Abstract: The precondition for the normal operation of the PV system is to complete the pre-charge and establish DC bus voltage. To complete the process fast and more stably， the topological structure and charging characteristics of DC boost system for photovoltaic system are studied in this paper. On this basis， the paper proposes that the proportional controller can be used to reduce the order of the equivalent model in the controlled charging stage. At the same time， the steady-state error is eliminated by the integral action of the capacitor on the current. Compared to the proportion and integral control of the voltage outer loop used in the soft start of multi-terminal MMC， it reduces the order of the equivalent system model and does not need slope controller for DC reference voltage. It is beneficial to the system stability and the selection of control parameters during the charging process. Finally， the correctness and effectiveness of the proposed control strategy is verified by MATLAB/Simulink simulation.

Key words: photovoltaic, DC collection, modular multilevel converter（MMC）, model order reduction, soft start