Cable selection parameter

The selection of cable is important

(1) Voltage of Cable:

§?The Nominal voltage is to be expressed with two values of alternative current Uo/U in V (volt)

§?Uo/U : Phase to earth voltage

§?Uo : Voltage between conductor and earth

§?U : Voltage between phases (conductors)

§?(i ) Low-tension (L.T.) cables — up to 1100 V

§?(ii ) High-tension (H.T. ) cables — up to 11,000 V to 33,000V

§?(iii ) Extra high-tension (E.H.V.) cables — from 66 kV to 500 KV

§??A low-voltage system usually has a solidly earthed neutral so that the line-to-earth voltage cannot rise higher than (line volts) ÷ √ 3. Cables for low-voltage use are insulated for 600V RMS score to earth and 1000V rms core to core.

§?High-voltage cables used in Shell installations are rated 19000/3300V or 3810/6600V or 6600/11000V, Phase/Phase.

§?In selecting the voltage grade of cable, the highest voltage to earth must be allowed for. For example, on a normal 6.6kV unearthed system, a line conductor can achieve almost 6.6kV to earth under earth-fault conditions, to withstand this, a cable insulated for 6600/11000V must therefore be used.

?(2) Current carrying capacity:

§?The current carrying capacity of a cable is called Ampacity. Ampacity is defined as the maximum amount of electrical current a conductor or device can carry before sustaining immediate or progressive deterioration and is the rms electric current that a device or conductor can continuously carry while remaining within its temperature rating

?(3) Short Circuit?values:

§?the “short-circuit current rating” is the maximum short-circuit current that a component can withstand. Failure to provide adequate protection may result in component destruction under short circuit conditions.

§?Short circuits and their effects must be considered in selecting cables. These cables should have a short circuit rating which is the highest temperature the cable can withstand during an electrical short circuit lasting up to about half a second.

?(4) Type of Conductor:

§?Type of Conductor Material Copper or Aluminum is the main criteria for the selection of Cable

?(5) No of Core:

§?No of Core selection depends upon Power System.

§?For Single Phase Power Supply We can use 2 core Cable for Three-phase supply we can use 3.5 Core or 4 Core Cable for HV supply We may be using Single Core Cable.

?(6)?Voltage drop:

§?It is a primary concern when installing lengths of cables is voltage drop. The amount of voltage lost between the originating power supply and the device being powered can be significant. All cables have resistance, and when current flows in them this results in a volt drop.

?(7) Type of Insulation:

§?Type of Cable Insulation Material like PVC, XLPE, Rubber

§?PVC Cable is cheaper than XLPE Cable

?(8) Method of Installation:

§?If we lay cable in the Ground Armor cable is required but If we lay cable in the cable tray We may be used an unarmored cable to reduce the cost of cable.

§? lay cable on cable tray than shielded cable is required.

§?Mutual heating effect due to cable group laying is also considered while selecting a cable. When multiple cables are nearby, each contributes heat to the others and diminishes the amount of external cooling affecting the individual cable conductors. Therefore cable de rating is a necessary consideration for multiple cables nearby.

?(9) Shielded Cable or unshielded Cable

§?The choice of a shielded cable or non-shielded cable is depend upon some criteria.

§?An area such as a production/factory floor where heavy equipment is being used is a prime example of a place where we might consider a shielded cable.

§?Grounding can also be a concern in some installations. If a shielded cable is used to connect equipment from two different circuits, a ground loop can occur causing noise on a network line. If the ground voltage difference is great enough it may even cause damage.

§?Terminations of the shielded cable must also be made with care, to provide for a smooth dielectric transition from the shielded condition to the unshielded condition

§?the substantial space required if shielded cables were used. Shielded cables require a significant amount of space at each end of the cable for the installation of the stress cone kit. Also, the minimum bending radius for shielded cables is twelve times cable outside diameter, whereas the minimum bending radius for unshielded cables is only eight times outside diameter (and even less with extra-flexible appliance connection cables used in controllers).

§?The two factors, high cost, and ample space requirements preclude the use of shielded cable in switchgear

?(10) Economics:

§?It is also an important factor in selecting the type of cable.

§?It is to be kept in mind that the cost of the cable should not be such large that it causes loss and another cable may fetch the same results in low cost and loss.

(11) Environmental conditions:

§?Cable operates at its best when it is installed in its optimum environmental conditions.

§?For example, Elastomeric Cable is applied in trailing, coal cutter, windmill, panel wiring, battery cable, and such other areas. XLPE cables work well in areas where moisture content is good. Thus, the proper cable should be selected so the system becomes more efficient.

?(12) Applications:

§?Low voltage cables with both PVC and XLPE insulation are suitable for indoor and outdoor applications.

§?Armored cables are not recommended for tray applications, as they are heavy and extra loads are exerted on the tray.

§?Unarmored cables are not recommended for direct buried applications, except if the quoted cables are designed and produced to pass direct burial test requirements.

§?A PVC jacket is a very stable material against a wide range of chemicals, while HDPE jacketed cables can serve better in wet locations.

?Cable Core Colors Identification

§?Single core – Natural

§?Two core – Red, Black

§?Three core – Red, Yellow, and Blue

§?Four core – Red, Yellow Blue, and Black

§?Five core – Red, Yellow Blue, Black, and Green

?Abbreviation for PVC & XLPE Cable

§??A = Aluminum Conductor.?

§?Y = PVC Insulation or PVC Sheath

§?2X = Cross-linked Polyethylene Insulation?

§?W = Round Steel Wire Armoring?

§?WW = Double Round Steel Wire Armoring

§?F = Formed Steel Wire (Strip) Armoring

§?FF = Double Formed Steel Wire (Strip) Armouring

§?C = Metallic Screening (Usually of Copper)?

§?CE = Metallic Screening (Usually of Copper) over each core.

§?Gb = Holding Helix Tape (of Steel)?

§?Wa = Aluminum Round Wire & Aluminum Formed Wire (Strip) Fa Armouring.

Example:

§?AYY- Aluminum Conductor, PVC Insulated, PVC Outer Sheathed Heavy Duty Cables.

§?AYWY- Aluminum Conductor, PVC Insulated, Round Steel Wire Armored, and PVC Outer Sheathed

§?AYFY- Aluminum Conductor, PVC Insulated, Flat Steel Wire (Strip) Armored and PVC Outer Sheathed

§?AYCY-?Aluminum Conductor, PVC Insulated, Metallic Screened, and PVC Outer Sheathed

§?A2XCY-?Aluminium Conductor, XLPE Insulated, Metallic Screened, and PVC Outer Sheathed

?Cable Application Standard:

§?IS7098(P-3) IEC 60502 ” EHV XLPE insulated cables

§?IS7098(P-1 &2) XLPE insulated cables up to 33KV

§??IS1554 PVC insulated cables

STRUCTURE OF AN UNDERGROUND CABLE SYSTEM

Overhead lines can frequently be seen traversing the landscape from a long way away. This is one area in which underground cables can score. Despite this, this transmission technology isn’t completely invisible: above and to either side of the underground cable trench, a protective strip must be kept free of deep-rooted trees and shrubs, and no buildings are allowed to be built. What’s more, an underground cable route comprises a large number of components: the underground cables themselves, cable joints, cable transfer stations, and, in the case of AC lines, reactive-power-compensation equipment. The last of the above consists mainly of reactors, which are already installed in several of Amprion’s substations. Reactors are similar to large transformers.

We only ever switch them into the grid if the voltage on a line is too high. The reactors correct the power factor and lower the voltage on the line again. If the opposite is the case, that is, the voltage on the line is too low, correction equipment, such as capacitor banks, is put to use.

1.?Cable sheath?– Protects the cable, against moisture in particular

2.?Wire screen?– Controls the electric field and discharges fault currents

3.?Insulating layer?– Insulates the electric conductor

4.?Electric conductor?– Conducts the current

Underground cables as a transmission medium

Underground cables essentially consist of a conductor, an insulating system, a wire screen, and a sheath. At the core is an electric conductor; in the case of extra-high-voltage (EHV) lines, this is usually made of copper. The conductor is enclosed by an insulating system (dielectric) whose main component is made of plastic, for example. An outer copper wire screen discharges fault currents and keeps the electric field in the cable. The outer cable sheath protects the cable against moisture.

Cable joints as connectors

Owing to the obvious transportation limitations, cross-country EHV cables can only be supplied to the respective installation locations in sections currently around 1,000 to 1,300 meters in length. Bridges and other structures impose limits on the size of the cable drums that can be transported by truck. As a result, when laying these cables, the sections have to be connected using cable joints. To be able to conduct electrical measurements whenever necessary, some joints allow the connections inside to be accessed from the outside.

Cable transfer stations for connecting to the grid

At those locations where the cables are introduced into the ground or emerge out of it again, we build what is known as “cable transfer stations”.

These are required to connect the underground cables to the overhead lines. Such a station is very much like a small substation. Longer cable sections in the AC grid also require reactors to correct the power factor. What’s more, they require additional switchgear, which in turn takes up more space and means the transfer station as a whole requires more space.

Insulation media of underground cables

Practically all underground cables used in today’s AC grid implement a plastic insulation medium (dielectric). Thanks to the new cable joint design, which enables the joints to be prefabricated and makes them faster to install on-site, as well as the better-operating characteristics compared with all other types of insulation, underground cables with a plastic dielectric have succeeded in establishing themselves. The insulation material itself is usually made of polyethylene (PE) that is cross-linked in a thermochemical process to produce what’s known as XLPE. This enables the cable system to be operated at higher temperatures. However, we’re talking here about a single-layer insulation medium that has to be elaborately repaired whenever it is found to be faulty using repair joints. Plastic-insulated cable systems are available on the market for applications ranging from medium voltage right up to 500 kilovolts.