A practical solution for grid connected dispersed generation from renewable sources: DC connection

Abstract

The exploitation of renewable sources is expected to become the most promising solution to the environmental issues and to the permanent increase of prices of the fossil energy. Since most of renewable energy sources are widely distributed and the energy converters generally do not provide electric power at main grid frequency and voltage, a DC bus is suggested to be used for integrating them into a DC grid and further transfer it into an AC grid (usually available at a certain distance). Due to differences among the technical parameters of the power supply equipment, their outputs have to be firstly conditioned and then connected in parallel to a DC bus via appropriate power electronics. The aim of this paper is to suggest practical solutions for RES integration into distribution networks. The proposed DC link is simulated and different methods for load sharing and droop control is discussed. The standard model is a 6 kV, 1 MW electrical hydro generator with six phases YDelta connected, and having the output rectified by six phase diodes and paralleled to a 10 kV DC cable underground line. Three levels DC-AC, front-end bi-directional converters are used as interface with the AC grid. The connection of the low voltage grid (connecting the output of different power sources) to the medium voltage DC link is done by means of a DC-DC quasi-resonant converter provided with an internal high frequency insulation transformer. For the further connection of the DC link to the medium voltage AC distribution grid a solution employing IGBT four legs, front-end converter is proposed. A low voltage DC grid model, in the tens of kW range, 400/230 V AC - 720/320 V DC is under commissioning