Energy margin from time domain simulation using partial energy function

The value of the time domain simulation technique for power system transient stability analysis can be greatly enhanced if it can indicate the degree of stability by producing a stability index. Often referred to as energy margin, the stability index offers additional insight into the transient stability problem being studied, and has the capability of speeding up transient stability limit derivation. In transient stability analysis, system separation largely depends on the energy of the critical machines that tend to separate from the rest of the system. It is this energy that has to be properly accounted for in evaluating system stability. This paper proposes the use of the partial energy function, corresponding to the critical machines, in conjunction with a relevant pseudo-fault-on trajectory, for computing energy margins for plant mode systems from conventional time domain simulations. The proposed method is successfully tested on a practical system consisting of 3596 buses and 383 generators, 25 of which are modelled in full detail. In comparison with the system transient energy function, the partial energy function requires much less CPU time while yielding almost the same energy margins