Because of their crucial role in protecting power electronic equipment against over-voltages and transients, metal oxide varistors (MOV) represent a key component of the electrical system. Although improper heat dissipation and internal temperature rises can lead to a significant decrease in performance or even to a wear-out failure of the MOV, there are very few studies in the literature describing the thermal modeling of varistors. Thus, in this paper, an instantaneous loss and thermal modeling procedure for MOV under a standard current surge mission profile is proposed. Initially, the specification and characteristics of a study-case varistor are presented, followed by a detailed description of the electro-thermal modeling procedure. Afterwards, the thermal behavior of the MOV is investigated and the differences between the heating and cool-down thermal models are highlighted. Finally, in order to validate the proposed electrothermal models, the temperature of the varistor is measured with an infrared camera. The experimental measurements are in well agreement with the simulation results, and thus, providing an initial validation of the proposed modeling procedure

Blaabjerg, Frede

Iannuzzo, Francesco

Jensen, Per Thåstrup

Otto, Susanne

Vernica, Ionut

Wang, Huai

English

VBN (Videnbasen)  Aalborg Universitets forskningsportal

Aalborg UniversitetLoss and Thermal Modeling of Metal Oxide Varistors (MOV) Under Standard CurrentSurge Mission ProfileVernica, Ionut; Jensen, Per Thåstrup; Wang, Huai; Iannuzzo, Francesco; Otto, Susanne;Blaabjerg, FredePublished in:2019 IEEE Energy Conversion Congress and Exposition, ECCE 2019DOI (link to publication from Publisher):10.1109/ECCE.2019.8913287Publication date:2019Document VersionAccepted author manuscript, peer reviewed versionLink to publication from Aalborg UniversityCitation for published version (APA):Vernica, I., Jensen, P. T., Wang, H., Iannuzzo, F., Otto, S., & Blaabjerg, F. (2019). Loss and Thermal Modelingof Metal Oxide Varistors (MOV) Under Standard Current Surge Mission Profile. In 2019 IEEE Energy ConversionCongress and Exposition, ECCE 2019 (pp. 7113-7117). Article 8913287 IEEE Press.https://doi.org/10.1109/ECCE.2019.8913287General rightsCopyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright ownersand it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.            - Users may download and print one copy of any publication from the public portal for the purpose of private study or research.            - You may not further distribute the material or use it for any profit-making activity or commercial gain            - You may freely distribute the URL identifying the publication in the public portal -Take down policyIf you believe that this document breaches copyright please contact us at vbn@aub.aau.dk providing details, and we will remove access tothe work immediately and investigate your claim.Downloaded from vbn.aau.dk on: January 11, 2026Loss and Thermal Modeling of Metal OxideVaristors (MOV) Under Standard Current SurgeMission ProfileIonut, Vernica1, Per Thåstrup Jensen2, Huai Wang1, Francesco Iannuzzo1, Susanne Otto2, and Frede Blaabjerg11 Department of Energy Technology, Aalborg University, 9220 Aalborg Øst, Denmark2 FORCE Technology, 2970 Hørsholm, DenmarkEmail: iov@et.aau.dk, ptj@force.dk, hwa@et.aau.dk, fia@et.aau.dk, suo@force.dk, and fbl@et.aau.dkAbstract—Because of their crucial role in protecting powerelectronic equipment against over-voltages and transients, metaloxide varistors (MOV) represent a key component of the electricalsystem. Although improper heat dissipation and internal temper-ature rises can lead to a significant decrease in performance oreven to a wear-out failure of the MOV, there are very few studiesin the literature describing the thermal modeling of varistors.Thus, in this paper, an instantaneous loss and thermal modelingprocedure for MOV under a standard current surge missionprofile is proposed. Initially, the specification and characteristicsof a study-case varistor are presented, followed by a detaileddescription of the electro-thermal modeling procedure. After-wards, the thermal behavior of the MOV is investigated and thedifferences between the heating and cool-down thermal modelsare highlighted. Finally, in order to validate the proposed electro-thermal models, the temperature of the varistor is measured withan infrared camera. The experimental measurements are in wellagreement with the simulation results, and thus, providing aninitial validation of the proposed modeling procedure.Index Terms—Metal oxide varistor, surge current, power lossmodel, thermal model.I. INTRODUCTIONNowadays, with the ever-growing number of power semi-conductor devices used in mission-critical applications, costconstrains and reliability requirements are becoming more andmore stringent. In many applications (e.g., pump drives, streetlightning, home appliances, etc.), over-voltages and transientsgenerated by lightning strikes or non-ideal grid conditions,are among the main causes of failure in semiconductor com-ponents [1]. These failures will inherently lead to an overalllife-cycle cost increase of the electrical system, due to theexpenses associated with maintenance and downtime.In order to protect the power electronic components andequipment against voltage and current surges, the metal oxidevaristors (MOV) are connected in parallel with the necessarysub-assembly, and thus forming a low-resistance shunt duringover-voltage occurrences [2].However, in order for the MOV to operate correctly, theabsorbed electrical energy must be converted into thermalenergy and dissipated quickly into the ambient [3]. Otherwise,the increase in temperature will lead to the MOV operatingoutside its operating temperature limit, which can result ina decrease in performance, or even a failure. Similarly, thenegative impact of rising temperature on the lifetime of MOVhas been shown in [4].Thus, an accurate thermal modeling of the MOV underactual operating conditions becomes nearly crucial. Unfortu-nately, there are little to no studies throughout the literatureinvestigating the instantaneous thermal behavior of MOV.Consequently, in order to address this issue, the power lossand thermal modeling of a MOV operating under standardcurrent surge mission profile is proposed and investigated inthis paper. Initially, the varistor specifications and its operatingrequirements are introduced. Afterwards, a brief description ofthe electro-thermal modeling procedure is presented and theresulting thermal behavior of the varistor under the given cur-rent surge mission profile is highlighted. Finally, experimentalmeasurements are used in order to validate the simulationresults and the thermal modeling procedure.II. SPECIFICATIONS AND TYPICAL REQUIREMENTS FORMETAL OXIDE VARISTORSIn this paper, a disk type varistor with epoxy resin coating isused as study-case. As shown in Fig. 1, the zinc oxide (ZnO)plate has a thickness of 2.9 mm and a diameter of 14 mm, andaccording to the manufacturer datasheet, the nominal operatingtemperature of the varistor ranges from -40 oC up to 85 oC[5].Fig. 1. General structure of a ZnO varistor.Fig. 2. Temperature dependency of ZnO thermal conductivityand thermal resistance [6].Fig. 3. Temperature dependency of ZnO specific heat andthermal capacitance [6].As presented in [6], the thermal properties of the ZnOcompound vary strongly with temperature. Despite the ZnOcore inside the body of the varistor reaching very high tem-perartures (most likely, larger than 1000 K) during a surgecurrent pulse, the temperature rise seen on the surface ofthe component is much less. This is mainly due to the slowpropagation of the heat to the surface and due to the surgeenergy distribution over the volume of the varistor. Thus,based on the data from [6], the thermal conductivity andthermal capacity characteristics of ZnO can be calculated as afunction of temperature, and are plotted in Fig. 2 and Fig. 3,respectively.In order to correctly include the temperature dependencyFig. 4. Surge current pulse waveform defined by IEC 60060[8].of the varistor characteristics into its thermal model, thethermal resistance (Rth) and thermal capacitance (Cth) haveto be determined. Thus, from the given characteristics and thegeometrical dimension of the varistors, the thermal resistancecan be calculated according to the following equation,Rth =tA · kth(1)where t is the thickness of the varistor, and kth is the thermalconductivity. Similarly, the thermal capacitance is computedbased on the formula given below,Cth = qth · V (2)where V is the volume of the varistor, and qth is the thermalcapacity.The resulting thermal resistance and thermal capacitancecharacteristics can be utilized to describe the thermal behaviorof the MOV as a function of temperature, within a RC Fosterthermal network [7], and are shown in Fig. 2 and Fig. 3,respectively.In order to operate correctly, the MOV must be able topresent a maximum energy absorption capability defined bythe IEC 60060 standard [8] and to withstand a current surgepulse described by the waveform shown in Fig. 4. The currentpulse rises from 10% of its nominal value to its peak valuewithin 8 µs (Ts) and then decreases to half of its peak valuein 12 µs (Tr−Ts). The mission profile that will be consideredwithin this paper will consist of five consecutive current surgepulses (as shown in Fig. 4), which will be repeated at a fixedinterval of 8 seconds.III. ELECTRO-THERMAL MODELING PROCEDUREAccording to the mission profile-based thermal characteri-zation of power electronic components [9–11], the proposedmodeling concept for the electro-thermal behavior of MOV isshown in Fig. 5. The current pulse mission profile (I), togetherFig. 5. Generic electro-thermal modeling procedure for MOV.with the test voltage (V ) will represent the inputs to the powerloss model, which will calculate the total losses generatedby the varistor under the given test conditions. Afterwards,the power losses (Ploss) and the local ambient temperature(Ta,local) are included into the thermal model, and translatedto the temperature of the varistor.Additionally, the resulting varistor temperature (T ) will rep-resent a feedback to the thermal model, in order to characterizethe temperature dependency of the ZnO compound thermalresistance and thermal capacitance (according to the thermalproperties presented in Fig. 2 and Fig. 3).The average power losses dissipated by the varistor canbe determined based on the instantaneous surge current pulse(Ipeak = 1050 A) and the voltage drop (V = 1350 V) acrossFig. 6. Average power loss dissipated by the ZnO varistorunder the 5-pulse 1050 A current surge mission profile.the varistor during the current flow.Thus, the following equation can be utilized:Ploss =1Tpulse∫ t1t0v(t) · i(t)dt (3)where Tpulse is the current pulse duration, t0 is the start timeof the current pulse, t1 is the end time of the current pulse, v(t)is the instantaneous voltage drop across the varistor, and i(t)is the instantaneous surge current pulse. The resulting powerlosses generated by the MOV under the given mission profileand operating conditions are presented in Fig. 6.With respect to the MOV thermal model, due to the thermalproperties of the ZnO compound [6], two thermal networkswith different thermal time constants are employed in order tomodel the varistor heating and cool-down phases. An overviewof the varistor thermal model is shown in Fig. 7.Even though during both temperature cycles (heating/cool-down) the RC network is built based on a first-order transferfunction, the values of the thermal resistance and thermal ca-Fig. 7. Proposed dynamic thermal modeling for MOV underheating and cool-down phases.Fig. 8. Thermal behavior of the ZnO varistor under the givencurrent surge mission profile.pacitance will be different, according to the varistor’s thermalphase.During the heating phase, the thermal resistance and ther-mal capacitance are updated continuously according to theirtemperature dependency curves shown in Fig. 2 and Fig. 3,which are included as a look-up table.On the other hand, during the cool-down phase of thevaristor, the thermal network is built based on the empiricalthermal resistance (Rth,exp) and capacitance (Cth,exp) values,which have been observed and fitted throughout experimentaltesting.By employing the thermal model presented in Fig. 7, thepower losses generated by the varistor under the given currentsurge mission profile are used in order to determine its thermalbehavior. Considering a local ambient temperature (Ta,local)of 31 oC, the resulting thermal dynamics are shown in Fig. 8,Fig. 9. Experimental recording using IR camera of varistorthermal behavior under given surge mission profile.Fig. 10. Comparison between the experimental and simulationtemperature swings of the ZnO varistor.from which it can be observed that a high dynamic temperatureswing occurs during the current pulse period, followed by aslow cool-down phase. As previously mentioned, this thermalbehavior is due to the internal thermal properties of the ZnOcore of the varistor.IV. EXPERIMENTAL VERIFICATIONThe thermal behavior of the MOV has been experimentallytested under the given study-case mission profile. Five currentpulses, 40 microseconds each, have been applied to the varistorwith an 8-second interval between them. A peak currentamplitude of 1050 A has been considered, while the voltagedrop across the varistor has been set to approximately 1350 V.The temperature has been recoded via an infrared (IR) cameraas shown in Fig. 9, and the experimental thermal dynamicsof the varistor are shown together with the simulation resultsin Fig. 10. From it, it can be inferred that the thermalmeasurements are in good agreement with the simulationresults, thus providing an initial validation of the proposedthermal model.V. CONCLUSIONSIn this paper an electro-thermal modeling procedure fora metal oxide varistor (MOV) has been proposed and in-vestigated. Initially, the main specifications of the varistorand some of its typical application requirements have beenintroduced. Afterwards, the power loss and thermal loadingmodels of the varistor have been discussed, demonstratingthe dynamic model employed in order to characterize thevaristor during its heating and cool-down phases. The lossand thermal dynamics of the MOV have been analyzed understandard surge-current mission profiles. Finally, experimentalresults have been provided in order to validate the proposedthermal model and the different thermal time constants ofthe heating and cool-down phases. The simulation results arein well agreement with the experimental measurements, thusan initial validation of the proposed thermal model has beenprovided. Further testing and validation are required in order toverify the robustness of the proposed electro-thermal modelingprocedure, and to facilitate the reliability modeling of MOV.REFERENCES[1] TDK, ”SIOV metal oxide varistors. General technicalinformation.” Available online [2019]: https://www.tdk-electronics.tdk.com/download/185716/8954d4a78154a9da5c70a7119fa03e86/siov-general.pdf[2] Vishay BCComponents, Tech. Note 29079, 2013.[3] S. M. Meysam, and M. Mirzaie, ”Thermal balancediagram modelling of surge arrester for thermal stabilityanalysis considering ZnO varistor degradation effect,”IET Gener. Transm. Distrib., vol. 10, no. 7, pp. 1570-15581, 2016.[4] Y. Wen, and C. Zhou, ”A novel method for predictingthe lifetime of MOV,” IEEE Trans. Power Del., vol. 19,no. 4, pp. 1688-1691, 2004.[5] Datasheet: S14K250 Leaded Varis-tor, Available online [2019]:https://www.digchip.com/datasheets/parts/datasheet/2135/S14K250-pdf.php[6] H. Morkoc, and U. Ozgur, Zinc Oxide: Fundamentals,materials and device technology, 2009, Wiley-VCH Ver-lag GmbH & Co KGaA, Weinheim.[7] Y. A. Cengel, and R. H. Turner, Fundamentals of ther-mal fluids sciences (2nd edition), McGraw-Hill Interna-tional Edition, 2015.[8] IEC 60060, ”High-voltage and high-current test tech-niques,” European Committee for Electrotechnical Stan-dardization, Belgium, Feb. 2015.[9] K. Ma, M. Liserre, T. Kerekes, and F. Blaabjerg, ”Ther-mal loading and lifetime estimation of for power devicesconsidering mission profiles in wind power converter,”IEEE Trans. Power Electron., vol. 30, no. 2, pp. 590-602, 2015.[10] I. Vernica, H. Wang, and F. Blaabjerg, ”Design for reli-ability and robustness tool platform for power electronicsystems – Study case on motor drive applications,” Proc.of IEEE APEC, 2018, pp. 1799-1806.[11] H. Wang, P. Davari, H. Wang, D. Kumar, F. Zare, and F.Blaabjerg, ”Lifetime estimation of DC-link capacitors inadjustable speed drives under grid voltage unbalances,”IEEE Trans. Power Electron., vol. 34, no. 5, pp. 4064-4078, 2019.

Loss and Thermal Modeling of Metal Oxide Varistors (MOV) Under Standard Current Surge Mission Profile

Crossref

    Aalborg UniversitetLoss and Thermal Modeling of Metal Oxide Varistors (MOV) Under Standard CurrentSurge Mission ProfileVernica, Ionut; Jensen, Per Thåstrup; Wang, Huai; Iannuzzo, Francesco; Otto, Susanne;Blaabjerg, FredePublished in:2019 IEEE Energy Conversion Congress and Exposition, ECCE 2019DOI (link to publication from Publisher):10.1109/ECCE.2019.8913287Publication date:2019Document VersionAccepted author manuscript, peer reviewed versionLink to publication from Aalborg UniversityCitation for published version (APA):Vernica, I., Jensen, P. T., Wang, H., Iannuzzo, F., Otto, S., & Blaabjerg, F. (2019). Loss and Thermal Modelingof Metal Oxide Varistors (MOV) Under Standard Current Surge Mission Profile. In 2019 IEEE Energy ConversionCongress and Exposition, ECCE 2019 (pp. 7113-7117). [8913287] IEEE Press. IEEE Energy ConversionCongress and Exposition https://doi.org/10.1109/ECCE.2019.8913287General rightsCopyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright ownersand it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.            ? Users may download and print one copy of any publication from the public portal for the purpose of private study or research.            ? You may not further distribute the material or use it for any profit-making activity or commercial gain            ? You may freely distribute the URL identifying the publication in the public portal ?Take down policyIf you believe that this document breaches copyright please contact us at vbn@aub.aau.dk providing details, and we will remove access tothe work immediately and investigate your claim.Loss and Thermal Modeling of Metal OxideVaristors (MOV) Under Standard Current SurgeMission ProfileIonut, Vernica1, Per Thåstrup Jensen2, Huai Wang1, Francesco Iannuzzo1, Susanne Otto2, and Frede Blaabjerg11 Department of Energy Technology, Aalborg University, 9220 Aalborg Øst, Denmark2 FORCE Technology, 2970 Hørsholm, DenmarkEmail: iov@et.aau.dk, ptj@force.dk, hwa@et.aau.dk, fia@et.aau.dk, suo@force.dk, and fbl@et.aau.dkAbstract—Because of their crucial role in protecting powerelectronic equipment against over-voltages and transients, metaloxide varistors (MOV) represent a key component of the electricalsystem. Although improper heat dissipation and internal temper-ature rises can lead to a significant decrease in performance oreven to a wear-out failure of the MOV, there are very few studiesin the literature describing the thermal modeling of varistors.Thus, in this paper, an instantaneous loss and thermal modelingprocedure for MOV under a standard current surge missionprofile is proposed. Initially, the specification and characteristicsof a study-case varistor are presented, followed by a detaileddescription of the electro-thermal modeling procedure. After-wards, the thermal behavior of the MOV is investigated and thedifferences between the heating and cool-down thermal modelsare highlighted. Finally, in order to validate the proposed electro-thermal models, the temperature of the varistor is measured withan infrared camera. The experimental measurements are in wellagreement with the simulation results, and thus, providing aninitial validation of the proposed modeling procedure.Index Terms—Metal oxide varistor, surge current, power lossmodel, thermal model.I. INTRODUCTIONNowadays, with the ever-growing number of power semi-conductor devices used in mission-critical applications, costconstrains and reliability requirements are becoming more andmore stringent. In many applications (e.g., pump drives, streetlightning, home appliances, etc.), over-voltages and transientsgenerated by lightning strikes or non-ideal grid conditions,are among the main causes of failure in semiconductor com-ponents [1]. These failures will inherently lead to an overalllife-cycle cost increase of the electrical system, due to theexpenses associated with maintenance and downtime.In order to protect the power electronic components andequipment against voltage and current surges, the metal oxidevaristors (MOV) are connected in parallel with the necessarysub-assembly, and thus forming a low-resistance shunt duringover-voltage occurrences [2].However, in order for the MOV to operate correctly, theabsorbed electrical energy must be converted into thermalenergy and dissipated quickly into the ambient [3]. Otherwise,the increase in temperature will lead to the MOV operatingoutside its operating temperature limit, which can result ina decrease in performance, or even a failure. Similarly, thenegative impact of rising temperature on the lifetime of MOVhas been shown in [4].Thus, an accurate thermal modeling of the MOV underactual operating conditions becomes nearly crucial. Unfortu-nately, there are little to no studies throughout the literatureinvestigating the instantaneous thermal behavior of MOV.Consequently, in order to address this issue, the power lossand thermal modeling of a MOV operating under standardcurrent surge mission profile is proposed and investigated inthis paper. Initially, the varistor specifications and its operatingrequirements are introduced. Afterwards, a brief description ofthe electro-thermal modeling procedure is presented and theresulting thermal behavior of the varistor under the given cur-rent surge mission profile is highlighted. Finally, experimentalmeasurements are used in order to validate the simulationresults and the thermal modeling procedure.II. SPECIFICATIONS AND TYPICAL REQUIREMENTS FORMETAL OXIDE VARISTORSIn this paper, a disk type varistor with epoxy resin coating isused as study-case. As shown in Fig. 1, the zinc oxide (ZnO)plate has a thickness of 2.9 mm and a diameter of 14 mm, andaccording to the manufacturer datasheet, the nominal operatingtemperature of the varistor ranges from -40 oC up to 85 oC[5].Fig. 1. General structure of a ZnO varistor.Fig. 2. Temperature dependency of ZnO thermal conductivityand thermal resistance [6].Fig. 3. Temperature dependency of ZnO specific heat andthermal capacitance [6].As presented in [6], the thermal properties of the ZnOcompound vary strongly with temperature. Despite the ZnOcore inside the body of the varistor reaching very high tem-perartures (most likely, larger than 1000 K) during a surgecurrent pulse, the temperature rise seen on the surface ofthe component is much less. This is mainly due to the slowpropagation of the heat to the surface and due to the surgeenergy distribution over the volume of the varistor. Thus,based on the data from [6], the thermal conductivity andthermal capacity characteristics of ZnO can be calculated as afunction of temperature, and are plotted in Fig. 2 and Fig. 3,respectively.In order to correctly include the temperature dependencyFig. 4. Surge current pulse waveform defined by IEC 60060[8].of the varistor characteristics into its thermal model, thethermal resistance (Rth) and thermal capacitance (Cth) haveto be determined. Thus, from the given characteristics and thegeometrical dimension of the varistors, the thermal resistancecan be calculated according to the following equation,Rth =tA · kth(1)where t is the thickness of the varistor, and kth is the thermalconductivity. Similarly, the thermal capacitance is computedbased on the formula given below,Cth = qth · V (2)where V is the volume of the varistor, and qth is the thermalcapacity.The resulting thermal resistance and thermal capacitancecharacteristics can be utilized to describe the thermal behaviorof the MOV as a function of temperature, within a RC Fosterthermal network [7], and are shown in Fig. 2 and Fig. 3,respectively.In order to operate correctly, the MOV must be able topresent a maximum energy absorption capability defined bythe IEC 60060 standard [8] and to withstand a current surgepulse described by the waveform shown in Fig. 4. The currentpulse rises from 10% of its nominal value to its peak valuewithin 8 µs (Ts) and then decreases to half of its peak valuein 12 µs (Tr−Ts). The mission profile that will be consideredwithin this paper will consist of five consecutive current surgepulses (as shown in Fig. 4), which will be repeated at a fixedinterval of 8 seconds.III. ELECTRO-THERMAL MODELING PROCEDUREAccording to the mission profile-based thermal characteri-zation of power electronic components [9–11], the proposedmodeling concept for the electro-thermal behavior of MOV isshown in Fig. 5. The current pulse mission profile (I), togetherFig. 5. Generic electro-thermal modeling procedure for MOV.with the test voltage (V ) will represent the inputs to the powerloss model, which will calculate the total losses generatedby the varistor under the given test conditions. Afterwards,the power losses (Ploss) and the local ambient temperature(Ta,local) are included into the thermal model, and translatedto the temperature of the varistor.Additionally, the resulting varistor temperature (T ) will rep-resent a feedback to the thermal model, in order to characterizethe temperature dependency of the ZnO compound thermalresistance and thermal capacitance (according to the thermalproperties presented in Fig. 2 and Fig. 3).The average power losses dissipated by the varistor canbe determined based on the instantaneous surge current pulse(Ipeak = 1050 A) and the voltage drop (V = 1350 V) acrossFig. 6. Average power loss dissipated by the ZnO varistorunder the 5-pulse 1050 A current surge mission profile.the varistor during the current flow.Thus, the following equation can be utilized:Ploss =1Tpulse∫ t1t0v(t) · i(t)dt (3)where Tpulse is the current pulse duration, t0 is the start timeof the current pulse, t1 is the end time of the current pulse, v(t)is the instantaneous voltage drop across the varistor, and i(t)is the instantaneous surge current pulse. The resulting powerlosses generated by the MOV under the given mission profileand operating conditions are presented in Fig. 6.With respect to the MOV thermal model, due to the thermalproperties of the ZnO compound [6], two thermal networkswith different thermal time constants are employed in order tomodel the varistor heating and cool-down phases. An overviewof the varistor thermal model is shown in Fig. 7.Even though during both temperature cycles (heating/cool-down) the RC network is built based on a first-order transferfunction, the values of the thermal resistance and thermal ca-Fig. 7. Proposed dynamic thermal modeling for MOV underheating and cool-down phases.Fig. 8. Thermal behavior of the ZnO varistor under the givencurrent surge mission profile.pacitance will be different, according to the varistor’s thermalphase.During the heating phase, the thermal resistance and ther-mal capacitance are updated continuously according to theirtemperature dependency curves shown in Fig. 2 and Fig. 3,which are included as a look-up table.On the other hand, during the cool-down phase of thevaristor, the thermal network is built based on the empiricalthermal resistance (Rth,exp) and capacitance (Cth,exp) values,which have been observed and fitted throughout experimentaltesting.By employing the thermal model presented in Fig. 7, thepower losses generated by the varistor under the given currentsurge mission profile are used in order to determine its thermalbehavior. Considering a local ambient temperature (Ta,local)of 31 oC, the resulting thermal dynamics are shown in Fig. 8,Fig. 9. Experimental recording using IR camera of varistorthermal behavior under given surge mission profile.Fig. 10. Comparison between the experimental and simulationtemperature swings of the ZnO varistor.from which it can be observed that a high dynamic temperatureswing occurs during the current pulse period, followed by aslow cool-down phase. As previously mentioned, this thermalbehavior is due to the internal thermal properties of the ZnOcore of the varistor.IV. EXPERIMENTAL VERIFICATIONThe thermal behavior of the MOV has been experimentallytested under the given study-case mission profile. Five currentpulses, 40 microseconds each, have been applied to the varistorwith an 8-second interval between them. A peak currentamplitude of 1050 A has been considered, while the voltagedrop across the varistor has been set to approximately 1350 V.The temperature has been recoded via an infrared (IR) cameraas shown in Fig. 9, and the experimental thermal dynamicsof the varistor are shown together with the simulation resultsin Fig. 10. From it, it can be inferred that the thermalmeasurements are in good agreement with the simulationresults, thus providing an initial validation of the proposedthermal model.V. CONCLUSIONSIn this paper an electro-thermal modeling procedure fora metal oxide varistor (MOV) has been proposed and in-vestigated. Initially, the main specifications of the varistorand some of its typical application requirements have beenintroduced. Afterwards, the power loss and thermal loadingmodels of the varistor have been discussed, demonstratingthe dynamic model employed in order to characterize thevaristor during its heating and cool-down phases. The lossand thermal dynamics of the MOV have been analyzed understandard surge-current mission profiles. Finally, experimentalresults have been provided in order to validate the proposedthermal model and the different thermal time constants ofthe heating and cool-down phases. The simulation results arein well agreement with the experimental measurements, thusan initial validation of the proposed thermal model has beenprovided. Further testing and validation are required in order toverify the robustness of the proposed electro-thermal modelingprocedure, and to facilitate the reliability modeling of MOV.REFERENCES[1] TDK, ”SIOV metal oxide varistors. General technicalinformation.” Available online [2019]: https://www.tdk-electronics.tdk.com/download/185716/8954d4a78154a9da5c70a7119fa03e86/siov-general.pdf[2] Vishay BCComponents, Tech. Note 29079, 2013.[3] S. M. Meysam, and M. Mirzaie, ”Thermal balancediagram modelling of surge arrester for thermal stabilityanalysis considering ZnO varistor degradation effect,”IET Gener. Transm. Distrib., vol. 10, no. 7, pp. 1570-15581, 2016.[4] Y. Wen, and C. Zhou, ”A novel method for predictingthe lifetime of MOV,” IEEE Trans. Power Del., vol. 19,no. 4, pp. 1688-1691, 2004.[5] Datasheet: S14K250 Leaded Varis-tor, Available online [2019]:https://www.digchip.com/datasheets/parts/datasheet/2135/S14K250-pdf.php[6] H. Morkoc, and U. Ozgur, Zinc Oxide: Fundamentals,materials and device technology, 2009, Wiley-VCH Ver-lag GmbH & Co KGaA, Weinheim.[7] Y. A. Cengel, and R. H. Turner, Fundamentals of ther-mal fluids sciences (2nd edition), McGraw-Hill Interna-tional Edition, 2015.[8] IEC 60060, ”High-voltage and high-current test tech-niques,” European Committee for Electrotechnical Stan-dardization, Belgium, Feb. 2015.[9] K. Ma, M. Liserre, T. Kerekes, and F. Blaabjerg, ”Ther-mal loading and lifetime estimation of for power devicesconsidering mission profiles in wind power converter,”IEEE Trans. Power Electron., vol. 30, no. 2, pp. 590-602, 2015.[10] I. Vernica, H. Wang, and F. Blaabjerg, ”Design for reli-ability and robustness tool platform for power electronicsystems – Study case on motor drive applications,” Proc.of IEEE APEC, 2018, pp. 1799-1806.[11] H. Wang, P. Davari, H. Wang, D. Kumar, F. Zare, and F.Blaabjerg, ”Lifetime estimation of DC-link capacitors inadjustable speed drives under grid voltage unbalances,”IEEE Trans. Power Electron., vol. 34, no. 5, pp. 4064-4078, 2019.

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Loss and Thermal Modeling of Metal Oxide Varistors (MOV) Under Standard Current Surge Mission Profile

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