CE marking is not a sticker you buy — it is a set of obligations someone must carry. On an imported transformer, knowing who that "someone" is protects you from a costly surprise.
For a power transformer, CE marking means the product meets the applicable EU directives (Low Voltage, EMC, Ecodesign) and that a responsible economic operator holds a technical file and issues an EU Declaration of Conformity (DoC). The mark is the visible tip; the file and the DoC are the substance.
A compliant delivery comes with: the EU Declaration of Conformity, the type and routine test reports (to IEC / EN 60076), the nameplate data, and the technical documentation needed by your installer and design office. This is exactly what lets your customs broker clear the unit without friction.
A frequent trap: clearing customs and being compliant are two separate things. Paying duties and VAT gets the crate through the border; it says nothing about CE, Tier 2 or WEEE. Both must be satisfied — see Tier 2 explained and the full EU compliance page. For the import mechanics themselves (EORI, VAT, Incoterms), the detailed walkthrough lives on the French import guide.
General information, not legal advice; the exact allocation of responsibilities depends on your setup.