35 kV Oil-Immersed Transformers: What Buyers Should Lock Before Factory Design

A 35 kV oil-immersed transformer connects utility, industrial and renewable-energy systems across a wide range of capacities. Because each project has different network and site conditions, the factory design should not start from voltage and kVA alone.

Clear technical data at the beginning reduces drawing revisions, protection changes and civil rework. The purchase specification should identify the guaranteed performance and the physical interfaces the supplier must meet.

Electrical Data That Drives the Design

Confirm rated power, frequency, primary and secondary voltage, highest system voltage, insulation level, vector group, neutral arrangement and tap range. The network study should provide the required impedance and short-circuit withstand duty.

Loss evaluation should distinguish no-load loss, load loss, auxiliary consumption and the economic value assigned to each. A low purchase price may not be the lowest lifecycle cost when the transformer is energized continuously.

Mechanical and Accessory Checklist

• Define ONAN or ONAF cooling duty, temperature-rise limits and fan-control stages.
• Confirm bushing type, terminal orientation, cable boxes or bus connections and phase spacing.
• Specify conservator or sealed-tank arrangement, oil-preservation system and pressure protection.
• List gauges, alarms, contacts, marshalling cabinet, CTs and online monitoring points.
• Provide transport limits, lifting and jacking points, total mass and shipping configuration.
• Coordinate foundation loads, oil containment, fire separation, earthing and maintenance clearances.

Approve Documents Before Materials Are Committed

The review package should include a general arrangement, nameplate data, terminal plan, control schematic, accessory list, foundation loads and guaranteed-loss schedule. Comments must be resolved consistently across electrical and civil drawings.

Factory test scope, witness points, report format and spare parts should be agreed before production. This keeps the final inspection focused on requirements that were defined, priced and engineered from the start.

Next Step

SOVIN’s three-phase oil-immersed distribution transformer supports project-specific voltage, tapping, impedance and accessories. Provide the single-line diagram and site data with the inquiry.

Technical reference: IEC 60076 series and the project’s applicable grid-code and substation requirements.