What keeps your transformer going?

• Paper Mechanical strength
• Dielectric strength
• Dielectric spacing

Oil

• Dielectric strength
• Cooling
• Protects the paper

But Oil oxidizes and creates oxidation by-products which in turn destroy the paper. To maximize 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 (sludge) in suspension.
• Hence Must keep NN <0.2 to avoid DepositingSludge

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 recognize 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.