Flexible Universal Branch Model For Steady State Operational Analysis And Optimisation Of Hybrid Ac Dc Grids
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Flexible Universal Branch Model For Steady State Operational Analysis And Optimisation Of Hybrid Ac Dc Grids
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Author : Abraham Alvarez-Bustos
language : en
Publisher:
Release Date : 2021
Flexible Universal Branch Model For Steady State Operational Analysis And Optimisation Of Hybrid Ac Dc Grids written by Abraham Alvarez-Bustos and has been published by this book supported file pdf, txt, epub, kindle and other format this book has been release on 2021 with categories.
Steady State Analysis And Optimal Power Routing Of Standalone Unbalanced Hybrid Ac Dc Microgrids
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Author : Mahmoud Ahmed Allam Sayed Alsanbawy
language : en
Publisher:
Release Date : 2018
Steady State Analysis And Optimal Power Routing Of Standalone Unbalanced Hybrid Ac Dc Microgrids written by Mahmoud Ahmed Allam Sayed Alsanbawy and has been published by this book supported file pdf, txt, epub, kindle and other format this book has been release on 2018 with Electric generators categories.
The concept of ac microgrids was introduced to integrate distributed generators (DGs) and loads within one entity that can operate autonomously or connected to a utility grid. Furthermore, dc microgrids have received increasing attention as a potential solution to deliver power from DGs to modern dc loads with reduced conversion stages. Moreover, hybrid ac/dc microgrids have been introduced as a paradigm combining the benefits of the two types of microgrids by interconnecting them through interlinking converters (ICs). Steady-state analysis is essential for planning and operation studies of electrical power systems. However, conventional analysis approaches cannot be applied to hybrid ac/dc microgrids due to their distinctive features, such as droop characteristics, lack of a slack bus, and coupling between the ac and dc variables. Additionally, the unbalanced nature of ac microgrids adds to the complexity of modeling and analysis in such networks. Therefore, this thesis is focused on developing steady-state modeling and analysis framework for standalone unbalanced hybrid ac/dc microgrids. First, a steady-state analysis tool for unbalanced hybrid ac/dc microgrids is developed. The ac subgrid's components are modeled in phase coordinates. Furthermore, the dc subgrid's components are modeled and the coupling between the ac and dc variables is formulated. The models of the various system elements are incorporated into a unified power flow formulation, which is solved using a Newton-Trust Region (NTR) method. The developed power flow algorithm is verified through comparisons with time-domain simulations of test microgrids. The analysis tool is used to analyze a larger hybrid ac/dc microgrid through case studies. The case studies shed light on some challenges of these microgrids, namely, imposed limitations on microgrid loadability due to unbalanced ac subgrid's loading, effect of IC settings on microgrid operation, and trade-off between proportional loading of the ac and dc subgrids and proportional power-transfer sharing among ICs. Second, based on the identified microgrid loadability limitation of unbalanced microgrids, a novel adaptive power routing (APR) scheme is proposed to maximize the microgrid loadability. The proposed scheme allows independent control of active and reactive powers flowing through IC phases, so that power can be routed among the ac subgrid's phases. The DPR scheme is integrated into an optimal power flow (OPF) formulation with the objective of minimizing load shedding. A supervisory controller is proposed to solve the OPF problem by adjusting the DG and IC settings. Several case studies are conducted to show the ineffectiveness of conventional supervisory controllers in resolving the loadability issue, and to verify the success of the proposed controller in solving the problem. Third, a power flow approach based on sequence component analysis of the ac microgrid's elements is adopted for faster convergence and improved modeling accuracy as compared to conventional approaches in phase coordinates. This approach breaks down the system model into positive-, negative-, and zero-sequence subsystems that can be solved in parallel for enhanced performance. The positive-sequence power flow is solved using a Newton-Raphson (NR) method, while the negative- and zero-sequence voltages are obtained by solving linear complex equations. The approach is verified through comparisons with time-domain simulations. In addition, the algorithm is utilized to investigate the operation of droop-controlled DGs in larger-scale isochronous unbalanced ac microgrids, and to examine its limit-enforcement abilities at the same time. The algorithm demonstrates significant improvements in terms of accuracy and convergence time when compared against the conventional NTR-based approach in phase coordinates. Finally, the power flow approach developed in the third part is extended to include the IC's and dc subgrid's models so that it can be applied to hybrid ac/dc microgrids. A power flow algorithm is proposed to solve the ac and dc power flows independently in a sequential manner, while maintaining the correlation between the two. The algorithm is verified through comparisons with time-domain models of test hybrid microgrids. Case studies are introduced to test the algorithm's effectiveness in enforcing the DG and IC limits in the power flow solution under various conditions. The algorithm also shows enhanced accuracy and solution speed with respect to the tool developed in the first stage.
Toward The Integration Of Dc Microgrids Into A Hybrid Ac Dc Paradigm
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Author : Amr Abdelnaeem Ismail Said
language : en
Publisher:
Release Date : 2016
Toward The Integration Of Dc Microgrids Into A Hybrid Ac Dc Paradigm written by Amr Abdelnaeem Ismail Said and has been published by this book supported file pdf, txt, epub, kindle and other format this book has been release on 2016 with Electric power distribution categories.
The recent penetration of distributed generation (DG) into existing electricity grids and the consequent development of active distribution networks (ADNs) have prompted an exploration of power distribution in a dc microgrid paradigm. Although dc power distribution has been implemented in aircraft, ships, and communication centres, the technology is still at an early stage and must be investigated with respect to technical feasibility when applied to distribution systems. In particular, the operation of a dc microgrid in both grid-connected and islanded modes and its integration into an existing ac infrastructure are subject to significant challenges that impede the practical realization of dc microgrids. On one hand, because the dc voltage profile is coupled with the injected active power at the system buses, it is seriously influenced by the intermittent nature of renewable resources such as solar and wind energy. In islanded operating mode, the presence of system resistance leads to a further trade-off between an appropriate system voltage profile and a precise power management scheme. On the other hand, the development of hybrid ac/dc microgrids introduces a fresh operational philosophy that enhances power sharing among ac and dc subgrids through the coupling of ac and dc steady-state variables. With these challenges as motivation, the primary goal of this thesis was to develop effective power management schemes and a steady-state analysis tool that can enable the reliable integration of dc microgrids into a smart hybrid ac/dc paradigm. Achieving this objective entailed the completion of three core studies: 1) the introduction of a robust control scheme for mitigating voltage regulation challenges associated with dc distribution systems (DCDSs) that are characterized by a high penetration of distributed and renewable generation, 2) the proposal of a supervisory control strategy for precise DG output power allocation that is based on DG rating and operational costs yet guarantees an appropriate voltage profile for islanded dc microgrids, 3) the development of an accurate and comprehensive power flow algorithm for analyzing the steady-state behaviour of islanded hybrid ac/dc microgrids, and 4) the optimization of hybrid ac/dc microgrids configuration. As the first research component, a novel multi-agent control scheme has been developed for regulating the voltage profile of DCDSs that incorporate a large number of intermittent energy sources. The proposed control scheme consists of two sequential stages. In the first stage, a distributed state estimation algorithm is implemented to estimate the voltage profile in DCDSs, thus enhancing the interlinking converter (IC) operation in regulating the system voltages within specified limits. If the IC alone fails to regulate the system voltages, a second control stage is activated and executed through either equal or optimum curtailment strategy of the DG output power. A variety of case studies have been conducted in order to demonstrate the effectiveness, robustness, and convergence characteristics of the control schemes that have been developed. The second element of this research is a multi-agent supervisory control that has been created in order to provide precise power management in isolated DC microgrids. Two aspects of power management have been considered: 1) equal power sharing, which has been realized via a proposed distributed equal power sharing (DEPS) algorithm, and 2) optimal power dispatch, which has been achieved through a proposed distributed equal incremental cost (DEIC) algorithm. Both algorithms offer the additional advantage of affording the ability to restore the average system voltage to its nominal value. Real-time OPAL-RT simulations have demonstrated the effectiveness of the developed algorithms in a hardware-in-the-loop (HIL) application. The third part of the research has introduced a sequential power flow algorithm for hybrid ac/dc microgrids operating in islanded mode. In contrast to the conditions in grid-connected systems, variable rather than fixed ac frequencies and dc voltages are utilized for coordinating power between the ac and dc microgrids. The primary challenge is to solve the power flow problem in hybrid microgrids in a manner that includes consideration of both the absence of a slack bus and the coupling between the frequency and dc voltage though ICs. In the proposed algorithm, the ac power flow is solved using the Newton-Raphson (NR) method, thereby updating the ac variables and utilizing them accordingly in a proposed IC model for solving the dc problem. This sequential algorithm is iterated until convergence. The accuracy of the algorithm has been verified through detailed time-domain simulations using PSCAD/EMTDC, and its robustness and computational cost compare favourable with those of conventional algorithms. The final part highlights the implementation of the developed steady-state models in obtaining an optimum hybrid microgrid configuration. The system configuration could be manipulated by changing the DG droop settings as well as the network topological structure. The contribution of both approaches has been investigated, through an optimum power flow (OPF) formulation, in improving the system loadability as the primary measure of the hybrid microgrid performance.
Multi Terminal Direct Current Grids
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Author : Nilanjan Chaudhuri
language : en
Publisher: John Wiley & Sons
Release Date : 2014-09-09
Multi Terminal Direct Current Grids written by Nilanjan Chaudhuri and has been published by John Wiley & Sons this book supported file pdf, txt, epub, kindle and other format this book has been release on 2014-09-09 with Technology & Engineering categories.
A generic DC grid model that is compatible with the standard AC system stability model is presented and used to analyse the interaction between the DC grid and the host AC systems. A multi-terminal DC (MTDC) grid interconnecting multiple AC systems and offshore energy sources (e.g. wind farms) across the nations and continents would allow effective sharing of intermittent renewable resources and open market operation for secure and cost-effective supply of electricity. However, such DC grids are unprecedented with no operational experience. Despite lots of discussions and specific visions for setting up such MTDC grids particularly in Europe, none has yet been realized in practice due to two major technical barriers: Lack of proper understanding about the interaction between a MTDC grid and the surrounding AC systems. Commercial unavailability of efficient DC side fault current interruption technology for conventional voltage sourced converter systems This book addresses the first issue in details by presenting a comprehensive modeling, analysis and control design framework. Possible methodologies for autonomous power sharing and exchange of frequency support across a MTDC grid and their impact on overall stability is covered. An overview of the state-of-the-art, challenges and on-going research and development initiatives for DC side fault current interruption is also presented.
The New Ac Dc Hybrid Microgrid Paradigm
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Author : Abdelsalam Eajal
language : en
Publisher:
Release Date : 2018
The New Ac Dc Hybrid Microgrid Paradigm written by Abdelsalam Eajal and has been published by this book supported file pdf, txt, epub, kindle and other format this book has been release on 2018 with Electric power distribution categories.
AC/DC hybrid microgrids (HMGs) represent a promising architecture that allows the hosting of innovative dc energy resources, such as renewables, and modern dc loads, such as electric vehicles, thereby reducing the number of conversion stages and offering other technical and cost benefits. Such advantages have prompted power distribution planners to begin investigating the possibility of hybridizing existing ac grids and designing new ac/dc hybrid clusters, referred to as microgrids, as a step toward an envisioned smart grid that incorporates multiple ac/dc microgrids characterized by "plug-and-play" features. Despite their potential, when either islanded or interfaced with the main grid, HMGs create challenges with respect to system operation and control, such as difficulties related to precise power sharing, voltage stability during a contingency, the control and management of power transfer through the interlinking converters (ICs), and the coordination of local distributed energy resources (DERs) with the hosting main grid. An understanding of HMGs and their operational philosophy during islanding will assuredly pave the way toward the realization of a future smart grid that includes a plug-and-play feature and will alleviate any operational challenges. However, the planning and operation of such islanded and hybrid systems are reliant on a powerful and efficient power flow analysis tool. To this end, this thesis introduces a novel unified, generic, flexible power flow algorithm for islanded/isolated HMGs. The developed algorithm is generic in the sense that it includes consideration of the unique characteristics of islanded HMGs: a variety of possible topologies, droop controllability of the DERs and bidirectionality of the power flow in the ICs. The new power flow formulation is flexible and permits the easy incorporation of any changes in the DER operating modes and the IC control schemes. The developed algorithm was validated against a detailed time-domain model and applied for the analysis of a variety of operational and control aspects in islanded HMGs, including the problem of imprecise power sharing and droop control of the ICs. The proposed load flow program can form the basis of and provide direction for further studies of islanded HMGs. This thesis also presents a deeper look at the problem of inaccurate active and reactive power sharing in islanded droop-based HMGs and proposes a unified and universal power sharing scheme that can simultaneously ensure precise power sharing in both ac and dc subgrids. Test results demonstrate the capability of the developed scheme with respect to achieving exact power sharing not only among DERs in proportion to their ratings but also among ICs that interface adjacent ac and dc microgrids. The developed unified power sharing scheme would assist system planners with the effective design of droop characteristics for DERs and ICs, which would result in enhancements such as the avoidance of converter overloading and the achievement of precise load sharing. Another operational aspect that was thoroughly investigated for this thesis is the possibility of voltage instability/collapse in islanded HMGs during contingencies. This research unveiled the possibility of voltage instability in HMGs that include constant power loads and a mix of synchronous-based and converter-based generating units. As indicated by the voltage stability analysis presented here, despite the fact that healthy microgrids have far-reaching loadability boundaries, the voltage at some ac/dc load buses can unexpectedly collapse during abnormal conditions. The analysis also revealed that fine tuning the droop characteristics of DERs and ICs can enlarge the voltage stability margin and safeguard the entire microgrid against collapse during contingencies, all without the sacrifice of a single load. A final component of this thesis is the proposal of a two-stage stochastic centralized dispatch scheme for ac/dc hybrid distribution systems. The developed dispatch scheme coordinates the operation of a variety of DERs, such as distributed generators and energy storage systems. It also ensures the coordinated charging of electric vehicles and models the degradation of their batteries that occurs due to the vehicle-to-grid action. The energy coordination problem has been formulated as a two-stage day-ahead resource scheduling problem: the intermittent supply; the variable demand, which includes electric vehicles; and the fluctuating real-time energy price are all modelled as random variables. The first stage produces day-ahead dispatch decisions for the dispatchable DG units. For a set of possible scenarios over the next 24 h, the second stage determines appropriate corrective decisions with respect to the import/export schedule, storage charging/discharging cycles, and electric vehicle charging/discharging patterns. The simulation results demonstrate the effectiveness of the developed scheme for optimally coordinating the various components of future ac/dc hybrid smart grids. Despite its substantial merits and value as a host for ac and dc technologies, a smart grid with HMGs creates previously unexperienced operational challenges for system planners and operators. The work completed for this thesis could help pave the way for the realization of ac/dc hybrid smart grids in years to come.
Optimal Planning And Operation Of Ac Dc Hybrid Distribution Systems
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Author : Haytham Mohamed Abdelrahman Mohamed Ahmed
language : en
Publisher:
Release Date : 2017
Optimal Planning And Operation Of Ac Dc Hybrid Distribution Systems written by Haytham Mohamed Abdelrahman Mohamed Ahmed and has been published by this book supported file pdf, txt, epub, kindle and other format this book has been release on 2017 with Distributed generation of electric power categories.
Recent years have been marked by a significant increase in interest in green technologies, which have led to radical changes in the way electric power is generated and utilized. These changes have been accompanied by greater utilization of DC-based distributed generators (DGs), such as photovoltaic (PV) panels and fuel cells, as well as DC-based load demands, such as electric vehicles (EVs) and modern electronic loads. In addition to accommodating these technologies, future distribution systems (DSs) will also need to support the integration of additional battery storage systems with renewable DGs. A further factor is the number of policies that have been implemented in Ontario, Canada, with the goal of encouraging the use of clean energy. The first, the feed-in-tariff (FIT) program, was introduced to promote the application of renewable DGs, including PV panels and wind DGs, and a second, new program that offers incentives for switching to EVs has been announced recently. The result is that future DSs must include additional DC loads and DC-based DGs along with their present AC loads and energy resources. Future DSs should thus become AC-DC hybrids if they are to provide optimal accommodation of all types of AC and DC loads and DGs. These considerations accentuate the need for reliable techniques appropriate for the planning and operation of future hybrid DSs. This thesis presents new directions for the planning and operation of AC-DC hybrid DSs. The main target of the research presented in this thesis is to optimally accommodate the expected high penetration of DC loads and DC-based DGs in future DSs. Achieving this target entailed the completion of four consecutive parts: 1) developing a unified load flow (LF) model for AC-DC hybrid DSs, 2) introducing an energy management scheme (EMS) for the optimal operation of AC-DC hybrid DSs, 3) introducing a planning model to determine the optimal AC-DC network configuration that minimizes the costs of the hybrid DS, and 4) developing a reliability-based planning technique for the simultaneous optimization of the DS costs and reliability. The first part of this research introduces a novel unified LF model for AC-DC hybrid DSs. The LF model can be applied in hybrid DSs with a variety of configurations for AC/DC buses and AC/DC lines. A new classification of DS buses is introduced for LF analysis. 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. The 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 DGs. The effectiveness and accuracy of the developed LF model has been verified against the steady-state solution produced by PSCAD/EMTDC software. The second part presents a two-stage EMS that can achieve optimal and reliable operation for AC-DC hybrid DSs. The first stage introduces a network reconfiguration algorithm to determine the optimal day-ahead reconfiguration schedule for a hybrid DS, while considering the forecasted data for load demands and renewable DGs. The objective of the reconfiguration algorithm is the minimization of DS energy losses. The second stage introduces a real-time optimal power flow (OPF) algorithm that minimizes the DS operation costs. In addition, a load-curtailment-management (LCM) technique is integrated with the OPF algorithm in order to guarantee optimal and reliable DS operation in the case of abnormal operating conditions. The third part presents a novel stochastic planning model for AC-DC hybrid DSs. Taking into account the possibility of each line/bus being AC or DC, the model finds the optimal AC-DC hybrid configuration of buses and lines in the DS. It incorporates consideration of the stochastic behavior of load demands and renewable DGs. The stochastic variations are addressed using a Monte-Carlo simulation (MCS). The objective of the planning model is the minimization of DS investment and operation costs. The developed planning model has been employed for finding the optimal configuration for a suggested case study that included PV panels, wind DGs, and EV charging stations. The same case study was also solved using a traditional AC planning technique in order to evaluate the effectiveness of the hybrid planning model and the associated cost-savings. The last part of this research introduces a stochastic multi-objective optimization model for the planning of AC-DC hybrid DSs. The introduced model determines the optimal AC-DC network configuration that achieves two objectives: 1) minimizing system costs, and 2) maximizing system reliability. Network buses and lines can become either AC or DC in order to achieve the planning objectives. The model features an MCS technique for addressing stochastic variations related to load demands and renewable DGs. The developed model has been tested using a case study involving a hybrid DS that included a variety of types of loads and DGs. Solving the same case study using a traditional AC planning technique provided verification of the benefits offered by the developed model, whose efficacy was confirmed through a comparison of the AC and hybrid Pareto fronts. The developed planning framework represents an effective technique that can be used by DS operators to identify the optimal AC-DC network configuration of future hybrid DSs.
Dynamics Robust Control And Power Management Of Voltage Source Converters In Hybrid Multiterminal Ac Dc Grids
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Author : Masoud Davari
language : en
Publisher:
Release Date : 2016
Dynamics Robust Control And Power Management Of Voltage Source Converters In Hybrid Multiterminal Ac Dc Grids written by Masoud Davari and has been published by this book supported file pdf, txt, epub, kindle and other format this book has been release on 2016 with Electricity categories.
The electric energy sector is moving toward extensive integration of clean and renewable energy sources, energy storage units, and modern loads via highly efficient and flexible multiterminal dc grids integrated within the traditional ac grid infrastructure in both transmission and distribution levels. A voltage-source converter (VSC) is the main technology enabling the interconnection of dc and ac grids. In such demanding applications, effective and robust integration of ac and dc grids, in the presence of coupling nonlinear dynamics, parametric uncertainties, and disturbances, is crucial to maintain the stability and robust performance of the overall ac/dc dynamic system. Motivated by this objective, this thesis addresses the dynamics, robust control, and power management of VSCs in hybrid multiterminal ac/dc grids. Firstly, a robust multi-objective dc-link voltage controller is developed for a bi-directional VSC regulating the dc-link voltage of a multiterminal dc grid; i.e., the VSC operates as a dc-voltage power-port. The proposed controller ensures excellent tracking performance, robust disturbance rejection, and robust stability against operating point and parameter variation with a simple fixed-parameter low-order controller. Secondly, the dynamics and control of VSCs considering the instantaneous power of both ac- and dc-side filters and dc grid uncertainties are addressed in the this thesis. The proposed controller ensures excellent tracking performance, robust disturbance rejection, and robust performance against operating point and parameter variation with a simple fixed-parameter controller. Thirdly, this thesis presents a natural-frame variable-structure-based nonlinear control system for the master VSC applied in multiterminal grids to overcome problems associated with conventional dc-link voltage controllers, which are suffering from stability and performance issues, mainly attributed to the small-signal-based control design approach and the use of cascaded control structure based on the power balance framework that yields unmodeled nonlinear dynamics. Fourthly, this thesis presents a robust vector-controlled VSC that facilitates full converter power injection at weak and very weak ac grid conditions (i.e., when the short-circuit capacity ratio is one). The controller overcomes problems related to the stability and performance of conventional vector-controlled VSCs integrated into very weak ac grids (high impedance grids) because of the increased coupling between the converter and grid dynamics, via the phase-locked loop (PLL). As a result, a detailed ac-bus voltage dynamic model, including the PLL dynamics, is developed and validated in this thesis. Then, the model is used to design a robust optimal ac-bus voltage controller to stabilize the dynamics under operating point variation and grid impedance uncertainty. Fifthly, this thesis addresses the challenges associated with a dc-voltage-controlled VSC interfacing a wind turbine into a dc grid, which is gaining widespread acceptance under weak grid connection or isolated operation. Under weak grid connection or isolated operation, the machine-side VSC regulates the dc-link voltage via changes in the generator speed. However, several control difficulties are yielded; important problems are: 1) the nonlinear plant dynamics with a wide range of operating point variation; 2) the control lever is mainly the generator speed, which complicates the dc-link voltage control dynamics; 3) the presence of uncertain disturbances associated with dynamic loads (e.g., power-converter-based loads) connected to the dc grid and wind speed variation; and 4) the presence of parametric uncertainty associated with the equivalent dc-link capacitance due to connecting/disconnecting converter-based loads. Finally, this thesis presents a robust power sharing and dc-link voltage regulation controller for grid-connected VSCs in dc grids applications to overcome difficulties and problems related to the dynamics and stability of a grid-connected VSC with dc power sharing droop control. Major difficulties are: 1) ignoring the effect of the outer droop loop on the dc-link voltage dynamics when the dc-link voltage controller is designed, which induces destabilizing dynamics, particularly under variable droop gain needed for optimum economic operation, energy management, and successful network operation under converter outages and contingencies; 2) uncertainties in the dc grid parameters (e.g., passive load resistance and equivalent capacitance as viewed by the dc side of the VSC); and 3) disturbances in the dc grid (i.e., power absorbed or injected from/to the dc grid), which change the operating point and the converter dynamics by acting as a state-dependent disturbance. A theoretical analysis and comparative simulation and experimental results are presented in this thesis to show the validity and effectiveness of the developed models and proposed control structures.
Operational Control And Analysis Of A Hybrid Ac Dc Microgrid
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Author : Hasan Alsiraji
language : en
Publisher:
Release Date : 2018
Operational Control And Analysis Of A Hybrid Ac Dc Microgrid written by Hasan Alsiraji and has been published by this book supported file pdf, txt, epub, kindle and other format this book has been release on 2018 with Electric power distribution categories.
In light of the growing demand for electrical power around the globe, the need to increase electrical power generation in order to diminish total carbon emissions has led to the installation of renewable resources to replace traditional generators. Most of today's microgrids are AC microgrids, whose advantages and shortcomings with respect to control techniques and stability assessment have been demonstrated through extensive studies reported in the literature. These considerations have led to the recent proposal and investigation of DC microgrids, accompanied by the introduction of the hybrid AC/DC microgrid as a means of combining the advantages and benefits of both types of microgrid. However, since a hybrid microgrid is viewed as a weak system with low inertia, controlling and assessing the performance of a hybrid microgrid constitutes a high-priority issue that requires further investigation. The lack of inertia of power electronics converters, especially in an islanded hybrid microgrid, poses a threat to stability and control. For these reasons, effective stability analysis has become a necessity with respect to the implementation of hybrid microgrids. Because of these challenges, the emulation of synchronous machine (SM) inertia and damping is now viewed as necessary for enhancing the effect of a VSC on an active distribution system and for facilitating its participation in voltage and frequency support. Improving the stability and performance of a hybrid microgrid therefore requires the introduction of a form of inertia into a hybrid microgrid. This research first proposes the incorporation of a novel form of virtual inertia into a hybrid microgrid using virtual synchronous machine (VSM) control of the intertying converter (IC) controller. The second proposal of this research is to employ the VSM control to establish autonomous control of the IC. A first research component, a novel control strategy for the Intertying converter in hybrid AC/DC microgrid has been proposed to ensure the benefit of a virtual synchronous machine (VSM) control algorithm in the hybrid AC/DC microgrid. The VSM controller application in hybrid AC/DC microgrid is capable to enable an IC converter to support the AC-side voltage and frequency as well as the DC-side voltage. The proposed control application of the VSM is chosen based on a comprehensive assessment of VSM control algorithms that are exist in the literature. Moreover, proposing an autonomous operation control of the VSM intertying converter based on dual droop characteristics which is quite different compared to using only current controller. The autonomous operation of the intertying converter based on dual droop control is modified and proposed to be capable to feed the VSM controller (swing equation) to ensure accurate power exchange management between the AC and DC sub-subsystems. The most important portion for the hybrid microgrid system is the stability study due to that fact that the behavior of the system when it is subjected to a temporary disturbance is the main concern. In hybrid microgrid, the disturbances take place continuously because of the load changing endlessly. Satisfying the hybrid microgrid operation during the disturbances conditions must be achieved in order to supply the demand. Therefore, the second part of the research introduces a generic small-signal state space model of the hybrid AC/DC microgrid system, and built to carry out the stability analysis. The development of the small-signal state-space model for the entire hybrid AC/DC microgrid was developed to investigate the overall system stability under different operating points. The final part of this thesis reveals three serious issues of operating hybrid AC/DC microgrid; some of these issues are temporary take a place based on the system operating conditions. In hybrid AC/DC microgrid, an Intertying converter (IC) becomes harmonics voltage source due to the antiparallel diodes and the shunt capacitor at its DC side. The nonlinearity behavior of ICs introduces another operation issue that is circulating current in case of parallel ICs. Reconnecting an IC after abnormal operation condition or schedule maintenance requires an extra challenging synchronization control due the variation of the AC subgrid voltages and frequency; which is the third issue. This part proposes a solution for all these issues by developing a new control strategy that combines the VSM control concept with a dual based droop control. The developed VSM controller on the IC solves these issues. The test system used in this research, which is simulated in a PSCAD/EMTDC environment, consisted of simulated voltage source converters with two AC voltage levels; while the stability analysis is conducted in MATLAB environment.
Modelling And Control Strategies For Hybrid Ac Dc Grids
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Author : Adedotun Jeremiah Agbemuko
language : en
Publisher:
Release Date : 2019
Modelling And Control Strategies For Hybrid Ac Dc Grids written by Adedotun Jeremiah Agbemuko and has been published by this book supported file pdf, txt, epub, kindle and other format this book has been release on 2019 with categories.
The proposals for large-scale deployment of renewable energy sources is leading to the need for more cross-border interconnections creating a pan-European power system, at the EU level. Such interconnections may take advantage of existing AC systems, but also of the DC links or DC grids under development; hence, hybrid AC/DC grids. Although hybrid AC/DC transmission grids are far from being truly large-scale, they are beginning to evolve as key components of future transmission infrastructures. By hybrid AC/DC grids, power electronic interfaces are invariably implied. As a consequence, several challenges are beginning to emerge at the network level due to the increasing adoption of these devices.The dynamics and interactions that may appear in such integrated systems are relatively unknown since they will interconnect at least two subnetworks of AC and DC characteristics. Understanding how several classes of interaction could occur is key to proper design of controllers to mitigate them. Further, changes to how subnetworks should be securely operated in unison is required. Therefore, a rethought is necessary in the presence of these devices. Nevertheless, the answer to the question of how to detect detrimental behaviours? Is central to any proposed solution.Currently, traditional methods for modelling and analysis are showing inadequacies. Lack of consistent methods to model and analyse the phenomena often result to complicated solutions as existing literature suggests. Even more important are models that are tractable, flexible, and technology agnostic to allow abstraction of the underlying challenges. Hence, methods to better understand and assess the mechanism of interactions at system level, that may impact secure operation are required. In addition, methods that are intuitive and efficient to detect sources of interactions, and isolate them as rapidly as possible are preferred. This goes to the heart of flexibility and tractability. Therefore, this thesis presents methodologies and strategies for modelling and control of large-scale hybrid AC/DC transmission grids from a systematic perspective, with the consideration of the controllable devices.This thesis employs several potent high-level methodologies that possess physical connotation, are technology agnostic, and provide tractability for control. Subsequently, recommended control strategies are easy to adopt as their physical significance can be established. The principal findings of this thesis are that, system interactions between subnetworks are dependent on the broad characteristics of the each subnetwork. Thus, manipulating any of these characteristics subject to considerations, improves the overall behaviour. Within each subnetwork, interactions depend mainly on the dynamics of existing controllers, and the interconnection between several devices. Then, questions on how to detect and mitigate interactions as efficiently as possible, while incorporating the most relevant behaviour is answered. Furthermore, the conflict of control requirements of each subnetwork of the hybrid AC/DC network is highlighted. Thus, information about these requirements are leveraged to achieve overall compromises without jeopardizing minimum performance. To conclude, following the assessment of detrimental interactions and their corresponding mechanism, control strategies that take these into consideration are proposed and demonstrated.
Modeling And Control Coordination Of Power Systems With Facts Devices In Steady State Operating Mode
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Author : Nguyẽ̂n Văn Liêm
language : en
Publisher:
Release Date : 2008
Modeling And Control Coordination Of Power Systems With Facts Devices In Steady State Operating Mode written by Nguyẽ̂n Văn Liêm and has been published by this book supported file pdf, txt, epub, kindle and other format this book has been release on 2008 with Electric power distribution categories.
This thesis is devoted to the development of new models for a recently-implemented FACTS (flexible alternating current transmission system) device, the unified power flow controller (UPFC), and the control coordination of power systems with FACTS devices in steady-state operating mode. The key objectives of the research reported in the thesis are, through online control coordination based on the models of power systems having FACTS devices, those of maximising the network operational benefit and restoring system static security following a disturbance or contingency. Based on the novel concept of interpreting the updated voltage solutions at each iteration in the Newton-Raphson (NR) power-flow analysis as dynamic variables, the thesis first develops a procedure for representing the unified power flow controllers (UPFCs) in the steady-state evaluation. Both the shunt converter and series converter control systems of a UPFC are modeled in their dynamical form with the discrete time variable replaced by the NR iterative step in the power-flow analysis. The key advantage of the model developed is that of facilitating the process of UPFC constraint resolution during the NR solution sequence. Any relative priority in control functions pre-set in the UPFC controllers is automatically represented in the power-flow formulation. Although the developed UPFC model based on the dynamic simulation of series and shunt converter controllers is flexible and general, the number of NR iterations required for convergence can be large. Therefore, the model is suitable mainly for power system planning and design studies. For online control coordination, the thesis develops the second UPFC model based on nodal voltages. The model retains all of the flexibility and generality of the dynamic simulation-based approach while the number of iterations required for solution convergence is independent of the UPFC controller dynamic responses. Drawing on the constrained optimisation based on Newton s method together with the new UPFC model expressed in terms of nodal voltages, a systematic and general method for determining optimal reference inputs to UPFCs in steady-state operation is developed. The method is directly applicable to UPFCs operation with a high-level line optimisation control (LOC) for maximising the network operational benefit. By using a new continuation technique with adaptive parameter, the algorithm for solving the constrained optimisation problem extends substantially the region of convergence achieved with the conventional Newton s method. Having established the foundation provided by the comprehensive models developed for representing power systems with FACTS devices including the UPFC, the research, in the second part, focuses on real-time control coordination of power system controllers, with the main purpose of restoring power system static security following a disturbance or contingency. At present, as the cost of phasor measurement units (PMUs) and wide-area communication network is on the decrease, the research proposes and develops a new secondary voltage control where voltages at all of the load nodes are directly controlled, using measured voltages.