High-gain interleaved DC-DC converters for renewable energy systems

Abstract

The interest for renewable energy has substantially increased in the last decades and the solar photovoltaic (PV) and fuel cell (FC) have become attractive energy sources with their advantageous features such as declining manufacturing cost and improving efficiency. With the continuous development of PV power plants, the current PV system architectures are challenged by requiring power converters with a higher power rating and a higher voltage level at the point of common coupling. This can lead to higher ratio transformers or more transformation stages to be used for the connection of the PV farm with the grid. In case of FC systems, their use is significantly growing in the area of stationary and automotive applications. FC-based energy systems require a boost stage to interface with utility grids, due to the low voltage of FC modules. Although a conventional boost converter can step-up the low FC voltage to the DC bus voltage level required by the inverter, it needs very high duty ratio which degrades the converter performance and efficiency. To address this problem, high-gain interleaved DC-DC converters are suggested in this thesis. Three different types of DC-DC converters are studied: Interleaved switched-capacitor (ISC) converter, bidirectional interleaved switched-capacitor (BISC) converter, and soft-switched interleaved (SSI) converter. The high gain capability of SSI converter allows it to be connected directly to a medium voltage DC bus. This will facilitate direct connection of a large PV system to MV AC grid only using a single transformation stage. In addition, the interleaved configuration increases the flexibility of the converter allowing for either higher voltage gain and/or current rating, thus increasing the power rating of the converter. The SSI converter can also be used by a stationary FC power conditioning system. The high gain characteristic is suitable to boost the low DC voltage of FC modules to the DC voltage level required by the inverter. Moreover, the converter is able to handle high FC current owing to its interleaved configuration besides reducing the FC current ripple and improving the performance and lifespan of FC module.