by João Miranda Lemos
Modelling and simulation is an important tool to study the impact of new control systems and structures in industrial processes. In this framework, INESC is cooperating with CPPE (Group EDP), the main Portuguese company devoted to the production of electricity, in order to model thermoelectric power units.
The main project developed so far is concerned with a 125MW group burning fuel-oil, now in operation for 20 years, which will be renewed in the near future. The objective was to understand the limits of performance of the plant with the existing control systems, and to forecast the possible improvements achievable by using new ones.
After an initial analysis of the problem, which included careful planning, experiments were conducted for two days in the plant. A data acquisition system consisting of a portable personal computer with two 16 channel data acquisition boards was installed in the plant to record 28 variables at a sampling rate of 3Hz. Some variables that were not instrumented were read manually during steady state periods. A number of important experiments were conducted, namely observation of the open loop response of the drum boiler level, and emergency replacement of one feed-water pump by another.
The data collected was then used to calibrated computer models of parts of the unit (condenser and water preheating subsystem and drum boiler level). These models were based on physical equations, polynomial fittings of the nonlinearities and dynamical regression models of ARMAX type identified from data. The mathematical symbolic manipulation package MATHEMATICA was used to eliminate algebraic loops in the model. Mathematical model fitting and identification were performed with MATLAB. The final model was implemented in MATRIXX, a modelling CAD system in which the model is described as an interconnection of blocks representing mathematical operations. Once the models were developed, they were used to simulate a number of situations, concerning different conditions of operation and control structures. The results and conclusions thus obtained served as a basis for the team of engineers preparing the renewal of the unit.
It is important to say that in all the phases of the project there was a tight cooperation between the researchers of INESC and the experts of CPPE. INESC brought to the project its experience on data acquisition, mathematical modelling, identification of dynamic systems, software for simulation and control design. CPPE contributed with its knowledge of Power Plants, in particular the dynamics of thermoelectric units, and experiment planning and conduction. The integration of both teams, which led to technology transfer in both senses, was made possible due to a past of several years of cooperation in other projects between INESC and EDP (from which CPPE issued).
The experience obtained in this area, namely the methodology of experiment planning, data collection, modelling and simulation, can be transposed to other types of industrial processes, in order to achieve good results in situations far from ideal.
The data acquisition system in stalled in the plant controll room