Fang Yang Yuanzhang Sun Lin Cheng Jianyun Zhang Locating observers and controllers for severely disturbed power system using mode matrices. 623-626 2008 APCCAS https://doi.org/10.1109/APCCAS.2008.4746100 conf/apccas/2008 db/conf/apccas/apccas2008.html#YangSCZ08