Previously, you found your transformer had excess harmonics on the load side. The next step is to look at the primary side. The easiest place is probably at the panel supplying this transformer. That may be your service panel or a distribution panel downstream of it.

If you find high 5th and 7th harmonics, you may have a problem with power factor correction capacitors. Also, look at highly inductive loads, such as motors. If you have large motors that start across the line, installing a VFD that corrects for power factor and harmonics is pretty much a no brainer for power quality and cable integrity reasons alone. As you address whatever problems you find, be sure you work to the applicable IEEE standards, especially IEEE-519 (the harmonics standard) and IEEE-142 (the Green Book).

Once you have the primary side within spec, look at harmonics on the load side again. If they are still excessive, make a list of all inductive loads served by that transformer and address them one at a time. The typical culprit is a fan or pump motor. Conduct insulation and vibration tests; replace and align as needed. If high harmonics persist, install an electronic drive. Consider putting the motors on their own transformer/panel and/or changing the larger ones to 3-phase with electronic drives.