How to evaluate Spectral Density of DC component vs. AC component during computing PSD?
PSD is the spectral power density and also has the DC component, but in practice finite length of window inevitably produce leaking (because of side-lobes), so to prevent masking of small amplitudes near zero frequency we have to remove DC component.

Functions like pwelch don't remove DC component but it's better to remove it manually and after PSD calculations find it manually.