If you’ve ever wondered why hydraulic oil with anti-wear (AW) additives is dangerous for transformers, here’s the short, direct answer:
AW additives—especially zinc-based ones—chemically attack cellulose paper and accelerate aging, moisture formation, and acidity inside the transformer.
This ultimately shortens insulation life, lowers dielectric strength, and dramatically increases the risk of failure.
In this article, I’ll break down how this happens, why transformer insulation reacts so bad... moreIf you’ve ever wondered why hydraulic oil with anti-wear (AW) additives is dangerous for transformers, here’s the short, direct answer:
AW additives—especially zinc-based ones—chemically attack cellulose paper and accelerate aging, moisture formation, and acidity inside the transformer.
This ultimately shortens insulation life, lowers dielectric strength, and dramatically increases the risk of failure.
In this article, I’ll break down how this happens, why transformer insulation reacts so badly to AW additives, and what alternatives industries should use instead.
AW Additives Are Designed for Hydraulic Systems — Not Electrical Insulation
Hydraulic oils often use Zinc Dialkyldithiophosphate (ZDDP) as their main anti-wear additive. ZDDP works perfectly in mechanical systems by forming a protective anti-wear layer on metal surfaces.
But transformers are not mechanical systems.
They are electrical + chemical systems, and ZDDP behaves very differently inside them:
It decomposes under heat
It produces corrosive by-products
It reacts with moisture and oxygen
It forms sludge and acidic compounds
Transformers are extremely sensitive to contamination. Even small amounts of AW additives disrupt the delicate chemical equilibrium of transformer oil and paper insulation.
How AW Additives Damage Transformer Paper Insulation
Transformer insulation is primarily cellulose-based kraft paper. Cellulose breaks down when exposed to acids, moisture, and oxidation catalysts.
Unfortunately, AW additives contribute to all three.
AW Additives Accelerate Cellulose Aging
When ZDDP decomposes inside hot transformer oil, it creates:
Phosphoric acids
Sulfur-containing compounds
Oxidation accelerators
These chemicals directly attack cellulose fibers, leading to:
Polymer chain scission
Shrinkage
Loss of tensile strength
Reduced dielectric strength
Effectively, the insulation ages several times faster than it would under normal conditions.
AW Additives Increase Acidity (TAN) in the Oil
Transformers require ultra-low acidity oil because acids dramatically speed up paper degradation.
AW additives are unstable at transformer temperatures (60–110°C).
As they break down, they increase the oil’s acidity (TAN).
Higher acidity = Faster insulation damage
Acid destroys cellulose and leads to:
Loss of mechanical strength
Higher moisture content
Increased risk of partial discharge
Accelerated sludge formation
For transformer operators, this means shortened transformer lifespan.
AW Additives Produce Sludge That Blocks Oil Flow
Another major issue is sludge.
As AW additives oxidize, they form sticky degradation products that:
Accumulate inside windings
Block cooling ducts
Reduce heat dissipation
Promote hot-spot overheating
Overheated cellulose deteriorates exponentially faster.
A transformer that could run 30–40 years might see its insulation fail in one-third of its normal life.
AW Additives Contain Metals That Increase Conductivity
ZDDP contains zinc and phosphorus, both of which:
Raise the oil’s electrical conductivity
Increase the risk of electrical discharge
Reduce dielectric breakdown voltage
Transformer oil must remain extremely pure and non-conductive.
Even tiny contamination from AW additives can lead to:
Bubble formation
Partial discharge
Insulation breakdown
Catastrophic failure under load
Moisture Formation: The Silent Killer
The decomposition of AW additives also produces water (via oxidation reactions).
Moisture is the number one enemy of transformer insulation:
It reduces dielectric strength drastically
It triggers accelerated hydrolysis of cellulose
It causes insulation brittleness
It leads to internal arcing under high voltage
Once moisture rises above 2%, transformer paper begins irreversible degradation.
Why Transformers Only Use Inhibited or Uninhibited Mineral Oil
Because of the extreme sensitivity of cellulose insulation, transformer oils are formulated to:
Avoid metals like zinc
Avoid sulfur-based antiwear additives
Maintain ultra-low acidity
Resist oxidation
Protect cellulose long-term
Remain chemically stable for decades
That’s why standards like IEC 60296 and ASTM D3487 explicitly prohibit AW-additive-type oils in transformers.
Hydraulic oil = additives for metal surface protection
Transformer oil = additives for cellulose and dielectric protection
The chemistry is completely different.
What Happens If AW Hydraulic Oil Is Accidentally Filled Into a Transformer?
This is a real-world problem, often caused by maintenance mistakes.
Consequences include:
Rapid increase in TAN (acidity)
Depressed BDV (breakdown voltage)
Increased moisture content
Sludge deposition in windings
Loss of paper mechanical strength
Risk of catastrophic dielectric failure
Utilities usually must:
Drain oil fully
Perform multi-stage flushing
Refill with fresh transformer-grade oil
Test paper DP (degree of polymerization)
In severe cases, insulation damage is irreversible.
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