background

Technical FAQ

Technical FAQ

Difference between Earth and Unearth system of Cable:
Earth system:

An earthed system is the one in which the star point of a generator or transformer is solidly connected to the mother earth.

In three phases earthed system, phase to earth voltage is root 3 (1.732) times less than phase to phase voltage. Therefore, voltage stress on cable to armor is 1.732 times less than voltage stress between conductors to conductor.

 

Unearthed System:

In bigger installation e.g. generators of bigger capacities like 500MVA fault level are very high and in case of an earth fault, heavy current flows intothe fault and this lead to damage of the transformer or generator.

To reduce this fault current, star point of the system is connected through a resistance.

If an earth fault occurs on one phase, the voltage of the faultwith respect to earth appears across the resistance. Therefore, the voltage of the other two healthy phases with respect to earth rises by 1.7 times. If the insulation of these phases is not designed for these increased voltages, they may develop earth fault.

Since the phases are not grounded in this kind of installation, there are chances of getting shock while operating the system.

Hence the insulation level for cable for unearthed system has to be more than insulation level for earthed system. Also, an unearthed system is not suitable for smaller generators and transformers especially for houses

For instance, let us imagine an earthed star connected system. An electrical gadget is connected to phase L1. Short circuit occurs on Phase L2. The equipment on the two good phases is good, because the star point has been connected to earth. If the star point was not earthed, the other two phases will raise their voltage by an additional root 3 (1.732), so the connected Gadget will start humming and eventually burnt into flames.

Voltage grade (Uo/U) where Uo is phase to earth voltage and U is phase to phase voltageEarthed system has an insulation grade of kV/1.732x kV
For earthed system (Uo/U): 1.9/3.3 kV, 3.8/6.6 kV, 6.35/11 kV, 12.7/22 kV and 19/33 kV
Unearthed system has insulation grade kV/kV
3 phase 3 wires system generally comes with unearthed grade cable and 3 phase 4 wire systems can be used as earthed grade cables.

 

Contribution of semi conducting compound on conductor and core insulation:

The outer layer of conductor forms uneven surface at the meet points of each wires increases electrical stress points in the electric field developed by conductor. In order to neutralize this field at the crest point semiconducting compound is applied by extrusion, thus the concentration of stress or field strength is reduced leading to avoidance of local dielectric breakdown. It is to be noted that any imperfection in conductor surface will develop which will cause failure of cable over the time. Similarly, a thin thickness layer of semi conducting material in form of extrusion is also used on XLPE insulation which helps in distributing the electrical stress perpendicular to the axis ofcable core. This process is quite critical as any imperfection at any point on conductor layer (semi conducting material on conductor) and insulation layer (semi conducting layer on XLPE) will develop high stress at the point initiating failure with time. In short, it is called triple extrusion process under single cross head. As per BIS 7098 (P-2), this process is followed for cables for 3.3 KV(UE) grade and above.

In order to make semi conducting compound, a high-quality carbon black having smooth and fine particles are used. Now a day, two type of carbon black is being manufactured. First type is manufactured in furnace by burning the mineral oil, while the second type is being processed by controlled pyrolysis of acetylene to produce acetylene black.

 

Copper tape on the cables are used for following reasons:

Medium and high voltage power cables, in circuits over 3000 volts, usually have a shield layer of copper or aluminium tape or conducting polymer. Coefficient expansion of different inbuilt materials are in variance affecting its elongation characteristics and resulting in nonsymmetrical radial stress distribution with the insulation. In case unshielded insulated cable is within close vicinity of earthpotential or grounded object, the electrostatic field around the conductor likely to concentrate at the contact point occurring corona discharge eventually resulting to insulation failure or damage. This besides, leakage current and the capacitive current is likely to offer danger of electrical shock. Grounding this tape along with semiconducting layer diverts to the ground with adequate safety precaution while making termination with the end equipment. However, for single core cable in A.C circuit, aluminium armour serves this purpose and separate copper tape is usually not required. In brief, the purpose of insulation shield is to:

  • Obtain symmetrical radial stress distribution with the insulation.
  • The circulation of current induces by magnetic field from other cables in vicinity.
  • Eliminate tangential and longitudinal stresses on the surface of insulation.
  • Exclude from the dielectric field viz, fillers, binding tape (innersheath) etc, which are not forming part of insulation etc.
  • Reduction of electrical influences on cable surrounding in case of earth fault

 

Why Dry (Radiation) Curing:

Let us conclude "SIOPLUS" curing process is not Steam Curing process and is no way inferior to most acceptable curing process "DRY CURE" in this present days. In fact, Sioplas process requires very special care not only in compounding but also in storage of the compound particularly due to its susceptibility to rapid deterioration when exposed to atmospheric moisture. The Dry curing process is developed in cable industry with the conception of the word "Dry" in this context means that the curing process has been carried out in presence of high temperature and pressurised inert gas i.e. not in the presence of steam / water / ambient temperature. The heating zone of the curing tube is heated up to the maximum temperature by heating the tube with electric heaters fitted outside the tube. Temperature of the heating zone is set independently with the pressure. The cable is thus heated up by radiation heat to the optimum temperature which is maximum stable temperature for cross linked compound (XLPE).

The inert gas (Nitrogen) at a certain pressure is used as pressuring medium inside the tube and thereby reduces the chances of formation of voids inside the insulation.

The heating, pre-cooling and cooling zone of the continuous tube is formed into catenary shape generally called a Catenary Continuous Vulcaniser (CCV) and this tube is heated to the extruder. Thus, in this system of curing, the insulated core does not come into contact with steam / water.

In dry cure process, void and moisture are almost entire eliminated by utilising an inert gas instead of the saturated steam that is used for steam curing process. This means, the XLPE cable manufactured by Dry curing has better electrical characteristics.

XLPE cables produced in Dry Curing process in CCV line has the following advantages:

  • Homogeneous insulation
  • Free from micro voids and water trees
  • Higher AC breakdown strength
  • Higher impulse breakdown strength
  • Low temperature loss
  • Practically free from moisture content as the system is closed and DRY
  • It is a closed system for raw material up to the finished core stage hence highest level of purity can be maintained.
  • Highest level of degree of crosslinking can be achieved
  • Longer cable life

 

Why Sioplas?

Polyethylene with its excellent properties is one of the most important insulating materials used by the Cable Industry. However, similar to polyvinyl chloride. PE softens and deforms if cable temperatures rise too high. Cross-linking postpones this behaviour to higher temperatures by converting the material from a thermoplastic into a thermoset with no adverse effect on electrical properties. PE can be cross-linked by means of organic peroxide catalysts at elevated temperatures. For wire and cable applications this process requires use of rather elaborate and expensive continuous vulcanising equipment which must be operated at high pressure and temperatures. The sioplas cross-linking process which does not require the high pressure equipment is therefore attractive for several reasons: lower capital investment, higher operating speeds, simpler operating procedures and easier adaptability to cables having sector shapes. In addition, Sioplas vulcanizates exhibit significantly higher resistance to heat deformation than do peroxide XLPE compounds. Thus the Sioplas process holds promise as a true alternative to the conventional cross-linking methods.

 

What is Basic Chemistry?

In the Sioplas process PE is treated with vinylsilane compound under specified conditions and in the presence of additives, which lead to attachment, through the unsaturated vinyl group, of the silane to the PE molecules in a random fashion. The reaction is known as "grafting" to PE. Subsequent exposure of the grafted polymer, in the presence of a hydrolysis catalyst, to exactly proportional amounts of water causes the formation of silicon-oxygen-silicon bonds between the polymer molecules. These have the effect of forming a network a polymer chains and the product than shows the characteristics of a vulcanized material.

 

How Processing is done?

Compounds range utilizes the "Sioplas" system for cross linking polymers, which permits processing at high out put rates in normal thermoplastic extruders, without the limitations imposed by peroxide addition or irradiation techniques.

The silane cross linkable compounds are a two-component system consisting of a graft co-polymer and a catalyst master batch, which are premixed at the processing stage normally in the ratio of 95 parts to 5 parts master batch. Cross linking can take place under ambient conditions but is normally accelerated by the incorporation of the catalyst master batch and exposure of the finished article to elevated temperature and high humidity.

 

How Processing is done?
  • Low Voltage Power Cables
  • Medium Voltage Power Cables
  • Aerial Bunched Cables
  • High Temperature Pressure Pipe & Fittings
  • Heat Shrinkable Tubing's
  • Injection Mouldings

 

What is the structure of X-Linked Polyethylene by Sioplas process?

The cross linking of polyethylene by Irradiation, Peroxide and Silane is a now a well-proven and established process in the Cable Industry.

The reason Silane cross linked polyolefin's have better properties than those of irradiation or peroxides cross linked products are based on the structural differences. Because silane co-polymers are processed at lower temperature, the size and density of micro voids is smaller.

Although all cross-linking methods generate a three-dimensional network, the structure of this network depends on the cross-linking mechanism and technique. Both irradiation and peroxide cross linking form C-C cross links whereas is Silane cross linked polyethylene the four polymer chains are bond in each cross link point of the network by Si-O-Si bonds. The solxane cross-links may form three dimensional, tetrahedral, "bunch like" networks. Because of this structure, some properties of silane cross-linked are better than those of the materials with "plainer" C-C cross-links.

The better thermo mechanical properties and the low after cure deformation result from a dense, tetrahedral, network structure, in the moisture-cross linked silane co-polymers from the fact that the cross linking reaction occur on the shaped articles, in solid state.

 

What is the structure of X-Linked Polyethylene by Sioplas process?
  • Silane cross-linked has excellent mechanical and aging properties due to this polyfunctional network than peroxide cross-linked XLPE.
  • Silane cross-linked technology reduces the contamination level in the insulation. For silane grafting system, the amount of peroxide is about 20 times lower than in peroxide cross-linking. In silane co-polymers no peroxide traces are present in the product, so no void formation resulting from volatile decomposition product occur.
  • The silane cross linked XLPE is inherently more stable peroxide cross linked XLPE because Silane cross linked XLPE contains Si-O-Si linkage (bond energy 452 KJ/mol) whereas peroxide cross linked XLPE contains C-C links (bond energy 346 KJ/mol)
  • The single greatest problem in the use of XLPE is in insulation for MV/HV cables is the growth of "water tress" in the insulation, partial discharges in the voids, A C and impulse breakdown strength of the dielectric. For cables up to 33 KV, the max stress rarely exceeds 4 KV/mm and water trees are not generally a problem so that the remaining dielectric strength after "water trees" growth is more than the adequate. The residual moisture content in the insulation, number of voids per mm3 and size of voids under different conditions of manufacturing are shown below.