The document discusses various tests that are conducted on transformer oil to monitor its condition and extend the service life of transformers. It describes tests such as moisture content, breakdown voltage, resistivity, tan delta, dissolved gas analysis, furan analysis and other tests. The results of these tests are compared to limit values specified in standards to determine if the oil and transformer are healthy or require actions like drying, filtration or oil replacement. The color of oil is also used to indicate its quality from good to extremely bad. Degree of polymerization testing and solid insulation conditions are other aspects covered in the document.

1. Condition monitoring of transformer oil for
extending its service life
M. G. Morshad , ADGM / Electrical
TPS II ( 7 x 210MW) NLC India Ltd

2. Transformer Insulating system
Oil Paper
1. Acts as a Coolant
2. Act as an insulation
3. Protects the Paper from chemical
attack
4. Prevention of sludge buildup
5. Used as Diagnostic Tool
1. Mechanical Strength
2. Dielectric Strength
3. Dielectric Spacing

3. TRANSFORMER OIL TEST PACKAGE
(As per IS: 1866)
3

4. 4
Moisture
(Dielectric Status)
1. Decomposes solid insulation
2. Reduces BDV
3. Increases oil heating
Slug & Sediments
(Aging status)
1. Reduce cooling effects
2. Indicates decomposition
of solid insulation
DGA
(Internal status)
1. Reduce life
2. Unforeseen failure
3. Sever damages
Water ppm
BDV
Tan ∂
Resistivity
Test results
are within
the limit
Test for
1. Hot oil circulation
2. Drying out
Replace Oil
Transformer is healthy.
No action is required.
No
No
Yes
Yes
Test for
Test results
are within
the limit
IFT
Acid Number
Sludge
content
Oil Colour
Test results
are within
the limit
Yes
(CO2/CO)<5
O2 >2000
Number
(CH4, C2H4,
C2H2) > Limit
value
Test for
Conduct
Furan
Analysis
No
Test results
are within
the limit
Yes
Replace Oil
1. RLA Study & DP test
2. Refurbishing of
solid insulation
3. Disposal of
transformer
Duval
Triangle
Analysis
(H2, CH4, C2H4,
C2H6, C2H2) >
Limit value
Roger
Ratio
Analysis 1. Internal Inspection
2. Tightness checking
3. Spare parts
changing
No

5. Pale
yellow
Good
oil
Yellow
Proposition
‘A’ oil
Bright
yellow
Marginal
oil
Amber
Bad
oil
Brown
Very bad
oil
Dark
brown
Extremely
bad oil
Black
Oils in disastrous condition
Discarded
oil
OIL QUALITY WITH COLOUR

6. 6
50
40
30
20
10
0
10
20
30
40
50
60
Waterppminoil
Dryness of oil
Limit value : Water ppm in transformer oil
New oil
(72 – 170 KV Transformer)
(>170 KV Transformer)
(< 72 Transformer)No free
water
Dry Oil
1. The presence of water molecule in the oil is measured by Karl Fisher Titration
methods.
2. The limit value of water ppm ( part per million) need to be maintained as per
the guideline of IS 335 shown in the above graph

7. 7
Determination of moisture accumulation in solid insulation
through water ppm in oil

8. 8
30
40
50
60
80
0
10
20
30
40
50
60
70
80
90
0 10 20 30 40 50 60
BreakDownVoltageinKV
Water ppm in Transformer oil
Limit value : Break Down Voltage in KV
< 72.5 KV Transformer
72.5 -170 KV Transformer
> 170KV Transformer
BDV value of transformer oil mainly depends on water ppm in the oil and it decreases with
the increase of water ppm in oil .In such case BDV of oil is improved by reducing water ppm
in oil through filtration .
 BDV of oil may also decrease due to low resistivity of oil caused by degradation of oil or
contamination of oil with soluble polar particles. In such case oil needs to be replaced after
confirming low resistivity & IFT value and high tan delta & acidity value with colour of oil.
 BDV of oil is determined by applying AC
voltage across a gap of 2.5mm ,filled with
transformer oil to be tested .
 The voltage at which the spark is observed
between the gap , is the BDV of the oil.
 Average of six such BDV value is taken as
the actual BDV of the oil.

9. 9
35
25
15
6
2
0.8 0.5 0.2 0.1
0
5
10
15
20
25
30
35
40
Resistivity(x1012Ohms–cm)
Conductivity ( Flow of leakage current )
Limit value (at 90 Deg C) : Resistivity
(Very Good)
(Good)
(Min)
Resistivity is measured by
applying 500 V DC voltage
across the oil sample after
heating up the sample at
90 Deg C which is the
maximum allowable
operating temperature of
transformer

10. 10
+
+
+
+
+
+
-
-
-
-
-
-
-
+
+
+
-
+
+
-
+
+
+
- +
-
-
+
+
+
+
+
-
Leakage current
Charging
current
Charging
current
High AC Voltage
Effect on presence of polar particles in Transformer oil
The insulating properties of transformer oil decreases with the increase of soluble polar particles
such as water molecule, sludge & sediments, varnish, resin etc, in the oil.
The polar particle present in the oil gets ionized under the influence of high AC voltage. These
ionized particles gets attracted by the opposite polarity causing the flow of leakage current through
the oil (insulator). The intensity of the leakage current increase with increase of concentration of
polar particulates in the oil. Because of this reasons BDV, Resistivity & Tan delta value of the
transformer oil gets affected due to presence of polar particles such as water molecule, sludge &
sediments, varnish, resin etc in the oil.

11. 11
Charging current
Leakage current
Total current
O
BA
δ3
δ2
Limit value ( At 90 Deg C) : Tan Delta
0.002
0.2
1
0.001
0.01
0.1
1
TANDELTAVALUE
Concentration of polar particles
Angle value % Value
Ideal
TAN δ 0.000 AB/OB 0.000
δ0 0.0 Deg OC 0.% of OB
Normal
TAN δ1 0.002 AB/OB 0.002
δ1 0.1 Deg OC 0.2 % OB
Maximum
( > 72.5 KV
Transformer)
TAN δ2 0.200 AB/OB 0.200
δ2 11.3 Deg OC 20 % OB
Maximum
(< 72.5 KV
Transformer)
TAN δ3 1.000 AB/OB 1.000
δ3 45 Deg OC 100 % OB
δ1
Normal
Max > 72.5 KV
Transformer
Max < 72.5 KV
Transformer
Tan delta value is measured
by applying 1000 V AC
voltage across the oil
sample after heating up the
sample at 90 Deg C which
is the maximum allowable
operating temperature of
transformer

12. Interfacial Tension, Acid Number, Years in Service

14. Dissolved Gas Analysis (DGA)
 Due to normal aging or internal faults different types of gases
are generated which subsequently dissolve in oil.
 Hence dissolved gas analysis of the transformer oil gives indication of
the nature of fault inside the transformer.
 A sudden increase in key gases and the rate of gas production is more
important in evaluating a transformer than the accumulated amount of gas
 The advantages of dissolved gas analysis are -
14

15. Acceptance limits
as per IS: 9434 / 1992 , IS: 10593 / 1983 & IEC 599 –1978
GAS
LESS THAN FOUR
YEARS IN SERVICE
(PPM)
4-10 YEARS IN
SERVICE (PPM)
MORE THAN 10 YEARS
IN SERVICE (PPM)
HYDROGEN 100-150 200-300 200-300
METHANE 50-70 100-150 200-300
ACETYLENE 20-30 30-50 100-150
ETHYLENE 100-150 150-200 200-300
ETHANE 30-50 100-150 800-1000
CARBON
MONOXIDE
200-300 400-500 600-700
CARBON DIOXIDE 3000-3500 4000-5000 9000-12000
15

16. Fault Analysis - by Roger methods
CH4
H2
C2H6
CH4
C2H4
C2H6
C2H2
C2H4
Faults
0.1 0 0 0 Partial discharge
0 0 0 0 Normal deterioration
1 0 0 0 Over heating >150 Deg C
1 1 0 0 Over heating 150 – 200 Deg C
0 1 0 0 Over heating 200 – 300 Deg C
0 1 1 0 General condition of overheating
1 0 1 0 Circulating current / Overheating of joints
0 0 0 1 Flash over without power flow
0 1 0 1 Current breaking through tap changer
0 0 1 1 Arc with power flow
•Ratio less than 0.1 is designated as 0
•Ratio greater 1 is designated as 1
16

17. Duval Triangle
17

18. solid insulation
 Paper and cloth used in transformer is known as solid insulation.
 The main constituent of paper and cloth is fibrous material known as Cellulose
 Cellulose is an organic compound whose molecule is made up of a long chain of
glucose monomers (rings), typically numbering between 1000 and 1400
 With breaking down of cellulose chain, the number of glucose ring decreases in
the cellulose molecule
 Degree of polymerization (DP) is the average number of glucose rings in the
cellulose molecule and DP values state the aging status of the insulating paper
 DP value > 900 indicates good paper whereas DP value < 200 indicates bad
papers.
Glucose Glucose Glucose
18

19. 19

20. degree of polymerization (DP) values and aging of
transformer
Years
DP ValuesAge (Yrs) DP Values
0 >1000
1 975
12 700
22 450
25 390
35 125
20

21. Furan analysis
 The mechanical properties of insulating paper can be established by direct
measurement of its tensile strength or degree of polymerization (DP).
 Direct measurement of these properties is not practical for in-service transformers since
it requires removal of a few strips of paper from suspect sites.
 This procedure can conveniently be carried out during transformer repairs. The results of
these tests will be a deciding factor in rebuilding or scrapping a transformer.
 Since it is usually not practical (and often dangerous to the transformer) to obtain a
paper sample from a de-energised, in-service transformer an alternative method has been
found.
 When a cellulose molecule de-polymerises (breaks into smaller lengths or ring
structures), a chemical compound known as a furan is formed.
 By measuring the quantity and types of furans present in a transformer oil sample, the
paper insulation overall DP can be inferred with a high degree of confidence.
The types and concentration of furans in an oil sample can also indicate abnormal stress
in a transformer, whether intense, short duration overheating or prolonged, general
overheating.
 Furan analysis can be used to confirm Dissolved Gas Analysis where carbon monoxide
present indicates problems with solid insulation
21

22. Limiting values for furan analysis
22

23. 23
Thank you