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It is well known that network analyzers perform reliable estimations of the Total Harmonic Distortion (THD). A distorted signal is actually decomposed in real time into its harmonic components using the Fourier transform. The implemented algorithm requires a 200ms observation window in order to meet both the necessary accuracy and standard requirements. Though the adopted observation time is compatible with the dynamic behavior of large generators, it is however not acceptable for small distributed generators (DG). Moreover, DGs are usually intermittent energy sources connected to a distribution network through inverters with time constants of dozens of microseconds. For this reason, smart grids with a massive presence of distributed renewable generation have to be monitored by fast algorithms in order to guarantee high levels of power quality. Actually, a remarkable distortion of the voltage waveform, which may also be due to a malfunctioning inverter, can cause for instance an incorrect behavior of digital protection systems. Herein a fast algorithm for the estimation of voltage harmonics is introduced, which, contrarily to algorithms currently adopted for the discrete Fourier transform, can be implemented also in non-updated, low performing microprocessors. The proposed method uses a second order generalized integrator that refers to the SOGI-based Frequency Locked Loop (SOGI-FLL). A number of simulations were performed in order to validate the method, and the results showed that the time response of the algorithm is less than 20 ms, which means the method can be profitably used to improve smart grid power quality.