Research on detection and elimination methods of the micro grid harmonics based on the continuous wavelet transform

For the harmonics measurement in the micro grid accurately a detection method based on continuous Morlet wavelet transform is proposed in this paper. Combined with the high resolution in the frequency domain of continuous wavelet transform, the Morlet wavelet function is used to extract the local maxima lines in each frequency band. The relevance of the local maxima lines and harmonic frequency is used to separate the harmonics orders clearly. In order to figure out the harmonics source in the micro grid, the modulation process of the voltage source converter, which is usually adopted in grid connected applications, is analyzed in detail. Because of the switching impact of the converter, even small harmonic magnitudes can excite harmonics of large magnitudes or cause additional harmonics. To overcome the disadvantage of the L-filter, the LCL-filter with virtual resistor is designed to eliminate the harmonics. Filter capacitor voltage feedforward with virtual resistor is inner fast loop, while outer loop with appropriate grid side current ensures fast command tracking. The use of the virtual resistor alleviates the impact of filter resonance on the specific frequency and improves the system stability. This paper with theoretical and simulation results, demonstrates effectiveness of the purposed methods to detect and eliminate the grid side harmonics in the micro grid.