  This paper proposes a recursive, virtual admittance based, secondary
controller for DG units that improves the voltage profile in distribution
networks. First, the adaptation of the virtual admittance concept for the goal
of voltage regulation is explained. Then, a recursive secondary controller is
developed to periodically update the virtual admittance gains. The controller
is formulated as an optimization problem with current and stability limitations
as constraints. Measurements across the grid, transmitted through low-bandwidth
communications, are used to simplify the calculations, resulting in a recursive
algorithm. Weight vectors are included in the objective function to allow
participation flexibility of each converter. Results show that the primary
virtual admittance controller is able to mitigate over- and under-voltages in
steady state and under transient conditions. Subsequently, the secondary
controller is shown to further improve the voltage profiles across the grid.
Experimental results obtained from a laboratory environment, comprising three
DG units and a grid emulator, validate the functionality of the complete
control structure.
