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Power flow analysis of AC/DC hybrid microgrids
Abstract
The future smart grid can have an AC/DC hybrid structure that enables the integration of AC/DC energy supply and demand, thus permitting the formation of AC/DC hybrid microgrids (HMGs). The AC/DC HMG is a promising concept that provides the envisioned smart grid with the plug-and-play feature. Nevertheless, understanding such hybrid systems, especially during islanding, require an accurate and robust load flow program. To this end, this paper proposes a novel robust power flow algorithm for isolated AC/DC HMGs. The power flow problem is formulated as a least-squares minimization problem, and solved using the Levenberg-Marquardt algorithm. The accuracy of the proposed load flow algorithm is validated against time domain simulation results, while its robustness is demonstrated on a highly-resistive network. The proposed load flow algorithm is expected to reveal the light for further studies on AC/DC HMGs.
... Researchers investigated several power flow analysis methods in HMGs [39]- [43]. The sequential power flow method [39][40] and the unified power flow method [41]- [43] are among these methods. The sequential power flow method solves the AC and DC power flow equations independently at each iteration until convergence is reached. ...
... Step 6: Calculate the fitness objective function for all populations, then update the values of and using (40) and (41). ...
This paper provides a stochastic framework for the energy management of multi-area hybrid microgrids to identify the optimal daily power dispatch in the islanded mode of operation. The primary goal of this framework is to minimize operational costs and power outages for customers by effectively managing the power resources of the hybrid microgrid. These resources include renewable energy sources, battery energy storage systems, and electric vehicles. To obtain accurate results, the costs related to active and reactive powers of diesel generator units are considered in this study. In addition, the uncertainties in the, power generated from renewable energy sources, electric vehicles charging and microgrid demand are modeled using the Monte Carlo Simulation as a scenario generation technique. The number of generated scenarios is reduced using the Fast Forward Scenario reduction technique. The proposed problem is formulated as a constrained mixed-integer nonlinear programming optimization problem and is solved using the Transient Search Optimization algorithm. The obtained solution is compared against other meta-heuristic optimization techniques to ensure the reliability of the results. To investigate the effectiveness of the proposed framework, several case studies are simulated on an islanded hybrid microgrid. The simulation results show the improvement in the economic performance and the reliability of the hybrid MG. The results also indicate the impact of increasing penetration levels of renewable energy resources on the operation cost and power exchange between AC and DC areas.
... In a hybrid microgrid, higher R/X values have a significant impact on power flow calculations. To minimize this effect of highly resistive networks, the Levenberg-Marquardt algorithm is used in [34] where the power flow is visualized for a basic six-bus AC/DC network with a fifteen-fold increase in R/X value. The calculation of the Jacobian matrix at each iteration for the above-discussed techniques increases the computational time. ...
A fundamental strategy for utilizing green energy from renewable sources to tackle global warming is the microgrid (MG). Due to the predominance of AC microgrids in the existing power system and the substantial increase in DC power generation and DC load demand, the development of AC/DC hybrid microgrids (HMG) is inevitable. Despite increased theoretical efficiency and minimized AC/DC/AC conversion losses, uncertain loading, grid outages, and intermittent complexion of renewables have increased the complexity, which poses a significant threat toward system stability in an HMG. As a result, the amount of research on the stability, management, and control of HMG is growing exponentially, which makes it imperative to recognize existing problems and emerging trends. In this survey, several strategies from the most recent literature developed to address the challenges of HMG are reviewed. Power flow analysis, power sharing (energy management), local and global control of DGs, and a brief examination of the complexity of HMG’s protection plans make up the four elements of the review technique in this article. During critical analysis, the test system employed for validation is also taken into consideration. A comprehensive review of the literature demonstrates that MILP is a frequently employed technique for the supervisory control of HMG, whereas tweaking bidirectional converter control is the most common approach in the literature to achieve efficient power sharing. Finally, this review identified the limitations, undiscovered challenges, and major hurdles that need to be addressed in order to develop a sustainable control and management scheme for stable multimode HMG operation.
Microgrids (MGs) have emerged as a promising solution for providing reliable and sustainable electricity, particularly in underserved communities and remote areas. Integrating diverse renewable energy sources into the grid has further emphasized the need for effective management and sophisticated control strategies. This review explores the crucial role of control strategies in optimizing MG operations and ensuring efficient utilization of distributed energy resources, storage systems, networks, and loads. To maximize energy source utilization and overall system performance, various control strategies are implemented, including demand response, energy storage management, data management, and generation‐load management. Employing artificial intelligence (AI) and optimization techniques further enhances these strategies, leading to improved energy management and performance in MGs. The review delves into the control strategies and their architectures, and highlights the significant contributions of AI and emerging technologies in advancing MG energy management.
The existing Newton‐Raphson (NR)‐based load flow for AC–DC microgrids (MG) implements a decoupled approach where the AC and DC sub‐systems are modeled using separate variable sets. Therefore, such types of approach are not genuinely unified in nature. Here, in this article, we have attempted to formulate an NR‐based load flow for AC–DC MG based on single set of variables. The approach has been developed by transforming the whole AC–DC system in terms of AC variables alone, which involves the conversion of DC lines and buses into their respective AC equivalent. At the same time, the proposed transformation concept eliminates the problem related to application of NR‐based load flow with DC lines having their R/X ratio as infinity. Thus, a single Jacobian is sufficient enough to obtain the load flow solution. The proposed method applies to the grid‐connected mode and also to the islanded mode of operation by considering the absence of slack bus. The proposed technique has been verified against existing results obtained using other methods and proves itself competent and accurate. The method further has been formulated for an unbalanced three‐phase AC–DC network. The load flow analysis is a tool for planning and analysis of an electrical power system. The planning engineers and operators collect the network specific data like, line resistances (DC), line impedances (AC), load present at different buses, number and types of sources, generating capacities of local sources etc. The data collected are then processed under the algorithm build to analyze the voltages at bus terminals and the power flow through the line. Adjustments are incorporated as per the analysis to maintain the every quantity under universal electrical standards.
This article proposes a new energy management system (EMS) for hybrid AC/DC microgrids (MGs) in remote communities. The major goal of the proposed EMS is to minimize the daily operational costs while supplying sufficient clean water to the remote community, meeting customers’ preferences, and maintaining the technical operational limits of the MG. To achieve this goal, the proposed EMS performs real-time supervisory control on all local controllers of all system assets in both generation and demand sides. On the generation side, distributed renewable and nonrenewable generation units are integrated into the system. Renewable sources of energy are controlled through a maximum power point tracking algorithm for increased efficiency, while nonrenewable DG units are controlled through droop parameters to satisfy the demand. On the demand side, optimal operational schedules for residential appliances and water desalination units are developed to satisfy customers’ requirements and achieve minimum operating costs. Moreover, the EMS controls the AC/DC interlinking converters (ICs) allowing bidirectional flow of power to and from both AC and DC sections of the MG. For practical real-time operation, the proposed EMS utilizes an adaptive rolling-time horizon when responding to changes/disturbances in the system or in user inputs. The assets scheduling problem is modeled as a mixed integer nonlinear programing. Simulation studies were conducted on a hybrid AC/DC MG to demonstrate the effectiveness of the proposed microgrid EMS.
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).
An AC-DC hybrid microgrid is gradually becoming popular. For economic viability and environmental sustainability, an AC-DC microgrid should be operated optimally. This paper introduces an optimal power dispatch strategy for simultaneous reduction of cost and emission from generation activities in an AC-DC hybrid microgrid under load and generation uncertainties. The operational attributes of an AC-DC hybrid microgrid, and load and renewable generation uncertainties are incorporated in the optimal scheduling problem by using a customized power-flow technique, and by modeling uncertainties by Hong’s two-point estimate method respectively. The economic and environmental objectives are modeled in the fuzzy domain by fuzzy membership functions. A combination of particle swarm optimization and fuzzy max-min technique is then employed for obtaining the optimal solution. The static active power droop constants of the dispatchable units are the control variables. Simulation results on a 6-bus AC-DC
hybrid microgrid system demonstrate that optimal scheduling results in 4.26% reduction of operating cost and 13.91% reduction of emission in comparison to capacity based droop settings. Further, a comparison between the proposed method and the elitist multi objective GA indicates that the optimized solution lies on the Pareto-front, thereby validating the proposed technique.
This paper proposes a unified load flow (LF) model for AC-DC hybrid distribution systems (DSs). The proposed model can be applied in hybrid DSs with mixed configurations for AC/DC buses and AC/DC lines. A new classification of DS buses is also introduced for LF analysis. A set of generic LF equations has been derived based on comprehensive analysis of the possible AC-DC hybrid system configurations. Three binary matrices, which are used as a means of describing the configuration of the AC and DC buses and lines, have been employed in the construction of the unified power equations. These matrices enable a single configuration at a time to be activated in the power equations. The proposed LF model is generic and can be used for both grid-connected and isolated hybrid DSs. The new model has been tested using several case studies of hybrid DSs that include different operational modes for the AC and DC distributed generators (DGs). As a means of evaluating the effectiveness and accuracy of the proposed model, the LF solution was compared to the solution produced by PSCAD/EMTDC. A comparison of the results reveals the efficacy of the proposed model.
Today, conventional power systems are evolving to modern smart grids, where interconnected microgrids may dominate the distribution system with high penetration of renewable energy and energy storage systems. The hybrid AC/DC systems with DC and AC sources/loads are considered to be the most possible future distribution or even transmission structures. For such hybrid AC/DC microgrids, power management strategies are one of the most critical operation aspects. This paper presents an overview of power management strategies for a hybrid AC/DC microgrid system, which includes different system structures (AC-coupled, DC-coupled, and AC-DC-coupled hybrid microgrids), different operation modes, a thorough study of various power management and control schemes in both steady state and transient conditions, and examples of power management and control strategies. Finally, discussion and recommendations of power management strategies for the further research are presented.
An improved approach based on sequential method for the AC-DC power flow calculation is proposed in this paper. This approach solves the convergence problem caused by voltage violations at AC buses during the power flow calculation for the DC subsystems. The convergence property can be significantly improved by adjusting the converter transformer tap position flexibly. In order to adjust the tap position of the converter transformer flexibly, three mainly modifications are proposed. Firstly, the equations for whole DC systems are decoupled into individual DC systems so as to easily figure out which DC system's tap position needs adjustment. Secondly, the tap ratio of a converter transformer is selected as an alternative state variable to replace the cosine of the control angle when necessary. Thirdly, the Newton-Raphson method is utilized to solve DC subsystems instead of the method using the linear equations. Furthermore, a theoretical analysis of the advantages of the proposed approach is also presented. Numerical simulations and practical applications show that the proposed approach meet the requirement of different system operating conditions and has advantages in terms of convergence and speed. The proposed approach has been successfully integrated into the Energy Management System (EMS) for China Southern Power Grid.
The analysis of the small-signal stability of conventional power systems is well established, but for inverter based microgrids there is a need to establish how circuit and control features give rise to particular oscillatory modes and which of these have poor damping. This paper develops the modeling and analysis of autonomous operation of inverter-based microgrids. Each sub-module is modeled in state-space form and all are combined together on a common reference frame. The model captures the detail of the control loops of the inverter but not the switching action. Some inverter modes are found at relatively high frequency and so a full dynamic model of the network (rather than an algebraic impedance model) is used. The complete model is linearized around an operating point and the resulting system matrix is used to derive the eigenvalues. The eigenvalues (termed "modes") indicate the frequency and damping of oscillatory components in the transient response. A sensitivity analysis is also presented which helps identifying the origin of each of the modes and identify possible feedback signals for design of controllers to improve the system stability. With experience it is possible to simplify the model (reduce the order) if particular modes are not of interest as is the case with synchronous machine models. Experimental results from a microgrid of three 10-kW inverters are used to verify the results obtained from the model
This paper addresses power management and control strategies of a hybrid microgrid system that comprises ac and dc subgrids. Each subgrid consists of multiple distributed generation (DG) units and local loads. Both entities are interconnected by voltage source converters (VSCs) to facilitate a bidirectional power flow and increase the system reliability. The control of the interconnecting VSC can be achieved autonomously. However, it is shown that the autonomously controlled hybrid microgrid fails to operate following variations in the power generation characteristics of local DG units (such as droop coefficients, set points, or loss/connection of DG units, etc.) A centralized controller is therefore proposed and compared to the autonomous scheme. The centralized control strategy provides an accurate and optimized power exchange between both subgrids. Parallel operation of multiple interconnecting VSCs is considered so that the transmitted power is shared according to their power ratings. Small-signal stability analysis is conducted to investigate the influence of the communication delays on the system stability. A hierarchical control strategy has been proposed by setting the autonomous controller in a primary layer whereas the centralized controller is set into a secondary layer to generate a compensation signal. Time-domain simulations results are presented to show the effectiveness of the proposed techniques and the drawbacks of the conventional scheme.
Modern distributed sources can either be ac or dc. It is thus possible to form hybrid microgrids with both ac and dc sub-grids inter-tied by power converters. The resulting hybrid architecture allows loads to be flexibly placed, hence reducing the amount of power conversion needed. Hybrid microgrids can therefore be more efficient if controlled appropriately. For that, a droop scheme is now proposed for coordinating energy flows within the hybrid microgrids. The scheme operates autonomously and efficiently without demanding for fast communication links. The overall energy flow tuning and hybrid microgrids have already been tested in experiments.
This paper proposes a multi-option power flow approach for hybrid AC/DC grids. A unified AC-DC unit is introduced which can be used in two different cases: case a) AC grids with embedded VSC-based MTDC grids, and case b) asynchronous AC grids connected via a common VSC-based MTDC grid. In the proposed method for each MTDC grid (regardless of the number of converters), a new state variable is introduced to handle any kind of converter loss models. For the case (b), the AC-DC unit can be used in two different analyses, namely, a1) the separated analysis and a2) the integrated analysis. Both a1) and a2) can be used in the practical analysis of the real-size power systems. However, it is shown that the separated analysis not only offers a shorter computational time but it is also very suitable for the future connection between large existing AC systems and other AC systems or remote renewable energy sources through the DC grids. The proposed methodology is implemented in MATLAB software and verified using commercial software SIMPOW.
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.
Modern distributed sources and loads can either be ac or dc. It is thus possible to have ac and dc subgrids intertied to form hybrid microgrids with lesser power conversion, and hence higher efficiency. However, operation of hybrid microgrids to gain these advantages is not trivial, and is so far limited. It is, therefore, the theme here to develop a control scheme for the hybrid microgrids so that they can operate efficiently with progressive energy flow tuning and no requirement for fast communication links. A plug-in scheme is also proposed for progressive and selective charging/discharging of storages, which are likely to be present in the hybrid microgrids for energy smoothing and ride-through purposes. The overall scheme has already been verified in experiment.
It has been over 100 years since Thomas Edison built the first direct current (dc) electricity supply system on 4 September 1882, at Pearl Street in New York City. Many prominent events occurred in the electricity supply industry after that. The first one, ?the war of currents,? started in 1888. Thomas Edison and his dc distribution system were on one side, and George Westinghouse and Nikolai Tesla with the alternating current (ac) system were on other side. The war ?ended? in about 1891 when ac won as the dominant power supply medium. The key behind the ac win was the invention of the transformer that could easily step up medium voltage to high and extra-high voltage for long-distance power transfer from a remote ac generation station to load centers hundreds of kilometers away with lower transmission losses. Transformers can also step down high voltage back to low voltage at load stations to supply the low-voltage equipment. Since the end of the war, ac power systems have been developed and expanded at a tremendous speed from the initial small isolated networks, with each supplying only lighting and motor loads with a few hundreds of customers, to its current scale of super interconnected networks each supplying billions of customers over large geographic areas in one or several countries. The voltage levels and capacities of transmission networks have increased from the first commercialized three-phase ac system with only 2.4 kV, 250 kW in the town of Redlands, California, United States, to the first commercial long-distance, ultra-high-voltage, ac transmission line in China with 1,000 kV, 2,000 MW. Transmission distance has been increased from several miles to over thousands of kilometers (miles). With such major achievements, it is little wonder that the ac power system became the top engineering achievement of the 20th century. Does this mean that dc is gone? The answer is an unambiguous no. What has happened in the past 50 years, such as applications of adva- ced control technologies in conventional power system loads, the power electronics based high-voltage dc (HVdc) transmission, and the additional renewable power sources in low-voltage distribution system, calls for a rethink about dc and ac in electricity supply systems.
The advantages arising from dc distribution networks are related to the possibilities of achieving higher quality supply and easier reconfigurability of the system. This paper presents the concept of introducing some dc islands interconnected with the ac distribution network. This will make it easier to connect storage systems, electrical drives, power converters, and renewable sources (i.e., photovoltaic panels). In order to realize the aforementioned goals, a methodology for designing the control strategy of different power converters connected to a dc bus without a centralized system management control unit is proposed. In this way, a plug-and-play functionality for connecting new power converter interfaced elements in the dc network able to stabilize the voltage under different working conditions is realized. In order to prove the advantages obtainable with such a network, a 100-kW dc test facility has been realized at the laboratories of RSE. Different working conditions were tested by means both of numerical and experimental results, proving the effectiveness of the proposed strategy for voltage regulation, continuity of service, and the smart use of storage devices.
In this paper, a steady-state multi-terminal voltage source converter high voltage direct current (VSC MTDC) model is introduced. The proposed approach is extended to include multiple AC and DC grids with arbitrary topologies. The DC grids can thereby interconnect arbitrary buses in one or more non-synchronized AC systems. The converter equations are derived in their most general format and correctly define all set-points with respect to the system bus instead of the converter or filter bus, which is often done to simplify calculations. The paper introduces a mathematical model to include the converter limits and discusses how the equations change when a transformerless operation is considered or when the converter filter is omitted. An AC/VSC MTDC power flow is implemented using MATPOWER to show the validity of the generalized power flow model.
Most algorithms for the least-squares estimation of non-linear parameters have centered about either of two approaches. On the one hand, the model may be expanded as a Taylor series and corrections to the several parameters calculated at each iteration on the assumption of local linearity. On the other hand, various modifications of the method of steepest-descent have been used. Both methods not infrequently run aground, the Taylor series method because of divergence of the successive iterates, the steepest-descent (or gradient) methods because of agonizingly slow convergence after the first few iterations. In this paper a maximum neighborhood method is developed which, in effect, performs an optimum interpolation between the Taylor series method and the gradient method, the interpolation being based upon the maximum neighborhood in which the truncated Taylor series gives an adequate representation of the nonlinear model. The results are extended to the problem of solving a set of nonlinear algebraic e
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 paper investigates the opportunities and challenges associated with adopting a DC distribution scheme for industrial power systems. A prototype DC distribution system has been simulated to investigate the issues. One of the issues focused is the interaction between power converters that are used to convert AC to DC and DC to AC. Another challenging issue investigated is the system grounding. These issues become challenging mainly due to the neutral voltage shift associated with the power converters. The paper shows that converter interactions can be minimized with proper filtering and control on the converters. The paper also proposes a grounding scheme and shows that this scheme provides an effective solution by keeping the neutral voltages low under normal conditions and by limiting the fault currents during fault conditions. With these features, DC distribution provides very reliable and high-quality power.