the Major factors influencing change of electric properties of isolation of cable lines
The materials applied to creation of isolation of cable lines, are vulnerable enough to the various factors affecting it during all term of work. One of the most important factors influencing difficult on the character on the electroinsulating materials, the temperature is.
When the temperature increases, the chemical reactions occurring between a material of isolation, its internal turnings on, a circumambient, a moisture, are sped up. It reduces electrophysical properties of isolation that can lead to a puncture of the insulating gap or mechanical destruction of isolation.The quantity of a moisture which contains in a paper-oil insulation of cable lines, depends on its thickness and temperature conditions. It also makes considerable impact on electric strength and speed of a strain ageing of isolation. Process of penetration of a moisture in a paper-oil insulation can be presented by means of the diffusion equation under Fick's second law [64]. We will characterise saturation of a paper-oil insulation by a moisture by means of time constant of balance of concentration.
Time constant of balance of concentration τ, ch refers to to a time interval for which the moisture content percent in internal and external layers of a paper-oil insulation remains invariable.
In drawing 2.1 schedules of dependence of equilibrum time constant from temperature [65] are presented.
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Drawing 2.1 - Nomograms τ = f (θ) for vegetative and mineral oils at d = 3 mm and Sv = 3,5 %
Where A1, A2, A3 - constant factors, their values experimentally define for each of aspects of oils; d - a thickness of a paper, m; θ - current temperature, °s; Sv - concentration of a moisture in percentage (%) from weight bumazhno - oil isolation.
According to schedules at increase in temperature of maintenance speed of saturation of a paper-oil insulation a moisture increases, that further becomes the reason of decrease in its dielectric properties and will lead to a puncture.
The international council about the big electric systems of a high voltage (fra. CIGRE) recommended the formula (2.2) to define service life tcn, year of polymeric (cellulose) isolation [66], [67]:
Where C∏t, C∏0 - degree of polymerisation of a material of isolation, according to it
Critical value which is attained at the moment of a time t, and initial value at t0; D - the constant depending on a chemical compound of polymeric isolation; Ea - a chemical reaction activation energy, the Dzh/GRAMME-MOLECULE; R - a universal gas constant, J / (a gramme-molecule - °); θ - temperature, °s.
Magnitude of the joint venture is defined by approximation CHendonga [68], units:
Where c2fal - concentration furfuraldegida (2FAL) in a cellulose mass unit.
The formula (2.1) is inferred on the basis of empirical equation Arreniusa which will be co-ordinated with calculations of the theory of activated complex about communication of constant speed of a chemical reaction to with temperature at which the given reaction [69] proceeds, year-1.
Where K - the constant factor independent of temperature; θp - temperature in a reactor, °C; n - an exponent of temperature preexhibitors; Е0 - potential barrier, the Dzh/GRAMME-MOLECULE.
As growth eksponentsialnoj functions in (2.4) occurs much more sweepingly at temperature growth, than increase predeksponentsialnogo mnozhitelja, the formula (2.4) for the restricted interval of temperatures:
Let's convert and prologarifmiruem expression (2.5). As a chemical reaction time tp, year in inverse proportion to speed of its leakage to: where A, B - constant factors which are defined from accelerated tests on nagrevostojkost on [70].
Let's observe specimens of the insulating material of class-rooms nagrevostojkosti: Y (90 °C), And (105 °C), E (120 °C) and B (130 °C). On fig. 2.2 schedule Arreniusa is resulted
On an instance of the insulating materials of the given class-rooms nagrevostojkosti.
Drawing 2.2 - Schedule Arreniusa for samples of isolation of class-rooms nagrevostojkosti Y, And, E, B with application vosmigradusnogo rules at standard service life of 25 years
Using expression (2.6) we will count actual service life of isolation through standard value Tnorm, year:
Let's take advantage of rule Montzingera for conducting of extrapolation of results of accelerated tests on nagrevostojkost: excess of standard temperature on everyone Δθ =8°C (so-called «vosmigradusnoe a rule») cuts thermal service life of isolation by half. The given rule can be written down by rule Vant-Goffa which is used in the standard [71]:
Where Δti - an elementary interval of a time, for which actual temperature θ (t)
Hour can be accepted invariable; the T-period of a time of research, hour.
On the basis of schedules Arreniusa for samples of isolation of various class-rooms nagrevostojkosti the most considerable impact on a technical condition of isolation has been shown, that by the factor, making, the temperature is.
Let's make the analysis of agency of currents basic and the higher frequencies on isolation heating, and also currents of an asymmetrical three-phase system of vectors on return sequence, using the methods, allowing to apply superposition of symmetric and sinusoidal linear circuit diagrammes of replacement. Such methods switch on expansion into a Fourier series [72] and a method of symmetric components [73]. We will consider complex loading in knots of an electric network as a source of higher harmonics and asymmetry of phase magnitudes. We will accept elements SES linear and symmetric on phases. Analysing KE according to [74] we will observe asymmetrical regimes on a fundamental frequency, on higher harmonics them we will not consider.
Taking into consideration the assumptions accepted above, we will define an instantaneous value of total power losses at a leakage of a current of j th factor for a section of a multiphase network of an alternating current under the law of Joule-disposition to laziness [75], Vt where m - number of phases; j - an instantaneous value of a current of j th factor of heating, And; Rnj - a phase pure resistance to a current of j th factor of heating, the Ohm.
Heating is the inertia process which can be characterised the time constant gained as a result of formulation of the equation of thermal balance. For a case of non-stationary heating of a homogeneous body [66]: where Sekv - an equivalent thermal capacity of the body, which heating is made Dzh/°s; Θς - the body total temperature, which heating is made °s; αcp - average value of factor of a convective heat exchange on a chilled surface, Vt / (m2 - °); Som - the square of a surface of cooling, м2.
The total temperature θ∑ switches on ambient temperature θ0и temperature excess Δθ between a heated up body and medium, °s:
Ambient temperature change - slow enough process, therefore for a heating interval we consider value of temperature to constants:
Let's write down the equation (2.10) as follows:
Where T - time constant of heating of a body, hour; ∆θ ycm - the installed value of excess of temperature between a heated up body and a circumambient, °s.
The full solution of the non-uniform differential equation (2.13):
Funktsijakotoraja apparently from (2.14) depends it will be read off
The known. Integration constant Sint (2.15) will be equal in expression to concrete value for the time moment t = 0.
2.2
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