Investigation of a cascaded tripping incident

This paper investigates an incident that occurred in a grid connected aluminum smelting plant that resulted in cascaded tripping of power plant parts and eventual system instability. The incident was initiated by tripping of an interconnector due to the operation of its directional over-current relay. The results of the power system simulation studies are described to explain the transient performance of the system during the incident. Having identified the main area of the electrical system weakness, an under frequency load shedding scheme is designed and implemented in practice to maintain the system stability during the most severe loss of generation scenarios. The paper demonstrates how using a detailed dynamic simulation of the electrical network can be used to assist with post event analysis, and provides additional insight into the dynamic performance of the system which cannot be obtained using only SCADA or PMS data. The good correlation between measured and simulated data confirms the veracity of the model and gives confidence in the proposed under frequency load shedding scheme.