Abstract The quantification of reactive power reserves can be essential for transmission system operators for the secure operation of large power systems with high share of renewables. Reactive power support from renewable energy sources needs to be included in reactive power reserve estimation. The reactive power capability of variable renewable energy sources like wind power plants depends on active power production. The stochastic nature of wind speed, and thereby active power production, makes reactive power reserve estimation quite complex. This article proposes a novel methodology to estimate reactive power reserve of wind power plants, taking the uncertainties such as the active power fluctuation, the technical availability of wind turbines as well as the on-load tap changer position of wind power plant transformers into account. The methodology incorporates the definition and determination of robustness index for wind power plant as a reactive power source. The proposed methodology is evaluated on a test system, by assessing the impact of the robustness index on long-term voltage stability. The results show that the choice of an appropriate time period for assessment of wind power plant as a reactive power source is critical to have a high degree of confidence in reactive power reserve estimation. Furthermore, the investigation reveals the importance of considering the power fluctuation and availability in the estimation of the reactive power reserve. The presented case study shows that ignoring fluctuation and availability can lead to overestimation of maximum power transfer, resulting in wrong assessment of the power system security. The proposed methodology can be used by the transmission system operators to quantify the reactive power reserve in real-time and allow better integration of wind power plants in voltage stability assessment studies.