  A virtual power plant (VPP) facilitates the integration of distributed energy
resources (DERs) for the transmission-level operation. A challenge in operating
a VPP is to characterize the cost function over its feasible power transfer
region under DERs' uncertainties. To address this challenge, a characterization
method is presented in this paper for the intraday operation of a VPP based on
the concepts of nonanticipativity and robustness to DERs' uncertainties. The
characterization stems from designing a second-order cone programming (SOCP)
problem, based on which a feasible power transfer region across all time
periods is constructed by exploring boundary points at each time period and
establishing time coupling constraints. Furthermore, a cost function over the
feasible power transfer region is formulated as a convex piecewise surface
whose breakpoints are obtained by solving SOCP problems, together with a
constant compensation cost from a linear programming problem. Finally, to
alleviate the heavy computational burden brought by numerous DERs, an
approximation method is presented by identifying the critical DERs whose
uncertainties have dominant impacts. The effectiveness of the presented methods
is verified by the numerical experiments in a 3-bus system and the IEEE 136-bus
system.
