The multitudes of inverter-based distributed energy resources (DERs) can be envisioned as distributed reactive power (var) devices (mini-SVCs) that can offer var flexibility at the TSO-DSO interface. To facilitate this vision, a systematic methodology is proposed to derive an aggregated var capability curve of a distribution system with DERs at the substation level, analogous to a conventional bulk generator. Since such a capability curve will be contingent on the operating conditions and network constraints, an optimal power flow (OPF) based approach is proposed that takes inverter headroom flexibility, unbalanced nature of the system, and coupling with grid side voltage into account, along with changing operating conditions. Further, the influence of several factors such as compliance to IEEE 1547 on the capability curve is thoroughly investigated on IEEE 37 bus and 123 bus distribution test systems along with unbalanced DER proliferation. Validation with nonlinear analysis is presented along with demonstrating a scenario with T-D co-simulation.
