The individual sources are responsive to {TAIL ~ an error signal} representing the difference between a component of power common to the sources or frequency, if the power sources are {HEAD ~ AC generators}, and a reference and to individual signals representing a variable power component 
The individual sources are responsive to { TAIL ~ an error signal } representing the difference between a component of power common to the sources or frequency , if the power sources are { HEAD ~ AC generators } , and a reference and to individual signals representing a variable power component
OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH