Modeling of a multilevel voltage source converter using the fast time-domain method

Power system harmonics can increase system loses, excite resonant frequency, and cause equipment damage. It is therefore essential to develop accurate harmonic models for harmonic sources such as power converters. During last decades, renewable energy sources have become an important part of the worldwide concern with clean power generation. In response to the growing demand for medium and high power trends, multilevel converters (MCs) have been attracting growing considerations. As they become more spread-out, their harmonic impact on the system could be significant. In this paper, a fast time-domain method (FTDM) suitable for modeling a MC is proposed. The FTDM essentially model the harmonic of interest as a harmonic state and solve it together with the system differential equations. Since the solution is analytical, the FTDM is very accurate. Nevertheless, to model a MC, the FTDM requires to solve a very large exponential matrix. Hence, a Krylov subspace method is proposed to solve for the exponential matrix, resulted from the FTDM. Two case studies on two different modulations (in-phase disposition and phase-shifted modulations) are performed to validate the proposed method. As demonstrated in the paper, the computation time of the proposed method is much shorter compared to the brute-force time-domain simulators such as PSCAD/EMTDC. Moreover, when compared to other solvers, the proposed Krylov-based method yields the best in terms of calculation time.