A perturb and observe (PQ) method is the most commonly used MPPT (maximum power point tracing) algorithm for photovoltaic system.For achieving accurate tracking effect

based on the idea of PQ method equivalent to piecewise constant voltage method

the settled time formula of the PQ control MPPT system was deduced in this paper.The motion characteristics of MPPT system operating point were analyzed when the light intensity changes

and then four concepts such as wrong maintaining

right maintaining

misjudgment and good judgment

are proposed.It is summarized that 12 movement situations of PQ control MPPT system operating point correspond to a dozen different dynamic effects;finally the simulations and experimental results verify 4 typical motions.Based on 12 motions

the fair evaluation method for different implementation of the PQ method is presented

and then the method is used to study the fixed step and variable step method.Simulation results show that when a misjudgment occurs

the dynamic performance of a variable step size is less than the fixed step size.The results contribute to the improvement of the dynamic performance of the PQ method.