Optimization of a load shedding scheme

An integrated process facility recently went through an update of its existing load shedding scheme in the context of process changes planned for the coming years. Transient simulations indicated that by changing the load shedding philosophy, the frequency profile during power shortages could be significantly improved. This article discusses the plant's electrical/steam load balance issues and the development of a high speed load shedding scheme. Shortage of power in a network is invariably reflected in collapse of frequency. Traditionally, frequency decay patterns have been calculated using simplified methods. Simplifications have included the assumption of constant deceleration constant voltages, and constant generator power. The results obtained using the simplified methods were generally adequate for simple, slow-acting load shedding systems. In the case at hand, the loads assigned for shedding and those to be "saved" by load shedding are part of an integrated process plant sequence, where speed of operation and selectivity of the load shedding is crucial to the continuity of the oil-production process. In the search for ways to optimize the load shedding scheme, transient stability simulations were used to calculate frequency profile, taking into account generators' transient responses and those of the excitation and speed governor systems. This article describes the outcome of the analysis in terms of possible improvements in load shedding performance through the use of rate of change of frequency as the shedding indicator.