Enhancement of low-frequency system stability of 60-Hz railway power grids

Low-frequency stability is a major concern in electric-traction applications independent of the specific nominal grid frequency. The related low-frequency oscillations occur in two dual scenarios — either one or only few traction vehicles are operated far away from the next substation or a multiple of vehicles is operated at a strong grid. With the state-of-the-art control concept for the line-side converter, low-frequency oscillations can be observed in the range of 10 to 30 percent of the nominal grid frequency. In this paper, a multivariable control concept for four-quadrant line-side converters is presented which notably improves the stability. Additionally, the transient response of the line converter is enhanced. The superiority of the proposed control method is outlined using time-domain simulation which has been performed using a simulation approach for multi-terminal power-electronic applications.