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This study deals with the problem of voltage security in microgrids. In general, voltage security is an issue for power systems, where the lack of reactive power is a concern. For microgrids a complexity is imposed, since frequency and voltage level deviations may take place by the operation of droop-controlled inverters. This study incorporates the load margin calculation into a microgrid structure. For this purpose, a special power flow program is developed in order to consider the effects of the droop-controlled inverters. Simulation results are obtained with the help of an islanded distribution system with only generators coupled by droop-controlled inverters, so the proposed methodology may be tested and discussed. [ABSTRACT FROM AUTHOR]

A sliding-mode input–output linearization controller for the dc/dc zero-voltage switching (ZVS) CLL-T resonant converter is presented. The proposed controller significantly improves the transient response and disturbance rejection of the converter while preserving the closed-loop stability. An averaged large-signal dynamic model of the converter operating with a novel discrete ZVS modulation technique is developed and used as starting point for the controller design. Since the model derivation process does not include small signal approximations, it can be used for large-signal analysis, providing accurate predictions of the converter dynamic behavior. The combination of the proposed controller and modulation technique provides ZVS operation over a wide load range, narrowed regulating frequency range, robustness, and fast transient response against transient load changes. Experimental and simulation results are reported to validate the theoretical predictions and to confirm the superior performance of the nonlinear controller when it is compared with a conventional linear controller. [ABSTRACT FROM PUBLISHER]

This paper explores the impact of the output impedance on the active and reactive power flows between parallelized inverters operating with the droop method. In these systems, a virtual output impedance is usually added to the control loop of each inverter to improve the reactive power sharing, regardless of line-impedance unbalances and the sharing of nonlinear loads. The virtual impedance is usually implemented as the time derivative of the inverter output current, which makes the system highly sensitive to the output current noise and to nonlinear loads with high slew rate. To solve this, a second-order general-integrator (SOGI) scheme is proposed to implement the virtual impedance, which is less sensitive to the output current noise, avoids to perform the time derivative function, achieves better output-voltage total harmonic distortion, and enhances the sharing of nonlinear loads. Experimental results with two 2-kVA inverter systems under linear and nonlinear loads are provided to validate this approach. [ABSTRACT FROM PUBLISHER]

The power quality of a three-phase photovoltaic (PV) inverter drastically deteriorates in the presence of grid faults with unbalanced voltages. A ripple in the injected power and an increase in the current harmonic distortion are the main noticeable adverse effects produced by this abnormal grid situation. Several grid-fault control schemes are nowadays available for operating under unbalanced grid voltage. These control schemes usually have extreme power quality characteristics. Some of them have been conceived to completely avoid power ripple during unbalanced voltage sags, but at an expense of high current harmonic distortion. With other schemes, the harmonic distortion is totally eliminated but at an expense of high ripple in the injected power. This paper further explores the performance of PV inverters under unbalanced voltage sags. It has three theoretical contributions: 1) a generalized control scheme, which includes the aforementioned grid-fault controllers as particular cases; 2) a control strategy based on the use of continuous values for the control parameters. This original approach gives adjustable power quality characteristics that cannot be achieved with the previous control schemes; 3) three different control algorithms for calculating the continuous values of the control parameters. These contributions are experimentally validated with a digital signal processor-based laboratory prototype. [ABSTRACT FROM PUBLISHER]

The injection of sinusoidal current to an electrical grid is nowadays possible with the use of active power filters (APFs). With quasi-steady-state (QSS) nonlinear controllers, these power filters provide low grid-current-harmonic content with simple-control analog implementation. This paper focuses on the design of a QSS nonlinear controller for a single-phase APE A complete analysis based on the sliding-mode-control theory is given. From the analysis, a set of control-design guidelines is presented to select the gains and parameters of the control scheme. Selected experimental results are reported to validate the theoretical design procedure. [ABSTRACT FROM AUTHOR]

The injection of low-harmonic current to the electrical grid is nowadays regulated by international standards. In order to accomplish these standards, selective harmonic compensation is carried out in grid-connected photovoltaic inverters by means of resonant harmonic compensators. This paper gives a systematic design procedure for selecting the gains and parameters of these harmonic compensators. Other factors considered in the design process include frequency deviation, grid synchronization, and transient response. Both a design example meeting the requirements of grid interconnection and selected experimental results from a digital-signal-processor-based laboratory prototype are also reported. [ABSTRACT FROM AUTHOR]

This paper presents a sliding-mode control for a single-phase ac/ac quantum resonant converter. The closed-loop system improves regulation, sensitivity to disturbances, and transient response. The design of the closed-loop system is based on the sliding-mode control theory, and it is carried out with the help of an averaged large-signal model of the resonant converter. Simulation and experimental results are reported to validate the theoretical predictions. Essentially, the contribution of this paper is as follows: 1) the averaged large-signal model for the resonant converter; 2) the sliding-mode control scheme; and 3) the design guidelines for the control gains and parameters. [ABSTRACT FROM AUTHOR]

This paper presents a comparative study of sliding-mode control schemes for quantum series resonant inverters. The single-input and two-input resonant systems considered in this study are designed with identical specifications. The control performance is evaluated in aspects such as load transient response, sensitivity to disturbances, and efficiency. Other aspects such as control complexity and cost are also considered. Simulation and experimental results are reported in order to validate the theoretical predictions. Essentially, the study demonstrates the following: 1) The load transient response of the proposed two-input system is insensitive to load variations; 2) the single-input system exhibits higher efficiency, particularly at light load conditions, and 3) both control schemes have similar complexity and cost. [ABSTRACT FROM AUTHOR]

This paper presents a linear current control scheme for single-phase active power filters. The approach is based on an outer voltage loop, an inner current loop, and a resonant selective harmonic compensator. The design of the control parameters is carried out using conventional linear techniques (analysis of loop gain and other disturbance-rejection transfer functions). The performance of the proposed controller is evaluated and compared with two reference controllers: a basic control and an advanced repetitive control. In comparison with these controllers, the proposed control scheme provides additional attenuation to the harmonics coming from the load current, the grid voltage, and the reference signal, resulting in a grid current with lower harmonic distortion. Experimental results are reported in order to validate this paper. [ABSTRACT FROM AUTHOR]

A dynamic study of the dc/dc series-parallel resonant converter operating with a discrete self-sustained phase-shift modulation technique is presented. The study includes the synthesis of a suitable averaged large-signal dynamic model and the design of a nonlinear feedback controller based in the input-output linearization approach. The proposed controller and modulation type provide some outstanding features: zero-voltage switching operation for the whole load range, narrowed frequency variation range, fast transient response, and robustness in relation to external parameter variations. Experimental and simulation results are reported to validate the theoretical predictions and confirm the superior performance of the nonlinear controller when it is compared with a conventional linear controller. [ABSTRACT FROM AUTHOR]

In this paper, the control strategy for a flexible micro- grid is presented. The microgrid presented here consists of several line-interactive uninterruptible power supply (UPS) systems connected in parallel. The control technique is based on the droop method to avoid critical communications among UPS units. Thus, a flexible microgrid is obtained to operate in either grid-connected or islanded mode. A small-signal analysis is presented in order to analyze the system stability, which gives rules to design the main control parameters. Simulation and experimental results are presented, showing the feasibility of the proposed controller. [ABSTRACT FROM AUTHOR]

This paper deals with an important aspect in the operation of single-phase grid-connected photovoltaic inverters, i.e., injecting low harmonic current into the electrical grid. Specifically, we propose a linear current control scheme with a resonant harmonic compensator connected in series with a tracking regulator (the standard harmonic compensator location is in parallel with the tracking regulator). The series connection provides an efficient attenuation of the grid voltage background distortion, an accurate synchronization with the grid voltage, and a low computational time in relation to the standard control scheme. Experimental results from a digital signal processor-based laboratory prototype validate the features of the proposed control scheme. [ABSTRACT FROM AUTHOR]

As the grid code specifies, wind turbines have to remain connected to the grid at voltage levels far below the nominal values. The improvement of wind turbine performance under such conditions has become a problem of general concern. However, this performance usually relies on conventional linear controllers that operate at network faults far off the nominal point for which they were designed. As a consequence, wind turbines should operate with increasing converter currents, which may result in converter damage. This paper proposes a nonlinear controller for converter-based wind turbines that ensures that the currents are maintained within the design limits. The controller is based on feedback linearization theory and is applied to the system through a sliding mode approach. This controller is robust against system perturbations and uncertainties, and overcomes the usually complex implementation that is associated with feedback linearization controllers. [ABSTRACT FROM AUTHOR]

The aim of this work is the application of the feedback linearization theory to a single-phase shunt active power filter, since this technique has been successfully applied to other areas of power electronic. The active filter is linearized by means of a nonlinear transformation of the system model, deduced from the application of Tellegen's theorem to the system. After that, a sliding mode controller is proposed to impose a desired dynamic behavior on the system, giving robustness and insensitivity to parameter variations. Moreover, the proposed controller ensures proper tracking of the reference signals and simplifies the overall control design. The controller was implemented into a low cost DSP. Experimental and simulation results are provided. [ABSTRACT FROM AUTHOR]

This paper presents a simple low-cost control architecture for low-voltage hysteretic regulators supplying loads with low-to-medium current consumption. Only two sensed voltages, a passive filter network and a hysteretic comparator are required to implement the main control functions: the output voltage regulation and the adaptive voltage positioning. This paper also shows that other previously reported low-cost control solutions have an important design tradeoff between load transient response and efficiency. In the proposed controller, the closed-loop output impedance can be designed to be resistive and the switching frequency can be adjusted to be independent of the output impedance requirement so that the load transient response and the efficiency can be optimized separately. Experimental results validate the performance of the proposed controller (i.e., a load transient response with insignificant output voltage overshoot and selectable switching frequency independent of the output impedance requirement). [ABSTRACT FROM AUTHOR]

This paper presents a design methodology for voltage hysteretic regulators powering digital integrated circuits with low voltage, high current, and high slew rate current transients. The design approach optimizes the transient response during large consumption changes by imposing constant closed-loop output impedance. This paper also suggests a novel compensator network for the adaptive voltage positioning feedback loop, which leads to a robust transient response performance against load disturbances. The application of the design methodology to the proposed hysteretic controller provides the suitable control parameter values for optimal transient response. Simulation and experimental results validate the theoretical predictions for the proposed controller, particularly the constant output impedance operation and the robust transient response performance. [ABSTRACT FROM AUTHOR]

In this paper, a novel wireless load-sharing controller for islanding parallel inverters in an ac-distributed system is proposed. This paper explores the resistive output impedance of the parallel-connected inverters in an island microgrid. The control loops are devised and analyzed, taking into account the special nature of a low-voltage microgrid, in which the line impedance is mainly resistive and the distance between the inverters makes the control intercommunication between them difficult. In contrast with the conventional droop-control method, the proposed controller uses resistive output impedance, and as a result, a different control law is obtained. The controller is implemented by using a digital signal processor board, which only uses local measurements of the unit, thus increasing the modularity, reliability, and flexibility of the distributed system. Experimental results are provided from two 6-kVA inverters connected in parallel, showing the features of the proposed wireless control. [ABSTRACT FROM AUTHOR]

In this paper, a method for the parallel operation of inverters in an ac-distributed system is proposed. This paper explores the control of active and reactive power flow through the analysis of the output impedance of the inverters and its impact on the power sharing. As a result, adaptive virtual output impedance is proposed in order to achieve a proper reactive power sharing, regardless of the line-impedance unbalances. A soft-start operation is also included, avoiding the initial current peak, which results in a seamless hot-swap operation. Active power sharing is achieved by adjusting the frequency in load transient situations only, owing to which the proposed method obtains a constant steady-state frequency and amplitude. As opposed to the conventional droop method, the transient response can be modified by acting on the main control parameters. Linear and nonlinear loads can be properly shared due to the addition of a current harmonic loop in the control strategy. Experimental results are presented from a two-6-kVA parallel-connected inverter system, showing the feasibility of the proposed approach. [ABSTRACT FROM AUTHOR]

This paper deals with the design of the output impedance of uninterruptible power system (UPS) inverters with parallel-connection capability. In order to avoid the need for any communication among modules, the power-sharing control loops are based on the P/Q droop method. Since in these systems the power-sharing accuracy is highly sensitive to the inverters output impedance, novel control loops to achieve both stable output impedance and proper power balance are proposed. In this sense, a novel wireless controller is designed by using three nested loops: 1) the inner loop is performed by using feedback linearization control techniques, providing a good quality output voltage waveform; 2) the intermediate loop enforces the output impedance of the inverter, achieving good harmonic power sharing while maintaining low output voltage total harmonic distortion; and 3) the outer loop calculates the output active and reactive powers and adjusts the output impedance value and the output voltage frequency during the load transients, obtaining excellent power sharing without deviations in either the frequency or the amplitude of the output voltage. Simulation and experimental results are reported from a parallel-connected UPS system sharing linear and nonlinear loads. [ABSTRACT FROM AUTHOR]

The purpose of this paper is to explore the problem of designing proper sliding-mode controllers to regulate the output voltage of the dc-to-dc quantum series-parallel resonant converter. A control-oriented dynamic model, which appropriately describes the large-signal behavior of the power circuit by average state variables, is first developed. Using input-output feedback linearization, a control design methodology is then presented, which leads to a family of sliding surfaces that make the output voltage behave following a particular large-signal linear dynamics. Among these surfaces, the final configuration is selected taking into account control circuit simplicity as the basic premise. Besides exhibiting the absence of output-voltage errors in steady state, the control solution leads to robust operation with respect to parameter variations and external disturbances. Simulations and experimental results are reported to validate the expected features of the proposed control solution. [ABSTRACT FROM AUTHOR]

This paper presents a novel control strategy for parallel inverters of distributed generation units in an ac distribution system. The proposed control technique, based on the droop con trot method, uses only locally measurable feedback signals. This method is Usually applied to achieve good active and reactive power sharing when communication between the inverters is difficult due to its physical location. However, the conventional voltage and frequency droop methods of achieving load sharing have a slow and oscillating transient response. Moreover, there is no possibility to modify the transient response without the loss of power sharing precision or output-voltage and frequency accuracy. In this work, a great improvement in transient response is achieved by introducing power derivative-integral terms into a conventional droop scheme. Hence, better controllability of the system is obtained and, consequently, proper transient performance can be achieved. In addition, an instantaneous current control loop is also included in the novel controller to ensure proper sharing of harmonic components when supplying nonlinear loads. Simulation and experimental results are presented to prove the validity of this approach, which shows excellent performance as opposed to the conventional one. [ABSTRACT FROM AUTHOR]

This paper shows the analysis and design of a parallel-connected converter system using sliding mode control techniques. The design is particularized for a system that consists of N boost converters and a current feedback loop based on a proportional-integral compensator of the output voltage error. The paper emphasizes the advantages of the sliding-mode control over the classic design method based on small-signal models, thus providing an effective and robust means of controlling nonlinear multi-input converters. The design is based on the Utkin conditions, which permit us to know the regions under which a sliding mode exists. This fact allows us to design the compensator and to introduce some modifications in the control loop that avoids input-current overshoots during the system startup. Simple design expressions are obtained and verified with simulation and experimental results, thus showing the improvements achieved with the proposed modifications. [ABSTRACT FROM AUTHOR]

Hybrid microgrids constitute a promising solution for filling the electricity access gap that currently exists in rural areas; however, there is still relatively little information about their reliability and costs based on measured data in real working conditions. This article analyzes data obtained from the operation of a 9 kW hybrid microgrid in the fishermen's cove of Laguna Grande, Paracas, in the Ica region of Perú, which has been running for 5 years. This microgrid has been equipped with data acquisition systems that measure and register wind speed, solar radiation, temperatures, and all the relevant electric parameters. Battery dynamics considerations are used to determine the depth of discharge in a real-time operative situation. The collected data are used to optimize the design using the specialized software HOMER, incorporating state-of-the-art technology and costs as a possible system upgrade. This work aims to contribute to better understanding the behavior of hybrid rural microgrids using data collected under field conditions, analyzing their reliability, costs, and corresponding sensitivity to battery size as well as solar and wind installed power, as a complement to a majority of studies based on simulations. [ABSTRACT FROM AUTHOR]

Purpose: To compare clinical outcomes and costs of cataract surgery between patients operated with standard extracapsular extraction (ECCE) and those undergoing phacoemulsification. Setting: Patients from the Ophthalmology Department of a teaching hospital in Barcelona (Spain) scheduled for cataract surgery, not combined with any other ophthalmic procedure. Methods: A retrospective analysis has been performed on a database of 1046 patients undergoing ECCE and phacoemulsification. The outcome measures used were: surgical complications, visual acuity and costs of surgery and of follow-up. Overall rate of all complications and postoperative visual acuity were compared between the two groups, adjusting for age, preoperative visual acuity, medical and ocular comorbidity. Results: 31.9% of the patients (334) underwent phacoemulsification, and 68.1% (712) underwent ECCE. Patients undergoing phacoemulsification presented a frequency of intra- and postoperative complications lower than those undergoing ECCE (odds ratio 0.57, 95%Cl 0.37–0.87 and 0.66, 95%Cl 0.46–0.96, respectively), specifically for intraoperative iris trauma (3.1% vs 0.3%, p = 0.004), residual posterior capsular opacity (2% vs 0.3%, p = 0.035) and postoperative corneal edema (7.4% vs 3.6%, p = 0.016). Costs of intervention and follow-up were lower for phacoemulsification compared with ECCE (23.9% and 14%, respectively). But global costs were slightly higher for phacoemulsification (4.87%), due to supply costs, which were more than twice those ofECCE. Conclusions: Phacoemulsification, when performed by an experienced surgeon, has better clinical outcomes than planned extracapsular extraction, and costs may be lower since supply costs are expected to decrease as the phacoemulsification technique becomes more widespread. [ABSTRACT FROM AUTHOR]

Aims/Background: Many preoperative ophthalmic and medical tests lack proven effectiveness in the evaluation of cataract surgery patients. Variation in use has been described in the USA, but little information is available for countries with different health systems. The objectives of the study were to assess variation in ophthalmologists' use of preoperative testing on cataract surgery patients and to identify ophthalmologists' characteristics associated with it. Methods: A postal survey was made in a representative sample (response rate of 70%) of the Barcelona Medical Association of Ophthalmologists performing cataract surgery during 1993 (N = 89). The questionnaire included information about common preoperative, intraoperative and postoperative practice and sociodemographic and professional data. Appropriateness of preoperative ophthalmic tests was classified in 3 levels (indicated tests, tests with unproven benefit, and non-indicated tests) according to recommendations of the Cataract Management Guideline Panel [1] and the American Academy of Ophthalmology Preferred Practice Pattern [2]. Results: Almost all the ophthalmologists reported performing Fundus exam plus A-scan in the preoperative evaluation of cataract surgery patients (indicated tests). Up to 20% reported performing ‘frequently’ or ‘always’ some non-indicated or unproven benefit ophthalmic tests. Additionally, an average of 7 medical tests were ‘routinely’ included. Private practice and cataract subspeciality were the ophthalmologists' characteristics more frequently associated with the use of inappropriate ophthalmic tests (p < 0.05). Conclusions: In Barcelona, Spain, some ophthalmic tests of unproven effectiveness are frequently performed preoperatively on cataract surgery patients without ocular comorbidity. Also, a generalized utilization of several medical tests was reported. There is potential for some cost containment and more appropriate care of cataract patients without diminishing the quality of care, in a location with low surgery rates. [ABSTRACT FROM AUTHOR]

A microgrid is a group of interconnected loads and distributed energy resources that can fill the gap between the dependence on a bulk power grid and the transition to renewable energies. The islanded mode presents itself as the most interesting scenario, when local controllers should maintain the power quality standards based on several parameters. A tool specifically focused on the process of parameter tuning of the secondary consensus-based control for inverter-based islanded microgrids was proposed in this paper. One often-quoted drawback in this process is the great number of parameters that must be tuned, even for a very simple microgrid structure. To manage such a large number of parameters, the design of experiments was used in this study. The main motivation for this work was to present an optimized way to define the correct parameters for the secondary consensus control for inverter-based islanded microgrids. The study shows how experimental design methodology can be an efficient tool to tune microgrid parameters, which are typically multi-objective-based experiments. From the results, it is correct to state that the design of experiments is able to reach the optimal setting with a minimal number of experiments, which would be almost impossible to obtain with the trial-and-error method. [ABSTRACT FROM AUTHOR]

Due to their flexibility, Electric Vehicles (EVs) constitute an important asset for the integration of renewable energy sources in the Smart Grid. In particular, they should have a dual role: as a controllable load and as a mobile generator with a low inertia. To perform these tasks, chargers must provide the electronics with a power flow from the grid to the vehicle and vice versa. This bidirectionality can also be implemented in wireless chargers. The power converters, the compensation networks and the coil misalignment must be considered when designing the control of these systems. This paper presents a review about the proposed algorithms to control the active and the reactive power flow in a bidirectional wireless charger. [ABSTRACT FROM AUTHOR]

The power quality of the Electrical Power System (EPS) is greatly affected by electrical harmonics. Hence, accurate and proper estimation of electrical harmonics is essential to design appropriate filters for mitigation of harmonics and their associated effects on the power quality of EPS. This paper presents a novel statistical (Least Square) and meta-heuristic (Grey wolf optimizer) based hybrid technique for accurate detection and estimation of electrical harmonics with minimum computational time. The non-linear part (phase and frequency) of harmonics is estimated using GWO, while the linear part (amplitude) is estimated using the LS method. Furthermore, harmonics having transients are also estimated using proposed harmonic estimators. The effectiveness of the proposed harmonic estimator is evaluated using various case studies. Comparing the proposed approach with other harmonic estimation techniques demonstrates that it has a minimum mean square error with less complexity and better computational efficiency. [ABSTRACT FROM AUTHOR]

Increasing the unplanned penetration of Distributed Generators (DGs) has spurred active and reactive power losses in the distribution system. This article suggests using a novel Strawberry Plant Propagation Algorithm (SPPA) for planning the placement of the DGs with the aim of reducing the network (active) power losses and improving the overall voltage profile. The proposed method (SPPA) has been tested on 33 and 69 node radial systems in MATLAB. A cost analysis was also performed and compared with other contemporary methods. The results for the considered variables show the significance of the proposed method in comparison to various other counterparts, including the Mine Blast Algorithm and Particle Swarm Optimization. [ABSTRACT FROM AUTHOR]

Power quality disturbance (PQD) is an important issue in electrical distribution systems that needs to be detected promptly and identified to prevent the degradation of system reliability. This work proposes a PQD classification using a novel algorithm, comprised of the artificial bee colony (ABC) and the particle swarm optimization (PSO) algorithms, called "adaptive ABC-PSO" as the feature selection algorithm. The proposed adaptive technique is applied to a combination of ABC and PSO algorithms, and then used as the feature selection algorithm. A discrete wavelet transform is used as the feature extraction method, and a probabilistic neural network is used as the classifier. We found that the highest classification accuracy (99.31%) could be achieved through nine optimally selected features out of all 72 extracted features. Moreover, the proposed PQD classification system demonstrated high performance in a noisy environment, as well as the real distribution system. When comparing the presented PQD classification system's performance to previous studies, PQD classification accuracy using adaptive ABC-PSO as the optimal feature selection algorithm is considered to be at a high-range scale; therefore, the adaptive ABC-PSO algorithm can be used to classify the PQD in a practical electrical distribution system. [ABSTRACT FROM AUTHOR]