Loads In Microgrid Research Articles

Modeling of Double Stage Photovoltaic Inverter System with Fast Delayed Signal Cancellation for Fault Ride-Through Control Application in Microgrids

This research presents a secondary control for a grid-supporting microgrid with photovoltaics sources to guarantee grid code compliance and ancillary services. The secondary control accomplishes the fault ride-through, which implements a delayed signal cancellation (DSC) algorithm for negative sequence detection. Without mode switching, the proposed control strategy meets grid code requirements and ensures voltage regulation at the secondary level, which is active and more salient throughout the transient period of host grid disturbances. This control also ensures a constant supply of the microgrid’s sensitive local load while adhering to grid code requirements. Similarly, active power injection into the main grid is limited by progressively altering the MPPT operating point dependent on the depth of voltage sag to optimize reactive power injection to sustain grid voltage sag. The recommended secondary control is triggered by utilizing the DSC process’s detection algorithm to identify the occurrence of a fault in a tiny fraction of a half-cycle in a grid fault. Consequently, while satisfying microgrid load needs, the devised technique guaranteed that increases in DC-link voltage and AC grid current were controlled. MATLAB Simscape ElectricalTM and OPAL-RT Lab are used to do time-domain simulations of the model using the recommended secondary control systems.

Modified Topology and Modulation Technique for Z-Source Neutral-Point Clamped Inverter

Abstract The impedance network makes it possible to increase and decrease the voltage, which is not available in normal inverters (voltage and current sources).This paper presents a modified topology and modulation technique for a three-phase Modified Z-Source Neutral-Point Clamped (MZS-NPC) inverter. A modulation scheme for the proposed topology is designed based on maximum gain control method to achieve the maximum voltage gain by simple implementation and balancing the neutral point voltage of the dc link. In order to supply the desired voltage to the critical load in an islanded micro-grid, a closed-loop ac voltage controller is realized in fuel cell or photovoltaic applications based on the proposed inverter. The ability to reinforce and validity of topology operations and modulation techniques has been demonstrated by simulation. It should be noted that the simulations are implemented in MATLAB / Simulink software.

Improvement Strategy for Voltage Quality of Microgrid Based on Master-Slave Control

Due to the nonlinear and unbalanced load of micro-grid, the output voltage quality on the main inverter side is easily affected in the micro-grid based on Master-Slave control.So, this paper put up a strategy that compensating the harmonic current and negative-sequence current by subordinate inverter. This strategy makes the current that flow through main inverter only have fundamental positive sequence component, so as to achieve the aim of improving the quality of the output voltage. Finally, the simulation model is built in the PSCAD simulation environment, which shows the effectiveness of the strategy.

An Improved Hybrid Micro-Grid Load Frequency Control Scheme for an Autonomous System

This paper presents an improved hybrid micro-grid load frequency control scheme for an autonomous system. The micro-grid system comprises of renewable and non-renewable energy-based Power Generating Units (PGU) which consist of Solar Photovoltaic, WT Generator, Solar Thermal Power Generator, Diesel Engine Generator, Fuel Cell (FC) with Aqua Electrolizer (AE). However, power produce from renewable sources in microgrid are intermittent in supply, hence make it difficult to maintain power balance between generated power and demand. Therefore, Battery energy storage system, ultra-capacitor and flywheel energy storage systems make up the energy storage units. These separate units are selected and combined to form two different scenarios in this study. This approach mitigates frequency fluctuations during disturbances (sudden load changes) by ensuring balance between the generated power and demand. For each scenario, Moth flame optimization algorithm optimized Proportional-Integral controllers were utilized to control the micro-grid (to minimize fluctuations from the output power of the non-dispatchable sources and from sudden load change). The results of the developed scheme were compared with that of Quasi-Oppositional Harmony Search Algorithm for overshoot and settling time of the frequency deviation. From the results obtained, the proposed scheme outperformed that of the quasi-oppositional harmony search algorithm optimized controller by an average percentage improvement of 35.95% and 28.76% in the case of overshoot and settling time when the system step input was suddenly increased. All modelling analysis were carried out in MATLAB R2019b environment. Keywords—Frequency Deviation, Micro-grid, Moth flame optimization algorithm, Quasi-Oppositional Harmony Search Algorithm.

Peer-to-peer multi-energy distributed trading for interconnected microgrids: A general Nash bargaining approach

Energy sharing has significant potential in improving system energy utilization, improving economic efficiency, and reducing the impact on power system. The multi-microgrids distributed coordinated operation model is proposed in this paper to provide a reference for the coordinated operation of multi-microgrids considering factors such as electric and heat demand response, multi-energy interaction and uncertainties. First of all, the system operation framework is introduced and the function of each part is analyzed. Secondly, the multi-microgrids cooperative operation model is established, and generalized Nash bargaining method is used to assign total operating cost to ensure the enthusiasm of each microgrid to participate in cooperation. Then, considering the uncertainty of wind power and load in microgrids, a stochastic programming model based on conditional value-at-risk is established. Furthermore, alternate direction multiplier method is used to solve the proposed model to protect the data privacy of each microgrid participating in cooperation. Finally, through the analysis of calculation example, it is verified that the proposed model can reduce the operating cost of each microgrid and maintain enthusiasm of each microgrid for cooperation. In addition, the convergence and accuracy of algorithm are explained, and the influence of different connection modes and the interaction limit of electric and thermal power on the operating cost of microgrids is compared. Moreover, stochastic programming model also provides a reference plan for the trade-off between economic benefits and risk level of multi-microgrids system.

Optimal Battery Energy Storage System Scheduling within Renewable Energy Communities

In this work, a strategy for scheduling a battery energy storage system (BESS) in a renewable energy community (REC) is proposed. RECs have been defined at EU level by the 2018/2001 Directive; some Member States transposition into national legislation defined RECs as virtual microgrids since they still use the existing low voltage local feeder and share the same low-medium voltage transformer. This work analyzes a REC which assets include PV generators, BESS and non-controllable loads, operating under the Italian legislative framework. A methodology is defined to optimize REC economic revenues and minimize the operation costs during the year. The proposed BESS control strategy is composed by three different modules: (i) a machine learning-based forecast algorithm that provides a 1-day-ahead projection for microgrid loads and PV generation, using historical dataset and weather forecasts; (ii) a mixed integer linear programming (MILP) algorithm that optimizes the BESS scheduling for minimal REC operating costs, taking into account electricity price, variable feed-in tariffs for PV generators, BESS costs and maximization of the self-consumption; (iii) a decision tree algorithm that works at the intra-hour level, with 1 min timestep and with real load and PV generation measurements adjusting the BESS scheduling in real time. Validation of the proposed strategy is performed on data acquired from a real small-scale REC set up with an Italian energy provider. A 10% average revenue increase could be obtained for the prosumer alone when compared to the non-optimized BESS usage scenario; such revenue increase is obtained by reducing the BESS usage by around 30% when compared to the unmanaged baseline scenario.

Multi-objective load dispatch for microgrid with electric vehicles using modified gravitational search and particle swarm optimization algorithm

With the increasing proportion of electric vehicles in the automobile market, the negative impact of vehicle’s charging on the power system is gradually increasing. The charging-discharging model of vehicles and the multi-objective optimization model of the load dispatch for the microgrid are established. By combining gravitational search algorithm (GSA) and particle swarm optimization (PSO) algorithm, a hybrid modified GSA-PSO (MGSA-PSO) scheme is proposed to optimize the load dispatch of the microgrid containing electric vehicles. To improve the global search performance of the GSA algorithm, the proposed scheme introduces the global memory capacity of the PSO into the GSA. At the same time, the hybrid algorithm is improved by designing adaptive inertia vector, learning factor and chaotic initialization population. The load dispatch optimization are implemented and analyzed, including the unordered charging strategy, the ordered charging-discharging strategy, and the ordered charging-discharging strategy with distributed generations. The optimization results show that, under the same weight factor, the ordered charging-discharging strategy can reduce 13.38% of the total cost, 78.77% of the microgrid load variance and improve the safety and economy of the grid. In addition, reasonable scheduling of distributed power output power can further reduce the total cost by 14.06% and the load variance by 22.36%. Further, the effectiveness of the proposed scheme is proved by analyzing the influences of different numbers of electric vehicles and different charging models.

Dynamic modeling and transient stability analysis of distributed generators in a microgrid system

Increasing the penetration level of distributed generation units as well as power electronic devices adds more complexity and variability to the dynamic behaviour of the microgrids. For such systems, studying the transient modelling and stability is essential. One of the major disadvantages of most studies on microgrid modelling is their excessive attention to the steady state period and the lack of attention to microgrid performance during the transient period. In most of the research works, the behaviour of different microgrid loads has not been studied. One of the mechanisms of power systems stability studies is the application of state space modelling. This paper presents a mathematical model for connected inverters in microgrid systems with many variations of operating conditions. Nonlineal tools, phase-plane trajectory analysis, and Lyapunov method were employed to evaluate the limits of small signal models. Based on the results of the present study, applying the model allows for the analysis of the system when subjected to a severe transient disturbance such as loss of large load or generation. Studying the transient stability of microgrid systems in the standalone utility grid is useful and necessary for improving the design of the microgrid’s architecture.

Multifunction Capability of SiC Bidirectional Portable Chargers for Electric Vehicles

A SiC bidirectional portable charger is proposed to realize both bidirectional ac–dc and bidirectional dc–dc high-efficiency power conversion by multiplexing the same powertrain. Control strategies for different operation modes are proposed to realize the comprehensive functions of the portable charger. The proposed portable charger enables the EV to interact with the ac grid, dc microgrid, other EVs, and off-line loads efficiently and conveniently. The SiC portable charger adopts 300-kHz high-frequency isolation to improve the power density. The proposed vehicle-to-vehicle (V2V) mode adopts two-stage direct dc–dc power conversion so that it achieves higher efficiency compared with conventional V2V mode using an on-board charger (OBC). A 6.6-kW SiC bidirectional portable charger is built and the power density is 14.7 W/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> . The full-load efficiency of V2V mode is 96.1%, which is 4.7% improved compared with the conventional V2V mode. Bidirectional energy interaction between EV and the 380-V dc microgrid is also verified. The portable charger provides 36–400-V wide output voltage for dc loads with the efficiency above 95.7% at 6.6 kW.

A Center of Mass Determination for Optimum Placement of Renewable Energy Sources in Microgrids

The objective of this article is to determine the location and size of the renewable energy sources (RESs) to be installed in microgrid networks that have multiple distributed generators. Since RESs have faster dynamics and low inertia, microgrid sources could encounter large disturbances in their power production when RESs are deployed. When the disturbance magnitude exceeds a certain limit, the stable operation of the microgrid can be compromised. In this study, the impact of installing a certain RES capacity on a microgrid is numerically evaluated. A novel methodology is developed to accurately determine the location and size of a newly installed RES in a microgrid. The proposed methodology considers the complete system dynamics in the process of locating and sizing the installed RES. The stability of the microgrid is examined using a modified microgrid load flow model. The proposed method automatically checks for the optimum RES placement to ensure minimal effect on microgrid stability. The effectiveness of the proposed methodology is verified experimentally on a microgrid testbed.

On the Assessment of Cyber Risks and Attack Surfaces in a Real-Time Co-Simulation Cybersecurity Testbed for Inverter-Based Microgrids

The integration of variable distributed generations (DGs) and loads in microgrids (MGs) has made the reliance on communication systems inevitable for information exchange in both control and protection architectures to enhance the overall system reliability, resiliency and sustainability. This communication backbone in turn also exposes MGs to potential malicious cyber attacks. To study these vulnerabilities and impacts of various cyber attacks, testbeds play a crucial role in managing their complexity. This research work presents a detailed study of the development of a real-time co-simulation testbed for inverter-based MGs. It consists of a OP5700 real-time simulator, which is used to emulate both the physical and cyber layer of an AC MG in real time through HYPERSIM software; and SEL-3530 Real-Time Automation Controller (RTAC) hardware configured with ACSELERATOR RTAC SEL-5033 software. A human–machine interface (HMI) is used for local/remote monitoring and control. The creation and management of HMI is carried out in ACSELERATOR Diagram Builder SEL-5035 software. Furthermore, communication protocols such as Modbus, sampled measured values (SMVs), generic object-oriented substation event (GOOSE) and distributed network protocol 3 (DNP3) on an Ethernet-based interface were established, which map the interaction among the corresponding nodes of cyber-physical layers and also synchronizes data transmission between the systems. The testbed not only provides a real-time co-simulation environment for the validation of the control and protection algorithms but also extends to the verification of various detection and mitigation algorithms. Moreover, an attack scenario is also presented to demonstrate the ability of the testbed. Finally, challenges and future research directions are recognized and discussed.

Load prioritization technique to guarantee the continuous electric supply for essential loads in rural microgrids

Microgrid (MG) is one of the practical and best concepts to provide energy access to rural communities, where electric grid extension is not techno-economically feasible. Since the trend of load consumption is not uniform with a low load factor in a rural area, the required rating of the system becomes very high. Similarly, the generation is fixed for these MGs, whereas the load increases continuously over time. Such a system faces supply deficit issues triggering a high number of interruptions that may cause frequent blackouts. Hence, rolling blackout and load clipping techniques are preferred during the peak load period in most of the rural MGs. These issues lead to an unreliable power supply and low satisfaction level of the user. This paper presents the load prioritization technique to guarantee the continuous supply for the essential loads within the rural community. A day-ahead energy allocation technique is mathematically formulated and optimized to maximize the total hours of energy served. This technique maximized the hours of energy served to the load with higher priority followed by the load with lower priorities. From this study, it is found that the proposed strategy helps to improve the hours of energy served in the overall system, by improving the state of charge (SoC) level of the battery system. The result shows that the user satisfaction level has been improved by 5% through 100% of continuity for the essential loads.

An adaptive method for accurate sharing of harmonic and unbalanced loads in islanded microgrids

An adaptive method for accurate sharing of harmonic and unbalanced loads in islanded microgrids

Research on Optimal Operation Strategy of Distributed Energy Based on Distribution Network Planning

Distributed energy is an energy supply mode based on load demand side. It aims at maximizing energy utilization and environmental benefits, integrates and optimizes various energy demands and resource allocation of users, and adopts a new energy system with demand response design and modular allocation. Because most of the energy it consumes is renewable and clean, it is environmentally friendly. Energy community is an important part of active distribution system. The method of power grid planning is studied and analyzed, and the energy management model of energy community is established. On this basis, the active distribution system is studied comprehensively and solidly. The research results can effectively improve the economy and reliability of active distribution system, and promote the large-scale grid connection of renewable energy. By using the heat storage system to periodically transfer heat load, the contradiction between the ratio of electric load to heat load in microgrid and the electric heat ratio of cogeneration system can be alleviated, thus realizing efficient and economical electric heat dispatching.

A Secondary Control Method for Voltage Unbalance Compensation and Accurate Load Sharing in Networked Microgrids

Interconnection of microgrids (MGs) provides the opportunity of increasing loadability and enhancing reliability by allowing power exchange among individual MGs. Control of networked MGs requires coordination of distributed energy resources (DERs) within each MG such that the sum of the MG's load and exported power is shared proportionally among DERs while maintaining power quality. This paper proposes a new hierarchical control scheme for accurate load sharing and power quality enhancement in networked MGs with unbalanced load currents. The proposed scheme is comprised of local DER controllers, MG control units, and Networked MG control center. The MG control units adjust the power exchange at the value specified by the control center and regulate the voltage at the point of common coupling (PCC) by means of a secondary control signal. At the DER level, droop control is adopted to enable load sharing with fast dynamic response. The droop controller output is combined with the secondary control signal and a correction voltage which eliminates the effect of line impedances on the current sharing accuracy. This way, accurate sharing of individual phase currents is realized without requirement of data exchange among DERs. Simulation and experimental results showcase the effectiveness of the proposed scheme.

Control of Electrodialysis Desalination Systems as Smart Loads in Microgrids with High Penetration of Renewable Generation

Water desalination systems connected to microgrids with high penetration of renewable energy generation are frequently used to promote the development of remote areas. These microgrids often have power quality and even stability problems. This work shows that electrodialysis desalination systems can be managed as smart loads, that is, they can contribute to the power balance and voltage regulation of the microgrid without neglecting its main function of water desalination. For this, a model of multiple inputs and multiple outputs for a desalination system is proposed where the variables to control are the treated water concentration and the active and reactive powers demanded by the desalination system. Based on this model, a control law is proposed that allows to face the complexity of the non-linear system in a simple and precise way. The proposed control guarantees the low salt concentration of the drinking water and favors the energy balance of the microgrid, allowing better control of the power quality and greater penetration of renewable energy generation.

Mathematical modeling of islanded microgrid with static and dynamic loads

Increasing the penetration level of distributed generation units as well as power electronic devices adds more complexity and variability to the dynamic behavior of the microgrids. For such systems, studying the transient modelling and stability is essential. One of the major disadvantages of most studies on microgrid modelling is their excessive attention to the steady state period and the lack of attention to microgrid performance during the transient period. In most of the research works, the behavior of different microgrid loads has not been studied. One of the mechanisms of power systems stability studies is the application of state space modelling. These studies include the development of state space models of various components of the power system and then linearizing them around an equilibrium point. In this paper, a comprehensive method for modelling of islanded microgrid with dynamic and static loads is presented. The basic step of the proposed method is transformation to a dq0-based model. In order to find a complete and accurate model of islanded inverter-based microgrid, the sub-modules of generation, network and load must be modelled in local dq reference and then transferred to a common reference. The simulation results show the effectiveness of the proposed modelling approach for transient stability studies.

Frequency Response of Motor Drive Loads in Microgrids

Islanded microgrids require power reserve with fast primary-frequency response to operate stably. Primary-frequency response from generators or batteries incurs high costs. Alternatively, primary-frequency responsive loads reduce costs by allowing generators and batteries to operate more flexibly. In industrial microgrids, which comprise a significant portion of the microgrid market, electrical motors are a significant portion of the total load. However, existing demand response mechanisms are not suitable for motor loads in microgrids. In this article, a novel grid-supporting motor drive controller is proposed. This controller can be retrofitted to motor drive loads to provide fast primary-frequency response. A reduced-order analytical model of the motor drive load is derived, which can be directly integrated into system-wide frequency stability studies. Additionally, the primary-frequency response time of the motor drive is assessed, analytically and through EMT simulation, and is less than <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\text{100 ms}$</tex-math></inline-formula> , which is comparable to large battery energy storage systems. Finally, the behaviour of a microgrid system that includes two large motor drive loads is assessed during various large disturbances through EMT simulation.

Impedance-Shaping-Based Stability Control of Point-of-Load Converter Integrated With EMI Filter in DC Microgrids

The point-of-load (POL) converters integrated with the input <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LC</i> filter are essential to supply the ac load in the dc microgrid. The interaction between the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LC</i> filter and the POL converter will bring instability problems to the dc-link voltage. This paper proposes a series-connected virtual impedance regulator based on the detailed small-signal model of the POL converter that feeds a linear three-phase load. The proposed method can improve the stability by modifying only the phase of the input impedance of the POL converter within the interaction frequency range. Therefore, the impact of the regulator on the converter’s dynamic performance is minimized. The experiments show that the proposed stabilization methods exhibit good performance under different linear load scenarios.

Assessment of the Impact of Electric Vehicle Batteries in the Non-Linear Control of DC Microgrids

Several efforts need to be performed in transportation and energy production to mitigate the current environmental issues that are related to fossil fuel use. The implementation of DC microgrids and the use of electric vehicles seem to be an adequate solution. However, various technical challenges have to be addressed, like grid stability issues. Thus, this case report assesses the impact of an electric vehicle load in a DC microgrid, subject to nonlinear control theory. The EV battery pack is modeled and simulated. Subsequently, it is included as a load in an available model of nonlinear control of DC microgrids. The results demonstrate high stability with this new load and the feasibility of its implementation.