Transformer oil maintenance purification and regeneration

TRANSFORMER OIL MAINTENANCE
(OIL PURIFICATION /
RECONDITIONING - REGENERATION /
RECLAMATION)
By M.RAVEENDRA NATH, Senior Electrical Engineer

https://www.linkedin.com/in/raveendranath-macherla-9b303858/
WHAT IS THE IMPORTANCE OF OIL IN
TRANSFORMER?
INSULATION OIL SERVES AS INSULATION MEDIUM (DIELECTRIC), COOLANT AND
ARC-QUENCHER.
THE OIL INCREASES THE BREAKDOWN VOLTAGE OF THE INSULATION.
THE OIL DISSIPATE HEAT BY TRANSFERRING IT TO THE COOLING SYSTEM.
OXIDATION STABILITY OF THE OIL ALLOWS IT TO ENDURE HIGH TEMPERATURES
AND PREVENT SIGNIFICANT DAMAGE TO THE INSULATION SYSTEM.

IMPORTANT
PROPERTIES
FOR
INSULATION OIL
HIGH DIELECTRIC STRNGTH, TO WITHSTAND THE
ELECTRIC STRESS IMPOSED IN SERVICE
SUFFICIENT LOW VISCOSITY, SO THAT ITS ABILITY TO
CIRCULATE AND TRANSFER HEAT
RESISTANCE TO OXIDATION TO MAXIMIZE SERVICE
LIFE
ADEQUATE LOW-TEMPERATURE PROPERTIES DOWN
TO THE LOWEST TEMPERATURE EXPECTED AT THE
INSTALLATION SITE

ROUTINE TESTS
ON
TRANSFORMER
OIL (IN SERVICE)
MINIMUM ROUTINE TESTS REQUIRED TO MONITOR THE OIL AND
TO ENSURE THAT IT IS SUITABLE FOR CONTINUED SERVICE.
1) COLOUR AND APPEARANCE (ISO 2049)
2) BREAK DOWN VOLTAGE (ISO IEC 60156)
3) WATER CONTENT (IEC 60814)
4) ACIDITY (NEUTRALIZATION VALUE) (IEC 62021)
5) DIELECTRIC DISSIPATION FACTOR AND RESISTIVITY (IEC 60247)
6) INHIBITOR CONTENT (IEC 60666)

ADDITIONAL
ROUTINE TESTS
ON
TRANSFORMER
OIL (IN SERVICE)
ADDITIONAL TESTS, WHICH MAY BE USED TO OBTAIN
FURTHER SPECIFIC INFORMATION ABOUT THE QUALITY OF
THE OIL, AND MAY BE USED TO ASSIST IN THE EVALUATION
OF THE OIL FOR CONTINUED USE IN SERVICE.
1) SEDIMENT AND SLUDGE (IEC 60422)
2) INTERFACIAL TENSION (ASTM D971)
3) PARTICLES (IEC 60970)

SPECIAL TESTS
ON
TRANSFORMER
OIL (IN SERVICE)
SPECIAL TESTS, USED MAINLY TO DETERMINE THE SUITABILITY OF THE OIL FOR THE TYPE OF EQUIPMENT
IN USE AND TO ENSURE COMPLIANCE WITH ENVIRONMENTAL AND OPERATIONAL CONSIDERATIONS
1) OXIDATION STABILITY (IEC 61125)
2) FLASH POINT (ISO 2719)
3) COMPATIBILITY (IEC 61125)
4) POUR POINT (ISO 3016)
5) DENSITY (ISO 3675)
6) VISCOSITY (ISO 3104)
7) POLYCHLORINATED BIPHENYLS (PCB) (IEC 61619)
8) CORROSIVE SULPHUR (IEC 62535)
9) DIBENZYL DISULFIDE CONTENT (DBDS) (IEC 62697-1)
10) PASSIVATOR CONTENT (IEC 60666)

IEC 60296:
RECOMMENDED
LIMITS

IEC 60422:
RECOMMENDED
LIMITS

WHY TRANSFORMER OIL PROCESSING (FILTERTAION
/ PURIFICATION / REGENERATION) REQUIRED?
• CELLULOSE MATERIAL IS THE WEAKEST LINK IN THE INSULATION
SYSTEM. SINCE THE TRANSFORMER LIFE IS IN ESSENCE THE LIFETIME OF
ITS CELLULOSE INSULATION, AND SINCE CELLULOSE DEGRADATION IS
IRREVERSIBLE, CONTAMINANTS MUST BE REMOVED IMMEDIATELY,
BEFORE THEY DAMAGE THE CELLULOSE.
• PURIFICATION OF TRANSFORMER OIL, INCLUDING REGENERATION, IS A
METHOD OF EXTENDING TRANSFORMER LIFETIME.
• THE OBJECTIVE OF THIS PROCESS IS TO REMOVE AGING PRODUCT
FROM SOLID INSULATION AND OIL BEFORE THEY DAMAGE THE
INSULATION SYSTEM.

WHAT IS TRANSFORMER OIL PURIFICATION
/RECONDITIONING?
• TRANSFORMER OIL PURIFICATION IS A PROCESS TO REMOVE SLUDGE,
DISSOLVED MOISTURE AND DISSOLVED GASES FROM THE OIL.
• RECONDITIONING IS A PROCESS THAT ELIMINATES OR REDUCES PHYSICAL
CONTAMINATION BY MEANS OF PHYSICAL PROCESSES (FLITRATION,
DRYING, DEGASSING ETC.)
• RECONDITIONING REDUCES THE PARTICLE AND WATER CONTENT OF THE
OIL. THE PROCESS MAY ALSO REMOVE SOME DISSOLVED GASES AND OTHER
COMPONENTS SUCH AS FURANIC COMPOUNDS.
• THE PHYSICAL MEANS THAT ARE USED FOR REMOVING WATER AND SOLIDS
FROM OIL INCLUDE SEVERAL TYPES OF FILTRATION, CENTRIFUGING AND
VACUUM DEGYDRATION TECHNIQUES.

NUMBER OF CYCLES FOR RECONDITIONING
OF OIL
• IT IS GENERALLY NECESSARY TO PASS THE TOTAL VOLUME OF OIL
THORUGH THE PURIFIER NOT LESS THAN THREE (3) TIMES.
• THE FINAL NUMBER OF CYCLES WILL DEPEND ON THE DEGREE OF
CONTAMINATION, AND IT IS ESSENTIAL THAT THE PROCESS BE CONTINUED
UNTIL A SAMPLE TAKEN FROM THE BOTTOM OF THE ELECTRICAL
EQUIPMENT AFTER THE OIL HAS BEEN ALLOWED TO SETTLE FOR A FEW
HOURS, PASSES THE BDV TEST.

WHAT SHALL BE THE OIL TEMPERATURE DURING OIL
RECONDITIONING?
• IF VACUUM TREATMENT IS NOT EMPLOYED, IT IS ADVISABLE TO LIMIT TEMPERATURE TO
30°C. IF VACUUM TRETAMENT IS EMPLOYED, A HIGHER TEMPERATURE MAY BE
ADVANTAGEOUS. HOWEVER INITIAL BOILING POINT OF THE OIL BEING TREATED SHOULD
NOT BE EXCEEDED, TO AVOID UNDUE LOSSES OF LIGHTER FRACTIONS.
• IF THIS INFORMATION IS NOT AVAILABLE, IT IS RECOMMENDED THAT THE OIL SHOULD
NOT BE VACUUM TREATED AT TEMPERATURES OVER 85°C.
• GENERALLY, IT IS RECOMMENDED TO RAISE OIL TEMPERATURE 65°C TO GIVE LATENT HEAT
WHICH SEPARATES MOISTURE AND GASES. HEATING THE OIL WILL MAKE IT EASIER TO
FILTER BEACAUSE OF THE DECREASE IN OIL VISCOSITY.
• IT IS IMPORTANT TO USE AUTOMATIC OIL HEATING TO KEEP THE OIL TEMPERATURE
WITHIN LIMITS SO AS TO AVOID THERMAL OXIDATION AND DESTRUCTION OF OIL.

WHAT IS TRANSFORMER OIL RECLAMATION / REGENERATION?
• THIS IS A PROCESS THAT ELIMNATES OR REDCUES SOLUBLE AND INSOLUBLE POLAR
CONTAMINANTS FROM THE OIL BY CHEMICAL AND PHYSICAL PRCOESSING.
• RECLAMATION PROCESS DOES NOT AIM TO DRY TRANSFORMER INSULATION. IT IS
IMPOSSIBLE TO DRY THE INSULATION WITHIN THE TIME OF REGENERATION. HIGH
LEVEL OF DEHYDRATION REQUIRES SIGNIFICANT TIME.
• RECLAMATION OF OILS OF MODERATE TO HIGH ACIDITY WILL USUALLY RESULT IN
OILS WITH LOWER OXIDATION RESISTANCE THAN THE ORIGINAL NEW OIL.
• BEFORE PERFORMING A RECLAMATION PROCESS, A LABORATORY FEASIBILITY TEST
IS RECOMMENDED.

ON-LINE OR OFF-LINE REGENERATION
• REGENERATION OF OIL INSIDE OR OUTSIDE OF THE TRANSFORMER, WHILE
IT THE TRANSFORMER IS ON OR OFF, DEPENDS NOT ONLY ON TECHNICAL
PARAMETERS, SUCH AS GAS CONTENT ETC, BUT ALSO ON ECONOMIC
CONSIDERATIONS.

OPERATION PRINCIPAL OF OIL REGENERATION
OIL REGENERATION IS A PHYSICAL-CHEMICAL TREATMENT BASED ON
ADSORPTION PROCESS, i.e. OIL CIRCULATION THROUGH THE COLUMNS WITH
GRANULATED ADSORBENTS. AGING PRODUCTS ARE ADSORBED AND
RETAINED ON THE ACTIVE SURFACE INSIDE ADSORBNET PORES. THIS
TECHNOLOGICAL PROCESS IS BASED ON USE OF SYNTHETIC ADSORBENTS
WHICH ARE MORE ACTIVE IN COMPARISON TO NATURAL ADSORBENTS SUCH
AS BLEACHING CLAY, INFUSORIAL EARTH, FULLER’S EARTH.
THE MAIN ADVANTAGE OF SYNTHETIC ADSORBENTS IS POSSIBILITY OF THEIR
REACTIVATION. AFTER SATURATION, ADSORBENTS CAN BE REACTIVATED A
VIRTUALLY UNLIMITED NUMBER OF TIMES WITHOUT LOSING THEIR
ADSORPTION CAPACITY.

MATERIAL USED FOR OIL RECLAMATION
• FULLER’S EARTH IS AN ACTIVE MATERIAL CONTAINING BOTH INTERNAL AND EXTERNAL POLAR ACTIVE
SITES, WHICH ALLOW THE NON-POLAR COMPONENTS OF THE OIL TO PASS THROUGH WITHOUT RETENTION
BUT WHICH RETAINS THE POLAR CONTAMINANTS OR DEGRADATION COMPOUNDS DISSOLVED IN THE OIL.
• SEVERAL DIFFERENT CLAYS ARE AVAILABLE THAT HAVE PROVEN SUITABLE FOR THESE PURPOSES. THE
MOST WIDELY USED ARE OF THE SEPIOLITE, BENTONITE, ATTAPULGITE OR MONTMORILLONITE TYPE OF
WHICH FULLER’S EARTH IS THE MOST COMMONLY USED. THEY ARE CONSTITUTED OF SILICATE ANIONS
[SI2O5]n CONDENSED WITH OCTAHEDRAL LAYERS OF THE TYPE X(OH)2 WHERE X MAY BE MAGNESIUM,
ALUMINIUM, ETC.
• NORMALLY, FULLER’S EARTH IS TREATED TO INCREASE ITS SPECIFIC SURFACE AREA AND THE
CONCENTRATION AND POLARITY OF ITS LEWIS ACIDS. FULLER’S EARTH CAN BE USED ALONE OR IN
COMBINATION WITH OTHER CHEMICALS LIKE TRISODIUM PHOSPHATE, ACTIVATED CHARCOAL AND SODIUM
SILICATE.
• THE RETENTION OF CONTAMINANTS BY ADSORBENT ACTIVE SITES IS, GENERALLY, IMPROVED BY
TEMPERATURE, THUS THE PROCESS NORMALLY TAKES PLACE AT 60 °C TO 80 °C.

RENEWAL OF ADDITIVES AFTER OIL RECLAMATION
• ONCE OIL RECLAMATION IS PERFORMED FOR AGING OIL, IT IS INEVITABLE THAT THE
INHIBTORS IN THE OIL ARE AT LEAST PARTLY SPENT. IT IS THEREFORE RECOMMENDED
THAT THE ADDITIVES BE REPLACED IN THE RECLAIMED OIL AFTER THE RECLAIMING
PROCESS AND BEFORE THE EQUIPMENT IS RE-ENERGIZED.
• ADDITION OF TOO MUCH AROMATIC COMPOUNNDS (ANTI-OXIDANTS) REDUCE
DIELECTRIC STRENGTH AND INCREASE THE OIL’S ABILITY TO SOLVE MOST OF THE SOLID
INSULATION SUBMERGED IN OIL.
• THE MOST WIDELY USED ADDITIVES ARE 2,6-DI-TERT-BUTYL-PARACRESOL (DBPC) AND
2,6-DI-TERT-BUTYL-PHENOL (DBP).
• METAL PASSIVATORS WILL ALSO BE REDUCED OR REMOVED DUE TO THEIR POLAR
NATURE.

WHAT HAPPENS AFTER OIL REGENERATION?
• IF INSUALTION CONTAINS MOISTURE, THE LEVEL OF MOISTURE CONTENT ON THE
OIL IS EXPECTED TO INCREASE. MOISTURE WILL MOVE FROM INSULATION TO THE
OIL TO RESTORE THE BALANCE OF OPERATION TEMPERATURE BETWEEN
CELLULOSE AND OIL. DIELECTRIC STRENGTH WILL FALL SOMEWHAT WITH
INCREASE IN OIL MOISTURE CONTENT.
• IS A SIGNIFICANT INCREASE OF ACIDITY IS OBSERVED WITHIN A SHORT TIME
AFTER REGENERATION, THIS IS CAUSED BY SOLUTION OF CONTAMINANTS. THIS
MEANS THAT THE RESULT OF REGENERATION IS LARGELY DEPENDANT ON HOW
LONG THE PROCESS TAKES.

WHAT HAPPENS AFTER OIL REGENERATION?
ALL PHYSICAL, CHEMICAL AND ELECTRICAL OIL CHARACTERISTICS
WILL BE RESTORED TO THE LEVEL OF NEW-UNUSED OILS IN
ACCORDANCE WITH THE SPECIFICATIONS OF IEC 60296.
BELOW CHARACTERISTICS ARE GUARANTEED AFTER OIL
REGENERATION:
• COLOUR
• ACIDITY
• INTERFACIAL TENSION
• DIELECTRIC STRENGTH
• WATER CONTET
• DIELECTRIC DISSIPATION FACTOR
• PARTICLES

OIL REGENERATION / OIL REPLACEMENT VS
FURANIC COMPOUNDS
• OIL RESTORATION (PURIFICATION OR REGENERATION) OR REPLACEMENT OF
TRANSFORMER OIL DESTROYS FURAN COMPOUNDS, WHICH ARE USED TO MEASURE THE
DEGREE OF POLYMERIZATION (INSULATION CONDITION AND LIFETIME). FURAN ANALYSIS
SHALL BE DONE BEFORE THE PROCESS.
• AFTER PURIFICATION, A NEW BASE LINE FOR FURAN COMPOUNDS CONTRO IS
ESTABLISHED. FUTURE FURAN TEST MUST BE REFERENCED AGAINST THIS NEW BASE LINE.

RECOMMENDED
ACTIONS VS OIL
SYMPTOMS

SOURCES:
• IEC 60422 - 2013
• On site regeneration of transformer oil – Globe Core

Transformer oil maintenance purification and regeneration

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