The unbalanced power between positive and negative poles in a bipolar DC distribution network (DC-DN) generates an unbalanced current at the neutral line, which enlarges the power losses of the system and the voltage deviation of DC loads. An unbalanced power suppression strategy based on a series-parallel power flow controller (SP-PFC) is proposed in this paper. The SP-PFC is adopted as the interconnection between two different DC-DNs. The topology and operating modes of SP-PFC are analyzed. Subsequently, SP-PFC output voltage and line current expressions under constant power control are derived. The nonlinear relationship between the output voltage and line current is linearized at the operating point. On this basis, the influences of unbalanced load and receiving-end voltage on the SP-PFC are investigated. A small-signal model of bipolar DC-DN containing an SP-PFC is established, and the system stability is analyzed. A simulation model of the bipolar DC distribution network containing an SP-PFC is built up in MATLAB/Simulink, and the effectiveness of the SP-PFC in the suppression of unbalanced power is verified.

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The two main challenges of medium voltage direct current (MVDC) distribution network are the flexible control of power flow (PF) and fault protection. In this paper, the power flow controller (PFC) is introduced to regulate the PF and inhibit the fault current during the DC fault. The coordination strategy of series-parallel PFC (SP-PFC) and hybrid DC circuit breaker (DCCB) is proposed. By regulating the polarity and magnitude of SP-PFC output voltage during the fault, the rising speed of fault current can be suppressed so as to reduce the breaking current of hybrid DCCB. The access mode of SP-PFC to the MVDC distribution network and its topology are analyzed, and the coordination strategy between SP-PFC and hybrid DCCB is investigated. Moreover, the emergency control and bypass control strategies of SP-PFC are developed. On this basis, the mathematical model of SP-PFC in different fault stages is derived. With the equivalent model of SP-PFC, the fault current of the MVDC distribution network can be calculated accurately. A simulation model of the MVDC distribution network containing SP-PFC is established in MATLAB/Simulink. The fault current calculation result is compared with the simulation result, and the effectiveness of the proposed coordination strategy is verified.@en

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.@en

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.

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