Optimal Stochastic Day-Ahead Power Management of Hybrid AC-DC Microgrids

Due to the reappearance of DC loads in electrical systems and advanced improvement in energy storage systems (batteries) and environment-friendly properties of photovoltaics as a green energy supply, DC architecture is considered as a new solution for next-generation power distribution systems. Hybrid AC-DC microgrids (MG) can take advantage of DC and AC flows in a smart distribution system. The best strategy for the optimal operation of hybrid MGs is to minimize the converting energy between AC and DC sides such that DC loads are provided by photovoltaics, fuel cells, and the stored energy in batteries and AC loads are satisfied by AC-based sources including wind turbines (WTs) and diesel generators (DEs). Accordingly, this paper aims to scrutinize an optimal green power management strategy for hybrid AC-DC MGs from an economic viewpoint while considering photovoltaics as a prior source for the DC side and wind turbines for the AC side. Moreover, the uncertainties of renewable energy sources (RESs), DC and AC loads, and the correlation among them are investigated using the unscented transformation method

Modeling and impedance analysis of a single DC bus-based multiple-source multiple-load electrical power system

The impedance based stability assessment method has been widely used to assess the stability of interconnected systems in different application areas. This paper deals with the source/load impedance analysis of the droop-controlled multiple sources multiple loads system which is a promising candidate in the future more-electric aircraft (MEA). This paper develops a mathematical model of the PMSG-based variable frequency generation system, derives the output impedance of the source subsystem including converter dynamics and shows the effect of parameters variation on source impedance and load impedance. A dynamic droop controller is proposed to provide the active damping to the system. In addition, the impedance analysis is extended to a generalized single bus-based multiple sources multiple loads system in which power losses are also investigated. The aforementioned analytical result is confirmed by experimental results

Phase-Locked Loop Control In Low-Inertia Grid-Connected Voltage-Source Converters

As the integration of renewable energy on the grid increases, the number of voltage-source converters (VSC) installed also does. VSC controls both switch turn-on and turn-off, allowing a dc voltage source to be switched between phases. For the converter to accurately synchronize with the grid, a phase-locked loop (PLL) is used for the frequency measurements of the grid. However, the implementation of PLL with measurement delay introduces harmonics, noise, high frequency, and voltage oscillation to the system due to its dynamics. The dynamics introduced to the grid can be ignored under stiff grid conditions, but power from renewable sources decreases the grid inertia creating a weak-grid condition. Older grids accommodate this by using generators that compensate for the rate of change of frequency (RoCoF). Modern grids have less generator to accommodate the RoCoF, so there is a desideratum to implore a robust controller that responds quickly to the RoCoF, disturbance/ distortion rejection, and noise immunity to the grid. In recent literature, the effect of the PLL dynamics on a weak grid has been of great concern because of its unmodeled dynamics that destabilizes the converter under the weak-grid condition. This thesis proposes showing the impact of the weak-grid on the VSC as the dynamic of the grid changes. It also provides remedies to the grid instability and high-power injection levels. The detailed PLL dynamics model, including the ac-bus voltage dynamics with constant frequency, is developed and linearized. Even at a fixed frequency, various factors play a role in grid instability, and this tremendously affects the ability of the VSC to control the grid efficiently. The effect of the PLL gain under the weak-grid condition is analyzed

Application of synchronous grid-connected controller in the wind-solar-storage micro grid

U zadnje je vrijeme poraslo zanimanje za uporabu raspodjelnih generatora (DG) ne samo za uključivanje energije u mrežu već i za poboljšanje kvalitete energije. U ovom se radu predlaže metoda regulacije SVPWM (space voltage pulse width modulation) za sinkroni mrežno povezani upravljački uređaj u mikro mrežu pohrane energije vjetra-sunca. Ta se metoda zasniva na odgovarajućoj topologiji sinkronog upravljačkog uređaja. Mikro mreža pohrane energije vjetra-sunca upravlja se tako da se istovremeno uključi u mrežu dok se ne završi održavanje ili popravi ujednačeni izvor energije DG u nekoj točki, razlika u naponu koja postoji između DG i obje strane čvora, naglo povećanje električne energije kada se DG ponovo uključuje u mrežu. To također utječe na normalan rad drugog DG i na kvalitetu potrebnog opterećenja, a moglo bi čak i ozbiljno paralizirati čitavi sustav. Stoga se topološka struktura i kontrolne metode sinkronog upravljačkog uređaja detaljno analiziraju i prezentiraju se rezultati simulacije. Prezentirani rezultati pokazuju učinkovitost predložene metode sinkronog upravljačkog uređaja povezanog s rešetkom u mikro mrežu pohrane energije vjetra-sunca.Recently, there has been an increasing interest in using distributed generators (DG) not only to inject power into the grid, but also to enhance the power quality. In this study, a space voltage pulse width modulation (SVPWM) control method is proposed for a synchronous grid-connected controller in a wind-solar-storage micro grid. This method is based on the appropriate topology of the synchronous controller. The wind-solar-storage micro grid is controlled to reconnect to the grid synchronously while maintance or repair is being completed to the power source equation DG at some point, the voltage difference that exists between DG and both sides of the node, a large and sudden electrical surge when DG is reconnected to the grid. It also affects the normal work of the other DG and power quality of a necessary load, and could even seriously paralyze the whole system. So, the topological structure and control method of the synchronous grid-connected controller are discussed in detail and simulation results are presented. The results demonstrate the effectiveness of the proposed method in the synchronous grid-connected controller for the wind-solar-storage micro grid

Hybrid ac/dc microgrids. Part I : Review and classification of topologies

Microgrids have been widely studied in the literature as a possible approach for the integration of distributed energy sources with energy storage systems in the electric network. Until now the most used configuration has been the ac microgrid, but dc-based microgrids are gaining interest due to the advantages they provide over their counterpart (no reactive power, no synchronization, increasing number of dc devices, etc.). Therefore, hybrid ac/dc microgrids are raising as an optimal approach as they combine the main advantages of ac and dc microgrids. This paper reviews the most interesting topologies of hybrid ac/dc microgrids based on the interconnection of the ac and dc networks and the conventional power network. After performing a description and analysis of each configuration, a comparative evaluation has been performed to highlight the most important features of each one. The future trends identified during the study also show that several features such as the scalability, modeling or design require further research towards the integration of hybrid microgrids in the power network

Planning and Operation of Hybrid AC-DC Microgird with High Penetration of Renewable Energy Sources

A hybrid ac/dc microgrid is a more complex but practical network that combines the advantages of an AC and a DC system. The main advantage of this network is that it connects both alternating current and direct current networks via an interlinking converter (IC) to form a unified distribution grid. The hybrid microgrid (HMG) will enable the direct integration of both alternating current (AC) and direct current (DC) distributed generators (DGs), energy storage systems (ESS), and alternating current and direct current (DC) loads into the grid. The alternating current and direct current sources, loads, and ESS are separated and connected to their respective subgrids primarily to reduce power conversion and thus increase overall system efficiency. As a result, the HMG architecture improves power quality and system reliability. Planning a hybrid microgrid entails estimating the capacities of DGs while taking technical, economic, and environmental factors into account. The hybrid ac-dc microgrid is regarded as the distribution network of the future, as it will benefit from both ac and dc microgrids. This thesis presents a general architecture of a hybrid ac-dc microgrid, which includes both planning and design. The goal of the Hybrid ac-dc microgrid planning problem is to maximise social welfare while minimising total planning costs such as investment, maintenance, and operation costs. This configuration will assist Hybrid microgrid planners in estimating planning costs while allowing them to consider any type of load ac/dc and DER type. Finally, this thesis identifies the research questions and proposes a future research plan

Design and operation of modular microgrids

textMicrogrids are being considered as a solution for implementing more reliable and flexible power systems compared to the conventional power grid. Various factors, such as low system inertia, might make the task of microgrid design and operation to be nontrivial. In order to address the needs for operational flexibility in a simpler manner, this dissertation discusses modular approaches for design and operation of microgrids. This research investigates Active Power Distribution Nodes (APDNs), which is a storage integrated power electronic interface, as an interface block for designing modular microgrids. To perform both voltage/current regulation and energy management of APDNs, two hierarchical control frameworks for APDNs are proposed. The first framework focuses on maintaining the charge level of the embedded energy storage at the highest available level to increase system availability, and the second framework focuses on autonomous power sharing, and storage management. The detailed design process, control performance and stability characteristics are also studied. The performance is also verified by both simulation and experiments. The control approaches enable application of APDNs as a power router realizing distributed energy management. The decentralized configuration also increases modularity and availability of power networks by preventing single point-of-failures. The advantages of using APDNs as a connection interface inside a power network are discussed from an availability perspective by performing a comparison using Markov-based availability models. Furthermore, the operation of APDNs as power buffers is explored and the application of APDNs enabling modular implementation of microgrids is also studied. APDNs enable the system expansion process—i.e. connecting new loads to the original system—to be performed without modifying the configuration of the original system. The analysis results show that a fault-tolerant microgrid with an open architecture can be realized in a modular manner with APDNs. APDNs also enable simplified selectivity planning for system protection. The effect of modular operation on microgrids is also studied by using an inertia index. The index not only provides insights on how system performance is affected by modular operation of modular microgrids, but is also used to develop a simpler operation strategy to mitigate the effect of plug and play operations.Electrical and Computer Engineerin