Mehdi Savaghebi

Modeling, Analysis, and Design of Stationary-Reference-Frame Droop-Controlled Parallel Three-Phase Voltage Source Inverters

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.

Secondary Control Scheme for Voltage Unbalance Compensation in an Islanded Droop-Controlled Microgrid

The concept of microgrid hierarchical control is presented recently. In this paper, a hierarchical scheme is proposed which includes primary and secondary control levels. The primary level comprises distributed generators (DGs) local controllers. The local controllers mainly consist of power, voltage and current controllers, and virtual impedance control loop. The central secondary controller is designed to manage the compensation of voltage unbalance at the point of common coupling (PCC) in an islanded microgrid. Unbalance compensation is achieved by sending proper control signals to the DGs local controllers. The design procedure of the control system is discussed in detail and the simulation results are presented. The results show the effectiveness of the proposed control structure in compensating the voltage unbalance.

Autonomous Voltage Unbalance Compensation in an Islanded Droop-Controlled Microgrid

Recently, there has been an increasing interest in using distributed generators (DGs) not only to inject power into the grid but also to enhance the power quality. In this paper, a stationary-frame control method for voltage unbalance compensation in an islanded microgrid is proposed. This method is based on the proper control of DGs interface converters. The DGs are properly controlled to autonomously compensate for voltage unbalance while sharing the compensation effort and also active and reactive powers. The control system of the DGs mainly consists of active and reactive power droop controllers, a virtual impedance loop, voltage and current controllers, and an unbalance compensator. The design approach of the control system is discussed in detail, and simulation and experimental results are presented. The results demonstrate the effectiveness of the proposed method in the compensation of voltage unbalance.

Secondary Control for Voltage Quality Enhancement in Microgrids

In this paper, a hierarchical control scheme is proposed for enhancement of sensitive load bus (SLB) voltage quality in microgrids. The control structure consists of primary and secondary levels. The primary control level comprises distributed generators (DGs) local controllers. Each of these controllers includes a selective virtual impedance loop which is considered to improve sharing of fundamental and harmonic components of load current among the DG units. The sharing improvement is provided at the expense of increasing voltage unbalance and harmonic distortion. Thus, the secondary control level is applied to manage the compensation of SLB voltage unbalance and harmonics by sending proper control signals to the primary level. DGs compensation efforts are controlled locally at the primary level. The system design procedure for selecting proper control parameters is discussed. Simulation results are provided in order to demonstrate the effectiveness of the proposed control scheme.

An Optimal Energy Management System for Islanded Microgrids Based on Multiperiod Artificial Bee Colony Combined With Markov Chain

The optimal operation programming of electrical systems through the minimization of the production cost and the market clearing price, as well as the better utilization of renewable energy resources, has attracted the attention of many researchers. To reach this aim, energy management systems (EMSs) have been studied in many research activities. Moreover, a demand response (DR) expands customer participation to power systems and results in a paradigm shift from conventional to interactive activities in power systems due to the progress of smart grid technology. Therefore, the modeling of a consumer characteristic in the DR is becoming a very important issue in these systems. The customer information as the registration and participation information of the DR is used to provide additional indexes for evaluating the customer response, such as consumers information based on the offer priority, the DR magnitude, the duration, and the minimum cost of energy. In this paper, a multiperiod artificial bee colony optimization algorithm is implemented for economic dispatch considering generation, storage, and responsive load offers. The better performance of the proposed algorithm is shown in comparison with the modified conventional EMS, and its effectiveness is experimentally validated over a microgrid test bed. The obtained results show cost reduction (by around 30%), convergence speed increase, and the remarkable improvement of efficiency and accuracy under uncertain conditions. An artificial neural network combined with a Markov chain (ANN-MC) approach is used to predict nondispatchable power generation and load demand considering uncertainties. Furthermore, other capabilities such as extendibility, reliability, and flexibility are examined about the proposed approach.

Distributed Smart Decision-Making for a Multimicrogrid System Based on a Hierarchical Interactive Architecture

In this paper, a comprehensive real-time interactive energy management system (EMS) framework for the utility and multiple electrically coupled MGs is proposed. A hierarchical bi-level control scheme (BLCS) with primary and secondary level controllers is applied in this regard. The proposed hierarchical architecture consists of sub-components of load demand prediction, renewable generation resource integration, electrical power-load balancing, and responsive load demand. In the primary level, EMSs are operating separately for each microgrid (MG) by considering the problem constraints, power set-points of generation resources, and possible shortage or surplus of power generation in the MGs. In the proposed framework, minimum information exchange is required among MGs and the distribution system operator. It is a highly desirable feature in future distributed EMS. Various parameters such as load demand and renewable power generation are treated as uncertainties in the proposed structure. In order to handle the uncertainties, Taguchis orthogonal array testing approach is utilized. Then, the shortage or surplus of the MGs power should be submitted to a central EMS in the secondary level. In order to validate the proposed control structure, a test system is simulated and optimized based on multiperiod imperialist competition algorithm. The obtained results clearly show that the proposed BLCS is effective in achieving optimal dispatch of generation resources in systems with multiple MGs.

Modeling, analysis, and design of stationary reference frame droop controlled parallel three-phase voltage source inverters

Power electronics based microgrids consist of a number of voltage source inverters (VSIs) operating in parallel. In this paper, the modeling, control design, and stability analysis of three-phase VSIs are derived. The proposed voltage and current inner control loops and the mathematical models of the VSIs were based on the stationary reference frame. A hierarchical control for the paralleled VSI system was developed based on three levels. The primary control includes the droop method and the virtual impedance loops, in order to share active and reactive power. The secondary control restores the frequency and amplitude deviations produced by the primary control. And the tertiary control regulates the power flow between the grid and the microgrid. Also, a synchronization algorithm is presented in order to connect the microgrid to the grid. The evaluation of the hierarchical control is presented and discussed. Experimental results are provided to validate the performance and robustness of the VSIs functionality during Islanded and grid-connected operations, allowing a seamless transition between these modes through control hierarchies by regulating frequency and voltage, main-grid interactivity, and to manage power flows between the main grid and the VSIs.

Distributed Voltage Unbalance Compensation in Islanded Microgrids by Using a Dynamic Consensus Algorithm

In islanded microgrids (MGs), distributed generators (DGs) can be employed as distributed compensators for improving the power quality in the consumer side. Two-level hierarchical control can be used for voltage unbalance compensation. Primary level, consisting of droop control and virtual impedance, can be applied to help the positive sequence active and reactive power sharing. Secondary level is used to assist voltage unbalance compensation. However, if distribution line differences are considered, the negative sequence current cannot be well shared among DGs. In order to overcome this problem, this paper proposes a distributed negative sequence current sharing method by using a dynamic consensus algorithm. In clear contrast with the previously proposed methods, this approach does not require a dedicated central controller, and the communication links are only required between neighboring DGs. The method is based on the modeling and analysis of the unbalanced system. Experimental results from an islanded MG system consisting of three 2.2-kVA inverters are shown to demonstrate the effectiveness of the method.

Flexible System Integration and Advanced Hierarchical Control Architectures in the Microgrid Research Laboratory of Aalborg University

This paper presents the system integration and hierarchical control implementation in an inverter-based Microgrid Research Laboratory (MGRL) at Aalborg University, Denmark. MGRL aims to provide a flexible experimental platform for comprehensive studies of microgrids. The structure of the laboratory, including the facilities, configurations, and communication network, is first introduced. The complete control system is based on a generic hierarchical control scheme including primary, secondary, and tertiary control. Primary control loops are developed and implemented in digital control platform, while system supervision, advanced secondary, and tertiary management are realized in a microgrid central controller. The software and hardware schemes are described. Several example case studies are introduced and performed to achieve power quality regulation, energy management, and flywheel energy storage system control. Experimental results are presented to show the performance of the whole system.

Original article: Selective compensation of voltage harmonics in grid-connected microgrids

In this paper, a novel approach is proposed for selective compensation of main voltage harmonics in a grid-connected microgrid. The aim of compensation is to provide a high voltage quality at the point of common coupling (PCC). PCC voltage quality is of great importance due to sensitive loads that may be connected. It is assumed that the voltage harmonics are originated from distortion in grid voltage as well as the harmonic current of the nonlinear loads. Harmonic compensation is achieved through proper control of distributed generators (DGs) interface converters. The compensation effort of each harmonic is shared considering the respective current harmonic supplied by the DGs. The control system of each DG comprises harmonic compensator, fundamental power controllers, voltage and current proportional-resonant controller and virtual impedance loop. Virtual impedance is considered at fundamental frequency to enhance power control and also at harmonic frequencies to improve the nonlinear load sharing among DGs. The control system design is discussed in detail. The presented simulation results demonstrate the effectiveness of the proposed method in compensation of the voltage harmonics to an acceptable level.