Ali Elrayyah

A Novel Load-Flow Analysis for Stable and Optimized Microgrid Operation

This paper proposes a novel load-flow analysis (LFA) algorithm for droop-based islanded microgrids (DBIM). The standard LFA is not applicable for DBIM due to the absence of a node with fixed reference voltage. As the voltage in the islanded microgrid depends on the droop relations, these relations are included as part of the load-flow equations. The proposed LFA is used with particle swarm optimization to select the droop parameters that optimize the reactive power sharing. Voltage compensation terms are also suggested to improve voltage regulation. By using the proposed LFA, a modeling procedure is suggested to check the stability and stabilize the microgrid. The effectiveness of the proposed methods is verified through different simulation studies.

An Effective Smooth Transition Control Strategy Using Droop-Based Synchronization for Parallel Inverters

This paper proposes a smooth transition control strategy for voltage-source inverters between stand-alone (SA) and grid-connected (GC) modes of operation. In both GC and SA modes of operation, the inverters use the droop control method to regulate the real power flow without the need of any external communication between them. One of the distributed energy sources working in the microgrid system is designated as a dispatch unit (DU) to facilitate interconnection with utility grid and achieve smooth transition between the modes of operations. During transition modes, this selected unit takes extra responsibility to ensure the continuous power delivery to the load. For smooth transition from GC to SA mode of operation, the DU compensates for the grid current immediately after the grid disconnection, and then, other units pick up this responsibility gradually through their individual droop controllers. The DU also adjusts its output power to synchronize the inverters with the grid during the transition from SA to GC mode. The simulation and experimental results, as well as the stability analysis, are provided to verify the effectiveness of the proposed control technique.

Microgrid-Connected PV-Based Sources: A Novel Autonomous Control Method for Maintaining Maximum Power

This article studies the control configuration of a microgrid-connected photovoltaic (MCPV) source. In the control of an MCPV, maximum power point (MPP) tracking, droop control, and dc bus voltage regulation are the main required functions. To increase their penetration in the microgrid, MCPV sources have to participate in the microgrid?s frequency regulation. Consequently, MCPVs may be forced to depart from MPP for short periods of time. In this article, a control method is proposed to operate the MCPV in the MPP at all times except when there is a need to stabilize the frequency. The method achieves this objective autonomously without the need to change the control configuration. This method is explained, and its superiority over other controllers to achieve the same objective is investigated. The suggested control configurations are validated through simulation studies and experiments.

Construction of Nonlinear Droop Relations to Optimize Islanded Microgrid Operation

In this paper, nonlinear droop relations are suggested to optimize the operation of islanded microgrids. By using nonlinear droop, many aspects of the microgrid operation can be optimized, in addition to the inherent advantages of the droop control. In the proposed method, the droop relations are allowed to have any nonlinear shape as long as that shape satisfies certain characteristics required for the stability and proper microgrid operation. The procedure of constructing the nonlinear droop relations that minimize the operating cost of the microgrid and share the reactive power effectively among the sources is investigated and explained in detail. The selected droop structure is a combination of integer and fractional power functions whose parameters are selected using a two-stage particle swarm optimization algorithm. The effectiveness of the proposed method is verified through simulation and experimental studies.

Efficient Harmonic and Phase Estimator for Single-Phase Grid-Connected Renewable Energy Systems

Renewable energy sources (RESs) in any power system can participate in removing the harmonics from the line voltage, but the RESs need to estimate these harmonics first. In this paper, an efficient method to estimate grid harmonics is proposed to be used by single-phase RESs. The proposed method provides accurate estimation for the harmonics while it has lower computation complexity than the other existing methods. To make the harmonic estimation process fast and accurate, the harmonics in the sampled grid voltage is eliminated before passing it to the phase-locked-loop block used for estimating the grid phase. Another application of the proposed method is to transform single-phase voltage or current from the stationary reference frame into the dq rotating reference frame. In this application, the use of the proposed method eliminates the need of generating fictitious voltage or current waveforms orthogonal to the measured quantities. The elimination of need to generate fictitious waveforms speeds up the transformation transients and reduces the operations to less than half of those required by traditional methods. Simulation and experimental results verified that the algorithm achieves fast and accurate harmonic estimation of highly distorted grid voltage.

Modeling and Control Design of Microgrid-Connected PV-Based Sources

A method is presented to model and tune microgrid-connected solar photovoltaic sources (MCPVs). The proposed controller combines maximum power point tracking with droop controller to participate in voltage and frequency regulations of islanded microgrids. In this paper, the controllers required by the MCPV are developed and a modeling procedure is presented to tune the controllers. The effect of MCPV penetration level on the controllers design is reflected in the model and a method is proposed to stabilize the MCPV as it switches among operation modes. The effectiveness of the proposed method is verified by simulation and experimental studies.

A novel load flow analysis for particle-swarm optimized microgrid power sharing

This paper proposes a novel load flow analysis (LFA) for droop-based islanded microgrids (DBIM). The standard LFA cannot be used since no single node sets the reference voltage. As the voltage in the islanded microgrid depends on the droop relation, they are included them as part of load flow equations. The proposed LFA is used with particle swarm optimization to select the droop parameters that optimize the power sharing among sources in a microgrid. The proposed methods are found to be effective through simulation studies.

An Effective Dithering Method for Electromagnetic Interference (EMI) Reduction in Single-Phase DC/AC Inverters

A comprehensive method of electromagnetic emission interference (EMI) reduction in dc/ac inverters through periodic dithering of the pulse-width modulated (PWM) switching frequency is presented in this paper. The periodic dithering is the process of changing the switching frequency of the PWM signal that drives the switches of the power electronics inverter. The effects of the various parameters involved in the PWM dithering process have been analyzed in this paper. The important parameters involved in the dithering process are the frequency range used for dithering, the periodic time of the dithering signal, and the phase of the dithering signal. The effect of each of these parameters has been studied to determine their values that minimize the EMI. Additionally, analytical expressions for theoretical prediction of the EMI levels at different frequencies have been derived for performance analysis during the design stage. The theoretical predictions for the dithering signals have been found to accurately correlate with both the simulation and experimental results.

Control of microgrid-connected PV-sources

This paper studies the control configuration of a Microgrid Connected Photo Voltaic (MCPV) source. In the control of a MCPV, maximum power point (MPP) tracking, droop control and DC bus voltage regulation are the main required functions. To increase their penetration in the microgrid, MCPV sources have to participate in the microgrids frequency regulation. Consequently, MCPV may be forced to depart from MPP for short periods of time. In this paper, a control method is proposed to operate the MCPV in the MPP at all times except when there is a need to stabilize the frequency. The method achieves this objective autonomously without any need to change the control configuration. This method is explained and its superiority over other controllers to achieve the same objective is investigated. The parameters of the universal controller are selected such that the controller can achieve the required performance. The suggested control configurations are validated through simulation studies.

Optimized Droop Control Parameters for Effective Load Sharing and Voltage Regulation in DC Microgrids

Abstract Droop control method is a widely used technique for achieving load sharing in DC microgrid applications. Virtual output impedance (or droop gain) and voltage reference are the main control parameters typically selected based on the power ratings of the sources for proper load sharing. The performance of droop controller is affected significantly by the voltage drops across the transmission line impedances, resulting in a load sharing error and voltage degradation across the microgrid. In this article, a new optimization procedure is proposed to find the optimal droop parameters such that the effect of the line impedances is minimized. An optimization problem along with the required constrains is formulated as the combination of current sharing errors as well as the voltage degradation for various loading conditions. Particle swarm-based technique is then used to provide a solution for this optimization problem. The performance of the droop controller with the optimal droop parameters is verified through a simulation case study implemented on the MATLAB/Simulink environment.