Transformer Life
   
 
What keeps your Transformer going?
Paper Mechanical strength
Dielectric strength
Dielectric spacing
   
Oil
Dielectric strength
Cooling
Protects the paper
   
But
Oil oxidies and creates oxidation by-products which in turn destroy the paper. To maximise the life of the paper insulation, oxidation of the oil must be controlled.
   

Acidity trigger points
(NN=Neutralisation number)

   • SD MYERS NN=0.05 TO 0.1
   • TRANSGRID (NSW AUST) NN=0.1 TO 0.2
   • OVERSEAS PRACTICE IS TOWARDS NN=0.1

  
   
FACT: Oil at acidity (NN)=0.2 will not hold oxidation by-products (sluge) in suspension.
   
HENCE: Must keep NN <0.2 to avoid depositing
   
SLUDGE
   

FACT: Damage to paper insulation cannot be repaired

Sludge formation
   • deposit initially in cooler areas e.g. radiators and conservators
   • initial deposits are invisible
   • build-up, unless oil condition is corrected, is exponential

Degradation of transformer insulation is combination of three main factors:
   
Moisture
   • oil leaks (very important!)
   • faulty breathing
   • low loading

"MOISTURE KILLS TRANSFORMERS"
   
Temperature
   • Max. top oil=60 deg C
   • Every 4-6 deg C above 60 deg C halves the remaining paper life
   
Oil oxidation - is a chemical process which must and can (readily) be kept in check

 
   

Water in Transformer
Introduction: Those responsible for transformer maintenance generally recognize that water in transformers is a problem. However, the relationship of water to oil and cellulose is less understood, as is the impact of water on cellulose, over an extended period of time.

   • MOISTURE 4% DRY WEIGHT PAPER= ENTERING RISK ZONE
   • 5-6% DRY WEIGHT PAPER = CONSIDERABLE RISK
   • 7% DRY WEIGHT PAPER= FAILURE IMMINENT

   
MOISTURE KILLS TRANSFORMERS

Oil Leaks: It is very important to recognise the significance of an oil leak. Oil loss is consequential but on the cooling down cycle moisture drawn in.
   
Faulty breathing: A ready source of moisture
   
Low Loading: When a transformer is left "sitting around" or is very lightly loaded it will become wet
   
 
   
Acidity
   • 0.1=65% Residual paper tensile strength
   • 0.25=50% Residual paper tensile strength
   
End of life definition
A loss of the initial tensile strength of the cellulosic insulation is now the accepted "end of life" criteria.
   
Loss of life definition
Loss of mechanical or electrical strength when paper is neglected can never be regained.
   
Thermal life of paper
The thermal life of class 105 insulation (eg 65C rise design) is halved for each increase of 8 deg C or conversely, doubled for each decrease of 8 deg C.