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Working Group A3.06

Final Report of the 2004 - 2007International Enquiry on Reliabili ty of

High Voltage Equipment

Part 4 - Instrument Transformers

MembersM. Runde (NO) Convener, C. E. Slver (SE) Past Convener, A. Carvalho (BR),

M. L. Cormenzana (ES), H. Furuta (JP), W. Grieshaber (FR), A. Hyrczak (PL), D. Kopejtkova (CZ),J. G. Krone (NL), M. Kudoke (CH), D. Makareinis (DE), J. F. Martins (PT), K. Mestrovic (HR),

I. Ohno (JP), J. stlund (SE), K.-Y. Park (KR), J. Patel (IN), C. Protze (DE), J. Schmid (DE),J. E. Skog (US), B. Sweeney (UK), F. Waite (UK).

Corresponding Members

B. Bergman (CA), S. Tsukao (JP)

Copyright 2012Ownership of a CIGRE publication, whether in paper form or on electronic support only infers right of use for personal

purposes. Are prohibited, except i f explicitly agreed by CIGRE, total or partial reproduction of the publication for use other

than personal and transfer to a third party; hence circulation on any intranet or other company network is forbidden.

Disclaimer NoticeCIGRE gives no warranty or assurance about the contents of this publication, nor does it accept any responsibility, as to theaccuracy or exhaustiveness of the information. All implied warranties and conditions are excluded to the maximum extentpermitted by law.

ISBN: 978- 2- 85873- 205-0

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Findings and Commentary

Current transformers form the majority of the survey with around 64% of the serviceexperience. Voltage transformers form around 30% of the service experience, splitroughly equally between capacitor and magnetic designs. Combined current and voltagetransformers form a much smaller part of the survey. Compared with the previoussecond survey there is a significant drop in the number of combined current and voltage

transformers reported.

4.5.4 CT Arrangement of AIS CT Service Experience Distribution

The respondents were asked to divide their current transformer population into thearrangement or design of the current transformer. The eyebolt arrangement wasassumed to be similar to the hairpin arrangement and both are included together. Thisquestion only has a valid choice for AIS current transformers and combined instrumenttransformers. The results for combined instrument transformers are shown in Table4-18. Since over 87% of the population was either bar primary or other / unknown andthe residual population was so small no further analysis was performed on the CTarrangement of combined instrument transformers (indeed some of the CTarrangements for combined instrument transformers were though technically unlikely to

be used and may have been entered in error). The results for current transformers areshown in Table 4-19 and Table 4-20 and graphically in Figure 4-12 and Figure 4-13.

Table 4-18: AIS CCVT arrangement service experience (all countr ies)

Voltage class

Service experience [IT-years]

Hairpin Bar primary CascadeOptical /

ElectronicOther /

Unknown

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When comparing capacitor voltage transformers with magnetic voltage transformers, themagnetic voltage transformers appear to have a worse failure frequency that causes fireor explosion (although the number of failures are small so the result may not besignificant) and the major failure frequencies appear the same to the capacitor voltagetransformers. However the dominant country has mainly capacitor voltage transformersand generally a low failure frequency so when the data from the dominant country istaken out the failure frequencies for capacitor voltage transformers increase.

For current transformers, the reported major failure frequency appears less than that forvoltage transformers but the frequency for failures that cause fire or explosion appearshigher.

Combined instrument transformers generally reported a low major failure frequency.These results for combined instrument transformers were largely unaffected when thedata from the dominant country were removed. The relatively high failure frequencyreported for 300 to 500 kV combined instrument transformers is due to the small serviceexperience reported making the one failure reported look high.

The failure frequencies seen for current transformers and capacitor voltage transformerscould have been affected by the higher proportion of these instrument transformersbeing used at higher voltages where the failure frequencies are higher therefore

increasing the apparent failure frequency.

The comparison with the previous second survey shows that this third survey hasreported a reduction in the failure frequencies. The reduction has been particularlyprominent for magnetic voltage transformers, which in the second survey were theinstrument transformers with the highest failure frequency.

4.7.4 CT Arrangement of AIS CT Failure Frequency Distribution

The respondents were asked to divide their current transformers into the arrangement ordesign of the current transformer. Since this question only has a valid choice for AIScurrent transformers and AIS combined instrument transformers, the analysis waslimited to AIS designs. The optical or electronic responses have been combined with theother / unknown responses. As the majority of combined instrument transformer CT

arrangements were either primary bar or unknown with only a small number of other CTarrangements reported (see Section 4.5.4) the results for combined instrumenttransformers are not shown. The results for AIS current transformers are shown in Table4-86 to Table 4-89.

Table 4-86: AIS CT arrangement major failure frequency (all countr ies)

Voltage class

Failure frequency [per 100 IT-years]

Hairpin Bar primary CascadeOther /

Unknown

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