the Basic ways of the control of electric properties of isolation of cable lines

Cable lines at commissioning should pass tests, and while in service their condition is controlled in conformity
Acting standard deeds [27-29]. Following aspects of various checks and measurements enter Into these tests.

Insulanion resistance measurement. It is carried out at feeding into of cables in maintenance, after conducting of various repairs, and also while in service before and after cable test by the raised voltage.

These measurements are made by means of a megaohmmeter with an operating voltage, equal 2500 n is defined between current carrying veins and a metal outermost shell or between current carrying veins. Directly magnitude of an insulanion resistance of a cable is on a megaohmmeter scale through a period, equal 1 mines from the moment of voltage emersion on the leading-outs of a megaohmmeter affiliated to the tested cable. Conducting of such measurements demands stress relief from a working cable line that does the given way unsuitable for realisation of the constant control over a condition of isolation of cables.

Isolation tests by the raised detected voltage.

At conducting of these tests for cable lines the detected voltage raised concerning the worker moves. The raised detected voltage is put to each vein of a tested cable serially. During test conducting other veins and metal shells, for example, the reservation and cable shields should be earthed. It provides check of electric strength of isolation between current carrying veins and land, and also between phases. Duration of a finding of a cable under test voltage at approval tests after line installation makes 10 mines, and for a cable which is on stream, - 5 minutes

Conducting of tests by the raised detected voltage demands stress relief from the working cable line, besides the given way of the control demands presence high-voltage vyprjamitelnogo the device. Also these tests in itself can lead to decrease in quality of isolation and origination of the new relaxed places, as in cable isolation, and
Muffs because of that test voltage is considerable (from 4 to 6 times) exceeds a nominal linear stress of a tested line [30].

Therefore quite often there are situations when the cables successfully standing affecting of raised detected voltage, are damaged in a time soon after end of tests. It is necessary to note, that such tests confidently define only already enough deeply developed defects and the faults more often created by penetration of a moisture in isolation of cables and muffs for the various reasons [31, 32].

Also ineffectively such tests the defects originating because of a strain ageing of isolation, an event owing to long affecting on cable lines of various factors come to light. Besides, conducting of tests by the raised detected voltage of cable lines with isolation from the sewed polyethylene not only does not give the proved information on their condition, but also have thus negative affecting on their isolation [30].

Therefore for power cables with plastic isolation and, first of all, for cables with isolation from the sewed polyethylene, more expedient is the method of conducting of test of isolation by voltage SNCH [30].

The given test method is based on the application to a cable of test voltage of frequency of the downgraded level of 0,1 Hz and the is cosinusoidal-right-angled form (drawing 1.7).

At such form of voltage pulses of 0,1 Hz all process of change of polarity is carried out by frequency the same as and at an operating voltage of a commercial frequency of 50 Hz, having the sinusoidal form [33-35]. Or the mustered cable is loaded and tested like test by voltage by frequency of 50 Hz.

It is necessary to note, that tests by voltage of ultralow frequency allow to detect emersion of defects in isolation of cables without origination of the residual volume electric charges in a thickness of polyethylene isolation that distinguishes these test processes from

Analogous, connected with the application of constant voltage to the cable

Drawing 1.7 - the Form of voltage pulses at tests of cables on

To ultralow frequency

Similar accumulation of volume electric charges in various small neodnorodnostjah and ectogenic turnings on of firm polyethylene cable isolation at the application to it of the raised constant voltage is the reason of decrease in electric strength, and under the influence of adverse conditions can lead to an isolation puncture.

Tests with the application of voltage of ultralow frequency do not create harmful affectings on a condition of the insulating material. Therefore recently it is more and more wide, especially abroad, conduct tests of cables with isolation from the sewed polyethylene voltage of ultralow frequency [36, 37]. In Russia where cables with the bumazhno-impregnated isolation are most often applied to networks voltage to 35 kv inclusive, conducting of tests by voltage of ultralow frequency would lead largely to decrease of test voltage under the relation with tests by constant voltage [38, 39].

Advantages of a way of tests of isolation of cables voltage of ultralow frequency to the present time prove to be true various scientific researches and conducting of numerous tests in practical conditions. To the basic advantage of the given way it is necessary to refer to higher efficiency of tests of a cable simultaneously with lower level of the test voltage put to isolation which value does not exceed more than in 3 times magnitude of rated voltage of a cable.

This way of formation of test voltage in a combination to its value can lead to an electric puncture only in case of existence of the big defects in isolation of cables, first of all because of a high speed of development of the channel of a puncture, and does not create conditions for development of faults to less developed defects of isolation of cables at conducting of their tests. It is necessary to note, that standard deeds of [27-29] tests of cables acting now voltage of ultralow frequency do not provide.

Despite it, specifications on power cables released in our country with isolation from the sewed polyethylene order to make tests of cables after a lining and armature installation variable 3 Uo voltage of ultralow frequency of 0,1 Hz, Uoявляется value nominal voltage of a tested cable between a vein and the shield in a normal regime of maintenance [4o].

To raise reliability of an electric-power supply of users and simultaneously to lower number of the cables damaged at conducting of preventive tests of isolation by raised voltage, it is expedient to use not destroying test methods and diagnostics of cables under operating conditions. The given methods of diagnostic are based on periodic measurements of the most significant characteristics of isolation.

They allow to obtain adequate data about a condition of isolation of cables at present a time, not creating conditions for the sped up development of faults. Also these methods can be used for reception prognoznyh estimations of values of the residual service life of cables with long service life.

The similar information allows to make corrections to schedules of service of cable lines and to make the proved solutions on timely replacement of the cables having strongly developed defects or with practically produced resource of isolation [41-43].

Recently the considerable quantity of researches in the field of working out and practical application of effective methods of not destroying diagnostic of power cables under operating conditions is carried out. Considerable successes in these works are attained to Germany, the USA, Japan and in a number of other countries [44-46].

From the known developed methods have gained the greatest extending following methods of not destroying diagnostic of cables [47, 48]:

- Method of measurement of characteristics of partial categories in isolation;

- Method of measurement and the return voltage analysis;

- Method of measurement of a current of a relaxation in cables with isolation from the sewed polyethylene;

- Method of measurement of dielectric characteristics of isolation.

As shown above, initial origination of defects in isolation of cables under the influence of various factors, for example, electric fields, humidity, sharp oscillations of temperature, in an operating time creates inhomogeneity in the insulating materials that creates favorable conditions for emersion of the local electric discharges reducing electric strength of isolation, or partial categories. These categories can be detected on acoustical, optical, electromagnetic and to a thermal radiation. Some ways of monitoring procedure and diagnostic of defects of isolation of cables under characteristics of partial categories are known.

There are the ways based on use of characteristics of separate categories, such as spectral composition, the form, and also changes of the given parametres depending on character of defect [49-52]. Despite creation of several ways of allocation of the single pulses matching to partial categories from arrival signals, including application vejvlet -

Transformations, yet it was not possible to reach considerable progress in this area [53, 54].

The basic data on the given ways and their deficiencies are resulted in table 1.1 [43].

There is a contactless electromagnetic way of change of partial categories and definition of their characteristics [55, 56]. Use of the given way gives the chance to define such parametres of categories as intensity and frequency of repetition, distribution of quantity of pulses on magnitudes of intensity and other data, diagnostic variables of existing defects and prospective agency of defects on a condition of isolation of cables. But it is necessary to note, that on results of measurements of partial categories in the electromagnetic way having high sensitivity, various electromagnetic handicapes strongly influence.

Table 1.1 - Ways of monitoring procedure and diagnostic of defects

The insulation of cables using the characteristics of partial categories

The acoustical method of measurement of registration of partial categories and measurement of their characteristics is based on fixing of the elastic vibrations originating in the field of defect of the insulating material at the moment of an electric discharge. These oscillations have a frequency range from an infrasonic range (10-20 Hz) to an ultrasonic range (100-200 kgts). At the extending the originated elastic vibrations oscillate the acoustical radiation matching to an initial vibration spectrum in a circumambient. High-sensitivity microphones are applied to detection of acoustical radiation from partial categories. Directly contactless acoustical control can be made by passive or active way.

At an active way the acoustical pulse is created by the special equipment and sent on a tested cable. In case of defect presence in isolation the acoustical pulse is reflected from it and displaid by the radiation detector. From the accepted reflected signal after machining the information on an arrangement and defect parametres is taken. Such leading-outs are made on change of intensity of the accepted signal in comparison with initial, spectrum and phase forms [57, 58]. But now the active method has not gained the further development as not only differs complexity at scanning by an acoustical bunch of especially big insulators in sizes, but also possesses small sensitivity to detection of defects.

On a passive way registration of a pulsing audible signal from partial categories is made by microphones or pezopreobrazovateljami. In this case acoustical radiation from the category in a defect place is displaid by the directed acoustical antenna [59].

On the accepted radiation the rule of a place of the partial category on a surface or in material volume is defined. Passive acoustical methods of detection of such defects have found bolshee application, than active methods, especially with use active uzkonapravlennyh acoustical antennas [60­62].

The method of measurement and the return voltage analysis is based on research of processes of gymnastics of capacity of a cable by constant voltage of the downgraded value in comparison with an operating voltage. So for cables voltage 10 kv with the bumazhno-impregnated and polyvinylchloride isolation magnitude of the enclosed constant voltage makes 1 kv [41]. Such voltage does not create affecting on isolation of cables and muffs, but calls the polarisation processes in isolation.

At diagnostic conducting it agree to the given method the gymnastics of capacity of a tested cable by constant voltage magnitude 1 kv from the special constant-voltage source are made. Gymnastics process should proceed long enough (not less than 30 mines) to provide uniform gymnastics of all elements of capacity of a cable. Measurement of a strength of current of gymnastics of capacity on which the extent uvlazhnennosti insulation of a tested cable is defined is thus made.

Then the short-term discharge of capacity of a mustered cable through a special discharge resistor is carried out within several seconds. At this stage there is a category of the basic capacity of a cable, and a charge containing in capacity, created by the polarisation processes, isolation of a cable defining a condition, in the core remains because of considerable time constant of the category of this chain. Measurement of time responses of rebuilt (returnable) voltage in the course of a recharge of capacity of a cable is made after end of a short-term discharge of a tested cable.

When process of measurement of a return voltage comes to an end, spent the category of a tested cable through a discharge resistor before full disappearrance of a residual charge. Further all acts on isolation diagnostic repeat again, but already at gymnastics of capacity of a cable constant voltage 2 sq.

Upon termination of process the analysis of the gained time dependences of the return voltage characterising a condition and extent is made
Strain ageing of isolation of a mustered cable on which basis following characteristics are defined:

• the maximum value of a return voltage;

• a time of emersion of the maximum value of a return voltage;

• speed of a build-up of a return voltage;

• linearity factors on a relationship of the maximum magnitudes;

• speeds of increment of a return voltage at two magnitudes zarjadnogo voltage.

The more the found values of factors of linearity differ from analogous magnitudes, characteristic for isolation of a cable obviously deprived of defects, the above extent of a strain ageing of isolation of a mustered cable.

Generally under the information gained with use of such method, it is possible to size up following parametres of a tested cable:

- Condition and extent uvlazhnennosti insulation of mustered cables and multiple cable joints on values of a current of gymnastics of capacities and comparison of the gained results of measurement to available values for this line, gained at the previous diagnostics, and especially with results, characteristic for new isolation of cables;

- Intensity of processes of a strain ageing of isolation of a mustered cable (using such factors, as moistening, a thermal strain ageing of isolation) on certain values of the maximum magnitude, speed of a build-up of a return voltage and linearity factor;

- Condition and extent of a strain ageing of isolation following the results of comparison of results of measurements during diagnostic of a return voltage with earlier gained data on a mustered cable line and with the accessible results gained during analogous tests on new similar cables, and also the additional information on character of the detected defects in isolation of cables and the possible reasons of a puncture of a cable at the further maintenance.

The method of measurement of a current of a relaxation is used for diagnostic of cables with isolation from the sewed polyethylene [63]. The given method is based on measurement
Discharge current. Measurements are made after a preliminary charge of capacity of isolation of a cable by voltage 1 sq. Analysis of the gained results is made with application of schedules of a discharge current [34].

According to a multiple-unit equivalent circuit of cable isolation, the earlier on the discharge current schedule the peak will be observed, the the condition of controllable isolation and more its pollution and moistening is worse, hence, that isolation of controllable installation has a smaller resource. Thus it is necessary to consider, that value of amplitude of peak on the schedule is direct does not contain the big physical sense. The schedule should be studied as qualitative, comparing only a time of achievement of this peak. From two equal cables smaller safety factor of isolation at for what the peak crest value on the schedule is attained earlier. Also it is possible to compare isolation of phases of one cable.

The special equipment is applied to conducting of diagnostic by a method of measurement of a current of a relaxation, for example, universal combined system CDS [34]. This installation carries out diagnostic of cables with polyethylene isolation by a method of the analysis of an isothermal current of a relaxation, and diagnostic of cables with the bumazhno-impregnated isolation - a method of measurement and the return voltage analysis.

Isolation of a current carrying vein or phase of a cable concerning other current carrying veins and an outermost shell in a three-phase cable and one current carrying vein concerning an outermost shell in odnozhilnom a cable creates the capacity which isolation is sized up by dielectric losses in it, expressed in such parametre as a the loss tangent of a dielectric tg δ [62].

Absolute a value tg δ, measured at the impressed voltages close to the worker, and also its variation at change of test voltage and temperatures, characterises quality of initial insulating materials and a process of manufacture of tested cables. On available to results of measurement tgδизоляции cables under operating conditions at different put test voltage and frequencies (including
Ultralow) also it is possible to size up a condition and extent of a strain ageing of isolation of mustered cables.

It is necessary to consider, that magnitude tgδдает the average volume characteristic of a condition of controllable isolation as the active component of a current in it, created by dielectric losses in local defect, at measurement is compared with the general capacitor current of a controllable cable. Therefore measurement tg δ allows to detect authentically only the general or considerable deterioration of isolation. Local defects or the defects occupying rather small part of volume of insulation, and also the concentrated defects are detected by measurement tg δ hardly, and the more difficult, than it is more volume of isolation of a controllable cable.

Most the wide circulation was gained by the circuit design with a bridge of a Schering [62]. Various alternatives of circuit designs with the current comparator, with a wattmeter or the vectormeter are used also.

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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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