• Conductor (Circular/Milliken)

                        Plain Annealed Copper/Aluminum (Class 2 Stranded/Class 2 Segmental)

  • Semi Conductive Screen (Extruded/Tape)

                        Conductor Screen/Insulation Screen

  • Insulation

                        XLPE/XLPE WTR

  • Metallic Screen

                        Copper Wire Screen/Copper Tape Screen/ Aluminum Foil Laminate/Lead Alloy Sheath/ALC

  • Water Blocking Tapes

                        Semi Conductive Water Blocking Tape/ Non Conductive Water Blocking Tape

  • Separation Sheath (Bedding)

                        PVC/LSPVC/LSFOH/PE (HDPE)

  • Protection Sheath

                        Lead Alloy Sheath /Aluminum Corrugated Sheath/ALPE Sheath (Aluminum Copolymer)

  • Armour

                        Round Wire Armour (SWA/AWA)/Double Tape Armour (STA/ATA)

  • Outer Sheath

                        PVC/LSPVC/LSFOH/PE (HDPE)

– Extra High Voltage and High Voltage Cables are manufactured with copper, aluminum conductors and XLPE insulation.

– Extra High Voltage Cables are rated at voltages of 230 to 400 kV. Our High Voltage Cables are rated at voltages of 63 to 132 kV.

– High Voltage cables are used for bulk transmission of power over 63 KV from power plant sites to primary distribution networks or central grid. The cables can also be installed on the racks, direct buried or in conduits.

Conductor

  • Usually stranded copper (Cu) or Aluminum (Al) is used.
  • Copper is denser and heavier, but more conductive than aluminum.
  • Electrically equivalent aluminum conductors have a cross-sectional area approximately 1.6 times larger than copper, but half the weight.
  • The size of the copper / Aluminum conductor forming one of the cores of a cable is expressed in square millimeters (mm2), and the current rating of the cable is dependent upon the cross-sectional area of each core.
  • Multi-core Aluminum or copper conductor are produced by two shapes.

Conductors can generally be according to IEC 60288:

Circular Conductor (Class 2 – Stranded)

Multi-layers of stranded wires are assembled together to make circular shape.

  • To achieve a circular conductor, the number of strands follows a particular progression: 3, 7, 19, 37, 61, and 127 etc, the diameter of each strand being chosen to achieve the desired cross-sectional area of whole conductor.
  • Circular shape conductor is normally available used up to 200 mm2

Milliken Conductor (Class 2 – Segmental)

Five segments of compacted conductor in triangle shape of 72 degrees are assembled together with separation of non-metallic tapes to reduce the skin effect which reduce the AC conductor resistance.

  • Larger sizes have conductors with the strands laid up in a segmental formation; this cable achieves a better space factor and reduces the overall diameter of the cable. It also reduces the inductance of the cable due to decreased spacing between phases
  • Milliken conductor is normally available from 1000 mm2 and above.

Semi conductive Screen

Semi-conductor screen on conductor (Conductor Screen)

  • The main purpose of conductor screen is to maintain a uniformly divergent electric field, and to contain the electric field within the cable core.
  • Conductor screen is semi-conducting material because semi-conducting materials do not conduct electricity well enough to be a conductor but will not hold back voltage. It smoothest out the surface irregularities of the conductor. The conductor shield makes the voltage on the inside of the insulation the same.
  • Semiconducting screening materials are based on carbon black that is dispersed within a polymer matrix. The concentration of carbon black needs to be sufficiently high to ensure an adequate and consistent conductivity.
  • The incorporation must be optimized to provide a smooth interface between the conducting and insulating portions of the cable.
  • The smooth surface is important as it decreases the occurrence of regions of high electrical stress.

Control Electrical Field: Conductor screen is control the electric field within the insulation and thus the same voltage gradient across it. It also avoids any interaction of the electric stresses due to the voltages on different phase conductors within the same cable.

Reduce Voltage Stress: Conductor Screen helps to reduce voltage stress at the interface between the conducting and insulating components.

Uniform Electrical Field: A black semi-conducting tape is used to maintain a uniform electric field and minimize electrostatic stresses in MV/HV power cables.

  • The external surfaces of the conductor may not be smooth, particularly for stranded conductors, so this layer provides a smooth surface at the same potential as the conductor to keep the electric field consistent all the way around the surface. Without this layer, any small peaks or troughs could cause concentrations of electrical energy which could create small arcs, and over time could erode the insulation layer and cause failure of the cable.

Reduce Electrical Flux line around each core: It provide a cylindrical, smooth surface between the conductor and insulation

  • Semi-conducting compounds also have the effect of filling in the interstices of the conductor giving a smooth surface for the insulation. This reduces the electrical flux lines around each individual wire that make up the conductor, which can reduce the stress by 10-15%

To prevent electric field concentration, there is an interface of ultra-smooth semi-conductor XLPE between the conductor and the insulation.

The conductor screen is extruded over the conductor to smooth out the discontinuities left by stranding the conductor. It presents a smooth interface to the electric field in the insulation, thus maintaining a uniformly divergent electric field. It is semi conductive that is, a polymer containing conducting carbon black, so it is neither an insulator nor a conductor, on larger conductor size, tape shield is often used to prevent material “fall-in” between the strands during manufacture. An extruded shield is then applied over the tape shield to provide a smooth interface for the insulation.

Semi-conductor screen on insulation (Insulation Screen)

  • An extruded layer of semi conducting is applied over the insulation layer to insure that the electric stress is homogenous around the insulated core. The semi conducting layer shall be firmly bonded to the outer layer of the insulation layer.
  • The purpose of insulation screen is same as conductor screen.
  • The purpose of insulation screen is to reduce voltage stress at the interface between the conducting and insulating component.
  • A cylindrical, smooth surface between the insulation and metallic shield.
  • Insulation screen is a layer of black cross linked semi conductive compound of approx. 1mm thickness and is either fully bonded to the insulation layer, or can be “cold strippable” by hand.
  • When terminating or jointing the cables, it is necessary to remove a part of the insulation screen

This layer has the same function as the conductor screen:

Progressive transition from an insulating medium, where the electric field is non- null, to a conductive medium (here the metal cable screen) in which the electric field is null.

The insulation screen or shield is extruded over the insulation to Uniform the electrical field within the insulation, to maintain a uniformly divergent field and to aid in the removal of capacitive current.

Insulation

Insulation materials used in MV power cables have long included the mature technology of fluid-impregnated paper. They have been successfully used for over 100 years.

Today, extruded polymer insulation are the standard. Extruded polymers include PE (LDPE and HDPE), XLPE, WTR-XLPE and EPR. Extruded polymer are either thermoplastic or thermoset. Thermoplastic materials will deform upon subsequent heating, whereas thermoset materials will tend to maintain their form at operating temperatures.

XLPE is a thermoset material produced by the compounding of LDPE with a crosslinking agent such as dicumyl peroxide. Al Gilbert and Frank Precopio invented XLPE in March 1963 in the GE Research Laboratory located in Niskayuna, New York. In the process, the long-chain PE molecules “crosslink” during a curing process to form a material that has electrical characteristics that are similar to thermoplastic PE, but with better mechanical properties, particularly at high temperatures.

XLPE-insulated cables have a rated maximum conductor temperature of 90°C and an emergency rating of up to 140°C, depending on the standard used to rate XLPE-insulated cables. Cables insulated with XLPE also have a conductor short-circuit rating of 250°C. XLPE has excellent dielectric properties making it useful for a large range of voltage applications from 600 V to 500 kV.

  • Insulation main Purpose is to withstand the electrical field applied to the cable for its design life in its intended installed environment.
  • This will be an extruded layer of XLPE, Elastomer, Rubber or PVC applied over conductor screen under triple extrusion process along with conductor screen and insulation screen.
  • They are known as XLPE Cable. It is form of polyethylene with cross links.
  • XLPE creates by direct links or bonds between the carbon backbones of individual polyethylene chains forms the cross linked polyethylene structure.
  • The result of this linkage is to restrict movement of the polyethylene chains relative to each other, so that when heat or other forms of energy are applied to the basic network structure cannot deform and the excellent properties that polyethylene has at room temperature are retained at higher temperatures.
  • The cross linking of the molecules also has the effect of enhancing room temperature properties.
  • The useful properties of XLPE are temperature resistance, pressure resistance (stress rupture resistance), environmental stress crack resistance (esc), and resistance to UV light, chemical resistance, oxidation resistance, room temperature and low temperature properties.

Water Tree Retardant Cross-linked Polyethylene (WTR-XLPE)

As noted earlier, the phenomenon of water treeing can reduce the service life of XLPE cables. Water trees grow relatively slowly over a period of months or years. As they grow, the electrical stress can increase to the point that an electrical tree is generated at the tip of the water tree. Once initiated, electrical trees grow rapidly until the insulation is weakened to the point that it can no longer withstand the applied voltage and an electrical fault occurs at the water/electrical tree location. Many actions can be taken to reduce water tree growth, but the approach that has been most widely adopted is the use of specially engineered insulating materials designed to limit water tree growth. These insulation materials are called WTR-XLPE. These insulation materials, combined with the use of clean semi conductive shields and sound manufacturing processes have dispelled the concerns that many utilities had regarding the use of cables with a polymeric insulation.

Metallic Screen

The main function of the metallic screen is to nullify the electric field outside of the cable it acts as a second electrode of the capacitor formed by the cable. The screen needs to connect to earth at least at one point along the route. The capacitive charging current and induced circulating currents which are generated under normal operating conditions will be drained away through the screen. The screen also drains the zero-sequence short circuit currents under fault conditions; this function is used to determine the required size of the metallic screen.

The second function of the metallic screen is to form a radial barrier to prevent humidity from penetrating the cable insulation system. The extruded insulation system should not be exposed to humidity. When humidity and a strong electric field are present together, the insulation deteriorates by what is called water-treeing, which can eventually cause the insulation to fail.

  • Medium Voltage & High voltage cables have an earthed metallic screen over the insulation of each core.
  • This screen consists one or multi layers of a lapped conductive copper wires, copper tape or metallic foil, lead, aluminum helically with overlap over insulation screen.
  • The metallic shield needs to be electrically continuous over a cable length to adequately perform its functions of electrostatic protection, electromagnetic protection, and protection from transients, such as lightning and surge or fault currents.

When the voltage reaches tens or even hundreds of kV, a metallic screen is necessary.

Its main function is to nullify the electric field outside the cable. It acts as the second electrode of the capacitor formed by the cable.

To provide:

  • An electric screen (no electric field outside the cable)
  • Radial waterproofing (to avoid contact between the insulation and water)
  • An active conductor for the capacitive and zero-sequence short-circuit current
  • A contribution to mechanical protection.

Screens can be:

 

Copper Wire Screen

The metallic screen mostly consists of copper wires with a copper counter wound tape to ensure potential bonding. The section of the screen is dictated by the network in which the cable will be used.

The Copper screen on the MV cables are for following reasons: To relieve electrostatic stresses from the insulation of the core. To earth the leakage fault currents or the capacitive currents preventing damage to the insulation.

Advantages

  • Lightweight and cost-effective design.
  • High short-circuit capacity.
  • Easy to terminate.

Drawbacks

  • Low resistance of screen may necessitate need for special screen connections to limit the circulating current losses.
  • Does not form a complete moisture barrier unless water swell able tapes are used under and/or over the copper wires.

Copper Tape Screen

copper tape with overlap to ensure potential bonding.

Advantages

  • Lightweight and cost-effective design.
  • High short-circuit capacity.
  • Easy to terminate.

Drawbacks

  • Low resistance of screen may necessitate need for special screen connections to limit the circulating current losses.
  • Does not form a complete moisture barrier unless water swell able tapes are used under and/or over the copper wires.

Aluminum Foil Laminate

Advantages

  • Lightweight and cost effective design.
  • Moisture proof radial barrier.

Drawbacks

  • Low short circuit capacity.
  • More difficult to terminate – requires special screen connections.

Lead Alloy Sheath

Advantages

  • Waterproofing guaranteed by the manufacturing process.
  • Excellent resistance to corrosion and hydrocarbons (suitable for oil and gas plants).

Drawbacks:

  • Heavy and expensive.
  • Lead is a toxic metal whose use is being restricted in some countries.
  • Limited capacity for short circuits.

Water Blocking Tapes

  • Water blocking is used to prevent moisture migration.
  • Water blocking tapes or Swelling powder should be applied between the conductor strands to block the ingress of water inside the cable conductor (if required).
  • Water blocking methods to be considered are as follows:
  • Powders: Swell able powders are used as longitudinal water blocks in cables to prevent longitudinal water penetration. These powders swell and expand sufficiently upon contact with water to form a gel-like material to block the flow of water.
  • Water-Blocking Tapes: A water-blocking tape is usually a non-woven synthetic textile tape impregnated with, or otherwise containing, a swell able powder.
  • Sealed Overlap: To ensure a seal of the overlap, hot-melt adhesives can be used. These adhesives can be extruded or pumped into the overlap seam of a longitudinally formed metallic tape before the seam is closed during cable manufacture.

Separation Sheath (Bedding)

Cables incorporating an armour or Protection layer have an extruded bedding of polyvinyl chloride (PVC) or zero halogen material (LSFOH).

  • It could be also called inner sheath or inner jacket, which serves as a bedding under cable armouring or protection sheath to protect the laid up cores or as a separation sheath between armour and protection sheath.
  • Inner sheath is over laid up of cores.
  • It gives circular shape of the cable and it also provides Bedding for the armouring.
  • Inner sheath is provided by extrusion of thermoplastic over the laid up of cores.
  • Inner sheath is provided by wrapping at thermoplastic tape.
  • All multi-core cables have either extruded PVC inner sheath or thermoplastic wrapped inner sheath, which is compatible with insulation material and removable without any damage to insulation.

Protection Sheath

Lead sheath and Aluminum Copolymer are designed to be used in any applications where the cables may be subjected to solvent penetration or corrosive attack or hydrocarbons.

Lead Alloy Sheath

It is applied between two other sheaths and is the best protection against aggressive chemicals. This is an expensive solution, increases weight and bending radius. It presents poor vibration resistance and normally an armour is required to protect it from crushing.

ALPE (Aluminum Copolymer)

It is an alternative to Lead Sheath and is composed by a longitudinal overlapped aluminum copolymer coated tape bonded to HDPE jacket and additional special alloy of polyamide/polypropylene sheath.

  • Excellent protection against corrosion and humidity.
  • Excellent impact resistance that in some cases prevents the use of the armour

This protection has a lower weight compared to lead sheath, cables have a smaller diameter, with a reduction of costs.

ALPE is the right choice to protect the environment.

Armour

Metallic armour are used when cables have to be installed direct buried, or if mechanical protection is required.

Following points must be considered:

  • Required tensile load
  • Expected pressure on cable during service
  • Protection against rodent
  • Protection against accidental damage
  • Minimum required bending radius.

Material: metallic or non-magnetic Aluminum, Steel wire/strip.

  • The armour provides mechanical protection against crushing forces.
  • Armor also can serve as an Earth Continuity Conductor (ECC).
  • The armouring type could be:

Mechanical protection of the cable is provided by a single layer of wire / strip strands laid over the bedding. Steel wire or strip is used for 3-core or 4-core cables, but single-core cables have aluminum wire armouring.

  • When an electric current passes through a cable, it produces a magnetic field (the higher the voltage the bigger the field). The magnetic field will induce an electric current in steel armour (eddy currents), which can cause overheating in AC systems. The non-magnetic aluminum armour prevents this from happening.
  • Magnetic Material’s armouring for 3Ph System: With 3-core or 4-core cables the vector sum of the currents in the conductors is zero, and there is virtually no resultant magnetic flux. In multi-core, armoured cables have either single layer of Galvanized Steel Wire Armour or Galvanized steel strip applied over inner sheath with left hand lay.
  • Non-Magnetic Material’s armouring for 1Ph System: This is not so, however, for a single-core cable, where eddy-current heating would occur if a magnetic material was used for the armouring. The material has to be non-magnetic for armouring as in this case of return current is not passing through the same cable. Hence, it will not cancel the magnetic lines produced by current. These magnetic lines which are oscillating in case of AC systems will give rise to eddy currents in magnetic armoring and hence, armouring will become hot, and this may lead to failure of the cable. Hence, single core cables for use on AC systems are armoured with single layer of non-magnetic (aluminum) material.

Armouring is mostly of following type:

SWA: Single Galvanized Steel Wire Armour, with diameters according to relevant standards, coverage min. 90%. This armour assures a very good mechanical protection and tensile strength. An additional counterspiral tape increases solidity, if required. Steel wire armour, used in multi-core cables (magnetic).

AWA: Aluminum Wire Armour, with diameters according to relevant standards, coverage min. 90%. This armour assures a very good mechanical protection and tensile strength. An additional counterspiral tape increases solidity, if required. Aluminum wire armour, used in single-core cables (non-magnetic).

STA: Double Galvanized Steel Tape Armour, composed by two tapes with overlapped edge; thickness of each tape: 0.20 – 0.30 – 0.40 mm, according to cable diameter. It grants a coverage > 100%. Very good crush resistance, but fair tensile strength. Galvanized Steel Tape armour, used in multi-core cables (magnetic).

ATA: Double Aluminum Tape Armour, composed by two tapes with overlapped edge; thickness of each tape: 0.20 – 0.30 – 0.40 mm, according to cable diameter. It grants a coverage > 100%. Very good crush resistance, but fair tensile strength.

Aluminum Tape armour, used in single-core cables (non-magnetic).

Outer Sheath

  • It is the outer protection part of the cable against the surrounding environment.
  • Protected against water ingress, protection against termite, protection against UV and protection against differing soil compositions.
  • It is applied over armoring in case of armored cable and over inner sheath in case of unarmored cable called as ‘Outer Sheath.’
  • The standard sheath color is Black other colors such as Red, Light Blue can also be provided.
  • High-voltage cables are identified by outer sheaths colored red; a black sheath indicates a low-voltage cable

The following are the electrical property may be considered while selecting an outer Sheath Materials.

Many compounds can be used as internal/external protection of cables. Working condition need to be considered for the right choice.

PVC, PE and LSFOH are the most popular materials, but we have to consider that different grades are available to meet specific working conditions.

Anyway the following conditions have to be evaluated:

v Type of installation (indoor/outdoor, direct buried…)

  • Possible presence of humidity, oil, chemicals…
  • Behavior in case of a fire (fire propagation, fire resistance, emission of gases and smoke…)
  • Range of temperature
  • UV resistance in case of sun exposure

To optimize the behavior in case of fire, the sheath is made of LSFOH (Lows Smoke Zero Halogen) materials since, thanks to that, fire is not propagated, toxic or corrosive gases are not developed and a minimum quantity of white fumes are emitted.

Other materials can obviously be used in case of specific installation requirements, such as:

PVC, for example, where a higher resistance to oils and chemicals is required (but this material contains halogens, so it emits acid gas and smoke).

PE, when a higher resistance to water and moisture is required also resistant to environmental stress cracking (this material is no anti-flame, though).

In most HV and EHV cable applications, the metal sheath/neutral is itself protected by a polymeric over sheath. Due to the critical performance needed from the over sheath, there are a number of properties that are required, such as good abrasion resistance, good Processability, reasonable moisture-penetration properties, and good stress cracking resistance. Experience has shown that the material with the best composite performance is a PE-based over sheath Jacket materials include PVC, PE, LLDPE, MDPE, HDPE.

 

Applicable Standards

  • Basic Design: IEC 60840, IEC 62067
  • Conductor: IEC 60228
  • Flame Retardant: IEC 60332-1
  • Acid Gas Emission: IEC 60754-1
  • Smoke Density: IEC 61034-2
  • Halogen Free: IEC 60754-1 (LSFOH)