THERMAL CONSIDERATIONS
The losses in the windings and the core cause temperature rises in the materials. This is another important area in which the temperatures must be limited to the long-term capability of the insulating materials. Refined paper is still used as the primary solid insulation in power transformers. Highly refined mineral oil is still used as the cooling and insulating medium in power transformers. Gases and vapors have been introduced in a limited number of special designs. The temperatures must be limited to the thermal capability of these materials. Again, this subject is quite broad and involved. It includes the calculation of the temperature rise of the cooling medium, the average and hottest-spot rise of the conductors and leads, and accurate specification of the heat-exchanger equipment.
VOLTAGE CONSIDERATIONS
A transformer must withstand a number of different normal and abnormal voltage stresses over its expected life. These voltages include:
- Operating voltages at the rated frequency
- Rated-frequency overvoltages
- Natural lightning impulses that strike the transformer or transmission lines
- Switching surges that result from opening and closing of breakers and switches
- Combinations of the above voltages
- Transient voltages generated due to resonance between the transformer and the network
- Fast transient voltages generated by vacuum-switch operations or by the operation of disconnect switches in a gas-insulated bus-bar system
This is a very specialized field in which the resulting voltage stresses must be calculated in the windings, and withstand criteria must be established for the different voltages and combinations of voltages. The designer must design the insulation system to withstand all of these stresses.
