IEEE C57.12.00-2010 IEEE Standard for General Requirements for Liquid-Immersed Distribution, Power, and Regulating Transformers.
22.214.171.124 Cooling air temperature limit
When air-cooled, the temperature of the cooling air (ambient temperature) shall not exceed 40 °C, and the average temperature of the cooling air for any 24 h period shall not exceed 30°C.
126.96.36.199 Liquid temperature limit
The top liquid temperature of the transformer (when operating) shall not be lower than —20 °C. Liquid temperatures below —20 °C are not considered as usual service conditions.
188.8.131.52 Cooling water temperature limit
When water-cooled, the temperature of the cooling water (ambient temperature) shall not exceed 30 °C, and the average temperature of the cooling water for any 24 h period shall not exceed 25 °C. Minimum water temperature shall not be lower than I °C, unless the cooling water includes antifreeze, which is suitable for —20 °C operation.
The altitude shall not exceed 1000 rn (3300 ft).
4.1.4 Supply voltage
The supply-voltage wave shape shall be approximately sinusoidal. The phase voltages supplying a polyphasc transformer shall be approximately equal in magnitude and time displacement.
4.1.5 Load current
The load current shall be approximately sinusoidal. The harmonic factor shall not exceed 0.05 per unit. Harmonic factor is defined in IEEE Sid C57.l2.80.
4.1.6 Operation above rated voltage or below rated frequency
Transformers shall be capable of the Ibllowing:
a) Operating continuously above rated voltage or below rated frequency, at maximum rated kVA for any tap, without exceeding the limits of observable temperature rise in accordance with 184.108.40.206 when all of the tNlowing conditions prevail:
1) Secondary voltage and volts per hertz do not exceed 105% of rated values
2) Load power factor is 80% or higher
3) Frequency is at least 95% of rated value
IEEE C57.12.00 pdf download.