Friday, May 27, 2016

High Tension Cables and its types; HT cables

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:-

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.
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.