Dynamic Modelling

Dynamic modelling has assumed an important role in power plants design in particular
when control issues are considered. It represents a solid tool for management of
power plants, contributing to the training of the sta ff and the prevention of emergency
situations. In the last years new methodologies for dynamic modelling have been devel-
oped and collected into the programme language Modelica.

Modelica language

Modelica has been introduced in 1997 and it is supported by the Modelica Association, a non-pro fit organization. The main characteristics are:

A-casual approach: All the required equations to describe a physical component (i.e.
turbo-generator, pump, valve) are expressed in terms of a di fferential-algebraic-
equation (DAE) system, which is the natural mathematical structure for dynamic
models. Inputs and outputs of a model are not set “a priori”, so that the
model results completely independent from what will be connected to it. This is
an enormous advantage that increases the potentiality of the program and facil-
itates the work of the programmer. The a-casual approach marks a signi cant
di fference with object oriented program where the modelling is based on block
diagram structures with casual interactions (i.e. Simulink).
Hierarchy: It represents the possibility of encapsulating subsystem models. In other
words a component may internally consist of other components. The interactions
between the components are allowed by connectors. These are characterized by
two kinds of variables: non- flow variables (i.e. pressure, temperature) and flow
variables (i.e. mass flow, thermal energy flow). If the connectors are compatible, it
results possible to connect several components together, to build a new component.
Furthermore changes to a specific part of a model can be done without influencing
the rest of the model.
Inheritance: It gives the possibility to copy parts of the code of a component into
another component, allowing the utilization of pre-existing code. When program-
ming a new component, it is possible to write a base-component that contains
characteristics common to a group of components, so that it is possible to inherit
it. In Modelica multiple inheritance is allowed as well.
Multi-physics modelling: Modelica allows to implement models of complex systems
from diff erent domains i.e. thermal, hydraulic, mechanical, electrical, electronic
Reusability: The above mentioned characteristics are a strong incentive to the de-
velopment of re-usable libraries. Components already developed and stored in
standard libraries provided by the Modelica Association can be re-used.

Modelica needs the support of a modelling and simulation environment which can solve the system of equations, displaying the results in a fast and easily accessible way. The commercial and free simulation environments availabe can be found here.