Date of final exam: 10/02/1999

E-mail: casella@elet.polimi.it

Tutor: Prof.  N. Schiavoni Politecnico di Milano

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Modelling, simulation, and control of a geothermal power plant   ___________________________________________________________________________________________________________

Advisor:

Prof.  C. Maffezzoni, Politecnico di Milano

Summary of the thesis

In this thesis, the dynamic modelling of the geothermal fluid processing in the ENEL plant of Latera is considered, aimed at building a plant simulator, for the subsequent study of control strategies for the plant.
Some significant problems have been posed by the size and complexity of the plant, by the presence of innovative components (in particular the "reboiler", a plate column for the separation of carbon dioxide from the steam), by the use of two-phase fluids with incondensable gases, and by the presence of long pressurised liquid transport lines, needing distributed parameter modelling.
The steam-water processing plant is characterised by a high degree of complexity and interconnection, comprising five production wells, the system for fluid transport to the main plant, two interconnected circuits for the processing of the geothermal water and of the CO2-steam mixture. Therefore,its dynamic behaviour cannot be simply inferred from that of the single components, but is essentially determined by their interaction. Consequently, it is of paramount importance the availability of dynamic simulation tools considering the plant as a whole (including the control systems).
The models have been developed in view of the implementation in the ProcSim simulation environment, developed in through the years in the Department Control Lab, which is heavily based on decoupling principles to reduce the computational complexity of the simulation. All the components processing two-phase two-components fluid have been modeled from scratch, starting from first principles, since there were no known examples in the literature. The structure of the simulator has been defined with the following trade-off in mind: on one hand, to achieve a process model able to validate the control system performance against all the predicted manoeuvres and faults on the plant; on the other hand, to obtain a tool which was sufficiently simple and manageable by the user.
The simulation of the most significant section of the process in terms of number of state variables and equations (the reboiler circuit) has been also undertaken using the gPROMS dynamic simulation package, developed by the Imperial College of London. This permitted some interesting considerations on the difference between a simulation approach based on decoupling and a fully equation-oriented approach.
The study of the control strategies for the plant was of great interest due to the presence of innovative process part (reboiler, long transport lines), which were never experimented before in similar plants. The availability of a plant simulator allowed to verify the compatibility of the initially proposed architecture of the distributed control system, against all the possible perturbations on the system. This is particularly critical in the case of a pump trip in the exhaust fluid transport system, which could cause severe damage to the plant due to dangerous dynamic behaviour, if unadequately dealt with. The analysis must necessarily be carried out with a non-linear, distributed-parameter model. Subsequently, improvements on the control system architecture have been studied. In the case of the reboiler cycle control, the analysis suggested the opportunity of a radical change in the initially proposed control strategy, towards a solution which guarantees the maximum energetic efficiency for the whole plant. This required an optimisation study on a simplified plant model.

MAIN RESULTS

• Modelling of an innovative power generation process using a two-phase mixture of water and carbon dioxide, along whith the exhaust water reinjection plant, with a distributed parameter modelling approach.
• Building of a full engineering simulator of the plant, available for the evaluation of operation in off-design conditions, large transient simulation, control system structure analysis, tuning and validation. Last but not least, the simulator can be used as a training tool for the people involved in the startup and operation of the plant, before the plant is actually available.
• Analysis of the control problems in the process (more than 20 single loops), resulting into substantial improvement with respect to the solution proposed in the initial project.
• Setting of the control problem in the wider context of the energetic optimisazion of the process operation.

 

MINORS

Modelling and simulation of a large flexible structure by means of piezoelectric actuators

Advisor: Prof. N. Schiavoni, Dipartimento di Elettronica e Informazione, Politecnico di Milano

The aim of the research is the development of an accurate dynamic model of a large flexible structure (a modular truss suspended by coils in a free-free configuration), using embedded piezoelectric actuators, starting from a model of the structure without such devices. It has been shown that, in this particular case, standard modal truncation techniques, if used blindly, give rise to grossly wrong results, and are thus unadequate. A simple yet effective extension to the standard model is presented, resulting in a good match with the available experimental data. The model includes the non-linear hyseresis effect of the actuators. The implications of these results on the control system synthesis have been discussed. Moreover, the accuracy of the model has also been verified in closed-loop experiments. Subsequently, two control systems for vibration suppression have been studied, a centralised and a decentralised one, obtaining good results in simulation and, then, in the following experimental activity. Finally, a "hybrid" control system has been studied, integrating the formerly developed control system based on air jet thrusters with the piezo controllers, again with good results.
 

Synthesis of a logic controller by means of Petri nets

Advisor: Prof. L. Mezzalira, Dipartimento di Elettronica e Informazione, Politecnico di Milano

The firs part of the research consists in a literature review on Petri Nets, both in general and, in particular, when applied to logic control of industrial plants. Some extensions to standard Petri Nets are described, which are of interest for the application under examination. Subsequently, the synthesis of a logic controller for a particular plant is carried out: the higher level (coordinating control) is specified by means of a coloured, synchronised Petri Net, which is synthesised through iterated refinements using a methodology found in the literature. The correctness of the specification is checked using classical techniques such as the reachability graph and the analysis of invariants. Finally, the problem of translating the net into executable code for a low-cost PLC is dealt with. The study is not concerned with the low-level details of the implementation on a specific PLC, which would be too specific. The high-level translation algorithm of the net in a generic programming language is given instead, with general guidelines for the actual implementation on a specific industrial PLC.

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