Unbalanced Voltage Suppression in a Bipolar DC Distribution Network Based on DC Electric Springs

In this paper, a method is proposed to mitigate unbalanced voltage and reduce power losses due to neutral line current in a bipolar direct current (DC) distribution network by introducing a DC electric spring (DC-ES). The coupling relationship between positive and negative pole voltages are investigated given the resistance of neutral line in the bipolar DC distribution network. The coupling matrixes of the positive and negative pole voltages and the DC-ES output voltages are deduced. And this matrixes demonstrate that the DC-ES control system is composed of double inputs of pole voltages and double outputs of DC-ES voltages. The small-signal model of the bipolar DC distribution network with DC-ESs is derived further. Subsequently, the control system with double inputs and outputs is transformed into two independent control systems by introducing the decoupling control variables, thereby increasing its rapidity and anti-interference performance significantly. The proposed unbalanced voltage suppression method and decoupling control of DC-ESs are validated by simulations and experiments.