Power system time variance - LPTV model implementation and identification problems

In power system harmonic studies the power system non-linearity is sometimes observed. It mainly manifests as a time variance of power system properties as the non-linear system components working points are set by relatively stable system voltage waveforms. The LPTV (linear periodically time variant) power system model is a natural extention of the time invariant model, and better describes the power system voltage/current relations. As the LPTV model deals with parametric system description it is more difficult in implementation and identification. Two approaches to LPTV system description are presented in the paper. The interpolating methods are discussed which allow for efficient model identification in non-stationary power system conditions.