Use of autotransformers in triangular networks

The use of an autotransformer should be especially tested in all countries with triangular networks, including e.g. North America, Japan, Korea, Taiwan and others. Here it can lead to problems through the specific network forms and their grounding possibilities. The sketch below shows and describes some errors. Here, the blue graph represents the customer's network and the black graph represents the autotransformer.

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1. Failure of a primary-side phase. Here the star point can reach a higher potential uncontrolled. This can lead to the destruction of the loads and the transformer.

2. Network configuration: Centre tap grounded. This is a typical network in North America. For example, there are voltages of 3x 480 V or 2x 240 V to grounded centre point.

3. Network configuration: Corner grounded delta. Likewise typical network form in North America and Asia. Here, 3-wire cables are mostly used, whereby the green/yellow wire is used simultaneously for the outer conductor. This is by no means to be confused with the cables used in European countries for single-phase networks!

4. In many applications, the neutral conductor of the transformer is grounded. This is sometimes required in different standards and regulations. This earthed connection would then have a direct contact to the "centre tap grounded" or "corner grounded" point and thus a short circuit to the outer conductor!

5. Output-side consumers such as line filters, primary switching regulators, frequency converters (DC link) are usually equipped with Y capacitors. These cause the same error as described in point 4.