Why ZDDP and Sulfur Additives Don’t Belong in Transformers

Because AW additives—especially ZDDP, sulfurized compounds, and phosphate esters—chemically attack cellulose paper, accelerate aging, form corrosive by-products with moisture and copper, and destroy the electrical properties of transformer oil.

In simple terms:
AW additives are great for hydraulic pumps, but toxic for transformer insulation paper.

Below we break down the “why”, the mechanisms, the failure modes, and the engineering implications.



TYA Series Vacuum Hydraulic Oil Filtration System

What Are AW Additives, and Why Are They Good in Hydraulic Oil?

Hydraulic oils often use anti-wear additives such as:

1. ZDDP (zinc dialkyldithiophosphate)


2. Phosphate esters


3. Sulfur/phosphorus EP agents


4. Sulfurized compounds

These additives are designed to:

1. Form a protective film on metal surfaces


2. Reduce wear in high-pressure pumps


3. Improve load-carrying capability


4. Enhance oxidation resistance

In hydraulic systems, these additives are beneficial.
In transformers, they are destructive.

Why Do AW Additives Attack Transformer Paper Insulation?

Transformer solid insulation is primarily cellulose paper.
Cellulose is extremely sensitive to:

1. Acids


2. Sulfur compounds


3. Phosphorus compounds


4. Moisture


5. High temperatures


6. Metal ions (especially copper)


7. AW additives create all of these stressors.

Phosphorus and sulfur react chemically with cellulose

ZDDP and sulfur-phosphorus AW additives decompose under heat and electric fields, forming:

1. acids


2. sulfur species


3. phosphates


4. corrosive compounds

These substances catalyze cellulose depolymerization, reducing:

1. DP (degree of polymerization)


2. Mechanical strength


3. Long-term dielectric performance


4. Even small ppm levels can dramatically shorten insulation life.

AW additives + moisture = highly corrosive by-products

Transformer oil always contains some moisture (10–40 ppm).
When combined with AW additives, moisture accelerates the formation of:

1. acids


2. metal corrosion complexes


3. sludge


4. copper sulfide (Cu₂S)

This causes:

1. paper embrittlement


2. loss of tensile strength


3. accelerated aging


4. serious copper corrosion

Copper sulfide is particularly dangerous because it can migrate into the insulation and cause catastrophic dielectric failure.

AW additives destroy the electrical properties of transformer oil

Transformers require oil with:

1. extremely high dielectric strength


2. extremely low polarity


3. minimal contaminants

However, AW additives are polar molecules. They:

1. lower breakdown voltage


2. increase dielectric dissipation factor (tan δ)


3. increase conductivity


4. attract moisture


5. reduce impulse withstand capability

This fundamentally compromises the transformer’s insulation system.

What Damage Occurs Inside a Transformer?

Field experience shows consistent failure modes when AW-containing oils enter a transformer:

✓ Fast degradation of paper insulation

1. Rapid DP decline


2. Darkening of paper


3. Higher moisture content


4. Weaker mechanical strength

This translates directly into loss of transformer lifetime.

✓ Copper corrosion and sludge formation

AW additive by-products often lead to:

1. Cu₂S deposits (black copper sulfide)


2. sticky sludge


3. clogged oil ducts

Consequences:

1. Poor cooling


2. Higher hot-spot temperature


3. Accelerated insulation aging

✓ Reduced dielectric strength

AW additives dramatically lower:

1. oil breakdown voltage


2. PD (partial discharge) inception voltage

This increases the risk of insulation failure.

Why Can't Hydraulic Oil Replace Transformer Oil?

Even though both appear to be “oil”, their requirements differ entirely:

Property	Transformer Oil	Hydraulic Oil (AW)
Primary function	Insulation + cooling	Lubrication + wear protection
Chemical additives	Very low, must be non-reactive	ZDDP, sulfur, phosphorus, EP additives
Polarity	Extremely low	Moderate (due to additives)
Electrical performance	Critical	Not relevant
Compatibility with paper	High	Poor / destructive

Using hydraulic oil in a transformer is a major misuse that leads to premature failure.

Real-World Engineering Outcomes

Documented field cases show that AW additives can lead to:

1. Rapid insulation aging


2. Copper sulfide deposition


3. Sludge blocking cooling channels


4. Decreased breakdown voltage


5. Internal flashovers


6. Transformer de-rating or total loss

In severe cases, the transformer becomes irrecoverable, requiring full oil replacement and sometimes complete retirement.

Best Practices to Prevent AW Additive Damage

✓ Only use transformer oils meeting IEC 60296 or ASTM D3487.

No exceptions.

✓ Never use hydraulic, engine, gear, or turbine oils in transformers.

✓ Test every incoming oil batch for additives.

including:

1. FTIR additive scan


2. corrosive sulfur test


3. sulfur/phosphorus content


4. breakdown voltage

✓ Use dedicated transformer oil purification equipment

Avoid cross-contamination from hydraulic or lubricating oils.

Conclusion

AW additives break down into acidic, sulfur-based, and phosphorus-based compounds that aggressively degrade cellulose and destroy the dielectric properties of transformer insulation systems.