Specification of NGT ( Neutral Grounding Transformers )
NGT (Neutral Grounding Transformers)
Resistance grounding systems protect power transformers
and generators from damaging fault currents. Low
resistance grounding of the neutral limits the ground fault
current to a high level (typically 50 amps or more) in order
to operate protective fault clearing relays and current
transformers. These devices are then able to quickly clear
the fault, usually within a few seconds. The limited fault
current and fast response time also prevent over-heating
and mechanical stress on conductors.
If the system has a neutral which is available, a single
phase distribution transformer can be used in conjunction
with a loading resistor to provide high resistance
grounding. This is particularly well suited for grounding of
generators, in that it allows the system to operate like an
ungrounded system under normal conditions, while still
retaining the ability to limit ground fault current. The
primary of the transformer is connected from the system
neutral to ground. The loading resistor is connected
across the transformer secondary. The resistor is normally
sized to allow a primary ground fault current in the range
of 2 to 12 amps and is typically rated for either one minute
or continuous duty. The transformer should be sized
accordingly, with a primary rating equal to the system line
to neutral voltage and the secondary normally rated 240 or 120 volts.
When a ground fault occurs downstream of the grounding transformer, ground fault current flows through
the fault and back through ground to the grounding transformer. The loading resistor limits the current
flow in the secondary winding, which in turn limits the flow of the ground fault current back into the system
through the primary of the grounding transformer.
Post Glover’s GT product line is designed for applications on systems up to 15kV. Secondary voltage is
typically 240 volts, and resistors are designed according to customer specification. Typical construction is
a dry type transformer with a secondary resistor mounted in a common enclosure..
