Strong-Tracking Observation of Generator Relative Rotor Angle for Transient Control of Cross-Voltage-Level Back-to-Back Converter Stations

Transient control of back-to-back (BTB) converter stations requires a generator relative-rotor-angle signal that remains responsive to post-fault swings while resisting measurement noise. Noisy rotor-angle measurements can trigger premature control actions, whereas excessive filtering can delay exit decisions. This study develops a power-corrected strong-tracking observer for generator relative rotor angle. Starting from a reduced-order relative-angle dynamic model, the observer introduces an AC-power recovery correction coefficient and an active-power measurement constraint to form a three-state observation model. A strong-tracking factor and a Huber-type innovation saturation mechanism are incorporated to retain fast post-fault tracking while suppressing strong-noise disturbances. Offline and closed-loop PSCAD simulations on a three-machine nine-bus system with a 500 MVA BTB station show that, under two-degree rotor-angle measurement noise, the proposed observer improves full-period estimation accuracy relative to a standard extended Kalman filter and a two-state strong-tracking EKF, while preserving low fault-window errors across fault durations. Closed-loop tests further show that the observer avoids premature emergency-power-control exit caused by noisy direct input and delayed exit caused by low-pass-filter lag. Under five-degree noise across ten tested random seeds, the observer reduces the observed counts of delayed exit and instability in the considered PSCAD cases, providing a real-time input for transient control of the tested BTB system.