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Optimal power flow for converter-dominated AC/DC hybrid microgrids
Abstract
This paper presents a new optimal power flow (OPF) formulation based on loadability maximization for islanded converter-dominated AC/DC hybrid microgrids. Hybridizing AC and DC at the distribution level brings the merits of AC and DC together as a valuable future layout for AC and DC technologies. Nevertheless, most recent AC/DC distributed resources and loads are converter-based resulting in low inertia. Further, the future distribution systems will allow investors and customers to plug their energy resources in and out. Such systems with a high penetration of converters and plug-and-play capability will have their own operational philosophy. During islanding in particular, loadability maximization is more pronounced due to the limited resources and being susceptible to any sudden and slow load/supply variations. Thus, the target of the system operator might be to increase the system steady-state stability margin by running OPF. In this work, the AC/DC OPF problem is formulated as a nonlinear constrained optimization problem, and solved by Interior Point method. The newly formulated OPF algorithm is tested on a modified 38-bus AC/DC hybrid microgrid. The developed AC/DC OPF can be a powerful tool for system planners and operators to explore the technical and economic challenges related to hybridizing the AC distribution systems.
... The difference of both values in Equations (4) and (5) will be the input of the proportional controller, where the gain parameters are 1/R p1 for the frequency deviation input and 1/R p2 for the DC voltage deviation input. The difference of both control outputs generates the active power reference of IC P IC as described in Equation (6). For the generated P IC sign, power flow from DC to AC subgrids is assumed as the positive power injection, and the opposite way is the negative power injection, as shown in Figure 6: ...
... Since both parameters have different units, the measurements need to be normalized in per unit (p.u.) as given in Equations (4) and (5): (4) and (5) will be the input of the proportional controller, where the gain parameters are 1/ for the frequency deviation input and 1/ for the DC voltage deviation input. The difference of both control outputs generates the active power reference of IC as described in Equation (6). For the generated sign, power flow from DC to AC subgrids is assumed as the positive power injection, and the opposite way is the negative power injection, as shown in Figure 6: The droop characteristics of IC droop control in this paper are designed as in Figure 7. ...
... In other words, when power imbalance happens in the system, and the battery is able to charge or discharge the power based to the voltage deviation in DC subgrid, the reference of IC power injection is only determined by the deviation in AC subgrid frequency. Meanwhile, if battery is not able to operate because of its SOC level, IC is managed to share the power proportionally between both subgrids by balancing the frequency and DC voltage deviations as described in (6). Such scheme can be applied by varying the gain parameters of the IC droop control in accordance to the battery operation mode, as described in Equation (8): ...
A hybrid alternate current/direct current (AC/DC) microgrid consists of an AC subgrid and a DC subgrid, and the subgrids are connected through the interlink bidirectional AC/DC converter. In the stand-alone operation mode, it is desirable that the interlink bidirectional AC/DC converter manages proportional power sharing between the subgrids by transferring power from the under-loaded subgrid to the over-loaded one. In terms of system security, the interlink bidirectional AC/DC converter takes an important role, so proper control strategies need to be established. In addition, it is assumed that a battery energy storage system is installed in one subgrid, and the coordinated control of interlink bidirectional AC/DC converter and battery energy storage system converter is required so that the power sharing scheme between subgrids becomes more efficient. For the purpose of designing a tracking controller for the power sharing by interlink bidirectional AC/DC converter in a hybrid AC/DC microgrid, a droop control method generates a power reference for interlink bidirectional AC/DC converter based on the deviation of the system frequency and voltages first and then interlink bidirectional AC/DC converter needs to transfer the power reference to the over-loaded subgrid. For efficiency of this power transferring, a linear quadratic regulator with exponential weighting for the current regulation of interlink bidirectional AC/DC converter is designed in such a way that the resulting microgrid can operate robustly against various uncertainties and the power sharing is carried out quickly. Simulation results show that the proposed interlink bidirectional AC/DC converter control strategy provides robust and efficient power sharing scheme between the subgrids without deteriorating the secure system operation.
... The switching modes of the microgrid need better controlling techniques to maintain optimal power flow within the system. Optimal power flow controller play a huge role in microgrid systems to balance the switching between grid-connected and islanded modes in the system [5]. The OPF controller helps the system improve power flow management within the microgrid boundaries, and it can perform independently in any fault condition. ...
In recent years, there has been a substantial increase in energy production from renewable sources worldwide. In rural areas, microgrids are suitable to support the power supply for the appliances. Solar PV systems have been integrated with the grid to help reduce energy costs. This paper studies an optimal power flow (OPF) algorithm-based controller to enhance power system stability. In microgrid systems, if the solar PV cannot produce enough power to satisfy the load demand, the main grid supplies the necessary power to ensure continued operation. The process requires a controller for continual switching between grid-connected and islanded modes in microgrid. The OPF controller is designed to provide the optimal switching method, confirming a perfect transition between these modes. An AC/DC hybrid microgrid system with AC linear load is used to analyze the performance of the OPF controller in the system. The system contains an OPF controller, a linear load, solar PV with an MPPT controller, and a main grid. The OPF controller receives information from the microgrid system, processes it, and provides a reference value between 1 and 0 for the grid switch to control the microgrid’s switching mode. The proposed method was analyzed using MATLAB/Simulink, and the results confirmed the optimal power flow within the microgrid system.
... In [188], a new Dynamic Power Routing (DPR) technique is presented for HMGs operating in islanded mode, which, if integrated with a supervisory controller, will minimize load-shedding and considerably improve system reliability. In [189], the authors analyze the increase in stability in an HMG through the Optimum Power Flow (OPF) using the Interior Point (IP) method, which is employed to discover the issues (technical and economic) associated with an HMG and results in the optimization of system parameters and the interconnection of PCs to maximize load capacity. In [190], the profits of an HMG are improved by implementing the spot price method and, thus, achieving an economic benefit (increased revenue and reduced operating costs) in the operation of the HMG. ...
Microgrids (MGs) are systems that cleanly, efficiently, and economically integrate Renewable Energy Sources (RESs) and Energy Storage Systems (ESSs) to the electrical grid. They are capable of reducing transmission losses and improving the use of electricity and heat. However, RESs presents intermi ent behavior derived from the stochastic nature of the renewable resources available on site. This can cause power-quality issues throughout the electrical grid, which can be solved by different optimization techniques and/or control strategies applied to power converters. This paper offers a detailed review of the literature regarding three important aspects: (i) Power-quality issues generated in MGs both in islanded mode and grid-connected mode; (ii) Optimization techniques used in the MGs to achieve the optimal operating conditions of the Energy Management System (EMS); and (iii) Control strategies implemented in the MGs to guarantee stability, mitigation of power-quality issues, power balance, and synchronization with the grid. It is worth mentioning that in this paper, we emphasize hybrid MGs (HMGs) since they combine the benefits of AC-MGs and DC-MGs while increasing system reliability. As the utility grid moves toward an optimal design of MG structures, this paper will serve as a foundation for future research, comparative analysis , and further development of novel techniques regarding HMGs.
... In this setting, academics have begun to explore and propose strategies and models to maximize advantages while minimizing drawbacks. To overcome difficulties related to the design, operation, and process of renewable systems, several researches combine traditional optimization techniques with newer heuristic approaches (Zhao T. et al., 2017;Zhao Zy. et al., 2017;Allam et al., 2018;Eajal et al., 2017). PV systems require precise and reliable performance data in order to precisely assess power output and capacity in current operating circumstances. ...
The study paper focuses on solar energy optimization approaches, as well as the obstacles and concerns that come with them. This study discusses the most current advancements in solar power generation devices in order to provide a reference for decision-makers in the field of solar plant construction throughout the world. These technologies are divided into three groups: photovoltaic, thermal, and hybrid (thermal/ photovoltaic). As a result, this article begins by outlining the approach that will be employed to undertake this research. Following that, solar energy production methods are researched and their sub-classifications are described in order to establish their resource needs and features. Following that, a detailed conversation is held. Each technology's environmental and economic performance will be evaluated. Furthermore, a statistical analysis is conducted to emphasize the efficiency and performance of each solar technology, as well as to identify their global rankings in terms of power output. Finally, research trends in the development of solar power plants are presented. The credibility of the Photovoltaic system, types and limitations is the discussion under study system makes use of sun's energy to generate electricity with the help of varied procedural systems; stand-alone, hybrid or grid charged. Based on this research, it is possible to infer that the primary goals of optimization approaches are to reduce investment, operation and maintenance costs, and emissions in order to improve system dependability. This paper also includes a brief overview of several solar energy optimization problems and issues.
... In this setting, academics have begun to explore and propose strategies and models to maximize advantages while minimizing drawbacks. To overcome difficulties related to the design, operation, and process of renewable systems, several researches combine traditional optimization techniques with newer heuristic approaches (Zhao T. et al., 2017;Zhao Zy. et al., 2017;Allam et al., 2018;Eajal et al., 2017). PV systems require precise and reliable performance data in order to precisely assess power output and capacity in current operating circumstances. ...
The study paper focuses on solar energy optimization approaches, as well as the obstacles and concerns that come with them. This study discusses the most current advancements in solar power generation devices in order to provide a reference for decision-makers in the field of solar plant construction throughout the world. These technologies are divided into three groups: photovoltaic, thermal, and hybrid (thermal/ photovoltaic). As a result, this article begins by outlining the approach that will be employed to undertake this research. Following that, solar energy production methods are researched and their sub-classifications are described in order to establish their resource needs and features. Following that, a detailed conversation is held. Each technology's environmental and economic performance will be evaluated. Furthermore, a statistical analysis is conducted to emphasize the efficiency and performance of each solar technology, as well as to identify their global rankings in terms of power output. Finally, research trends in the development of solar power plants are presented. The credibility of the Photovoltaic system, types and limitations is the discussion under study system makes use of sun's energy to generate electricity with the help of varied procedural systems; stand-alone, hybrid or grid charged. Based on this research, it is possible to infer that the primary goals of optimization approaches are to reduce investment, operation and maintenance costs, and emissions in order to improve system dependability. This paper also includes a brief overview of several solar energy optimization problems and issues.
... The results demonstrate the enhancement of system reliability. In [33], the authors target the increase in steady-state stability by optimum power flow (OPF) by using the Interior Point (IP) method. The presented AC/DC optimal power flow (OPF) can be used to discover the challenges (technical and economic) associated with a hybrid microgrid. ...
The depletion of natural resources and the intermittence of renewable energy resources have pressed the need for a hybrid microgrid, combining the benefits of both AC and DC microgrids, minimizing the overall deficiency shortcomings and increasing the reliability of the system. The hybrid microgrid also supports the decentralized grid control structure, aligning with the current scattered and concentrated load scenarios. Hence, there is an increasing need to explore and reveal the integration, optimization, and control strategies regarding the hybrid microgrid. A comprehensive study of hybrid microgrid’s performance parameters, efficiency, reliability, security, design flexibility, and cost-effectiveness is required. This paper discusses major issues regarding the hybrid microgrids, the integration of AC and DC microgrids, their security and reliability, the optimization of power generation and load management in different scenarios, the efficient management regarding uncertainty for renewable energy resources, the optimal placement of feeders, and the cost-effective control methodologies for the hybrid microgrid. The major research areas are briefly explained, aiming to find the research gap that can further improve the performance of the grid. In light of the recent trends in research, novel strategies are proposed that are found most effective and cost-friendly regarding the hybrid microgrid. This paper will serve as a baseline for future research, comparative analysis, and further development of novel techniques regarding hybrid microgrids.
... For example, [18,19] focus only on grid-connected systems. Also, [20][21][22][23][24] only examine the power flow in the microgrids that [21][22][23][24] focus on power flow techniques in systems with multiple AC and DC subgrids. ...
This paper presents a unified power flow formulation for grid-connected and islanded distribution systems with any desired AC-DC structure. Load flow equations are formed using the generalized matrices. The definition of these matrices is based on graph theory and includes network structure and data of the lines, loads, power sources, converters, and efficiency of converters. The efficiency of the converters is modeled in the load flow program as a function of its operating current. By simulation in PLECS software, the efficiency curve of the converter is obtained. The proposed method is implemented on several AC-DC case studies in islanded and grid-connected operating modes. The results show that the proposed formulation of power flow simply considers the actual efficiency of the converter in the calculations and has less computational time than the available methods.
... The objective function in Eq. (26) includes the total distributed resources operating cost, C DR , the total grid power, P Grid , and the total system losses, P Loss [12][13][14][15][16][17]. P Grid and P Loss have weights, W Grid and W Loss , respectively. ...
As the interest in DC-based power system increases, the low-voltage DC (LVDC) distribution system is gaining attention. Until now, however, the related studies and developments are still not mature to commercialize the system. This corresponds to the protection system as well. In fact, previous studies have dealt with protection schemes targeting only the LVDC distribution system although the system is closely influenced by the low-voltage AC (LVAC) distribution system–hybrid AC/DC low-voltage (LV) distribution system. However, most literatures reporting the hybrid AC/DC LV distribution system do not focus on the protection scheme. Thus, in the work, the protective schemes for the hybrid AC/DC LV distribution system are developed and coordinated. The transient analysis on the events in both systems is first conducted; subsequently, the protection schemes for each system are developed. Finally, two proposed protection schemes are coordinated using the state diagram and their performances are verified through simulations using the electromagnetic transients program (EMTP).
The effect of load models on distributed generation (DG) planning in distribution system is investigated in this work. It is shown that load models can significantly affect the DG planning. Normally a constant power (real and reactive) load model is assumed in most of the studies. Such assumptions may lead to inconsistent and misleading results about deferral values, loss reduction, payback period, and other subsequent calculations. It has been demonstrated that DG planning based on such assumptions would not be effective after implementation. It is shown that load models can significantly affect the optimal location and sizing of DG resources in distribution systems. A comparative study of real and reactive power loss, real and reactive power intake at the main substation and MVA support provided by installing DG resources for different type of loads models has been performed.
Provides insight on both classical means and new trends in the application of power electronic and artificial intelligence techniques in power system operation and control. This book presents advanced solutions for power system controllability improvement, transmission capability enhancement and operation planning. The book is organized into three parts. The first part describes the CSC-HVDC and VSC-HVDC technologies, the second part presents the FACTS devices, and the third part refers to the artificial intelligence techniques. All technologies and tools approached in this book are essential for power system development to comply with the smart grid requirements. Discusses detailed operating principles and diagrams, theory of modeling, control strategies and physical installations around the world of HVDC and FACTS systems Covers a wide range of Artificial Intelligence techniques that are successfully applied for many power system problems, from planning and monitoring to operation and control Each chapter is carefully edited, with drawings and illustrations that helps the reader to easily understand the principles of operation or application Advanced Solutions in Power Systems: HVDC, FACTS, and Artificial Intelligence is written for graduate students, researchers in transmission and distribution networks, and power system operation. This book also serves as a reference for professional software developers and practicing engineers © 2016 by The Institute of Electrical and Electronics Engineers, Inc. All rights reserved.
A promising configuration for future smart grids is an AC/DC hybrid topology that enables the integration of AC/DC energy resources and modern loads, thus permitting the consequent formation of AC/DC hybrid microgrids (HMGs). An understanding of AC/DC HMGs and their operational premise during islanding will certainly pave the way toward the realization of a future smart grid that includes a plug-and-play feature. However, the planning and operation of such isolated and hybrid systems are reliant on a powerful and efficient power flow tool. To this end, this paper proposes a novel unified, generic, and flexible power flow algorithm for isolated AC/DC HMGs. The power flow subproblems related to AC and DC subgrids are described mathematically by a set of linear and nonlinear equations and are solved simultaneously using a Newton trust-region method. The proposed algorithm is generic in the sense that it includes consideration of the unique characteristics of islanded AC/DC HMGs: a variety of possible topologies, droop controllability of the distributed resources (DRs), and bidirectionality of the power flow in the interlinking converters (ICs). The new power flow formulation is flexible and permits the easy incorporation of any changes in DR operating modes and IC control strategies. The developed algorithm was tested and applied for analyzing selected operational and control aspects of isolated AC/DC HMGs, including inaccurate power sharing and interlinking converters characterized by differing control strategies. The proposed load flow program can form the basis of and provide direction for further studies of isolated AC/DC HMGs.
In this paper, a converter-based dc microgrid is studied. By considering the impact of each component in dc microgrids on system stability, a multistage configuration is employed, which includes the source stage, interface converter stage between buses, and common load stage. In order to study the overall stability of the above dc microgrid with constant power loads (CPLs), a comprehensive small-signal model is derived by analyzing the interface converters in each stage. The instability issue induced by the CPLs is revealed by using the criteria of impedance matching. Meanwhile, virtual-impedance-based stabilizers are proposed in order to enhance the damping of dc microgrids with CPLs and guarantee the stable operation. Since droop control is commonly used to reach proper load power sharing in dc microgrids, its impact is taken into account when testing the proposed stabilizers. By using the proposed stabilizers, virtual impedances are employed in the output filters of the interface converters in the second stage of the multistage configuration. In particular, one of the virtual impedances is connected in series with the filter capacitor, and the other one is connected at the output path of the converter. It can be seen that by using the proposed stabilizers, the unstable poles induced by the CPLs are forced to move into the stable region. The proposed method is verified by the MATLAB/Simulink model of multistage dc microgrids with three distributed power generation units.
An important consideration in the operation of droop-controlled islanded microgrid (IMG) systems is its maximum loadability. This paper proposes the consideration of the system maximum loadability in droop-controlled IMGs optimal power flow (OPF) problems. Three Droop-controlled IMG OPF problems are investigated: (I) The OPF problem for maximum loadability assessment, (2) The OPF for maximizing system loadability, and (3) The bi-objective OPF problem for loadability maximization and generation cost minimization. A detailed microgrid model is adopted to reflect the special features and operational characteristics of droop-controlled IMG systems in the proposed OPF problems formulations. Numerical simulation studies have been carried out to demonstrate the importance and consequences of considering the system maximum loadability in the operational planning of droop-controlled IMGs.
This paper investigates on power-sharing issues of an autonomous hybrid microgrid. Unlike existing microgrids which are purely ac, the hybrid microgrid studied here comprises dc and ac subgrids interconnected by power electronic interfaces. The main challenge here is to manage power flows among all sources distributed throughout the two types of subgrids, which is certainly tougher than previous efforts developed for only ac or dc microgrid. This wider scope of control has not yet been investigated, and would certainly rely on the coordinated operation of dc sources, ac sources, and interlinking converters. Suitable control and normalization schemes are now developed for controlling them with the overall hybrid microgrid performance already verified in simulation and experiment.
This study extends the methodologies presented in other study concerned with the modelling and stability analysis of autonomous microgrids. It elaborates on a continuation method for finding the equilibria states through the entire loading range of an autonomous microgrid. The continuation method is based on the conventional continuation power flow (CPF) employed in the analysis of large power systems. However, the dissimilarities between conventional, simplified models of large power systems employed in CPF and those of detailed dynamics employed in microgrids make it necessary to reinterpret the CPF methodology. The methodology followed employs modelling procedures of microgrids previously published so as to build the Jacobians required by the predictor step. The procedure is quite expeditious, speeding up the solutions. The study also discusses revisions conducted on the CPF to warrant convergence of the corrector step to the particular case of islanded microgrids.
As demonstrated by recent major blackouts, power grids and their associated markets play a vital role in the operation of our society. Understanding how electric generation, transmission, and delivery systems interact and operate is paramount to guaranteeing reliable sources of electricity. Electric Energy Systems offers highly comprehensive and detailed coverage of power systems operations, uniquely integrating technical and economic analyses. The book fully develops classical subjects such as load flow, short-circuit analysis, and economic dispatch within the context of the new deregulated, competitive electricity markets. With contributions from 24 internationally recognized specialists in power engineering, the text also presents a wide range of advanced topics including harmonic load flow, state estimation, and voltage and frequency control as well as electromagnetic transients, fault analysis, and angle stability. A well-needed and updated extension on classical power systems analysis books, Electric Energy Systems provides an in-depth analysis of the most relevant issues affecting the blood-line of our society, the generation and transmission systems for electric energy.
This paper proposes a hybrid ac/dc micro grid to re- duce the processes of multiple dc-ac-dc orac-dc-ac conversions in an individual ac or dc grid. The hybrid grid consists of both ac and dc networks connected together by multi-bidirectional converters. AC sources and loads are connected to the ac network whereas dc sources and loads are tied to the dc network. Energy storage systems can be connected to dc or ac links. The proposed hybrid grid can operate in a grid-tied or autonomous mode. The coordi- nation control algorithms are proposed for smooth power transfer between ac and dc links and for stable system operation under var- ious generation and load conditions. Uncertainty and intermittent characteristics of wind speed, solar irradiation level, ambient tem- perature, and load are also considered in system control and oper- ation. A small hybrid grid has been modeled and simulated using theSimulink intheMATLAB.The simulationresults showthatthe system can maintain stable operation under the proposed coordi- nation control schemes when the grid is switched from one oper- ating condition to another.
This document describes the GAMS interface to MINOS which is a general purpose nonlinear programming solver