Wind turbine generators can be operated in various types of system configurations. Once the configurations change, the system oscillation mode shapes change accordingly. The modelling of the full system is necessary for studying this issue, yet it is quite hard. In this paper, a simple approach is developed to study the mode shapes of wind power systems without the necessity of adopting the complex full-system models. The key is that the q-d axis model of electric power system is transformed into the single-axis model, so that it could integrate with the equivalent circuit model of drive train mechanism. After analyzing some of the system configurations organized by the well-known MOD-2 wind turbine generator unit, the proposed approach is found to be effective for analyzing various oscillation modes such as the local torsional oscillations, as well as the inter-unit and inter-area oscillations.