Power balancing control of a multi-terminal DC constructed by multiport front-to-front DC-DC converters

This paper studies the power balancing control of a multi-terminal high voltage direct current (HVDC) system constructed by a multiport front-to-front type DC-DC converter (MF2F). Dynamics of the inner AC circuit of the MF2F is analysed which points out the natural instability of the MF2F under un-balanced power orders. To solve the instability problem, an inner power balance loop and an external power balance loop are proposed at each port of the MF2F. The inner balance loop ensures power balance inside the MF2F and the external balance loop ensures power balance between a port and its external DC system. Power balance control is also designed for external converters that are under constant power control. Limits of the integrators and the total output of the balance loops are specifically set to automatically disable the balance controllers in normal operation and automatically enable the balance controllers during power imbalance. To isolate the DC faults at external DC systems, self-blocking modular multilevel converters are used at each port of the MF2F. Efficacy of the proposed power balancing control is verified by extensive simulations under AC faults and DC faults that cause un-balanced power orders. The proposed controllers can be used as generic controllers for MF2F.