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.
- Bidirectional power flow
- Distributed generator
- Droop controller
- Ac/dc hybrid microgrid
Figure 1. General Converter-Based Dg Control Structure.
EXPECTED SIMULATION RESULTS:
Figure 2. Dynamic Responses For Dc To Ac Power Flow Condition Without Ic Reactive Power Support.
Figure 3. Dynamic Responses For Dc To Ac Power Flow Condition With Ic Reactive Power Support.
Figure 4. Dynamic Responses For Ac To Dc Power Flow Condition.
Figure 5. Dynamic Responses For An Increase In The Ac Power Generation Capacity.
Figure 6. Dynamic Responses For An Increase In The Dc Power Generation Capacity.
Figure 7. Dynamic Responses For Ac To Dc Power Flow Condition In Case
Of (Pac V ) And (Qac !) Droop Control In The Ac Subgrid. Figure 16. Dynamic Responses For Ac To Dc Power Flow Condition In Case Of (Pac V ) And (Qac !) Droop Control In The Ac Subgrid.
The operating point including the amount and direction of the power flow between ac and dc subgrids in an HMG largely affects the stability. Thus, 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. It is observed that as the power flow from the ac to dc subgrid increases, the stability margin of the HMG may be reduced. This is mainly because when the power is exchanged from the ac to dc subgrid, the dynamics associated with the ac subgrid have greater influence on the HMG stability as compared to those of the dc subgrid. Moreover, an increase in the generation capacity of the ac subgrid increases the power flow from the ac to dc subgrid to supply the dc load power, which could degrade the stability of the HMG. Thus, it is technically advised to design the HMG such that the ac subgrid receives power from the dc subgrid. The stability analysis presented in this paper is not meant to emphasize that the amount and direction of the power transfer could always jeopardize the stability but rather, precaution should be exercised when transferring power from one subgrid to the other.
 S. Anand, B. G. Fernandes, and J. Guerrero, “Distributed control to ensure proportional load sharing and improve voltage regulation in low- voltage DC microgrids,” IEEE Trans. Power Electron., vol. 28, no. 4, pp. 1900_1913, Apr. 2013.
 R. Majumder, “Some aspects of stability in microgrids,” IEEE Trans. Power Syst., vol. 28, no. 3, pp. 3243_3252, Aug. 2013.
 E. A. A. Coelho, P. C. Cortizo, and P. F. D. Garcia, “Small-signal stability for parallel-connected inverters in stand-alone AC supply systems,” IEEE Trans. Ind. Appl., vol. 38, no. 2, pp. 533_542, Aug. 2002.
 F. Gao, S. Bozhko, A. Costabeber, C. Patel, P. Wheeler, C. I. Hill, and G. Asher, “Comparative stability analysis of droop control approaches in voltage-source-converter-based DC microgrids,” IEEE Trans. Power Electron., vol. 32, no. 3, pp. 2395_2415, Mar. 2017.
 J. M. Guerrero, L. GarciadeVicuna, J. Matas, M. Castilla, and J. Miret, “A wireless controller to enhance dynamic performance of parallel invert- ers in distributed generation systems,” IEEE Trans. Power Electron., vol. 19, no. 5, pp. 1205_1213, Sep. 2004.