Primary control of DC microgrids

Abstract

Due to the recent advancements in the electrical and electronic industries, the demand for direct current (DC) has increased significantly to power consumer equipment such as laptops, mobiles and LED lighting. In addition, numerous distributed generations such as renewable energy sources (RES) have been installed over the last decade so as to reduce the carbon emissions. Most of these energy sources such as photovoltaics and fuel cells generate DC power which is then converted into alternating current (AC) using inverters and connected to the grid. Hence, it would be much more efficient to generate the DC power and use it to directly power the DC loads since in this way the required power conversions are minimized. DC microgrids are gaining popularity since these are distribution networks which contain RES, energy storage systems and distributed DC loads connected together in a single system. DC microgrids result in high overall efficiency while ensuring that the DC network remains stable for varying load conditions. This project studies the operation of a low voltage (LV) DC microgrid which consists of two identical DC-DC step-down converters connected in parallel which share the power consumed by a common DC load. The implementation of the LV DC microgrid was mainly divided into three steps. The first step was to design and simulate: the LC output filter of the DC-DC converter; the inner control loops which consists of cascaded voltage and current loops; and the primary control loop to ensure equal load sharing between the two converters. The second stage consisted of implementing experimentally the power electronic converters (PECs) and the necessary signal conditioning circuitry to implement the aforementioned control loops and protection circuitry. The final step consisted of implementing the inner control loops and the droop control on a digital signal controller (DSC). Experimental and simulation results were obtained to verify the operation of the individual PECs and of the complete DC microgrid. The test results show that the designed DC-DC converters have an efficiency of approximately 93.5% at rated power. In addition, due to the droop control loop a compromise between the microgrid voltage regulation and the load current sharing was achieved after the DC microgrid voltage was established.