Control and operation of a DC microgrid

This thesis presents several aspects of the control and operation of a DC micro grid with variable generation and energy storage. These aspects mainly concern the dynamic performance improvement of energy storage interface converters, a standardized autonomous control strategy for a DC microgrid, and its stability and dynamics assessment method. Several predictive average current control schemes are proposed for a bi-directional DC- DC converter which is used to interface the energy storage device to the DC microgrid. By properly arranging the sampling and duty cycle updating points, optimal control laws are derived for enhanced dynamic performance. Robustness and static error correction are also analyzed. An autonomous 3-level hierarchical control strategy is proposed for DC microgrids with selective slack terminal(s) assigned to each operation level. The system operational status is acknowledged via the common DC voltage and the transitions between different operational levels are triggered by its variation. A 4-terminal DC micro grid system is established and specific control scheme is outlined to demonstrate the proposed strategy during various operating conditions. During grid-connected condition, a voltage droop based power sharing and coordination strategy among the slack terminals is proposed for power smoothing. A simulation study is carried out to preliminarily verify the effectiveness of the proposed control strategy. A stability and dynamic assessment method is proposed for converter interfaced DC networks. Based on the virtual impedance modelling method, a case study concerning DC voltage variation suppression in a multi-terminal DC network is performed using root locus and open-loop margin analysis. The accuracy of the modeling method and its effectiveness for assessing system stability and dynamic performance is validated using circuit simulation in time domain. A prototype DC micro grid is also established to validate the effectiveness of the control proposals and analytical method.EThOS - Electronic Theses Online ServiceGBUnited Kingdo