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Research on Microgrid-based Power Flow Analysis and Flexible Control Technologies

Author: DuanYuBing
Tutor: LiQingMin;WangHui
School: Shandong University
Course: High Voltage and Insulation Technology
Keywords: Microgrid Power flow Analysis Flexible control Islanding detection Energy storage Power quality
CLC: TM727
Type: PhD thesis
Year: 2011
Downloads: 125
Quote: 0
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Abstract


Due to gradual depletion of the available coal resources as well as the pending environmental crises, new power generation technologies based on high efficiency and clean energy resources are nowadays developing very quickly. With a growing proportion in power systems, the distributed generation (DG) can meet various load demands, reduce environmental pollution and improve the overall efficiency of energy transfer and conversion, however, it may also lead to a series of emerging issues when DG sources are connected to the power systems. To counteract the problems such as the impact on power grids caused by interconnection of wind power and photovoltaic generation, as to fully explore the advantages of DGs, the concept of Microgrid has been proposed. Microgrid being a significant and indispensible component of smart grid, its optimal operation depends on the power flow analysis. Furthermore, to account for the specific operational conditions of Microgrid, such as smooth switch of the operation modes, intelligent detection of the islanding state as well as flexible compensation for the power quality issues, optimal and flexible control of the Microgrid voltages and currents needs to be achieved based on power electronic converters, with a view to guaranteeing safe, stable and reliable operation of the Microgrid. The thesis focuses on some key Microgrid issues including power flow analysis, islanding detection, flexible control strategy and power quality problems, the innovative research on which aspects present further development of the Microgrid technologies.Power flow analysis of the Microgrid is the foundation to realize power distribution and optimal operation. According to physical structure of the Microgrid and variety of the microsources, a method for node classification is proposed, and based on the backward-forward sweep scheme, deterministic power flow models are established with consideration of the transfer performance of the microsource converters. Futher, an effective calculation methodology for probabilistic power plows is also developed by incorporating the intermittent and random characteristics of the microsources, which presents a methodological basis for analysis, assessment and optimization of the Microgrid. The effectiveness of the proposed approach is verified by a computation case.Effective islanding detection is critical to ensure reliable and smooth switching between different operation modes of the Microgrid. Traditional islanding detection methods for distributed generation, either active or passive, is no longer applicable for the Microgrid, hence new islanding detection technologies must be further developed. Based on negative sequence harmonic and reactive current decomposition, a hybrid islanding detection methodology is established with combination of a harmonic current distortion rate and a random disturbance of reactive current, which can quickly identify the islanding state of the Microgrid without blind area, and the simulation results show effectiveness and accuracy of the proposed scheme without any side effect on power quality.Bottom-level control of the Microsources forms the base of optimized control strategy for the Microgrid. However, the asymmetrical nature of various system loads and faults may seriously affect the control behavior of the converters regarding different operation modes of the Microgrid. Based on orthogonal transformation in theα-βdomain, a dynamic mathematical model is developed as to eliminate the effects on converter output current control caused by various unsymmetrical factors. Further, with effective current decoupling of the inner controller, an adaptive discrete-time slide mode control strategy for converter output currents is established. Simulation results have verified the feasibility and effectiveness of the control strategy.Energy storage device can provide a backup boost for voltage and frequency stability of the Microgrid, as well as act an important technical scheme to ameliorate power qualities. Through analysis of the impacts of wind power generation on the Microgrid voltage stability, it shows the fluctuations of active and reactive power are the principal culprits to cause voltage instability of the Microgrid. Hence, a voltage stability control method is put forward based on an energy storage topology, and the simulation case verifies the validity and effectiveness of the proposed strategy. To improve the Microgrid power quality, a technical solution is presented with energy storage-based compensation devices. Both ip-iq and p-q methods are combined for detecting harmonics and reactive currents, further, based on tracing control of the Microsource currents, a new scheme with much preferable performance for harmonic and reactive power compensation is proposed. The novel scheme renders advantages of simple structure, easy realization and high reliability, and both simulation and experimental studies have verified its feasibility.The research achievements and outcomes in this thesis present references of significance for developing Microgrid-based key technologies.

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CLC: > Industrial Technology > Electrotechnical > Transmission and distribution engineering, power network and power system > Transmission and distribution technology > The power grid
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