Grounding transformers (GT) differ from “common distribution transformers” (DT) mainly because they are utilized to create a return path for floor fault currents on a system which is normally isolated or efficiently un-grounded. This differentiates the design in a couple of ways.
Grounding transformers must be built to satisfy two primary criteria:
- They need to be able to carry the ongoing stage and neutral currents with out exceeding their temperature scores.
- They will have to be able to carry the fault existing devoid of excessive heating that deteriorates the conductors or adjacent insulation.
It is in the second parameter which most greatly separates grounding transformers from distribution transformers. DTs are built to have a fault present, which is confined by their impedance, for a greatest duration of 2 seconds per criteria. Whilst the GT should have a fault existing that is not minimal by its impedance, for durations exceeding the 2 next limitation. Frequently this time is 10 seconds or additional. The GT structure have to be these kinds of that at the close of this extended time period, the conductor temperature is underneath the crucial thermal restrict as discovered in the specifications.
DT: Key Fears
The DT primary issue is for heating brought about by loading. Radiators are added to the transformer to aid the insulating fluid regulate the constant state temperature rise, but these do not assist all through fault disorders. Warmth created for the duration of a fault comes about in this sort of a limited period of time (usually seconds) that the calculation assumes “all warmth is saved” in the conductor due to the fact heat dissipation does not come about rapidly enough to overcome the fast heating conductors. The GT takes this into account and is built these types of that the conductor can handle the fault heating devoid of relying on insulating oil for heat transfer throughout the fault.
Numerous GT specs acknowledge this and enable the continual state cooling to be calculated making use of the magnetizing existing and HV I2R decline ensuing from energizing the core only. This qualified prospects to some false impression that the DT is better cooled, but the opposite is through faults.
One more subtle distinction is the way the two units “see” faults. The DT generally sees a line to floor fault or probably, a line to line fault, but because the GT is supplying a return path to the network, it commonly sees a zero sequence fault which impresses the fault existing similarly on all 3 legs concurrently. To combat the forces created, GT conductors are generally copper for utmost power to cross segment ratio, and mainly because copper has a better thermal endure ability. GT coils are normally round on cruciform cores to get the highest sort security. Distribution transformers typically employ rectangular coil design which does not have the very same sort security available by the circular coil engineering.
