Investigation and Enhancement of Stability in Grid-Connected Active DC Distribution Systems With High Penetration Level of Dynamic Loads

Nowadays, grid-connected active dc distribution systems are gaining widespread acceptance due to the remarkable development of the dc technology, high penetration levels of dc loads, and the market availability of dc-based distributed generators. One of the main features of dc systems is the elimination of multiple conversion stages required for variable frequency ac loads, such as variable-speed drive applications; therefore, dc distribution systems are considered as an efficient and cost-effective choice for supplying such dynamic loads. Induction motors (IMs) equipped with open-loop constant voltage/frequency $(V/f)$ variable speed drives are considered as the workforce for many industrial loads due to their simplicity and satisfactory dynamic performance. However, $V/f$ IM drives exhibit poor stability dynamics, particularly, at low-speed operation, which might negatively interact with the dc distribution system, leading to further stability degradation. Therefore, this paper investigates the interaction dynamics and the performance of a grid-connected dc distribution system with a high penetration level of dynamic loads. A detailed small-signal model of the entire system is developed to characterize the overall system stability margins with the help of the eigenvalues and impedance-based analysis. Moreover, the uncertainties affecting the marginal stability such as motor operating speed, dc feeder length, and bus capacitance, are thoroughly addressed. It has been found that the dynamic loads in grid-connected dc distribution systems would exhibit instability issues due to various dynamic interactions; therefore, two different stabilizing compensation methods are proposed to mitigate the associating instability issues and enhance the system damping capability. Detailed time-domain non-linear simulations and experimental results are presented to validate the analytical results.