Subsynchronous Resonance (SSR) is one of the major obstacles for the wide spread of high degrees (60% and higher) of series capacitor compensation. Recently, a new obstacle, namely Subsynchronous Interaction (SSI) has been added to the list after the Zorillo Gulf wind farm incident in Texas in October 2009. SSI is due to the interaction between large Doubly-Fed Induction Generator (DFIG)-based wind farms and series capacitor compensated transmission systems. In integrated power systems incorporating series capacitor compensated transmission lines and high penetration of wind energy conversion systems, especially DFIG-based wind farms, SSR and SSI could occur concurrently as a result of some system contingences. Therefore, mitigating SSR and SSI is an important area of research and development targeting at developing practical and effective countermeasures. This thesis reports the results of digital time-domain simulation studies that are carried out to investigate the potential use of Full-Scale Frequency Converter (FFC) and DFIG-based wind farms for simultaneous mitigation of SSR and SSI. This is achieved through introducing supplemental control signals in the reactive power control loops of the grid side converters of the DFIG and/or the FFC wind turbines. In this context, two supplemental controls designated as Supplemental Controls 1 and 2 are examined. Supplemental Control 1 introduces a signal in the grid side converter of the FFC wind turbines to damp both SSR and SSI oscillations. On the other hand, Supplemental Control 2 introduces a signal in the grid side converter of the FFC wind turbines for damping SSR oscillations and another signal in the grid side converters of the DFIG wind turbines for damping SSI oscillations. Time-domain simulations are conducted on a benchmark model using the ElectroMagnetic Transients program (EMTP-RV). The results of the investigations have demonstrated that the presented two supplemental controls are very effective in mitigating the SSR and SSI phenomena at different system contingencies and operating conditions.