On the Potential of Subsynchronous Resonance of Voltage-Source Converters with the Grid.

This paper examines the possibility of subsynchronous resonance (SSR) between voltage-source converters (VSC) and the power grid under different configurations and in different applications. SSR between Type-III wind turbines and a weak grid with series compensation has been discussed in the literature. The purpose of this work is to understand whether a VSC operating as an inverter for Type-IV wind turbine and PV application could form SSR with the grid, especially a weak grid with or without series compensation. The analysis is based on analytical impedance models of the VSC and is performed in two steps. In the first step, dynamics of the dc bus voltage are ignored, allowing the use of relatively simple models of the VSC to be used. In the second step, the effects of dc bus dynamics on the inverter output impedance are considered. In each case, both an inductive grid and a capacitive grid (when series compensation is used) are considered, and design guidelines to avoid SSR and possible mitigation methods when SSR occurs are also presented. It is found that, with an ideal dc bus, SSR could happen but is unlikely if there is no series compensation in the grid. A soft dc bus, on the other hand, may form resonance with the grid through the inverter and, when such a resonance occurs, the inverter output current typically exhibits a supersynchronous as well as a subsynchronous resonance at frequencies that are symmetrical about the fundamental frequency. The impedance-based small-signal analysis is verified by detailed circuit simulation. [ABSTRACT FROM AUTHOR]