Aplicação do conceito de cargas cíclicas para dimensionamento de condutores em usinas fotovoltaicas

Abstract

Electricity generation facilities that use solar irradiance have gained space in the world electricity matrix and specially in Brazil, where there is abundant land and sun. During the project of this kind of installations, the Low voltage and Medium Voltage cable sizing (for direct buried or cable in duct method) involves the criteria of cable ampacity, which evaluates the cable ampacity according to the installation method and its characteristics. The selection of an optimized cable size is made by applying correction factors for soil temperature, cable grouping and soil thermal resistivity, so that there is a de-rated ampacity of the conductor. The cable is well designed when the electrical current is lower than its derated ampacity. This approach is adequate for a constant electrical current in the conductor. In the case of solar farms, the electrical current flowing in a conductor is not constant since the sun irradiation varies throughout the day, in other words the electrical current follows a cyclic profile in the period of a day, reaching its peak value around midday, when the solar irradiation is higher. In this case it is possible to proceed a cyclic analysis of the electrical current in a duration of 24 hours. Such analysis determines a new correction factor that translates the cyclic behavior of the current to be used together with the other factors in the cable sizing analysis. It provides some benefits like reducing the width of the cable’s trenches, or a large number of cables grouping, or even a reduction in the size of the selected cables, when compared to analysis of continuous loading.