Abstract:
|
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. |