U0 and {HEAD ~ P0} are a nominal voltage and {TAIL ~ nominal active power}, respectively, as measured prior to the contingency 
are
U0 and { HEAD ~ P0 } are a nominal voltage and { TAIL ~ nominal active power } , respectively , as measured prior to the contingency
OTH OTH HEAD HEAD HEAD HEAD HEAD REL OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH