IEEE C57.12.24-2009 IEEE Standard for Submersible, Three-Phase Transformers, 3750 kVA and Smaller: High Voltage, 34 500 GrdY/19 920 Volts and Below; L ow Voltage, 600 Volts and Below.
4.2 Submerged operation As the transformer enclosure may be subjected to flooding or to high water tables, the transformer shall be capable of continuous unattended operation while completely submerged under a head of 3 m (10 ft) of water over the top of the tank. Manual operation of certain accessories may require that the water level be lowered below the top of the transformer prior to operation. 5. Rating Data 5.1 Cooling class The cooling class shall be as specified in IEEE Std C57.12.00-2006. 5.2 Kilovolt-ampere ratings The kVA ratings shall be continuous and based on not exceeding an average winding temperature rise of 55 °C and a hottest-spot temperature rise of 70 °C. The temperature rise of the insulating liquid shall be measured near the top of the tank and shall not exceed 55 °C. The transformers shall have a temperature- rise insulation system of 65 °C. Nonsymmetrical unbalanced excitation or loading of wye-wye connected units may cause heating of their tanks in excess of that which would be produced by balanced conditions. To reduce the probability of this tank heating, such a unit shall be provided with a core construction that will not saturate when 33% zero sequence voltage is applied.
6. Tests 6.1 General All tests shall be performed in accordance with IEEE Std C57.12.OO-2006 and IEEE Std C57.12.90-2006. 6.2 Reference temperature The reference temperature to which load losses, impedance. regulation. and etilciency shall be corrected to 85 °C. No-load losses shall be corrected to 20 °C. 7. Construction 7.1 General [)evices such as separable connectors, switch handles, tap changers, plug-in terminators, and replaceable fuses, which are designed for operation after the transformer is in place. shall be located on the transformer cover so that they can be operated from above with hot-line tools. Mineral-oil-immersed transformers shall have dimensions not exceeding those as shown in Figure 1. The plan view of the transformer shall be in accordance with Figure 2 for the purpose of locating terminations and operating devices. Parking stands of series 304L or 316L stainless steel shall be provided in accordance with Figure 3.
IEEE C57.12.24 pdf download.