The fast start-up and load-increasing of power plants is a complex task involving several restrictions that have to be fulfilled simultaneously. An important restriction is the maximum allowed thermal stress of the steam generator pipes and the steam turbines caused by temperature gradients.

In this paper the start-up process is treated as a dynamic optimization problem. Any appropriate objective function can be used in this optimization problem. Examples include the minimization of fuel consumption, or the minimization of the time required to reach the desired load. The maximum allowable temperature and pressure gradients in major plant components appear as additional constraints.

A general methodology for solving these problems is presented: The dynamic process model, consisting of first-order ordinary differential equations (ODEs) and algebraic equations, is discretized over the time horizon using well established methods for the solution of ODEs. Thus, the continuous dynamic optimization problem is transformed into a large-scale non-linear parameter optimization problem with up to 20,000 optimization parameters and constraints. Such parameter optimization problems can be solved with appropriate sequential quadratic programming (SQP) methods that have become available lately.

The application of this method is demonstrated by optimizing the process of rapid load-increase in a single-pressure combined-cycle power plant on the base of a simplified model.

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