The increasing interest in integrating intermittent renewable energy sources into microgrids presents major challenges from the viewpoints of reliable operation and control. In this paper, the major issues and challenges in microgrid control are discussed, and a review of state-of-the-art control strategies and trends is presented; a general overview of the main control principles (e.g., droop control, model predictive control, multi-agent systems) is also included. The paper classifies microgrid control strategies into three levels: primary, secondary, and tertiary, where primary and secondary levels are associated with the operation of the microgrid itself, and tertiary level pertains to the coordinated operation of the microgrid and the host grid. Each control level is discussed in detail in view of the relevant existing technical literature.

Trends in Microgrid Control

There has been considerable interest in the development and applications of active filters because of the increasing concern over power quality, at both distribution and consumer levels, and the need to control reactive power and voltage stability at transmission levels. The existing approaches are classified and assessed to provide a framework of references for both researchers in this field and for generators, suppliers and consumers of electrical power who are, or may be, concerned about the problems associated with power quality and are considering installing active filters for their particular sets of problems.

Active power filters: a review

In the last years, the use of distributed uninterruptible power supply (UPS) systems has been growing into the market, becoming an alternative to large conventional UPS systems. In addition, with the increasing interest in renewable energy integration and distributed generation, distributed UPS systems can be a suitable solution for storage energy in micro grids. This paper depicts the most important control schemes for the parallel operation of UPS systems. Active load-sharing techniques and droop control approaches are described. The recent improvements and variants of these control techniques are presented.

/pdf/control-of-distributed-uninterruptible-power-supply-systems-5ed5mwl5to.pdf

Control of Distributed Uninterruptible Power Supply Systems

The use of an inverter with an output LC filter allows for generation of output sinusoidal voltages with low harmonic distortion, suitable for uninterruptible power supply systems. However, the controller design becomes more difficult. This paper presents a new and simple control scheme using predictive control for a two-level converter. The controller uses the model of the system to predict, on each sampling interval, the behavior of the output voltage for each possible switching state. Then, a cost function is used as a criterion for selecting the switching state that will be applied during the next sampling interval. In addition, an observer is used for load-current estimation, enhancing the behavior of the proposed controller without increasing the number of current sensors. Experimental results under linear and nonlinear load conditions, with a 5.5-kW prototype, are presented, verifying the feasibility and good performance of the proposed control scheme.

Model Predictive Control of an Inverter With Output $LC$ Filter for UPS Applications

The injected grid current regulator and active damping of the LCL filter are essential to the control of LCL-type grid-connected inverters. Generally speaking, the current regulator guarantees the quality of the injected grid current, and the active damping suppresses the resonance peak caused by the LCL filter and makes it easier to stabilize the whole system. Based on the proportional-integral (PI) and proportional-resonant (PR) compensator together with capacitor-current-feedback active-damping which are widely used for their effectiveness and simple implementations, this paper proposes a simple step-by-step controller design method for the LCL-type grid-connected inverter. By carefully dealing with the interaction between the current regulator and active damping, the complete satisfactory regions of the controller parameters for meeting the system specifications are obtained, and from which the controller parameters can be easily picked out. Based on these satisfactory regions, it is more convenient and explicit to optimize the system performance. Besides, the insight of tuning the controller parameters from these satisfactory regions is also discussed. Simulation and experimental results verify the proposed step-by-step design method.

Step-by-Step Controller Design for LCL-Type Grid-Connected Inverter with Capacitor–Current-Feedback Active-Damping

A buck pulsewidth modulated zero-current switching quasi-resonant converter (buck PWM ZCS QRC) operating at constant frequency is discussed. Operating principle and design-oriented analysis are presented with normalized design curves, design procedure, design example, simulations, and experimental results. The new topology, which can be considered as a particular one, is compared with the well-established buck frequency-modulated zero-current switching quasi-resonant converter (buck FM ZCS QRC) proposed by Fred C. Lee (1988). >

Buck quasi-resonant converter operating at constant frequency: analysis, design and experimentation

This paper presents an innovative tuning strategy for the design of multiple-loop lag-lead compensators for uninterruptible power supplies (UPSs) based on the H-infinity robust control theory. Taking into account an average model of a UPS, the parameters of a double-loop controller are synthesized such that the upper bound on the two-norm of the input-to-output operator is minimized while guaranteeing local stability of the closed-loop system for all admissible load variations. The stabilization conditions are cast in terms of linear matrix inequalities that are solved via standard software packages. To improve regulation dynamics, a newer technique to compute the root-mean-square value of the output voltage is also proposed. Practical results considering a commercial UPS system demonstrate the effectiveness of the proposed approach as a tool for tuning multiloop controllers

Multiple-Loop H-Infinity Control Design for Uninterruptible Power Supplies

Nowadays, hybrid filtering is an important alternative to improve line performance when supplying high-power nonlinear loads. This paper presents a simple and low cost control strategy for hybrid filter as an alternative to other more complex algorithms, in some specific applications. The proposed control strategy is recommended for a utility interface with three-phase diode bridge rectifier front-end due to its high displacement power factor. It has been tested by simulations (both steady-state and transient analysis) and by experimental verification, where good results are obtained. It can be seen that using this control strategy, acceptable harmonic levels according to IEEE-519 standard are obtained. Also a different connection of the passive filter is experimentally verified, allowing the reduction of passive filter capacitors.

http://ieeexplore.ieee.org/iel3/4860/13421/00616884.pdf

Simplified control strategy for hybrid active filters

This article presents the application of a data-driven control design to the output voltage regulation of dc–dc converters. Using only data collected on the plant, a proportional-integral-derivative (PID) controller is designed in order to achieve a desired closed-loop response, without deriving a mathematical model for the plant. Since an inverse response is typical on voltage signals of different dc–dc converters, the virtual reference feedback tuning (VRFT) method with flexible criterion is applied to estimate the controller gains, so the limitation due to the nonminimum phase zero, if it exists, is taken into account by the algorithm. We present appropriate choices for data collection, controller structure, and reference model definition in order to safely apply the VRFT method to design a controller for dc–dc converters. A practical application on a boost converter shows the effectiveness of the proposed method.

Virtual Reference Feedback Tuning Applied to DC–DC Converters

Hybrid filter topologies are becoming more important to improve harmonic performance in industrial systems. Hybrid filters are composed of active and passive filter associations taking advantage of both filter schemes. As a consequence of their complexity the analysis and design requires an optimization process to set up the most important parameters. In this paper the most important filter configurations are analyzed in the frequency domain. The analysis helps in the selection process of the main parameters, especially the value of the equivalent active filter impedance, which affects strongly the filter behavior. This study provides a good support in the analysis and design of hybrid filters.

F.B. Libano

Papers

Buck quasi-resonant converter operating at constant frequency: analysis, design and experimentation

Multiple-Loop H-Infinity Control Design for Uninterruptible Power Supplies

Simplified control strategy for hybrid active filters

Virtual Reference Feedback Tuning Applied to DC–DC Converters

Frequency characteristics of hybrid filter systems