OPERATING LIMITATIONS OF TRANSFORMERS

Transformer Losses (Heat)

The efficiency of power transformers is high, especially, for large transformers at full load. However, losses are present in all transformers. These losses may be classified as copper or I² R losses and core or iron losses.The thermal ratings of a transformer are determined by the following three factors:

• the amount of heat produced in the windings and connections;

• the amount of heat produced in the iron core; and

• how effectively the heat can be removed from the transformer

when the thermal rating of the transformer is reached. At this point, the heat being produced must equal the heat being removed or dissipated – thermal equilibrium.

 

1. Copper (or Winding) Losses

Copper losses are resistive and proportional to load current and are sometimes called load losses or I²R losses. As the transformer is loaded, heat is produced in the primary and secondary winding and connections due to I²R. At low loads, the quantity of heat produced will be small but as load increases, the amount of heat produced becomes significant. At full load, the windings will be operating at or near their design temperature. Figure  shows the relationship between load-current and the heat produced in transformer winding and connections.

2.Iron (or Core) Losses

The iron loss is due to stray eddy currents formed in the transformer core. lines of flux are formed around the current-carrying conductors. The majority of the flux is as indicated in the following Figure , flowing around the core.

Some of the flux however, will try to flow at angles to the core and will cause eddy currents to be set up in the core itself. The term eddy is used because it is aside from the main flow. To combat this effect, the core is laminated as illustrated in Figure . The laminations provide small gaps between the plates. As it is easier for magnetic flux to flow
through iron than air or oil, stray flux that can cause core losses is minimized.
Some of the flux however, will try to flow at angles to the core and will cause eddy currents to be set up in the core itself. The term eddy is used because it is aside from the main flow. To combat this effect, the core is laminated as illustrated in Figure . The laminations provide small gaps between the plates. As it is easier for magnetic flux to flow through iron than air or oil, stray flux that can cause core losses is minimized.

3.Transformer Temperature Limitations

For dry (air-cooled) transformers (that normally have their windings insulated with silicone resin), a temperature limit of 155°C is usually imposed. Allowing air to circulate through the windings and over the core cools these transformers. Assuming a maximum ambient temperature of 40°C, then the temperature rise is limited to 155° – 40° = 115°C. For oil-insulated transformers, there is usually a measurement of oil temperature and winding temperature provided. The simulated winding temperature is called hot-spot. It is derived by passing a representative amount of load current through a resistor located in the oil and measuring the resulting temperature

4.Current Limits

Current has two direct effects on the transformer: It produces heat in the windings of the transformer as we have just
discussed. It produces a voltage drop across the output winding proportional to the load current. As the transformer is loaded, the secondary voltage will fall due to the affects of winding resistance and reactance.

 

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