Transformer Oil Testing; IS:335 for Transformer oil; Transformer BDV values

Transformer are integral part of Electrical Systems. In same way Transformer oil is Integral part of Transformers. Although Dry type transformers are available in market , but still oil type transformers are most widely used and  preferred worldwide.

FUNCTIONS OF TRANSFORMER OIL
1.  Transformer oil provide dielectric strength to transformer insulation system.
2. It also provide efficient cooling to Transformer as heat generated in winding get dissipated through transformer oil.

PRODUCTION OF TRANSFORMER OIL                            

¬  Transformer oil is hydrocarbon product mainly  oil contains

(i)                  Naphthenic

(ii)                Paraffinic and

(iii)               Aromatics.

Following procedure is adopted for obtaining the TOBS (Transformer Oil Base stock) from crude oil which is buried from earth crust :-

(i)      Distillation
(ii) Acid treatment
(iii) Neutralization
(iv) Water wash
(v) Hot air blowing.

(vi) Clay treatment process on Transformer Oil Base Stock (TOBS) will give us the finished transformer oil.

(iii)    Finally after hot filtration operation transformer oil is ready for filling the transformers.

DETERIORATION ASPECTS OF TRANSFORMER OIL
There are so many reasons for deterioration of Transformer oil few of them are as listed below:-

(i)      Deterioration due to water .

(ii)    Deterioration due to the chemical decomposition.

(iii)   Deterioration due to the oxidation.

(iv)  Deterioration due to the contamination by gases.

(v)    Deterioration due to  the electrical stresses.

(vi)  Deterioration due to the thermal stresses.

(vii) Deterioration due to the effect of oxidation products.

(viii)Deterioration due to the physical contamination.

PRECAUTIONS INVOLVED IN  TRANSFORMER OIL SAMPLING.

¬  The utmost care should be taken to avoid contamination of samples with external impurities such as dust and moisture.

¬  Attention is drawn to the danger of sampling in rainy or foggy weather.

¬  The hands of the sampler should not come into contact with the samples.

¬  Care should be taken when sampling oil colder than the surrounding air, to avoid contamination by condensation.

TRANSFORMER OIL

Specifications for Testing of transformer oil.

¬  IS-335

¬  ASTM-D3487

¬  IEC-60296

¬  BS-148

¬  JIS-2320

IS-335 SPECIFICATION

APPEARANCE: 
Clear transparent and free from suspended matter and or sediments.

 DENSITY:
IS-1448 P(16)

HYDROMETER/THERMOMETER  

DESIGN VALUE—0.89 gms/ml @29.5°C max

 KINEMATIC VISCOCITY:
IS-1448 P(25)

CONSTANT TEMPERATURE BATH THERMOMETERS/STOP WATCH

DESIGN VALUE—27cSt @ 27 °C max 

¬  INTERFACIAL TENSION:IS-6104

INTERFACIAL TENSIO METER

DESIGN VALUE—0.04N/M max

¬  FLASH POINT:IS-1448 P(21)  

PENSKY MARTINS CLOSED CUP FLASH POINT TESTER/THERMOMETERS.

DESIGN VALUE—140 °C min

¬  POUR POINT:IS-1448 P(10)

CLOUD POINT&POUR POINT APPARATUS/THERMOMETERS        

DESIGN VALUE--   -6 °C max

¬  NEUTRALIZATION VALUE:IS-1448 P(2)

CONICALFLASKS/BURETTES/MEASURING CYLINDERS/BURETTE STANDS/ VOLUMETRIC FLASKS                        DESIGN VALUES—

A. TOTAL ACIDITY : 0.03mgKOH/gm           B.  INORGANIC ACIDITY: NIL

¬  ELECTRIC STRENGTH(BDV): IS-6792

 BDV TEST SET

 DESIGN VALUES

  A. UN FILTERD :30kv min                                  B. FILTERD : 60kv min

¬  DIELECTRIC DISSIPATION FACTOR (TAN DELTA):IS-6262 

TANDELTA TEST SET/OIL CELL HEATER/OIL CELL.

DESIGN VALUE : 0.002 max

¬  RESISTIVITY : IS-6013

RESISTIVITY TEST SET/OIL CELL HEATER /OIL CELL

DESIGN VALUES

A. 1500*10E12 Ohm-cm @27 °C min         

              B.  35*10E12 Ohm-cm @90 °C min

¬  PRESENCE OF ANTI OXIDATIVE INHIBITOR :IS13631                     

              MEASURING CYLINDERS/PIPPETES/ BURETTES/BURETTE STANDS

              DESIGN VALUE : NOT DETCTABLE

¬  WATER CONTENT : IS-13567                

              AUTO MATIC KARLE FISHER TITRATOR/WEIGHING BALANCE

              DESIGN VALUE : 50PPM max

¬  CORROSIVE SULPHUR : IS-335 ANNEX-B

 TEMPERATURE CONTROLLED OVEN/CONOCAL FLASKS/FORECEPS

 DESIGN VALUE : NON CORROSIVE

¬  OXIDATION STABILITY : IS-335 ANNEX-C 

OXIDATION STABILITY BLOCK/VACUUM PUMP/OXIDATION TUBES/G-4 FILTERS / BURETTES / BURETTE STANDS / CONICAL FLASKS/WEIGHING BALANCE.  

DESIGN VALUES

 A.NEUTRALIZATION VALUE:0.4mgKOH/gm

               B. SLUDGE % BY MASS :0.1max

¬  AGEING CHARACTERISTICS: IS-12177   

             AGEING OVEN /WEIGHING BALANCE/ VACCUME PUMP/ G-4 FILTERS/ BURETTES/BURETTE  STANDS /  CONICAL FLASKS.                              

    DESIGN VALUES           

A. RESISTIVITY@27DegC-2.5Ohm-cm min 

 B. RESISTIVITY@90DegC-0.2Ohm-cm min

C. TANDELTA@90DegC-0.2 max                    

D.TOTAL ACIDITY -0.05mgKOH/gm            

E. TOTAL SLUDGE –0.05%

NABL RECUREMENTS IN OIL LAB

¬  Equipment history cards

¬  Measurement uncertainty calculations.

¬  Regular calibrations of instruments from NABL accredited laboratories.

¬  Interlab comparison

Replicate testing

APPEARANCE TEST

¬  APPEARANCE

¬  Measurement uncertainty calculation for transformer oil appearance test

¬  Sources of uncertainty

a) Improper sampling

         b) Presence of sediments and suspended matter in the bottles used for sampling

         c)  Persons skills

       a) Improper sampling :

 To avoid the uncertainty due to improper sampling the following care should be taken

   1) Don’t use the cotton waste while sampling

   2) Don’t pickup the sampling in dust and foggy weather

   3) Clean the sampling point (pipe end) before sampling

b) Presence of sediments and suspended matter in the bottles used for sampling :

The test results may be effect due to the contamination of sediments and suspended matter, if any present in the sampling bottles. Bottles should be thoroughly clean before sampling by using the following cleaning agents as per the sequence

1) Tap water

2) Soap solution 

3) Tap water

 4) Hot distilled water

 5) Acetone and allow for oven drying at 110°C for 1hr. And then vacuum with the vacuum pump.    

Breakdown VOLTAGE TEST

Measurement uncertainty calculation for BDV test on transformer oil 

Sources of uncertainty

 1) Test results obtained              

2) BDV test set resolution                                                           

3) BDV test set calibration                                                           

4) Atmospheric condition

Results obtained .

Repeated measurements on sample  transformer oil sample                                                                                  a)  ( 68,74,72,60,68,64)   = 67.6kv

             b) (66,72,80,64,68,62)    = 68.6kv

             c) (68,70,70,68,80,76)    = 72.0kv

             d) (72,62,76,68,82,68)   = 71.3kv                                

              e) (64,66,78,72,68,70)   = 69.6kv

          Average value  = 69.82kv

St. Deviation= Ö(-2.22)²+(-1.22)²+(2.18)²+(1.48)²+(-0.22)²/4

                       = Ö(4.9284+1.4884+4.7524+2.1904+0.0484)/4

                      = 1.8308

Type A uncertainty = 0.818kv

2) BDV test set resolution :

      (B₁)uncertainty due to BDV test set resolution= 2 / 2 Ö3 = 0.577kv

3) BDV test set calibration :                                                                         

(B₂)uncertainty due to BDV test set calibration =1.9 / 2  =.95kv                                                                    

   Type B uncertainty = Ö(B₁)²+(B₂)²                                                                                               

                                               = Ö(0.572)²+(0.95)² = 1.1115kv                                                 

    Combined uncertainty = Ö(Type A)²+(Type B)²  

     Combined uncertainty = Ö(Type A)²+(Type B)²  

                                              = Ö (0.818)²+ 1.1115)²= 1.38kv

Note :4

Room temperature and relative Humidity maintained at 27°C and 44% RH respectively so uncertainty due to Atmospheric Conditions is not Considered

¬  Expended uncertainty = 2 x 1.38 = 2.76kv

¬  At 95% confidence level with respeet to 69.82kv

69.82 ±  2.76