TESTING AND ANALYSIS OF TRANSFORMER OIL

Delivered by: Er. Harkiranjit Kaur Asstt. Director /P&D ( TS) Dated 30.08.2004

TRANSFORMER FLUID IS POWERFUL

DIAGNOSTIC TOOL

1. Characterization : Which gives parameters that can be used to identify the oil.

2. Ageing status : Which gives parameters relevant to the ageing process.

3. Dielectric Status : Which gives parameters used to determine the dielectric safety margin and dielectric characteristics of the insulation spaces.

4. Degradation status : which gives parameters relevant faults failure and wear.

CHEMISTRY AND COMPOSITION OF OIL

Transformer oils are made from a mixture of Paraffinic (CP), Naphthenic (CN) and Aromatic (CA) molecules in varying proportions and all the three components are essential in certain proportion for good and long performance of the transformers .

Difference between Naphthenic oil and Paraffinic Base oil :

Pour point of Paraffinic base oil is more than Naphthenic base oil.

Naphthenic based oils are more easily oxidized as compared to paraffinic oils

the Naphthenic based oil has 25% reduced viscosity at 70ºCdue to which it act as a better coolant

AROMATIC MOLECULES:

It is that component which give the oil its oxidation and ageing stability.

It is considered maximum upto 8%.

CHARACTERSTICS OF OIL:

Sr.No

Characteristics Requirement as per ISS-335

1. Appearance Clear & transparent

2. Density at 29.5 deg. C (Max.)

0.89 g/cm3

3. Kinematic viscosity at 27 °C (Max)

27 cSt

4. Interfacial tension at 27 °C (Min.)

0.04 N/M

5. Flash Point (Min.) 140 °C

6. Pour Point (Max.) -6 °C

7. Neutralization valueTotal acidity (Max.)

0.03 mg KOH/g

8. Corrosive Sulphur Non-corrosive

9. Electric strength (Break down voltage) Min.New Unfiltered oil

30KV (rms) (Min.)If the above value is not obtained the oil shall be filtered.

After filtration 60KV(r.m.s) (Min.)

10. Dielectric dissipation factor (Tan-delta) at 90 ° C

0.002 (Max.)

11. Water content (Max.) 50ppm

12. Specific resistance (Resistivity) a) At 90 °C (Min.) b) At 27 °C (Min.)

35X1012 ohm-cm1500X1012 ohm-cm

13. Oxidation stability (after oxidation for 164 hours @ 100 °CNeutralization value after oxidation (Max.)Total sludge after oxidation (Max.)

0.40 mg KOH/g0.10% by weight

14. Ageing characteristics after accelerated ageing (open beaker method with copper catalyst) i.e. 96 hours at 115 °C

a. Specific resistance (resistivity) i) At 27 °C (min.)ii) At 90 °C (min.)

25X1012 ohm-cm0.2X1012 ohm-cm

b. Dielectric dissipation factor (Tan delta) at 90 °C

i) Total acidity, mg. KOH/gii) Total sludge value percent

by weight.

0.20 Max.

0.05 Max 0.05 Max

15. Presence of oxidation inhibitor Absent

PERMISSIBLE VALUES OF OIL CHARACTERISTICS FOR

IN SERVICE TRANSFORMER OIL AS PER ISS 1866:2000

Sr. No.

Name of Characteristics

Equipment voltage

Permissible limit for in service T/F

Oil ISS:1866-2000

1. Electric Strength (Break down voltage (KV) (Minimum)

A)>170KV B)>72.5KV-

170KVC) 72.5KV and

below

5040

30

2. Specific resistance (Resistivity at 90degC) ohm-cm (Minimum)

All voltages 0.1X1012

3. Neutralization value

All voltages 0.3mg

3. Water Content (PPM) (Max)

A)>170KVB)>72.5KV-170KVC) 72.5KV and below

2040No free moisture at Room Temperature

4. Dielectric dissipation factor max (Tan Delta) at 90°C

A)>170KVB&C) Upto 170KV

0.20.1

6. IFT at 27°C (m N/m) Min

All voltages 15

7. Flash Point °C - 15 ° C (Max.) decrease from initial value

CONDITION MONITORING OF TRANSFORMER OIL

ELECTRICAL PARAMETRS

DIELECTRIC STRENGTH

Popularly known as BDV. BDV of oil is the AC voltage which causes spark between two electrodes placed in the oil under test at the standard distance of 2.5mm.

Indicator of solid impurities and water content present in the oil

BDV of the oil within transformer goes down slowly as the water content and conducting impurities increase as a result of oxidation of oil

The minimum value of BDV is (30KVrms) for new unfiltered oil and 60KV(rms)(min.) after filtration

SPECIFIC RESISTANCE (resistivity)

Also called resistivity . It is the DC resistance of volume of oil of unit cross sectional area and unit length

It should be as high as possible .

An increase in temperature reduces the resistivity .

The value of resistivity is considered as 35x1012 ohm-cm at 90ºC and 1500x1012 ohm-cm at 27ºC. The resistivity of the used oil at 90ºC may be taken as 0.1x1012 ohm- cm(min.).

DIELECTRIC DISSIPATION FACTOR

Popularly known as Tan delta or loss factor Reveals presence of contaminants picked by oil ,derived

due to oxidation, poor manufacture or bad use of equipment.

High value of DDF leads to increased 1. heating leading to thermal runway 2. corrosion & oxidation 3. water solubility and emulsifying power. Low value of DDF– ensures freedom from moisture, polar

compounds, soluble impurities and minimum energy loss.

Limits (Max) at 90° C new oil 0.002 oil in service less than 170 KV 0.2 for upto 170KV class 1.0 SK VALUE SK value prescribes the behavior of insulating oils in the

presence of concentrated Sulfuric acid

It furnishes the information on the refining degree of mineral oil.

SK value % by weight – 4 to 8% (max). Under consideration as per IS

:

CHEMICAL PARAMETRS

WATER CONTENT Presence of water in oil adversely affect the

dielectric properties of the oil. Water present in oil is absorbed by the solid

insulation of the transformer. The water present in oil is measured in ppm and

measured with sophisticated instruments like Karl/Fischer

The max. value of water in new transformer oil may be taken as 50ppm followed by 20ppm in used oil.

ACIDITY/ NEUTRALISATION NUMBER : Measure of acidity in new & used mineral Insulating

oils Expressed in number of mg of KOH required to

neutralize 1g of oil. Limits 1) New oil -- --- 0.03mg KOH/g (max)

2) Service oil --- 0.3 mg KOH/g

CORROSION TEST: To detect any traces of free or combined sulphur

that may be present in an oil . The presence of sulphur promotes oxidation and

also corrodes the copper and silver metal points of the equipment.

Colour of copper after test : Orange red , Multi coloured ---New corrosive Black, grey or brown--- Corrosive New insulating oil is required to be non Corrosive.

OXIDATION STABILITY : Another method to determine the quality of New

insulating oil with respect to oxidative Degradation. IS consists in ageing 25g of the oil at 100ºC for 164

hours during which oxygen at the rate of one litre/ hour is bubbled into the oil.

The maximum limits fixed for new oils are acidity

0.4mg KOH/gm and sludge 0.1 percent by weight. PRESENCE OF OXIDATION INHIBITER: The Indian Standards IS:335 (1993) is applicable to

only unhibited oils .

Oil containing any amine or phenolic type of inhibitors is not accepted.

PHYSICAL PARAMETERS: DENSITY: The ratio of the weight of a given volume of the oil

to the weight of the same volume of water. TEST METHOD: IS: 1448 (Part 16) – 1997 Evaluating methods -Hydrometer method & Density bottle method Density range: 0.85 – 0.89 g/CC (max)

VISCOSITY : Measurement of resistance to gravity flow of a liquid,

the pressure head being proportional to its density UNIT : CENTISTOKES (cSt) Test method – is : 1448(Part 25) 01976 VISCOSITY – TEMP. RELATIONSHIP Low viscosity at low temp. Efficient heat removal from windings Prevents localized over heating. Kinematic viscosity at 27 °C (Max) is 27 cSt.

POUR POINT : TEST METHOD – IS-1448(P-10)-1970 The sample oil is cooled and the temperature at

which it does not flow when held horizontally in a tube for 5 seconds is noted.

High pour point- Increase in viscosity ,When exposed to weather, hinder convection currents & poor cooling of windings.

The limit of pour point (max) as per IS is -6°C.

FLASH POINT The minimum temp at which oil gives so much vapors

that this vapor when mixed with & forms and ignitable mixture and gives a momentary f lash on application of small pilot flame.

TEST METHOD – IS : 1448 (P:21)-1970 Low Value – Risk of fire in transformer Min. Limit as per IS: 335 – 140°C Max. reduction for in service oils as per IS 1866– 15 deg

C

INTERFACIAL TENSION: Measurement of molecular attractive force between

their unlike molecules at the interface Of oil and water. Unit = N/m TEST METHOD – IS:6104 – 1971 It is the force required to lift a planar ring of

platinum from the Oil. Limit as per IS: 335- 0.040 N/m (min) For oil in service IS: 1866 – 0.015 N/m (min)

SLUDGE: Sludge deposits itself on windings, tank walls and in

cooling ducts. The deposition of sludge in the oil ducts blocks the

free circulation of oil thereby/impairing cooling,

increased temperature and more sludge.

TESTS TO BE PERFORMED ON IN SERVICE OILS:

Tests for Electrical properties and DGA of the oil of the transformer of the age below 10 years should be done after every two years and of more than 10 years, it should be done every year. These tests are also required to

be carried out after every dehydration.

DIAGNOSTIC TESTS: Dissolved gas analysis in oil Furanic compounds in oil Metals in oil Water content in oil and relative saturation

DISSOLVED GAS ANALYSIS: The causes for the generation of gases are classified

as : Thermal degradation/Insulation Decomposition Arcing Partial discharge

Oxidation Vaporization

Major and Minor gases produced are:

Major Gas Minor Gas Nature of fault

C2H4 CH4 Thermal decomposition

CH4 H2 Hot spot

H2 , CH4 C2H6, C2H2, C2H4

Electrical discharge

H2 CH4, C2H6 Internal Corona

CO, CO2 - Cellulous insulation Decomposition

The sensitivity limits for gases dissolved in oil shall meet the following requirements :

Hydrogen : 5ppm Hydrocarbons : 1ppm (Methane, Ethane, Acetylene, Propane, Ethylene,

Propylene) CO,C O2 : 25ppm O2, N2 : 50ppm Before considering that any gas is present in significant

quantity, its concentration shall be at least 10 times these sensitivity level

As per Morgan and Morse, the recommended safe gas levels in oil immersed equipment are:

Gas Dissolved Gas Concentration

H2 <20n+50

CH4 <20n+50

C2H6 <20n+50

C2H4 <20n+50

C2H2 <5n+10

CO <25n+500

CO2 <100n+1500

TCG <105n+710

Here n = number of years in service of transformer TCG= Total Combustible Gas GAS GENERATION RATE :

Rate of gas generation is different for every transformer due to its unique loading condition & design.

Transformer Rating TCG/ppm/year >275KV >10MVA 350 <10MVA 250 500KV - 150

INTERPRETATION SCHEMES; IEC 599 Method: Code of range of Ratios C2H2 CH4 C2H4 C2H4 H2 C2H6 Ratios of Characteristic Gases

<0.1 0 1 0

0.1-1 1 0 0

1-3 1 2 1

>3 2 2 2

Case No. Diagnosis of

faults/Characteristic fault Typical examples

0 No fault 0 0 0 Normal ageing 1 Partial discharges 0 1 0

of low energy density Discharge in gas-filled cavities

2 Partial discharges 1 1 0 of high energy density

As above, but leading to tracking or per formation of solid insulation.

3 Discharge of low 1-2 0 1-2

energy

Continuous

sparking in oil

4 Discharge of high 1 0 2

energy Arcing breakdown of oil between coils to earth

5 Thermal fault of low 0 0 1Temp.150°C

General insulated conductor overheating.

6 Thermal fault of 0 2 0low temp. 150°C –300°C

Local overheating of the core

7 Thermal fault of 0 2 1Medium temp. 300°C –700°C

-do-

8 Thermal fault of 0 2 2High temp. 700°C

-do-

Rogers Method

CH4H2

C2H6CH4

C2H4C2H6

C2H2C2H4

Analysis

0 0 0 0 If CH4/H2 is =0 or 0.1 P.D. otherwise normal

deterioration 1 0 0 0 Slight overheating below

150°C

1 1 0 0 Slight overheating 150°C -200° C

0 1 0 0 Slight overheating 200°C -300°C

0 1 1 0 General conductor overheating

1 0 1 0 Circulating current overheated joints

0 0 0 1 Flash over without power flow.

0 1 0 1 Tap changer breaking current

0 0 1 1 Arc with persistent sparking

TDCG Method: ( Total Dissolved Combustible Gas (TDCG) Limits in PPM)Fault Gases :H2 CH4 C2H6 C2H4, C2H CO CO 100 120 65 50 35 350 2500

TCG=720(PPM) TDCG LIMITS ACTION

≤720 Satisfactory operation 721-1920 Normal ageing/ slight decomposition 1921-4630 Significant decomposition

>4630 Substantial decomposition

SAMPLING: Properly retrieving a sample of oil is important to ensure

that it is a true representative of oil in the equipment sample should be drawn when the equipment is warm. For Forced oil cooled transformers , the oil pumps should

be run for at least 10 minutes before sample is drawn . Two samples instead of one drawn at a time . Sample containers must be full , wrapped, labeled and

sealed. CELULOSE DEGRADATION : For a transformer with healthy cellulose insulation the

ratio of the CO2/CO is expected to be in the range of 3 to11 . CO2 and CO are also produced due to oil break down, moisture content and temperature of degradation and some times this ratio may lead to wrong indications

FURANIC COMPOUNDS: Test have been developed which detect oil soluble

breakdown products of the cellulose chain called furanic compounds.

The 5 compunds namely are: 5-Hydroxymethyl-2- furaldehyde 2-furfural alcohol 2-furfuralhyde 2-acetylfuran 5-methyle-2-furalhydeAmong these 2-furfural is the most significant

Relationship between DP(Degree of Polymerization) value and the amount of furanic concentration in the oil:

Lower DP values indicate increased furfural content when paper samples were subjected to heating over a long period.

Filtration of oil removes already existing furan temporarily and they re-appear in their original concentration levels after 6 months.

Limits of furan are as prescribed by CPRI are:

TOTAL FURAN (PPB) CONDITION

0-100 Normal

101-250 Questionable

251-1000 Deteriorated

1001-2500 Low reliable>2500 Replace the windings

ANALYSIS OF METALS IN OIL: Diagnostic or investigative tool when other system

indicate an incipient-fault condition. The most common metals dissolved in the oil could be

iron copper or aluminum depending upon the coil construction.

Since new oil in transformers should not have significant quantities of any dissolved or particulate metals any appreciable increase of metals in the oil could be an indicator of where a problem may exists

MAINTENANCE PROCEDURE FOR THE INSULATING OIL IN TRANSFORMERS :

Oil level of the transformer should be checked at frequent interval and leakage of oil should be investigated.

All leaks should be repaired as quick as possible . The oil for topping up should preferably be from the

some source as the original oil . Samples of oil should be taken at regular intervals

and tested Don’t rely upon the dielectric strength of the oil. Reconditioning by centrifugal separation or filtration

doesn’t remove the acidity from oil but will remove sludge, dust, dirt etc

Filter with fullers earth will help the reduce acidity in the oil and in addition improve the resistivity.

If BDV is below 40KV and 50KV for HV & EHV respectively the oil should be reconditioned by passing through filter machine. If acidity is increasing rapidly or exceeds above 0.5mg, KOH/gm of oil, the oil should be treated for improvement to 0.5 of less.

If the Tan delta of oil is increasing rapidly and exceeds above 0.01 to 0.2, oil should be treated for improvement to 0.01 or less.

CONCLUSIONS; All tests have to be carried out as per IS:1866

systematically and periodically, as a part of maintenance schedule.

The periodic testing will help O&M personnel to build up databank and case histories of Power Transformers.

Periodic testing will help to know the oil condition as well to know growing of any incipient faults by Dissolved Gas Analysis.