Auto-Transformers Advantages Disadvantages

Auto-transformer Advantages:

• Auto-transformers in comparison with the double winding transformers are generally advantageous of the voltage ratio is favorable from the point of view of the equivalent size
• The reduction in the equivalent power in relation to the throughput, reduction in the weight and size, reduction in the no load (iron losses) and on load losses (cooper losses), the reduction in the no load current and the short circuit impedance
• If P is the power of the transformer, linear dimensions of the auto transformer vary as P^0.25 and the weight and volume  vary as P^0.75 thus reducing the losses and weight of the transformer
• The short circuit impedance of the auto-transformer goes down with equivalent rating which is advantageous because of the lower voltage drop and better regulation.
• For connections between the two systems at very high voltage, where the power is to be transmitted in the order of GVA (Giga Volt-amperes), the use of auto transformer make it possible for high power equipment and can be easily transportable. This will not be possible with normal transformers

Auto-transformer Disadvantages:

• An electrical connection between the primary and secondary is not always desirable, particularly when the voltages levels on both the sides of transformer are quite different. If the neutral point is not solidly earthed, the lower voltage side can be subjected to high potential in the event of earth fault on high voltage side. In general practice, auto transformers are only used on distribution systems where the neutral is connected to the earth
• Auto-transformers are particularly sensitive to the atmospheric over-voltages. Therefore the auto-transformer requires much highs standard of insulation compared to normal transformer. Surge Arrester protection is required
• As the short circuit impendace values of the auto-transformer are low, short circuit currents can reach higher critical values.