The paper addresses the possibilities of application of discrete Short-time Fourier transform (STFT) and Fast Fourier transform (FFT) for detection of voltage dips in the distribution grid with high level of distributed generation in the concept of the future smart grids. Modelling and testing are carried out in the MATLAB/SimPowerSystems environment. Modified IEEE13 bus system with high level of distributed generation from wind and solar sources, with different control algorithms, was used. Some advantages of FFT is noted, especially when the task is very fast detection of voltage dips.