NEW BACKFILLING MATERIAL FOR UNDERGROUND POWER CABLE

Transcription

1 NEW BACKFILLING MATERIAL FOR UNDERGROUND POWER CABLE Abdul Hamid ELBAKLY Alexandia Electicity Distibution Company - Egypt Mail global net. com. eg ABSTRACT Cuent caying capacity [C.C.C.] of undegound powe cable is highly affected by the specific themal esistivity [S.T.R] of suounding mateial. It inceases with such mateial s [S.T.R.] deceasing and vice-vesa. [S.T.R.] of poous mateials which could be used as an envionment of powe cable depends on seveal factos like chemical composition, ganules suface aea and shape, ganule s content of metals, compaction, and moistue content. It is found [by measuing] small enough fo slightly compacted soft limestone, which could be easily used as cable s suounding in dy-not cold places, and gives high [C.C.C.] compaed with that attained by using natual sand in these places. Sand is vey good mateial fo this pupose when used in wet-cold places, but it is vey bad in dy - not cold places. INTRODUCTION Actually, undegound powe cables and it s suoundings, in addition to the envionment factos including soil popeties, must be teated as one unit. Any change in one of it's vaiables will cause elatively change in [C.C.C.] of the cable. Conducto mateial and it s coss section ae so impotant factos in [C.C.C.] detemination. But the effect of [S.T.R.] of cable suounding poous mateial is lage, specially that it s impovement expenses is elatively low. Lage coss section aea of such conducto will lead to less cuent density, while moe diminish in suounding mateial s [S.T.R.] will give moe incease in cable s [C.C.C.] without deceasing the cuent density. Fo XLPE insolated cable, coppe conductos, 1x 3x50 mm coss section, 12/20kv., at 50 cycle / second, cuent density is 4A/mm 2. Cuent density fo the same cable at the same conditions will be 1.1A/mm 2. if it s conducto s coss section enlaged to 1x3x630 mm 2.. Mateials used in cable manufactuing ae dea and of going exhausted souces by time, while those composing suoundings ae abundant. So, inceasing undegound powe cables [C.C.C.] must be acquied though choosing eliable suounding mateials at fist. We must not exaggeate in deeping cables, except in cases of which moe deeping is needed fo special conditions. That because moe depth means highe soil themal esistance ove the cable at the same [S.T.R.],and then less [C.C.C.], in addition to moe excavation cost. At hot seasons, tempeatue deceases with going down inside the gound skin at midday and will be fixed at a cetain depth.this depth depends on soil and climate factos. It is called damping depth. It is about 50 cm. in a site nea Caio. Table no [1] shows this damping depth. TABLE 1 - Daily tempeatue distibution in a site in Giza city Time Depth (cm) t.(h) At midday duing hot seasons, inceasing cable depth leads to :- * Fom gound suface to the tempeatue damping point depth: -[C.C.C.] inceases gadually due to soil tempeatue deceasing. -[C.C.C.] goes to decease gadually due to soil themal esistance inceasing which is popotional to cable depth and soil's [S.T.R.]. * Fom tempeatue damping point depth and going down : -[C.C.C.] inceases due to pemissible inceasing of moistue content. -[C.C.C.]goes to decease due to continued aising of soil themal esistance ove the cable by the eason of aising depth. That is when othe factos which effects on [C.C.C.] ae fixed. Then, [S.T.R.] of undegound powe cable s envionment plays vey impotant ole at detemining it's [C.C.C.]. AED_Elbakly_A1 Session 1 Pape No

2 FUNDAMENTALS Poous mateials ae composed of ganules, ai, and moistue. Ganules ae classified mechanically in goups, by it s diametes lengths. These goups ae mainly sand, silt, and clay. Sand is the poous mateial of which the ganules diametes between 2 and 0.02 mm.. Silt is fine. Clay is so fine [ ganules diametes ae less than 0.002mm.]. One cm 3 of sand contains ganules has total suface aeas of 45cm 2 in aveage. This total suface aea in the same volume of clay is about 8x10 6 cm 2.. So, clay ganules has vey good suface contact, which gives high chance fo heat tansfe by conduction. If this clay contains oganic mateials, chance of heat tansfe will be less by means of high [S.T.R.] of oganic mateials though it s ganules have vey lage total suface aea. When sand contains moistue, this moistue takes place of pat of ai between paticles, and then it's [S.T.R] goes to be less [S.T.R. of wate about 180 k 0 cm/w. compaed with about 4500 fo ai ]. Also, all poous mateials, it's [S.T.R.] goes to be less by containing moistue. Because the total suface aea in volume unit of sand is mall, the electostatic foces at the paticles sufaces and the wate sueface tension, by which paticles catchs moistue, will be vey small compaed with that in silt, loam, and clay. So, small amount of heat enegy will be enough to make sand dy, while in case of silt, loam, and clay, by the eason of lage paticles suface aeas, it is not easy to make it dy. It needs moe heat enegy to be so. In natue, soils ae not classified mechanically to sand, silt, loam, and clay only, but also to many othe foms like sandy silt, sandy clay, silty clay, clay loam, silty clay loam, sandy clay loam, silt loam, sandy loam, and loam sand. That is shown in fig. no. [1], fom which we can define the mechanical classification of any kind of soil by means of the pecentage of sand, silt, and clay composing it. Because clay is composed of so fine ganules and sand of bigge ones, silty clay, sandy clay, sandy silt and othe soils ae composed of ganules of diamete lengths between this and that. We expect that these soils have [S.T.R.] less than that of sand and moe than that of clay fom the mechanical classification view. In geneal, [S.T.R.] of poous mateial depends on it s chemical composition, mechanical classification, physical popeties, and it s contents of moistue and ai. Chemically, [S.T.R] will be lowe by inceasing the metallic content of soil compound, and vice - vesa. Oganic mateials have elatively high [S.T.R.]. Fo example, [S.T.R.] of wood is about 800k 0.cm/w.. Mechanically, [S.T.R.] will be lowe by inceasing the total suface aea of ganules in volume unit. This will be happened by suitable ganuling and good compaction. Physically, it will be lowe by means of it s ganules suface shape which give lage total ganules suface aea, and also the soil paticles stuctue. Fig 1 Mechanical classification of soils Refeing to these facts, [S.T.R] of natual sand in geneal is high in dy-not cold places, and deceases shaply by aising it's moistue content. It has vey low [S.T.R] when satuated of moistue. That is the eason of it s excellent themal popety to be used in places of continuous aining o aining at seasons of hot climate and peak electic loads. In the othe hand, [S.T.R] of dense ocks ae geneally small. But when it is cushed to make it suitable to be used as cable suounding, ai will take place of pat of it s volume, and then it s[ S.T.R.] will go to be highe. By good choosing of ganules diametes, adding moistue, and good compacting,it s [ S.T.R.] impoves. It will be still expensive. Limestone is found of the best popeties to be cable suounding mateial, in dy not cold places. LIMESTONE It is composed mainly of calcium cabonate, and is chemically stable. Thee is no mutual chemical eaction between it and alkalines o wate o polymes [ P.E.and P.V.C.]. Chemical eaction will be happened with acids,which gives poducts of neutal salts. In dy not cold places, soils ae alkalined, in which any chemical eaction with limestone will not happen. In wet- cold places soils ae acidic, which causes chemical eaction with limestone and poduces diffeent salts which has othe physical popeties. Fig.no [2] shows the natue of soils in diffeent places in the wold elative to tempeatue and wetness. Soils in dy-hot places ae in geneal alkalined,while soils in wet - cold places ae peat, which means that it is acidic [This point needs moe details which could not be included in this pape]. Calcium cabonates tansfes to calcium oxide if it s tempeatue aised to moe than 1000 c 0.. Max. allowed tempeatue of cable conducto is 250 c. 0 at shot cicuit. So, thee is no dange fom max. cable tempeatue at nomal opeation o shot cicuit upon chemical and physical popeties of limestone. AED_Elbakly_A1 Session 1 Pape No

3 DRY COLD FORMATION SALINIZATION ALKALIZATION WET COLD PEAT INDURATION PODZOLIZATION LATERIZATION PLINTHIZATION DRY WET HOT HOT FIG 2- natue of soils in places of diffeent climate Then, this mateial could be used in all places of dynot cold climate, and could not be used in wet - cold climate. FIG no [3] shows[by gay colou] the total aea in the wold in which limestone could be used,efeing to ain distioution duing seasons and tempeatue degees. At the nothen of the topics, all the places of ae o depived of ain duing summe ae in need to limestone, and also places of ae o depived of ain duing winte at southen of topics. That because the maximum tempeatue degee at nothen places is duing summe, while it is duing winte in southen places. The aea of which climate suitable fo using limestone in the wold is moe than half the total land aea. In all places inside this aea, limestone is an excellent cable tench backfille. It s (S.T.R.) measued at compaction of 1.58 gm./cm. 3. It was found between 24 and 32 k. 0 cm./w at dy state, and between 4, 8 at wet state. These values ae vey low. It is ( at wet state ) vey nea to that fo metals, which is 2.9 k. 0 cm. /w fo lead and 2.1 fo steel. Sandy silt, sandy clay, silty clay, and othe mixed soils in addition to clay, may contain an oganic mateials which makes (S.T.R.) high although it s own (S.T.R.) when it is fee of oganic mateials ae low. 1.58gm./cm. 3 is slight compaction fo soft limestone. It's compaction without outside pessue is in the level of 1.45 gm./cm. 3. Table no. [2] shows the [ S.T.R.] of mateials which could be used as a cable suounding. * We obseve that :- - Slag [ contains oganic mateials ] has vey high [S.T.R.]. - Dy sand has high[s.t.r ], which deceases shaply with moistue content. - Dense ocks have vey low [S.T.R.], but this value will be aised fo the cushed ocks. That while slightly compacted soft limestone [S.C.L.] has a vey low [S.T.R.]. Limestone is geneally abundant in so many places in the wold inside the total aea of dy not cold climate. TABLE 2-Specific themal esistivity of vaious types of soil Type of soil Slag fill, dy- -Nomal sand: humidity% 0 humidity%10 humidity %20 satuated with moistue -Eath, sandy: dy 8% humidity -Clay o loam -Eath, damp, o sand fill exposed to ain -Dense ock ( ganite, basalt, etc.) -Poous ock( Sandstone, soft limestone ) Specific themal esistivity [appox.] k o.cm. / w AED_Elbakly_A1 Session 1 Pape No FIG 3

4 CALCULATIONS In this section, we want to look fo the compaison between [C.C.C] of cables laid inside natual sand as a suounding mateial in dy - hot place, and it's [C.C.C.] when slightly compacted soft limestone [S.C.L.] used in the same place, when othe factos ae fixed. The following two fomulas ae used in case of single cable, low and medium voltages : - I Θ n R a c S t I = nr θ ac cuent caing capacity in ampee tempeatue ise of conducto in C 0 numbe of conductos a. c. esistance of each conducto at woking tempeatue, in ohm. cable's dielectic themal esistance plus soil themal esistance ove the cable in kº.cm./w 1 R = P1 Ln + p 2Π S h + Ln t h 2 2 S t 2 p 1 p 2 R h [S.T.R.] of the cable dielectic mateial in k 0. cm./w.. [S.T.R.] of the soil mateial in k 0. cm./w. cable adius in cm. conducto adius in cm.. cable depth, measued fom it's axis to the gound suface in cm. When θ, n, R ac, p 1, R,, and h ae fixed, and p 2 is vaied fom the value of dy natual sand at the fist calculation, to that of [S.C.L.] at the second calculation, cable s [C.C.C.] changed accodingly. It is found that [C.C.C.] aised at the second calculation by 115% of it s value which calculated at the fist time. This means that new [C.C.C.] of the cable became 215% of it s value which calculated when natual sand used. Using moe complicated fomulas, [C.C.C.] is calculated two times as illustated above fo 220 kv. ated voltage, cable, XLPE insulated, 12cm. oveall diamete, 1000mm. 2 conducto coss sec. aea, coppe conducto, laid in flat fomation of 30 cm. spacing at a depth 100 cm., when soil temp. was 35 c 0. Moe complicated fomulas ae used because in case of high voltage cables, dielectic losses must be taken into consideation. These calculations ae caied out at conditions of dy-hot climate as the natual sand ound the cable is dy by means of heat enegy losses of cable duing nomal opeation and at max. load, and also by the heat and dyness of climate. The esult of fist calculation [ when sand is used ] was 747A, while it was 1586A in second calculation [ when (S.C.L.) is used]. Table no [3] contains eduction factos of [C.C.C.] elated to seveal values of [S.T.R.]. Fom this table we find that in field of low and medium voltage up to 5.8/10k.v., using suounding mateial of [S.T.R.] 35k. 0 cm./w. instead of mateial of [S.T.R.] 310k o. cm./w, [C.C.C.] of the same cable aises by 114%. It is like the case of using [S.C.L.] instead of natual sand in dy not cold places. TABLE 3 - Reduction factos of [ C.C.C.] elated to[s.t.r.] of types of soils Specific themal esistivity Reduction facto k o.cm. / w [appox.] [ 0.6/ 1 to 5.8/10 kv. cables ] CONCLUSION In places of wet soil duing seasons of max. tempeatue and peak electic load, sand is vey good mateial when used as a cable suounding, eithe as an initial mateial in which tench is excavated, o as a backfille. That because of it's vey low [S.T.R.] at high moistue contents. othe poous mateials could be used if it's chemical, physical, and mechanical popeties ae suitable and [S.T.R.] ae low enough. In places of dy - not cold climate, sand is vey bad cable suounding mateial. That because of it s elatively high [S.T.R.] when moistue content is low, and because it s weak powe fo keeping moistue which is easy dissipated [in this case] by heat enegy lost fom cable conducto, spetial at times of max. electic load. In wet - cold places, soil is acidic. Chemical eaction between calcium cabonate and acids gives poducts of othe mateials which have diffeent physical popeties including [S.T.R.] fom that of calcium cabonate. At places of wet - cold climate, using limestone as a cable backfilling mateial is fobidden due to this eason. In places of dy not cold climate, soil is alkalined. Alkalines have not chemical eaction with calcium cabonate. Limestone is suitable backfilling mateial in these places fom the view of chemical popeties. Physically, [S.T.R.] of [S.C.L.] is between 24 and 32 k 0.cm./w.. It is the most suitable mateial which could be used as a cabl backfilling mateial in these places. It is vey good moistue absobe. Mechanically, it's ganules have not dange shap edges compaed with that of cushed dense ocks which is safe used in this pupose at compaction 2.1 gm / cm 3. [ cige ]. Moe compaction of soft limestone gives less [ S.T.R.] if equied. [ C.C.C.] of undegound powe cables inceases by lage pecentages when [ S.C.L.] is used instead of natual sand in places of dy not cold climate. It is about 215 % of the [C.C.C.] attained by using natual sand fo cables of voltages between 0.6/1 and 5.8/10kv., and slightly less pecentage fo exta high voltage [212% fo discussed case in this pape ]. Limestone is AED_Elbakly_A1 Session 1 Pape No

5 abundant and cheap in Egypt. Also in so many places in dy-not cold counties, and not abundant in wet-cold places!. Total expenses of backfilling item compaed with total expenses of the othe items of cables pojects is vey low eithe using sand o [S.C.L.], while it s effect on [C.C.C.] is vey lage. REFERENCES [1] A. Zaky & I. Megahed, 1972, " Electical Powe Engineeing " Al Maaif Establishment. Alexandia, Egypt, Fist edition. [2] O.Guthmann and othes, 1975, "Switchgea Manual " BROWN, BOVERI & CIE-AG, MANNHEIM. FRG, Fifth, evised edition. [3] Lotha Heinhold, 1979, " Powe Cables And Thei Application" Siemens Aktiengesellschaft, Repint. [4]Gwefel and othes,1987,"soil Science Fundamentals", Shenhaby Fo publishing cente, Alexandia, Egypt [5] Janik, 1985, " Hoticultual Science ", AL. DAR AL ARABIA LIL NASHR WA AL TAWZEIA, Nicosia, Cypus [6] V.F. Wolkenstein, 1971, " Poblems In Geneal Physics ", Mi Publishe, Moskow, USSR.. [7] A.M. Gad, 1999, " Civic And Empty Places Ways Engineeing ", Books Wold, Alexandia, Egypt. [8] Abdel Azise, and othes, 1995, " Chemisty, Fo Fist Stage Of The Geneal Seconday ", Minsty of taining and education, Egypt. [9] Sahan and othes, 1989, " Chemisty, Fo Fouth Class Of Seconday ", Minsty of taining, Kewit, Kewit [10] M.S. Abdel Hakim, " School Atlas ", Geogafical publishing cente, Scotland, United Kingdom. [11] A.J. de O.LIMA and othes, 2000, " Note Miguel Reale : A Moden Design Of A 345 kv.undegound Powe Line ", in Cige, Goup21, [12] R. Mahmoud and othes, 1985, Detemination Of Code Of Pactice Fo Waming Of Soil Below Feezing Rooms Suitable Fo Egyptian Soil at Diffeent Localities, Tech. Rep., Supeme Council Of Univesities, Caio, Egypt. [13] I. E.C., AED_Elbakly_A1 Session 1 Pape No