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| 1. Substantial reduction in size and weight. (Figure 4-9a) |

| 2. Ability to operate at high overloads for long periods of time without insulation deterioration. |

| 3. Elimination of fire hazard and potential contamination from failure, an environmental advantage (nitrogen is non-flammable). |

| The HTS conductors, capable of being made into windings, presently being employed consist of alloys of copper oxide and calcium with such exotic (and expensive) materials as barium, bismuth, titanium, and yttrium. Cooling is accomplished by cryocoolers that operate at very low temperatures where common gases liquefy. (Figure 4-9b) |

| The HTS transformer windings and insulation operate in ultra cold ranges of temperature (-253°C to -196°C) where insulation will not degrade. At this temperature, the resistance of HTS windings decreases markedly approaching zero, hence 12R losses in the windings also approach zero. Eddy current and hysteresis losses and other a-c losses still remain. |

| The savings in losses and the ability to overload are matched against the cost of the cryocooler and associated piping and controls. Overloading of present oil cooled transformers substantially hastens the degradation of the winding insulation, decreasing the life of the transformer. In the HTS unit, the overloading is controlled by increasing the refrigeration load (the use of liquid nitrogen), decreasing the overall efficiency of the unit during the period of overload. The nitrogen coolant may be dissipated harmlessly into the atmosphere, or may be recycled (at additional cost) to liquid form and reused. |

| HTS transformers that are safer, operationally superior, more environmentally friendly, and high in efficiency for longer lifetime operation, presently in the development stage, may very well meet the demand for replacement of the aging inventory of oil filled transformers. (Figure 4-9c) |

| Review.. |

| 1. The most important consideration in transformer operation is the prevention of overheating. Causes generally are overloads, poor ventilation, failure of cooling systems, and incipient faults; also failure of auxiliary accessory equipment. These may be uncovered through heat indicators, internal to give hot spot indication in the windings, or external through case temperature indicators. Ammeters, instant reading, maximum indicating, or recording, may give indications of transformer loading. Analysis of readings of watt-hour meters supplied from the transformer and the effect of ambient temperatures also give indications. Large substation-type installations may include more complex relay and alarm systems (Fig. 4-1). |

| 2. Periodic inspections, with the unit energized, are often limited to external conditions. Broken or cracked bushings, discoloration of bushings, terminals and leads, rust or corrosion of tank, signs of oil overflow, broken or ruptured diaphragms on conservators, unusual noises, level as well as color and clarity of oil in tank oil gauges, all serve to indicate the need for |

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