Computer simulation of the dynamic behaviour of electrical power systems, with particular reference to voltage instability and collapse phenomena.

The research deals with the study of voltage collapse phenomena at short, medium and long term. It analyzes the models of “generic” load, recently proposed in literature, which are described by two first order independent differential equations, whose purpose is to reproduce the recover of respectively active and reactive power, following a lowering of the voltage value at the charge bars. The research also investigates possible alternatives to these models. In the case of medium and long term, it analyzes also the interaction of the charges dynamics with the one of the different regulators on the net, and in particular with the dynamics of the under load tap changers, of the transformers and of the voltage regulators. The research activity, carried out in cooperation with the University of Padua, has shown that the generic dynamic non–linear load models of the first order, successfully employed in power stability studies, need further refinements when employed to predict the dynamics of loads having high percentage of induction motors, due to the fact that they do not take into account the inherent coupling between active and reactive power absorbed by induction motors [1,2]. On the other hand, it has been highlighted that they give adequate results for long term voltage stability studies for limited voltage variations at the load busses: typically distribution network buses fed through an LTC transformer [4]. In [4] it is also shown the ability of generic load models to take into account the effects of distribution LTC: by introducing in the GNLD load models also the tap changer regulating the voltage at the load bus, the long-term voltage collapse of the system is qualitatively satisfactorily reproduced. In [3] it is shown that a further improvement of GNLD performances can be achieved by implementing in them a more accurate load Q-V characteristic, by means of polynomial functions. The modelling activity has been carried out by means of the modular code “Legocad” developed at ENEL, whose library is enlarged in the framework of the present research. Experimental tests are planned at one ENEL substation for the validation of the developed models are planned.
 

Models and software tools for the solution of optimization problems faced by the electricity industry in a competitive market.

The research deals with the development of a method for the solution of the Unit Commitment (UC) problem in a competitive market with demand-side bidding (DSB) explicitly considered; in order to allow customers to play a proactive role in the price determination process, the DSB provides the opportunity for them to submit bids for load reductions in specific periods. The behaviour of DSB inclusion in electricity auctions simulations obtained with a specially developed Lagrangian relaxation scheme that effectively takes advantage of the structure of the problem is analysed. The results of the research have been published in a chapter of the volume "The next generation of unit commitment models" [5]. The research activity concerns also the critical analysis of optimization algorithms applied to the UC problem. For this specific topic, the research, carried out in cooperation with the Department of Informatics of the University of Pisa, is financed by ‘CNR Agenzia 2000’. A simple and effective Lagrangian relaxation approach for the solution of the optimal short term UC problem in hydrothermal power generation systems is presented in [7]. The proposed approach, based on a disaggregated Bundle method for the solution of the dual problem, with a new warm starting procedure, achieves accurate solutions in few iterations; the adoption of a disaggregated Bundle method not only improves the convergence of the proposed approach but also provides information that are suitably exploited for generating a feasible solution of the primal problem and for obtaining an optimal hydro scheduling. A comparison between the proposed Lagrangian approach and other ones, based on sub-gradient and Bundle methods, is presented for a simple yet reasonable formulation of the Hydrothermal Unit Commitment problem. The research is aimed also at comparing the results obtained for different types of Lagrangian relaxation based UC problems, with those obtained through the so-called ‘Taboo search’ heuristic technique. In [6] both approaches are shown to be satisfactory for the problem of interest; further, their comparison provides insights for the construction of ‘hybrid’ techniques which would incorporate the best features of both approaches, particularly useful in the development of computer models for competitive electricity markets. The developed UC code can be utilized as a tool to assist a generating company in choosing the most convenient bidding strategies for a day-ahead electricity energy market, as many of the factors influencing the most convenient bidding strategies, e.g. production capacity of the company, types of the generating units, number, size and characteristics of the competitors, market rules, are inaccurately known. The optimization model used is aimed to be independent, as far as possible, of the specific power exchange rules, which appear to be in perpetual evolution. In [8], results obtained for a realistic set of hydro-thermal power plants of a generating company are presented and discussed in order to emphasize the impact of the method on the bidding strategy choice of the company. Either price takers and oligopolistic companies are considered in the analysis.
 

Power system restoration after a blackout, in a competitive electricity market.

In Italy, in particular, the restoration of a power system after a blackout has made use up to now of the black-start capability of selected hydro power stations (named ‘early-restoration plants’) through pre-established paths called ‘restoration lines’. With the unbundling and liberalization of the electrical energy systems, some studies have been recently carried out for assessing the possible contribution to the restoration plan from thermal units (see Production). Additional topics to consider while developing defense and restoration plans in deregulated regional/national systems are addressed in this research activity, with the aim of defining the skeleton of an intelligent supporting, and training, aid for independent system operator (ISO) control center personnel. This important issue is the object of a research collaboration among three Italian Universities (Bologna, Genova e Pisa), within the framework of a National research project financed by the Ministry of University and Scientific Research (MIUR) on " The management of the ancillary services in a deregulated market of electric energy". The first results obtained are presented in [9,10].
 

NEMP effects on transmission networks.

The research, carried out within the framework of an international research collaboration between the University of Bologna and the Swiss federal Institute of Technology (Laboratoire de Reseaux d’énergie électrique) is aimed at evaluating the electromagnetic effects of a high altitude nuclear burst on power transmission systems. Part of the activity has been devoted to the characterization of the NEMP-type source of interference [11,12,14]; part is devoted to the development of models for the appraisal of the NEMP-to-transmission lines coupling, in order to estimate the induced overvoltages and overcurrents. The developed models are based on the transmission line approximation [15, 16]. Part of the activity is devoted to the experimental validation of the proposed models: this is accomplished by means of NEMP simulators and reduced-scale network models, in cooperation with the Swiss Federal Institute of Technology [17-19]. This topic has been object of part of a PhD thesis [20]. The models are developed either in the frequency and in the time domain, and the analysis of the advantages and disadvantages of both approaches is reported in [21].