- Email: [email protected]
The Inconveniences Related to Accelerated Thermal Ageing of Cables
Available online at www.sciencedirect.com Available online at www.sciencedirect.com
ScienceDirect ScienceDirect
Transportation Research Procedia 00 (2019) 000–000 Available online at www.sciencedirect.com Transportation Research Procedia 00 (2019) 000–000
ScienceDirect
www.elsevier.com/locate/procedia www.elsevier.com/locate/procedia
Transportation Research Procedia 40 (2019) 90–95 www.elsevier.com/locate/procedia
13th International Scientific Conference on Sustainable, Modern and Safe Transport 13th International 2019), Scientific Conference on Sustainable, and Safe Transport (TRANSCOM High Tatras, Novy Smokovec –Modern Grand Hotel Bellevue, (TRANSCOM 2019),Slovak High Tatras, Novy Smokovec – Grand Hotel Bellevue, Republic, May 29-31, 2019 Slovak Republic, May 29-31, 2019
The Inconveniences Related to Accelerated Thermal Ageing of The Inconveniences Related to Accelerated Thermal Ageing of Cables Cables Zuzana Šaršounová* Zuzana Šaršounová*
Dept. of Electrotechnology, Czech Technical University in Prague, Technická 2, Praha, Czech Republic Dept. of Electrotechnology, Czech Technical University in Prague, Technická 2, Praha, Czech Republic
Abstract Abstract
For the purpose of reliability estimation and lifetime determination, electrical equipment is often accelerated aged For the purpose of reliability estimation and lifetime electrical equipment is difficult often accelerated aged according to Arrhenius approach. The correct setup ofdetermination, accelerated thermal ageing is a very process where according to Arrhenius approach. The correctofsetup of accelerated thermal difficult with process the key problem is the correct determination the ageing temperature thatageing has to is beainvery accordance the where used the key problem the correct determination of the tendency ageing temperature that has be in process accordance with as thepossible. used materials. Due toistime and economic demands, is to accelerate the to ageing as much materials. Dueitto and economic demands, tendency to accelerate the ageingthermal processageing as much as possible. In this paper, is time described practically problemsthe that happenisduring cable accelerated if the ageing In this paper,isitdisproportionately is described practically temperature high. problems that happen during cable accelerated thermal ageing if the ageing temperature is disproportionately high.
© 2019 The Authors. Published by Elsevier B.V. © 2019 The Authors. Published by Elsevier B.V. © 2019 The Authors. Published byof Elsevier B.V. committee of the 13th International Scientific Conference on Sustainable, Peer-review under responsibility the scientific Peer-review under responsibility of the scientific committee of the 13th International Scientific Conference on Sustainable, Peer-review under responsibility of the scientific Modern and Safe Transport (TRANSCOM Modern and Safe Transport (TRANSCOM2019). 2019).committee of the 13th International Scientific Conference on Sustainable, Modern and Safe Transport (TRANSCOM 2019). Keywords: Arrhenius approach; cable; thermal ageing Keywords: Arrhenius approach; cable; thermal ageing
1. Introduction 1. Introduction Nowadays, practically all means of transportation contain kilometers of cables. Manufactured cars have more than practically of allvarious means of transportation contain kilometers of cables. Manufactured have more than oneNowadays, and a half kilometers cables, in plain (e.g. airbas A 380), the length of cable routs iscars approximately 500 one and a half kilometers of various cables, in plain (e.g. airbas A 380), the length of cable routs is approximately 500 km. The importance of safety long-term operation is obvious and further it can be demonstrated on the other vehicle km. The importance of safety long-term operation is obvious and further it can be demonstrated on the other vehicle examples such as trains, space shuttles and/or submarines. examples such as trains, space shuttles and/or submarines.
* Corresponding author. Tel.: + 420 2 2435 2353; E-mail address:author. [email protected] * Corresponding Tel.: + 420 2 2435 2353; E-mail address: [email protected] 2352-1465 © 2018 The Authors. Published by Elsevier B.V. Peer-review©under responsibility of the scientific committee 2352-1465 2018 The Authors. Published by Elsevier B.V. of the 13th International Scientific Conference on Sustainable, Moder n and Safe Transport (TRANSCOM 2019). Peer-review under responsibility of the scientific committee of the 13th International Scientific Conference on Sustainable, Moder n and Safe Transport (TRANSCOM 2019). 2352-1465 2019 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the scientific committee of the 13th International Scientific Conference on Sustainable, Modern and Safe Transport (TRANSCOM 2019). 10.1016/j.trpro.2019.07.015
2
Zuzana Šaršounová / Transportation Research Procedia 40 (2019) 90–95 Sarsounova / Transportation Research Procedia 00 (2019) 000–000
91
The ageing process is a chemical reaction that is depended on many factors, mainly on environmental condition (such as operation temperature, humidity, chemical pollution) and material composition; these are polymers in the case of the cable system. During the design process, cables should be tested if they are capable to be in operation in a required lifetime – it is unconditional for safety-critical cable systems. To demonstrate that the cable is in-laboratory accelerated aged. The most easier way how to proceed accelerated thermal ageing is to use higher temperatures. To calculate the time to equivalent damage at different temperatures the Arrhenius equation is usually used (1).
−EA k = A e RT
(1)
k - rate constant, A - preexponential factor, EA - activation energy, R - gas constant, T - temperature With regard to the exponential character of Arrhenius equation, it means that with a relatively small increase of temperature (or equivalently of activated energy) it is possible to achieve the huge increase of reaction rate. In most cases, only 10° C increase is enough for doubling or four times increase of reaction rates. Of course, there are more precise equations, e.g. Logan (1997). Nevertheless, it is in almost all cases used the Arrhenius equation for prediction of service lifetime of cables; not in its original form (1), but in a modified one (2). This equation grounds the rapid analytical methods of the thermal lifetime evaluation and its advantage is, that do not require the knowledge of the integral conversion function Budrugeac and Segal (1992).
t1 = t2 e
E A ( T2 − T1 ) k B T1T2
(2)
t1 - time of accelerated ageing, t2 - required service life, EA - activation energy (in eV), T1 - temperature of accelerated ageing, T 2 - service temperature, kB - Boltzmann constant. The test time (t1) in thermal chamber at elevated temperature, which should simulate the long-term service ageing, depends on the service conditions, testing temperature and activation energy. Applying this equation, we assume that the short-term simulation of thermal ageing at higher temperature caused the same degradation as the service longterm ageing at a lower temperature and that the cable has at the end of the testing the same mechanical and electrical properties. This is the very base for isoconversion method of analysis, in which the relations at the same base are used for kinetic analysis Ozawa et al. (1995-1996). 2. Testing Temperature For reliable simulation of long-term thermal ageing, the temperature of accelerated ageing should not be too far from the service temperature. The standards IEC 216 and IEC 610 recommend using not more the 25 °C difference. But if one would simulate 40 years of operation at 60 °C at the test temperature of 85 °C it would be an extremely long process. Hence, the test temperature is usually higher. The maximum allowed temperature is limited by the range of chemical stability or by any thermodynamical transition in the cable material, like glass transition (T g), melting point or material softening. If between the test and service temperature occurs such a phenomenon it is very difficult, if not impossible, to extrapolate the data from higher to lower temperature. The testing would be in this case not very reliable. A very good tool, which can easy and fast find out if such a process appears in the temperature of interest is differential scanning calorimeter (DSC). DSC records all processes, which are joined with heat consumption or evaluation. Since all considered transitions or processes are either exothermic (oxidative degradation, etc.)
Zuzana Šaršounová / Transportation Research Procedia 40 (2019) 90–95 Sarsounova / Transportation Research Procedia 00 (2019) 000–000
92
3
or endothermic (melting, Tg-point, etc.) Budrugeac and Segal (1992) and Höhne et al. (1996), the benefit of using DSC is quite evident. 3. Activation Energy Activation energy is not constant during the ageing period as seen in Fig 1. However, Arrhenius approach requires a single value of activation energy for calculation. Arrhenius approach expects, that the activation energy determined for one specific point of the conversion (specific property value) is temperature independent. This applies if the material ages with the same mechanism, i.e. in the case of thermal aging if the limit temperature is not exceeded. Each material contained in tested cable has different activation energy (EA). The differences in EA have to be taken into account. For example: maximum allowed testing temperature of cable sample is 105 °C. It means that for cable jacket with EA = 1.17 eV, it is 115 days thermal ageing but for core insulation (EA = 1.00 eV) it is 231 days. In practice, we encounter overaged components. Activation energy we can receive from the producer, database, literature. We can use conservative (low) value or determine it Bartoníček et. al (1999).
Activation energy (kJ/mol)
160
140
120
100
80
60 0
20
40
60
80
100
Conversion (%) Fig. 1. Activation energy is not constat, it may change during the ageing (Graph published with the kind permission of V. Plaček).
4. Examples of Thermal Ageing Problems from Practice The methodology for determining the time and temperature of accelerated aging has been outlined in this paper. In practice, we meet usually some problems connect with this approach and accelerated thermal ageing (Fig. 2a). 4.1. Different Thermal Expansivity After ageing of optic breakout cable, it was observed cracks in cable jacket as seen in the Fig. 2b. Although the jacket was in very good mechanical condition: elastic, not brittle, the cracks were in some loops (cables were placed in the chamber in loops). After visual inspection, the reason for the cracks was determined as different thermal expansivity. Probably, the ripcord cut the jacket – ripcord is a parallel cord of strong yarn that is situated under the jacket for its removal.
4
Zuzana Šaršounová / Transportation Research Procedia 40 (2019) 90–95 Sarsounova / Transportation Research Procedia 00 (2019) 000–000
a
93
b
Fig. 2. (a) Cable samples prepared on the drum in a thermal chamber. In the top, there are jacket and insulation samples destined for mechanical tests, Konečná (2017). (b) An example of jacket crack of optic breakout cable. During thermal ageing at a higher temperature, the cracks are appeared due to different thermal expansivity.
4.2. Higher Simulation/operation Temperature Simulation temperature can be higher if it is demanded to speed up the process of accelerated ageing. By increasing the temperature, it is increased ageing oxidation process, the number of radicals is raised. Radicals enter into different chemical reactions with a polymer chain. Subsequently, polymer chains are split, which means that polymers loss its mechanical properties, become brittle, crack and its electrical properties are impaired. Temperature fluctuations in the cable may cause internal stresses until cracks occur (cracks in insulation seen in Fig. 3a and in jacket in Fig. 3b). At elevated temperature, migration of stabilizers, plasticizers or other polymer additives also occurs.
a
b
Fig. 3. (a) An example of insulation cracks influenced by oxidation degradation at elevated temperature. (b) Cracked PVC jacket influenced by elevated temperature and loss of plasticizers.
4.3. Joule Heating Self-heating due to ohmic heating (Joule heating) is a common stressor for power cables, that should be taken into account during accelerated ageing. Cables are warming up by Joule’s heat, which increases operation temperature. In the case of communication cables, this factor should not be prevailing. Power cables, which are continuously operating, can be warming up to 10 °C above ambient temperature due to Joule’s heating and cable degradation speeds up.
94
Zuzana Šaršounová / Transportation Research Procedia 40 (2019) 90–95 Sarsounova / Transportation Research Procedia 00 (2019) 000–000
5
4.4. Influence of Additives It is very important for a quality product to maintain the quality of the cables production. Whatever changes in the composition of the polymers and their manufacturing technology can have significant consequences for cable lifetime, especially in extreme conditions of the means of transport. Used polymers contain a lot of additives, pigments, catalysts that shall improve polymer quality. However, not all catalysts are compatible with specific polymer granules. Final cable can have very different properties and it has very often an influence on the fire resistance of cable and lifetime. 4.5. Other Environmental Stressors There are also some other stressors like humidity, electrical field, chemicals, mechanical stress, fire, etc. These stressors are usually not a part of cable accelerated ageing simulation. The seismic and vibration factors/influences are usually not tested. However, if the cables are qualified with connector junctions, then vibration test is necessary – verification that vibration cannot cause disconnection / unscrewing of cable. The vehicles have special vibration profiles depending on the type of vehicles (trains, cars, etc.). 5. Conclusion The cables are arteries of vehicles. It is necessary to develop and keep the required quality during the whole lifetime. Sensors, controls unit, monitoring systems – all these parts are connected with cables thereby the cables are safety important parts of the vehicle. To simulate long term operation, cables are subjected to thermal ageing at elevated temperature. Conditions of accelerated ageing are based on Arrhenius approach. For calculation of required ageing time, the operation conditions (time, temperature), ageing temperature, activation energy are needed. Several approximations and limitations, associated with using of such a method, cause that the predicted service lifetime need not be fully justified. To overcome these limitations, it is important to know all the assumptions, simplifications, sources of uncertainty and to manage them. We should do: • For accelerated thermal ageing use Arrhenius approach. • Do not use too high ageing temperature. • Take into account how activation energy was determined. • At best use value determined from an important functional property. • Eliminate activation energy determined from product evaluation. • Do not forget, that activation energy can be determined with a specific accuracy. Use lower 95 % confidential level. • Use safety margin • On-going qualification, condition monitoring. Acknowledgements This work was supported by the Grant Agency of the Czech Technical University in Prague, grant No. SGS 16/224/OHK3/3T/13. Moreover, the author acknowledges the support of colleague Vít Plaček from Nuclear Research institute ÚJV Řež, a. s., Czech Republic. References Logan, S. R., 1997. Grundlagen der chemischen Kinetik (German Edition). Wiley-VCH. Budrugeac P., Segal E., 1992. Thermal analysis in the evaluation of the thermal lifetime of solid polymeric materials. Thermochimica Acta 211 131-136 IEC 216: Guide for the determination of thermal endurance properties of electrical insulating materials IEC 610: Principal aspects of functional evaluation of electrical insulation systems: Ageing mechanisms and diagnostic procedures
6
Zuzana Šaršounová / Transportation Research Procedia 40 (2019) 90–95 Sarsounova / Transportation Research Procedia 00 (2019) 000–000
95
Ozawa T., Kaneko T., Sunose T., 1995-1996. Historical review on research of kinetics in thermal analysis and thermal endurance of electrical insulating materials. Part I: J. Therm. Anal.44 (1995) 205-216, Part II: J. Therm. Anal. 47 (1996) 1205-1220 Höhne G., Hemminger W., Flammersheim H. J., 1996. Differential scanning calorimetry. An introduction for practitioners. Springer-Verlag, Berlin. Bartoníček, B., Hnát, V., Plaček, V., 1999. Comparison of the activation energies as determined by DSC and by conventional methods. J. Thermal. Anal. 56, p. 799-804. Konečná, Z., 2017. Cable Qualification for Nuclear Power Plants. Proceedings of the International Student Scientific Conference Poster, pp. 1-4
ScienceDirect ScienceDirect
Transportation Research Procedia 00 (2019) 000–000 Available online at www.sciencedirect.com Transportation Research Procedia 00 (2019) 000–000
ScienceDirect
www.elsevier.com/locate/procedia www.elsevier.com/locate/procedia
Transportation Research Procedia 40 (2019) 90–95 www.elsevier.com/locate/procedia
13th International Scientific Conference on Sustainable, Modern and Safe Transport 13th International 2019), Scientific Conference on Sustainable, and Safe Transport (TRANSCOM High Tatras, Novy Smokovec –Modern Grand Hotel Bellevue, (TRANSCOM 2019),Slovak High Tatras, Novy Smokovec – Grand Hotel Bellevue, Republic, May 29-31, 2019 Slovak Republic, May 29-31, 2019
The Inconveniences Related to Accelerated Thermal Ageing of The Inconveniences Related to Accelerated Thermal Ageing of Cables Cables Zuzana Šaršounová* Zuzana Šaršounová*
Dept. of Electrotechnology, Czech Technical University in Prague, Technická 2, Praha, Czech Republic Dept. of Electrotechnology, Czech Technical University in Prague, Technická 2, Praha, Czech Republic
Abstract Abstract
For the purpose of reliability estimation and lifetime determination, electrical equipment is often accelerated aged For the purpose of reliability estimation and lifetime electrical equipment is difficult often accelerated aged according to Arrhenius approach. The correct setup ofdetermination, accelerated thermal ageing is a very process where according to Arrhenius approach. The correctofsetup of accelerated thermal difficult with process the key problem is the correct determination the ageing temperature thatageing has to is beainvery accordance the where used the key problem the correct determination of the tendency ageing temperature that has be in process accordance with as thepossible. used materials. Due toistime and economic demands, is to accelerate the to ageing as much materials. Dueitto and economic demands, tendency to accelerate the ageingthermal processageing as much as possible. In this paper, is time described practically problemsthe that happenisduring cable accelerated if the ageing In this paper,isitdisproportionately is described practically temperature high. problems that happen during cable accelerated thermal ageing if the ageing temperature is disproportionately high.
© 2019 The Authors. Published by Elsevier B.V. © 2019 The Authors. Published by Elsevier B.V. © 2019 The Authors. Published byof Elsevier B.V. committee of the 13th International Scientific Conference on Sustainable, Peer-review under responsibility the scientific Peer-review under responsibility of the scientific committee of the 13th International Scientific Conference on Sustainable, Peer-review under responsibility of the scientific Modern and Safe Transport (TRANSCOM Modern and Safe Transport (TRANSCOM2019). 2019).committee of the 13th International Scientific Conference on Sustainable, Modern and Safe Transport (TRANSCOM 2019). Keywords: Arrhenius approach; cable; thermal ageing Keywords: Arrhenius approach; cable; thermal ageing
1. Introduction 1. Introduction Nowadays, practically all means of transportation contain kilometers of cables. Manufactured cars have more than practically of allvarious means of transportation contain kilometers of cables. Manufactured have more than oneNowadays, and a half kilometers cables, in plain (e.g. airbas A 380), the length of cable routs iscars approximately 500 one and a half kilometers of various cables, in plain (e.g. airbas A 380), the length of cable routs is approximately 500 km. The importance of safety long-term operation is obvious and further it can be demonstrated on the other vehicle km. The importance of safety long-term operation is obvious and further it can be demonstrated on the other vehicle examples such as trains, space shuttles and/or submarines. examples such as trains, space shuttles and/or submarines.
* Corresponding author. Tel.: + 420 2 2435 2353; E-mail address:author. [email protected] * Corresponding Tel.: + 420 2 2435 2353; E-mail address: [email protected] 2352-1465 © 2018 The Authors. Published by Elsevier B.V. Peer-review©under responsibility of the scientific committee 2352-1465 2018 The Authors. Published by Elsevier B.V. of the 13th International Scientific Conference on Sustainable, Moder n and Safe Transport (TRANSCOM 2019). Peer-review under responsibility of the scientific committee of the 13th International Scientific Conference on Sustainable, Moder n and Safe Transport (TRANSCOM 2019). 2352-1465 2019 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the scientific committee of the 13th International Scientific Conference on Sustainable, Modern and Safe Transport (TRANSCOM 2019). 10.1016/j.trpro.2019.07.015
2
Zuzana Šaršounová / Transportation Research Procedia 40 (2019) 90–95 Sarsounova / Transportation Research Procedia 00 (2019) 000–000
91
The ageing process is a chemical reaction that is depended on many factors, mainly on environmental condition (such as operation temperature, humidity, chemical pollution) and material composition; these are polymers in the case of the cable system. During the design process, cables should be tested if they are capable to be in operation in a required lifetime – it is unconditional for safety-critical cable systems. To demonstrate that the cable is in-laboratory accelerated aged. The most easier way how to proceed accelerated thermal ageing is to use higher temperatures. To calculate the time to equivalent damage at different temperatures the Arrhenius equation is usually used (1).
−EA k = A e RT
(1)
k - rate constant, A - preexponential factor, EA - activation energy, R - gas constant, T - temperature With regard to the exponential character of Arrhenius equation, it means that with a relatively small increase of temperature (or equivalently of activated energy) it is possible to achieve the huge increase of reaction rate. In most cases, only 10° C increase is enough for doubling or four times increase of reaction rates. Of course, there are more precise equations, e.g. Logan (1997). Nevertheless, it is in almost all cases used the Arrhenius equation for prediction of service lifetime of cables; not in its original form (1), but in a modified one (2). This equation grounds the rapid analytical methods of the thermal lifetime evaluation and its advantage is, that do not require the knowledge of the integral conversion function Budrugeac and Segal (1992).
t1 = t2 e
E A ( T2 − T1 ) k B T1T2
(2)
t1 - time of accelerated ageing, t2 - required service life, EA - activation energy (in eV), T1 - temperature of accelerated ageing, T 2 - service temperature, kB - Boltzmann constant. The test time (t1) in thermal chamber at elevated temperature, which should simulate the long-term service ageing, depends on the service conditions, testing temperature and activation energy. Applying this equation, we assume that the short-term simulation of thermal ageing at higher temperature caused the same degradation as the service longterm ageing at a lower temperature and that the cable has at the end of the testing the same mechanical and electrical properties. This is the very base for isoconversion method of analysis, in which the relations at the same base are used for kinetic analysis Ozawa et al. (1995-1996). 2. Testing Temperature For reliable simulation of long-term thermal ageing, the temperature of accelerated ageing should not be too far from the service temperature. The standards IEC 216 and IEC 610 recommend using not more the 25 °C difference. But if one would simulate 40 years of operation at 60 °C at the test temperature of 85 °C it would be an extremely long process. Hence, the test temperature is usually higher. The maximum allowed temperature is limited by the range of chemical stability or by any thermodynamical transition in the cable material, like glass transition (T g), melting point or material softening. If between the test and service temperature occurs such a phenomenon it is very difficult, if not impossible, to extrapolate the data from higher to lower temperature. The testing would be in this case not very reliable. A very good tool, which can easy and fast find out if such a process appears in the temperature of interest is differential scanning calorimeter (DSC). DSC records all processes, which are joined with heat consumption or evaluation. Since all considered transitions or processes are either exothermic (oxidative degradation, etc.)
Zuzana Šaršounová / Transportation Research Procedia 40 (2019) 90–95 Sarsounova / Transportation Research Procedia 00 (2019) 000–000
92
3
or endothermic (melting, Tg-point, etc.) Budrugeac and Segal (1992) and Höhne et al. (1996), the benefit of using DSC is quite evident. 3. Activation Energy Activation energy is not constant during the ageing period as seen in Fig 1. However, Arrhenius approach requires a single value of activation energy for calculation. Arrhenius approach expects, that the activation energy determined for one specific point of the conversion (specific property value) is temperature independent. This applies if the material ages with the same mechanism, i.e. in the case of thermal aging if the limit temperature is not exceeded. Each material contained in tested cable has different activation energy (EA). The differences in EA have to be taken into account. For example: maximum allowed testing temperature of cable sample is 105 °C. It means that for cable jacket with EA = 1.17 eV, it is 115 days thermal ageing but for core insulation (EA = 1.00 eV) it is 231 days. In practice, we encounter overaged components. Activation energy we can receive from the producer, database, literature. We can use conservative (low) value or determine it Bartoníček et. al (1999).
Activation energy (kJ/mol)
160
140
120
100
80
60 0
20
40
60
80
100
Conversion (%) Fig. 1. Activation energy is not constat, it may change during the ageing (Graph published with the kind permission of V. Plaček).
4. Examples of Thermal Ageing Problems from Practice The methodology for determining the time and temperature of accelerated aging has been outlined in this paper. In practice, we meet usually some problems connect with this approach and accelerated thermal ageing (Fig. 2a). 4.1. Different Thermal Expansivity After ageing of optic breakout cable, it was observed cracks in cable jacket as seen in the Fig. 2b. Although the jacket was in very good mechanical condition: elastic, not brittle, the cracks were in some loops (cables were placed in the chamber in loops). After visual inspection, the reason for the cracks was determined as different thermal expansivity. Probably, the ripcord cut the jacket – ripcord is a parallel cord of strong yarn that is situated under the jacket for its removal.
4
Zuzana Šaršounová / Transportation Research Procedia 40 (2019) 90–95 Sarsounova / Transportation Research Procedia 00 (2019) 000–000
a
93
b
Fig. 2. (a) Cable samples prepared on the drum in a thermal chamber. In the top, there are jacket and insulation samples destined for mechanical tests, Konečná (2017). (b) An example of jacket crack of optic breakout cable. During thermal ageing at a higher temperature, the cracks are appeared due to different thermal expansivity.
4.2. Higher Simulation/operation Temperature Simulation temperature can be higher if it is demanded to speed up the process of accelerated ageing. By increasing the temperature, it is increased ageing oxidation process, the number of radicals is raised. Radicals enter into different chemical reactions with a polymer chain. Subsequently, polymer chains are split, which means that polymers loss its mechanical properties, become brittle, crack and its electrical properties are impaired. Temperature fluctuations in the cable may cause internal stresses until cracks occur (cracks in insulation seen in Fig. 3a and in jacket in Fig. 3b). At elevated temperature, migration of stabilizers, plasticizers or other polymer additives also occurs.
a
b
Fig. 3. (a) An example of insulation cracks influenced by oxidation degradation at elevated temperature. (b) Cracked PVC jacket influenced by elevated temperature and loss of plasticizers.
4.3. Joule Heating Self-heating due to ohmic heating (Joule heating) is a common stressor for power cables, that should be taken into account during accelerated ageing. Cables are warming up by Joule’s heat, which increases operation temperature. In the case of communication cables, this factor should not be prevailing. Power cables, which are continuously operating, can be warming up to 10 °C above ambient temperature due to Joule’s heating and cable degradation speeds up.
94
Zuzana Šaršounová / Transportation Research Procedia 40 (2019) 90–95 Sarsounova / Transportation Research Procedia 00 (2019) 000–000
5
4.4. Influence of Additives It is very important for a quality product to maintain the quality of the cables production. Whatever changes in the composition of the polymers and their manufacturing technology can have significant consequences for cable lifetime, especially in extreme conditions of the means of transport. Used polymers contain a lot of additives, pigments, catalysts that shall improve polymer quality. However, not all catalysts are compatible with specific polymer granules. Final cable can have very different properties and it has very often an influence on the fire resistance of cable and lifetime. 4.5. Other Environmental Stressors There are also some other stressors like humidity, electrical field, chemicals, mechanical stress, fire, etc. These stressors are usually not a part of cable accelerated ageing simulation. The seismic and vibration factors/influences are usually not tested. However, if the cables are qualified with connector junctions, then vibration test is necessary – verification that vibration cannot cause disconnection / unscrewing of cable. The vehicles have special vibration profiles depending on the type of vehicles (trains, cars, etc.). 5. Conclusion The cables are arteries of vehicles. It is necessary to develop and keep the required quality during the whole lifetime. Sensors, controls unit, monitoring systems – all these parts are connected with cables thereby the cables are safety important parts of the vehicle. To simulate long term operation, cables are subjected to thermal ageing at elevated temperature. Conditions of accelerated ageing are based on Arrhenius approach. For calculation of required ageing time, the operation conditions (time, temperature), ageing temperature, activation energy are needed. Several approximations and limitations, associated with using of such a method, cause that the predicted service lifetime need not be fully justified. To overcome these limitations, it is important to know all the assumptions, simplifications, sources of uncertainty and to manage them. We should do: • For accelerated thermal ageing use Arrhenius approach. • Do not use too high ageing temperature. • Take into account how activation energy was determined. • At best use value determined from an important functional property. • Eliminate activation energy determined from product evaluation. • Do not forget, that activation energy can be determined with a specific accuracy. Use lower 95 % confidential level. • Use safety margin • On-going qualification, condition monitoring. Acknowledgements This work was supported by the Grant Agency of the Czech Technical University in Prague, grant No. SGS 16/224/OHK3/3T/13. Moreover, the author acknowledges the support of colleague Vít Plaček from Nuclear Research institute ÚJV Řež, a. s., Czech Republic. References Logan, S. R., 1997. Grundlagen der chemischen Kinetik (German Edition). Wiley-VCH. Budrugeac P., Segal E., 1992. Thermal analysis in the evaluation of the thermal lifetime of solid polymeric materials. Thermochimica Acta 211 131-136 IEC 216: Guide for the determination of thermal endurance properties of electrical insulating materials IEC 610: Principal aspects of functional evaluation of electrical insulation systems: Ageing mechanisms and diagnostic procedures
6
Zuzana Šaršounová / Transportation Research Procedia 40 (2019) 90–95 Sarsounova / Transportation Research Procedia 00 (2019) 000–000
95
Ozawa T., Kaneko T., Sunose T., 1995-1996. Historical review on research of kinetics in thermal analysis and thermal endurance of electrical insulating materials. Part I: J. Therm. Anal.44 (1995) 205-216, Part II: J. Therm. Anal. 47 (1996) 1205-1220 Höhne G., Hemminger W., Flammersheim H. J., 1996. Differential scanning calorimetry. An introduction for practitioners. Springer-Verlag, Berlin. Bartoníček, B., Hnát, V., Plaček, V., 1999. Comparison of the activation energies as determined by DSC and by conventional methods. J. Thermal. Anal. 56, p. 799-804. Konečná, Z., 2017. Cable Qualification for Nuclear Power Plants. Proceedings of the International Student Scientific Conference Poster, pp. 1-4