A Voltage Interpolation Method in Inverter Modeling for Fast Electromagnetic Transient Simulations

The electromagnetic transient (EMT) simulation of a power system involving power-electronics converters requires a fairly small time-step size to take into account switching of 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 represents each arm of an inverter by a voltage source, and its output voltage is modified by interpolation at an instance of switching. For the interpolation, the proposed method uses the trapezoidal method of integration which is widely-used in off-line simulation programs, while the TAM uses the primitive backward Euler method. 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 reduces computation time by a factor of 15 for the off-line simulation of a single-phase inverter with reasonable reproduction of harmonics.