  This paper proposes a new strategy for optimal grid frequency regulation (FR)
in an interconnected power system where regional ac grids and an offshore wind
farm are linked via a multi-terminal high voltage direct-current (MTDC)
network. In the proposed strategy, decentralized H-infinity controllers are
developed to coordinate the operations of ac synchronous generators and hybrid
MTDC converters, thus achieving optimal power sharing of interconnected ac
grids and minimizing frequency deviations in each grid. To develop the
controllers, robust optimization problems are formulated and solved using a
dynamic model of the hybrid MTDC-linked grids with model parameter uncertainty
and decentralized control inputs and outputs. The model orders of the resulting
controllers are then reduced using a balanced truncation algorithm to eliminate
unobservable and uncontrollable state variables while preserving their dominant
response characteristics. Sensitivity and eigenvalue analyses are conducted
focusing on the effects of grid measurements, parameter uncertainty levels, and
communication time delays. Comparative case studies are also carried out to
verify that the proposed strategy improves the effectiveness, stability, and
robustness of real-time FR in MTDC-linked grids under various conditions
characterized mainly by load demands, communications systems, and weighting
functions.
