A Methodology for Seamless Hot-Swap of Converters in DC Microgrids

DC microgrids with paralleled sources require a seamless addition and removal of a converter module without shutting down the system. This replacement of a converter is known as a hot-swap event. In a democratic current sharing scheme, a critical issue is an overshoot in the grid voltage when a converter is suddenly added and an undershoot when a converter is suddenly removed. In this paper, by analyzing the start-up and shut-down operation of a converter module during the hot-swap, the reason for these excursions in the grid voltage is explained using an analytical model. It is shown that by changing the voltage controller's bandwidth, overshoot and undershoot can be reduced, but there's a trade-off between the voltage controller's bandwidth and the stability of the system. To overcome this trade-off, an algorithm that provides an additional degree of freedom is proposed in this paper. This algorithm without disturbing the system's stability prevents the overvoltage and the undervoltage by exponentially changing the current reference of the converter being added or removed. The stability analysis along with the circuit simulation results using the proposed algorithm are shown for a wide range of operating conditions and different system parameters.