Karar Al-Nagar

Aswan University, Electrical Engineering, Faculty Member

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I am working as an Assistant Professor at Electrical Engineering department, Aswan Faculty of Engineering, Aswan University.
Phone: 00201069386771
Address: Egypt, Aswan

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diana ramirez

Grecia Castillo

Escuela Superior Politécnica De Chimborazo

Rosabel Roig-Vila

University of Alicante / Universidad de Alicante

Sonia Beatriz Concari

Universidad Nacional de Rosario (UNR) Argentina

José Octavio Islas Carmona

Universidad Autónoma del Carmen

Maria Elena Del Valle

EAE Business School

Ricardo-Patricio Medina-Chicaiza

Silvia J . Pech-Campos

University of Castilla-La Mancha

Javier Collado Ruano

Universidad Nacional de Educación (UNAE) de Ecuador

Miguel Fernández-Bermejo

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Papers by Karar Al-Nagar

Probabilistic Approach for Hosting High PV Penetration in Distribution Systems via Optimal Oversized Inverter With Watt-Var Functions

IEEE Systems Journal, 2021

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Machine Learning Based Method for Estimating Energy Losses in Large-Scale Unbalanced Distribution Systems with Photovoltaics

International Journal of Interactive Multimedia and Artificial Intelligence, 2020

In the recent years, the penetration of photovoltaics (PV) has obviously been increased in unbala... more In the recent years, the penetration of photovoltaics (PV) has obviously been increased in unbalanced power distribution systems. Driven by this trend, comprehensive simulation tools are required to accurately analyze large-scale distribution systems with a fast-computational speed. In this paper, we propose an efficient method for performing time-series simulations for unbalanced power distribution systems with PV. Unlike the existing iterative methods, the proposed method is based on machine learning. Specifically, we propose a fast, reliable and accurate method for determining energy losses in distribution systems with PV. The proposed method is applied to a large-scale unbalanced distribution system (the IEEE 906 Bus European LV Test Feeder) with PV grid-connected units. The method is validated using OpenDSS software. The results demonstrate the high accuracy and computational performance of the proposed method.

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Three-level control strategy for minimizing voltage deviation and flicker in PV-rich distribution systems

International Journal of Electrical Power & Energy Systems, 2020

Abstract Voltage deviation (VD) and voltage flicker (VF) are considered common operational proble... more Abstract Voltage deviation (VD) and voltage flicker (VF) are considered common operational problems associated with high photovoltaic (PV) penetrated distribution systems. In this paper, an optimal control strategy is proposed for minimizing VD and VF in PV-rich distribution systems. The control strategy is based on proposed analytical expressions that minimize both voltage problems by optimizing the smart functions of the PV inverters and control devices simultaneously. The proposed analytical expressions are formulated based on voltage sensitivities with respect to the active and reactive power injections of PV. Specifically, a three-level control strategy with different time resolutions is proposed to significantly alleviate voltage deviation/flicker while minimizing PV active power curtailments and tap movements for transformers. These control levels are (1) local control (LC), (2) area control (AC), and (3) coordinated control (CC). LC provides rapid local control actions to minimize VD and VF, AC minimizes VD within the corresponding area individually, and CC plays a vital role to coordinate between the various control units. The proposed control strategy is assessed using high PV penetration with realistic high-resolution very-variable solar radiation datasets (10 ms). To demonstrate the accuracy and efficiency of the proposed analytical expressions, the calculated results have been compared with existing methods. Results demonstrate that the proposed control strategy effectively coordinates between the various voltage control units while minimizing VD and VF.

A Novel Smart Grid State Estimation Method Based on Neural Networks

International Journal of Interactive Multimedia and Artificial Intelligence, 2018

The rapid development in smart grids needs efficient state estimation methods. This paper present... more The rapid development in smart grids needs efficient state estimation methods. This paper presents a novel method for smart grid state estimation (e.g., voltages, active and reactive power loss) using artificial neural networks (ANNs). The proposed method which is called SE-NN (state estimation using neural network) can evaluate the state at any point of smart grid systems considering fluctuated loads. To demonstrate the effectiveness of the proposed method, it has been applied on IEEE 33-bus distribution system with different data resolutions. The accuracy of the proposed method is validated by comparing the results with an exact power flow method. The proposed SE-NN method is a very fast tool to estimate voltages and re/active power loss with a high accuracy compared to the traditional methods.

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Optimal Allocation of Inverter-Based WTGS Complying with their DSTATCOM Functionality and PEV Requirements

IEEE Transactions on Vehicular Technology, 2020

Recently, the integration of inverter-based wind turbine generation systems (WTGS) and plug-in el... more Recently, the integration of inverter-based wind turbine generation systems (WTGS) and plug-in electric vehicles (PEV) has remarkably been expanded into distribution systems throughout the world. These distributed resources could have various technical benefits to the grid. However, they are also associated with potential operation problems due to their stochastic nature, such as high power losses and voltage deviations. An optimization-based approach is introduced in this paper to properly allocate multiple WTGS in distribution systems in the presence of PEVs. The proposed approach considers 1) uncertainty models of WTGS, PEV, and loads, 2) DSTATCOM functionality of WTGS, and 3) various system constraints. Besides, the realistic operational requirements of PEVs are addressed, including initial and preset conditions of their state of charge (SOC), arriving and departing times, and various controlled/uncontrolled charging schemes. The WTGS planning paradigm is established as a bi-level optimization problem which guarantees the optimal integration of multiple WTGS, besides optimized PEV charging in a simultaneous manner. For this purpose, a bi-level metaheuristic algorithm is developed for solving the planning model. Intensive simulations and comparisons with various approaches on the 69-bus distribution system interconnected with four PEV charging stations are deeply presented considering annual datasets. The results reveal the effectiveness of the proposed approach.

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Optimal Voltage Control Strategy for Voltage Regulators in Active Unbalanced Distribution Systems Using Multi-Agents

IEEE Transactions on Power Systems, 2019

The rapid increase in the installation of renewable energy sources, particularly solar photovolta... more The rapid increase in the installation of renewable energy sources, particularly solar photovoltaic (PV) sources associated with unbalanced features of distribution systems (DS), disturbs the classic control strategy of voltage regulation devices and causes voltage violation problems. This paper proposes an effective control strategy for voltage regulators in the DS based on the voltage sensitivity using a multi-agent system (MAS) architecture. The features of the unbalanced distribution system (UDS) with the PV and different types and configurations of voltage regulators are considered in the proposed strategy. The novelty of the proposed method lies in realizing both the control optimality of minimizing voltage violations and the flexibility to accommodate changes in the DS topology using an MAS scheme. An advantageous feature of using the MAS scheme is the robust control performance in normal operation and against system failure. Simulation studies have been conducted using IEEE 34node and 123-node distribution test feeders considering high PV penetration and different sun profiles. The results show that the proposed voltage control strategy can optimally and effectively manage the voltage regulators in the UDS, which decrease their operation stresses and minimize the overall voltage deviation.

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Optimal Placement and Sizing of Uncertain PVs Considering Stochastic Nature of PEVs

IEEE Transactions on Sustainable Energy, 2019

Recently, the penetration of photovoltaic (PV) units and plug-in electric vehicles (PEVs) has bee... more Recently, the penetration of photovoltaic (PV) units and plug-in electric vehicles (PEVs) has been quickly increased worldwide. Due to the intermittent nature of PV and the stochastic nature of PEVs, several operation problems can be noticed in distribution systems, including excessive energy losses and voltage violations. In this paper, an optimization-based algorithm is proposed to accurately determine the optimal locations and capacities of multiple PV units in the presence of PEVs to minimize energy losses while considering various system constraints. The proposed algorithm considers the uncertainty of PV and loads, and the stochastic nature of PEVs. Furthermore, the operational constraints of PEVs are incorporated in the optimization model: 1) arrival and departure times, 2) initial state of charge (SOC), 3) minimum preset state of charge by the owner, and 4) the time-of-use electricity tariff, and 5) different charging control schemes. The optimal PV planning model is formulated as a two-layer optimization problem that ensures an optimal PV allocation while optimizing PEV charging simultaneously. A two-layer metaheuristic method is developed to solve the optimization model considering annual datasets of the studied distribution systems. The results demonstrate the efficacy of the proposed algorithm.

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Sensitivity-based and optimization-based methods for mitigating voltage fluctuation and rise in the presence of PV and PHEVs

International Transactions on Electrical Energy Systems, 2017

Voltage fluctuation and voltage rise are common issues associated with the massive integration of... more Voltage fluctuation and voltage rise are common issues associated with the massive integration of photovoltaic (PV) technologies in distribution systems. In this paper, sensitivity‐based and optimization‐based methods are proposed for mitigating voltage fluctuation and rise in the presence of plug‐in hybrid electric vehicles (PHEVs). The concept of these methods is to optimize the reactive power of PV inverter and active power from charging station of PHEVs for matching a target voltage profile. The sensitivity‐based method is based on the first‐order power flow sensitivities around the desired voltage profile. An optimization model is used in optimization‐based to minimize the mismatches between the fluctuating and required voltage profiles. The charging/ discharging operation of PHEVs and reactive power of PV inverter are simultaneously controlled for compensating power fluctuation during cloud transients and load fluctuations. The results demonstrate the effectiveness of the proposed methods compared with existing methods.

Fast-yet-Accurate Energy Loss Assessment Approach for Analyzing/Sizing PV in Distribution Systems using Machine Learning

IEEE Transactions on Sustainable Energy, 2018

The penetration of photovoltaic (PV) has obviously been increased in distribution systems through... more The penetration of photovoltaic (PV) has obviously been increased in distribution systems throughout the world. To sufficiently assess the energy losses with PV, comprehensive simulations with high time-resolution data are required. These simulations have a heavy computational burden, which makes it difficult to analyze distribution systems and evaluate PV impacts with fine resolutions. To cope with this issue, most related works down-sample, cluster, or quantize the full data to reduce the computational time on the expense of the accuracy. In this paper, we propose a fast yet accurate energy-loss assessment approach in distribution systems using machine learning. The unique feature of the proposed approach is that it uses all data to estimate losses, which yields accurate results close to the exact solutions in a very short time. The simulation results demonstrate that the proposed approach extremely reduces the computational time of energy-loss estimation with high accuracy rates. The speedup of the proposed approach with respect to power flow simulations for a yearlong at a 30-s time resolution is 28 691 (99.9965$\%$ reduction in computational time). The effectiveness of the proposed approach is also illustrated by applying it to optimize the PV size for minimizing energy losses.

Accurate photovoltaic power forecasting models using deep LSTM-RNN

Neural Computing and Applications, 2017

Photovoltaic (PV) is one of the most promising renewable energy sources. To ensure secure operati... more Photovoltaic (PV) is one of the most promising renewable energy sources. To ensure secure operation and economic integration of PV in smart grids, accurate forecasting of PV power is an important issue. In this paper, we propose the use of long short-term memory recurrent neural network (LSTM-RNN) to accurately forecast the output power of PV systems. The LSTM networks can model the temporal changes in PV output power because of their recurrent architecture and memory units. The proposed method is evaluated using hourly datasets of different sites for a year. We compare the proposed method with three PV forecasting methods. The use of LSTM offers a further reduction in the forecasting error compared with the other methods. The proposed forecasting method can be a helpful tool for planning and controlling smart grids.

Optimal scheduling of electric vehicles considering uncertain RES generation using interval optimization

Electrical Engineering, 2017

The penetration of renewable energy sources (RES) has been increased throughout the world. The ma... more The penetration of renewable energy sources (RES) has been increased throughout the world. The main characteristic of RESs is that their generating powers are intermittent and unpredictable. This paper presents an interval optimization method to optimally schedule electric vehicles (EV) with considering the uncertainty of RES generation and loads. For this purpose, the RES generation (including photovoltaic and wind power) and loads are considered as interval parameters, and the charging/discharging power of EV is expressed as an interval variable to be optimally computed. The capability of RES inverters to regulate voltages is also considered in the interval optimization model. The objective function is to minimize the network active power losses and total voltage magnitude deviation with considering overall system constraints. The proposed method is tested on a 33-bus distribution system with uncertain RESs and loads, and the optimal day-ahead scheduling of EV is performed. Different case studies are carried out to test the effectiveness of the proposed method. It is demonstrated that the proposed interval optimization method can accurately represent the uncertain problem, and it provides further information compared with the deterministic optimization.

Efficient sequential power flow approach based on regression and correction models for active distribution systems

IET Renewable Power Generation, 2017

This study proposes efficient methods for sequential power flow (SPF) analysis of distribution sy... more This study proposes efficient methods for sequential power flow (SPF) analysis of distribution systems with intermittent photovoltaic (PV) units and fluctuated loads. The proposed methods are based on machine learning techniques; more specifically, they use a regression trees (RTs) algorithm to construct a model for voltage estimation. This model is trained using synthetic data generated by a number of PV generation and load demand scenarios. The SPF methods that utilise iterative techniques have a high computational burden. In turn, the proposed method, which is called SPF-RT, is fast and accurate. Furthermore, the authors combine SPF-RT with a correction method to develop a new method, called SPF-RTC, which significantly reduces the estimation error of the RT model. The proposed methods are tested using a 33-bus distribution test system interconnected with two PV units. To assess the performance of the proposed methods, they conducted several experiments at different resolutions of day/year data. The proposed methods are compared with the iterative SPF methods and validated using the OpenDSS software. The simulation results demonstrate that the proposed methods outperform the other methods in terms of the computational speed. The SPF-RT and SPF-RTC methods are useful for real-time assessment of distribution systems with PV units.

Unbalanced distribution power-flow model and analysis of wind turbine generating systems

International Transactions on Electrical Energy Systems, 2012

The paper presents an unbalanced three-phase power-flow model for wind turbine generating systems... more The paper presents an unbalanced three-phase power-flow model for wind turbine generating systems (WTGSs). The model takes into account voltage unbalance factor which exists at the point of common coupling. The developed model is integrated with the unbalanced forward/backward sweep power-flow method. The model comprises of three main components: they are the wind turbine, induction generator, and interface transformer to the grid. Due to their design symmetry, the generator and the transformer are modeled using symmetrical sequence networks. The results show that the developed model has robust convergence characteristics. The solution of the IEEE unbalanced radial feeders shows that the injected powers per phase due to the WTGS are not equal and strongly dependent on the voltage unbalance factor. The results also show that the simplified models based on positive sequence network lead to inaccurate overall power-flow solution.

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Optimal voltage control in distribution systems using PV generators

International Journal of Electrical Power & Energy Systems, 2011

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Implementation of three-phase transformer model in radial load-flow analysis

Ain Shams Engineering Journal, 2013

This paper presents an efficient approach for developing three-phase transformer admittance matri... more This paper presents an efficient approach for developing three-phase transformer admittance matrices in the radial power-flow analysis. The proposed transformer model overcomes the singularity problem of the nodal admittance submatrices of ungrounded transformer configurations. This has been achieved by applying symmetrical components modeling. The classical (6 • 6) transformer nodal admittance matrix written in phase components is converted to sequence components instead of the (3 • 3) admittance submatrices. In this model, the phase shifts accompanied with special transformer connections are included in the radial power-flow solution process without any convergence problems. The final model of the transformer is represented by a generalized power-flow equation written in phase components. The developed equation is applicable for all transformer connections. The transformer model is integrated into the radial power-flow and tested using the IEEE radial feeders. The results have shown that the developed transformer model is very efficient and the radial power-flow has robust convergence characteristics.

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Efficient three-phase power-flow method for unbalanced radial distribution systems

Melecon 2010 - 2010 15th IEEE Mediterranean Electrotechnical Conference, 2010

this paper presents an efficient three-phase power flow algorithm for distribution network analys... more this paper presents an efficient three-phase power flow algorithm for distribution network analysis. A new transformer model with various connections is implemented in the forward/backward sweep power flow method. The developed method provides an effective solution to the singularity problem of the nodal admittance submatrices appeared in some transformer configurations. Different load models and capacitor banks are also implemented with any number of phases and any connection. The proposed load flow has been tested using both the IEEE 4 and 34 node test feeders. The obtained results show that the proposed load flow is very efficient and the numerical solution is identical to that provided with the IEEE data.

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Analysis of hybrid photovoltaic and wind energies connected to unbalanced distribution systems

2010 IEEE International Conference on Power and Energy, 2010

Abstract This paper presents a steady state analysis of allocation of photo-voltaic and wind gene... more Abstract This paper presents a steady state analysis of allocation of photo-voltaic and wind generation units in electrical distribution networks. A complete steady state models for PV energy power generation systems for power flow applications is applied. In addition, a new model for the induction generator for wind generation unit will be presented. These models are driven without any assumption and by taking into consideration complete generation system equivalent circuits parameters. It is noticeable that the input data for these driven ...

Induction generator model for unbalanced distribution power-flow analysis

2010 IEEE Energy Conversion Congress and Exposition, 2010

Abstract The paper presents an accurate procedure for modeling induction generator in steady stat... more Abstract The paper presents an accurate procedure for modeling induction generator in steady state unbalanced distribution power-flow analysis. The proposed induction generator model is based on the steady-state model of the induction machine. The model is developed without any assumptions. The unbalanced factor of the voltage at the machine bus is fully exploited. The model is integrated into the forward-backward sweep power-flow algorithm. Calculated results of induction generator connected to unbalanced radial feeders show ...

Sizing and locating distributed generations for losses minimization and voltage stability improvement

2010 IEEE International Conference on Power and Energy, 2010

Abstract The paper presents analysis of distribution system connected with distributed generation... more Abstract The paper presents analysis of distribution system connected with distributed generations. The study addresses aspects related to optimal sizing and location of DG units for losses minimization and voltage stability improvements. Many cases have investigated to highlight the relationship between the optimum size and location for losses minimization and the optimum size and location for stability improvements. The student version of the AMPL software is used in the proposed study. The objective function is formulated with full ...

Optimal Distributed Generation allocation in distirbution systems employing ant colony to reduce losses

2008 43rd International Universities Power Engineering Conference, 2008

Distribution system planners usually provide dedicated feeders to its different class of customer... more Distribution system planners usually provide dedicated feeders to its different class of customers, each of whom has its own characteristic load pattern which varies hourly and seasonally. A more realistic modeling should be devised by considering the daily and seasonal variations in the aggregate load patterns of different class of customers. This paper addresses a new methodology to provide integrated solution for the optimal allocation of distributed generations and network reconfiguration considering load patterns of customers. The objectives considered are to maximize annual energy loss reduction and to maintain a better node voltage profile. Bat algorithm (BA) is a new bio-inspired search algorithm which has shown an advance capability to reach into the promising region, but its exploration is inadequate. The problem is solved by proposing the improved BA (IBA). The proposed method is investigated on the benchmark IEEE 33-bus test distribution system and the results are very promising.

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Probabilistic Approach for Hosting High PV Penetration in Distribution Systems via Optimal Oversized Inverter With Watt-Var Functions

IEEE Systems Journal, 2021

Download

Machine Learning Based Method for Estimating Energy Losses in Large-Scale Unbalanced Distribution Systems with Photovoltaics

International Journal of Interactive Multimedia and Artificial Intelligence, 2020

Download

Three-level control strategy for minimizing voltage deviation and flicker in PV-rich distribution systems

International Journal of Electrical Power & Energy Systems, 2020

A Novel Smart Grid State Estimation Method Based on Neural Networks

International Journal of Interactive Multimedia and Artificial Intelligence, 2018

Download

Optimal Allocation of Inverter-Based WTGS Complying with their DSTATCOM Functionality and PEV Requirements

IEEE Transactions on Vehicular Technology, 2020

Download

Optimal Voltage Control Strategy for Voltage Regulators in Active Unbalanced Distribution Systems Using Multi-Agents

IEEE Transactions on Power Systems, 2019

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Optimal Placement and Sizing of Uncertain PVs Considering Stochastic Nature of PEVs

IEEE Transactions on Sustainable Energy, 2019

Download

Sensitivity-based and optimization-based methods for mitigating voltage fluctuation and rise in the presence of PV and PHEVs

International Transactions on Electrical Energy Systems, 2017

Fast-yet-Accurate Energy Loss Assessment Approach for Analyzing/Sizing PV in Distribution Systems using Machine Learning

IEEE Transactions on Sustainable Energy, 2018

Accurate photovoltaic power forecasting models using deep LSTM-RNN

Neural Computing and Applications, 2017

Optimal scheduling of electric vehicles considering uncertain RES generation using interval optimization

Electrical Engineering, 2017

Efficient sequential power flow approach based on regression and correction models for active distribution systems

IET Renewable Power Generation, 2017

Unbalanced distribution power-flow model and analysis of wind turbine generating systems

International Transactions on Electrical Energy Systems, 2012

Download

Optimal voltage control in distribution systems using PV generators

International Journal of Electrical Power & Energy Systems, 2011

Download

Implementation of three-phase transformer model in radial load-flow analysis

Ain Shams Engineering Journal, 2013

Download

Efficient three-phase power-flow method for unbalanced radial distribution systems

Melecon 2010 - 2010 15th IEEE Mediterranean Electrotechnical Conference, 2010

Download

Analysis of hybrid photovoltaic and wind energies connected to unbalanced distribution systems

2010 IEEE International Conference on Power and Energy, 2010

Induction generator model for unbalanced distribution power-flow analysis

2010 IEEE Energy Conversion Congress and Exposition, 2010

Sizing and locating distributed generations for losses minimization and voltage stability improvement

2010 IEEE International Conference on Power and Energy, 2010

Optimal Distributed Generation allocation in distirbution systems employing ant colony to reduce losses

2008 43rd International Universities Power Engineering Conference, 2008

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