8MVA Substation Transformer with Dual 480V Secondaries for a North American Project

Winley Electric manufactured a custom 8MVA three-phase oil-immersed transformer for a North American power project. From a 34.5kV grounded-wye supply, the transformer provides two separate 4MVA, 480V delta secondary windings.

The project specification called for two 4MVA secondary windings rather than one combined 8MVA secondary output. Each winding therefore needed its own connection path and terminal identification.

If the two circuits were not clearly separated in the physical layout, field wiring, inspection and future maintenance would become more difficult.

At 4MVA and 480V, each secondary winding is rated at 4811.4A. This placed much greater demands on the low-voltage interface than the voltage rating alone might suggest.

The customer needed a terminal arrangement that could accommodate the required current capacity while maintaining adequate electrical clearance, mechanical support and practical access for field conductors.

Winley configured the transformer with a YNd1d1 vector group. The 34.5kV high-voltage winding is grounded wye, while the two 480V secondary windings are delta connected.

The two low-voltage windings are brought out through separate X and Y terminal groups, allowing each 4MVA circuit to be identified and connected separately.

The low-voltage side uses six live-front porcelain bushings rated for 1.2kV class, 45kV BIL and 6500A, together with NEMA eight-hole spade terminals.

The X and Y terminal groups are side mounted on opposite sides of the transformer and enclosed within the terminal arrangement, providing space for the large field connections while keeping the two circuits visibly separated.

During final assembly, the production and inspection teams checked the transformer against the approved drawings, connection diagram and accessory schedule.The inspection covered terminal labels, control wiring, monitoring circuits and auxiliary terminal connections.

Special attention was given to the identification and separation of the X and Y secondary terminal sets so that the finished transformer matched the approved dual-secondary connection arrangement.

Transformer operating information is available through oil temperature and winding temperature indicators, a liquid level gauge and pressure-related monitoring or protection devices.

The relevant contacts and control wiring are routed to labeled terminal blocks inside the auxiliary terminal box for field-system integration.

Factory testing was completed on May 13, 2026. The report records separate winding-resistance and voltage-ratio results for both the X and Y secondary windings across the tap positions, with the results marked qualified.

Selected factory records for losses, impedance, dielectric withstand and voltage regulation.

Insulating-oil records: DGA qualified and average breakdown voltage 43.8kV.

After testing, the detachable radiator assemblies were removed and packed separately in protective frames.

The transformer body was placed on timber supports inside the container and secured for transportation. Photographs of the packing and loading process were retained with the shipment records.

Left: protected radiator assemblies. Right: transformer body positioned and restrained for export.

The project delivered a completed and factory-tested 8MVA transformer built around a specific dual-secondary requirement.

The finished unit combines a 34.5kV grounded-wye primary with two 4MVA, 480V delta secondary windings, separate X and Y high-current terminals, primary-side protection and measurement equipment, and auxiliary monitoring connections.

After completion of manufacturing and testing, the transformer and its radiator assemblies were prepared and loaded for export shipment.

This case provides a practical reference for projects that need two high-current 480V secondary circuits supplied from one medium-voltage transformer.

For custom substation transformer projects, Winley Electric can review the required voltage, capacity, vector group, secondary winding arrangement, terminal configuration, accessories and documentation before design approval.