Paolo Pelacchi

Control techniques of Dispersed Generators to improve the continuity of electricity supply

It is expected that dispersed generation (DG) will play an increasing role in electric power systems in the near future. Among the benefits that DG can give to the power system operators and to the electricity customers, one of the most attractive is the possibility of improving the continuity of power supply. DG plants can be designed to supply portions of the distribution grid in the event of an upstream supply outage. Techniques for controlling DG plants that use feedback of only locally measurable variables are presented. This solution allows correct system operation and switching between parallel and isolated modes without needing online communication of control signals between the generators. The control technique is described with particular reference to inverter-interfaced systems (micro-turbines, fuel cells). Simulations of sample cases including different size and type of generators are presented.

On the use of simplified reactive power flow equations for purposes of fast reliability assessment

In the present paper, a new method, derived from the linearization of the reactive power flow equations, is presented and discussed. The proposed technique is able to quickly calculate voltages at load busses, as well as reactive power injections at generation busses, using the results of a preliminary ordinary DC load flow. Due to the extremely reduced computational time, this technique can be successfully used for fast reliability assessments, when a huge number of load flow scenarios, generated by a probabilistic approach like Monte Carlo techniques, must be analyzed in terms of voltage violations; the algorithm is also promising for Optimal Reactive Power Flow procedures. Two case studies, based on the IEEE RTS-96 and Poland transmission and sub-transmission network, confirm a speed-up of more than 6 times respect to the Newton-Raphson algorithm, while keeping the mean squared error of voltages within 0.5% and the maximum voltage error below 1.5% of the voltage limits.

Sizing and Siting of Large-Scale Batteries in Transmission Grids to Optimize the Use of Renewables

Power systems are a recent field of application of complex network research, which allows performing large-scale studies and evaluations. Based on this theory, a power grid is modeled as a weighted graph with several kinds of nodes and edges, and further analysis can help in investigating the behavior of the grid under critical conditions. Among the crucial aspects of a power network, those concerning flow limits and power flow distribution are gaining relevance due to the increasing introduction of large-scale renewable energy generation facilities. Storage systems are a key element in having a more sustainable but still reliable grid. This paper focuses on a new research challenge regarding the siting of the storage on the transmission grid and its appropriate sizing. The problem is tackled by considering realistic configurations based on the IEEE-RTS-96 bus and data coming from the Italian transmission operator, and evaluating novel economic and complex network-based metrics on these configurations. Power flows are modeled in a linear way, and the representative optimization problem is expressed as a linear programming problem. The results show the potential benefits of storage in transmission lines and indicate that the siting has a minor role in the optimal operation of the system.

Hybrid energy systems for static applications

Through a couple of examples this paper describes how hybrid systems can play an important role in todays energy systems. They are valuable and efficient solutions in both isolated systems and for large interconnected networks. Different technologies for storage and generation as well as different operation techniques are needed for each application, but the benefits that can be achieved with respect to traditional solution are important in easing the transition towards a sustainable energy system.

Parallel computing of sequential MonteCarlo techniques for reliable operation of Smart Grids

Probabilistic methods based on Monte Carlo techniques are successfully employed, both in vertically integrated utilities and in deregulated markets, to assess the long-term adequacy of power systems. Unfortunately, the computational complexity of many time-consuming algorithms has constituted for years a crucial barrier to the use of probabilistic methods also for short-term reliability assessment and decision-support. Nowadays, the availability of cheap and fast computers discloses new opportunities also for on-line reliability calculations. In particular, parallel computing seems to have good chances to be successfully applied to the on-line operation of Smart Grids. In the present paper, a technique of parallel computing, applied to a sequential Monte Carlo algorithm that simulates a power system in a multi-core machine, is proposed and described. A case study based on the IEEE Reliability Test System RTS-96 is shown and discussed, with particular emphasis to the speed-up and accuracy obtained as a function of number of employed cores.

Assessment of the accuracy of electricity price forecasts on the basis of a mixed deterministic-probabilistic approach

Nowadays it is essential for an electricity market player to have not only tools for price forecasting, but also methods for quantifying the accuracy of such forecasts, in order to perform an optimum risk management of his physical/financial portfolio. This paper proposes a simple method to assess the accuracy of electricity price forecasts, based on the probability distributions of the main factors affecting the price. In particular, the method consists in the identification of the main drivers of the electricity spot price, the estimation of their probability distributions and finally their combination in order to determine the probability distribution of the corresponding electricity price. The sensitivity analysis between the price and its drivers is carried out by means of a deterministic electricity market simulator. The validity of the proposed methodology has been tested on the present scenario of the Italian electricity market (year 2004).

Technical-economical simulations of a deregulated electricity market: the setting of power flow limits in the interconnection lines

One of the key functions of any independent system operator is checking that the scheduled power flows do not exceed the operating limits of the transmission grid. If these limits are overcome zonal prices arise and this fact produces a major barrier to the free trade of energy. The operating limits of the transmission grid are generally fixed to ensure a given level of security in the power system. Nowadays the problem is simply solved by verifying that, after an interconnection line has gone out of service, the remaining lines are able to transport the power flowing in the faulty line (first contingency security criterion). In the present paper a different approach is proposed, based on probabilistic assumptions. The methodology developed under this approach is based on the consideration that the total costs for the customers (energy, system auxiliary services and undelivered energy) must be minimized. The methodology is applied to a case study where two zones linked by two interconnection lines are present.

Electromechanical dynamics of overhead electric power line conductors analysed through the Modelica language models

In the last years the use of high temperature low sag conductors has become more and more frequent in electric power system. Such conductors have an inner layer in polymeric and/or fibre material, with reduced thermal coefficient and weight with respect to conventional ones. For this reason, under short circuit conditions they can experience large displacements due to the electromagnetic forces induced by the fault currents. Such displacements can influence phase-to-ground or phase-to-phase clearances, thus reducing the system security. In this paper, a possible way to model the dynamic electromechanical behaviour of overhead electric power lines conductors is presented and discussed, using lumped parameter models implemented through Modelica language.

Simulation of deregulated electrical systems to set balancing reserve margins

In a deregulated electric system, ISO is always responsible for system reliability and security. For this purpose it must provide some system services and, primarily, an adequate amount of power for spinning reserve and of power and energy for balancing operations (nonspinning and replacement reserve). The number of sessions of the day-of spot market, when present, is strongly related to the power and energy for balancing operations and, consequently, to the real time market. In fact, after a load-frequency control process, the spinning reserve power margin must be restored in order to guarantee the assigned availability to the system; this is obtained by using the margins for balancing operations; this type of reserve can be restored only when the nearest session of day-of spot market becomes active. For this reason, the more frequent the day-of spot market is, the less there is the need for balancing reserve. The evaluation of the reserve can be carried out using probabilistic techniques; in this paper a simulation method, based on Monte Carlo technique, is described and a case study regarding a complex generating system is shown.