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Inverters (dc/ac converters) are commonly used to connect distributed energy resources (DERs) to the grid. Hence, the penetration rate of inverters is growing in distribution systems along with DERs. Depending on their development and penetration level, grid requirement standards for DERs are revised. New grid codes allow DERs to control their output reactive power to provide reactive power ancillary service. In this paper, the main contribution is reactive power management of inverter-based DERs (IBDER) through a practical procedure, which controls the available volt-ampere (headroom) of inverters to improve voltage profile and distribution system operation when confronting with short-term voltage magnitude variations. In this regard, a two-level control strategy (namely, central and local control levels) is proposed. In the central controller, reactive power management of IBDERs is optimized in short time intervals for mitigating voltage variations. The optimized data is sent to the local controllers of IBDERs to increase the speed of reaction to voltage variations in each time interval. The improvement of distribution systems operation with a high penetration rate of IBDERs is illustrated by performing the simulation of the proposed control strategy on a smart distribution test system. [ABSTRACT FROM AUTHOR]

Modern distribution grids and smart buildings comprise unbalanced single-phase, three-phase, DC, and constant power loads and battery energy storage systems (BESSs). This paper presents a hybrid single/three-phase multiobjective automated/autonomous intelligent load (MAIL1) control for smart buildings. The main focus is to autonomously and automatically respond to grid disturbances and power imbalances and unbalances by adaptive demand adjustment of various loads. Also, it can realize regulation objectives of a supervisory operator to enable fast building demand control to participate in performance-based frequency regulation markets. Moreover, it is a unified load control strategy for both grid-connected and islanded hybrid single/three-phase AC/DC grids with smooth transition between them without a need for the strategy reconfiguration. The MAIL1 renders an adaptive and continuous demand for single-phase AC and DC loads. The MAIL1 is based on a fundamentally new controller for single-phase voltage source converters (VSCs) by introducing single-phase DC-source synchronous current converter technology, with extra functionalities such as auto-synchronization with single-phase grids and virtual inertia. Extensive real-time hardware-in-loop results are presented to demonstrate the MAIL1 performance. [ABSTRACT FROM AUTHOR]

This paper presents a hybrid rectifier consisting of a 12-pulse rectifier in parallel with a Vienna rectifier. In the proposed structure, the 12-pulse rectifier supplies the bulk power to the load, whereas the Vienna rectifier shapes the input current to reduce its harmonic distortion. The unidirectional power flow of the Vienna rectifier results in undesirable distortion around the current zero crossing, which deteriorates current shaping in this region. To overcome this problem, besides current shaping, the Vienna rectifier is forced to participate in the active power. In this regard, the share of output power in each rectifier module must be calculated. To shape the input current, the reference current for the Vienna rectifier is generated using instantaneous power theory, and current tracking is carried out using the finite control set model predictive control. A detailed analysis of how to select the output power sharing ratio based on system losses and input current distortion is presented. The theoretical analysis is verified by using simulation and experimental results obtained from a 1 kW laboratory setup. [ABSTRACT FROM AUTHOR]

This paper presents a new multicell topology based on flying capacitor (FC) structure suitable for multilevel converters. The proposed topology employs the concept of phase-shifted carrier pulse width modulation (PWM) with modified modulating signals to generate appropriate PWM signals. The modulation strategy along with all possible switching states are explained. The implementation of the proposed PWM technique is shown by the help of illustrating waveforms. To show the advantages of the proposed structure, different performance criteria are compared with other FC multicell (FCM) structures. Simulation results are used to further evaluate the potential merits of the proposed topology. It is shown that the proposed structure is superior to other FCM-based structures from different standpoints. Specifically, it reduces the number of high-frequency switches, the number of FCs, and at the same time improves the quality of output voltage. A laboratory-type experimental setup is used to validate the modulation strategy and other theoretical findings. [ABSTRACT FROM AUTHOR]

This study is concerned with presenting control strategy for interlink converters in hybrid microgrids. When both ac microgrids (AC-MGs) and dc microgrids are in the vicinity of each other, they can be interconnected via ac-dc converters, known as ICs, making it possible to exchange energy between two grids. Common methods employed in IC control strategy use frequency variation and CCM in their structure. This method requires the frequency variation at IC ac-side to be large, leading to poor power quality and stability of AC-MG. Furthermore, CCM does not directly regulate the IC ac-side voltage, which again deteriorates the IC ac-side voltage quality. The proposed strategy is based on droop method, similar to what is used in the autonomous control of sources in an AC-MG. This technique uses voltage-controlled method and does not need frequency measurement in its structure. This considerably improves the voltage quality of AC-MG. The derivation of appropriate droop characteristic is explained. Time-domain simulation is used to verify the performance of the proposed method. The small-signal stability of the system is studied. It is shown how by introducing a proper virtual impedance, the small-signal stability of the system can be improved. [ABSTRACT FROM AUTHOR]

Abstract—This article proposes a method that enables ideal sharing of reactive power among converter-based micro-sources in a microgrid with a decentralized control strategy. Power sharing must be properly performed among micro-sources to avoid circulating currents or overloading. Using conventional droop characteristics to achieve power sharing is not satisfactory, as system asymmetry will greatly impact the quality of sharing. In the proposed method, droop characteristic parameters are modified upon a change in system operating point such that ideal sharing takes place after the modification process. Simulation results are presented to show the validity of analysis. [ABSTRACT FROM PUBLISHER]

SUMMARY This paper is concerned with determining the compensation capability of an interline dynamic voltage restorer (IDVR) under various balanced and unbalanced voltage sag scenarios. An IDVR consists of two or more DVRs with a common dc link which enables active power exchange among DVRs. This feature increases the compensation range of an IDVR as compared with separate but otherwise similar DVRs with no energy storage. The steady-state modeling and analysis of an IDVR is briefly explained in this paper. The expressions governing power requirement for compensating various voltage sags are derived. Consequently, the constraints associated with an IDVR operation are obtained and an algorithm is developed to calculate the compensation range of an IDVR and two similar storage-less DVRs. Illustrative graphs demonstrate the operating characteristics of an IDVR and compare these characteristics with those of two similar storage-less DVRs. Copyright © 2013 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The first part of this article reviews the concepts of apparent power and power factor in single-phase circuits with non-negligible line impedances. The study is based on the well-known definition that the apparent power is the maximum load power subject to constant line losses and constant load voltage. In reality, however, all transmission/distribution lines have some finite impedance, and thus, the constant voltage condition cannot be maintained at the load terminals. In this article, the constant load voltage condition is relaxed and allowed to change, as long as it remains within acceptable range of application. This is a scenario that better reflects the realistic situations. Modified expressions for the apparent power and power factor are derived and discussed. The second part of the article presents a new concept called the current factor as the ratio of the minimal current to the actual current while keeping the load power constant. The current factor coincides with the power factor when the line impedances are neglected. The article explains how the current factor can be superior to the power factor when the line impedances are not negligible. The current factor is then extended to non-sinusoidal current factor for a non-linear load and is examined. [ABSTRACT FROM AUTHOR]