  In multi-infeed hybrid AC-DC (MIDC) systems, the asynchronous interconnection
between regional grids, the complicated system dynamics and possible cascading
failures have an enormous effect on the frequency stability. In order to deal
with the frequency instability problems in emergency situations, this paper
proposes a decentralized emergency frequency control strategy based on
coordinated droop for the MIDC system. First, a P-f droop control for LCC-HVDC
systems is introduced and the coordinated droop mechanism among LCC-HVDC
systems and generators is designed. Then, to reasonably allocate the power
imbalance among LCC-HVDC systems and generators, an optimal emergency frequency
control (OEFC) problem is formulated, and the optimal droop coefficients are
selected in a decentralized approach, which can deal with various control
objectives. A Lyapunov stability analysis shows that the closed-loop
equilibrium is locally asymptotically stable considering the LCC-HVDC dynamics.
The effectiveness of the proposed emergency control strategy is verified
through simulations.
