  Increasing penetration of Photovoltaic (PV) generation brings an opportunity,
and sometimes necessity, for this new resource to provide ancillary services
such as frequency support. Recent efforts toward this goal focused mainly on
the large-scale PV plants with identical subsystems but cannot handle
distributed PV systems with diversities. Therefore, directly applying them to
distributed PV requires repetitive control design for each unit, and thus
implies huge design efforts. In this paper, we propose a novel frequency
support control scheme focusing on the distributed PV to overcome the above
limitation. Considering the diversities, we first derive a reduced-order
aggregate model to represent the overall dynamic behaviors of a group of
distributed PV. Using this model, we can then design one single frequency
support controller for a large number of distributed PV without excessive
design computational efforts. This controller aims at controlling the PV total
output to provide frequency support to the grid. We also supplement it with the
proposed inversion method to obtain individual PV's control signals from the
aggregate one. The proposed reduced-order aggregate model is validated against
a group of distributed PV systems represented by the detailed nonlinear models.
We also demonstrate the effectiveness of proposed control scheme, including
both controller and the inversion method, through time-domain simulations using
a standard test system.
