A state estimation based approach to obtain an external network equivalent and track topology changes is developed in this work. The proposed approach enables real-time detection and identification of topology-based external events. Following an orthogonal parameter estimator, an index related to the measurement residuals is tracked to detect external disturbances, which are classified based on reference values and actual system data. The method makes use of boundary bus voltages, external currents, an initial snapshot of the network topology, as well as the sensitivity of external changes to the equivalent network. Tests on the IEEE 300-bus and Brazilian 6092-bus systems highlight its capabilities in detecting and identifying shunt, single and double line outages and insertion events.