Oil Immersed Transformer: Complete Technical Guide to Oil-Type Transformers

An oil immersed transformer is a power transformer in which the windings and core are immersed in insulating mineral oil that simultaneously provides dielectric insulation and heat dissipation for the active parts. This guide explains the two main construction families of oil-type transformers — hermetically sealed units and expansion tank (conservator) units — along with their internal components, cooling methods, protection features, and the criteria buyers should use to select the right unit for medium voltage distribution networks. The content is written for procurement engineers, EPC contractors, and utility buyers sourcing transformers for projects across the Middle East and Gulf region including Egypt and Saudi Arabia, North Africa, CIS countries, and Sub-Saharan Africa, where humid coastal climates, dusty inland environments, and maintenance-limited rural sites all influence the choice of transformer construction.

Why Oil Is Used as the Insulating and Cooling Medium

In an oil immersed transformer, the oil performs two functions at once: it insulates the live conductive parts from each other and from the tank, and it transfers heat generated by copper losses and core losses away from the windings toward the tank walls, where it dissipates into the surrounding air. The oil used is a high-grade mineral insulating oil, specially refined to deliver strong dielectric strength, good thermal stability, and efficient heat transfer. Because the oil is in direct contact with the winding insulation paper, its condition — moisture content, oxidation level, dielectric strength — has a direct impact on the transformer’s insulation life. This is the core reason why the two construction types described below (hermetically sealed and expansion tank) exist: each represents a different strategy for managing the oil’s exposure to the outside atmosphere over the transformer’s service life.

Two Main Types of Oil-Type Transformers

Hermetically Sealed Transformer

A hermetically sealed transformer is filled with transformer oil under vacuum and then completely sealed from the external atmosphere. There is no expansion tank (conservator) open to outside air; instead, the tank is built with flexible corrugated wave walls that expand and contract to absorb the thermal expansion and contraction of the oil as load and ambient temperature change. Because the oil never contacts outside air, humidity, dust, or oxygen, the risks of oil oxidation, moisture absorption, and progressive insulation degradation are significantly reduced. This is currently the most widely used oil-filled transformer design in distribution networks, valued for its lower cost, high reliability, and strong performance in outdoor and harsh operating environments.

Because the system is fully sealed, hermetically sealed transformers do not need an expansion tank, a Buchholz relay, or a silica gel breather — components that conventional open-oil designs require to manage moisture ingress.

Expansion Tank (Conservator) Type Transformer

In an expansion tank type transformer, the main tank is connected to a conservator tank that sits above it and remains open to the atmosphere through a breathing path. As the oil heats up during operation, it expands and flows into the conservator; as it cools, the oil level drops back. Because this is an open system, it requires additional protective components — a Buchholz relay to detect gas accumulation and internal faults, and a silica gel breather to dry the air drawn in and out of the conservator as oil volume changes. Despite these protective devices, moisture and contaminants can gradually enter the oil over the transformer’s life, which can reduce insulation quality and shorten service life compared with a fully sealed design.

Structure and Main Components

A hermetically sealed oil immersed transformer is built from the following core components:

• Copper or aluminum windings
• Grain-oriented silicon steel core
• High-quality mineral transformer oil for insulation and cooling
• Corrugated wave-wall tank (absorbs oil expansion without a conservator)
• Off-load or on-load tap changer
• High voltage and low voltage cable bushings
• Electronic protection relay, typically fitted above 400 kVA
• Pressure relief valve
• Oil temperature indicator with contacts
• Wheels for transportation

Because the hermetic design eliminates the need for an expansion tank, Buchholz relay, and silica gel breather, the overall structure is simpler, which reduces routine maintenance requirements and improves long-term operational reliability.

Cooling Methods

Oil immersed transformers rely on natural or forced circulation of oil and air to remove heat generated by copper and core losses. Hermetically sealed transformers are typically cooled using the ONAN (Oil Natural Air Natural) method, where the corrugated wave walls of the tank increase the effective cooling surface area and allow natural convection of surrounding air to dissipate heat. For higher power ratings or increased load conditions, ONAF (Oil Natural Air Forced) cooling, which adds fans to improve airflow across the tank surface, can be applied to boost heat transfer capacity.

Effective cooling is not a secondary consideration — if internal temperature rises too high, it can damage the paper insulation system and degrade oil quality, directly shortening transformer life.

Cooling Code Reference

Comparison Table: Hermetically Sealed vs Expansion Tank Type

Typical Power Ratings and Applications

Hermetically sealed transformers are commonly manufactured up to 2500 kVA for distribution network applications. They are widely preferred in concrete kiosk substations up to 1600 kVA. These ratings make the design suitable for:

• Public and private distribution substations
• Concrete kiosk-type prefabricated substations
• Industrial plants requiring outdoor, low-maintenance transformers
• Utility networks in humid coastal or dusty inland climates
• Remote or maintenance-difficult project sites where a Buchholz relay and breather servicing routine is impractical

Protection and Monitoring

Above 400 kVA, hermetically sealed transformers are usually equipped with an electronic protection relay. This relay continuously monitors gas accumulation, oil temperature, and internal tank pressure, and issues alarm and trip signals to the medium voltage switchgear to protect the transformer against developing faults before they escalate into failures. This monitoring layer, combined with the pressure relief valve and oil temperature indicator built into the standard component list, gives buyers a layered protection scheme without relying on the mechanical Buchholz relay used in conservator-type units.

Selection Criteria for Buyers

When specifying an oil immersed transformer for a distribution or industrial project, buyers should evaluate:

• Environmental exposure: humid, coastal, or dusty sites favor the hermetically sealed design because it isolates the oil from ambient moisture and contamination.
• Maintenance access: remote or low-maintenance-capacity sites benefit from the simpler hermetic structure that removes the breather and Buchholz relay servicing burden.
• Power rating range: confirm the required kVA rating fits within the manufacturer’s hermetically sealed product range, noting that this construction is commonly offered up to 2500 kVA.
• Substation type: for concrete kiosk substations, verify compatibility with ratings up to 1600 kVA, a common configuration in this segment.
• Cooling class: ONAN is standard; specify ONAF with fans if the load profile or ambient temperature demands additional forced-air cooling capacity.
• Protection scheme: confirm whether an electronic protection relay is included, particularly for units above 400 kVA.
• Winding material: copper or aluminum, selected based on load profile, cost target, and thermal performance requirements.
• Tap changer type: off-load or on-load tap changer, depending on whether voltage regulation must occur under energized conditions.

Efficiency and Service Life

Hermetically sealed oil-type transformers are widely regarded as the most popular choice for outdoor and industrial applications due to their robustness, high efficiency of approximately 99%, and long operational life. This efficiency level, combined with the reduced moisture and oxidation risk of the sealed design, gives utilities and industrial operators a dependable long-term asset with a lower total cost of ownership compared to open-oil designs requiring ongoing breather and relay maintenance.

Standards and Compliance

In Turkey, hermetically sealed transformers must comply with the TEDAŞ MLZ/99.032E technical specification. Buyers sourcing for export markets should confirm the applicable national or utility-specific technical specification for their project country, alongside the manufacturer’s own quality and testing documentation, since local grid codes and substation standards vary across the Middle East, Gulf, North Africa, CIS, and Sub-Saharan African markets.

Related guides

See also our guides on gas-insulated switchgear, ring main units and metal-clad switchgear.