  This paper proposes a new semi-analytical approach for online time-domain
power system simulation. The approach applies the differential transformation
method (DTM) to the power system differential equation model to offline derive
a semi-analytical solution (SAS) having symbolic variables about time, the
initial state and system conditions. When simulation is online needed for a
contingency under the current system condition, the SAS can be evaluated in
real time to generate simulation results. Compared to the Adomian decomposition
method in obtaining a power system SAS, an SAS derived by the DTM adopts a
recursive form to avoid generating and storing its complete symbolic
expression, which makes both derivation and evaluation of the SAS more
efficient especially for multi-machine power systems. The optimal order of a
DTM-based SAS is studied for the best time performance of simulation. The paper
also designs a parallel computing strategy for power system simulation using
the DTM-based SAS. Tests on the IEEE 10-machine 39-bus system demonstrate
significant speedup of simulation using the proposed approach compared with the
Runge-Kutta method.
