In a photovoltaic（PV） DC collection system

DC-DC converters with independent input on the PV side can improve the voltage level by cascading on the output side. But when the illumination is uneven

there is a problem of voltage imbalance on the output side of the cascade DC-DC converter. The voltage equalization unit can be set on the output side of the DC-DC converter to achieve output side voltage equalization

but the introduction of the voltage equalization unit will significantly increase the output current ripple of the DC collection system. To this end

we propose a shift-phase combined level modulation strategy for the equalizing unit. By controlling the voltage phase shift between the voltage equalization units of cascaded DC-DC converters

the cascaded output side voltage is controlled to be closer to the DC grid voltage

thus reducing the output current ripple. Then an LC filter is introduced to filter the voltage spike at the cascaded output side to ensure the voltage stability at the cascaded output side. Finally

the proposed control strategy is applied in the simulation of the PV DC collection system. The results show that the proposed strategy can effectively reduce the ripple of grid-connected current while achieving voltage balance at the output side. The research can provide theoretical basis for the stable operation and power optimization of the PV DC collection system.