Dynamics of the auxiliaries of thermal power stations and construction of relevant model for its implementation in training simulators for system operators

The research activity is carried out in the framework of a collaboration with ENEL and aims at developing a dynamic model of the electrical auxiliaries’ system of a 320 MW thermoelectric steam group, to be included in a real time simulator for power-plant operator training. The model, composed by a set of modules, each representing the dynamic behaviour of a power system component, allows to suitably simulate both the transient phenomena with time constants over a second (such as voltage r.m.s. value and power consumption variations), and the operator manoeuvres, as well as protection interventions. In order to simulate with sufficient fidelity the starting-up of the group or certain severe faults, the model of the auxiliaries includes the modules of the main generator (and its exciter) and that of the generator connection to the main transformer up to the HV busses. The main results have been presented at national and international conferences [1-4]. The training simulator has been enlarged by developing and implementing in it an additional task-model to reproduce the re-start of the power station from remote hydro stations after a complete or partial loss of the transmission system. Such an additional task-model includes the models of the hydro generating units, of the speed governors and of the voltage regulators of the remote hydro station, that of the power restoration lines and relevant loads and that of the electrical system of the thermal power station with relevant regulators. The developed simulator can be used either autonomously, for personnel education on the phenomenology of the various phase of the start-up process, and, suitably interfaced with the real-time simulator, for personnel training aimed at prompt and reliable accomplishment of delicate restoration manoeuvres [4].
 

Dynamic behaviour of steam power plants repowered by gas turbines during autonomous start-up manoeuvres after a blackout

The research activity is carried out in collaboration with ENEL-CESI and, more recently, with the Czech system operator, and aims at verifying the thermoelectric power plant capability to autonomously start-up and restore the electricity service after a black-out, which means, in turn, at assessing and improving the black-start capabilities of these power plants. In particular, the research focuses on steam groups repowered by gas turbines. A dynamic simulator has been built with reference to a 320 MW steam group (SPP), equipped with a one-through boiler, and a 120 MW gas turbine (GT) of a typical Italian power station. The results obtained with the simulator are compared with the experimental data obtained during some power plant manoeuvres. The simulator has allowed the design of a load scheduler control system which co-ordinates the load requests to the GT and the SPP generators of the repowered unit during the black-startup manoeuvre. The manoeuvre is made easier by the presence of the bypass valve that diverts the gas turbine exhaust flow from the exchanger, making the two sections (gas and steam) completely independent in this phase. The crucial role played by this additional control system when the repowered plant performs a black-startup is thoroughly analyzed. It is shown that such a coordinating control unit makes the black-start manoeuvre possible and renders it more reliable and prompt [5-7]. In [8] the results obtained with the developed simulator are compared with those achieved with the simulator of the Czech ISO and with some experimental tests carried out at a thermal power station in the Czech republic.
 

Distributed generation

Part of the activity deals with the analysis of dynamic behaviour after typical perturbations (e.g. short circuit) of distribution systems of in presence of distributed generators connected to the network with power electronic converters, such as photovoltaic plants, micro turbines and fuel cells. In particular the activity deals with the critical assessment of the adequacy of power system protections. The comparison between the effects on the network of the presence of distributed generators (either synchronous and asynchronous) directly connected to it or through static converters is presented in [10]. The analysis in [10] has been carried out with reference to the short circuit currents and protection coordination. Another research topic deals with the analysis and design of small hydroelectric power plants (tens of kW), equipped by pumps used as turbines (without regulator) and located at the bottom of groundwater recharging wells, instead of more conventional energy dissipaters [9].