630 kVA Oil-Cooled Transformer for 6 kV Industrial Park Distribution

Why 6 kV Distribution Suits Industrial Parks

Industrial parks group many small and medium consumers inside a limited area, so the load density is high but the individual demands are moderate. Distributing at 6 kV keeps cable sizes and switchgear compact while limiting voltage drop over the short distances involved, and it allows a single incoming substation to feed a ring of load points. Compared with a low-voltage distribution scheme, the higher voltage reduces current for the same power, which cuts copper content in cables and reduces losses along the feeders. The 6 kV level also matches the medium-voltage equipment and motors that many plants already use internally.

Why 630 kVA Is a Common Block Size

A 630 kVA unit is large enough to serve a workshop, a warehouse with refrigeration and handling equipment, or a group of small tenants through a single low-voltage board, yet small enough to be delivered on a standard truck and installed in a compact substation. Its impedance voltage, usually near four and a half percent in this rating class, keeps the secondary fault current within the breaking capacity of ordinary moulded-case and air circuit breakers. The rating also divides neatly when the park grows, because additional blocks can be added along the ring without redesigning the incoming capacity.

Matching the Transformer to Ring Main Units

In an industrial park the transformer is normally associated with a ring main unit that provides the incoming cable connections, the tee-off to the transformer and the protection for the transformer circuit. The transformer primary is fed through a dedicated switch-fuse or circuit-breaker tee-off, and the cable box of the transformer must be arranged to suit the cable type and direction of entry used in the ring. Because the ring can be operated with either end energised, the transformer must withstand the short-circuit level of the whole park and its protection must coordinate with the ring main unit so that a fault on one block does not trip the entire ring.

Protection, Earthing and Metering

Typical protection includes overcurrent and earth-fault relays on the primary side, a Buchholz relay on conservator designs, a pressure relief device and a winding temperature indicator that can trip or alarm. The tank and the low-voltage neutral are connected to the park earthing system, and the earthing arrangement of the low-voltage side must be decided because it affects the earth-fault protection settings. Metering is usually installed on the medium-voltage side for billing the tenant and on the low-voltage side for internal energy management, and current transformers must be selected with a burden that matches the meters and relays.

Loss Evaluation and Life-Cycle Cost

Two transformers with the same rating can differ substantially in the energy they consume. No-load loss is present continuously once the unit is energised, while load loss grows with the square of the current and therefore with the loading pattern of the block. Purchasers normally apply a capitalization formula that assigns a monetary value to each kilowatt of loss, so a unit with a better core grade and a lower loss specification can be justified even at a higher purchase price. Maintenance cost, expected service life and the cost of a forced outage should be part of the same comparison, because an industrial park loses production as well as energy when a transformer fails.

Frequently Asked Questions

Q: Why is 630 kVA preferred over a smaller unit in industrial parks?
It matches the load of a typical workshop block, keeps secondary fault current manageable and allows the park to expand in convenient steps without new incoming capacity.

Q: Can the transformer be fed from either direction of the ring?
Yes, ring operation is normal, so the unit and its protection must be rated for the full fault level of the park in both directions of power flow.

Q: Which cooling class is usual at this rating?
ONAN cooling is standard, with ONAF added where the site needs more output from the same frame or where the ambient temperature is high.

Q: How should losses be compared between suppliers?
Convert no-load and load losses into a capitalized cost using an evaluation factor that reflects the expected loading and the cost of energy over the service life.

Q: What protection is essential for the transformer circuit?
Overcurrent and earth-fault protection on the primary, plus Buchholz and pressure relief devices on oil-filled designs, together with winding temperature monitoring.