This document provides information on parameters for condition monitoring of transformer oil, including tests to assess oil degradation. Key parameters to check include flash point, dielectric dissipation factor, specific resistance, moisture content, sludge percentage, breakdown voltage, and dissolved gas analysis. Tests like interfacial tension, neutralization number, and acidity help indicate chemical deterioration in the oil. Guidelines are provided on maintenance of test records and interpretation of test results to determine if the oil is suitable for continued use, requires reconditioning, or needs replacement.

1. CONDITION MONITORING OF
TRANSFORMER OIL

2. Parameters for condition monitoring of oil service
Two ways are available to an operating engineer
1. To make periodic oil tests to establish trends and classify
them.
2. To conduct dissolved gas analysis to assess the internal
condition of transformers
PARAMETERS TO BE CHECKED ARE:
1. FLASH POINT
2. DIELECTRIC DISSIPATION FACTOR
3. SPECIFIC RESISTANCE
4. NEUTRALISATION.
5. MOISTURE CONTENT
6. SLUDGE %
7. BREAK DOWN VOLTAGE
8. D.G.A.

3. PHYSICAL CONTAMINATION
1. Dust, fibre, metallic, particles, other solid impurities.
2. Dissolution of varnish.
3. Free and dissolved water.
CHEMICAL DETERIORATION
Oxidation resulting in acids sludges and polar
impurities.
CONTAMINATION OF GASES
a) Dissolved air from atm. Nitrogen, co2
b) Generated in oil, methane, ethane, acetylene,
ethylene etc.Before the oil is put in the
transformer, its properties should be fully
ensured.

4. CHEMICAL DECOMPOSITION OF
HYDROCARBONS
Catalyst
RH R* + H
(Hydrocarbon) (Free radical)
R* + O RO2
*
(Peroxy radical)
RO2
* + RH RO2
*H+ R*
(Hydro per oxide)

5. 2 RO2
*
R* + RO2
*
2 R*
Oxidation products
(Higher Temperature metal
Ions as catalyst favours
the reaction)
Hydrocarbon RH first produces a free radical R. The radical R
now combines with the available oxygen giving peroxide RO2
.
First product of oxidation peroxides reacts with any
hydrocarbon (RH) giving hydro peroxide RO2
H and another free
radical R. this type of chain propagation continues till the chains
are broken adding suitable inhibitor.

6. SERVICE OIL TESTS
The service oil tests to be conducted are furnished below:
Service oil tests as per IS: 1866 – 2000
Sl. No. TESTS
INFORMATION PROVIDED BY
TESTS
1
Interfacial Tension Sludge present in the oil.
2
Neutralisation Number Acid present in the oil
3 Moisture content
(ppm)
Reveals total water content or
cellulosic deterioration.
4
Flash point
Sudden drop in flash point indicates of
unsatisfactory working condition of
transformer.
5 Sludge Indicated deterioration

7. 6
Dielectric Dissipation
Factor
Reveals presence of moisture, resins,
varnishes or their products of oxidation
in oil
7 Dielectric strength
Conductive contaminants and moisture
present in the oil.
8 Resistivity Indicative of conducting impurities.
9 Dissolved Gas Analysis
Reveals ppm of combustible gases
dissolved in the oil to assess the internal
condition of the transformer.

8. A Typical Method to Maintain Test Record for Transformer Oil
TRANSFORMER DATA OIL DATA
Location……………………… Oil supplier……………………
Manufacture’s Name………… Code Designation of Oil………
Manufacture’s Number………. Inhibited or
Un-inhibited…………………….
User’s reference………………
Number of phases……………. Type of Inhibitor
If present……………………………..
Voltage Ratio…………………
Outdoor or Indoor…………….
Type of cooling………………. Quantity of oil in
The system………………………….
With or without Conservator……
Type of Breather…………

9. INSTALLATION DATA
Filled on………………………………….
Commissioned on………………………..
Maximum load…………………………...
Maximum oil temperature………………..
Normal load………………………………
Normal Oil Temperature…………………

10. MAINTENANCE OF DATA
1. Date of sampling……………………..
2. Topping up done up………………….
Type and quantity of oil added………
3. Note on abnormal operations………...
4. Oil filtered on………………………...
5. Breather reconditioned on……………
6. Physical observation…………………

11. 7. Tests conducted:
a. Interfacial Tension.
b. Neutralisation Number
c. Moisture content (ppm)
d. Flash point
e. Sludge
f. Dielectric Dissipation Factor
g. Dielectric Strength
h. Resistivity

12. TABLE 1: APPLICATION AND INTERPRETATION OF
TESTS ON OIL IN TRANSFORMERS
AS PER IS: 1866 – 1983
Sl.
N
o.
Characteristic
Equipment
voltage
Permissible
limit,
satisfactory for
use
Action if outside
permissible limit
1
Specific
resistance at
90° c ohm-
cm, min
All
voltages
0.1 * e12
Recondition it the
value of dielectric
dissipation factor
permits, reclaim if
not.
2
Dielectric
dissipation
factor at 90° c,
max
> 145 kv
< 145 kv
0.2
1.0
Reclaim

13. Sl.
No
Characteristic
Equipment
voltage
Permissible limit,
satisfactory for use
Action if outside
permissible limit
3
Neutralization
value,
mg / KOH / g
All
voltages
0.5 Reclaim
4
Precipitate
sludge
All
voltages
No perceptible
sludge
Reclaim
5 Flash point
All
voltages
Decrease in flash
point 15°c (max) of
the initial value,
minimum value
125° c
Reclaim after
checking causes.

14. Sl.
N
o
Characteristi
c
Equipment
voltage
Permissible
limit,
satisfactory for
use
Action if
outside
permissible
limit
6
Interfacial
tension at
27°c, n/m,
min
All
voltages
0.015 Reclaim.
7
Electric
strength
145 kv +
above
< 145 kv
> 72.5 kv
< 72.5 kv
50kv min
40 kv min
40 kv min
30 kv min
Recondition
Recondition
Recondition
Recondition
8
Water
content ppm
> 145 kv
< 145 kv
25 ppm
35 ppm
Recondition
Recondition

15. LIMITING VALUES IS: 1866 – 1983
Test Equ. Voltage Method Limit
Electric strength
> 145 kv
< 145 > 72.5
kv< 72.5 kv
Is: 6792
50
40
30
Water content
ppm, max
> 145 kv
< 145 kv Is: 335
25
35
Specific
resistance @ 90,
10 E12 ohm, cm
All voltage Is: 61 0.1
Tan delta @90,
max
> 145 kv
< 145 kv
Is: 6262
0.2
1.0

16. ACIDITY mg
KOH / g, Max
ALL
VOLTAGE
IS: 144 0.5
IFT, N/m. Min
ALL
VOLTAGE
IS: 6104 0.015
FLASH POINT
Deg C, Min
ALL
VOLTAGE
IS: 1448
Max Dec 15
125
SEDIMENT
AND / OR
PRECIPITABLE
SLUDGE
ALL
VOLTAGE
IS: 1666 NIL
FOR TRANSFORMERS IN SERVICE

17. Properties Condition Remedies
Breakdown
voltage
Low
Filtration under vacuum
+ temperature
Moisture High
Dielectric
dissipation
factor
High
Filtration as above if the
cause is moisture or other
Volume
resistivity
Low
IFT Low Reclamation
GENERAL

18. NV HIGH
FP LOW
INVESTIGATE THE
CAUSE
SLUDGE % HIGH DE-SLUDGING

19. SUGGESTED LIMITS FOR IN SERVICE OILS GROUP I BY VOLTAGE CLASS,
BASED ON IS: 1866-1983
LIMITS
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
VOLTAGE CLASS
PROPERTIES
145 KV & 72.5 KV BELOW
ABOVE & < 145KV 72.5 KV
ELECTRIC STRENGTH ≥ 50 ≥ 40 ≥ 30
(MIN) (MIN) (MIN)
NEUTRALISATION VALUE ≤ 0.1 ≤ 0.1 ≤ 0.1
(mgKOH/g) (MAX) (MAX) (MAX)
INTERFACIAL TENSION N/m ≥ 0.015 ≥ 0.015 ≥ 0.015
(MIN) (MIN) (MIN)
DIELECTRIC DISSIPATION ≤ 0.2 ≤ 1.0 ≤ 1.0
FACTOR AT 90° C (MAX) (MAX) (MAX)
WATER CONTENT, ppm ≤ 25 ≤ 35 ≤ 35
(MAX) (MAX) (MAX)
SPECIFIC RESISTANCE
Ohm-cm 0.1x1012
≥ 0.1x1012
≥ 0.1x1012

20. Suggested limits for in service oils group ii by voltage class
(reconditioning) based on
IS: 1866-1983
LIMITS
VOLTAGE CLASS
PROPERTIES - - - - - - - - - - - - - - - - - - - - - - - - - - - - - --------------
145 KV & 72.5 KV BELOW
ABOVE & < 145KV 72.5 KV
Electric strength < 50 < 40 < 30
Neutralisation 0.1 0.1 0.1
Value (mg.KOH/g) UPTO 0.5 UPTO 0.5 UPTO 0.5
Interfacial < 0.015 < 0.015 < 0.015
Tension N/m (MIN) (MIN) (MIN)
Dielectric dissipation > 0.2 > 1.0 > 1.0
Factor At 90° C
Water Content, ppm > 25 > 35 > 35
Specific resistance < 0.1 x1012
< 0.1 x1012
< 0.1x1012
Ohm-cm

21. TABLE IV
Suggested values for in-service oils group II by voltage class
(reconditioning) based on IS: 1866 (1983)
LIMITS
Voltage Class
Properties
145 Kv & 72.5 Kv Below
Above & < 145kv 72.5 Kv
Electric Strength < 50 < 40 < 30
Neutralisation Value 0.1 0.1 0.1
(mg KOH/g) Upto 0.5 Upto 0.5 Upto 0.5
Interfacial Tension < 0.015 < 0.015 < 0.015
N/m
Dielectric Dissipation > 0.2 > 1.0 > 1.0
Factor at 90° C
Water Content, ppm > 25 > 35 > 35
Specific Resistance < 0.1 < 0.1 < 0.1
Ohm-Cm

22. GROUP III OILS:
To categorize under group III, the parameters should be
well beyond the limits proposed in table IV. Such oils
should be initially filtered under vacuum and temperature
to verify whether the properties improve or not. If
properties like Dielectric Dissipation Factor, Interfacial
Tension do not improve on filtration, then there is a case
for oil to be replaced.

23. Properties of oils evaluated in transformers of a utility
Sl.
No
Voltage
class
BDV Tan Delta Resistivity IFT Acidity
1 33 / 11 18 0.0321 0.914E12 30 0.1
2 66 / 11 38 0.0392 0.046E12 30 0.3
3 66 / 11 47 0.29 0.666E12 10 0.5
4 220 / 11 27 0.0108 2.75E12 30 0.1
5 220 / 11 35 0.263 0.912E12 30 0.1

24. Properties of oils evaluated in transformers of a utility
Sl.
No
Voltage
class
BDV
Tan
Delta
Resistivity IFT Acidity
16 220 / 66 45 0.0032 6E12 30 0.1
17 66 / 11 47 0.0063 5.86E12 30 0.2
18 66 / 11 11 0.014 2.68E12 30 0.1
19 66 / 11 8 0.039 2.18E12 30 0.1
20 66 / 11 24 0.023 1.76E12 25 0.2
21 66 / 11 13 0.011 6.68E12 30 0.5

25. Properties of oils evaluated in transformers of a
utility oil to be replaced/reclaimed
Capacity
in MVA
Voltage in
KV
BDV
Tan
Delta
Resistivity Acidity IFT
5
33 / 11 29 1.000 0.0177 0.6
10
5
33 / 11
-
1.050 0.0151 0.6
10
5 110 / 33 /
11
-
0.820 0.0194 0.6
15
5
33 / 11 24 0.270 0.0643 0.8
10
5
33 / 11 29 1.050 0.0168 0.8
15
5
33 / 11 37 0.720 2.3700 0.8
15
5
33 / 11 44 1.170 0.0150 0.8
10

26. Properties of oils evaluated in transformers of a
utility – oil to be replaced/reclaimed
Capacity
in MVA
Voltage in
KV
BDV
Tan
Delta
Resistivity Acidity IFT
5 33 / 11 44 0.990 0.0175 0.5 10
5 33 / 11 30 0.370 0.0452 0.8 15
5 33 / 11 39 0.290 0.0589 0.8 15
5 33 / 11 33 0.190 0.0905 0.8 10
5 33 / 11 29 0.680 0.0227 0.8 15
Contd…

27. 5 33 / 11 63 0.880 0.0187 0.6 15
5 33 / 11 27 0.775 0.0222 0.6 10
5 33 / 11 22 1.100 0.0157 0.6 10
5 33 / 11 18 0.620 0.0283 0.8 10
5 33 / 11 15 0.700 0.0243 0.6 10

28. CONCLUSIONS
a. In view of the importance of service oil tests, all tests have to
be carried out as per IS: 1866 systematically and periodically,
as a part of maintenance schedule, as no single test is a reliable
indicator.
b. The periodic testing will help O & M personnel to build up
databank and case histories of power Transformers.
c. Periodic testing will help to know the oil condition as well to
know growing of any incipient faults by Dissolved Gas
Analysis.
d. The preventive maintenance followed by all utilities will result
not only savings in O & M costs but also prevent any
premature breakdown / Failures besides improving the overall
system reliability.

29. Test required to assess the level of oil
degradation
Screening tests: - Laboratory evaluation can be minimised by
performing screening tests at site.
The tests are:
1. Appearance
i. Colour
ii. Visible particles
iii. Haziness or cloudiness
2. Acidity by colour
CPRI acidity testing kit
3. Interfacial tension test
By field method
4. Quality index
By Myers method

30.
PORTABLE ACIDITY TESTING KIT
A portable acidity testing kit was developed by cpri to quickly
check the acidity of transformer oil at site
Procedure:
A) 1.1 ml of oil in a test tube.
B) Add 1ml of rectified spirit.
C) Shake well
D) Add 1 ml. of alkaline solution
E) Add few drops of universal indicator
F) Match the colour with the colour chart to arrive at the acidity
value.
Advantages: Test can be performed by semi skilled personnel at
the site of transformer / remote areas.

31.
SHAPES OF SPOTS
OIL ALONE INDICATOR ALONE
NEW OIL + INDICATOR SERVICE OIL + INDICATOR
SHAPES OF INDICATOR SPOT ARE DEPENDENT ON IFT OF OIL

32.
Recommended limits of unused mineral oil filled in new
power transformer
Property Highest voltage equipment (kV)
<72.5 72.5 to 170 > 170
Appearance
Clear, free from sediment and suspended
matter
Density @ 29.5° C (g / cc), max 0.89 0.89 0.89
Flash point, (° C), max 140 140 140
Neutralisation value, mg /
KOH /g
0.03 0.03 0.03

33.
Water content, ppm, min 20 15 10
Interfacial tension, N/m, min 35 35 35
Dielectric Dissipation Factor
@ 90° C, 40-60 Hz², max
0.015 0.015 0.010
Resistivity, ohm-cm, min 6 6 6
Breakdown voltage, kV, min 40 50 60
Oxidation stability of
uninhibited oil
Neutralisation value,
Mg.KOH/g, min
Sludge (% by Mass), max
oxidation stability of inhibited
oil - induction period
0.4 0.4 0.4
0.1 0.1 0.1
Similar values before filling

34.
POSSIBLE CASES OF FAILURES
1. Failure in appearance:
i. Colour.
ii. Rusty particles.
Prolonged storage in the case of drums contamination from
the tankers and deliberate adulteration
• Failure in physical properties
i. Low flash point
ii. High pour point
iii. High / low viscosity.
Prolonged storage combined effect of thermal and oxidative
degradations and chemical reactions

35. 3. Failure in electrical properties
i. Low dielectric strength
ii. Low resistivity
iii. High dissipation factor
Prolonged storage ingress of moisture and bad sampling
4. Non attainment of IR values
i. Improper design / construction
ii. Poor quality of construction and solid
iii. Insulating materials