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API Summer School 2016.
Maintenance Management, Condition Monitoring and Diagnostics
Gary Winsor Manager – Network Performance, Ausgrid Ph +61 2 9269 7202 [email protected]
Today
Objectives of maintenance
What is reliability?
Determining Maintenance task periods
Aging and Failure mechanisms
Measurement techniques
Repair/replace decision making
Discussion
2
Objectives of maintenance
• Definition – All activities necessary to retain an item in or return it to a serviceable
condition. • Objectives
– Preserve inherent levels of safety and reliability designed into equipment
– Restore safety and reliability to their inherent level when deterioration has occurred
– Obtain the information to improve all processes associated with the system lifecycle
– Do the above at minimum cost of ownership • Maintenance actions address the consequence of failures
– Risk management of unplanned failure • Valid maintenance actions must be:
– applicable to the failure mode cause – cost - effective in managing the consequence of the failure mode
3
Objectives of maintenance
Traditional Concepts • Maintenance is a post design activity • Maintenance is viewed as a variable business overhead -
based only on available budgets. • Maintenance is something to be done when there is no
capital work. • Maintenance requirements are based on technical
excellence. • Maintenance system is not dynamic;
– does not change with changing business needs. • Maintenance requirements based on OEM’s
recommendations. Q. The Risk to who or what is minimised ?
4
Objectives of maintenance
Key issues… • How much maintenance work is enough? • How can we produce the same outcome at a lesser cost,
or a better outcome at a lesser or same cost? • What is the design basis of our maintenance, and should
that be documented? • How do we deal with budget cuts? • How should maintenance be resourced? • How often should these questions be asked?
5
What is Reliability?
Reliability is an inherent characteristic of design.
Manufacture, construction / installation, operations and maintenance cannot improve reliability beyond its inherent value
6
What is Reliability?
7
Design Influence
Organisational Influence
Availability
Reliability
Maintainability
Supportability
UP DOWN
TOTAL
Influence
Ao = R/(R+M+S)
R= MTBF M= MTTR S= MLDT
A0 = Uptime Total Time
FAILURE
What is Reliability? Equipment failure modes possess one of six reliability characteristics
1. Bathtub curve. Infant mortality - useful life - rapid wear out 2. Rapid wear out after long useful life
3. Gradual wear out over entire life
4. No infant mortality followed by
indefinite useful life
5. Indefinite useful life
6. Infant mortality followed by indefinite
useful life 8
What is Reliability? 1968
4 %
2 %
5 %
7 %
14 %
68 %
2001
2 %
10 %
17 %
9 %
56 %
6 %
1982
3 %
17 %
3 %
6 %
42 %
29 %
9
What is a Random Failure pattern?
Reliability
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9 1
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9 2
2.1
2.2
2.3
2.4
t/MTBF
R Number of survivors at t = MTBF
62%
-t / MTBF R = e
10
What is Reliability?
β=1 β < 1 β > 1
Weibull Distribution
11
What is Reliability?
12
What is Reliability?
Transformer Major Failure - Probability Density Vs Age
0.0%
0.5%
1.0%
1.5%
2.0%
2.5%
3.0%
3.5%
4.0%
4.5%
5.0%
1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49 52 55 58 61 64 67 70 73 76 79 82 85 88 91 94 97 100
103
106
109
112
115
Years132kV Bushing ST 66kV Bushing 33kV Zone Bushing33kV Zone Endbox 132kV Bushing Zone 132kV Bushing Zone Nom 38MVA
13
What is Reliability?
Transformer Major Failure - Probability Density Vs Age
0.0%
0.5%
1.0%
1.5%
2.0%
2.5%
3.0%
3.5%
4.0%
4.5%
5.0%
1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49 52 55 58 61 64 67 70 73 76 79 82 85 88 91 94 97 100
103
106
109
112
115
Years132kV Bushing ST 66kV Bushing 33kV Zone Bushing33kV Zone Endbox 132kV Bushing Zone 132kV Bushing Zone Nom 38MVA
Transformer Major Failure - Cummulative Risk vs Age
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49 52 55 58 61 64 67 70 73 76 79 82 85 88 91 94 97 100
103
106
109
112
115
Years132kV Bushing ST 66kV Bushing 33kV Zone Bushing33kV Zone Endbox 132kV Bushing Zone 132kV Bushing Zone Nom 38MVA
14
What is Reliability?
Cumulative
0.0000
0.0100
0.0200
0.0300
0.0400
0.0500
0.0600
0.0700
0.0800
b 1 180 360 540 720 900 1080 1260 1440 1620 1800 1980
Panasonic LCR127PPanasonic LCR127R2PPanasonic LCR12V65PPanasonic LCR127R2P1YUASA NP7-12
15
Manufacturer 1 Manufacturer 2 Manufacturer 3 Manufacturer 4 Manufacturer 5
Which item would you select and what maintenance period?
Determining Condition Monitoring Task periods
Random point to commence degrading
CF interval must be consistent
CF interval of useful duration
Failure mode parameter is practical to monitor
MTBF >> T (CF Interval)
Failure detection probability constant over CF interval
100%
0% Time
Resistance to failure
Functional Failure Point
Warning Period (CF)
Conditionally failed
16
100%
0% Time
Resistance to failure
Conditional Defect Point
Standards Decision
Degrading Asset Condition
Functional Failure Point
Task Period < Warning period and Task Effectiveness of 0.95
Warning (CF) Period
19 Conditional Failures
20 Items 1 Functional Failure
17
Determining Condition Monitoring Task periods
400 items
TO FAILURE RESISTANCE
100%
Preventive Task Done here
CONDITIONAL FAILURE
FAILURE
20 x 19
20
19
1
399 Captured
Failure Detection Probability 0.95
Effectiveness Task 0.95 Strategy (α) 0.9975
18
Determining Condition Monitoring Task periods
Determining Mtce Task periods
Non Safety Critical Failures - Condition Monitoring Tasks
nTperiod Task CF
=
( ) ( )
( )θ
θ
−
⎥⎥⎥
⎦
⎤
⎢⎢⎢
⎣
⎡
−−
=1
1
ln
ln * CCT
C * MTBF-
ln
n where
CF
i
pfnpm
So what is the task period?
19
n =
ln
- MTBF Tcf
Ci
(Cnpm - Cpf) ln(1-θ)
ln(1-θ)n =
ln
- MTBF Tcf
Ci
(Cnpm - Cpf) ln(1-θ)
ln(1-θ)
20
n =
ln
- MTBF Tcf
Ci
(Cnpm - Cpf) ln(1-θ)
ln(1-θ)n =
ln
- MTBF Tcf
Ci
(Cnpm - Cpf) ln(1-θ)
ln(1-θ)
Schedule Cost Risk Curve
So Why?
22
Functional failures Σ Risk Cost Condition Monitoring Failure Modes Risk Cost – 10% Latitude
Inspection period resulting from reduction to Opex Current Inspection period with latitude
Σ (365 * Pop / MTBF) * ((1- θ) (CF / x) )
Note MTBF in Days
So Why?
23
Condition Monitoring activities
• Can be periodic or continuous • On-line or off-line • High voltage assets subject to in-
service stresses: – Electrical – Thermal – Environmental – Mechanical
• Normal operating stresses – Power frequency voltage, heat
from losses, vibration, …. • Abnormal stresses
– Lightning, switching surges, mechanical forces from through faults, contamination
Off-line
• Better control of test conditions
• Location of defects possible
On-line
• No outage required
• Asset subject to in-service levels of electrical and mechanical stresses
24
Aging and Failure modes
• So what do we see? • Conductors
– High resistance joints • heat generated from I2R
losses – Arcing
• Heat generation • Increase in
Electromagnetic radiation – High localised field strength
• Discharges • Increase in
Electromagnetic radiation
• Insulation – Reduction in dielectric strength – Reduction in insulation
resistance – Increase in dielectric loss – Increase in partial discharge – Increase in Electromagnetic
radiation
25
Measurement techniques
• More common ones are: – Current
• Excitation • Waveshape • Frequency spectrum
– Conductor / contact resistance
– Impedance – Capacitance (and
permittivity) – Dielectric loss
• Insulation resistance • DDF • Dielectric response
– Partial discharge – Radio frequency emission – Transfer function and frequency response. – Sweep FRA
• winding displacement – Conductor / winding resistance reveals
• High resistance joints • Shorted turns • Contact condition (& timing)
– Cameras • Visual Only • Thermal • Corona
• SF6 26
Measurement techniques
Remember the resistance and insulation resistance is temperature dependent
27
Measurement techniques
• Measure the tangent angle δ to determine the condition of the insulation
• Tan δ = IR / IC (mW/VAR)
• As δ is usually small tan δ ≈ δ
• In past used DLA in milli-radians
• USA measures cos θ = IR / I
– Units %
– Close to tan δ for small angles
– Correction required for larger δ
Remember DDF is temperature dependent
28
Switchboard DDF
• To eliminate temperature effect of DDF in busbar results
• Correction factor to be in a range close to 1.7 times per 10 deg
• Fitting this in an exponential gives the curve:
DDFT2 = DDFT1 * e –k*(T1-T2) where k = 0.05
• Individual bushing contribution via phase to phase comparison: • This can eliminate the temperature component by
comparing all 3 phases (all measured at the same temperature).
• If DDF of all phases increase at same rate, the likelihood of single bushings in all 3 phases increasing substantially is slim.
• Reasonable level of comfort if all are similar increases
Power Transformers
• Periodic Monitoring
– Visual inspection – Insulating oil Analysis – Thermography
• On-line diagnostics
– Ultrasonic discharge survey
• Off-line condition assessment
– IR,PI, Winding resistance, DDF
• On and Offline diagnostics – Insulating oil analysis (Furans) – Dielectric response – Frequency Response Analysis
• Visual Inspection – Often undervalued – Humans are an excellent CM
Tool – Sight – Smell – Sound – Touch
30
Failure Finding – In Service Trip Checks
• Proves the entire trip circuit will work when required.
• Must not be post operator isolation – Needs to reflect how the CB would perform in
service – Operator isolation will work lube through mechanism
• Will identify poor circuit breaker trip performance – Degraded lubes – Wrong lube – Trip time growth (emerging issues)
31
Determining failure finding task periods
32
Repair / Replace decision making
As asset managers, how can we objectively compare the
economic merits of various technical solutions available to
repair an asset against the investment require to replace it?
33
Economic life
Age
Cum
ulat
ive
Cos
t
Acquisition cost
Realised Risk
Costs with realised risk
Replace Asset
34
Repair / Replace decision making
Cumulative Cost vs Age
Replacement considering unrealised risk
Age
Cum
ulat
ive
Cos
t
Cumulative Cost vs Age
Realised Risk
Costs with realised risk
Replace Asset
Costs with unrealised risk
Replace Asset
35
Why Replace Assets? - Spend limits
• How much do we spend on an existing asset?
• When is it more economical to replace it?
• Use spend limits (1)
Spend Limit = Remaining life of old asset x (Annualized cost of new asset – Annualized future cost of old asset)
Where A = Acquisition cost p = discount factor (1/[1+r]) i = year of life n = age of asset at disposal G(i) = M(i) + R(i) M(i) = maintenance cost in year i R(i) – risk cost in year I S(n) – disposal cost in year n
⎥⎦
⎤⎢⎣
⎡⎥⎦
⎤⎢⎣
⎡−+= ∑∑
=
=
=
=
ni
i
ini
i
ni pnSpiGpAEAC11
/)()( (2)
36
The model
37
Case studies – Repair when?
$0
$500,000
$1,000,000
$1,500,000
$2,000,000
$2,500,000
$3,000,000
$3,500,000
1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49 51 53 55 57 59 61 63 65 67 69
year
Existing TX cummulative costsOptimum EAC - existing Tx
38
Case studies – clear case Replace
$0
$500,000
$1,000,000
$1,500,000
$2,000,000
$2,500,000
$3,000,000
$3,500,0001 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49 51 53 55 57 59 61 63 65 67 69
year
Existing TX cummulative costsOptimum EAC - existing Tx
39
Questions
References
• Reliability-Centered Maintenance, US Dept of Commerce (National Technical Information Service), AD-A066 579, Nolan and Heap, United Airlines, 1978
• Navair 00-25-403 Guidelines for the Naval Reliability Centered Maintenance Analysis Process
• US MIL-STD-2173 Reliability-Centered Maintenance Analysis for Naval Aircraft Weapons Systems and Ground Equipment
• IEC 60812, Procedure for a failure modes effects analysis • US MIL-STD-1629:1974 Procedure for a failure modes effects and
criticality analysis • Ausgrid Maintenance Requirements Analysis Manual (AM-STG-10005) • S Buncombe & G Winsor, Repair / Replace Decision Making Practices ,
ICOMS 2007 • N Hastings & B Sharp, Spend-Limits And Asset Management, ICOMS 2004 • P Buckland & N Hastings, The Replacement Decision for Long Assets,
ICOMS 2001
41