Cascaded Droop and Inverse Droop Regulation for Two-Layer Coordinated Power Flow Control in Series-Connected Power Cells

Abstract

To interconnect string power cells into high voltage grid in a distributed manner, a cascaded power flow regulator is proposed for simultaneous control of the injected power to the grid and flexible power sharing among string power cells. First, the traditional frequency and voltage magnitude droop control at the central controller behaves as the outer power control loop for an accurate regulation of the system power flow to main grid. Then, by setting the frequency reference and voltage magnitude reference from outer power control loop as base values for each power cells local controller, inner voltage magnitude droop for apparent power S control and the frequency inverse droop for P/E control are implemented at each power cell local controller to achieve well-decoupled real and reactive power sharing among series-connected power cells. With this control architecture, the power cells and the entire grid-tied system can obtain the flexible power regulation in a fully voltage control manner. Accordingly, seamless grid-tied to islanding operation mode transfer and power sharing of the system with other converters in autonomous islanding operation can be easily obtained.