Observing time-varying unbalances and asymmetries at harmonic frequencies

Traditional harmonic distortion analysis assumes balanced and steady-state conditions. Moreover, harmonic components are often associated with certain sequences (positive, negative or zero). However, in reality unbalance causes the existence of all sorts of sequences for every frequency. Additionally, time-varying conditions complicate analysis and require appropriate models. Many of the time-varying unbalances, especially at higher frequencies, have not been discussed in detail before. This paper uses the Slide Window Recursive Fourier Transform to compute the time-varying harmonic components and then calculates and plots the positive, negative and zero sequences and parameters for each frequency. Recorded signals from a real system are used for this analysis. These time-varying parameters can then be used to further investigate the nature of some transient phenomena and for providing information for protection and control applications as the grid becomes smarter and more complex.