phase active filters. In addition, it enables the use of low-capacity film capacitors in NPC converters. The proposed modulation takes the carrier-based modulation strategy as a basis. It is formulated following a generalized approach that makes it expandable to n-phase NPC converters. In addition, the NP voltage is controlled directly using a closed-loop algorithm that does not rely on the use of the linear control regulators or the additional compensators used in other modulation algorithms. Therefore, no tuning of parameters is required and it performs optimally for any operating conditions and kind of loads, including unbalanced and nonlinear loads. Although the high-frequency harmonic content of the output voltages may increase, the weighted total harmonic distortion generated by the proposed strategy is similar to that of a standard sinusoidal pulse width modulated strategy. The proposed modulation algorithm has been tested in a four-leg NPC converter prototype performing as a three-and four-phase system and operating with balanced and unbalanced loads.
Ma, L.; Kerekes, T.; Rodriguez, P.; Jin, X.; Teodorescu, R.; Liserre, M. IEEE transactions on power electronics Vol. 30, num. 9, p. 5331-5340 DOI: 10.1109/TPEL.2014.2387152 Data de publicació: 2015-01-09 Article en revista
Photovoltaic systems technological development is driven by the request for higher efficiency and safety. These concerns influence also the choice of the power converter stage. Several topologies have been proposed and many of them are available commercially. Among them, the neutral point clamped (NPC) and derived topologies offers high efficiency, low leakage current, and low EMI. However, one main disadvantage of the NPC inverter is given by an unequal distribution of the losses in the semiconductor devices, which leads to an unequal distribution of temperature that can affect lifetime. By using the active NPC (ANPC) topology, where the clamping diodes are replaced by bidirectional switches, the power losses distribution problem is alleviated. The modulation strategy is a key issue for losses distribution in this topology. In this paper, two known strategies are discussed and a new PWM strategy, namely the adjustable losses distribution is proposed for better losses distribution in the ANPC topology. Simulations and experimental results help in evaluating the modulation strategies
Mirhosseini, M.; Pou, J.; Agelidis, V.; Robles, E.; Ceballos, S. IEEE transactions on power electronics Vol. 29, num. 12, p. 6255-6259 DOI: 10.1109/TPEL.2014.2328657 Data de publicació: 2014-12-01 Article en revista
Camacho, A.; Castilla, M.; Miret, J.; Guzman, R.; Borrell, A. IEEE transactions on power electronics Vol. PP, num. 11, p. 6224-6234 DOI: 10.1109/TPEL.2014.2301463 Data de publicació: 2014-11-01 Article en revista
Grid faults are one of the most severe problems for network operation. Distributed generation power plants can help to mitigate the adverse effects of these perturbations by injecting the reactive power during the sag and the postfault operation. Thus, the risk of cascade disconnection and voltage collapse can be reduced. The proposed reactive power control is intended to regulate the maximum and minimum phase voltages at the point of common coupling within the limits established in grid codes for continuous operation. In balanced three-phase voltage sags, the control increases the voltage in each phase above the lower regulated limit by injecting the positive sequence reactive power. In unbalanced voltage sags, positive and negative sequence reactive powers are combined to flexibly raise and equalize the phase voltages; the maximum phase voltage is regulated below the upper limit and the minimum phase voltage just above the lower limit. The proposed control strategy is tested by considering a distant grid fault and a large grid impedance. Selected experimental results are reported in order to validate the behavior of the control scheme.
Grid faults are one of the most severe problems for network operation. Distributed generation power plants can help to mitigate the adverse effects of these perturbations by injecting reactive power during the sag and the post-fault operation. Thus, the risk of cascade disconnection and voltage collapse can be reduced. The proposed reactive power control is intended to regulate the maximum and minimum phase voltages at the point of common coupling within the limits established in grid codes for continuous operation. In balanced three-phase voltage sags, the control increases the voltage in each phase above the lower regulated limit by injecting positive sequence reactive power. In unbalanced voltage sags, positive and negative sequence reactive powers are combined to flexibly raise and equalize the phase voltages; maximum phase voltage is regulated below the upper limit and the minimum phase voltage just above the lower limit. The proposed control strategy is tested by considering a distant grid fault and a large grid impedance. Selected experimental results are reported in order to validate the behavior of the control scheme.
Castilla, M.; Miret, J.; Camacho, A.; Garcia de Vicuña, J.; Matas, J. IEEE transactions on power electronics Vol. 29, num. 11, p. 6139-6150 DOI: 10.1109/TPEL.2013.2296774 Data de publicació: 2014-11-01 Article en revista
Three-phase inverters equipped with voltage support control schemes have been successfully used to alleviate the negative impact of voltage imbalance on electric power systems. With these schemes, dc-link voltage ripple and current harmonics are significantly reduced by processing the positive and negative sequence components separately. However, the design methods for tuning the parameters of these control schemes have one or more of the following limitations: 1) the design is a very time-consuming task, 2) it is conservative, 3) it does not guarantee the specifications for all the considered situations, and 4) the system can be unstable in some abnormal conditions. As an alternative, this paper presents a design method based on the analysis of oscillations in nonlinear systems. The method proceeds by first developing simple and accurate models of the power system, second it reveals the system features through an in deep analysis of the derived models, and third it introduces a systematic design procedure for tuning the parameters of the control schemes. As an example, a voltage support control scheme for a three-phase inverter operating under an unbalanced voltage sag is designed and validated experimentally.
Rivera, S.; Kouro, S.; Wu, B.; Alepuz, S.; Malinowski, M.; Cortés, P.; Rodriguez, J. IEEE transactions on power electronics Vol. 29, num. 10, p. 5592-5604 DOI: 10.1109/TPEL.2013.2294711 Data de publicació: 2014-10-01 Article en revista
This paper presents a generalized multilevel direct power control (ML-DPC) scheme for grid-connected multilevel power converters. The proposed method extends the original DPC operating principle by considering only the closest subset of two-level voltage vectors to the present switching state. The implementation of this principle requires the power derivatives for feedback, which can present numerical problems when applied experimentally, mainly due to high measurement noise sensitivity. Therefore, a derivative estimator is proposed based on the converter-grid model in the synchronous reference frame. In addition, a virtual flux observer is developed to achieve synchronization and improve robustness in the presence of grid voltage harmonics. The proposed method is applicable to any multilevel converter topology of any number of levels. In this paper, simulations and experimental results are presented for a seven-level cascaded H-bridge converter.
Multilevel flying capacitor (FC) converters provide natural capacitor voltage balance under phase-shifted pulse width modulation (PS-PWM). However, natural balancing may not be robust enough to maintain the voltages at the reference values, especially under certain transient conditions. Furthermore, natural balancing dynamics depend on the load and it may be very slow in some practical applications. Therefore, 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 the 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 controller. The crossed effect between FC currents and duty cycles is considered and is used for optimal FC voltage balancing. Simulation and experimental results verify that the proposed voltage-balancing method is very robust to different operating conditions, such as load transients, linear/nonlinear and unbalanced loads
This letter reports a method for the initial charging of capacitors in grid-connected flying capacitor (FC) multilevel converters. A resistor is inserted between each phase of the FC converter and the grid. A voltage balancing algorithm is activated from the beginning of the process and the FC converter generates proper output voltages to achieve balanced charging of both the dc-bus capacitor and the FCs. The proposed initial charging method achieves low voltage and current stress on the power devices and the passive components. The method is simple to implement and can be applied to an FC with any number of levels. Experimental results demonstrate the effectiveness of the proposed method.
Capella, G.; Pou, J.; Ceballos, S.; Konstantinou, G.; Zaragoza, J.; Agelidis, V. IEEE transactions on power electronics Vol. 30, num. 3, p. 1121-1125 DOI: 10.1109/TPEL.2014.2338357 Data de publicació: 2014-03-01 Article en revista
This letter presents a novel implementation of pulse width modulation that improves the quality of the line-to-line output voltages in interleaved multiphase voltage-source inverters (VSIs). In multiphase VSIs with n interleaved parallel-connected legs, the best single-phase output voltage is achieved when the carriers are evenly phase shifted. However, switching among nonadjacent levels can be observed at regular intervals in the line-to-line voltages, causing bad harmonic performance. With the proposed method, switching in the line-to-line voltages happens exclusively between adjacent levels. The modulator utilizes two sets of n evenly phase-shifted carriers that are dynamically allocated. Because of its generality, the proposed implementation is valid for any number of phases and any number of legs in parallel. A MATLAB/Simulink model has been set up for simulation purposes. Selected experimental results obtained from a three-phase VSI made up with two and three legs in parallel per phase are reported, confirming the enhancement attained with the proposed implementation.
Montesinos-Miracle, D.; Massot-Campos, M.; Bergas, J.; Galceran-Arellano, S.; Rufer, A. IEEE transactions on power electronics Vol. 28, num. 8, p. 3970-3979 DOI: 10.1109/TPEL.2012.2231702 Data de publicació: 2013-08 Article en revista
This paper compares different cascaded and multilevel
topologies to interface supercapacitors to a dc bus in regenerative
braking applications. It is shown that the modular multilevel
dc/dc converter (MMC) can benefit from both reduced voltage
and increased frequency across the inductor to reduce its weight
and volume when using phase shifting modulation. The proposed
controlmethod is able to balance supercapacitor voltage while providing
precise output current control. The converter topology and
control method are validated with simulations and experimental
Miret, J.; Camacho, A.; Castilla, M.; Garcia de Vicuña, J.; Matas, J. IEEE transactions on power electronics Vol. 28, num. 11, p. 5252-5262 DOI: 10.1109/TPEL.2013.2246190 Data de publicació: 2013-05 Article en revista
Voltage sags are one of the main problems in transmission and distribution grids with high penetration of distributed generation. This paper proposes a voltage support control scheme for grid-connected power sources under voltage sags. The control is based on the injection of reactive current with a variable ratio between positive and negative sequences. The controller determines, also, the amount of reactive power needed to restore the dropped voltage magnitudes to new reference values confined within the continuous operation limits required in grid codes. These reference values are chosen in order to guarantee low current injection when fulfilling the voltage support objective. Selected experimental results are reported in order to validate the effectiveness of the proposed control.
Interturn faults in permanent magnet synchronous
machines may have very harmful effects if not early identified. This study deals with the detection of such faults when the machine operates under varying speed conditions. The performance of two methods is analyzed and compared, i.e., the analysis of the third harmonic of the stator currents and the first one of the zero-
sequence voltage components. The Vold–Kalman filtering order tracking algorithm is introduced and applied to track the harmonics of interest when the machine operates under a wide speed range and different load levels. This study also presents two reliable fault indicators especially focused to detect stator winding interturn faults under nonstationary speed conditions. Experimental results endorse the methodology proposed, showing its potential
to carry out a reliable fault diagnosis scheme
Senturk, O.; Helle, L.; Rodriguez, P.; Munk-Nielsen, S.; Teodorescu, R. IEEE transactions on power electronics Vol. 27, num. 7, p. 3195-3206 DOI: 10.1109/TPEL.2011.2182661 Data de publicació: 2012-07 Article en revista
Wind turbine power capability is an essential set of
data for both wind turbine manufacturers/operators and transmission
system operators since the power capability determines
whether a wind turbine is able to fulfill transmission system reactive
power requirements and how much it is able to provide
reactive power support as an ancillary service. For multimegawatt
full-scale wind turbines, power capability depends on converter
topology and semiconductor switch technology. As power capability
limiting factors, switch current, semiconductor junction temperature,
and converter output voltage are addressed in this study
for the three-level neutral-point-clamped voltage source converter
(3L-NPC-VSC) and 3L Active NPC VSC (3L-ANPC-VSC) with
press-pack insulated gate bipolar transistors employed as a gridside
converter. In order to investigate these VSCs’ power capabilities
under various operating conditions with respect to these
limiting factors, a power capability generation algorithm based on
the converter electrothermal model is developed. Built considering
the VSCs’ operation principles and physical structure, the model is
validated by a 2MV·A single-phase 3L-ANPC-VSC test setup. The
power capability investigations regarding a sample grid code’s reactive
power requirement showthat 3L-ANPC-VSC results in 32%
better power capability than 3L-NPC-VSC at the switching frequency
of 1050 Hz. Furthermore, 3L-ANPC-VSC with 57% higher
switching frequency (1650 Hz) and 33% smaller switching ripple
filter can yield close power capability compared to 3L-NPC-VSC
with 1050 Hz.
A synchronous buck converter based multiphase architecture
is evaluated to determine whether or not the most
widespread voltage regulator (VR) topology canmeet the power delivery
requirements of next-generation computer processors. The
applied analysis methodology relies on accurate device models for
circuit simulations, where the power MOSFETs are central due to
their primary relevance to power losses. The method is referred
to as virtual design loop and aims at optimizing the overall system
performance with minimum empirical efforts. This is successfully
applied to the development of a power MOSFET technology offering
outstanding dynamic and static performance characteristics
in the application. From a system perspective, the limits of power
density conversion will be explored for this and other emerging
technologies that promise to open up a new paradigm in power
Rodriguez, P.; D. Bellar, M.; Muñoz-Aguilar, R. S.; Busquets-Monge, S.; Blaabjerg, F. IEEE transactions on power electronics Vol. 27, num. 3, p. 1055-1060 DOI: 10.1109/TPEL.2011.2172224 Data de publicació: 2012-03 Article en revista
This letter introduces a new series of multilevel (ML)
converters based on the ML clamping concept. By applying this
technique, a ML clamping unit (MCU) conveys additional levels
for synthesizing the output waveforms of a diode-clampedML dc–
ac power converter. The basic building block of the ML clamping
scheme is the ML clamping cell, which is composed of a pair of dc
sources associated with one single-pole/triple-throw type of switch
arrangement. The number of series-connected ML-clamping cells
will set the number of levels of the synthesized waveform. By depending
on the MCU arrangements, different converter configurations
can be derived, namely, the common clamping and the
modular types. Both approaches can be accomplished by employing
classical three-level neutral-point-clamped (NPC) technology.
Thus, the overall structure of the resultant converter is kept simple,
wherein the number of parts count can be reduced, when
compared with conventional NPC counterparts. In this letter, emphasis
is given on the common clamping converter configuration.
The attractiveness of the proposed ML concept is evidenced
by conducting a comparative cost analysis. Experimental results
are presented as a practicality proof of the proposed ML power
Pou, J.; Zaragoza, J.; Ceballos, S.; Saeedifard, M.; Boroyevich, D. IEEE transactions on power electronics Vol. 27, num. 2, p. 642-651 DOI: 10.1109/TPEL.2010.2050783 Data de publicació: 2012-01-09 Article en revista
Rodriguez, P.; Luna, A.; Muñoz-Aguilar, R. S.; Etxebarría, I.; Teodorescu, R.; Blaabjerg, F. IEEE transactions on power electronics Vol. 27, num. 1, p. 99-112 DOI: 10.1109/TPEL.2011.2159242 Data de publicació: 2012-01 Article en revista
Grid synchronization algorithms are of great importance
in the control of grid-connected power converters, as fast and
accurate detection of the grid voltage parameters is crucial in order
to implement stable control strategies under generic grid conditions.
This paper presents a new grid synchronization method for
three-phase three-wire networks, namely dual second-order generalized
integrator (SOGI) frequency-locked loop. The method is
based on two adaptive filters, implemented by using a SOGI on
the stationary αβ reference frame, and it is able to perform an excellent
estimation of the instantaneous symmetrical components of
the grid voltage under unbalanced and distorted grid conditions.
This paper analyzes the performance of the proposed synchronization
method including different design issues. Moreover, the
behavior of the method for synchronizing with highly unbalanced
grid is proven by means of simulation and experimental results,
demonstrating its excellent performance.
Miret, J.; Castilla, M.; Camacho, A.; Garcia de Vicuña, J.; Matas, J. IEEE transactions on power electronics Vol. 27, num. 10, p. 4262-4271 DOI: 10.1109/TPEL.2012.2191306 Data de publicació: 2012 Article en revista
Nowadays, the majority of the photovoltaic (PV)
power sources are connected to the public grid. One of the main
connection problems occurs when voltage sags appear in the grid
due to short circuits, lightning, etc. International standards regulate
the grid connection of PV systems, forcing the source to remain
connected during short-time grid-voltage faults. As a consequence,
during the voltage sag, the source should operate with increasing
converter currents to maintain the injection of the generated power.
This abnormal operation may result in nondesired system disconnections
due to overcurrents. This paper proposes a controller for
a PV three-phase inverter that ensuresminimum peak values in the
grid-injected currents, as compared with conventional controllers.
From the system analysis, a design method is presented in order
to set the parameters of the control scheme. Selected experimental
results are reported in order to validate the effectiveness of the
Reyes, M.; Rodriguez, P.; Vazquez, S.; Luna, A.; Teodorescu, R.; Carrasco , J. IEEE transactions on power electronics Vol. 27, num. 9, p. 3934-3943 DOI: 10.1109/TPEL.2012.2190147 Data de publicació: 2012 Article en revista
In the last few years, restrictive grid codes have arisen to ensure the performance and stability of electrical networks, which experience amassive integration of renewable energy sources and distributed generation systems that are normally connected to the grid through electronic power converters. In these codes, the injection of positive- and negative-sequence current components becomes necessary for fulfilling, among others, the low-voltage ride-through requirements during balanced and unbalanced grid faults. However, the performance of classical dq current controllers, applied to power converters, under unbalanced grid-voltage conditions is highly deficient, due to the unavoidable appearance of current oscillations. This paper analyzes the performance of the double synchronous reference frame controller and improves its structure by adding a decoupling network for estimating and compensating the undesirable current oscillations. Experimental results will demonstrate the validity of the proposed decoupled DSRF controller
Rocabert, J.; Azevedo, G.; Luna, A.; Guerrero, J.M.; Candela, J.; Rodriguez, P. IEEE transactions on power electronics Vol. 26, num. 10, p. 2993-3005 DOI: 10.1109/TPEL.2011.2116126 Data de publicació: 2011 Article en revista