D.M. Vilathgamuwa

Design, analysis, and real-time testing of a controller for multibus microgrid system

This paper concentrates on the design and analysis of a controller for multibus microgrid system. The controller proposed for use with each distributed generation (DG) system in the microgrid contains inner voltage and current loops for regulating the three-phase grid-interfacing inverter, and external power control loops for controlling real and reactive power flow and for facilitating power sharing between the paralleled DG systems when a utility fault occurs and the microgrid islands. The controller also incorporates synchronization algorithms for ensuring smooth and safe reconnection of the micro and utility grids when the fault is cleared. With the implementation of the unified controller, the multibus microgrid system is able to switch between islanding and grid-connected modes without disrupting the critical loads connected to it. The performance of this unified controller has been verified in simulation using a real-time digital simulator and experimentally using a scaled laboratory prototype.

Pulse-width modulation of Z-source inverters

Z-Source inverters have recently been proposed as an alternative power conversion concept as they have both voltage buck and boost capabilities. These inverters use a unique impedance network, coupled between the power source and converter circuit, to provide both voltage buck and boost properties, which cannot be achieved with conventional voltage-source and current-source inverters. To facilitate understanding of Z-source inverter modulation, this paper presents a detailed analysis, showing how various conventional pulse-width modulation strategies can be modified to switch a voltage-type Z-source inverter either continuously or discontinuously, while retaining all the unique harmonic performance features of these conventional modulation strategies. This paper starts by analyzing the modulation requirements of a single-phase H-bridge Z-source inverter, and subsequently extends the analysis to cover the more complex three-phase-leg and four-phase-leg Z-source inverters, with carrier-based implementation reference equations derived for all the inverters. The theoretical and modulation concepts presented have been verified both in simulation and experimentally.

Microgrid power quality enhancement using a three-phase four-wire grid-interfacing compensator

This paper presents a three-phase four-wire grid-interfacing power quality compensator for microgrid applications. The compensator is proposed for use with each individual distributed generation (DG) system in the microgrid and consists of two four-phase-leg inverters (a shunt and a series), optimally controlled to achieve an enhancement of both the quality of power within the microgrid and the quality of currents flowing between the microgrid and the utility system. During utility grid voltage unbalance, the four-phase-leg compensator can compensate for all the unwanted positive-, negative-, and zero-sequence voltage-current components found within the unbalanced utility. Specifically, the shunt four-leg inverter is controlled to ensure balanced voltages within the microgrid and to regulate power sharing among the parallel-connected DG systems. The series inverter is controlled complementarily to inject negative- and zero-sequence voltages in series to balance the line currents, while generating zero real and reactive power. During utility voltage sags, the series inverter can also be controlled using a newly proposed flux-charge current-limiting algorithm to limit the flow of large fault currents between the micro- and utility grids. The performance of the proposed compensator has been verified in simulations and experimentally using a laboratory prototype.

Design of a Robust Grid Interface System for PMSG-Based Wind Turbine Generators

A robust and reliable grid power interface system for wind turbines using a permanent-magnet synchronous generator (PMSG) is proposed in this paper, where an integration of a generator-side three-switch buck-type rectifier and a grid-side Z-source inverter is employed as a bridge between the generator and the grid. The modulation strategy for the proposed topology is developed from space-vector modulation and Z-source network operation principles. Two PMSG control methods, namely, unity-power-factor control and rotor-flux-orientation control (Id = 0), are studied to establish an optimized control scheme for the generator-side three-switch buck-type rectifier. The system control scheme decouples active- and reactive-power control through voltage-oriented control and optimizes PMSG control for the grid- and generator-side converters independently. Maximum power point tracking is implemented by adjusting the shoot-through duty cycles of the Z-source network. The design considerations of the passive components are also provided. The performances and practicalities of the designed architecture have been verified by simulations and experiments.

Protection of Microgrids During Utility Voltage Sags

Microgrids are systems with clusters of microgenerators, which are installed for distributed power generation. When interfaced to the utility grid, microgrids are exposed to common utility power-quality disturbances. In particular, during utility-voltage sags, large line currents can flow along distribution feeders connecting the micro- and utility grids. To limit this flow of large line currents and, hence, protect the microgrids, this paper proposes two current-limiting algorithms, namely, the RL feedforward and flux-charge-model feedback algorithms, for controlling a series inverter connected between the micro- and utility grids during utility voltage sags. Both methods function by inserting a large virtual RL or L impedance in series with the distribution feeder to limit the line-current flow. Detailed descriptions, controller designs, and comparisons of both algorithms are presented. Phasor analyses of both methods are also presented to show how the inserted RL or L values can be optimally tuned to improve the inverter damping performance and minimize its injected voltages and circulating power under all operation conditions. Lastly, both methods have been tested in simulation and in experiments using an emulated laboratory microgrid system

A Novel Technique to Compensate Voltage Sags in Multiline Distribution System—The Interline Dynamic Voltage Restorer

The dynamic voltage restorer (DVR) provides a technically advanced and economical solution to voltage-sag problem. As the voltage-restoration process involves real-power injection into the distribution system, the capability of a particular DVR topology, especially for compensating long-duration voltage sags, depends on the energy storage capacity of the DVR. The interline DVR (IDVR) proposed in this paper provides a way to replenish dc-link energy storage dynamically. The IDVR consists of several DVRs connected to different distribution feeders in the power system. The DVRs in the IDVR system share a common energy storage. When one of the DVR compensates for voltage sag appearing in that feeder, the other DVRs replenish the energy in the common dc-link dynamically. Thus, one DVR in the IDVR system works in voltage-sag compensation mode while the other DVRs in the IDVR system operate in power-flow control mode. In principle, IDVR can operate effectively when constituent DVRs are electrically (not necessarily physically) far apart. Closed-loop load voltage and current-mode-control techniques are used as the control strategy in the two modes of operation. Experimental results obtained for a laboratory prototype of the IDVR are presented to show the effectiveness and the efficacy of the proposed IDVR system to improve power quality

A Sensor Fault Detection and Isolation Method in Interior Permanent-Magnet Synchronous Motor Drives Based on an Extended Kalman Filter

Interior permanent-magnet synchronous motors (IPMSMs) become attractive candidates in modern hybrid electric vehicles and industrial applications. Usually, to obtain good control performance, the electric drives of this kind of motor require one position, one dc link, and at least two current sensors. Failure of any of these sensors might lead to degraded system performance or even instability. As such, sensor fault resilient control becomes a very important issue in modern drive systems. This paper proposes a novel sensor fault detection and isolation algorithm based on an extended Kalman filter. It is robust to system random noise and efficient in real-time implementation. Moreover, the proposed algorithm is compact and can detect and isolate all the sensor faults for IPMSM drives. Thorough theoretical analysis is provided, and the effectiveness of the proposed approach is proven by extensive experimental results.

Development of a Comprehensive Model and a Multiloop Controller for

This paper presents the modeling and design of a closed-loop controller for a Z-source inverter. The Z-source inverter is a recently proposed single-stage power converter, and it is capable of operating in both buck and boost modes. Hence, this inverter gives an economical solution for power conversion in distributed generation (DG) applications, particularly by eliminating the need for a two-stage conversion. Moreover, applications such as DG demand quality output waveforms, and additionally, when the system is subjected to input- and load-side disturbances, their effects need to be minimized. This can be achieved with closed-loop controlling. Toward this end, the system is modeled first with large- and small-signal modeling techniques, and relevant transfer functions are derived. The dc-side of the Z-source inverter shows a non-minimum-phase characteristic, and the output voltage of a Z-source impedance network shows a significant overshoot and undershoot, following a step change in the input due to energy resettling. These effects could be transferred to the ac-side, giving rise to the undershoot and overshoot in the ac output as well. Hence, the proposed controllers should be able to minimize such effects. The ac- and dc-sides are considered separately when designing the controllers. An indirect controller is employed in the dc-side, whereas the ac-side controller is designed in the synchronous reference frame. The modulation index, shoot-through time, and saturation levels are appropriately selected so that the dc-side effects are prevented from propagating into the ac-side. The simulation results are obtained using a state-space-averaged inverter model, and an experimental prototype is built in a laboratory to prove the efficacy of the proposed algorithm. Simulation and experimental results show good reference-tracking and disturbance-rejection properties, validating the desired functionality of the proposed controller.

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This paper describes a transformerless self-charging dynamic voltage restorer (DVR) series compensation device used to mitigate voltage sags. A detailed analysis on the control of the restorer for voltage sag mitigation and dc-link voltage regulation are presented. A nonlinear element is shown to exist in the regulator, the activation of which can adversely affect its stability. Active cancellation for this element is recommended. Simulation and experimental results are presented for a 1-kVA prototype to validate the analysis as well as demonstrate the DVRs performance under both voltage restoration and self-charging operating conditions.

-Source Inverter DG Systems

Distributed generation (DG) systems are usually connected to the grid using power electronic converters. Power delivered from such DG sources depends on factors like energy availability and load demand. The converters used in power conversion do not operate with their full capacity all the time. The unused or remaining capacity of the converters could be used to provide some ancillary functions like harmonic and unbalance mitigation of the power distribution system. As some of these DG sources have wide operating ranges, they need special power converters for grid interfacing. Being a single-stage buck-boost inverter, recently proposed Z-source inverter (ZSI) is a good candidate for future DG systems. This paper presents a controller design for a ZSI-based DG system to improve power quality of distribution systems. The proposed control method is tested with simulation results obtained using Matlab/Simulink/PLECS and subsequently it is experimentally validated using a laboratory prototype.