HT
cables are integral part of electrical systems. Electricity is always
transferred through HT cable/ Conductors. So it becomes very important to know
about HT cables. HT cables are usually XLPE i.e. cross linked polyethylene cables.
Let’s discuss about HT cables used in
electrical systems.
There
are following types of High voltage cables
There
are two common HT cables
are used:
(a)
Single
conductor:-
These
cables consisting of one conductor per cable or three cables for a three-phase
system.
(b)
Three
conductor
These cable consisting of three cables
sharing a common jacket.
There are fe'w visible differences between HT cables but HT cables
are same there are following parts of HT cables:
(i)
Conductor
(ii)
Strand
shield
(iii)
Insulation
(iv)
Insulation
shield system (semi-con & metallic)
(v)
Jacket.
These components are vital and must be needed to be understood in
order to make a dependable termination.
Let’s discuss about each component in details:-
HIGH VOLTAGE CABLE COMPONENTS
Conductor
The current carrying components are Made of copper or aluminum. Conductor
used with modern solid dielectric cables come in four basic configurations:
Concentric Stranding
(Class B)
Not commonly used in modern shielded power cables due to the
penetration of the extruded strand shielding between the conductor strands,
making the strand shield difficult to remove during field cable preparation.
Compressed Stranding
This is common conductor configuration these are Compressed to 97%
of concentric conductor diameters. This compression of the conductor strands
blocks the penetration of an extruded strand shield, thereby making it easily
removable in the field. For sizing lugs and connectors, sizes remain the same
as with the concentric stranding.
Compact Stranding
Compacted to 90% of concentric conductor diameters. Although this
conductor has full ampacity ratings, the general rule for sizing is to consider
it one conductor size smaller than concentric or compressed. This reduced
conductor size results in all of the cable’s layers proportionally reduced in a
diameter, a consideration when sizing for molded rubber devices.
Solid Wire
This conductor is not commonly used in industrial shielded power
cables.
Strand Shielding
The semi-conductive layer between conductor and insulation which
compensates for air voids that exist between conductor and insulation.
Air is a poor insulation, having a nominal dielectric strength of
only 76 volts per mil, while most cable insulation have dielectric strengths
over 700 volts/mil. Without strand shielding an electrical potential exists
that will over-stress these air voids. As air breaks down or ionizes, it goes
into corona (partial discharges). This forms ozone which chemically deteriorates
cable insulation. The semi-conductive strand shielding eliminates this
potential by simply “shorting out” the air.
High voltage power
cable
Modern cables are generally constructed with an extruded strand
shield.
Insulation.
A third layer consisting of many different variations such as
extruded solid dielectric, or laminar (oil paper or varnish cambric). Its
function is to contain the voltage within the cable system. The most common
solid dielectric insulations in industrial use today are:
(i)
Polyethylene
(ii)
cross-linked
polyethylene (XLP)
(iii)
ethylene
proplyene rubber (EPR)
Each is preferred for different properties such as superior
strength, flexibility, temperature resistance, etc. depending upon the cable
characteristics required. The selection of the cable insulation level to be
used in a particular installation shall be made on the basis of the applicable
phase to-phase voltage and the general system category as outlined below.
a. 100 Percent Level –
Cables in this category may be applied where the system is
provided with relay protection such that ground faults will be cleared as
rapidly as possible, but in any case within one minute. While these cables are applicable
to the great majority of the cable installations which are on grounded systems,
they may be used also on other systems for which the application of cable is
acceptable provided the above clearing requirements are met in completely
deenergizing the faulted section.
b. 133 Percent Level –
This insulation level corresponds to that formerly designated for
ungrounded systems. Cables in this category may be applied in situations where
the clearing time requirements of the 100 percent level category cannot be met,
and yet there is adequate assurance that the faulted section will be
de-energized in a time not exceeding one hour. Also, they may be used when
additional insulation strength over the 100 percent level category is
desirable.
c. 173 Percent Level –
Cables in this category should be applied on systems where the
time required to de-energize a grounded section is indefinite. Their use is
recommended also for resonant grounded systems. Consult the manufacturer for
insulation thicknesses.
Insulation Shield System
The outer shielding which is comprised of two conductive
components: a semi-conductive layer (semi-con) under a metallic layer (see
cable types for common shield varieties). The principal functions of the
insulation shield system are to:
1. Confine the dielectric field within the cable
2. Obtain a symmetrical radial distribution of voltage stress within the
dielectric
3. Protect the cable from induced potentials
4. Limit radio interference
5. Reduce the hazard of shock
6. Provide a ground path for leakage and fault currents.
The SHIELD MUST BE GROUNDED for the cable to perform these functions. This is for the following reason stated below:-
For safe and reliable operation, High tension cables shields and metallic sheaths of power cables must be grounded. If grounding wasn't done than shields would operate at a potential considerably above ground. This would be hazardous to touch and would cause rapid degradation of the jacket or other material intervening between shield and ground. Usually this is caused by the capacitive charging current of the cable insulation that is on the order of 1 mA/ft of conductor length.
For safe and reliable operation, High tension cables shields and metallic sheaths of power cables must be grounded. If grounding wasn't done than shields would operate at a potential considerably above ground. This would be hazardous to touch and would cause rapid degradation of the jacket or other material intervening between shield and ground. Usually this is caused by the capacitive charging current of the cable insulation that is on the order of 1 mA/ft of conductor length.
