ion of dc pollution flashover performance of porcelain, glass and composite

IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 23, NO. 2, APRIL 2008 1183

Comparison of DC Pollution Flashover Performancesof Various Types of Porcelain, Glass, and

Composite InsulatorsXingliang Jiang, Jihe Yuan, Lichun Shu, Zhijin Zhang, Jianlin Hu, and Feng Mao

Abstract—Based on the artificial pollution tests, the flashoverperformance of various types of dc porcelain and glass suspensioninsulators as well as composite long-rod insulators were analyzedand compared. The test results show that there is a nearly linearrelation between the pollution flashover voltage and the disc-typeinsulator string length. The flashover voltage gradients of the in-sulators are affected by their materials and shed shapes. The an-tipollution performances of glass insulators are superior to thoseof porcelain insulators with the same profile. The flashover voltagegradients of composite insulators are higher than those of porcelainor glass ones. The exponent characterizing the influence of salt de-posit density on the pollution flashover voltage is dependent on theprofile and the material of insulators, and the values of the com-posite insulators’ exponents are smaller than those of porcelain orglass insulators, namely, the influence of the pollution on the com-posite insulators is relatively less. The effectiveness of leakage dis-tances of porcelain or glass insulators is less than 0.9 while that ofcomposite insulators is higher than 0.9.

Index Terms—DC insulator, effectiveness of leakage distance,material, pollution flashover performance, type.

I. INTRODUCTION

THE first 800-kV ultra-high-voltage (UHV) dc transmissionline, the Yun-Guang Line, was constructed in December

2006 in China. The line will be polluted with industrial con-taminants, coastal fog, natural dust, bird feces, etc. The serviceexperience shows that pollution flashover is one of the main nat-ural calamities harming the high-voltage (HV) and extremelyhigh voltage (EHV) transmission lines in China. One key issueof the design of the 800-kV UHV dc transmission line is to se-lect appropriate insulators for these conditions. The scientificexternal insulation selection is significant to techno-economicand secure operation, which can save construction cost and, atthe same time, reduce the risk of pollution flashover.

There has been much investigation on ac and dc pollutionflashover performances of various types of insulators. More con-taminants accumulate on dc insulators because of the static elec-tric field of dc voltage, which is 1.2–1.5 times higher than that onac insulators under the same atmospheric environment [1], [2].

Manuscript received February 26, 2007; revised May 30, 2007. This work wassupported in part by the National Natural Science Foundation of China underGrant 90210026 and in part by the China Southern Power Grid Co. Ltd. Paperno. TPWRD-00819-2006.

The authors are with the Key Laboratory of High Voltage and Elec-trical New Technology of Ministry of Education, College of ElectricalEngineering, Chongqing University, Chongqing 400030, China (e-mail:[email protected]; [email protected]; [email protected]; [email protected]; [email protected]; [email protected]).

Digital Object Identifier 10.1109/TPWRD.2007.908779

Furthermore, dc arc without zero crossing is steadier than ac arcand will develop for a longer time. The arc floating led by steadydc arc will bridge the sheds of insulators, which reduces thepollution flashover voltages of dc insulator strings and causeseffective leakage distances of insulators less than geometricalleakage distances. Under the same pollution and wetting con-ditions, the dc pollution flashover voltages are 20%–30% lowerthan those of ac, moreover, the dc pollution flashover voltageswill decrease more than ac voltages with an increase of the pol-lution degree [3]–[5].

Under the same environmental condition, the surface pollu-tion severity depends on the shapes and materials of insulators.The pollution severity on the top surface is very low and almostthe same for every type of porcelain or glass insulator cleanedby wind and rain. On the other hand, the pollution severity onthe bottom surface is rather different, depending on the insulatorprofile. For example, the accumulative pollutant on the bottomsurfaces is lower for outer-rib types whose sheds are designedfor relative smoothness and better aerodynamic performance,while the pollutant of the fog-type insulator is about 1.3–1.5times higher than that of the outer-rib-type insulator due to poorself-cleaning ability [6]. The pollutant showed a tendency ofbeing twice that than that of the porcelain insulator for com-posite insulators because of the adhesive nature of the housingmaterials [7]. The investigation indicated that the fog-type in-sulator may probably be used in coastal areas while the aero-form-type insulator is used in dry and larger winds and sandsareas [8].

The electric performance of polluted insulators is related tothose profiles and materials, and the flashover voltage is affectedby the shapes of insulators. The research in [9] show that theeffectiveness of the leakage length is generally a functionof the dimensions , and , and is mainly affected by theratio as well as the pollution level. For the insulators withthe typical profile, the effect of on could be ignored andthe empirical equation is expressed as the function

(1)

where is a constant depending on the pollution level.Presently, there is very little information on design and con-

struction, and there is no operational experience of the UHVdc transmission lines in the world. Therefore, there is almostno reference data for the design of the external insulation. Inthis paper, the dc pollution flashover performances of varioustypes of porcelain and glass disc (cap-and-pin) as well as SIRcomposite long-rod insulators are investigated and compared to

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1184 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 23, NO. 2, APRIL 2008

Fig. 1. Profiles of specimens.

TABLE IPARAMETERS OF PORCELAIN AND GLASS DISC INSULATORS

meet the demands of the insulation selection of 800-kV UHVdc transmission lines.

II. TEST SPECIMENS, SETUPS, AND PROCEDURES

A. Test Specimens

The specimens are the standard porcelain disc insulator, twotypes of dc porcelain disc insulators, two types of dc glass discinsulators, four types of short samples of FXBW-500/160 dcSIR composite long rod insulators with different profiles andone type of short sample of FXBZ- 800/400-dc SIR composite

long-rod insulator, which are denominated Type A-J, respec-tively. Their profiles and dimensions as well as some parametersare shown in Fig. 1 and Tables I and II.

B. Test Setups

The dc artificial pollution tests of various types of porcelain,glass, and composite insulators have been carried out in the mul-tifunction artificial climate chamber with a diameter of 7.8 mand a height of 11.6 m, in which the power supply is lead througha 330-kV wall bushing [10].

A dc high voltage of up to 600 kV is supplied by a cas-cade rectifying circuit controlled by the thyristor voltage–cur-rent feedback system, which ensures a dynamic voltage drop of


JIANG et al.: COMPARISON OF DC POLLUTION FLASHOVER PERFORMANCES 1185

TABLE IIPARAMETERS OF SHORT SAMPLES OF SIR COMPOSITE LONG-ROD INSULATORS

TABLE IIIFLASHOVER VOLTAGES OF FIVE TYPES OF PORCELAIN AND GLASS DISC INSULATORS STRINGS

less than 5% when the load current is 0.5 A. The test powersupply satisfies the requirements commended by IEEE Stan-dard-4-1995 [11] and IEC 61245–1993 [12].

C. Test Procedures

Referring to the test standards [12]–[14], the test proceduresin the paper were as follows.

Preparation: Before the tests, all specimens were carefullycleaned so that all of the traces of dirt and grease were re-moved and were dried naturally. The surfaces of the shortsamples of composite long-rod insulators were coated by avery thin layer of dry kieselguhr to destroy the hydropho-bicity which would be at the degree of WC4 or WC5.Since the layer of kieselguhr was very thin, the effect of



the kieselguhr on a nonsoluble deposit density could be ne-glected [13].Artificial polluting. In 1 h after the aforementioned prepa-ration, the solid-layer method was applied to the pollutionlayer on the specimens where sodium chloride and kiesel-guhr were electric and inert materials, respectively [12].Arrangement: All insulator strings are I-string. The testedcomposite insulators were fixed to two corona rings onthe ends and all specimens were hung up vertically in thechamber. The diameter of the corona rings is 24 cm usedfor the type F-I shorter composite insulators and 38 cm fora type J longer composite insulator. The minimum clear-ances between any part of the specimens and any earthedobject were larger than 3.5 m, which satisfied the require-ments in [12].Wetting. After 24-h natural drying, the pollution layer onthe insulators was wetted by the steam fog, and the in-putting intensity of fog was . The hy-drophobicity of the composite insulators would gain a cer-tain recovery to WC3 or WC4 after 24 h of natural drying,which would be different from the tests that the hydropho-bicity was still WC4 or WC5 after only 1 h of drying.The pollution layer on the insulators could be wetted com-pletely after 7–15-min time. In the whole wetting process,the temperature in the chamber was controlled lower than35 C.Flashover tests. After the pollution layer was wetted com-pletely, dc voltage was applied to the specimens and in-creased at a constant rate of 3 kV/s up to flashover.A positive polarity-applied voltage generally resulted inhigher flashover voltage than did negative polarity for disc-type insulators. But no substantial polarity differences havebeen experienced with long-rod insulators in tests [14]. Sothe negative dc voltages were introduced in this paper.There were 18 flashover tests for every porcelain or glassdisc insulator string and there were between 19 and 27flashover tests for the composite long-rod insulators atevery pollution severity. The 50% flashover voltage(in kilovolts) was the average value of all flashover volt-ages. The standard error (in percentage) was computedfrom the observed flashover voltages.

III. POLLUTION FLASHOVER PERFORMANCES

OF VARIOUS TYPES OF INSULATORS

Salt deposit density (SDD) (in mg/cm ) has been widely usedto characterize the outdoor pollution severity. A large number oftest results show that the relation between the pollution flashovervoltage and SDD can be expressed as the following [15],[16]:

(2)

where is a coefficient related to the shape and material of in-sulator; is an exponent characterizing the influence of SDDon and is related to the profile and material of insulators.The value of depends on the conditions of partial arc burning;thus, the environmental conditions, the test methods, the pollu-tion materials, and the materials of insulators will influence it.

Fig. 2. Flashover voltage gradients of dry arc distance� of a type A-E porce-lain and glass disc insulator versus SDD.

A. Flashover Performances of Porcelain and Glass DiscInsulators

Artificial pollution tests of five types of disc insulatorsstrings of five to 23 units have been carried out, and the 50%flashover voltages at various insulator string lengths andvarious SDDs are shown in Table III. The standard error ofevery flashover voltage is smaller than 9%.

From Table III, it can be concluded that there is a nearly linearrelation between and the insulator string length up to 23units. The flashover voltage gradients of dry arc distance(in kilovolts per meter) and the flashover voltage gradients ofleakage distance (in kilovolts per meter) can be gained byusing (3) and (4)

(3)

(4)

The relations between , and SDD are shown in Figs. 2and 3. Based on (2), the curves in Figs. 2 and 3 are expressed as(5) and (6).

Based on (5) and (6), Figs. 2 and 3, , , and the exponentare related to the profile and material of the insulator. For

example, the exponent of the standard porcelain insulator typeA is the lowest of five types of disc insulators

Type AType BType CType DType E

(5)

Type AType BType CType DType E.

(6)



Fig. 3. Flashover voltage gradients of leakage distance� of type A-E porce-lain and glass disc insulator versus SDD.

TABLE IVFLASHOVER VOLTAGES OF FIVE TYPES OF SHORT

SAMPLES OF COMPOSITE LONG-ROD INSULATORS

The type B porcelain insulator has the same profile as a typeD glass insulator, but its flashover voltage gradients , arelower than those of the type D insulator, and its exponent ishigher than that of the type D insulator. There is a similar rulefor the type C porcelain insulator and type E glass insulator.The rule shows the advantage of glass insulators over porcelaininsulators under the pollution condition.

B. Flashover Performances of Composite Long-Rod Insulators

The 50% pollution flashover voltages of five types ofcomposite long-rod insulators at various SDDs are shown inTable IV. All are less than 7.5%. Since there are experimentaluncertainties in the pollution test method, has certain dis-persion even though there were enough tests at every flashovervoltage.

The flashover voltage gradients of dry arc distance and theflashover voltage gradients of leakage distance of five typesof short samples of composite long-rod insulators are shown inFigs. 4 and 5. According to the curves in Figs. 4 and 5, the

Fig. 4. Flashover voltage gradients of dry arc distance � of a type F-J com-posite long-rod insulator versus SDD.

Fig. 5. Flashover voltage gradients of leakage distance � of a type F-J com-posite long-rod insulator versus SDD.

relations between their , , and SDD can be denoted asfollows:

Type FType GType HType IType J

(7)

Type FType GType HType IType J.

(8)



Fig. 6. Effectiveness of leakage distances of various types of insulators.

Based on (7) and (8), , , and the exponent are alsorelated to the profile and material of the insulator shed. Com-pared with other types of composite insulators, Type I has thehighest , , and the least ; therefore, the antipollution per-formance of type I is superior than the other types of insulators.Type F and type I have the same shed profile and different ma-terials, but they have different and ; thus, the materials ofthe sheds have an effect on the pollution performance of com-posite insulators.

C. Comparison of Flashover Performances of Porcelain,Glass, and Composite Insulators

Compared with the pollution flashover voltage gradients ofvarious types of insulators in Figs. 2–5, the pollution perfor-mances of the composite long-rod insulators are superior tothose of the porcelain or glass disc insulators. The values of thecomposite insulators’ exponents vary between 0.27 and 0.30,which are less than those of porcelain or glass insulators varyingbetween 0.30 and 0.36. Therefore, the influence of SDD on aporcelain or glass disc insulator is higher than that on a com-posite long-rod insulator.

The reason for better characteristics of the composite insula-tors is the hydrophobicity on the surface of its synthetical ma-terial and the hydrophobicity transfer [17], [18]. Based on thepollution flashover tests on various types of dc insulators, theflashover voltages of composite insulators are still 30% higherthan those of glass insulators even though the composite insu-lators have lost their hydrophobicity [2]. Therefore, compositelong-rod insulators have an advantage with porcelain or glassdisc insulators in the serious pollution area.

IV. EFFECTIVENESS OF LEAKAGE DISTANCES

OF VARIOUS TYPES OF INSULATORS

Based on GB/T 16434 (1996) [19], the effectiveness ofleakage distances—the effective utilizable coefficient ofleakage distances—is determined by comparing of the

nonstandard insulator with that of the standard insulator underthe same SDD, as follows:

(9)

where is the effectiveness of leakage distances of insulators;(in kilovolts per meter) is the flashover voltage gradient of

the leakage distance of the standard insulator.The effectiveness of the leakage distances not only shows

the utilization ratio of the leakage distance, but also shows theprofile and material characteristics of various types of insula-tors. Choosing type A as the standard insulator, of other typesof insulators under various SDDs can be calculated based on (6)and (8) and are expressed as (10), which can be plotted visuallyin Fig. 6

Type BType CType DType EType FType GType HType IType J.

(10)

Equation (10) and Fig. 6 show that the effectiveness of leakagedistance of a type B-E porcelain and glass disc insulator isless than 0.9 and is reduced with an increase of SDD, whichsuggests that the four types of porcelain and glass insulators donot fully utilize their leakage distances. The higher the pollu-tion level is, the lower this utilization factor becomes. On thecontrary, the composite long-rod insulators utilize their leakagedistances better; for example, of types F, H, and I are higherthan 1.0. Moreover, of types F, H, I, and J increase with an in-crease of SDD, which shows that the worse the pollution degreeis, the more obvious the advantage of the composite insulatorsis.

Based on the aforementioned comparison and analysis, thecomposite long-rod insulators have better antipollution perfor-mances than the porcelain or glass disc insulators do of the samedry arc distance in the more serious pollution area.

V. COMPARISON OF SELECTIONS OF INSULATORS

FOR 800-kV DC TRANSMISSION LINES

Based on the analysis of many flashover and withstand testsof polluted insulators, there is a standard error between the 50%flashover voltage and the 50% withstand voltage (inkilovolts) by the using up-and-down method, namely

(11)

where is 10% based on the test results.At an SDD of 0.05 mg/cm , of -kV composite

long rod insulator type J is 113.5 kV/m. Based on (11), the 50%withstand voltage gradient of dry arc distance (kV/m) oftype J is 102.2 kV/m, which agrees closely with the test result

kV/m) of EPRI [20].



The pollutant quantities on the top surface and the bottomsurface of the natural polluted insulators are different, so anonuniformity factor is introduced for porcelain insulators andexpressed as [21]

(12)

where is the nonuniformity factor; is the top-to-bottomratio of the pollutant on the insulator surface. is 1:5 at anSDD of 0.05 mg/cm . Due to the absence of the nonuniformityfactor for composite insulators, (12) for porcelain insulators isstill used in this paper.

The highest operating voltage is 1.02 times the rated voltage(in kilovolts) of the 800-kV dc transmission line, so the

basic dry arc distance is 9.0 m at an SDD of 0.05 mg/cm byusing (13)

(13)

where is 1.266 at an SDD of 0.05 mg/cm andis the standard error considered the security of the lines.

of porcelain disc insulator type C and glass disc insulatortype E are 92.3 and 94.9 kV/m, respectively. If the type C ortype E insulator is selected for 800-kV dc transmission lines,their basic dry arc distances are 11.1 and 10.8 m based on thesame method as before. Obviously, the basic dry arc distancesare longer than that of a type J composite long-rod insulator.

In the 1990s, composite insulators were widely used for500-kV ac and dc transmission lines in China [22]. The oper-ating experience shows that they have excellent antipollutionperformance. However, the aging and corrosion of compositeinsulators is the more severe problem under a dc electrical field.So the better profile and material of shed are also needed.

Based on the aforementioned comparison, it is recommendedto select composite long-rod insulators with the appropriate pro-file and material for the construction of 800-kV UHVdc trans-mission lines in this paper.

VI. CONCLUSION

Based on the dc artificial pollution tests of various types ofporcelain, glass, and composite insulators, the following con-clusions could be obtained.

1) There is a nearly linear relation between the dc pollu-tion flashover voltage and the disc-type insulator stringlength.

2) Compared with the porcelain disc insulators, the glassdisc insulators with the same profiles have better antipol-lution performances.

3) The pollution flashover performances of the compositelong-rod insulators are affected by their materials andshed shapes.

4) The pollution flashover gradients of the compositelong-rod insulators are superior to those of the porcelainor glass disc insulators. The effectiveness of leakagedistances of the porcelain and glass insulators is lessthan 0.9 while that of the composite insulators is higherthan 0.9. With the increase of salt deposit density, theeffectiveness of leakage distances of the compositeinsulators will increase and that of the porcelain andglass insulators will decrease.

ACKNOWLEDGMENT

The authors would like to thank B. Wang, Y.Du, L. Luo, R.Xue, Z. Wen, and Y. Zhang for their work on the test of thisdocument. They also thank H. Long for her work on the originalversion of this document.

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Xingliang Jiang was born in Hunan Province,China, on July 31, 1961. He received the M.Sc.and Ph.D. degrees from Chongqing University,Chongqing, China, in 1988 and 1997, respectively.

His employment experiences include theShaoyang Glass Plant, Shaoyang, Hunan Province;Wuhan High Voltage Research Institute, Wuhan,Hubei Province; and the College of ElectricalEngineering, Chongqing University. His researchinterests include high-voltage external insulation andtransmission-line icing and protection. He published

his first monograph: Transmission Line’s Icing and Protection in 2001.Dr. Jiang has published many papers about his professional work. He received

the Second-Class Reward for Science and Technology Advancement from theMinistry of Power in 1995, Beijing Government in 1998, Ministry of Educationin 1991 and 2001, respectively, the first-class Reward for Science and Tech-nology Advancement from the Ministry of Power in 2004; the Third-Class Re-ward for Science and Technology Advancement from the Ministry of Powerin 2005; the Second-Class Reward for Science and Technology Advancementfrom the Ministry of Technology in 2005; the First-Class Reward for Scienceand Technology Advancement from the Ministry of Education in 2007; and theFirst-Class Reward for Science and Technology Advancement from ChongqingCity in 2007.

Jihe Yuan was born in Hebei Province, China, onJanuary 30, 1977. He received the B.Sc. and M.Sc.degrees from Chongqing University, Chongqing,China, in 2000 and 2004, respectively, where he iscurrently pursuing the Ph.D. degree in the Collegeof Electric Engineering, Chongqing University.

He was an Associate Engineer with HandanPower Supply, Hebei Electric Power Company,Hebei Province, China, from 2000 to 2001. Hisresearch interests include high-voltage technology,and external insulation and transmission-line icing.

Lichun Shu was born in Chongqing, China, in Feb-ruary 1964. He received the B.Sc., M.Sc., and Ph.D.degrees in engineering from Chongqing University,Chongqing, in 1985, 1988, and 2002, respectively.

In 1988, he was an Assistant Professor atChongqing University where he became a Lecturerand Associate Professor from 1992 to 1999. He thenbecame a Professor in 2000. In 2001–2002, he wasa Visiting Professor with the Research Group onAtmospheric Environment Engineering (GRIEA) ofthe Université du Québec à Chicoutimi, Chicoutimi,

QC, Canada. He has worked mainly in the field of high-voltage externalinsulation. He is author and coauthor of several scientific publications.

Zhijin Zhang was born in Fujian Province, China, inJuly 1976. He received the B.Sc. and M.Sc. degreesfrom Chongqing University, Chongqing, China, in1999 and 2002, respectively, where he is currentlypursuing the Ph.D. degree.

He has been a Teacher in the College of ElectricalEngineering, Chongqing University, since 1999. Hismain research interests include high voltage, externalinsulation, numerical modeling, and simulation. He isthe author or coauthor of several technical papers.

Jianlin Hu was born in Hubei Province, China,in January 1978. He received the B.Sc. and M.Sc.degrees from Chongqing University, Chongqing,China, in 2001 and 2003, respectively, where he iscurrently pursuing the Ph.D. degree in the Collegeof Electrical Engineering, Chongqing University.

He has been a Teacher in the College of ElectricalEngineering, Chongqing University, since 2003. Hismain research interests include high-voltage externalinsulation

Feng Mao was born in Jiangsu Province, China,in October 1981. He graduated from ChongqingUniversity, Chongqing, China, where he receivedthe B.Sc. and M.Sc. degrees in 2004 and 2007,respectively.

His main research interests include high-voltagetechnology and external insulation.


ion of dc pollution flashover performance of porcelain, glass and composite

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