Hermetically Sealed vs Conservator Type Transformers – Which to Specify

Oil expands when it heats and contracts when it cools. Every oil-immersed transformer must accommodate that volume change without admitting air, because oxygen and moisture are what age the insulation. Two constructions solve the problem differently, and the choice affects maintenance, service life and the accessories the transformer can carry.

How Each Works

Construction Difference

Comparison

The Breather

The breather is the component that determines whether a conservator transformer ages well or badly. As oil contracts on cooling, air is drawn into the conservator. The breather removes moisture from that air using silica gel or a similar desiccant.

When the desiccant is saturated it stops absorbing, and moisture passes into the oil. Water in oil reduces dielectric strength and accelerates the degradation of paper insulation, which is what ultimately ends a transformer's life.

• Silica gel changes colour as it saturates — typically orange or blue when dry, colourless or pink when spent
• Replacement or regeneration interval depends on climate and loading; humid environments consume desiccant faster
• Maintenance-free dehydrating breathers with automatic regeneration are available where site visits are difficult

Protection Differences

The Buchholz relay sits in the pipe between tank and conservator, and detects gas accumulation or oil surge — both indicators of an internal fault. It is one of the most effective protection devices available for an oil-filled transformer, and it is only possible on a conservator unit.

Hermetically sealed transformers use different protection:

• Pressure relief device — releases pressure in the event of a serious internal fault
• Sudden pressure relay — detects a rapid pressure rise, functionally similar in purpose to a Buchholz
• Oil temperature indicator with alarm and trip contacts
• Winding temperature indicator on larger units

The protection is adequate but less informative. A Buchholz relay accumulates gas over time and allows a slowly developing fault to be identified through gas collection and analysis. A sealed unit gives no equivalent early warning short of a full DGA sample — see Dissolved Gas Analysis (DGA).

Corrugated Wall and Radiator Construction

Hermetically sealed transformers accommodate oil expansion through elastic deformation of the tank. Two arrangements are used:

When to Specify Hermetically Sealed

• Remote or unmanned installations where routine maintenance visits are impractical
• Pole-mounted and pad-mounted units in distribution networks
• Humid, coastal or dusty environments where breather maintenance would be demanding
• Space-constrained installations where the additional height of a conservator is a problem
• Ratings up to around 2500 kVA where no on-load tap changer is required
• Where minimising lifetime maintenance cost is a priority

When to Specify Conservator Type

• Ratings above 2500 kVA, where sealed construction becomes impractical
• Where an on-load tap changer is required
• Where Buchholz protection is specified by the utility or the project standard
• Critical installations where oil sampling and filtering during service is part of the maintenance regime
• Where a maintenance team is already established on site
• Where the transformer will be subject to extended condition monitoring over a long service life

Practical Notes

• The tank type is fixed at manufacture. A sealed transformer cannot be converted to conservator construction, or the reverse, without substantial rework.
• Sealed units are filled and sealed under vacuum at the factory. Any site work that breaks the seal requires proper vacuum treatment to restore the insulation condition.
• Where a sealed transformer is specified for a high ambient temperature site, confirm the expansion capacity is calculated for the actual temperature range. The tank must accommodate the full oil volume change without excessive pressure.
• Conservator units for outdoor installation in cold climates need attention to breather selection — some desiccant types perform poorly below freezing.
• For both types, thermal performance and insulation life depend on the temperature the windings actually reach. See Transformer Insulation Thermal Classes and Temperature Limits.