A New Control Structure for Multi-Terminal dc Grids to Damp Inter-Area Oscillations

This article analyzes the control structure of the multi-terminal dc (MTDC) system to damp ac system interarea oscillations through active power modulation. A new control structure is presented that maximizes the relative controllability without the need for communication among the dc terminals. In point-to-point high voltage dc (HVDC) transmission, the active power modulation of the two terminals occurs in opposite directions. In this case the control direction is given and only
needs to be phase compensated to align for maximal damping. In the case of MTDC systems the control direction interrelates with the active power modulation share of the dc terminals and the relative controllability depends on this.
The new control structure eliminates the need of communication between the dc terminals by performing dc voltage feedback loop shaping. This makes it possible to modulate the power in one terminal and let the other terminals react on the dc voltage change. Through loop shaping, where the feedback gain varies in the frequency plane as compared to the regular droop design, the control direction is aligned with the direction of highest relative controllability. The loop shaping takes place without influencing the high frequency or steady-state gain. Simulations in the Nordic32 test system show the validity of the proposed controller and its structure.