SOLUTION

• The transformers used to step down the transmission voltage to the distribution voltage are called distribution transformers.
• Hence the voltage ratio of such transformers is very high.
• Similarly, the isolation between primary and secondary is necessary.
• But the autotransformers are economical only when the voltage ratio is less than 2.
• Similarly, autotransformers cannot provide isolation between primary and secondary.
• An autotransformer is never used as a distribution transformer because the lack of isolation can cause dangerously high voltages in a customer’s location if the neutral opens.
• Autotransformers are generally used for transforming one transmission voltage to another when the ratio is 2:1 or less.
• They are used in a transmission substation to transform from one high-voltage to another high-voltage or from a transmission voltage to a substation-voltage.
• They are normally connected in wye with the neutral solidly grounded, having a delta-connected tertiary for harmonics suppression.