I´m trying to get a time series stationary. The sample size is rather large >100 000. In this example I have only used 14 600 of these samples. I have differentiated the series and removed some seasonal patterns that I expected and found through a power spectrum.
Looking at the periodogram after this shows no specific patterns. The log was also taken before the transformations to reduce the variance dependency.
I have attached a plot of the ACF after all this was done. My question is if this series can be regarded as stationary? If not, any suggestions how I could transform it to make it stationary?