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DC microgrids are the new trend for renewable energy distributed systems due to their high efficiency and more suitability to new load appliances. However, some problems are still open to discussion as it is an emerging concept. In a DC microgrid, a very important issue consists on an enhanced control of the DC bus voltage. This control should be reliable especially towards power flow variations which can be caused by distributed generation sources or by abrupt load demand. These oscillations are also caused by special load type called Constant Power Loads (CPL) when connected to the DC bus voltage due to the negative impedance they induce between source converter and load side. To overcome the above mentioned problems, this paper investigates two internal types of control in the primary level control to ensure a constant DC bus voltage with good performances firstly when a CPL is connected and secondly when a power flow change occurs from the source side or even the load one. The first control is based on cascaded PI regulators and the second one is a nonlinear control. Both control performances are compared for different load points using PSIM software. Simulation and experimental results are presented and discussed. [ABSTRACT FROM AUTHOR]