As the traditional converters in distributed photovoltaic-energy storage direct current(DC)generation system will be limited when the power increases or is at higher step-up ratios

four-phase parallel converters are adopted in the proposed system

which can broaden the range of output voltage and suppress oscillation of photovoltaic output voltage. For photovoltaic four-phase parallel boost converters

an improved extremum seeking algorithm is designed to achieve maximun power point tracing and improve the system rapidity and stability in light step change. For energy storage four-phase parallel bidirectional converters

a novel nonlinear control law based on the flatness principle is proposed

which is a simple solution to smooth the output power and stabilize the system with a load mutation or control parameter perturbation. Compared with the conventional proportional-integral control method

the nonlinear differential flatness control algorithm has broader degrees of freedom

fewer steady-state errors and better system stability as proved by simulation and experimental results. This work is supported by National Natural Science Foundation of China(No. 51407114)

Shanghai Natural Science Foundation(No. 15ZR1418200)and Shanghai Engineering Research Center of Green Energy Grid-Connected Technology(No. 13DZ2251900).