Network Harmonic Impedance Measurement: Practical Challenges and Possible Solutions

Network harmonic impedance is an important parameter for the calculation of harmonic emission limits and inverter-grid system stability studies. This work discusses the invasive measurement approach for network harmonic impedance, practical challenges in the measurement, and existing solutions. A methodology aimed at improving the accuracy of measured network harmonic impedance is then proposed and discussed. The proposed approach makes use of an interleaved approach of network perturbation, synchronization of pre- and post-perturbation data segments, and averaging. Through field measurements, the proposed method is verified and the effects of measurement window length are discussed. It is shown that the length of the measurement window influences the obtained impedance frequency response, especially in the harmonic frequency band due to network variations.