This paper analyses the fault current characteristics of a flexible DC grid with a power flow controller (PFC) and proposes a coordinated control method for suppression of fault currents between the PFC and the DC circuit breaker (DCCB). The equivalent circuits of the PFC during different stages of the DC fault are analyzed. Moreover, the influence of the output voltage of the PFC on the fault current is studied. After that, the current suppression characteristics of the current limiting reactor (CLR) and PFC are investigated. Due to the output voltage of the PFC can be flexibly adjusted during the DC fault, the fault current can be suppressed significantly by adjusting the output voltage of the PFC when the DC fault is detected. This paper adopts a bang-bang control for PFC during the fault. Finally, a simulation model was built in Matlab/Simulink to verify the fault current characteristics of the DC grid containing the PFC, and the coordinated control of the PFC was verified. ... Read more

For a bipolar meshed DC distribution grid, the number of DC transmission lines is greater than converter stations. Therefore, the power flow control (PFC) needs to be introduced to increase the control flexibility of the DC grid. Besides, the unbalanced power between the positive and negative poles in bipolar DC girds may result in a large unbalanced current in the neutral line, causing the pole voltages to deviate from the rated value and increasing the power losses in the bipolar network. This paper aims to realize the PFC of DC distribution grids while suppressing the unbalanced voltage/power of the positive and negative poles. The application of a series parallel-connected PFC in bipolar DC distribution systems is investigated. Subsequently, the relationship between the output voltage of the PFC and the unbalanced power of the bipolar DC distribution grids is derived under the two different control modes of PFC. The simulation model is built in MATLAB/Simulink. The results show that when constant voltage control is applied, the PFC can suppress the unbalanced current in the transmission line, and it can also realize flexible control of the DC power flow. ... Read more