i want to implement upfc in ieee-14 bus system to improve the reliability as well as voltage profile and improvements in losses both active & reactive .
You can define the weakest bus in any system for stability studies for example there are many indexes can determine the weakest bus such as PV, PQ, λV, and many other stability indexes.
There are more than ten different indices available in the literature to identify the weak bus and branch. You can refer the paper " M.V. Suganyadevi · C.K. Babulal , Estimating of loadability margin of a power system by comparing Voltage Stability Indices" for further understanding.