API Summer School 2016.

Maintenance Management, Condition Monitoring and Diagnostics

Gary Winsor Manager – Network Performance, Ausgrid Ph +61 2 9269 7202 gwinsor@ausgrid.com.au

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

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What is Reliability?

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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

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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

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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

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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

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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

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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?

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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?

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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

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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

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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

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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

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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

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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)

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Determining failure finding task periods

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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?

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Economic life

Age

Cum

ulat

ive

Cos

t

Acquisition cost

Realised Risk

Costs with realised risk

Replace Asset

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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

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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

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