A Novel Coordinated Control Strategy for LCC/VSC Hybrid DC Distribution Power System

For the multi-terminal DC distribution power system (DPS), line commutated converter (LCC) and voltage source converter (VSC) can be used in different terminals to take full advantage of large capacity of LCC and control flexibility of VSC. However, there are usually a large number of DC loads and renewable energy sources connected to the DC distribution system, which will bring large, rapid and frequent power fluctuation. It is difficult to maintain the power balance and DC bus voltage due to the slow response of LCC and small capacity of VSC. To solve it, this paper proposes a steady-state power allocation ratio for LCC and VSC as well as the switching strategy. By this way, the large-capacity LCC can be timely put into operation to suffer most power and save the investment, or shut down to avoid the discontinuous current at low load condition, thus that the DC voltage fluctuation can be reduced. To further decrease the voltage fluctuation due to small inertia of DC DPS, a dynamic inertia coordinated control strategy of LCC/VSC based on virtual DC generator (VDG) method is also proposed in this paper. By designing a reasonable low-pass filter (LPF), the slow LCC responds to most of low-frequency armature current in VDG, and small-capacity VSC responds to a small low-frequency and all high-frequency components. Hence, the inertia increases fully utilizing the characteristics of LCC and VSC. Through the above steady-state and transient control strategy, the DC bus voltage can be stabilized well, while disturbance to the AC system can also be effectively reduced. Finally, a simulation model of a dual-terminal DC DPS is built to verify the proposed strategy.