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