Abstract
The emergence of microgeneration has recently lead to the concept of microgrid, a network of LV consumers and producers able to export electric energy in some circumstances and also to work in an isolated way in emergency situations. Research on the organization of microgrids, control devices, functionalities and other technical aspects is presently being carried out, in order to establish a consistent technical framework to support the concept. The successful development of the microgrid concept implies the definition of a suitable regulation for its integration on distribution systems. In order to define such a regulation, the identification of costs and benefits that microgrids may bring is a crucial task. Actually, this is the basis fora discussion about the way global costs could be divided among the different agents that benefit from the development of microgrids. Among other aspects, the effect of microgrids on the reliability of the distribution network has been pointed out as an important advantage, due to the ability of isolated operation in emergency situations. This paper identifies the situations where the existence of a microgrid may reduce the interruption rate and duration and thus improve the reliability indices of the distribution network. The relevant expressions necessary to quantify the reliability are presented. An illustrative example is included, where the global influence of the microgrid in the reliability is commented.

Abstract
The main purpose of this paper is the development of an optimization model to design grounding grids in electrical substations. The design of a grounding grid in a substation is formulated as a mixed-integer linear programming problem. The developed optimization model incorporates the constructive Characteristics, as well as the technical and security requirements inherent to the construction, installation and operation of these grids. The model includes variables defining the grid characteristics according to the configurations admitted by the designer, which are selected amongst a set of pie-selected grounding designs. The definition of these configurations includes the geometry of the grid, the depth at Which the conductors will be installed and the radius of the conductor. A finite number of configurations can be generated before running the optimization process by considering all the variables in accordance with the IEEE Std 80-2000. The optimization problem also includes safety constraints related with the maximum allowed touching and step voltages, which are defined according to the fibrillation discharge limits. These fibrillation discharge limits are defined by IEEE Std 80-2000 for low frequencies for high frequencies. the limits are not the same as in 50 Hz). The model also includes the equivalent impedance of the transmission line supplying the substation where it will be located the grounding grid to be designed. As a result, the problem outputs define the most adequate grounding grid among the possible pre-selected configurations. This selection is driven by the total investment and installation costs, corresponding to the objective of the optimization model. To illustrate the interest of this research, the paper includes a case study based oil a real Situation, as all example of a potential application of this approach for engineering grounding design. Finally, it should also be referred that the scope of application of this methodology is potentially very wide given that it is in accordance with the specifications defined by the IEEE Std 80-2000.

Abstract
Power flow calculations are one of the most important computational tools for planning and operating electric power systems. After the stabilization of the deterministic power flow calculation methods, the need to capture uncertainty in load definition lead first to the development of probabilistic models, and later to fuzzy approaches able to deal with qualitative declarations and other non-probabilistic information about the value of the loads. Present fuzzy power flow (FPF) calculations use typically incremental techniques, in order to obtain a good approximation of the fuzzy state variables. However, these models and procedures are not entirely satisfactory for the evaluation of the adequacy of the electric transmission system, since they are not completely symmetric. In this paper, we show how to perform the detailed calculation of the state variables of the FPF problem in an exact and symmetrical way, by means of solving multiple optimization problems. The procedure is illustrated using the IEEE 118 test system.

Abstract
There are already several power systems coping with large amounts of wind power. Hi h penetration of wind power has impacts that have to be manage through proper plant interconnection, integration, transmission planning, and system and market operations. This report is a summary of case studies addressing concerns about the impact of wind power.s variability and uncertainty on power system reliability and costs. The case studies summarized in this report are not easy to compare due to different methodology and data used, as well as different assumptions on the interconnection capacity available. Integration costs of wind power need to be compared to something, like the production costs or market value of wind power, or integration cost of other production forms. There is also benefit when adding wind power to power systems: it reduces the total operating costs and emissions as wind fossil fuels. Severalissues that impact on the amount of wind power that can be integrated have been identified. Large balancing areas and aggregation benefits of large areas help in reducing the variability and forecast errors of wind power as well as help in pooling more cost effective balancing resources. System operation and working electricity markets at less than day-ahead time scales help reduce forecast errors of wind power. Transmission is the key to aggregation benefits, electricity markets and larger balancing areas. From the investigated studies it follows that at wind penetrations of up to 20 % of gross demand (energy), system operating cost increases arising from wind variability and uncertainty amounted to about 1.4 ./MWh. This is 10 % or less of the wholesale value of the wind energy.

Abstract

Abstract
This paper describes the methodology developed to build estimates of electricity prices having the horizon of one week. This approach uses artificial neural networks and includes a particular treatment of weekends and national holidays as a way to improve the quality of the results. The developed approach was tested using data obtained from the Spanish market operator for the time period of 2006 to 2008. The obtained value of MAPE - Mean Absolute Percentage Error - was 12,62% for workdays and 10,73% for holidays and weekends. The obtained results show that this study has interest to the market agents in question, since realistic forecasting was achieved.