Studies of large-scale DC connected photovoltaic power system based on multi-modular cascaded DC-DC converter

DC collection and transmission is one of the major development directions of large-scale photovoltaic (PV) power system. In order to achieve low-cost, high-efficiency and long-distance transmission of PV power, this paper adopted a DC grid-connected topology by using multi-modular cascaded DC-DC converters, forming an input-independent and output-series (IIOS) system. In this system, an LLC type DC-DC converter based on IGCT and magnetically integrated transformer was proposed in this paper, which can significantly increase the capacity, efficiency and power density of DC-DC converter. And the equalization unit based on half-bridge topology was adopted for the case of uneven irradiation of the PV inputs. In addition, this paper proposed an equalization control method based on the Nearest Level Modulation (NLM), which greatly simplifies the design of the acquisition and control system. Furthermore, the parametric design of a 50MW PV system was presented, and the simulation was carried out to verify the feasibility and advantages of the system in this paper. The proposed IIOS system is more suitable for high-capacity, long-distance PV power transmission, which can serve as a reference for the development of large-scale photovoltaic systems.