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Vladimirski L.L., Solomonik Е.А. (HVDC Electric Power Transmission Research Institute, Russia, Saint-Petersburg), Yarmarkin M.K, (Petersburg Power Engineering Institute for upgrading power industry personnel, Russia, Saint-Petersburg) Flashover Characteristic of Polluted and Wetted Polymeric Insulators. 1. Introduction According to Russian standards for insulation selection [1], polymeric insulators (PI) selection must be executed in compliance with their flashover characteristic under artificial pollution and wetting. Standards for insulation selection (testing voltage U50%,и specific surface conductivity χи) for the regions with the different pollution severity (СЗ) using document [1] and document IEC 60815 are presented in our report 36-WG11/Gdansk/204/[2]. According to these standards all new PI (line insulators, support insulators, bushing, the apparatus covers and so on ) which are designed in Russia, pass acceptance tests aimed to determine the maximum pollution severity level (PL) which doesn’t prevent them from using. Most of newly designed insulators pass these tests successfully and without unnecessary margin of electric strength, however in some cases originally planned LP for insulators being tested needed to be reduced. As the creepage distance isn’t standardized formally in the document [1], so corresponding demands are being written down into technical requirements for insulators and as a rule they are established to be not less than standardized for porcelain and glass insulators for the same PL. Yet lately some manufactures of PI referring to high flashover voltages which they got for polluted and wet conditions, started to produce PI which creepage distance is notably shorter then creepage distance of traditional insulators, which are used in the same conditions. Field experience of PI with shorter creepage distance in comparison with standardized creepage distance is of undoubted interest, however some data which we have show that PI with shorter creepage distance than for traditional porcelain and glass insulators have more failures. Because of devising the document IEC 60815-3 on the ground of work carried out by HVDC Electric Power Transmission Research Institute (NIIPT) and taking into account the fact that flashover characteristics of PI in polluted and wetted conditions determine their dimensions and finally determine their durability in the electric failure aspect, further in this report two main points are briefly reviewed: • flashover characteristics of PI (separately for line and post insulators) in conditions of pollution and wetting of their surface; • some peculiarities of tested PI in conditions of their artificial pollution and wetting.
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2. Flashover characteristics of PI under pollution and wetting; 2.1 Flashover characteristics of PI under the artificial and natural pollution Under the pollution and wetting the normal long-duration condition determines the dimensions of their electrical installations insulation. It means the impact of the long-duration operating voltage in conjunction with pollution and wetting of an insulator’s surface. The standardized in Russia requirements for electric strength for the strings of suspended cap and pin insulators ([1]) and for the line rod polymeric insulators under pollution (National Standard [3]) coincide completely. The accuracy of this approach is confirmed by the field exploitation of insulation made of traditional and polymeric insulators on high voltage lines. Flashover voltage under the artificial wetting and pollution of new line PI is much higher than flashover voltage of cap and pin glass insulators and porcelain insulators with the same pollution severity level. It is caused by the successful design of line PI (small diameter of the core) and it was reviled in NIIPT during the comparative testing in conditions of artificial pollution of a big number of different kinds of line PI 35-330kV. During the exploitation, the PI are under the climatic, electric and mechanical influence, which may cause their rapid ageing or may lead to the fatal damages, electrical breakdown. This may cause failure of entirety when cracks and exfoliation appear, it may also cause the appearance of conductor traces, tracks and erosion of the surface, the decreasing of hydrophobic behavior etc. First of all an electrical influence on insulators is determined by the operating voltage protractedly applied to insulator and besides by the overvoltages (switching and lightning) that appear during the exploitation. The ageing of the polymeric housing is determined by the big number of the influential factors, and the life duration of the covering material - by the speed of the process of ageing under the combined influence of these factors. The ageing (the process of fatal changing of the properties of the material of the coverage with time) is determined by the main following types of destruction: thermal-oxidative, hydrolytic and by the destruction caused by the solar radiation. The influence of cold, heat, moisture, solar radiation, voltage and other outdoor factors on PI causes changes in electrical, mechanical and other properties of insulation materials. Usually, the intensity of the influence increases, if several factors influence the materials simultaneously. Taking into account the process of unavoidable ageing of PI housing, it is important to determine the parameters of PI and cap and pin glass and porcelain insulators and to compare it one to another. The main of this parameters is the flashover voltage under the artificial pollution of the same pollution severity level. On the base of the information about the pollution intensity and flashover characteristics of PI, that were placed on the test benches or on the high voltage lines under the operation, figure 1 shows the dependence of the flashover voltage of PI LK 70/110-3 from the specific surface conductance of the pollution layer, that formed in the natural conditions. It is necessary to note, that the insulators of this type were polluted in different natural-climatic zones, which differed in
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conditions of wetting and polluting. Testing were carried out with applying of different test devices and measurement methods. It is the reason of a big variety of experimental data.
Figure 1. The dependence of the flashover voltage of PI LK 70/110-3 from the specific surface conductance under of natural and natural pollution in Kirgizstan, Tadgikistan, Turkmenistan, Uzbekistan, Ukraine, Estonia, Russia The comparison of the flashover strength (the specific flashover voltage through the insulation height Eh) of PI LK 110/70-3 under the artificial and natural pollution is shown on figure 2. It is shown, that flashover voltages under the artificial and natural pollution in the areas of 0.5-2 µSm are practically the same, and with further increasing of specific surface conductance (æ), insulators with natural pollution layer have lower value of Eh.
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Figure 2 Comparison of the flashover strength (specific flashover voltage through insulation height Eh) of PI of the LK-model under the artificial and natural pollution). 1- artificial pollution 2- natural pollution
Figure 3 shows the correlation of parameters Eh of the LK insulators and cap and pin PS-70-model insulators under of artificial and natural pollution. There is shown, that the advantage of rod PI under the natural pollution is significantly less than the advantage of cap and pin glass insulators under the artificial pollution.
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Figure 3. Correlation of parameters Eh of the LK insulators (LK 70/110) and cap and pin insulators (PS-70) under the artificial and natural pollution. 1- artificial pollution 2- natural pollution. 3The future accumulation of the information concerning flashover voltages of PI under natural pollution will allow to take into account the real flashover voltage and the pollution severity level of PI. For the traditional glass and porcelain insulators this approach to the selection insulation levels is carried out and used into practice, when the maps of pollution severity levels according to the results of the researches are being compiled. 2.2 Flashover characteristics of PI under the artificial pollution. Flashover characteristics of post PI under the polluting and wetting were studied in NIIPT to a smaller extent in comparison to the flashover characteristics of line insulators. The main requirements for PI in Russia are standardized in the document [4]. The insulation level of the outdoor substation equipment must be coordinated with the insulation level of suspended line insulation, because the reliability of the outdoor substation insulation is the most important element of the electrical network system. This requirement is contained in the main document in force on the selection of the insulation of the electrical installation [1]. According to this document, the requirements for specific effective creepage distance of the high voltage lines insulators and the requirements for the outdoor substation insulation, considaring in the regions with the same pollution severity level are
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accepted the same. So the values of æн and U50%н for the post PI which are standardized in the documents [1, 4] do not differ from the requirements for line insulators. NIIPT determined flashover characteristics of many types of post PI and polymeric tire for the voltage as high as 110 kV under the artificial pollution. It was done in accordance with test procedure of post PI and the requirements of the National Standard [4]. The diameter of the core of PI was 5.5-16 sm, the creepage distance was 197-248 sm. The diameter of the core of polymeric tires was 32.5 sm, the creepage distance was 293 sm. Support insulators and porcelain tires with well known operating characteristics and application area were used to compare them with PI. The simultaneous testing of PI and porcelain insulators with the same configuration and dimensions allowed to check the accuracy of the standardized criteria (æн and U50%н ) for the choosing the insulators and allowed to check the repeatability of the accepted test procedure. To evaluate the area of the usage (according to pollution severity level) of the polymeric and porcelain insulators, comparison of their flashover voltages was carried out. For the comparison were used the porcelain rod insulators for 110kV with the height about of 91sm. The results of comparison tests lead to the followed conclusion: • flashover voltages of the post PI having the same length of insulation part on average is 60% bigger than these ones of the porcelain post insulators and 35% bigger than the voltages of the porcelain antipollution-type insulators. Several facts contributed to it: • the diameter of PI and their surface are smaller than these ones of the porcelain insulators. • the ratio of the coefficient of form to insulation height of insulator for PI ( kf/Hins =7,5-10 м-1) are two times bigger than the same ratio for porcelain insulators (kf/Hins =4-4.5 м-1) • partial restoration of hydrophobicity for insulators with silicon covering due to desorption of the low-molecular components to the surface, covered with a layer of pollution. The comparison of flashover voltages of polymeric and porcelain tires (which haven’t such big structural differs as polymeric and polymeric post insulators) shows, that polymeric tires under the artificial pollution are not much better than porcelain tires. 1. Method of artificial polluting and wetting of PI. Insulators testing with alternating voltage in the conditions of artificial polluting and wetting must be carried out according to the documents [3-5] using the method of preliminary polluting and preliminary wetting. Application of voltage must be produced by the method of the long application (LA) of voltage and wetting (the preferable method), or by the method of the impact application IA) of the testing voltage to the preliminary wetted insulator. To determine the value of 50% flashover voltage with the given standardized value of the testing specific surface conductivity, it is recommended to get the dependence of 50% flashover voltage
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from the specific surface conductivity in the area of its given standardized value. In NIIPT this dependence during the tests is determined necessarily. The tests of insulators which are to work in the regions with 3-4 pollution severity level can be carried out without the imitation of HV conductors and earthed elements. The insulators are considered to pass the test, if the value of the received 50% flashover voltage with the given value of the testing surface conductivity is not less than the standardized one and also no considerable or critical electrical damages of the insulators occurred. In Russia the followed composition as a polluting substance is to be used: 250 g of ceramic substance (neutral); 1000 g of flowing water, the necessary (for the necessary conductivity) amount of sodium chloride (NaCl) of industrial purity. Only ceramic mass, allowed to be used in the production of the porcelain insulators (ceramic electro technical stuff, sub-group 110-120, National Standard 20419), allowed to be used for this test too. To prepare water suspension of the polluting substance the water with specific electrical conductivity not higher than 500 µSm·sm-1 at the temperature of 20°C. If the conductivity of the flowing water is higher than 500 µSm·sm-1, ti is recommended to use demineralized water. To get the necessary pollution severity level, the electrical conductivity of the prepared suspension must be determined and then the insulator or its part must be polluted. The necessary value of electrical conductivity can be reached by the salt regulation in the suspension. The insulators which are to be tested must be carefully cleared from dirt and fat before the pollution layer will be applied. After the clearance the insulators must be washed with the stream of the tap water and then must be dried. If after applying the layer of pollution, spottiness takes place on the insulator, it is necessary to wash and clear its surface again. Than it is required to make one or several repeated pollutions, each of them must be washed away again. If after this procedure a homogenous layer will be got, the tests can be started. As a rule it is enough to repeat polluting and washing away 2-3 times to get the insulator surface which is ready for practical homogenous pollution. In case after the mentioned procedures you failed to get a homogenous layer of pollution, then it is required to rub the dry surface of the insulator with the powder made of ceramic substance. The process of applying the powder must be carried out by the way of rubbing of the insulator with a soft fabric carrying some powder. Then the insulator is polluted for testing as it will be explained below. The insulators prepared for polluting are to be covered with a layer of artificial pollution by the means of spraying of the water suspension of the polluting substance on the insulator surface. The direction of the nozzle of the pulverizer must be regulated in such a way to provide layer which is homogenous enough on the whole surface of the insulator. The required pollution density may be received by the means of the repeated layer-by-layer pollution application. An average pollution density evaluated in mg of dry substance per the square sm of surface for the insulators which are under the test simultaneously must be 3±0.6
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mg/sm2. The evenness of the pollution layer within the bounds of each insulator must be of such value that the surface density of the pollution layer in any place of the surface wouldn’t differ from the average value more than on ±25%. The surface pollution density (γ) is determined by the division of the mass of the polluting substance (mg) cleared away from the certain part of the tested insulator surface by the square of the cleared surface (sm2). The pollution severity level of the tested insulator is determined by the specific surface conductivity (æ) measured on the tested (application of the voltage by the means of LA way) or on the monitoring insulator application of the voltage, which is in the same conditions as the tested one. The surface conductivity of insulators must be determined according to the formula: G=I/U, where U- is the value of applied to insulator voltage, I- is the value of the leakage current of the tested insulator or of the monitoring insulators. The specific surface conductivity must be determined by the means of multiplying of the value of the surface conductivity of the pollution layer by the form factor determining according to the [5]. The specific surface conductivity of a pollution level must be brought in correspondence with the temperature 20°C according to the [5]. The insulators are considered to have the same pollution severity level if their specific surface conductivity is (0.85÷1.15)·æ, where æ- is a standardized value for testing. While using monitoring insulators (LA) the general number of measurements of æ must be not fewer then 10, at the same time not more then 2 separate measurements of æ are allowed to undertake on each monitoring insulator. The determination of the conductivity of the pollution layer must be undertaken on the monitoring insulators during their constant wetting process, up to the state of saturation, and must be repeated to determine the maximum measured value. Each measurement of conductivity of a pollution layer must be done by the means of applying the voltage of about 5kV (effective value) to the insulator on the meter of the creepage distance, and by the means of measuring of the currant, running through the wetted layer. At the same time the voltage must be applied with an impact, and the leakage current must be measured during the initial half-cycle, when the changed of its value and form (amplitude and harmonicity) do not occur. Using tested insulators (ПТД) their surface conductivity is determined by the means of applying of the testing voltage with an impact (at the moment of the saturation with the moisture of the pollution layer). This voltage is 0.8-1.2 of 50% flashover voltage and of the value of the leakage current during not longer than 0.1sec after the voltage applying (before the strong changes) of the amplitude and harmonicity). The leakage current must be measured according to the [5]. The tested insulator must be wetted with a help of steam generator which provides even allocation on the whole surface of the insulator. The tests must be carried out in the testing chamber filling with steam (fog) or on the outdoor testing site in the conditions of wetting with the upstream of the steam. In the latter case to limit the volume of air around the tested insulator and keeping stability of wetting, a tent made of polyethylene tape can be used.
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During the process of testing the initial temperature of the insulator mustn’t differ from the temperature of the environment more than ±5°C. The temperature of the environment during the tests must be limited with the temperature of 5-30°C. When you place the tested insulator on the open site the fog should be generated as steam by the means of heating water in the steam generator. It must be transmitted to the insulator with a low speed through the nozzles of a big diameter. The nozzles must be situated under the tested insulator at the level of the flour at distance not less than 1.5m from the tested object. Steam supply mustn’t be directed on the insulator, that means that the nozzles must be placed on some distance from the axis of the insulator and nozzles must situate evenly around it. The tested insulator must be wetted in such a way that the visible fog would surround it as even as possible. The stability of the wetting from test to test during the testing must be controlled by the pattern of change of the surface conductivity with time. During the period of practicing the testing method, a standard curve of changing of the conductivity of the pollution layer with time with constant wetting of the tested insulator with the given pollution severity level must be received. During the testing it is necessary to monitor the changes in the conductivity of the pollution layer with time and to compare with the standard curve. To provide the coincidence of the measured curve with the standard one it is required to regulate the consumption of the steam supply or the direction of the steam streams around the tested insulator. The generating of the fog around the tested insulator must be developed up to the end of each separate test with constant stable supply which can be estimated by the monitoring of the steam pressure. If you use IA way of testing then the insulator with a dry pollution layer which was prepared for testing as it was described above, must be contained into the testing chamber or on the test site not earlier than in 24 hours and not later than in 36 hours after the polluting. The same insulator, polluted by the method of preliminary polluting, is be tested (be wetted) only one time, excluding wetting while you are measuring the specific surface conductivity. For the next applying of the voltage (of the wetting) the other insulator with the same pollution severity level must be used. It is allowed to retest the same insulator without changing of the pollution layer if after the previous voltage applying (wetting) the damage of the pollution layer isn’t monitored and the specific surface conductivity measured before another scheduled voltage appliance differs not more than on 10% of the value measured before the first voltage appliance. The consumption of steam supply should be rather high and stable in order the conductivity of the pollution layer reaches its maximum value in 8-15 minutes after the beginning of generating the steam. The maximum value of the conductivity of the pollution layer measured before the testing should coincide with the given value of the specific surface conductivity.
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References 1. The regulations of arrangement of electric installations, 7th edition. Chapter 1.9 “The Rules of Insulation of Electric Installations”. Moscow, 2002. 2. 36-WG11/Gdansk/204, 07/03/05. Vladimirski L.L., Solomonik E.A. (NIIPT), Yarmarkin M.K. (PEIPK) About the standards for the insulators according to the discharge voltage. 3. GOST 28756-90. Line suspended rod polymeric insulators. General technical requirements. 4. GOST Р52 082.2003. Post polymeric insulators of the outdoor installations for the voltage of 6-220kV. General technical requirements. 5. GOST 10390-86. Electrical equipment for the voltage higher than 3kV. Methods of testing of outdoor polluted insulation.
