Analysis of Electric Stress in High Voltage Cables Containing Voids

Transcription

1 Analysis of Electic Stess in High Voltage Cables Containing s Pashant S. Patel M.E. Electical (Powe System), Electical Engg. Dept. Paul Institute of Engg.and Tech., Limda, Waghodia Vial S. Chaudhai Assistant Pofesso, Electical Engg. Dept. Paul Institute of Engg.and Tech., Limda, Waghodia Hien M. Patel Assistant Pofesso, Electical Engg. Dept. Faculty of Tech. &Engg.,The MSU of Baoda, Abstact High voltage cables ae used fo the tansmission and distibution pupose fo electic powe. In high voltage cables, the main eason fo the beakdown is patial dischage due to the voids inside the insulation. Electostatic stess vaies due to the pesence of void inside the insulation mateial. So, fo the analysis of high voltage cable insulation, analysis of stess distibution o electic field distibution with void effect is desiable. The stess distibution in the insulation containing a void is influenced by the size of the void, shape of the void, location of the void, dielectic pemittivity of insulation mateial and opeating voltage. This pape pesents the esults of all kind of these vaiations. Finite element analysis techniue is used in this wok. void so the pesence of void leads to insulation beakdown[5]. The electic field and stess inside the void is highe so the possibility of beakdown inceases. In this pape, electic stess within the insulation with void is calculated. This pape consides stess analysis with diffeent size of the void, shape of the void, location of the void, dielectic pemittivity of insulation mateial and opeating voltage. II. ELECTOSTATIC STESS IN A SINGLE COE CABLE It is known by the theoy that in a single coe cable of having adius of conducto and inne adius of insulation, the potential gadient g at a distance x fom the cente of the conducto within the dielectic mateial is Keywods, Maximum void stess(e max ), Coss linked polyethylene (XLPE) I. INTODUCTION In moden wold, wide ange of high voltage cables of impoved design and pefomance ae used. The age of the cable is consideed by the status of the insulation. The cable insulation has to be continuously exposed to the vaiety of stesses. One of the majo poblems, which shoten the life of the cables, is the pesence of void of ai o wate inside the insulation mateial of the cables [1]. Due to void, bubble o defect, patial dischage is caused that can be esulted in educed life of the cable. The voids may be fomed duing manufactuing, fabication, installation, and enegization o opeation pocess. The void can be fomed in the powe cables insulation like Pape oil insulation, coss linked polyethylene (XLPE) and Ethylene popylene ubbe (EP) [5]. Pape cables ae elatively esistant to patial dischage activity but XLPE cables ae not[2]. So, XLPE cables ae moe concen fo this effect. In the study of patial dischage activity, electostatic stess within the void should be calculated and analyzed [4]. The beakdown stength of the void is lowe than the insulation because of the lowe pemittivity of the g = Whee, ξ x is the electic field intensity, is the chage pe unit length, 2πεx = ξ x (1) ԑ is the pemittivity of the dielectic mateial. So, the potential of the conducto will be, V = ξ x dx = 2πεx dx = log 2πε e Since, = ξ 2πεx x (Fom e. (1)). So, ξ x = V xlog e Hee, x' is the only vaiable in the euation, the maximum stess in dielectic mateial occus at the minimum value of the adius (hee, x=) [6]. So, (2) (3) (4) IJETV3IS

2 ξ max = V log e III. METHODS FO CALCULATING ELECTIC STESS AND FIELD VALUES Electic fields o stess calculation euies the solution of Laplace s and Poisson s euations with the bounday conditions satisfied. The solution of these euations can be done by eithe analytical o numeical methods. In many cases, the situation is so complex that the analytical solutions ae difficult o impossible. The analytical method takes much time fo solving the euations. So, numeical methods ae commonly used fo engineeing applications. Some widely used numeical methods ae: Finite Diffeence Method (FDM), Finite Element Method (FEM), and Chage Simulation Method (CSM) [5]. Among these methods Finite Element Method is most suitable fo its some advantages. So, this method is used hee fo the calculation of the electic stess. A. Finite Element Method Fo almost all fields of engineeing, this method is useful. This method divides the whole egion into small finite elements and calculates fo each element and so whole egion is consideed. Finite element method is suitable fo estimating fields at highly cuved and thin electode sufaces with diffeent dielectic mateials. This method is moe useful fo unifom o weakly non-unifom fields and which can be epesented by two dimensional geometies. Fo two dimensional as well as thee dimensional poblems, Finite element method is easie to apply and euies less time fo computation. It is capable of woking with egula o iegula geometies[5]. This method is best suited fo the electo static poblems. Vaious types of compute softwae ae also available those ae using finite element method fo solving vaious types of poblems. So, fo the electic field and stess analysis Finite element method is consideed hee fo the analysis. Finite Element Methods Magnetics (FEMM) softwae is used hee fo the calculation. Finite Element Method Magnetics (FEMM) softwae is a finite element package. This softwae is used fo solving 2-dimensional plana and axisymmetic poblems in electostatics, cuent flow, heat flow and low feuency magnetics. It is a vey convenient pogam. The poblem is boken down in lage numbe of egions with finite elements of simple geomety. Hee tiangles ae consideed fo discetization. The potential values ae intepolated at the thee vetices of the tiangle and the solution is obtained fo each element. Automatic mesh geneation is povided in this softwae. (5) diamete is consideed between the conducto suface and the insulation oute suface. The cable configuation and its stess with mesh ceation ae as shown in the figue 2. Fom Fig.1and Fig.2, it can be seen that moe stess is concentated at the conducto suface due to the voltage applied to it. of ai of.6cm is consideed in the insulation as shown in the Fig.2. The stess distibution fom the conducto suface to the insulation oute suface is obtained as shown in the Fig.3 and 4. Fig. 1. Fig. 2. Cable without void XLPE cable with void IV. CABLE CONFIGUATION AND STESS DISTIBUTION The cable configuation in pesent wok consists of inne conducto of coss sectional aea of 8.55 cm 2 (1.65 cm adius) and oute insulation of thickness of 2.85cm is consideed so that the atio of conducto adius to the whole cable adius is The mateial of Coss Linked Polyethylene (XLPE) is consideed as the insulation mateial with elative pemittivity of 2.5. The cable configuation of a conducto at 66 KVand the insulation mateial kept at gound potential fo the analysis in this pape. The atificial void of ai (pemittivity of 1) of.6 cm Fig. 3. Stess distibution without void IJETV3IS

3 Stess (MV/m) Stess (MV/m) Fig. 4. Stess distibution with void If the void is not pesent in the insulation, the stess is deceasing fom conducto suface to the insulation oute suface as shown in Fig.3. The maximum stess (E max ) is obtained at closest to the conducto suface. When void is pesent in the insulation, the stess at the void location is inceased as shown in Fig.4.The maximum stess (E max ) acoss the void is 3.25 MV/m. Electic field at the void is highe due to the lowe pemittivity than the insulation mateial and so that insulation mateial has to withstand moe stess at the location of the void. Beakdown stength of void (ai) is lowe than the insulation mateial due to the lowe pemittivity of it. So, the beakdown stength of the insulation mateial is deceased due to the void inside it. When the maximum stess (E max ) at the void exceeds cetain level of it, the electon avalanche occus and it is the main component of the patial dischage event. Patial dischage can lead to beakdown of the insulation. The maximum void stess (E max ) values ae consideed fo the beakdown stength of the mateial. So that the analysis of stess values in cable insulation with void and without void is necessay fo patial dischage study and life of the cable. The maximum stess (E max ) inside the void changes accoding to the change in some paametes like diamete o size of the void and shape of the void defect. It also depends on the location of the void inside the insulation mateial [2] [3]. Due to the change in opeating voltage of the conducto, the maximum void stess is vaied and the elative pemittivity of the insulation mateial is also esponsible fo that. By these diffeent paametes the exact idea of the case of the maximum stess (E max ) due to the void can be obtained. All these cases ae analyzed and esults ae discussed hee in this wok. V. SIMULATION ESULTS AND DISCUSSION A. Effect of diffeent void sizes (Diamete) In nomal case of Fig.2 the void size (diamete) of.6cm is consideed and the stess value is plotted. The Coss Linked Polyethylene (XLPE) insulation mateial with elative pemittivity of 2.5 is taken into consideation. The maximum void stess (E max ) of 3.25 MV/m is obtained in that case. When the size (diamete) of the void is changed, thee will be a change in maximum void stess (E max ) values. This change is shown in the plot of Fig.5 Fig. 5. Effect of diffeent void sizes (Diamete) The esult indicates that the maximum void stess (E max ) is stonge when the diamete of the void inceases and so that the beakdown stength of the mateial will deceased. If the diamete of the void inceases, the thickness of the insulation deceases with espect to the size of the void. It means that by deceasing the thickness of insulation the void stess will be stonge and vice vesa. B. Effect of distance of void fom conducto suface The maximum void stess (E max ) with espect to the oientation of the void inside the insulation is shown in the Fig.6. In this case, the oientation of the void is consideed by the distance between the conducto suface and void cente. XLPE insulation mateial is consideed. In nomal case of Fig.2 the void cente is in the cente of the conducto suface and the insulation oute suface. Fig Stess Size of void Size of void (cm) Stess Location of void Location of void cente fom conducto (cm) Effect of distance of void fom conducto suface Stess Size of void Stess Location of void The plot gives the value of maximum void stess (E max ) by changing distance of the void fom the conducto suface. It can be seen that the maximum void stess (E max ) deceases with inceasing the distance of the void cente fom the conducto suface. stess is stonge with the void neae to the conducto suface and if the void is fa away fom the conducto suface the maximum void stess IJETV3IS

4 Stess (MV/m) Stess (MV/m) will be less [3]. So the void neae to the conducto suface is dangeous fo the beakdown of the cable insulation. C. Effect of shape of void In this case 2-D shapes of thee majo types of voids ae consideed fo the analysis. Spheical, Elliptical and Cylindical shapes of voids ae taken inside the insulation. The maximum void stess (E max ) value is obtained fo each shape and compaed with the othe shapes Fig. 7. Effect of shape of void The esult indicates that the cylindical void has highest maximum void stess (E max ) value than othe two shapes of void. Elliptical void has lowe maximum void stess value than othe two shapes. So they ae less dangeous compaed to othe two shapes. And the spheical void has the maximum void stess (E max ) lowe than the cylindical void and highe than the elliptical void shape. In these nomal shapes of voids the cylindical voids ae vey dangeous compaed to othe two types. D. Effect of pemittivity of insulation mateial of cable In nomal case Coss Linked Polyethylene (XLPE) with elative pemittivity of 2.5 is consideed as the insulation mateial. In that case the maximum void stess (E max ) of 3.25 MV/m was obtained. Now maximum void stess (E max ) values ae calculated with the change of elative pemittivity of the insulation mateial and compaed with each othe o maximum void stess values ae consideed with changing the mateial with diffeent pemittivity. Fig.8 shows the esult of this case Fig. 8. stess Shape of Spheical Elliptical Cylindical Stess elative Pemittivity elative Pemittivity Effect of pemittivity of insulation mateial of cable stess Shape of Stess elative Pemittivity Fom this plot, it can be seen that the maximum void stess (E max ) inceases with inceasing the elative pemittivity of the insulation mateial of the cable. The pemittivity of ai void is vey low so by inceasing the value of elative pemittivity of insulation mateial the diffeence between the pemittivity of void and mateial inceases. So, the elative pemittivity of void is deceased as compaed to the pemittivity of insulation mateial when inceasing the pemittivity of insulation mateial. So, the stess is highe at the lowe pemittivity. This esults in highe maximum void stess (E max ) inside the insulation. And if the pemittivity of void inceases the stess will be deceased. E. Effect of opeating voltage of conducto The opeating voltage of 66kV is consideed in nomal case. Now the values of maximum void stess (E max ) ae obtained and plotted by changing the opeating voltage of the conducto as shown in Fig Stess Conducto voltage Conducto Voltage (KV) Fig. 9. Effect of opeating voltage of conducto By the above figue it can be seen that by inceasing the opeating voltage of the conducto without changing othe paametes, the maximum void stess (E max ) inceases. So, the chance of beakdown of insulation inceases. Fom this esult it can be said that fo the same size of cable with the same size of void, highe opeating voltages esults in inceased maximum void stess. So that effective conducto size and insulation thickness should be consideed fo the change in highe opeating voltages. VI. CONCLUSION Stess Conducto voltage The Finite Element Method can be easily used to calculate the maximum stess inside the void. Fo the beakdown of cable insulation and patial dischage activity, the calculations of the maximum stess inside the voids ae the impotant. When the size (diamete) of the void is inceased, the incease in maximum void stess is obtained. The maximum void stess deceases as the distance of void fom the conducto suface inceases. The maximum void stess is highe in the cylindical type of void than spheical and elliptical shape. stess inceases while inceasing the elative pemittivity of the insulation mateial used. While inceasing the opeating voltage with same size of cable, the maximum void stess will incease. IJETV3IS

5 VII. EFEENCE 1. H. S. B. Elayyan and M. H. Abdeazza, Electic Field Computation in Wet Cable Insulation Using Finite Element Appoach, IEEE Tansactions on Dielectics and Electical Insulation, Vol. 12, No. 6; Decembe H. N. O, T.. Blackbun, B. T. Phung, H. Zhang and. H. Khawaja, Investigation of Electic Field Distibution in Powe Cables with s, IEEE, Kaa, Ö. Kalendeli and K. Madikyan, Effect of Dielectic Baies to the Electic Field Of od-plane Ai Gap. 4. Mohamed Alshaif, Caledonian Univesity - UK Glasgow, Pete A WALLACE, Caledonian Univesity - UK Glasgow, and Donald M HEPBUN, Caledonian Univesity - UK Glasgow, Patial Dischage esulting Fom Intenal Degadation In Undegound Mv Cables: Modeling And Analysis, IEEE Tansactions, William A Thue, Electical Powe Cable Engineeing, Macel Dekke, INC NEW YOK BASEL, 2nd Edition, C.L.Wadhwa, Electical Powe Systems, New Age Intenational Publishes, 6th Edition, Febuay 212. IJETV3IS