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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.