  The unique problems and phenomena in the distributed voltage control of
large-scale power distribution systems with extremely-high DER-penetration are
targeted in this paper. First, a DER-explicit distribution network model and
voltage sensitivity are derived. Based on that, a voltage inference method is
implemented to fill the gap of measurement insufficiency in the grid-edge
areas. Then, autonomous Q control being implemented in each local area, a
$\overline{Q}$-coordinated P control is designed to coordinate the reactive and
real power controls. All the algorithms have been tested in standard and
synthetic systems, and have expected results. Moreover, an open-source software
platform, which integrates the modeling of communication networks, DER
controls, and power networks, is developed to enable the distributed control
and optimization algorithms in the grid simulation of the large-scale
distribution systems.
