Predicting Resonance Frequencies in Distribution Networks with Grid-Connected Inverters

Increase in the amplitude of current harmonics at a resonance frequency could lead to high voltage harmonics in the distribution network due to resonance. As these voltage harmonics could potentially damage the insulation of network elements, preventive actions need to be taken. To take preventive actions, investigating the possibility of resonance occurrence in network is necessary. For that purpose, the resonance frequencies of the network can be estimated by employing the equivalent circuits of circuit elements, such as cables or transformers. This indicates that the accuracy of these equivalent circuits significantly affects the estimated resonance frequencies. Consequently, this paper studies the equivalent circuits of grid elements in terms of resonance frequency calculation. Also, the impacts of skin effect and parallel capacitor in cable, and the high frequency model of transformers on the estimated resonance frequencies are analysed. These investigations show that skin effect changes the value of parameters at different frequencies nonlinearly, which further complicates the estimation of resonance frequencies. Therefore, Reduced Least Squares (RLS) method is developed to estimate resonance frequencies based on Least Squares method. Finally, the RLS method is verified by the results of different case studies.