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Power sharing amongst sources can be improved in an islanded microgrid by utilizing the information of the network’s impedance. In this paper, an impedance based model reference adaptive system (Z-MRAS) scheme is proposed to estimate the impedance angle for converter based distributed generators (DGs) in the islanded mode. The focus is to enhance the proportional apparent power sharing between the DGs operated using generalized droop control (GDC). The performance of GDC depends on the effective tie-line impedance between the DGs and the proposed MRAS scheme provides the feedback of the estimated impedance angle. The accuracy of the impedance angle estimation is established for various tie-line $X/R$ ratios, and this results in enhanced power sharing performance for balanced, harmonic, unbalanced, dynamic and constant power load cases as well as parallel line and underground cable cases. The proposed method is validated through off-line simulations in MATLAB and controller hardware in loop (CHIL) using real time simulator (Opal-RT) including cases of meshed networks and DG plug and play for a modified CIGRE network. [ABSTRACT FROM AUTHOR]

Several drawbacks of the conventional and inverse droop control based decentralized techniques for islanded microgrids are being addressed actively, over the years by various researchers. One of the prominent issues is the inaccurate power sharing (reactive power in the conventional droop and real power in the inverse droop) among the distributed generators (DGs) due to the feeder/line impedances. Virtual impedance (VI) is a popular technique to overcome impedance mismatches but the challenge is in the setting of the appropriate value. This paper presents an adaptive decentralized technique for adjusting the virtual impedance in the controller of a DG, based on its output current, without the need of communication, extra sensors or network parameter/load estimations. The methodology is tested through simulations for a wide variety of cases including unbalanced, harmonic, constant power and induction motor loads, DG plug and play functionality and performance in larger networks (modified 13 and 33 bus systems) under meshed configurations. The range for parameter stability is verified through modelling and small signal eigenvalue analyses. Further, improved power sharing performances using the proposed scheme is validated through implementation on a laboratory experimental setup. [ABSTRACT FROM AUTHOR]

The control algorithms proposed to improve load sharing, stability, and power quality in microgrids need rigorous testing. Real-time emulation is one of the proven approaches to provide realistic testing environments for the development of products and control algorithms. Typically, for microgrid system-level testing, individual component-level (source or load) emulators are employed: hardware test bed clusters. These use multiple converters, increasing control complexity and consuming space. In this article, we propose a flexible microgrid system emulator, based on a single converter for the testing of a complete microgrid, at any node selected by the user. This is unlike the case of emulators proposed in the literature till date, which use a large number of converters to emulate the system dynamics, wherein each converter is used to mimic the behavior of one particular component, such as a single source or a single load. The modeling and control aspects are discussed, and the accuracy of the model developed and the working of the emulator are established through simulations and experiments on a laboratory prototype, respectively, considering the case of a simple two-distributed-generator-based microgrid for various scenarios. [ABSTRACT FROM AUTHOR]

Dynamic frequency regulation and effect of penetration of static and inertial sources on system stability are important issues for islanded microgrid power quality and reliability. This paper presents a novel strategy of utilizing an inverter-based source as a voltage source inverter or virtual synchronous generator (VSG). Electromechanical and power modes are critical for small signal stability of an isolated microgrid having static and inertial sources. Interaction of these modes is analyzed through eigenvalue analysis of microgrid model and differential equations describing respective modes. Inertia is important for providing fault current, determining steady state and transient stability, and better system frequency profile. A novel technique is proposed to include inertia virtually to the inverter-based sources by adding swing equation. Furthermore, inverter-based sources with traditional and modified droop controls and VSGs are compared with respect to inertia, energy, and stability. The proposed control and stability comparison are verified through experimental microgrid setup having three inverter-based sources, which can be alternately operated as VSGs. [ABSTRACT FROM AUTHOR]

Voltage-sourced converter (VSC)-based static synchronous compensators (STATCOMs) are used for voltage regulation in transmission and distribution systems. Unlike PWM-controlled STATCOMs, angle-controlled STATCOMs are switched at line frequency to limit the system losses. In recent years, angle-controlled STATCOMs have been deployed by utilities for the purpose of transmission system voltage regulation, voltage stability improvement, and increasing operational functionality. Despite the superior feature on voltage waveform quality and efficiency, the practical angle-controlled STATCOMs suffer from the over-current (and trips) and possible saturation of the interfacing transformers caused by negative sequence current during unbalanced conditions and faults in the utility. This paper specifically proposes a control structure to improve the angle-controlled STATCOMs performance under unbalanced conditions and faults. The main improvement is a substantial decrease in the negative sequence current and dc-link voltage oscillations under power system faults by the proposed control. This eliminates the need to redesign the STACOM power components to operate under fault current and dc-link voltage oscillations. The proposed control structure is designed based on adding appropriate oscillations to the conventional angle-controller output that is the control angle by which the VSC voltage vector leads/lags the line voltage vector. Since this control structure uses two angles for controlling the VSC output voltage, it is called dual angle control (DAC). PSCAD/EMTDC and experimental results verify the validity of the proposed control structure under unbalanced system conditions and faults. The experiments were conducted on a transient network analyzer, a unique hardware-based flexible ac transmission system simulator which was designed to study system faults and transients for a 2 × 100 MVA STATCOM field installation. [ABSTRACT FROM AUTHOR]

This paper studies microgrids where loads are supplied by parallel-connected inverters controlled by decentralized active power/voltage frequency and reactive power-/voltage-magnitude droop control laws. A paralleled ac system, such as a multiinverter microgrid, is susceptible to circulating currents due to differences in voltage magnitude, frequency, phase angle, or dc offset. Circulating currents due to differences in voltage magnitude and dc offset have been known issues reported in literature. However, an in-depth analysis of the problem is required to ascertain the deviation of the system-operating condition from the desired condition. This paper provides a mathematical model that predicts the effect of voltage-magnitude offsets on reactive power sharing between inverters. Simulation and experimental results verify the accuracy of the analytical results obtained from the mathematical model. We examine the effect of dc-circulating currents and propose a simple capacitor emulation control law implemented in software to eliminate dc-circulating currents. This solution is a possible alternative for hardware implementation to eliminate dc-circulating currents. The effectiveness of the capacitor emulation control law has been verified through experimental results. [ABSTRACT FROM PUBLISHER]