dga scoring system.pdf

7/21/2019 DGA Scoring System.pdf

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Knowledge Is PowerSMApparatus Maintenance and Power Management

for Energy Delivery

DGA Scorin S stem

John Lapworth,Doble PowerTest Ltd


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Dissolved Gas Analysis (DGA)

Ideal condition monitoring tool

Sensitive, broadband Cheap, on-line

Sampled by site staff and analysed by oil

laboratoryNeed to apply engineering experience for

interpretation

DGA Scoring System


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DGA Interpretation

Type and severity of fault - gas signature

Key gases

Loading, operational history

Development of fault

Action levels

Trend analysis

Scoring system

DGA Scoring System


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DGA - Key Gases

Fault Primary Secondary

ar a sc arge y rogen

Dielectric(Arcing/sparking)

HydrogenAcetylene

Ethylene

e ec r c(Power arc)

y rogen e aneEthylene

Acetylene

Temperature Ethane Monoxide

Thermal hotspot Ethylene Methane

OverheatedCellulose

CarbonMonoxide

CarbonDioxide

DGA Scoring System


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Difficulties of Using DGA

Deciding on an appropriate sampling interval

Ensuring accuracy not compromised by :

Poor sampling method

Deciding when a result is abnormal

Deciding on seriousness of a fault

DGA Scoring System


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Existing DGA Interpretation Schemes

Gas signatures and ratios

E.G. Rogers, IEC 599, Duval etcTrend anal sis

Normal and action levels

Expert systems

We will discuss pros and cons of these

DGA Scoring System


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Rogers Ratios

DGA Scoring System


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Gas Signatures and Ratios

Thermodynamic basis

Focus on diagnosing fault

e.g. Rogers Normal Deterioration(0000) code is rare

Triple zero syndrome

DGA Scoring System


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Trend Analysis

Increasing gas levels considered indication of

eve op ng au , u ...

Increase after de-gassing to equilibrium levels

Gas levels constant for inactive fault

Different interpretation for H2 and C2H2

May not have enough results for good trend

DGA Scoring System


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Normal and Action Levels

What level is significant ...

Statistical approach is problematic: What exceedance should be used - 90, 95 or

97% ?

important ?

unhealthy

DGA Scoring System


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Expert Systems

E. . Kohonen net cluster anal sis techni ue

Attempt to classify similar results

earc or en n orma on

Identify changes of stateRely on human expert to ascribe significance

DGA Scoring System


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New DGA Interpretation Scheme

Based on IEEE C57.104-1991 Key Gas Method

Combustible gas signature - bar chart

Percentage total combustible gas (%TCG)

CO, H2, CH4, C2H4, C2H6, C2H2

Used in IEEE to distinguish faults Used here to differentiate normal and abnormal

Faulty, normal and heavily loaded transformers

DGA Scoring System


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IEEE C57.104-1991

DGA Scoring System


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IEEE C57.104-1991

DGA Scoring System


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Faulty Transformers

100

60

80

CG

40

P

ercentage

0

20

TransformerA B C D E F G

CO H2 CH4 C2H4 C2H6 C2H2

DGA Scoring System


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Normal Transmission Transformers

100

60

80

CG

40

P

ercentage

0

20

Transformer1 2 3 4 5


DGA Scoring System


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Normal Generator Transformers

100

60

80

CG

40

P

ercentage

0

20

TransformerI II III IV V


DGA Scoring System


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New DGA Interpretation

High [ COCO ] ( 60-90 %TCG ) = normal

( Higher levels may be abnormal )

High [ C2H4C2H4 ] ( > 10 %TCG ) = abnormal

Significant [ H2H2 ] and [ C2H2C2H2 ] = serious

Significant [ CH4CH4 ] and [ C2H6C2H6 ] = not serious

DGA Scoring System


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Scoring System

NormalNormal < 60< 60

All normal results can be ignored

>>

Only suspect results need to be investigated

SeriousSerious > 100> 100

Winding and dielectric faults

Some can be discounted

DGA Scoring System


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Scoring System

Quality function

Importance of gas signature

Strength function

Importance of gas levels

DGA Scoring System


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Benefits of Scoring System

Normal results can be screened out (ignored)

Consistent, objective assessment of results

Resource focussed on deserving cases

Can ignore unusualcases

DGA Scoring System


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Use in Condition Assessment

CO ND ITIO N ASSESSMENT TEST RESULTS

Dielectric

9 8 3 2 4 8

DGA RFI MAG TTR PF AE PD

10 10

Thermal / Life

3 4 4 4 2 4 4DGA FFA H2O IRES ACID PF POL DP

5 10 10 5 10

8 4

FRA LRT CAP

SUMMARY

Dielectric

1 2 3 4 5 6 7 8 9 10

Thermal / Life

1 2 3 4 5 6 7 8 9 10

Mechanical

1 2 3 4 5 6 7 8 9 10

O VERALL

1 2 3 4 5 6 7 8 9 10

DGA Scoring System


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Application 1 - Core Earth Fault

30 year old 400 kV 750 MVA auto-transformer

Suspected core / frame to earth circulating

curren au

Core earth brought out and resistor inserted

Main tank de-gassed after work

Buchholz alarms subsequently at high loads

DGA Scoring System


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DGA History

Sample date H2 CO CO2 CH4 C2H4 C2H6 C2H2

16/4/96 10 140 757 14 16 5 0.4

14/8/97 11 271 1,463 27 28 9 0.7

Core earth brought out and resistor fitted. Main tank de-gassed

20/4/98 14 64 576 19 18 4 0.6

.

22/7/98 40 112 823 62 64 12 2.9

23/7/98 38 100 1,393 87 131 23 5.1

31/7/98 109 180 1,142 125 152 25 5.7

DGA Scoring System


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DGA Signature

100

80

G

40

Pe

rcentageT

20

0

16/04/96 14/08/97 20/04/98 22/07/98 23/07/98 31/07/98


DGA Scoring System


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DGA Score

140

100

120

60

80

DGAScore

40

0

20

DGA Scoring System


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Evidence

DGA Scoring System


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Application 2 - Puzzling Results

High CH4CH4 and C2H6C2H6in many new transformers

Some also have high H2H2-high scores

Seasonal dependence -H2H2 peaks in late

summer

Differenttransformer designs

Same oil

DGA Scoring System


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New Transformer DGA

250

150

200

100

PPM

50

01/01/94 01/01/95 01/01/96 31/12/96 31/12/97 31/12/98 31/12/99 30/12/2000

0

H2 CH4 C2H6

DGA Scoring System


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Effect of Oil Type on DGA

Oil type can change symptoms exhibited

Temperature and time effects-

How can we allow for this ? Need to know oil type

Need information on assin ro erties

Tailor scoring parameters for oil type

DGA Scoring System


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DGA Scoring Conclusions

Scoring system has been valuable

More consistent and objective assessments

Resources focussed where deserved

Applicable to wider class of transformers ?

Further work required to confirm this

Oil type needs to be taken into account

Further work to understand effects on gassing

Vehicle for improving understanding ?

DGA Scoring System

dga scoring system.pdf

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