Catalin Gavriluta Ignacio Candela Alvaro Luna Antonio Gómez Expósito Pedro Rodríguez Hierarchical Control of HV-MTDC Systems With Droop-Based Primary and OPF-Based Secondary. 1502-1510 2015 6 IEEE Trans. Smart Grid 3 https://doi.org/10.1109/TSG.2014.2365854 db/journals/tsg/tsg6.html#GavrilutaCLER15