Bi-level Control Structure with Accurate Real and Reactive Powers Sharing for Low Voltage Microgrids with Highly Resistive Line Impedances

Microgrid is a power system incorporating renewable energy sources and energy storage systems in a small geographic area. Since the microgrid covers small area its voltage rating is low. Therefore, the low voltage microgrid has highly resistive line impedances unlike a conventional power system with highly inductive line impedances. Thus, the power decoupling assumption based on the highly inductive power lines is no longer valid and the primary level droop control and higher level controls have to be redesigned. This paper proposes a bi-level control structure, combining the secondary level control with the primary level droop control, for the highly resistive low voltage microgrids with distributed photovoltaic generation sources and energy storage systems. The primary level droop control implements P -V and Q - f strategy. Then, the primary level droop control frequency and voltage references are set by the secondary level control for the frequency and average voltage restorations, state of charge balancing and reactive power sharing between renewable energy sources and energy storage systems. Presented simulation results carried out on the RTDS real-time digital simulator verify the proposed control structure on a microgrid with switching level converter models.