IEEE C57.12.24-2009 pdf free download.American National Standard for Transformers— Underground-Type Three-Phase Distribution Transformers, 2500 kVA and Smaller; High Voltage, 34 500 GrdY/19 920 Volts and Below; Low Voltage, 480 Volts and Below— Requirements.
1 Scope 1.1 This standard is intended to set forth characteristics relating to performance, limited electrical and mechanical interchangeability, and safety of the equipment described, and to assist in the proper selection of such equipment. 1.2 This standard describes certain electrical and mechanical characteristics and takes into consideration certain safety features of three phase, 60-Hz, liquid-immersed, self-cooled, underground- type distribution transformers rated 2500 kVA and smaller with high voltages of 34 500GrdY/1 9 920 volts and below and with low voltages of 480 volts and below. These transformers are generally used for step- down purposes from an underground primary cable supply and are suitable for occasional submerged operation. 2 References When an American National Standard referred to in this document is superseded by a revision approved by the American National Standards Institute, Inc., the revision shall not apply. The referenced standard and the specific referenced edition shall be the applicable referenced standard until the new version of the referenced document is incorporated by formal action or appropriate revision of the citing standard. ANSI C57.1 2.70-1 978 (R 1 987), Terminal Markings and Connections for Distribution and Power Transformers ANSI C84.1 – 1 995, Electrical Power Systems and Equipment – Voltage Ratings (60 Hertz) ANSI/ASME B1 .1 -1 989, Unified Inch Screw Threads (un and unr thread form) ANSI/IEEE 1 00-1 996, Dictionary of Electrical and Eectronics Terms ANSI/IEEE 386-1 995, Separable Insulated Connector Systems for Power Distribution Systems Above 600 V ANSI/IEEE C57.1 2.00-1 993, General Requirements for Liquid-immersed Distribution, Power, and Regulating Transformers. ANSI/IEEE C57.1 2.32 -1 994, Submersible Equipment – Enclosure Integrity ANSI/IEEE C57.1 2.80-1 978 (R 1 992), Terminology for Power and Distribution Transformers
3.1.2 Transformers conforming to this standard shall be suitable for continuous operation at rated kVA, provided that the temperature of the cooling air (enclosure ambient temperature) does not exceed 50°C and the average temperature of the cooling air does not exceed 40°C for any 24-hour period. For loading beyond rated kVA, refer to loading guide ANSI/IEEE C57.91 . 3.1.3 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- percent zero sequence voltage is applied. 3.2 Voltage ratings and tap ratings Voltage and tap ratings shall be in accordance with Tables 1 and 2, respectively.
5.3 Tolerance on impedance on a tap The percent departure of the actual impedance on any tap from the actual impedance at the rated voltage shall not be greater than the total tap voltage range expressed as a percentage of the rated voltage. 6 Tests 6.1 General All tests shall be performed in accordance with ANSI/IEEE C57.1 2.00 and C57.1 2.90. 6.2 Reference temperature The reference temperature to which load losses, impedance, regulation, and efficiency are corrected shall be 85°C. 7 Construction 7.1 General The transformer shall include primary bushing wells and secondary terminations. 7.1.1 Devices such as separable connectors, switch handles, 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. 7.1.2 Mineral-oil-immersed transformers shall have dimensions not exceeding those as shown in Figure 1 .
7.1.3 The transformer tank, cover, and all external appurtenances shall be of corrosion resistant material or shall be otherwise rendered corrosion resistant. 7.1.4 The transformer base structure shall consist of bars parallel to the long axis of the transformer. The base structure shall provide a 40mm (1 1 /2-inch) minimum clearance from the floor to the tank bottom, with corners left clear for jacking. 7.1.5 The plan view of the transformer shall be in accordance with Figure 2 for the purpose of locating terminations and operating devices.