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Do I need to connect the neutral and ground my HPS three-phase autotransformer?

If the application needs a neutral (including 3 phase 4 wire systems), the autotransformer must be ordered with the optional neutral terminals (“3L0U” suffix).

This option will provide the customer with a common (H0/X0) neutral connection point that is connected by the factory to the middle point of the Y winding configuration.

When selecting this option, both the Line and Load side neutral cables must be connected to the respective neutral terminals in order to ensure the proper operation of the autotransformer.

HPS does not recommend that the transformer H0/X0 point be grounded locally.

When an autotransformer without neutral connections is selected, typically the neutral is grounded at the source transformer secondary and is properly referenced throughout the whole installation and carried through to the end load downstream the autotransformer.

When installing an autotransformer with neutral connections problems can occur when the X0 point of the autotransformer is grounded locally. In such cases a multiple grounding situation may occur which would be against the electrical codes in North America.

In the above case typically the upstream transformer secondary is grounded at the X0 point of the Y secondary (GND1), in the meantime grounding the X0 point of the autotransformer would create a secondary ground (GND2). Since the two grounds are typically in two different locations, likely far away from each other they will be at different ground potentials.

This situation can create a number of issues including:

With the two grounds at different potentials, if the autotransformer center point (X0) is used as a neutral, the line voltages compared to that local neutral would be unbalanced. The extent of the unbalance would depend on the extent of the potential difference between the two grounds (GND1 and GND2). This unbalance could cause issues with the equipment connected to the autotransformer.

Grounding the X0 of the autotransformer will force the center point of the Y to be always at a certain potential, defined by the local ground. However the voltages of the lines coming into the autotransformer are referenced to the ground point of the upstream transformer. The likely scenario is that the two grounds will be at different potentials which will result in conflicting reference points at the autotransformer. The autotransformer and the electrical system will try to resolve the conflict and equalize the two ground points. The only way that can happen is by having ground current flowing between the two grounds.  Depending on how much of a difference in voltage potential there is between the two grounds and also depending on the ground resistances, there can be a significant current flow through the wye center points.  Adding to this fact that the impedance of an autotransformer is typically low, there could be enough current through the autotransformer to burn out one or more coils of the autotransformer.

The effects and resulting problems that occur due to improper grounding can be unpredictable and manifest themselves differently in time. Ground potentials can greatly vary depending on environmental conditions.  After installing an autotransformer and grounding the center point of the wye (X0) problems may not surface initially.  However, there is a chance that after a rainstorm or some other event, all of a sudden the user experiences high ground currents just because the grounding conditions have changed.  These problems could be very intermittent in nature and hard to diagnose.

When an autotransformer with neutral connections is requested, we do not recommend the grounding of the X0 point and recommend that the customer and installing contractor should refer to the local electrical code requirements for grounding and the short circuit protection of a three phase autotransformer.