DGA Scoring System.pdf

Know Kn owle ledg dge e Is Is Po Powe wer  r 

SM SM

 Apparatus Maintenance and Power Management Management for Energy Delivery 

DGA Scorin S stem John Lapworth, Dobl Do ble e Pow Power erTe Test st Lt Ltd d

Dissolved Gas Analysis (DGA)  Ideal

condition monitoring tool



Sensitive, broadband



Cheap, on-line

 Sampled

by site staff and analysed by oil laboratory

 Need

to apply engineering experience for interpretation

Dissolved Gas Analysis (DGA)  Ideal

condition monitoring tool



Sensitive, broadband



Cheap, on-line

 Sampled

by site staff and analysed by oil laboratory

 Need

to apply engineering experience for interpretation

DGA Interpretation 

Type and sever severity ity of fault - gas signa signature ture



Key gases



Loading, operational history



Developmentt of fault Developmen

  Action

levels



Trend analysis



Scoring system

DGA - Key Gases Fault ar a sc arge Dielectric (Arcing/sparking) e ec r c (Power arc)

Primary y rogen Hydrogen Acetylene y rogen

Secondary Ethylene

Temperature Thermal hotspot

Ethane Ethylene

Monoxide Methane

Overheated Cellulose

Carbon Monoxide

Carbon Dioxide

e ane Ethylene Acetylene

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

Existing DGA Interpretation Schemes  Gas 

signatures and ratios

E.G. Rogers, IEC 599, Duval etc

 Trend

anal sis

 Normal  Expert

and action levels

systems

We will discuss pros and cons of these

Rogers Ratios

Gas Signatures and Ratios  Thermodynamic  Focus



basis

on diagnosing fault

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



‘Triple zero syndrome’

Trend Analysis  Increasing

gas levels considered indication of

eve op ng au , u ...  Increase  Gas



after de-gassing to equilibrium levels

levels constant for inactive fault

Different interpretation for H2 and C2H2

 May

not have enough results for good trend

Normal and Action Levels  What

level is significant ...

 Statistical 

approach is problematic:

What exceedance should be used - 90, 95 or 97% ?



important ? ‘

’

‘ ‘ unhealthy ’ 

’

 

Expert Systems  E.

. Kohonen net cluster anal sis techni ue

 Attempt

earc  Identify  Rely

to classify similar results or

en n orma on

changes of state

on human expert to ascribe significance

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

IEEE C57.104-1991

IEEE C57.104-1991

Faulty Transformers 100

80

   G    C   e   g   a    t   n   e   c   r   e    P

60

40

20

0  A

B

CO

C

H2

D Transformer  CH4

C2H4

E

F

C2H6

C2H2

G

Normal Transmission Transformers 100

80

   G    C 60   e   g   a    t   n   e   c   r   e    P

40

20

0 1

2

3

4

5

Transformer  CO

H2

CH4

C2H4

C2H6

C2H2

Normal Generator Transformers 100

80

   G    C   e   g   a    t   n   e   c   r   e    P

60

40

20

0 I

II

CO

H2

III Transformer  CH4

C2H4

IV

C2H6

V

C2H2

New DGA Interpretation 

High [ CO ] 

( 60-90 %TCG ) = normal 

( Higher levels may be abnormal )



High [ C2H4 ] ( > 10 %TCG ) = abnormal 



Significant [ H2 ] and [ C2H2 ] = serious



Significant [ CH4 ] and [ C2H6 ] = not serious

Scoring System 

Normal   All

<

60

normal results can be ignored >

 

Only suspect results need to be investigated

Serious

> 100



Winding and dielectric faults



Some can be discounted

Scoring System  ‘Quality’ 

function

Importance of gas signature

 ‘Strength’ 

function

Importance of gas levels

Benefits of Scoring System 

Normal results can be screened out (ignored)



Consistent, objective assessment of results



Resource focussed on deserving cases



Can ignore ‘unusual’ cases

Use in Condition Assessment CO N D ITI O N ASSESSMEN T TEST RESULTS Dielectric 9 8 3 2 DGA RFI MAG TTR 10 10

4 PF

8 AE

PD

2 ACID

4 PF 5

4 POL 10

Ther mal / Life 3 DGA 5

4 FFA 10

4 H2O 10

4 IRES

8 FRA

LRT

4 CAP

2

3

4

5

6

7

8

9

10

Ther mal / Life 1 2

3

4

5

6

7

8

9

10

Mechanical 1

2

3

4

5

6

7

8

9

10

2

3

4

5

6

7

8

9

10

DP

SUMMARY

Dielectric 1

O VERALL 1

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 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 Signature 100

80

   G    T   e   g   a    t   n   e   c   r   e    P

40

20

0 16/04/96

14/08/97 CO

20/04/98 H2

CH4

22/07/98 C2H4

23/07/98 C2H6

C2H2

31/07/98

DGA Score 140

120

100

  e   r 80   o   c    S    A    G    D 60

40

20

0

Evidence

Application 2 - Puzzling Results  High

CH4 and C2H6 in C2H6  many new transformers

 Some

also have high H2 - high H2  scores

 Seasonal

dependence - H2  H2 peaks in late

summer   Different transformer  Same

oil 

designs

New Transformer DGA 250

200

150

   M    P    P 100

50

0 01/01/94

01/01/95

01/01/96

31/12/96

H2

31/12/97

CH4

31/12/98

C2H6

31/12/99

30/12/2000

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