CONDITION MONITORING OF TRANSFORMER OIL 

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 POINT2. DIELECTRIC DISSIPATION FACTOR3. SPECIFIC RESISTANCE4. NEUTRALISATION.5. MOISTURE CONTENT6. SLUDGE %7. BREAK DOWN VOLTAGE8. D.G.A.

PHYSICAL CONTAMINATION1. 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.

CHEMICAL DECOMPOSITION OF HYDROCARBONS

 

Catalyst

RH R* + H

(Hydrocarbon) (Free radical)

  R* + O RO2*

(Peroxy radical)

 RO2* + RH RO2*H+ R*

(Hydro per oxide)

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 RO2H and another free radical R. this type of chain propagation continues till the chains are broken adding suitable inhibitor.

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 pointSudden drop in flash point indicates of unsatisfactory working condition of transformer.

5 Sludge Indicated deterioration

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 AnalysisReveals ppm of combustible gases dissolved in the oil to assess the internal condition of the transformer.

A Typical Method to Maintain Test Record for Transformer Oil

 TRANSFORMER DATA OIL DATALocation……………………… 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…………

INSTALLATION DATA

Filled on………………………………….

Commissioned on………………………..

Maximum load…………………………...

Maximum oil temperature………………..

Normal load………………………………

Normal Oil Temperature…………………

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…………………

7. Tests conducted:

a. Interfacial Tension.b. Neutralisation Numberc. Moisture content (ppm)d. Flash pointe. Sludgef. Dielectric Dissipation Factorg. Dielectric Strengthh. Resistivity

TABLE 1: APPLICATION AND INTERPRETATION OF TESTS ON OIL IN TRANSFORMERS AS PER IS: 1866 – 1983

Sl. No.

Characteristic Equipment voltage

Permissible limit,

satisfactory for use

Action if outside permissible limit

1Specific resistance at 90° c ohm-cm, min

All voltages 0.1 * e12

Recondition it the value of dielectric dissipation factor permits, reclaim if

not.

2Dielectric dissipation factor at 90° c, max

> 145 kv< 145 kv

0.21.0 Reclaim

Sl. No Characteristic Equipment

voltagePermissible limit,

satisfactory for useAction if outside permissible limit

3Neutralization 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.

Sl. No

Characteristic

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 min40 kv min40 kv min30 kv min

ReconditionReconditionReconditionRecondition

8 Water content ppm

> 145 kv< 145 kv

25 ppm35 ppm

ReconditionRecondition

LIMITING VALUES IS: 1866 – 1983

Test Equ. Voltage Method Limit

Electric strength > 145 kv< 145 > 72.5 kv< 72.5 kv

 Is: 6792

504030

Water content ppm, max

> 145 kv< 145 kv

 Is: 335

2535

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

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: 1448Max Dec 15 125

SEDIMENT AND / OR PRECIPITABLE SLUDGE

ALL VOLTAGE IS: 1666 NIL

FOR TRANSFORMERS IN SERVICE

Properties Condition Remedies

Breakdown voltage Low

Filtration under vacuum + temperature

Moisture High

Dielectric dissipation factor

HighFiltration as above if the cause is moisture or other

Volume resistivity Low

IFT Low Reclamation

GENERAL

NV HIGH  

FP LOW INVESTIGATE THE CAUSE

SLUDGE % HIGH DE-SLUDGING

SUGGESTED LIMITS FOR IN SERVICE OILS GROUP I BY VOLTAGE CLASS, BASED ON IS: 1866-1983

LIMITS _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

VOLTAGE CLASSPROPERTIES

145 KV & 72.5 KV BELOWABOVE & < 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 ≤ 1.0FACTOR 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

Suggested limits for in service oils group ii by voltage class (reconditioning) based on

IS: 1866-1983LIMITS

VOLTAGE CLASSPROPERTIES - - - - - - - - - - - - - - - - - - - - - - - - - - - - - --------------

145 KV & 72.5 KV BELOWABOVE & < 145KV 72.5 KV

Electric strength < 50 < 40 < 30

Neutralisation 0.1 0.1 0.1Value (mg.KOH/g) UPTO 0.5 UPTO 0.5 UPTO 0.5Interfacial < 0.015 < 0.015 < 0.015Tension N/m (MIN) (MIN) (MIN)

Dielectric dissipation > 0.2 > 1.0 > 1.0

Factor At 90° C

Water Content, ppm > 25 > 35 > 35Specific resistance < 0.1 x1012 < 0.1 x1012 < 0.1x1012 Ohm-cm

TABLE IV

Suggested values for in-service oils group II by voltage class (reconditioning) based on IS: 1866 (1983)

LIMITS

Voltage ClassProperties

145 Kv & 72.5 Kv BelowAbove & < 145kv 72.5 Kv

Electric Strength < 50 < 40 < 30Neutralisation Value 0.1 0.1 0.1(mg KOH/g) Upto 0.5 Upto 0.5 Upto 0.5Interfacial Tension < 0.015 < 0.015 < 0.015N/m Dielectric Dissipation > 0.2 > 1.0 > 1.0Factor at 90° CWater Content, ppm > 25 > 35 > 35Specific Resistance < 0.1 < 0.1 < 0.1Ohm-Cm

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.

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

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

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

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…

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

CONCLUSIONSa. 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.

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. Colourii. Visible particlesiii. Haziness or cloudiness

2. Acidity by colourCPRI acidity testing kit

3. Interfacial tension test By field method

4. Quality indexBy Myers method

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.

SHAPES OF SPOTS

 OIL ALONE INDICATOR ALONE

NEW OIL + INDICATOR SERVICE OIL + INDICATOR

SHAPES OF INDICATOR SPOT ARE DEPENDENT ON IFT OF OIL

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

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, minSludge (% by Mass), max oxidation stability of inhibited oil - induction period

   0.4 0.4 0.40.1 0.1 0.1  Similar values before filling

POSSIBLE CASES OF FAILURES1. 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

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