The Effect of Mode of Control on Voltage Stability in DC Micro-grids Operating with Constant Power Loads

The penetration of renewable energy systems has helped trigger the shift to DC distribution systems. Switching converters are the building blocks of DC distribution systems and these individual blocks have their unique control. Manufacturers of these building blocks tend to overlook the effect of coupling converters together that have closed loop control, and integration engineers face the challenge of voltage instability during integration. Load converters operating with closed-loop control behave as constant power loads and have a destabilizing negative incremental resistance that diminishes the damping of the system at the DC bus they are connected. The mode of control also influences how far the action of constant power loads drives the system into instability. This work compares the average current mode control and voltage mode control applied to small DC network to see which control mode is better to implement for DC distribution grids. Simulations of a small single bus DC distribution system comprising of two constant power loads and a source converter were carried out with the objective of comparing the extent of bus voltage deterioration when using either control modes. The simulation results show that for the same system average current mode control is better at maintaining voltage stability compared voltage mode control.