Unified Transmission-Distribution Modeling of Virtual Power Plants for Grid Reliability Studies and Grid-Edge Flexibility

(VPP), DERs provide grid-edge flexibility but may introduce reliability challenges. Traditional grid planning tools are not well-suited to evaluate the full locational value that VPPs can provide to transmission and distribution networks.

This paper presents a unified transmission–distribution (T&D) modeling framework that enables grid planners to evaluate T&D reliability and identify VPP congestion relief benefits.

The approach treats VPPs as dispatchable, network-constrained resources analogous to conventional generation while retaining distribution system detail to capture locational impacts.

Production cost modeling is integrated with a balanced transmission model and three-phase distribution feeder representation, enabling hourly or sub-hourly simulations that reflect timevarying behavior of DERs such as weather dependency, energy limits, and price responsiveness. Soft network limits convert congestion into locational price signals that drive

VPP responses; unbalanced power flow studies validate voltage and thermal performance.

The framework is demonstrated through a case study on the Vermont power system, which has one of the highest DER concentrations in the United States. N-1 contingency analysis shows that targeted VPP dispatch can relieve transmission congestion without adverse distribution system impacts. This work illustrates a pathway for studying VPP benefits for transmission congestion management.