Presentation Transformer Oil

TESTING AND ANALYSIS OF TRANSFORMER OIL

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

TR ANS FOR MER FL UI D IS PO WE RFUL D IAGNO STIC TOOL 

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Ch ar acte riz ati on : W hich giv es par ameters th at ca n be u sed to i dentif y th e oi l . Ag ei ng stat us : Which gi ves p ar ameters relev an t to th e agein g pr oce ss. Dielectri c Stat us : Which giv es p ar ameters use d to dete rm in e th e diel ectri c safet y mar gin and dielectr ic char acte ris tics of th e insul ati on spaces . Degr ad ati on status : which g ives par amete rs relevant faults f ai lu re and we ar.

CH EMI STR Y A ND CO MPO SI TIO N O F 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 : 

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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: 

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It is that component which give the oil its oxidation and ageing stability. It is considered maximum upto 8%.

CH AR AC TER STICS OF OIL: S Ch ara cterist ics r.N o 1. Appearance

Req uir emen t as per ISS- 335 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 0.04 N/M (Min.)

5.

Flash Point (Min.)

140 °C

6.

Pour Point (Max.)

-6 °C

7.

Neutralization value Total 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.)

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

0.002 (Max.)

10.

11.

Water content (Max.)

50ppm

12.

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

35X1012 ohm-cm 1500X1012 ohm-cm

13.

Oxidation stability (after oxidation for 164 hours @ 100 °C Neutralization value after oxidation (Max.) 0.40 mg KOH/g Total sludge after oxidation (Max.) 0.10% 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) ii) At 27 °C (min.) iii) At 90 °C (min.) b. Dielectric dissipation factor (Tan delta) at 90 °C ii) Total acidity, mg. KOH/g iii) Total sludge value percent by weight.

15.

Presence of oxidation inhibitor

25X1012 ohm-cm 0.2X1012 ohm-cm 0.20 Max. 0.05 Max 0.05 Max Absent

PERMISSIBLE V ALU ES O F O IL CHA RAC TER IS TICS F OR IN SER VIC E TRAN SF ORMER OIL A S P ER IS S 1866:20 00 Sr. No.

1.

Name of Ch ara cteri stics

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

Equ ipmen t vo ltag e

Pe rm is sib le limit for in serv ice T/ F Oi l ISS:1866-2000

A)>170KV B)>72.5KV170KV C) 72.5KV and below

50 40 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)>170KV B)>72.5KV-170KV C) 72.5KV and below

20 40 No free moisture at Room Temperature

4.

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

A)>170KV B&C) Upto 170KV

0.2 0.1

6.

IFT at 27°C (m N/m) Min

All voltages

15

7.

Flash Point °C

-

15 ° C (Max.) decrease from initial value

COND ITION M ONITORING OF TRANSF ORM ER OIL ELECT RI CAL PARAME TR S DIELECTR IC ST RE NGTH  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

SP ECI FIC RE SI STAN CE (res istiv ity) 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.).

DI ELECTR IC DI SS IPATIO N FACTOR 

Popularly known as Tan delta or loss factor

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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.

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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 VAL UE  SK value prescribes the behavior of insulating oils in the presence of concentrated Sulfuric acid 

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It furnishes the information on the refining degree of mineral oil. SK value % by weight – 4 to 8% (max). Under consideration as per IS

: CH EMIC AL PARA MET RS 

WATE R CON TE NT

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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.

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AC ID ITY/ N EUTR AL ISATION N UMB ER : 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

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CO RROSI ON TE ST : 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.

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OXIDATIO N ST AB ILI TY 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. PR ES EN CE OF O XI DATI ON INHIB ITER : The Indian Standards IS:335 (1993) is applicable to only unhibited oils

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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)

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VISCOSI TY : 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.

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POUR POI NT : 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.

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FL AS H 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

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INTE RFA CI AL TE NSI ON : 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)

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SLU DGE: 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.

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TEST S TO BE PERFO RM ED ON IN SERV ICE OIL S: 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.

DIAG NOSTIC TEST S:

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

DIS SO LVED GAS AN AL YS IS :

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The causes for the generation of gases are classified as : Thermal degradation/Insulation Decomposition Arcing Partial discharge

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Oxidation Vaporization

Major and Minor gase s p rod uced are: Ma jor Gas

Min or 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

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Cellulous insulation Decomposition

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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

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

Gas

Dis solv ed Gas Con ce ntration

H2

<20n+50

CH4

<20n+50

C2H6

<20n+50

C2H4

<20n+50

C2H2

<5n+10

CO

<25n+500

CO2

<100n+1500

TCG

<105n+710

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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;

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IEC 599 Method: Cod e of rang e o f Rati os C2H2 CH4 C2H4 C2H4 H2 C2H6 Rati os of Ch ara cteri stic Gas es

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<0.1

0

1

0

0.1-1

1

0

0

1-3

1

2

1

>3

2

2

2

Case No.

Diagnosis of

0

No fault

0

0 0

Normal ageing

1

Partial discharges 0

1 0

Discharge in gasfilled cavities

Typical examples

faults/Characteristic fault

of low energy density 2

Partial discharges 1 1 of high energy density

0

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

3

Discharge of low 1-2 0 1-2

Continuous sparking

energy

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 1 Temp.150°C

General insulated conductor overheating.

6

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

Local overheating of the core

7

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

-do-

8

Thermal fault of High temp. 700°C

-do-

0 2 2

Rogers Method

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CH4 H2

C2H6 CH4

C2H4 C2H6

C2H2 C2H4

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 CH 4 C2H6 C 2H 4, C 2H CO CO 100 120 65 50 35 350 2500 TCG=720(PPM) TDC G LIMITS AC TION ≤720 Satisfactory operation 721-1920 Normal ageing/ slight decomposition 1921-4630 Significant decomposition 

>4630

Substantial decomposition

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SAM PL ING: 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. CELULO SE DEG RAD AT ION : 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

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FU RAN IC COM PO UN DS: 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-furalhyde Amon g thes e 2-f ur fu ral is the mos t sign ifi ca nt

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Rel ati on shi p betw ee n DP( Deg re e of Pol ymeri zatio n) value and the amo un t of furan ic co nce ntration in th e oi l: 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

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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

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MAIN TEN AN CE PR OCEDU RE FO R THE INSU LA TING OIL IN TRANSFO RMER S : 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

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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.

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CO NCLUSIONS; Al l tes ts have to be carri ed out as per IS:1866 systema tica lly and peri od ical ly , as a p art of main tenance sched ul e. The peri od ic tes tin g wil l h elp O&M pers on nel to bui ld up data ban k and cas e hi stori es of Pow er Trans formers . Pe riod ic tes ti ng will help to know the o il con diti on as well to k now grow ing of an y inci pi ent faults b y Dis solv ed Gas An alys is.