24 kV Oil-Immersed Transformer: Where It Fits and What to Specify

24 kV sits between the two voltages most distribution networks are built around. It is a standard IEC primary voltage, and it turns up on industrial estates, on mine and processing sites, and on utility networks that were originally built to 22 kV and later uprated.

Where 24 kV is specified

Industrial estates and processing plants with their own primary distribution at 24 kV.
Utility networks that moved from 22 kV to 24 kV and kept the rest of the system.
Mining, water and wastewater sites where the primary voltage is set by the site, not by the grid.
Export projects where the local standard voltage is 24 kV rather than 11 kV or 33 kV.

How it differs from 11 kV and 33 kV

The transformer does the same job; what changes is the insulation. IEC 60038 groups 24 kV systems under a highest voltage for equipment of 36 kV, so the winding insulation, clearances and bushings are specified to that level rather than to the 12 kV or 36 kV grouping you would use for 11 kV or 33 kV. Insulation levels are selected to IEC 60076-3 against that grouping.

The practical consequence is on the enquiry: a 24 kV unit is not a 22 kV unit with a different label, and it is not a 33 kV unit either. Quote the actual system voltage and the highest voltage for equipment, and the insulation is specified to match.

Oil-immersed or cast resin at 24 kV

Oil-immersed: the usual choice outdoors. Sealed or with a conservator, ONAN cooling, and an off-circuit tap changer as standard.
Cast resin dry type: chosen where the unit goes indoors, in a basement, or where oil is not permitted on the site.
Either way, at 24 kV the insulation and the termination arrangement matter more than at 11 kV — say whether it is cable or bushing connected.

What to send with the enquiry

Rating in kVA, and the primary and secondary voltages on both sides.
Highest voltage for equipment on the primary system.
Frequency, vector group and the impedance the board needs.
Oil-immersed or cast resin, and whether it sits indoors or out.
Tap changer type and range, ambient range, and altitude if the site is above 1,000 m.
Cable or bushing termination, and the standard the unit has to comply with.

Send the system voltage, the rating and the site conditions and we will come back with a configuration, a footprint and a delivery schedule.

Send us the specification

Rating, primary and secondary voltage, frequency, quantity and the application. We answer within one working day.

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