Yes it is very possible to use boundary angles or boundary power but It is more or less like stability problem. Why do you want to introduce these boundary variables?

Dear Aamir

Please take a look for these attachments

Good luck

Regards

Article Boundary Detection and Enhancement Strategy for Power System...

https://web.eecs.umich.edu/~hiskens/publications/ps_aug01_1.pdf

http://orbit.dtu.dk/ws/files/158592279/Improved_Voltage_Stability_Boundary_Monitoring_by_Accounting_for_Variations_in_Thevenin_Voltage.pdf

http://www.sapub.org/global/showpaperpdf.aspx?doi=10.5923/j.eee.20130301.01

You can add wherever needed according to your objective , but this will add more complexity to your problem to solve . Also by adding this you solution results may not satisfy other costraints