A well-specified transformer lasts 30 to 40 years. The ones that die earlier are almost always victims of the same causes, and most are neutralised at specification stage, not at the repair shop.
A transformer rarely dies suddenly: it ages. Every hour spent above its design temperature degrades the insulation (paper, resin), and the degradation is exponential: as an order of magnitude, a few extra degrees held continuously can roughly halve the service life. The usual culprits: chronic overload (often born of undersizing; see the kVA / kW confusion), insufficient room ventilation, fouled radiators, underestimated ambient temperature.
Drives, servers, UPS systems and charging points distort the current. Those harmonics create extra losses the kW meter never shows: a transformer "half loaded" on paper can run as hot as at full load. The remedy is known: a K-factor matched to the load.
Water is insulation's enemy: it lowers the oil's dielectric strength and accelerates paper ageing. It enters through tired gaskets, a saturated breather, or interventions closed up badly. On dry-type units, the threats are conductive dust and condensation on the encapsulated windings. In marine, dusty or humid environments this is handled at order stage: IP rating, anticorrosion treatment, suitable resin; the "site conditions" field of the configurator exists exactly for that.
Lightning strikes on the network and switching overvoltages fatigue the insulation in jolts. The classic protections, well-placed surge arresters and insulation coordination (BIL levels per IEC / EN 60076-3), belong to the substation design and are planned with your design office.
A banal, avoidable cause: a poorly torqued LV lug heats, oxidises, heats further, until the incident. Torque tightening at commissioning, then infrared thermography checks during maintenance rounds, remove most of the risk.
These protections sit in the "Protections / accessories" field of the configurator: specifying them at order stage costs little; regretting them costs a transformer. And for the fleet already in service, the spare parts line keeps the watchdogs alive.
Orders of magnitude and general good practice; the exact maintenance plan depends on the unit, its load and your operating environment.
In the order of 30 to 40 years for a unit correctly sized and operated within its rated conditions. Chronic overheating is what shortens it: a few extra degrees held continuously can roughly halve the insulation's life.
Gas evolution, pressure and temperature on an oil-immersed unit, with two levels: alarm first, then trip. It is the standard French protection for hermetically filled distribution transformers.
Dissolved gas analysis (DGA) is the transformer's blood test: a periodic sample reveals the gases produced by a developing internal fault, often weeks or months before the outage. It is one of the most effective preventive tools.
Sizing on real kVA with margin, declaring the site's true ambient temperature, guaranteeing the room's ventilation, and ordering the watchdogs (DGPT2 or probes) with the unit. Specifying them at order stage costs little; regretting them costs a transformer.