dc pollution

2132 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 25, NO. 4, OCTOBER 2010

Study on DC Pollution Flashover Performance ofVarious Types of Long String Insulators Under

Low Atmospheric Pressure ConditionsZhijin Zhang, Xinliang Jiang, Yafeng Chao, Ling Chen, Caixin Sun, and Jianlin Hu

Abstract—In this paper, the dc pollution flashover performanceof various types of porcelain, glass, and composite insulators isinvestigated. It also presents analysis of the dc flashover processof polluted insulator string at high altitude using insight fromhigh-speed photography. The research results indicate that therelationship between the dc pollution flashover voltage and thestring length of insulators is basically linear, the characteristicexponents describing the influence degree of air pressure on pollu-tion flashover voltage vary between 0.35 and 0.77 and are relatedto the insulator types and pollution degree, etc., the characteristicexponents describing the influence degree of pollution on flashovervoltage vary between 0.24 and 0.36 and are related to the insulatortypes and air pressure, etc. Based on the flashover phenomenausing the insight from high-speed photography, a new physicalmodel explaining the flashover mechanism for a polluted insulatorstring at high altitude is introduced, which can be expressed asan electrical circuit consisting of a surface arc of length � andair-gap arc of length � in series with a resistance representingthe wet pollution layer. In addition, the exponent describingthe influence degree of air pressure on flashover voltage for thepolluted insulator is discussed.

Index Terms—DC, external insulation, flashover performance,insulator string, low air pressure, pollution.

I. INTRODUCTION

E NVIRONMENTAL pollution can cause the outdoorinsulators to become progressively coated with dirt and

chemicals in the long term. In the presence of wet atmosphericconditions, the contamination particles on the insulator surfacewill dissolve into the water and provide a continuous con-ducting path between the high-voltage electrode and groundwhich makes a leakage current flow on the polluted layer of theinsulator. The formation of dry bands on the surface becauseof the influence of the leakage current will result in arcing. Thearcs may suddenly elongate across the wet surface, leading toa total flashover [1].

Manuscript received September 23, 2009; revised January 13, 2010. Firstpublished August 23, 2010; current version published September 22, 2010.This work was supported in part by the National Basic Research Program ofChina (973 Program) (No. 2009CB724503) and in part by the foundation ofChongqing University. Paper no. TPWRD-00714-2009.

The authors are with the State Key Laboratory of Power TransmissionEquipment and System Security and New Technology, Chongqing University,Chongqing 400044, China (e-mail: [email protected]; [email protected]; [email protected]; [email protected]; [email protected];[email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TPWRD.2010.2049132

To ensure the safe and reliable operation of dc transmissionlines, a great number of research on dc flashover performanceof polluted insulators has been conducted at home and abroad[1]–[34]. The artificial pollution tests of HVDC insulators, ini-tiated by CIGRE and completed with the cooperation of corre-sponding IEEE working groups, show that dc flashover voltageof the polluted insulators is about 20% lower than ac flashovervoltage under the same conditions [2]. References [3] and [4]also give similar conclusions. References [5][6]–[7] give thelong-term research results of operation performance of com-posite insulators under dc voltage, and the results show that thepeak leak current of composite insulators’ energized dc voltageis obviously lower than those of energized ac. The research onpollution accumulation conducted by Sweden ASEA Co. andAmerican BPA Co., etc. show that the amount of accumulatedpollution on insulator surface under dc voltage is 20% 50%heavier than that under ac voltage under the same condition [8].The research [9] on the polarity effect of composite insulatorconducted by French researchers shows that the difference of dcpollution flashover voltage between negative and positive po-larity is less than 3%, so the polarity effect can be neglected.Similar results are obtained by Chongqing University in Chinain [10]. Researchers have conducted dc artificial pollution testson different types of insulators and the results show that underthe condition of moderate pollution, the flashover voltage ofcomposite insulators is 30% higher than that of glass insulatorstring; the more serious the pollution is, the greater superiorityof composite insulators can be shown; the composite insulators’dc pollution flashover performance is better than its ac pollu-tion flashover performance [11], [12]. U.S. The Electric PowerResearch Institute (EPRI) conducted the dc pollution flashovertests on five types of composite insulators with different shapes(the length is about 3.5 m). The results show that the flashovergradient is 124 kV/m when the salt deposit density (SDD)is mg/cm , and the configuration of composite insulatorshas a great influence on dc pollution flashover voltage [9].

As is known to all, the effect of high altitude on transmission-line insulation has considerable influence on the hardware se-lection and operating conditions. Several investigations, mostlyexperimental [13]–[18], have already addressed the problem ofaltitude effects on pollution flashover of high-voltage insula-tors. The results show that the relationship between the flashovervoltage and air pressure for polluted insulators can be expressedas follows:

(1)

0885-8977/$26.00 © 2010 IEEE

ZHANG et al.: STUDY ON DC POLLUTION FLASHOVER PERFORMANCE 2133

where is the flashover voltage of polluted insulators at a lowpressure , is the flashover voltage of polluted insulators atnormal atmosphere pressure 101.32 kPa, and is the ex-ponent describing the influence degree of air pressure which, ingeneral, depends on the type of voltage stress, insulator profile,and pollution severity.

Rudakova and Tikhodeev [14] reviewed the Russian literatureon the subject, including field, laboratory, and vacuum chambertests and found that the exponent is in the range offor dc. The experimental results [19] of Ishii et al., which wereobtained with half-scale insulator, showed that the exponentis 0.35 for negative dc. DC pollution tests were performed ondifferent types of insulators in the pressure range of 98.6kPa, and the results showed that varies in the range of

for different pollution severity and insulator types [20]. Theis for cap-and-pin insulators and for

post insulators according to the experimental tests [21], [22].Despite the aforementioned experiments, the literature on the

air pressure dependence of flashover mechanism for insulatorsshows very little reported work. Some attempts to model theflashover mechanisms on polluted insulators beginning with re-sults obtained with arcs struck between metallic electrodes areanalyzed by Wilkins [23] and by Novak [24]. Based on revisedvalues for thermal properties of air at high temperature by con-sidering the effect of ambient pressure on the physical param-eters of the dielectric recovery equation, Rizk introduced themodel of altitude effects on the ac flashover of polluted high-voltage insulators [25].

The need for China’s rapid socioeconomic development forelectrical power promotes the fast development of the electricalpower industry. The unbalanced distribution of energy and loadcenter promotes the constructions of long-distance bulk HVDCpower transmission projects. But two-thirds of the territory ofChina is plateau and mountainous areas with an altitude that ishigher than 1 km. Besides, the pollution is another main threat tothe power system in China. So the HVDC projects in China facethe compounding effects of pollution and high altitude. With thechange of altitude/air pressure, the pollution flashover perfor-mance will be different.

For example, the 800-kV dc Yun-Guang line (from YunnanProvince to Guangdong Province) has a 268 km run throughareas higher than 2 km, and the highest area reaches about 2.7km and the pollution level of equivalent salt deposit density(ESDD) is about mg/cm .

But present research cannot provide enough references tothe construction and operation of the 800-kV HVDC line. Soknowing how to deal with the combined influence of high alti-tude and pollution on the flashover performance of the insula-tors on ultra-high voltage (UHV) dc transmission lines is a keytechnical problem faced by Chinese UHV projects. To solve theproblem of the pollution flashover in order to ensure the 800kV transmission line’s safe and reliable operation, it is importantand urgent to study the pollution flashover performance of UHVdc transmission-line insulators at high altitude sites. This paperalso presents the analysis of the dc flashover process of pollutedinsulator at high altitude using insight from high-speed photog-raphy. It then introduces a new physical process to explain theflashover performance for the polluted insulator at high altitude.

II. TEST FACILITIES, SPECIMENS, AND TEST PROCEDURES

A. Test Facilities

All tests were performed in the multifunction artificial cli-mate chamber with a diameter of 7.8 m and a height of 11.6 m,and the power supply is led by a 330-kV wall bushing. The tem-perature of climate chamber can be lowered to C by therefrigeration system, and the atmospheric pressure can be de-pressed to 30 kPa by vacuum pump. Furthermore, in the climatechamber, strong ultraviolet light can be simulated, and the windvelocity can be adjusted from 0 m/s to 12 m/s.

The dc power supply is a 600-kV/0.5-A cascade rectifyingcircuit controlled by the thyristor voltage-current feedbacksystem which ensures that the voltage ripple factor is less than3.0% when load current is 0.5 A, the dynamic voltage dropis less than 5%, and the relative voltage overshoot, due toload-release, is less than 8%. The test power supply satisfiesthe requirements recommended by [26]–[30].

B. Test Specimens

The specimens are porcelain, glass, and composite insulators,as shown in Fig. 1. Their main parameters are shown in Table I.In Table I, is the length of insulator in millimeters; is thecreepage distance in millimeters, and is the diameter of shedin millimeters. SIR is the abbreviation of silicone rubber.

C. Test Procedures

1) Polluting Manners: Before the tests, all samples werecarefully cleaned so that all traces of dirt and grease were re-moved and dried naturally. For composite insulators, the sur-faces of the samples were coated by a very thin layer of drykieselguhr to destroy the hydrophobicity which would be at thedegree of WC4 or WC5. Since the layer of kieselguhr was verythin, the effect of the kieselguhr on the nonsoluble deposit den-sity (NSDD) could be neglected.

The insulators were polluted by quantitative coating withpasting method. Sodium chloride and kieselguhr were used tosimulate conductive and inert materials, respectively. First, therequired amount of sodium chloride and kieselguhr were calcu-lated and weighed according to the specified SDD, NSDD, andthe surface areas of the specimens, and the errors of the weightof sodium chloride and kieselguhr were less than 1% and

10%, respectively. Then, the sodium chloride and kieselguhrwere mixed to slurry with appropriate volume of deionizedwater . In 1 h after the pretreatment, thespecimens were polluted by fully stirred suspension. After 24h of natural drying, the specimens were suspended into theclimate chamber.

It is well known that the nonuniformity between the uppersurface and the lower surface of a suspension-type insulator af-fects the pollution flashover voltage. The SDD and NSDD of theupper surface are equal to that of the lower surface of all insu-lators in this paper.

2) Arrangement: The minimum clearances between any partof the samples and any earthed objects met the requirementsof [31].

3) Wetting: The polluted insulators were wetted by steamfog. The steam fog was generated by a 1.5 t/h boiler, the nozzles


Fig. 1. Profiles of test insulators. (a) Type A: XP-160. (b) Type B: XZP-210.(c) Type C: LXZP-210. (d) Type D: LXZP-300. (e) Type E: FXBW-500/160.(f) Type F: FXBZ-�800/400.

TABLE IPARAMETERS OF INSULATORS

were perpendicular to the axis of test insulator, and the distancebetween them was greater than 3.5 m. The input rate of fog is

, and the temperature in the chamber wascontrolled between C C. The wetness degree of thepollution layer on the insulators was determined by measuringthe layer conductivity. The test voltage was applied to the sam-ples immediately when the conductivity reached the maximumvalue.

The input of steam fog would raise the air pressure in thechamber, and the air pressure difference between the inside andthe outside of chamber would accelerate the rate of input of fog,so the following measures were taken when testing dc pollutionflashover performance of the insulator string under lower airpressure:

TABLE IICOMPARING THE RESULTS BETWEEN � AND �

The air pressure was reduced to a level lower thanthe simulated/target value. Then, the valve of fog generator wasturned on to let the steam fog in. In order to keep the fog sprayrate at the target value, the opening degree of the valve at thiscondition was of that at normal air pressure. Theflashover test was conducted when the pressure in the chamberreached the target value and the pollution layer was sufficientlywetted.

4) Determination of Flashover Voltage: In this study, aneven-rising voltage method was adopted and the step-up ratewas 3 kV/s. Flashover tests were carried out on stringsof insulators and times per string at the same pollutiondegree. The flashover voltages, whose deviation was less than10% compared to the mean value of those flashover voltages,was defined as valid flashover voltages. The average value ofvalid flashover voltage was defined as average flashover voltageof insulator string at that pollution degree

(2)

(3)

where is the average flashover voltage of the insulator (inkilovolts), is the pollution flashover voltage for the time (inkilovolts), is the number of valid flashover voltages, andis the relative standard deviation of the test results.

To evaluate the method, the up and down method was alsoused in this paper with the voltage step being 5% of the ex-pected 50% withstand voltage , and the results are shownin Table II.

The conclusions can be drawn that the and of the in-sulator decrease with the increase of SDD and the value of ishigher, about 4 6% than that of under the same conditions.

Numerous experimental results indicate that the relationshipbetween the dc pollution flashover voltage and SDD can be ex-pressed as follows [2]–[12]:

(4)

where is the coefficient that is related to the insulator types,materials, air pressure and voltage types; SDD is the salt depositdensity mg/cm ; is the characteristic exponent describing theinfluence degree of SDD on pollution flashover voltage and itis related to the insulator types, air pressure, and voltage types,etc.


TABLE IIIRELATIONSHIP BETWEEN DC POLLUTION FLASHOVER VOLTAGE �� AND STRING LENGTH ��

TABLE IVPOLLUTION FLASHOVER VOLTAGES OF INSULATORS AT LOW AIR PRESSURE

Fitting the test results in Table II with (4), the values ofare 0.29 and 0.30 for the even-rising voltage method and upand down method, respectively. Thus, according to the resultsof these two test methods, the change law of the or onthe SDD is basically identical. The even-rising voltage methodwas applied because of its high efficiency in this paper.

III. DC POLLUTION FLASHOVER PERFORMANCE OF INSULATOR

STRINGS UNDER LOW AIR PRESSURE

A. Test Results

The dc pollution flashover performances of various insula-tors were investigated according to the aforementioned test pro-cesses; the results are shown in Tables III and IV.

The conclusions according to Tables III and IV are as follows.1) The dispersion of the test results is small and the relative

standard deviations of these results are all less than7%.

2) The dc pollution flashover voltage of insulators decreaseswith an increase of SDD and a decrease of air pressure.

3) With the increase of SDD, the influence of air pressure ondc pollution flashover voltage of the insulator is reduced.Take the string of 21 units XP-160 (Type A), for example,Table IV, the air pressure decrease from 98.6 kPa to70.1 kPa, the decreasing amplitudes of are 44.1 kVand 18.4 kV when SDD is mg/cm and SDD is0.15 mg/cm , respectively.

4) There is a nearly linear relation between and theinsulator string length up to 21 units at high altitude.The reason is that the polluted insulator string can beequivalent to series resistances under dc voltage, andthe resistance performance of the polluted insulators ismainly determined by the resistance of the surface pol-lution layer which depends on the physical properties ofthe contaminants themselves; it is not related to the airpressure.


Fig. 2. Relationships between � of polluted insulators and �� (a) SDD� �� mg/cm . (b) �� mg/cm .

TABLE VVALUES OF � AND � FOR VARIOUS INSULATORS UNDER DIFFERENT ��

B. Relationship Between Flashover Voltage and air Pressure

The general conclusion is that dc and ac discharge voltageof the polluted insulators decreases with the decrease of the airpressure. Fitting the test results in Table IV with (1), the fittingcurves are shown in Fig. 2, and the values of and for variousinsulators under different SDDs are shown in Table V.

The conclusions according to Fig. 2 and Table V are as fol-lows.

1) The value of is and is related to insulatortypes and SDD.

Fig. 3. Relationships between � of polluted insulators and �� (a) � �

98.6 kPa. (b) � � 70.1 kPa.

2) For the influence degree of air pressure on the dc pollutionflashover voltage, Type B is the smallest and type A is thegreatest.

C. Relationship Between the Flashover Voltage and Pollution

DC pollution flashover voltage is related to the pollutionseverity as shown in Table IV. DC pollution flashover voltagedecreases with the increase of pollution degree.

Fitting the test results in Table IV according to (4), the valuesof and of various insulators under different air pressures areshown in Table VI and the fitting curves are shown in Fig. 3.

The conclusions according to Fig. 3 and Table VI are asfollows.

1) The value of of dc pollution flashover voltage is.

2) The value of is related to air pressure and decreases withthe decrease of air pressure. The reason is that there is aphenomenon of arc floating during the process of dc pollu-tion flashover, and the higher the altitude/lower air pressureis, the heavier the arc floating will occur (i.e., the greaterproportion of air-gap arcs in partial arcs will be. The per-formance difference of the air-gap arcs and surface arcsmakes the characteristic exponent decrease.


TABLE VIFITTING VALUE OF �, � ACCORDING TO THE TEST RESULTS OF WITH (4)

Fig. 4. Creepage flashover gradient of various types of insulators �� mg/cm �.

3) The value of is related to insulator types, as shown inTable VI, for the influence degree of pollution on the dcpollution flashover voltage, Type B is the largest and TypeF is the smallest. It means that type F is superior in theheavy polluted locations because of composite insulatorsunder pollution condition arising from the smaller diameterof the sheds which, in turn, limits the leakage current.

Definine the insulators’ creepage flashover gradient as theratio of pollution flashover voltage to the creepage distance

, namely, , define the insulators’ string lengthflashover gradient as the ratio of pollution flashover voltageto the length of insulator , namely, . The and

of various types polluted insulators are shown in Figs. 4 and5 according to the test results and the basic technical parametersof the insulators in Table I.

The conclusions according to Figs. 4 and 5 are as follows: Theand are different for various types of insulators. Under

the same pollution degree and the same pressure, the andof the composite insulators is higher than that of the porce-

lain and the glass insulators. Type E has the maximal creepageflashover gradient, which indicates that the utilization rate of theleakage distance of this composite insulator is the best.

IV. DC FLASHOVER PROCESS OF THE POLLUTED INSULATOR

AT HIGH ALTITUDE

To reveal the question as to how air pressure affects the dcpollution flashover performance of the insulator string, the dc

Fig. 5. Insulators’ string length flashover gradient of various types of insulators�� mg/cm � .

flashover process of the 5-unit XP-160 polluted insulator stringat high altitude is obtained through the high-speed framingcamera of HG-100 K while setting the imaging rate 1000frames/s, as shown in Fig. 6.

Fig. 6 shows that the dc flashover process of the polluted in-sulator string at high altitude includes four stages as follows: 1)wetness of the contamination layer on the insulator surface, 2)formation of dry bands, 3) burning and elongate of the partialarc, and 4) finally flashover. The phenomenon of extinguishingand re-burning the arc is not evident during the process of dcpartial arc elongation.

Some of the partial arcs may deviate from the surface of pol-luted insulator string at high altitude and form the air-gap arc,which may become more serious with the increment of altitude.That is to say, the partial arcs include two main parts: air-gaparc and surface arc during the flashover process for the pollutedinsulator string at high altitude.

The existence of the air-gap arc during the flashover processfor the polluted insulator string at high altitude makes the dis-tance of discharge path shorter than the total leakage distance ofthe insulator string. It takes about 6 s from the burning of par-tial arc to flashover for the 5-unit XP-160 insulator string at theair pressure of 63.6 kPa, that is to say, the average propagationvelocity of the partial arc is lower than 10 m/s at high altitude.

According to the aforementioned insight of the dc flashoverprocess of polluted insulator at high altitude from high-speedphotography, the physical flashover process of the polluted in-sulator string at high altitude can be simply described, as shownin Fig. 7.

Based on the Obenaus model, a new physical model ex-plaining the flashover mechanism for polluted insulator at highaltitude can be put forward in this paper. It can be simply ex-pressed as a circuit consisting of a surface arc of length andair-gap arc of length in series with a resistance representingthe wet pollution layer supplied by a constant voltage.

In order to reveal the mechanisms underlying the decrease inflashover voltage of an insulator at high altitude, the air-gap arcand surface arc characteristics at high altitude (especially thedifference between them, which had been mentioned by D.A.Swift in his discussion of [25]) must be determined.

Many studies have been carried out to determine the arc char-acteristics [23]–[25], [32]–[34]. The voltage gradient-current


Fig. 6. Pollution flashover process of five units of XP-160 �� kPa� �� mg/cm .

Fig. 7. Physical process of the polluted insulator string at high altitude.

characteristics of an arc at standard and low air pressurecan be expressed as follows:

(5)

where is the voltage gradient along arc (in volts per cen-timeter; is the current through arc (in amperes); is the airpressure corresponding to determine the arc characteristics (inkPa); is the normal atmosphere pressure 101.32 kPa;

, , and are the constants of the arc characteristics.Using a plane triangular glass sample, we investigated the

dc flashover on a polluted surface under high altitude condi-tions and obtained the surface arc characteristics shown as fol-lows [35]:

Positive arc (6)

Negative arc (7)

Using spheres with the diameter of 20 mm, we determined thedc air-gap arc characteristics at low air pressure and obtained the

characteristics as follows [35]:

Negative (8)

The voltage balance equation expressing the pollutionflashover process at high altitude with a static voltage is given

(9)

where is the dc voltage applied to the insulator string; is theleakage current passing through the polluted insulator string;is the surface resistance of the pollution layer; is the lengthof the surface arc; is the length of the air-gap arc; is thelength of residual pollution layer of insulator string; , ,and are the constants of the surface arc characteristics; ,

, and are the constants of the air-gap arc characteristicsand represents the electrode fall voltage. The total length of

, , and is shorter than the total leakage distance ofunits of the insulator string.The surface resistance of the pollution layer is

(10)

where is the equivalent diameter of the polluted insulatorand is the surface conductivity. The equivalent diameter ofthe insulator, which is expressed in centimeters, is defined [33]as

(11)

where is the form factor of the insulator and is the numberof units in an insulator string

(12)


where is the insulator diameter as a function of the positionon the insulator.

Usually, it is very difficult to obtain the general solution of amultiple power function. To reveal the flashover mechanism ofthe polluted insulator string, the best approach is to discuss theextreme condition.

1) Suppose the partial arcs are all surface arcs (i.e., 0)during the flashover process for the polluted insulator string athigh altitude. Then, (9) can be changed as follows:

(13)

Suppose is constant, using , the minimumvoltage to maintain an arc with certain length can be de-rived. When

(14)

(15)

From , the critical arc length can be obtainedby numerical solution

(16)

Therefore, the critical current and critical flashover voltageare

(17)

(18)

Equation (18) shows that the exponent describing the influ-ence degree of air pressure is under the condition.

2) Suppose the partial arcs are all air-gap arcs (i.e., 0)during the flashover process for the polluted insulator string athigh altitude. Then, (9) can be changed as follows:

(19)

where is the ratio of the distance of the discharge path to thetotal leakage distance of insulator strings and .

Using the aforementioned method, the critical current andcritical flashover voltage can be obtained as follows:

(20)

(21)

Equation (21) shows that the exponent describing the influ-ence degree of air pressure is under the condition.

TABLE VIIVALUES OF CUR

During the flashover process for the polluted insulator stringat high altitude, the air-gap arc and surface arc may occur at thesame time. Thus, the exponent describing the influence degreeof air pressure is

(22)

Taking 0.50, 0.52, 0.92, 0.28 [35],into (22), the following can be reached:

(23)

Thus, it can be seen that the exponent describing the in-fluence degree of air pressure is in the range of 0.33 0.72 ac-cording to (23). The value of depends on the ratio of the lengthof the air-gap arc to the length of the surface arc during theflashover process for the polluted insulator string at high alti-tude. The main reason that makes the of insulators different isthe partial arc fluttering degree difference for insulators.

V. DETERMINATION OF STRING LENGTH

ON -KV UHV LINE

In the test, the pollution on insulators is even, but pollutionon insulators in service is uneven. So the experimental resultsshould be corrected [9] by

(24)

where is a coefficient which varies between 0.29 and 0.47,the value of takes 0.38 according to major experiences in theworld; is the pollution uneven coefficient which is definedas the ratio of SDD on the upper shed surface to that on the lowershed surface The values of are shown in Table VII.

For the 800-kV dc UHV transmission line, its rating voltageis 800 kV and its maximum operating voltage is

816 kV. Considering the dispersion and certain safety margin,the target withstand voltage of its suspension insulator string is

(25)

where is the standard deviation of experimental results, andtakes 7% according to the design and operation experience.

So the target withstand voltage is 1033 kV. Experimentalresearch results show that the discrepancy of average flashovervoltage and 50% flashover voltage is a standard deviation.

A. String Length for the Porcelain or Glass Insulator

If the porcelain or glass insulator is used in the 800-kV trans-mission line, the number of insulator units can be determined by

(26)


TABLE VIIINUMBER OF SUSPENSION INSULATOR UNITS OBTAINED BY EXPERIMENT

TABLE IXSTRING LENGTH OF THE INSULATOR OBTAINED BY EXPERIMENT (in meters)

where is the average flashover voltage of the insulatorper unit obtained by experiment; is the string length; and ittakes the minimum integer larger than or equal to the calculatedresult depending on whether the calculated result is decimal. Soaccording to the experimental results, the number of insulatorunits for the 800-kV line is shown in Table VIII.

B. String Length for Composite Insulator

If the composite insulator is used in the 800-kV transmis-sion line, the sting length of insulator can be determined by

(27)

where is the insulators’ string length flashover gra-dient by experiment, and is the string length (in meters). Soaccording to the experimental results, the string length for the

800-kV line is shown in Table IX.According to the calculation results, the string length is about

10.88 m for XZP-210 or 8.43 m for the composite insulatorin the regions with an air pressure of 89.8 kPa (about heightaltitude of 1000 m) and below and the SDD of mg/cm . Thecomposite insulator has advantages in the selection of externalinsulation in heavy pollution areas.

VI. CONCLUSION

Based on the dc pollution flashover performance tests per-formed on different types of porcelain, glass long insulatorstrings, and the composite long-rod insulator in an artificial cli-mate chamber where high altitude was simulated, conclusionsare obtained as follows.

1) From the viewpoint of engineering applications, the rela-tionship between pollution flashover voltage of the insu-lator under high-altitude conditions and string length is ba-sically linear when .

2) DC pollution flashover voltage decreases with the in-crease of pollution level, and the characteristic exponent

describing the influence degree of SDD on pollutionflashover voltage is related to insulator type, air pres-sure, etc. For the types of , the values of are

, , , ,, and , respectively.

3) DC pollution flashover voltage decreases with the decreaseof air pressure, and the characteristic exponent describingthe influence degree of air pressure on pollution flashovervoltage is related to insulator type, pollution degree, etc.For the types of , the values of are ,

, , , , and, respectively.

4) The influence degree of pollution and air pressure on the dcflashover voltage of the insulator is related to the materialof the insulator. The composite insulator has advantages inthe selection of external insulation in heavy pollution areas.

5) From the dc flashover process of the polluted insulator athigh altitude by a high-speed photograph, it can be obvi-ously seen that the partial arcs include two main parts: 1)air gap arc and 2) surface arc.

6) A new physical model explaining the flashover mechanismof the polluted insulator at high altitude is introduced byusing a circuit consisting of a surface arc of length andair-gap arc of length in series with a resistance rep-resenting the wet pollution layer supplied by a constantvoltage.

7) According to mathematic solution, the exponent de-scribing the influence degree of air pressure is given,which is in the range of . The value ofdepends on the ratio of the length of air to the length ofsurface arc during the flashover process for the pollutedinsulator string at high altitude.

ACKNOWLEDGMENT

The authors would like to thank all members of the externalinsulation research team in Chongqing University for their hardwork to obtain the experimental data in this paper.

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Zhijin Zhang was born in Fujian Province, China,in 1976. He received the B.Sc., M.Sc., and Ph.D.degrees from Chongqing University, Chongqing,China, in 1999, 2002, and 2007 respectively.

Currently, he is an Associate Pofessor of theCollege of Electrical Engineering at ChongqingUniversity, Chonqing, China. His main researchinterests include high voltage, external insulation,numerical modeling, and simulation. He is the authoror coauthor of several technical papers.

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

His employment experiences include theShaoyang Glass Plant, Shaoyang; Hunan Province;Wuhan High Voltage Research Institute, WuhanHubei Province; and College of Electrical Engi-neering, Chongqing University, Chongqing China.His special fields of interest include high-voltageexternal insulation, transmission line’s icing and

protection. He published his first monograph-Transmission Line’s Icing andProtection in 2001, and has published more than 80 papers about his profes-sional work. He received the Second-Class Reward for Science and TechnologyAdvancement from the Ministry of Power in 1995; Beijing Government in1998; Ministry of Education in 1991 and 2001, respectively; the First-ClassReward for Science and Technology Advancement from the Ministry of Powerin 2004; the Third-Class Reward for Science and Technology Advancementfrom the Ministry of Power in 2005; the Second-Class Reward for Scienceand Technology Advancement from the Ministry of Technology in 2005; theFirst-Class Reward for Science and Technology Advancement from Ministryof Education in 2007; and the First-Class Reward for Science and TechnologyAdvancement from Chongqing City in 2007.

Yafeng Chao was born in Hubei Province, China,on February 19, 1982. He received the M.Sc. degreefrom Chongqing University, Chonqing, China, in2008, and is currently pursuing the Ph.D. degreein the College of Electric Engineering, ChongqingUniversity, Chonqing, China.

His main research interests include high-voltagetechnology, external insulation, and transmis-sion-line icing. He is the author or coauthor ofseveral technical papers.

Ling Chen was born in Chongqing, China, inNovember 1982. He received the B.Sc. degreefrom Harbin University of Science and Technology,Harbin, China in 2005 and the M.Sc. degree fromXihua University, Sichuan, China, in 2008, and iscurrently pursuing the Ph.D. degree in the Collegeof Electric Engineering, Chongqing University,Chonqing.

His research interests include high voltage andtransmission-line icing and deicing.


Caixin Sun was born in Chongqing, China, on De-cember 13, 1944. He graduated from Chongqing Uni-versity, Chonqing.

Currently, he is Professor and Doctorate Advisorof the College of Electrical Engineering, ChongqingUniversity. His current research includes electricalexternal insulation technology in complex climaticenvironments, online detection of insulation condi-tion, and insulation fault diagnosis for high-voltageapparatus and high voltage techniques applied inbiomedicine.

Mr. Sun is a member of the Chinese Academy of Engineering.

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.

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

dc pollution

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