Dynamic behaviour of material states in steam power plant control

Abstract

The enormous price increase in petroleum and the limited reserves of other fossil fuels have resulted in the increased use of nuclear power plants for the basic load of electricity supply. Consequently, conventional steam power plants have been used to meet the variable and peak loads in electricity generation. The control of fast and large load changes is becoming more and more important for conventional power plant operations. An economical control of the nonstationary operation (start-up, shut-down and large load changes) is possible only by approaching and maintaining the maximum allowable values of the material state variables (temperature θ, strain ϵ and stress σ) at the critical points of the power plant components. To achieve this primary control goal, the dynamic behaviour of material states must be well understood. A direct measurement of the changes in material states especially due to thermal stresses, caused by temperature gradients, is not possible. The material states can be calculated, however, by means of mathematical models describing the nonstationary temperature distribution. The working fluid temperature, pressure and mass flow rate are the input variables for these mathematical models. This paper presents a linear mathematical model for the investigation of the dynamic behaviour of material states. Another mathematical model is also developed for calculating the thermal stresses for large temperature and load changes. Thus, the steam generator models so far describing only the states of the working fluid can be extended to the material states. The basic idea is demonstrated for an insulated thick wall tube containing a working fluid. The simulation of the mathematical models is performed.