insulator shed profile

8/8/2019 Insulator Shed Profile

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Abstract--The selection of suitable shed profiles for station

insulators with silicone rubber housings is an important subjectfor the successful operation of the HVDC and UHVDC systems.This subject has been discussed on a Seminar with a group of

invited experts on external insulation. The participantsrecognized the lack of research results in this area. However,considering the urgent engineering needs for the coming UHVDCprojects, after a review of the operational experiences of theexisting 500-600 kV HVDC stations, especially the stations inChina, the participants concluded the Seminar with preliminaryrecommendations on the decisive parameters of suitable shedprofiles and a test method for shed profile selection. Theserecommendations are considered to be applicable to verticallyinstalled HVDC station insulators with silicone rubber housingand with relative large diameters. At the same time, theparticipants encourage and invite utilities, manufacturers, andresearch institutes to be active on this subject and to providemore reliable test results.

Index TermsHVDC insulators, Silicone rubber insulators,Insulator testing.

I. INTRODUCTION

ore and more insulators with silicone rubber housingshave been and will be used in HVDC stations because

of, among other advantages, their superior pollutionperformance. The selection of suitable shed profiles for thistype of station insulators is an important subject for thesuccessful operation of the HVDC and specially UHVDCsystems. However, there are no available standards andrecommendations on this subject. At the same time, there is an

urgent need for engineers both in utility and manufacturers tobe able to determine the shed profiles for the coming UHVDCprojects. With this background in mind, a Seminar took place

The authors are participants of the Seminar on the Selection of ShedProfiles for Composite HVDC Station Insulators. The participants of thismeeting are listed with no special order:

W.M. Ma (BDCC of SGCC, China), B. Luo (TRC of CSG, China), Z.Y.Su (CEPRI, China), Z. P. Dang (XECRI, China), Z.C. Guan (TsinghuaUniversity, China), X.D. Liang (Tsinghua University, China), U. strm(ABB HVDC, Sweden), D. Wu (ABB HVDC, Sweden), E.Y. Long (ABBCNCRC, China), H.G. Sun (ABB CNCRC, China).

Contact persons:Zhi-Yi Su, CEPRI, Qinghe, Beijing, China. E-mail: [email protected] Wu, ABB HVDC, Ludvika, Sweden. E-mail: [email protected]

at Tsinghua University on 22nd Nov. of 2005, with a group ofinvited experts on external insulation from Beijing WanglianHVDC Engineering Technology (BDCC) of State Power GridCooperation of China (SGCC), Technology Research Center(TRC) of China Southern Power Grid (CSG), China Electric

Power Research Institute (CEPRI), Xian Electro-CeramicResearch Institute (XECRI), Tsinghua University, ABBHVDC Sweden, and ABB Corporate Research in China(CNCRC).

This seminar for invited experts was entitled as Seminar onthe Selection of Shed Profiles for Composite HVDC StationInsulators. During this all-day seminar, the participantsdiscussed at great length on the issues related to the profileselection, the parameters that need to be recommended, theranges of these parameters, as well as the test methods that canbe used to verify the profile selection. This paper present theMinute of Meeting formed on this seminar as well as some

related discussions.It should be noted, significantly, after this seminar, severalmanufactures have designed their composite station insulatorsfor UHVDC applications. Several research groups are now, atthe same time of writing this article, performing experimentalstudies on shed profile of composite station insulators.

II. REVIEW OF OPERATIONAL EXPERIENCES OF EXISTING 500& 600 KV HVDC PROJECTS

The key issues reviewed on the Seminar are brieflysummarized below.

A. Wall bushings

The wall bushings were the equipment that had the highestflashover rate in the converter stations. It was identifiedthrough research work that the main cause of these flashoverswas uneven wetting (uneven-rain). To apply the hydrophobiccoatings on porcelain insulators or to use the silicone rubbercomposite bushings have lead to a more uniform voltagedistribution along the insulator during uneven rain. Theproblem of uneven wetting has therefore been solved withgood operational experience.

B. Station post insulators

Flashover on station post insulators was rare in earlier

operational experience. For some of the station post insulatorswith an under dimensioned creepage distance, severe partial

Preliminary Recommendations on the SuitableShed Profile for HVDC Station Insulators with

Silicone Rubber HousingW.M. Ma, B. Luo, Z.Y. Su, Z.P. Dang, Z.C. Guan, X.D. Liang,

U. strm, D. Wu, E. Y. Long, and H.G. Sun

M


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discharge activities were observed. This problem has beensolved through the application of hydrophobic coatings orbooster sheds.

The porcelain station post insulators used in the Three-Gorges to Changzhou and Three-Gorges to Guangdong

projects have had a satisfactory operational experience. Theshed profile of these insulators is given in Fig. 1.

Fig. 1. The shed profile of porcelain station post insulators used in ThreeGorges to Changzhou and Three Gorges to Guangdong projects

C. Vertically installed bushings

According to flashover statistics, half of the flashovers

occurred on this type of insulators were in rain. Certainimprovement in performance has been achieved through theapplication of hydrophobic coatings.

TABLE INUMBER OF FLASHOVERS ON DIFFERENT PORCELAIN INSULATORS UNDER FOG

OR RAIN CONDITIONS (DATA FROM MORE THAN 40 STATIONS WORLD-WIDE)

Position/insulator type Fog RainHorizontal wall bushings 1 39Vertically installed stationinsulators of different types

28 28

D. Silicon- rubber composite insulators

The superior pollution performance of silicone-rubberinsulators is widely recognized. In the inland regions, silicone-rubber insulators can operate safely even with a shortercreepage distance than that of porcelain insulators. Such agood experience has been reported in North America andfurther demonstrated by vast experiences in China. It is sorequired in China that the silicone-rubber insulators to be usedin polluted areas should be made of silicone-rubber materialsthat have both high tracking resistance and goodhydrophobicity transferring ability.

E. Flashover statistic of insulators in HVDC stations

A review of operational experience of insulators in HVDCstations reveal clearly that the flashover rate can be controlled

to reach a very low level, e.g. 0.04 per pole per year asindicated in Fig. 2. The flashover rate has no direct relations tothe system voltage level. A good performance of externalinsulation can be achieved.

Fig. 2. Number of flashovers occurred per pole per year in 47 HVDCstations worldwide.

III. ASPECTS NEED TO BE CONSIDERED

Various aspects that need to be considered when selectingshed profiles and specifying related test methods for compositeinsulators used in 800 kV DC stations are listed below.

1. The natural conditions that have a major influenceon the shed profile selection.

2. The decisive parameters of insulator shed profile.

3. The influence of hydrophobicity on the shedprofile selection.

4. Operational experience of porcelain insulators.5. Operational experience of insulators in HVAC

systems.6. Operational experience of insulators in HVDC

systems.7. Other constrains.8. Test methods.

IV. RECOMMENDATIONS

A. The natural conditions that have a major influence on the

shed profile selection

For vertically installed composite insulators, both postinsulator and bushings, the flashover caused by heavy rainoccurred after relative long time of pollution accumulation isconsidered as the major condition that will determine the shedprofile selection. This is because that operational experiencehas shown that the typical pollution flashover is not the mainproblem for silicone rubber insulators. However, some of theexperts pointed out that the low air pressure at areas of a highaltitude would impair the pollution performance. Therefore,the pollution performance might still be a major influencecondition for the shed profile selection of composite insulators

to be used in the areas of a high altitude. Another constrain for


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the shed profile selection, which may only be applicable forthe insulators in parallel construction, is the aerodynamicproperty of the insulator under strong wind.

B. The influence of hydrophobicity on the shed profile

selection

Insulators with a good hydrophobicity give better rain andpollution performance than hydrophilic insulators. However,the influence of the degree of hydrophobicity on the profileselection needs to be further studied. The recommendationsgiven on this Seminar is base on the assumption that theinsulator is able to retain certain degree of hydrophobicity onits surface during the operation.

C. Recommendations on the decisive parameters of insulator

shed profiles

The recommendation applies to vertically installed silicone-rubber station insulators of relative large diameters. For the

alternating profile (one large and one small shed, see Fig. 3.)the recommended parameters are listed below:

Fig. 3. Decisive parameters of the shed profiles for vertically installedsilicone-rubber composite station insulators

1. Spacing between the large sheds: S 65 mm2. Ratio between shed overhang and shed spacing:

P/S 1.253. Difference between overhangs of the large and the

small sheds: P-P1 should be larger than thatrequired for AC insulators (i.e. 15 mm); and it isrecommended to use a larger value e.g. 20 mm ormore.

4. Upper-side inclination angle: > 10o; under-sideinclination angle: > 3o. The designer isrecommended neither to use too small in orderto avoid the backflow of the water drops, nor touse too large in order to avoid poor naturalwashing effect. Shed profiles with under-rib arenot recommended.

5. For insulators with a shed profile that does not

fulfill above recommendations, it can still beadopted if it is proved to have an equally goodperformance as those insulators that fulfill therecommendations in the tests specified below.

D. Test method

The heavy rain test under polluted conditions.1) Test voltage

It is recommended to use the up-and-down method in orderto obtain a voltage level that corresponding to 50% flashoverprobability, U50, and the standard deviation, . It is requiredthat the withstand voltage, defined by U50(1-3), should behigher or equal to the maximum rated operational voltage.

For a test object with a reduced size from the real equipmentand with the aim of the test as to verify the shed profile, thetest voltage applied shall be proportional to the real length.The effect of the difference in insulator length shall beevaluated in the final evaluation of the test results.

2) Pollution level

The SDD and NSDD levels used in the test shall be that inthe Project Technical Specification. Kieselguhr will be used asthe inert material.

3) Rain intensity and conductivity

Rain intensity: 3-5 mm/min, in 45o.Conductivity of the water: 100 m (refer to IEC standard

rain).4) Test procedure

The pollution should be applied uniformly on insulators.After pollution application, insulators are allowed to recoverits hydrophobic property for a period of four days.

The insulator to be tested is then installed at the testposition and with the test voltage applied before starting therain. The test will continue until a flashover has occurred onthe insulator or the risk of flashover is considered to beminimal. This can be determined by observing the leakagecurrent.

V. DISCUSSIONS ON THE TEST METHOD

Some details of the test method were discussed among theinvolved experts during and even after the Seminar. Some ofthe important issues are briefly introduced below for thereference of the reader.

The pollution level is proposed on the Seminar as the samelevel in each specific Project Specification. Actually, thehighest pollution level and the heavy rain may not appear atthe same time of a year. Therefore, the proposal on theSeminar may be considered as a conservative approach.

A problem that may occur during the test is the pollutionlayer being washed away too quick after starting the rain. Innatural conditions, the pollution accumulation is often a slowprocess. The pollution may be better encapsulated by thesilicone rubber surface than that in laboratory. Therefore, itwill probably take a longer time for the pollution to be washedaway in natural conditions. One possible solution is to addsome type of glue in the pollution slurry in order to slow downthe washing process. The possible drawback of adding glue is


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that we are not so clear on its interaction with silicone rubbersurface. This is the subject need to be further investigated.

Regarding the pollution distribution, although it is true thatthere will be more pollution at the under-side of the insulatorsheds than at the upper-side, a uniform pollution layer will be

easier to achieve in the laboratory. Therefore, uniformpollution is recommended here.

As to the degree of hydrophobicity recovery that will beallowed for the test objects, there are also some considerations.In natural conditions, it is reasonable to expect that the surfaceof the insulator will retain or recover to certain degree ofhydrophobic property before rain. Therefore, the test shouldalso be performed on insulators that have recovered some ofits hydrophobicity after pollution application. However,different surface materials may recover with different speedunder the same pollution. To allow a full recovery for allinsulators may take a long time, i.e. a few days or even weeks.

This will make the test very time consuming. Certaincompromise is necessary here. Therefore, a period of four dayswas recommended on the Seminar based on laboratoryexperiences.

The rain conductivity is proposed on the Seminar to becorresponding to IEC standard. However, it is recognized bythe experts that the rain conductivity is not the most criticalparameter in the test severity. Therefore, water with otherconductivity may also be used as long as the conductivity ofthe water is under control.

VI. CONCLUSIONS

Considering the fact that the information available on thissubject is very limited in literature, the recommendations givenfrom the participants of this Seminar is preliminary. However,these recommendations represent the common understandingof the participants at this stage based on the operationalexperience and limited test results. The recommendation willprovide a reference position for the utility and manufacturers.At the same time, the participants encourage and inviteutilities, manufacturers, and research institutes to be active onthe subject of shed profile selection for HVDC compositestation insulators. This subject will be better understood ifmore reliable test results will be obtained on both conditions,i.e., heavy rain under polluted conditions and low air pressure

in combination with pollution flashover.

VII. BIOGRAPHIES

Weimin Ma was born in China in 1966. He received his Ph.D. from WuhanInstitute of Hydraulic and Electric Engineering. His working experiencesinclude HVDC, overhead power transmission line design and high voltagetesting. He is main author and contributor to the Technical Specification andsystem design of many of the large scale HVDC projects in China, such as3GC, 3GG and 3GS, etc. In 2000 He became the Chief Engineer of BeijingWanglian HVDC Engineering Technology Co. Ltd (BDCC).

Bing Luo was born China in 1966. He received his Ph.D. from TsinghuaUniversity in 1996. His working experiences include work at Mianyang

Electric Power Supply Bureau and Guangzhou Electric Power Supply Bureau.In 2005, he joined China Southern Power Grid (CSG) and worked at HVDC

Department of Technology Research Center (TRC). He is member of PowerIndustry HVDC Transmission Technical Committee of China.

Zhiyi Su was born in China in 1946. He has graduated from Beijing ElectricPower Institute in China in 1970. His working experiences include the testing,research, design consult on operation safety of external insulation of HVAC

and HVDC equipment and transmission line. In 2000, He became the deputyChief Engineer of High-voltage Department of China Electric PowerResearch Institute (CEPRI). He is the vice Director of Power IndustryInsulator Technical Committee of China. He is also the Secretary-General ofPower Industry HVDC Transmission Technical Committee of China.

Zhenping Dang was born in China in 1966. He received his M.Sc.fromXian Jiaotong University. Since 1988, his research areas have mainly beenmaterials, production techniques, and test methods of HVAC and HVDCinsulators and bushings. He is now the deputy Chief Engineer of XianElectro-Ceramic Research Institute (XECRI) of China. He has been membersof Chinese national standard committee for insulators for three terms from1995 and contributed to most of the todays national standards for compositeinsulators.

Zhicheng Guan was born in China in 1944. He received his Ph.D. from

Tsinghua University in China in 1984. He has a long working experience andis active in the area of high voltage technology. He is now the dean ofgraduate school at Shenzhen, Tsinghua Unversity, Vice president of TsinghuaUniversity Council, president of China Electrotechnical Society and vicepresident of Chinese Society for Electrical Engineering.

Xidong Liang was born in China in 1962. He received the B.Sc., M.SC., andPh.D. degrees from Tsinghua University, Beijing, China, all in high voltageengineering in 1984, 1987 and 1991, respectively. He did his research inUMIST, England as a visiting scholar from 1991 to 1992. Then he wasassociate professor in Tsinghua University from 1993 and full professor from1997. He is member of CIGRE SC B2, and member of working groups of IECand CIGR dealing with composite insulators. His special fields of interestinclude outdoor insulation, composite insulator and its materials.

Dong Wu, was

born in China in 1952, He received his Ph.D. form the RoyalInstitute of Technology in Sweden in 1985. His working experiences include

R & D activities at China Electric Power Research Institute (CEPRI), researchwork at the Royal Institute of Technology and STRI in Sweden in the areas ofhigh voltage technology and power electronics. In 1998, He joined ABBHVDC in Sweden. In 2001, He was appointed as Senior Specialist on ElectricInsulation Systems at ABB HVDC in Sweden. He is active in IEC and CigreWorking Groups on external insulation.

Urban strm was born in Sweden in 1946. He received his M.Sc. fromUniversity of Uppsala, Sweden in 1973. In 1974 he joined ABB (ASEA)HVDC in Sweden. His working experiences include R & D and Managingwork on thyristor valve, converter transformer, various HVDC converterstation equipments, and external insulation. He is now project manager forUHVDC development at ABB HVDC.

Emily-Ying Long was born in China in 1972. She received her B. Sc, M. Sc,and Ph. D from Tsinghua University, in 1995, 1997, and 2000 respectively,all in electrical engineering. From 2000 to 2004, she worked as a seniorengineer of HVDC control and protection at Beijing HVDC EngineeringConsultant Company (BDCC), involved in Three Gorges-Changzhou, ThreeGorges-Guangdong, Three Gorges-Shanghai HVDCs and Northwest toCentral China Back to Back Link. She is now with ABB Corporate Researchin China, working in the field of high voltage engineering.

Huigang Sun was born in China in 1976. He received his M.Sc. from Xi'anJiaotong University, China in 2002. He began his PhD study in ChongqingUniversity, China from 2006. In 2002, he joined ABB high voltage factory inBeijing as a R&D engineer. He began to work in ABB Corporate ResearchChina from 2005. Now he works mainly in high voltage and insulation field,and is responsible for several high voltage projects

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insulator shed profile

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