Coordination of Generator Underexcitation Protection with Underexcitation Limiter to Prevent Generator Tripping During Power Transformer Energization: Asaluyeh Power Plant Case

In Asaluyeh BOO power plant, during energizing the Pars substation 500MVA transformer or each unit of main transformer, it is possible to have a load rejection (tripping) for one of in-service units. This sudden outage is due to the grid voltage decrease during transient mode caused by power transformer inrush current. Regarding to the grid low short-circuit level in this region, and the reactive power fluctuations caused by grid and power plants maneuvers, one of the units is working in the underexcitation area. Eventually, the underexcitation protection trips the generator and Gas Circuit Breaker (GCB) and leads to load rejection. In this paper, this problem is investigated using fault analysis, factors affecting the faults occurrences and dynamic tests performed at different load levels. MATLAB and DIgSILENT software are used in order to simulate the system. Based on simulation results, correct settings for underexcitation protection (Ansi 40G) and Underexcitation Limiter (UEL) are respectively proposed, to prevent units unintended outage and their negative effects on grid transients.