Coordinated hierarchical voltage control for flexible DC distribution systems

•The changes of line and control parameters can lead to the instability of the system.•The dead-zone based PID control is proposed to carry out fast power demand response.•The coordinated hierarchical voltage control (CHVC) strategy is proposed.•Numerical examples verify the feasibility and the effectiveness of CHVC. The poor voltage stability and weak damping of flexible DC distribution system bring great challenges to its safe and stable operation, and its power imbalance also has a negative impact on AC distribution system. In this paper, a novel dead-zone based PID control strategy and a coordinated hierarchical voltage control (CHVC) paradigm are presented to improve the voltage stability and the damping of flexible DC distribution systems. The dead-zone based PID control is performed by a PID controller with a dead zone. It can not only carry out fast power demand response, but also prolong the service life of the battery. The CHVC strategy coordinates the battery converter control and the interconnected voltage source converter (VSC) droop control. It can not only improve the overall dynamic and steady-state characteristics of the system, but also reduce the influence of the DC system unbalanced power on the AC system. Furthermore, the effects of DC line parameters, the intensity of interconnected AC distribution systems and outer loop control parameters on the stability of the system are also analyzed, which can provide a theoretical basis for the selection of system hardware and control parameters. Numerical examples verify the feasibility and the effectiveness of CHVC.