improvements in medium voltage mov blocks & evaluation … · improvements in medium voltage...

Improvements in Medium Voltage MOV Blocks & Evaluation of their Surge Withstand Capability

MINORU TSUKAZAKI, NAOYUKI TSUKAMOTO

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Improvement in Medium Voltage Blocks AND

Evaluation of their Surge Withstand Capability

Naoyuki Tsukamoto([email protected]) Minoru Tsukazaki([email protected])

Otowa Electric Co., Ltd ,Japan Otowa Proprietary information

2 Otowa Proprietary information

Contents

1.  Explanation of Metal Oxide Varistor(MOV) and Surge Arrester for distribution line in Japan

2.  Diagram for manufacturing high-performance MOV and surge arrester.

3.  Ideal structure of MOV

4.  Key factor for MOV manufacturing process

5.  Impulse current withstand Test result


OTOWA ELECTRIC CO.,LTD

Lightning technology center

MOV manufacturing in KOBE works

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4ｋV

２０ｋV

Surge arresters Surge protec-ve devices　

Metal oxide varistors (MOVs)

Performances -‐  Non linear characteris-cs -‐  Surge withstand Capability -‐  Life (Ageing performance) 　

Otowa Proprietary information

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For 6.6kV distribu-on line　

Typical Surge arrester for distribution line in Japan



Conventional Type

Developed type

Type Internal gap Internal gap

MOV

Norminal Size Diameter[mm] 32 32

Height[mm] 27 38

V1mA/mm[V] 200 300

The number of stuck (MOV 2 1

comparison of Construction of surge arrester and MOV


Table. JEC-2371 standard Item Criteria

AC sparkover voltage More than 13.9kV Lightning impulse sparkover voltage (1.2/50µs) Less than 33kV

Residual voltage (2500A,8/20µs) 　Less than 33kV

Long duration current impulse withstand (2ms) 75A×18times

Lightning impulse current withstand (4/10µs) 25kA×2times

Standards for distribution line !  This internal gap type surge arrester is uncommon in the world, and it is

govered by Japan origin standard !  Applicable standards ・JEC standard(JEC-203,JEC217,JEC2371・・・) 　　　 →These are published by Japanese electrotechnical committee ・Electric power company standards →This is published by each Electric power company.

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Diagram for manufacturing high-performance MOV and surge arrester.


!  Non-linear characteristics !  Superior long-term stability against the surge

current and the charging deterioration property. !  Superior surge withstand capability

Requirements for High-performance MOVs


Ideal structure of MOV


Key factor for MOV manufacturing process

Process Development item Expected effect

Granulated powder

!  Material pulverization of additives

!  Suitable dispersant !  Suitable spray condition

!  Improvement the homogeneously of pressed body

!  Uniform density

Sintering !  Adopt the new sintering temperature profile

!  Improvement of energy efficiency

!  Improvement of homogeneity of varistor voltage

Grazing insulating material

!  Establish suitable grazing condition

!  Selecting the suitable glass material

!  Control the characteristic variation


Impulse current withstand Test result

Fig.4 4/10us waveform










Withstand capability for 4/10us and 8/20us

! Withstand capability of developed product is much higher than that of conventional one.

!  Energy withstand capability is almost equal for the 2 waveforms. This implies that the failure mode in this test is not dependent on the shape of the waveforms, but is dependent on the waveforms energy content.


Repetitive withstand energy for 8/20µs waveform

! All specimens passed 5 consecutive impulses of 600J/cc. Above 800 J/cc failure occurred at 5th impulse.

!  The failure mode in this repetitive impulse testing was mechanical cracking, and is different from the failure mode of single shot testing. Progressive accumulation of energy is believed to be the cause of this mechanical failure mode.


V1mA drop on repetitive test by 8/20µs waveform

!  V1mA variability due to 600 J/cc and 1000 J/cc is very similar. Nevertheless, all specimens punctured after 2 shots in case of 1000 J/cc.

! V1mA Variability at 600J/cc level off after 3shots.


Withstand capability for 10/350us

!  Short-mode failure occurred for long tail waveforms. !  For long waveform, surge current concentration at the failure point caused

melting by Joule heat. !  Long waveform energy withstand variability has a wider range compared to

short duration waveforms. !  We presume that this failure was caused by a combination of factors to be

identified. We will continue to investigate this failure mechanism.

Failure point

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Conclusion


!  We developed a 300V/mm MOV with much higher performance than 200V/mm MOV. In addition, we confirmed that this 300V/mm MOV has a significant performance margin above target requirements.

!  In order to achieve high performance MOV, we considered the essential requirements of “High-performance surge arrester”. To realize the ideal MOV, both micro and macro aspects of the MOV have to be considered.

!  Surface flashover was observed in single shot 4/10µs and 8/20µs waveforms.　Mechanical crack was caused by energy accumulation in 8/20µs repetitive test.

!  When we compare energy withstand test results, energy withstand of 4/10µs and 8/20µs were similar. In contrast, there is no correlation between 8/20µs and 10/350µs results, because the failure mode for each waveform is markedly different.

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Conclusion


!  We will continue to investigate the detailed failure mechanisms and countermeasures for the10/350µs failure mode.

!  While this development was focused on developing a specific MOV for

applications in Japan, the development process and design considerations are applicable to other high performance MOV development.

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