Correlations of major failure modes 122 

• Failed subassembly versus environmental stresses, service conditions, origin and primary cause (see chapter 5.8.2)

• Primary cause versus failure mode, origin and failed subassembly (see chapter 5.8.3) • Service conditions versus failure mode and maintenance philosophy (see chapter

5.8.4)

In the 2nd _{GIS survey [13] there was not provided any correlation analysis among different}

major failure characteristics. Therefore in chapter 5.8 there are no subchapters with a comparison with the previous survey. In the 2nd _{survey, there was provided only detail}

correlation of different characteristics and main components involved in the GIS CB-bay major failure there. These comparisons are included in separate equipment technical brochures [2], [3] and [4] and in chapter 5.7.11.

5.8.1 Correlations of major failure modes

Major failure modes offered the following categories:

• Failing to perform requested operation, function resp., (being locked incl.) • Loss of electrical connections integrity in primary (e.g. fails to carry current) • Loss of electrical connections integrity in secondary

• Dielectric breakdown in normal service (without switching operation) • Dielectric breakdown in connection with switching operation

• Loss of mechanical integrity (mechanical damages of different parts, big SF6 leakage incl.)

• Other

The following chapter exposed only the two main prevailing categories (see chapter 5.7.1), i.e. “Failing to perform request operation” and “Dielectric breakdown”. In all the other categories the number of failures is too small.

123

5.8.1.1 Major failure modes in correlation with failed subassembly

Table 5-122 shows the major failure mode subdivided into the two main categories “failing to perform request operation” and “dielectric breakdown” in correlation with failed subassembly. The table shows absolute and relative data. As the results in the table are evident (prevailing characteristics marked by red numbers) it has been decided that graphical interpretation is not necessary.

Table 5-122: Distribution of major failure modes in correlation with failed subassemblies (all data absolute and relative values)

Failed subassembly

Major failure mode Failing to perform requested

operation, function resp., (being locked incl.)

Dielectric breakdown (normal and switching

operation)

Abs. % Abs. %

Component in primary circuit 7 3,1 76 93,8

Component in control, auxiliary or

monitoring circuit 112 49,3 0 0,0

Component in operating mechanism 106 46,7 1 1,2

Component in IT secondary circuit 1 0,4 0 0,0

Unknown or other 1 0,4 4 4,9

Total 227 100,0 81 100,0

Findings and commentary

Failure mode “Failing to perform requested operation or function” can have its basis in any part of the equipment, i.e. all categories of failed subassemblies can be responsible for. The survey shows that components in operating mechanisms and components in control, supply and auxiliary circuit are the most frequent ones and they split the responsibility by approximately half to half.

Failure mode “Dielectric breakdown” can also appear in all parts of the equipment. However the absolutely prevailing failed subassemblies were components in primary circuits (94%).

5.8.1.2 Major failure modes in correlation with origin

Table 5-123 shows the major failure modes subdivided into the two main categories “failing to perform request operation” and “dielectric breakdown” in correlation with origin of failure. The table shows absolute and relative data. Figures 5-87 show the relative distribution of these major failure modes in pie charts.

124

Table 5-123: Distribution of major failure modes in correlation with the failure origins (absolute and relative values)

Origin

Major failure mode Failing to perform

requested operation, function resp., (being

locked incl.)

Dielectric breakdown (normal and switching

operation)

Abs. % Abs. % Mechanical in other than

operating mechanism 15 6,6 19 23,5

Mechanical in operating

mechanism (earthed) 74 32,6 1 1,2

Electrical in main circuit 1 0,4 50 61,7

Electrical in secondary circuit 100 44,1 3 3,7

Unknown 37 16,3 8 9,9

Total 227 100,0 81 100,0

Figures 5-87: Relative Distribution of major failure modes in correlation with failure origins

Findings and commentary

“Failing to perform requested operation or function” major failure mode origins share approximately the same division as failed components responsible. If subassemblies in operating mechanisms and components in control, monitoring and auxiliary circuit split the responsibility by approximately half to half then mechanical problems in operating mechanisms and electrical problems in secondary circuits should share the responsibility approximately by half and half, too. It is like so but there is quite a big portion of “unknown” origins reported which most probably means that the utility did not make an investigation of the major failure origin, and/or the respondent did not know the investigation results.

7%

33%

0% 44%

16%

MaF Mode "Failing to perform requested operation" in Correlation

with Failure Origin

23%

1%

62%

4% _10%

MaF Mode "Dielectric breakdown (normal and switching operation)" in Correlation with Failure Origin

Mechanical in other than operating mechanism Mechanical in operating mechanism (earthed) Electrical in main circuit Electrical in secondary circuit Unknown

125

“Dielectric breakdowns” mode origins split between electrical in main circuit (62%) and mechanical in other than operating mechanism (24%). However it is necessary to mention that origin determination needs a really deep detailed investigation. Dielectric breakdown is an electrical problem, however its origin can be mechanical in main circuit (e.g. void in GIS spacers) caused by mechanical imperfection in solid insulation materials as well as mechanical in other parts e.g. in kinematic chain or SF6 gas sealing system. On the other hand an example of pure electrical origin is a breakdown caused by any kind of overvoltage stresses. The subassembly is in majority of cases a component in primary circuits.

5.8.1.3 Major failure modes in correlation with primary cause

Table 5-124 shows the major failure (MaF) mode subdivided into “failing to perform requested operation” and “dielectric breakdown” in correlation with primary cause of failure. The table shows absolute and relative data. Figures 5-88 show the relative distribution of these two MaF modes in dependence on primary cause in pie charts.

Table 5-124: Distribution of major failure modes in correlation with failure primary causes (relative and absolute values)

Primary cause

Major failure mode Failing to perform requested

operation, function resp., (being locked incl.)

Dielectric breakdown (normal and switching

operation)

Abs. % Abs. % Cause introduced during a period

before putting into service 29 12,8 37 45,7

Cause other than wear introduced

during service 19 8,4 27 33,3

Wear/Ageing & Corrosion 130 57,3 9 11,1

Unknown other causes 49 21,6 8 9,9

Total 227 100,0 81 100,0

Figures 5-88: Relative distribution of major failure modes in correlation with failure primary causes 13% 8% 57% 22% MaF Mode "Failing to perform requested operation" by primary cause 46% 33% 11% 10%

MaF Mode "Dielectric breakdown (normal and

switching operation)" by primary cause

Cause introduced during a period before putting into service

Cause other than wear introduced during service

Wear/Ageing & Corrosion

126

Findings and commentary

As expected, more than a half of “failing to perform requested operation” was caused by wear, ageing and corrosion. Unfortunately there is again a relatively large portion with unknown causes (22 %).

The “dielectric breakdown” failure mode (for both under normal and switching operation) cause was introduced during a period before putting into service in about 50 % of the cases. That is a surprising result as the required IEC tests before putting into operation should be sufficient to detect the most failures, e.g. material, manufacturing, transport and erection defects. Another relatively large portion of dielectric breakdowns modes causes are failures caused by other than wear phenomenon introduced during service (mainly different overstresses). Portion of ageing signs is relatively small and represents only 11%. It leads to a possible conclusion that ageing concerns more mechanical than electrical equipment properties.