Martin StoesslR&D Manager

Siemens Transformers Austria - Weiz

1892

1979

2007

2

Transformer reliability

Ability to

• perform its required functions

• under stated conditions

• for a specified period of time

Failure

• termination of the ability

It is often reported as a probability

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Transformer Reliability - based on ANSI C57. 117 (1986)

Definitions:• Population:

Transformers that have given common specific characteristics

• Failure:

Termination of a transformer to perform its specific functions

• Failure with forced outage:

Failure of a transformer that requires its immediate removal from the system for more than 1 day in conjunction with internal measures

• Failure Rate FR [%] = (nF / SY) 100

The ration of the number of failures with forced outage of a given population over a given period of time to the number of accumulated service years for all transformers in that period of time

• Mean Time Between Failures MTBF [years] = 1 / FR

Mean Time between failures (MTBF) = 1 / FR (years)

N: Number of units in service within evaluation period (floating 10 years)

SY: Number of service years accumulated with [N] units in service

nF: Number of failures with forced outage of a population within the evolution period

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Transformer Reliability - based on ANSI C57. 117 (1986)

FRe and MTBF as a degree of Reliability:Failure rate and reliability are related in the following way:

R = e-λt

t ... time in year

λ … failure rate in failures per transformer-years of service (λ=nF/SY)

e ... 2.718

• MTBF is considered to be the reciprocal of failure rate for purpose of estimating reliability:

MTB = 1 / λ

• MTBF and reliability are related in the following way:

R = e-t/MTBF

• Example:

Given a constant MTBF of 500 years

The reliability R of a transformer surviving t years of service without a failure would be shown as below: λ = 0.002

t R

1 0.9980

5 0.9900

10 0.9802

20 0.9608

30 0.9418

For a given MTBF of 500 years the probability of a transformer surviving 20 years of service without failure is 96.08%

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CIGRE activities related toTransformer Reliability

SC A2 Reliability Advisory Group 2006

• “Bathtub” curve

• Reliability databases

WG A2.37: Reliability Surveys

• difficult to get reliable data

WG A2.43: TR Bushing reliability (Um≥72,5 kV)

• Started in June 2010

Plans to work on topics like post mortem and end of life decision.

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Reliability of main accessories

A2 session papers in Paris 2010 showed that

• accessories cause a significant proportion of the power transformer major failures.

• new bushing and tap-changer technologies were introduced recently

• these technologies and their impact on maintenance have to be reviewed

Future task of CIGRE Study Committee A2

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Operation conditions

Thermal

stress

Electrical

stress

Ambient

stress

Mechanical

stress

Transformer

operating stress

of the electrical insulation system

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Electrical insulation system

transformer insulation life [1]

total time between initial state (new) and the final state when due to normal service

• thermal ageing

• dielectric stress

• short-circuit stress

• mechanical movement

could result in a high risk of electrical failure

[1] IEC 60076-7 Loading guide for oil-immersed power transformers

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Operation conditions

Thermal

stress

Electrical

stress

Ambient

stress

Mechanical

stress

Transformer

operating stress

of the electrical insulation system

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Thermal stresses and ageing

Caused by the heating due to no-load and load losses

• Chemical

• Polymerization

• Depolymerization

• Physical

• Diffusion

• Expansion/contraction

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Thermal ageing retardation

Control possibilities

• Reduce temp. rises by new cooling stage control (overall loss of life optimization)

• Hot spot factor determination and control

• Proper material selection

• Initial moisture content

• Air sealing

• Forced cooling

• Design know how

[1] IEC 60076-7 Loading guide for oil-immersed power transformers

[1]

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Manufacturer activities

• Continue verification cooling calculations

• Interfaces for dynamic calculation

• Cooling calculation for alternative insulation fluids

• Repair experience

• Post mortem analysis

• Material tests

• Supplier control

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Operation conditions

Thermal

stress

Electrical

stress

Ambient

stress

Mechanical

stress

Transformer

operating stress

of the electrical insulation system

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Electrical stresses and ageing

Caused by the electrical field and the interaction within the insulation system

• Increased local temperatures

• Partial Discharge

• X-Wax

• Leakage current

• Creeping distance

• Electrical treeing

• Static electrification

could appear

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Electrical ageing retardation

Control possibilities

• Proper material selection

• Supplier quality control

• “PD free” transformers

• Design and processingknow how

• Calculation possibilities

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Manufacturer activities

• Continuous adaptation of the transients package to needs of design departments, and to customer

• Testing and evaluation of new insulation components and materials

• Basic reproducible test and comparative investigations

• Optimize technology to ease production and deliver PD-free products

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Operation conditions

Thermal

stress

Electrical

stress

Ambient

stress

Mechanical

stress

Transformer

operating stress

of the electrical insulation system

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Ambient stress

Caused by different ambient stresses the inner & outer systems of a Transformer could be damaged

• Humidity

• High/extreme low ambient temperature

• Altitude

• Pollution

• Wind

• Earthquake

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Operation conditions

Thermal

stress

Electrical

stress

Ambient

stress

Mechanical

stress

Transformer

operating stress

of the electrical insulation system

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Mechanical stress

Caused by acceleration during

• Transport

• Installation

• Operation

inner & outer systemsof a Transformer might be damaged

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Operation conditions

• Real case studies shows a combination of stresses as well as interactions

Thermal

stress

Electrical

stress

Ambient

stress

Mechanical

stress

Transformer

operating stress

of the electrical insulation system

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Real case study #1

Sealed GSU Transformer

Manufactured in 1952

Power: 8 MVA

Rating: 115,5 / 5,25 kV

Measurement of Degree of Polymerization (DP-value)in 1982 (after 30 years!!!)

• Phase 1: HV-winding, shielding ring: DP = 884

• Phase 2: HV-winding, leads: DP = 902

• Phase 3: HV-winding, winding insulation: DP = 721

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#1: Actual Investigations 2011 (after around 60 years of operation)

• HV leads ~ 300

• LV leads ~ 100

• DP-distribution over winding height

• DP average = 300 – 400

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Real case study #2

Sealed GSU Transformer

Manufactured in 1966Power:103 //51,5 / 51,5 MVA

Rating:403// 16 / 16 kV

Measurement of

T1Q: 642 T1R: 449

T1S: 449 T1T: 704

• Samples were taken from LV – flexible connector

• Tests were performed according ISO 14453:1997

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#2: Refurbishment Result in 2006

Measurement of De-polymerization (DP – value)

• We expect a transformer rest – life time of >13 years under the used load – conditions for all 4 transformers.

• This assessment is based on the results of the performed measurements, the

fact that the transformers are

new sealed and that the

insulation oil is purified.

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In order to create a save energy supply worldwide

we as power transformer manufacturersdesign, build and deliver

reliable transformers as a fundamental responsibility

Thank you for your attention!