3259_Transformer Specifications.pdf

3-5 PAD-MOUNTED DISTRIBUTION TRANSFORMERS. (UFC 3-550-01)

3-5.2 Configuration.

Use dead-front construction for pad-mounted transformers unless not available within system parameters. Use pad-mounted transformers, separately protected with vacuum fault interrupter equipped switches for 34.5 kV systems. Three-phase pad-mounted transformers must be loop-feed capable with 6 bushings. Provide two-position, oil-immersed, load break switches that are appropriate for the application. If the transformer might be used as part of a loop-feed design, provide three switches to permit closed transition loop feed and sectionalizing. If the transformer will be installed at the end of a radial supply with no intention of future loop feed capability, provide a single on-off switch. Provide a spare conduit in the high voltage section extending 5 ft (1.5 m) out from the transformer pad.

3-5.3 Size and Secondary Voltage.

Do not use pad-mounted transformers with secondary currents exceeding 3,000 amperes because of the size and quantity of secondary conductors. Transformers rated above 1,000 kVA serving 208Y/120 volt loads and above 2,500 kVA serving 480Y/277 volt loads must be in a secondary unit substation configuration. UFC 3-550-01 1 September, 2016 9 Minimize double transformations to reduce energy consumption and to minimize items of equipment. Provide two oil-filled pad-mounted transformers in lieu of one 480Y/277 volt service if the required 208Y/120 volt load using dry-type transformers exceeds 40 percent of the 480 volt service transformer capability. Connect equipment at the highest available voltage to minimize the capital cost and energy losses of transformation equipment.

3-5.4 Winding Configuration.

Provide delta-wye connections for three phase systems.

3-5.5 Surge Protection.

Provide bushing-mounted elbow type arresters at the ends of all radials and in normally open locations in loops. Provide arresters for all voltage levels above 5 kV.