Abstract
The bidirectional power flow through the interlinking converter (IC), in ac/dc hybrid microgrids (HMGs) consisting of distributed generators (DGs) with droop controllers, plays an important role on the stability of such systems during islanding. This paper investigates the impact of the power flow direction on the small-signal stability of islanded droop-based HMGs. Firstly, a linearized state-space model of an HMG is developed. Secondly, eigenvalue analysis is carried out to realize the dominant modes, which are the rightmost eigenvalues. Thirdly, participation factor analysis is performed to identify the system and control parameters that effect stability the most. Lastly, sensitivity analysis is conducted to determine the critical droop gains and stability margin. It is observed from the eigenvalue and sensitivity analysis that the dominant modes of HMGs become more stable as more power flows from dc to ac subgrid. On the contrary, an increase in the power flow from ac to dc subgrid degrades the HMG stability. Additionally, the sensitivity of the dominant modes to changes in ac and dc droop gains is studied under bidirectional power flow through the IC to ascertain their impact on the stability margins. Finally, time-domain simulations, in MATLAB/Simulink, suggest that more generation on the dc subgrid would enhance the overall HMG stability margin during islanding.
Highlights
Hybrid microgrids (HMGs) are evolving with the ability to exchange power between ac and dc subgrids
In the light of the autonomous operation of HMGs, the interlinking converter (IC) adopt voltage source converters (VSCs) that employ a droopbased control strategy to decide on the direction of the active power flow between ac and dc subgrids
This paper investigated the impact of the power flow on the stability of HMGs formed by the interconnection of ac and dc subgrids through bidirectional ICs
Summary
Hybrid microgrids (HMGs) are evolving with the ability to exchange power between ac and dc subgrids. In [13], a control strategy utilizing ICs for autonomous operation of HMGs is introduced to manage the power flow between subgrids. In the light of the autonomous operation of HMGs, the ICs adopt voltage source converters (VSCs) that employ a droopbased control strategy to decide on the direction of the active power flow between ac and dc subgrids. This paper proposes a system-level systemic approach, which contributes with: 1) The eigenvalue analysis of the overall system to realize the dominant eigenvalues; 2) The participation factor analysis to identify the system and control parameters that affect the dominant eigenvalues the most; and 3) The sensitivity analysis to determine the critical droop gains in order to ensure stable and oscillatory-free bidirectional power flow. It is worth pointing out that droop-based DGs are either renewable-based DGs with storage or dispatchable DGs such as micro turbines and fuel cells [21], [22]
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