Analysis of limit cycle oscillations in maximum power point tracking algorithms

This paper analyses the limit cycle oscillations (LCOs) that characterize some of the most commonly adopted maximum power point tracking (MPPT) digital algorithms. The paper identifies necessary conditions for the LCO existence and clarifies the relationship between some of its characteristics, namely amplitude and frequency, and some relevant system parameters, e.g. DC-DC converter control strategy, adopted MPPT algorithm and iteration frequency, A/D converter resolution. The theoretical study is based on the derivation of an equivalent small signal model of a complete photovoltaic system, where the MPPT algorithm is modelled by its describing function. The final goal is to allow the designer to predict the frequency and amplitude of the LCO and so to drive the design of key system parameters. The correctness of the results has been verified performing computer simulations as well as experimental tests on a 1 kWp PV generator.