1. What is the Neutral Point?
a Star (Wye, Y) configuration, one end of each of the three windings is tied
together to a common junction. This junction is known as the Neutral Point.
Mathematical/Vector Definition: In a perfectly balanced three-phase
system, the vector sum of the three phase voltages is zero:
$$V_a + V_b + V_c = 0$$
Consequently, in a balanced state, the potential at the neutral point relative to
the ground is theoretically 0V.
Physical Function: The neutral point serves as a reference datum for
the system. When a conductor is extended from this point, it becomes
the Neutral Wire, allowing the system to provide two different voltages
simultaneously (e.g., $400\text{V}$ Line-to-Line and $230\text{V}$
Phase-to-Neutral).
2. Is it Necessary to Ground the Neutral Point?
A. The Case for Grounding (The Necessity)
Personnel Safety: Grounding ensures that if a winding's insulation
fails and touches the transformer tank (enclosure), a high-current path
is created. This triggers protective devices (circuit breakers/relays) to
trip immediately, isolating the fault.
Voltage Stabilization: In an ungrounded system, if one phase faults
to ground, the voltage of the other two "healthy" phases rises to the
Line-to-Line voltage (roughly $1.732$ times the normal phase voltage).
This stresses the insulation of all connected equipment. A grounded
neutral limits this voltage shift.
Transient Overvoltage Suppression: Grounding provides a path to
dissipate lightning strikes or switching surges, preventing high-voltage
transients from oscillating within the windings.
B. The Case for Ungrounded Systems
Continuity of Service: In specific industries (e.g., chemical plants,
continuous glass manufacturing, or certain hospital wings), an
ungrounded system allows the process to continue running even during
a "single phase-to-ground fault" because the fault current is extremely
low. However, this requires sophisticated monitoring to locate the fault
before a second phase faults, which would cause a catastrophic short
circuit.
3. Common Neutral Grounding Methods
Based on IEEE C57.32 and IEC standards, the neutral can be grounded in
several ways:
1. Solidly Grounded: The neutral is connected directly to the earth grid.
It is the most cost-effective method and provides the best voltage
stability but results in very high fault currents.
2. Resistance Grounding: A Neutral Grounding Resistor (NGR) is
inserted between the neutral and the ground.
o Low Resistance Grounding (LRG): Limits fault current
(typically to $100\text{A}$–$400\text{A}$) to reduce equipment
damage while still allowing enough current for protective relays
to detect the fault.
o High Resistance Grounding (HRG): Limits current to very low
levels (usually $< 10\text{A}$). It provides the "no-trip" benefit of
an ungrounded system while suppressing transient
overvoltages.
3. Reactance Grounding: Uses an inductor (reactor) to limit the fault
current. This is often used in generator neutrals or high-voltage utility
networks.
4. Reference Standards
IEEE Std 142 (The Green Book): The primary authority on grounding
for industrial and commercial power systems.
IEC 60364: Defines the various grounding arrangements, known as
TN, TT, and IT systems (where 'T' stands for Terre/Direct Ground, and
'I' stands for Isolated/Ungrounded).
