Comparative Methods for Flexibility Estimation and Load/Loss Analysis in TSO/DSO Coordination with DERs

The modern power system is undergoing vast transformation due to the growing penetration of of Distributed Energy Resources (DERs) such as Solar, Wind and other DGs within distribution networks. This paradigm shift asks for coordinated efforts between Transmission System Operators (TSOs) and Distribution System Operators (DSOs) to maintain grid stability and resilience. This paper compares the Linear DistFlow and Forward-Backward Sweep methods for estimating flexibility at the Transmission System Operator - Distribution System Operator (TSO-DSO) interface in an IEEE 33-bus distribution system. Using Monte Carlo simulations over 10,000 scenarios, both methods evaluate flexibility potential with distributed energy resources (DERs) such as Diesel Generators (DGs), Wind, and Photovoltaics (PV). Results indicate distinct performances in terms of load and loss reduction, flexibility range, and computational efficiency, offering insights into each method's effectiveness in modern distribution systems.