10 kV 100 kVA Three Phase Oil Immersed Distribution Transformer for Building and Utility Supply

Purpose of a 100 kVA Transformer on a 10 kV Network

A 10 kV, 100 kVA three-phase oil-immersed distribution transformer is the smallest three-phase unit that is still built on a full laminated core and tank construction rather than a moulded design. It is installed on pole-mounted platforms, in pad-mounted enclosures and inside small substation rooms to supply a single commercial building, a factory workshop bay, a hotel, a hospital wing or a cluster of apartments through a 0.4 kV distribution board.

Because the unit is small, its losses are often treated as negligible during planning; in reality a 100 kVA transformer running at 40 % average load can consume several per cent of the energy it delivers over a year, so efficiency at part load matters more than the nameplate rating.

Construction Features That Drive Efficiency and Lifetime

The core is a wound or stacked core made of grain-oriented silicon steel in 0.23 mm and 0.27 mm gauges. IEC 60404-8-7 is the specification that governs those grades, and it fixes the guaranteed maximum specific total loss at 1.7 T and 50 Hz, quoted per kilogramme of strip. Selecting a lower-loss grade directly reduces no-load loss, which is the component that runs 8,760 hours a year regardless of load.

Windings use copper conductor throughout. HV windings are wound from enamelled round wire with interleaved insulation cylinders; LV windings use copper foil or a flat conductor to reduce the radial build and improve the space factor. Insulating pressboard and crepe paper are used for cylinders, end blocks and lead support, and the complete core-and-coil assembly is dried, then impregnated and filled under vacuum so that moisture is removed from the cellulose insulation before any voltage is applied.

Ratings and Customisation Options

Standard ratings cover a 10 kV or 11 kV primary and a 0.4 kV secondary, with Dyn11 as the usual vector group. Buyers that need a different arrangement can specify:

Alternative HV levels: 6 kV, 6.3 kV, 10.5 kV, 20 kV.

Off-circuit tap changer with +/-2 x 2.5 % or +/-2 x 5 % tapping range.

Oil-level gauge, pressure relief valve, gas relay and dial or glass thermometer.

Corrosion-resistant paint systems, galvanised tank, or a stainless steel enclosure for coastal and humid sites.

Anti-vibration mounts where the unit is fixed to a building structure.

Site Conditions and Installation Rules

IEC 60076-1 sets the reference service conditions for outdoor units: a maximum ambient of 40 degrees C, a minimum of -25 degrees C, and a monthly average no greater than 30 degrees C. Where a site exceeds those values - a 55 degrees C desert yard, a cold-store plant room at -40 degrees C - the rating must be re-derived rather than assumed. Seismic restraint is normally specified as an acceleration limit applied at the base, for example below 3 m/s2 horizontally and 1.5 m/s2 vertically, with the tank bolted through slotted feet to allow thermal movement.

The installation area should be free of heavy pollution, explosive or corrosive gas and conductive dust, with natural ventilation on at least two opposite sides so that the corrugated walls can reject heat by convection and radiation.

Testing and Condition Monitoring

Every unit is submitted to the routine test sequence of IEC 60076-1: winding resistance measurement, voltage ratio and phase-relation check, no-load loss and no-load current, load loss and short-circuit impedance, applied-voltage dielectric test and induced overvoltage test. Insulating oil is sampled and tested for power-frequency breakdown as described by IEC 60156, with ASTM D1816 as an accepted alternative method for the same property. Periodic dissolved-gas analysis in service is the earliest warning of localised overheating or a developing discharge, and the loading guide IEC 60076-7 provides the temperature basis for deciding how long an overload may be carried.

Frequently Asked Questions

Q: Can a 100 kVA transformer be loaded above its rating?
A: Yes, within the temperature-rise limits of IEC 60076-2 and under the cyclic loading rules of IEC 60076-7. A short daily peak is tolerable because insulation ageing is driven by hot-spot temperature and time, not by the nameplate figure alone.

Q: How much oil does a unit of this size hold?
A: Tank volume is dictated by the tank geometry and cooling surface rather than by the rating alone. It is stated on the outline drawing for each order, and the sealed corrugated design holds less oil than a conservator type because the expansion space is provided by the wall deflection.

Q: Is a low-voltage circuit breaker required inside the enclosure?
A: The transformer protects itself against internal faults through the gas relay and pressure relief device, but LV faults are cleared by the downstream breaker. The LV breaker must be selected for the prospective short-circuit current at the transformer terminals.

Q: What does the temperature rise figure of 60 K apply to?
A: It is the permissible rise of the top oil above ambient measured during the temperature-rise test defined in IEC 60076-2. The average winding rise is limited to 65 K over the same test, and the two limits together set the rated operating condition.

Q: How often should the insulating oil be tested?
A: Typical utility practice is an oil sample for dielectric strength and moisture at commissioning, then again within the first year, and every two years afterwards by dissolved-gas analysis. The sample should be drawn from the bottom valve using clean, dry apparatus.

Q: Which tap setting suits a 10.5 kV feeder on a 10 kV nameplate?
A: The off-circuit tap changer is set so that the LV voltage lies in the required band at the expected HV level. On a five-position +/-2 x 2.5 % changer, a 10.5 kV feeder is normally served from the +2.5 % position.