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The Differences between Grounding Transformers and Distribution Transformers

Grounding transformers (GT) differ from "standard distribution transformers" (DT) because they are used to establish a return path for ground fault currents on a system which is otherwise isolated or effectively un-grounded. This differentiates the construction in a couple of ways.
Grounding transformers must be designed to meet two basic criteria:
  1. They must be able to carry the continuous phase and neutral currents without exceeding their temperature ratings.
  2. They must be able to carry the fault current without excessive heating that deteriorates the conductors or adjacent insulation.
It is in the second parameter which most widely separates grounding transformers from distribution transformers. DTs are designed to carry a fault current, which is limited by their impedance, for a maximum duration of 2 seconds per standards.  Whereas the GT must carry a fault current that is not limited by its impedance, for durations exceeding the 2 second limitation. Often this time is 10 seconds or more. The GT design must be such that at the end of this extended time period, the conductor temperature is below the critical thermal limit as identified in the standards.

DT: Main Concerns
The DT main concern is for heating caused by loading. Radiators are added to the transformer to help the insulating fluid control the steady state temperature rise, but these do not help during fault conditions. Heat generated during a fault happens in such a short period of time (usually seconds) that the calculation assumes "all heat is stored" in the conductor because heat dissipation does not occur fast enough to combat the rapidly heating conductors. The GT takes this into account and is designed such that the conductor can handle the fault heating without relying on insulating oil for heat transfer during the fault.

Many GT specifications recognize this and allow the steady state cooling to be calculated using the magnetizing current and HV I2R loss resulting from energizing the core only. This leads to some misconception that the DT is better cooled, but the opposite is during faults.

Another subtle difference is the way the two devices "see" faults. The DT typically sees a line to ground fault or maybe, a line to line fault, but since the GT is providing a return path to the network, it typically sees a zero sequence fault which impresses the fault current equally on all three legs simultaneously. To combat the forces generated, GT conductors are always copper for maximum strength to cross section ratio, and because copper has a higher thermal withstand capability. GT coils are always circular on cruciform cores to gain the maximum form stability. Distribution transformers often utilize rectangular coil construction which does not have the same form stability offered by the circular coil technology.
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