How does a distribution transformer work?
The HV winding and the LV winding sit on the same laminated steel core. Alternating current in the HV winding creates an alternating magnetic flux in the core, and that flux induces a voltage in the LV winding. The voltage ratio equals the turns ratio. For an 11 kV to 400 V Dyn11 unit, each HV delta winding sees 11,000 V and each LV star winding sees 231 V.
PractitionerThe LV winding usually sits next to the core and the HV winding outside it, because the LV needs less insulation to the earthed core. Off-circuit taps (typically ±2 × 2.5%) are on the HV winding, where the current is lower.
Oil-immersed or cast resin?
| Oil-immersed (ONAN) | Cast resin (AN/AF) | |
|---|---|---|
| Where used | Utility and outdoor substations, package substations | Inside buildings, near load centres, high-rises, hospitals |
| Fire risk | Oil is flammable. Needs an oil pit, fire separation and often detection or suppression | Self-extinguishing (class F1), no liquid |
| Losses and cost | Lower losses and lower cost | Higher cost, higher no-load losses |
| Overload | Good short-term overload because of the oil's thermal mass | Fans (AF) can add 30 to 40% capacity |
| Maintenance | Oil tests, breather, gauges | Cleaning, connection torque checks, temperature sensors |
What does a vector group mean?
The capital letter is the HV connection (D delta, Y star), the small letter is the LV connection (d, y, or z for zigzag), n means the neutral is brought out, and the number is the phase shift in 30° steps, read like a clock. HV phase A is at 12 o'clock. In Dyn11 the LV phase a points to 11 o'clock, so LV leads HV by 30°. Transformers can only run in parallel if they have the same clock number.
How do you size a transformer?
- Work out the maximum demand: connected load × diversity, divided by power factor to get kVA.
- Add future growth and choose a maximum planned loading, often 75 to 80% in hot climates.
- Pick the next standard size, then check the LV fault level against the LV switchgear rating.
- Check the ambient: in the GCC, specify for the real site ambient and the substation ventilation.
- Compare losses over the life of the unit. Paying more for lower losses usually pays back.
ExpertThe LV fault current is roughly Isc = In / (uk + Sn/Ssource). A 2500 kVA, 6.25% unit at 400 V gives about 53 to 58 kA depending on the MV source, which drives up the LV switchgear cost. Two smaller transformers on separate LV boards can keep fault levels down.
Why does hot-spot temperature matter?
Paper insulation ages faster as it gets hotter. Under IEC 60076-7, ageing doubles for every 6 K above 98 °C at the hottest spot in the winding. A transformer that runs its hot spot at 110 °C all day ages about four times faster than normal. That is why GCC substations need good ventilation and why peak summer loads matter more than the annual average.
Regional notes
GCC
Dyn11, 11 kV/400 V is standard, with sizes such as 1000 and 1500 kVA common in utility substations. Design for ambient up to 50 °C with reduced temperature rise, use only utility-approved makes, and give cast resin units good room ventilation.
Saudi Arabia
60 Hz, with 13.8 kV and 33 kV primaries. Check SEC specifications for the LV voltage and approved designs.
Australia and New Zealand
Distribution transformers must meet the MEPS in AS 2374.1.2 (GEMS in Australia, EECA rules in New Zealand). Pad-mount units are common, often 11 kV or 22 kV to 433/250 V on no-load tap.
Asia
Follow the utility specification (TNB, SP Group, Indian DISCOMs). India also has BEE star-rated loss levels for distribution transformers.