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In isolated ac microgrids, multiple controllable distributed energy resources (DERs) may simultaneously participate in load frequency control (LFC). To improve system frequency dynamics and reduce the frequency deviation for such a multiple-DER microgrid,this paper presents a novel LFC method based on an optimal reset control (ORC) scheme. The proposed ORC comprises a baseline linear controller and resetting elements, i.e., the integrator outputs, determined by a linear quadratic regulation problem. The overshoot in frequency regulation induced by integral control is reduced, and the settling time is shortened. Besides, the simple structure facilitates its real-time application, making ORC a simple yet effective solution for the LFC of microgrids. By coordinating the reset controllers of all controllable DERs, the ORC further improves the overall frequency control performance. The asymptotic stability of the closed-loop system is theoretically proved. Finally, the effectiveness of the presented LFC method is validated by switch-level simulation studies. [ABSTRACT FROM AUTHOR]

Threshing wheat (Triticum aestivum LO at high speeds is the main reason behind abnormal seedlings and vigor reduction of the seeds. This problem is expected to be severe in headstripper combines with successive impact Ioaclings of stripping and threshing units. The airo of this study was to simulate the effects of impact velocities (IV), number of impact loadings (NL), and seed moisture content (MC) on percentage of physical damage (PPD) and percentage of Ioss in germination (PLG) to wheat seeds. Modeling the correlation between dependent ancI independent variables was performed using mathematical and artificial neural networks (ANN). The result showed that all the three independent variables significantly influenced PPD and PLG (P = 0.01). Increasing the IV from 5 to 30 m [s.sup.-1] caused an increase in PPD and PLG from 0.17 to 35.8% and from 0.37 to 19.9%, respectively. It was found that the seeds with higher MC could better withstand physical and physiological damage than those with Iower MC. With an increase in NL from 1 to 3 times, the mean values of PPD and PLG were increased by 2.9 and 2.6 times, respectively. An ANN model with two hidden layers, trained with a back-propagation algorithm, successfully learned the relationship between the input and output variables. In comparison with regression models, ANN performecI better when predicting PPD and PLG to wheat seeds.

Deployment of photovoltaic generation in smart grids has introduced overvoltage problems. Cyberattackers can take advantage of this situation and cause intentional overvoltages in distribution systems using stealthy false data injections (FDIs). To protect distribution grids from these FDIs, the first step is to model cyberattacks that can cause overvoltages. An accurate nonlinear optimization model is developed for stealthy FDI cyberattacks that can result in overvoltages on targeted nodes in distribution grids without being detected by bad data detection algorithms. Compared to the existing research, which mainly focused on dc state estimation for transmission systems, the proposed model formulates cyberattacks that cause overvoltages using ac state estimation in distribution grids. The proposed nonlinear model is then approximated to a convex optimization model using second-order cone programming relaxation formulation to guarantee that a global optimum exists for the model. Several case studies are considered to validate the effectiveness of the proposed attack model. It is shown that by limiting the FDIs on targeted buses to 20% of their nominal load, multiple buses can experience severe overvoltages in a distribution grid. [ABSTRACT FROM AUTHOR]

A cyberattack detection model based on supervised deep neural network is proposed to identify random false data injection (FDI) on the tank's level measurements of a water distribution system. The architecture of the neural network, as well as various hyper-parameters, is modified and tuned to acquire the highest detection performance using the smallest size of training data set. The efficacy of the proposed detection model against various activation functions including sigmoid, rectified linear unit, and softmax is examined. Regularization and momentum techniques are applied to update the weights and prohibit overfitting. Moreover, statistical metrics are presented to evaluate the performance and effectiveness of the proposed model in the presence of a range of measurement noise levels. The proposed model is tested for three attack scenarios composed for the battle of the attack detection algorithms. Results confirm that the size of the data sets required to train the neural network (NN) to accomplish the highest levels of accuracy and precision is significantly decreased as the number of hidden layers is increased. The trained 4- and 5-layer deep neural networks are able to detect the readings' FDIs with 100% precision and accuracy in the presence of 30% background noise in the sensory data. [ABSTRACT FROM AUTHOR]

Several methods have been developed to measure the friction coefficient of grains and seeds as well as their angle of repose, independently. However, it seems that one single method capable of simultaneous measurement of these two important parameters of grain products is of importance. In this study, simultaneous measurement of brown rice grain friction coefficient and angle of repose is performed by using rotating cylinders method. The effects of rotational speed, cylinder diameter and filling degree on rice grains, also lower and higher angle of repose along with their friction properties were considered. Results showed that proper speed for reaching slumping and rolling motions depends on cylinder diameter and filling degree varying from 0.5 to 9.7 rpm and 2 to 34.6 rpm, respectively. An insignificant difference in the range of 1° to 4° was observed between lower and upper angles of repose which indicates the reliability of the proposed method to determine the dynamic angle of repose. Results also revealed that rotational speed of cylinders did not significantly affect the lower and upper angles of repose; however, they were not affected by filling degree. Finally, the friction coefficient of rice grains was affected by cylinder diameter and filling degree. The findings of this study may be applied in the design and mathematical modeling of grains' motions in rotary dryers, mixers, and silos. [ABSTRACT FROM AUTHOR]

This article proposes a framework as a remedial action scheme (RAS) for close to real-time system operation when system operator becomes aware of possibility of decentralized outages due to simultaneous attacks on several market retailers. The upper level of the framework is an offline RAS where system operator immediately implements a customized reconfiguration preventing the decentralized outages. Due to immediate implementation of upper-level reaction, it is possible to face decentralized congestions in the new topology having minimum reliance on the attacked market retailers. Since severity of congestions is less than that of outages, the lower level of the framework is implemented as an online RAS. In this level, eligible nonattacked market retailers are invited into a secondary market to compete for generation/consumption redispatch and contribute into alleviation of congestions with the purpose of increasing their income. The effectiveness of the proposed framework is validated on a well-known 136-bus distribution system. [ABSTRACT FROM AUTHOR]

Distinct characteristics of transmission and distribution power grids typically necessitate different control algorithms, especially at the primary control level where quick responses are required. Accordingly, various droop control schemes have been developed due to specific types of power grid impedances. Therefore, controllers at higher levels often have to be redesigned whenever a different power grid is considered, hence, incurring higher cost and effort. As a way to overcome that problem, in this work, a unified secondary controller using distributed control theory is designed for different droop schemes associated with distributed battery storage in different grid conditions. The control design follows the consensus theory, originally designed for multi-agent systems, to control the frequency and voltage at the point of common coupling (PCC), synchronize energy levels, and proportionally share active and reactive powers of battery storage systems. A sufficient condition for the upper bound of communication delays between storage systems is derived to ensure system stability. Several scenarios are studied using a modified IEEE 118-bus benchmark to support the theoretical results of the proposed approach. [ABSTRACT FROM AUTHOR]

Energy, building, and water networks are three interlinked critical infrastructures that need to be operated cooperatively to maximize the smart grid’s economic benefits. In this paper, a mixed-integer linear programming (MILP) formulation is proposed to approach the economic dispatch (ED) problem for smart grids embedded with interdependent water and energy networks. Energy management of various building applications is considered by intelligently controlling the indoor temperature during occupied and unoccupied hours. To optimize the demand of water distribution system, pump’s nonlinear scheduling and hydraulic factors and daily water usage of buildings are added to the proposed model. Piecewise linear approximation of univariate and bivariate nonlinear functions is used to convert the nonlinear problem to an MILP formulation. Several case studies were conducted to examine the impact of indoor temperature settings of the buildings, speed of pumps, battery efficiency, and end of day (EoD) battery and tank constraints on economic dispatch of the microgrid system. [ABSTRACT FROM AUTHOR]

Hybrid storage system composed of battery energy storage systems (BESSs) and supercapacitors is a promising solution to mitigate the high frequency power fluctuations of pulsed power loads (PPLs) in medium voltage DC (MVDC) shipboard power systems. Due to the presence of multiple storage units, a power sharing algorithm needs to be considered within the energy management system (EMS) in the shipboard power system. In this paper, an optimal power flow problem is formulated for MVDC shipboard power systems with hybrid energy storage systems. Battery energy storage systems (BESSs) and conventional generation units are enhanced with virtual resistive droop controllers to share the steady-state power fluctuations. Supercapacitors are enhanced with virtual capacitive droop controllers to share the high frequency fluctuations in the load. The optimal flow accounts for minimizing the operational cost of the generation units in the shipboard power system considering the constraints of network, load balance, voltage profile, and power/energy limits. Second-order cone programming (SOCP) relaxation is used to approximate the nonconvexity of the optimal flow formulation and necessary conditions for global optimality of the solution are discussed. Results confirm the effectiveness of the proposed formulation in load support and managing the energy between storage units. [ABSTRACT FROM AUTHOR]

This paper investigates the design and analysis of a robust nonlinear controller for voltage source converters (VSCs) connected to weak AC grids. Feedback linearization method is used to develop the control inputs that regulate the active power and output voltage of the grid-connected converter. The frequency-domain model of an LCL filter in $dq$ -frame is used to develop the nonlinear control blocks for small-signal stability analysis. Impact of phase-locked loop (PLL) is added to the control block through feedforward loops. It is shown that unlike vector controlled converters, the proposed controller is not negatively affected by the PLL gains and a broader range of operation is achieved. In addition, the designed controller is robust against sudden active power commands from 0 to 1 p.u., and is capable of fault ride through when AC grid is very weak. The robustness of the proposed controller is evaluated using eigenvalue analysis of the linearized closed-loop model and time-domain simulations of a grid-connected VSC in various cases. [ABSTRACT FROM AUTHOR]

Developing attack models is the first step to understand cyberattacks in smart grids and develop countermeasures. In this article, a three-level nonlinear programming formulation is proposed for false data injection (FDI) cyberattacks that could result in multiple transmission line congestions without being detected by conventional bad data detection (BDD) algorithms. The model is then converted to a mixed integer linear programming (MILP) formulation to guarantee a global optimum exists. A detection framework based on recursive least-square estimation (RLSE) is developed that can successfully detect the stealthy FDIs. The developed model with the detection framework is validated through various case studies in IEEE 118-bus benchmark. [ABSTRACT FROM AUTHOR]

This article proposes a distributed consensus-based demand response control for permanent magnet synchronous generator (PMSG)-based wind turbines using standalone distributed battery energy storage systems (BESSs). The proposed controller cooperatively regulates the output power of individual wind turbines and BESSs in a wind farm to deliver active and reactive powers to the load in varying wind speed conditions. Capability curves of the wind turbines and BESSs are considered in the design. In addition, a virtual leader is designed to regulate the voltage and frequency of the combined PMSG-BESS units at the point of common coupling. Simulations on modified IEEE 14-bus power system are performed to validate the proposed design. [ABSTRACT FROM AUTHOR]

In this study, physical properties of grain and cluster straw including geometric dimensions, moisture absorption and aerodynamic properties of two wheat cultivars were investigated. The effect of cultivar on width, thickness, geometric diameter, spheroid coefficient and mass density was significant at 1% probability level whilst there were no significant effect on grain projected area, length and weight. Moisture absorption of the grains was rapid during the first 30–40 min and then turned to zero. The terminal velocity of wheat grain and straw in three shapes was measured by calculating the projected area in horizontal, lateral and perpendicular directions and then, the drag coefficient was obtained by the equilibrium of the gravity and drag forces at the terminal velocity. Results also showed that increasing moisture content resulted in an increase in the terminal velocity and a decrease in the drag coefficient. Results obtained in this study can be used in designing wheat- cluster straw separation and processing. [ABSTRACT FROM AUTHOR]

Renewable energy sources normally provide low inertia, which allows a little time for controlling the system. The inertia emulation has recently been introduced for photovoltaic (PV) generation to fight against the small inertia and improve the energy efficiency. The main purpose of this article is to analyze the stability of a virtual inertia-based three-phase two-stage PV system for smart grid applications. Dynamics of DC/DC converter, DC/AC converter, AC side filter, virtual inertia controller, and controllers of the DC/DC and DC/AC converters are considered in the modeling procedure. Eigenvalue analysis is performed to evaluate the sensitivity of the design parameters on stability of a grid-connected virtual inertia PV system. Time-domain simulations using MATLAB/Simscape Power System toolbox are used to validate the eigenvalue analysis results. [ABSTRACT FROM AUTHOR]

This paper proposes a day-ahead market framework for congestion management in smart distribution networks. The presented scheme provides a platform for collaboration between distribution-level market operator (DMO) and data traffic operator (DTO) to alleviate congested feeders such that data transmission traffic between market participants is effectively managed in a smart grid. In addition, a decentralized mechanism is developed for collaboration of electric vehicle (EV) aggregators with common clients to take advantage of EVs not only as flexible loads but also as mobile distributed storage (MDS) for congestion management. Moreover, the proposed framework outlines an administrative action for distribution system operator (DSO) to support the market when the decentralized competitions among distributed generation (DG) aggregators and EV aggregators do not fully relieve a serious congestion. The proposed day-ahead congestion management scheme is validated on an unbalanced 136-bus distribution system massively integrated with wind turbine DGs (WTDGs), photovoltaic DGs (PVDGs), diesel-engine DGs (DEDGs), and EVs. [ABSTRACT FROM AUTHOR]

Voltage source converter (VSC) based high-voltage direct current (HVDC) system is an emerging technology in high-power applications. Previous research has shown that VSCs, with commonly used vector current control (VCC) technique, cannot transmit more than 0.4 p.u. of active power when connected to a very weak ac grid. This paper presents a virtual synchronous generator (VSG) control strategy integrated with VCC to allow stable power transfer for very weak ac grid integration. Impedance analysis is used to analyze the designed controller in various scenarios. In addition, sensitivity analysis is performed to study the effect of virtual inertia gains on system stability. Time-domain simulation results using MATLAB/Simscape Power System toolbox show the effectiveness of the developed controller in very weak ac grid integration of VSC-HVDC systems. [ABSTRACT FROM AUTHOR]