Abstract
With the increase of dc based renewable energy generation and dc loads, the medium voltage dc (MVDC) distribution network is becoming a promising option for more efficient system integration. In particular, large-capacity photovoltaic (PV)-based power generation is growing rapidly, and a corresponding power conversion system is critical to integrate these large PV systems into MVDC power grid. Different from traditional ac grid-connected converters, the converter system for dc grid interfaced PV system requires large-capacity dc conversion over a wide range of ultra-high voltage step-up ratios. This is an important issue, yet received limited research so far. In this paper, a thorough study of dc-dc conversion system for a medium-voltage dc grid-connected PV system is conducted. The required structural features for such a conversion system are first discussed. Based on these features, the conversion system is classified into four categories by series-parallel connection scheme of power modules. Then two existing conversion system configurations as well as a proposed solution are compared in terms of input/output performance, conversion efficiency, modulation method, control complexity, power density, reliability, and hardware cost. In-depth analysis is carried out to select the most suitable conversion systems in various application scenarios.
Highlights
With the increase of dc based renewable energy generation and dc loads, the medium voltage dc (MVDC) distribution network is becoming a promising option for more efficient system integration
Ac and dc hybrid power distribution networks have been found more suitable for the development of modern urban distribution networks due to the following considerations [1,2,3]: 1 A DC grid is more compatible with distributed dc power sources and dc loads (LED lighting, electric vehicles, etc.) with reduced power-conversion losses; 2 Voltage stability and power quality of distribution networks can be improved, since the harmonics in ac partitions can be reduced by the dc link; 3 DC cable is a more efficient transmission medium that eases the shortage of urban power transmission corridors
For integrating large-scale and low dc voltage PV energy into a grid, the medium voltage dc grid is a promising choice. These MVDC grid-connected PV systems feature the following advantages over traditional low-voltage ac grid-connected PV systems [7,8,9,10]: 1 Simple scheme - a dc grid is free from issues of reactive power compensation and harmonic suppression that affect an ac grid; 2 Capacity advantage - higher voltage leads to larger capacity, which benefits the economic operation of the system; 3 Efficiency advantage - only dc-dc conversion is required, and the conversion losses are lower than those of multi-stage dc-to-ac conversion
Summary
Ac and dc hybrid power distribution networks (shown in Fig. 1) have been found more suitable for the development of modern urban distribution networks due to the following considerations [1,2,3]: 1 A DC grid is more compatible with distributed dc power sources (photovoltaic generation, battery energy storage, etc.) and dc loads (LED lighting, electric vehicles, etc.) with reduced power-conversion losses; 2 Voltage stability and power quality of distribution networks can be improved, since the harmonics in ac partitions can be reduced by the dc link; 3 DC cable is a more efficient transmission medium that eases the shortage of urban power transmission corridors. For integrating large-scale and low dc voltage PV energy into a grid, the medium voltage dc grid is a promising choice These MVDC grid-connected PV systems feature the following advantages over traditional low-voltage ac grid-connected PV systems [7,8,9,10]: 1 Simple scheme - a dc grid is free from issues of reactive power compensation and harmonic suppression that affect an ac grid; 2 Capacity advantage - higher voltage leads to larger capacity, which benefits the economic operation of the system; 3 Efficiency advantage - only dc-dc conversion is required, and the conversion losses are lower than those of multi-stage dc-to-ac (dc-dc and dc-ac) conversion. For large-capacity PV systems integrated into MVDC grids: 1 A topological scheme based on a modular parallel-series connection is analyzed and classified.
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