Decentralized and Per-Unit Primary Control Framework for DC Distribution Networks with Multiple Voltage Levels

For a DC distribution network which contains multiple-voltage-level sub-networks interlinked by DC transformers, there is currently no primary control method to realize both of bidirectional voltage support between the different DC buses and power sharing between the energy storage systems (ESSs) in different sub-networks at the same, where the controllers of the DC transformers do not rely on the characteristics of the ESSs. To solve this problem, this paper proposes a decentralized and per-unit primary control framework for a multiple-voltage-level DC distribution network. In the proposed framework, the DC transformers are controlled by decentralized and per-unit difference-of-v2 controllers to regulate the buses voltages to be identical in the per-unit view, which only need the local information of the DC transformers and make the multiple-voltage-level DC distribution network to be regarded as a single-per-unit-voltage-level one. Besides, the (ESSs) in the sub-networks are regulated by per-unit p-v2 droop controllers to support the buses voltages and share the power. Under the proposed framework, the power sharing among the ESSs in the different sub-networks can be realized. Furthermore, under contingencies of ESSs malfunctions, as long as one ESS in a subnetwork remains to serve, the whole buses voltages of the DC distribution network can be supported, which means that the DC buses can provide bidirectional voltage support for each other. The feasibility of the proposed framework is illustrated by theoretical analyses and verified by hardware-in-loop tests.