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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. Index Terms--Nonlinear controller, series-parallel resonant converter (SPRC), zero-voltage switching (ZVS).

In this paper, the control strategy for a flexible microgrid 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. Index Terms--Distributed generation (DG), droop method, microgrids, uninterruptible power systems.

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. Index Terms--Current harmonic content, grid-connected inverter, linear current regulator, photovoltaic (PV) power system.

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. Index Terms--Active power filter, feedback linearization, sliding mode control.

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). Index Terms--Computer power supplies, design methodology, hysteretic control, voltage regulators.

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. Index Terms--Computer power supplies, design methodology, hysteretic control, voltage regulators.

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. Index Terms--Distributed generation (DG), droop method, inverters, microgrids.

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. Index Terms--DC-AC power conversion, pulsewidth-modulated (PWM) inverters, uninterruptible power systems (UPSs).

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. Index Terms--Feedback linearization, quantum modulation, resonant converter, sliding-mode control.

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 control 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. Index Terms--Distributed generation (DG), droop control method, nonlinear loads.

This paper shows the analysis and design of a parallel-connected converter system using sliding mode control techniques. The design is particularised 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 emphasises 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. Index Terms--DC--DC power conversion, power control, variable-structure systems.

In this article, a novel cascade model-predictive control for a three-phase unity power factor rectifier is presented. Unlike the traditional control techniques for rectifiers, the proposed method uses a cascade dual-model-predictive control for the inner and outer control loops. The three-phase currents of the rectifier are controlled with the inner control loop, while the desired output voltage is achieved with the outer control loop. In this proposal, the output power is used as a feedforward to enhance the transient dynamics of the output dc voltage. Several advantages can be highlighted such as a fixed switching frequency, and output voltage variations are avoided when a sudden change in the load or a voltage sag appears. 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 article. [ABSTRACT FROM AUTHOR]

The design of sliding mode control schemes for quantum resonant converters is introduced by means of two different approaches. First, an easy-to-use procedure for devising nonlinear control structures is established, using Lyapunov's well-known stability criteria. Second, an alternative method that provides linear sliding surfaces is also developed, considering reaching, existence, and stability conditions. The application of both control design techniques is illustrated in detail by means of three selected examples. The advantages and drawbacks of the resulting control circuits are examined. Simulation and experimental results corroborate the expected features of the close-loop quantum converters. Index Terms--Linear sliding surfaces, Lyapunov-based control, quantum resonant converters.

Grid faults are one of the most severe perturbations in power systems. During these extreme disturbances, the reliability of the grid is compromised and the risk of a power outage is increased. To prevent this issue, distributed generation inverters can help the grid by supporting the grid voltages. Voltage support mainly depends on two constraints: the amount of injected current and the grid impedance. This paper proposes a voltage support control scheme that joins these two features. Hence, the control strategy injects the maximum rated current of the inverter. Thus, the inverter takes advantage of the distributed capacities and operates safely during voltage sags. Also, the controller selects the appropriate power references depending on the resistive–inductive grid impedance. Therefore, the grid can be better supported since the voltage at the point of common coupling is improved. Several voltage objectives, which cannot be achieved together, are developed and discussed in detail. These objectives are threefold: to maximize the positive sequence voltage; to minimize the negative sequence voltage; and to maximize the difference between positive and negative sequence voltages. A mathematical optimal solution is obtained for each objective function. This solution is characterized by a safe peak current injection, and by the optimization of the voltage profile in any type of grid connection. Therefore, the proposed control scheme includes advanced features for voltage support during voltage sags, which are applicable to different power facilities in different types of networks. Due to system limitations, a suboptimal solution is also considered, analyzed, and discussed for each of the optimization problems. Experimental results are presented to validate the theoretical solutions. [ABSTRACT FROM AUTHOR]