Due to the complexity of detailed models, the single-generator equivalent model (SEM) of a wind farm is commonly used to facilitate the stability analysis. However, the adequacy of the SEM for stability analysis in direct-drive wind farms with VSC-HVDC systems is still uncertain. Therefore, this article analyzes the SEM adequacy in two aspects: the oscillation modes analysis and the sub-synchronous oscillation (SSO) stability enhancement by optimizing wind farm parameters. Firstly, various critical oscillation modes are characterized according to the frequency, and a comparison of these modes between the two-generator equivalent model and the SEM is presented. The effect of single-generator aggregation on the critical oscillation modes is discussed. Then, the impact of permanent magnet synchronous generator (PMSG) parameters on the inside-wind-farm/wind-farm-grid SSO modes is explored. It is revealed that with the change of PMSG parameters, the damping effect of the two SSO modes is identical. This implies that the SEM can be used to improve the SSO stability of the multi-machine system by optimizing the PMSG parameters. PSCAD/EMTDC simulations are further performed to verify the theoretical analysis.