scispace - formally typeset
Search or ask a question
Author

H. Mokhtari

Bio: H. Mokhtari is an academic researcher from Sharif University of Technology. The author has contributed to research in topics: AC power & Control theory. The author has an hindex of 14, co-authored 25 publications receiving 895 citations. Previous affiliations of H. Mokhtari include University of Toronto & Islamic Azad University.

Papers
More filters
Journal ArticleDOI
TL;DR: In this paper, the authors proposed a new control strategy for the islanded operation of a multi-bus medium voltage (MV) microgrid consisting of several dispatchable electronically-coupled distributed generation (DG) units.
Abstract: This paper proposes a new control strategy for the islanded operation of a multi-bus medium voltage (MV) microgrid. The microgrid consists of several dispatchable electronically-coupled distributed generation (DG) units. Each DG unit supplies a local load which can be unbalanced due to the inclusion of single-phase loads. The proposed control strategy of each DG comprises a proportional resonance (PR) controller with an adjustable resonance frequency, a droop control strategy, and a negative-sequence impedance controller (NSIC). The PR and droop controllers are, respectively, used to regulate the load voltage and share the average power components among the DG units. The NSIC is used to effectively compensate the negative-sequence currents of the unbalanced loads and to improve the performance of the overall microgrid system. Moreover, the NSIC minimizes the negative-sequence currents in the MV lines and thus, improving the power quality of the microgrid. The performance of the proposed control strategy is verified by using digital time-domain simulation studies in the PSCAD/EMTDC software environment.

134 citations

Journal ArticleDOI
TL;DR: The proposed modulation method can be used to obtain the maximum reactive power in the input of a matrix converter in applications such as wind turbine and microturbine generators, where the input reactive power control is necessary.
Abstract: In this paper, a new simple and complete technique of modeling and analysis of a matrix converter is presented based on the singular value decomposition (SVD) of modulation matrix. The proposed modeling method yields a new limitation between the matrix converter gain and its input power factor, which is more relaxed as compared to the limits reported so far in the literature. The SVD of the modulation matrix leads to a unified modulation technique which achieves the full capability of a matrix converter. It is shown that this approach is general and all other modulation methods established for a matrix converter are specific cases of this technique. The proposed modulation method can be used to obtain the maximum reactive power in the input of a matrix converter in applications such as wind turbine and microturbine generators, where the input reactive power control is necessary.

132 citations

Journal ArticleDOI
TL;DR: Simulation results indicate that the signal processing unit can provide the required signals for APF to perform filtering/compensation within the transient period of 2 to 3 cycles.
Abstract: A signal processing system for extraction of harmonic and reactive current components is introduced and its performance is evaluated. The extraction system is adopted as part of the control system of a single-phase active power filter (APF) to provide the required signals for harmonic filtering and reactive power compensation. Performance of the overall system is evaluated based on digital time-domain simulation studies. The APF control system including the signal processing algorithms are implemented in Matlab/Simulink Fixed-Point Blockset to accommodate bit-length limitation which is a crucial factor in digital implementation. The power system including the APF, load and the supply system are simulated with the PSCAD/EMTDC software to which the Matlab-based control model is interfaced. The simulation results indicate that the signal processing unit can provide the required signals for APF to perform filtering/compensation within the transient period of 2 to 3 cycles.

112 citations

Journal ArticleDOI
TL;DR: In this article, a droop paralleling strategy is proposed to control the power sharing of parallel uninterruptible-power-supply (UPS) systems, where the controller is a proportional-resonant controller which ensures good transient response and steady-state objectives.
Abstract: In this paper, a novel droop method to control the power sharing of parallel uninterruptible-power-supply (UPS) systems is presented. In a clear-cut contrast to the previously reported works, the controller is a proportional-resonant controller which ensures good transient response and steady-state objectives. Furthermore, the need for sensing the output current for the power-sharing control is removed by implementing it in a software routine. This brings a less complicated and less expensive structure in which the number of feedback sensors is reduced from three to two. The droop paralleling strategy is based on the drop in the inverter output frequency and amplitude. The application of proportional-resonant controllers based on internal model principle is also extended to parallel inverters, and its superior performance over the well-known proportional-integral-derivative controller is presented. In order to show the performance of the proposed system, two parallel-connected UPS systems are analyzed, and two types of linear and nonlinear loads are considered. The nonlinear load is compliant with IEC 62040-3 standard for class I UPS. The results show that the reduction of sensors does not result in any error, and the performance of the control system is excellent. To verify the proposed concept, the proposed strategy is implemented within two-625VA UPS systems. Several test results are provided for linear and nonlinear loads which validate the simulations results. The results indicate that the proposed parallel inverter structure offers a better system in terms of the number of feedback sensors, performance, and complication.

109 citations

Journal ArticleDOI
TL;DR: In this paper, a control strategy for an islanded medium voltage microgrid to coordinate hybrid power source (HPS) units and to control interfaced multilevel inverters under unbalanced and nonlinear load conditions is presented.
Abstract: This paper presents a control strategy for an islanded medium voltage microgrid to coordinate hybrid power source (HPS) units and to control interfaced multilevel inverters under unbalanced and nonlinear load conditions. The proposed HPS systems are connected to the loads through a cascaded H-bridge (CHB) multilevel inverter. The CHB multilevel inverters increase the output voltage level and enhance power quality. The HPS employs fuel cell (FC) and photovoltaic sources as the main and supercapacitors as the complementary power sources. Fast transient response, high performance, high power density, and low FC fuel consumption are the main advantages of the proposed HPS system. The proposed control strategy consists of a power management unit for the HPS system and a voltage controller for the CHB multilevel inverter. Each distributed generation unit employs a multiproportional resonant controller to regulate the buses voltages even when the loads are unbalanced and/or nonlinear. Digital time-domain simulation studies are carried out in the PSCAD/EMTDC environment to verify the performance of the overall proposed control system.

95 citations


Cited by
More filters
Journal ArticleDOI
TL;DR: This paper reviews the status of hierarchical control strategies applied to microgrids and discusses the future trends.
Abstract: Advanced control strategies are vital components for realization of microgrids. This paper reviews the status of hierarchical control strategies applied to microgrids and discusses the future trends. This hierarchical control structure consists of primary, secondary, and tertiary levels, and is a versatile tool in managing stationary and dynamic performance of microgrids while incorporating economical aspects. Various control approaches are compared and their respective advantages are highlighted. In addition, the coordination among different control hierarchies is discussed.

1,234 citations

Journal ArticleDOI
TL;DR: The modeling, control design, and stability analysis of parallel-connected three-phase VSIs are derived and a hierarchical control scheme for the paralleled VSI system control architecture is developed.
Abstract: Power-electronics-based microgrids (MGs) consist of a number of voltage source inverters (VSIs) operating in parallel. In this paper, the modeling, control design, and stability analysis of parallel-connected three-phase VSIs are derived. The proposed voltage and current inner control loops and the mathematical models of the VSIs are based on the stationary reference frame. A hierarchical control scheme for the paralleled VSI system is developed comprising two levels. The primary control includes the droop method and the virtual impedance loops, in order to share active and reactive powers. The secondary control restores the frequency and amplitude deviations produced by the primary control. Also, a synchronization algorithm is presented in order to connect the MG to the grid. Experimental results are provided to validate the performance and robustness of the parallel VSI system control architecture.

610 citations

Journal ArticleDOI
TL;DR: In this article, a virtual impedance design and implementation approach for power electronics interfaced distributed generation (DG) units is presented, where the virtual impedances can be placed between interfacing converter outputs and the main grid.
Abstract: This paper presents a virtual impedance design and implementation approach for power electronics interfaced distributed generation (DG) units. To improve system stability and prevent power couplings, the virtual impedances can be placed between interfacing converter outputs and the main grid. However, optimal design of the impedance value, robust implementation of the virtual impedance, and proper utilization of the virtual impedance for DG performance enhancement are key for the virtual impedance concept. In this paper, flexible small-signal models of microgrids in different operation modes are developed first. Based on the developed microgrid models, the desired DG impedance range is determined considering the stability, transient response, and power flow performance of DG units. A robust virtual impedance implementation method is also presented, which can alleviate voltage distortion problems caused by harmonic loads compared to the effects of physical impedances. Furthermore, an adaptive impedance concept is proposed to further improve power control performances during the transient and grid faults. Simulation and experimental results are provided to validate the impedance design approach, the virtual impedance implementation method, and the proposed adaptive transient impedance control strategies.

543 citations

Journal ArticleDOI
TL;DR: In this article, the authors present a comprehensive literature review of the latest research and development in the field of microgrid as a promising power system, which shows a broad overview on the worldwide research trend on microgrid which is most significant topic at present.
Abstract: The concept of integration of distributed energy resources for formation of microgrid will be most significant in near future. The latest research and development in the field of microgrid as a promising power system through a comprehensive literature review is presented in this paper. It shows a broad overview on the worldwide research trend on microgrid which is most significant topic at present. This literature survey reveals that integration of distributed energy resources, operation, control, power quality issues and stability of microgrid system should be explored to implement microgrid successfully in real power scenario.

431 citations

Journal ArticleDOI
TL;DR: A comprehensive literature review about electric motor energy analysis is presented in this article, where different types of losses that occur in a motor have been identified and ways to overcome these losses explained and an energy audit that helps to identify motor energy wastages have been discussed extensively.
Abstract: The industrial sector is the largest users of energy around the world. Industrial motor uses a major fraction of total industrial energy uses. This paper describes a comprehensive literature review about electric motor energy analysis. This paper compiles latest literatures in terms of thesis (MS and PhD), journal articles, conference proceedings, web materials, reports, books, handbooks on electrical motor energy use, losses, efficiency, energy savings strategies. Different types of losses that occur in a motor have been identified and ways to overcome these losses explained. An energy audit that helps to identify motor energy wastages have been discussed extensively. As motors are the major energy users, different energy savings strategies such as use of high-efficient motor, variable speed drive (VSD), and capacitor bank to improve the power factor to reduce their energy uses have reviewed. Different policy measures (i.e. regulatory, voluntary and incentives based) to save motor energy use have been reviewed and presented in this paper. In this review, computer tools that can be used to analyze electric motors energy used has been discussed. Cost parameters to carry out economic analysis have been shown as well. Moreover, payback period for different energy savings strategies have been identified.

398 citations