Improvements in Medium Voltage MOV Blocks & Evaluation of their Surge Withstand Capability MINORU TSUKAZAKI, NAOYUKI TSUKAMOTO

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1 Improvements in Medium Voltage MOV Blocks & Evaluation of their Surge Withstand Capability MINORU TSUKAZAKI, NAOYUKI TSUKAMOTO

2 Improvement in Medium Voltage Blocks AND Evaluation of their Surge Withstand Capability Naoyuki Minoru Otowa Electric Co., Ltd,Japan 1

3 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 2

4 OTOWA ELECTRIC CO.,LTD MOV manufacturing in KOBE works Lightning technology center

5 Metal oxide varistors (MOVs) Surge arresters Surge protec-ve devices Performances - Non linear characteris-cs - Surge withstand Capability - Life (Ageing performance) 20kV 4kV 4

6 Typical Surge arrester for distribution line in Japan For 6.6kV distribu-on line 5

7 comparison of Construction of surge arrester and MOV MOV Norminal Size Conventional Type Developed type Type Internal gap Internal gap Diameter[mm] Height[mm] V1mA/mm[V] The number of stuck (MOV 2 1 6

8 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. Table. JEC-2371 standard Item AC sparkover voltage Lightning impulse sparkover voltage (1.2/50µs) Residual voltage (2500A,8/20µs) Long duration current impulse withstand (2ms) Lightning impulse current withstand (4/10µs) Criteria More than 13.9kV Less than 33kV Less than 33kV 75A 18times 25kA 2times 7

9 Diagram for manufacturing high-performance MOV and surge arrester. Requirements for High-performance MOVs! Non-linear characteristics! Superior long-term stability against the surge current and the charging deterioration property.! Superior surge withstand capability 8

10 Ideal structure of MOV 9

11 Key factor for MOV manufacturing process Process Development item Expected effect Granulated powder Sintering Grazing insulating material! Material pulverization of additives! Suitable dispersant! Suitable spray condition! Adopt the new sintering temperature profile! Establish suitable grazing condition! Selecting the suitable glass material! Improvement the homogeneously of pressed body! Uniform density! Improvement of energy efficiency! Improvement of homogeneity of varistor voltage! Control the characteristic variation 10

12 Impulse current withstand Test result Fig.4 4/10us waveform Fig.4 Fig.4 4/10us 4/10uswaveform waveform Fig.5 8/20us waveform Fig.5 Fig.5 8/20us 8/20uswaveform waveform Fig.6 10/350us waveform Fig.6 Fig.6 10/350us 10/350uswaveform waveform 11

13 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. 12

14 Repetitive withstand energy for 8/20µs waveform! All specimens passed 5 consecutive impulses of 600J/cc. Above 800 J/cc failure occurred at 5 th 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. 13

15 V1mA drop on repetitive test by 8/20µs waveform! V 1mA 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.! V 1mA Variability at 600J/cc level off after 3shots. 14

16 15 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

17 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. 16

18 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. 17

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