The differences between grounding transformers and distribution transformers
Grounding Transformers (GTs) differ from “standard distribution transformers” (DTs) in that they are used to establish a return path for ground fault currents in a system that is otherwise isolated or effectively ungrounded. This differentiates the build in a couple of ways.
Grounding transformers must be designed to meet two basic criteria:
- They must be able to carry the DC phase and neutral currents without exceeding their temperature ratings.
- 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 where the grounding transformers are most separated from the distribution transformers. DTs are designed to carry a fault current, which is limited by its impedance, with a maximum duration of 2 seconds according to standards. While the GT must carry a fault current that is not limited by its impedance, for durations that exceed 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 period, the conductor temperature is below the critical thermal limit identified in the standards.
DT: Main Concerns
DT’s main concern is heating caused by charging. Radiators are added to the transformer to help the insulating fluid control steady-state temperature rise, but these do not help during fault conditions. The heat generated during a fault occurs in such a short period of time (usually seconds) that the calculation assumes “all the heat is stored” in the conductor because heat dissipation does not occur fast enough to combat the conductors getting hot. quickly. The GT takes this into account and is designed so that the driver can handle fault heating without relying on insulating oil for heat transfer during fault.
Many GT specs recognize this and allow the steady state cooling to be calculated using the magnetizing current and HV I2R loss resulting from powering only the core. This leads to the misconception that the DT cools better, but the opposite occurs during faults.
Another subtle difference is the way the two devices “see” faults. The DT usually sees a line-to-ground fault or maybe, a line-to-line fault, but since the GT provides a return path to the grid, it usually sees a zero-sequence fault that prints the fault current equally on all three legs simultaneously. To combat the forces generated, GT conductors are always copper for maximum resistance to cross-section ratio, and because copper has a higher thermal resistance capacity. GT coils are always circular on cruciform cores for maximum shape stability. Distribution transformers often use a rectangular coil construction which does not have the same shape stability as circular coil technology.