Unearthed and Earthed Grade Cable
sreedhar thokala
when we look into the cable datasheet :
" TECHNICAL DETAIL FOR HAVELLS 3.8/6.6 THREE CORES, AL/COPPER COND.,
XLPE INSULATED, ARMOURED CABLES "
We see the above in the contents,
we understand that 3.8/6.6KV mean 3.8 KV (UE) and 6.6 KV (E),
that again mean the same cable can be used when the system is
unearthed upto 3.8KV level only we have to use and is the system is
earthed we can use upto 6.6kv voltage level.
I want to understand, what is meant by earthed system and unearthed system.
thanks
Sreedhar
manish tiwari
Follow IEEE 141:1993 (Red Book)
Resistance-grounded systems employ an intentional resistance connection between the electric
system neutral and ground. This resistance appears in parallel with the system-to-ground
capacitive reactance, and this parallel circuit behaves more like a resistor than a capacitor.
Resistance-grounded systems can take the forms of
a) High-resistance grounded systems
b) Low-resistance grounded systems
Investigations recommend that high-resistance grounding should be restricted to 5 kV class
or lower systems with charging currents of about 5.5 A or less and should not be attempted on
15 kV systems (Walsh 1973 [B37]), unless proper ground relaying is employed. The reason
for not recommending high-resistance grounding of 15 kV systems is the assumption that the
fault will be left on the system for a period of time. Damage to equipment from continued
arcing at the higher voltage can occur. If the circuit is opened immediately, there is no
problem.
In a high-resistance connection (R <= Xco/3, where R is the intentional resistance between the
electric system neutral and ground, and Xco/3 is the total system-to-ground capacitive reactance),
the overvoltage-producing tendencies of a pure capacitively grounded system will be
sufficiently reduced. In a low-resistance grounded system, phase-to-ground potentials are rigidly
controlled, and sufÞcient phase-to-ground fault current is also available to operate
ground-fault relays selectively.Xco is difÞcult to determine in a high resistance grounded system
without testing (Bridger 1983 [B7]), thus 5 A to 10 A is recommended for the phase-to ground
fault current limitation.
The ohmic value of the resistance should be not greater than the total system-to-ground
capacitive reactance (Xco/3). The neutral resistor current should be at least equal to or greater
than the system total charging current. For details on obtaining and testing the value of the
total system charging current. (See Bridger 1983 [B7].)
High-resistance grounding provides the same advantages as ungrounded systems yet limits
the steady state and severe transient overvoltages associated with ungrounded systems. Continuous
operation can be maintained. Essentially, there is minimal phase-to-ground shock
hazard during a phase-to-ground fault since the neutral is not run with the phase conductors
and the neutral is shifted to a voltage approximately equal to the phase conductors. There is
no arc ßash hazard, as there is with a solidly grounded system, since the fault current is limited
to approximately 5 A.
Another beneÞt of high-resistance grounded systems is the limitation of ground fault current
to prevent damage to equipment. High value of ground faults on solidly grounded systems
can destroy the magnetic iron of the rotating machinery. Small winding faults on solidly
grounded systems may be readily repaired without replacing the magnetic iron. However, not
having to replace the lamination with equipment installed on high-resistance grounded systems,
when a phase-to-ground fault occurs, is a benefit.
High-resistance grounded systems should require immediate investigation and clearing of a
ground fault even though the ground-fault current is of a very low magnitude (usually less
than 10 A). This low magnitude of continuous fault current can deteriorate adjacent insulation
or other equipment. As long as a phase-to-ground fault does not escalate into an additional
phase-to-ground fault on a different phase, resulting in a phase-to-phase fault and operating
the protective device, the continuous operation can continue. It is essential to monitor and
alarm on the Þrst phase-to-ground fault. If the fault impedance is zero, solidly connected to
ground, the high-resistance system takes on the characteristics of a solidly grounded system
until the fault is located and repaired.
The key to locating a ground fault on a high-resistance grounded system is the ability to
injection a traceable ground signal to the faulted system. This fault-indicating system permits
fault location with the power system energized. An oversized, large opening, special clamp
on type ammeter is used. Some skill is required in Þnding the location of the fault. (See GET-
35548 [B35] for additional information.)
High-resistance grounding will limit to a moderate value the transient overvoltages created
by an inductive reactance connection from one phase to ground or from an intermittentcontact
phase-to-ground short circuit. It will not avoid the sustained 73% overvoltage on two
phases during the presence of a ground fault on the third phase. Nor will it have much effect
on a low-impedance overvoltage source, such as an interconnection with conductors of a
higher voltage system, a ground fault on the outer end of an extended winding transformer or
step-up autotransformer, or a ground fault at the transformer-capacitor junction connection of
a series capacitor welder.
Low-resistance grounding requires a grounding connection of a much lower resistance. It is
common to have 5 kV and 15 kV systems low-resistance grounded. The resistance value is
selected to provide a ground-fault current acceptable for relaying purposes. The generator
neutral resistor is usually limited for large generators to a minimum of 100 A and to a maximum
of 1.5 times the normal rated generator current (Johnson 1945 [B28]). Typical current
values used range from 400 A (to as low as 100 A) on modern systems using sensitive toroid
or core balance current transformer ground-sensor relaying and up to perhaps 2000 A in the
larger systems using residually connected ground overcurrent relays. In mobile electric
shovel application, much lower levels of ground-fault current (50 to 25 A) are dictated by the
acute shock-hazard considerations. One final consideration for resistance-grounded systems
is the necessity to apply overcurrent devices based upon their “single-pole” short-circuit
interrupting rating, which can be equal to or in some cases less than their “normal rating.”
Regards,
Manish
Manish D Tiwari
manish tiwari
In earthed system the start point / neutral point of the system is earthed, whereas in unearthed system the neutral of the system in not earthed.
In earthed system, when there is earth fault, the voltage of the faulty phase will become zero and the voltage between the healthy phases and ground will be the same Vph/√3 (3.8kv/√3 or 6.6kV/√3)
In case of unearthed system, the voltage between the healthy phases and ground will become phase to phase voltage i.e. Vph-ph (3.8kv or 6.6kV).
Hence the insulation level of the cables for unearthed system need to be higher.
Hence in you example 3.8kV is UE and 6.6kV is E. so you can use 3.8kV (UE) /6.6KV (E) cable only for unearthed system upto 3.8kV ……….. or for earthed system upto 6.6kV
Also note that if the system star point is grounded through resistance or impedance, then also the system shall be considered as unearthed system.