Methods
of Earthing
In
theory, the main power network does not have to be earthed and sometimes
arguments are put forward that an unearthed network may be more reliable. In
some cases this can be true, but generally an unearthed networks can be unreliable
due to influence of electrical or some other factors.
There
are a number of ways in which an electrical power system can be operated –
- Unearthed or insulated system- High impedance earthed and- Low impedance earthed arrangement
Unearthed
system/IT earthing system
In
this kind of installation, there is no formal connection to earth because there
may be some high impedance connections for instrumentation e.g coil of a
measuring device.
Earthed
vs Unearthed Electrical cable
In
some generators and transformers, the star point is solidly grounded. This is
called Earthed Electrical system.
In a
three phase earthed system, phase to earth is 1.732(sqrt 3) is
higher than
phase to phase voltage.
 |
| solidly earthed system |
In
unearthed system (with neutral not grounded) phase to ground voltage can be equal
to phase to phase voltage. In such case, the insulation level of conductor to
armor should be equal to insulation level of conductor to conductor.
Unearthed
System
In
bigger installation e.g. generators of bigger capacities like 500MVA fault
level are very high and in case of an earth fault, heavy current flows into the
fault and this lead to damage of the transformer or generator.
To
reduce this fault current, star point of the system is connected through a resistance.
If an earth fault occurs on one phase, the voltage of the fault with respect to
earth appears across the resistance.
Therefore,
the voltage of the other two healthy phases with respect to earth rises by 1.7
times. If the insulation of these phases is not designed for these increased
voltages, they may develop earth fault.
Since
the phases are not grounded in this kind of installation, there are chances of
getting shock while operating the system. Hence the insulation level for
cable for unearthed system has to be more than insulation level for earthed
system.
Also,
an unearthed system is not suitable for smaller generators and transformers especially
for houses.
 |
| Unearthed system |
For
instance
Imagine an earthed star connected system. Imagine a TV is connected to phase L1. Image a short circuit occurs on Phase L2. The equipment on the two good phases is good, because the star point has been connected to earth.If the star point was not earthed, the other two phases will raise their voltage by an additional root 3 (1.732), so the connected TV will sizzle and eventually burnt into flames.
The greatest
difference arises in the voltage grade (Uo/U), which is:
|
Earthed
System
|
Unearthed
System
|
|
1.9/3.3kV,
3.3/6.6kV, 6.35/11kV, 12.7/22kV and 19/33kV
|
3.3/3.3kV
and 11/11kV
|
In an exception to the above,
the cables of 6.35/11kV for an earthed system can also be used in the place of
6.6/6.6 kV for an unearthed system. This is because each core of the cable has
the insulation level to withstand 6.6kV due to which between core to core
insulation level will be 6.6kV+6.6kV = 11kV.
Earthed System
An earthed system is the one in
which the star point of a generator or transformer is solidly connected to the
mother earth.
In three phases earthed system,
phase to earth voltage is root 3 (1.732) times less than phase to phase
voltage. Therefore voltage stress on cable to armor is 1.732 times less than
voltage s tress between conductors to conductor.
Differences
- Cabling
So, 11kV earthed cable can be used
in place of 6.6kV unearthed system since the cable manufacturing process is the
same. The size of the cable will depend on the current rating and voltage
level. So,
Voltage grade (Uo/U) where Uo is
phase to earth voltage and U is phase to phase voltage
Earthed system has an insulation
grade of kV/1.75x kV
For earthed system (Uo/U):
1.9/3.3 kV, 3.8/6.6 kV, 6.35/11 kV, 12.7/22 kV and 19/33 kV
Unearthed system has insulation
grade kV/kV
3 phase 3 wires system generally
comes with unearthed grade cable and 3 phase 4 wire systems can be used as
earthed grade cables.
- Preferable for MV Transmission
For MV transmission, earthed cable will be more economical,
but unearthed cable offers more insulation. This is because, if an earth fault
occurs in the underground system, the voltage between the healthy phases and
the ground will be equal to phase to phase voltage - 11kV or 6.6kV and higher
insulation level is required. The voltage of the healthy phases rises by nearly
1.7 times resulting in an earth fault since the insulation of these phases is
not designed for increases voltage. It is advisable to opt for an unearthed
cable so that the core insulation has enough strength.
- Insulation Level
Unearthed cable requires more insulation
strength than earthed cable. If a fault occurs in the phase to ground voltage
is √3 time the normal phase to ground voltage. So, if an earthed rated cable is
used in an unearthed system, it may result in an insulation puncture. Hence, it
is essential to use unearthed rated cable in such situations, especially in the
case of 6.6kV systems where resistance type earthing is used.
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