Reactive Power Management in Islanded Microgrid—Proportional Power Sharing in Hierarchical Droop Control




A microgrid (MG) is a local energy system consisting of a number of energy sources (e.g., wind turbine or solar panels among others), energy storage units, and loads that operate connected to the main electrical grid or autonomously. MGs provide flexibility, reduce the main electricity grid dependence, and contribute to changing large centralized production paradigm to local and distributed generation. However, such energy systems require complex management, advanced control, and optimization. Moreover, the power electronics converters have to be used to correct energy conversion and be interconnected through common control structure is necessary. Classical droop control system is often implemented in MG. It allows correct operation of parallel voltage source converters in grid connection, as well as islanded mode of operation. However, it requires complex power management algorithms, especially in islanded MGs, which balance the system and improves reliability.





The control solution, called droop control, includes many VCL sources and provides possibility to many different RES interconnection. The idea of droop control is based on active and reactive power related to voltage frequency and amplitude droop on coupled impedances. Unfortunately, classical droop control method with proportional droop coefficients does not provides proper reactive power sharing between converters connected to common ac bus. In classical approach, the equal reactive power sharing (ERPS) can be obtained only when active powers are equal and droop coefficients are well chosen. When active powers are changing, the reactive power sharing cannot be controlled causing overload or reactive power circulation between converters. Moreover, the important issue in droop control is static trade-off between voltage regulation and reactive power



          In islanded mode of operation there is the need to manage reactive power sharing and allow RESs work with maximum active power. Hence, the new reactive power sharing algorithm was proposed in this paper, based on the analysis of power sharing between converters in MG. The novel solution prevents the reactive power circulation and disconnection or damage of any converter in MG. Moreover, it allows to converters operation with MPPT, causing better exploitation of each RES and keeping apparent power of each unit below nominal level as long as possible.

Because of short switching period of power electronics converters in RES, the algorithm was developed for implementation in hierarchical control structure, providing parallel calculations in each PCU. Simulation analysis was performed, where the three solutions of power control in islanded MG were compared what confirms the correct operation of developed algorithm and shows the advantage of proportional power sharing over others solution presented in literature.



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Adam Milczarek, Student Member, IEEE, Mariusz Malinowski, Fellow, IEEE, and Josep M. Guerrero, Fellow, IEEE, “Reactive Power Management in Islanded

Microgrid—Proportional Power Sharing in Hierarchical Droop Control”, IEEE TRANSACTIONS ON SMART GRID, VOL. 6, NO. 4, JULY 2015.