Research On Flexible DC Distribution Svstem Unbalanced Control

Voltage source converter(VSC) based flexible DC distribution net is well-known for itsaccessibility of absorbing distributed generators(DGs) like photovoltaicgeneration system and wind turbine generation system, and DC loads like electric vehicle charging stations and data centers, which makes it a great supplement to present AC distribution net. The impact on DC side from AC short circuit faults can be reduced by DC distribution system, which improves the power quality to the loads of the DC side. However, specific controllers are needed to be designed to fulfill this function, so as to promote the performance of DC distribution system under unbalanced AC voltages.This paper first analyzes the mechanism and the impact of AC asymmetric faults. And then the unbalanced control strategy of VSC is analyzed. The power transmission limit issue is discussed in the last part of the paper. On the basis of the deduction of analytical expressions of the AC currents, the relations between asymmetric AC voltages and the transmission power limit of the AC side is deduced with consideration of the overcurrent constraints. Then a numerical method to calculate the transmission power limit of the DC side is proposed. The transmission power limits of VSCs of both the AC and DC sides are necessary for power distribution in multi-terminal DC distribution network. To suppress the fluctuation of the DC voltage, it is necessary to control the active power of VSCs within the transmission power limitof the DC side.All analysis mentioned above and methods proposed are verified by electromagnetic transient simulation software PSCAD.