M 1 M 6 M 7 M 5 M 5a M 6a M 7a M sw M hw M CO M 7b M 5b unit. M 4 M 4a M 4b measurement Phasor M 3 M 3a M 3b PMU, PMU. the M 2 of M 2a M 2b Modeling 3.7 FIGURE