Analysing partial shading of PV modules by circuit modelling

In most cases, solar cells within a PV module are connected in series in order to generate a high voltage. The series-connection is a limiting factor if a PV module is partially shaded, for example by nearby buildings, passing clouds or wildlife. In a string of cells connected in series, the cell that generates the smallest current limits the current of the whole string. Additionally, shaded cells operate in reverse bias and may, for example, show “hot spots” in case they are locally shunted. Bypass diodes are usually introduced to reduce such effects. The bypass diode will ensure the operation of the module with partially or fully shaded cells, at the price of a reduced voltage [1]. However, the number of bypass diodes in a module is typically limited, so that shading of one single cell will still affect a significant percentage of the cells in the module. In this paper, distributed circuit simulations of a PV module under partial shading conditions are presented. The circuit is modeled from one-diode elements and implemented in LTspice IV [2]. The influence of different grades of shading on the current-voltage characteristics and the output power of the module are investigated. Also, different possible configurations of bypass diodes are evaluated. Finally, a time-dependent model of a PV module is constructed to simulate the modules' behavior if a shadow moves across it. The shadow's moving direction is also taken into consideration. It is observed that the choice of bypass diode configuration has a strong influence on the performance of a PV module under partial shading conditions.