This invention presents a modeling approach for unified harmonic and dynamic stability assessment of systems subject to unbalanced faults. The approach involves building a generalized dynamic circuit representation using operational calculus and translating the voltage and current relationship at the fault location into the relationship of three sequence subsystems. The final circuit model is an interconnected sequence network with impedances expressed in the Laplace domain associated with voltage and current dynamic phasors. An example case of an induction motor served by a grid through a series compensated line is used to illustrate the modeling procedure. The analysis results generated by the dynamic circuit corroborate the electromagnetic transient simulation results which demonstrate that sub-synchronous oscillations can be mitigated when a single line to ground fault is applied at the motor terminal. This invention provides insights into why imbalance can improve damping of the subsynchronous mode. Overall, the invention presents a potentially powerful approach for analyzing and addressing harmonics issues, and stability issues in systems subject to unbalance, which can occur in both low and high voltage AC systems.
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