A combined zone-3 relay blocking and sensitivity-based load shedding for voltage collapse prevention

A typical power system voltage collapse scenarios is often ended with the undesirable operation of the Zone-3 distance relay of the transmission lines. This paper presents a protection scheme to avoid power system voltage collapse using a combined method of distance relay´s Zone-3 blocking scheme and a sensitivity-based load shedding selection. The Zone-3 distance relay blocking is based on the proper differentiation between transmission line overloading and line faulted conditions, using a fast estimation of power flow based on Line Outage Distribution Factor (LODF) and Generation Shift Factor (GSF). The Zone-3 distance relay of the transmission line would be blocked if the power flow change over the line is determined to be due to an overload so that more time would be available for the system to take necessary control actions. One of the important control actions is the emergency load shedding. A method based on the calculated sensitivities GSF to identify the most effective load shedding positions and amounts is proposed. The proposed method has been implemented in the Advanced Real-Time Interactive Simulator for Training and Operation (ARISTO) software with the Nordic 32-bus test system. ARISTO offers the possibility to test the proposed scheme since it can be seen as the virtual power system with all live information. The analyses of power system voltage collapse scenarios with and without the proposed scheme implemented have shown the effectiveness of the scheme to prevent the voltage collapses.