Harmonic modelling of voltage source converters for HVDC stations

Interest in the use of VSCs for HVDC substations is increasing rapidly due to their economical and technical advantages. Most of the work reported in the open literature has been on PWM control and VSC topologies, and in HVDC commissioning, construction and operation. Thus far, little emphasis has been placed on VSC harmonic generation and its impact on the power system, since it is assumed that the use of PWM control yields low harmonic distortion and that there is no risk of interacting with other power plant components. But in real systems this preconceived assumption is not always met. The harmonic frequencies generated by the PWM control and the multiple resonant impedance points exhibited by transmission lines and cables, may lead to unwanted interactions between the HVDC stations and the power system causing resonance problems at harmonic frequencies. In order to predict such problems, a suitable HVDC station model for harmonic propagation is required. In this paper, a new and comprehensive harmonic model of a VSC is developed using first principles. The VSC model is used as the basic building block with which harmonic models for HVDC-VSC back-to-back and HVDC-VSC transmission station are assembled. The response of the newly developed harmonic models are in good agreement with the steady-state responses of time domain models implemented in PSCAD/EMTDC.