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Audel electrical course for apprentices and journeymen - Rosenberg P.

Rosenberg P. Audel electrical course for apprentices and journeymen - Wiley & sons , 2004. - 424 p.
ISBN: 0-764-54200-1
Download (direct link): audelelectricalcourseforapprentices2004.pdf
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100-kVA, 480-volt, one-phase transformer has an impedance of 5%. How many amperes of fault current will be available upon short circuit?
100 kVA X 1000 „„„„ n
-------------------= 208.3 amperes (full load)
480 V
100%
I (short-circuit) =---------------;----X I (full load)
Percent Impedance
100%
ISc = ^7^ X 208.3 = 20 X 208.3 = 4166 amperes 5%
If the transformer above were a three-phase transformer, then
100 kVA X 1000
480 V X 1.73
120.4 amperes
100%
ISc = X 120.4 = 20 X 120.4 = 2408 amperes
5%
Transformer Facts 289
Transformers in Parallel
Transformers may be paralleled if more capacity is required over that which one transformer will supply. There are very definite guides that must be followed:
1. The combined kVA rating of the transformers shall be large enough to handle the load.
2. The transformers must be connected so that their polarities are the same. (Polarities will be covered a little later.)
3. The transformers shall have identical voltage and frequency ratings.
4. The impedances and percentages of resistance and reactance shall be identical.
5. They shall have the same identical tap ratings.
6. With three-phase banks, the phase rotations shall be identical, such as phase A', B', and C to phases A', B', and C, and not phases A', B', and C to phases A', C, and B, etc. More will be covered on phase rotation in a future chapter.
7. The phase banks shall have the same phase displacement angles, such as delta-delta, wye-wye, delta-wye, or wye-delta. If a delta-delta is paralleled with a delta-wye, there will be a 30° phase displacement. This will be explained in a later chapter.
Transformer Insulation
Most types of insulation must be impregnated with an insulating varnish, which
1. Improves resistance to moisture
2. Improves temperature ratings
3. Improves dielectric strength
4. Fills in small voids
5. Gives mechanical strength
6. Molds the assembly into one solid mass to prevent movement
7. Also aids in keeping down the noise from lamination vibrations
There are a number of insulation classes. The insulation class picked should take into consideration the application and location of the transformer that is to be used. The inside of the winding usually gets the hottest and there is about 10° Celsius (centigrade) difference
290 Chapter 26
between this hot-spot temperature and the average temperature. A 10% rise in temperature cuts the insulation life in half.
NEMA and ANSI insulation classes are
105°C rise—Class A 130°C rise—Class B 155°C rise—Class F 180°C rise—Class H
Types of Transformers
There are three basic types of transformers:
1. Dry Type: This is the type that is not immersed in a liquid dielectric.
2. Oil-filled: This has the core and windings immersed in a high grade of insulating mineral oil, which serves as a coolant and dielectric. Carbons and water are heavier than this oil, so they settle to the bottom. When oil is drawn off for checking its dielectric strength, the test sample is taken from the bottom of the tank.
3. Askarel-filled: Askarel is no longer manufactured because of its toxicity. (Askarel contained PCBs.) Nonetheless, you may come across an older transformer filled with askarel, which acts as a coolant and dielectric as does oil. Water and carbons are lighter than askarel, and thus rise to the top. Therefore, test samples are taken from the top of the transformer.
Quite often transformers are tightly sealed and nitrogen gas is added under a low pressure to keep outside air and moisture from entering the case, which prevents contamination.
Effects of Altitudes
Transformers are designed and rated for altitudes from sea level to 3300 feet. As altitudes increase, the atmosphere becomes less dense, resulting in less effective cooling. Therefore, transformer rated capacity must be derated 0.3% for each 330 feet over 3300 feet of altitude. This is 1.0% derating for each 1000 feet over 3300 feet.
Ambient Temperature
Ambient temperature is the temperature in the location where the transformer will be located. High ambient temperatures will raise the insulation temperatures and shorten the life of a transformer.
Transformer Facts 291
The kVA rating of a transformer should be derated 0.4% for each 1°C over 30°C average ambient temperature for 150°C insulation.
Transformer Polarity
Transformers are often marked either “Additive Polarity” or “Subtractive Polarity.” The leads on the high side are usually marked H1, H2, H3, etc., and the leads on the low side are marked with an X, such as X0, Xv X2, X3, etc. This is done for phasing out, paralleling, and connecting transformers for different voltages. Sometimes the markings may have been changed or marked incorrectly, so it may become necessary to check the polarity.
Figure 26-6 gives the illustration of additive polarity. Figure 26-6A shows the actual direction of the windings. Note that both the high and low sides are wound in the same direction, and in Figure 26-6B the arrows show the direction of the impressed voltage on the high side and direction of the induced voltage on the low side.
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