An improved distributed secondary control for multi-voltage-level DC distribution networks with virtual impedance arrangement

The distributed secondary control of the DC distribution network is becoming increasingly important in modern power systems to achieve accurate power sharing of each distributed unit. Here, an improved secondary control strategy for energy storage systems (ESSs) in the multi-voltage-level DC distribution network (MVL-DCDN) is proposed. At the primary control level, the interlinked DC transformers (DCT) are controlled by difference-of-v(2) to support bidirectional voltage making the MVL-DCDN regarded as a single-voltage-level one using only the local information of DCT. At the secondary control level, it can be observed that the voltage correction term and virtual impedance correction term have a coupling relationship. In order to tackle this coupling effect, a distributed secondary control is imposed in an MVL-DCDN based on improved p-v(2) droop control. Concretely, this proposed secondary control strategy contains a power sharing controller, a virtual impedance controller, an average voltage restoration controller, and a reference voltage consensus controller. With the proposed method, this coupling effect can be eliminated to ensure the steady operation of the virtual impedance. Furthermore, a steady-state analysis is performed to discuss the unique solution of the DC distribution network and conduct hardware-in-loop experiments to demonstrate that this method achieves effective results.