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Hassan Bevrani

University of Kurdistan - Iran, Electrical and Computer Engineering, Faculty Member

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Hassan Bevrani received PhD degree in electrical engineering from Osaka University in 2004. He is a full professor and the Program Leader of Micro/Smart Grids Research Center (SMGRC) at the University of Kurdistan. Over the years, he has worked with Osaka University, Kumamoto University, Doshisha University, Nagoya University (Japan), Queensland University of Technology (Australia), Kyushu Institute of Technology, Centrale Lille (France), and Technical University of Berlin (Germany). Currently, he is a visiting professor at the Osaka University and experienced research fellow of AvH foundation. He is the author of 6 international books, 15 book chapters, and more than 300 journal/conference papers. He has been the gust editor of 4 volumes of Elsevier Energy Procedia and Energy Reports journals. His current research interests include Smart grid operation and control, power system stability, Microgrid dynamics and control, and Intelligent/robust control applications in power electric industry.
Phone: +98-87-33660073
Address: University of Kurdistan, Smart/Micro Grids Research Center, Sanandaj, PO Box 416, Kurdistan, Iran

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Papers by Hassan Bevrani

Nonlinear and linear robust control of switching power converters

Proceedings of the 1999 IEEE International Conference on Control Applications (Cat. No.99CH36328), 1999

Switching power converters are nonlinear and time variant systems. The classical controller desig... more Switching power converters are nonlinear and time variant systems. The classical controller design for a switching power converter based on the linear model derived by one of conventional modelling methods. However the parameter and load variations and input unregulated voltage are not taken into consideration simultaneously in a controller design.

MICROGRID CONTROLS

by SMGRC UOK and Hassan Bevrani

Currently, economical harvesting of electrical energy on a large scale considering the environmen... more Currently, economical harvesting of electrical energy on a large scale considering the environmental issues is undoubtedly one of the main challenges. As a solution, microgrids (MGs) promise to facilitate the wide penetration of renewable energy sources (RESs) and energy storage devices into the power systems, reduce system losses, and greenhouse gas emissions and increase the reliability of the electricity supply to the customers.

PMSG Control for a Stand-Alone Gas Engine Generator Using Active Rectifier and VSG-Controlled Inverter

by SMGRC UOK and Hassan Bevrani

The engine generator system with a diode rectifier causes harmonic currents in the generator whic... more The engine generator system with a diode rectifier causes harmonic currents in the generator which can affect generator efficiency and may produce torque oscillations. Using an active rectifier instead of a diode rectifier helps us to improve the current waveforms. In this paper, an active rectifier is used for a stand-alone gas engine generation system with a permanent magnet synchronous generator (PMSG). The generator side converter and the load side converter can be controlled separately to achieve high performance and reliability of the system. In the proposed control framework, the generator side converter is controlled by means of a current vector control method in a cascade structure with the synchronous reference frame (dq-frame). In the proposed control scheme, the dc link voltage is controlled by the generator side converter. For load side converter control, the concept of virtual synchronous generator control method is adopted to support a smooth power transient during the load changes. To verify the usefulness of the proposed control structure, using PSCAD software (version 4.2.1), the system transient responses with both a diode rectifier and an active rectifier are investigated under loading and load removal cases. Moreover, for the system with an active rectifier, the transient response of the system with different vector control strategies of PMSG is also investigated.

A Novel Control Scheme for Multi-Terminal Low-Frequency AC Electrical Energy Transmission Systems Using Modular Multilevel Matrix Converters and Virtual Synchronous Generator Concept

by SMGRC UOK and Hassan Bevrani

This paper proposes a new control scheme for the low frequency AC transmission (LFAC) system aimi... more This paper proposes a new control scheme for the low frequency AC transmission (LFAC) system aiming at extending the point-to-point configuration to form a multi-terminal electrical energy network. The multi-terminal low frequency ac (MT-LFAC) system configuration is based on the use of modular multilevel matrix converters (M3Cs) and virtual synchronous generator (VSG) control. The M3C is the next ac/ac converter generation, which is used as an interface with the conventional AC network and the LFAC electrical energy system. Application of VSG control is proposed to enable proper power sharing, to provide synchronization of each terminal, and frequency stabilization, thus, to offer multiterminal forming capability. Two different operation modes are applied in the system to damp the frequency deviation after a dynamic perturbation, which provides additional stabilization feature to the VSG. Frequency restoration mode and commanded mode of power sharing are applied as dynamic states to validate the robustness of the VSG control system. Besides, to solve the negative impact of low X/R ratio in the LFAC electrical energy system, we enhance the VSG control by proposing a virtual-impedance-based solution, which increases the output total impedance on the low frequency side and prevents the coupling between P and Q. The operation of the proposed system is examined by simulation results with a precise model of M3Cs in the PSCAD/ EMTDC software environment (version 4.2.1, Winnipeg, MB, Canada).

Frequency Response Improvement in Microgrids: A Fuzzy-based Virtual Synchronous Generator Approach

by SMGRC UOK and Hassan Bevrani

The use of renewable energy sources in microgrids has grown dramatically in recent years. The abs... more The use of renewable energy sources in microgrids has grown dramatically in recent years. The absence of a rotational mass in these microgrids and their interfaces leads to a lack of inertia and consequently, frequency and voltage instability. To cope with these dilemmas, the virtual synchronous generator (VSG) has been introduced as an effective solution. This paper first focuses on modeling a VSG using basic electrical equations. It, then, proffers a transient fuzzy controller augmented on virtual inertia's topology. Inspired by the FACTS' performance, the privileged specifications such as STATCOM fluctuation damping ability for major perturbations at transient times are appended to the VSG scheme by a fuzzy controller. This controller is implemented with a difference in voltage angle and its derivative, as well as in frequency and its derivative. The modified coefficients of both active and reactive powers are outputs of the fuzzy system. Using the proposed fuzzy controller, the transient response of VSG-based microgrids is improved. Simplicity and ability to improve the transient response are the principal specifications of the proposed configuration. Simulation results confirm the improvement of the presented method by the introduced augmented VSG control mechanism. Article Info

A Unified Modeling Method of Virtual Synchronous Generator for Multi-Operation-Mode Analyses

by SMGRC UOK and Hassan Bevrani

To provide inertia support for the grid, virtual synchronous generator (VSG) control of inverter-... more To provide inertia support for the grid, virtual synchronous generator (VSG) control of inverter-interfaced distributed generators (IIDGs) becomes a focus of worldwide attention. However, a VSG-based IIDG behaves differently in the grid-connected mode, the islanded-single-DG mode, and the islanded-multi-DG mode, whereas the mathematical and physical interpretations of this phenomenon are not well studied. In this paper, we propose a unified modeling method of VSG-based IIDG to analyze its different dynamic performance in each operation mode. The proposed unified formulas can obtain the state-space models of islanded-single-DG mode and islanded-multi-DG mode from that of grid-connected mode for any VSG control method. With the obtained models, for several different types of VSG control in different operation modes, we analyze the distribution and sensitivity of the closed-loop poles and investigate the step responses both analytically and experimentally. These analyses reveal the intrinsic differences and correlations of the dynamics of VSG-based IIDG between each operation mode. These intrinsic features are valid independent from the applied VSG control scheme, thus a test method to evaluate the parameters and performance of an unknown IIDG is derived. The findings of this paper provide important instructions for engineers to model, design and test multi-operation-mode distributed generators.

STABILITY AND INERTIA RESPONSE IMPROVEMENT OF BOOST CONVERTERS INTERLACED WITH CONSTANT POWER LOADS

by SMGRC UOK, Mehran Jami, and Hassan Bevrani

In dc microgrids, the distributed sources and loads with different electrical characteristics are... more In dc microgrids, the distributed sources and loads with different electrical characteristics are typically interconnected to the main bus through power electronic converters. The existence of interfaced converters creates two major problems: 1) the load-side converters and their associated loads, usually considered as constant power loads (CPLs), introduce destabilizing effects into the system; 2) the source-side converters do not possess any inertia or damping properties but reduce the overall inertia of the system. To overcome the stability problems caused by CPLs and low inertia, this paper proposes an active damping strategy based on a linear feedback. The proposed strategy measures the inductor current implemented in source-side boost converter in order to enhance the damping of the dc microgrids with CPLs. The control input-to-output voltage transfer function of a boost converter loaded with a CPL is inherently nonlinear, unstable and a non-minimum phase system that makes its control very difficult. In addition, the synthetic inertia of dc bus is enhanced by adding the virtual inertia control to the inner current control loop that is fast enough to emulate inertia and damping coefficient concept. In order to study the stability of dc MG with CPLs, a comprehensive small-signal model is derived and then, an acceptable range of inertia response parameters is determined by using the system's root locus analysis. Performance of the proposed control structure is demonstrated through numerical simulations.

Decentralized Scalable Robust Voltage Control for Islanded AC Microgrids with General Topology

by SMGRC UOK, Marjan Shafiee Rad, and Hassan Bevrani

This paper introduces a scalable decentralized control strategy for the satisfaction of robust st... more This paper introduces a scalable decentralized control strategy for the satisfaction of robust stability and robust performance of islanded inverter-interfaced microgrids with the general structure. The proposed control method maintains the stability and the desired performance of the closed-loop system against microgrid topology changes, plug-and-play functionality of DGs, and local load variations. The robust control scheme is formulated in the form of a dynamic output feedback controller for a linear time-invariant system with polytopic-type uncertainty and then converted to an LMI-based convex optimization problem. The capability of the suggested controller is evaluated under several scenarios simulated in MATLAB/SimPowerSystems Toolbox.

Interconnected Autonomous AC Microgrids via Back-to-Back Converters-Part II: Stability Analysis

by SMGRC UOK and Hassan Bevrani

In this paper, the stability of voltage source converter-based autonomous AC microgrids (MGs), wh... more In this paper, the stability of voltage source converter-based autonomous AC microgrids (MGs), which are interconnected through back-to-back converters (BTBCs), is analyzed. The small-signal stability analysis is based on a detailed, comprehensive and generalized small-signal modeling of the AC interconnected MGs (IMGs), which is possible for any number of MGs and interconnections. The large-signal stability of the IMGs is investigated for the case of the initial BTBC DC voltage as a part of paper contribution. A new margin/criterion is determined for the initial DC voltage in different situations of the BTBC operation. According to the proposed criterion, a fundamental difference between very weak MGs and conventional strong grids in the BTBC voltage stability is addressed. Using eigenvalue analysis and participation matrix, the main participating state variables and corresponding parameters in the dominant critical modes are recognized for an equilibrium point. Sensitivity analysis involves changing initial values of the state variables, parameters, and forcing functions to study their different values and find acceptable ranges of the parameters. Particularly, the considerable contribution of the BTBC in the critical modes is found out by analyzing the initial DC voltage, DC voltage controller and PLLs. In order to observe possible unstable situations and verify the transient studies, real-time simulations are provided for two and three MGs interconnected through BTBCs using OPAL-RT digital simulator. The IMGs can be robustly stable only by specifying the stabilizing ranges of the sensitive parameters to the critical modes and selecting their appropriate values.

Model Predictive Control for Indirect Boost Matrix Converter Based on Virtual Synchronous Generator

by SMGRC UOK and Hassan Bevrani

Indirect boost matrix converter is potentially a great alternative to a back-to-back converter fo... more Indirect boost matrix converter is potentially a great alternative to a back-to-back converter for permanent magnet synchronous generators based distributed generation since it can achieve a voltage-boost functionality without utilizing a bulky DC-link capacitor. Despite the success of the indirect boost matrix converter topology, there still exist some issues in the relevant control structure that must be resolved appropriately. First, the existing controls are grid-following controls, which is incapable of islanded operation. Secondly, the exiting controls generate a highly distorted current waveform, which needs to be suppressed by a passive damping resistor. Moreover, without an energy storage element, the distributed generations have no short-time power reserve unit for providing an inertial power to support the utility. In order to solve these issues, a novel approach based on a modified virtual synchronous control and a finite control set model predictive control scheme is proposed in this paper. The former is adopted to ensure proper operations in both grid-connected and islanded modes and to emulate the virtual inertial response by drawing inertial power from the input source. The latter utilizes multi-controls of real-time variables to avoid complicated coupling between the input side and the output side controls and to grant the indirect boost matrix converter with the capability of providing active filter resonance damping. Comparative studies between the proposed control and its existing counterpart are conducted with several simulations in PSCAD/EMTDC software to demonstrate the superior performances of the proposed strategy. Finally, the proposed control is verified in a scale-down experiment testbed. INDEX TERMS AC-AC converters, distributed power generations, indirect matrix converter, power control, power system stability, predictive control, virtual synchronous generator.

A Dual VSG-Based M3C Control Scheme for Frequency Regulation Support of a Remote AC Grid Via Low-Frequency AC Transmission System

by SMGRC UOK and Hassan Bevrani

This paper proposes a low-frequency ac (LFAC) transmission system capable of providing frequency ... more This paper proposes a low-frequency ac (LFAC) transmission system capable of providing frequency regulation support for a remote ac grid aiming at enhancing its frequency stability. The LFAC transmission system using modular multilevel matrix converters (M3Cs) for a direct ac-ac conversion is receiving noticeable attention as an alternative solution for a long-distance transmission system. One of its potential applications is the system connection between a large ac grid and a remote ac grid, whereas the latter usually suffers from a fluctuant frequency due to lack of inertia. The core of this work is addressing a novel control scheme of the remote ac grid-side M3C, which makes both interactions with the LFAC system and the remote ac grid behave like a synchronous generator using a dual virtual synchronous generator (VSG) control scheme. This control scheme can enhance the frequency regulation in the remote ac grid, which the existing control schemes cannot provide. Average dc capacitor voltage versus active power (V ave-P) droop control is proposed to coordinate the power flow between the VSG controls applied in both sides of the M3C. To demonstrate the problem of the existing control scheme, as well as the benefits offered by the proposed control scheme, transient performance including load variation and fault events in the remote ac grid is studied and examined in the EMTDC/PSCAD software environment. INDEX TERMS Frequency control, low-frequency ac transmission, modular multilevel matrix converter, power control, power grids, power transmission, remote grid, virtual synchronous generator.

Intelligent Demand Response Contribution in Frequency Control of Multi-area Power Systems

by SMGRC UOK and Hassan Bevrani

Frequency control is one of the most important issues in a power system due to increasing size, c... more Frequency control is one of the most important issues in a power system due to increasing size, changing structure and the complexity of interconnected power systems. Increasing economic constraints for power system quality and reliability and high operational costs of generation side controllers have inclined researchers to consider demand response as an alternative for preserving system frequency during off-normal conditions. However, the main obstacle is calculating the accurate amount of load related to the value of disturbances to be manipulated, specifically in a multi-area power system. Dealing with this challenge, this paper makes an attempt to find a solution via monitoring the deviations of tie-line flows. The proposed solution calculates the magnitude of disturbances and simultaneously determines the area where disturbances occurred, to apply demand response exactly to the involved area. To address communication limitations, the impact of demand response delay on the frequency stability is investigated. Furthermore, this paper introduces a fuzzy-PI-based supervisory controller as a coordinator between the demand response and secondary frequency control avoiding large frequency overshoots/undershoots caused by the communication delays. To evaluate the proposed control scheme, simulation studies are carried out on the 10-machine New England test power system. Index Terms-Frequency control, fuzzy-PI coordinator, multi-area power system, regional demand response, time delay. NOMENCLATURE , tie i P ∆ Total tie-line power change between area i and other areas. ∆f Frequency deviation in Hz. ij T Tie-line synchronizing torque coefficient. D Load damping coefficient. H Equivalent inertia constant. Li P ∆ Load change in area. i ∆ m P Mechanical power change. ∆ mni P Mechanical power change of n th governor-turbine of area. i R Speed-droop characteristic.

Decentralized Frequency Control of AC Microgrids: an Estimation-Based Consensus Approach

by SMGRC UOK and Hassan Bevrani

In this paper, a decentralized secondary control (SC) based on the active power estimation (APE) ... more In this paper, a decentralized secondary control (SC) based on the active power estimation (APE) is presented. This achievement is realized by employing the unique feature of frequency as a global variable in autonomous AC microgrids (MGs). The APE is merely based on the droop coefficient of P −ω characteristics. The decentralized SC, utilizing a consensus protocol, restores the MG frequency to the nominal value while maintaining accurate power-sharing of the droop mechanism. The consensus protocol is estimation-based and does not require communication infrastructure. In addition to the proposition of stability analysis method, experimental results with four distributed generation units (DGUs) also verify the effectiveness of the proposed SC structure.

Interconnected Autonomous AC Microgrids via Back-to-Back Converters-Part I: Small-Signal Modeling

by SMGRC UOK and Hassan Bevrani

In this paper, a set of autonomous AC Microgrids interconnected by back-to-back converters is tak... more In this paper, a set of autonomous AC Microgrids interconnected by back-to-back converters is taken into account, where they are supplied fully using voltage source converter-based distributed energy resources. A comprehensive and generalized small-signal model of the interconnected autonomous microgrids as a large-scale system is proposed using the inter-connection method. The modeling is based on detailed module models to show the impact of each module on the dynamic modes, especially the dominant critical modes. It is generalized and scalable due to separate modeling of modules as well as using unlimited and expandable interconnecting. The proposed interconnection method deals with all electrical and control connections between individual modules including feedback , feed-forward, augmentation, and the order of module inputs and outputs. The model is validated employing Prony analysis method and using output results of an OPAL-RT real-time simulator. Using the proposed modeling method, the small-signal stability analysis and controller design can be realized simply for interconnected microgrids with any number of microgrids and different structures. Typically for two interconnected microgrids, all dynamic modes and participant state variables in different frequency ranges are identified using the eigenvalue analysis and participation matrix in MATLAB.

An Emergency Active and Reactive Power Exchange Solution for Interconnected Microgrids

by SMGRC UOK and Hassan Bevrani

In this paper, an important application of interconnected microgrids (IMGs) is presented in order... more In this paper, an important application of interconnected microgrids (IMGs) is presented in order to handle an emergency condition of individual microgrids (MGs), which causes an intolerable voltage magnitude or frequency deviation. In this method, the active and reactive powers are exchanged via an interlinking back-to-back converter (BTBC) and in accordance with voltage magnitude and/or frequency differences of individual MGs. In order to increase the IMG reliability and overcome the communication challenges, only local voltages and frequencies of both AC sides of the BTBC are used to determine the required active and reactive powers to be exchanged. A generalized droop control is employed to consider the impact of voltage magnitude and frequency differences on exchanging active and reactive powers separately for any type of interlinking lines. In this emergency power exchange method, a logical control is employed to detect the emergency and the normal conditions. The former results in the IMG formation, while the latter form the individual MGs operation again. The real-time results, obtained from OPAL-RT simulator, practically show advantages of the proposed method including voltage and frequency supports, postponing the load shedding, plug-and-play capability, good power sharing between the IMGs and method compatibility with the controls of individual MGs.

Model Validation of Power Electronics-based Networked Micro-grids by Prony Analysis

by SMGRC UOK and Hassan Bevrani

In this paper, a complete small signal model of two networked microgrids (NMGs) is provided, whic... more In this paper, a complete small signal model of two networked microgrids (NMGs) is provided, which can be used in the stability analysis, stability margin derivation and controller design. This model is validated by comparing it with the simulated NMGs using the Prony analysis. In this method, the modes, calculated by eigenvalue analysis and contributing to a particular state variable, are identified using the participation matrix and compared with the natural frequencies of its waveform. The state variable waveform is locally measured from the simulated NMGs in the SimPowerSystems environment of MATLAB and its natural frequencies are calculated using Prony analysis. It is concluded that Prony analysis estimates the natural frequencies well and there is a good similarity between them and the calculated modes, which shows the modeling validation.

Enhanced Performance of a Stand-Alone Gas-Engine Generator Using Virtual Synchronous Generator and Energy Storage System

by SMGRC UOK, Jia Liu, and Hassan Bevrani

Gas engine generators have been recognized as a set of beneficial generating units for producing ... more Gas engine generators have been recognized as a set of beneficial generating units for producing electric power owing to their high efficiency, low emissions and appropriate fuel price. However, the major drawback of gas engine generators is the weak transient performance during sudden load change. The reason is that the engine's mechanical system cannot quickly respond to the demanded torque. Therefore, a change in load causes a large engine speed deviation, especially in stand-alone mode. If the engine speed deviation exceeds the allowable limits, the system operation will be shut down. To fix this drawback, this paper introduces the virtual synchronous generator control method combined with an additional energy storage system to the stand-alone gas engine generator. In this paper, the applied virtual synchronous generator control scheme is presented. Compared to the constant-voltage constant-frequency control method, the benefit of virtual synchronous generator control method for the gas engine generator is evaluated by simulation study in PSCAD software. The energy-variables-based feedback control structure is adopted for the control of energy storage system. The required capacity of energy storage system is estimated. To verify the effectiveness of proposed control structure, simulation is conducted under load connection and load removal cases. Finally, an experimental study is carried out for the same configuration of simulation model using a real gas-engine generator set up. INDEX TERMS Gas engine generator, virtual synchronous generator, energy storage system, transient response.

Modeling and Stability Analysis of Back-to-Back Converters in Networked Microgrids

by SMGRC UOK, mobin naderi, Hassan Bevrani, and Rasool Heydari

This paper provides a small-signal model and stability analysis of interconnected AC microgrids (... more This paper provides a small-signal model and stability analysis of interconnected AC microgrids (MGs) connected through back-to-back converters (BTBCs). The proposed model-ing method of the networked microgrids (NMGs) is derived and it is generalized for any number of NMGs through BTBCs. Different BTBC control parts are analyzed to study their impact on the NMG stability. The eigenvalue analysis and participation matrix are employed to identify dynamic modes of BTBC DC voltage controller. For two NMGs, main participating state variables and corresponding parameters in the dominant low-frequency modes (LFMs) are recognized, then acceptable ranges of the parameters are calculated using the sensitivity analysis (SA). The contribution of the control BTBC parameters including PLL and DC voltage controller parameters in the small-signal stability margin are shown. In addition, to show the BTBC control impact on the NMGs stability in the time domain, simulation results are provided for the two NMGs in SimPowerSystems/Matlab environment.

Decentralized Model Predictive Control of DC Microgrids with Constant Power Load

by SMGRC UOK and Hassan Bevrani

This paper introduces a decentralized model predic-tive controller (DMPC) to ensure power sharing... more This paper introduces a decentralized model predic-tive controller (DMPC) to ensure power sharing and regulate dc bus voltage in dc microgrids (MGs) with constant power load (CPL). The proposed method replaces the conventional primary layer of dc MGs, i.e., inner loops and droop control, with a single optimal controller. A predictive automatic model of the system is realized for prediction purposes and to be used in the cost function. The control objectives are then incorporated in the cost function to attain an optimal state switching in each sampling time, hence regulating dc bus voltage and accurate sharing of current among the MG. The proposed solution provides the system with a fast dynamic response and zero steady state error. The effectiveness of proposed control is verified via hardware-in-the-loop real time experiments, and the results are compared with the conventional primary control.

Interconnected microgrids frequency response model: An inertia-based approach

by SMGRC UOK, Sharara Rehimi, and Hassan Bevrani

Preserving the frequency stability of new important blocks in smart grids such as interconnected ... more Preserving the frequency stability of new important blocks in smart grids such as interconnected microgrids, is a serious challenge. Inertia can act as a significant stability/performance index for frequency regulation in the power grids. Renewable energy sources have low inertia and mostly need a special control mechanism to emulate an inertia and inject to the system. In this work, a frequency response model is introduced for AC interconnected microgrids based on the inertia concept. The absence of a suitable model for study on frequency and tie-line power control in the interconnected microgrids, limits research activities in this field. A clear classification has not been done considering the inherent dynamic behavior of all distributed generators types, which may help researchers to introduce more simple strategies to provide better dynamic performance. This paper addresses a new approach for study on inertia-based simple frequency response model for interconnected microgrids. The outcome can be useful for frequency and tie-line power control analysis and synthesis in interconnected microgrids.