Transformer Theory



Transformer Theory

Test Circuit


Thermal Analysis



Power transformers transform voltages. The voltage comes from the hydroelectric or nuclear power plant at 150 kilovolts (kV) and may need to be stepped down to 50kV for a specific town. The voltage will need to be stepped down again to 120 volts (V) for each house. There may be transformers to step down the voltage at various places in the lines to reduce dangers associated with large voltages. Or transformers may be used at the end of a long line to step the voltage back up to what is needed. Each one of these transformers could develop a fault and stop working correctly.

Voltage transformers use magnetic flux to convert voltages as seen in Figure 1.

Transformer theory image
Figure 1. Ideal Transformer

Wire is wound around an iron core for a certain number of turns. When voltage is applied, a specific flux is created that flows through the iron as well as through wire wound on another iron leg of the transformer. A current is created in this other wire, and a voltage can be measured here. The flux (ø) will remain the same, but the voltages, currents (I), and number of turns (N) of wire vary according to the following ratio and as seen in Figure 1:

Transformer Equation
A fault can occur when two wires fuse together. This causes a short in the circuit. A transformer could have 240V across 480 turns of copper wire. Suddenly instead of the 2 amperes (A) that was running across each turn, all of the current would be in the 2 winding short and would be 1200 A. Heat is generated through power loss. Power loss is measured in watts—voltage times current. 240 V times 1200 A is 288 kW. The goal in most electrical circuits is to minimize power loss. Since 288 kW is 288,000 watts, this is very hot. This type of heat can cause a transformer that was not built to this specification to explode. Most transformers are built to withstand a certain amount of heat, but after a number of years in the elements, unexpected events can occur to cause faults in even the best transformer. These faults cause the transformer to short and often explode.
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Last updated: 4/26/07.
University of Idaho, Moscow, Idaho.