The changing inertia profile caused by the increased penetration of inertialess renewable energy sources has raised concerns about power system frequency control. The kinetic energy of the turbines in wind turbine generators (WTGs) can be utilized to support power system frequency during contingencies. In this paper, we investigate the frequency support capability of WTGs operating at the maximum power point (MPP). The requirements to prevent secondary frequency dips, provoked from switching between normal operating mode and the frequency support mode, are formulated. A time-variable droop characteristic is proposed for frequency support from WTGs, which is quite effective in preventing large frequency excursions and facilitates smooth recovery of the kinetic energy of WTGs. The performance of the proposed method is examined in different operating conditions of WTGs in a single bus model of power systems, as well as a regional 14-generator model of the Australian National Electricity Market. The results show that with the time-variable droop characteristic, the frequency nadir following a contingency can be largely improved and simultaneously, the WTGs can smoothly regain their kinetic energy and continue operating at the MPP.