Hybrid Phase Angle Monitoring Using PMU and Power Flow Models

Measurement Units (PMUs) enable accurate, time-synchronised angle monitoring; however, limited deployment reduces observability at system boundaries, while EMS power flow and state estimation results may deviate from actual system behaviour, particularly in weakly observable areas. This paper proposes a hybrid approach combining PMU measurements with

EMS model results to extend phase angle monitoring and assess model accuracy. Three estimation modes are considered: PMU–PMU as a measurement-based reference, Model–

Model relying on EMS estimates only, and PMU–Model as a hybrid configuration for partially instrumented corridors. The method is demonstrated on selected Croatian 400 kV corridors using minute-averaged PMU data and EMS-derived values from SCADA/state estimation. The results show that EMS-based Δθ trends generally follow the PMU reference under stressed operating conditions, with typical deviations within approximately 1° (95th percentile), while occasional short-duration larger discrepancies are linked to temporary state estimator reliability issues. In addition, the rate of change of Δθ (RoCoΔθ) is introduced as a quasi-dynamic screening indicator to highlight significant corridor ramping behaviour. The proposed approach supports practical corridor monitoring and model validation using existing EMS infrastructure and limited PMU deployment.