Engineer's Notes: distribution transformers in buildings
Sizing a transformer the right way
A building transformer is sized from the design demand, not the connected load. Apply diversity, add a growth allowance, then derate for the site. In the GCC the 50 °C design ambient alone takes about 10 % off an oil-filled unit, so a 1000 kVA transformer behaves like a 900 kVA one on a summer afternoon. Aim to run at no more than about 80 % of nameplate.
Cast resin or oil?
Cast resin (dry type) is the default inside occupied buildings and basements because there is no liquid to leak or burn. Oil-filled and ester-filled units have lower losses and cost less, and are common in utility, kiosk and padmount substations. Natural ester (K class) raises the fire point above 300 °C and is biodegradable.
Fault level and protection
The transformer impedance sets the LV fault level. A 1000 kVA, 5 % unit gives roughly 28 kA at the LV terminals, which drives the breaking capacity of every board downstream. On the HV side a fuse-switch protects units up to about 1600 kVA at 11 kV. Larger units use a circuit breaker with a relay (see HV-507).
Installation and the room
Plan the room for delivery: door width, a floor rated for the mass, a trench with room for cable bending, an oil bund holding at least 110 % of the oil, and ventilation that removes the full losses. Lift only from the lifting lugs and never by the bushings. The full site procedure is covered in HV-517.
Frequently asked questions
How do I size a distribution transformer for a building?
Start from the connected load, apply a diversity factor and a growth allowance to get the design demand, derate for site ambient and altitude, divide by the number of units that share the load, and pick the next standard or utility rating. Keep the design loading at or below about 80 % so the unit runs cool and has room for growth.
Should I choose a cast resin or an oil-filled transformer?
Cast resin dry-type units are preferred inside occupied buildings because they have no liquid to leak or burn. Oil-filled units are cheaper to run, have lower losses and suit outdoor or separated substations, but need oil containment, fire separation and more authority checks.
Why is Dyn11 the standard vector group?
The delta HV winding traps triplen harmonics and isolates zero-sequence currents, and the star LV winding gives a neutral for TN earthing and single-phase loads. Dyn11 means the LV voltage leads the HV voltage by 30 degrees. Units can only be paralleled with the same vector group.
How much ventilation does a transformer room need?
The air flow must remove the transformer losses. Air flow in m3/s equals losses in kW divided by (1.13 x 1.005 x temperature rise in K). For natural ventilation a common rule is inlet free area = 0.18 x losses in kW divided by the square root of the height between inlet and outlet, with the outlet about 10 % larger. Hot GCC rooms usually need fans or air conditioning.
What size oil bund does a transformer need?
A common rule is that the bund or sump holds at least 110 % of the transformer oil volume. Check the utility and environmental authority, as some require more or require a connection to an oil interceptor.
Topics: transformer sizing · ambient derating · IEC 60909 LV fault level · transformer losses and efficiency · Ecodesign Tier 2 · AS 2374.1.2 MEPS · Dyn11 · off-circuit tap changer · paralleling · IEC 60076-7 hot-spot and ageing · cast resin vs oil · substation room ventilation · oil bund · transformer installation
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