There are several points to consider when selecting a transformer.
1. kVA Rating
The kVA rating must be sufficient to handle the load. Consideration should also be given to possible future load growth. Some companies handle growth using a large number of various size transformers, installing them with a rating close to the actual load requirements, and then changing the bank when the load grows. While this is the way electric utilities do it, most industrial plants do not have the necessary stock of spares to operate in this manner, and the practice does not lend itself to the use of substations. Of course, transformer ratings can be increased by forced cooling.
The transformer kVA, impedance, and voltage rating determine the short-circuit current interruption requirements for the protective device. Remember that replacing a transformer with a larger unit to handle a larger connected load also impacts the interrupting requirements.
2. Voltage Ratings and Ratios
The transformer should be selected to give the proper voltage at the load terminals. This voltage is the system voltage desired, not the equipment utilization voltage. For instance, if motors are rated for 460 Volts, the transformer no-load voltage should be 480 Volts. This permits a voltage drop in the feeders to the point of use.
3. Voltage Taps
Most modern transformers have taps in the windings that make it possible to slightly change the turns ratio. These taps do not materially affect the voltage drop through the transformer; they merely change the voltage level. The standard for the taps in transformers used in industrial systems is two 2-1/2% taps both above and below rated voltage. Tap changing must be carried out with the transformer disconnected from the circuit. However, load tap changing transformers are available at additional cost.
Taps are intended to adjust the transformer to the primary voltage level actually present. Therefore, if the input voltage is 2-1/2% above the nominal system voltage, the tap should be set to provide 2-1/2% more primary turns in order to keep the secondary voltage at the design level during operation. Taps are not intended to be used to raise or lower the secondary voltage from the design or rated values.
Some transformers are supplied with several taps on the secondary windings. These permit the choice of any one of the secondary terminal voltages appropriate to that particular tap. However, this is not what is referred to as "changing the taps" on a transformer. Some auto transformers are designed to provide continuous adjustment of the secondary voltage by using wires wound around a toroidal core and providing a sliding contact to bared regions on these secondary turns. This is equivalent to having a secondary tap for every turn.
4. Types of Construction
Liquid-filled transformers may be filled with either transformer oil or an insulating liquid specifically designed for transformers. These liquids perform two functions, serving as heat transfer mediums and as insulation. Dry-type transformers are available in either ventilated or sealed enclosures.
The physical location of the transformer is of primary importance in determining the type of transformer to be used for a particular application. The characteristics of a given location may preclude the use of certain types or may make one type more desirable than another. In general, liquid-filled and sealed dry-type transformers are suitable for indoor and outdoor locations. Oil-filled transformers are suitable only for outdoor locations, and may not be used indoors because of the flammability of the oil (unless located in a suitable fireproof vault). Where a non-flammable oil is used, a containment area should be provided.
Ventilation and atmospheric conditions are important factors in locating ventilated dry-type transformers. They are designed for installation in dry locations with a clean, dry supply of cooling air. Atmospheric conditions are not critical in the location of liquid-filled or sealed dry-type transformers since their core and coils are sealed.
Pad-mounted transformers and transclosures, which have the transformer, switch and connectors in a single metal enclosure, are becoming increasingly popular in industry as well as on utility systems. Several slow-burning silicone-based liquids are also being used as a cooling and insulating medium in transformers. These have a flash point of 305°C and can be used under some conditions without a vault in Type 1 or Type 2 buildings. A non-flammable tetrachloroethylene liquid can be used in transformers with aluminum windings (instead of copper) and with no current transformers internal to the transformer case in accordance with NEC 450-24.
Some other transformer liquids, while designated as "less flammable," require competent engineering advice before considering their use.
5. Insulation and Impulse Levels
Transformers may be subject to overvoltages from lightning or switching surges and should be appropriately protected. These surge handling capabilities are described by the impulse level; the amount of a momentary voltage surge which the insulation can withstand. Dry-type transformers with their basic insulation being air, have approximately one-half the impulse level handling capability of similar kVA liquid-filled transformers. Therefore, more thought must be given to dry-type transformer situations.
Pole-top distribution transformers are frequently exposed to these voltage surges. That is why these transformers have the insulation on their windings strengthened to withstand lightning (and other) surges. The strength of this insulation is given by the Basic Impulse Level (BIL) number. Units with high BIL numbers can be obtained at a cost premium. As an example, typical pole-top insulators for 9-13 kV would have a BIL of 112 kV since that is their flash-over voltage. The transformer used on such a line should have a BIL greater than this value.
Another insulation parameter is the insulation between primary and secondary windings. This is essential in the application of instrument transformers, and in applications which require isolation between circuits (such as the filament transformers for high-voltage X-Ray tubes). Auto transformers have the primary and secondary windings interconnected; therefore, they do not have any circuit isolation.
6. Impedance
Low-voltage transformers are generally designed to conform with the impedance's as shown in the TRANSFORMER IMPEDANCE Table in the Appendix. These values are subject to standard tolerances of plus-or-minus 7 percent of the nominal impedance values listed. Manufacturers will design transformers to meet other impedance requirements if the standard values are not adequate, but this naturally adds to the cost.
Transformers with lower impedance have lower voltage drop, but will also allow higher available fault currents. Some devices, such as welders and arc furnaces, use transformers that are specifically designed to have relatively high percent impedance's. This is done to reduce the disturbances on the primary power system when these devices are operating.
7. Single-Phase or Three-Phase Transformers
The recent trend is to use three-phase transformers rather than three single-phase transformers to make up a three-phase bank. Three-phase transformers have an excellent service record, cost less to install, and require less space. If three single-phase transformers connected delta-delta are used, the bank can still operate three-phases in the event one transformer fails. However, the capacity is reduced to about 57 percent of the original capacity and the voltages are unbalanced. Transformer failures are rare, and the extra space and labor needed to connect a bank of three instead of one three-phase unit may not be justified.
8. Wye-Delta Arrangements
Another consideration when ordering transformers is the manner in which they are connected to the system. For example, when using three-phase transformers, specify how the primary and secondary windings are to be connected, whether they are to be delta or wye, and the polarity.
9. Interrupting Rating
Today's utility transformers are capable of delivering very large currents to a building. Short circuits can cause high currents and electrical arcs that generate tremendous heat and large mechanical forces in a building, which in turn could seriously damage or destroy apparatus and conductors in a very short time if they are not promptly interrupted. This increased short circuit potential over the values assumed to be adequate a few years ago should be considered whenever any transformer is changed out for a larger one.
