TIME FRAME MEASUREMENTS IMPACT ON PROBABILISTIC BEHAVIOUR OF PHOTOVOLTAIC SYSTEMS Buletinul AGIR nr. 3/2012 ● iunie-august 1 TIME FRAME MEASUREMENTS IMPACT ON PROBABILISTIC BEHAVIOUR OF PHOTOVOLTAIC SYSTEMS Professor Giuseppe Marco TINA 1 , Professor Gilles NOTTON 2 , Lecturer Ciprian NEMES 3 1 University of Catania, Italy, 2 University of Corsica, France 3 Technical University of Iasi, Romania REZUMAT. Pentru un sistem electric cu panouri fotovoltaice este important să se evalueze şi să se înţeleagă modul în care intervalele de măsură a radiaţiei solare pot afecta capacitatea sistemului de a genera și de a acoperi necesarul de sarcină. Nivelul puterii generat de un sistem fotovoltaic se evaluează pe baza valorilor medii orare ale gradului de nebulozitate. Obiectivul principal al acestei lucrări este de a evalua efectele diferitelor intervale de măsură a radiaţiei solare asupra indicatorilor de adecvabilitate ai unui sistem ce integrează surse fotovoltaice. Cuvinte cheie: energie solară, nebulozitate orară, indicatori de adecvabilitate. ABSTRACT. For a generation system including photovoltaic systems is important to assess and understand how different time frames for the irradiation measurements can impact the ability of the system generating capacity to meet the total system load. The electrical output power generated by a photovoltaic system is evaluated using the hourly average clearness index values. The main objective of this paper is to evaluate the effects of different time frames, over that the measurement data are recorded, on the probabilistic behaviour of a grid-connected photovoltaic system. Key words: solar energy, hourly clearness index, adequacy indices. 1. INTRODUCTION During the last decades, a growing interest in renewable energy resources has been observed. Even if electric energy from renewable energy sources brings various benefits to power system, due to the variable and intermittent behaviour of many renewable resources, their integration into electric grid leads to new challenges in the system operation. The operation of power systems with renewable energy sources, such as photovoltaic systems, has to consider the stability of the electrical power system, which is based on a reliable power generation that is permanently balanced by the load. Due the variation of the demanded load or the generated power by non- programmable generation units, for system balancing, the Independent System Operator (ISO) has to buy energy resources on the ancillary services markets. The relevant time for these markets is about 15 minutes, whereas the energy markets work on an hourly base. Therefore, for a generation system including photovoltaic systems, to evaluate the real energy needs is important to assess and understand how different time frames from the irradiation measurements can impact on power system adequacy. The adequacy associated to a generation power system is a measure of the ability of system generating capacity to meet the total system load. Thus, our study is conducted in order to find the main adequacy indices, associated to a grid-connected photovoltaic system. Generally, the electrical output power generated by a photovoltaic system is evaluated using the hourly clearness index values. The main objective of this paper is to evaluate the effects of intra- hourly time frames, over that the measurement data are recorded, on the photovoltaic system adequacy indices over a long time interval. The paper is organized as follows. In order to evaluate the adequacy indices, a brief description of the evaluation technique used in the paper is presented in section 2. Furthermore, the probability density functions associated to electrical output power and to load demand are also presented in this section, highlighting the main parameters that affect these functions. In section 3, a measurement database over 10 minutes time frame is analysed and involved in order to develop another two database, over 30 and 60 minutes time frames. Furthermore, these three database over mentioned time frames are involved in statistical analyses related to evaluate the upper, average and lower values of clearness index. In section 4 a numerical analysis is developed in order to calculate the adequacy indices, based on the 10, 30 and 60 minutes time frames and three scenarios related to load profile. Finally, the main conclusions are given in section 5.