Hermetically sealed transformer selection
Select an oil-immersed transformer from the actual load, system voltage, installation environment and site interfaces—not from a model number alone. This guide helps prepare the transformer specification for a new installation or a replacement unit.
Start with the project type
A new transformer and a replacement transformer begin with different information. Choose the path that matches the work on site.
New transformer specification
Define the transformer from the load schedule, system voltage, duty, installation conditions and project standards.
- Load schedule and single-line diagram
- Primary and secondary voltage
- Duty cycle and installation environment
Replacement transformer
Match the electrical duty and physical interfaces of the existing unit, then review any operating problems or required upgrades.
- Existing nameplate and test report
- Outline, foundation and terminal position
- Site photos and known operating issues
Transformer technical data required for selection
Complete data speeds up review, but an inquiry can start with the information already available.
Rated power and load profile
Required kVA, continuous or cyclic duty, starting current, overload and future load allowance.
Primary and secondary voltage
System voltage, output voltage, frequency, phase, vector group and permissible voltage variation.
Impedance and tapping
Required impedance, off-circuit or on-load tapping, tap range and regulation steps.
Ambient temperature and altitude
Maximum and minimum temperature, installation altitude, humidity, dust, salt mist and hazardous-area classification.
Installation and interfaces
Indoor or outdoor location, foundation, available space, cable or bus connection and terminal orientation.
Cooling and accessories
Cooling method, temperature indication, pressure protection, oil level monitoring and control-system contacts.
When hermetically sealed construction is a good fit
A hermetically sealed transformer uses a sealed oil system to limit contact between insulating oil and the outside atmosphere. The construction is evaluated against the electrical duty, temperature rise, tank design and site environment—not selected by the name alone.
Sealed oil system
For projects that require controlled oil preservation without direct oil-to-atmosphere breathing.
Demanding environment
For indoor or outdoor service where moisture, dust, salt mist or access conditions affect equipment configuration.
Project-specific interfaces
For installations where dimensions, bushings, protection devices and control contacts must match the surrounding system.
Send the data you already have
You do not need a finished transformer specification to begin. Send the available electrical and site information; the missing selection data can then be confirmed in sequence.
Contact engineeringEngineering review for new, replacement and retrofit transformer projects.- 01Single-line diagram or system drawing
- 02Load schedule and voltage data
- 03Existing transformer nameplate
- 04General arrangement or foundation drawing
- 05Site and installation photographs
- 06Project standards and accessory requirements
Common transformer selection questions
How is an oil-immersed transformer sized?
Sizing begins with the connected and operating load, duty pattern, starting current, diversity, expected growth and permissible temperature rise. Rated power should not be chosen from connected load alone.
What is needed for a replacement transformer?
At minimum, provide the existing nameplate, primary and secondary voltage, rated power, impedance, vector group, outline dimensions, foundation details and terminal positions. Site photos help confirm interfaces.
Can selection start without a complete data sheet?
Yes. A single-line diagram, nameplate, load list or site photograph is enough to start an engineering review. Missing parameters can be identified before a final specification is prepared.