The electromagnetic transient (EMT) simulation of a power system involving power-electronics converters requires a fairly small time-step size to consider switching of the converters, thus leading to a heavy computational burden. To accelerate such simulations, this paper generalizes the time average method (TAM), originally developed for real-time simulations, so that it becomes suitable to off-line EMT simulations. For obtaining accurate current waveforms with a large time step, the TAM and the proposed method represent each leg of an inverter by voltage sources, and its output voltage is modified by interpolation at an instance of switching. The original TAM was intended for the primitive backward Euler method. This paper contributes to generalize it for the trapezoidal integration method which is widely used in off-line simulation programs. In addition, the proposed method uses a simple formula to identify the switching instance for the implementation on off-the-shelf PCs, rather than a hardware counter in an FPGA as used in the TAM. This paper shows that the proposed method enables to extend the time step by a factor of five without deteriorating the accuracy. A case study demonstrates reduction of computational time by a factor of three for the off-line simulation of a single-phase grid-connected inverter with reasonable reproduction of harmonics.
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