A power quality survey has been performed in several Swedish medium voltage (MV) substations and a study of the long-term power quality levels under normal operation has been done. The measurements have been performed during several years and the study includes seasonal variations as well as weekly and daily variations. This paper will focus on harmonic voltage distortion (THD and individual harmonics). An analysis has been done looking into metrological data to see any possible correlation with variations in harmonic distortion. Conclusions show that there are some significant differences over the years and between days for all sites. Long term measurements are needed to gain information on which levels one can expect, measuring for only a few weeks or month can give an over or underestimation of the actual levels. Results also shows that the magnitudes of harmonic distortion are well below the permitted level at all time at the selected sites.