Abstract
Abstract As the requirement of expensive and unreliable high band-width communication infrastructure is obviated, decentralized droop-like control method has been considered for power sharing implementation in autonomous microgrids (MGs). To this end, the power network is regarded as a communication link and voltage variables (magnitude and frequency) as control signals. This, however, reduces the stability margin of islanded MGs due to the interaction of droop controllers through the power network. Lack of inertia of droop-controlled power converters and low X/R ratio of interconnecting power lines intensify this interaction which may lead to the instability of Networked MGs (NMG). On the other hand, the existing parallel-based small signal model of MGs is inadequate to represent this interaction, as the adopted common-based reference frame (RF) is not applicable in islanded NMGs. This issue is investigated in this work in details and, inspired from power flow equations, a local RF is proposed to improve the small-signal model accuracy. Droop controllers are also correlated through the power flow equations to properly model their interaction through the power network. Moreover, the state-space model is developed in a fully decentralized approach which does not rely on any converter for any specific role. Eigenvalue analysis and simulation in MATLAB\SIMULINK platforms are executed to evaluate the effectiveness and accuracy of the proposed model.
Highlights
As the requirement of expensive and unreliable high band-width communication infrastructure is obviated, decentralized droop-like control method has been considered for power sharing implementation in autonomous microgrids (MGs)
The MG’s power network, common bus and common loads are modeled in the common reference frame (CRF) and the small signal models of individual voltage source inverters’ (VSIs) are transformed to the CRF; noting that the phase angle of the CRF varies in the islanded mode, these transformations haven’t considered the dynamics of the CRF in the literature
In order to demonstrate the interaction of droop controllers, simulation results for a meshed MG is compared with a parallel MG consisting of three distribution generation (DG) units
Summary
The microgrid (MG) concept has been proposed to gain the full advantages of renewable energy resources (RESs) penetration into power systems [1]-[2]. State-space models of all the components are modified to be incorporated in the model; Based on the modified model presented in this work, it is found that, in addition to droop gains, the X/R ratio of feeder impedance in the NMG (cross-coupling effect), power network topology, phase angle difference as consequence of increasing load demand, cut-off frequency of the low pass filters, and interaction of droop controllers are the most important factors influencing the stability margin of NMGs, while electromagnetic transient of power lines is not effective in critical mode oscillations and can be ignored; The proposed model is a reduced-order model (Highlight3) that models power network via power flow equations and ignoring power line dynamics.
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