NUMERICAL SIMULATION OF PHOTOVOLTAIC GENERATORS WITH SHADED CELLS

A model which is able to describe the relationship between current and voltage of a solar cell in generation region as well as the breakdowns at positive and negative voltages is given. A set of suitable numerical algorithms to compute the currents for a given voltage at the solar cell and vice versa is described. A general model for the description of solar generators is proposed which gives all voltages and currents as well as the voltage and current at the output of the solar generator itself. This new model is also implemented in a numerical algorithm. For the description of the electrical behaviour of a solar cell during generating operation the two diodes model is commonly used, see e.g. Overstraeten [5]. For cells which are driven in the negative voltage range another model is necessary, which describes the breakdown region at high negative voltages. Negative voltages at solar cells can occur at non-uniform illuminated photovoltaic generators, especially during partially shading of the generator. One model was described by Rauschenbach [9]. A more precise model, based on the one diode model was given by Bishop [2]. This model can be adapted to the two diodes model and offers optimal conditions for the description of the solar cell characteristics. Several simulation programs for photovoltaic generators were developed during the last two decades. Owing to the development of more powerful computers, they became more complex and more efficient. However, these programs still offer limited possibilities to simulate the electrical behaviour of photovoltaic generators under mismatched conditions or partial shading. For this purpose, a more general electrical model which allows simulations at all conditions has been developed. Numerical methods, such as described by Press at. al. [6] are used to obtain a solution for this model.