The three-phase oil-immersed transformer is the most common power distribution transformer in utility and industrial networks. Its fully oil-filled, sealed corrugated tank absorbs the thermal expansion of the insulating oil with the elasticity of the tank walls, which keeps the oil permanently sealed and free from atmospheric moisture. Understanding how the core, windings, tank and protective devices work together helps buyers specify the right unit and helps maintenance teams identify weak points during inspection.
The Magnetic Core: The Heart of the Transformer
The core provides the magnetic circuit that couples the primary and secondary windings. It is built from thin grain-oriented silicon steel laminations, each coated with insulating varnish or an insulating oxide layer to limit hysteresis and eddy-current losses. The laminations are stacked to form three limbs, one per phase, with the windings of each phase installed on the same limb. Core quality directly affects no-load loss, which is why transformer specifications and standards place tight limits on core steel grade and stacking design.
Windings: Primary and Secondary Coils
The winding, also called the coil, is divided into the primary winding, connected to the supply, and the secondary winding, connected to the load. Both are wound from copper or aluminum conductor and insulated with high-strength insulating materials. In a three-phase oil-immersed transformer, the cylindrical high-voltage and low-voltage windings of each phase are concentrically arranged on the same core limb. Copper windings give lower losses and higher overload capacity, while aluminum windings reduce cost and weight; the choice is a specification decision based on loss capitalization and budget.
Oil Tank and Oil Conservator
The core and windings are installed inside the oil tank and completely immersed in mineral transformer oil, which provides electrical insulation and heat transfer. Larger units add radiators or heat pipes on the tank exterior to increase the cooling surface. The oil conservator is a cylindrical vessel mounted horizontally above the tank and connected to it by piping; it accommodates the expansion and contraction of the oil as temperature changes, and is typically sized at roughly 8 to 10 percent of the main tank oil volume. Modern units use a capsule-type conservator in which a flexible rubber bag isolates the oil from ambient air, so that the oil never absorbs moisture or oxygen from the atmosphere.
Explosion-Proof Protection and Bushings
If a serious internal fault occurs, the oil vaporizes rapidly and pressure inside the tank rises. The explosion-proof device, a rupture disc or frangible vent mounted on the tank, releases the pressure before the tank bursts, and the associated relay initiates an alarm or trips the breaker. The bushing is the insulated terminal that carries the winding leads through the tank lid; it both insulates the guide rod from the tank and holds the rod in position. High-voltage and low-voltage bushings are sized for their respective voltage classes, and their porcelain or composite sheds provide the necessary creepage distance.
Component Overview Table
| Component | Function | Typical failure symptom |
|---|---|---|
| Silicon steel core | Magnetic circuit, low no-load loss | Overheating, high no-load current |
| Copper or aluminum windings | Voltage transformation | Short circuit, insulation breakdown |
| Corrugated oil tank | Contains oil, absorbs thermal expansion | Oil leakage at welds or corrugations |
| Capsule-type conservator | Compensates oil volume, blocks air contact | Capsule rupture, moisture ingress |
| Explosion-proof device | Pressure relief on internal fault | Rupture disc blown after fault |
| Bushings | Insulated terminals through the tank lid | Tracking, flashover, oil seepage |
Why the Sealed Design Matters in Service
Transformer oil degrades when it absorbs moisture and oxygen: the dielectric strength drops and sludge forms, reducing cooling and insulation performance. The sealed corrugated tank and capsule conservator together prevent that degradation, which is why sealed units require less oil maintenance than older open-breather designs. For procurement, this means checking the sealing arrangement, the oil preservation system type, and the expected oil maintenance interval when comparing quotations.
Frequently Asked Questions
Q1. Why is transformer oil used instead of air insulation?
Oil provides both insulation and cooling in one medium. Its dielectric strength is far higher than air, and circulation through the tank and radiators removes heat from the core and windings. This allows a much more compact and powerful transformer than an equivalent air-insulated design.
Q2. What is the function of the corrugated tank wall?
The corrugations act as elastic bellows. As the oil heats and expands, the tank walls flex slightly to absorb the volume change, keeping the transformer fully sealed without a free oil surface exposed to air.
Q3. What does the oil conservator do?
It provides a reservoir that absorbs oil volume changes with temperature. In a capsule-type conservator, a rubber bag separates the oil from atmospheric air, preventing moisture absorption and oxidation while the oil level in the main tank stays constant.
Q4. How does the explosion-proof device protect the transformer?
During a severe internal fault, oil vaporizes and tank pressure spikes. The rupture disc or frangible vent opens at a calibrated pressure, releasing the pressure before the tank can burst, while the protective relay simultaneously signals an alarm or trips the circuit breaker.
Q5. What should a buyer check on the nameplate of a three-phase oil-immersed transformer?
Verify the rated power in kVA, voltage ratio, vector group, impedance voltage, no-load and load losses, cooling class, oil type and total oil volume. These parameters determine whether the unit matches the network requirements and the enclosure or plinth preparation on site.