Introduction
In HT electrical distribution, the system can be earthed or unearthed
The selection of unearthed or earthed cable depends on distribution system. If such system is earthed, then we have to use cable which is manufactured for earthed system. (which the specifies the manufacturer). If the system is unearthed then we need to use cable which is manufactured for unearthed system
The unearthed system requires high insulation level compared to earthed system
For earthed and unearthed XLPE cables, the IS 7098 part2 1985 does not give any difference in specification. The insulation level for cable for unearthed system has to be more.
Earthed System
Earlier
the generators and transformers were of small capacities and hence the
fault current was less. The star point was solidly grounded. This is
called earthed system
in three phases earthed system, phase to earth voltage is 1.732 times less than phase to phase voltage. Therefore voltage stress on cable to armor is 1.732 times less than voltage stress between conductors to conductor
Where in unearthed system, (if system neutral is 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
In an earthed cable, the three phase of cable are earthed to a ground. Each of the phases of system is grounded to earth
.
Example: 1.9/3.3 KV, 3.8/6.6 KV system
Unearthed System
Today generators of 500MVA capacities
are used and therefore the fault level has increased. In case of an
earth fault, heavy current flows into the fault and this lead to damage
of generators and transformers. To reduce the fault current, the star
point is connected to earth through a resistance. If an earth fault
occurs on one phase, the voltage of the faulty phase 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. This is called unearthed system
In
an unearth system, the phases are not grounded to earth .As a result of
which there are chances of getting shock by personnel who are operating
it
Example: 6.6/6.6 KV, 3.3/3.3 KV system
Unearthed cable has more insulation strength as compared to earthed cable. When fault occur phase to ground voltage is √3 time the normal phase to ground voltage. So if we used earthed cable in unearthed System, It may be chances of insulation puncture
So unearthed cable are used. Such type of cable is used in 6.6 KV systems where resistance type earthing is used
Nomenclature
In simple logic the 11 KV earthed cable is suitable for use in 6.6 KV unearthed system. The process of manufacture of cable is same.
The size of cable will depend on current rating and voltage level.
- Voltage Grade (Uo/U) where Uo is Phase to Earth Voltage & U is Phase to Phase Voltage.
- Earthed system has insulation grade of KV / 1.75 x 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 of KV / KV.
- For Unearthed System (Uo/U): 3.3/3.3 kV and 11/11 kV.
- 3 phase 3 wire system has normally Unearthed grade cables and 3 phase 4 wire systems can be used earthed grade cables, insulation used is less, and cost is less
Thumb Rule
As a thumb rule we can say that 6.6KV unearthed cable is equal to 11k earthed cable i.e 6.6/6.6kv Unearthed cable can be used for 6.6/11kv earthed system.
Because each core of cable have the insulation level to withstand 6.6kv so between core to core insulation level will be 6.6kV+6.6kV = 11kV
For
transmission of HT, earthed cable will be more economical due to low
cost where as unearthed cables are not economical but insulation will be
good.
Generally 6.6 kV and 11kV systems are
earthed through a neutral grounding resistor and the shield and armor
are also earthed, especially in industrial power distribution applications. Such a case is similar to an unearthed application but with earthed shield (sometimes called solid bonding ).
In
such cases, unearthed cables may be used so that the core insulation
will have enough strength but current rating is de-rated to the value of
earthed cables.
But it is always better to mention the type of
system earthing in the cable specification when ordering the cables so
that the cable manufacturer will take care of insulation strength and de
rating.
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