Harmonic impedance is an important parameter for electrical power system analyses. This parameter can be applied in some important application such as island and resonance detection. This work presents an impedance estimation technique implemented in real time. The method uses a small current disturbance injection in the harmonic frequency and uses Sliding Window Recursive Discrete Fourier Transform (SWRDFT) to calculate magnitude and phase for each component of current and voltage. The small current disturbance is a Modified Morlet Wavelet waveform and a sequential of small waves, each one centered in a harmonic frequency is injected in the network. In order to quantify the impact of the sequential waves injection in the power quality a new factor is defined, named Root Mean Square Ratio (RMSR). The estimation technique was implemented in real time using the dSPACE computer, and the electrical network, whose harmonic impedance at the point of common coupling is required, was simulated in the Real Time Digital Simulator (RTDS).