The modelling and quantitative analysis of power electronics controllers based on voltage source converters, for harmonic studies in power systems, are the main objectives of this book. A complete steady-state model, using the harmonic domain, for the voltage source converter based on PWM switching techniques is given. As a modular approach, the voltage source converter model is used to build up models for the STATCOM, HVDC-VSC stations and UPFC. A state-space representation of systems, based on time-dependent Fourier series, is proposed for dynamic harmonic analysis. In this new state-space, the new state variables are the harmonic coefficients of the original state variables. State-space equations for the STATCOM and HVDC-VSC station models are obtained and presented in this book. Finally, a harmonic power flow method, based on the instantaneous power flow balance and Newton-Raphson method, is also developed in this book. The proposed method support all the models developed in the harmonic domain.