By simulating the operating dynamics of synchronous generators (SGs), the use of virtual synchronous generators (VSGs) can help overcome inverter-based generators’ shortcomings of low inertia and minimal damping for grid-forming applications. VSGs’ stability are very important for their solar and wind electricity applications. Currently, the related research primarily focuses on VSGs and their applications for microgrids. There has been little research to explore how VSGs effect low frequency oscillations in power transmission systems. This paper describes a small-signal model of a VSG-SG interconnected system, which is suitable for studying low frequency oscillation damping in a power transmission grid. Based on this model, the effects of VSGs on low frequency oscillations are compared with the effects of SGs to reveal the mechanism of how VSGs influence damping characteristics. The influence of each VSG control loop on oscillations is also analyzed in this paper. Then, the low frequency oscillation risks with different types of VSGs are described. Finally, experiments on a real-time laboratory (RT-LAB) platform are conducted to verify the small-signal analysis results.