Asynchronous motor protection against dynamic instabilities

This paper deals with the dynamic behavior of the electric power system of a Venezuelan oil refinery (El Palito Refinery, Puerto Cabello, Venezuela), which includes a 6000-hp asynchronous motor working as a motor or generator (M/G) coupled to the power recovery unit (PRU) shaft at the fluidized catalytic cracking plant (FCC). Problems associated with inappropriate maintenance at the external power utility, whose system is linked with the national interconnected grid, have decreased the electric power system reliability at the refinery. Critical failure events, up to third-order contingency events, have been occurring at the refinery frequently over the past few years. Once these kind of events take place, the 6000-hp motor tends to balance the system generating or consuming electric power, but due to the large equivalent inertia in the external utility, the 6000-hp motor trips before the refinery normal operating conditions have been restored. Then, both the FCC and PRU shut down, the FCC Plant being a vital process unit within an oil refinery. This paper involves a dynamic study of those critical failure events in order to properly set protective relaying schemes and, thus, ensure a safe and reliable operation of the refinery electric power system, while establishing dynamic stability limits for the M/G.