Isolation of cable lines and its basic electric parametres

Cable lines serve for transfer and distribution of electric energy. As their basic element power cables which mechanically include serve: veins on which the current proceeds, isolation, shells and protective overlying stratas.

Also as a part of a design there can be shields, zero veins, veins of a protective earthing and fillers.

Necessary electric resistance of veins of a cable rather each other and the earthed shell or land is provided with isolation. Isolation of cables is divided on had directly on a current carrying vein (isolation of veins) and imposed between the isolated veins or zone isolation.

To the insulating shielding materials applied in manufacture of cables, rigid demands are made. They should have high mechanical strength to stand without faults numerous bends at disposing on drums and performance of works at a lining, to provide necessary durability as at origination of faults of the cable lines especially laid in ditches or blocks, there are long breaks in an electric-power supply, and also long enough and expensive works on a finding and elimination of faults are necessary.

The insulating materials also necessarily should have considerable electric strength that allows to reduce a thickness of applied isolation and other shells and to reinforce thus tap of warmth from current carrying veins in a circumambient. It provides increase is long an admissible current at the same cross-section, and also good flexibility of a cable, facilitating thereby conducting of installation works. Also recently all in bolshem quantity in manufacture of cables the materials providing are applied
pozharobezopasnost, suppression of ignition and ecological purity of isolation.

Now for cable isolation various elastic dielectrics are used: the rubber, a special paper, various synthetic films, polyvinylchloride plastic compound, the thermoplastic and sewed polyethylene, and also a paper-oil insulation [1].

The Paper-oil insulation of cables is carried out by tapes from a cable paper preliminary impregnated by a mix of oil and rosin. At manufacturing of cables with an operating voltage from 1 to 10 kv on each phase the isolation is put, in addition from above already isolated veins the general or zone isolation takes places.

The electric field structure in the cables containing zone isolation, has difficult character. This results from the fact that power lines of force in separate places of cross-sections of a cable are directed not on the square to layers of an isolating paper. Thereof inevitably there is a tangential component of electric field in a paper-oil insulation, therefore the most dangerous place from the point of view of origination of punctures in such isolation are mezhdufaznye gaps.

The basic deficiency of the paper impregnated isolation of cables is its big ability to absorb moisture, therefore cables with such isolation conclude in various shells for protection against moistening at storage, installation and maintenance. Besides, processes of manufacturing of such cables have low productivity and difficult character.

Use of various plastic in the capacity of insulation of power cables provides considerable decrease in complexity of their manufacture. Plastic cable isolation is imposed on current carrying veins by expression process on special pressah. It provides the big productivity, than drawing of a paper insulation by a method of a winding of veins of a cable by preliminary prepared tapes.

Also raises productivity absence of drying and paper insulation impregnating. Besides, application of plastic for
Isolation accomplishments are provided with weight reduction of cables with the same cross-section of current carrying veins, facilitates installation works when on lining routes there is a big difference between levels of passage of cables.

Functioning of isolation of cable materials is in many respects defined by its electric parametres, conductance, polarisation, dielectric losses, various aspects of a puncture of isolation and an electric strain ageing refer to to the main things from which [1].

Conductance of cable isolation is created thanks to existence in it of a small amount of free charges to which refer to elektrony and ions. Formation of free charges occurs under the influence of many reasons - the raised values of temperature, a finding in electric field free carrying agents of charges etc. Existing in isolation can form an electric current because of following major factors:

- The electric field affecting isolation;

- Gradient of temperatures in isolation cross-section;

- Joint action of electric field and gradient of temperatures.

Conductance of cable isolation is sized up specific volume and superficial provodimostjami or specific volume and a skin resistance [1]. Values of these magnitudes depend on the materials applied to manufacturing of isolation, working conditions and other factors.

Polarisation of cable isolation can be several aspects: electronic elastic, ionic elastic, dipolnaja and migratory. It is characterised by the electric moment of unit volume or polarisation P which is defined under the following formula [1]:

P = εo (ε - 1) E, (1.1)

Where ε0 - inductivity in vacuum; ε - relative inductivity of a material of which it is made insulation; E - stress level of electric field in which there is an isolation material.

Dielectric losses in cable isolation are created at the expense of sorbtion
Powers from electric field owing to processes of conductance, the polarisation occurring to small speed, ionisation of air occluded gases in a material of isolation [1]. The given losses are sized up by a dielectric loss angle δ and a the loss tangent of a dielectric tg δ. If to present cable isolation in the form of the joint of two equivalent elements - resistance and capacities value tg δ can be defined as:

tg δ = P/Q, (1.2)

Where P - the power of dielectric losses matching to equivalent resistance; Q - a reactive power matching to equivalent capacity.

Isolation test represents sharp increment of an electric current through it because of formation of the spending channel. This phenomenon originates for the several reasons, will be observed more low in more details.

Prominent features of cable isolation are:

- Difficult character of dependence of electric strength from stress level of a field at long affecting of voltage;

- Strong dependence of electric strength on mechanical, thermal and other factors, such as moistening, pollution, etc. as a result of which act processes of a strain ageing and isolation destruction develop;

- Practically for all aspects of cable isolation, including combined, origination of an electric puncture is irreversible.

These properties of cable isolation do not allow to define authentically its electric strength before commissioning by supply of adjustable test voltage till the puncture moment since after similar test the cable will appear unsuitable to the further work in electric networks. Therefore electric strength of isolation of cables at maintenance is sized up by measurements of indirect parametres [2]:

- Values of a dielectric loss angle tg δ;

- Intensity of the partial categories originating in neodnorodnostjah of isolation;

- Magnitudes of insulanion resistance Ru3.

On a leakage of processes in isolation the big agency renders stress level of electric field and character of its distribution [3]. With stress level of a field are connected both electric, and thermal processes because the electrical field energy partially dissipates in dielectric, it will be converted to heat and calls additional heating of isolation of a cable. This heating reduces electric strength, reinforces processes of a strain ageing and increases tg δ insulation [4-6].

trehzhilnye cables with neekranirovannymi the isolated veins and cables with veins, cross-section differing from a circle, have difficult electric field [7]. Often detailed calculations of electric field do not make, and define the maximum value of stress level of the given field. Most easier given problem dares for cables with round veins, the calculation circuit design is resulted in drawing 1.1.

Drawing 1.1 - the Circuit design of calculation of value of the maximum stress level of electric field in trehzhilnyh cables with round veins (1...3) the Maximum stress level of the electric field existing in isolation of such cables, it is possible to define approximately under the following formula [7]:

Where silt - mezhdufaznoe or a linear stress, kv; - a thickness zhilnoj
Insulation, mm; r0 - vein radius, mm.

The maximum value of stress level of a field in any point and it is possible to count, using the formula from the theory of electric field for parallel cylinders, cable sheath agency thus is not considered:

For values N=1,1... 2,0 results of calculations gained on (1.1), exceed analogous results of scalings on (1.2) no more, than on 4 %.

In trehzhilnyh the cables having sector current carrying veins, calculation of stress level of electric field in isolation carry out for three characteristic points: a, cи to (drawing 1.2) [8].

Drawing 1.2 - the Circuit design of a cable with sector veins

Stress level of a field in a point and pays off similarly as for odnozhilnogo a cable, instead of vein radius the radius of a rounding of a sector vein (radius on shaft JAsk) [7] it is used:

Where Δ - a thickness of isolation of a current carrying vein, mm; - a thickness of zone isolation of a cable, mm.

For calculation of stress level of electric field at an internal rib of a vein in a point to instead of vein radius the radius of a rounding of an internal or central rib of sector of a vein (rk) is used, and a thickness of isolation in this case accept equal to distance from a point to to a cable axis (on an axis of a cable the electric field potential is equal to null):

The approximate value of stress level of electric field cили between external ribs of two veins at voltage Uл can be found in a point by formula (1.4) for two parallel cylinders, using in it instead of r0радиус rc, and instead of R - the sum rc + Δ.

For the cables having separately ekranirovannye sector veins, stress level of electric field in a point and pays off by formula:

In points cи to Eatgk it is defined under the following formula:

Where r - radius of a rounding of sector in observed points cили to.

If the round vein of a cable not ekranirovana also is twisted from round prowhirtle plates (for the big cross-sections) in this case local maximum stress level of a field it is counted so:

Where - number of prowhirtle plates in an external layer of a current carrying vein.

By formula (1.9) authentic results when in an outdoor layer of a current carrying vein is available not less than 12 prowhirtle plates are gained.

With a paper-oil insulation it is possible to present cables consisting of separate layers of a paper, oil and air. Such layers can be observed at calculations of stress level of electric fields as consistently connected three among themselves the capacitor [9].

Plastic isolation of cables can have the diversified forms of turnings on [10]. In the presence of similar turnings on in cable isolation there is a uniformity infringement weeding, that creates considerable difficulties for performance of calculations of a field. Depending on inductivity of available turning on stress level of electric field will increase in turning on or by its peripheries.

At high inductivity of turning on concerning isolation power lines of force will concentrate in turning on, i.e. stress level of electric field in an observed place will increase. Hence, will increase also and electrical field energy density.

Thus, presence of various turnings on in thickness of isolation can create local increase in stress level of electric field that leads to increase in density of an electrical field energy. Such increment of stress level of electric field is equivalent to potential difference growth in point of connection.

If the electrical field energy density becomes equal to certain critical value partial destruction of structure of atoms of isolation will begin. This process will be accompanied by the big allocation of energy in various aspects:

- Electromagnetic radiation;

- Heat release increase;

- Pressure growth;

- Origination of mechanical affecting on turning on boundary lines.

If to present, that turning on in isolation is in the form of an ellipsoid of revolution with a spinning ax h, directed along a direction of homogeneous electric field in isolation Esr, in volume of such turning on Evk it is possible to consider that field homogeneous - drawing 1.3.

Drawing 1.3 - the Lay-out diagramme ellipsoidalnogo turnings on in cable isolation

Stress level of a field in turning on it is possible to count as follows [7]:

Where Esr - stress level of a homogeneous field in the absence of turning on; ε1и ε2 - accordingly inductivities of medium and turning on.


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A source: Kudelina Darya Vasilevna. the METHOD, MODELS And ALGORITHMS FOR the AUTOMATED CONTROL of the CONDITION of ISOLATION of CABLE LINES. The dissertation on competition of a scientific degree of a Cand.Tech.Sci. Kursk - 2017. 2017

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  2. the Major factors influencing change of electric properties of isolation of cable lines
  3. Mathematical models of change of electric properties of isolation of cable lines
  4. Physical processes in isolation at maintenance of cable lines
  5. the Basic electric parametres of isolation and methods of their control
  6. 4.1 Definition of a reheat temperature of isolation of cable lines
  7. Mathematical model of definition of a condition of isolation of cable lines
  8. 4.2 Estimation of a condition of isolation of cable lines on the basis of application of an illegible neural network
  9. Kudelina Darya Vasilevna. the METHOD, MODELS And ALGORITHMS FOR the AUTOMATED CONTROL of the CONDITION of ISOLATION of CABLE LINES. The dissertation on competition of a scientific degree of a Cand.Tech.Sci. Kursk - 2017, 2017
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