In recent years, since DC grids provide higher power supply reliability, higher equipment redundancy, more adaptable power supply mode and more secure power flow control, the medium voltage DC (MVDC) grid is becoming a strategic industry that is widely developed and focused in the world. Particularly, power generation based on large-capacity photovoltaic (PV) is growing rapidly, and a corresponding power conversion system is critical for integrating large PV systems to medium voltage DC grid. In order to improve the reliability and efficiency of PV generation system, the high voltage gain and high efficiency for large capacity DC conversion are required. However, researches on this issue have been relatively few and limited so far. An in-depth analysis and study about high step-up ratio conversion system for MVDC grid connected with PV system is carried out in this paper. Specifically, the characteristics suitable for high step-up ratio conversion system in structure are discussed. According to these characteristics, the conversion systems fall into four categories by series-parallel connection scheme of power modules. Then, three conversion system solutions are identified and compared in terms of input/output performances, conversion efficiency, modulation method, control complexity, power density, reliability and hardware cost. Through thorough analysis, how to select the most suitable conversion system in various applications is also discussed.