Numerical backup protection scheme based on alienation indices of voltage and current measurements practically applied to synchronous generators

Differential current relays are the primary protection for detecting fault disturbances within the protection zone of three-phase synchronous generator stator windings. This paper presents a numerical backup protection scheme, based on the alienation algorithm, which serves as a redundant protection when the main differential current relay fails to isolate stator winding faults. The proposed method quantifies a set of alienation indices derived from fifteen Pearson correlation coefficients, calculated for the three-phase voltages and currents measured at the load terminals of the generator stator windings. A novel set of tripping characteristic curves, formulated from the alienation indices, enables the implementation of secondary protection functions. The method effectively detects fault disturbances and voltage/current imbalance conditions in the three-phase generator signals. The experimental validation is performed using a motor-generator set equipped with voltage and current transformers installed at the generator load terminals. The system hardware is emulated through a DAC and personal computer, while the alienation algorithm is implemented and processed using the LABVIEW software environment. Extensive experimental investigations were carried out to validate the proposed method under various fault scenarios affecting the generator. The results demonstrate the alienation algorithm's notable simplicity, effectiveness, reliability, and efficiency. Quantitatively, the protection scheme achieved security and dependability levels exceeding 99.80%, while its reliability and accuracy surpassed 99.60%. Moreover, the proposed scheme reduces the protection malfunction rate to only 0.37%. Compared to the coherence-based alienation method for analyzing voltage and current signals, the correlation-based alienation method provides superior response speed, improved performance metrics, greater accuracy, and a more reliable assessment of asymmetry and disturbance levels. An alienation tripping-time curve is created for the backup protection scheme, enabling time-delayed tripping under fault conditions, and its operating speed can be flexibly adjusted to meet specific protection requirements.