IEEE ISIE 2006, July 9-12, 2006, Montreal, Quebec, Canada

Remote Online Monitoring Systemfor Suspension Insulator Strings

Bo Li Xiujie Wang Liu NianDept. of Electrical Engineering; Sichuan University, 610065

Chengdu, Sichuan, ChinaE-mail: libolina@,]63.com

Abstract-Suspension insulator strings are used in high voltagetransmission lines and substations widely and they are importantfor the power system on insulation. This paper describes someinsulation ability of suspension insulator strings and completessome exploration work to the remote online monitoring system forsuspension insulator strings of high voltage transmission lines andsubstations. A new method of combinative leakage current andpulse current for insulators is introduced. This monitoring systemcan monitor the leakage current and pulse current of suspensioninsulator strings effectively, and it can make use of computer andcommunication technology to process the Real-time signal whichis able to reflect the insulation state with the relevant data ofinsulators. The leakage current of suspension insulator strings canincarnate the grade of insulators contamination. Accordingly, thepulse current of them can embody the characteristic of internalinsulation. Base on these methods, the system has the real-timeearly warning function, and the pre-warning work can be realizedfor the pollution flashover. The experimental equipment is alsoinstalled in the High Voltage Laboratory and the software of thissystem is programming. We will test the system and improve itcontinuously to achieve that this system can be used in field early.This can help the operators obtaining the data of insulatorsindoors, cleaning and maintaining the insulators designedly, andreducing the maintenance expenses of effectively.

I. INTRODUCTION

Suspension insulator strings occupy very important positionin the insulation of the power system. In terms of material, theyhave three types, porcelain insulators, glasses insulators andpolymer insulators. The various kinds of suspension insulatorsoperating in the transmission lines and substations are exposedin the open air for a long time, and undergo the rainfall,contamination, ice and snow. Therefore, they can easily getpollution flashover under the lightning over-voltage and theswitching over-voltage.

Flashover, aroused by contamination, on high voltagetransmission lines and substations is another dominated causeof outage except the lightning. However, the contaminationflashover of suspension insulators is the more harmful fault topower system, and the loss is very great for the operation ofelectrical power systems. Therefore, the all parts of the world

have almost taken place pollution flashover on power system,and result in the serious economic losses.

In the modern power system, more and more organicsynthesis insulators with the antipollution and anticorrosiveability are used. At the same time, the method of seasonalwashing the surface of insulators is also used in the practice.But, as short of knowing the status in the operating insulators,the washing on the online insulators is not obvious sometimeswhen winter with ice and snow, and the method is verydangerous for operation high voltage transmission lines andsubstations [1].

II. BACKGROUND KNOWLEDGE FOR REMOTELIVE-LINE MONITORING SYSTEM

There are four stages from contamination depositing toflashover occurring.

A. Deposition ofcontamination andNatural cleaningIn the effect of natural power, the contamination in the air

and rainfall fall the suspension insulators. The ingredients ofcontamination are different according to the pollution sourceand the area. The contamination deposited on the surface ofinsulators mainly comes from power, iron, steel, cement plantand chemical plant, as well as the exhaust of automobile andthe people daily living contamination and so on. Additionally,in the coastal area, the contamination is mainly from thecontamination of sea breeze or rainfall. Under the serviceconditions, the deposited contamination is possibly blowndown by the wind or washes off by rain and heavy dew. Therepetition of the processes above gradually makes thecontamination of the surface of suspension insulator stringscome into being.

B. WettingOnly under the moist situation, the contamination has the

obvious influence to the flashover voltage of insulators. As thewetting effect of fog, rain drops, dew, thawy snow and ice andmoist weather, the conductive channel forms in thecontamination layers.

C. The forming of the dry-area and the production of someelectrical-arc

Moist contamination is drying slowly as Joule-heat effect,then, it forms dry-area on the surface of insulators. Thedistribution of pressure on the surface of the insulators will

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change. If the pressure which is between two ends of dry-areaexceeds a certain value, part-discharge will take place andelectrical-arc will come forth. This is the third step of flashover.

D. Flashover behaviorAs contaminated status of the surface of insulators, the status

of moist contamination and the pressure of dry-area such as,the form of part-discharge and its development are different,some weaken gradually and the other arouse flashover.

Obennaus present a pollution flashover model in which theexterior electric-arc and the resistance of remainedcontamination are series-wound [1]. the maintained equation ofelectrical arc is as follow:

U = AXI -n + IR(X) (1)

Where X is the length of electrical-arc, I is the surface current,R(X) is resistance of contamination where the length of theelectrical-arc is X, and A, n are the static constants ofcharacteristics of the electrical-arc.

Fig. 1. Leakage current and pulse current sensor and RTU in the tower. TheRTU can acquire the signal and sent it to the center computer by RadioFrequency (RF) communication, mobile communication network or powerline carrier etc.

When the suspension insulators are installed, the equation offlashover voltage is as follow:

U1 E11 +E212 (2)

Where El is the flashover electric field (virtual value) of thesurface, which is caught in; E2 is flashover electric field of theair gap;Le is the length of surface, which is caught in; 12 is thelength of the air gap.

III. METHODS FOR REMOTE ON-LINEMONITORING SYSTEM

A. Methodsfor data acquisition system (DAS)Over the years, various methods for monitoring insulators on

high voltage transmission lines and substations have been used.Some are active methods while others are passive, each onewith its own advantages and disadvantages. They can be sortedinto two types of electrical method and non-electrical method.

Ultraviolet imaging method, infrared temperature method,ultrasonic method and digital image processing method are themainly non-electrical methods. Each one has some advantagefactors and disadvantage [2].

Ultraviolet imaging method detects the ultraviolet radiationwhich is emitted by the part discharge of insulators to judge theinsulators well or not. However, it needs detecting when thepart discharge took place and this method is influenced by the

Fig.2.The leakage current and pulse current sensor on the tower. This paperacquire the signal of the leakage current and pulse current by using differenttypesensr toavod pick-up pulse current from leakage current or leakage

current from pulse current.

environment.Infrared temperature method can measure the temperature

which is conducted by part discharge and some other factors.However, the sunshine, wind, humidity and environmentaltemperature are the mainly influencing factor for this method.

Ultrasonic method can detect the rupture of insulators and itis primarily used in factory or laboratory.

Digital image processing method can indicate thehydrophobicity of insulators. However, it will be need ofhelicopter to complete the measurement, if it is used intransmission lines. This could increase the cost [3].

It is obvious to us these methods above are not suit forremote monitoring system of suspension insulator strings.These could not be widely applied in the field.On the other hand, more and more electrical method are

reported by the papers.Electrical method is as follow:1. Potential detection2. Resistance detection3. Electric field detection4. Leakage current detection

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5. Pulse current detection

1) Potential Detection

Buzz method, this method which detects the potential is toput a shot-circuit across an insulator and listen for a sparkingnoise, is mentioned in the previous paper [4]. However, thismethod easily arise change of potential distribution of insulatorstrings and it is a very dangerous method to operators whoneed to climb tower to measure. According to above, we canconclude that buzz method is not suit for remote on-linemonitoring.

2) Resistance Detection

Resistance detecting method is an insulators monitoringmethod by measure the resistance of the insulators. Thismethod was used mainly to test insulators before they left

Fig.3. Adaptive noise cancellation for the testing system whichis disposed the noise signal in the leakage current and leakagecurrent signal.factory by megohm meters. It can be used in on-line detectionafter making some technical improvement. Therefore, we canconclude that resistance detecting method is not be used inremote on-line monitoring.

3) Electric Field Detection

Electric field measurement is a method which is utilized thedifference between the electric field of normal insulators andfaulty insulators. The difference is that the equi-potential lineof normal insulators is smooth and there is salient on the equi-potential line of fault insulators. This method is not suit forremote online monitoring. First, this method need operator toclimb tower to complete them work. Second, it is a risk workfor operator. Farther more, it is influenced by naturalqualification.

4) Leakage Current Detection

Leakage current detection is a method to analyze the state ofinsulators by means of measuring the leakage current ofsuspension insulator strings, registering the value of leakagecurrent, counting the number of peak in vary value range.Obviously, this method is devoid of full monitoring tosuspension insulator strings, and there is no enough time foroperator to maintain.

5) Pulse Current Detection

Pulse current measurement mainly measures the coronapulse while there are fault insulators in the suspension insulatorstrings, part discharge pulse which is led by fissure oninsulators and leakage current pulse before flashover. Thismethod can monitor inner and exterior multi-parameter ofsuspension insulator strings [4, 5, 6].

There are four influencing factors on the resolution, theamount of insulators on one string, the corona inceptionvoltages of the insulators close to line, the resistance of thefaulty insulators and the location of the faulty insulator on thestring [7].

B. Methodsfor dataprocessingThe main data processing methods are frequency

Fig.4. The structure map of RTU. The remote terminal unit (RTU) iscomposed of the filter, the signal magnifying unit, the A/D change unit, theSingle Chip Microprocessor unit, and the remote inquire unit.

characteristics [8], liner stochastic [9] and neural network-basesystem (NNS) [10, 11]. Although these methods can work inpractical operation, it is not fully satisfied for pick-up wavelet.

IV. NEW REMOTE ON-LINE MONITORINGSYSTEM FOR SUSPENSION INSULATORSTRINGS

Compared the methods above, this paper adopted anassociative method of monitoring leakage current and pulsecurrent to remote on-line monitoring the suspension insulatorstrings at the same time. Only in this way can we well exert thetwo detection methods above. Fig. 1 shows the arrangement ofthe remote monitoring system in tower. The leakage currentsensor and pulse current sensor are installed in one suspensioninsulator string and the remote terminal unit is fixed on thetower. The sensor secondary is connected to the RTU througha coaxial cable.The leakage current which is determined by instantaneous

practice climb electricity distance, weather and filthy degree isvery weak. In addition, the leakage current, which includepartial information of the operating state of insulator, isrelatively easily measured. This method is suit for continuouson-line monitoring, and it includes the information of theinsulator with the four processes, which reflect the completeprocess of insulators. On the other hand, pulse currentreflects the inner insulated status of insulators, such as thecorona pulse while there are fault insulators in the suspensioninsulator strings, part discharge pulse which is led by fissure oninsulator and leakage current pulse before flashover etc.

Therefore, we adopt the integrated methods includingleakage current and pulse current, to assess the online state ofthe insulators in the remote online monitoring system. Inaddition, this method is applicable for remote onlinemonitoring suspension insulator strings on the transmissionlines and substations.

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The remote live-line monitoring system of suspensioninsulator strings is mainly composed of the sensor group, theremote terminal unit (RTU), the transform unit, the monitoringcomputer and the relevant equipment used for informationexchange.

This paper acquire the signal of the leakage current andpulse current by using different types of sensor to avoid pick-up pulse current from leakage current or leakage current frompulse current. The signal acquiring unit is as shown as fig.2.This monitoring system utilizes the adaptive noise cancellationto dispose the noise signal in the leakage current and leakagecurrent signal. First, the principium of this system is, as showas fig.3, set one or several sensors around the suspensioninsulator strings to collect the noise signal of environment.Second, the noise signal is eliminated from the leakage current

RTU iiti

For transmission line insulators, it selects Radio Frequency(RF) communication, mobile communication network orpower line carrier etc. to achieve the data communication fromRTU to central computer. RF is convenient, as shown as fig.5,but it is easily influenced by geography and the transmissiondistance. For the substation insulators, it directly adoptsRS232/RS485 to achieve that. The RTU is connected to thecentral computer by a coaxial cable through RS485, as shownas fig.6.The signal from the microprocessor unit is sent to the central

computer in the control room to process. The data processing isfinished by the software, which is selects wavelet transform toanalyze. The data, which is processed and conserved by thecentral computer, can be printed, shown, inquired and pre-warned, as shown as fig.5. Here, the pre-warning will berealized through adopting fuzzy sentencing to the alert methodin analysis on the remote terminal signal.Some practicable experience is mentioned in the early paper.

On one hand, paper [7] has studied the corona pulse current.The amplitude of the negative pulses, which is conducted bycorona pulse of the fault insulators, is larger than normal

RiT.J

Fig.5. The constitution of the communication system between RTU andcentral computer on transmission lines. This model can be used in RF ormobile communication network.

and leakage current signal. In addition, ambient temperature,relative humidity sensors are installed on the every severaltowers to detect the weather condition.

The remote terminal unit (RTU) is composed of the filter,the signal magnifying unit, the A/D change unit, the SingleChip Microprocessor unit, and the remote inquire unit. Thestructure map ofRTU is shown as the fig.4. The power supplyconsists of a 12V, 18W solar cell installed at the middle of thetower together with the RTU box. A 105Ah battery, which ischarged by the solar cell, is used as a backup during nights andcloudy days, and gives up to 5 days operation [6]. On the otherhand, the RTU is set in saving power model. RTUintermittently collect the current signal and intermittently sentthe signal to the central computer to reduce the wastingelectrical-power.

Fig.6. The constitution of the communication system between RTU andcentral computer. MAX489 is the drive of the signal transmission from RTUto central computer.insulaters. We can easily pick out the fault insulater from oneinsulater strings. In addition, the lower resistance of faultyinsulater, the higher voltage drop, it will produce more coronadischarge. So, it is safed to conclude that pulse currentmonitoring method is effective. On the other hand, the leakagecurrent monitering method has used in the practice wildly. Theinterrelated technology is very self-contained. It can count theamount of peaks in different peak level, such as 50-150mA,150-250mA, 250-350mA, 350-45OmAand >450mA. It canindicate the states of the insulaters well and truly.This paperacquire the signal of the leakage current and pulse current byusing different types of sensor to avoid pick-up pulse currentfrom leakage current or leakage current from pulse current. It'svery convenient to obtain the leakage current singal and pulsecurrent singal. It can be analysed by the computer informationtechnology, such as nerval net work and fuzzy theory.

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Niftl

V. HIGH VOLTAGE LABORATORY TESTS

The experimental equipments are installed and the softwareof this system is programming. We will test the system in thehigh-voltage laboratory and improve it continuously to achievethat this system can be used in field early. This system willoperate under different level voltage to test its validity.

[9] Felix Amarh, George G. Karady, Raji Sundararajan, "Linear stochasticanalysis of polluted insulator leakage current," IEEE Transactions onPower Delivery, USA, Vol. 17, No.4, Oct. 2002 pp. 1063 -1069

[10] S.Kumagai, N.Yoshimura, "Leakage current analysis for monitoring theconditions of polymer insulators," Electrical Insulating Materials, Tokyo,Japan, Nov. 2001, pp.55-58

[11] M.A.R.M., Fernando, S.M. Gubanski, "Analysis of leakage current waveforms for field-aged and new composite insulators," Electrical Insulationand Dielectric Phenomena, 1997. IEEE 1997 Annual Report.,Conference on,, Swedan, Vol.1, Oct. 1997, pp.350 - 353

VI. CONCLUSIONS

The remote live-line monitoring system of suspensioninsulator strings is mainly composed of the sensor group,remote terminal unit, transform unit, central computer, andrelevant equipment for the information exchanging, and thedata processing is finished by the software, which operate onthe central computer mainly.To high voltage transmission lines and substations, the ones

that have adopted the online real-time monitoring system canmonitor comprehensively for the integrative performance ofthe insulators very much. It can also master its insulating statereal-time. This can help to clean and maintain the insulatordesignedly, and reduce the maintenance expenses of theinsulator effectively.

BIOGRAPHIES

Bo Li was born in 1981 in the provinceof Sichuan in China. He graduated fromHarbin University of science andtechnology and received his bachelor'sdegree. He was a graduate of SichuanUniversity, China. Major in ConditionMonitoring and Fault Diagnosistechnology of electrical equipmentE-mail:libolina@163.com

ACKNOWLEDGMENT

This work was supported by the Sichuan Electric PowerBuilding Company in Chengdu City, Sichuan Province, China;and we greatly thank the support of the High-voltagelaboratory, School of Electrical and Information Engineeringof Sichuan University, Chengdu, China.

REFERENCES

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[3] M. Berg, R. Thottappillil, and V. Scuka. "Hydrophobicity Estimation ofHV Polymeric Insulating Materials, Development of aDigital ImageProcessing Method," IEEE Transaction and Electrical Insulation,Uppsala, Sweden, vol. 8 No.6, December 200 1, pp. 1098-1107

[4] Vaillancourt, G.H.; Bellerive, J.P.; St-Jean, M.; Jean, C. "New live linetester for porcelain suspension insulators on high-voltage power lines,"Power Delivery, IEEE Transactions on, Canada,Vol.9, Issue 1, Jan. 1994,pp.208 - 219

[5] Richards, C.N, Renowden, J.D, "Development of a remote insulatorcontamination monitoring system Power Delivery," IEEE Transactionson Power Delivery, USA,Vol.12, Issue 1, Jan. 1997 pp.389 - 397

[6] J.L.Fierro-Chavez, I.Ramirez-Vazquez, G.Montoya-Tena, "On-lineleakage current monitoring of 400 kV insulator strings in polluted areas,"Generation, Transmission and Distribution, IEE Proceedings, Mexico,Vol. 143, No. 6, Nov. 1996,pp.560 - 564

[7] Cheng Rong Li; Qian Shi; Yang Chun Cheng; Guo Lin; Huang Xingquan,"Identifying faulty insulator strings by detecting insulator corona pulsecurrent," Power System Technology 1998. Proceedings. POWERCON'98. 1998 International Conference on, Beiing, China, Vol. 1, Aug. 1998,pp.529- 532

[8] Suda, T. "Frequency characteristics of leakage current waveforms of anartificially polluted suspension insulator," IEEE Transactions onDielectrics and Electrical Insulation, Tokyo, Japan, Vol.8, No.4, Aug.2001, pp.705 - 709

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