TRANSFORMER DESIGN CONSIDERATIONS

TRANSFORMER DESIGN CONSIDERATIONS

Frequency

The transformer cannot change the frequency of the supply. If the supply is 60 hertz, the output will also be 60 hertz.

Impedance

The impedance (or resistance to current flow) is important and used to calculate the maximum short circuit current which is needed for sizing, circuit breakers and fuses. Impedance is expressed as a percent. This percentage represents the amount of normal rated primary voltage which must be applied to the transformer to produce full rated load current when the secondary winding is short circuited. The maximum short circuit current that can be obtained from the output of the transformer is limited by the impedance of the transformer and is determined by the multiplying the reciprocal of the impedance timed the full load current . Thus, if a transformer has 5% impedance, the reciprocal of .05 is 20 and maximum short circuit current is 20 times the full load current.

Insulation System and Temperature Rise

All Federal Pacific FH class transformers are designed with 220 ^oC insulation systems. The standard units are rated X0 ^oC rise. The insulation system classification represents the maximum temperature permitted in the hottest spot in the winding when operated in a 40 ^oC maximum ambient. The hotspot temperature is determined by adding the maximum value for each of the following: 40 ^oC maximum ambient
150 ^oC maximum average winding rise
30 ^oC maximum hot spot in winding220 ^oC ultimate temperature at hot spot The temperature rise commonly associated with transformers is the temperature of the conductor inside the coil and does not apply to the outside surface. The wiring compartment is ventilated and cooled enough to permit the use of 60 ^oC cable for connections. Some customers will specify 220 ^oC insulation with 80 ^oC or 115 ^oC rise to get overloaded capability, better efficiency, and longer life. These transformers are designed to operate with a lower rise per the following example at 80 ^oC rise 40 ^oC maximum ambient
80 ^oC maximum average winding rise
30 ^oC maximum hot spot in winding
30 ^oC thermal overload 30%
220 ^oC ultimate temperature at hot spot There may be differences between the voltage ratings of transformers and the rated voltage of some utilization equipment. Some equipment may be rated 230, 460, or 575 volts to allow drop due to impedance of wire, circuit breakers, ect. The respective transformer secondary voltages would be rated 240, 480, and 600 volts which are the system or source voltage . If you are asked to quote on a transfer rated 460V primary, 115/230V secondary would be proper to quote a transformer rated 480V primary, 120/240V secondary

Basic Impulse Insulation Levels (BIL)

Outdoor electrical distribution systems are subject to lightning surges. Even if the lightning strikes the line some distance from the transformer, voltage surges can travel down the line and into the transformer. High voltage switches and circuit breakers can also create similar voltage surges when they are opened and closed. Both types of surges have steep wave fronts and can be very damaging to electrical equipment . To minimize the effects of these surges, the electrical system is protected by lighting arresters but they do not completely eliminate the surge from reaching the transformer. The basic impulse level (BIL) of the transformer measures its ability to withstand these surges. All 600 volt and below transformers are related 10 KV BIL. The 2400 and 4160 volt transformers are rated 25 KV BIL.

Transformer Sound/Noise

A Humming is an inherent characteristic of transformers due to the vibration caused by alternating flux in the magnetic core. Sound levels will vary according to transformers due to the vibration caused by alternating flux in the magnetic core. Sound levels will vary according to transformer size.Attention to installation methods can help reduce any objectionable noise. When possible ,locate the transformer in an area where the ambient sound will be equal or greater than the noise of the transformer sound level. Avoid locating units in corners. Make connections with flexible conduits and couplings to prevent transmitting vibration to other equipment. Larger units should be installed on flexible mountings to isolate the transformer from the building structure. Sound Level In Decibles

KVA 150 Degrees Celcius Rise K-1	Nema ST-20 Average
0-9	40
10-50	45
51-150	850
151-300	55
301-500	60
501-700	62
701-1000	64

We know that the air is thinner at higher altitudes which, in turn, reduces its ability to cool the transformer. Therefore, standard dry-type self cooled transformers are designed to operate with normal temperature rise at heights through 3300 feet above sea level. If the operation is at higher altitudes, the rating should be reduced 0.3% for each 330 feet above 3300 feet.