THREE PHASE TRANSFORMER APPLICATION BASIC AND TUTORIALS

For central stations of medium size, three-phase transformers are rarely superior to single-phase, except when the large sizes can be applied, in which case the transformers are normally installed in substations or central stations.

The chief reason for this is the nonflexibility of a three-phase transformer. It is usually purchased for a particular size and type of load, and if that load should be changed, the transformer, representing a comparatively heavy investment, remains on the hands of the central station, whereas a single-phase transformer of one-third the size could usually be adapted for some other service.

This feature becomes of less importance as the central station increases its size, and three-phase transformers for purely power service are now being used by a considerable number of the large central stations in the United States.

The three-phase transformer costs less to install, and the connections are simpler, points that are of importance in connection with outdoor installations. The fact that a failure of a three-phase transformer would interrupt service more than the failure of one single-phase transformer in a bank of three is of little importance because of the comparatively few failures of modern transformers.

On the other hand, especially for 2200-V service, the single-phase transformer has been carried to a high degree of perfection and is manufactured in much larger quantities, so that better performance is usual and in some cases initial cost is lower.

Three-phase distribution transformers are used extensively in underground city network service on account of the smaller space required by them in the manhole, their higher efficiency, and their lower initial cost.

For overhead service for pole or platform mounting, three single-phase units are more common on account of the ease of handling and mounting the smaller-sized units.

POWER TRANSFORMER INSULATION CLASS SYSTEM BASIC AND TUTORIALS

The type of insulation used in dry-type–transformer design and construction has a definite bearing on the size and operating temperature of the unit. Currently four classes of insulation, each having a separate NEMA specification and temperature limit, are being used.

A look at these will facilitate selection of the proper unit to meet prescribed installation and operating conditions.

1. Class 130 insulation-system transformers. When properly applied and loaded in an ambient not over 40 C, these transformers will operate at not more than a 60 C temperature rise on the winding.

These units can be used as control-type transformers when higher temperatures might affect other temperature-sensitive devices in the enclosure or as distribution transformers in locations (textile mills, sawmills, etc.) where combustible flyings might be present in the surrounding atmosphere.

2. Class 150 insulation-system transformers. These units have a higher-temperature insulating system and are physically smaller and about half the weight of Class A units of corresponding rated capacities.

When properly loaded to rated kilovolt-amperes and installed in an ambient not over 40 C, Class 150 units will operate at a maximum 80 C rise on the winding. For years dry-type distribution transformers have been of the Class 150 type.

3. Class 200 insulation-system transformers. These units also have a high-temperature insulating system and, when properly loaded and applied in an ambient not over 40 C, will operate at no more than 130 C rise on the winding.

The units are smaller in size than similarly rated Class 150 units and currently are available from a number of manufacturers in ratings of 25 kVA and lower, both single- and three-phase design. One manufacturer designs in-wall, flush-mounted dry-type transformers as Class 200 units.

4. Class 220 insulation-system transformers. These units are insulated with a high temperature system of glass, silicone, and asbestos components and are probably the most compact ones available. When properly loaded and applied in an ambient not over 40 C, Class 220 transformers will operate at a maximum 150 C rise on the winding.

This class of insulation is used primarily in designs in which the core and coil are completely enclosed in a ventilated housing. Generally, this stipulation covers units with ratings of 30 kVA and larger. Some experts recommend that the hottest spot on the metal enclosure be limited to a maximum rise of 40 C above a 40 C ambient.

It should be noted that Class 150 insulation is being replaced with Class 200 or 220 insulation in transformers of recent design.

Another significant factor which concerns all dry-type transformers is that they should never be overloaded. The way to avoid this is to size the primary or secondary overcurrent device as close as possible to the full load primary or secondary current for other than motor loads.

If close overcurrent protection has not been provided, loads should be checked periodically. Overloading a transformer causes excessive temperature, which, in turn, produces overheating. This results in rapid deterioration of the insulation and will cause complete failure of the transformer coils.