How to read a transformer nameplate

The rating plate is the transformer's passport. Commissioning engineers, maintenance teams and anyone planning a replacement or parallel connection rely on it, so it pays to know what each value means.

## Rating and voltages
- Rated power (kVA or MVA) is the continuous apparent power at the stated cooling class and ambient conditions.
- Rated voltages are the no-load voltages of the HV and LV windings, for example 33/0.433 kV.
- Tappings list the tap range and step, for example ±5 % in 2.5 % steps, and whether the tap changer is off-circuit or on-load.

## Vector group
The vector group describes how the windings are connected and the phase shift between them. In Dyn11, the HV winding is delta (D), the LV winding is star (y) with the neutral brought out (n), and the LV voltage leads the HV voltage by 30°, shown as the 11 o'clock position. Transformers can only run in parallel if their vector groups are compatible.

## Impedance voltage
Shown as a percentage, the impedance voltage is the voltage needed to circulate rated current with the LV winding short-circuited. It limits fault current, sets voltage regulation and must match closely for parallel operation.

## Insulation levels
Values such as LI 75 AC 28 kV give the lightning impulse withstand voltage and the power-frequency withstand voltage of a winding. They follow from the system's highest voltage and the standard used.

## Cooling and temperature rise
The four-letter cooling code describes the internal and external cooling medium and how each circulates. ONAN means mineral oil with natural circulation, cooled by natural air flow; ONAF adds fans. The plate also states permitted top-oil and winding temperature rises.

## Masses and standards
Total mass, oil mass and transport mass help with lifting and logistics. The standard (for example IEC 60076 or IS 2026) defines how every value on the plate was tested.

General guidance. Always follow the rating plate and documentation supplied with the specific unit.