2004 EEEIPES Transmission B Distribution Conference L Exposition: Latin America 1

Electro-thermal Simulation of ZnO Arresters for Diagnosis Using Thermal Analysis

E. T. Wanderley Net0 and E. G. da Costa, UFCG M. J. A. Maia, T. C. L. Galindo and A. H. S . Costa, CHEW

Abstrucr - Surge arresters are equipments of greater importance for the protection of electrical systems. Their choice for a specific substation line or equipment must folIow a rigid selection and testing. The maintaining of these equipments by the realization of regular monitoring also plays a major rule for the system integrity - an arrester failure may result in an its explosion or system turn off. Thermographic inspection is one of the most used techniques for monitoring high voltage devices. It registers the temperature gradient along the equipments surface, indicating overheat. For zinc oxide fZnO) arresters, this technique presents a limitation. Because of the low thermal conductivity of the porcelain or polymeric housing there is no direct relation between the temperatures on the housing surface and the temperatures on varistors. A solution for this limitation iS the development of computational routines to make this correlation. This paper presents a program based on finite difference techniques that makes the heat transfer over dl the arrester indicating the temperature OI any desired point. It considers both ceramic and polymeric housed arresters and can be used to simulate electrical tests de8 ned in technical normative and to estimate the inner arrester temperatures when the outside temperatures are measured. A comparison between thermal dissipation for porcelain and polymeric insulated arresters is also presented.

Index Terms - Arrester, monitoring, simulation, thermograph, varistor, zinc oxide.

I. INTRODUCTION RRESTERS are protection equipments connected A between phase and ground. They limit the voltage level

in equipments like transformers, avoiding them to be submitted to a voltage inadequate to their operation. They are characterized by a non-linear current flow through their terminals, presenting a low leakage current for continuous operation and a high level of conduction when submitted to overvoltages - in this case, the energy related to the high current conduction must be dissipated by joule effect.

ZnO arresters present a very simple structure. They are composed basically of an insulating housing, made of porcelain or polymeric material (usually silicone rubber), and an inner active column, composed of the ZnO varistors and thermal dissipating elements. The varistors are the main components of the equipment, They provide the desired non- linear characteristics and present a strong relation with temperature, which is associated with its energy absorption capacity [ 1,2 1. The main differences between polymeric and porcelain insulated arresters are the constructive designs and

the housing mechanical properties. Porcelain insulated equipments present an air gap between varistors and housing (Fig. 1). They also present a greater mechanical and electric resistivity. Polymeric insulated arresters usually do not present an air gap between ZnO and housing (Fig 2). In most cases, the polymeric material is molded directly on the varistor columns,

Top c~ver pate \

Fig. 1 -Constructive design of a porcelain insulated arrester [9].

Aluminium end fitting

Silicone Nbber housing

. ZnO varistor

FRP-fods

Fig. 2 -Constructive design of a polymeric insulated arrester [9].

11. NORMATIVE TESTS AND THERMOGRAPHIC INSPECTION

A. Normntive Tests Arresters must be submitted to tests defined by norms so

that it can be considered adequate for the use in a power substation [ 13,141. These tests usually measure the capacity of the arrester to stand short-circuit requests with the correct operation of the pressure-relief device (in the case of ceramic housing) or with a fragmentation previously evaluated (in the case of polymeric housing devices).

Another test usually applied to arresters is the use of a stressing operation cycle for a long time interval (5,000 hours) called accelerated weather ageing cycle under operation voltage. This test evaluates the behavior of the equipment by the simulation of the environmental conditions for which it

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would be submitted during its lifetime. The execution of these tests demands a previous study and

a significant cost. It is necessary the utilization of a new sample arrester and the participation of specialized technicians in a laboratory. A digital model is abie to execute these normative tests presenting an estimation of the final results. The presentation of these previous results represents a saving in resources and time, accelerating the realization of the real tests in the sample arrester,

B. Tkrmographic Evaluation

Different monitoring techniques can be used with surge arresters, One of these techniques is the thermography. Thermographic inspection is usually used with most equipments in substations and with transmission lines to detect overheating patterns which may indicate failures in the equipment [3]. Besides, any changing in the arrester behavior results in temperature changing [SI. Meantime, using thermographic inspection results in the analysis of only the extemal surface of the equipments. In the case of arresters, it can indicate some kind of problem, although it is not possible to know what is happening to its varistor column, which represents its functional part.

The knowledge of the varistors behavior inside the insulating housing is important to detect which kind of problem is happening to the arrester. One alternative to analyze the arrester behavior is the usage of electro-thermal models that represent the whole equipment structure and the physicaI phenomena involved. With the aid of an adequate model, it is possible to estimate the temperatures in the ZnO elements starting from the outside arrester surface temperatures obtained from the thermographic inspection. Besides, this knowledge and the employment of simulation are also extremely useful for the realization of electrical tests that usually precedes arrester acquisition by electrical companies.

111. THE ARRESTER ELECTRO-THERMAL MODEL ZnO arresters can be modeled in different ways - by

analogy with electrical circuitry, by section models or by computational simulation. This last one is the most used modeling, being more accurate, flexible and fast [ 1 11.

Digital simulation makes use of mathematical equations that represents the electric and thermal phenomena related to the arrester operation. These phenomena include mainly the energy absorption in the active components of the arrester and the dissipation of this same energy by heat transfer.

A . Fundamentals 1) Energy absorption An arrester absorbs electrica1 energy transforming it into

thermal energy. The varistors power absorption can be calculated by the measurement of the voltage applied to the arrester and the current running through its terminals. If this power is maintained for a certain period of time, the total energy absorbed can be also calculated. In laboratory, these

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values can be obtained by the execution of tests with actual high voltage arresters.

For the normative tests, a sequence of different voltage entrances is used so that in digital simulation, equations representing different functions for the applied voltage are developed - periodic waveforms, steps, impulses, voltage surges, allowing the usage of a wide variety of power entrance for the arrester model.

It is important to notice that the arrester also absorbs thermal energy by other means like solar radiation and proximity with heat sources. Although they represent only a small portion of the total energy absorbed by the arrester, they must be correctly represented in the modeling so that it will be the closest possible to real systems.

2) Energy dissipation

The electrical energy absorbed by the arrester is converted in thermal energy by joule effect. This energy can now be dissipated to the environment by means of conduction, convection and radiation. Each one of these heat transfer processes are used for different regions of the arrester. Inside solid elements, the conduction plays a major role, while convection and radiation takes place in regions where air or other gaseous substances are present.

One of the most desired characteristics of an arrester is its thermal stability. It has to return to its normal operation conditions after the occurrence of any electrical request so that it is able to face another possible subsequent surge 111. When a varistor is degraded, it looses its thermal stability reaching a process of thermal runaway, characterized by the continuous increase of current and temperature resulting in the varistor cracking or even the arrester explosion. Thermal runaway can also happen to equipments improperly chosen to specific operations. A degraded or improper arrester must be replaced before compromising the integrity of the electrical system in which it is connected [9], that is the importance of preliminary testing and regular monitoring.

The heat transfer flow for dissipation depends mainly on the kind of materials used. ZnO varistors presents a high thermal conductivity level, so that the energy dissipated in this elements are easily conducted to their borders. Leaving the ZnO column, the kind of housing used will now determine how the heat transfer happens. Porcelain housed arresters present a very low heat transfer duty. It happens because they have a large air gap between the ZnO column and the housing. As the air presents a very low conductivity, the heat transfer happens by means of convection, which is a slow process. Besides that, the porcelain also presents a lower value of conductivity when compared to silicon rubber.

B. Finite Differences Method The finite differences technique allows the analysis of the

temperature variation in a bulk solid in both radial and axial directions. An arrester model using this method can be used to determine the heat flow and thermal stability of an arrester

submitted to different operational states [7,8]. The method is based on the construction of a net

representing the desired arrester. Each element of the net is considered as an elementary unity for which all the thermal, mechanic and electric proprieties are valid.

The efficiency of this method depends on the quality of de generated net. The most complex is the geometry of the analyzed object, the less efficient will be the generated net. To avoid this problem, the arrester representation is made by approximating the round borders like the ones of the housing sheds by orthogonal borders. It allows an optimized use of the finite differences technique but results in an accuracy loss because of the approximation done for the sheds. A simple example of the finite elements is shown in Fig. 4 [ 1 1,6].

I+M

Fig. 3 -Example of finite elements with heat transfer by conduction mechanism.

In Fig. 3 , the element I transfer its heat by conduction to the elements 1+1, I - I , I+M and I-M. Another possibility is the one shown in Fig. 4, In this picture there is an air gap between the elements I and I+I, representing part of the ZnO varistor and part of the porcelain housing respectively. For this configuration, the heat transfer made between these two elements is made by convection and radiation.

Fig. 4 - Example of finite elements with heat transfer by conduction, convection and radiation mechanisms.

For the complete object being studied, some elements will behave as heat sources, so that the heat transfer flow can be calculated for all the other elements. For the arrester, two different options are possible. In the first case, the sources are the elements receiving the electric power that is converted do thermal power. In the second case, the sources are the finite elements containing the housing surface, for which the temperatures are measured with the thermograph. The finit differences method can now be used to analyze the behavior of the arrester when submitted to a voltage level, or to estimate the inner temperatures for monitoring purposes.

Iv. LA~ORATORY TESTS AND SIMULATION

A. Experimental Set An experimental set with actual arresters samples was used

so that the results estimated in digital simulation could be compared with the results measured in laboratory. The sample arrester, a high voltage source, a resistive divider, a compensating capacitor, a digital oscilloscope and a digital computer compose this set. A thermographer and contact thermometers are also used for the tests. A schematic diagram is shown in Fig, 5.

The resistive divider provides a voltage signal proportional to system voltage so that the oscilloscope can be used. The capacitor branch is used to compensate the capacitive current generated by the arrester. The oscilloscope registers the values of voltage and current and a computational program developed in laboratory stores and processes these values and shows the amount of energy absorbed by the arresters for each changing in the value of the applied voltage. At the same time, thermal measurements are made with thermographs so that the values of the temperatures obtained can be compared with the values calculated in the simulation.

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Fig. 5 -Experimental set.

B. Methodology For the tests realized in laboratory, a continuous

overvoltage is applied to the arrester resulting in overheating of the ZnO varistors. Contact bimetallic thermometers are used in different parts of the arrester, including the inner column. The oscilloscope registers the applied voltage and the leakage current, sending these values to the computer. Termographic pictures are obtained for each ten minutes so that the evaluation of the arrester external behavior is also analyzed.

When the tests are concluded, the values of voltage, current and temperature are used for the electrothermal simulation. Different cases can be executed, according to the users requirements. Voltage and current can be used as entry data so that a thermal evaluation of the whole arrester can be estimated or, the values of the measured surface temperatures can be used to anticipate the inner temperatures.

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V. RESULTS

120 w Different test cases were executed with the simulation program in order to analyze the results. The first case tested represents the arrester cooling after an overvoltage. The second case represents the execution of normative tests usually made for the electrical companies to determine the feasibility of the arrester usage in their substations. The third case represents the estimative of the arrester inner temperatures when the outer temperatures are fixed (e.g., when the outer temperatures are measured with an thennographer). For all the cases, the results involving porcelain housed arrester could be compared with the results obtained with real arresters in laboratory tests. For the porcelain cases, the simulation and laboratory tests results match with a very small error. For the simulation involving polymeric arresters, additional tests should be made in laboratory although the results shown here presents the behavior expected for these equipments.

A . Arrester cooring after an overvoltage For this case, an overvoltage is applied to the arrester until

its inner temperature increases to a previously determined level. The voltage source is disconnected and the arrester cools down. The heat transfer can be seen in pictures bellow. The Fig. 6 and 7 show the temperature on the surface of a varistor in the middle of the active columns after the temperature of 127°C is reached. An important point to be noticed is that the cooling for the polymeric housed arrester is faster because of the internal air gap absence.

Fig. 8 and 9 show the temperature along the length of the arresters when the maximum temperature is reached on the ZnO surface. Because of the air gap, the difference between the ZnO surface and the porcelain surface is extremely large when compared with the polymeric case, This difference is the main obstacle for a correct evaluation of termographic images of porcelain-housed arresters. For the polymeric case, the internal housing surface is in direct contact with the ZnO column surface, so there is no distinction for the temperatures on these surfaces.

1 4 0 , .-----l

120 4

2o I O ! i

0 100 200 300 400 500

Time (mln)

Fig. 6 -Cooling curve for a varistor in a porcelain housing arrester when submitted to an overvoltage.

1 ,

4 a I 2 16 20 24

Time (min)

Fig. 7 - Cooling curve for a varistor in a poIymeric housing arrester when submitted to an overvoltage.

-- I

8 3 5

f 30

LY , 0,O 0.2 0,4 0,6 0,8 1,0 1,2 1,4

Lenght (m)

-2nOSUrfaaat +porcelain intemalsurhoe --Pordalnw3w" surt3a

Fig. 8 -Comparison between ZnO column surface temperature and porcelain housing surface temperature.

26,6 4 0 0,2 OA 0,s 0,s

Lenght (m)

--r Zno surface - Pdpenc a e m d sufface

Fig. 9 -Comparison between ZnO columns surface temperature and polymeric housing surface temperature.

B. Normative tem For these simulations, a sequence of pulses and

overvoltages defined by technical norms are applied to the arrester and its thermal behavior is analyzed. This is the kind of test usually made with real arrester samples to evaluate its

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viability for a specific site. Fig. 10 and 1 1 present the results the simulation software for the analysis of the arrester when a for a normalized ANSI test. Fig. 10 depicts the temperature thermographer is used for equipment inspection. for points on ZnO surface and porcelain internal and external AS shown in Fig. 12, even when a porcelain outside surface at 54 cm from the base. In Fig. 11, the points are about tempemme of 3O0C is measured by a themographer, the 60 cm from the base. varistor already reached about 53OC emphasizing the absence

of control of the inner arrester temperature. 1

0 0 100 200 300 400 500

Time (min)

-Z-DSulrace+Pm lntemalsurface -~-Pm.E~temalsrrrface

Fig. 10 -ANSI test for a porcelain housed arrester.

55

- 5 0 Y

E M

E30

y" g35

-

25

20 0.2 0,4 0.6 0,8 1 1.2 1.4

Arresterlengbt (m)

+Estimated temperature +~hed t e m p "

Fig. 12 - Temperature evaluation on varistor column surface of a porcelain housed arrester when the outer surface temperature is fixed.

so

70 t = g o

I + 4 0

g 50

30

20 o 100 zoo 300 400 500 600 7w 800

T i m (mn)

+ Z d surtace -9-kkxsirg e x t m surface

Fig. 11 -ANSI test for a polymeric housed arrester.

Results emphasize the better thermal dissipation of polymeric housing arrester. For the used ANSI test, the temperature pattem in the chosen point of polymeric housed arrester follows narrowly the applied voltage pattern. For the porcelain case, the difficult to transfer energy is visible in Fig. 10. The temperature doesn't follow the pattem of the applied voltage, demonstrating a high thermal resistivity of the porcelain housing - the temperature increases and decreases in a very low duty.

Other interesting observation is the proximity between temperatures in silicon rubber surface and varistors surface for polymeric case and the distance between temperatures on porcelain surface and varistors surface for porcelain cases. These results also indicate a better dissipation for polymeric case mainly due to the absence of intemal air gap.

C. Inner temperature estimation

For this case, the temperature on the housing surface is fixed in about 30°C so that the temperature in the ZnO varistors can be estimated. I t could represent the application of

VI. CONCLUSION Arresters are equipments of hndamental importance to

substations and transmission systems. Being connected directly between electric system and ground they are submitted to an electrical leakage current which increases when facing an electrical surge, overvoltage or when the arrester varistors are degraded.

The main consequence of increasing the leakage current is the elevation of the varistors temperature, what can lead to their complete degradation and cracking. To avoid the problems related above, arresters behavior must be studied before their installation in the site, determining their feasibility to that specific use. They must also be submitted to preventive inspection from time to time when aIready installed. The analysis of thermographic images is one of the most used inspection procedures in substations.

An electrothermal simulation computational program was presented. This program is capable of simulating the heating over all the arrester when a sequence of voltage waveforms is applied on its terminals - what is useful to analyze the arrester behavior when submitted to extreme electrical solicitations or norm tests. It is also useful to estimate the inner arrester temperature when the temperatures in the arrester housing surface are known - this is a tool for predicting the temperatures in the varistors surface when a thermal image is analyzed.

Simulations of different cases were executed showing the usefilness of the program. The simulation results also compare the behavior of porcelain housing and polymeric housing arresters when facing similar electrical conditions, showing that arresters with polymeric housing are Iess affected by overheating.

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M. J. de A. Maia received his B.Sc. degree in electrical engineering in 1978 from the Universidade Federal da Paraiba. Since 1978, he has been with the Companhia Hidro Elittrica do si0 Francisco, Recife, PE, Brazil. His research interests include power system, surge arresters, transmission and distribution of electric energy.

VII. ACKNOWLEDGMENT

Authors would like to thank Max Norat from C E S F for being always ready to help with technical infomation and equipment lending.

VTII. REFERENCES

W. G. Carlson, T. K. Gupta, A. S w m t a ~ . “A Procedure for Estimating the Lifetime of Gapless Metal Oxide Surge Arresters”. IEEE Trans. on PowerSystems, vol. 1, n. 2, p , 67-73, April, 1986. T. K. Gupta. “Application of zinc oxide varistors”. J. Am. Ceram. Soc.,

2. Korendo and M. Florkowski, “‘Thermography-based diagnostics of power equipment”, IEE Power Engineering Journal, pp. 3342, Feb. 2001. M. V. Lat. “Thermal properties of metal oxide surge arresters”, IEEE Transactions on Power Apparatus and Systems, Vol. 102, No 7, pp.

C. Heinrich, V. Hinrichsen. “Diagnostics and Monitoring of Metal- Oxide Surge Arresten in High-Voltage Networks - Comparison of Existing and Newly Developed Procedures”. IEEE Transactions on PowerDelivery, Vol. 16, No 1, January 2001. E. G da Costa, S. R. Naidu, A. G de Lima. “Electrothermal model for complete metal-oxide surge arresters”. IEE Proceedings - Generation, Transmission and Distribution, Vol. 148,N” 1, pp. 29-33, January 2001. C. R. Maliska, “Transfdacia de Calor e Mecinica dos Fluidos Computacional”. LTC, Rio de Janeiro, RJ, 1995. J, P. Holman, “Transferhcia de calor”. Mc Graw-Hill do Brad, Slo Paulo, 1983. ABB Switchgear, “Physical properties of zinc oxide varistors”. ABB Power Technology Products AB, 2001 V. Hinrichsen, “Metal Oxide Surge Arresters ~ Fundamentals”, Siemens - Power Transmission and Distribution Power Voltage Division, Berlin, 2001 E. T. Wanderley Neto. “Pdra-Roios de &do de Zinco: Fundanentos, CaracterizuqGo e Monitoramento ’‘. Research project presented at Federal University of Campina Grande, February 2003. E. G . da Costa, “Anilise do Desempenho de Para-bios de bxido de Zinco”, Ph.D Dissertation, Electrical Engineering Department, Federal University of Paraiba, 1999. IEEE standard for metal oxide surge arresters for ac power circuits.

Metal-oxide surge arresters withour gaps for ax. systems, IEC 99-4, SURGE ARRESTERS Part 4. First edition, 1991-1 I.

Vol. 73,ND7, p. 1817-1840, 1990.

2194-2202, July 1983.

ANSiflEEE C62.11-1987.

IX. BIOGRAPHIES

Esticio Tavares Wanderley Neto was born in cam pi^ Grande, Brazil, on September 15, 1977. He graduated from the Federal University of Paraiba, Campina Grande, and obtained his Maters degree from the Federal University of Campina Grande in 2003 His expenence includes a professional apprenticeship at the Companhia Hidrelktnca do SE0 Francisco - CHESF in 2000. Esticio is now applying for his PhD. degree from the Federal University of Campina Grande in the area of zinc oxide arresters

S

Edson C. da Costa was bom In Brazil, 1954 He graduated as Electncal Engineer in 1978, received his Masters degree in 1981 and his Ph D title In 1999, at Federal University of Paraiba His interests are power systems, electnc fields, partial discharges, arresters and insulators. His IS professor at the Elrctncal Engineenng Department of Federal University of Campina Grande since 1978

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