- The document discusses grounded and ungrounded neutral systems in power systems. - In an ungrounded system, the neutral is isolated from ground which can cause overvoltages and issues with fault detection. - Grounded systems connect the neutral to ground to limit voltages and improve safety, reliability and fault detection. - Common methods for grounding the neutral include solid grounding, resistance grounding, reactance grounding and Peterson coil grounding. The selection depends on system size and protection requirements.

1. LENDI INSTITUTE OF ENGINEERING AND TECHNOLOGY
Jonnada, Andhra Pradesh- 535005
UNIT- VI (PART-2)
PROTECTION AGAINST OVER
VOLTAGE AND GROUNDING
Department of Electrical and Electronics Engineering

2. SYLLABUS
Department of Electrical and Electronics Engineering
∟Generation of over voltages in power systems
∟Protection against lightning over voltages
∟Valve type and zinc oxide lighting arresters
∟Insulation coordination
∟BIL
∟Impulse ratio
∟Standard impulse test wave
∟Volt-time characteristics
∟Grounded and ungrounded neutral systems
∟Effects of ungrounded neutral on system performance
∟Methods of neutral grounding
―Solid
―Resistance
―Reactance
―Arcing grounds and grounding Practices

3. Grounded and ungrounded neutral systems
Department of Electrical and Electronics Engineering
Neutral Grounding
• In neutral grounding system, the neutral of the system or rotating system or transformer is
connected to the ground.
• The neutral grounding is an important aspect of power system design because the performance of
the system regarding short circuits, stability, protection, etc., is greatly affected by the condition of
the neutral.
• A three phase system can be operated in two possible ways
1. With ungrounded neutral
2. With a grounded neutral

4. Grounded and ungrounded neutral systems
Department of Electrical and Electronics Engineering
1. With ungrounded neutral
• In an ungrounded neutral system, the neutral is not
connected to the grounded.
• In other words, the neutral is isolated from the ground.
• Therefore, this system is also known the isolated
neutral system or free neutral system shown in the
figure.

5. Grounded and ungrounded neutral systems
Department of Electrical and Electronics Engineering
2. With grounded neutral
• In neutral grounding system, the neutral of the system
is connected to the ground.
• Because of the problems associated with ungrounded
neutral systems, the neutrals are grounded in most of
the high-voltage systems.

6. Grounded and ungrounded neutral systems
Department of Electrical and Electronics Engineering
Some of the advantages of neutral grounding are as follows
1. Voltages of phases are limited to the line-to-ground voltages.
2. Surge voltage due to arcing grounds is eliminated.
3. The overvoltages due to lightning discharged to ground.
4. It provides greater safety to personnel and equipment.
5. It provides improved service reliability.

7. Grounded and ungrounded neutral systems
Department of Electrical and Electronics Engineering
Effects of ungrounded neutral on system performance
1. System Voltage Increase: When the earth’s fault occurs in line then for a healthy line voltage will
increase three times if its ungrounded system.
2. Protection Complicacy: In this system earth fault is not easy to sense and troubleshooting will
become complicated.
3. Arcing Ground: Sudden temporary fault can be caused by the failure of a branch creates an arc
between the overload line and the ground. Arc extinguished and can restrict in a repeated regular
manner. This is called arcing ground.
4. Static Induced Voltage: Overvoltage due to the static induced charges are not conducted to the
earth.

8. Grounded and ungrounded neutral systems
Department of Electrical and Electronics Engineering
Preventive Maintenance for Earthing System
1. The earth resistance must need to be checked a minimum two times in a year when it’s a dry
season.
2. Earthing system should be checked periodically when there is any kind of wear or loose
connection with earth continuity conductor (ECC) or earthing lead or the earthing bus bar. Any
type of looseness must be eliminated.
3. It may need chemical treatment to the soil if the earth’s resistance is not sufficient or not desired
level.
4. It may need to pour sufficient water to the soil around the earth electrode through a funnel
of earthing system pit periodically if the soil is very dry.

9. Grounded and ungrounded neutral systems
Department of Electrical and Electronics Engineering
The methods commonly used for grounding the system neutral are
1. Solid grounding (or effective grounding)
2. Resistance Grounding
3. Reactance Grounding
4. Peterson-coil grounding (or resonant groundings)
The selection of the type of grounding depends on the size of the unit, system voltage and protection
scheme to be used.
Method of Neutral Grounding

10. Grounded and ungrounded neutral systems
Department of Electrical and Electronics Engineering
1. Solid grounding (or effective grounding)
• A power system is said to be effectively grounded or solidly grounded when the neutral of a
generator, power transformer or grounding transformer are directly connected to the ground
through a conductor of negligible resistance and reactance.
• A part of a system or system is said to be solidly grounded when the positive-sequence impedance
of the system is greater or equal to the zero sequence resistance, and positive sequence reactance is
three times greater than or equal to the zero sequence reactance.

11. Grounded and ungrounded neutral systems
Department of Electrical and Electronics Engineering
• Consider a system having three phases a, b and c shown in the
figure.
• If the single-ground-fault occur in phase a the voltage of the phase
becomes zero. However, the remaining two phases b and c will still
have the same voltages as before shown in the figure.
• When the fault occurs in the system, in addition to the charging
current the power source also feeds the fault current.
• For the solidly neutral grounded system, it is necessary that the
ground fault current should not exceed 80% of the three-phase
fault.
• It is usually used for keeping the fault current within safe limits.

12. Grounded and ungrounded neutral systems
Department of Electrical and Electronics Engineering
2. Resistance grounding
• In this type of neutral grounding, the neutral of the system is
connected to ground through one or more resistance.
• Resistance grounding limits the fault currents.
• It protects the system from transient overvoltages.
• Resistance grounding decreases the arcing grounding risk and
permits ground-fault protection.
• The value of resistance used in the neutral grounding system should
neither be very high nor be very low shown in the figure.
• A very low resistance makes the system to the solidity grounded,
whereas a very high resistance makes the system ungrounded.

13. Grounded and ungrounded neutral systems
Department of Electrical and Electronics Engineering
• The value of resistance is chosen such that the ground-fault current is limited, but still sufficient
ground current flows permit the operation of ground faults protections.
• In general, the ground fault may be limited up to 5% to 20% of that which occur with a three-phase
line.

14. Grounded and ungrounded neutral systems
Department of Electrical and Electronics Engineering
3. Reactance grounding
• In reactance grounded system, a reactance is inserted
between the neutral and ground to limit the fault
current as shown in the figure.
• To minimize transient overvoltages, the ground fault
current in a reactance grounded system should not be
less than 25% of the three phase fault current.
• This is considerably more than the minimum current
desirable in resistance grounded systems.

15. Grounded and ungrounded neutral systems
Department of Electrical and Electronics Engineering
4. Arcing grounding
Definition: Arcing ground is the surge, which is produced if the neutral is not connected to the earth.
• The phenomenon of arcing ground occurs in the ungrounded three-phase systems because of the
flow of the capacitive current.
• The capacitive current is the current flow between the conductors when the voltage is applied to it.
The voltage across the capacitances is known as the phase voltage.
• During the fault, the voltage across the capacitance reduces to zero in the faulted phase, while in
the other phases the voltage is increased by the factor of √3 times.

16. Grounded and ungrounded neutral systems
Department of Electrical and Electronics Engineering
Arcing Ground Phenomena
• In a three phase line, each phase has a capacitance on earth.
• When the fault occurs on any of the phases, then the capacitive fault current flows into the
ground.
• If the fault current exceeds 4 – 5 amperes, then it is sufficient to maintain the arc in the ionised
path of the fault, even though the fault has cleared itself.
• With the formation of the arc, the voltage across it becomes zero, and therefore the arc is
extinguished.
• The potential of the fault current restored due to which the formation of a second arc takes places.
• The phenomenon of intermitting arcing is called the arcing grounding.

17. Grounded and ungrounded neutral systems
Department of Electrical and Electronics Engineering
• The alternating extinction and reignition of
the charging current flowing in the arc build
up the potential of the other two healthy
conductors due to the setting of the high-
frequency oscillations.
• The high-frequency oscillations are
superimposed on the network and produce
the surge voltage as high as six times the
normal value.
• The overvoltage damages the healthy
conductor at some other points of the system.

18. Grounded and ungrounded neutral systems
Department of Electrical and Electronics Engineering
How to Eliminates Arcing Ground?
• The surge voltage due to arcing ground can
remove by using the arc suppression coil or
Peterson coil. The arc suppression coil has an iron
cored tapped reactor connected in neutral to
ground connection.
• The reactor of the arc suppression coil extinguishes
the arcing ground by neutralising the capacitive
current.
• The Peterson coil isolates the system, in which the healthy phases continue supplies power and avoid
the complete shut down on the system till the fault was located and isolated.

19. Grounded and ungrounded neutral systems
Department of Electrical and Electronics Engineering
5. Peterson Coil Grounding
• Peterson coil is an iron core reactor connected between transformer neutral and ground.
• It is used for limiting the capacitance earth fault current which is flowing when the line ground
fault occurs in the line.
• The coil is provided with the tapping so that it can be adjusted with the capacitance of the system.
• The reactance is selected so that the current through the reactor is equal to the small line charging
current which would flow into the line-to-ground fault.
• Peterson coil is rated for a short time of about 5 minutes, or it is designed to carry its rated current
continuously.
• It reduces the transient fault which occur due to lightning and also minimized the single line-to-
ground voltage drops.

20. Grounded and ungrounded neutral systems
Department of Electrical and Electronics Engineering
Consider an LG fault in phase B at a point F as
shown in the figure. The line-to-ground voltage of
phase B becomes zero. The voltage of the phases R
and Y increase from phase values to line values.
The resultant of ICR and ICY is IC.

21. Grounded and ungrounded neutral systems
Department of Electrical and Electronics Engineering
For balanced conditions
If IC is equal to IL there will be no current through the ground, and there will be no tendency of the
arcing grounds to occur.
With the help of Peterson coil neutral grounding, arc resistance is reduced to such a small value that it
is usually self-extinguishing.
Therefore, Peterson coil is also known as a ground fault neutralizer or arc suppression coil.

22. Thank you
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