Mathematical Modelling and Numerical Simulation of the Dynamic Behaviour of Thermal and Hydro Power Plants

The global economic and social development process has brought about increasing capacities of electric energy production, transportation and distribution. This fact has required both the development of the national power systems and the interconnection of most of them, leading to real expanded continental power systems. But technological development and territorial expansion have generated new problems concerning their running and monitoring. Planning, designing, leading and running such huge systems has turned to very complex activities, indissolubly linked with their operating stability. The problem of power system stability has got new spatial and temporal dimensions implying the reconsideration of the means and methods of analysis through revising and expanding mathematical modelling in order to get a most accurate numerical simulation of the operating regime1. The operating experience shows that the synchronism of synchronous generators in networking power plants can be lost even at a few minutes after a disturbance has appeared. In this case the phenomena are much more complex and they refer to slow power oscillations on the interconnecting electric lines among large areas which lead to a decrease in frequency and loss of synchronism among these regions. Such phenomena can appear due to the poor performance of the frequency exchange power control and the unsatisfactory answer of the slow action of the governing elements as, for example, those of the boilers, turbines, charging valves, feeding pumps, hydro units etc. In this respect, they speak about Long Term Dynamic (LTD) stability or slow phenomena stability2. From the point of view of time scale analysis, the phenomena which are manifest in Long Term Dynamic processes are minute long, comprising a part of the time allotted to the variation of consumers` electric loading and the values of the time constants of the boilers and steam turbines as well as those of the primary installations of the hydro units. Therefore it is necessary to increase the number of the system elements whose mathematical modelling has to be considered in simulation, so that the main components of the power system be included starting from the thermal, hydro and mechanical primary installations up to the consumers, including the characteristics of the respective elements and the functional relationships between the input and the output values and the assembly as a whole.