Mohsin Adnan
By= Muhammad Mohsin Adnan
Difference between Neutral Ground Resistor and Neutral Earthing
Resistor:
Neutral Ground Resistor (NGR):
An NGR is a device that limits the current flow from the generator neutral to the ground during a
ground fault. An NGR reduces the fault current by introducing a resistance in series with the
neutral-ground connection. This resistance is usually adjustable and can be set to a value that
allows the protective relays to detect and isolate the fault. To calculate the resistance value, we
need two parameters the rated voltage and current of the equipment and by using the formula
𝑉
𝑅=
√3𝐼
Working Principle:
The NGR is connected between the neutral point of a transformer or generator and the ground.
NGRs reduce the fault currents to a maximum pre-determined value that avoids a network
shutdown and damage to equipment yet allows sufficient flow of fault current to activate protection
devices to locate and clear the fault.
Key Notes:
NGR only protects from phase to ground faults, not phase to phase faults.
Importance of Installing NGRS:
By installing NERs on the distribution system and controlling fault currents and transient over-
voltages, the following benefits can be realized:
• Elimination or reduction of physical damage to equipment
• Extended life of connected distribution equipment such as transformers
• Reduced operation and maintenance expenses
• Simplification and fast isolation and clearing of the original fault
• Improvement in network security and reduction in unplanned shutdowns
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Mohsin Adnan
Methods of Neutral grounding:
High or low resistance grounding:
1. High resistance grounding is used in commercial systems that require continuous
operation even after a fault occurs, such as continuous process industries. NGR typically
reduces the current to a low value, 10 Amps or less, without tripping the circuit breakers.
2. Low resistance grounding is used in large MV/HV electrical networks where there is a high
level of capital equipment and network interruptions have a significant economic impact.
These NGRs are generally sized to permit only 200A to 2500A of fault current to flow. The
allowed current level is enough to operate protective devices yet not enough to create major
damage at the fault point.
Solidly Earthed Grounded:
• Solidly earthed grounding NGRs are typically used in LV applications of 600V or less and
connect the neutral point to earth. These systems reduce the problem of transient over-
voltage but do not limit the fault current during a fault event.
Neutral Earthing Transformer (NET):
A Neutral Earthing Transformer (NET) is a type of transformer used in electrical power systems to
create a neutral point for grounding purposes. This transformer is used in systems that do not have
an accessible neutral point, such as delta-connected systems. The NET helps in managing fault
currents and stabilizing system voltages during ground faults.
Working Principle of Neutral Earthing Transformer:
The NET operates by creating a neutral point in an ungrounded system, typically using specific
transformer configurations such as zigzag or wye-delta connections.
• A zig-zag transformer has windings arranged in a specific zig-zag pattern, which provides a
neutral point without affecting the line-to-line voltages of the system. The zig-zag
connection cancels out line-to-line voltages but allows fault currents to flow to ground
during a fault. When a ground fault occurs, the fault current flows from the faulted phase
through the neutral point provided by the zig-zag transformer and then to ground. The
impedance of the zig-zag transformer limits the fault current to a safe level, protecting the
system and equipment.
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Mohsin Adnan
• In a wye-delta transformer configuration, the primary winding is connected in a wye
configuration with the neutral grounded, while the secondary winding is connected in a
delta configuration. During a ground fault, the fault current flows through the neutral point
and the ground path. The transformer stabilizes the voltage and limits the fault current
through its inherent impedance.
Importance of Neutral Earthing Transformer
The NET plays a crucial role in ensuring the stability and safety of power systems. Its key functions
include:
1. Creating a Neutral Point: In systems without a neutral point, the NET provides a neutral for
grounding, which is essential for the operation of protective devices.
2. Limiting Fault Currents: The NET controls the magnitude of fault currents during ground
faults, preventing damage to equipment and reducing safety hazards.
3. Stabilizing System Voltage: By providing a stable reference point, the NET helps maintain
voltage stability during ground faults, avoiding excessive voltage swings.
4. Enhancing Protection: The presence of a neutral point allows for the effective operation of
protective relays and circuit breakers, ensuring timely fault detection and isolation.
5. Maintaining System Continuity: The NET allows the system to continue operating during
single line-to-ground faults, preventing complete shutdowns and ensuring continuous
power supply.
Key Differences between NGR and NET:
NGR NET
A resistor specifically designed to limit current. A transformer designed to create a neutral
point and manage fault currents.
Used in systems where the neutral point is Used in systems without a neutral point, such
accessible, and the goal is to limit the fault as delta-connected systems, to create a
current by introducing resistance. neutral for grounding purposes.
Primarily focused on current limitation, with Provides better voltage stabilization by creating
some effect on voltage stabilization due to a stable neutral point and managing fault
controlled fault current. current flow.
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