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This paper proposes a wavelet-based monitor (WBM) for grid-impedance estimation, which combines a wavelet-based transient detection scheme (WB-TDS) and a wavelet-based grid impedance estimator (WB-GIE). The WB-TDS employs the analysis of the wavelet coefficients energy for detecting grid impedance changing, whereas the WB-GIE estimates the grid-impedance by using the real-time stationary discrete wavelet packet transform (RT-SDWPT) associated with signal injection scheme. During a grid impedance changing, the WB-TDS triggers the WB-GIE for injecting an interharmonic into the power grid to estimate its current impedance. This method mitigates the THD generated by the continuous signal injection employed in the existing techniques. The WB-GIE identifies the phase grid impedance resistance and reactance accurately in balanced or unbalanced conditions. Due to its inherent characteristics, WBM is suitable to be inserted into the adaptive power flow control of distributed generation systems. Experimental results obtained from a grid-connected photovoltaic generation laboratory setup validated the proposed method and demonstrated its effectiveness.

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