Synchronous machine reference frames

PRESENT-DAY economics indicate the practicability of long-distance a-c power transmission in many localities for distances greater than 300 miles. One of the important characteristics of such a transmission system is its steady-state stability limit. That is, at some particular steady-state load and system condition, when the synchronous machine is still able to supply more power across the line, the system nevertheless loses synchronism. This is interpreted physically to mean that the generator has no longer enough synchronizing torque to keep it in synchronism against small disturbances that continually arise.