Generalized voltage droop strategy for power synchronization control in multi-terminal DC grids - an analytical approach

Vector current control based on Phase-locked loop, regardless of its popularity in control of grid-connected converters, performs under the limitation of the short-circuit capacity of the connected ac system. A previously proposed method, power-synchronization control (PSC), by L. Zhang et al. has been demonstrated good performance in HVDC links especially in case of weak ac grid interconnection. This method utilizes the internal synchronization mechanism, analogous to the operation of a synchronous machine in ac grids. By using this technique, the voltage source converter (VSC) avoids the instability caused by a standard PLL in a weak ac-system connection. Moreover, a VSC terminal can give the weak ac system strong voltage support, just like a common synchronous machine does. In this paper, generalized voltage droop (GVD) strategy is developed based on PSC to provide more flexibility in control paradigm of MTDC grids and smoother transition among different operation modes of converter stations. GVD is implemented at the primary layer of a two-layer hierarchical control structure of MTDC grid, and brings about some new features such as fixed power control and fixed dc voltage control, additional to the conventional voltage droop characteristics. The mode transition can be achieved according to the set point given by the secondary layer of the control framework. Analytical justifications are given to demonstrate the effectiveness in each control mode and the capability in soft mode transition.