Substation Neutral Earthing

Distribution systems are earthed to create a reference point for the
system voltage, to facilitate the detection and discriminative isolation
of faults involving contact to earth, and to limit over-voltages under
transient conditions.
A neutral earthing system is a system in which the neutral is connected
to earth, either solidly, or through a resistance or reactance of value
sufficient to materially reduce transients, and to give sufficient current
for selective earth fault protection devices to operate.
Sensitive fault detectors allow the reduction of fault currents to very
low values.

Unearthed neutrals were used in the past due to the fact that the first
earth fault did not require tripping of the system. An unscheduled
shutdown on the first earth fault was
particularly undesirable for industries
that were based on continuous
processes and where continuation
of the supply even under single fault
conditions was necessary.
Although achieving the initial goal,
the unearthed system provided no
control of transient over-voltages.

Neutral earthing systems are similar to fuses in that they do nothing until
something in the system goes wrong. Then, like fuses, they protect personnel and
equipment from damage.
Damage comes from two factors, how long the fault lasts and how large the fault
current is. Earth fault relays trip breakers and limit how long a fault lasts and
neutral earthing resistors limit how large the fault current is.
There are Five Methods of Neutral Earthing:
•Unearthed neutral system
•Solid neutral earthed system
•Resistance neutral earthing system
o Low resistance earthing
o High resistance earthing
•Resonant neutral earthing system
•Transformer earthing system

Unearthed Systems
In a unearthed neutral system there is no internal connection between the
conductors and earth. However capacative coupling exists between the system
conductors and the adjacent earthed surfaces. Consequently, the “unearthed
system” is, in reality, a “capacative earthed system” by virtue of the distributed
capacitance.
As a result, this series resonant
L-C circuit can create over-voltages
well in excess of line-to-line voltage
when subjected to repetitive re-strikes
of one phase to earth.
This in turn, reduces insulation life
resulting in possible equipment failure
(Fig.1).

Under normal operating conditions, this distributed capacitance causes no
problems. In fact, it is beneficial because it establishes, in effect, a neutral point
for the system. As a result, the phase conductors are stressed at only line-to-
neutral voltage above earth potential.
But problems can rise in earth fault conditions.
A earth fault on one line results in full
line-to-line voltage appearing throughout the
system, between the conductors and earthed
surfaces.
Thus, a voltage 1,73 times the normal voltage
is present on all insulation in the system.
This situation can often cause failures in
transformers, due to insulation breakdown.

Solidly Earthed Systems
In a solidly earthed system, the neutral point is connected directly to earth, either
directly or via a virtual neutral transformer. Generally all LV systems are solidly
earthed. For MV and HV systems, solid earthing is the cheapest method but has a
number of serious drawbacks.
•High fault currents with resultant damage to plant
•High current will cause tripping of all phases
Neutral earthing resistors are used to limit the fault current in transformers When
a phase to earth occurs, the fault current is limited only by the soil resistance.
This current, which can be very high, can damage the windings.
Low voltage networks are generally solidly earthed and resistive neutral earthing
is generally only applied to MV and HV lines.

Resistive Earthed Systems
The main reasons for limiting the phase to earth current by resistance earthing
are:
•To reduce burning and melting effects in faulted electrical equipment such
as switchgear, transformers, cables.
•To reduce mechanical stresses in circuits/equipment carrying fault currents.
•To reduce electrical-shock hazards to personnel caused by stray earth fault.
•To reduce the arc blast or flash hazard.
•To reduce the momentary line-voltage dip.
•To secure control of the transient over-voltages
while at the same time.
•To improve the detection of the earth fault in a
power system.
Fig. 2: A solidly earthed system [4].

Earthing resistors are generally connected between earth and the neutral of
substation transformers, (Fig. 3), to limit maximum fault current to a value which
will not damage equipment, while allowing sufficient fault current to
operate earth fault protective relays. Although it is possible to limit fault currents
with high resistance neutral earthing resistors, earth short circuit currents may be
extremely reduced and protection devices may not sense the fault.
Fig. 3: Resistive neutral earthing [4].
It is common to limit single phase fault
currents with low resistance neutral earthing
resistors to approximately the rated current of
the transformer. In addition, limiting fault
currents to predetermined maximum values
permits selective coordination of protective
devices, which minimises system disruption
and allows for quick location of faults.

Earthing resistance can be classified into high and low value types. The resistance
is also categorised on the basis of time they can withstand the fault current.
Typical durations are 1 s, 10 s, 1 min and 10 min rating. The extended time rating
resistor is used in systems where the reliability of the system is critical. In these
situations, a high resistance which can sustain the fault for a long period is used.
When an earth fault occurs of one phase, an alarm is generated. However, the
system continues to run until the next scheduled shutdown.
Earth fault current flowing through either type of resistor when a single phase
faults to earth will increase the phase-to-earth voltage of the remaining two
phases. As a result, conductor insulation and surge arrestor ratings must be based
on line-to-line voltage. This temporary increase in phase-to-earth voltage should
also be considered when selecting two and three pole breakers installed on
resistance earthed low voltage systems

High Resistance Neutral Earthing Systems
High resistance earthing systems are designed for limiting phase-to-earth fault
currents in distribution systems by using a neutral earthed resistor between the
transformer or generator neutral and earth. With this type of system earthing
there is no need to trip the corresponding circuit breaker in the case of a phase-
to-earth fault.
The system will only provide an
alarm while the faulted feeder remains
operational until the fault is located and
cleared. This functionality is required in
certain electrical applications where
the risks associated to interrupting the
electrical service are higher than the
risk of letting the system run with a
phase-to-earth fault limited by the resistor. Fig. 4: Delta star transformer earthing [4].

Besides preventing the feeder from tripping by limiting the fault current, some
of the advantages of a High Resistance earthing system include:
•Transient overvoltages are reduced
•Arc flash risks are diminished
•Fault location is easily accomplished
The disadvantage is that, in case of single earth fault, the voltage at the two other
healthy phases tends to reach the value of the line-to-line voltage, depending on
the relation between zero and direct sequence impedance seen from the fault.
This rise in phase voltage increases the probability of a second earth fault in a
different phase and a different feeder.
In this case, a phase-to-earth-to-phase fault current will flow with a
magnitude limited by:
•The total impedance of the earth path
•The possible occurrence of an arc flash

The neutral earthing resistor cannot limit the magnitude of this fault because the
resistor is outside of its trajectory. The fault will evolve until being finally cut off
by the overcurrent protection of the feeders involved, and the risks associated to
a sudden interruption will not be avoided. Second Earth Fault Protection
Systems have been developed to prevent this situation by tripping only one of
the feeders, the one with the lowest priority, in the event of a second phase-to-
earth fault, leaving the rest of the system operational with only a phase-to-earth
fault limited in magnitude by the neutral earthing resistor.
Coordination with circuit breakers’ overcurrent protection and priority settings
are important considerations. If the second phase-to-earth fault in a different
phase occurs in the same feeder that had the initial fault the second earth fault
protection system will not operate, leaving the responsibility for tripping or
disconnecting the feeder to the circuit breaker’s overcurrent protection or fuses.

Fig. 5: Zig-zag transformer neutral earthing [4].
Low Resistance Neutral Earthing
Systems
Low resistance earthing 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 NERs are generally sized to
limit the fault current to a level
enough to operate protective devices
yet not enough to create major
damage at the fault point.

Earthing via a Transformer or Neutral Electromagnetic Coupler (NEC)
Where the neutral point is not available, an artificial earth can be created by
using a transformer. The earthing transformer is used to provide a path to an
unearthed system or when the system neutral is not available for some reason,
for example, when a system is delta connected. It provides a low impedance path
to the neutral and also limits the transient overvoltage when the earth faults
occur in the system. The earthing of the system can be done in the following
way:
Delta-Star Earthing Transformer
In a case of delta-star earthing transformer, the delta side is closed to provide a
path for zero-sequence current. The star winding must be of the same voltage
rating as the circuit that is to be earthed, whereas the delta voltage rating can be
chosen to be any standard voltage level.

Fig. 6: The Petersen coil system [4].
Zig-zag Transformer
A zig-zag transformer can be used for
providing earthing on the transformer. It
provides insulation between the earth and
the component so that the system
component may not be affected by the fault
currents. The zig-zag transformer cancels
out the harmonics of the power system. It
also protects the power system by reducing
the stress of the voltage under a fault
condition. The zig-zag transformer has no
secondary winding. It is a three-limbed
(branched) transformer where each limb has
two identical windings. One set of windings
is connected in the star to provide the
neutral point.

Resonant Earthed Neutral
Fault currents can also be reduced by
earthing the neutral through an inductive
impedance. Adding inductive reactance from
the system neutral point to earth is an easy
method of limiting the available earth fault
from something near the maximum 3-phase
short circuit capacity to a relatively low
value.
To limit the reactive part of the earth fault
current in a power system a neutral point
reactor can be connected between the
transformer neutral and the station earthing
system.
Fig. 7: Plunger type NERC (Trench).

Petersen Coil Earthing
A Peterson coil is an adjustable iron cored reactor used to neutralise the capacitive earth fault current in
a power system. When a phase-to-earth fault occurs in unearthed 3-phase systems, the phase voltage of
the faulty phase is reduced to the earth potential as the capacitance of the faulty line is discharged at the
fault location, the phase-to-earth voltage of the other two phases rises by √3 times. A charging current
occurs between these phase-to-earth capacitances, which will continue to flow via the fault path while it
remains.
A modern steplessly adjustable Petersen coil consists of an iron-cored reactor connected between the
star point of the substation transformer and earth in a three-phase system. In the event of a fault, the
capacative earth fault current (Ir +Iy) is now neutralised by the current in the reactor (Ir) as this is equal
in magnitude, but 180° out-of-phase. Petersen coils adjust automatically to compensate for the earth
fault current. Fig. 7 shows a plunger type adjustable Petersen coil.
The value of the inductance in the Petersen coil needs to match the value of the network capacitance
which may vary, as and when switching in the network is carried out. Modern coil controllers
continually monitor the zero-sequence voltage and detect any change which occurs. When there is a
change in the network capacitance, the controller auto-tunes the Petersen coil to this new level to ensure
that it is tuned to the correct point to immediately neutralise any earth fault which may occur. This rapid
earth fault current limiting occurs automatically without any further intervention from the system [2].

The Petersen coil may also be referred to as an arc
suppression coil (ASC).
Technology and Design
Liquid neutral earthing resistors (LNER)
The design of a liquid neutral earthing resistor
is a large tank containing an electrolyte
solution (distilled water with a small amount of
electrolytic powder).(Fig.8) The outer case of
the tank is connected solidly to the earth point.
An inner electrode which is insulated from the
tank, provides the connection to the
transformer neutral point.
At commissioning, small amounts of
electrolyte are added to the water to increase
the conductivity of the solution until it reaches
the calibrated resistance level. The final result
is a high current carrying capacity fluid with a
high resistance, in a very robust and low
maintenance package.

The LNER has a fixed structure as
opposed to the more familiar liquid starter
resistors or rheostats, and is therefore
simpler to construct and calibrate.
The amount of liquid in the tank provides
a high heat absorption capacity. Problems
with LNERs include a wide tolerance on
ohmic values , and the necessity to
recalibrate at regular intervals.
Fig. 9: Solid earthing resistor (Postglover).
Solid Earthing Resistors
Solid neutral earthing resistors consist of coils of resistive material wound on insulators. The
resistor uses unforced air cooling, and carful design is required to ensure that temperature limits
are not exceeded. Resistive material is usually stainless steel or some other alloy.
The solid earthing resistor may include a current transformer to drive a protection device. The
current transformer is required to withstand the fault current. However in the case of resistive
earthing, the fault current is greatly reduced and the design of the CT is not so severe.

Substation Neutral Earthing

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