The use of an autotransformer should be particularly checked in all countries with delta grids, such as North America, Japan, Korea, Taiwan and others. Problems can arise here due to the specific grid shapes and their earthing options. The diagram below illustrates and describes some fault cases. The blue diagram shows the customer’s grid and the black diagram shows the autotransformer.
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1. failure of a phase on the primary side. Here the star point can reach a higher potential in an uncontrolled manner. This can lead to the destruction of the loads and the transformer.
2nd type of net: Center tap grounded. This is a typical grid in North America. For example, this results in voltages of 3x 480V or 2x 240V to the grounded center point.
3rd net shape: Corner grounded delta. Also typical mains form in North America and Asia. Here, 3-wire cables are usually used, with the green/yellow wire also being used for the outer conductor. Under no circumstances should this be confused with the cables used in European countries for single-phase networks!
4 In many applications, the neutral conductor of the transformer is earthed. This is sometimes also required in various standards and regulations. This earth connection would then have a direct contact to the “center tap grounded” or “corner grounded” point and thus a short circuit to the outer conductor!
5. output-side loads such as mains filters, primary switching regulators, frequency converters (intermediate circuits) are usually fitted with Y capacitors. These cause the same error as described in point 4.
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