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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]

Renewables interfaced through inverters in microgrids, may be controlled using centralized, decentralized or distributed approaches. The decentralized droop control involves mimicking the performance of a synchronous generator of the power system, so as to share load power proportionately amongst parallel connected sources. When the conventional droop control is used, only the fundamental real powers can be shared in proportion to the source ratings, which does not ensure satisfactory performances in the presence of unbalanced/ asymmetrical loads. Several schemes have been worked upon to achieve proportional load power-sharing with power quality aspects: in the presence of harmonic and unbalanced loads. This paper proposes five modifications for the popular virtual conductance vs. negative sequence power (G vs. Q-) approach, aimed at improving the sharing of the unbalanced currents amongst the sources to improve the power quality and microgrid system reliability. The eigenvalue analysis for determining the parameter range for assuring stability and comparisons with fixed virtual conductance values are discussed. The proposed modifications are discussed and demonstrated through simulations on a two inverter-based distributed generator (DG) system, a modified 13 bus system, and using experimental results on a lab-scale microgrid. • The work presented focuses on the power quality of unbalance in islanded AC microgrid systems and proposes five modifications for the popular G vs. Q- approach, aimed at improving the sharing of the unbalanced currents amongst the parallel connected sources. • The proposed modifications are based on local measurements and do not need communication, network and load estimations • The eigenvalue based small signal analysis for determining the parameter range for assuring stability and comparisons with fixed virtual conductance values are presented. • The proposed modifications are discussed and demonstrated through simulations on a two-inverterbased distributed generator (DG) system, a modified 13 bus system, and using experimental results on a lab-scale microgrid. • An overall improvement in the range of 26.0% to 61.0% was observed across the different cases (enhanced unbalanced power-sharing: before and after activating the proposed control). [ABSTRACT FROM AUTHOR]

Locating the stator interturn (SIT) fault on the motor winding structure adds an important feature in the fault diagnosis. This motivates to study the effects of SIT fault location on the induction machine. In this paper, a simple yet accurate stationary reference frame q-d-0 model of SIT-faulted induction machine, including the fault location parameter, is developed. The fundamental components of winding functions (WFs) are used to calculate the machine inductances for the proposed model. These inductances in stationary reference frame q-d-0 variables are rotor-position-independent expressions and functions of fault severity and fault location. The proposed model is as accurate as the multiple-coupled-circuit model for fault location study. It does not require the recalculation of machine inductances in each integration step to solve the model. The experimental validation of the model is presented. The steady-state analysis based on the proposed model indicates that the inclination of the current vector locus and the phase angle of the negative-sequence current phasor are the strong indicators of the SIT fault location. [ABSTRACT FROM PUBLISHER]

Multi-machine single-inverter induction motor drives are attractive in situations in which all machines are of similar ratings and operate at approximately the same load torques. The advantages include their small size compared to multi-machine multi-inverter systems, lower weight, and overall cost. This article presents a consolidated study and comparison of vector control methods applied to multi-machine drives along with relevant simulation and experimental results. The mean control method and its modifications prove to be superior to the other control approaches. [ABSTRACT FROM AUTHOR]

This paper presents a method of emulating electrical loads using power electronic converters. The loads include machines such as induction motors and their associated mechanical load and also more complex machine systems such as wind-driven generators. The load emulator is, effectively, a dynamically controllable source or sink which is capable of bidirectional power exchange with either a grid or another power electronic converter system. Using load emulation, the feasibility of connecting a particular machine to a grid under various load conditions can be examined without the need for any electromechanical machinery. This paper considers the case of a power electronic voltage source inverter (VSI) emulating a three-phase induction motor connected to a three-phase ac grid. The VSI is operated in a mode where the current drawn from the ac grid is controlled by closed-loop control. The consistency of the experimental results with the simulation results proves the ability of the emulator and the proposed testing approach. [ABSTRACT FROM AUTHOR]

There has been a growing demand for using active power filters (APFs) for grid-connected converter systems based on nonconventional energy sources such as solar, wind, and fuel cells. In addition to power quality conditioning, the APF can also be used for bidirectional active power exchange with the three-phase four-wire grid and, therefore, acts as a multifunctional compensator. This paper presents a four-leg shunt multifunctional grid-connected power quality compensator. The compensator reference currents are generated using the instantaneous p-q-r theory. This paper proposes a simple modification to the conventional theory to eliminate the source neutral current from the three-phase four-wire power system. A digital controller is used to implement dead-beat current control. The outputs of the digital current controller are used as reference voltages for a 3-D space vector modulator (3D-SVM). A new approach for pulse width modulation waveform generation in 3D-SVM is proposed. All the intermediate steps are discussed from the viewpoint of implementation on a digital signal processor field-programmable gate array platform. The multifunctional compensator is analyzed, simulated, and tested experimentally for the most practical conditions in the power system, and the results are presented. [ABSTRACT FROM AUTHOR]

Renewable energy sources are typically interfaced to the main grid at the distribution level, through power electronic converter interfaces. Testing these microgrid systems is a crucial aspect which is gaining increased interest and importance among researchers. This article proposes a system emulator, for testing microgrids, with a special focus on replicating unbalanced network conditions. The proposed system uses a single back-to-back bidirectional converter for real-time emulation, and the user has the option to chose any node of their interest to study the dynamics. This is different from the ones proposed to-date - most of them focus on balanced systems, and use several converters to replicate the dynamics of the microgrid system, in which each converter mimics one entity at a time (source or load). A system with two generators is emulated for different control strategies and types of unbalances in the network. The crucial aspects related to the modelling and control are presented in detail supported by simulations and experimental validation on a prototype developed. [ABSTRACT FROM AUTHOR]

Strategies to implement emulation of large power electronic converters with different topologies using a generic multiport decomposition technique with different levels by exploiting the parallel nature of field programmable gate array (FPGA) are explained in this article. Switches are modeled as on/off resistors and a Python-based software tool is developed to precompute the system matrices corresponding to each switch combination. Converters like three-phase five-level-flying-capacitor are modeled in C using the precomputed data, converted to VHDL and implemented on ZCU104 and ZedBoard using Vivado and verified against the corresponding MATLAB-Simulink model. Also, the real-time performance is analyzed by connecting it for the closed-loop speed control of induction motors of various specifications by changing the load torque and reference speed and found out that the latency is less than 6 μs, which is sufficient for the wide range of variable frequency drive applications. [ABSTRACT FROM AUTHOR]

Microgrids are subjected to limits on system voltage and frequency deviations for transient and steady state operation. Similar to a conventional power system with rotating generators, inertia and damping of frequency are necessary for frequency profile and stability of an isolated microgrid. This work proposes an approach to analyze inertia and frequency damping coefficient for a system based on the fact that sources relatively close to each other observe similar transients in frequency within an isolated microgrid. This is validated through bode plots and simulation. The approach allows defining the system response to transients in frequency in terms of individual source response using standard frequency mode parameters such as damping and inertia. Further, to optimize the overall system response so that sources share the frequency regulation requirement, a novel strategy is proposed to design individual source inertia and damping coefficient based on the source capacity. An experimental setup consisting of a microgrid system having three inverter-based sources working as virtual synchronous generators operating in parallel has been set up to verify the analysis. [ABSTRACT FROM PUBLISHER]

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]

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]

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]

Generation is shifting from a centralized power generating facility having large synchronous generators to distributed generation involving sources of smaller capacity. Most of these sources require inverters on the front end while being connected to the grid. Lower available kinetic energy, coupled with less short-circuit current ratio compared to large synchronous generators, compromises the transient stability of the microgrid when isolated from the main grid. Sources in the microgrid use droop control to share power according to their capacity without any form of communication. This paper proposes a novel controller for inverters to improve the frequency response of microgrid under disturbances involving large frequency deviations. It also discusses design of various parameters defined for the proposed control. The microgrid, which has two inverters and two synchronous generators, is simulated using Simulink/MATLAB software to test the proposed control strategy. [ABSTRACT FROM PUBLISHER]

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]

The time integral of the output voltage vector of a three-phase inverter is often termed the inverter flux vector. This paper addresses the control of a three-phase four-leg sine-wave output inverter having an LC filter at its output, by controlling the flux vector in three dimensions. Flux vector control has the property that an output filter resonance is actively damped by an output voltage control loop alone. Furthermore, an inverter switching action inherently regulates the output voltage rapidly against dc-bus voltage variations. The flux vector control of sine-wave output inverters finds several applications in three-phase four-wire systems. This paper presents a flux modulation method for threephase four-leg inverters feeding unbalanced and nonlinear loads. All the necessary steps for the digital implementation of the flux modulator are presented. The switching behavior of the modulator has been evaluated, which is useful for the variable fundamental frequency applications of the inverters. To provide experimental validation, the modulator is implemented as a part of the control system for a stand-alone three-phase four-leg inverter with an LC filter at its output. Control system details are also provided. Experimental results indicate the effectiveness of the modulator and control system in providing balanced voltages at the output of the LC filter even under highly unbalanced conditions with nonlinear loads. The resonance damping and voltage regulation properties of the modulator are also apparent from the experimental results. [ABSTRACT FROM AUTHOR]

The submodule capacitor voltage sensing and balancing is necessary for proper operation of modular multilevel converters (MMC). Voltage controlled oscillator is used for capacitor voltage measurement in the reported literature where voltage sensor is required to be calibrated. In this article, a voltage sensing method that does not need calibration of individual voltage sensors is used for capacitor voltage measurement. Moreover, a simplified sorting algorithm is employed for capacitor voltage balancing. A method of sorting the capacitor voltages that selects the submodules to be inserted directly based on logic equations is used as compared to the conventional methods of arranging them in ascending or descending order. This facilitates to simplifying the sorting technique and the scheme is easy to implement using any controller. Furthermore, to simplify the overall MMC control, the decentralized control arrangement is developed. Fairly good capacitor voltage balancing is obtained. The proposed balancing and sensing schemes of submodule capacitors are validated on a three-phase seven-level MMC hardware prototype with six submodules per arm. The prototype is tested for different operating conditions such as precharging, circulating current control, and grid connected operation of MMC. [ABSTRACT FROM AUTHOR]